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Keywords:

  • apoptosis;
  • autophagy;
  • deafness;
  • IGF-I;
  • miRNA;
  • notch signaling;
  • otic precursors;
  • SOX2

Abstract

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

This is a review of the biological processes and the main signaling pathways required to generate the different otic cell types, with particular emphasis on the actions of insulin-like growth factor I. The sensory organs responsible of hearing and balance have a common embryonic origin in the otic placode. Lineages of neural, sensory, and support cells are generated from common otic neuroepithelial progenitors. The sequential generation of the cell types that will form the adult inner ear requires the coordination of cell proliferation with cell differentiation programs, the strict regulation of cell survival, and the metabolic homeostasis of otic precursors. A network of intracellular signals operates to coordinate the transcriptional response to the extracellular input. Understanding the molecular clues that direct otic development is fundamental for the design of novel treatments for the protection and repair of hearing loss and balance disorders. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.


AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

The inner ear is a structurally complex sensory system formed by the organs responsible for hearing and balance. The inner ear is located inside the temporal bone and it is formed by soft canals and cavities, named the membranous labyrinth, filled by endolymph and encased by the bony labyrinth. Between the membranous labyrinth and the bony labyrinth there is a space filled by a different fluid the perilymph.

In mammals, a coiled structure, the cochlea, is responsible for hearing and contains the organ of Corti, the sensory receptor, where the mechanosensory hair cells transduce the sound stimuli and generate the electrochemical signal response that otic neurons will transmit to the brain (Raphael and Altschuler, 2003; Fig. 1). The cochlea is a complex and integrated system, the damage of a specific cell type can lead to the damage of other cochlear elements and to hearing loss. Genetic and environmental factors can damage cochlear cells causing deafness (Eisen and Ryugo, 2007; Liu and Yan, 2007; Raviv et al., 2010; Fetoni et al., 2011). The cochlea has three major functional parts: the lateral wall, the organ of Corti, and the spiral ganglion (Fig. 1). The lateral wall, with the spiral ligament and the stria vascularis, is essential to the normal physiology of hearing (Jin et al., 2007). The stria vascularis is responsible for endolymph production (Takeuchi et al., 2000), a specialized extracellular fluid with intracellular characteristics like high K+ and low Na+ and Ca2+ concentrations (Wangemann, 2006; Jin et al., 2008; Patuzzi, 2011). The melanocytes or intermediate cells of the stria vascularis play an important role to preserve the cochlear function in stress situations (Murillo-Cuesta et al., 2010), and their alteration causes deafness (Cohen-Salmon et al., 2007).

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Figure 1. Anatomy of the inner ear. A: Schematic view of the inner ear showing the cochlear and vestibular parts and the sensory areas, crista, macula, and organ of Corti (shadowed). B: Whole mount preparation of the cochlea showing the basal turn, the apex, the round window (RW), and the oval window (OW). Note the pigmented stria vascularis through the lateral wall (melanin granules at intermediate cell layers). Scale bar 0.5 mm. C: Low magnification of a midmodiolar section from a mouse cochlea. Note the scala vestibule (SV) limiting by the Reissner's membrane (RS) with the scala media (SM) where the auditory receptor is located (red boxes), the scala tympani (ST) with the basilar membrane (BM), and the spiral ganglion (SG, in the osseous Rosenthal's canal). The lateral wall (LW) is directly in contact with the osseous otic capsule. Scale bar 0.5 mm. D: The organ of Corti containing the neurosensorial cells (inner hair cell, IHC; outer hair cells, OHC), the nonsensorial cells (DC: Deiter cells; HC: Hensen cells; CC: Claudius cells; PC: pillar cells), the tectorial membrane (TM), the spiral limbus (SL), the basilar membrane (BM), the tunnel of Corti (asterisk), and the myelinated cochlear nerves fibers (CNF). Scale bar 50 μm (E and F) Synaptophysin (Syn, in E) labeling of presynapses in the IHC and efferent fibers arriving at the IHC and OHC, Neurofilament 200 kD (NF, in F) labeling of the afferent fibers of the organ of Corti and the synapse region. Scale bar 50 μm. (G) Phalloidin histochemistry of the organ of Corti, labeling F-actin in viable sensory epithelium (stereocilia and cuticular plate of hair cells, reticular lamina, and pillar cells). Scale bar 50 μm. H: Semithin section showing the cytoarchitecture of the spiral ganglion (SG). The inset shows electron microphotograph of the ganglionar cells, surrounded by the Schwann cells (SC). The most abundant neurons in the SG, the type I cells, present a myelin sheath, with external compact myelin (CM) and internal loose myelin (LM). Scale bar 5–0.1 μm (inset) and 30 μm. (I) Detail of the mouse lateral wall showing the stria vascularis with the marginal cells (MC) close to the scala media, the intermediate cells (IC), and the basal cells (BC). The spiral ligament (SpL) is the most lateral part that is close to the otic capsule of the cochlea. Scale bar 50 μm. J: Kir4.1 (KCNJ10, an inwardly rectifying K+ channel) expression in the stria vascularis of an aging mouse. Note the relative loss of expression in some patches in the stria (asterisks). This channel is related with the production of the endocochlear potential, thus with an important role in auditory physiology. Scale bar 50 μm. K: Na+-K+-ATPase expression in the stria vascularis, another functional marker of striatal healthiness that are related with ion homeostasis and auditory physiology (Patuzzi, 2011). Scale bar 50 μm. L and M: The sensory epithelium of the vestibular inner ear: the macula and the cristae gross anatomy. L: Cytoarchitecture of a semithin section of the macula (L) and cristae ampullaris (M) showing the morphological characteristics of these vestibular receptors. Note the arrangement of the hair cells (HC) with the stereocilia (asterisks), the supporting cells (SC), the basement membrane (arrows), the otoconial membrane (OM) with otolites, and the vestibular nerve fibers (VNF). Scale bar 50 μm. N and O: Detail of the macula (N) and cristae ampullaris (O) showing the myosin VIIa expression (red, labeling sensory hair cells) and neurofilament 200 kD expression (green, labeling macula nerve fibers). Arrows show the afferent calyx of type I hair cells.

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The organ of Corti is located within the cochlear duct or scala media laying on the basilar membrane along its entire length from the base to the apex of the cochlea. This organ consists of two types of functional hair cells and support cells (Kimura, 1975; Santi and Tsuprun, 2001). The hair cells are the receptors that transduce the mechanical stimulus to an electrochemical signal by means of stereocilia arranged in a “V or W”-shaped bundle (Hudspeth, 2008). There are two types organized in parallel rows, one row of inner hair cells (IHC) and three rows of outer hair cells (OHC) separated by the Tunnel of Corti. The stereocilia of OHC are embedded in the Tectorial Membrane, an extracellular component that covers the organ of Corti throughout the cochlea. Support cells, Deiter's cells, Hensen's cells, Claudius's cells, participate in regulating the ionic and nutrients homeostasis (Forge and Wright, 2002; Chang et al., 2008). Alterations in their functions are a frequent cause of hearing impairment (Lefebvre and Van de Water, 2000).

The spiral ganglion is located within the cochlear modiolus and it is formed by the cell bodies of bipolar neurons that connect the hair cells of the organ of Corti with the brain (Nayagam et al., 2011). The dendritic ends of spiral neurons innervate the hair cells, type I neurons are the most abundant subtype (95%) and innervate the IHC. Type II neurons innervate several OHC. The axons of spiral neurons leave the spiral ganglion and pass through the basis of the modiolus to form the cochlear division of the cochleo-vestibular nerve towards the cochlear nuclei at the brainstem. Sound information progresses in a complex multisynaptic, parallel, and ascendant pathway from the cochlea through the brainstem nuclei to the auditory cortex (Webster et al., 1992; Fig. 2). The cochlear nuclei, olivar complex, nucleus of lateral lemniscus, inferior colliculus, and medial geniculate complex are part of the rely nuclei that transmit the information to the cortex, where the auditory information is processed in multiple areas. The tonotopic organization present at the cochlea is maintained along the pathway. In addition, neurons from the brainstem (superior olivary complex) also contact hair cells in a centrifugal control mechanism of the auditory pathway.

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Figure 2. The auditory pathway. A: Schematic view of the relay nuclei of the multisynaptic and complex ascendant auditory pathway. B: Example showing the ABR register of normal (+/+) and deaf mutant (−/−) mice. The auditory brainstem response (ABR) reflects the evoked potential response of auditory activity in the auditory nerve and subsequent fiber tracts and nuclei within the auditory brainstem.

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The vestibular system contains the balance receptors with highly specialized mechanoreceptor hair cells (Goldberg, 1991; Fig. 1). Three sensory organs called “cristae” located at the base of the semicircular canals are responsible of balance perception, whereas the two “maculae” of the sacculus and utriculus detect linear and angular acceleration (Highstein and Fay, 2004).

To sum up, the mammalian inner ear contains six sensory patches: the coiled cochlear duct or organ of Corti, which is the auditory receptor, the maculae, and the three cristae corresponding to the three semicircular canals are responsible of balance perception. Sensory patches are connected to the brain nuclei by the fibers of the spiral and vestibular ganglions that form the eighth cranial nerve.

DEVELOPMENT OF THE VERTEBRATE INNER EAR

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

The sensory organs of the inner ear have a common embryonic origin at the otic placode. The cells that constitute the adult inner ear originate from the embryonic otic placode with the exceptions of the melanocytes of the stria vascularis and the ganglionar Schwann cells that are of neural crest origin (D'Amico-Martel and Noden, 1983; Rubel and Fritzsch, 2002; Fritzsch et al., 2011). Cranial placodes are regions of the ectoderm that generate a wide variety of cell types, including elements of the sense organs and most of the sensory neurons of the cranial nervous system. The different placodes derive from a common preplacodal region, which surrounds the neural plate (Streit, 2007). Before the otic placode becomes visible, the ectodermal cells that will form it undergo a genetic program to express preplacodal transcription factors as the Dlx family, Sox9a and Foxi1 (Ekker et al., 1992; Groves and Bronner-Fraser, 2000; Solomon and Fritz, 2002; Liu et al., 2003). There is also induction from the underlying mesenchyme that produces growth factors of the fibroblast growth factor family (FGF10, FGF19, or FGF15 depending on the species) and from the hindbrain, which secretes FGF3 (Maroon et al., 2002; Léger and Brand, 2002; Wright and Mansour, 2003).

After induction, the otic placode invaginates beneath the surface ectoderm to form the otic cup, which pinches off in birds and mice, or cavitates in fish to produce the otic vesicle or otocyst (Fig. 3; Haddon and Lewis, 1996; Sanchez-Calderon et al., 2007). As development continues, the otic vesicle undergoes reshaping processes that modify the simple epithelial sac and transform it into a sophisticated fluid-filled labyrinth composed by interconnected chambers where the hair and supporting cells from the different sensory epithelia are located forming a complex cell mosaic (Anniko, 1983; Kelly and Chen, 2009). Simultaneously to these changes, the inner ear recruits nearby mesenchymal cells that will form the bony capsule that surrounds the labyrinth (Chang et al., 2002).

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Figure 3. Inner ear development of vertebrates. A: Scheme of the early development of the vertebrate inner ear. The inner ear originates from the otic placode, a thickening of the head ectoderm that invaginates to form the otic cup and separates from the ectoderm to form the otocyst or otic vesicle, an ellipsoid-shaped structure able to generate most of the cell types of the adult inner ear. In parallel to otic vesicle shaping, the neuronal precursors (yellow) delaminate from the otic cup ventral region, migrate and differentiate to produce the postmitotic otic neurons (red) that form the acoustic-vestibular ganglion. (Modified from Ref. Varela-Nieto et al., 2004) B: Schematic drawing of the inner ear of adult vertebrates. From left to right: zebrafish, birds, and mouse. The origin and early developmental stages of the inner ear are very similar in vertebrates, although ear development in mammals is delayed with respect to that of birds and, in zebrafish, the otic cup cavitates to form the otocyst. All vertebrates show the general vestibular (light purple) and auditory sensory components (dark purple), but with considerable differences, of which the most notable is that the auditory region is a coiled cochlea in mammals that contain the organ of Corti, and a straight cochlear duct in birds called the basilar papilla. Zebrafish auditory function is carried out by the saccule and the lagena. Abbreviations: A: anterior; ac: anterior crista; bp: basilar papilla; co: cochlea; D: dorsal; l: lagena; lc: lateral crista; M: medial pc: posterior crista; s: saccule; u: utricle.

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Otic neuroblasts delaminate from the otic cup and otic vesicle to form the acoustic-vestibular ganglion (AVG), which contains the neural precursors of the spiral auditory and vestibular ganglia that form a single ganglion at early developmental stages. Otic neurons extend their processes connecting the sensory epithelium to the brainstem nuclei through the eighth cranial nerve (Hemond and Morest, 1991; Adam et al., 1998; Rubel and Fritzsch, 2002; Figs. 2, 3A).

The general structure of the inner ear is comparable in all vertebrates (Fig. 3B) but the rate at which the inner ear develops is highly variable among species. The mechanisms coordinating cell proliferation and survival with differentiation remain poorly understood during embryonic development and also in postnatal regeneration. Here, we will review the biological processes and the main signaling pathways required to generate the different otic cell types. The contribution of cell survival, apoptosis, proliferation, differentiation, and autophagy will be discussed, with particular emphasis on the actions of insulin-like growth factor I (IGF-I). There are excellent recent reviews focused on complementary aspects of otic sensory development (Kelley, 2007; Kelly and Chen, 2009; Kwan et al., 2009; Chatterjee et al., 2010; Fritzsch et al., 2011).

THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

Multipotent progenitor cells from the otic epithelia will generate the three lineages of prosensory (the future hair and supporting cells), proneural (future auditory and vestibular neurons), and nonsensory (other otocyst-derived cells) cells (Fig. 4). A common origin for neuronal and sensory cells has also been proposed (Kelley, 2006; Raft et al., 2007). Neural specification starts with the activation of a transcriptional program of expression of proneural genes, being Neurog1 required for the specification of the neuronal fate. Gain of function studies have demonstrated the formation of ectopic neurons in regions where Neurog1 is over-expressed (Perron et al., 1999; Kim et al., 2001), whilst Neurog1 inactivation causes loss of sensory ganglia neurons and also of hair cells (Ma et al., 2000), suggesting that at least in some areas exist common progenitors for both cell lineages. Accordingly, SOX2, a marker of the prosensory region is also expressed in the proneural domain. A key common feature of the SOX family of transcription factors is their ability to maintain self-renewal state and pluripotency of progenitors (Wegner and Stolt, 2005; Takahashi and Yamanaka, 2006). In the zebrafish inner ear, however, SOX2 is not required for the initial development of hair cells (Millimaki et al., 2010). Lineage tracing in the chicken otocyst showed that although some neuroblast or sensory cells can be derived from a common progenitor, this is not very frequent. Therefore, these data indicate that most otic neurosensory cells have either a proneural or a prosensory origin (Satoh and Fekete, 2005; Kelley, 2006). Further work is still required to understand this very early stage of cell fate specification that may vary among species.

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Figure 4. Cell specification in the otic vesicle. Three cell lineages are generated from the otic vesicle: prosensory, neural, and nonsensory. Auditory and vestibular neurons are generated from a common neuronal progenitor. Hair and supporting cells derive from the prosensory progenitors. The drawing summarizes some of the factors required to produce cellular diversity. Among others to be identified, SOX2, Islet-1, the Notch pathway, Eya1, BMP4, and the miR-200 miRNA family are required to generate the pool of prosensory cells. The Notch signaling pathway is also playing a role in the differentiation of hair cells. Ids release the interacting partners of Atoh1, and the Ahoh1 positive cells will express JAG2 and Dll1, which by activating the Notch signaling targets Hes1 and Hes5 in the surrounding cells, direct the supporting cell fate. The expression of Neurog1, Islet-1, and SOX2 lead the neural cell fate. Tbx1 is required to delimit the neurogenic domain in the otocyst. The sequential activation of the proneural genes NeuroM and NeuroD will promote the formation of the vestibular and auditory neurons. IGF-I/IGF1R signaling modulates cell survival and proliferation, although a direct role in cell fate specification has not been shown yet.

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The Notch Signaling Pathway

Several studies have shown that Notch signaling participates in the earliest induction stages of the prosensory domain. Members of the canonical Notch pathway Notch1, Lunatic fringe (Lfng), Delta-like1 (Dll1), and Hes5 are differentially expressed in the otic placode/cup (Adam et al., 1998; Cole et al., 2000; Groves and Bronner-Fraser, 2000; Daudet and Lewis, 2005; Jeon et al., 2011). The nonprosensory region expresses Hes1 and the Notch ligand Jagged1 (Jag1), but not Lfng and Dll1. In the chicken basilar papilla, ectopic activation of Notch signaling induced sensory patches in nonsensory regions of the cochlea (Daudet and Lewis, 2005). Accordingly, the pharmacological inhibition of Notch in vitro during prosensory specification blocks the prosensory phase of development in the mammalian cochlea (Hayashi et al., 2008). In addition, the conditional activation of the Notch pathway in otic regions at early mouse developmental stages induces prosensory markers in the whole otic epithelium (Hartman et al., 2010). These results support a model where early activation of Notch promotes the prosensory character in specific regions of the developing otocyst. Nonetheless, there is still some controversy. Basch et al., (2011) showed that ectopic activation of Notch signaling did not induce ectopic sensory patches in nonsensory regions of the cochlea suggesting that Notch signaling is not sufficient for prosensory specification in the mouse cochlea. In addition, they show that conditionally inactivation of RBPjk, the mediator of Notch signaling, is not followed by a reduction on the prosensory markers, but instead by a shorter life of hair and supporting cells.

Bone Morphogenetic Protein 4 (BMP4)

BMP4 is a member of the TGFβ superfamily of secreted signaling molecules that has been proposed to play a key role in the specification of prosensory patches (Oh et al., 1996; Wu and Oh, 1996; Cole et al., 2000). Although BMP4 expression is in agreement with an inductive sensory role, functional in vitro studies have provided opposing results (Li et al., 2005; Pujades et al., 2006). This controversy evidences the complexity of BMP4 actions in different developmental scenarios.

The Fibroblast Growth Factor (FGF) Family

The FGF family is composed by a large number of ligands and four receptors (FGFR) implicated in the regulation of cell differentiation, proliferation, growth, motility, and survival (Wright and Mansour, 2003). FGF signaling plays a key role during vertebrate inner ear development and participates in the early induction of the otocyst (Schimmang, 2007), in prosensory specification (Pirvola et al., 2002; Sanchez-Calderon et al., 2007; Millimaki et al., 2007), otic neurogenesis and neuritogenesis (Nicholl et al., 2005; Wei et al., 2007), and in pillar cell differentiation (Doetzlhofer et al., 2009; Sanchez-Calderon et al., 2010). FGF ligands present redundant actions and complex interactions, therefore a combination of experimental approaches and the study of mouse, chicken and zebrafish animal models are being used to further understand and unravel FGF actions in inner ear development (Kelly and Chen, 2009).

The HMG-Box Transcription Factor SOX2

The high mobility group (HMG)-box transcription factor SOX2 is a marker of the prosensory region, as well as of the otocyst proneural domain (Fig. 5). Studies in the chicken embryo evidenced that inductive signals regulate directly SOX2 expression in the neural tube and that SOX2 is responsible for neural fate acquisition in proliferating precursors (Rex et al., 1997; Bylund et al., 2003; Graham et al., 2003). Early in development during otic vesicle stages, SOX2 and other SOX proteins such as SOX3 are expressed by proliferating cells in the prosensory domain. SOX2 wide expression in the prosensory domain becomes restricted to the supporting cells as development continues (Kiernan et al., 2005; Neves et al., 2007). Indeed, several genes that are initially expressed in the prosensory domain are afterwards restricted to the supporting cells (Jag1, Lfng, and p27kip). These spatiotemporal patterns emphasize the different and even opposing functions that certain factors may have at different developmental stages, which greatly difficult the identification of specific prosensory markers (Kelley, 2007).

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Figure 5. Spatiotemporal expression patterns of SOX2 and B-RAF during vertebrate inner ear development. In early development, SOX2 expression (red throughout the figure) marks, in chicken and mouse, the prosensory and the sensory epithelia. SOX2 is initially expressed in all prosensory and sensory regions (AD, A′ and B′), later the expression in hair cells diminishes (C′ and D′) and is restricted to supporting cells (boxed area in E′). B-RAF (green throughout the figure) although shows a basal expression at very early developmental stages in most tissues, is expressed in abundance in sensory regions of chicken and mouse (A–D, A′ and B′). B-RAF is also highly expressed in hair cells (E and F, C′ and D′ asterisk and arrows) although the expression in the mouse OHC diminishes at the postnatal P30 stage (boxed area in E′, asterisk and arrows). While B-RAF is intensively expressed in otic neurons of the AVG and the SG, SOX2 is expressed in nonoverlapping regions of glial cells in chicken (B and D) and mouse (F′, arrows in the boxed area). Both SOX2 and B-RAF are also expressed in the vestibular component in nonoverlapping regions (B-RAF higher expression is found in hair cells, while SOX2 is abundantly expressed in supporting cells. The boxed areas show higher magnifications of the selected regions. The bracket in B′ indicates the sensory region. The bracket in G′–J′ indicates the differential expression of B-RAF and SOX2 in the macula. Abbreviations: AVG: acoustic-vestibular ganglion; bp: basilar papilla; IHC: inner hair cells; OHC: outer hair cells; ms, macula sacculi; mu: macula utriculi; sc: supporting cells; sg: spiral ganglion. Orientation: D, dorsal; M, media. Scale bars: 200 μm applies to A′ and C′; 100 μm applies to A–E, E′, and G′–I′: 50 μm applies to B′ and F′; 25 μm applies to D′ and to E′ inset. (Partially reproduced from Ref. Magariños et al., 2010).

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Two mutant mice deficient for Sox2 have been described, Light coat and circling (Lcc) and Yellow submarine (Ysb). Both mutants show defects in hearing and balance (Kiernan et al., 2005), the cochleae of Lcc mice lack neurons in the spiral ganglion (Puligilla et al., 2010) and neither hair cells nor supporting cells differentiate (Kiernan et al., 2005). In addition, mutations in human SOX2 cause sensorineural deafness (Hagstrom et al., 2005). Due to the missing expression of Sox2 in Jag1 mouse mutants, an interesting hypothesis is that the Notch pathway element Jag1 could induce the expression of Sox2 (Dabdoub et al., 2008).

Insulin-Like Growth Factor I

IGF-I belongs to the family of polypeptides of insulin that plays a central role in embryonic development and adult nervous system homeostasis by endocrine, autocrine, and paracrine mechanisms (Murillo-Cuesta et al., 2011, 2012). IGF-I is secreted by the developing chicken otocysts and it is expressed in the mouse inner ear throughout development (Camarero et al., 2002; Sanchez-Calderon et al., 2010). Igf1r is expressed in the sensory patches of otocysts from HH19 chicken (Aburto et al., 2012) and E15.5 mouse (Sanchez-Calderon et al., 2010) embryos. One of the earliest cell fate determination steps in the specification of the proneurosensory field is the expression of the bHLH proneural gene Neurog1 (Adam et al., 1998; Fritzsch et al., 2010). Neurog1 activates the proneural genes NeuroD and NeuroM, all have the potential to activate the expression of Igf1 and Igf1r genes. An appealing hypothesis is that upon prosensory specification by other growth factors and morphogenetic proteins, Igf1 expression is upregulated by proneural genes to master early neurogenesis. IGF-I actions and IGF1R expression are mediated by an intracellular signaling network (Murillo-Cuesta et al., 2011, 2012; Fig. 6), and precedes that of neurotrophins and TrkA expression in neurons (Li et al., 2009). Indeed, otic neurons gradually reduce their expression of the Igf1r, while they increase the neurotrophin receptor TrkC (Aburto et al., 2012), suggesting that IGF-I is a key trophic factor during the otic neuronal progenitor phase of early inner ear development.

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Figure 6. Biological programmes that generate otic cell diversity. AC: Programmed cell death in the developing inner ear. Apoptosis occurs in vivo in selected regions (arrows) of the developing otic vesicle (C). Cryostat section of a HH18 chicken embryo visualized by TUNEL (green throughout the figure). Blocking apoptosis in organotypic cultures of otic vesicles with BOC caused a reduction in the AVG size (arrow) and the otic epithelium appeared thickened (arrowhead, A and B). Otic vesicles were isolated from HH18 chicken embryos, made quiescent, and cultured for 20 hr in serum-free culture medium without additions (0S, A) or in the presence of BOC (50 μM; B). (DF) Growth factors are required to induce the survival of specific cell populations in the otic vesicle. Otic vesicles were culture without additions 0S (D), IGF-I (10 nM; E) or in the presence of LY (25 μM; F). TUNEL-levels decreased markedly in the presence of IGF-I and increased dramatically with the PI3K/AKT inhibitor. GI: Cell proliferation of otic progenitors is required for correct morphogenesis and AVG neurogenesis. Proliferation was measured by the incorporation of BrdU (red) during 1 hr. Cultured otic vesicles in 0S medium (G), with IGF-I (10 nM; H), or in the presence of Sor (5 μM; I) showed that IGF-I promoted otic proliferation, whilest Sor impaired BrdU incorporation. Consequently, the size of Sor-treated otic vesicles was severely reduced. Reproduced from Ref. Magariños et al., 2010. JL: IGF-I drives otic neurogenesis by adjusting the neuroblast/neurons ratio. Otic vesicles cultured in 0S medium (J), containing IGF-I (10 nM; K) or LY (25 μM; L) were double-immunostained for TuJ-1 (red) and Islet-1 (green). Islet-1-positive population increases in the presence of IGF-I (J–K, dashed areas). In the presence of LY, there are fewer Islet-1-positive neuroblasts in the AVG, while the remaining AVG presents generalized TuJ-1 expression (L) Reproduced from Ref. Aburto et al., 2012. MO: Autophagy is involved in inner ear development. HH18 cultured otic vesicles were incubated 20 hr in the 0S condition (M) or with the 3-MA (10 mM; N) and then cultured with An-V (red). Apoptotic cell death was visualized by TUNEL (green) and An-V, an “eat me” flag for the non-professional macrophages. When autophagy is inhibited (N), TUNEL labeling increases and An-V is reduced, suggesting that apoptotic cell clearance is impaired. The otocysts present aberrant features evidenced by TuJ-1 labeling (red; O). Representative images are shown that were obtained from compiled confocal microscopy projections of the otic vesicles. Orientation: AN, anterior; DO, dorsal. Scale bar, 150 μm. Abbreviations: An-V, Annexin-V; B AVG, Acoustic-vestibular ganglion; BOC, Boc-D-FMK (pan-caspase inhibitor); LY, LY294002 (PI3K/AKT inhibitor); Sor, Sorafenib (RAF-MEK-ERK inhibitor); 3-MA, (3-methyladenine).

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BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

Hair and supporting cells are originated after the specification of the prosensory domain (Driver and Kelley, 2009). During early development, it seems to be a default program by which the first cells specified are the hair cells that in turn, by lateral inhibition, block their fate to neighbor cells. Atoh1, a basic helix–loop–helix (bHLH) transcription factor is a key player in hair cell production and development. Although its expression pattern is somehow controversial (Bermingham et al., 1999; Lanford et al., 2000; Chen et al., 2002), loss and gain of function studies have proven that Atoh1 is necessary and sufficient to induce hair cell formation in sensory and nonsensory cells (Bermingham et al., 1999; Zheng and Gao, 2000; Kawamoto et al., 2003; Woods et al., 2004; Jones et al., 2006). It is interesting to note that the solely expression of Atoh1 is able to generate hair cells in nonsensory patches, suggesting that the sensorial competence is not restricted to certain regions of the otic epithelium (Kelley, 2006).

Before hair cell formation starts there is a broad expression of a family of proteins, the Ids (Jones et al., 2006) that prevent hair cell differentiation by the inactivation of Atoh1. Ids are inhibitors of differentiation that contain a HLH domain that competes with Atoh1 for its dimerization partners E47 and E2A. The Ids do not have a DNA binding domain, but they can sequester the factors required by Atoh1 to form a functional heterodimer (Norton, 2000). The expression of the Id family is reduced in hair cells as development continues, consequently releasing the factors necessary for Atoh1 activity, and thus allowing the differentiation of hair cells (Jones et al., 2006). Once hair cells have differentiated, the surrounding cells become supporting cells by a lateral inhibition mechanism. The Notch pathway that had had a previous role in prosensory specification is also implicated in individual cell differentiation. The Atoh1 positive cells begin to express two Notch ligands Jagged2 (Jag2), and Dll1 (Lanford et al., 1999; Morrison et al., 1999; Hartman et al., 2007). This leads to Notch1 activation and to intracellular expression of Notch targets such as Hes1, Hes5, Hey1, and Hey2 in the future supporting cells (Lanford et al., 2000; Zheng et al., 2000; Zine et al., 2001; Murata et al., 2006; Hayashi et al., 2008; Li et al., 2008; Doetzlhofer et al., 2009). There are also other factors that act as Atoh1 antagonist such as SOX2 and Prox1 (Zheng et al., 2000; Dabdoub et al., 2008; Doetzlhofer et al., 2009).

In summary, Notch activity through evolution is required for sensory development to specify the prosensory regions in the otic cup and otocyst, and later to promote cell type differentiation to hair and supporting cells (Eddison et al., 2000; Daudet and Lewis, 2005).

The Role of MicroRNAs in Inner Ear Development

MicroRNAs (miRNA) are small noncoding RNA, which bind to the three untranslated region of target mRNA to suppress their translation (Kloosterman and Plasterk, 2006). miRNA have been implicated in multiple biological processes across phyla including development and disease (Bartel, 2009; Lewis and Steel, 2010). miRNA started to be an inner ear topic when the expression of the miR-183 family (composed by miR-182, miR-96, and miR-183) was reported in the inner ear of zebrafish embryos (Wienholds et al., 2005). This family is highly conserved throughout evolution and it is expressed in ciliated neurosensorial organs (Pierce et al., 2008). These miRNA are expressed in hair cells in chicken, and mouse, and in lower levels in mouse sensory neurons of acoustic and vestibular structures (Darnell et al., 2006; Weston et al., 2006; Li and Fekete, 2010). Indeed, mutations in miR-96 cause hearing loss in mice and men (Lewis et al., 2009; Mencía et al., 2009). The dynamic expression of the miR-183 family in the mouse begins at the otocyst stage and has the higher expression in differentiating hair cells, suggesting that this family is involved in hair cell differentiation and maturation (SacheLi et al., 2009). Another miRNA family, the miR-200 (miR-200a, miR-200b, miR-200c, miR-141, and miR-429) has been associated to inner ear prosensory specification. The miR-200 family is expressed in the inner ear epithelia of zebrafish, chicken, and mouse (Wienholds et al., 2005; Darnell et al., 2006; Weston et al., 2006). Because of their known targets and interacting pathways, it has been suggested that it plays a key role in establishing the prosensory epithelial domains (Soukup, 2009).

The importance of miRNAs in the inner ear is further evidenced by the conditional deletion of the enzyme Dicer specifically in the otic placode. These mutants show a severe phenotype similar to that found in the Sox2 null mouse, with profound defects in inner ear neurogenesis and sensory epithelial histogenesis (Kiernan et al., 2005; Friedman et al., 2009; Soukup et al., 2009). The emerging role of miRNAs in inner ear development suggests that they may be of use in novel therapeutic strategies aimed to hair cell regeneration (Beisel et al., 2008).

DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

The AVG contains cells derived from the otic epithelia that will transit through different stages to become postmitotic neurons finally. Otic neurogenesis can therefore be separated into different cellular stages that are characterized by the distinct expression of a combination of molecular markers. After the regionalization of the otic cup/vesicle by the expression of a defined set of genes including among others, Neurog1, LFng, Dl1, Sox2, and Sox3 that specify the neural fate, neuroblasts migrate from the proneural region of the otic cup toward the closely mesenchymal space. The expression of the proneural genes NeuroD and NeuroM define the following stage in neural differentiation: the epithelial neuroblast population (Chae et al., 2004; Sanchez-Calderon et al., 2007). This population will migrate in a process that has been intensely studied in chicken and mouse embryos (D'Amico-Martel and Noden, 1983; Alvarez et al., 1989; Hemond and Morest 1991; Davies, 2007, 2011). Migration begins at the otic cup stage and reaches its maximal rate in the early otic vesicle stage (Figs. 3A, 7A). The new formed ganglionic neuroblast population can be identified by their location, round shape, and by the expression of a defined set of transcription factors, neurofilaments, and neurotrophins receptors. Including the transcription factor Islet-1 (Hobert and Westphal, 2000), which begins to be expressed before migration by epithelial neuroblasts (Adam et al., 1998; Camarero et al., 2003; Li et al., 2004). Ganglionic neuroblasts retain the capacity to undergo cell division to expand their population and, accordingly, express receptors for IGF-I and cell cycle activators as FoxM1. Once neuroblasts exit from the cell cycle, they differentiate into postmitotic neurons that begin to project their processes toward their peripheral and central targets (Whitehead and Morest, 1985; Fekete and Campero, 2007). The immature post-mitotic neurons show a reduced expression of early neural markers, and start to gradually express another set of genes related to neuritogenesis and cell cycle exit, as type III tubulin (TuJ1), G4, and the cyclins inhibitor p27Kip (Sanchez-Calderon et al., 2007, 2010). Finally, the mature otic neurons generate action potentials, express synaptic receptors and neurotransmitters, and reach the most differentiated and mature state (Raphael and Altschuler, 2003). Mature neurons express a whole set of genes related to neuronal trophic support as the neurotrophins receptors TrkB and TrkC (Pirvola et al., 1997; Brumwell et al., 2000; Kim et al., 2001).

Autophagy in the Developing Inner Ear

Autophagy is a self-degradative process of the cellular cytosolic constituents that is crucial for balancing sources of energy in response to different extracellular stimuli and for preventing the accumulation of misfolded proteins or damaged organelles (Qu et al., 2007; Glick et al., 2010; Ravikumar et al., 2010). Autophagy in vertebrates has been shown to have key roles during development (Levine and Klionsky 2004; Mizushima and Levine 2010; Montero and Hurlé 2010). In the inner ear, autophagy genes are essential for the vestibular function in mice (Mariño et al., 2010) and dead cells with autophagic features have been observed in the damaged cochlea (Taylor et al., 2008). Autophagy plays a role during normal inner ear development (Aburto et al., in press) and its inhibition by treatment with 3-methyladenine (3-MA; Rubinsztein et al., 2007) dramatically impairs neurogenesis (Fig. 6). An appealing hypothesis is that the degradation by autophagy of the otic cells owns components provide the energy required for the migration of the neuronal precursors. In this context, it would be very interesting to examine early inner ear development in Atg4b−/− and Atg5−/− deficient mice, which show different degrees of vestibular defects (Mariño et al., 2010).

Vestibular Versus Auditory Neuronal Cell Fate

Otic neuroblasts are the common progenitors of auditory and vestibular neurons that will have defined traits and innervate different brain centers and establish precise connections with their peripheral sensory targets. There are no precise molecular markers of this specification process. The sequential expression of NeuroM and NeuroD by migrating neuroblasts could determine specific stages of maturation; in the chicken embryo, evidence suggests that these genes do not identify different neuron identities but rather a temporal appearance of vestibular and auditory neurons (Bell et al., 2008). Studies in mouse null mutants have also shown the temporally regulated expression of Neurog1 to first generate vestibular neurons and secondly cochlear neurons (Koundakjian et al., 2007).

Neuritogenesis and Axonal Pathfinding

The innervation of specific sensory structures by otic neurons is built in a stereotyped fashion (Fekete and Campero, 2007; Koundakjian et al., 2007; Appler and Goodrich, 2011). It has been suggested that otic neurons send their projections by retracing back to the original delamination place (Carney and Silver, 1983; Bruce et al., 1997). However, there are some long distance connections that would require the release of molecular cues. Though the mechanisms and factors affecting axon guidance in the inner ear are still under study and discussion, its basis are probably similar to those behind the wiring of the central nervous system. Indeed, most of the molecules that have been reported to play a role in ear innervation are well-known chemoattractants, as brain-derived neurotrophic factor, neurotrophin-3 (NT-3), Shh, and the FGF family (Fariñas et al., 2001a, b; Fritzsch et al., 2004; Fantetti and Fekete, 2011), or else chemorepellent cues such as Semaphorin3/Npn1, Eph/Ephrins, and Slit/Robo (Webber and Raz, 2006; Fekete and Campero, 2007; Battisti and Fekete, 2008). It is also interesting to underline that molecules that reorganize the cytoskeleton such as the integrins (Davies, 2007, 2011), autophagy and pathways traditionally implicated in axon outgrowth such as the RAF-MEK-ERK pathway, have been also reported to act in early otic neuritogenesis (Zhong et al., 2007; Magariños et al., 2010; Fig. 7). Autophagy is emerging as a new player in neurite generation and degeneration (Yang et al., 2007; Koike et al., 2008; Plowey et al., 2008). Unc-51, the homolog for Atg1 in C. elegans, is required for axonal guidance (Ogura and Goshima, 2006) and it has been recently demonstrated that neural soma survival is required for adequate axonal maintenance and regeneration (Rodríguez-Muela et al., 2011).

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Figure 7. Early otic neurogenesis. A: Neurogenic markers of otic neurogenesis. Schematic representation of stages of development of otic neurons. Postmitotic neurons depend on neurotrophins, but the factors expanding neuroblast populations are still to be fully defined. Abbreviations: Del, delaminating; IN, immature neuron; MP, multipotent progenitor; NBe epithelial neuroblast; NBg, ganglionar neuroblast; ON, otic neuron. Adapted from Sanchez-Calderon et al., 2007. B: In vivo early otic neurogenesis. (a) Cryostat section of a HH19 chicken embryo inmunostained for the transcription factor Islet-1 (green) and the neuron marker G4 (magenta). Panel b corresponds to the boxed area in a, showing a higher magnification of the AVG. C: Schematic representation of the AVG ex vivo culture. The AVG can be explanted from the embryo at HH19+. The figure shows a schematic drawing of a HH19+ chicken embryo showing the otic vesicle and the AVG location and of an AVG immediately after dissection (0 hr) and after 24 hr in culture. Factors and drugs can be added to the serum-free culture medium to study their effects on AVG neuritogenesis. D: Signaling during early otic neuritogenesis. (a and b) Inhibition of both IGF-I signaling pathways, RAF-MERK-ERK and PI3K/AKT impaired otic neuritogenesis. (a) AVG explants were cultured in the 0S medium, or with Sor (2.5 μM) and immunostained for G4 (red) and Islet-1 (green). Sor-treated AVG have shorter processes without affecting the size of the AVG soma. Reproduced from Ref. Magariños et al., 2010. (b) AVG cultured in 0S or in the presence of LY (25 μM) and immunostained for TuJ-1 (magenta) and Islet-1 (green) showed that both the neuronal soma area and the length of the neurites of the LY culture are smaller. Reproduced from Ref. Aburto et al., 2010. Representative images are shown from at least six otic vesicles per condition obtained in at least three independent experiments. Compiled confocal microscopy projections of AVG are shown. (c) Inhibition of autophagy alters AVG neurogenesis. AVG explants cultured in the 0S condition or with 3-MA were immunostained for G4 (green). Scale bar: 300 μm. Fluorescence images were obtained from the compiled projections of confocal images of otic vesicles and acoustic-vestibular ganglia. Abbreviations: B AVG: Acoustic-vestibular ganglion; LY: LY294002 (PI3K/AKT inhibitor); Sor: Sorafenib (RAF-MEK-ERK inhibitor); 3-MA: (3-methyladenine).

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Currently, the best available therapy for treating hearing loss is the electrical stimulation of the spiral ganglion neurons with cochlear implants. Therefore, there is a need to improve our knowledge on how auditory neurons survive, differentiate, and interact with their targets to provide new avenues for the therapy of inner ear disorders.

IGF-I SIGNALING IN INNER EAR DEVELOPMENT

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

IGF-I is fundamental for the regulation of cochlear development, growth, and differentiation and its mutations are associated with hearing loss in mice and men (Walenkamp and Wit, 2006; Murillo-Cuesta et al., 2011, 2012). Early hearing loss in Igf1 null mice is due to neuronal loss and poor innervation of the sensory hair cells a phenotype associated to sensorineural deafness (Cediel et al., 2006). With ageing the stria vascularis degenerates and cellular traits of metabolic deafness, appear earlier than in the wild type littermates (Riquelme et al., 2010). Recent studies have also shown that low levels of IGF-I correlate with different human syndromes showing hearing loss (Murillo-Cuesta et al., 2011, 2012; Rodriguez de la Rosa et al., 2011) and with presbycusis (Riquelme et al., 2010).

IGF-I actions are mediated by a complex network of intracellular molecules. IGF-I binding to IGF1R results in its autophosphorylation in tyrosine residues (LeRoith et al., 1995; Laviola et al., 2008), recruitment of insulin receptor substrates leading to the activation of two main downstream pathways: the PI3K-AKT and the RAF-MEK-ERK phosphorylation cascade. During inner ear development, the generation of cellular diversity requires the strict regulation of biological processes that are coordinated by the concerted action of extrinsic and intrinsic factors. Otic epithelial cells have to escape apoptosis, survive, nurture themselves by autophagy, proliferate, migrate, and differentiate to generate the diversity of cell types of the adult inner ear. To this end, IGF-I acts differentially over its signaling pathways: the PI3K-AKT and the RAF-MEK-ERK cascade (Fig. 6).

Apoptosis Versus Survival

Programmed cell death plays an important role during the earliest morphogenetic events of inner ear development (Lang et al., 2000; León et al., 2004). Apoptosis contributes to the regulation of cell number in the epithelial and ganglionar neuroblast populations, and it serves to remove aberrant cells. Total blockage of apoptosis with the caspase inhibitor Boc-D-FMK increases the size of otic vesicles, caused an abnormal thickening in all the otic epithelium and also caused a reduction in the AVG size (Fig. 6A–C) (Aburto et al., 2012). These data suggest that apoptosis is required for morphogenesis and neurogenesis fulfilling a complementary role in the epithelial neurogenic area to allow the neuroblasts to physically detach from the epithelium. Otic vesicle cells require the actions of growth factors and neurotrophins to survive. For example, blockade of endogenous IGF-I activity in otic vesicle explants, increases cell death and inhibits the formation of the AVG, which shows that IGF-I is essential to promote survival in the otic precursors (Fig. 6D,E; Camarero et al., 2003). Accordingly, IGF-I reduces caspase-3 activation and TUNEL staining. However, the associated cell survival is spatiotemporally regulated so that it results in the persistence of cell death of specific populations. Cell death mainly occurs in postmitotic neurons that down-regulate the expression of high affinity IGF-I receptors. To promote the survival of neural progenitors, IGF-I acts through the PI3K-AKT pathway as indicated by the loss of epithelial and ganglionic neuroblast after blockade of either PI3K or AKT activity with the inhibitor LY294002 (Kong and Yamori, 2008; LY; Fig. 6F,J–L). Therefore, IGF-I/AKT signaling is fundamental for survival of proliferative otic neuroblasts and for the maintenance of the undifferentiated state. This suggests a crucial role of this pathway in establishing the final number of neurons and the timing at which neuron generation proceeds during otic development.

Cell Proliferation

There is previous evidence that IGF-I can promote proliferation of neural progenitors (Aberg et al., 2003; Mairet-Coello et al., 2009), and of neural stem cells in culture (Arsenijevic et al., 2001). Depending on the cell type and context, these actions of IGF-I are mediated by distinct signaling pathways (Varela-Nieto et al., 2003; Ye and D'Ercole, 2006; Magariños et al., 2010; Murillo-Cuesta et al., 2011). The basic mechanism underlying these actions is the capacity of IGF-I to promote G1/S cell cycle progression by regulating cyclin kinase activation via the activation of specific signaling pathways (Hodge et al., 2004; Mairet-Coello et al., 2009). In chicken otocyst cultures, IGF-I promotes otic proliferation as assessed by complementary cellular and molecular approaches (Frago et al., 1998, 2003; Sanz et al., 1999; Magariños et al., 2010). The activation of the RAF-MEK-ERK cascade is essential for cellular proliferation and it plays a fundamental role in the G1/S transition (Schreck and Rapp, 2006; Chambard et al., 2007), accordingly, blockade of the RAF pathway with Sorafenib (Sor; Schreck et al., 2006) produces a dramatic decrease in otocyst cell proliferation (Fig. 6G–I; Magariños et al., 2010) and increases the level of apoptotic cells. Nevertheless, RAF activity is essential for the progression of otocyst cell proliferation but not for cell survival because IGF-I rescues otic progenitors by activating the PI3K-AKT pathway even in the presence of the RAF kinase inhibitor Sor.

Therefore, IGF-I orchestrates cell proliferation and survival in the otic vesicle through distinct pathways, although cross talk between signaling pathways also occurs, as reported in other cell contexts (Moelling et al., 2002; Magariños et al., 2010). The Igf1 deficient mouse showed a significant reduction of the activated forms of the proteins ERK1/2 and AKT in the cochlea in prenatal stages and a great increase of the activated p38α MAP kinase. In summary, IGF-I deficiency in the cochlea decreases the activity of the signaling pathways that regulate cell survival and proliferation, and increases those involved in the cellular response to stress (Sanchez-Calderon et al., 2010).

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

Hearing and balance sensory organs have a common embryonic origin. Otic progenitor cells will transit trough stages of commitment and differentiation to generate sensory cells, support cells, and neurons for the vestibular and spiral ganglions. This transit is exquisitely regulated by extracellular signals. Extracellular signals as growth factors and morphogenetic molecules will elicit a network of intracellular signals leading to the expression of master transcription factors that will generate gene expression spatiotemporal patterns. Future therapies for hearing loss and balance disorders will benefit from the increasing knowledge on the molecular clues that direct otic development.

Acknowledgements

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED

The authors thank the Image Unit (IIBM, Madrid) for their technical support. The anti-Islet-1 monoclonal antibody was developed by Drs T.M. Jessel and J. Dodd and it was obtained from the Developmental Studies Hybridoma Bank, maintained at the Iowa University, Department of Biological Sciences. They warmly thank the critical comments and generous sharing of results of our colleagues at the Neurobiology of Hearing group.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. AN INTRODUCTION TO THE ANATOMY OF THE ADULT INNER EAR
  4. DEVELOPMENT OF THE VERTEBRATE INNER EAR
  5. THE SPECIFICATION OF THE PROSENSORY COMPETENT DOMAIN
  6. BIRTH AND DIFFERENTIATION OF HAIR AND SUPPORTING CELLS
  7. DEVELOPMENTAL OTIC NEUROGENESIS AND INNERVATION
  8. IGF-I SIGNALING IN INNER EAR DEVELOPMENT
  9. CONCLUSIONS
  10. Acknowledgements
  11. LITERATURE CITED