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Errata: Erratum Volume 51, Issue 3, 160, Article first published online: 16 August 2011
Toshihisa Hatta, MD, PhD, Department of Anatomy, Kanazawa Medical University, Uchinada, Ishikawa 920-0293, Japan. Email: firstname.lastname@example.org
Melanocortins, adrenocorticotropic hormone (ACTH) and α-, β-, and γ-melanocyte-stimulating hormone (MSH) are produced in the placenta and secreted into embryos/fetuses. ACTH concentrations are higher in fetal plasma than in maternal plasma and peak at mid-gestation in rats, whereas ACTH production starts in the anterior lobe of the fetal pituitary at later stages. Melanocortin receptors (MC1-5R), receptors for ACTH and α-, β- and γ-MSH, are expressed in various adult organs. The specific function of these receptors has been well examined in the hypothalamic-pituitary-adrenocortical (HPA) axis and the HPA axis-like network in the skin, and anti-inflammatory effects for white blood cells have also been investigated. MC2R and/or MC5R are also expressed in the testis, lung, kidney, adrenal, liver, pancreas, brain and blood cells at different stages in mouse and rat embryos/fetuses. Melanocortins in embryos and fetuses promote maturation of the HPA axis and also contribute to the development of lung, testis, brain and blood cells. Recently, a unique ACTH function was revealed in fetuses: placental ACTH, which is secreted by the maternal leukemia inhibitory factor (LIF), and induces LIF secretion from fetal nucleated red blood cells. Finally, the maternal LIF-placental ACTH-fetal LIF signal relay regulates the LIF level and promotes neurogenesis in fetuses, which suggests that ACTH acts as a signal transducer or effector for fetal development in the maternal-fetal signal pathway.
Adrenocorticotropic hormone (ACTH) is derived from post-translational processing of the precursor molecule proopiomelanocortin (POMC) in the anterior lobe of the pituitary. Alpha-melanocyte-stimulating hormone (α-MSH) and corticotropin-like intermediate lobe peptide (CLIP; ACTH18-39) are processed in the hypothalamus, the intermediate lobe of pituitary and the skin (Eberle 1988; Valverde et al. 1995; Raffin-Sanson et al. 2003). During pregnancy, the placenta produces and secretes ACTH (Jailer and Knowlton 1950a; Jailer and Knowlton 1950b; Assali and Hamermesz 1954), α-MSH (Krieger et al. 1980; Liotta and Krieger 1980), β-lipotrophin (β-LPH) (Nakai et al. 1978; Krieger et al. 1980) and β-endorphin (β-end) (Nakai et al. 1978; Krieger et al. 1980) into embryos/fetuses before the fetal pituitary gland starts to secrete these POMC-derived peptides. Moreover, melanocortin-2 receptor (MC2R) and melanocortin-5 receptor (MC5R), which are the receptors for ACTH and α-MSH, are expressed in the peripheral tissues in mouse fetuses (Nimura et al. 2006). It is well known that ACTH induces glucocorticoid production via MC2R. In fetuses, glucocorticoids are important for fetal organogenesis, in particular the deficiency of either glucocorticoid or MC2R due to be the immaturity of lung histogenesis (Bolt et al. 2001). The roles of ACTH in embryos/fetuses may be different from those in adults. However, there have been few studies on the role of ACTH and/or POMC derivatives during development. Recently, Simamura et al. (2010) revealed that placental ACTH, which is induced for fetal secretion by maternal leukemia inhibitory factor (LIF), induces LIF secretion from fetal nucleated red blood cells (nRBC). These authors suggest that placental ACTH acts as a transmitter and/or regulator for a unique transplacental LIF signaling pathway, the palindromic LIF-ACTH-LIF pathway from mother to fetus. In the present article, we review placental ACTH, focusing on (i) the processing of POMC derivatives produced in the placenta; (ii) the expression of the melanocortin receptor (MCR) family during development; and (iii) the role of ACTH in organogenesis and histogenesis.
PRODUCTION OF POMC DERIVATIVES IN THE PLACENTA
Melanocortins, ACTH and α-MSH are derived from POMC by post-translational processing (Bertagna et al. 1983, 1986, 1988; Hadley and Haskell-Luevano 1999). In the anterior lobe of the pituitary, four cleavage sites are used, all of which are of the Lys-Arg type, by prohormone convertase 1/3 (PC1/3) (Fig. 1). Indeed, six peptides (N-terminal peptide, jointing peptide (JP), ACTH and β-LPH, and a small amount of γ-LPH and β-end) are produced in corticotrophic cells (Fig. 1) (Liotta and Krieger 1980; Liotta et al. 1984). Post-translational processing in the placenta is similar to that in the intermediate lobe of the pituitary or the hypothalamus, and all cleavage sites are used for the production of peptides as follows: γ-MSH from the N-terminal; α-MSH (ACTH1-13) and CLIP (ACTH18-39) from ACTH cleaved by PC2; and γ-LPH, β-MSH, β-end (1–31) and β-end (1–27) from β-LPH. The amount of α-MSH is less than the amount of ACTH in the pituitary. In contrast to the pituitary, the amount of placental α-MSH produced is almost equal to the amount of ACTH. Three different Pomc mRNA species have been detected in the placenta, with sizes estimated at 1380, 1200 and 800 bases (Raffin-Sanson et al. 1999; Grigorakis et al. 2000). Thus, placental Pomc mRNA is suggested to be alternatively spliced.
The placenta produces ACTH, the amount of which is less than that of pituitary ACTH and similar to those of other pituitary-like peptides, placental lactogen, chorionic gonadotropin and prolactin (Krieger 1982). Human fetal plasma ACTH and amniotic ACTH levels are higher than those in maternal plasma, and the ACTH concentration rises throughout pregnancy (Raffin-Sanson et al. 1999, 2000). In rats, the ACTH concentration in fetal plasma is higher (10–30 nM) than that in maternal plasma (undetectable level). However, placental ACTH transfer from fetus to mother, or vice versa, does not occur (Winters et al. 1974; Tuimala et al. 1976). Intriguingly, a surge of plasma ACTH levels in rat fetuses appears at 14 days post coitus (dpc) (Fig. 2a) (Simamura et al. 2010), which corresponds to approximately weeks 5 to 9 of gestation in humans. Following a surge of fetal plasma ACTH levels, a surge of fetal LIF level appears in fetal cerebrospinal fluid (CSF) (Simamura et al. 2010). Thus, fetal LIF plays important roles in the brain development of fetuses, which are discussed later. Then, the mouse ACTH level decreases after birth (O'Shaughnessy et al. 2003; Simamura et al. 2010). Therefore, it has been suggested that ACTH plays important roles in the development of embryos at this stage.
MELANOCORTIN RECEPTOR FAMILY EXPRESSION IN FETUSES
The five melanocortin receptors (MC1-5Rs), receptors for ACTH and α-, β- and γ-MSH were cloned and belong to the Gsα class, the G protein that mediates receptor-stimulated cAMP generation (Cone 2000). Although ACTH has affinity for all MCRs, MC2R, which is expressed in the adrenal gland, is specific to ACTH. Alpha-MSH has affinity for MC1R, MC3R and MC4R (Chhajlani and Wikberg 1992; Chhajlani et al. 1993; Roselli-Rehfuss et al. 1993; Alvaro et al. 1996) and has its highest affinity for MC5R (Bednarek et al. 2007). MC1R and MC5R are expressed in a wide variety of tissues, and MC3R and MC4R are mainly expressed in the central nervous system of adults (Table 1). Mouse embryonic stem (ES) cells express MC5R instead of other MCRs that have not been detected by RT-PCR (Ogawa et al. 2004). The expression of MC2R and MC5R has been examined in the mouse after 11.5 dpc by immunostaining (Nimura et al. 2006). MC2R is expressed in the gonads, nephrons, blood cells, brain, spinal cord and choroid plexus. Interestingly, MC2R in the brain and spinal cord is expressed until 13.5 dpc and becomes undetectable after 14.5 dpc. MC5R is also expressed in many kinds of tissues in fetuses and adults. Its expression in the telencephalon is especially high only during the terminal stage of gestation. Although it remains unclear why MC2R is expressed at an earlier stage and MC5R later in the developing brain, MC2R may contribute to the organogenesis and MC5R to the histogenesis of the brain. In red blood cells, both MC5R and MC2R have been detected after 11.5 dpc in rats by immunostaining (Simamura et al. 2010). In the placenta, MC1R, MC2R and MC3R are expressed in trophoblasts (Gantz et al. 1993; Thornwall et al. 1997; Izumi et al. 2004). The role of these receptors in the placenta is unclear.
Table 1. Expression of melanocortin receptor family in embryo/fetus and adult
Expression in embyro/fetus and placenta
Expression in adult
Expressions of melanocortin receptors include detections by immunohistochemistry, in situ hybridization or RT-PCR.
ROLE OF ACTH IN THE DEVELOPMENT OF EMBRYOS/FETUSES
Maintaining pluripotency in ES cells
Cultured mouse ES cells require a feeder cell layer with fetal calf serum and LIF for self-renewal (Niwa 2001). However, the critical factor for suppression of ES cell differentiation is not LIF (Thomson et al. 1998). Ogawa et al. (2004) revealed that ES cells cultured with 10 µM of ACTH form the same number of colonies as those cultured with 0.3% FCS. Moreover, ACTH induces proliferation on the development of the inner cell mass (Lorthongpanich et al. 2008). These findings suggest that ACTH is required to maintain pluripotency and suppress differentiation or accelerate proliferation in the inner cell mass. It remains unknown which type of cell in the inner cell mass or trophoblast secretes ACTH physiologically during the blastocyst stage.
Fetal testes expressed both MC2R and MC5R mainly in the spermatogonium and in some mesenchymal cells (Nimura et al. 2006). Testosterone production in fetal and neonatal Leydig cells can be induced in vitro by ACTH treatment. However, no ACTH effects on mouse testes older than 20 days after birth were observed (O'Shaughnessy et al. 2003). AtT20 cell-implanted fetuses caused morphological changes in the testis, including pyknotic nuclei and a decrease in the number of undifferentiated spermatogonia, but not Leydig and Sertoli cells (Nimura et al. 2008). These findings suggest that ACTH has specific effects on testicular development.
In mice, glucocorticoid is produced not only in the developing fetal adrenal but also in the lung. Glucocorticoid accelerates fetal lung maturation, and premature delivery increases the risk of respiratory distress syndrome in neonates (Bolt et al. 2001). ACTH is detected by immunohistochemistry in the fetal lung in the mouse. Genes related to the HPA axis, corticotropin releasing hormone (Crh), corticotropin releasing hormone-binding protein (Crhbp), corticotropin releasing hormone receptor 1 (Crhr1), corticotropin releasing hormone receptor 2 (Crhr2), Mc2r, and nuclear receptor subfamily 3 (Nr3c1, glucocorticoid receptor) have also been detected in fetal lung by in situ hybridization and quantitative RT-PCR at 15.5–17.5 dpc in mice (Simard et al. 2010). Although immunostaining has shown early MC2R expression from 11.5 to 14.5 dpc, but undetectable after 15.5 dpc (Nimura et al. 2006), more precise studies are needed to resolve the discrepancy between PCR and immunostaining concerning the determined periods of MC2R expression. These findings suggest that the autocrine/paracrine mechanism involved in the pathway of ACTH/MC2R- glucocorticoid/glucocorticoid receptors, might regulate the histogenesis of fetal lungs.
Glucocorticoid has been known to promote pancreatic development in rat fetuses (Gesina et al. 2004; Gesina et al. 2006). Recently, MC2R expression was determined in the epithelia of the pancreatic ducts and developing glands, including both exocrine and endocrine lineage cells in mice (Kawamoto et al. 2010). AtT20 cells-implanted fetuses caused elevation of endocrine cells in the pancreas. In the AtT20 cells-implanted fetuses, α- and β-cells did not change in number, whereas the other unidentified cells increased in number in the pancreas. It is suggested that ACTH plays roles in the maintenance of stem cells or inhibition of apoptosis, because the ratio of pyknotic cells, including apoptotic cells is decreased in the pancreas of AtT20 cells-implanted fetuses.
ROLE OF MATERNAL-FETAL LIF-ACTH-LIF SIGNAL PATHWAY IN THE DEVELOPMENT OF EMBRYOS/FETUSES
LIF–ACTH-LIF signal contributes to the brain development
It is well known that MC3R and MC4R are expressed in the adult hypothalamus, which regulates energy homeostasis (Table 1) (Ollmann et al. 1997; Vaisse et al. 1998; Yeo et al. 1998; Chen et al. 2000). Recently, some groups have reported that maternal nutritional status induces epigenetic changes in Pomc expression, Pomc/Mc4r expression and the glucocorticoid receptor and affects the inflammatory pathway in the hypothalamus of offspring (Grayson et al. 2010; Stevens et al. 2010). Moreover, α-MSH and ACTH inhibit the production of tumor necrosis factor-α, interleukin-6 and nitric oxide from cultured microglial cells due to lipopolysaccharide plus interferon-γ treatment (Delgado et al. 1998). However, it is unclear whether MCRs other than MC3R and MC4R are expressed in adult brains. MC2R and MC5R are expressed in mouse fetal brains (Table 1). It has been reported that MC2R and POMC deficient mice lead to neonatal lethality in three-quarters of the mice and hypoplasia in their adrenal gland (Coll et al. 2004; Chida et al. 2007). MC5R deficient mice exhibited less aggressiveness than wild type on pheromone-regulated behavior (Morgan et al. 2004). Whereas, direct functions of MC2R or MC5R in the fetal brain remain unclear, because there have been no reports about in development in the brain of deficient mice.
In rat fetuses, ACTH indirectly promotes the proliferation of neural stem/progenitor cells in the dorsal telencephalon (Simamura et al. 2010). It is well known that LIF participates in neural stem cell self-renewal (Hatta et al. 2002; Oshima et al. 2007) and differentiation to astrocytes (Nakashima et al. 1999a; Nakashima et al. 1999b). The LIF surge in fetal CSF appears at 14 dpc in mice or at 15 dpc in rats, when the chronological change of the neural proliferation in the ventricular zone of cerebral neocortex increases (Hatta et al. 2006; Simamura et al. 2010). They revealed that LIF is secreted from fetal nRBC by placental ACTH stimulation, which might be one of the major sources of fetal LIF. These are discussed more fully in the next section.
Contributions of fetal RBCs to LIF-ACTH-LIF signal pathway
In adult blood cells, MCRs are expressed in monocytes/macrophages, neutrophils and T cells. Their main function is an anti-inflammatory effect mediated by α-MSH and ACTH (Star et al. 1995; Catania et al. 1996; Andersen et al. 2005). However, there have been few reports about MCR expression in fetal blood cells, including hematopoietic stem cells and hematopoietic progenitor cells. Rodent fetal blood cells express MC2R and/or MC5R, and their functions are quite unique in fetal development. Fetal nRBCs coexpress MC2/5R and LIF (Hatta et al. 2006; Simamura et al. 2010). The source of fetal LIF has been unclear for a long time. LIF is essential and has effects on blastocyst implantation (Bhatt et al. 1991; Stewart et al. 1992), placentation, maintenance of pregnancy (Kojima et al. 1995; Sawai et al. 1995) and hematopoiesis (Metcalf 1997; Ratajczak et al. 1997). It also has significant effects on neural stem cell self-renewal (Hatta et al. 2002; Oshima et al. 2007) and differentiation to astrocytes (Nakashima et al. 1999a; Nakashima et al. 1999b). Interestingly, LIF secretion from rat nRBCs at 14.5–15.5 dpc is induced by ACTH and α-MSH in vitro. Maternal LIF injection induces ACTH secretion from the placental syncytiotrophoblast into fetuses. In rats, ACTH production in the fetal pituitary is not yet started at 14.5–15.5 dpc, and LIF does not pass through the placenta from mother to embryo/fetus. Moreover, the maternal LIF-placental ACTH-fetal LIF signal pathway increases bromodeoxyuridine uptake in the neural stem and progenitor cells in the ventricular zone of the fetal forebrain (Fig. 2b) (Simamura et al. 2010). An ACTH surge in the fetal plasma appears at 14.5 dpc (Fig. 2a) followed by a LIF surge in the fetal cerebrospinal fluid at 15.5 dpc, which occurs earlier than the period when the fetal cerebral cortex appears in rat embryos after 16 dpc. However, placental trophoblasts and fetal nRBCs do not react to exogenous LIF injection into dams except at around 15 dpc, which suggests that the functional fetal-maternal signal window regulates the LIF-ACTH-LIF signal pathway. Thus, neurogenesis may be one of the targets of the maternal-fetal neuro-immuno-endocrine network via the melanocortin systems during development (Fig. 3).
ACTH and α-MSH, have been known to be produced in the placenta and secreted into embryos/fetuses. However, melanocortin effects on stem cells and embryos/fetuses have not been well studied because high sensitivity immunohistochemical methods for detecting MCRs have not been established. Recently, it was revealed that MC2R and MC5R are expressed in various fetal organs. Embryonic and fetal melanocortins exhibit a variety of functions involving the proliferation and inhibition of ES cell differentiation, HPA axis maturation, induction of lung maturation and histogenesis of testis and pancreas. Moreover, placental ACTH and α-MSH induce LIF secretion from nRBCs and the LIF surge in fetal CSF, which would be caused by a surge of plasma ACTH secreted from nRBC in rat fetuses at 14 dpc, promotes neuron production in the fetal forebrain. This model shows that placental ACTH acts as the maternal-fetal LIF signal transducer to regulate organ development in fetuses. MC2R and MC5R are differentially expressed depending on the organ and developmental stage. It is strongly suggested that the differential expression pattern of five melanocortin receptors is related to significant physiological implications in organ development.
This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (22390216, 21791045, 22659289, 19659414, 15209034), by a Grant for Project Research from the High-Tech Research Center of Kanazawa Medical University (H2009-14, H2010-14) and by grants for promoting research from Kanazawa Medical University (C2010-2, S2009-10, S2009-2, S2010-8)