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Keywords:

  • Lrp;
  • Wnt;
  • dopaminergic;
  • neurogenesis, precursor;
  • midbrain

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Wnts are known to bind and activate multiple membrane receptors/coreceptors and to regulate dopaminergic (DA) neuron development and ventral midbrain (VM) morphogenesis. The low density lipoprotein receptor–related protein (Lrp6) is a Wnt co-receptor, yet it remains unclear whether Lrp6 is required for DA neuron development or VM morphogenesis. Lrp6 is expressed ubiquitously in the developing VM. In this study, we show that Lrp6−/− mice exhibit normal patterning, proliferation and cell death in the VM, but display a delay in the onset of DA precursor differentiation. A transient 50% reduction in tyrosine hydroxylase–positive DA neurons and in the expression of DA markers such as Nurr1 and Pitx3, as well as a defect in midbrain morphogenesis was detected in the mutant embryos at embryonic day 11.5. Our results, therefore, suggest a role for Lrp6 in the onset of DA neuron development in the VM as well as a role in midbrain morphogenesis. Developmental Dynamics 239:211–221, 2010. © 2009 Wiley-Liss, Inc.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Wnts comprise a family of 19 secreted lipid-modified glycoproteins that regulate a myriad of biological processes including midbrain and dopaminergic (DA) neuron development (McMahon and Bradley, 1990; Thomas and Capecchi, 1990; Castelo-Branco et al., 2003; Prakash et al., 2006; Andersson et al., 2008). We have previously shown that canonical Wnt signaling, leading to the stabilization of cytosolic β-catenin (Logan and Nusse, 2004), is involved in the differentiation of postmitotic DA precursors into DA neurons (Castelo-Branco et al., 2004), and Wnt5a, which activates Rac1, is involved in DA differentiation and midbrain morphogenesis (Andersson et al., 2008). Wnt1 plays an essential role in the development of the mid-/hindbrain region and in the establishment of the DA progenitor domain in the ventral midbrain (VM; McMahon and Bradley, 1990; Thomas and Capecchi, 1990; Danielian and McMahon, 1996; Panhuysen et al., 2004; Prakash et al., 2006).

Wnt signaling is transduced by a receptor complex consisting of the seven-pass transmembrane Frizzled (Fzd) receptors and the low density lipoprotein receptor (LDLR) -related protein (Lrp) 5 or 6 (Tamai et al., 2000; Mao et al., 2001; Cong et al., 2004). Initially, observations that Drosophila mutants for arrow, an ortholog of the mammalian Lrp6, phenocopy the wingless (the Wnt1 Drosophila ortholog) mutants, supported Lrp as an exclusive canonical Wnt signaling component (Wehrli et al., 2000). However, the involvement of Lrp6 in modulating planar cell polarity (PCP) and convergent extension (CE) has recently been described in both Xenopus (Tahinci et al., 2007) and mice (Bryja et al., 2009), indicating that Lrp6 could potentially regulate DA neuron development through multiple signaling mechanisms. In support of this hypothesis, we recently found that Wnt5a, a Wnt that induces PCP signals, also regulates DA neuron development in vivo (Andersson et al., 2008). These results suggest that Lrp6 could potentially modulate multiple aspects of DA neuron development through different Wnt ligands. In this study, we asked whether Lrp6 is required for midbrain or DA neuron development in vivo, by analyzing VM, progenitor and neuron development and midbrain morphogenesis in Lrp6−/− mice.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Lrp Coreceptors Are Expressed in the Developing VM

Wnts transduce their signal through a ternary complex formed by receptors of the Fzd and Lrp families (Logan and Nusse, 2004). The developing VM is known to express several Fzd receptors (Rawal et al., 2006; Fischer et al., 2007), while the ubiquitous central nervous system expression of Lrp coreceptors has been shown in both Xenopus and mouse (Houston and Wylie, 2002; Zhou et al., 2004a). We first confirmed these results during rat and mouse VM development by quantitative polymerase chain reaction (qPCR) and in situ hybridization. Lrp5 and Lrp6 transcripts were detected in the developing VM by qPCR (Fig. S1A,B), and in situ hybridization confirmed that Lrp5 and Lrp6 were ubiquitously expressed at embryonic day (E) 11.5 (Supp. Fig. S1C, which is available online).

Lrp6−/− Mice Do Not Display Patterning, Proliferation, or Cell Death Defects in the Ventral Midbrain

Several developmental phenotypes associated with dysregulation of Wnt signaling have been described in Lrp6−/− mice, including a deletion of the dorsocaudal midbrain and cerebellar defects (Pinson et al., 2000). Interestingly, whereas the isthmus was clearly less well-defined at a dorsal level in E9.5 and E10.5 Lrp6−/− mice (Fig. 1A,B, and Pinson et al., 2000), in situ hybridization revealed no difference in the ventral expression of Otx2, Engrailed (En1), Lmx1b, Sonic Hedgehog (Shh) or Wnt5a in the midbrain of E9.5 (Fig. 1A) or E10.5 (Fig. 1B) Lrp6−/− mice. Previous observations by Pinson et al. were confirmed by decreased or lost dorsal expression of midbrain/hindbrain marker genes, such as En1 and Fibroblast growth factor 8 (Fgf8), and concomitant loss of dorsal mid-/hindbrain tissue at E12.5 (Fig. 1C). However, ventral expression of En1 and Fgf8 at the midbrain–hindbrain boundary (MHB) was normal (Fig. 1C). As previously reported (Pinson et al., 2000), many Lrp6−/− embryos displayed neural tube defects, including exencephaly. When patterning was examined in exencephalic Lrp6−/− mice, Shh (marker for floor plate [FP]/basal plate [BP]), Lmx1b (FP and roof plate [RP]) and Wnt3a (RP) were expressed in the correct structures at E9.5 (Supp. Fig. S2A) and E10.5 (Supp. Fig. S2B). Thus, despite the dorsal mid–hindbrain defects in Lrp6−/− mice, no discernible patterning defects manifested in the VM.

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Figure 1. Patterning of the mid–hindbrain region is not affected in Lrp6−/− mice at embryonic day (E) 9.5 or E10.5, before the onset of dopaminergic (DA) neurogenesis. A,B: The E9.5 (A) and E10.5 (B) mice were probed for Otx2 (forebrain/midbrain marker), En1 (midbrain/hindbrain marker), Lmx1b (ventral midbrain and roof plate marker), Shh (floor plate marker), and Wnt5a (ventral midbrain marker). Expression of each marker was found in the correct domain. However, Lrp6−/− mice were usually smaller than WT littermates, and the isthmus (as previously reported) was less morphologically defined (A,B). C: Sagittal sections of E12.5 mice revealed normal expression of En1 and Fgf8 in ventral domains but a reduced or absent expression in dorsal domains concomitant with a loss of dorsal tissue at the midbrain-hindbrain boundary.

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In agreement with our findings on patterning, we did not observe any decrease in the proliferation of VM precursors at E11.5, as assessed by EdU (5-ethynyl-2′-deoxyuridine) incorporation (Fig. 2A,B) immunostaining for the cell cycle marker phospho-histone-3 at E11.5 (Fig. 2C,D), or by BrdU (5-bromo-2-deoxyuridine) incorporation at E10.5, E11.5, E12.5 or E15.5 (Supp. Fig. S3). Moreover, the number of cleaved/active caspase-3 immunoreactive cells, a marker of cells undergoing apoptosis, was similarly low in wild-type and mutant VM at E11.5 (Supp. Fig. S4) and E13.5 (data not shown). The expression level and distribution of the neural stem/progenitor cell marker nestin (Fig. 2D,E) and the mRNA levels of the DA progenitor cell marker, aldehyde dehydrogenase 2 (AHD2; Fig. 2F), expressed from E9.5 onward (Wallen et al., 1999), were not altered in the mutant VM. These results suggested that deletion of Lrp6 does not alter normal patterning, proliferation, or cell survival in the VM, including the DA lineage.

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Figure 2. Proliferation and early dopaminergic (DA) markers are unchanged in Lrp6−/− mice. A: The proliferative capacity of embryonic day (E) 11.5 ventral midbrain (VM) precursors was not affected in Lrp6−/− mice, as assessed by EdU (5-ethynyl-2′-deoxyuridine) staining in coronal sections of the ventral midbrain of 2-hr EdU-pulsed embryos at E11.5. B: Quantification of EdU+ cells did not reveal a significant difference between Lrp6−/− and wild-type mice within the Glast-expressing floor plate. C–E: Quantification of phospho-histone-3 (PH3+) cells (C) after immunostaining of the VM at E11.5 for PH3 and Nestin (D), and Nestin quantitative polymerase transcription polymerase chain reaction (qPCR; E) showed no difference in Lrp6−/− mice compared with wild-type (WT). F: Similarly, qPCR for the DA progenitor marker AHD2 revealed no difference at E11.5. V, ventricle; PS, pial surface.

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Altered VM Morphology in Lrp6−/− Mice

We have previously shown that Wnt5a is required for the appropriate invagination of the VM ventricular zone (VZ) and medial hinge-point formation (Andersson et al., 2008), in that loss of Wnt5a leads to a U-shaped, rather than V-shaped, VM. Consequently, Wnt5a−/− mice sometimes display neural tube closure defects (Qian et al., 2007; Andersson et al., 2008). Lrp6−/− mice also present with neural tube closure defects such as exencephaly (Pinson et al., 2000; Bryja et al., 2009; Andersson et al., 2009), which is rescued by loss of Wnt5a in a dose-dependent manner (Bryja et al., 2009). We therefore asked how the loss of Lrp6 itself affects VM VZ morphology.

In contrast to the flattened VM (VZ) medial hinge-point previously reported in Wnt5a−/− mice, Lrp6−/− mice generally displayed a much more acute VM VZ angle of circa 40°, compared with wild-type mice which displayed a circa 135° VM VZ angle (P = 0.013, N = 3, unpaired t-test). This resulted in a narrow V-shaped VM VZ in the Lrp6−/− mice (Fig. 3A,B).

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Figure 3. Loss of Lrp6 results in an acute ventral midbrain (VM) ventricular zone (VZ) invagination angle. A: The angle of invagination was measured on DAPI (4′,6-diamidine-2-phenylidole-dihydrochloride) -stained coronal VM sections. B: At embryonic day (E) 11.5, wild-type mice displayed an angle of circa 135°, which was significantly different from the 40° angle seen in Lrp6−/− mice (unpaired t-test; N = 3; P = 0.012).

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Delayed DA Differentiation in Lrp6−/− Mice

Ngn2 is a basic helix–loop–helix transcription factor required for DA neurogenesis (Kele et al., 2006). At E11.5, Ngn2 expression in the midbrain FP defines the DA progenitor domain, whereas Ngn1 in the adjacent BP defines the oculomotor (OM) and red nucleus (RN) progenitor domains (Kele et al., 2006). Interestingly, both Ngn1 and Ngn2 were expressed in the expected domains, despite the Lrp6−/− brains sometimes being smaller (Fig. 4A). This result was further confirmed by qPCR for Ngn2 (Fig. 4B).

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Figure 4. Ventral midbrain expression of Ngn1 and Ngn2 is normal in Lrp6−/− mice. A:Ngn1 and Ngn2 were expressed normally in the midbrain of Lrp6−/− mice at embryonic day (E) 11.5, as assessed by in situ hybridization. B: qPCR did not reveal a statistically significant difference in Ngn2 mRNA levels in the VM.

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Nurr1, a nuclear receptor expressed in postmitotic cells in the VM DA lineage (DA precursors and neurons), is known to be required for the differentiation of DA precursors and the acquisition of the DA phenotype (Zetterstrom et al., 1997; Castillo et al., 1998; Le et al., 1999). At E11.5, we found a 40% decrease in the number of Nurr1+ cells (from 647.7 ± 77.38 in the wild-type (WT) to 391.0 ± 102.8; Fig. 5A,B), and a 60% decrease in Nurr1 mRNA levels (Fig. 5C). However, these defects were partially recovered as early as E13.5 (Fig. 5D–F). We next examined whether the reduction in Nurr1 expression and in cell numbers were the result of a delayed marker acquisition or accompanied by delayed differentiation into tyrosine hydroxylase–positive (TH+) DA neurons.

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Figure 5. Postmitotic cells in the dopaminergic lineage are reduced at embryonic day (E) 11.5, but recover by E13.5, in Lrp6−/− mice. A,B: The number of Nurr1+ precursors was assessed by immunohistochemistry and was reduced from 647 ± 77.38 in wild-type to 391 ± 102.8 in the Lrp6−/− mice (paired t-test; N = 3; P = 0.0136). C: qPCR analysis of ventral midbrains (VMs) from Lrp6−/− mice revealed a similar decrease in Nurr1 mRNA levels (paired t-test; N = 6; P = 0.0111). D–F: This reduction was rescued by E13.5 in the mutants as assessed by immunohistochemistry and qPCR.

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Pitx3 is a transcription factor expressed during DA differentiation (Smidt et al., 1997) that is required for DA neuron maintenance and survival (Hwang et al., 2003; Nunes et al., 2003; van den Munckhof et al., 2003; Smidt et al., 2004; Maxwell et al., 2005). We found that Pitx3 expression was greatly reduced, as assessed by in situ hybridization at E12.5 (Fig. 6A) and quantitative reverse transcription PCR (qRT-PCR) at E11.5 (Fig. 6B), confirming that the DA neuron differentiation process was impaired.

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Figure 6. Delayed onset of dopaminergic (DA) differentiation in Lrp6−/− mice. A,B: In situ hybridization revealed a drastic reduction of Pitx3 expression in the Lrp6−/− mice at embryonic day (E) 11.5 (A), which was confirmed by qPCR (paired t-test; N = 5; P = 0.0228) (B). Coronal ventral midbrain (VM) sections of E11.5 wild-type (WT) and Lrp6−/− mice revealed a reduced number of tyrosine hydroxylase–positive (TH+) DA neurons in the mutants. C: However, overall neuronal differentiation (as assessed by β-tubulin III [TUJ-1]) was not affected. D: TH+ cell numbers were reduced from 362.5 ±30.38 in WT to 175.5 ±24.4 in Lrp6−/− mice at E11.5 (paired t-test; N = 4; P = 0.0038). E: This reduction was confirmed by qPCR, showing a 50% reduction (paired t-test; N = 6; P = 0.0120). F: A partial recovery in the number of TH+ DA neurons was detected at E13.5 in Lrp6−/− mice. G,H: TH+ cell numbers were still somewhat reduced in Lrp6−/− mice (G), a small difference that could not be detected by qPCR (H).

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We next examined the number of TH+ DA neurons at E11.5, and observed a 50% reduction in the number of TH+ cells (362.5 ± 30.38 in WT, 175.5 ± 24.4 in Lrp6−/−; Fig. 6C,D), with no apparent change in the total population of neurons in the VM (β-tubulin III [TUJ-1] -positive cells; Fig. 6C). TH mRNA levels were also significantly lower in Lrp6−/− mice, as assessed by qPCR (Fig. 6E) and in situ hybridization (data not shown). However, at E13.5, the decrease in the number of TH+ cells in the Lrp6−/− mice was attenuated, and a reduction of only 25% was detected (Fig. 6F,G). No statistically significant difference in TH mRNA levels was detected by qPCR at this stage (Fig. 6H). Moreover, at E17.5, the numbers of TH+ cells were normal in the substantia nigra and in the ventral tegmental area, and their innervation of the striatum was also normal in Lrp6−/− mice (data not shown).

These data suggest that the alteration in the number of Nurr1 and TH+ cells during E11.5–E13.5 was the result of a delay in the differentiation of DA precursors and DA neurons in the mutant VM.

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

In this study, we examined whether the Wnt coreceptor, Lrp6, is required for VM DA neuron development or midbrain morphogenesis. Whereas the function of Lrp6 in the developing ventral midbrain has not yet been described, Lrp6 has previously been found to be necessary for isthmus and dorsocaudal midbrain development (Pinson et al., 2000). We report here that Lrp6 is required for the timely onset of DA differentiation in the VM and normal VM morphogenesis, but not for the proper patterning, growth, and survival of VM tissue.

Wnt1 (a β-catenin–activating Wnt) has been shown to be essential for both dorsal and ventral midbrain development, including DA neuron development (McMahon and Bradley, 1990; Thomas and Capecchi, 1990; Danielian and McMahon, 1996; Panhuysen et al., 2004; Prakash et al., 2006). Moreover, we have previously shown that other Wnts can also contribute to the development of DA neurons in vivo and in vitro (Castelo-Branco et al., 2003; Andersson et al., 2008). When the specific contribution made by Lrp6 to DA neuron development was examined in vivo, we found that the Lrp6 receptor was required for the timely onset of DA differentiation in the VM. Ngn2 and Ahd2 mRNA levels were normal in Lrp6−/− mice, indicating that the delay in differentiation occurs after onset of expression of these markers in DA progenitors. Indeed, the most significant difference that we detected was a decrease in the expression of Pitx3, a gene with an important role in DA differentiation (Hwang et al., 2003; Nunes et al., 2003; van den Munckhof et al., 2003; Smidt et al., 2004; Maxwell et al., 2005). This defect was accompanied by a decrease in the number of Nurr1+ precursors and TH+ DA neurons, as well as lower expression of Nurr1 and TH mRNAs at E11.5 in the VM of Lrp6−/− mice. These effects were specific to the DA lineage, in that no differences in cell death, proliferation, or patterning were observed in the mutant VM, despite the fact that these processes are also regulated by Wnts (McMahon and Bradley, 1990; Thomas and Capecchi, 1990; Pinson et al., 2000; Castelo-Branco et al., 2003; Viti et al., 2003; Panhuysen et al., 2004; Ciani and Salinas, 2005). Thus, our results suggest a role for Lrp6 in the differentiation of DA neurons during early stages of their development.

Compared with Wnt1−/− mice, the DA differentiation defect in Lrp6−/− mice was transient, while Wnt1−/− mice show a severe and permanent defect (McMahon and Bradley, 1990; Thomas and Capecchi, 1990; Prakash et al., 2006). Moreover, the decrease in expression of Pitx3 was more severely affected in the Wnt1−/− mice (Prakash et al., 2006) than in the Lrp6−/− mice. These results suggest the presence of compensatory mechanisms that permit a recovery of postmitotic DA precursors and neurons in the Lrp6−/− mice. One possibility is that Lrp5, another canonical Wnt coreceptor expressed in the VM, may be able to compensate for the absence of Lrp6. In support of this hypothesis, Lrp5+/−;Lrp6−/− mice exhibit a much more severe phenotype than Lrp6−/− mice, but die before DA neurogenesis (Kelly et al., 2004), thus precluding the analysis of DA neuron development in these mice. Future experiments using midbrain-specific deletions of these genes would help to further elucidate the specific contribution of Lrp receptors to Wnt signaling in the VM.

Region-specific defects in the Lrp6−/− mice have also been observed in brain areas other than the VM. Neuronal development is severely affected in the dorsal thalamus (with ablation of Shh and Wnt5a expression), and in the dentate gyrus of Lrp6−/− mice (Zhou et al., 2004b), while other hippocampal and neocortical cell types are not affected (Zhou et al., 2004b). These data, together with our results showing a developmental impairment of a ventral neuronal cell type in the midbrain, suggest a function of Lrp6 as a regulator of neuronal development in specific cell lineages. Interestingly, in the first study by Zhou et al. (Zhou et al., 2004a), a disruption of thalamocortical projections was described. We therefore examined the innervation of the striatum of Lrp6−/− mice at E17.5, aiming at detecting a possible permanent defect in the nigrostriatal pathway, but did not find any alterations.

While the mild similarity of the Lrp6−/− mice to the Wnt1−/− mice could be expected, based on their similar roles in signaling to β-catenin (Huang and He, 2008), the VM VZ morphogenic phenotype of Lrp6−/− mice in relation to Wnt5a−/− mice was more surprising. Lrp6 has long been exclusively viewed as a coreceptor for Wnt/β-catenin signaling (Wehrli et al., 2000; He et al., 2004), although recent reports have challenged this view (Tahinci et al., 2007; Bryja et al., 2009). We have previously shown that loss of Wnt5a leads to a flattened VM VZ invagination and a rostrocaudally shortened, but laterally expanded DA population (Andersson et al., 2008), morphogenic defects typical of disrupted convergent extension (CE; Ybot-Gonzalez et al., 2007). While the decrease in DA cell numbers in the Lrp6−/− mice precluded a detailed analysis of the distribution of DA neurons, analysis of the VM VZ invagination revealed a more narrow, or acute, angle of invagination. It is interesting to note that loss of Wnt5a or of Lrp6 have opposite effects on VM morphology, and that loss of Lrp6 results in neural tube closure defects that are rescued by loss of Wnt5a (Bryja et al., 2009). Overall, this indicates that Wnt5a and Lrp6 functionally oppose each other in VM morphogenesis. Our previous results have shown that Lrp6 physically interacts with Wnt5a and can oppose Wnt5a in regulation of CE (Bryja et al., 2009); however, we have also found that Wnt5a and Lrp6 synergize in some organs or systems (Andersson et al., 2009). Therefore, further studies are warranted to assess whether loss of Lrp6 results in gain-of-function of Wnt5a signaling at the level of DAergic neuron differentiation. At the level of VM progenitor proliferation, loss of Wnt5a resulted in an increase in proliferation at E11.5 (Andersson et al., 2008), but we did not detect any difference in proliferation in the Lrp6−/− mice.

In summary, our results demonstrate that Lrp6−/− mice display a phenotype that is similar to Wnt1−/− mice in DA neuron differentiation and opposite to Wnt5a−/− mice in midbrain morphogenesis, and that Lrp6 is necessary for the timely onset of DA neuron differentiation in the developing VM. Our results also suggest that other co-receptors may mediate some of the multiple functions regulated by Wnts in the midbrain and specifically in DA neurons.

EXPERIMENTAL PROCEDURES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Lrp6 Mutants and Genotyping

Lrp6+/− mice (Pinson et al., 2000; a kind gift from William Skarnes, The Sanger Institute) were housed, bred, and treated in accordance with the ethical approval for animal experimentation granted by Stockholms Norra Djurförsöks Etiska Nämnd (in Sweden), or by the HMGU Institutional Animal Care and Use Committee (in Germany). WT and heterozygous mice were identified with genotyping PCR reactions with the previously described primers Lrp6 -U1 and Lrp6 -D1 (Kelly et al., 2004), and mice with the gene trap insertion were recognized with the following primer set: CD4mix forward: 5′-GCACGGATGTCTCAGAT CAAGAGG-3′ and CD4mix reverse: 5′-CGGGATCATCGCTCCCATATATG- 3′, with an annealing temperature of 63°C and an amplicon of 108 bp. For DNA extraction, ear or embryonic tissues were boiled at 95°C for 40 min in 100–200 μl of 25 mM NaOH/0.2 mM ethylenediaminetetraacetic acid (ED TA), after which an equal volume of 40 mM Tris HCl pH 5 was added to neutralize the solution. A total of 3 μl of this solution was used in PCRs, which were performed as described for the qPCR, but without SYBR Green and for 30 cycles.

Noon of the day of plug was taken as E0.5.

Immunohistochemistry and Image Acquisition

E11.5, E13.5, and E17.5 mice were fixed with 4% paraformaldehyde (PFA) overnight and immersed in a 20% sucrose gradient. Samples were then rapidly frozen in Tissue-Tek O.C.T Compound (Labonord, France) on dry ice. Serial sagittal and coronal sections (14 μm thick) were collected on microscope slides (StarFrost, Germany) and stored at −80°C. For immunohistochemistry, slides were thawed and incubated for 10 min with 4% PFA. After three 15 min washes with phosphate-buffered saline (PBS) the slides were blocked with PBTG (PBS with 0.1–0.3% Triton X and 5% goat serum) for 30 min. Primary antibody (rabbit α-TH [1:250-Pelfreeze], rabbit α-Nurr1 [1:1,000-Santa Cruz], rabbit α-active-caspase III [1:100-Cell Signaling] rabbit α-phospho-histone-3 [1:100-Cell Signaling], guinea pig anti-Glast [1:2000, Chemicon], or mouse α-nestin [Rat401, 1:100-Developmental Studies Hybridoma Bank, Iowa]) in PBTG was incubated at 4°C overnight. After three washes with PBS, the sections were again blocked with PBTG for 30 min and incubated for 1–2 hr with secondary antibody (cyanine-2, cyanine-3, or rhodamine-coupled horse-α-mouse IgG 1:200, goat α-rabbit IgG 1:200, or donkey anti-guinea pig 1:500 (Jackson Laboratories)). Slides were then washed three times with PBS for 15 min, counterstained with Hoechst 33258 or DAPI (4′,6-diamidine-2-phenylidole-dihydrochloride; Invitrogen) for 1–20 min, and mounted in PBS/glycerol (1:4). Images were acquired at room temperature with a confocal laser scanning microscope (Zeiss 510, argon (488 nm) and helium-neon (543 and 633 nm) lasers) and Zeiss LSM Viewer software. Images were processed with Adobe Photoshop version 7.0 or CS4. Figure panels were assembled using Adobe Illustrator CS4 or Photoshop CS4.

In Situ Hybridization

WT and Lrp6−/− mouse embryos at E11.5, E12.5 and E15.5 were fixed overnight in 4% PFA at 4°C and embedded in paraffin. Serial sections of 8 μm were processed for radioactive in situ hybridization using [S35]-UTP labeled antisense riboprobes. Hybridization was carried out at 56°C in 50% formamide according to a modified protocol of Dagerlind et al. (Dagerlind et al., 1992). Sections were counterstained with Cresyl Violet (Sigma-Aldrich, Sweden). Probes for in situ hybridization were as follows:, Otx2 (Simeone et al., 1992), En1 (Davis and Joyner, 1988), Lmx1b (Chen et al., 1998), Shh (Echelard et al., 1993), Wnt5a (Yamaguchi et al., 1999), Fgf8 (Martinez et al., 1999), Ngn1, Ngn2 (Cau et al., 1997), TH, Pitx3 (kindly given by Jordi Guimera; Brodski et al., 2003), Lrp5, Lrp6 (PCR-products for regions 2884–3444bp for Lrp5 NM_008513 and 2941–3446bp for Lrp6 NM_008514 [C. Kokubu]), and Wnt3a (Parr et al., 1993).

BrdU and EdU Detection

For proliferation assays, the BrdU Detection kit II and protocol (Roche, Germany) or EdU (Invitrogen) was used with slight modifications. Pregnant mice were injected peritoneally with 5-bromo-2-deoxyuridine (BrdU, 10 μg/g body weight) or 5-ethynyl-2′-deoxyuridine (EdU, Invitrogen, 10 μg/g body weight) 2 hr before sacrificing. For BrdU, embryos/brains were incubated overnight in 4% PFA at 4°C, dehydrated through ethanol and rotihistol and paraffin embedded. Paraffin sections (8 μm) were deparaffinized in rotihistol, rehydrated, cooked in sodium citrate (0.01 M) for 5 min, washed with PBS and incubated 1 hr in blocking solution (PBS with 10% fetal calf serum, 0.05% Triton X-100). Next, slides were incubated overnight at 4°C with anti-BrdU (dilution 1:10 in PBS with 0.05% Triton X-100). After three washes with PBS, sections were incubated 2 hr with the secondary anti-mouse biotinylated antibodies (1:500, Jackson ImmunoResearch). After three washes in PBS, slides were incubated 30 min with ABC solution (ABC-kit, Vectastain, Vector Laboratories) and then diaminobenzidine staining (DAB, Sigma-Aldrich, Sweden) until signal was seen. Slides were washed twice with PBS, dehydrated and mounted with Roti-Histokit (ROTH, Germany).

For EdU staining, embryos/brains were fixed in 4% PFA for 4 hr, washed twice with PBS, and then incubated in 30% sucrose overnight at 4°C. Embryos/brains were then embedded in optimum cutting temperature (O.C.T.) embedding compound on dry ice and 14-μm sections were collected on a cryostat. Slides were rehydrated in PBS before EdU detection, which was performed as described previously (Salic and Mitchison, 2008). In brief, slides were incubated for 30 min with a solution composed of 100 mM Tris (pH 8.5), 1 mM CuSO4, 10 μM Alexa 488 azide (Invitrogen) and 100 mM ascorbic acid. Slides were then washed with PBS and either mounted in glycerol or used for subsequent immunohistochemistry.

Quantification of Immunohistochemical Data or EdU and Statistical Analyses

Quantitative immunohistochemical data represent means ± standard error of the mean. All the sections where VM was present were counted for each animal and three to four pairs of mice (WT and Lrp6 mutant) were analyzed. Statistical analysis was performed using Prism 4 software (Graph Pad, San Diego) with paired t-test (for littermates) and significance was assumed at the level of P < 0.05 (*P < 0.05; ** 0.01 < P < 0.001; ***P < 0.001).

For EdU quantification, 3 sections of ventral midbrain were randomly chosen per animal (N = 3 for WT and Lrp6−/− mice) and EdU was quantified within a 2,000μm2 box within the Glast-expressing floor plate (Fig. 2C). Graphs represent means of three animals per genotype ± standard error of the mean, statistical analyses were performed in Microsoft Excel using a two-tailed unpaired t-test and significance was assumed at the level of P < 0.05.

Measurement of Ventral Midbrain Ventricular Zone Invagination

WT and Lrp6−/− E11.5 brains were sectioned coronally and stained with DAPI, and images were collected as described above. The angle of VM VZ invagination was measured in ImageJ (Rasband, 1997–2009), as depicted in Figure 3A, in three random sections of ventral midbrain per animal (N = 3 for WT and Lrp6−/− mice). The graph represents means of three animals per genotype ± standard error of the mean, statistical analyses were performed in Microsoft Excel using a two-tailed unpaired t-test and significance was assumed at the level of P < 0.05 (*P < 0.05).

Reverse Transcription

Total RNA was isolated from pools of VM dissected from E10.5, E11.5, E13.5, E15.5, and postnatal day (P) 1 rats, or from Lrp6−/− or WT E11.5 VMs (n = 6) or E13.5 VMs (n = 3), using RNeasy extraction kit (Qiagen, Hilden, Germany). For RT, 0.25–1 μg of total RNA was initially treated with 1 unit of RQ1 RNAse-free DNAse (Promega, Madison, WI) for 40 min. The DNAse was inactivated by the addition of 1 μl of EDTA 0.02 M and incubated at 65°C for 10 min. A total of 1.5 μg random primers (Invitrogen) were then added, and the mixture was incubated at 65°C for 5 min. Each sample was then divided equally into two tubes, a cDNA reaction tube and a negative control tube (RT−). A master mix containing 1× First-Strand Buffer (Invitrogen), 0.01 M dithiothreitol (DTT; Invitrogen), and 0.5 mM dNTPS (Promega) was then added to both cDNA and RT− tubes and incubated at 25°C for 10 min, followed by a 2-min incubation at 42°C. Supercript II reverse transcriptase (200 units, Invitrogen) was then added only to the cDNA tubes and all samples were incubated at 42°C for 50 min. Superscript II was inactivated by incubation for 10 min at 70°C. Both cDNA and RT- were then diluted 10 times, for further analysis.

Primer Design and Quantitative PCR

Genbank cDNA sequences were used to design gene specific primers in Primer Express 2.0 (PE Applied Biosystems, CA). The specificity of PCR primers was determined by BLAST run of the primer sequences. The oligonucleotide sequences for the primers are displayed in Table 1 and their annealing temperature is 59/60°C, unless otherwise indicated. Apart from Quantum RNA classic 18S internal standard (Ambion, Austin, TX), all primers were purchased from DNA Technologies, Denmark.

Table 1. Oligonucleotide Sequences of Primers
mRNAAnnealing temperature (°C)Sequence (5′-3′)
Lrp5 forward61GACATCTACAGCCGGACACTGTTC
Lrp5 reverse TGGACATTGATAGTGTTGGTGGC
Lrp6 forward59GCTACAAATGGCAAAGAGAATGC
Lrp6 reverse CAGTATACAAGCCATGACCAAACA
TH forward62AGTACTTTGTGCGCTTCGAGGTG
TH reverse CTTGGGAACCAGGGAACCTTG
Nestin forward59GTCAGATCGCTCAGATCCTGGA
Nestin reverse CCAGACTAAGGGACATCTTGAGGT
AHD2 forward59GGAAGAAAGAAGGAGCCAAACTG
AHD2 reverse ACTTCATGATTTGTTGCACTGGTC
Pitx3 forward65TTCCCGTTCGCCTTCAACTCG
Pitx3 reverse GAGCTGGGCGGTGAGAATACAGG

qPCR reactions were performed twice for a particular gene, in triplicate (or duplicate) for each sample. Each PCR reaction had a final volume of 25 μl and was derived from 75-μl (50-μl) master mixes containing 3 μl (2 μl) of 10×-diluted cDNA or RT−. Each PCR reaction consisted of 1× PCR buffer (Invitrogen), 3 mM MgCl2 (Invitrogen), 0.2 mM dNTPs (Promega, Madison), 0.3 μM of each of the forward and reverse primers, 0.5 unit Platinum Taq DNA polymerase (Invitrogen) and 1× SYBR Green (Molecular Probes, Leiden, The Netherlands). The following thermo cycling program was used: 94°C for 2 min and then for 35–40 cycles 94° C for 30 sec, 60°C for 30 sec, 72° C for 15 sec, and at 80°C for 5 sec (for SYBR Green detection), on the ABI PRISM 5700 Detection System (PE Applied Biosystems, Foster City, CA). Alternatively, the Platinum Quantitative PCR SuperMix-UDG (Invitrogen) was used, according to the manufacture's instructions (but with a 4× dilution from the original mastermix, instead of 2×). Random PCR products were also run in a 2% agarose gel to verify the size of the amplicon.

Standard curves were generated for every real-time PCR run and were obtained by using serial three-fold dilutions of a sample containing the sequence of interest (reverse transcribed RNA, plasmid containing sequence or genomic DNA). Their plots were used to convert Cts (number of PCR cycles needed for a given template to be amplified to an established fluorescence threshold) into arbitrary quantities of initial template for a given sample. The expression levels were then obtained by subtracting the RT− value for each sample from the corresponding cDNA value (when appropriate), and subsequently normalized by the value of the housekeeping gene, 18S, obtained for every sample in parallel assays. The 18S assays were run at the beginning and in the middle of assays, to verify the integrity of the samples.

Statistical analysis of the qPCR results was performed by paired t-test. Significance for all tests was assumed at the level of P < 0.05 (*P < 0.05; **P < 0.001; ***P < 0.0001).

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

We thank Drs. Anita Hall, Ola Hermanson, Kenji Imai, and Ulrich Heinzmann for critical reading and advice and Lottie Jansson-Sjostrand, Lenka Bryjova, Lena Amaloo, Johnny Söderlund, Claudia Tello, and Susanne Laass for additional assistance. We thank Drs. Kathy Pinson (Berkeley) and William Skarnes (The Sanger Institute), for the Lrp6−/− mice. E.A. was funded by grants from the Swedish Foundation for Strategic Research (INGVAR and CEDB), Swedish Research Council (DBRM), Norwegian Research Council, Karolinska Institutet, Michael J. Fox Foundation, and European Commission (Eurostemcell). G.C.B. is supported by the Portuguese Fundação para a Ciência e Tecnologia, Karolinska Institute and Calouste Gulbenkian Foundation. The Work of W.W. and N.P. is supported by BMBF National Genome Research Network (NGFN+ Functional Genomics of Parkinson Disease), Virtual Institute on Neurodegeneration and Ageing, the Initiative and Networking Fund in the framework of the Helmholtz Alliance of Systems Biology and of Mental Health in an Ageing Society, Bayerischer Forschungsverbund ‘ForNeuroCell’, European Union, and Deutsche Forschungsgemeinschaft.

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  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. RESULTS
  5. DISCUSSION
  6. EXPERIMENTAL PROCEDURES
  7. Acknowledgements
  8. REFERENCES
  9. Supporting Information

Additional Supporting Information may be found in the online version of this article.

FilenameFormatSizeDescription
DVDY_22094_sm_SuppFig1.tif764KSupporting Information Figure 1. Temporal regulation of Lrp5 and Lrp6 expression during midbrain development. A,B: Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis revealed that Lrp5 (A) is expressed at constant levels in the developing ventral midbrain (VM), whereas Lrp6 (B) shows a peak of expression at embryonic day (E) 11.5. The dopaminergic (DA) neurogenic period in the rat ventral midbrain (VM) is highlighted with a box. (Error bars represent SEM of technical replicates as qRT-PCR was performed on pools of VMs). C: Detection of TH, Lrp5, and Lrp6 expression on sagittal sections of E11.5 wild-type mouse embryos by in situ hybridization. Lrp5 and Lrp6 are ubiquitously expressed in the brain, including the VM domain containing TH+ DA neurons. au, arbitrary units; te, telencephalon; ov, optic vesicle; di, diencephalon; mes, mesencephalon; hb, hindbrain.
DVDY_22094_sm_SuppFig2.tif3956KSupporting Information Figure 2. Lrp6−/− mice with exencephaly display normal ventral patterning, despite gross morphological dorsal defects. A,B: Shh (floor plate [FP] and basal plate [BP] marker), Lmx1b (FP and roof plate [RP] marker), and Wnt3a (RP marker) in situ hybridization of embryonic day (E) 9.5 (A) and E10.5 (B) wild-type (WT) and exencephalic Lrp6−/− mice revealed that patterning of the ventral domains is maintained, even though dorsal structures are grossly disrupted in the mutants. A: At E9.5, the ventricular invagination at the midline appears sharper and Shh and Lmx1b expression in the FP/BP might be broadened in Lrp6−/− mice. B: This difference is not seen in the mutants at E10.5.
DVDY_22094_sm_SuppFig3.tif2320KSupporting Information Figure 3. Proliferation, as assessed by BrdU (5-bromo-2-deoxyuridine) incorporation, is unchanged in Lrp6−/− mice. The proliferative capacity of embryonic day (E) 11.5 ventral midbrain (VM) precursors was not affected in Lrp6−/− VM as assessed by immunohistochemistry against BrdU in sagittal sections at E10.5, E11.5, E12.5, and E15.5.
DVDY_22094_sm_SuppFig4.tif1597KSupporting Information Figure 4. Cell death is not affected in Lrp6−/− mice. Cleaved caspase 3+ cells were virtually absent in wild-type and Lrp6−/− ventral midbrain (VM) at embryonic day (E) 11.5. All images were acquired with the same settings. Inset in each image is another region of the same brain showing positive staining.

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