Shox2 is required for the proper development of the facial motor nucleus and the establishment of the facial nerves
© Rosin et al. 2015
Received: 9 March 2015
Accepted: 12 June 2015
Published: 9 July 2015
Axons from the visceral motor neurons (vMNs) project from nuclei in the hindbrain to innervate autonomic ganglia and branchial arch-derived muscles. Although much is known about the events that govern specification of somatic motor neurons, the genetic pathways responsible for the development of vMNs are less well characterized. We know that vMNs, like all motor neurons, depend on sonic hedgehog signaling for their generation. Similarly, the paired-like homeobox 2b (Phox2b) gene, which is expressed in both proliferating progenitors and post-mitotic motor neurons, is essential for the development of vMNs. Given that our previous study identified a novel role for the short stature homeobox 2 (Shox2) gene in the hindbrain, and since SHOX2 has been shown to regulate transcription of islet 1 (Isl1), an important regulator of vMN development, we sought to determine whether Shox2 is required for the proper development of the facial motor nucleus.
Using a Nestin-Cre driver, we show that elimination of Shox2 throughout the brain results in elevated cell death in the facial motor nucleus at embryonic day 12.5 (E12.5) and E14.5, which correlates with impaired axonal projection properties of vMNs. We also observed changes in the spatial expression of the vMN cell fate factors Isl1 and Phox2b, and concomitant defects in Shh and Ptch1 expression in Shox2 mutants. Furthermore, we demonstrate that elimination of Shox2 results in the loss of dorsomedial and ventromedial subnuclei by postnatal day 0 (P0), which may explain the changes in physical activity and impaired feeding/nursing behavior in Shox2 mutants.
Combined, our data show that Shox2 is required for development of the facial motor nucleus and its associated facial (VII) nerves, and serves as a new molecular tool to probe the genetic programs of this understudied hindbrain region.
In the hindbrain, motor neuron progenitors are born in a region-specific manner, neighboring the floor plate, in response to their level of exposure to different morphogenetic gradients [1, 2]. These neuronal progenitors differentiate in a rhombomere-specific pattern, which requires the correct spatiotemporal expression of the homeotic (Hox) gene clusters [3–6]. Motor neuron progenitors in the hindbrain can differentiate into somatomotor neurons (sMNs), which innervate the skeletal muscles of the body, or visceromotor neurons (vMNs), which innervate autonomic ganglia (general vMNs) and branchial arch-derived muscles (special vMNs) [3, 7, 8]. Facial (VII) motor neurons, which originate from rhombomeres 4/5, belong to the vMN class of motor neurons. At embryonic day 11.25 (E11.25), facial motor neurons begin to migrate caudally to the pial side of the hindbrain to form the facial motor nucleus, which subsequently segregates into two lobes [6, 9, 10]. Despite the elucidation of the genetic mechanisms that specify sMN identity, the corresponding pathways responsible for the establishment of vMNs are less well understood. Furthermore, even though sonic hedgehog (SHH) signaling is known to be required for the development of all motor neurons [7, 11–15], the upstream regulators of Shh expression and function during late embryonic and early postnatal development, especially within the vMNs of the facial motor nucleus, remain largely unknown .
The paired-like homeobox 2b (Phox2b) gene is expressed in both proliferating progenitors and post-mitotic motor neurons and is required for the development of vMNs [17, 18]. Similarly, the homeodomain transcription factor islet1 (ISL1), which is expressed in both post-mitotic sMNs and vMNs, plays a broad role in motor neuron identity and specification [7, 8, 17, 19, 20]. In the absence of Isl1, mice die during embryogenesis around E11.5. This embryonic lethality is associated with an increase in cell death in the hindbrain and neural tube, suggesting that loss of Isl1 function results in the death of cells that were fated to differentiate into motor neurons . In the trigeminal and dorsal root ganglia (DRG), Isl1 appears to play distinct roles during early (i.e. neurogenesis) versus late (i.e. subtype specification) development . In cranial ganglia, Isl1 is required for cell survival and Isl1 mutants show thinner blunted facial (VII) ganglia . However, the precise role of Isl1 in these cell types and in their respective hindbrain nuclei, such as the facial motor nucleus, remains poorly documented.
The adult facial motor nucleus contains seven subnuclei; the lateral, dorsolateral, dorsal intermediate, ventral intermediate, dorsomedial and ventromedial nuclei are located within the main nucleus, while the seventh, the dorsal accessory nuclei, is located above the main nucleus [23, 24]. Neurons innervating the nasolabial musculature are located in the lateral facial motor nucleus, while neurons supplying the auricular musculature are located in the medial facial motor nucleus [23, 24]. Together the lateral, intermediate and medial subnuclei comprise 43, 27.1, and 28.7%, respectively, of the motor neurons in the adult facial motor nucleus, with the dorsal accessory nuclei contributing the final 1.2% .
Studies with mouse knockout models have shown that Shox2 is required for normal development of the humerus and femur [25–27], the anterior palate , the temporomandibular joint of the jaws , the sinoatrial valves and pacemaker region of the heart [30, 31], and TrkB-positive mechanosensory neurons of the dorsal root ganglia . Most recently, Shox2 has been shown to play an important role in development of the inferior colliculus and cerebellum . We suspected a function for Shox2 during facial motor nucleus development given our previous study demonstrating a novel role for Shox2 in the hindbrain , and since SHOX2 has been shown to regulate transcription of Isl1 , an important regulator of vMN development [7, 8, 19, 22]. In the current study we show that conditional inactivation of Shox2 in the brain results in impaired axonal projections of vMNs and a concomitant loss of medially located neurons in the facial motor nucleus postnatally. Furthermore, we demonstrate that Shox2-mutant neonates display impaired feeding behavior, perhaps due to facial paralysis as a result of improper development and function of the facial nerves.
Shox2 expression in the face and facial motor nucleus during development
Nestin-Cre Shox2-mutant pups exhibit defects in feeding behavior
To determine the functional role of Shox2 in the facial motor nucleus, we used Nestin-Cre  to delete the Shox2 gene conditionally in neural tissue throughout the central nervous system (CNS). We crossed Nestin-Cre; Shox2 +/− mice with Shox2 flox/flox mice  to obtain conditional mutant Nestin-Cre; Shox2 flox/− embryos and neonates. Shox2 expression was eliminated in the brain of Nestin-Cre Shox2-mutant embryos, including the facial motor nucleus (Additional file 3: Figure S3C–D, dashed-circle). All Nestin-Cre Shox2-mutant pups died between 18 to 26 h following birth. This limited our characterization of Shox2 function to embryonic and neonatal stages. P0 Nestin-Cre; Shox2 flox/− mutant pups displayed reduced activity and impaired feeding behavior as early as 6 h following birth (Additional file 4: Movie 1), which correlated with a lack of milk in the stomachs of Nestin-Cre; Shox2 flox/− mutant pups (Additional file 3: Figure S3E, arrows). Since Shox2 is required for the proper development of the palate , we analyzed the anterior and posterior palate of Nestin-Cre; Shox2 flox/− mutant pups. We did not observe any obvious defects or malformations in the palate of Nestin-Cre; Shox2 flox/− mutant pups (Additional file 3: Figure S3F–K), suggesting that the absence of milk in Shox2-mutant neonatal stomachs was not due to cleft palate malformations. Nestin-Cre; Shox2 flox/− mutant pups also displayed tremors 16–26 h following birth (Additional file 4: Movie 1), which could be related to defects in neural circuitry or result from a loss of Shox2 function in other regions of the hindbrain.
Facial (VII) nerve branching defects in Nestin-Cre Shox2-mutant animals
Loss of Shox2 in the facial motor nucleus results in elevated apoptosis
Shox2 is required for normal expression of markers of vMN identity in the facial motor nucleus
Loss of Shox2 results in severe developmental disruptions to the facial motor nucleus
In contrast to the subtle changes at E14.5, postnatally we observed a drastic reduction in the size of the facial motor nucleus and changes to the overall organization of motor neurons in Nestin-Cre; Shox2 flox/− mutant animals (Figure 6), consistent with the phenotype displayed in Figure 4. Specifically, lacZ staining of P0 control and Nestin-Cre; Shox2 lacZ/flox sagittal sections of the facial motor nucleus demonstrate that the nucleus is reduced in size in mice lacking Shox2 (Figure 6b). The expression levels of Isl1, Phox2b, Cntn2, Tubb3, Periph, Slc18a3 and Slit2 were all strikingly reduced in the P0 facial motor nucleus of Nestin-Cre; Shox2 flox/− mutant animals as compared to controls (Figure 6e–r, dashed-circle). Since conditional inactivation of Shox2 in the brain was previously shown to result in the down-regulation of Shh expression in dorsal-residing Purkinje cells of the cerebellum , we next investigated whether loss of Shox2 function in the facial motor nucleus interfered with the expression of Shh. The expression of both Shh and patched 1 (Ptch1) were absent in the facial motor nucleus of Nestin-Cre; Shox2 flox/− mutant animals (Figure 6s–v).
In this study, we used a Nestin-Cre driver and a conditional knockout strategy to determine the contribution of Shox2 to the development of the facial motor nucleus. We found that mice lacking Shox2 function failed to develop proper vMNs in the medial subnuclei normally present in the facial motor nucleus, which was accompanied by truncated or absent facial (VII) nerve projections. Our results have uncovered an unexpected and novel role for Shox2 in the development of the facial motor nucleus and facial (VII) nerves. Specifically, Shox2 is required for cell survival and perhaps plays a region-specific role in the development and/or maintenance of normal transcriptional programs of vMNs of the facial motor nucleus. Ultimately, in the absence of Shox2, pups lack facial (VII) nerves and display abnormal feeding behavior.
Shox2 is required for cell survival in the facial motor nucleus
Nestin-Cre-mediated deletion of Shox2 in the brain resulted in a significant increase in the number of apoptotic cells present at E12.5 and E14.5 in the facial motor nucleus of Shox2-mutants. Given that truncated and abnormal facial (VII) nerve projections could be observed in Nestin-Cre; Shox2 flox/− mutants from E11.5 onward, apoptosis could: (1) contribute to the disruptions in the axonal projection properties of vMNs in the facial motor nucleus, or (2) result from a lack of peripheral trophic support for the facial (VII) nerves in Shox2 mutants. Similarly, the changes observed in the size of the P0 facial motor nucleus also likely results from elevated cell death embryonically in Shox2-mutant animals. However, given that there are ~6,400 neurons that comprise the facial motor nucleus [23, 24], future analysis is needed to quantify cell death accurately over a developmental time course to determine if the levels of cell death we observe in Shox2-mutant animals is sufficient to account for the disruptions observed in the facial motor nucleus in the absence of Shox2. This is similar to what has been documented in Isl1 mutants, as loss of Isl1 is associated with increased cell death in the trigeminal, DRG, hindbrain and neural tube prior to E14.5, while later developmentally Isl1 functions during subtype specification [19, 21, 22, 38]. Accordingly, if Shox2 is required to maintain proper Isl1 expression in vMNs of the facial motor nucleus, similar to what has been observed during heart development , and Isl1 is required to induce the subsequent differentiation of adjacent neurons, similar to what has been observed in the neural tube , this could explain the elevated cell death observed in neurons adjacent to the facial motor nucleus at P0. Pattyn et al.  have shown that unlike PHOX2B and ISL1, NKX6 proteins (NKX6.1 and NKX6.2) are not required for the establishment of vMN identity, yet NKX6 proteins are necessary to prevent vMNs from trans-differentiating into improper neuronal subtypes. Similar to Shox2 mutants, mice lacking Nkx6 function display impaired axonal projection properties of vMNs and facial nerve disruptions . Together, the Shox2-mutant model could provide an additional example to support the argument of Pattyn et al. that vMN differentiation requires multiple inputs rather than one dominant determinant . Alternatively, necrosis in the facial motor nucleus region could be responsible for the observed ectopic increase in the number of apoptotic cells in tissue immediately surrounding the facial motor nucleus.
Shox2 may play a regional specific role in the transcriptional programs of vMNs during development of the facial motor nucleus
In this study we have shown that Shox2 is expressed in the facial motor nucleus and provide evidence that Shox2 plays an important role during facial motor nucleus development. We have demonstrated that loss of Shox2 function results in the down-regulation or loss of Isl1, Phox2b, Cntn2, Tubb3, Periph, Slc18a3 and Slit2 in the facial motor nucleus. Since loss of Isl1 function in the trigeminal and DRG have been shown to result in changes in the expression of Cntn2, Tubb3 and Periph , and given that Phox2b-deficiency influences Slit2 expression , we propose that the observed changes in gene expression in Shox2 mutants could result from direct or indirect regulation of Isl1 and/or Phox2b by SHOX2. Since Isl1 can rescue Shox2-mediated bradycardia, and Isl1 has been shown to be a transcriptional target of SHOX2 in vitro , it would not be surprising if SHOX2 regulates Isl1 in the facial motor nucleus.
As conditional inactivation of Shox2 in the brain was previously shown to result in the down-regulation of Shh expression in dorsal-residing Purkinje cells of the cerebellum , we also investigated whether loss of Shox2 function in the facial motor nucleus interfered with expression of Shh. Similar to what was observed in the cerebellum, the expression of both Shh and Ptch1 were reduced or lost in the facial motor nucleus of Shox2-mutant animals, suggesting that Shox2 acts upstream of SHH signaling, or that the neurons that would normally express Shh underwent apoptosis earlier embryonically in Shox2-mutant animals. Despite the known critical function of Shh in motor neuron generation [7, 11–15], the role of Shh in the later development of the vMNs of the facial motor nucleus remains largely unknown. In rats, SHH appears to play a role in the regeneration of the facial motor nucleus and facial (VII) nerves following nerve injury . Therefore, as the facial motor nucleus normally undergoes massive cell death from E19 to P10 , the late-onset of Shh expression in the facial motor nucleus could be localized to specific vMNs for cell survival. This hypothesis will be tested in future studies. Together, it appears that Shox2 is necessary for proper Isl1 and Phox2b expression levels in the developing facial motor nucleus, and is subsequently required for late-onset Shh expression; whether this results from facial (VII) nerve disruption and embryonic apoptosis in Shox2 mutants or occurs via direct or indirect regulation of Isl1, Phox2b and/or Shh has yet to be determined.
Since the phenotype observed in the facial motor nucleus of Shox2 mutants, including the changes in gene expression, were most prominent in the medial and intermediate subnuclei, factors other than SHOX2 must contribute to the development and survival of the lateral subnuclei of the facial motor nucleus. The observed phenotype could reflect a regional specific requirement for Shox2 in the facial motor nucleus or indicate that there are populations of vMNs within the facial motor nucleus that do not express Shox2. The latter hypothesis is supported by results depicted in Figure S2F, H, J, as the Shox2 expression domain appears more restricted then those of Isl1 and Phox2b; therefore, future analysis will be required to examine whether SHOX2−/ISL1+ and/or SHOX2−/PHOX2B+ cell populations exist in the facial motor nucleus. Alternatively, if vMNs destined for the lateral subnuclei of the facial motor nucleus are specified first, this could explain why they are the least disrupted group of nuclei apparent in Shox2 mutants. However, considering the severe disruptions to facial (VII) nerve development observed in Shox2 mutants, including those that innervate the nasolabial musculature that are supplied by neurons located in the lateral subnuclei of the facial motor nucleus [23, 24], it is also possible that Shox2 is required for the proper development of the lateral facial motor nuclei. This is further supported by changes in the expression of Cntn2, Tubb3, Shh and Ptch1 in the lateral facial motor nucleus. Taken together, it appears that Shox2 may play a role in the transcriptional programs of vMNs during development of the facial motor nucleus; whether this occurs via a regional requirement for Shox2 in the facial motor nucleus or influences the development of the entire facial motor nucleus in an equal manner has yet to be determined.
Disruptions in the development of the facial motor nucleus influences the axonal projections of vMNs of the facial (VII) nerves in Shox2-mutant animals
As Shox2-mutant neonates display impaired feeding behavior, we wondered if this was due to a loss of a subset of axons in the nerves present in the face or whether it reflected a more substantial innervation deficit. Our findings demonstrate that the loss of Shox2 causes severe truncations and disruptions to the development of the facial (VII) nerves, which could result from: (1) a lack of peripheral trophic support for the facial (VII) nerves, (2) vMN cell death, (3) facial motor nucleus defects that influence the axonal projections of vMNs and target acquisition of the facial (VII) nerves, or a combination thereof. It should be mentioned, however, that disruptions in peripheral trophic support for the facial (VII) nerves would have to result from Shox2’s autonomous function in the facial (VII) nerves themselves, and not as a result of Shox2 playing a role as a peripheral target, since loss of Shox2 expression in the face in Wnt1-Cre; Shox2 flox/− mutant animals does not disrupt facial (VII) nerve development.
Given that the face of Shox2-mutant neonates are severely devoid of innervation, the behavioral abnormalities exhibited by Shox2-mutant animals could be attributed to facial paralysis. Disruption of Hoxb1 function has been shown to result in facial nerve defects in mice, where the phenotype is similar to that of human patients with Bell’s Palsy or Moebius Syndrome . Similarly, other Hox-mutant mouse models have displayed disruptions in facial (VII) nerve development, which correlate with facial impairments . Moreover, Isl1 mutants display disruptions in the development of the facial (VII) nerves, however similar to Shox2 mutants, early postnatal lethality has limited the behavioral analysis of these animals [21, 22, 38]. To determine if disruption to the facial motor nucleus is the cause of the early postnatal lethality of Nestin-Cre; Shox2 flox/− animals, future studies could employ more restricted hindbrain Cre lines to specifically remove Shox2 expression from the facial motor nucleus. If regionally restricted loss of Shox2 in facial motor nucleus does not result in postnatal lethality, future studies could examine Shox2-mutant behavior later postnatally and in the adult to test whether Shox2-mutant animals have control over their face or are experiencing facial paralysis.
Taken together, these results demonstrate that elimination of Shox2 in the brain results in disruptions in the development of the facial (VII) nerves and the facial motor nucleus. The present study demonstrates that in the absence of Shox2 the murine face develops with severe innervation deficits, ultimately highlighting the importance of Shox2 to proper facial motor nucleus development. The Shox2-mutant model can now be exploited to further our understanding of normal neuronal development and circuit formation; processes that are likely dysregulated in human facial paralysis disorders.
To generate conditional Shox2 knockout animals, mice bearing the Nestin-Cre transgene , in addition to a wild-type and deleted Shox2 allele, were crossed to mice carrying two floxed Shox2 alleles . Shox2 mutants were positive for Nestin-Cre and carried one deleted and one floxed Shox2 allele. Littermate control animals carried either one or two functional copies of Shox2, as no changes in the gross morphology of the facial motor nucleus or facial nerves were observed in heterozygotes. We designated embryonic day 0.5 (E0.5) to be noon on the day a plug was detected. Mice were maintained on a mixed C57BL/6-129/Sv background. Mice carrying the deleted and floxed Shox2 alleles were described previously , while mice carrying the Nestin-Cre transgene were from the Jackson Laboratory (Bar Harbor, ME), and described previously . BAC RP23-105B3-lacZ transgenic animals were described previously , while Shox2 lacZ mice are described below. All experiments performed at the University of Calgary were approved by the Life and Environmental Sciences Animal Care Committee. Experiments were conducted on three embryos or neonates of a particular genotype at each gestational and postnatal time point described, unless otherwise stated.
Generation and characterization of Shox2 lacZ mice
Shox2 lacZ mice were produced by pronuclear injection of a 15.4 kb NarI fragment (Additional file 1: Figure S1A) of BAC RP23-103D17-lacZ that has a lacZ cassette inserted in the Shox2 gene as previously described . One of seven transgenic lines carrying this transgene showed an unexpected expression pattern that recapitulated all of the expression domains of the endogenous Shox2 gene (compare Additional file 1: Figure S1D–S1E), despite evidence that the regulatory regions controlling expression in most of these domains is not within the transgene sequence . This result suggested that the transgene in this line, designated Shox2 lacZ may have inserted into the Shox2 locus. Although rare, targeting of transgenic constructs generated by pronuclear injection has been reported [41, 42]. We confirmed a loss of Shox2 function by a complementation test and targeting was shown by long range PCR (Additional file 1: Figure S1B). For the former, the Shox2 lacZ allele was combined with the Shox2 − allele. Mice with the genotype Shox2 lacZ/− died at birth with defects similar to Shox2 null animals, showing non-complementation (the limb and palate phenotypes are shown in Additional file 1: Figure S1F–K, respectively). Long-range PCR was performed with a forward primer corresponding to a sequence 4.5 kb upstream of Shox2 (327 bp upstream of the 5′ limit of the transgene) and a reverse primer within the lacZ sequence. These primers amplified a 5.2 kb PCR product from DNA of Shox2 lacZ mice as expected if the transgene were inserted at the Shox2 genomic locus (Additional file 1: Figure S1B). Furthermore, western blotting failed to detect SHOX2 protein in lysates from E11.5 limb buds of Shox2 lacz/lacZ embryos while lysates from wild-type littermate embryos showed clear SHOX2 signal (data not shown). Therefore we conclude that the Shox2 lacZ allele is a null or severely hypomorphic allele. The precise molecular nature of the transgene insertion at its 3′ end has not yet been determined.
Embryos with lacZ transgenes were stained with X-gal according to standard techniques . ImageJ was used to determine the relative size of the facial motor nucleus in lacZ stained control and NestinCre; Shox2 lacZ/flox sagittal sections at E14.5 and P0 (Figure 4t–u).
In situ hybridization
Whole-mount in situ hybridization (WISH) with a riboprobe was previously described . In situ hybridization (ISH) on cryosections was previously described . The Shox2 , Shh  riboprobes have been described elsewhere. PCR primers used for generating the Ptch1 riboprobe are as follows: Fwd: GGGAGGAAATGCTGAATAAAGCC and Rev: CCAGGAGGAAGACATCATCCACAC, while the primers used for generating the Isl1, Cntn2, Tubb3, Periph, Slc18a3, Phox2b, and Slit2 riboprobes were obtained from the Allen brain atlas website (http://www.brain-map.org). DIG labeled riboprobes were detected with an AP-conjugated anti-DIG antibody (Roche) and BM Purple AP (Roche) was used as the color development substrate. All samples/slides used for comparisons were processed together. Following staining, tissue sections were mounted using Aqua Poly/Mount (Polysciences Inc.). ISH images were taken with a Leica MZ12.5 stereomicroscope using the associated Leica software. Brightness and/or contrast of the entire image was adjusted using Adobe Photoshop CS5.1 if deemed appropriate.
Dissected embryos and postnatal brains were placed in ice-cold PBS, and fixed in 4% paraformaldehyde (PFA) overnight at 4°C. The tissue was washed in PBS and equilibrated in 30% sucrose/PBS overnight at 4°C. Both embryos and postnatal brains were embedded in Tissue Freezing Medium (TFM, Triangle Biomedical Sciences) and cryosectioned (16–20 μm sections). For immunohistochemistry (IHC), cryosections were rehydrated in PBS, washed with PBT (0.1% Triton-X), blocked using 5% normal goat serum (NGS) for 2 h at room temperature, and exposed to anti-active Caspase-3 (1:400, BD Pharmingen or 1:500, Promega) at 4°C overnight. Slides were then washed with PBT and exposed to secondary antibody (1:200, alexa 488 goat-anti-rabbit, Life Technologies) for 2 h at room temperature. Sections were mounted using Aqua Poly/Mount (Polysciences Inc.). Fluorescent IHC images were captured on a Zeiss LSM710 confocal microscope.
Antibodies recognizing neurofilaments (2H3) were used to visualize nerves in embryos or postnatal day 0 (P0) pup heads (skin was removed at P0). Samples were fixed at least 4 h in Dent’s fixative (4:1, Methanol:DMSO). Embryos and P0 heads were processed according to standard techniques previously described . The 2H3 antibody was developed by T. Jessell/J. Dodd  and was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa, Department of Biology, Iowa City, IA 52242.
Quantification methods and statistical analysis
Quantitative results for cell counts are represented by mean scores ± S.E.M. and were analyzed by two-tailed unpaired t tests using Prism 3 for Macintosh (GraphPad Software, La Jolla, CA, USA).
JMR, DMK and JC conceived and designed the experiments. JMR performed the experiments and prepared the manuscript. JMR, DMK and JC edited the manuscript. All authors read and approved the final manuscript.
The authors thank C. Schuurmans and N. Klenin for the α-Caspase3 antibody, S. Neufeld for the Ptch1 in situ probe, S. Abassah-Oppong and I. Skuplik for BAC RP23-105B3-lacZ mice and embryos, B. Bobick for Wnt1Cre; Shox2 +/− animals and assistance during the analysis of the Shox2 lacZ transgenic line, S. Pattar and C. Ostler for their contributions to the analysis of the Shox2 lacZ transgenic line, R. Hampton for animal care, and D. Hansen, E. Turner and T. Cox for critical discussion. J.C is supported as an Alberta Heritage Foundation for Medical Research (AHFMR) Scholar. J.M.R. is supported by fellowships from NSERC and the CIHR Training Program in Genetics, Child Development and Health at the University of Calgary. D.M.K is supported by the Canadian Institute of Health Research (CIHR). This work was supported by the Canadian Institute of Health Research (CIHR) (MOP-93562).
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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