T-type calcium channels belong to the family of low voltage activated Ca²⁺ (LVA) channels encoded by three α1 subunit genes: α1G (Ca_v3.1), α1H (Ca_v3.2), and α1I (Ca_v3.3) (Perez-Reyes, 1999). LVA channels are involved in cell excitability due to their low voltage threshold for activation (Cain and Snutch, 2010), and shape neuronal firing patterns because of their unique gating properties (Fox et al., 1987; Perez-Reyes et al., 1998; Vassort et al., 2006; Powell et al., 2009; Isope et al., 2012). Unfortunately, determination of the individual contribution of each T-type subunit to these physiological processes has been hampered by the lack of specific subtype pharmacological blockers, and the fact that in native systems more than one T-type channel is often expressed (Perez-Reyes, 2003). Also, their highly conserved primary amino-acid sequence has made the production of antibodies specific for individual subunits extremely challenging (Cribbs et al., 1998). Nevertheless, the expression and distribution of the mRNA transcripts and protein for each subunit in the brain and dorsal root ganglia (DRGs) of rats has been described (Talley et al., 1999; McKay et al., 2006).

In order to separate the functions of these three different channel subunits, an important first step is to determine exactly where and when each individual subunit is expressed. We are interested in the Ca_v3.2 ion channel’s role in the somatosensory system. Some studies show a role for this subunit in peripheral nociception (Bourinet et al., 2005; Choi et al., 2006; Wen et al., 2006), while others have emphasized a role of this channel in the physiology of an ultrasensitive subpopulation of mechanoreceptors in the DRG, the D-hair receptors (Shin et al., 2003; Dubreuil et al., 2004; Wang and Lewin, 2011; Lechner and Lewin, 2013; François et al., 2015) and recently also C-fiber low-threshold mechanoreceptors (C-LTMRs) (François et al., 2015). To resolve issues of where and when Ca_v3.2 channels are expressed we used gene targeting to generate two knock-in strains: the Ca_v3.2^EGFP and the Ca_v3.2^Cre. The first expresses EGFP and the second Cre recombinase under the control of the Ca_v3.2 gene promoter. The Ca_v3.2^EGFP strain allowed us to examine the spatiotemporal expression of Ca_v3.2 ion channels since detection of EGFP represents a measure of Ca_v3.2 gene promotor activity. We determined the onset of the Ca_v3.2 gene expression during development in the nervous system and characterized the type of neurons expressing the Ca_v3.2 channel in the DRG at E18.5 and in the brain of adult mice. The Ca_v3.2^Cre strain was used in conjunction with the Tau^mGFP reporter mouse which is specific for the nervous system or the Rosa26^LacZ strain which is a ubiquitous reporter line. In this way we could determine cell types that had expressed the Ca_v3.2 ion channel at any point throughout development. Moreover, the reporter line used here also allowed us to visualize those cells that express or had expressed Ca_v3.2 both in vitro and in vivo. Using this approach, we observed that the expression of the Ca_v3.2 gene was absent in certain neural crest lineages giving rise to slowly adapting mechanoreceptors (SAMs) in the sensory lineage. We also show that viral-based gene transfer in adult Ca_v3.2^Cre mice is a powerful tool to label specific sensory receptor subtypes.

This study was carried out in accordance with the recommendations of the German animal welfare laws and protocols for euthanasia and virus injections were approved by German federal authorities (State of Berlin).

We generated the Ca_v3.2^EGFP knock-in mouse in order to unambiguously visualize cells expressing the Ca_v3.2 channel. We observed strong EGFP staining in the brains of Ca_v3.2^EGFP adult animals, e.g., in the dentate gyrus (DG) of the hippocampus (Figure 1A and zoom in), and in axons located in the internal capsule (Figure 1A). The GAP43 membrane targeting signal attached to the EGFP molecule explains why the majority of the staining is observed in mossy fiber axons coming from dentate granule neurons (Mosevitsky, 2005). The positive staining observed in the internal capsule is presumably derived from the ascending or descending axons that run between the cerebral cortex and the pyramids of the medulla. Previous reports demonstrated that the Ca_v3.2 channel is also expressed in other brain regions of rats such as the olfactory system, insidium griseum in the basal forebrain, amygdala, hypothalamus, reticular thalamic nucleus and layer V of the neocortex (Talley et al., 1999; McRory et al., 2001; Chen et al., 2010). We considered the possibility that a low expression level of Ca_v3.2 in these cells made the immunodetection of EGFP difficult in these brain regions. To test this idea, we used real time qPCR and Western blotting to detect the levels of EGFP mRNA and protein present in these tissues in our knock-in mice (Figures 1B,C). Real time qPCR from total RNA of P0 Ca_v3.2^EGFP homozygous animals showed EGFP expression in hippocampus, olfactory bulbs, cerebellum, neocortex, thalamus, and brainstem (Figure 1B). This experiment revealed that the absolute level of EGFP molecules present in the hippocampus was, on average, higher compared to the other brain regions. We could also detect EGFP protein in all the tested brain regions using Western blotting; subsequent densitometry analysis of the blots also showed higher EGFP levels in the hippocampus compared to other brain regions (Figure 1C). It should be noted that all the analysis of Ca_v3.2^EGFP were carried out on homozygous mice as we had difficulty detecting EGFP in Ca_v3.2^EGFP/+ mice. Homozygous Ca_v3.2^EGFP mice were likely null for Ca_v3.2 channel expression as we noted that some mice showed premature death which is consistent with the presence of cardiac disease described in mice with a targeted deletion of this gene (Chen et al., 2003).

Considering that the Ca_v3.2^Cre strain allows a more sensitive detection of the Ca_v3.2 channel promoter activity when crossed with a reporter line, we next examined adult Ca_v3.2^Cre;Rosa26^LacZ mice. In these mice, β-gal is expressed in cells in which Cre is driven by the Ca_v3.2 promoter. In the brains from Ca_v3.2^Cre;Rosa26^LacZ mice we observed β-gal positive cells in the cortex, hippocampus, cerebellum, and brainstem (Figure 2A). In the hippocampus, positive β-gal cells were detected in the pyramidal layers of fields CA1, CA2, and CA3. In addition, scattered cells were observed in the hilus of the DG (Figure 2A and zoom in). In the cortex, β-gal positive cells were observed in layers I–VI in the retrosplenial, somatomotor and perirhinal areas, and in layer V in the somatosensory areas (Figure 2A and zoom in). Strong staining was also observed in cells of the amygdala in the striatum (Figure 2A and zoom in), in the brainstem some β-gal⁺ cells were found in the latero-dorsal thalamic nucleus (Figure 2A), and in the medulla in the hindbrain (Figure 2B and zoom in). In the cerebellum, the expression of β-gal seems to be restricted to only a few of the Purkinje cells within the cerebellar cortex (Figure 2B and zoom in).

The difference in the expression pattern of the Ca_v3.2 channel inferred from Ca_v3.2^EGFP mice and the Ca_v3.2^Cre could be due to an early activation of the Ca_v3.2 during development or to relatively weak Ca_v3.2 promoter activity in many cells. We crossed the Ca_v3.2^Cre knock-in mouse with the Tau^mGFP reporter line which is specific for the nervous system and analyzed the β-gal staining pattern in whole embryos at different stages of development. We observed positive β-gal staining in Ca_v3.2^Cre;Tau^mGFP embryos as early as E11.5. At this stage, strong nuclear β-gal staining is detected in the hind brain, DRGs and spinal cord, and only a few positive cells were observed in the forebrain and midbrain (Figure 3A and zoom in). In addition, we noticed that there was no positive β-gal staining in the sympathetic ganglia (Figure 3A, arrow head). At E12.5, the number of β-gal positive cells in the same brain areas, DRGs and spinal cord had increased enormously (Figure 3B and zoom in), and remained constant until E18.5 (Figure 3C and zoom in). Thus, using this reporter line, we can demonstrate an early onset of Ca_v3.2 promoter activity and that the Ca_v3.2 ion channel is not expressed in the sympathetic ganglia at any point during development. One advantage of using the Tau^mGFP reporter line is that mGFP is a membrane targeted protein and thus permits the tracing of axonal projections. This feature allowed us to analyze the innervation of the sensory end organs in the skin of Ca_v3.2^Cre;Tau^mGFP animals. We used glabrous and hairy skin of the hindpaw of adult mice because of the diversity of end organs in these skin areas (Li et al., 2011; Heidenreich et al., 2012; Wende et al., 2012; Lechner and Lewin, 2013). Double immunofluorescence staining against mGFP and the Schwann cell marker S100 showed that Meissner’s corspuscles and hair follicles were innervated by mGFP⁺ afferents (Figures 4A,B). Meissner corpuscles are innervated by Aβ-myelinated fibers classified as rapidly adapting mechanoreceptors (RAMs) (Lumpkin and Caterina, 2007; Wende et al., 2012), and hair follicles can be innervated by Aβ- and/or Aδ-myelinated fibers that are also classified as RAMs or D-hair receptors (Figure 4, scheme) (Li et al., 2011; Lechner and Lewin, 2013). In addition, using cytokeratin 20 to visualize Merkel cells we found that mGFP⁺ fibers in Ca_v3.2^Cre;Tau^mGFP animals were not associated with Merkel cells. We attempted to visualize mGFP⁺ fibers together with Merkel cells in three different mice and we could easily detect mGFP⁺ fibers associated with local hairs, but we never found such fibers in the Merkel disks of the hairy skin (Figure 4C). Touch domes are also found in the glabrous skin, but here as in hairy skin we found no evidence of mGFP⁺ fibers associated with these structures (Figure 4D). In the same skin regions we could always detect GFP free nerve endings, all double positive for the pan neuronal marker PGP9.5, which probably belongs to nociceptive afferent endings (Figure 4E). Merkel cells are innervated by Aβ-fibers with a slowly adapting (SA type I) response to mechanical stimuli (Figure 4, scheme) (Iggo and Muir, 1969). Thus, the lack of mGFP⁺ fibers in this end organ suggests that SA type I fibers arise from neural crest cells (NCCs) or progenitors that do not express the Ca_v3.2 gene during development.

In the DRG, the majority of the neurons appear to be positive for mGFP in Ca_v3.2^Cre;Tau^mGFP animals (Supplementary Figure 2), which is consistent with the great diversity of sensory endings that we observed in the skin. We further characterized the sensory neuron subtypes expressing mGFP in these animals using double immunofluorescence for other standard markers of sensory subtypes in the DRG. Many of the mGFP⁺ neurons were positive for NF200 suggesting that they contain myelinated Aβ- and Aδ-myelinated fibers, this is consistent with the innervation of hair follicles and Meissner’s corpuscles by mGFP⁺ afferents observed in the skin (Supplementary Figure 2A) (Bourane et al., 2009; Luo et al., 2009). We could also show that non-peptidergic nociceptors labeled with IB4, a marker for high-threshold C-unmyelinated fibers, were mGFP⁺ (Silverman and Kruger, 1990), as well as those sensory neurons positive for Tyrosine hydroxylase (TH), used to label putative low-threshold C-unmyelinated mechanoreceptors (C-LTMRs) (Supplementary Figure 2A) (Li et al., 2011). Thus, the data suggested that the Ca_v3.2 ion channel was expressed in mechanoreceptors and nociceptors in the DRG. To confirm this, we sought to characterize the electrophysiological properties of these mGFP⁺ sensory neurons in cell culture.

An initial step to test the electrophysiological properties of these cells was to confirm that mGFP⁺ neurons could be visualized amongst cultures of DRG neurons isolated from Ca_v3.2^Cre;Tau^mGFP mice (Supplementary Figures 2B,C). We took advantage of the strong mGFP⁺ signal to carry out an electrophysiological analysis of these neurons using the whole cell patch clamp technique. We examined mGFP⁺ cells 18 and 24 h post-isolation and recorded the AP in the current clamp mode after injection of current via the recording electrode. Increasing square wave pulses starting with 80 pA with a duration of 80 ms were used and such pulse protocols are sufficient to evoke APs is DRG neurons cultured from both wild type and Ca_v3.2^Cre;Tau^mGFP mice (Wang and Lewin, 2011). We could classify the recorded mGFP⁺ neurons (n = 11) on the basis of their AP shape into putative mechanoreceptors or nociceptors. We measured the duration of the half peak amplitude, recovery time after hyperpolarization, and presence or absence of a hump on the falling phase of the AP. Our results highlighted the existence of two populations of cells with distinctive AP properties (Supplementary Figure 2B). Approximately 50% of the mGFP⁺ positive cells had narrow, humpless APs with short half peak duration and short hyperpolarization durations characteristic of mechanoreceptors (Djouhri et al., 1998; Petruska et al., 2000; Lawson, 2002; Lechner et al., 2009). The remaining 50% of the EGFP positive cells had APs with broad half peak durations and longer after-hyperpolarization durations characteristic of nociceptors (Supplementary Figure 2C).

We could determine the onset of the Ca_v3.2 gene promoter activity using the Ca_v3.2^Cre;Tau^mGFP strain. However, this strain does not allow us to observe the dynamic activity of the promoter during development, for this reason we again used homozygotes Ca_v3.2^EGFP knock-in mouse to study the spatiotemporal pattern of the Ca_v3.2 gene promoter in the spinal cord and DRG at different embryonic stages namely E9.5, E13.5, E16.5, and E18.5 (Figure 5). The EGFP protein could be detected in stages as early as E9.5 at the roof plate, in some cells at the dorsal ventricular zone of the spinal cord and in a fraction of the migrating NCCs that later form the DRG at E10.5. Positive EGFP cells were also observed in the epithelium (Figure 5A, white arrow). After the DRG formed at E13.5, EGFP expression was detected in a few cells within the ganglia, and a very strong fluorescent signal was observed in cells in the ventral spinal cord (Figure 5B). The number of EGFP⁺ cells in the DRG increased throughout developmental and reached approximately 7% at E16.5 (Figure 5C). The percentage of EGFP⁺ cells remained constant at E18.5 (Figure 5D). Surprisingly, we could not detect EGFP⁺ cells in the DRG and spinal cord of P0 Ca_v3.2^EGFP animals using antibodies against EGFP, but we could detect mRNA expression at this postnatal stage using qPCR (data not shown). However, very shortly before birth at E18.5 we found that around 7% of the DRG cells were GFP⁺ in Ca_v3.2^EGFP knock-in mice. The percentage of EGFP⁺ cells in the DRG at E18.5 matches well with the reported number of cells shown to express Ca_v3.2 mRNA in the adult mouse DRG (Talley et al., 1999; Shin et al., 2003). However, our data indicate fewer Ca_v3.2 positive neurons at E18.5 than detected using a recently published Ca_v3.2 knock-in mouse in which a EGFP sequence was inserted within the Ca_v3.2 protein coding sequence (François et al., 2015).

We decided to characterize the molecular features of the small population of EGFP⁺ neurons observed in the DRG at E18.5 in our Ca_v3.2^EGFP knock-in mice (Supplementary Figure 3). Co-immunostaining analyses of EGFP and molecular markers for sensory neuron subtypes such as the Trk receptors, revealed that about 45 ± 13% of the EGFP⁺ cells are TrkB⁺, which has already been shown to be a good marker of D-hair receptors (Shin et al., 2003; Li et al., 2011). However, not all TrkB⁺ were positive for EGFP, 55 ± 2.8% of the EGFP⁺ neurons were found to be TrkC⁺, and only 13 ± 4.1% were TrkA⁺ (Supplementary Figures 3A,B). We did not observe co-labeling of EGFP with IB4 (data not shown), indicating that non-peptidergic nociceptors do not express Ca_v3.2 (Silverman and Kruger, 1990; Molliver et al., 1997; Stucky and Lewin, 1999), and 28 ± 19% of the EGFP⁺ neurons were putative C-LTMRs (data not shown) as observed by co-labeling with a TH antibody (Li et al., 2011).

In order to characterize more precisely the sensory neurons in the adult DRG that express Ca_v3.2 in the mature somatosensory system we used a viral reporter approach. The AAV-flex-tdTomato particles were injected into the sciatic nerve of adult heterozygous Ca_v3.2^Cre mice. Four to eight weeks after viral infection we obtained robust and intense labeling of a small population of sensory neurons with tdTomato in the lumbar DRGs. We performed a molecular analysis of this population by co-labeling these cells with various molecular markers like NF200, TrkB, TrkC, IB4, calcitonin-gene related peptide (CGRP), and TH. The vast majority (77 ± 4.3%) of tdTomato⁺ cells expressed NF200 and also 76_- ± 6.9% of virally transduced Ca_v3.2^Cre cells were TrkB⁺. A smaller percentage of tdTomato⁺ cells were found to be CGRP⁺ (33 ± 6.6%) (Figures 6A,B). No co-labeling of tdTomato was found with TrkC (images not shown) or TH (Figures 6A,B). We also never observed IB4-positive tdTomato⁺ neurons. Control experiments in which AAV-flex-tdTomato was injected into the sciatic nerve of adult Advillin^Cre mice (Zurborg et al., 2011) revealed that all examined neuronal subtypes but TH⁺ cells expressed tdTomato (Supplementary Figure 4). This result suggest that the AAV serotype 9 does not transduce TH⁺ primary sensory neurons. Another possibility could be that TH⁺ neurons do not express Advillin since Advillin^Cre expression has been shown not to be present in up to 18% of DRG neurons (Zurborg et al., 2011).

Viral-based gene transfer proved to be a robust way to visualize the sensory-endings of Ca_v3.2 expressing DRG neurons in both the hairy and glabrous skin (Figures 7A,B). In the glabrous skin, there were no tdTomato⁺ fibers associated with free nerve endings, Meissner’s corpuscles or Merkel cells (Figure 7A). Similarly, in hairy skin, neither free nerve endings (data not shown) nor sensory fiber endings innervating Merkel cells were tdTomato⁺ (Figure 7A). However, tdTomato⁺ lanceolate endings, but not circumferential endings, were found surrounding hair follicles which were co-labeled with antibodies against NF200 and TrkB (Figures 7A,B).

Interestingly, we could also visualize the central projections of Ca_v3.2 expressing DRG neurons in the spinal cord dorsal horn after intraneural AAV-flex-tdTomato injection (Figures 7D–F). Sciatic nerve injections of AAV-flex-tdTomato injections resulted in labeling of fiber terminals in the superficial and deep dorsal horn laminae. It is known that D-hair mechanoreceptor afferents terminate exclusively within the deep dorsal horn with typical flame shaped arbor with terminals not penetrating into lamina II (Light and Perl, 1979). However, genetic tracing experiments using a TrkB^tauEGFP mouse have suggested penetration of terminals into lamina II (Li et al., 2011). The projection to the deep dorsal horn found after intraneural injection of the virus was consistent with a specific labeling of D-hair mechanoreceptors which show a highly restricted medio-lateral distribution that reflects the somatotopy of the sciatic nerve projection. In contrast, the medio-laterally broad projection of tdTomato⁺ fibers in the superficial dorsal horn labeled from the sciatic nerve is not consistent with a projection from D-hair mechanoreceptors. This projection is reminiscent of the projection of singly labeled nociceptors from deep tissue (Ling et al., 2003) and this population may represent the peptidergic nociceptors found to be double positive for CGRP and tdTomato in the ganglia (Figures 6A,B). Considering that in the skin, no tdTomato⁺ free nerve endings were found, we hypothesized that the lamina I projection must arise from CGRP⁺ fibers innervating muscle. Indeed in Ca_v3.2^Cre animals in which the AAV-flex-tdTomato virus was injected into the sciatic nerve we could detect GGRP⁺ fibers within the gastrocnemius muscle (Figure 7C). To further test this idea, we performed AAV-flex-tdTomato injections into the saphenous nerve which contains only cutaneous afferents as opposed to the sciatic nerve which is a mixed nerve. Injection of AAV-flex-tdTomato into the saphenous nerve did not result in labeling of fibers terminating in lamina I but only in labeling of fibers projecting to deeper dorsal horn with morphologies typical of D-hair mechanoreceptors (Figure 7F). Thus, it can be concluded that CGRP⁺ Ca_v3.2 expressing neurons innervate muscle or joint, but not the hairy skin.

T-type calcium currents have been recorded in spinal interneurons and deep dorsal horn neurons (Ikeda, 2003; Rivera-Arconada and Lopez-Garcia, 2015) and so we also asked whether the Ca_v3.2^Cre promoter was also active in the spinal cord. We made intrathecal injections of AAV-flex-tdTomato in adult heterozygous Ca_v3.2^Cre mice which resulted in labeling of many dorsal horn and ventral horn neurons (Supplementary Figures 5A,B). Thus, Ca_v3.2^Cre mice appear to be a useful tool to analyze specific populations of spinal neurons that express the Ca_v3.2 ion channel in the adult.

Here, we describe the generation and characterization of two knock-in mouse strains to accurately determine the temporal and spatial expression pattern of the Ca_v3.2 channel gene in the mouse nervous system. By using antibody detection of EGFP in the Ca_v3.2^EGFP mouse, and crossing the Ca_v3.2^Cre strain with the Tau^mGFP or the Rosa26^LacZ mouse reporter lines, we show that the Ca_v3.2 gene is mainly expressed in the DG of the adult brain. We have, however, concentrated our analysis on the expression of the Ca_v3.2 gene in the somatosensory system as many studies have shown a role for this important calcium channel in regulating sensory neuron excitability as well as the gating of sensory information in the spinal cord (Ikeda, 2003; Shin et al., 2003; Bourinet et al., 2005; Heppenstall and Lewin, 2006). Using our two new mouse models we were able to identify sensory neuron progenitors expressing the Ca_v3.2 channel gene during development. We show that NCCs expressing the Ca_v3.2 channel at E9.5 specifically give rise to a large population (∼90%) of DRG neurons (Supplementary Figure 2A). However, NCCs giving rise to sympathetic neurons and a specific population of SAMs that innervate Merkel cells in the skin never express the Ca_v3.2 channel gene. Using our Ca_v3.2^EGFP we could show that a much smaller population of DRG neurons express the Ca_v3.2 gene in late embryonic development. Thus, we observed that the Cav3.2 channel is expressed in about 7% of the DRG cells, which were mainly TrkB⁺ and TrkC⁺, with only a small percentage expressing TrkA. In a further analysis using viruses containing a Cre-dependent reporter in our Ca_v3.2^Cre mice we could label and characterize the nature of adult sensory neurons that express Ca_v3.2. The results of these experiments revealed that there are two major populations of Ca_v3.2 expressing sensory neurons; first low threshold Aδ-fiber mechanoreceptors (D-hair receptors) that exclusively innervate hair follicles and a population of peptidergic nociceptors that innervate muscle, but not the skin. We could not demonstrate expression of Ca_v3.2 in tyrosine hydroxylase positive sensory neurons which have been postulated to be C-fiber low threshold mechanoreceptors. It is not clear at this time whether the failure to label putative C-LTMRs was due to technical limitations of the AAV virus reporter approach.

In the brain, the Ca_v3.2 channel shapes neuronal firing patterns of hippocampal and thalamic neurons (Crunelli et al., 2005; Cain and Snutch, 2010; Deleuze et al., 2012; Tringham et al., 2012). Strong expression of the Ca_v3.2 gene has been reported in neurons of the CA1, CA3 fields, and in granule cells of the hippocampus (Talley et al., 1999; McKay et al., 2006). Also, expression of the Ca_v3.2 and Ca_v3.3 subunit genes has been observed in the reticular thalamic nucleus (RT) (Talley et al., 1999; McKay et al., 2006), and here Ca_v3.2 has been proposed to modulate tonic firing regularity and spike frequency during burst firing (Liao et al., 2011). Using our Ca_v3.2^EGFP knock-in mice we could show that EGFP was predominantly expressed in the DG, and in the internal capsule of the brain. Surprisingly, we did not observe activation of the Ca_v3.2 promoter in the RT of the Ca_v3.2^EGFP or the Ca_v3.2^Cre mouse, even though the Ca_v3.2^Cre knock-in mouse allowed us to determine promoter activity in brain regions where the Ca_v3.2 channel was poorly expressed. Instead, we detected activation of the Ca_v3.2 promoter in the lateral-dorsal nucleus of the thalamus (Figure 2A). Nevertheless, the CNS expression data from our reporter mice broadly agrees with those found using conventional in situ hybridization techniques (Talley et al., 1999).

In the developing somatosensory system, Ca_v3.2 gene expression has a very early onset and displays a highly dynamic spatiotemporal expression pattern. In Ca_v3.2^EGFP embryos, EGFP expression was already observed in some of the trunk NCCs at E9.5. However, since NCCs give rise to a wide variety of cell types including sensory neuron lineages, sympathetic neurons, melanocytes, and Schwann cells (Serbedzija et al., 1990; Jessen and Mirsky, 2005; Takahashi et al., 2013), it was interesting to observe that in Ca_v3.2^Cre;Tau^mGFP embryos, β-gal activity was restricted to DRG neurons and was not found in sympathetic neurons. Non-neuronal cell populations generated from NCCs such as Schwann cells, satellite cells, and melanocytes are expected not to be labeled in Ca_v3.2^Cre;Tau^mGFP embryos (Opdecamp et al., 1997; Jessen and Mirsky, 2005; Takahashi et al., 2013), but we also did not find evidence of EGFP positive glial cells in sections from the Ca_v3.2^EGFP embryos.

At E13.5, when the three waves of DRG neurogenesis are complete (Ma et al., 1999), Ca_v3.2 promoter activity was detected in only a few sensory neurons in the DRG of Ca_v3.2^EGFP embryos. At E16.5 the number of EGFP⁺ cells was found to be around 7% of the total DRG and appeared to remain constant until E18.5. The steady and unchanging expression of the Ca_v3.2 promoter suggests that the transcriptional regulation of the Ca_v3.2 channel is a result of early sub-type specification. Indeed, E16.5 sensory neurons with the electrophysiological properties of mechanoreceptors appear already mature at this stage (Lechner et al., 2009), consistent with the idea that Ca_v3.2 channel is an early marker of this population.

Characterization of the Ca_v3.2-positive sub-population in Ca_v3.2^EGFP embryos at E18.5 revealed that the majority of the EGFP⁺ cells at this stage express TrkB and TrkC, and only a low percentage of them express TrkA. There is high probability that EGFP⁺/TrkB⁺ and EGFP⁺/TrkC⁺ cells in the DRG developed from the early TrkB/C sensory linage expressing Runx3/Shox2 transcription factors (Levanon et al., 2002; Kramer et al., 2006; Scott et al., 2011), and not from the early Ret (eRet⁺/MafA⁺) population (Bourane et al., 2009; Luo et al., 2009). This idea is supported by a recent study on the transcription factor cMaf in sensory neurons where cMaf acts upstream of MafA, and loss of function alleles cause specific alterations in the physiological properties of Aβ-fiber mechanoreceptors without effects on Aδ-fibers. Interestingly, the development of Ca_v3.2 positive sensory neurons was unchanged in cMaf mutants (Wende et al., 2012). TrkA⁺ DRG cells develop in two waves, the first wave gives rise to Aδ-fibers and is generated from the Ngn2-dependent precursor pool that are Cux2-positive (Bachy et al., 2011). The second wave is generated from the Ngn1-dependent wave of precursors which are Cux2-negative (Averill et al., 1995). Because the EGFP⁺/TrkA⁺ cells were IB4-negative, and previous studies had shown that the Ca_v3.2 channel is expressed in medium size DRG cells (Shin et al., 2003), it may be that these EGFP⁺/TrkA⁺ neurons developed from the Ngn2-dependent wave of neurogenesis.

The function of the Ca_v3.2 channel in undifferentiated DRG precursor cells is unclear. However, it has been reported that LVA Ca²⁺, but not HVA Ca²⁺ currents can be recorded in undifferentiated precursor cells of the chick DRG, while both conductances are observed in differentiated sensory neurons (Gottmann and Lux, 1990; Gottmann et al., 1991). Our study definitively identifies the Ca_v3.2 channel as one of the T-type α-subunits expressed in sensory precursor cells. Loss of function alleles of the Ca_v3.2 gene have not, however, been reported to affect the specification of sensory lineages (Wang and Lewin, 2011).

Expression of the Ca_v3.2 channel has been observed in small to medium size DRG and has been proposed as a molecular marker for D-hair receptors (Shin et al., 2003; Dubreuil et al., 2004; Aptel et al., 2007; François et al., 2015). In addition, the Ca_v3.2 channel has been shown to be required for the normal temporal coding of moving stimuli by D-hair receptors (Wang and Lewin, 2011). Thus, the presence of EGFP⁺/TrkB⁺ DRG neurons was expected since signaling of NT-4 through TrkB is required for D-hairs survival in mature mice (Stucky et al., 1998, 2002). Expression of TrkB has been observed in D-hair receptors by Shin et al. (2003), and more recently, this receptor has been proposed as a molecular marker for D-hair receptors (Li et al., 2011). In that study, Li et al. (2011) generated the TrkB^tauEGFP knock-in mouse to visualize TrkB expressing cells in the DRG. They showed that in thoracic DRG from TrkB^tauEGFP mice, EGFP⁺ cells were medium size neurons, did not express the TH, IB4 or c-Ret, and in hairy skin they innervated longitudinal lanceolate endings associated with zigzag and awl/auchene hair follicles, and electrophysiological characterization classified them as D-hair receptors (Li et al., 2011). Using a Cre-dependent viral reporter approach, we now show that our Ca_v3.2^Cre mice can be used to label a highly specific population of cutaneous receptors that are identical to TrkB⁺ D-hair mechanoreceptors. Thus, infection of sensory axons with a AAV-flex-tdTomato virus labeled medium sized sensory neurons that exclusively make lanceolate endings on hair follicles in the hairy skin, glabrous skin was completely free of any innervation from Ca_v3.2⁺ endings (Figures 7A,B). What was even more striking was that these Ca_v3.2⁺ fibers also appeared to provide a specific and organized central projection into the deep dorsal horn as expected from previous work (Light and Perl, 1979) (Figures 7D–F).

Previous studies do not support the idea that TrkB is found uniquely in Aδ-fibers. Indeed, in the mouse there is a subpopulation of Aβ-fiber RA low-threshold mechanoreceptors that innervate Meissner corpuscles and/or lanceolate endings which are TrkB⁺/MafA⁺/cRet⁺ (Bourane et al., 2009; Luo et al., 2009; Wende et al., 2012). Also, lack of BDNF, the other TrkB high affinity ligand, affects the mechanical threshold of SA-mechanoreceptors, suggesting that TrkB receptors are also expressed in SA-mechanoreceptors innervating Merkel cells and/or Ruffini corpuscles (Carroll et al., 1998). In contrast we show here that the use of our Ca_v3.2^Cre in combination with viral reporters injected into cutaneous nerves provides a remarkably specific method to label and manipulate D-Hair mechanoreceptors.

The Ca_v3.2 channel has also been proposed to play a role in nociception (Todorovic et al., 2001; Bourinet et al., 2005; Nelson et al., 2005; Choi et al., 2006). However, our Ca_v3.2^EGFP knock-in mouse shows that only a small percentage of EGFP⁺ DRG cells could be classified as peptidergic nociceptors since they express TrkA and are IB4-negative. We have now identified the nature of the Ca_v3.2⁺ nociceptive population within the DRG that have molecular properties of nociceptors and these CGRP⁺ neurons innervate muscle (Figure 7C) and have projections in superficial laminae that are typical for muscle nociceptors (Ling et al., 2003) (Figure 7E). Recently, it has been proposed that another sensory neuron type with unmyelinated C-fiber axons the so-called C-LTMR may also express Ca_v3.2 (François et al., 2015; Reynders et al., 2015). This sensory neuron population have been shown to be TH⁺ (Li et al., 2011), but we were not able to observe sensory neurons in Ca_v3.2^Cre infected with reporter constructs that were double positive for TH and dtTomato (Figure 6). It is not clear whether our failure to detect Ca_v3.2⁺ sensory neurons with molecular features of C-LTMRs is due to an inability of the AAV9 virus to infect TH⁺ cells.

The reported changes in pain behavior after ablation of Ca_v3.2 channels in sensory neurons should now been seen in the context that a substantial number of sensory neurons innervating the muscle express this channel. The importance of muscle nociceptors in chronic pain has often been underestimated although activation of such neurons can provoke powerful central sensitization (McMahon et al., 1993; Lewin et al., 2014). We also injected our reporter virus centrally and observed a substantial number of labeled dorsal and ventral horn interneurons in Ca_v3.2^Cre mice (Supplementary Figure 5). This finding emphasizes the fact that pharmacological or genetic manipulation of Ca_v3.2 channels may modulate pain behavior by acting at sites within the spinal cord. The tools we have developed here should make possible a more detailed dissection of the role of such interneurons in pain and hyperalgesia.

In summary, our genetic models provide a comprehensive picture of Ca_v3.2 channel expression in the developing and mature nervous system. Our study shows that the Ca_v3.2 gene is active in early, committed migrating NCCs that give rise to a restrictive lineage of sensory neurons, but not sympathetic neurons. The expression of Ca_v3.2 gene is rapidly restricted to early post-mitotic neurons some of which are identical to D-hair low-threshold mechanoreceptors already at E13.5. However, we also show that a small population of muscle nociceptors also express Ca_v3.2 channels in the adult DRG. Ca_v3.2 is the only ion channel so far identified whose expression in the neural crest defines a sub-population of committed sensory precursors.

YABS and AK designed the knock-in constructs. YABS generated knock-in animals, performed crossings with reporter lines, immunostainings and electrophysiological characterization. JH carried out viral injection, immunostainings and two-photon microscopy. VB helped YABS in generating Ca_v3.2 reporter lines. YABS, JH, and GL planned experiments and analyzed data. YABS, JH, and GL wrote the paper.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.