[1] 12w Runx3-Regulated Expression of Two Ntrk3 Transcript Variants in Dorsal Root Ganglion Neurons

[1] 254w Somatosensation is divided into proprioception and cutaneous sensation. Dorsal root ganglion (DRG) neurons project their fibers toward peripheral targets including muscles and skin, and centrally to the spinal cord. Proprioceptive DRG neurons transmit information from muscle spindles and Golgi tendon organs to the spinal cord. We previously showed that Runt-related transcription factor 3 (Runx3) is expressed in these neurons and their projections to the ventral spinal cord and muscle spindles are lost in Runx3-deficient (Runx3 2/2 ) mouse embryos. Although Runx3 is likely to contribute to the fate decision and projection of proprioceptive DRG neurons, the precise roles for Runx3 in these phenomena are unknown. To identify genes regulated by Runx3 in embryonic DRGs, we performed microarray analyses using cDNAs isolated from wild-type and Runx3 2/2 DRGs of embryonic day (E) 12.5 and selected two transcript variants of the tyrosine kinase receptor C (TrkC) gene. These variants, Ntrk3 variant 1 (Ntrk3-v1) and variant 2 (Ntrk3-v2), encode fulllength and truncated receptors of neurotrophin-3, respectively. Using double in situ hybridization, we found that most of Ntrk3-v1 mRNA expression in E14.5 DRGs depended on Runx3 but that more than half of Ntrk3-v2 mRNA one were expressed in a Runx3-independent manner. Furthermore, our data revealed that the rate of Ntrk3-v1 and Ntrk3-v2 colocalization in DRGs changed from E14.5 to E18.5. Together, our data suggest that Runx3 may play a crucial role in the development of DRGs by regulating the expression of Ntrk3 variants and that DRG neurons expressing Ntrk3-v1 but not Ntrk3-v2 may differentiate into proprioceptive ones. V

[1] 187w Runt-related transcription factors (Runx) are characterized by their Runt domains. In mammals, they are subdivided into three subfamilies, that is, Runx1, 2, and 3 (Levanon and Groner, 2004;Inoue et al., 2008;Ito, 2008). Runx1 is involved in the differentiation of blood cells and is known as a causative gene of acute myeloid leukemia (Okuda et al., 1996;Ito, 2008;Ichikawa et al., 2013). Runx2 is essential for bone formation and acts as a causative gene for cleidocranial dysplasia (Komori et al., 1997;Otto et al., 1997;Ito, 2008). Runx3 is expressed in the gastrointestinal mucosa epithelium and has been shown to act as a tumor suppressor gene in the gastric cancer (Li et al., 2002;Ito, 2008). In addition, both Runx1 and 3 are expressed in the nervous system, especially in dorsal root ganglion (DRG) neurons at early stages of development. They play a pivotal role in the formation of subtypes and projections of DRG neurons (Inoue et al., 2007(Inoue et al., , 2008;;Levanon et al., 2002;Chen et al., 2006aChen et al., , 2006b;;Kramer et al., 2006;Yoshikawa et al., 2007Yoshikawa et al., , 2013;;Nakamura et al., 2008;Lallemend et al., 2012;Yang et al., 2013).

[2] 181w DRG neurons are responsible for the transmission of somatic information that is classified into nociception, mechanoreception, and proprioception. Nociceptive and mechanoreceptive DRG neurons project their fibers to the dorsal horn of the spinal cord and to the skin. Nociceptive DRG neurons express receptors for neurotrophic factors such as tyrosine kinase receptor A (TrkA) and Ret. Mechanoreceptive DRG neurons express various receptors for neurotrophic factors such as TrkB, TrkC, GDNF family receptor a1-4 (GFRa1-4), and Ret (Inoue et al., 2002;Luo et al., 2009;Honma et al., 2010;Abdo et al., 2011;Lallemend and Ernfors, 2012;Yoshikawa et al., 2013). Conversely, proprioceptive DRG neurons express TrkC and project their fibers to the ventral horn and intermediate zone of the spinal cord, Golgi tendon organs and to muscle spindles, transmitting the information necessary for coordinated movement, such as tension of the muscle and the position of the limbs (Inoue et al., 2002;Lallemend and Ernfors, 2012). Previous studies showed that nonpeptidergic nociceptive DRG neurons express Runx1 and that proprioceptive DRG neurons express Runx3 (Levanon et al., 2002;Kramer et al., 2006;Yoshikawa et al., 2007;Inoue et al., 2008;Lallemend and Ernfors, 2012).

[3] 240w The role of Runx3 in DRG neuronal development has been analyzed by studying mouse embryos deficient for the Runx3 gene (Runx3 2/2 ). Several studies have revealed that Runx3 2/2 embryos have deficiencies in the projections of their proprioceptive DRG neurons to both peripheral and central targets (Inoue et al., 2002;Levanon et al., 2002). More recently, by analyses using double knockout mouse embryos deficient for Bax and Runx3 genes, the defective projections of proprioceptive DRG neurons are not due to cell death, but due to changes in the fate of DRG neurons (Nakamura et al., 2008;Lallemend et al., 2012). Proprioceptive DRG neurons differentiate from precursor neurons coexpressing TrkC and TrkB, and Runx3 is involved in this differentiation (Kramer et al., 2006;Inoue et al., 2008). After embryonic day (E) 11.5, the precursor neurons coexpressing TrkC and TrkB are suppressed in their TrkB expression by Runx3 and they differentiate into TrkC-positive/TrkBnegative proprioceptive DRG neurons (Kramer et al., 2006;Inoue et al., 2007;Abdo et al., 2011). The fate of proprioceptive DRG neurons is likely to be converted into that of mechanoreceptive DRG neurons in Runx3 2/2 mouse embryos and the projection of the proprioceptive fibers thus might be lost in these embryos. However, the expression of the TrkC protein is partly maintained in Runx3 2/2 mouse DRGs (Nakamura et al., 2008). This observation led us to identify genes that contribute to the fate and projection of proprioceptive DRG neurons in the downstream of Runx3.

[4] 60w In this study, as a first step to elucidate the Runx3regulated mechanism that controls the cell fate and projection of proprioceptive DRG neurons, we examined the expression pattern of downstream genes of Runx3 in DRG neurons. We found that Runx3 is involved in the development of DRG neurons by regulating the expression of two transcript variants of the TrkC gene.

[1] 272w Microarray analyses were performed using DRGs from E12.5 wild-type (n 5 15) and Runx3 2/2 (n 5 15) mouse embryos. Based on previous studies on neural development, we obtained more than 20 genes that showed a difference in expression amount more than twice between wild-type and Runx3 2/2 mouse DRGs (Table 1). We then focused on two Ntrk3 genes encoding the TrkC receptor for neurotrophic factor neurotrophin-3 (NT3; Table 1). Neurotrophic tyrosine kinase receptor type3 variant 1 (Ntrk3-v1, NCBI Reference Sequence: NM_008746) and Neurotrophic tyrosine kinase receptor type3 variant 2 (Ntrk3-v2, NCBI Reference Sequence: NM_182809) are transcript variants of Ntrk3 genes. The results of these microarray analyses showed that the relative expression values (log 2 ratios) of Ntrk3-v1 in Runx3 2/2 mouse DRGs were 22.71 and that those of Ntrk3-v2 were 21.26 compared with the wild-type values (Table 1). Previous experiments have revealed that NT3 acts as a chemoattractant for DRG neurites in vitro and is required for the innervation of TrkCpositive DRG fibers toward their targets in vivo (Genc ¸et al., 2004;Usui et al., 2012). These results clearly suggest that the NT3/TrkC signaling pathway may be involved in the projection of proprioceptive DRG neurons. Although both of the cDNA sequences maintain a homology from extracellular to transmembrane domains, the sequences of their intracellular ones are different (Supporting Information Fig. 1; Menn et al., 1998). The Ntrk3-v1 gene has a tyrosine kinase domain, whereas the Ntrk3-v2 gene lacks this domain. Thus, Ntrk3-v1 and Ntrk-v2 proteins are considered to be a full-length TrkC with tyrosine kinase activity (full-length TrkC) and an incomplete TrkC (truncated TrkC), respectively (Ichinose and Snider, 2000;Cronk et al., 2002).

[2] 304w By performing in situ hybridization using specific cRNA probes for Ntrk3-v1 and Ntrk3-v2, we examined the Ntrk3-v1 and Ntrk3-v2 mRNA expression profiles in E14.5 wild-type and Runx3 2/2 DRGs (Fig. 1A-D). In these experiments, we used FLUlabeled cRNA probes to detect both Ntrk3-v1 and Ntrk3-v2 signals. Islet-1/2, a marker of pan-DRG neurons, was seen in the nucleus of DRG neurons, whereas Ntrk3-v1 and Ntrk3-v2 were expressed in the cytoplasm and the nucleus, respectively (Fig. 1A-D). We confirmed that several other cRNA probes for Ntrk3-v1 and Ntrk3-v2 (probes synthesized against other gene regions of Ntrk3-v1 and Ntrk3-v2) also showed the same expression patterns in the cytoplasm and the nucleus (data not shown). The percentage of Ntrk3-v1 and Ntrk3-v2 mRNA-positive DRG neurons among neurons expressing Islet-1/2 was calculated for both wild-type and Runx3 2/2 DRGs. We observed that 5.98 and 0.34% of DRG neurons expressed Ntrk3-v1 mRNA in E14.5 wild-type and Runx3 2/2 DRGs, respectively (Fig. 1A, B, E). Conversely, Ntrk3-v2-positive neurons were observed with 5.45% in E14.5 wild-type DRGs, and with 3.21% in Runx3 2/2 ones (Fig. 1C-E). Therefore, the percentage rates of Ntrk3-v1-and Ntrk3-v2-positive neurons among total DRG neurons in Runx3 2/2 embryos as compared with those for wild-type ones were 5.7 and 58.8%, respectively. Additionally, using qRT-PCR, we performed rate-of-change analyses of the expression of Ntrk3-v1 and Ntrk3-v2 mRNAs in DRG neurons under Runx3 deficiency and confirmed that Ntrk3-v1 and Ntrk3-v2 expression levels in Runx3 2/2 DRGs showed a significant reduction compared with those in wild-type ones, although the degree of reduction in the level of Ntrk3-v2 mRNA was smaller than that in Ntrk3-v1 mRNA (Supporting Information Fig. 2A-C). Together, these results suggest that most of the Ntrk3-v1 mRNA expression in E14.5 DRGs depended on Runx3 and that more than half of the Ntrk3-v2 mRNA expression was independent of Runx3 in these neurons.

[3] 80w Next, we examined the colocalization of Ntrk3-v1 and Ntrk3-v2 using a double in situ hybridization procedure. To detect Ntrk3-v1 and Ntrk3-v2 signals, we used FLU-and DIG-labeled cRNA probes, respectively. Ntrk3-v1 was expressed in the cytoplasm of DRG neurons, whereas Ntrk3-v2 and DAPI were seen in the nucleus (Fig. 2A-H). We observed that 63.1% of Ntrk3-v1 mRNA-positive DRG neurons coexpressed Ntrk3-v2 mRNA and that 67.6% of Ntrk3-v2 mRNA-positive DRG neurons coexpressed Ntrk3-v1 mRNA in E14.5 wild-type embryos (Fig. 2A-D, I, J).

[4] 92w To further characterize Ntrk3-v2 and Ntrk3-v1 colocalization at later development, we examined the colocalization of Ntrk3-v1 and Ntrk3-v2 mRNA-positive DRG neurons in wild-type DRGs at E18.5 when proprioceptive DRG neurons have finished their projection to the ventral horn of the spinal cord. We observed that 89.1% of Ntrk3-v2 mRNA-positive DRG neurons coexpressed Ntrk3-v1 mRNA and 43.5% of Ntrk3-v1 mRNA-positive DRG neurons coexpressed Ntrk3-v2 mRNA in E18.5 wild-type embryos (Fig. 2E-I, K). These results showed that the colocalization rate of Ntrk3-v1 and Ntrk3-v2 mRNAs in DRG neurons had changed from E14.5 to E18.5.

[5] 228w Using the double in situ hybridization procedure, we also examined the colocalization of Ntrk3-v1 or Ntrk3-v2 with Runx3 in E14.5 wild-type DRGs. To detect Runx3 signals, we used the FLU-labeled cRNA probe; and for detection of Ntrk3-v1 and Ntrk3-v2 signals, DIG-labeled cRNA probes. We observed that 92.0% of Ntrk3-v1 mRNA-positive DRG neurons and 53.8% of Ntrk3-v2 mRNA-positive ones coexpressed Runx3 mRNA (Fig. 3A-D, I-L, Q, R). These results showed that the rate of colocalization of Ntrk3-v1 and Runx3 in E14.5 wild-type DRGs was nearly equal to the reduction rate of Ntrk3-v1-positive neurons in E14.5 Runx3 2/2 ones (Fig. 1E). These results further showed that the rate of colocalization of Ntrk3-v2 and Runx3 in wild-type DRGs was also equal to the reduc-tion rate of Ntrk3-v2-positive neurons in Runx3 2/2 mouse ones at E14.5 (Fig. 1E). Next, we examined the colocalization of Ntrk3-v1 or Ntrk3-v2 with Runx3 in E18.5 wild-type DRGs and found that 46.2% of Ntrk3-v1 mRNA-positive DRG neurons and 24.0% of Ntrk3-v2 mRNA-positive DRG neurons coexpressed Runx3 (Fig. 3E-H, M-Q, S). These results showed that the colocalization rates of Ntrk3-v1 or Ntrk3-v2 with Runx3 in DRGs changed from E14.5 to E18.5. In other words, our results revealed that the rate of Ntrk3-v2 mRNApositive and Runx3-negative DRG neurons increased at E18.5 and that Ntrk3-v1-positive and Runx3-negative DRG neurons emerged at the same stage. Figure 3. Runx3-Regulated Expression of Ntrk3s

[6] 177w Our study showed that Ntrk3-v1 and Ntrk3-v2 exhibited different expression patterns in colocalization with Runx3. Whereas the majority of Ntrk3-v1 colocalized with Runx3 in E14.5 wild-type DRG neurons, about half of Ntrk3-v2 did not colocalize with Runx3 (Fig. 3Q, R). This result is consistent with the fact that more than half of the Ntrk3-v2-positive DRG neurons were present in E14.5 Runx3 2/2 mouse embryos (Fig. 1E). Furthermore, our study revealed that almost all Runx3-and Ntrk3-v2-double-positive DRG neurons coexpressed Ntrk3-v1 at E14.5 (Fig. 3R). Taken together, these results suggest that the subpopulation of Ntrk3-v2-positive DRG neurons lost in Runx3 2/2 embryos might be proprioceptive DRG neurons coexpressing Runx3 and Ntrk3-v1 and that another subpopulation, which was not lost in Runx3 2/2 embryos, might be mechanoreceptive DRG neurons. In addition, the colocalization of Ntrk3-v1 or Ntrk3-v2 with Runx3 showed a different pattern between E14.5 and E18.5. In particular, about half of the Ntrk3-v1-positive DRG neurons did not express Runx3 at E18.5 (Fig. 3S). These results imply that the Ntrk3-v1 expression may be regulated by Runx3 in a stage-specific manner.

[7] 239w What type of neurons coexpress Ntrk3-v1 and Ntrk3-v2 in each stage? Furthermore, what function do Ntrk3-v1 and Ntrk3-v2 have in each stage? TrkB, a receptor for brain-derived neurotrophic receptor (BDNF), is present as two isoforms, full-length TrkB (TrkB.tk 1 ) and truncated TrkB (TrkB.t1;Fenner, 2012). On the basis of these isoforms, three types of dimers, TrkB.tk 1 homodimers, TrkB.t1 homodimers, and TrkB.t1-TrkB.tk 1 heterodimers, are present in the brain (Fenner, 2012). TrkB.t1 homodimers are known to regulate the concentration of extracellular BDNF, change the morphology of neurons and glial cells, and activate the intracellular signaling (Fenner, 2012). Conversely, TrkB.t1-TrkB.tk 1 heterodimers have a dominant-negative function to inhibit the action of the TrkB.tk 1 signaling (Fenner, 2012). In the case of TrkC, it is not yet clear whether Ntrk3-v1 and Ntrk3-v2 form a heterodimer as in the case of TrkB, but it is highly possible that Ntrk3-v2 forms a heterodimer with Ntrk3-v1 and functions as a dominant-negative receptor. Because most of the Ntrk3-v1-and Ntrk3-v2double-positive neurons coexpressed Runx3 at E14.5, they were supposedly proprioceptive DRG neurons. However, most of these double-positive neurons did not coexpress Runx3 at E18.5, suggesting that they might not be proprioceptive DRG neurons. At E18.5, DRG neurons have almost completed their projections to the targets and begun synapse formation (Mears and Frank, 1997;Ozaki and Snider, 1997). We cannot exclude the possibility that colocalization of Ntrk3-v1 and Ntrk3-v2 may be involved in the synapse formation of DRG neurons.

[8] 55w It is well known that hierarchical transcription factors are used to specify DRG sensory neurons. Although the expression of Ntrk3-v1 and Ntrk3-v2 may be controlled by Runx3, it is unclear whether Runx3 binds directly to the regulatory region of the Ntrk3 gene or acts through some other transcription factors. Further studies will clarify this issue.

[1] 122w In this study, as a first step to search for molecules that control the cell fate and projection of proprioceptive DRG neurons in the downstream of Runx3, we examined the expression of two transcript variants of the TrkC gene, Ntrk3-v1 and Ntrk3-v2. In mouse embryonic DRG neurons, we examined whether the expression of Ntrk3-v1 and Ntrk3-v2 depended on Runx3 or not. At E14.5, corresponding to the period when proprioceptive DRG fibers project to the intermediate zone, Ntrk3-v1 expression in Runx3 2/2 DRGs was low, whereas more than half of the Ntrk3-v2 expression was observed in these DRGs in a Runx3-independent manner. Thus, these results suggest that the development of DRG neurons may be controlled through the Runx3-regulated expression of TrkC transcript variants.

[1] 80w The generation and characterization of Runx3-deficient mice were performed as described previously (Inoue et al., 2002). Noon of the vaginal plug date was considered as embryonic day 0.5 (E0.5). After anesthetic treatment with isoflurane, E12.5-18.5 mouse embryos were excised. Genotyping was performed by polymerase chain reactions (PCRs) using primer pairs as described previously (Nakamura et al., 2008). All experiments using mice were performed according to the Guide for the Care and Use of Laboratory Animals of the University of Tsukuba.

[2] 54w From E12.5 wild-type and Runx3 2/2 mouse embryos, DRGs (from cervical to lumbar levels) were dissected, and the samples were analyzed by use of an Agilent Expression Array (Agilent Technologies, California). Log 2 ratios representing the difference in the expression level were obtained on the basis of the expression level in wild-type mouse DRGs.

[3] 319w In vitro transcription from the cDNA fragments of mouse Neurotrophic tyrosine kinase receptor type3 variant 1 (Ntrk3-v1, NM_008746, 1977-2638;662 bp), mouse Neurotrophic tyrosine kinase receptor type3 variant 2 (Ntrk3-v2, NM_182809, 1686-2713; 1028 bp), and mouse Runx3 (NM_019732, 944-1938;995 bp, Usui et al., 2012) was performed with a digoxigenin (DIG) or fluorescein (FLU) RNA-labeling kit (Roche Diagnostics, Basel, Switzerland). Lumbar segments 1 and 2 of E14.5 wild-type or Runx3 2/2 mouse embryos were dissected out and frozen in Tissue-Tek OCT compound (Sakura Finetech Japan, Tokyo, Japan). Transverse sections (10-mm thickness) were cut by using a cryostat and collected onto silane-coated glass slides (Agilent Technologies). After fixation with paraformaldehyde/0.1 M phosphate buffer for 10 min, the sections were dehydrated with 100% methanol for 1 h and acetylated with triethanolamine (Sigma-Aldrich, Missouri) for 10 min. Hybridization was carried out for 18 h at 728C with DIG-labeled cRNA probes (500 ng/mL). Prior to detection, sections were washed in 23 SSC for 1 h at 728C and for 30 min at room temperature. After incubation with normal goat serum (Vector Laboratories, Peterborough, UK) for 2 h at room temperature, hybridization was detected by treatment with sheep anti-DIG-AP, fab fragments (Roche Diagnostics, 1:10,000), sheep anti-FLU-AP, fab fragments (Roche Diagnostics; 1:10,000), sheep anti-DIG-POD, fab fragments (Roche Diagnostics; 1:1000), or sheep anti-FLU-POD, fab fragments (Roche Diagnostics; 1:1000) for 2 days at 48C. To detect alkaline phosphatase, we used the 3hydroxy-N-2 0 -biphenyl-2-naphthalenecarboxamide phosphate ester (HNPP) Fluorescent Detection Set (Roche Diagnostics). To detect peroxidase, we used the TSA Plus Cyanine 3 System (PerkinElmer, Massachusetts; 1:100) or TSA Plus Biotin Kit (PerkinElmer; 1:100) and Alexa Fluor 488-labeled streptavidin (Life Technologies, California; 1:1000). To calculate the ratio of positive cells, we obtained the total number of DRG neurons by Islet-1/2 staining described below and then calculated the percentage of various mRNA-expressing cells among the total number of DRG neurons per section. Data were statistically analyzed by using Student's unpaired t-test.

[4] 71w After in situ hybridization, sections were incubated with a rat monoclonal antibody against Islet-1/2 (39.4D5, Developmental Studies Hybridoma Bank, Iowa; 1:200) overnight at 48C. Then, they were incubated with an Alexa Fluor 488labeled goat anti-mouse IgG antibody (Life Technologies; 1:1000) at room temperature for 1 h. The sections were sealed with VECTASHIELD Mounting Medium with 4 0 ,6diamidino-2-phenylindole dihydrochloride (DAPI, Vector Laboratories) and photographed using an AxioVision (Carl Zeiss, Jena, Germany).

[5] 179w Total RNA was prepared from DRGs of E14.5 wild-type and Runx3 2/2 mouse embryos from thoracic to lumbar levels using an RNeasy Plus Micro Kit (Qiagen, Limburg, Netherlands). The cDNA was synthesized using a Quanti-Tect Reverse Transcription Kit (Qiagen). Using SYBR Premix Ex Taq II (Takara Bio, Shiga, Japan), real-time quantitative reverse transcription PCR (qRT-PCR) was carried out for 50 cycles by Thermal Cycler Dice Real Time System (TP800; Takara Bio). All of the reactions were conducted in duplicate for each cDNA sample and four animals from each genotype were used. The following PCR primers were used: Ntrk3-v1 forward, 5 0 -TGGTCTTTGAATAC ATGAAGCATGG-3 0 , reverse, 5 0 -AGGCTATCTGACTGG CGATGTG-3 0 (amplicon size, 150 bp; Takara Bio); Ntrk3-v2, forward, 5 0 -GAACCTTTGGAACCCAGTGGAA-3 0 , reverse, 5 0 -CACACATATTGACCAGAGACCAGGA-3 0 (amplicon size; 138 bp; Takara Bio); 18S rRNA, forward, 5 0 -ACTCAAC ACGGGAAACCTCA-3 0 , reverse, 5 0 -AACC AGACAAAT CGCTCCAC-3 0 (Hokkaido System Science Co., Hokkaido, Japan). The 18S rRNA expression level was determined for each sample to normalize the data. Data were statistically analyzed using Student's unpaired t-test.

[1] 312w Proprioceptive DRG neurons have been reported to use TrkC for responding to NT3-induced chemoattraction and extend their fibers to the muscle spindle and ventral horn of the spinal cord expressing NT3 (Fariñas et al., 1996;Chen et al., 2002). This axonal guidance mechanism is based on the observation that projections of proprioceptive DRG fibers to the ventral horn are lost in both NT3 2/2 and Ntrk3 2/2 embryos (Ernfors et al., 1994;Patel et al., 2003;Genc ¸et al., 2004;Usui et al., 2012). We previously examined the developmental patterns of all TrkC expression including Ntrk3-v1 and Ntrk3-v2 using anti-TrkC antibody (antibody against the extracellu-lar domain of TrkC). In Runx3 2/2 mouse embryos, in spite of the loss of proprioceptive DRG neurons, TrkC-expressing neurons are not completely lost, indicating that TrkC expression is partially maintained even in Runx3 2/2 mouse DRGs (Nakamura et al., 2008). From these results, it is quite difficult to explain the mechanism of the projection of proprioceptive DRG neurons by only the NT3/TrkC signaling pathway. The Ntrk3-v2 receptor without tyrosine kinase activity (truncated TrkC) is considered to be dominant-negative for the Ntrk3-v1 receptor with tyrosine kinase activity (full-length TrkC; Palko et al., 1999). Conversely, it is highly possible that the NT3/Ntrk3-v2 signaling has functions different from those of the NT3/Ntrk3-v1 signaling, because Ichinose and Snider (2000) showed that the total length of neurite outgrowth is maintained in Ntrk3-v2-transfected wild-type DRG neurons but that their outgrowth patterns are different from those of Ntrk3-v1transfected ones. Furthermore, knockout mice for the entire extracellular domain of TrkC, which lack the functions of both Ntrk3-v1 and Ntrk3-v2, exhibit more serious abnormalities in the development of Merkel cells and projections of mechanoreceptive DRG neurons to Merkel cells than those in Ntrk3-v1deficient mice lacking the tyrosine kinase domain of TrkC. These results suggest a different function of Ntrk3-v2 in the projection of mechanoreceptive DRG neurons (Cronk et al., 2002).

[2] 126w In the present results obtained by in situ hybridization, the expression of Ntrk3-v1 was almost lost, but that of Ntrk3-v2 was seen, in E14.5 Runx3 2/2 DRG neurons (Fig. 1E). Thus, at E14.5, corresponding to the stage of intensive axonal projection, the Ntrk3-v1 protein may be expressed in a Runx3-dependent manner in proprioceptive DRG neurons, but Ntrk3-v2, in a Runx3-independent manner in presumptive mechanoreceptive DRG neurons. Therefore, fibers of proprioceptive DRG neurons may be guided toward the ventral horn of the spinal cord via the NT3/Ntrk3-v1 signaling pathway under the control of Runx3. Knockdown experiments for Ntrk3-v1 in wild-type DRG neurons or rescue experiments for Ntrk3-v1 in Runx3 2/2 DRG neurons may further reveal the precise role for Ntrk3-v1 in the projection of proprioceptive DRG neurons.