Differential expression of TrkC catalytic and noncatalytic isoforms suggests that they act independently or in association

Abstract

Members of the trk gene family encode neurotrophin receptors. The trkC locus encodes multiple neurotrophin-3 catalytic and noncatalytic receptor isoforms. We report the molecular cloning and characterization of mouse cDNAs encoding two noncatalytic TrkC receptors: novel isoforms designated as TrkC NC1 and TrkC NC2, the mouse homologue of the TrkC truncated form previously identified in rat (Tsoulfas et al. [1993] Neuron 10:975-990; Valenzuela et al. [1993] Neuron 10:963-974). We extensively analyzed the transcription pattern of these two noncatalytic isoforms and that of the catalytic isoforms by Northern blotting and in situ hybridization. We did not detect trkC NC1 transcripts in embryos, but we found that trkC NC1 expression is restricted to specific areas in adult brain. In contrast, trkC NC2 transcripts are readily detected early during embryogenesis and are expressed predominantly in adult brain and gonads. We also provide the first evidence for the existence of TrkC NC2 protein by using polyclonal antibodies that specifically recognize this isoform. By using in situ hybridization, we show for the first time that trkC NC2 transcripts are found in differentiating fields of maturing neurons and in mature neurons of laminar structures of adult brain. We also report a similarity of localization between trkC NC2 transcripts and markers of oligodendrocyte progenitors in the embryonic spinal cord. Furthermore, our results also show that trkC NC2 and trkC catalytic transcripts could be either codistributed (in the central and peripheral nervous system) or independently expressed, especially outside the nervous system. These results suggest that the TrkC NC2 isoform acts either independently or in association with its catalytic counterpart. Finally, we show that TrkC NC2 is expressed in dendrites of pyramidal neurons of hippocampus and cerebral cortex. We propose that this receptor is involved in proliferation of oligodendrocyte progenitors, neuronal differentiation, and synaptic plasticity and that it may also play a fundamental role in mediating neurotrophin-3 effects outside the nervous system.

The noncatalytic TrkCNC2 receptor is cleaved by metalloproteases upon neurotrophin-3 stimulation

“…Since these deficiencies are not observed in TrkC catalytic receptor mutants, it suggests that NT-3 effects are mainly mediated during heart development by noncatalytic TrkC receptors that are highly expressed in the cardiovascular system (Menn et al, 1998). Taken together, these data suggest that the TrkCNC2 receptor acts either alone or in association with its catalytic counterpart.…”

“…Finally, to rule out the possibility that generation of p20 TrkCNC2 was merely a consequence of ectopic receptor expression in fibroblasts, we investigated its presence in lysates of cortical neurons cultured in NT-3-and serumdeprived medium, and of adult mouse brain, previously shown to express endogenous TrkCNC2 protein (Menn et al, 1998(Menn et al, , 2000. Interestingly, we detected the presence of p20 TrkCNC2 , in addition to that of the mature full-length TrkCNC2 receptor (gp105 TrkCNC2 ), by immunoprecipitation followed by Western blot analysis with anti-NC2 serum ( Figure 5).…”

“…Antibodies used were directed against either the specific NC2 cytoplasmic domain (anti-NC2), the catalytic TK domain (anti-K) or the extracellular domain of the TrkC receptor (anti-Ext). Specificity of these immune sera was assessed in previous studies (Menn et al, 1998(Menn et al, , 2000. Both NC2 and Ext antisera identified two major protein species with apparent molecular masses of 110 and l00 kDa, likely to represent the mature form and the partially glycosylated precursor, respectively ( Figure 1a).…”

“…Splicing also occurs in the intracellular domain, as described for TrkB and TrkC. Thus, we can distinguish catalytic isoforms that exhibit tyrosine kinase (TK) activity from noncatalytic forms in which the TK domain was substituted by specific cytoplasmic sequences (Klein et al, 1990;Middlemas et al, 1991;Lamballe et al, 1993;Tsoulfas et al, 1993;Valenzuela et al, 1993;Garner and Large, 1994;Shelton et al, 1995;Menn et al, 1998).…”

“…We previously reported that the noncatalytic TrkCNC2 receptor is highly expressed either alone or with its catalytic counterpart, in the central and peripheral nervous system (Menn et al, 1998). In vivo studies showed that trkC NC2 transcripts were abundant in regions of neuronal maturation.…”