Christine R. Matheson

The TrkB family of transmembrane proteins serve as receptors for brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-4/5, and possibly NT-3, three members of the neurotrophin family of neurotrophic factors. In order to understand the potential roles played by these receptors, we have examined the distribution of the TrkB receptor proteins in the adult rat brain by using immunohistochemistry. Several different antisera, directed against either synthetic peptides corresponding to different regions of TrkB or a recombinant fusion protein comprising part of the extracellular domain, were generated. Each of these antisera was directed to epitopes found on all known TrkB isoforms (both the tyrosine kinase-possessing isoform and the truncated kinase-lacking isoforms). In addition, a commercially available antibody to the intracellular domain of TrkB was also used. Widespread and distinct staining was observed on the surface of neuronal cell bodies, axons, and dendrites in many structures, including the cerebral cortex, hippocampus, dentate gyrus, striatum, septal nuclei, substantia nigra, cerebellar Purkinje cells, brainstem and spinal motor neurons, and brainstem sensory nuclei. Staining was also observed in the pia matter, on a subpopulation of ependymal cells lining the cerebral ventricle wall, and other nonneuronal cells. The expression pattern of TrkB receptor protein suggests that TrkB plays a broad role in the central nervous system. In addition, the detection of TrkB immunoreactivity on cell bodies and dendrites is consistent with recent models suggesting that neurotrophins may be derived from presynaptic and/or autocrine sources in addition to the classical postsynaptic target. J Comp Neurol 378:135–157, 1997. 1997 © Wiley-Liss, Inc.

Several recently reported investigations have shown that a member of the neurotrophin family of neuronal growth factors, brain-derived neurotrophic factor (BDNF), supports motoneurons in vitro and rescues motoneurons from naturally occurring and axotomy-induced cell death (Oppenheim et al., 1992b; Sendtner et al., 1992b; Yan et al., 1992; Koliatsos et al., 1993; Henderson et al., 1993). In the current study, we have explored the issue of whether BDNF and other neurotrophins act to regulate motoneuron survival during development and asked whether synthesis of motoneuron transmitter enzymes is also regulated. We first examined whether spinal motoneurons in newborn animals could retrogradely transport iodinated neurotrophins from their targets in a specific, receptor-mediated manner. We found that motoneurons readily transported NGF, BDNF, and neurotrophin-3 (NT-3). The retrograde transport of one factor could be completely or largely blocked by excess of unlabeled homologous factor, but only partially blocked by excess of unlabeled heterologous factors. Since previous studies have shown that these three neurotrophins bind to the low-affinity NGF receptor, p75NGFR, with similar affinity, our data suggest that the retrograde transport of neurotrophins by motoneurons may be mediated by additional components, such as the trk family of proto-oncogenes. Consistent with this hypothesis, we demonstrate here that motoneurons express mRNA for two members of the trk family, trkB and trkC. Furthermore, both trkB and trkC were expressed by E13, consistent with a role for BDNF and NT-3 in regulating important developmental events involving motoneurons such as naturally occurring cell death. In order to determine which members of the neurotrophin family influence motoneuron survival and to assess the generality of their effects, we evaluated the abilities of NGF, BDNF, and NT-3 to save both spinal and cranial motoneurons after neonatal axotomy. Locally applied BDNF saved 40-70% of motoneurons which would ordinarily die after axotomy in lumbar and cranial motor pools, depending on the treatment protocol employed. NT-3 also exhibited some ability to rescue motoneurons and saved 20-25% of motoneurons which would die in the absence of treatment. Finally, we asked whether neurotrophins could influence synthesis of transmitter enzymes by motoneurons as well as their survival after axotomy. Locally applied BDNF and NT-3 could partially prevent the decrease of protein contents in L4 and L5 ventral roots which normally follows sciatic nerve transection. However, treatment with these neurotrophins did not prevent the decrease in choline acetyltransferase (ChAT) activity in L4 and L5 ventral roots which results from this procedure.(ABSTRACT TRUNCATED AT 400 WORDS)