Influences of neurotrophins on mammalian motoneurons in vivo

Abstract

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)

Differential responses to the application of exogenous NT‐3 are observed for subpopulations of motor and sensory neurons depending on the presence of skeletal muscle

“…Neurotrophin-3 knockout experiments have noted no loss of MNs in the LMC and MMC (Yan et al, 1993;Snider, 1994;Huang and Reichardt, 2001;Patel et al, 2003) and facial motor nucleus neuron loss is shown to be nonsignificant, i.e., no obvious deficits were detected, in the brainstem of newborn NT-3Ϫ/Ϫ mice . Whereas certain in vitro studies have demonstrated inadequate promotion of chick motoneuron survival through the use of NT-3 and BDNF, the application of these same neurotrophic factors on cultured fetal rat motoneurons demonstrated a survival-promoting effect for these agents (reviewed in Li et al, 1994).…”

“…Neurotrophin-3, like other neurotrophins of the same family, binds to the low affinity p75 NTR receptor protein (where NTR ϭ neurotrophin receptor; Yan et al, 1993). The highaffinity receptor most commonly associated with NT-3 is the tyrosine kinase TrkC receptor, although NT-3 is known to interact with both TrkA and TrkB in certain cell culture systems (Barbacid, 1994;Snider, 1994).…”

“…It should be noted that recent evidence suggests that it may no longer be accurate to use the terms high-and low-affinity receptors due to the possibility that the two receptor types interact in forming a tertiary complex (Barker, 2007). TrkC is the only Trk receptor expressed in the large proprioceptive neurons of the DRG by E9.5 and is expressed throughout the CNS by E11.5 (Barbacid, 1994) and, along with TrkB, is expressed in SC MNs by E13.5 (Yan et al, 1993). Importantly, retrograde labeling has enabled investigators to confirm that both adult and developing MNs transport NT-3 from the target muscle to the cell body of respective neurons, in vivo (Yan et al, 1993, Chalazonitis et al, 2001).…”

“…TrkC is the only Trk receptor expressed in the large proprioceptive neurons of the DRG by E9.5 and is expressed throughout the CNS by E11.5 (Barbacid, 1994) and, along with TrkB, is expressed in SC MNs by E13.5 (Yan et al, 1993). Importantly, retrograde labeling has enabled investigators to confirm that both adult and developing MNs transport NT-3 from the target muscle to the cell body of respective neurons, in vivo (Yan et al, 1993, Chalazonitis et al, 2001). The results of several studies suggest that both neurotrophin and Trk receptor are transported, by means of endocytosis, to the cell body and that chemical signals may in fact be exerting an effect before the neurotrophin (or NT/receptor complex) arrives to the cell body.…”

“…Others have demonstrated that application of exogenous NT-3 serves to rescue both SC and cranial motor and proprioceptive neurons from axotomy-induced and deafferentation cell death in developing chicks and in adult mice (Yan et al, 1993;Li et al, 1994). In the current study, we examine the ability of NT-3 to rescue neurons and increase neuron numbers during EPCD and NPCD, respectively.…”