Differential effects of the trophic factors BDNF, NT-4, GDNF, and IGF-I on the isthmo-optic nucleus in chick embryos

The isthmo-optic nucleus (ION) of chick embryos is a model system for the study of retrograde trophic signaling in developing CNS neurons. The role of brain-derived neurotrophic factor (BDNF) is well established in this system. Recent work has implicated neurotrophin-4 (NT-4), glial cell line-derived neurotrophic factor (GDNF), and insulin-like growth factor I (IGF-I) as additional trophic factors for ION neurons. Here it was examined in vitro and in vivo whether these factors are target-derived trophic factors for the ION in 13- to 16-day-old chick embryos. Unlike BDNF, neither GDNF, NT-4, nor IGF-I increased the survival of ION neurons in dissociated cultures identified by retrograde labeling with the fluorescent tracer DiI. BDNF and IGF-I promoted neurite outgrowth from ION explants, whereas GDNF and NT-4 had no effect. Injections of NT-4, but not GDNF, in the retina decreased the survival of ION neurons and accelerated cell death in the ION. NT-4-like immunoreactivity was present in the retina and the ION. Exogenous, radiolabeled NT-4, but not GDNF or IGF-I, was retrogradely transported from the retina to the ION. NT-4 transport was significantly reduced by coinjection of excess cold nerve growth factor (NGF), indicating that the majority of NT-4 bound to p75 neurotrophin receptors during axonal transport. Binding of NT-4 to chick p75 receptors was confirmed in L-cells, which express chick p75 receptors. These data indicate that GDNF has no direct trophic effects on ION neurons. IGF-I may be an afferent trophic factor for the ION, and NT-4 may act as an antagonist to BDNF, either by competing with BDNF for p75 and/or trkB binding or by signaling cell death via p75.     

Neurotrophic factor regulation of developing avian oculomotor neurons: Differential effects of BDNF and GDNF

Neurotrophic factors support the development of motoneurons by several possible mechanisms. Neurotrophins may act as target‐derived factors or as afferent factors derived from the central nervous system (CNS) or sensory ganglia. We tested whether brain‐derived neurotrophic factor (BDNF), neurotrophin 3 (NT‐3), neurotrophin 4 (NT‐4), and glial cell line–derived neurotrophic factor (GDNF) may be target‐derived factors for neurons in the oculomotor (MIII) or trochlear (MIV) nucleus in chick embryos. Radio‐iodinated BDNF, NT‐3, NT‐4, and GDNF accumulated in oculomotor neurons via retrograde axonal transport when the trophic factors were applied to the target. Systemic GDNF rescued oculomotor neurons from developmental cell death, while BDNF and NT‐3 had no effect. BDNF enhanced neurite outgrowth from explants of MIII and MIV nuclei (identified by retrograde labeling in ovo with the fluorescent tracer DiI), while GDNF, NT‐3, and NT‐4 had no effect. The oculomotor neurons were immunoreactive for BDNF and the BDNF receptors p75^NTR and trkB. To determine whether BDNF may be derived from its target or may act as an autocrine or paracrine factor, in situ hybridization and deprivation studies were performed. BDNF mRNA expression was detected in eye muscles, but not in CNS sources of afferent innervation to MIII, or the oculomotor complex itself. Injection of trkB fusion proteins in the eye muscle reduced BDNF immunoreactivity in the innervating motoneurons. These data indicate that BDNF trophic support for the oculomotor neurons was derived from their target. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 295–315, 1999     

Neurotrophic factor regulation of developing avian oculomotor neurons: differential effects of BDNF and GDNF.

Neurotrophic factors support the development of motoneurons by several possible mechanisms. Neurotrophins may act as target-derived factors or as afferent factors derived from the central nervous system (CNS) or sensory ganglia. We tested whether brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), neurotrophin 4 (NT-4), and glial cell line-derived neurotrophic factor (GDNF) may be target-derived factors for neurons in the oculomotor (MIII) or trochlear (MIV) nucleus in chick embryos. Radio-iodinated BDNF, NT-3, NT-4, and GDNF accumulated in oculomotor neurons via retrograde axonal transport when the trophic factors were applied to the target. Systemic GDNF rescued oculomotor neurons from developmental cell death, while BDNF and NT-3 had no effect. BDNF enhanced neurite outgrowth from explants of MIII and MIV nuclei (identified by retrograde labeling in ovo with the fluorescent tracer DiI), while GDNF, NT-3, and NT-4 had no effect. The oculomotor neurons were immunoreactive for BDNF and the BDNF receptors p75(NTR) and trkB. To determine whether BDNF may be derived from its target or may act as an autocrine or paracrine factor, in situ hybridization and deprivation studies were performed. BDNF mRNA expression was detected in eye muscles, but not in CNS sources of afferent innervation to MIII, or the oculomotor complex itself. Injection of trkB fusion proteins in the eye muscle reduced BDNF immunoreactivity in the innervating motoneurons. These data indicate that BDNF trophic support for the oculomotor neurons was derived from their target.     

Effects of castration on the immunoreactivity to NGF, BDNF and their receptors in the pelvic ganglia of the male rat

Nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) and are members of the neurotrophin family, a family of neurotrophic factors that also includes neurotrophin (NT) 3 and NT4/5. Neurotrophins have essential roles in the survival, development and differentiation of neurons in the central and peripheral nervous systems. Neurotrophins exert their effects by binding to corresponding receptors which are formed by the tyrosine protein kinases TrkA, TrkB and TrkC, and the low affinity neurotrophic receptor (p75NTR). In the present study, using immunohistochemistry and quantitative analysis, we have investigated immunoreactivity to BDNF, NGF, TrkB, p75NTR and TrkA in the pelvic ganglia of normal and castrated rats. Neurons of the pelvic ganglia expressed both these neurotrophins and their receptors. After castration the immunoreactivity persisted. However, the number of BDNF- and p75NTR-IR cells statistically significant decreased after castration. These results suggest that castration modulates the expression of neurotrophins and their receptors in pelvic autonomic neurons.     

Positive and negative interactions of GDNF, NTN and ART in developing sensory neuron subpopulations, and their collaboration with neurotrophins

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN) and neublastin/artemin (ART) are distant members of the transforming growth factor beta family, and have been shown to elicit neurotrophic effects upon several classes of peripheral and central neurons. Limited information from in vitro and expression studies has also substantiated a role for GDNF family ligands in mammalian somatosensory neuron development. Here, we show that although dorsal root ganglion (DRG) sensory neurons express GDNF family receptors embryonically, they do not survive in response to their ligands. The regulation of survival emerges postnatally for all GDNF family ligands. GDNF and NTN support distinct subpopulations that can be separated with respect to their expression of GDNF family receptors, whereas ART supports neurons in populations that are also responsive to GDNF or NTN. Sensory neurons that coexpress GDNF family receptors are medium sized, whereas small-caliber nociceptive cells preferentially express a single receptor. In contrast to brain-derived neurotrophic factor (BDNF)-dependent neurons, embryonic nerve growth factor (NGF)-dependent nociceptive neurons switch dependency to GDNF, NTN and ART postnatally. Neurons that survive in the presence of neurotrophin 3 (NT3) or neurotrophin 4 (NT4), including proprioceptive afferents, Merkel end organs and D-hair afferents, are also supported by GDNF family ligands neonatally, although at postnatal stages they lose their dependency on GDNF and NTN. At late postnatal stages, ART prevents survival elicited by GDNF and NTN. These data provide new insights on the roles of GDNF family ligands in sensory neuron development.     

Bicaudal‐D1 regulates the intracellular sorting and signalling of neurotrophin receptors

We have identified a new function for the dynein adaptor Bicaudal D homolog 1 (BICD1) by screening a siRNA library for genes affecting the dynamics of neurotrophin receptor‐containing endosomes in motor neurons (MNs). Depleting BICD1 increased the intracellular accumulation of brain‐derived neurotrophic factor (BDNF)‐activated TrkB and p75 neurotrophin receptor (p75^NTR) by disrupting the endosomal sorting, reducing lysosomal degradation and increasing the co‐localisation of these neurotrophin receptors with retromer‐associated sorting nexin 1. The resulting re‐routing of active receptors increased their recycling to the plasma membrane and altered the repertoire of signalling‐competent TrkB isoforms and p75^NTR available for ligand binding on the neuronal surface. This resulted in attenuated, but more sustained, AKT activation in response to BDNF stimulation. These data, together with our observation that Bicd1 expression is restricted to the developing nervous system when neurotrophin receptor expression peaks, indicate that BICD1 regulates neurotrophin signalling by modulating the endosomal sorting of internalised ligand‐activated receptors.     

Target‐derived and locally derived neurotrophins support retinal ganglion cell survival in the neonatal rat retina

Brain‐derived neurotrophic factor (BDNF) and neurotrophin‐4/5 (NT‐4/5) protein and mRNA are found in the neonatal rat retina and also in target sites such as the superficial layers of the superior colliculus. Both neurotrophins support neonatal retinal ganglion cell survival in vitro. In vivo, injections of recombinant BDNF and NT‐4/5 reduce naturally occurring cell death as well as death induced by removal of the contralateral superior colliculus. In the latter case, the peak of retinal ganglion cell death occurs about 24 h postlesion. We wished to determine: whether a similar time‐course of degeneration occurs after selective removal of target cells or depletion of target‐derived trophic factors, and whether ganglion cell viability also depends on intraretinally derived neurotrophins. Retinal ganglion cell death was measured 24 and 48 h following injections of kainic acid or a mixture of BDNF and NT‐4/5 blocking antibodies into the superior colliculus and 24 h after intraocular injection of the same antibodies. Retinotectally projecting ganglion cells were identified by retrograde labeling with the nucleophilic dye diamidino yellow. We show that collicular injections of either kainic acid or BDNF and NT‐4/5 blocking antibodies significantly increased retinal ganglion cell death in the neonatal rat 24 h postinjection, death rates returning to normal by 48 h. This increase in death was greatest following collicular injections; however, death was also significantly increased 24 h following intravitreal antibody injection. Thus retinal ganglion cell survival during postnatal development is not only dependent upon trophic factors produced by central targets but may also be influenced by local intraretinal neurotrophin release. © 2004 Wiley Periodicals, Inc. J Neurobiol 60: 319–327, 2004     

BMP‐2 and cAMP elevation confer locus coeruleus neurons responsiveness to multiple neurotrophic factors

The locus coeruleus (LC) is a major target of several neurodegenerative disorders, including Parkinson's and Alzheimer's diseases. However, very little is known of the trophic requirements of LC neurons. In the present work, we have studied the biological activity of neurotrophic factors from different families in E15 primary cultures of LC neurons. In agreement with previous results, neurotrophin‐3 (NT‐3) and also glial cell line‐ derived neurotrophic factor (GDNF) increased the number of embryonic LC noradrenergic neurons in the presence of serum. In serum‐free conditions, none of the factors tested, including NT‐3, GDNF, neurturin, basic fibroblast growth factor (bFGF), or bone morphogenetic protein‐2 (BMP‐2), promoted the survival of tyrosine hydroxylase (TH)‐immunoreactive neurons at 6 days in vitro. However, when BMP‐2 was coadministered with any of these factors the number of LC TH‐positive neurons increased twofold. Similar results were obtained by cotreatment of LC neurons with forskolin and NT‐3, bFGF, or BMP‐2. The strongest effect (a fourfold increase in the number of TH‐positive cells) was induced by cotreatment with forskolin, BMP‐2, and GDNF. Thus, our results show that LC neurons require multiple factors for their survival and development, and suggest that activation of LC neurons by bone morphogenetic proteins and cAMP plays a decisive role in conferring noradrenergic neuron responsiveness to several trophic factors. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 291–304, 2002; DOI 10.1002/neu.10034     

Interactions between beta-neuregulin and neurotrophins in motor neuron apoptosis

Neuregulins play a major role in the formation and stabilization of neuromuscular junctions, and are produced by both motor neurons and muscle. Although the effects and mechanism of neuregulins on skeletal muscle (e.g. regulation of acetylcholine receptor expression) have been studied extensively, the effects of neuregulins on motor neurons remain unknown. We report that neuregulin-1beta (NRGbeta1) inhibited apoptosis of rat motor neurons for up to 7 days in culture by a phosphatidylinositol 3 kinase-dependent pathway and synergistically enhanced motor neuron survival promoted by glial-derived neurotrophic factor (GDNF). However, binding of neurotrophins, including brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), to the p75 neurotrophin receptor (p75NTR) abolished the neuregulin anti-apoptotic effect on motor neurons. Inhibitors of the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase prevented motor neuron death caused by co-incubation of NRGbeta1 and BDNF or NGF, as well as by trophic factor deprivation. Motor neuron apoptosis resulting from both trophic factor deprivation and exposure to NRGbeta1 plus neurotrophins required the induction of neuronal nitric oxide synthase and peroxynitrite formation. Because motor neurons express both p75NTR and neuregulin erbB receptors during the period of embryonic programmed cell death, motor neuron survival may be the result of complex interactions between trophic and death factors, which may be the same molecules acting in different combinations.     

BDNF and NT4/5 promote survival and neurite outgrowth of pontocerebellar mossy fiber neurons

The neurotrophins nerve growth factor (NGF), brain‐derived neurotrophic factor (BDNF), neurotrophin‐3 (NT3), and NT4/5 are all found in the developing cerebellum. Granule cells, the major target neurons of mossy fibers, express BDNF during mossy fiber synaptogenesis. To determine whether neurotrophins contribute to the development of cerebellar afferent axons, we characterized the effects of neurotrophins on the growth of mossy fiber neurons from mice and rats in vitro. For a mossy fiber source, we used the basilar pontine nuclei (BPN), the major source of cerebellar mossy fibers in mammals. BDNF and NT4/5 increased BPN neuron survival, neurite outgrowth, growth cone size, and elongation rate, while neither NT3 nor NGF increased survival or outgrowth. In addition, BDNF and NT4/5 reduced the size of neurite bundles. Consistent with these effects, in situ hybridization on cultured basilar pontine neurons revealed the presence of mRNA encoding the TrkB receptor which binds both BDNF and NT4/5 with high affinity. We detected little or no message encoding the TrkC receptor which preferentially binds NT3. BDNF and NT4/5 also increased TrkB mRNA levels in BPN neurons. In addition to previously established functions as an autocrine/paracrine trophic factor for granule cells, the present results indicate that cerebellar BDNF may also act as a target‐derived trophic factor for basilar pontine mossy fibers. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 254–269, 1999     

Prevention of thrombin‐induced motoneuron degeneration with different neurotrophic factors in highly enriched cultures

Previous reports have shown that neuronal and glial cells express functionally active thrombin receptors. The thrombin receptor (PAR‐1), a member of a growing family of protease activated receptors (PARs), requires cleavage of the extracellular amino‐terminus domain by thrombin to induce signal transduction. Studies from our laboratory have shown that PAR‐1 activation following the addition of thrombin or a synthetic thrombin receptor activating peptide (TRAP) induces motoneuron cell death both in vitro and in vivo. In addition to increasing motoneuron cell death, PAR‐1 activation leads to decreases in the mean neurite length and side branching in highly enriched motoneuron cultures. It has been suggested that motoneuron survival depends on access to sufficient target‐derived neurotrophic factors through axonal branching and synaptic contacts. However, whether the thrombin‐induced effects on motoneurons can be prevented by neurotrophic factors is still unknown. Using highly enriched avian motoneuron cultures, we show here that alone, soluble chick skeletal muscle extracts (CMX), brain‐derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), and glial cell line–derived neurotrophic factor (GDNF) significantly increased motoneuron survival compared to controls, whereas nerve growth factor (NGF) did not have a significant effect on motoneuron survival. Furthermore, cotreatment with muscle‐derived agents (i.e., CMX, BDNF, GDNF) significantly prevented the death of motoneurons induced by α‐thrombin. Yet, non–muscle‐derived agents (CNTF and NGF) had little or no significant effect in reversing thrombin‐induced motoneuron death. CMX and CNTF significantly increased the mean length of neurites, whereas NGF, BDNF, and GDNF failed to enhance neurite outgrowth compared to controls. Furthermore, CMX and CNTF significantly prevented thrombin‐induced inhibition of neurite outgrowth, whereas BDNF and GDNF only partially reversed thrombin‐induced inhibition of neurite outgrowth. These findings show differential effects of neurotrophic factors on thrombin‐induced motoneuron degeneration and suggest specific overlaps between the trophic and stress pathways activated by some neurotrophic agents and thrombin, respectively. © 1999 John Wiley & Sons, Inc. J Neurobiol 38: 571–580, 1999     

Brain‐derived neurotrophic factor selectively regulates dendritogenesis of parvalbumin‐containing interneurons in the main olfactory bulb through the PLCγ pathway

Molecular mechanisms of neurotrophin signaling on dendrite development and dynamics are only partly understood. To address the role of brain‐derived neurotrophic factor (BDNF) in the morphogenesis of GABAergic neurons of the main olfactory bulb, we analyzed mice lacking BDNF, mice carrying neurotrophin‐3 (NT3) in the place of BDNF, and TrkB signaling mutant mice with a receptor that can activate phospholipase Cγ (PLCγ) but is unable to recruit the adaptors Shc/Frs2. BDNF deletion yielded a compressed olfactory bulb with a significant loss of parvalbumin (PV) immunoreactivity in GABAergic interneurons of the external plexiform layer. Dendrite development of PV‐positive interneurons was selectively attenuated by BDNF since other Ca²⁺‐binding protein‐containing neuron populations appeared unaffected. The deficit in PV‐positive neurons could be rescued by the NT3/NT3 alleles. The degree of PV immunoreactivity was dependent on BDNF and TrkB recruitment of the adaptor proteins Shc/Frs2. In contrast, PLCγ signaling from the TrkB receptor was sufficient for dendrite growth in vivo and consistently, blocking PLCγ prevented BDNF‐dependent dendrite development in vitro. Collectively, our results provide genetic evidence that BDNF and TrkB signaling selectively regulate PV expression and dendrite growth in a subset of neurochemically‐defined GABAergic interneurons via activation of the PLCγ pathway. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Comparison of the effects of HGF,BDNF, CT‐1, CNTF,and the branchial arches on the growth of embryonic cranial motor neurons

In the developing embryo, axon growth and guidance depend on cues that include diffusible molecules. We have shown previously that the branchial arches and hepatocyte growth factor (HGF) are growth‐promoting and chemoattractant for young embryonic cranial motor axons. HGF is produced in the branchial arches of the embryo, but a number of lines of evidence suggest that HGF is unlikely to be the only factor involved in the growth and guidance of these axons. Here we investigate whether other neurotrophic factors could be involved in the growth of young cranial motor neurons in explant cultures. We find that brain‐derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and cardiotrophin‐1 (CT‐1) all promote the outgrowth of embryonic cranial motor neurons, while glial cell line‐derived neurotrophic factor (GDNF) and neurotrophin‐3 (NT‐3) fail to affect outgrowth. We next examined whether HGF and the branchial arches had similar effects on motor neuron subpopulations at different axial levels. Our results show that HGF acts as a generalized rather than a specific neurotrophic factor and guidance cue for cranial motor neurons. Although the branchial arches also had general growth‐promoting effects on all motor neuron subpopulations, they chemoattracted different axial levels differentially, with motor neurons from the caudal hindbrain showing the most striking response. © 2002 Wiley Periodicals, Inc. J Neurobiol 51: 101–114, 2002     

Glial cell line–derived neurotrophic factor (GDNF) promotes the survival of axotomized retinal ganglion cells in adult rats: Comparison to and combination with brain‐derived neurotrophic factor (BDNF)

Adult rat retinal ganglion cells (RGC) undergo degeneration after optic nerve transection. Studies have shown that exogenously applied neurotrophic factors such as brain‐derived neurotrophic factor (BDNF) can attenuate axotomy‐induced as well as developmental RGC death. Here, we examined whether glial cell line–derived neurotrophic factor (GDNF), a known neurotrophic factor for dopaminergic neurons and motor neurons, could provide neurotrophic support to RGC in adult rats. We determined whether RGC could retrogradely transport GDNF from their target tissue. After injection into the superior colliculus of adult rats, ¹²⁵I‐GDNF was retrogradely transported to contralateral eyes but not to ipsilateral eyes. The transport of ¹²⁵I‐GDNF could be blocked by coinjection of excess unlabeled GDNF, indicating that it was receptor mediated. We tested whether intravitreally applied GDNF could prevent axotomy‐induced RGC degeneration. The RGC were prelabeled with Fluorogold (FG) and axotomized by intraorbital optic nerve transection. GDNF, BDNF (positive control), cytochrome c (negative control), or a GDNF/BDNF combination was injected intravitreally on days 0 and 7. On day 14, FG‐labeled RGC were counted from whole‐mount retinas. We found that, similar to BDNF, GDNF could significantly attenuate the degeneration of RGC in a dose‐dependent fashion. Furthermore, the combination treatment of GDNF and BDNF showed better protection than either factor used individually. Our data indicate that GDNF is a neurotrophic factor for the adult rat RGC. GDNF, like BDNF, may be useful for the treatment of human RGC degenerative diseases. © 1999 John Wiley & Sons, Inc. J Neurobiol 38: 382–390, 1999     

Axonal outgrowth from adult mouse nodose ganglia in vitro is stimulated by neurotrophin‐4 in a Trk receptor and mitogen‐activated protein kinase‐dependent way

The actions of neurotrophic factors on sensory neurons of the adult nodose ganglion were studied in vitro. The ganglia were explanted in an extracellular matrix–based gel that permitted observation of the growing axons. Neurotrophin‐4 (NT‐4) was a very efficient stimulator of outgrowth of axons from the nodose ganglion and had almost doubled the outgrowth length when this was analyzed after 2 days in culture. Brain‐derived neurotrophic factor also stimulated outgrowth, but to a lesser degree, whereas NT‐3 gave only weak stimulatory tendencies. Nerve growth factor and glial cell line–derived neurotrophic factor both lacked stimulatory effects. NT‐4 is known to act via TrkB receptors, and the presence of these on growing nodose neurons was demonstrated immunohistochemically. In line with a Trk‐mediated growth effect, the NT‐4 stimulation was abolished by K252a, a selective inhibitor of neurotrophin receptor–associated tyrosine kinase activity. K252a had no effect on the unstimulated preparation. NT‐4 treatment led to activation of the mitogen‐activated protein kinase and inhibition of the latter pathway by PD98059 significantly reduced the NT‐4 stimulated outgrowth, whereas the drug had no effect on the unstimulated growth. In conclusion, the data suggest that NT‐4 can serve as a powerful growth factor for neurons of adult nodose ganglia and that the growth stimulation involves TrkB‐ and mitogen‐activated protein kinase. © 2000 John Wiley & Sons, Inc. J Neurobiol 45: 142–151, 2000     

Sympathetic neuronal survival induced by retinal trophic factors

Neuronal survival in the vertebrate peripheral nervous system depends on neurotrophic factors available from target tissues. In an attempt to identify novel survival factors, we have studied the effect of secreted factors from retinal cells on the survival of chick sympathetic ganglion neurons. Embryonic day 10 sympathetic neurons undergo programmed cell death after 48 h without appropriate levels of nerve growth factor (NGF). Retina Conditioned Media (RCM) from explants of embryonic day 11 retinas maintained for 4 days in vitro supported 90% of E10 chick sympathetic neurons after 48 h. Conditioned medium from purified chick retinal Muller glial cells supported nearly 100% of E10 chick sympathetic neurons. Anti‐NGF (1 μg/mL) blocked the survival effect of NGF, but did not block the trophic effect of RCM. Neither BDNF nor NT4 (0.1–50 ng/mL) supported E10 sympathetic neuron survival. Incubation of chimeric immunoglobulin‐receptors TrkA, TrkB, or TrkC had no effect on RCM‐induced sympathetic neuron survival. The survival effects were not blocked by anti‐GDNF, anti‐TGFβ, and anti‐CNTF and were not mimicked by FGFb (0.1–10 nM). LY294002 at 50 μM, but not PD098059 blocked sympathetic survival induced by RCM. Further, the combination of RCM and NGF did not result in an increase in neuronal survival compared with NGF alone (82% survival after 48 h). The secreted factor in RCM is retained in subfractions with a molecular weight above 100 kDa, binds to heparin, and is unaffected by dialysis, but is heat sensitive. Our results indicate the presence of a high‐molecular weight retinal secreted factor that supports sympathetic neurons in culture. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 13–23, 2002     

The nerve growth factor receptor: a multicomponent system that mediates the actions of the neurotrophin family of proteins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) are members of a family of structurally related proteins termed neurotrophins that promote the growth and survival of neurons in the central and peripheral nervous systems. Each of these proteins bind to at least two membrane receptors. One is the low affinity nerve growth factor receptor (p75), which binds each member of the neurotrophin family. The other is one of a family of tyrosine kinase receptors —trkA binds only NGF, the relatedtrkB receptor binds BDNF and NT-3, andtrkC binds NT-3 alone. This article reviews kinetic and biochemical information on p75 and its relationship to thetrk gene products.     

Intrastriatal administration of human immunodeficiency virus‐1 glycoprotein 120 reduces glial cell‐line derived neurotrophic factor levels and causes apoptosis in the substantia nigra

Uninfected neurons of the substantia nigra (SN) degenerate in human immunodeficiency virus (HIV)‐positive patients through an unknown etiology. The HIV envelope glycoprotein 120 (gp120) causes apoptotic neuronal cell death in the rodent striatum, but its primary neurotoxic mechanism is still under investigation. Previous studies have shown that gp120 causes neurotoxicity in the rat striatum by reducing brain‐derived neurotrophic factor (BDNF). Because glial cell line‐derived neurotrophic factor (GDNF) and BDNF are neurotrophic factors crucial for the survival of dopaminergic neurons of the SN, we investigated whether gp120 reduces GDNF and BDNF levels concomitantly to induce apoptosis. Rats received a microinjection of gp120 or vehicle into the striatum and were sacrificed at various time intervals. GDNF but not BDNF immunoreactivity was decreased in the SN by 4 days in gp120‐treated rats. In these animals, a significant increase in the number of caspase‐3‐ positive neurons, both tyrosine hydroxylase (TH)‐positive and ‐negative, was observed. Analysis of TH immunoreactivity revealed fewer TH‐positive neurons and fibers in a medial and lateral portion of cell group A9 of the SN, an area that projects to the striatum, suggesting that gp120 induces retrograde degeneration of nigrostriatal neurons. We propose that dysfunction of the nigrostriatal dopaminergic system associated with HIV may be caused by a reduction of neurotrophic factor expression by gp120. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

A small peptide mimetic of brain‐derived neurotrophic factor promotes peripheral myelination

The expression of the neurotrophins and their receptors is essential for peripheral nervous system development and myelination. We have previously demonstrated that brain‐derived neurotrophic factor (BDNF) exerts contrasting influences upon Schwann cell myelination in vitro – promoting myelination via neuronally expressed p75NTR, but inhibiting myelination via neuronally expressed TrkB. We have generated a small peptide called cyclo‐d PAKKR that structurally mimics the region of BDNF that binds p75NTR. Here, we have investigated whether utilizing cyclo‐d PAKKR to selectively target p75NTR is an approach that could exert a unified promyelinating response. Like BDNF, cyclo‐d PAKKR promoted myelination of nerve growth factor‐dependent neurons in vitro, an effect dependent on the neuronal expression of p75NTR. Importantly, cyclo‐d PAKKR also significantly promoted the myelination of tropomyosin‐related kinase receptor B‐expressing neurons in vitro, whereas BDNF exerted a significant inhibitory effect. This indicated that while BDNF exerted a contrasting influence upon the myelination of distinct subsets of dorsal root ganglion (DRG) neurons in vitro, cyclo‐d PAKKR uniformly promoted their myelination. Local injection of cyclo‐d PAKKR adjacent to the developing sciatic nerve in vivo significantly enhanced myelin protein expression and significantly increased the number of myelinated axons. These results demonstrate that cyclo‐d PAKKR promotes peripheral myelination in vitro and in vivo, suggesting it is a strategy worthy of further investigation for the treatment of peripheral demyelinating diseases.