neurotrophic factors and their receptors - university of liverpool

Neurotrophic factors and their receptors

Mariano Barbacid

Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, USA

Several new members of the nerve growth factor family of neurotrophins,

which comprises nerve growth factor itself, brain-derived neurotrophic factor

and neurotrophins-3, -4 (also known as neurotrophin5) and -6, have been

isolated in recent years. Their signaling receptors have been identified as the

Trk family of tyrosine protein kinases, thus facilitating the dissection of the

signaling pathways responsible for mediating their trophic properties. More

recently, the advent of gene targeting has made it possible to generate strains

of mice lacking neurotrophins and their receptors. Analysis of the phenotypes

of these mutant animals has provided detailed information on the role that

neurotrophins and their receptors play in the ontogeny of the mammalian

nervous system.

Current Opinion in Cell Biology 1995, 7:148-l 55

Introduction

Growth factors play a critical role in the development and maintenance of the mammalian nervous system. Some of these factors (e.g. platelet-derived growth factor and fibroblast growth factor) have pleiotropic activities and are often studied in systems other than the nervous system [l-3]. Other factors have a more restricted activity on neuronal cells and are classified as ‘neurotrophic factors’. Among them, the most in- tensively studied include the ciliary neurotrophic factor (CNTF) [4], the recently discovered glial-derived neurotrophic factor (GDNF) [5*] and the members of the nerve growth factor (NGF) family, now known as ‘neurotrophins’ [6,7]. Each of these neurotrophic factors exerts its biological activities through a different class of receptors. Neurotrophins signal through the Trk family of tyrosine protein kinases [8-lo]. In addition, neurotrophins interact with a second receptor, ~75, a member of the tumor necrosis factor receptor superfamily [ll]. CNTF signals though a multimeric receptor complex which includes binding and signaling elements [12,13]. The binding subunit is specific for CNTF and does not have transmembrane or cytoplasmic domains, but instead anchors to the plama membrane by a glycosyl phosphatidylinositol linkage [14]. The signaling subunits include gp130, a transmembrane glycoprotein previously found in various cytokine receptors [15], and the p-subunit of the receptor for leukemia inhibitory factor [16]. The cytoplasmic domains of both of these molecules interact with members of the Jak family of cytoplasmic tyrosine kinases, which are likely to be the primary

mediators of CNTF activity [12,13]. CNTF shares these signaling subunits with a variety of cytokine receptors, including the receptors for leukemia inhibitory factor, interleukin-6, oncostatin M and, possibly, interleukin-1 1 [12,13]. How these growth factors can elicit such different biological responses by activating the same signaling receptor subunits is a subject of intense study at this time [12,13]. Finally, the receptor for GDNF has not as yet been identified; however, the limited structural similarities between GDNF and the transforming growth factor-p superfamily [5*] raise the possibility that GDNF may signal through multimeric serine/threonine kinase receptors.

During the past year, several important developments have occured in the field of neurotrophins and their receptors. The generation of strains of mice carrying targeted mutations in the genes encoding several neurotrophins, as well as in those encoding the ~75 and the Trk kinase receptors, has provided a wealth of information regarding the role that these molecules play in the ontogeny of the mammalian nervous system [17]. In consideration of the space limitations, this review will focus on neurotrophins and their receptors. Other exciting developments regarding CNTF and its signal transduction pathways have been recently summarized in several reviews [4,12,13] and will not be discussed here. GDNF, initially thought to be specific for dopaminergic neurons [5-l, appears to have a broader specificity, as it can also support the survival of motorneurons [18]. Further studies are needed to define the full range of neurotrophic activities of this novel growth factor.

148

Abbreviations

BDNF-brain-derived neurotrophic factor; CNTF-ciliary neurotrophic factor; EGFGpidermal growth factor; GDNF-glial-derived neurotrophic factor; MAP-mitogen-activated protein; NCF-nerve growth factor;

NT-neurotrophin; PLC-phospholipase C.

0 Current Biology Ltd ISSN 0955-0674

Neurotrouhic factors and their receutors Barbacid 149

The NCF family of neurotrophins

This family of growth factors includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-3 and NT-4 (also known as NT-4/5 and NT-5) [2]. A molecule recently isolated from the teleost fish Xiphopho- rus, NT-6, probably represents a novel member of this f&nily [19*]. These neurotrophins bind specifically to the three known Trk tyrosine kinase receptors, T&A, TrkB and TrkC [8-lo]. Whereas NGF binds to T&A, BDNF and NT-4 interact with TrkB (Fig. 1). NT-3 appears to be more promiscuous and binds to each of the Trk receptors, but its primary biological responses are me- diated by TrkC (see below). In addition, each of these neurotrophins binds with low affinity to ~75, a receptor structurally unrelated to the Trks and whose precise role in mediating neurotrophin activity remains to be elucidated (see below) [ll] (Fig. 1). Whether NT-6 binds to any of these receptors is not known at the present time.

NGF recruitment ?

Qo

NGF BDNF NT-4 NT-3

TrKA TrkB

Signal transduction

TrKC

0 1995 Current Opinion in Cell Biology

Fig. 1. Functional interactions between neurotrophins and their cognate receptors. Each of the neurotrophins binds with similarly low affinity (nanomolar range) to ~75. The neuronal defects observed in p75 null mice, however, suggest a functional interaction of this receptor with NGF (solid arrow) but not with BDNF or NT-3 (dashed arrows). The interactions of NCF with TrkA and of BNDF and NT-4 with TrkB are highly specific. NT-3 primarily activates TrkC receptors; however, this neurotrophin can also interact with the TrkA and TrkB receptors (see text).

The three-dimensional structure of NGF has been re- solved by X-ray crystallography (at 2.3A resolution; [20]) and shown to consist of two identical subunits. Based on previous sequence analysis, each subunit has 118 amino acids residues and is generated by cleav- age of a precursor polypeptide of 252 residues [1,7]. The NGF protomers are elongated and contain three twisted anti-parallel p-sheet segments linked by turns. These protomers are linked to each other by a ‘cysteine knot’, a novel structure also found in transforming growth factor-b and platelet-derived growth factor B-subunit homodimers, and display a twofold symmetry with parallel long axes [20]. The structure of the other neurotrophins remains to be elucidated; however, their close similarities to each other (47 residues are identical in all four mammalian neurotrophins) suggest that they are likely to have similar tertiary structures.

Site-directed mutagenesis studies have provided important information regarding those residues critical for the

interaction of neurotrophins with the p75 and TrkA receptors. The interaction of NGF with p75 is medi- ated by a positively charged region that consists of at least three lysine residues located in two spatially adjacent loops (I and V) [21]. These positively charged residues are conserved in NT-3 and NT-4, but not in BDNF, which contains three consecutive positively charged residues in loop V that may compensate for the absence of the two lysines present in loop I in the other neurotrophins [22]. These differences may ex- plain the somewhat distinct dissociation kinetics shown by BDNE

The interaction of NGF with TrkA is more complex and requires cooperativity of multiple contact sites derived horn at least five different domains within the NGF molecule [21,22]. These sites are concentrated on one side of the NGF dimer with both protomers contributing some of the critical residues. Overall, the TrkA binding site appears as a flat, continuous surface ap- proximately parallel to the twofold axis of the molecule. Because of its twofold symmetry, the active NGF dimers have two potential T&A binding sites which may facili- tate the formation of T&A receptor homodimers, a step required for receptor activation [23].

Historically, the biological activity of the neurotrophins has been studied using explants of primary neurons or neuronal progenitors in culture, as well as certain im- mortal cell lines such as PC12 and MAH cells [6,24]. In the case of NGF, pioneering in viva studies using neutralizing antibodies (either passively administered into the placenta or generated by immunization of pregnant rats with NGF) provided the first conclusive evidence for a role of neurotrophins during the development of the mammalian nervous system [6]. These studies have been described in detailed elsewhere [6,24] and will not be discussed here. In this review, I will focus on recent results obtained with ‘knockout’ mice devoid of either NGF, BDNF or NT-3 (see below).

Trk receptors

The structural features of the Trk family of receptors have been reviewed recently [25]. The unique combination of structural motifs, particularly in their extracellular, ligand-binding domains, indicate that they constitute a novel receptor subfamily (Fig. 2). Their kinase catalytic domain is most closely related to a series of novel orphan receptors that include the mammalian RORl, ROR2, DDR (also known as Ptk-3, Nep or T&E), Tyro-10 and TKT as well as the Torpedo RTK tyrosine kinases [l 11.

Several Trk receptor isoforms have been identified 111,251. Two TrkA kinases differ by the presence of six amino acid residues (VSFSPV, in the single letter code for amino acids) located in the extracellular domain near the transmembrane region [26,27]. Both isoforms appear to have similar biological properties, but they have different patterns of expression. Whereas the TrkA isoform carrying the VSFSPV sequence is primarily expressed in

150 Cell reaulation

Signal peptide

Cysteine cluster 1

Leucine-rich motif

Cysteine cluster 2

Ig-like domains

Transmembrane

Juxtamembrane

Kinase domain

COOH Tai I

TrkA TrkB TrkC

Signal peptide

Cysteine repeats

Transmembrane

Cytoplasmic domain

P’5

0 1995 Current Opinion in Cell Biology

Fig. 2. Neurotrophin receptors. Representation of the main structural motifs of the Trk and p75 receptors [lo,1 1,251. Only one TrkB and one TrkC non-catalytic receptor isoform is depicted. Likewise, only one of the three TrkC tyrosine kinase isoforms containing additional sequences in the kinase domain is shown (see text). The black box in one of the TrkA isoforms represents the VSFSPV sequence.

neuronal cells, the other isoform has been found in cells of non-neuronal origin [26,27]. Four TrkC kinase isoforms may exist on the basis of analysis of t&C cDNA clones. These isoforms differ from the canonical TrkC tyrosine kinase by the presence of 14 (TrkC K14), 25 (TrkC K25) or 39 (TrkC K39) additional amino acid residues in the middle of their kinase catalytic domains [28-301. The additional residues present in the TrkC K14 and TrkC K25 isoforms are unique and do not contain informative structural motifs. TrkC K39 contains the combined 25 and 14 amino acid long sequences (in this order) of TrkC K25 and TrkC K14.

A second class of Trk receptor isoforms lacking a tyrosine kinase catalytic domain has also been described [l l] (Fig. 2). These receptors have the same extracellular and transmembrane domains as the Trk kinases described above, but they have distinct and unique cytoplasmic domains that are devoid of distinctive structural motifs. So far, two T&B [31,32] and four TrkC [29,30] non-catalytic receptors have been identified (Fig. 2). Among these putative receptors, TrkB.Tl (also known as gp95trkB) has been identified at the protein level [31]. It remains to be determined whether other Trk non-catalytic receptors are expressed in t&o.

The role of these receptors remains to be determined. It has been proposed, however, that gp95trkB may play a role in BDNF transport (or clearing) from the blood and the spinal fluid to (or from) the brain, on the basis of the high levels of expression of this receptor in the choroid plexus and the lining of the ventricles [31]. More recently, it has been speculated that non-catalytic

receptors may play a role in recruiting circulatory neurotrophins in a manner similar to that proposed for the p75 receptor (see below).

Signal transduction

As indicated above, the Trk family of tyrosine protein kinases is responsible for mediating neurotrophin signaling. PC12 cells lacking TrkA receptors do not respond to NGF in spite of containing abundant p75 receptors; however, transfection of TrkA restores full NGF- responsiveness to these cells [33]. Likewise, transfection of either Trkl3 or TrkC kinases (but not the TrkC K14 or K25 isoforms) renders PC12 cells responsive to either BDNF (and NT-4) or NT-3, respectively. Moreover, activation of Trk kinase receptors by their cognate ligands in proliferating cells such as rodent frbroblasts elicits a mitogenic response that leads to their malignant trans- formation [25]. Trk kinase receptors become activated by a two-step process that involves their ligand-medi- ated dimerization followed by autophosphorylation of their tyrosine residues [22], a mechanism common to other tyrosine kinase receptors [34]. To date, several Trk substrates have been identified [35]. They include phospholipase C (PLC-y, phosphatidylinositol-3 kinase and the adaptor protein She. Other potential substrates for the Trk receptor are the Ras GTPase activating protein and the mitogen-activated protein (MAP) kinase ERKl (but not the highly related ERK2 kinase). It is not known, however, whether these two proteins are directly phosphorylated by the Trk receptors [35].

Neurotrophic factors and their receptors Barbacid 151

PLC-‘y binds to a conserved tyrosine residue located in the short carboxy-terminal tail characteristic of the Trk receptor family. Phosphatidylinositol-3 kinase inter- acts with a neighboring tyrosine residue located at the carboxyl terminus of the kinase domain. The physiological significance of these interactions remains ob- scure, however, as Trk receptors carrying Tyr+Phe mutations in these residues retain their ability to transform NIH3T3 cells and to cause PC12 cells to differentiate [36*,37*]. The She binding site has been mapped to a conserved tyrosine residue located in the juxtamembrane domain [37*]. Mutation of this residue decreases significantly the mitogenic (NIH3T3 cells) and differentiative (PC12 cells) activity of the Trk receptors. Complete ablation of Trk signaling, however, requires the additional mutation of the tyrosine residue responsible for PLC-y binding [36*,37*]. The TrkC kinase isoforms, TrkC K14 and TrkC K25, have normal autocatalytic kinase activity but they do not phosphorylate PLC-), or phosphatidylinositol-3 kinase [28]. These receptors mediate NT-3 dependent signal transduction, as determined by the induction of c-Fos expression and DNA synthesis, but they cannot elicit full mitogenic or PC12 differentiation responses. These results, taken together, indicate that neurotrophin activity may require activation of multiple signaling pathways.

One of these pathways includes the well characterized signaling elements of the Ras/Raf/MAP kinase pathway [38]. Addition of NGF to wild-type, but not to Trk-deficient, PC12 cells results in the rapid activation of Ras as well as of the downstream Raf and MAP kinases [35]. Moreover, expression of oncogenic Ras and Raf proteins in PC12 cells results in their neuronal differentiation in the absence of NGE A second pathway utilized by Trk receptors may involve neuron-specific signaling elements such as the recently identified SNT protein [39]. This 90 kDa protein binds to ~13, a subunit of the cell cycle regulatory complex that includes the cdc2 kinase and cyclin. SNT is rapidly phosphorylated on tyrosine residues upon treatment of PC12 with NGF, but not upon treatment with mitogenic factors such as epidermal growth factor (EGF).

Neurotrophins and their Trk receptors: in vivo

function

The generation of mice carrying germ line mutations in the genes encoding NGF [40”], BDNF [410*,42**] and NT-3 [43**,44**] by homologous recombination in embryonic stem (ES) cells has opened up a unique op- portunity to study the role of these neurotrophins in the development and maintenance of the mammalian nervous system. Similar studies involving knockout mice carrying mutations in the catalytic domains of each of the Trk kinase receptors have also provide critical information regarding the role of these receptors in viva [45**-47**]. Moreover, the striking similarities between the phenotypes of mice defective for each Trk receptor and its cognate neurotrophin (Table 1) represent the

most compelling evidence to date supporting the concept that the Trk receptors mediate most, if not all, of the biological activities of the neurotrophins [17].

Mice defective for either NGF [40**] or TrkA receptors [46**] display severe sensory defects characterized by a complete loss of nociceptive activity (Table 1). These mice fail to react to deep pinpricks in their whisker pads and rear paws [40”,46**]. In addition, they exhibit deficiencies in thermoception as they can stay on top of a 60°C hot plate for at least 10s. Neuro- anatomical examination of the TrkA- and NGF-null mice revealed extensive neuronal cell loss in trigeminal, dorsal root and sympathetic ganglia [40**,46**]. In the dorsal root ganglia, the vast majority of the missing neurons correspond to those of small size, a population known to be NGF-dependent. The sympathetic ganglia are severely shrunken and contain only a few neurons [40”,46”].

A similar correlation can be found between the phenotypes of mice targeted in the genes encoding NT-3 [43**,44**] and TrkC [47**] (Table 1). These mice also display severe sensory defects, but of a nature distinct h-om those observed in NGF- and T&A-defective mice. Mice lacking NT-3 or its cognate TrkC kinase receptors have normal nociception, but they are defective in proprioception, the sensory activity responsible for localiz- ing the limbs in space [43**,44**,47**]. As a consequence, these targeted mice display abnormal movements of an athetotic nature that result in highly abnormal limb pos- tures. This sensory defect is due to the complete absence of Ia muscle afferents, the projections derived from large proprioceptive dorsal root ganglion neurons that connect primary endings of muscle spindles in the periphery to motor pools in the ventral region of the spinal cord [43**,44**,47**]. TrkC and NT-3 mutant mice have limited life spans most probably because of additional neuronal defects; however, a few TrkC (but not NT-3) mutant mice have survived for over six months. The ap- parently more severe phenotype of the NT-3 targeted mice might simply be attributed to differences in strain or in the animal colony However, the TrkC defective mice may express non-catalytic TrkC receptor isoforms, as the gene targeting took place within their t&C tyrosine kinase sequences. Whether mutant mice carrying a completely inactivated t&C gene display the shorter life span of the NT-3 knockouts remains to be investigated.

Comparison of the phenotypes of mutant mice deficient in BDNF [41**,42**] and in its TrkB kinase receptor [45**] has been less straightforward (Table 1). TrkB mutants display a severe phenotype that results in the death of most animals in the first postnatal week due to their inability to feed [45**]. BDNF-defective mice also have short life spans; however, many animals survive at least two weeks. These mice display defective movement coordination and balance, with head bobbing and spin- ning followed by long periods of inactivity. This defect is likely to be due to atrophy and loss of vestibular ganglion neurons, which results in defective innervation of the inner ear [41**,42**]. No such defects were observed

152 Cell regulation

Table 1. Summary of the defects observed in mice targeted in genes encoding various neurotrophins and their receptors.*

Knockout strain

Phenotype p75 NGF TrkA BDNF TrkB NT-3 TrkC

Sensory activity Nociception Partial Very low Very low Normal Normal Normal Normal Balance Normal Normal Necmal . Impaired** ND-~:]: Normal Normal Proprioception Normal Normal Normal Normal Normal Impaired Impaired

PNS defects~::[: Superior cervical ganglion Normal 5% 5% Normal Normal 50% 75% Trigeminal ganglion Normal 30% 30% 60% 40% 40% ND Nodose-petrosal ganglion ND Normal Normal 40% 10% (a) 60% ND Vestibular ganglion ND ND ND 15% ND:]: 80% ND Dorsal root ganglia Smaller 30% 30% 70% 70% 35% 80% la Afferents ND Normal Normal Normal Normal Lost Lost

CNS defects Facial motor neurons ND ND ND Normal 30% Normal ND Spinal cord motor neurons ND ND ND Normal 70% Normal ND Cholinergic projections ND Reduced (b) Reduced ND ND ND ND

*Data obtained from [40°'~7°',57]. **Characters in bold indicate significant differences found in mice defective for a neurotrophin and its cognate Trk receptor. LND, not determined. :[:Animals die too early. :[::[:Approximate percentage of remaining neurons. (a)JT Erickson, DT Katz, personal communication. (b)H Phillips, personal communication. PNS, peripheral nervous system; CNS, central nervous system.

in the TrkB mutant mice as they do not survive long enough to undergo maturation of their vestibular neurons (Table 1). The TrkB-defective mice display more extensive neuronal cell loss in the motor facial nucleus (50% cen loss compared with none in the BDNF mutant) and/or in the nodose-petrosal complex (>90% cell loss compared with ~40°A in BDNF mutant mice) ([45°°]; JT Erickson, RJ Smeyne, M Barbacid, DM Katz, unpub- lished data). Neurons in the nodose-petrosal complex control sensory information from cardiovascular, respi- ratory and gastrointestinal systems and therefore their absence may account for the early death of the TrkB mutant mice. These results are not entirely surprising, as the TrkB tyrosine kinase is also a receptor for NT-4. Whether double knockout BDNF and NT-4 mice display the same phenotype as the TrkB-defective animals remains to be determined. Likewise, it is not known whether mice that also lack expression of the TrkB non- catalytic isoforms display additional neuronal defects.

The p75 neurotrophin receptor

p75 is a cell surface glycoprotein that belongs to the tumor necrosis factor receptor superfam~y [11] (Fig. 1). The p75 receptor binds each of the neurotrophins with similarly low nanomolar affinity [11]; however, its precise biological function remains to be elucidated. A significant body of evidence indicates that p75 does not mediate neurotrophin signaling, at least as it relates to neuronal survival and/or differentiation [11,48]. For instance, PC12 cells expressing high levels of p75 receptors

in the absence of TrkA do not differentiate in response to NGE Moreover, p75 is expressed in many cell types that do not respond to neurotrophins.

There are several reports, however, that suggest that p75 may mediate some aspects of signal transduction [11]. For instance, ectopic expression of wild-type, but not mutant, p75 receptors in PC12 cells lacking NGF receptors led to the induction oftyrosine phosphorylation (of as yet unidentified substrates) and c-fos expression [11]. In addition, wild-type PC12 cells expressing a chimeric EGF-p75 receptor whose cytoplasmic sequences are derived from p75 can undergo neuronal-like differentiation in the presence of EGF [11]. More recently, it has been reported that p75 mediates the activation of the sphin- gomyelin cycle [49] and the glycosyl-phosphatidylinositol/inositol phosphoglycan pathway [50] as well as Schwann cell migration [51] and matrix invasion by melanoma cells [52]. Finally, two recent reports indicate that unliganded p75 may mediate signal transduction in apoptotic pathways [53,54]. NGF binding appears to block this apoptotic activity, perhaps contributing to the well known effect of NGF in neuronal survival.

A more generally accepted view is that p75 may facil- itate NGF signaling through TrkA receptors [11,48]. For instance, it has been proposed [11] that p75 in- teracts and/or cooperates with the signaling Trk kinases to generate 'high-affinity' binding sites thought to be responsible for mediating neurotrophin function. More recently, it has been shown that transfection of p75 into TrkA-expressing MAH sympathoadrenal cells

Neurotrophic factors and their receutors Barbacid 153

enhances their response to NGF [55]. Likewise, block- ing p75 binding sites in PC12 cells with either a p75 monoclonal antibody or with BDNF decreases their response to NGF [56].

Role of p75 receptors in vivo

The most relevant information regarding the function of the p75 receptor in neurotrophin signaling has been provided by analysis of mutant mice carrying a targeted p75 gene [57]. These mice display sensory and sympathetic defects, thus demonstrating that the p75 receptor is indeed required for proper neuronal development (Table 1). Interestingly, these mice do not display the defects characteristic of mice lacking BDNF (or TrkB receptors) and NT-3 (or TrkC receptors). These observations indicate that ~75, despite serving as a receptor for BDNF and NT-3 in vitro, may not be involved in mediating their biological activities in viva (Fig. 1). Al- ternatively, the absence of p75 might be compensated for by other molecules such as the TrkI3 and TrkC non- catalytic receptors.

The observed defects in the p75-null mice appear to be limited to NGF-dependent neurons, mainly sensory (nociceptive) and sympathetic neurons [58*]. Moreover, these defects are much more limited than those observed in either NGF or TrkA mutant mice (Table 1). For instance, NGF- and T&A-null mice have lost most (>95%) of their sympathetic neurons, whereas, in contrast, the p75 null mice have normal sympathetic ganglia as well as normal innervation patterns to all their target tissues with the exception of the pineal and the sweat glands [57,58’].

These observations raise the possibility that TrkA signaling may require p75 receptors in only a subset of NGF-dependent neurons. A more plausible interpreta- tion of these results, however, is that p75 plays a role in recruiting circulating NGF molecules rather than in NGF signaling [48]. In support of this hypothesis, p75-defective sensory and sympathetic neurons survive well in culture in the presence of NGF [59*,60], but they need four times more NGF to achieve the same response as wild-type neurons [59*,60]. Considering the rapid dissociation kinetics of NGF binding to p75 [6], it is possible that expression of p75 receptors either in neurons or in adjacent glial cells may increase the lo- cal concentration of diffusible NGE If so, the lack of p75 receptors will only have phenotypic consequences in those neurons for which availability of NGF is limiting. Crossing p75-null mice with transgenic strains overex- pressing NGF should provide the experimental tools to test this hypothesis.

Conclusions

Most of the information we had to date regarding the role of neurotrophic factors has been derived corn in vitro studies. The advent of gene targeting has provided new

means to study their function in viva. Characterization of strains of transgenic mice lacking neurotrophic factors and their receptors is making it possible to define pre- cisely the physiological role of each of these molecules, at least during development. Some of the results obtained (i.e. the observation of sympathetic and sensory defects in NGF- and T&A-null mice) were partly expected on the basis of earlier studies using immunological approaches to neutralize NGF activity. Others, such as the exquisite specificity of the NT-3-TrkC pathway in proprioception or the limited defects displayed by the p75- null mice, were unexpected. Another important lesson learned from these gene-targeted mice is the differen- tial role that neurotrophins play in the peripheral versus the central nervous systems. In the peripheral nervous system, ablation of neurotrophin or Trk receptor genes results in massive neuronal cell death, whereas the central nervous system neurons of these mutant mice appear to be, for the most part, unaffected in spite of widespread neurotrophin and receptor expression. These observations illustrate the complex mechanisms involved in the development and survival of the nervous system and pre- dict the existence of additional neurotrophic factors that await discovery.

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M Barbacid, Department of Molecular Biology, Bristol-Myers Squibb Pharmaceutical Research Institute, PO Box 4000, Prince- ton, NJ 08543-4000, USA.

neurotrophic factors and their receptors - university of liverpool

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