Microarray-assisted fine mapping of quantitative trait loci on chromosome 15 for susceptibility to seizure-induced cell death in mice

Prior studies with crosses of the FVB/NJ (FVB; seizure-induced cell death-susceptible) mouse and the C57BL/6J (B6; seizure-induced cell death-resistant) mouse revealed the presence of a quantitative trait locus (QTL) on chromosome 15 that influenced susceptibility to kainic acid-induced cell death (Sicd2). In an earlier study, we confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced cell death through the creation of the FVB.B6-Sicd2 congenic strain, and created three interval-specific congenic lines (ISCLs) that encompass Sicd2 on chromosome 15 to fine-map this locus. To further localise this Sicd2 QTL, an additional congenic line carrying overlapping intervals of the B6 segment was created (ISCL-4), and compared with the previously created ISCL-1-ISCL-3 and assessed for seizure-induced cell death phenotype. Whereas all of the ISCLs showed reduced cell death associated with the B6 phenotype, ISCL-4, showed the most extensive reduction in seizure-induced cell death throughout all hippocampal subfields. In order to characterise the susceptibility loci on Sicd2 by use of this ISCL and identify compelling candidate genes, we undertook an integrative genomic strategy of comparing exon transcript abundance in the hippocampus of this newly developed chromosome 15 subcongenic line (ISCL-4) and FVB-like littermates. We identified 10 putative candidate genes that are alternatively spliced between the strains and may govern strain-dependent differences in susceptibility to seizure-induced excitotoxic cell death. These results illustrate the importance of identifying transcriptomics variants in expression studies, and implicate novel candidate genes conferring susceptibility to seizure-induced cell death.

Congenic strains provide evidence that a mapped locus on chromosome 15 influences excitotoxic cell death

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation is unknown. Prior studies with crosses of the FVB/NJ (seizure-induced cell death susceptible) mouse and the seizure-induced cell death resistant mouse, C57BL/6J, showed the presence of three quantitative trait loci (QTLs), named seizure-induced cell death 1 (Sicd1) to Sicd3. To better localize and characterize the Sicd2 locus, two reciprocal congenic mouse strains were created. While the B6.FVB-Sicd2 congenic mouse was without effect on modifying susceptibility to seizure-induced excitotoxic cell death, the FVB.B6-Sicd2 congenic mouse, in which the chromosome (Chr) 15 region of C57BL/6J was introgressed into FVB/NJ, showed reduced seizure-induced excitotoxic cell death following kainate administration. Phenotypic comparison between FVB and the congenic FVB.B6-Sicd2 strain confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced excitotoxic cell death. Interval-specific congenic lines (ISCLs) that encompass Sicd2 on Chr 15 were generated and were used to fine-map this QTL. Resultant progeny were treated with kainate and examined for the extent of seizure-induced cell death in order to deduce the Sicd2 genotypes of the recombinants through linkage analysis. All of the ISCLs exhibited reduced cell death associated with the C57BL/6J phenotype; however, ISCL-2 showed the most dramatic reduction in seizure-induced cell death in both area CA3 and in the dentate hilus. These findings confirm the existence of polymorphic loci within the reduced critical region of Sicd2 that regulate the severity of seizure-induced cell death.

Variation in Galr1 expression determines susceptibility to exocitotoxin-induced cell death in mice

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation in differential susceptibility is unknown. Previously, we identified a highly significant quantitative trait locus (QTL) on chromosome 18 that influenced susceptibility to kainic acid-induced cell death (Sicd1). Comparison of susceptibility to seizure-induced cell death between reciprocal congenic lines for Sicd1 and parental background mice indicates that genes influencing this trait were captured in both strains. Two positional gene candidates, Galr1 and Mbp, map to 55 cM, where the Sicd1 QTL had been previously mapped. Thus, this study was undertaken to determine if Galr1 and/or Mbp could be considered as candidate genes. Genomic sequence comparison of these two functional candidate genes from the C57BL/6J (resistant at Sicd1) and the FVB/NJ (susceptible at Sicd1) strains showed no single-nucleotide polymorphisms. However, expression studies confirmed that Galr1 shows significant differential expression in the congenic and parental inbred strains. Galr1 expression was downregulated in the hippocampus of C57BL/6J mice and FVB.B6-Sicd1 congenic mice when compared with FVB/NJ or B6.FVB-Sicd1 congenic mice. A survey of Galr1 expression among other inbred strains showed a significant effect such that 'susceptible' strains showed a reduction in Galr1 expression as compared with 'resistant' strains. In contrast, no differences in Mbp expression were observed. In summary, these results suggest that differential expression of Galr1 may contribute to the differences in susceptibility to seizure-induced cell death between cell death-resistant and cell death-susceptible strains.

Effect of age on kainate-induced seizure severity and cell death

While the onset and extent of epilepsy increases in the aged population, the reasons for this increased incidence remain unexplored. The present study used two inbred strains of mice (C57BL/6J and FVB/NJ) to address the genetic control of age-dependent neurodegeneration by building upon previous experiments that have identified phenotypic differences in susceptibility to hippocampal seizure-induced cell death. We determined if seizure induction and seizure-induced cell death are affected differentially in young adult, mature, and aged male C57BL/6J and FVB/NJ mice administered the excitotoxin, kainic acid. Dose response testing was performed in three to four groups of male mice from each strain. Following kainate injections, mice were scored for seizure activity and brains from mice in each age group were processed for light microscopic histopathologic evaluation 7 days following kainate administration to evaluate the severity of seizure-induced brain damage. Irrespective of the dose of kainate administered or the age group examined, resistant strains of mice (C57BL/6J) continued to be resistant to seizure-induced cell death. In contrast, aged animals of the FVB/NJ strain were more vulnerable to the induction of behavioral seizures and associated neuropathology after systemic injection of kainic acid than young or middle-aged mice. Results from these studies suggest that the age-related increased susceptibility to the neurotoxic effects of seizure induction and seizure-induced injury is regulated in a strain-dependent manner, similar to previous observations in young adult mice.

Modulation of cell death by mouse genotype: differential vulnerability to excitatory amino acid-induced lesions

Previously we have demonstrated that mature inbred strains of mice differ significantly in their response to kainate-induced cell death. While both C57BL/6 and FVB/N mice exhibit similar seizure activity in response to kainate, only C57BL/6 mice can be characterized as resistant to kainate-induced cell death. To examine further the molecular pharmacological basis for this strain difference in hippocampal sensitivity, we assessed the ability of the ionotropic glutamate receptor agonists, kainic acid (KA), N-methyl-D-aspartate (NMDA), ibotenic acid (IBO), and quinolinic acid (QUIN), to promote excitotoxic damage. We examined seizure-related behavior and subsequent neurotoxicity in C57BL/6 and FVB/N mice following intrahippocampal administration of the kainate receptor agonist, KA, the NMDA receptor agonists NMDA or QUIN, or the NMDA and metabotropic glutamate receptor agonist, IBO. The time course and extent of cell death in mice were evaluated using Nissl and selective silver stains, and Fluoro-Jade, a fluorescent marker for dying neurons. In the present study, FVB/N mice were exquisitely sensitive to injection of KA at all doses, while susceptibility in C57BL/6 mice was dose dependent. In contrast, while hippocampal damage was present in both strains at all doses of QUIN, the extent of cell damage was significantly less in C57BL/6 mice at low doses (30 and 60 mM). Similarly, IBO administration resulted in differences in the extent of cell death when administered at the highest dose (126 mM). No strain-dependent differences in cell loss were observed following NMDA lesions. These results provide further evidence that susceptibility to excitotoxin-induced cell death is highly strain dependent and is kainate and NMDA receptor dependent.

Seizure-induced neuronal death is associated with induction of c-Jun N-terminal kinase and is dependent on genetic background

Previous studies have shown that expression of c-Jun protein, as well as the c-Jun amino-terminal kinase (JNK) group of mitogen-activated protein kinases, may play a critical role in the pathogenesis of glutamate neurotoxicity. In order to define the molecular cascade that leads to c-Jun activation following excitotoxic injury and delineate whether induction of protein synthesis is related to cell death signaling cascades or those changes associated with increased seizure activity, we examined the expression of JNK-1, as well as its substrate, c-Jun and N-terminal phosphorylated c-Jun following kainic acid (KA) administration in two strains of mice. In the present study, we assessed the immunohistochemical expression of these proteins at time points between 2 h and 7 days, in excitotoxic cell death-resistant (C57BL/6) and -susceptible (FVB/N) mouse strains that were systemically injected with saline or kainic acid. No strain-related differences in the immunohistochemical expression of any of the proteins were observed in intact control mice. However, following KA administration, the magnitude and period of induction of JNK-1 protein was associated with impending cell death, while increased phosphorylation of c-Jun protein was associated with resistance to cell death. In contrast, expression of c-Jun protein does not appear to be a reliable indicator of impending cell death, as it was expressed in resistant and vulnerable subfields in mice susceptible to kainate injury. These results provide the first evidence that JNK-1 expression may be involved in producing the neuronal cell death response following excitotoxin-induced injury.

Genetic dissection of the signals that induce synaptic reorganization

Synaptic reorganization of mossy fibers following kainic acid (KA) administration has been reported to contribute to the formation of recurrent excitatory circuits, resulting in an epileptogenic state. It is unclear, however, whether KA-induced mossy fiber sprouting results from neuronal cell loss or the seizure activity that KA induces. We have recently demonstrated that certain strains of mice are resistant to excitotoxic cell death, yet exhibit seizure activity similar to what has been observed in rodents susceptible to KA. The present study takes advantage of these strain differences to explore the roles of seizure activity vs cell loss in triggering mossy fiber sprouting. In order to understand the relationships between gene induction, cell death, and the sprouting response, we assessed the regulation of two molecules associated with the sprouting response, c-fos and GAP-43, in mice resistant (C57BL/6) and susceptible (FVB/N) to KA-induced cell death. Following administration of KA, increases in c-fos immunoreactivity were observed in both strains, although prolonged induction of c-fos was present only in the hippocampal neurons of FVB/N mice. Mossy fiber sprouting following KA administration was also only observed in FVB/N mice, while induction of GAP-43, a marker associated with mossy fiber sprouting, was not observed in either strain. These results indicate that: (i) KA-induced seizure activity alone is insufficient to induce mossy fiber sprouting; (ii) mossy fiber sprouting may be due to the loss of hilar neurons following kainate administration; and (iii) induction of GAP-43 is not a necessary component of the sprouting response that occurs following KA in mice.

Genetic approaches to neurotrauma research: opportunities and potential pitfalls of murine models

Genetic strategies provide new ways to define the molecular cascades that regulate the responses of the mammalian nervous system to injury. Genetic interventions also provide opportunities to manipulate and control key molecular steps in these cascades, so as to modify the outcome of CNS injury. Most current genetic strategies involve the use of mice, an animal that has not heretofore been used extensively for neurotrauma research. Therefore, one purpose of the present review is to consider how mice respond to neural trauma, focusing especially on recent information that reveals important differences between mice and rats, and between different inbred strains of mice. The second aim of this review is to provide a brief introduction to the opportunities, caveats, and potential pitfalls of studies that use genetically modified animals for neurotrauma research.

Differential regulation of astrocytic mRNAs in the rat striatum after lesions of the cortex or substantia nigra

This study evaluates the time course of expression of three astrocytic mRNAs, glial fibrillary acidic protein (GFAP), apolipoprotein E (ApoE), and clusterin, in the rat striatum (ST) following a unilateral lesion of either the cortex (CX) or the substantia nigra (SN), using Northern blot and in situ hybridization analyses. We found that while there was a time-dependent increase in astrocytic GFAP mRNA in the deafferented ST following both the CX and the SN lesions, the time course of the response was different between the two lesion paradigms. Specifically, the increase in GFAP mRNA in striatal astrocytes after the SN lesion was rapid and transient returning to control levels by 10 days postlesion, while the response was long lasting and remained increased until at least 27 days after the CX lesion. In addition, the mRNA response for both ApoE and clusterin was differentially regulated in response to the two lesions. Specifically, both clusterin and ApoE mRNAs were rapidly increased in the ST following the CX lesion while both mRNAs remained unchanged following the SN lesion. Data from this study extend information derived from previous investigations on the multifunctional role of astrocytes in the response to brain injury. Specifically, our data support the notion that while the time course of the GFAP response in striatal astrocytes may vary between lesion paradigms, the upregulation of GFAP is part of a generalized response of reactive astrocytes to diverse brain injuries. By comparison, upregulation of the mRNAs for the lipoproteins clusterin and ApoE are lesion specific and may play a role in the transport of recycled myelin lipids from dying axons to actively growing axons and dendrites in reactive synaptogenesis.

Signals that regulate astroglial gene expression: induction of GFAP mRNA following seizures or injury is blocked by protein synthesis inhibitors

Previous studies have revealed that a single electroconvulsive seizure (ECS) strongly induces glial fibrillary acidic protein (GFAP) expression in astrocytes in the hippocampal dentate gyrus. The signals that trigger this induction are not known, but circumstantial evidence suggests the hypothesis that GFAP expression may be induced as a result of the induction of growth factor expression by dentate granule cells that also occurs as a result of the ECS and other types of seizures. The present study tests one prediction of this hypothesis by evaluating whether increases in GFAP mRNA levels after ECS are blocked by inhibiting protein synthesis at various times after the ECS. We report that the upregulation of GFAP expression following ECS is blocked by protein synthesis inhibitors given 5 min before or up to 12 h after a single ECS. This temporal gradient suggests an intermediate step involving the increased expression of a protein growth factor.

Genetic determinants of susceptibility to excitotoxic cell death: implications for gene targeting approaches

Recent studies have sought to identify the genes involved in excitotoxic neurodegeneration. Here we report that certain strains of mice, including strains that are used for gene targeting studies, do not exhibit excitotoxic cell death after kainic acid seizures. Kainic acid produced excitotoxic cell death in the CA3 and CA1 subfields of the hippocampus in 129/SvEMS and FVB/N mice, in the same pattern as described in rats. C57BL/6 and BALB/c mice exhibited excitotoxic cell death only at very high doses of kainate, and then only in a very restricted area, although they exhibited comparable seizures. Hybrids of 129/SvEMS x C57BL/6 mice created using embryonic stem cells from 129/SvEMS mice also did not exhibit excitotoxic cell death. These results demonstrate that C57BL/6 and BALB/c strains carry gene(s) that convey protection from glutamate-induced excitotoxicity. This differential susceptibility to excitotoxicity represents a potential complication for gene targeting studies.

Genetic influences on cellular reactions to brain injury: activation of microglia in denervated neuropil in mice carrying a mutation (Wld(S)) that causes delayed Wallerian degeneration

This study examines the relationship between the appearance of degenerative changes in synaptic terminals and axons and the activation of microglia in denervated neuropil regions of normal mice of the C57BL/6 strain and mutant mice (Wld(S)), in which Wallerian degeneration is substantially delayed. The time course of degenerative changes in synaptic terminals and axons was assessed using selective silver staining. Microglial cells were identified by immunostaining for Mac-1, a monoclonal antibody to the CR3 complement receptor, and by histochemical staining for nucleoside diphosphatase (NDPase). Increased argyrophilia, indicative of degenerative changes, was evident as early as 1 day postlesion in normal mice, but was not seen until 6-8 days in mice with the Wld(S) mutation. Microglial activation in normal C57BL/6 mice was evident by 24 hours postlesion, as evidenced by increased immunostaining for Mac-1, increased histochemical staining for NDPase, and morphological changes indicative of an activated phenotype (short, thick processes). Quantitative evaluation of immunostaining for Mac-1 revealed that peak activation occurred between 2 and 6 days postlesion with a return to a quiescent phenotype by 12 days. In contrast, the microglial response was significantly delayed and prolonged in mice bearing the Wld(S) mutation. Activated microglia were not seen within the deafferented area until 6 to 8 days postlesion and peak activation occurred between 12 and 20 days postlesion. These data suggest that the response of microglia in denervated neuropil zones is triggered by the same types of degenerative changes that cause increased argyrophilia as detected by selective silver staining methods.

Dendritic remodeling of dentate granule cells following a combined entorhinal cortex/fimbria fornix lesion

This study examined the time course of dendritic reorganization of dentate granule neurons of the hippocampus following the loss of input from both the fimbria fornix (FF) and the entorhinal cortex (EC). We used the Golgi-Cox stain to assess the morphology of dentate granule neurons at six postlesion time points (4, 8, 14, 30, 45, and 60 days) and dendritic measures included total dendritic length, number of segments, number of branch points, and spine density. We found that as early as 4 days postlesion, total dendritic length and number of segments were significantly decreased with the greatest change occurring in the distal parts of the dendritic arbor located in the outer molecular layer of the dentate gyrus. Dendritic measures related to segment number and dendritic length returned to 70% of intact values by 30 days postlesion and were not significantly different from unlesioned rats at 45 and 60 days postlesion. In contrast, the recovery of spine density was transient. Spine density in the outer molecular layer of the dentate gyrus decreased by 60% at 4 days postlesion and returned to 87% of intact values by 30 days postlesion. However, there was a second loss of dendritic spines along the distal portion of the dendrite between 30 and 60 days postlesion. These data provide evidence that the ability of granule neurons to recover a dendritic morphology similar to that of unlesioned rats is impaired following the combined EC/FF lesion and that the "secondary loss" of dendritic spine density on granule neurons may significantly limit the chances of the hippocampus reforming a synaptic circuitry that could lead to functional recovery after the EC/FF lesion.

Comparison of RPTP zeta/beta, phosphacan, and trkB mRNA expression in the developing and adult rat nervous system and induction of RPTP zeta/beta and phosphacan mRNA following brain injury

The receptor protein tyrosine phosphatase (RPTP) zeta/beta and a major isoform, phosphacan, a chondroitin sulfate proteoglycan that contains the RPTP zeta/beta extracellular domain but not the transmembrane and intracellular phosphatase domains, are expressed abundantly in the nervous system, primarily by astroglia. Because of similarities in the expression patterns of RPTP zeta/beta and the receptor tyrosine kinase TrkB, we investigated whether RNAs encoding these proteins were co-localized during development, which would suggest that these molecules might functionally interact in vivo. By in-situ hybridization, we noted extensive areas of overlap in the expression of trkB and RPTP zeta/beta mRNAs in the developing peripheral and central nervous systems. Analysis with a probe specific for the catalytic TrkB isoform suggested that RPTP zeta/beta and non-catalytic trkB mRNAs were co-expressed in particular regions of the nervous system while the catalytic trkB and RPTP zeta/beta transcripts were also, but to a lesser extent. RPTP zeta/beta and phosphacan expression were extremely similar, differing particularly in the level of expression in the ventricular and subventricular zones, hippocampus, and ependyma. Furthermore, both RPTP zeta/beta and phosphacan mRNAs were found in several subsets of neurons as well as astrocytes. Following CNS injury, we observed robust induction of RPTP zeta/beta mRNA in areas of axonal sprouting, and of both RPTP zeta/beta and phosphacan mRNAs in areas of glial scarring, implying that the encoded proteins and the cell adhesion molecules and extracellular matrix proteins to which they bind may contribute to recovery from injury and perhaps regulation of axonal regrowth in the nervous system.

Lesion-induced sprouting of commissural/associational axons and induction of GAP-43 mRNA in hilar and CA3 pyramidal neurons in the hippocampus are diminished in aged rats

Removal of synaptic connections following a partial deafferentation lesion results in a sprouting of remaining afferents that terminate near the denervated area. However, while the ability to form new synapses in response to injury has been reported in both young and aged rats, previous studies have suggested that the injury-induced response in the hippocampus of aged rats may be delayed and/or not as extensive as compared to young adults. Given that growth associated proteins are central for the regulation of neurite outgrowth during both development and regeneration, we were interested in determining if the magnitude and time course of the sprouting response of hippocampal neurons to deafferentation might correlate with induction of growth associated proteins and whether these parameters could be modulated with age. For our studies we used the Holmes fiber stain to determine the expansion of the C/A fiber plexus following denervation and compared the time course of the sprouting response with that observed by in situ hybridization for the neurite outgrowth proteins, growth associated protein-43 (GAP-43), superior cervical ganglion-10 (SCG-10), and neurofilament-68 (NF-68) at various time points after the lesion for each age group. We found that the commissural/associational (C/A) fiber plexus expanded by 45% in young adult rats at 30 and 45 d postlesion and was accompanied by a significant increase in expression of GAP-43 mRNA in both ipsilateral and contralateral hilar and CA3 pyramidal neurons, the cell bodies of origin for the C/A pathway. In contrast, a dampened sprouting response was observed in aged rats at all time points postlesion and coincided with a lack of induction of any of the growth-associated proteins. These results suggest that GAP-43 is involved in outgrowth of C/A axons in the hippocampus in response to a partial deafferentation lesion. However, factors that stimulate neurite outgrowth and upregulate GAP-43 mRNA in response to a partial deafferentation lesion diminish with age.

Enhanced but delayed axonal sprouting of the commissural/associational pathway following a combined entorhinal cortex/fimbria fornix lesion

From previous lesion studies of the hippocampus it has been reported that axons of the commissural/associational pathway expand their termination zone in the molecular layer of the dentate gyrus by 20-25% in response to loss of input from the entorhinal cortex. However, although much is known about the response of the commissural/associational pathway with regard to extent, latency, and speed of the reinnervation response following an entorhinal cortex lesion, little is known about how the loss of additional afferent systems might modulate this response. To address this issue, we examined at 14, 30, and 45 days postlesion, the sprouting of commissural/associational afferents following either a unilateral fimbria fornix transection, a unilateral entorhinal cortex lesion, or combined lesions of both the entorhinal cortex and the fimbria fornix. Loss of septal innervation to the hippocampus was assessed using the cholinesterase stain, whereas sprouting from the commissural/associational pathway was determined from Holmes fiber-stained sections. In addition, the Timms stain was used to examine the time course of the loss of terminal fields of the various zinc-containing afferent systems within the hippocampus. Following the removal of input to the hippocampus via the fimbria fornix transection, there was no evidence of sprouting of the commissural/associational fibers into the deafferented portion of the dentate gyrus. In contrast, rats receiving an entorhinal cortex lesion showed a significant increase (28%) in the width of the commissural/associational fiber plexus that was present by 14 days postlesion. By comparison, the magnitude of the expansion of the commissural/associational fiber plexus was significantly larger after lesioning both the entorhinal cortex and the fimbria than after the entorhinal cortex lesion alone (45% vs. 28%). In addition, the expansion of the commissural/associational fiber plexus was not increased at 14 days postlesion but was significantly increased at 30 days postlesion. The delay in the sprouting of the commissural/associational pathway coincided with the time course of loss of zinc-containing fibers in the outer molecular layer of the dentate gyrus as assessed with the Timms stain. These results suggest that the magnitude and time course for the sprouting of axons from the commissural/associational pathway into the partially deafferented hippocampus of the adult rat is lesion dependent and that the effect of the loss of input from the entorhinal cortex can be modulated and enhanced by the concomitant depletion of input from the fimbria fornix.

Induction of noncatalytic TrkB neurotrophin receptors during axonal sprouting in the adult hippocampus

Brain-derived neurotrophic factor (BDNF) and its signal transducing receptor, the TrkB tyrosine protein kinase, are expressed at high levels in the hippocampus of the adult brain, suggesting a role for BDNF mechanisms in neuronal plasticity. To test this hypothesis, we used defined lesions of perforant path and fimbria-fornix, two major hippocampal afferents, to remove synapses on dendrites of dentate gyrus granule cells and pyramidal cells of Ammon's horn and induce synaptic rearrangements. These combined lesions remove afferent connections from entorhinal cortex and septum and produce massive sprouting of axons of the commissural/associational pathways into the molecular layer of the hippocampal dentate gyrus. At days 1, 3, and 6, the lesions decreased BDNF mRNA expression ipsilaterally to approximately 50% of control, with complete recovery at 14 d. The lesions did not alter trkB mRNA levels in neuronal layers of the hippocampus; however, they resulted in a pronounced induction of trkB mRNA expression in hippocampal non-neuronal cells 6-14 d after lesioning. The induction corresponded in time and place to the synaptic reorganization in the lesioned hippocampus. The mRNA species newly induced by the lesions corresponded to those transcripts encoding the noncatalytic TrkB receptor isoform that lacks the cytoplasmic protein kinase domain. Expression of mRNAs coding for neurotrophin-3 and the TrkC tyrosine protein kinase were not altered by the lesions. The findings suggest that truncated noncatalytic TrkB molecules expressed on the surface of glial cells play an important role in plasticity of the adult brain, possibly regulating the concentration of bioactive neurotrophins or the responsiveness of neurotrophin receptors. Alternatively, they may play a role in presenting neurotrophin molecules to growing axons.