Biomedical subjects

M Barbacid

Publications and source records attributed to M Barbacid.

At least 37 records · Page 2Linked to original sources

PubMed1998

Ephrin-A5 (AL-1/RAGS) is essential for proper retinal axon guidance and topographic mapping in the mammalian visual system.

Ephrin-A5 (AL-1/RAGS), a ligand for Eph receptor tyrosine kinases, repels retinal axons in vitro and has a graded expression in the superior colliculus (SC), the major midbrain target of retinal ganglion cells. These properties implicate ephrin-A5 in the formation of topographic maps, a fundamental organizational feature of the nervous system. To test this hypothesis, we generated mice lacking ephrin-A5. The majority of ephrin-A5-/- mice develop to adulthood, are morphologically intact, and have normal anterior-posterior patterning of the midbrain. However, within the SC, retinal axons establish and maintain dense arborizations at topographically incorrect sites that correlate with locations of low expression of the related ligand ephrin-A2. In addition, retinal axons transiently overshoot the SC and extend aberrantly into the inferior colliculus (IC). This defect is consistent with the high level of ephrin-A5 expression in the IC and the finding that retinal axon growth on membranes from wild-type IC is inhibited relative to that on membranes from ephrin-A5-/- IC. These findings show that ephrin-A5 is required for the proper guidance and mapping of retinal axons in the mammalian midbrain.

Animals↗

PubMed1998

Topographic guidance labels in a sensory projection to the forebrain.

Visual connections to the mammalian forebrain are known to be patterned by neural activity, but it remains unknown whether the map topography of such higher sensory projections depends on axon guidance labels. Here, we show complementary expression and binding for the receptor EphA5 in mouse retina and its ligands ephrin-A2 and ephrin-A5 in multiple retinal targets, including the major forebrain target, the dorsal lateral geniculate nucleus (dLGN). These ligands can act in vitro as topographically specific repellents for mammalian retinal axons and are necessary for normal dLGN mapping in vivo. The results suggest a general and economic modular mechanism for brain mapping whereby a projecting field is mapped onto multiple targets by repeated use of the same labels. They also indicate the nature of a coordinate system for the mapping of sensory connections to the forebrain.

Aging↗

PubMed1998

Corneal innervation and sensitivity to noxious stimuli in trkA knockout mice.

Most primary sensory neurones depend on neurotrophins for survival. Mutant mice in which TrkA, the high-affinity receptor for nerve growth factor (NGF), has been inactivated lack nociceptive neurones in sensory ganglia and do not respond to noxious stimuli. The cornea of the eye is innervated by trigeminal neurones that are activated by noxious mechanical, thermal and chemical stimuli. In the human cornea, these stimuli evoke only sensations of pain. We have analysed the innervation pattern and the response to noxious stimulation of the cornea of trkA (-/-) mutant mice. Corneal nerves were stained with the gold chloride impregnation method. Corneal sensitivity to noxious stimuli was assessed by counting blinking movements evoked by von Frey hairs, topical application of saline at different temperatures and application of acetic acid and capsaicin at different concentrations. In the cornea of trkA (-/-) mutant animals, we observed a drastic reduction in the number of nerve trunks and branches in the corneal stroma. Furthermore, quantitative analysis of the number of thin nerve terminals revealed a marked decrease in the corneal epithelium of trkA (-/-) mice when compared to those present in wild type and trkA (+/-) animals. The blinking response of trkA (-/-) mice to mechanical, thermal and chemical noxious stimuli was also significantly reduced. These results indicate that the population of corneal sensory neurones is markedly depleted in trkA (-/-) mutant mice. However, a small portion of corneal sensory neurones survive in these mice suggesting that they may be NGF independent. On the basis of our results, we propose that these surviving cells are polymodal nociceptive neurones, sensitive to mechanical stimulation, noxious heat and acid.

Acids↗

PubMed1997

Cbl-b, a member of the Sli-1/c-Cbl protein family, inhibits Vav-mediated c-Jun N-terminal kinase activation.

We have used the yeast two-hybrid system to identify proteins that interact with Vav, a GDP/GTP exchange factor for the Rac-1 GTPase that plays an important role in cell signaling and oncogenic transformation. This experimental approach resulted in the isolation of Cbl-b, a signal transduction molecule highly related to the mammalian c-cbl proto-oncogene product and to the C. elegans Sli-1 protein, a negative regulator of the EGF-receptor-like Let23 protein. The interaction between Vav and Cbl-b requires the entire SH3-SH2-SH3 carboxy-terminal domain of Vav and a long stretch of proline-rich sequences present in the central region of Cbl-b. Stimulation of quiescent rodent fibroblasts with either epidermal or platelet-derived growth factors induces an increased affinity of Vav for Cbl-b and results in the subsequent formation of a Vav-dependent trimeric complex with the ligand-stimulated tyrosine kinase receptors. During this process, Vav, but not Cbl-b, becomes highly phosphorylated on tyrosine residues. Overexpression of Cbl-b inhibits the signal transduction pathway of Vav that leads to the stimulation of c-Jun N-terminal kinase. By contrast, expression of truncated Cbl-b proteins and of missense mutants analogous to those found in inactive Sli-1 proteins have no detectable effect on Vav activity. These results indicate that Vav and Cbl-b act coordinately in the first steps of tyrosine protein kinase receptor-mediated signaling and suggest that members of the Sli-1/Cbl family are also negative regulators of signal transduction in mammalian cells.

Animals↗

PubMed1997

A role for TrkA during maturation of striatal and basal forebrain cholinergic neurons in vivo.

Nerve growth factor (NGF), acting via the TrkA receptor, has been shown to regulate the survival and maturation of specific neurons of the peripheral nervous system. Furthermore, exogenous NGF has potent actions on TrkA-expressing cholinergic neurons of the basal forebrain (BFCNs) and striatum. However, initial analysis of mice lacking NGF or TrkA revealed that forebrain cholinergic neurons were present in these animals through the fourth postnatal week. Because of the potential effects of NGF/TrkA interactions on these developing neurons, we have analyzed quantitatively the striatal and basal forebrain cholinergic neurons in trkA knock-out mice. By postnatal day (P) 7/8, forebrain cholinergic neurons are smaller in trkA (-/-) mice than those in wild-type littermate controls. However, cholinergic neuron number and fiber density in the hippocampus, a target region of BFCNs, are grossly intact. Interestingly, by P20-P25 trkA knock-outs contain significantly fewer (20-36%) and smaller cholinergic neurons in both the striatum and septal regions, as compared with controls. Cholinergic fiber density within the hippocampus also is depleted in knock-outs by the end of the second postnatal week. Contrary to some predictions, despite expression of p75(NTR) in the absence of trkA in BFCNs of these knock-out mice, many cells, although smaller, are still alive at P25. Our data suggest that, in the absence of NGF/TrkA signaling, striatal cholinergic neurons and BFCNs do not mature fully and that BFCNs begin to atrophy and/or die surrounding the time of target innervation.

Animals↗

PubMed1997

Differential dependency of cutaneous mechanoreceptors on neurotrophins, trk receptors, and P75 LNGFR.

The impact of null mutations of the genes for the NGF family of neurotrophins and their receptors was examined among the wide variety of medium to large caliber myelinated mechanoreceptors which have a highly specific predictable organization in the mystacial pad of mice. Immunofluorescence with anti-protein gene product 9.5, anti-200-kDa neurofilament protein (RT97), and anti-calcitonin gene-related product was used to label innervation in mystacial pads from mice with homozygous null mutations for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), the three tyrosine kinase receptors (trkA, trkB, trkC), and the low-affinity nerve growth factor receptor p75. Specimens were sacrificed at birth and at 1, 2, and 4 weeks for each type of mutation as well as at 11 weeks and 1 year for p75 and trkC mutations, respectively. Our results demonstrate several major concepts about the role of neurotrophins in the development of cutaneous mechanoreceptors that are supplied by medium to large caliber myelinated afferents. First, each of the high-affinity tyrosine kinase receptors, trkA, trkB, and trkC, as well as the low-affinity p75 receptor has an impact on at least one type of mechanoreceptor. Second, consistent with the various affinities for particular trk receptors, the elimination of NGF, BDNF, and NT-3 has an impact comparable to or more complex than the absence of their most specific high-affinity receptors: trkA, trkB, and trkC, respectively. These complexities include potential NT-3 signaling through trkA and trkB to support some neuronal survival. Third, most types of afferents are dependent on a different combination of neurotrophins and receptors for their survival: reticular and transverse lanceolate afferents are dependent upon NT-3, NGF, and trkA; Ruffini afferents upon BDNF and trkB; longitudinal lanceolate afferents upon NGF, trkA, BDNF, and trkB; and Merkel afferents on NGF, trkA, NT-3, trkC, and p75. NT-4 has no obvious detrimental impact on the mechanoreceptor development in the presence of BDNF. Fourth, NT-4 and BDNF signaling through trkB may suppress Merkel innervation and NT-3 signaling through trkC may suppress Ruffini innervation. Finally, regardless of the neurotrophin/receptor dependency for afferent survival and neurite outgrowth, NT-3 has an impact on the formation of all the sensory endings. In the context of these findings, indications of competitive and suppressive interactions that appear to regulate the balance of innervation density among the various sets of innervation were evident.

Animals↗

PubMed1997

A giant protein that stimulates guanine nucleotide exchange on ARF1 and Rab proteins forms a cytosolic ternary complex with clathrin and Hsp70.

We have recently identified an unusually large human protein that stimulates guanine nucleotide exchange on ARF1 and Rab proteins (EMBO J 15: 4262-4273, 1996). This protein, designated p532 based on its predicted molecular weight (EMBO J 15: 5738, 1996), contains multiple structural domains including two regions of seven internal repeats highly related to the cell cycle regulator RCC1, a guanine nucleotide exchange factor for the small GTP-binding protein Ran, seven beta-repeat domains characteristic of the beta subunit of heterotrimeric G proteins, three putative SH3 binding sites, a putative leucine-zipper and a carboxy-terminal HECT domain characteristic of E3 ubiquitin-protein ligases. Some of these domains are known to be involved in protein-protein interactions, suggesting the existence of p532-interacting proteins. To identify some of these proteins, we used the carboxy-terminal RCC1-like domain (RLD) of p532 in the yeast two-hybrid system. We report here the isolation of a clone that encodes the last 654 amino acid residues of the clathrin heavy chain. This interaction involves amino acid residues 1315-1557 of the clathrin heavy chain and the carboxy, but not the amino-terminal RLD of p532. p532 has been located in the cytosolic fraction as well as in the Golgi apparatus. The interaction between p532 and clathrin only occurs in the cytosol and is mediated by the formation of an ATP-dependent ternary complex with the heat shock protein, Hsp70. These observations suggest that p532 is involved in membrane transport processes.

ADP-Ribosylation Factor 1↗

PubMed1997

Aberrant axonal projections in mice lacking EphA8 (Eek) tyrosine protein kinase receptors.

We have generated mice homozygous for a mutation that disrupts the gene encoding EphA8, a member of the Eph family of tyrosine protein kinase receptors, previously known as Eek. These mice develop to term, are fertile and do not display obvious anatomical or physiological defects. The mouse ephA8/eek gene is expressed primarily in a rostral to caudal gradient in the developing tectum. Axonal tracing experiments have revealed that in these mutant mice, axons from a subpopulation of tectal neurons located in the superficial layers of the superior colliculus do not reach targets located in the contralateral inferior colliculus. Moreover, ephA8/eek null animals display an aberrant ipsilateral axonal tract that projects to the ventral region of the cervical spinal cord. Retrograde labeling revealed that these abnormal projections originate from a small subpopulation of superior colliculus neurons that normally express the ephA8/eek gene. These results suggest that EphA8/Eek receptors play a role in axonal pathfinding during development of the mammalian nervous system.

Animals↗

PubMed1997

TrkB signaling is required for postnatal survival of CNS neurons and protects hippocampal and motor neurons from axotomy-induced cell death.

Newborn mice carrying targeted mutations in genes encoding neurotrophins or their signaling Trk receptors display severe neuronal deficits in the peripheral nervous system but not in the CNS. In this study, we show that trkB (-/-) mice have a significant increase in apoptotic cell death in different regions of the brain during early postnatal life. The most affected region in the brain is the dentate gyrus of the hippocampus, although elevated levels of pyknotic nuclei were also detected in cortical layers II and III and V and VI, the striatum, and the thalamus. Furthermore, axotomized hippocampal and motor neurons of trkB (-/-) mice have significantly lower survival rates than those of wild-type littermates. These results suggest that neurotrophin signaling through TrkB receptors plays a role in the survival of CNS neurons during postnatal development. Moreover, they indicate that TrkB receptor signaling protects subpopulations of CNS neurons from injury- and axotomy-induced cell death.

Animals↗

PubMed1997

Ablation of the CDK inhibitor p57Kip2 results in increased apoptosis and delayed differentiation during mouse development.

p57(Kip2) is a paternally imprinted gene that encodes a potent inhibitor of several cyclin/Cdk complexes. p57(Kip2) is primarily expressed in terminally differentiated cells, associates with G1 Cdks, and can cause cell cycle arrest in G1 phase. To investigate the role of p57(Kip2) in vivo, we have ablated the p57(Kip2) gene by homologous recombination in ES cells and generated mice devoid of p57(Kip2) expression. Most p57(Kip2) null mice die after birth and display severe developmental defects with varying degrees of penetrance. As expected, heterozygous mice that inherit a maternal, but not a paternal, targeted allele exhibit similar deficiencies and neonatal death. Developmental defects of p57(Kip2) mutant mice include cleft palate and gastrointestinal abnormalities ranging from an inflated GI tract to loss of the jejunum and ileum. These tissues display a significant increase of apoptotic cells in the absence of p57(Kip2). Most p57(Kip2) mutant mice have short limbs, a defect attributable to abnormal endochondral ossification caused by delayed cell cycle exit during chondrocyte differentiation. A similar defect has been observed in mice lacking p107 and p130, thus suggesting that p57(Kip2) might be an upstream regulator of these Rb-related proteins. The p57(Kip2) locus has been implicated in the Beckwith-Wiedemann syndrome and in the development of sporadic Wilms' tumors and lung carcinomas. To date, we have not observed neoplastic development even in those p57(Kip2) mutant mice that have survived for >5 months of age. These findings indicate that p57(Kip2) has an important role during mouse development that cannot be compensated by other Cdk inhibitors.

Abnormalities, Multiple↗

PubMed1997

Genetic analysis of the role of Eph receptors in the development of the mammalian nervous system.

The Eph family of tyrosine protein kinase receptors and their cognate ligands, a family of membrane-bound molecules known as ephrins, have been implicated in diverse biological processes, such as axon guidance, the formation of topographic neural projections, axon fasciculation, the regionalization of the central nervous system, and cell migration. The recent generation of mutant mice lacking some of these receptors has made possible the dissection of the role that these molecules play in the development of the mammalian nervous system.

Animals↗

PubMed1997

Mice with a targeted disruption of the neurotrophin receptor trkB lose their gustatory ganglion cells early but do develop taste buds.

The alleged ability of taste afferents to induce taste buds in developing animals is investigated using a mouse model with a targeted deletion of the tyrosine kinase receptor trkB for the neurotrophin BDNF. This neurotrophin was recently shown to be expressed in developing taste buds and the receptor trkB has been shown to be expressed in the developing ganglion cells that innervate the taste buds. Our data show a reduction of geniculate ganglion cells to about 5% of control animals in neonates. Degeneration of ganglion cells starts when processes reach the central target (solitary tract) but before they reach the peripheral target (taste buds). Degeneration of ganglion cells is almost completed in trkB knockout mice before taste afferents reach in control animals the developing fungiform papillae. Four days later the first taste buds can be identified in fungiform papillae of both control and trkB knockout mice in about equal number and density. Many taste buds undergo a normal maturation compared to control animals. However, the more lateral and caudal fungiform papillae grow less in size and become less conspicuous in older trkB knockout mice. No intragemmal innervation can be found in trkB knockout taste buds but a few extragemmal fibers enter the apex and end between taste had cells without forming specialized synapses. Taste buds of trkB knockout mice appear less well organized than those of control mice, but some cells show similar vesicle accumulations as control taste bud cells in their base but no synaptic contact to an afferent. These data strongly suggest that the initial-development of many fungiform papillae and taste buds is independent of the specific taste innervation. It remains to be shown why others appear to be more dependent on proper innervation.

Animals↗

PubMed1997

Severe sensory deficits but normal CNS development in newborn mice lacking TrkB and TrkC tyrosine protein kinase receptors.

Analysis of mice carrying targeted mutations in genes encoding neurotrophins and their signalling Trk receptors has provided critical information regarding the role that these molecules play in the mammalian nervous system. In this study we generated mice defective in both TrkB and TrkC tyrosine kinase receptors to determine the biological effects of these receptors in the absence of compensatory mechanisms. trkB(-/-);trkC(-/-) double-mutant mice were born at the expected frequency, indicating that TrkB and TrkC signalling are not required for embryonic survival. However, these double-mutant mice had a significantly shorter lifespan and displayed more severe sensory defects than their single-mutant trkB(-/-) and trkC(-/-) littermates. The most dramatic sensory deficit observed in trkB(-/-);trkC(-/-) mutant mice was the absence of vestibular and cochlear ganglia. Interestingly, these mice developed inner ear sensory epithelia in spite of the complete absence of sensory innervation. Analysis of the CNS in trkB(-/-);trkC(-/-) mutant mice revealed a well formed hippocampus, cortex and thalamus. Moreover, the pattern of expression of several neuronal markers appeared normal in these animals. These observations suggest that neurotrophin signalling through TrkB and TrkC receptors is essential for the development of sensory ganglia; however, it does not play a major role in the differentiation and survival of CNS neurons during embryonic development.

Animals↗

PubMed1996

TrkA, but not TrkC, receptors are essential for survival of sympathetic neurons in vivo.

Neurotrophins and their signaling receptors, the Trk family of protein tyrosine kinases, play a major role in the development of the mammalian nervous system. To determine the precise stages that require Trk receptor signaling during development of the sympathetic system, we have analyzed the superior cervical ganglion (SCG) of embryonic and postnatal mice defective for each of the known Trk receptors. Transcripts encoding TrkC are detected in early sympathetic development, before the coalescence of the SCG. trkA expression appears at E13.5, becoming robust from E15.5 onward. In contrast, trkC expression decreases significantly after E15.5 and remains detectable only in a small subpopulation of cells. No significant trkB expression could be detected in the SCG at any developmental stage. Ablation of TrkA receptors does not affect neurogenesis, expression of neuronal markers, or initial axonal growth. However, these receptors are absolutely necessary for the survival of sympathetic neurons after E15.5 and for proper innervation of their distal targets. In contrast, mice defective for either TrkC or TrkB tyrosine kinase receptors do not display detectable defects in their SCGs. These results illustrate the differential roles of the Trk family of receptors during SCG development and define a critical role for TrkA signaling in the survival, but not differentiation, of SCG neurons. Moreover, these observations raise the possibility that at least some SCG neurons become neurotrophin-dependent before complete target innervation.

Animals↗

PubMed1996

Mice lacking brain-derived neurotrophic factor exhibit visceral sensory neuron losses distinct from mice lacking NT4 and display a severe developmental deficit in control of breathing.

The neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT4) act via the TrkB receptor and support survival of primary somatic and visceral sensory neurons. The major visceral sensory population, the nodose-petrosal ganglion complex (NPG), requires BDNF and NT4 for survival of a full complement of neurons, providing a unique opportunity to compare gene dosage effects between the two TrkB ligands and to explore the possibility that one ligand can compensate for loss of the other. Analysis of newborn transgenic mice lacking BDNF or NT4, or BDNF and NT4, revealed that survival of many NPG afferents is proportional to the number of functional BDNF alleles, whereas only one functional NT4 allele is required to support survival of all NT4-dependent neurons. In addition, subpopulation analysis revealed that BDNF and NT4 can compensate for the loss of the other to support a subset of dopaminergic ganglion cells. Together, these data demonstrate that the pattern of neuronal dependencies on BDNF and NT4 in vivo is far more heterogeneous than predicted from previous studies in culture. Moreover, BDNF knockout animals lack a subset of afferents involved in ventilatory control and exhibit severe respiratory abnormalities characterized by depressed and irregular breathing and reduced chemosensory drive. BDNF is therefore required for expression of normal respiratory behavior in newborn animals.

Alleles↗

PubMed1996

p619, a giant protein related to the chromosome condensation regulator RCC1, stimulates guanine nucleotide exchange on ARF1 and Rab proteins.

We report the identification of a novel human gene, designated p619, that encodes a polypeptide of 4861 amino acid residues, one of the largest human proteins known to date. The p619 protein contains two regions of seven internal repeats highly related to the cell cycle regulator RCC1, a guanine nucleotide exchange factor for the small GTP binding protein, Ran. In addition, p619 possesses seven beta-repeat domains characteristic of the beta-subunit of heterotrimeric G proteins, three putative SH3 binding sites, seven polar amino acid-rich regions, a putative leucine zipper and a carboxy-terminal HECT domain characteristic of E3 ubiquitin-protein ligases. p619 is expressed ubiquitously in mouse and human tissues and overexpressed in several human tumor cell lines. Subcellular localization studies indicate that p619 is located in the cytosol and in the Golgi apparatus. Localization of p619 in the Golgi is altered by Brefeldin A. The carboxy-terminal RCC1-like domain of p619 interacts specifically with myristoylated ARF1, a small GTP binding protein also located in the Golgi. Moreover, the second RCC1-like motif located at the amino-terminus of p619 stimulates guanine nucleotide exchange on ARF1 and on members of the related Rab proteins, but not on other small GTP binding proteins such as Ran or R-Ras2/TC21. These observations suggest that p619 is a Brefeldin A-sensitive Golgi protein that functions as a guanine nucleotide exchange factor for ARF1 and, possibly, for members of the Rab family of proteins.

ADP-Ribosylation Factor 1↗

PubMed1996

Rac-1 dependent stimulation of the JNK/SAPK signaling pathway by Vav.

The protein product of the human vav oncogene, Vav exhibits a number of structural motifs suggestive of a role in signal transduction pathways, including a leucine-rich region, a plekstrin homology (PH) domain, a cysteine-rich domain, two SH3 regions, an SH2 domain, and a central Dbl homology (DH) domain. However, the transforming pathway(s) activated by Vav has not yet been elucidated. Interestingly, DH domains are frequently found in guanine nucleotide-exchange factors for small GTP-binding proteins of the Ras and Rho families, and it has been recently shown that, whereas Ras controls the activation of mitogen activated kinases (MAPKs), two members of the Rho family of small GTPases, Rac 1 and Cdc42, regulate activity of stress activated protein kinases (SAPKs), also termed c-jun N-terminal kinases (JNKs). The structural similarity between Vav and other guanine nucleotide exchange factors for small GTP-binding proteins, together with the recent identification of biochemical routes specific for members of the Ras and Rho family of GTPases, prompted us to explore whether MAPK or JNK are downstream components of the Vav signaling pathways. Using the COS-7 cell transient expression system, we have found that neither Vav nor the product of the vav proto-oncogene, proto-Vav, can enhance the enzymatic activity of a coexpressed, epitope tagged MAPK. On the other hand, we have observed that, whereas proto-Vav can slightly elevate JNK/SAPK activity, oncogenic Vav potently activates JNK/SAPK to an extent comparable to that elicited by two guanine-nucleotide exchange factors for Rho family members, Dbl and Ost. We also show that point mutations in conserved residues within the cysteine rich and DH domains of Vav both prevent its ability to activate JNK/SAPK and render Vav oncogenically inactive. In addition, we found that coexpression of the Rac-1 N17 dominant inhibitory mutant dramatically diminishes JNK/SAPK stimulation by Vav, as well as reduces the focus-forming ability of Vav in NIH3T3 murine fibroblasts. Taken together, these findings provide the first evidence that Rac-1 and JNK are integral components of the Vav signaling pathway.

3T3 Cells↗

PubMed1996

Synchronous onset of NGF and TrkA survival dependence in developing dorsal root ganglia.

Determinations of dorsal root ganglion (DRG) neuron loss in nerve growth factor (NGF) and neurotrophin-3 (NT-3) null mutant mice have supported the concept that neurons can switch neurotrophin dependence by revealing that many neurons must require both of these factors acting either sequentially or simultaneously during development. The situation is complex, however, in that NT-3(-/-) mutant mice show far greater neuron loss than mice deficient in the NT-3 receptor TrkC, suggesting that NT-3 may support many DRG neurons via actions on the NGF receptor TrkA. To assess the possibility of ligand-receptor cross-talk as a developmental mechanism, we have compared the onset of survival dependence of lumbar DRG neurons on NT-3, TrkC, NGF, and TrkA signaling in mice deficient in these molecules as a result of gene targeting. At embryonic day 11.5 (E11.5), virtually all lumbar DRG cells express TrkC mRNA and many require NT-3 and TrkC signaling for survival. In contrast, although many lumbar DRG cells also express TrkA at E11.5, there is little survival dependence on TrkA signaling. By E13.5, most lumbar DRG cells have downregulated TrkC mRNA. The onset of survival dependence on NGF and TrkA-signaling is concurrent and of equal magnitude at E13.5, demonstrating that NT-3 alone does not support DRG neurons via TrkA, nor can NT-3 compensate for the loss of NGF. We conclude that many murine DRG cells require NT-3 activation of TrkA is unimportant to these early NT-3 survival-promoting actions. We suggest that the discrepancy in cell loss between NT-3(-/-) and trkC(-/-) mutants is attributable to the ability of NT-3 to support DRG neurons via TrkA in the artificial situation where TrkC is absent.

Animals↗