Trk receptors: roles in neuronal signal transduction

A developmental switch in the response of DRG neurons to ETS transcription factor signaling

Two ETS transcription factors of the Pea3 subfamily are induced in subpopulations of dorsal root ganglion (DRG) sensory and spinal motor neurons by target-derived factors. Their expression controls late aspects of neuronal differentiation such as target invasion and branching. Here, we show that the late onset of ETS gene expression is an essential requirement for normal sensory neuron differentiation. We provide genetic evidence in the mouse that precocious ETS expression in DRG sensory neurons perturbs axonal projections, the acquisition of terminal differentiation markers, and their dependence on neurotrophic support. Together, our findings indicate that DRG sensory neurons exhibit a temporal developmental switch that can be revealed by distinct responses to ETS transcription factor signaling at sequential steps of neuronal maturation.

By expressing ETS transcription factors at different developmental stages of dorsal root ganglion development, the authors show that late onset of ETS expression is essential for normal sensory neuron differentiation

Dose and age-dependent axonal responses of embryonic trigeminal neurons to localized NGF via p75NTR receptor

Nerve growth factor (NGF) and related neurotrophins are target-derived survival factors for sensory neurons. In addition, these peptides modulate neuronal differentiation, axon guidance, and synaptic plasticity. We tested axonal behavior of embryonic trigeminal neurons towards localized sources of NGF in collagen gel assays. Trigeminal axons preferentially grow towards lower doses of localized NGF and grow away from higher concentrations at earlier stages of development, but do not show this response later. Dorsal root ganglion axons also show similar responses to NGF, but NGF-dependent superior cervical ganglion axons do not. Such axonal responses to localized NGF sources were also observed in Bax-/- mice, suggesting that the axonal effects are largely independent of cell survival. Immunocytochemical studies indicated that axons, which grow towards or away from localized NGF are TrkA-positive, and TrkA-/- TG axons do not respond to any dose of NGF. We further show that axonal responses to NGF are absent in TG derived from mice that lack the p75 neurotrophin receptor (p75NTR). Collectively, our results suggest that localized sources of NGF can direct axon outgrowth from trigeminal ganglion in a dose- and age-dependent fashion, mediated by p75NTR signaling through TrkA expressing axons.

A role for brain-derived neurotrophic factor in B cell development

In the present study, we demonstrated a significant reduction of B lymphocytes in the blood, spleen and bone marrow of BDNF deficient mice. The observed developmental block in bone marrow B cell development was linked specifically to the Pre-BII stage. B lymphocytes express the BDNF receptors p75NTR and TrkB(gp95), while no BDNF expression was found. However, a strong BDNF expression was demonstrated in bone marrow stromal cells. An increase of intracellular free calcium [Ca2+]i in B lymphocytes after BDNF application confirms a direct responsiveness of B lymphocytes to BDNF. In conclusion, these results suggest a role of BDNF for normal B lymphocyte development through paracrine effects in the bone marrow.

Exercise activates the phosphatidylinositol 3-kinase pathway

Physical exercise is known to enhance psychological well-being and coping capacity. Voluntary physical exercise in rats also robustly and rapidly up-regulates hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels, which are potentiated following a regimen of chronic antidepressant treatment. Increased BDNF levels are associated with enhanced activity of cyclic AMP response element binding protein (CREB). So far, relatively little is known about the intracellular signaling mechanisms mediating this effect of exercise. We wished to explore the possibility that exercise and/or antidepressant treatment activate the hippocampal phosphatidylinositol-3 (PI-3) kinase pathway, which mediates cellular survival. In young male Sprague-Dawley rats, we examined the effects of 2 weeks of daily voluntary wheel-running activity and/or tranylcypromine (n = 7 per group) on the levels of the active forms of protein-dependent kinase-1 (PDK-1), PI-3 kinase, phospho-thr308-Akt, phospho-ser473-Akt, and phospho-glycogen synthase kinase-3beta (GSK3beta; inactive form), as well as BDNF, activated CREB, and the phospho-Trk receptor, in the rat hippocampus, and compared these with sedentary saline-treated controls. Immunoblotting analyses revealed that in exercising rats, there was a significant increase in PI-3 kinase expression (4.61 times that of controls, P = 0.0161) and phosphorylation of PDK-1 (2.73 times that of controls, P = 0.0454), thr308-Akt (2.857 times that of controls, P = 0.0082), CREB (60.27 times that of controls, P = 0.05), and Trk (35.3 times that of controls, P < 0.0001) in the hippocampi of exercising animals; BDNF was also increased (3.2 times that of controls), but this was not statistically significant. In rats receiving both exercise and tranylcypromine, BDNF (4.51 times that of controls, P = 0.0068) and PI-3 kinase (4.88 times that of controls, P = 0.0103), and the phospho- forms of Trk (13.67 times that of controls, P = 0.0278), thr308-Akt (3.644 times that of controls, P = 0.0004), GSK-3beta (2.93 times that of controls, P = 0.026), and CREB (88.97 times that of controls, P = 0.0053) were significantly increased. These results suggest that the exercise-induced expression of BDNF is associated with the increased expression of several key intermediates of the PI-3 kinase/Akt pathway, which is known for its role in enhancing neuronal survival.

Extracellular signal-regulated kinase 1/2 mediates survival, but not axon regeneration, of adult injured central nervous system neurons in vivo

Neurotrophins play important roles in the response of adult neurons to injury. The intracellular signaling mechanisms used by neurotrophins to regulate survival and axon growth in the mature CNS in vivo are not well understood. The goal of this study was to define the role of the extracellular signal-regulated kinases 1/2 (Erk1/2) pathway in the survival and axon regeneration of adult rat retinal ganglion cells (RGCs), a prototypical central neuron population. We used recombinant adeno-associated virus (AAV) to selectively transduce RGCs with genes encoding constitutively active or wild-type mitogen-activated protein kinase kinase 1 (MEK1), the upstream activator of Erk1/2. In combination with anterograde and retrograde tracing techniques, we monitored neuronal survival and axon regeneration in vivo. MEK1 gene delivery led to robust and selective transgene expression in multiple RGC compartments including cell bodies, dendrites, axons and targets in the brain. Furthermore, MEK1 activation induced in vivo phosphorylation of Erk1/2 in RGC bodies and axons. Quantitative analysis of cell survival demonstrated that Erk1/2 activation promoted robust RGC neuroprotection after optic nerve injury. In contrast, stimulation of the Erk1/2 pathway was not sufficient to induce RGC axon growth beyond the lesion site. We conclude that the Erk1/2 pathway plays a key role in the survival of axotomized mammalian RGCs in vivo, and that activation of other signaling components is required for axon regeneration in the growth inhibitory CNS environment.

The single AmphiTrk receptor highlights increased complexity of neurotrophin signalling in vertebrates and suggests an early role in developing sensory neuroepidermal cells

Neurotrophins (Nt) and their tyrosine kinase Trk receptors play an essential role in the development and maintenance of the complex vertebrate nervous system. Invertebrate genome sequencing projects have suggested that the Nt/Trk system is a vertebrate innovation. We describe the isolation and characterisation of the amphioxus Trk receptor, AmphiTrk. Its ancestral link to vertebrate Trk receptors is supported by phylogenetic analysis and domain characterisation. The genomic structure of AmphiTrk strongly suggests that a ProtoTrk gene emerged by means of exon-shuffling prior to the cephalochordate/vertebrate split. We also examined the physiological response of AmphiTrk to vertebrate neurotrophins, and found that despite 500 million years of divergence, AmphiTrk transduces signals mediated by NGF, BDNF, NT3 and NT4. Markedly, AmphiTrk is able to activate survival and differentiation pathways, but fails to activate the PLCgamma pathway, which is involved in synaptic plasticity in higher vertebrates. AmphiTrk is expressed during amphioxus embryogenesis in sensory neural precursors in the epidermis, which possesses single migratory cells. We propose that the duplication and divergence of the Nt/Trk system, in tandem with recruitment of the PLCgamma pathway, may have provided the genetic basis for a key aspect of vertebrate evolution: the complexity of the nervous system.

Sequential activation of p75 and TrkB is involved in dendritic development of subventricular zone-derived neuronal progenitors in vitro

Dendritic arbor development of subventricular zone-derived interneurons is a critical step in their integration into functional circuits of the postnatal olfactory bulb. However, the mechanism and molecular control of this process remain unknown. In this study, we have developed a culture model where dendritic development of purified subventricular zone cells proceeds under serum-free conditions in the absence of added growth factors and non-neural cells. We demonstrate that the large majority of these cells in culture express GABA and elaborate dendritic arbors with spine-like protrusions but they do not possess axons. These neurons expressed receptors for neurotrophins including p75, TrkB and TrkC but not TrkA. Application of exogenous neurotrophins, including brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and nerve growth factor (NGF), to cultures stimulated dendritic growth and led to more complex dendritic arbors during the initial 3 days in culture. Our results suggest that these effects are independent of Trk receptors and mediated by the p75/ceramide signaling pathway. We also show that brain-derived neurotrophic factor is the only neurotrophin that is able to influence late-phase dendritic development via TrkB receptor activation. These results suggest that dendritic arbor development of subventricular zone-derived cells may be regulated by neurotrophins through the activation of p75 and the TrkB receptor signaling pathways in a sequentially defined temporal pattern.

Mood stabilizer valproate promotes ERK pathway-dependent cortical neuronal growth and neurogenesis

Manic-depressive illness has been conceptualized as a neurochemical illness. However, brain imaging and postmortem studies reveal gray-matter reductions, as well as neuronal and glial atrophy and loss in discrete brain regions of manic-depressive patients. The roles of such cerebral morphological deficits in the neuropathophysiology and therapeutic mechanisms of manic-depressive illness are unknown. Valproate (2-propylpentanoate) is a commonly used mood stabilizer. The ERK (extracellular signal-regulated kinase) pathway is used by neurotrophic factors to regulate neurogenesis, neurite outgrowth, and neuronal survival. We found that chronic treatment of rats with valproate increased levels of activated phospho-ERK44/42 in neurons of the anterior cingulate, a region in which we found valproate-induced increases in expression of an ERK pathway-regulated gene, bcl-2. Valproate time and concentration dependently increased activated phospho-ERK44/42 and phospho-RSK1 (ribosomal S6 kinase 1) levels in cultured cortical cells. These increases were attenuated by Raf and MEK (mitogen-activated protein kinase/ERK kinase) inhibitors. Although valproate affects the functions of GSK-3 (glycogen synthase kinase-3) and histone deacetylase (HDAC), its effects on the ERK pathway were not fully mimicked by selective inhibitors of GSK-3 or HDAC. Similar to neurotrophic factors, valproate enhanced ERK pathway-dependent cortical neuronal growth. Valproate also promoted neural stem cell proliferation-maturation (neurogenesis), demonstrated by bromodeoxyuridine (BrdU) incorporation and double staining of BrdU with nestin, Tuj1, or the neuronal nuclei marker NeuN (neuronal-specific nuclear protein). Chronic treatment with valproate enhanced neurogenesis in the dentate gyrus of the hippocampus. Together, these data demonstrate that valproate activates the ERK pathway and induces ERK pathway-mediated neurotrophic actions. This cascade of events provides a potential mechanism whereby mood stabilizers alleviate cerebral morphometric deficits associated with manic-depressive illness.

Essential role of TRPC channels in the guidance of nerve growth cones by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) is known to promote neuronal survival and differentiation and to guide axon extension both in vitro and in vivo. The BDNF-induced chemo-attraction of axonal growth cones requires Ca2+ signalling, but how Ca2+ is regulated by BDNF at the growth cone remains largely unclear. Extracellular application of BDNF triggers membrane currents resembling those through TRPC (transient receptor potential canonical) channels in rat pontine neurons and in Xenopus spinal neurons. Here, we report that in cultured cerebellar granule cells, TRPC channels contribute to the BDNF-induced elevation of Ca2+ at the growth cone and are required for BDNF-induced chemo-attractive turning. Several members of the TRPC family are highly expressed in these neurons, and both Ca2+ elevation and growth-cone turning induced by BDNF are abolished by pharmacological inhibition of TRPC channels, overexpression of a dominant-negative form of TRPC3 or TRPC6, or downregulation of TRPC3 expression via short interfering RNA. Thus, TRPC channel activity is essential for nerve-growth-cone guidance by BDNF.

The neurotrophin-3 receptor TrkC directly phosphorylates and activates the nucleotide exchange factor Dbs to enhance Schwann cell migration

During the development of the peripheral nervous system, Schwann cells, the myelin-forming glia, migrate along axons before initiating myelination. We previously demonstrated that endogenous neurotrophin-3 (NT3) acting through the TrkC tyrosine kinase receptor enhances migration of premyelinating Schwann cells. This signaling pathway is mediated by the c-Jun N-terminal kinase (JNK) cascade regulated by the Rho GTPases Rac1 and Cdc42. However, missing is the link between TrkC and the GTPases. Here, we show that a guanine-nucleotide exchange factor (GEF), Dbl's big sister (Dbs), couples with TrkC to activate Cdc42 in Schwann cells. Furthermore, TrkC directly phosphorylates Dbs, thereby inducing the Cdc42-GEF activity. Taken together, activation of TrkC triggers Schwann cell migration by regulating Dbs upon direct tyrosine phosphorylation, providing a mechanism whereby a membrane receptor tyrosine kinase can induce the activation of Rho GTPase-GEFs.

Multi-tasking by the p75 neurotrophin receptor: sortilin things out?

Signalling by the p75 neurotrophin receptor has been implicated in diverse neuronal responses, including increased differentiation or survival, inhibition of regeneration, and initiation of apoptotic cell death. These numerous roles are matched by, but are not yet correlated with, a multiplicity of extracellular ligands and intracellular interactors. Membrane proteins such as sortilin, a member of the Vps10p family of sorting receptors, and the glycosylphosphatidylinositol-linked Nogo receptor (NgR) and the associated adaptor lingo 1 have recently been added to the list of p75-interacting modulators. Other studies have described intramembranal cleavage of p75 and the potential nuclear targeting of cleavage fragments or of the complete receptor after it has been internalized into a putative signalling endosome. These findings suggest that some of the diversity in p75 activities might be due to differential subcellular localization and transport of p75 receptor complexes. We therefore argue that cell-biology-driven approaches are now required to make sense of p75 signalling.

Transactivation of Trk neurotrophin receptors by G-protein-coupled receptor ligands occurs on intracellular membranes

Neurotrophins, such as NGF and BDNF, activate Trk receptor tyrosine kinases through receptor dimerization at the cell surface followed by autophosphorylation and intracellular signaling. It has been shown that activation of Trk receptor tyrosine kinases can also occur via a G-protein-coupled receptor (GPCR) mechanism, without involvement of neurotrophins. Two GPCR ligands, adenosine and pituitary adenylate cyclase-activating polypeptide (PACAP), can activate Trk receptor activity to increase the survival of neural cells through stimulation of Akt activity. To investigate the mechanism of Trk receptor transactivation, we have examined the localization of Trk receptors in PC12 cells and primary neurons after treatment with adenosine agonists and PACAP. In contrast to neurotrophin treatment, Trk receptors were sensitive to transcriptional and translational inhibitors, and they were found predominantly in intracellular locations particularly associated with Golgi membranes. Biotinylation and immunostaining experiments confirm that most of the transactivated Trk receptors are found in intracellular membranes. These results indicate that there are alternative modes of activating Trk receptor tyrosine kinases in the absence of neurotrophin binding at the cell surface and that receptor signaling may occur and persist inside of neuronal cells.

Enhanced Ras activity promotes spine formation in synRas mice neocortex

The effect of enhanced Ras activity upon dendritic spine formation has been investigated on pyramidal cells in the adult neocortex of synRas mice over-expressing Val12-Ha-Ras specifically in postmitotic neurons. For quantitative analysis, commissural neurons of layers II/III were retrogradely labelled with dextran amine. In synRas mice, spine frequency on several orders of basal dendrites and apical oblique branches was significantly increased. Whereas density on basal dendrites was overall augmented, on apical dendrites, distal regions were most affected. The proportions of different morphological types of spines remained unchanged. It is concluded that Ras might be critically involved in mechanisms that mediate and regulate dendritic spine formation in the adult brain.

p75NTR – live or let die

During neuronal development, neurotrophins are essential factors that promote survival, differentiation and myelination of neurons. The trophic signals are relayed to the cells via binding to Trk receptor tyrosine kinases and the p75 neurotrophin receptor. Paradoxically, the p75 neurotrophin receptor also ensures rapid and appropriate apoptosis of neonatal neurons not reaching their proper targets and transmits death signals to injured neurons. Until recently, the mechanisms by which the p75 neurotrophin receptor governs these opposing functions have remained elusive. By the identification of new ligands and cytosolic interacting partners, receptor cleavage products and coreceptors, some of these mechanisms are now being unraveled. Here, we review recent progress in delineating the molecular networks that enable p75(NTR) to dictate life and death.

Neudesin, a novel secreted protein with a unique primary structure and neurotrophic activity

We identified a novel secreted protein and named it neudesin. Mouse neudesin of 171 amino acids is unique with no primary structural similarity to any known proteins. The neudesin protein produced in cultured cells was secreted efficiently into the culture medium. Mouse neudesin mRNA was expressed abundantly in the developing brain and spinal cord in embryos, but was expressed widely in postnatal tissues including brain, heart, lung, and kidney. Mouse neudesin mRNA was expressed in neurons but not glial cells of the brain. The protein exhibited significant neurotrophic activity in primary cultured mouse neurons but not mitogenic activity in primary cultured mouse astrocytes. Neudesin activated the mitogen-activated protein (MAP) and phosphatidylinositol-3 (PI-3) kinase pathways. The activity of neudesin was inhibited by the inhibitor pertussis toxin for Gi/Go-protein but not by inhibitors for receptor tyrosine kinases. These results indicated that the activity was mediated via the activation of the MAP and PI-3 kinase pathways, potentially by the activation of a Gi/Go-protein-coupled receptor. Human neudesin of 172 amino acids with high similarity ( approximately 91% identity) to mouse neudesin was also identified. The human neudesin gene was mapped to chromosome 1p33. The identification of neudesin, a novel secreted protein with a unique primary structure and neurotrophic activity, will provide new insights into the development and maintenance of neuron

BDNF modulates GABAA receptors microtransplanted from the human epileptic brain to Xenopus oocytes

Cell membranes isolated from brain tissues, obtained surgically from six patients afflicted with drug-resistant temporal lobe epilepsy and from one nonepileptic patient afflicted with a cerebral oligodendroglioma, were injected into frog oocytes. By using this approach, the oocytes acquire human GABAA receptors, and we have shown previously that the "epileptic receptors" (receptors transplanted from epileptic brains) display a marked run-down during repetitive applications of GABA. It was found that exposure to the neurotrophin BDNF increased the amplitude of the "GABA currents" (currents elicited by GABA) generated by the epileptic receptors and decreased their run-down; both events being blocked by K252A, a neurotrophin tyrosine kinase receptor B inhibitor. These effects of BDNF were not mimicked by nerve growth factor. In contrast, the GABAA receptors transplanted from the nonepileptic human hippocampal uncus (obtained during surgical resection as part of the nontumoral tissue from the oligodendroglioma margins) or receptors expressed by injecting rat recombinant alpha1beta2gamma2 GABAA receptor subunit cDNAs generated GABA currents whose time-course and run-down were not altered by BDNF. Loading the oocytes with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate-acetoxymethyl ester (BAPTA-AM), or treating them with Rp-8-Br-cAMP, an inhibitor of the cAMP-dependent PKA, did not alter the GABA currents. However, staurosporine (a broad spectrum PK inhibitor), bisindolylmaleimide I (a PKC inhibitor), and U73122 (a phospholipase C inhibitor) blocked the BDNF-induced effects on the epileptic GABA currents. Our results indicate that BDNF potentiates the epileptic GABAA currents and antagonizes their use-dependent run-down, thus strengthening GABAergic inhibition, probably by means of activation of tyrosine kinase receptor B receptors and of both PLC and PKC.

Cyclic AMP controls BDNF-induced TrkB phosphorylation and dendritic spine formation in mature hippocampal neurons

Synaptic actions of brain-derived neurotrophic factor (BDNF) are 'gated' by cyclic AMP (cAMP), but the underlying molecular mechanisms remain unclear. Here we report that cAMP regulates BDNF function in mature hippocampal neurons by modulating the signaling and trafficking of its receptor TrkB. cAMP gated the TrkB tyrosine kinase with three characteristic features: BDNF-induced TrkB phosphorylation was attenuated by inhibitors of cAMP signaling, it was potentiated by cAMP analogs, and activation of the cAMP pathway alone had no effect. In addition, cAMP facilitated trafficking of TrkB to dendritic spines, possibly by promoting its interaction with synaptic scaffolding protein PSD-95. Norepinephrinergic and dopaminergic agonists, which elevate intracellular cAMP concentration, also enhanced TrkB phosphorylation and its translocation to spines. cAMP gated long-term modulation by BDNF of spine density, but not the number of primary dendrites. These results reveal a specific role of cAMP in controlling BDNF actions in the brain, and provide new insights into the molecular mechanism underlying cAMP gating.

A neurotrophin axis in myeloma: TrkB and BDNF promote tumor-cell survival

Multiple myeloma (MM) is a B-cell neoplasm that is characterized by the clonal expansion of malignant plasma cells and is frequently associated with chromosomal translocations placing an oncogene under the control of the immunoglobulin heavy chain enhancer. Despite these pathogenic translocations, MM cells remain dependent on external cues for survival. We present evidence that brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family of growth factors, and its high-affinity receptor, tropomyosin receptor kinase B (TrkB), contribute to these survival cues. MM cells express TrkB, and respond to BDNF by activating mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase-a PI3K target (PI3K/Akt) signaling cascades. Addition of BDNF protects human MM cell lines (HMCLs) from apoptosis induced by dexamethasone or bortezomib and prolongs the survival of primary MM cells cultured alone or with human bone marrow (BM) stroma. As BDNF and TrkB are expressed by osteoblasts, stromal cells, and endothelial cells within the BM microenvironment, a BDNF-TrkB axis may be critical to the interactions of MM with bone and stroma that contribute to MM tumor progression. Finally, BDNF is expressed by malignant plasma cells isolated from a subset of patients with MM, as well as by most HMCLs, suggesting a potential role for this neurotrophin axis in autocrine as well as paracrine support of MM.

TRKing signals through the Golgi

The subcellular localization of transmembrane receptors and other signaling proteins has emerged as a key component in the regulation of the intensity and specificity of their activity. Recent research indicates that immature TrkA neurotrophin receptors are transactivated in the Golgi apparatus after stimulation of neuropeptide pituitary adenylate cyclase-activating polypeptide PAC1 receptors or adenosine A(2A) receptors. Transactivation occurs independently of the TrkA extracellular ligand, nerve growth factor (NGF), through a signaling pathway that is distinct from that used in the transactivation of other receptor tyrosine kinases and has consequences distinct from those elicited by NGF at the plasma membrane.

Overexpression of Tropomysin-Related Kinase B in Metastatic Human Pancreatic Cancer Cells

Purpose: Pancreatic adenocarcinoma is currently the fourth leading cause of cancer death in the United States, and most pancreatic cancers develop locally advanced disease or metastasis at the time of diagnosis. The mechanisms by which it invades and metastasizes are not known.

Experimental Design: To identify the genes involved in pancreatic cancer metastasis, we analyzed the gene expression profiles between highly metastatic Colo357L3.6pl and parental Colo357FG pancreatic cancer cell lines using cDNA microarrays and confirmed differential gene expression by reverse transcription-PCR, Western blotting, and immunologic analysis of 54 samples from pancreatic cancer patients. The correlation with clinical outcome was also examined. The possible signaling pathways involved with tropomyosin-related kinase B (TrkB) were analyzed.

Results: Our findings showed that TrkB was overexpressed in the highly metastatic Colo357L3.6pl cells, which correlated with perineural invasion (P = 0.026), positive retroperitoneal margin (P = 0.0005), and shorter latency to development of liver metastasis (Cox proportional hazard ratio, 0.3; 95% confidence interval, 0.1-0.8; P = 0.01) in patient samples. Extracellular signal-regulated kinases 1 and 2 were activated and Elk-1 and AP-1 DNA binding activity was induced in Colo357L3.6pl cells. Furthermore, interleukin 8 and vascular endothelial growth factor were more strongly expressed in Colo357L3.6pl than Colo357FG cells, and these findings were confirmed in Colo357L3.6pl and Colo357FG orthotopic tumors.

Conclusion: These results suggest that overexpression of TrkB and activation of mitogen-activated protein kinase and AP-1, which may in turn induce the expression of vascular endothelial growth factor and interleukin 8, may mediate the cardinal clinical features of locally aggressive growth and metastasis of pancreatic cancer. Our results also imply that TrkB receptor may be a novel therapeutic target for pancreatic cancer.

Neurotrophic factor receptor expression and in vitro nerve growth of geniculate ganglion neurons that supply divergent nerves

We investigated which neurotrophic factors may contribute to the divergence of two peripheral nerves emanating from the geniculate ganglion. We compared receptor mRNA profiles of the neurons that supply the nerves, and also the growth of their neurites in response to neurotrophic factors in culture. Three mRNAs, Gfra2, TrkA, and TrkC, were differentially expressed. Only one ligand, Neurturin, promoted substantially different nerve regrowth from the nerves, and therefore may contribute to nerve divergence. Three receptor mRNAs were expressed in 100% of the neurons: TrkB, TrkB.T2 (kinase-lacking isoform), and NCAM-140. Ligands for these Trks and FRalpha-1 promoted more outgrowth than ligands for the other receptors. NT-3 and BDNF synergistically promoted outgrowth. Finally, receptors are coexpressed at random rates, arguing against the existence of neuronal subtypes defined by a combinatorial code of these receptors.

Retinal ganglion cell neurotrophin receptor levels and trophic requirements following target ablation in the neonatal rat

Superior colliculus (SC) ablation in neonatal rats results in a rapid increase in retinal ganglion cell (RGC) death. This injury-induced death is reduced by exogenous brain-derived neurotrophic factor or neurotrophin-4/5 (NT-4/5), but the protective effect of these molecules is transient, delaying but not preventing neuronal loss. We sought to discover why neurotrophins only temporarily reduce RGC death after target ablation, focusing on changes in neurotrophin receptor expression and possible changes in growth factor dependency. In unlesioned rats, receptor tyrosine kinase B (trkB) immunohistochemistry revealed no change in the number of trkB positive cells in the RGC layer 24 h after intraocular NT-4/5 injection. However, after SC lesions there were significantly less immunoreactive cells and, surprisingly, even fewer immunoreactive cells in NT-4/5 injected eyes. Semi-quantitative confocal analysis of immunofluorescence intensity revealed an increase in trkB staining in the RGC layer in unlesioned rats 24 h after NT-4/5 injection, whereas in SC-lesioned animals exposed to NT-4/5 there was a significant decrease in staining. To determine whether injured neonatal RGCs can switch their trophic requirements, different doses of ciliary neurotrophic factor were given intraocularly, either alone or combined with NT-4/5. We also tested an SC-derived chondroitin sulfate proteoglycan that has been reported to promote neonatal RGC survival. None of these interventions reduced lesion-induced RGC death 24 or 36 h after SC ablation. In summary, we show that developing RGCs do not shift their trophic dependence to other survival factors following injury; rather, the application of neurotrophins causes a down-regulation of the cognate trkB receptor, presumably altering the long-term responsiveness of neonatal RGCs to exogenous neurotrophins. These data highlight the difficulty in promoting long-term neuronal survival when using one-off administration of recombinant growth factors.

GSK-3β Regulates Phosphorylation of CRMP-2 and Neuronal Polarity

Neurons are highly polarized and comprised of two structurally and functionally distinct parts, an axon and dendrites. We previously showed that collapsin response mediator protein-2 (CRMP-2) is critical for specifying axon/dendrite fate, possibly by promoting neurite elongation via microtubule assembly. Here, we showed that glycogen synthase kinase-3beta (GSK-3beta) phosphorylated CRMP-2 at Thr-514 and inactivated it. The expression of the nonphosphorylated form of CRMP-2 or inhibition of GSK-3beta induced the formation of multiple axon-like neurites in hippocampal neurons. The expression of constitutively active GSK-3beta impaired neuronal polarization, whereas the nonphosphorylated form of CRMP-2 counteracted the inhibitory effects of GSK-3beta, indicating that GSK-3beta regulates neuronal polarity through the phosphorylation of CRMP-2. Treatment of hippocampal neurons with neurotrophin-3 (NT-3) induced inactivation of GSK-3beta and dephosphorylation of CRMP-2. Knockdown of CRMP-2 inhibited NT-3-induced axon outgrowth. These results suggest that NT-3 decreases phosphorylated CRMP-2 and increases nonphosphorylated active CRMP-2, thereby promoting axon outgrowth.

Two developmental switches in GABAergic signalling: the K+-Cl- cotransporter KCC2 and carbonic anhydrase CAVII

GABAergic signalling has the unique property of 'ionic plasticity', which is based on short-term and long-term changes in the Cl(-) and HCO(3)(-) ion concentrations in the postsynaptic neurones. While short-term ionic plasticity is caused by activity-dependent, channel-mediated anion shifts, long-term ionic plasticity depends on changes in the expression patterns and kinetic regulation of molecules involved in anion homeostasis. During development the efficacy and also the qualitative nature (depolarization/excitation versus hyperpolarization/inhibition) of GABAergic transmission is influenced by the neuronal expression of two key molecules: the chloride-extruding K(+)-Cl(-) cotransporter KCC2, and the cytosolic carbonic anhydrase (CA) isoform CAVII. In rat hippocampal pyramidal neurones, a steep up-regulation of KCC2 accounts for the 'developmental switch', which converts depolarizing and excitatory GABA responses of immature neurones to classical hyperpolarizing inhibition by the end of the second postnatal week. The immature hippocampus generates large-scale network activity, which is abolished in parallel by the up-regulation of KCC2 and the consequent increase in the efficacy of neuronal Cl(-) extrusion. At around postnatal day 12 (P12), an abrupt, steep increase in intrapyramidal CAVII expression takes place, promoting excitatory responses evoked by intense GABAergic activity. This is largely caused by a GABAergic potassium transient resulting in spatially widespread neuronal depolarization and synchronous spike discharges. These facts point to CAVII as a putative target of CA inhibitors that are used as antiepileptic drugs. KCC2 expression in adult rat neurones is down-regulated following epileptiform activity and/or neuronal damage by BDNF/TrkB signalling. The lifetime of membrane-associated KCC2 is very short, in the range of tens of minutes, which makes KCC2 ideally suited for mediating GABAergic ionic plasticity. In addition, factors influencing the trafficking and kinetic modulation of KCC2 as well as activation/deactivation of CAVII are obvious candidates in the ionic modulation of GABAergic responses. The down-regulation of KCC2 under pathophysiological conditions (epilepsy, damage) in mature neurones seems to reflect a 'recapitulation' of early developmental mechanisms, which may be a prerequisite for the re-establishment of connectivity in damaged brain tissue.

Regulation of dendritic growth by calcium and neurotrophin signaling

The development of cortical dendrites is regulated by both activity-dependent and activity-independent signaling. Activity-dependent dendritic growth involves calcium-dependent gene expression. Both CREB and CREST are transactivators that contribute to calcium-dependent dendritic growth. Dendritic development is also regulated by extracellular factors such as neurotrophins. Neurotrophin-dependent dendritic growth is mediated by the MAP kinase and PI 3-kinase pathways. Selective responsiveness to activity cues and neurotrophins may contribute to morphological diversity in the nervous system.

R-Ras3/(M-Ras) is involved in thermal adaptation in the critical period of thermal control establishment

Neuroanatomically, the body temperature is balanced by the preoptic anterior hypothalamus (PO/AH) and controlled by thermosensitive neurons. Hot or cold exposure during the critical period of temperature control development causes a plastic change in the ratio between hot- and cold-sensitive cells and can modulate temperature tolerance. In this project, mRNA fingerprinting was used to identify the proteins involved in thermal adaptation in 3-day-old chicks. Fifteen genes were induced, among which were NADH dehydrogenase, protocadherin, anolase alpha, 14-3-3epsilon, and R-Ras3. The role of each of these genes is potentially interesting and requires detailed evaluation, but since the present working hypothesis assumed neuronal remodeling, we concentrated on the role of R-Ras3/(M-Ras), which is uniquely expressed in the brain and whose physiological role has not been described. In the present study, R-Ras3 expression during thermal conditioning was investigated by several molecular techniques and its mRNA was found to be induced in the PO/AH with a tenfold peak after 12 h of heat conditioning and a fourfold increase after 6 h of cold conditioning. To improve our understanding of thermal adaptation-related signal transduction, we screened for changes in the expression of transcription factors that were implicated with the Ras gene family, and found that both jun mRNA expression and Jun phosphorylation were induced after 30 min of temperature conditioning. Taken together, the present findings correlate the R-Ras3-jun pathway with thermal-control establishment.

PLC? signaling underlies BDNF potentiation of Purkinje cell responses to GABA

Brain-derived neurotrophic factor (BDNF) regulates neuronal survival, neurite outgrowth, and excitatory synaptic transmission. We reported recently that acute BDNF exposure decreased gamma-aminobutyric acid (GABA) responses in cultured mouse cerebellar granule cells through tyrosine receptor kinase B (TrkB) receptor-mediated signaling. In the present study, we extend this work to investigate BDNF-induced modulation of GABA responses and GABA(A) receptor-mediated synaptic events in cerebellar slices. Thin (200 microm) parasagittal slices of cerebellum were prepared from postnatal Day 7 and 14 mice. Purkinje cells and granule cells, both of which express TrkB-like immunoreactivity, were identified for whole-cell recording. BDNF promptly enhanced GABA responses in Purkinje cells but, consistent with our previous finding in culture, attenuated those recorded in granule cells. In Purkinje cells, BDNF exposure shifted rightward the cumulative peak amplitude distribution for miniature inhibitory postsynaptic currents (mIPSCs) without changing the mIPSC frequency. BDNF-induced potentiation of Purkinje cell responses to GABA was blocked by TrkB-Fc (receptor body that sequesters BDNF), K252a (inhibitor of TrkB receptor autophosphorylation), U73122 (inhibitor of phospholipase-Cgamma [PLCgamma]), KN62 (specific inhibitor of calcium/calmodulin-dependent kinase), KT5720 (specific cyclic AMP-dependent kinase inhibitor), and by intracellular dialysis of Rp-cyclic AMP or BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N, N',N'-tetraacetic acid). Overall, our results indicate that BDNF acutely potentiates GABA(A) receptor function in cerebellar Purkinje cells via the TrkB receptor-PLCgamma signal transduction cascade. In addition, we propose that cyclic AMP-mediated intracellular signaling mechanisms may facilitate manifestation of the BDNF-induced modulatory outcome.

Deafferentation and neurotrophin-mediated intraspinal sprouting: a central role for the p75 neurotrophin receptor

Axonal plasticity in the adult spinal cord is governed by intrinsic neuronal growth potential and by extracellular cues. The p75 receptor (p75(NTR)) binds growth-promoting neurotrophins (NTs) as well as the common receptor for growth-inhibiting myelin-derived proteins (the Nogo receptor) and so is well situated to gauge the balance of positive and negative influences on axonal plasticity. Using transgenic mice lacking the extracellular NT-binding domain of p75(NTR) (p75-/- mice), we have examined the influence of p75(NTR) on changes in the density of primary afferent (calcitonin gene-related peptide-expressing) and descending monoaminergic (serotonin- and tyrosine hydroxylase-expressing) projections to the dorsal horn after dorsal rhizotomy, with and without concomitant application of exogenous nerve growth factor and NT-3. We found that, in intact p75-/- mice, the axon density of all populations was equal to or less than that in wild-type mice but that rhizotomy-induced intraspinal sprouting was significantly augmented. Monoaminergic axon sprouting was enhanced in both nerve growth factor- and NT-3-treated p75-/- mice compared with similarly treated wild-type mice. Primary afferent sprouting was particularly robust in NT-3-treated p75-/- mice. These in vivo results illustrate the interactions of p75(NTR) with NTs, with their respective tropomyosin-related kinase receptors and with inhibitory myelin-derived molecules. Our findings illustrate the pivotal role of p75(NTR) in spinal axonal plasticity and identify it as a potential therapeutic target for spinal cord injury.

Activation of p90 Rsk1 is sufficient for differentiation of PC12 cells

We investigated the role of Rsk proteins in the nerve growth factor (NGF) signaling pathway in PC12 cells. When rat Rsk1 or murine Rsk2 proteins were transiently expressed, NGF treatment (100 ng/ml for 3 days) caused three- and fivefold increases in Rsk1 and Rsk2 activities, respectively. Increased activation of both wild-type Rsk proteins could be achieved by coexpression of a constitutively active (CA) mitogen-activated protein kinase (MAPK) kinase, MEK1-DD, which is known to cause differentiation of PC12 cells even in the absence of NGF. Rsk1 and Rsk2 mutated in the PDK1-binding site were not activated by either NGF or MEK1-DD. Expression of constitutively active Rsk1 or Rsk2 in PC12 cells resulted in highly active proteins whose levels of activity did not change either with NGF treatment or after coexpression with MEK1-DD. Rsk2-CA expression had no detectable effect on the cells. However, expression of Rsk1-CA led to differentiation of PC12 cells even in the absence of NGF, as evidenced by neurite outgrowth. Differentiation was not observed with a nonactive Rsk1-CA that was mutated in the PDK1-binding site. Expression of Rsk1-CA did not lead to activation of the endogenous MAPK pathway, indicating that Rsk1 is sufficient to induce neurite outgrowth and is the only target of MAPK required for this effect. Collectively, our data demonstrate a key role for Rsk1 in the differentiation process of PC12 cells.

BDNF, but not NT-4, is necessary for normal development of Meissner corpuscles

Meissner corpuscles are rapidly adapting cutaneous mechanoreceptors depending for development on TrkB expressing sensory neurons, but it remains to be established which of the known TrkB ligands, BDNF or NT-4, is responsible of this dependence. In this study we analyze Meissner corpuscles in the digital pads of mice with target mutations in the genes encoding for either BDNF or NT-4, using immunohistochemistry and transmission-electron microscopy, and they were identified based on their morphology and expression of S100 protein. All wild-type animals as well as NT-4(-/-) animals and BDNF and NT4 heterozygous animals have Meissner corpuscles that are normal in number and size. However, Meissner corpuscles are absent the BDNF(-/-) mice. These results suggest that BDNF is the only TrkB ligand involved in the development of Meissner corpuscles in murine glabrous skin, and it probably regulates the development of the sensory neurons that innervate Meissner corpuscles.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Connective tissue growth factor CCN2 interacts with and activates the tyrosine kinase receptor TrkA

Connective tissue growth factor (CTGF) is implicated as a factor promoting tissue fibrosis in several disorders, including diabetic nephropathy. However, the molecular mechanism(s) by which it functions is not known. CTGF rapidly activates several intracellular signaling molecules in human mesangial cells (HMC), including extracellular signal-related kinase 1/2, Jun NH(2)-terminal kinase, protein kinase B, CaMK II, protein kinase Calpha, and protein kinase Cdelta, suggesting that it functions via a signaling receptor. Treating HMC with CTGF stimulated tyrosine phosphorylation of proteins 75 to 80 and 140 to 180 kD within 10 min, and Western blot analysis of anti-phosphotyrosine immunoprecipitates identified the neurotrophin receptor TrkA (molecular weight approximately 140 kD). Cross-linking rCTGF to cell surface proteins with 3,3'-dithiobis(sulfosuccinimidylpropionate) revealed that complexes formed with TrkA and with the general neurotrophin co-receptor p75(NTR). rCTGF stimulated phosphorylation of TrkA (tyr 490, 674/675). K252a, a known selective inhibitor of Trk, blocked this phosphorylation, CTGF-induced activation of signaling proteins, and CTGF-dependent induction of the transcription factor TGF-beta-inducible early gene in HMC. It is concluded that TrkA serves as a tyrosine kinase receptor for CTGF.

Each sensory nerve arising from the geniculate ganglion expresses a unique fingerprint of neurotrophin and neurotrophin receptor genes

Neurons in the geniculate ganglion, like those in other sensory ganglia, are dependent on neurotrophins for survival. Most geniculate ganglion neurons innervate taste buds in two regions of the tongue and two regions of the palate; the rest are cutaneous nerves to the skin of the ear. We investigated the expression of four neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4, and five neurotrophin receptors, trkA, trkB, trkC, p75, and truncated trkB (Trn-B) in single sensory neurons of the adult rat geniculate ganglion associated with the five innervation fields. For fungiform papillae, a glass pipette containing biotinylated dextran was placed over the target papilla and the tracer was iontophoresed into the target papilla. For the other target fields, Fluoro-Gold was microinjected. After 3 days, geniculate ganglia were harvested, sectioned, and treated histochemically (for biotinylated dextran) or immunohistochemically (for Fluoro-Gold) to reveal the neurons containing the tracer. Single labeled neurons were harvested from the slides and subjected to RNA amplification and RT-PCR to reveal the neurotrophin or neurotrophin receptor genes that were expressed. Neurons projecting from the geniculate ganglion to each of the five target fields had a unique expression profile of neurotrophin and neurotrophic receptor genes. Several individual neurons expressed more than one neurotrophin receptor or more than one neurotrophin gene. Although BDNF is significantly expressed in taste buds, its primary high affinity receptor, trkB, was not prominently expressed in the neurons. The results are consistent with the interpretation that at least some, perhaps most, of the trophic influence on the sensory neurons is derived from the neuronal somata, and the trophic effect is paracrine or autocrine, rather than target derived. The BDNF in the taste bud may also act in a paracrine or autocrine manner on the trkB expressed in taste buds, as shown by others.

Overcoming the inhibitors of myelin with a novel neurotrophin strategy

Myelin inhibitors activate a p75(NTR)-dependent signaling cascade in neurons that not only inhibits axonal growth but also prevents neurotrophins (NT) from stimulating growth. Most intriguingly, in addition to Trk receptors, neurotrophins also bind to p75(NTR). We have designed a "mini-neurotrophin" called B(AG) to activate TrkB in the absence of p75(NTR) binding. We find that B(AG) is as effective as the natural TrkB ligands (brain-derived neurotrophic factor (BDNF) and NT-4) at promoting neurite outgrowth from cerebellar neurons. Furthermore, the neurite outgrowth responses stimulated by BDNF and B(AG) are inhibited by a common set of reagents, including the Trk receptor inhibitor K252a, as well as protein kinase A and phosphoinositide 3-kinase inhibitors. However, in contrast to BDNF, B(AG) promotes growth in the presence of a myelin inhibitor or when antibodies directly activate the p75(NTR) inhibitory pathway. On the basis of this observation, we postulated that the binding of BDNF to the p75(NTR) might compromise the ability of BDNF to stimulate neurite outgrowth in an inhibitory environment. To test this, we used NGF, and an NGF-derived peptide, to compete for the BDNF/p75(NTR) interaction; remarkably, in the presence of either agent, BDNF acquired the ability to promote neurite outgrowth in the presence of a myelin inhibitor. The data suggest that in an inhibitory environment, the BDNF/p75(NTR) interaction compromises regeneration. Agents that activate Trk receptors in the absence of p75(NTR) binding, or agents that inhibit neurotrophin/p75(NTR) binding, might therefore be better therapeutic candidates than neurotrophins.

Before the loss: neuronal dysfunction in Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) disease is an autosomal recessive disorder caused by mutations in either the NPC1 or HE1 genes. Hallmarks of this presently incurable disease include abnormal intracellular accumulation of cholesterol and glycosphingolipids, progressive neuropathology and neurodegeneration, and premature death. There have been increased efforts to understand the effects of NPC disease on neurons of the brain, in part due to the recent development of improved research tools and reagents, and in part due to the rapidly growing appreciation of the importance of cholesterol and lipoproteins in the brain during neuronal development, function, and degeneration. Here, we highlight fundamental aspects of neurons that appear to be affected by NPC disease, including their morphology, metabolism, intracellular transport, electrical signaling, and response to environmental factors, and suggest other potentially important areas for future investigation. This provides a framework for acquiring additional insight to this disorder and shaping new therapeutic approaches to NPC disease.

Ternary complex with Trk, p75, and an ankyrin-rich membrane spanning protein

Neurotrophins play many critical roles in regulating neuronal plasticity, survival, and differentiation in the nervous system. Neurotrophins recognize two different receptors, the Trk receptor tyrosine kinase and the p75 neurotrophin receptor, which are associated closely. Several adaptor proteins are associated with each receptor. An ankyrin-rich membrane spanning protein (ARMS), originally identified as a substrate for protein kinase D (Kidins220) and as a p75 interacting protein, serves as a novel downstream target of Trk receptor tyrosine kinases. Kidins220/ARMS is co-expressed frequently with Trk and p75 and represents the only membrane-associated protein known to interact with both receptors. We report here that a ternary complex can be formed between Trk, p75, and Kidins220/ARMS. The extracellular domains of the TrkA and the p75 receptors are necessary for their association, whereas the juxtamembrane region of p75 was responsible for the interaction with Kidins220/ARMS. Interestingly, increasing the level of Kidins220/ARMS expression resulted in a decreased association of TrkA with p75. These findings thus suggest that Kidins220/ARMS plays an important role in regulating interactions between Trk and p75 neurotrophin receptors.

Depolarization activates ERK and proline-rich tyrosine kinase 2 (PYK2) independently in different cellular compartments in hippocampal slices

In the hippocampus, extracellular signal-regulated kinase (ERK) and the non-receptor protein proline-rich tyrosine kinase 2 (PYK2) are activated by depolarization and involved in synaptic plasticity. Both are also activated under pathological conditions following ischemia, convulsions, or electroconvulsive shock. Although in non-neuronal cells PYK2 activates ERK through the recruitment of Src-family kinases (SFKs), the link between these pathways in the hippocampus is not known. We addressed this question using K(+)-depolarized rat hippocampal slices. Depolarization increased the phosphorylation of PYK2, SFKs, and ERK. These effects resulted from Ca(2+) influx through voltage-gated Ca(2+) channels and were diminished by GF109203X, a protein kinase C inhibitor. Inhibition of SFKs with PP2 decreased PYK2 tyrosine phosphorylation dramatically, but not its autophosphorylation on Tyr-402. Moreover, PYK2 autophosphorylation and total tyrosine phosphorylation were profoundly altered in fyn-/- mice, revealing an important functional relationship between Fyn and PYK2 in the hippocampus. In contrast, ERK activation was unaltered by PP2, Fyn knock-out, or LY294002, a phosphatidyl-inositol-3-kinase inhibitor. ERK activation was prevented by MEK inhibitors that had no effect on PYK2. Immunofluorescence of hippocampal slices showed that PYK2 and ERK were activated in distinct cellular compartments in somatodendritic regions and nerve terminals, respectively, with virtually no overlap. Activation of ERK was critical for the rephosphorylation of a synaptic vesicle protein, synapsin I, following depolarization, underlining its functional importance in nerve terminals. Thus, in hippocampal slices, in contrast to cell lines, depolarization-induced activation of non-receptor tyrosine kinases and ERK occurs independently in distinct cellular compartments in which they appear to have different functional roles.

Regulation of late-phase LTP and long-term memory in normal and aging hippocampus: role of secreted proteins tPA and BDNF

Long-lasting forms of memory are generally believed to be mediated by protein synthesis-dependent, late-phase long-term potentiation (L-LTP). L-LTP exhibits at least two distinctive characteristics compared with early phase LTP (E-LTP): synaptic growth and requirement of gene transcription and new protein synthesis. In this review, we discuss the cellular and molecular mechanisms underlying the structural and functional changes of hippocampal synapses during L-LTP, in the context of long-term memory. We describe experiments that reveal the critical role of cAMP/protein kinase A and MAP kinase pathways, and the downstream transcription factor CREB. Because transcription-dependent long-term changes are input specific, we also discuss the role of "local protein synthesis" and "synaptic tagging" mechanisms that may confer synapse specificity. We then focus on brain-derived neurotrophic factor (BDNF) and tissue plasminogen activator (tPA), two secreted proteins that have been repeatedly implicated in L-LTP. Biochemical and molecular biology experiments indicate that the expression and secretion of both factors are enhanced by strong tetanic stimulation that induces L-LTP as well as by training in hippocampal-dependent memory tasks. Inhibition of either tPA or BDNF by gene knockout and specific inhibitors results in a significant impairments in L-LTP and long-term memory. Further work will be required to address the relationship between BDNF and tPA in various forms of synaptic plasticity, and the mechanisms by which BDNF/tPA achieves synapse-specific modulation. Finally, we discuss how the aging process affects L-LTP and long-term memory.

The p75 neurotrophin receptor appears in plasma in diabetic rats-characterisation of a potential early test for neuropathy

AIMS/HYPOTHESIS

This study tested the premise that immunoreactivity representing the p75 neurotrophin receptor (p75(NTR)) appears in plasma of diabetic rats in association with the early stages of neuronal dysfunction or damage. We also examined whether treatment beneficial to neuropathy might reduce the p75(NTR) immunoreactivity.

METHODS

Plasma proteins were fractionated by SDS-PAGE and immunoblots exposed to p75(NTR) antibody, using receptor protein from cultured PC12 cells as an external standard. Rats were made diabetic with streptozotocin for various periods and exsanguinated. Plasma glucose, HbA(1)c and plasma proteins were determined. We also studied plasma samples from diabetic mice lacking the gene coding for p75(NTR), as well as the effect of sciatic nerve crush on healthy male Wistar rats.

RESULTS

Plasma p75(NTR) immunoreactivity began to exceed normal levels at 8 weeks after induction of diabetes, and was significantly raised at 10 (p<0.05) and 12 weeks (p<0.001). Treatment between 8 and 12 weeks with insulin, fidarestat (an aldose reductase inhibitor), nerve growth factor and neurotrophin 3 all normalised the plasma p75(NTR) immunoreactivity. Plasma from p75(NTR) (-/-) mice contained no such immunoreactivity, though it was present in plasma from wild-type mice. Following nerve crush, p75(NTR) immunoreactivity appeared in plasma of non-diabetic mice, indicating that this can be a result of nerve trauma.

CONCLUSIONS/INTERPRETATION

These observations suggest that plasma p75(NTR) immunoreactivity may serve as an early indicator of neuronal dysfunction or damage in diabetes. The time course of its appearance relates well to that of early neuropathy and its response to interventions that are neuroprotective suggests that it might mirror neurological status.

A neural signaling triumvirate that influences ageing and age-related disease: insulin/IGF-1, BDNF and serotonin

The ageing process and its associated diseases all involve perturbed energy metabolism, oxidative damage, and an impaired ability of the organism and its cells to cope with adversity. We propose that some specific signaling pathways in the brain may be important determinants of health during ageing. Among such specific signaling modalities are those activated in neurons by insulin-like growth factors (IGFs), brain-derived neurotrophic factor (BDNF) and serotonin. This triumvirate may be particularly important because of their cooperative influence on energy metabolism, food intake, stress responses and cardiovascular function. The health benefits to the periphery and central nervous system of dietary restriction and exercise may be mediated by this triumvirate of signals in the brain. At the molecular level, BDNF, serotonin and IGFs can all stimulate the production of proteins involved in cellular stress adaptation, growth and repair, neurogenesis, learning and memory and cell survival. The importance of this triumvirate is emphasized when it is seen that their general roles in energy metabolism, stress adaptation and disease resistance are conserved among diverse organisms consistent with important roles in the ageing process.

Nerve growth factor signaling regulates motility and docking of axonal mitochondria

Axonal transport is thought to distribute mitochondria to regions of the neuron where their functions are required. In cultured neurons, mitochondrial transport responds to growth cone activity, and this involves both a transition between motile and stationary states of mitochondria and modulation of their anterograde transport activity. Although the exact cellular signals responsible for this regulation remain unknown, we recently showed that mitochondria accumulate in sensory neurons at regions of focal stimulation with NGF and suggested that this involves downstream kinase signaling. Here, we demonstrate that NGF regulation of axonal organelle transport is specific to mitochondria. Quantitative analyses of motility show that the accumulation of axonal mitochondria near a focus of NGF stimulation is due to increased movement into bead regions followed by inhibition of movement out of these regions and that anterograde and retrograde movement are differentially affected. In axons made devoid of F-actin by latrunculin B treatment, bidirectional transport of mitochondria continues, but they can no longer accumulate in the region of NGF stimulation. These results indicate that intracellular signaling can specifically regulate mitochondrial transport in neurons, and they suggest that axonal mitochondria can respond to signals by locally altering their transport behavior and by undergoing docking interactions with the actin cytoskeleton.

Interaction of Brn3a and HIPK2 mediates transcriptional repression of sensory neuron survival

The Pit1-Oct1-Unc86 domain (POU domain) transcription factor Brn3a controls sensory neuron survival by regulating the expression of Trk receptors and members of the Bcl-2 family. Loss of Brn3a leads to a dramatic increase in apoptosis and severe loss of neurons in sensory ganglia. Although recent evidence suggests that Brn3a-mediated transcription can be modified by additional cofactors, the exact mechanisms are not known. Here, we report that homeodomain interacting protein kinase 2 (HIPK2) is a pro-apoptotic transcriptional cofactor that suppresses Brn3a-mediated gene expression. HIPK2 interacts with Brn3a, promotes Brn3a binding to DNA, but suppresses Brn3a-dependent transcription of brn3a, trkA, and bcl-xL. Overexpression of HIPK2 induces apoptosis in cultured sensory neurons. Conversely, targeted deletion of HIPK2 leads to increased expression of Brn3a, TrkA, and Bcl-xL, reduced apoptosis and increases in neuron numbers in the trigeminal ganglion. Together, these data indicate that HIPK2, through regulation of Brn3a-dependent gene expression, is a critical component in the transcriptional machinery that controls sensory neuron survival.

Neurotrophins induce short-term and long-term changes of cortical neurotrophin expression

Neuronal activity, hormones, transmitters, physical exercise and enrichment influence cortical neurotrophin expression. Neurotrophins then elicit structural and physiological changes, and regulate gene expression. This prompted the hypothesis that neurotrophins themselves are involved in regulating neurotrophin expression. Here we investigated the mRNA expression level of brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), NT-3 and nerve growth factor (NGF) as well as the tyrosine receptor kinases TrkB and TrkC receptor in response to BDNF, NT-4, NT-3 and NGF pulses in organotypic cortex cultures. Single neurotrophin pulses evoked a dramatic up- or down-regulation of some, but not all four, neurotrophin mRNAs, even within 3-24 h, indicating an immediate impact on neurotrophin transcription. Most strikingly, neurotrophin pulses during the first 10 days in vitro (DIV) potentiated the expression of some neurotrophin mRNAs at 20 DIV, suggesting that early trophic factor experience influences the expression levels seen later in development. The NT-3 mRNA expression, for example, was consistently promoted by NGF and BDNF, suggesting that these two factors help to maintain the low level of NT-3 found in adult cortex. Rapid bidirectional changes characterized the NT-4 mRNA expression. A single pulse of NT-4 transiently increased NT-4 mRNA, whereas a BDNF pulse transiently reduced NT-4 mRNA. Surprisingly, NGF strongly potentiated BDNF mRNA and in particular NT-4 mRNA. By contrast, TrkB mRNA remained constant at ages or time points at which other mRNAs amplified from the very same cDNA libraries revealed dramatic increases or decreases. Our study suggests the existence of a complex regulatory neurotrophin network controlling the expression of other neurotrophins.

Nesca, a novel adapter, translocates to the nuclear envelope and regulates neurotrophin-induced neurite outgrowth

We provide the first characterization of a novel signaling adapter, Nesca, in neurotrophic signal transduction. Nesca contains a RUN domain, a WW domain, a leucine zipper, a carboxyl-terminal SH3 domain, and several proline-rich regions. Nesca is highly expressed in the brain, is serine phosphorylated, and mobilizes from the cytoplasm to the nuclear membrane in response to neurotrophin, but not epidermal growth factor, stimulation in a MEK-dependent process. Overexpression studies in PC12 cells indicate that Nesca facilitates neurotrophin-dependent neurite outgrowth at nonsaturating doses of nerve growth factor (NGF). Similarly, short interfering RNA studies significantly reduce NGF-dependent neuritogenesis in PC12 cells. Mutational analyses demonstrate that the RUN domain is an important structural determinant for the nuclear translocation of Nesca and that the nuclear redistribution of Nesca is essential to its neurite outgrowth-promoting properties. Collectively, these works provide the first functional characterization of Nesca in the context of neurotrophin signaling and suggest that Nesca serves a novel, nuclear-dependent role in neurotrophin-dependent neurite outgrowth.

Suppression of anoikis and induction of metastasis by the neurotrophic receptor TrkB

Metastasis is a major factor in the malignancy of cancers, and is often responsible for the failure of cancer treatment. Anoikis (apoptosis resulting from loss of cell-matrix interactions) has been suggested to act as a physiological barrier to metastasis; resistance to anoikis may allow survival of cancer cells during systemic circulation, thereby facilitating secondary tumour formation in distant organs. In an attempt to identify metastasis-associated oncogenes, we designed an unbiased, genome-wide functional screen solely on the basis of anoikis suppression. Here, we report the identification of TrkB, a neurotrophic tyrosine kinase receptor, as a potent and specific suppressor of caspase-associated anoikis of non-malignant epithelial cells. By activating the phosphatidylinositol-3-OH kinase/protein kinase B pathway, TrkB induced the formation of large cellular aggregates that survive and proliferate in suspension. In mice, these cells formed rapidly growing tumours that infiltrated lymphatics and blood vessels to colonize distant organs. Consistent with the ability of TrkB to suppress anoikis, metastases--whether small vessel infiltrates or large tumour nodules--contained very few apoptotic cells. These observations demonstrate the potent oncogenic effects of TrkB and uncover a specific pro-survival function that may contribute to its metastatic capacity, providing a possible explanation for the aggressive nature of human tumours that overexpress TrkB.

Brain-derived neurotrophic factor and trkB signaling in parasympathetic neurons: relevance to regulating alpha7-containing nicotinic receptors and synaptic function

Parasympathetic neurons do not require neurotrophins for survival and are thought to lack high-affinity neurotrophin receptors (i.e., trks). We report here, however, that mRNAs encoding both brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tropomyosin-related kinase B (trkB) are expressed in the parasympathetic chick ciliary ganglion (CG) and that BDNF-like protein is present in the ganglion and in the iris, an important peripheral target of ciliary neurons. Moreover, CG neurons express surface trkB and exogenous BDNF not only initiates trk-dependent signaling, but also alters nicotinic acetylcholine receptor (nAChR) expression and synaptic transmission. In particular, BDNF applied to CG neurons rapidly activates cAMP-dependent response element-binding protein (CREB), and over the long-term selectively upregulates expression of alpha7-subunit-containing, homomeric nAChRs (alpha7-nAChRs), increasing alpha7-subunit mRNA levels, alpha7-nAChR surface sites, and alpha7-nAChR-mediated whole-cell currents. At nicotinic synapses formed on CG neurons in culture, brief and long-term BDNF treatments also increase the frequency of spontaneous EPSCs, most of which are mediated by heteromeric nAChRs containing alpha3, alpha5, beta4, and beta2 subunits (alpha3*-nAChRs) with a minor contribution from alpha7-nAChRs. Our findings demonstrate unexpected roles for BDNF-induced, trk-dependent signaling in CG neurons, both in regulating expression of alpha7-nAChRs and in enhancing transmission at alpha3*-nAChR-mediated synapses. The presence of BDNF-like protein in CG and iris target coupled with that of functional trkB on CG neurons raise the possibility that signals generated by endogenous BDNF similarly influence alpha7-nAChRs and nicotinic synapses in vivo.

p75 neurotrophin receptor signaling regulates growth cone filopodial dynamics through modulating RhoA activity

The mechanisms by which neurotrophins regulate growth cone motility are unclear. We investigated the role of the p75 neurotrophin receptor (p75NTR) in mediating neurotrophin-induced increases in filopodial length. Our data demonstrate that neurotrophin binding to p75NTR is necessary and sufficient to regulate filopodial dynamics. Furthermore, retinal and dorsal root ganglion growth cones from p75 mutant mice are insensitive to neurotrophins but display enhanced filopodial lengths comparable with neurotrophin-treated wild-type growth cones. This suggests unoccupied p75NTR negatively regulates filopodia length. Furthermore, p75NTR regulates RhoA activity to mediate filopodial dynamics. Constitutively active RhoA blocks neurotrophin-induced increases in filopodial length, whereas inhibition of RhoA enhances filopodial lengths, similar to neurotrophin treatment. BDNF treatment of retinal neurons results in reduced RhoA activity. Furthermore, p75 mutant neurons display reduced levels of activated RhoA compared with wild-type counterparts, consistent with the enhanced filopodial lengths observed on mutant growth cones. These observations suggest that neurotrophins regulate filopodial dynamics by depressing the activation of RhoA that occurs through p75NTR signaling.

Functionally intact glutamate-mediated signaling in bipolar cells of the TRKB knockout mouse retina

In the juvenile trkB knockout (trkB−/−) mouse, retina synaptic communication from rods to bipolar cells is severely compromised as evidenced by a complete absence of electroretinogram (ERG)b-wave, even though the inner retina appears anatomically normal (Rohrer et al., 1999). Since it is well known that theb-wave reflects light-dependent synaptic activation of ON bipolar cellsviatheir metabotropic glutamate receptor, mGluR6, we sought to analyze the anatomical and functional integrity of the glutamatergic synapses at these and other bipolar cells in thetrkB−/−mouse. Although rod bipolar cells from wild-type juvenile mice were determined to be immunopositive for trkB, postsynaptic metabotropic and ionotropic glutamate receptor-mediated pathways in ON and OFF bipolar cells were found to be functionally intact, based on patch electrode recordings, using brief applications (“puffs”) of glutamate or its analog, 2-amino-4-phosphonobutyric acid (APB), a selective agonist for mGluR6 receptors. Ionotropic glutamate receptor function was assayed in OFF-cone bipolar and horizontal cells by applying exogenous glutamatergic agonists in the presence of the channel-permeant guanidinium analogue, 1-amino-4-guanidobutane (AGB). Electron-microscopic analysis revealed that the ribbon synapses between rods and postsynaptic rod bipolar and horizontal cells were formed at the appropriate age and appear to be structurally intact, and immunohistochemical analysis did not detect profound defects in the expression of excitatory amino acid transporters involved in glutamate clearance from the synaptic cleft. These data indicate that there does not appear to be evidence for postsynaptic deficits in glutamatergic signaling in the ON and OFF bipolar cells of mice lacking trkB.