Trk receptors: roles in neuronal signal transduction

A unique pathway for sustained neurotrophin signaling through an ankyrin-rich membrane-spanning protein

A major question in cell biology is how molecular specificity is achieved by different growth factor receptors that activate apparently identical signaling events. For the neurotrophin family, a distinguishing feature is the ability to maintain a prolonged duration of signal transduction. However, the mechanisms by which neurotrophin receptors assemble such a sustained signaling complex are not understood. Here we report that an unusual ankyrin-rich transmembrane protein (ARMS+kidins220) is closely associated with Trk receptor tyrosine kinases, and not the EGF receptor. This association requires interactions between transmembrane domains of Trk and ARMS. ARMS is rapidly tyrosine phosphorylated after binding of neurotrophins to Trk receptors and provides a docking site for the CrkL-C3G complex, resulting in Rap1-dependent sustained ERK activation. Accordingly, disruption of Trk-ARMS or the ARMS-CrkL interaction with dominant-negative ARMS mutants, or treatment with small interference RNA against ARMS substantially reduce neurotrophin-elicited signaling to ERK, but without any effect upon Ras or Akt activation. These findings suggest that ARMS acts as a major and neuronal-specific platform for prolonged MAP kinase signaling by neurotrophins.

Calcium Activation of ERK Mediated by Calmodulin Kinase I

Elevated intracellular Ca(2+) triggers numerous signaling pathways including protein kinases such as the calmodulin-dependent kinases (CaMKs) and the extracellular signal-regulated kinases (ERKs). In the present study we examined Ca(2+)-dependent "cross-talk" between these two protein kinase families. Using a combination of pharmacological inhibitors and dominant-negative kinases (dnKinase), we identified a requirement for CaMKK acting through CaMKI in the stimulation of ERKs upon depolarization of the neuroblastoma cell line, NG108. Depolarization stimulated prolonged ERK and JNK activation that was blocked by the CaMKK inhibitor, STO-609; this inhibition of ERK activation by STO-609 was rescued by expression of a STO-609-insensitive mutant of CaMKK. However, activation of ERK by epidermal growth factor or carbachol were not suppressed by inhibition of CaMKK, indicating specificity for this "cross-talk." To identify the downstream target of CaMKK that mediated ERK activation upon depolarization, dnKinases were expressed. The dnCaMKI completely suppressed ERK2 activation whereas dnAKT/PKB or nuclear-targeted dnCaMKIV, other substrates for CaMKK, were not inhibitory. ERK activation upon depolarization or transfection with constitutively active (ca) CaMKI was blocked by dnRas. Additionally, depolarization of NG108 cells promoted neurite outgrowth, and this effect was blocked by inhibition of either CaMKK (STO-609) or ERK (UO126). Co-transfection with caCaMKK plus caCaMKI also stimulated neurite outgrowth that was blocked by inhibition of ERK (UO126). These data are the first to suggest that ERK activation and neurite outgrowth in response to depolarization are mediated by CaMKK activation of CaMKI.

Alsin Is a Rab5 and Rac1 Guanine Nucleotide Exchange Factor

ALS2 is the gene mutated in a recessive juvenile form of amyotrophic lateral sclerosis (ALS2). ALS2 encodes a large protein termed alsin, which contains a number of predicted cell signaling and protein trafficking sequence motifs. To gain insight into the overall function of alsin and to begin to evaluate its role in motor neuron maintenance, we examined the subcellular localization of alsin and the biochemical activities associated with its individual subdomains. We found that the Vps9p domain of alsin has Rab5 guanine nucleotide exchange activity. In addition, alsin interacted specifically with and acted as a guanine nucleotide exchange factor for Rac1. Immunofluorescence and fractionation experiments in both fibroblasts and neurons revealed that alsin is a cytosolic protein, with a significant portion associated with small, punctate membrane structures. Many of these membrane structures also contained Rab5 or Rac1. Upon overexpression of full-length alsin, the overexpressed material was largely cytosolic, indicating that the association with membrane structures could be saturated. We also found that alsin was present in membrane ruffles and lamellipodia. These data suggest that alsin is involved in membrane transport events, potentially linking endocytic processes and actin cytoskeleton remodeling.

Paracrine and autocrine functions of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in brain-derived endothelial cells

Brain-derived neurotrophic factor (BDNF) is expressed by endothelial cells. We investigated the characteristics of BDNF expression by brain-derived endothelial cells and tested the hypothesis that BDNF serves paracrine and autocrine functions affecting the vasculature of the central nervous system. In addition to expressing TrkB and p75NTR and BDNF under normoxic conditions, these cells increased their expression of BDNF under hypoxia. While the expression of TrkB is unaffected by hypoxia, TrkB exhibits a base-line phosphorylation under normoxic conditions and an increased phosphorylation when BDNF is added. TrkB phosphorylation is decreased when endogenous BDNF is sequestered by soluble TrkB. Exogenous BDNF elicits robust angiogenesis and survival in three-dimensional cultures of these endothelial cells, while sequestration of endogenous BDNF caused significant apoptosis. The effects of BDNF engagement of TrkB appears to be mediated via the phosphatidylinositol (PI) 3-kinase-Akt pathway. Modulation of BDNF levels directly correlate with Akt phosphorylation and inhibitors of PI 3-kinase abrogate the BDNF responses. BDNF-mediated effects on endothelial cell survival/apoptosis correlated directly with activation of caspase 3. These endothelial cells also express p75NTR and respond to its preferred ligand, pro-nerve growth factor (pro-NGF), by undergoing apoptosis. These data support a role for neurotrophins signaling in the dynamic maintenance/differentiation of central nervous system endothelia.

Structure of nerve growth factor complexed with the shared neurotrophin receptor p75

Neurotrophins are secreted growth factors critical for the development and maintenance of the vertebrate nervous system. Neurotrophins activate two types of cell surface receptors, the Trk receptor tyrosine kinases and the shared p75 neurotrophin receptor. We have determined the 2.4 A crystal structure of the prototypic neurotrophin, nerve growth factor (NGF), complexed with the extracellular domain of p75. Surprisingly, the complex is composed of an NGF homodimer asymmetrically bound to a single p75. p75 binds along the homodimeric interface of NGF, which disables NGF's symmetry-related second p75 binding site through an allosteric conformational change. Thus, neurotrophin signaling through p75 may occur by disassembly of p75 dimers and assembly of asymmetric 2:1 neurotrophin/p75 complexes, which could potentially engage a Trk receptor to form a trimolecular signaling complex.

Sensitization of recombinant vanilloid receptor-1 by various neurotrophic factors

The vanilloid receptor (VR1) is a central integrator molecule of nociceptive stimuli. In this study, we have measured the effects of various neurotrophins (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and -4) on recombinant rat VR1-mediated intracellular calcium rise in response to capsaicin in VR1/C6 cells. Our results clearly show that all neurotrophins sensitize the VR1 to capsaicin. Furthermore, using K252a, an inhibitor of tyrosine kinases, we present that actions of neurotrophins are mediated by the trk (A, B, C) receptors expressed in these cells. These data argue for the putative roles of neurotrophins in inducing inflammatory (thermal) hyperalgesia via VR1.

Mechanisms of neurotrophin receptor vesicular transport

Accumulating evidence has indicated that neurotrophin receptor trafficking plays an important role in neurotrophin-mediated signaling in developing as well as mature neurons. However, little is known about the molecular mechanisms and the components of neurotrophin receptor vesicular transport. This article will describe how neurotrophin receptors, Trk and p75 neurotrophin receptor (p75NTR), are intimately involved in the axonal transport process. In particular, the molecules that may direct Trk receptor trafficking in the axon will be discussed. Finally, potential mechanisms by which receptor-containing vesicles link to molecular cytoskeletal motors will be presented.

Microarray analysis of developmental plasticity in monkey primary visual cortex

We performed microarray gene expression analyses on the visual cortex of Old-World monkeys (Cercopithicus aethiops) in an effort to identify transcripts associated with developmental maturation and activity-driven changes during the visual critical period. Samples derived from normal animals and those subjected to monocular enucleation (ME) were hybridized to human Affymetrix HG-U95Av2 oligonucleotide microarrays (N = 12) and the results were independently validated by real-time quantitative RT-PCR. To identify genes exhibiting significant expression differences among our samples, the microarray hybridization data were processed with two software packages that use different analytical models (Affymetrix MicroArray Suite 5.0, dChip 1.2). We identified 108 transcripts within diverse functional categories that differed in their visual cortical expression at the height of the critical period when compared to adults. The expression levels of four transcripts were also globally modulated following ME during the critical period. These transcripts are particularly sensitive to ME during the critical period but are not significantly modulated in ME adults. Three of the ME-driven genes (NGFI-B, egr3, NARP) are known immediate-early genes (IEG) while the other (DUSP6) is a phosphatase that can regulate IEG expression. The putative biological significance of the ME-driven and developmentally regulated genes is discussed with respect to the critical period for activity-dependent visual cortical neuroplasticity.

Perturbed signal transduction in neurodegenerative disorders involving aberrant protein aggregation

Aggregation of abnormal proteins, both inside and outside of cells, is a prominent feature of major neurodegenerative disorders, including Alzheimer's, Parkinson's, polyglutamine expansion, and prion diseases. Other articles in this special issue of NeuroMolecular Medicine describe the genetic and molecular factors that promote aberrant protein aggregation. In the present article, we consider how it is that pathogenic aggregation-prone proteins compromise signal transduction pathways that regulate neuronal plasticity and survival. In some cases the protein in question may have widespread and relatively nonspecific effects on signaling. For example, amyloid beta-peptide induces membrane-associated oxidative stress, which impairs the function of various receptors, ion channels and transporters, as well as downstream kinases and transcription factors. Other proteins, such as polyglutamine repeat proteins, may affect specific protein -protein interactions, including those involved in signaling pathways activated by neurotransmitters, neurotrophins, and steroid hormones. Synapses are particularly sensitive to abnormal protein aggregation and impaired synaptic signaling may trigger apoptosis and related cell death cascades. Impairment of signal transduction in protein aggregation disorders may be amenable to therapy as demonstrated by a recent study showing that dietary restriction can preserve synaptic function and protect neurons in a mouse model of Huntington's disease. Finally, emerging findings are revealing how activation of certain signaling pathways can suppress protein aggregation and/or the cytotoxicity resulting from the abnormal protein aggregation. A better understanding of how abnormal protein aggregation occurs and how it affects and is affected by specific signal transduction pathways, is leading to novel approaches for preventing and treating neurodegenerative disorders.

Expression of constitutively active p21H-rasval12 in postmitotic pyramidal neurons results in increased dendritic size and complexity

The small G protein p21Ras is a critical molecular switch for relaying neurotrophic actions and is essential for normal functioning and plasticity of the nervous system. In this study, the morphogenetic effects of p21Ras were investigated on neurons in vivo. Morphological changes of layers II/III and Vb commissural pyramidal neurons of the primary somatosensory cortex were analyzed in transgenic mice expressing permanently active p21H-RasVal12 in postmitotic neurons. Pyramidal cells were retrogradely labelled with biotinylated dextran amine and subsequently traced using Neurolucida. Compared with wild-type mice, transgenic animals showed a significant increase in the surface area and volume of basal dendrites on the proximal and intermediate segments in layers II/III and on further distal segments in layer V. In addition, the surface area and volume of the trunk and of the proximal segments of oblique branches of apical dendrites were enlarged in both layers. Sholl analyses of basal and apical dendrites showed a significant increase in dendritic complexity of layer V neurons. A positive correlation was observed between the size of the basal dendrite and the neuronal soma size in the transgenic population, indicating that growth-promoting effects of p21H-RasVal12 affect both cellular compartments in parallel. However, the dendritic surface correlated with the number of tips and dendritic stem diameter in both wild-type and transgenic populations, demonstrating that these relations represent rather conservative design principles in dendritic morphology. The data presented here suggest an important role of p21Ras-dependent signaling in the final differentiation and maintenance of dendritic morphology.

Involvement of adenylate cyclase and tyrosine kinase signaling pathways in response of crayfish stretch receptor neuron and satellite glia cell to photodynamic treatment

Neuroglial interactions are most profound during development or damage of nerve tissue. We studied the responses of crayfish stretch receptor neurons (SRN) and satellite glial cells to photosensitization with sulfonated aluminum phthalocyanine Photosens. Although Photosens was localized mainly in the glial envelope, neurons were very sensitive to photodynamic treatment. Photosensitization gradually inhibited and then abolished neuron activity. Neuronal and glial nuclei shrank. Some neurons and glial cells lost the integrity of the plasma membrane and died through necrosis after the treatment. The nuclei of other glial cells but not neurons become fragmented, indicating apoptosis. The number of glial nuclei around neuron soma increased, probably indicating proliferation for enhanced neuron protection. Adenylate cyclase (AC) inhibition by MDL-12330A, or tyrosine kinase (TK) inhibition by genistein, shortened neuron lifetime, whereas AC activation by forskolin or protein tyrosine phosphatases (PTP) inhibition by sodium orthovanadate prolonged neuronal activity. Therefore, cAMP and phosphotyrosines produced by AC and TK, respectively, protected SRN against photoinactivation. AC inhibition reduced photodamage of the plasma membrane and subsequent necrosis in neuronal and glial cells. AC activation prevented apoptosis in photosensitized glial cells and stimulated glial proliferation. TK inhibition protected neurons but not glia against photoinduced membrane permeabilization and subsequent necrosis whereas PTP inhibition more strongly protected glial cells. Therefore, both signaling pathways involving cAMP and phosphotyrosines might contribute to the maintenance of neuronal activity and the integrity of the neuronal and glial plasma membranes. Adenylate cyclase but not phosphotyrosine signaling pathways modulated glial apoptosis and proliferation under photooxidative stress.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

Selective regulation of 14-3-3? in primary culture of cerebral cortical neurons and astrocytes during development

The 14-3-3 proteins exist predominantly in the brain and may play regulatory roles in cellular processes of growth, differentiation, survival, and apoptosis. The biological functions, however, of the various 14-3-3 isoforms (beta, epsilon, eta, gamma, and zeta) in the brain remain unclear. We have reported previously upregulation of 14-3-3gamma in ischemic astrocytes. In the present study, we report selective regulation of 14-3-3eta in cultured cerebral cortical neurons and astrocytes during in vitro development. In cultured neurons, gene expression levels of 14-3-3eta increase with culture age (0-10 days). Brain-derived neurotrophic factor and neurotrophin-3 upregulate 14-3-3eta gene expression. In cultured astrocytes, 14-3-3eta is downregulated with culture age (1-5 weeks). The gene expression level of 14-3-3eta is not affected by scratch injury in astrocytes or by ischemia in neurons. These data suggest a possible role of 14-3-3eta in growth and differentiation of neurons and astrocytes, indicating an intricate mechanism governing coordinated and well-controlled developmental events in the brain to ensure normal neural functions.

Neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase pathway

During development and nerve injury, complex interactions between glial cells and neurons are essential for establishing proper nerve function. Neurotrophins play multiple roles in the developing nervous system, including cell survival, growth, and differentiation. Here we show that migration of Schwann cells, isolated from sciatic nerves, is significantly enhanced by neurotrophin 3, but not by nerve growth factor or brain-derived neurotrophic factor. The neurotrophin-3-induced cell migration was also observed in Schwann cells isolated from sciatic nerves of p75NTR-/- mice, indicating that neurotrophin 3 enhances cell migration through TrkC. This effect was blocked by K252a, an inhibitor of the Trk receptor family. Additionally, the neurotrophin-3-induced cell migration depended on Rho GTPases (Rac1 and Cdc42) and c-Jun N-terminal kinase. We obtained the same results with Cos-7 cells expressing TrkC. Taken together, these results suggest that neurotrophin 3 activation of TrkC induces Schwann cell migration through the c-Jun N-terminal kinase signaling pathway.

Constitutive expression of p21H-Rasval12in neurons induces increased axonal size and dendritic microtubule density in vivo

The small G protein p21Ras is a key signal transducer mediating cellular growth and proliferation responses to extracellular stimuli. We investigated by electron microscopy the effects of augmented p21Ras activity on neuronal processes and microtubule arrangement in vivo. We used transgenic mice with a neuron-specific overexpression of p21H-RasVal12, which starts postnatally around Day 15. Axonal and dendritic diameters and the numerical density of dendritic microtubules were analyzed at postnatal Day 12 before the onset of transgene expression and in adult mice. In adult transgenic mice, calibers of both axons (corpus callosum) and dendrites (layers II/III of somatosensory cortex) were enlarged by about 57% and 79%, respectively. The increase in dendritic calibers was associated with an increment in the amount of microtubules. Even in dendrites of equivalent diameters, the number of microtubules was higher in transgenic mice compared to that in wild-type mice suggesting an elevated microtubule density. Changes in process diameters or microtubule density were not observed at postnatal Day 12 before relevant transcription of transgenic p21H-RasVal12. The present results extend previous findings on neuronal hypertrophy as a consequence of p21H-RasVal12 expression and suggest a profound influence on the dendritic microtubule network.

Model of autism: increased ratio of excitation/inhibition in key neural systems

Autism is a severe neurobehavioral syndrome, arising largely as an inherited disorder, which can arise from several diseases. Despite recent advances in identifying some genes that can cause autism, its underlying neurological mechanisms are uncertain. Autism is best conceptualized by considering the neural systems that may be defective in autistic individuals. Recent advances in understanding neural systems that process sensory information, various types of memories and social and emotional behaviors are reviewed and compared with known abnormalities in autism. Then, specific genetic abnormalities that are linked with autism are examined. Synthesis of this information leads to a model that postulates that some forms of autism are caused by an increased ratio of excitation/inhibition in sensory, mnemonic, social and emotional systems. The model further postulates that the increased ratio of excitation/inhibition can be caused by combinatorial effects of genetic and environmental variables that impinge upon a given neural system. Furthermore, the model suggests potential therapeutic interventions.

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.

Marcadores Biológicos da Depressão:

Introdução. A depressão é um dos mais prevalentes transtornos psi­quiátricos e tem sido associada a alterações nas vias de sinalização que regulam a neuroplasticidade e a sobrevivência celular. Objetivo. O objetivo deste estudo foi revisar a relação entre neurotrofinas e mo­delos experimentais de depressão, bem como a forma em que os me­dicamentos antidepressivos podem estar atuando na estimulação da produção dos fatores neurotróficos. Método. Uma revisão de litera­tura através de pesquisa nas bases de dados MEDLINE e SCIELO, utilizando a combinação dos seguintes descritores: modelo animal de depressão, fatores neurotróficos e BDNF, e suas respectivas traduções no idioma inglês. Resultados. Foram apresentados dados que revela­ram que o fator neurotrófico derivado do cérebro apresenta um papel crucial na regulação da atividade sináptica e plasticidade, relacionando a depressão maior com danos nas vias de sinalização celular. Entre­tanto, para a transcrição desta neurotrofina é necessário a ativação de mecanismos de sinalização, incluindo cálcio, CREB, MEK, MeCP2, CaMKII e hormônios. Conclusão. Após esses argumentos foi possível rever o conceito sobre a fisiopatologia e os mecanismos envolvidos para o desenvolvimento da doença e com isso sugerir possíveis estra­tégias a fim de desenvolver novos fármacos para melhorar a eficácia do tratamento.