Faculty Bibliography

Neurotrophin activation of Trk receptors elicits diverse effects on neuronal survival, differentiation, and synaptic plasticity. One of the central questions is how specificity is encoded in neurotrophin receptor signaling and actions. A unique downstream protein is the Ankyrin-Repeat Rich Membrane Spanning (ARMS)/Kinase D-interacting substrate-220 kDa (Kidins220), a very abundant scaffold protein in the hippocampus. To determine the roles of ARMS/Kidins220 in hippocampal neurons, we have analyzed the effects of synaptic activity upon the regulation and distribution of ARMS/Kidins220. At early times in vitro (<7 DIV), synaptic activity was low and ARMS/Kidins220 levels were high. As synaptic activity and markers for synapse maturation, such as PSD-95, increased, ARMS/Kidins220 significantly decreased to a plateau by later times in vitro (>12 DIV). Immunocytochemistry showed ARMS/Kidins220 to be concentrated at the tips of growing processes in immature cultures, and more diffusely distributed in older cultures. To examine the apparent inverse relationship between activity and ARMS/Kidins220 levels, neuronal firing was manipulated pharmacologically. Chronic exposure to TTX increased ARMS/Kidins220 levels, whereas bicuculline caused the opposite effect. Moreover, using shRNA to decrease ARMS/Kidins220 levels produced a corresponding increase in synaptic activity. We find that ARMS/Kidins220 may function in neuronal development as an indicator and potentially as a homeostatic regulator of overall synaptic strength in hippocampal neurons

Neurotrophins are potent survival factors for developing and injured neurons. However, they are not being used to treat neurodegenerative diseases because of difficulties in administration and numerous side effects that have been encountered in previous clinical trials. Their biological activities use Trk (tropomyosin-related kinase) transmembrane tyrosine kinases. Therefore, one alternative approach is to use transactivation pathways such as adenosine 2A receptor agonists, which can activate Trk receptor signaling independent of neurotrophin binding. However, the relevance in vivo and applicability of these transactivation events during neurodegenerative and injury conditions have never been extensively studied. Here we demonstrate that motoneuron survival after facial nerve lesioning is significantly enhanced by transactivation of Trk receptor tyrosine kinases by adenosine agonists. Moreover, survival of motoneurons directly required the activation of the BDNF receptor TrkB and an increase in Akt (AKT8 virus oncogene cellular homolog) activity. The ability of small molecules to activate a trophic response by using Trk signaling provides a unique mechanism to promote survival signals in motoneurons and suggests new strategies for using transactivation in neurodegenerative diseases

Previous studies suggest that brain-derived neurotrophic factor and its receptor TrkB are critically involved in the therapeutic actions of antidepressant drugs. We have previously shown that the antidepressants imipramine and fluoxetine produce a rapid autophosphorylation of TrkB in the rodent brain. In the present study, we have further examined the biochemical and functional characteristics of antidepressant-induced TrkB activation in vivo. We show that all the antidepressants examined, including inhibitors of monoamine transporters and metabolism, activate TrkB rapidly in the rodent anterior cingulate cortex and hippocampus. Furthermore, the results indicate that acute and long-term antidepressant treatments induce TrkB-mediated activation of phospholipase-Cgamma1 (PLCgamma1) and increase the phosphorylation of cAMP-related element binding protein, a major transcription factor mediating neuronal plasticity. In contrast, we have not observed any modulation of the phosphorylation of TrkB Shc binding site, phosphorylation of mitogen-activated protein kinase or AKT by antidepressants. We also show that in the forced swim test, the behavioral effects of specific serotonergic antidepressant citalopram, but not those of the specific noradrenergic antidepressant reboxetine, are crucially dependent on TrkB signaling. Finally, brain monoamines seem to be critical mediators of antidepressant-induced TrkB activation, as antidepressants reboxetine and citalopram do not produce TrkB activation in the brains of serotonin- or norepinephrine-depleted mice. In conclusion, our data suggest that rapid activation of the TrkB neurotrophin receptor and PLCgamma1 signaling is a common mechanism for all antidepressant drugs

Clearance of fibrin through proteolytic degradation is a critical step of matrix remodeling that contributes to tissue repair in a variety of pathological conditions, such as stroke, atherosclerosis, and pulmonary disease. However, the molecular mechanisms that regulate fibrin deposition are not known. Here, we report that the p75 neurotrophin receptor (p75(NTR)), a TNF receptor superfamily member up-regulated after tissue injury, blocks fibrinolysis by down-regulating the serine protease, tissue plasminogen activator (tPA), and up-regulating plasminogen activator inhibitor-1 (PAI-1). We have discovered a new mechanism in which phosphodiesterase PDE4A4/5 interacts with p75(NTR) to enhance cAMP degradation. The p75(NTR)-dependent down-regulation of cAMP results in a decrease in extracellular proteolytic activity. This mechanism is supported in vivo in p75(NTR)-deficient mice, which show increased proteolysis after sciatic nerve injury and lung fibrosis. Our results reveal a novel pathogenic mechanism by which p75(NTR) regulates degradation of cAMP and perpetuates scar formation after injury

The growing realization that neurotrophins, such as brain-derived neurotrophic factor (BDNF), are crucial in modulating synaptic plasticity has broadened the spectrum of their trophic actions. At the same time, it has become clear that Abeta peptides derived from amyloid precursor protein (APP) have dramatic effects on synaptic transmission before the onset of the neurodegenerative disease. Because neurotrophins and Abeta are responsible for affecting both synaptic and cognitive function, it is likely that their mechanisms of action will be related and might even intersect. This review highlights several recent findings that suggest trophic factors and APP use similar pathways to control neuronal activity

Localization of Trk neurotrophin receptors is an important factor in directing cellular communication in developing and mature neurons. One potential site of action is in lipid raft membrane microdomains. Although Trk receptors have been localized to lipid rafts, little is known about how these neurotrophin receptors are directed there or how localization to these membrane microdomains regulates Trk signaling. Here, we report that the TrkB brain-derived neurotrophic factor (BDNF) receptor specifically localized to intracellular lipid rafts in cortical and hippocampal membranes in response to BDNF and that this process was critically dependent on the tyrosine kinase Fyn. BDNF-induced TrkB accumulation at lipid rafts was prevented by blocking the internalization of TrkB. BDNF stimulation also resulted in the association between endogenous TrkB and Fyn. Moreover, in neurons derived from Fyn knock-out mice, the translocation of TrkB to lipid rafts in response to BDNF was compromised, whereas the corticohippocampal region of Fyn mutants displayed lower amounts of TrkB in lipid rafts in vivo. In support of a role for lipid rafts in neurotrophin signaling, inhibiting TrkB translocation to lipid rafts, either by using Fyn knock-out neurons or lipid raft-disturbing agents, prevented the full activation of TrkB and of downstream phospholipase C-gamma. These results indicate that the lipid raft localization of TrkB receptors is regulated by Fyn and represents an important factor in determining the outcome of BDNF signaling in neurons.

Neurotrophins are trophic factors that regulate important neuronal functions. They bind two unrelated receptors, the Trk family of receptor-tyrosine kinases and the p75 neurotrophin receptor (p75). p75 was recently identified as a new substrate for gamma-secretase-mediated intramembrane proteolysis, generating a p75-derived intracellular domain (p75-ICD) with signaling capabilities. Using PC12 cells as a model, we studied how neurotrophins activate p75 processing and where these events occur in the cell. We demonstrate that activation of the TrkA receptor upon binding of nerve growth factor (NGF) regulates the metalloprotease-mediated shedding of p75 leaving a membrane-bound p75 C-terminal fragment (p75-CTF). Using subcellular fractionation to isolate a highly purified endosomal fraction, we demonstrate that p75-CTF ends up in endosomes where gamma-secretase-mediated p75-CTF cleavage occurs, resulting in the release of a p75-ICD. Moreover, we show similar structural requirements for gamma-secretase processing of p75 and amyloid precursor protein-derived CTFs. Thus, NGF-induced endocytosis regulates both signaling and proteolytic processing of p75

It is well established that motor neurons depend for their survival on many trophic factors. In this study, we show that the precursor form of NGF (pro-NGF) can induce the death of motor neurons via engagement of the p75 neurotrophin receptor. The pro-apoptotic activity was dependent upon the presence of sortilin, a p75 co-receptor expressed on motor neurons. One potential source of pro-NGF is reactive astrocytes, which up-regulate the levels of pro-NGF in response to peroxynitrite, an oxidant and producer of free radicals. Indeed, motor neuron viability was sensitive to conditioned media from cultured astrocytes treated with peroxynitrite and this effect could be reversed using a specific antibody against the pro-domain of pro-NGF. These results are consistent with a role for activated astrocytes and pro-NGF in the induction of motor neuron death and suggest a possible therapeutic target for the treatment of motor neuron disease