Faculty Bibliography

Receptors of diverse primary structure and with diverse ligands have been reported to be capable of stimulating apoptosis in the absence of ligand binding. These receptors are called dependence receptors, and the newest member of this family appears to be the Sonic hedgehog receptor Patched, which has been reported to stimulate apoptosis when expressed in the absence of its ligand. The signaling mechanisms that account for this type of receptor activity are unknown. Several theories behind how dependence receptors may trigger cell death are described

The subventricular zone (SVZ) is the most active site for the production of new neurons in the adult mouse brain. Neural stem cells in the adult SVZ give rise to neuroblasts that travel via the rostral migratory stream (RMS) to the olfactory bulb, where they differentiate into interneurons. The enzyme telomerase has been identified in other population of stem cells and is necessary for the synthesis of telomeric DNA to prevent chromosomal shortening, end-to-end fusions, and apoptosis during successive rounds of cell division. However, previous studies have failed to detect telomerase in the adult mammalian brain. Here we demonstrate that telomerase is expressed by all brain regions shortly after birth, but becomes restricted to the SVZ and olfactory bulb in the adult mouse brain. Cultures of neural precursor cells or of migratory neuroblasts purified from the SVZ were each found to possess telomerase activity. After elimination of migrating neuroblasts and immature precursor cells in vivo by treatment with cytosine-beta-D-arabinofuranoside (Ara-C), telomerase activity was still detectable in the remaining SVZ, which includes a population of neural stem cells. Following withdrawal of Ara-C, telomerase activity subsequently increased with a time course that parallels regeneration of the SVZ network and RMS. Finally, intracranial surgery alone, which has previously been shown to increase the number of cells in the SVZ, produced higher telomerase levels in the SVZ. We conclude that telomerase is active in neural precursor cells of the adult mouse and suggest that its regulation is an important parameter for cellular proliferation to occur in the mammalian brain

Trafficking of trophic factors in axons can occur in a retrograde and anterograde direction. Recent findings bring further support for a vesicle-based process for retrograde transport but raise new questions that need to be pursued. Unraveling the exact mechanisms that account for retrograde transport of neurotrophins and their receptors will reveal the cellular requirements for propagating trophic signals over long distances

Members of the JNK pathway are organized together by virtue of interactions with JNK interacting protein 1 (JIP1), a scaffold protein. Here we have investigated the possibility that JIP1 may also affect the catalytic activity of Akt1, a serine/threonine kinase that has been implicated in multiple cellular processes, including survival and proliferation. JIP1 expression enhanced Akt1 kinase activity in a dose-dependent manner following serum starvation in 293 cells. Cellular activation of Akt1 following stimulation with low concentrations of insulin-like growth factor (IGF-1) was elevated in the presence of JIP1. JIP1 expression also prolonged Akt1 stimulation after a short IGF-1 pulse. The mechanism of JIP1-mediated Akt1 activation involved JIP1 protein binding to the Akt1 pleckstrin homology domain, which in turn promoted the phosphorylation of the activation T-loop of Akt1 by phosphoinositide-dependent kinase-1. These results suggest that, in certain cellular contexts, JIP1 may act as an Akt1 scaffold, which regulates the enzymatic activity of Akt1. This study also indicates that JIP1 expression can exert signaling effects independent of JNK activity

The generation of biologically active proteins by regulated intramembrane proteolysis is a highly conserved mechanism in cell signaling. Presenilin-dependent gamma-secretase activity is responsible for the intramembrane proteolysis of selected type I membrane proteins, including beta-amyloid precursor protein (APP) and Notch. A small fraction of intracellular domains derived from both APP and Notch translocates to and appears to function in the nucleus, suggesting a generic role for gamma-secretase cleavage in nuclear signaling. Here we show that the p75 neurotrophin receptor (p75NTR) undergoes presenilin-dependent intramembrane proteolysis to yield the soluble p75-intracellular domain. The p75NTR is a multifunctional type I membrane protein that promotes neurotrophin-induced neuronal survival and differentiation by forming a heteromeric co-receptor complex with the Trk receptors. Mass spectrometric analysis revealed that gamma-secretase-mediated cleavage of p75NTR occurs at a position located in the middle of the transmembrane (TM) domain, which is reminiscent of the amyloid beta-peptide 40 (Abeta40) cleavage of APP and is topologically distinct from the major TM cleavage site of Notch 1. Size exclusion chromatography and co-immunoprecipitation analyses revealed that TrkA forms a molecular complex together with either full-length p75 or membrane-tethered C-terminal fragments. The p75-ICD was not recruited into the TrkA-containing high molecular weight complex, indicating that gamma-secretase-mediated removal of the p75 TM domain may perturb the interaction with TrkA. Independent of the possible nuclear function, our studies suggest that gamma-secretase-mediated p75NTR proteolysis plays a role in the formation/disassembly of the p75-TrkA receptor complex by regulating the availability of the p75 TM domain that is required for this interaction

Expression of the p75 neurotrophin receptor (p75(NTR)) in primary melanomas is associated with deeply invasive lesions. In turn, there is expression of high levels of neurotrophins at the invasion front of normal tissue adjacent to brain metastases, thus implicating this growth factor-receptor system in melanoma tumorigenesis. The neurotrophins nerve growth factor (NGF) and neurotrophin-3 (NT-3) are potent chemotactic agents for human melanoma cells which express p75(NTR)in vitro. Here we show that the actin-bundling protein fascin specifically interacts with p75(NTR) in an NGF-dependent manner by co-immunoprecipitation and colocalization in melanoma cells that express the two proteins endogenously. In addition, expression of a fascin point mutant at the serine phosphorylation site (serine 39) regulating actin binding abrogates neurotrophin-induced migration. These results suggest a causal role for NGF-mediated dephosphorylation of serine 39 on fascin in mediating actin binding and subsequent melanoma cell migration