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David R Kaplan

Publications and source records attributed to David R Kaplan.

29 records · Page 2Linked to original sources
PubMed2003

SHP-1 negatively regulates neuronal survival by functioning as a TrkA phosphatase.

Nerve growth factor (NGF) mediates the survival and differentiation of neurons by stimulating the tyrosine kinase activity of the TrkA/NGF receptor. Here, we identify SHP-1 as a phosphotyrosine phosphatase that negatively regulates TrkA. SHP-1 formed complexes with TrkA at Y490, and dephosphorylated it at Y674/675. Expression of SHP-1 in sympathetic neurons induced apoptosis and TrkA dephosphorylation. Conversely, inhibition of endogenous SHP-1 with a dominant-inhibitory mutant stimulated basal tyrosine phosphorylation of TrkA, thereby promoting NGF-independent survival and causing sustained and elevated TrkA activation in the presence of NGF. Mice lacking SHP-1 had increased numbers of sympathetic neurons during the period of naturally occurring neuronal cell death, and when cultured, these neurons survived better than wild-type neurons in the absence of NGF. These data indicate that SHP-1 can function as a TrkA phosphatase, controlling both the basal and NGF-regulated level of TrkA activity in neurons, and suggest that SHP-1 regulates neuron number during the developmental cell death period by directly regulating TrkA activity.

Animals↗
PubMed2003

TrkB receptor signaling regulates developmental death dynamics, but not final number, of retinal ganglion cells.

We investigated the effects of endogenous neurotrophin signaling on the death-survival of immature retinal ganglion cells (RGCs) in vivo. Null mutation of brain-derived neurotrophic factor [(BDNF) alone or in combination with neurotrophin 4 (NT4)] increases the peak rate of developmental RGC death as compared with normal. Null mutation of NT4 alone is ineffective. Null mutation of the full-length trkB (trkBFL) receptor catalytic domain produces a dose-dependent increase in the peak RGC death rate that is negatively correlated with retinal levels of trkBFL protein and phosphorylated (activated) trkBFL. Depletion of target-derived trkB ligands by injection of trkB-Fc fusion protein into the superior colliculus increases the peak rate of RGC death compared with trkA-Fc-treated and normal animals. Adult trkBFL+/- mice have a normal number of RGCs, despite an elevated peak death rate of immature RGCs. Thus, target-derived BDNF modulates the dynamics of developmental RGC death through trkBFL activation, but BDNF/trkB-independent mechanisms determine the final number of RGCs.

Animals↗
PubMed2003

Semaphorin 3F antagonizes neurotrophin-induced phosphatidylinositol 3-kinase and mitogen-activated protein kinase kinase signaling: a mechanism for growth cone collapse.

Peripheral nerve growth is regulated by the coordinated action of numerous external stimuli, including positively acting neurotrophin-derived growth cues and restrictive semaphorin cues. Here, we show that Semaphorin 3F (Sema 3F) can antagonize nerve growth factor (NGF)-stimulated TrkA (tyrosine receptor kinase A) signaling in sympathetic neurons, thereby apparently contributing to growth cone collapse. Sema 3F suppressed NGF-induced activation of the phosphatidylinositol 3 (PI3)-kinase-Akt and MEK (mitogen-activated protein kinase kinase)-ERK (extracellular signal-regulated kinase) pathways, both of which we show to be required to maintain growth cone structure. Sema 3F-induced growth cone collapse was partially reversed by sustained activation of the PI3-kinase and MEK pathways, which was achieved by overexpression of the Gab-1 (growth-associated binder 1) docking protein. These data indicate that a novel mechanism used by Sema 3F to collapse growth cones in sympathetic neurons is to dampen neurotrophin signaling, providing an intracellular mechanism for cross talk between positive and negative axon growth cues.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Cell cycle regulators in the neuronal death pathway of amyotrophic lateral sclerosis caused by mutant superoxide dismutase 1.

There is growing evidence for involvement of members of the cyclin-dependent kinase (Cdk) family in neurodegenerative disorders and in apoptotic death of neurons subjected to various insults. After our recent report that a deregulation of Cdk5 activity by p25 may contribute to pathogenesis of amyotrophic lateral sclerosis (ALS), we further examined the possible involvement of other Cdks in mice expressing a mutant form of superoxide dismutase (SOD1(G37R)) linked to ALS. No substantial changes in Cdk2 or Cdk6 distribution and kinase activities were detected in spinal motor neurons from SOD1(G37R) mice when compared with normal mice. Of particular interest was the upregulation and mislocalization of Cdk4, a regulator of the G1-S checkpoint of the cell cycle, in motor neurons of SOD1(G37R) mice. The increase of Cdk4 activity in SOD1(G37R) mice was associated with an increase in nuclear Cdk4, cyclin D1, its coactivator, and with the abnormal phosphorylation of the retinoblastoma (Rb) protein at Cdk phosphorylation sites. Pharmacological treatment of SOD1(G37R) mice with minocycline, a compound that attenuates microgliosis and slows down disease, lessened the dysregulation of Cdk5/Cdk4 and the phosphorylation of Rb. Interestingly, phospho-Rb was immunoprecipitated with anti-Cdk4 but not with anti-Cdk5 antibodies, suggesting a key role for Cdk4 in the phosphorylation of Rb. Remarkably, the overexpression of a transgene coding for human neurofilament H, a phosphorylation sink for deregulated Cdk5 activity by p25, resulted in a reduction in levels of nuclear Cdk4 and Rb phosphorylation. These results indicate that a cell cycle signaling at the neuronal G1-S checkpoint subsequent to Cdk5 deregulation may constitute a critical step of the neuronal death pathway in ALS caused by mutant SOD1.

Amino Acid Substitution↗
PubMed2003

Signaling mechanisms underlying dendrite formation.

During development, the nervous system is confronted with a problem of enormous complexity; to progress from a large number of 'disconnected' neurons to a network of neuronal circuitry that is able to dynamically process sensory information and generate an appropriate output. To form these circuits, growing axons must make synapses with targets, usually the dendrites of postsynaptic neurons. Although a significant amount is known about the signals that regulate and guide developing axons, we are only now starting to understand how environmental cues like growth factors and activity regulate the formation and maintenance of dendrites in the developing and mature nervous system.

Animals↗
PubMed2002

Brain-derived neurotrophic factor activation of TrkB protects neuroblastoma cells from chemotherapy-induced apoptosis via phosphatidylinositol 3'-kinase pathway.

Neuroblastoma (NB) tumors expressing high levels of brain-derived neurotrophic factor (BDNF) and TrkB are associated with poor 5-year survival outcomes. Our previous studies indicated that BDNF blocked the cytotoxic effects of vinblastine on NB cells. Here we evaluated the ability of BDNF to decrease the chemosensitivity of NB cells to a number of common chemotherapeutic agents. Two SH-SY5Y NB cell lines (TB3 and TB8) expressing TrkB under the control of a tetracycline (Tet)-repressible promoter element were generated, and used to assess apoptosis resulting from treatment with cisplatin, doxorubicin, etoposide, and vinblastine. BDNF treatment of high TrkB-expressing TB8 (Tet-) and TB3 (Tet-) cells blocked drug-induced cell death in a dose-dependent manner. Only high-dose BDNF (100 ng/ml) could block the effects of chemotherapy in low TrkB-expressing cells. The ability of BDNF to rescue the cells from chemotherapeutic agent-induced cell death was inhibited by treatment with the Trk tyrosine kinase inhibitor K252a or the phosphatidylinositol 3'-kinase (PI3K) inhibitor LY294002, but not by the mitogen-activated protein kinase kinase inhibitor PD98059 or the peritoneal lymphocyte gamma inhibitor U73122, indicating that both TrkB and PI3K activities are required for the survival-promoting effects of BDNF. BDNF also protected TrkB-expressing NGP and KCNR NB cells from chemotherapeutic agent-induced cell death, and LY294002 inhibited this protection. These results suggest that TrkB and BDNF can contribute to the chemoresistance of poor prognosis tumors, and that suppression of PI3K activity might improve the ability of these agents to induce the death of NB tumors.

Animals↗
PubMed2002

The Rb-family protein p107 inhibits translation by a PDK1-dependent mechanism.

The Rb family of proteins, which consists of Rb, p107 and p130, are critical regulators of cell proliferation. In addition to their inhibitory effects on cell cycle progression, Rb-family proteins repress transcription by RNA polymerases I and III, and may therefore restrain cell growth. However, it is not known if Rb, p107 or p130 have direct effects on protein synthesis. Here we report that ectopic expression of p107 in rat fibroblasts markedly attenuates the stimulation of mRNA translation and global protein synthesis by serum growth factors. This effect is associated with a reduction in the phosphorylation and activation of the serine-threonine kinases Akt1 and p70 S6 kinase (S6K1), two downstream targets of phosphoinositide-dependent kinase 1 (PDK1). We show that overexpression of p107 interferes with the recruitment of PDK1 to the plasma membrane in response to growth factors. Overexpression of PDK1 restores the defect in translation elicited by p107. These results suggest that p107 restricts cell growth by interfering with the phosphoinositide 3-kinase (PI3K) signaling pathway.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2002

Suppression of Rac activity induces apoptosis of human glioma cells but not normal human astrocytes.

Tumors of glial origin such as glioblastoma multiforme (GBM) comprise the majority of human brain tumors. Patients with GBM have a very poor survival rate, with an average life expectancy of <1 year. We asked whether we could identify a survival pathway in high-grade glioma and oligodendroglioma cells that when suppressed, would induce apoptosis of these tumor cells but not of normal human adult astrocytes. To identify these pathways, we selectively suppressed the activity of a number of proteins (Ras, Rac1, Akt1, RhoA, c-jun, and MEK1/2) hypothesized to play roles in cell survival. We found that suppression of Rac1, a small GTP-binding protein, inhibited survival and produced apoptosis in three human glioma cell lines (U87, U343, and U373). Serum induced the activity of Rac1 and the activity or phosphorylation state of p21-activated kinase 1 and c-Jun NH(2)-terminal kinase (JNK), two intracellular targets of Rac1. Suppression of Rac1 also induced apoptosis in 19 of 21 short-term cultures of human primary cells from grades II and III oligodendroglioma and grade IV glioblastoma that varied in p53, epidermal growth factor receptor, epidermal growth factor receptor vIII, MDM2, and p16/p19 mutational or amplification status. In contrast, inhibition of Rac1 activity did not induce apoptosis of normal primary human adult astrocytes. In both established glioma cell lines and primary glioma cells, apoptosis induced by the inhibition of Rac was partially rescued by activated mitogen-activated protein kinase kinase 1, an activator of JNK, suggesting that JNK functions downstream of Rac1 in glioma cells. These results indicate that Rac1 regulates a major survival pathway in most glioma cells, and that suppression of Rac1 activity stimulates the death of virtually all glioma cells, regardless of their mutational status. Agents that suppress Rac1 activity may therefore be useful therapeutic treatments for malignant gliomas.

Adenoviridae↗
PubMed2002

N-myc promotes survival and induces S-phase entry of postmitotic sympathetic neurons.

In most postmitotic neurons, expression or activation of proteins that stimulate cell cycle progression or DNA replication results in apoptosis. One potential exception to this generalization is neuroblastoma (NB), a tumor derived from the sympathoadrenal lineage. NBs often express high levels of N-myc, a proto-oncogene that can potently activate key components of the cell cycle machinery. Here, we show that in postmitotic sympathetic neurons, N-myc can induce S-phase entry while protecting neurons from death caused by aberrant cell cycle reentry. Specifically, these experiments demonstrate that expression of N-myc at levels similar to those in NBs caused sympathetic neurons to reenter S-phase, as monitored by 5-bromo-2-deoxyuridine incorporation and expression of cell cycle regulatory proteins, and rescued them from apoptosis induced by withdrawal of their obligate survival factor, nerve growth factor. The N-myc-induced cell cycle entry, but not enhanced survival, was inhibited by coexpression of a constitutively hypophosphorylated form of the retinoblastoma tumor suppressor protein, suggesting that these two effects of N-myc are mediated by separate pathways. In contrast, N-myc did not cause S-phase entry in postmitotic cortical neurons. Thus, N-myc both selectively causes sympathetic neurons to reenter the cell cycle and protects them from apoptosis, potentially contributing to their transformation to NBs.

Adenoviridae↗