Insulin prevents apoptosis of external granular layer neurons in rat cerebellar slice cultures

Progress toward understanding the molecular mechanisms of neurotrophic factor signalling

Understanding the mechanism of action of the neurotrophic factors is central to unravelling of the mysteries of some of the neurodegenerative disorders. In this review we will discuss recent advances in our understanding of neurotrophic factor signalling in primary cultured neurons, in particular those from the superior cervical and dorsal root ganglia, as well as cerebellar granule cells, cortical neurons and oligodendrocytes.

A review of experimental methylmercury toxicity in rats: neuropathology and evidence for apoptosis

As an animal model for examining the pathogenicity of human organic mercury intoxication, rats have been used for the reproduction of human neurologic diseases. Rats experimentally exposed to methylmercury chloride showed clinical signs of neurologic dysfunction characterized by ataxic behavior. Neuropathology of the diseased animals consisted of lesions such as: (a) degeneration of the peripheral nerve and sensory root nerve with preservation of the motor root nerve; (b) degeneration of the posterior funiculus of the spinal cord; and (c) degeneration of cerebellar granule cells with preservation of Purkinje cells. These findings suggest the human neuropathology of this toxicity. The degeneration was characterized by nerve fiber damage or neuronal cell death accompanied by astrocytic gliosis and activated macrophages or microglias. For the cerebellar granule cells, the mechanism of neuronal cell death was shown to be apoptosis. This fact was verified by histologic and ultrastructural findings as well as by in situ nick-end labeling and electrophoretic methods. Evidence of apoptosis involvement in cerebellar degeneration would provide a new viewpoint from which to analyze the selected degeneration of the nervous system in neurotoxicology.

Activation of mitogen-activated protein kinase and p70S6 kinase is not correlated with cerebellar granule cell survival

We have investigated the role of mitogen-activated protein (MAP) kinase in the survival of cerebellar granule cells in primary culture. Brain-derived neurotrophic factor (BDNF) and insulin, but not epidermal growth factor (EGF), promoted the survival of P6 cerebellar granule neurons. BDNF promoted a sustained activation of MAP kinase, whereas that induced by EGF was only transient. Insulin promoted a small but transient activation of MAP kinase that was completely blocked by PD98059, an inhibitor of MAP kinase kinase activation. PD98059 had no effect on the insulin- or BDNF-induced survival of cerebellar granule cells. We also investigated the role of p70S6 kinase in survival. The activation of p70S6 kinase by EGF was transient, whereas BDNF and insulin promoted a sustained activation of p70S6 kinase. Rapamycin, which blocked p70S6 kinase activation, had no effect on the BDNF- or insulin-induced survival of cerebellar granule cells. We conclude that sustained activation of MAP kinase is not correlated with the survival response of cerebellar granule cells; indeed insulin-mediated survival is independent of MAP kinase. Survival of cerebellar granule cells is also independent of the activation of p70S6 kinase.

Quantitative analysis of long-term survival and neuritogenesis in vitro: cochleovestibular ganglion of the chick embryo in BDNF, NT-3, NT-4/5, and insulin

The dynamics of survival and growth were examined for cochleovestibular ganglion (CVG) cells maintained in long-term cultures. CVG cells were explanted from chick embryos after 90 h of incubation into a defined-medium containing BDNF, NT-3, or NT-4/5 and an insulin, transferrin, selenium, and progesterone supplement. Explant survival and neuritogenesis was measured for 23 to 24 days in vitro. All three neurotrophins prolonged CVG survival in a dose-dependent manner although insulin acted as a cofactor. In 0.872 microM insulin-containing medium the ED50 for BDNF and NT-3 was 100 pg/ml, whereas the ED50 for NT-4/5 was 600-1200 pg/ml. However, at later ages in vitro, survival decreased with concentrations of BDNF greater than 2 ng/ml. In insulin-free medium, concentrations of 5-200 ng/ml of BDNF or 30-200 ng/ml of NT-4/5 maintained the survival of explants at a rate that was equivalent to or less than the survival rate of cultures treated with insulin but not with neurotrophin. In contrast, NT-3-treated explants in insulin-free medium did not survive the duration of the experiment. Dose-dependent effects of BDNF and NT-3 on explant neuritogenesis were reflected as an initial delay in outgrowth, whereas NT-4/5 had no effect. Insulin regulation of neuritogenesis was suggested when outgrowth decreased in the presence of an antibody to the insulin receptor. These data suggest that while all three of these neurotrophins protect the CVG from death the long-term consequences of cofactors and certain dose levels should be considered when treating CVG cells in vivo.