Functional characterization of two splice variants of rat bad and their interaction with Bcl-w in sympathetic neurons

Neuronal cell death is in many cases regulated by competitive interactions between pro- and antiapoptotic proteins of the Bcl-2 family. In this study we have identified two splice variants of the rat proapoptotic molecule Bad, which differ in their carboxy-terminal regions. Both splice variants of Bad interacted with the antiapoptotic molecule Bcl-w as shown by yeast two-hybrid assay and by co-immunoprecipitation experiments from transfected cells. mRNA expression for the two variants of bad were detected in all neonatal and adult rat tissues tested. Overexpression of either of the two isoforms of Bad in nerve growth factor (NGF)-maintained sympathetic neurons by microinjection induced the cell death of these neurons, which was neutralized by co-expression of Bcl-w. Overexpression of Bcl-w in sympathetic neurons also counteracted death induced by NGF deprivation, which was not reduced by co-expression of either of the two Bad variants. The results suggest that Bcl-w, Bad-alpha, and Bad-beta may participate in the regulation of apoptosis in the sympathetic nervous system.

Differential expression of bcl-w and bcl-x messenger RNA in the developing and adult rat nervous system

The bcl-2 family of proteins comprises both anti-apoptotic and pro-apoptotic members, which play a pivotal role in regulating cell death. Bcl-w is a recently identified member of this family, which was shown to inhibit apoptosis in haemopoietic cell lines. However, the function and expression patterns of bcl-w in the nervous system have so far not been described. We have cloned complementary DNA corresponding to rat bcl-w and analysed the expression of bcl-w messenger RNA during rat brain development, using RNA blotting and in situ hybridization techniques. We also compared the expression patterns of bcl-w with those of bcl-xL. During brain development, the levels of bcl-w messenger RNA were found to increase, with highest expression located to specific regions of the mature brain, such as hippocampus, cerebellum, piriform cortex and locus coeruleus. Bcl-w messenger RNA was expressed by neurons, as shown with double labeling with neuronal markers. In contrast to bcl-w, bcl-xL messenger RNA expression levels were highest during early development, particularly in cortex, hippocampus, thalamus, spinal cord and dorsal root ganglia. During postnatal development the expression of bcl-xL messenger RNA decreased and were only detected at low levels in the adult nervous system. As shown earlier for bcl-2, the expression of bcl-w and bcl-x messenger RNA in cultured cerebellar granule cells was not altered by the deprivation of neurotrophic factors. The present results suggest that bcl-w may play an important role in the mature nervous system.

Neurotrophins and cerebellar development

Neurotrophins are structurally related molecules which regulate the survival and differentiation of various populations of neurons during development. In the cerebellum, the neurotophins and their Trk receptors are expressed at a relatively high level, suggesting an important function for these factors during development. There is also a tight age-dependent and spatial regulation of the molecules in the various cerebellar neurons. Previous studies have shown that BDNF and NT-3 have distinct biological effects on survival and differentiation of cerebellar granule neurons and Purkinje cells. Aside from acting as survival and differentiation factors, the neurotrophins could also have more subtle effects on neuronal function. It is also becoming increasingly evident, not the least from studies in neurotrophin deficient and in cerebellar mutant mice, that the neurotrophins act in concert with other factors and molecules in controlling neuronal development. We will here review some of the recent developments in the neurotrophin field with regard to cerebellum and also discuss what is known about the signaling event following stimulation of cerebellar neurons with BDNF and NT-3. The characterization of specific maturation stages and of genes which are involved and regulated by neurotrophins in developing cerebellum will help us to understand the processes of neuronal survival and differentiation in general.

Bcl-2 regulates the levels of the cysteine proteases ICH and CPP32/Yama in human neuronal precursor cells

Members of the Bcl-2 family are major regulators of cell death and survival. Bcl-2 has been shown to heterodimerize with the death-inducing protein Bax, but the mechanism of action of Bcl-2 is not fully understood. Here we show, using the human NT-2 neuronal cell line, that overexpression of Bcl-2 leads to dramatic down-regulation of the cysteine proteases ICH and CPP32/Yama, which are directly involved in cell death. In addition, the nuclear enzyme poly(ADP-ribose) polymerase was cleaved in control cells but not in cells overexpressing Bcl-2 following induction of apoptosis. The mRNA levels of ICH and CPP32/Yama were differentially affected by Bcl-2 overexpression, suggesting both transcriptional and post-transcriptional effects of the protein. These results demonstrate novel mechanisms of action of Bcl-2 in influencing the expression of death effectors such as the cysteine proteases. The relative levels of Bcl-2 and of various cysteine proteases ultimately determine survival and death of different cells, including neurons.

Cell density increases Bcl-2 and Bcl-x expression in addition to survival of cultured cerebellar granule neurons

The proto-oncogene bcl-2 and its family members, bcl-x and bax are recognized as major regulators of cell death and survival. Although Bcl-2 and Bcl-x are expressed in brain, little is known how they are regulated in neurons. Here we have studied the expression of bcl-2, bcl-xL and bax mRNA in rat cerebellar granule neurons cultured under conditions which influence neuron survival. Insulin-like growth factor-1 and brain-derived neurotrophic factor supported the survival of these neurons, but affected neither the expression of bcl-2, bcl-xL nor bax mRNA. In contrast, bcl-2 and bcl-xL mRNAs were up-regulated in cerebellar granule neurons plated at high density exhibiting an increased neuronal survival. Western blots showed that cell density also increased Bcl-2 protein level. However, conditioned medium from dense cultures did not affect the level of bcl-2 mRNA nor survival of the neurons. This suggests that cell density promotes survival and regulates Bcl-2 expression in cerebellar granule neurons through a signaling pathway different from known neurotrophic factors.