Dominant-negative c-Jun promotes neuronal survival by reducing BIM expression and inhibiting mitochondrial cytochrome c release

c-Jun mediates axotomy-induced dopamine neuron death in vivo

Expression of the transcription factor c-Jun is induced in neurons of the central nervous system (CNS) in response to injury. Mechanical transection of the nigrostriatal pathway at the medial forebrain bundle (MFB) results in the delayed retrograde degeneration of the dopamine neurons in the substantia nigra pars compacta (SNc) and induces protracted expression and phosphorylation of c-Jun. However, the role of c-Jun after axotomy of CNS neurons is unclear. Here, we show that adenovirus-mediated expression of a dominant negative form of c-Jun (Ad.c-JunDN) inhibited axotomy-induced dopamine neuron death and attenuated phosphorylation of c-Jun in nigral neurons. Ad.c-JunDN also delayed the degeneration of dopaminergic nigral axons in the striatum after MFB axotomy. Taken together, these findings suggest that activation of c-Jun mediates the loss of dopamine neurons after axotomy injury.

Caspase-mediated cleavage of the Ca2+/calmodulin-dependent protein kinase-like kinase facilitates neuronal apoptosis

This study was designed to identify the role of a recently identified Ca(2+)/calmodulin-dependent protein kinase (CaMK)-like kinase (CaMKLK) in neuronal apoptosis. For this purpose, we studied proteolytic cleavage of CaMKLK by caspases in vitro and in neuronal NG108 cells. In addition, we have investigated the effect of overexpression of wild type and mutant CaMKLK proteins on staurosporine- and serum deprivation-induced apoptosis of NG108 cells. We found that CaMKLK is a substrate for caspase-3 and -8, both in vitro and in NG108 cells during staurosporine- and serum withdrawal-induced apoptosis. Substitution of an aspartic acid residue at position 62 in an asparagine residue within a putative caspase cleavage site completely blocked cleavage of CaMKLK, strongly indicating that (59)DEND(62) is the caspase recognition site. Overexpression of an Asp(62) --> Asn CaMKLK mutant protected NG108 cells from staurosporine-induced apoptosis to a similar extent as Bcl-x(L). In contrast, overexpression of wild type CaMKLK did not lead to protection. Moreover, microinjection of Asp(62) --> Asn CaMKLK protected NG108 cells from serum deprivation-induced apoptosis, while overexpression of a caspase-generated noncatalytic N-terminal CaMKLK fragment exacerbated apoptosis. Together, our data suggest that cleavage of CaMKLK and generation of the noncatalytic N-terminal domain of CaMKLK facilitate neuronal apoptosis.

BH3-only Bcl-2 family members are coordinately regulated by the JNK pathway and require Bax to induce apoptosis in neurons

The Bcl-2 family of proteins are key regulators of programmed cell death. A distinct subfamily of BH3-only molecules has been identified, but their exact mechanism of action remains unclear. Here we show that the BH3-only Bcl-2 family members, Dp5/Hrk and Bim, are induced upstream of the Bax checkpoint in neuronal apoptosis in a manner that shows significant dependence on JNK signaling. We also show that Dp5 and other BH3-only proteins kill cerebellar granule neurons in a Bax-dependent manner. These studies demonstrate that BH3-only members do not act independently in their proapoptotic activities but rather require the action of multidomain proapoptotic Bcl-2 family members to produce cell death.

Transcription-dependent and -independent control of neuronal survival by the PI3K-Akt signaling pathway

The PI3K-Akt signaling pathway plays a critical role in mediating survival signals in a wide range of neuronal cell types. The recent identification of a number of substrates for the serine/threonine kinase Akt suggests that it blocks cell death by both impinging on the cytoplasmic cell death machinery and by regulating the expression of genes involved in cell death and survival. In addition, recent experiments suggest that Akt may also use metabolic pathways to regulate cell survival.