Although the aberrant activation of cell cycle proteins has a critical role in neuronal death, effectors or mediators of cyclin D1/ cyclin-dependent kinase 4 (CDK4)-mediated death signal are still unknown. Here, we describe a previously unsuspected role of LIM kinase 2 (LIMK2) in programmed necrotic neuronal death. Downregulation of p27 Kip1 expression by Rho kinase (ROCK) activation induced cyclin D1/CDK4 expression levels in neurons vulnerable to status epilepticus (SE). Cyclin D1/CDK4 complex subsequently increased LIMK2 expression independent of caspase-3 and receptor interacting protein kinase 1 activity. In turn, upregulated LIMK2 impaired dynamic-related protein-1 (DRP1)-mediated mitochondrial fission without alterations in cofilin phosphorylation/expression and finally resulted in necrotic neuronal death. Inhibition of LIMK2 expression and rescue of DRP1 function attenuated this programmed necrotic neuronal death induced by SE. Therefore, we suggest that the ROCK-p27 Kip1 -cyclin D1/CDK4-LIMK2-DRP1-mediated programmed necrosis may be new therapeutic targets for neuronal death. The cell cycle is essential for a vital process, including proliferation, differentiation and survival of various cells. Cell cycle progression is regulated by two classes of proteins, the cyclins and the cyclin-dependent kinases (CDKs). Among them, cyclin D1 forms a complex with CDK4 and inactivates retinoblastoma protein (Rb) through phosphorylation resulting in activating E2 promoter-binding factor (E2F) family of transcription factors. Active E2F induces various gene transcriptions involving the cell cycle. 1 In post-mitotic neurons, some pathological conditions upregulate cyclin D1/CDK4 complex expression to induce activation of E2F members that contributes to increased transcription of proapoptotic molecules. 2,3 However, it is unclear whether cyclin D1 expression directly induces neuronal death for a number of following reasons. First, due to a time lag between cyclin D1/CDK4 expression and the onset of DNA fragmentation, most cyclin D1-positive neurons show TUNEL negativity. 4,5 Second, cyclin D1 level is unaltered in cultured embryonic neurons following apoptosis induction. 6 Third, Rb phosphorylation is rarely observed in vivo, 4 although cyclin D1/CDK4 complex phosphorylates Rb protein in apoptotic cultured neurons. 6 Fourth, cyclin D1 induction following ischemia is associated with regeneration and resistance to apoptosis rather than a mediator of apoptosis. 5,7,8 Fifth, effectors or mediators of cyclin D1/CDK4-mediated death signal are still unknown. Finally, neuronal death induced by various insults is morphologically necrotic rather than apoptotic. 9-12 Therefore, it is likely that some essential factors are missing in the cyclin D/CDK4-mediated neuronal death pathway.LIM kinases (LIMK1 and LIMK2) phosphorylate cofilin that is a stimulus-responsive mediator of actin dynamics. [13][14][15] Interestingly, non-phosphorylated cofilin targets mitochondrial membranes in response to apoptotic stimuli for cytochrome c releas...

Poly(ADP-ribose) polymerase-1 (PARP1) plays a regulatory role in apoptosis, necrosis and other cellular processes after injury. Status epilepticus (SE) induces neuronal and astroglial death that show regional-specific patterns in the rat hippocampus and piriform cortex (PC). Thus, we investigated whether PARP1 regulates the differential neuronal/glial responses to pilocarpine (PILO)-induced SE in the distinct brain regions. In the present study, both CA1 and CA3 neurons showed PARP1 hyperactivation-dependent neuronal death pathway, whereas PC neurons exhibited PARP1 degradation-mediated neurodegeneration following SE. PARP1 degradation was also observed in astrocytes within the molecular layer of the dentate gyrus. PARP1 induction was detected in CA1–3-reactive astrocytes, as well as in reactive microglia within the PC. Although PARP1 inhibitors attenuated CA1–3 neuronal death and reactive gliosis in the CA1 region, they deteriorated the astroglial death in the molecular layer of the dentate gyrus and in the stratum lucidum of the CA3 region. Ex vivo study showed the similar regional and cellular patterns of PARP1 activation/degradation. Taken together, our findings suggest that the cellular-specific PARP1 activation/degradation may distinctly involve regional-specific neuronal damage, astroglial death and reactive gliosis in response to SE independently of hemodynamics.

Cellular ionic homeostasis, fundamentally K þ homeostasis, has been implicated as a critical regulator of apoptosis. The intracellular K þ efflux on apoptotic insult and suppression of apoptosis by high concentration of extracellular K þ or after inhibition of this efflux by K þ channel blockers have established the crucial role of K þ in turning on the apoptotic machinery. Several contrasting observations have reported the antiapoptotic effect of intracellular K þ concentration to be the result of inhibition of cytochrome c release from mitochondria, but the exact inhibitory mechanism remains obscure. However, here we show the blockage of K þ efflux during apoptosis did not affect cytochrome c release from the mitochondria, still completely inhibited the formation of the apoptosome comprising Apaf-1, cytochrome c, caspase-9 and other accessories. As a consequence of this event, procaspase-9, -3, -8 and other death-related proteins were not processed. Furthermore, physiological concentrations of K þ also inhibited the processing of procaspase-3 by purified caspase-8 or -9, the nucleosomal DNA fragmentation by purified DFF40/CAD and the nuclear fragmentation to varying extents. Altogether, these findings suggest that the efflux of K þ is prerequisite not only for the formation of the apoptosome but also for the downstream apoptotic signaltransduction pathways.

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