Interaction of Akt-phosphorylated SRPK2 with 14-3-3 Mediates Cell Cycle and Cell Death in Neurons

Abstract

Terminally differentiated neurons are unable to reenter the cell cycle. Aberrant cell cycle activation provokes neuronal cell death, whereas cell cycle inhibition elevates neuronal survival. However, the molecular mechanism regulating the cell cycle and cell death in mature neurons remains elusive. Here we show that SRPK2, a protein kinase specific for the serine/arginine (SR) family of splicing factors, triggers cell cycle progression in neurons and induces apoptosis through regulation of nuclear cyclin D1. Akt phosphorylates SRPK2 on Thr-492 and promotes its nuclear translocation leading to cyclin D1 up-regulation, cell cycle reentry, and neuronal apoptosis. In addition, SRPK2 phosphorylates SC35 and, thus, inactivates p53, resulting in cyclin D1 up-regulation. 14-3-3 binding to SRPK2, regulated by Akt phosphorylation, inhibits these events. We find that SRPK2 is phosphorylated in ischemia-attacked brain, correlating with the observed increase in cyclin D1 levels. Hence, phosphatidylinositol 3-kinase/Akt mediates the cell cycle and cell death machinery in the nervous system through phosphorylation of SRPK2.In the central nervous system the nascent neuroblasts leave ventricular zone or subventricular zone and migrate to the destination where they differentiate and become permanently post-mitotic cells (1). It is well established that terminally differentiated neurons are unable to reenter the cell cycle, but accumulating evidence has demonstrated the up-regulation of cell cycle regulatory proteins in degenerating neurons of Alzheimer disease (AD) 2 brain (2). Elevated levels of Cdc2, Cdk4, p16, Ki-67, cyclin B1, and cyclin D have been found in pathologically affected or vulnerable neurons in AD (3-7). Moreover, several of these regulators have been observed in vulnerable neurons before lesion formation (8). Together these findings suggest that the activity of certain cell cycle regulators plays a critical upstream role in the AD neurodegenerative process. Greene and co-workers (9 -12) have shown that drugs that block cell cycle advances are efficient in preventing the death of PC12 cells as well as sympathetic neurons. Dominant negative forms of the Cdk4 and Cdk6 preventing the cell death induced by camptothecin (a topoisomerase inhibitor) are effectively blocked by the G 1 /S blockers, such as deferoxamine and mimosine, as well as by the cyclin-dependent kinases (Cdk) inhibitors flavopiridol and olomoucine. In addition, they show that neurons treated with DNA-damaging agents such as UV irradiation or camptothecin also require cyclin D and Cdk4/6 activity to induce neuronal death (13). Thus, this evidence supports that mature neurons might retain certain elements of the cell cycle and have the capability of reactivating additional aspects of the replication mechanism when under stresses. Any events that force a mature neuron back into the cell cycle are lethal rather than mitogenic for the neuron.SRPK, a family of cell cycle-regulated protein kinases, phosphorylate serine/arginine (SR) domain-containing protein...

SOX9 modulates the expression of key transcription factors required for heart valve development

“…Loss of Cops5 in embryonic limb results in shortened limbs due to impaired chondrogenesis and Sox9 levels are decreased in mutant long bones (Bashur et al, 2014) suggesting a potential feedback loop between SOX9 and COPS5. SRPK2 (SRSF protein kinase 2) can promote proliferation and cell cycle progression by enhancing cyclin D1 levels (Jang et al, 2009), and AKT2 regulates progression of cell cycle via phosphorylation of its targets including cyclin-dependent kinase inhibitors and maintaining protein stability of MYC and D-type cyclins via GSK3β . EED (embryonic ectoderm development), HDAC1 and HDAC2 (histone deacetylase 1 and 2) are epigenetic regulators associated with cell proliferation (Bracken et al, 2003;Kelly and Cowley, 2013).…”

The N-terminal Fragment from Caspase-cleaved Serine/Arginine Protein-specific Kinase2 (SRPK2) Translocates into the Nucleus and Promotes Apoptosis

“…Incubation of recombinant caspases with in vitro-translated SRPKs demonstrates that they are in vitro substrates for caspases-8 and -9 (8). Recently, we have shown that SRPK2 triggers cell cycle progression in post-mitotic neurons and induces apoptosis through up-regulation of nuclear cyclin D1 (9). Ablation of SRPK2 abrogates cyclin A1 expression in leukemia cells and arrest cells at G1 phase.…”

“…Ablation of SRPK2 abrogates cyclin A1 expression in leukemia cells and arrest cells at G1 phase. Knocking down of SRPK2 induces caspase-3 activation in cortical neurons (9). SRPK2 overexpression increases leukemia cell proliferation and elicits primary cortical neuronal cell death (9,10), indicating that SRPK2 is a critical player in regulating cell survival.…”

Proteomics of tissue factor silencing in cardiomyocytic cells reveals a new role for this coagulation factor in splicing machinery control

“…Regulation of splicing 477 factors by Akt is not heretical [67]; activated Akt has been involved in directly modulating 478 serine/arginine-rich proteins (SR proteins) [68]; more recently, hnRNP L has been recently 479 identified as a direct substrate of Akt, thus influencing the alternative splicing of caspase-9 [69], 480 and down-regulation of splicing factors has been also observed in U373 glioma cells after Akt 481 silencing [70]. Furthermore, SR-specific protein kinases (SRPKs) are also directly regulated by 482 Akt [71]; lastly, a phosphoproteomic screen of Akt isoforms identified 25 RNA processing 483 proteins including splicing factors as Akt isoform-dependent targets [72]. 484…”