Abstract
A-T (ataxia telangiectasia) is a genetic disease caused by a mutation in the Atm (A-T mutated) gene that leads to neurodegeneration. Despite an increase in the numbers of studies in this area in recent years, the mechanisms underlying neurodegeneration in human A-T are still poorly understood. Previous studies demonstrated that neural stem cells (NSCs) isolated from the subventricular zone (SVZ) of Atm -/- mouse brains show defective self-renewal and proliferation, which is accompanied by activation of chronic p38 mitogen-activated protein kinase (MAPK) and a lower level of the polycomb protein Bmi-1. However, the mechanism underlying Bmi-1 down-regulation and its relevance to defective proliferation in Atm-/- NSCs remained unclear. Here, we show that over-expression of Bmi-1 increases self-renewal and proliferation of Atm-/- NSCs to normal, indicating that defective proliferation in Atm-/- NSCs is a consequence of down-regulation of Bmi-1. We also demonstrate that epidermal growth factor (EGF)-induced Akt phosphorylation renders Bmi-1 resistant to the proteasomal degradation, leading to its stabilization and accumulation in the nucleus. However, inhibition of the Akt-dependent Bmi-1 stabilizing process by p38 MAPK signaling reduces the levels of Bmi-1. Treatment of the Atm-/- NSCs with a specific p38 MAPK inhibitor SB203580 extended Bmi-1 posttranscriptional turnover and H2A ubiquitination in Atm-/- NSCs. Our observations demonstrate the molecular basis underlying the impairment of self-renewal and proliferation in Atm-/- NSCs through the p38 MAPK-Akt-Bmi-1-p21 signaling pathway.
Highlights
ATM (A-T mutated) kinase plays a critical role in the regulation of cell cycling, DNA repair, and cellular redox status
We examined the mRNA levels of the CDK inhibitors p21, p16, and p19, which are regulated by Bmi-1, and observed elevated levels of these transcripts in Atm-/- neural stem cells (NSCs)
Cells were counterstained by DAPI (49-6-Diamidino-2-phenylindole), which identifies the nuclei of the NSCs. (C) After epidermal growth factor (EGF) starvation of Atm+/+ NSCs for 4 hours, EGF was added back to the cultures for 3 hours and analyzed for levels of phospho-Akt, Akt, and Bmi-1 by Western blotting
Summary
ATM (A-T mutated) kinase plays a critical role in the regulation of cell cycling, DNA repair, and cellular redox status. The number of NSCs is the result of a tightly controlled balance between self-renewal, proliferation, differentiation and death. Further investigation into the mechanisms underlying Bmi-1 down-regulation in Atm-/NSCs is necessary to better understand the role of ATM in regulating self-renewal and proliferation of NSCs. Until now, the identity of upstream effectors that control the level or the function of Bmi-1 remains unclear. We show that posttranscriptional regulation of Bmi-1 via proteasomal degradation causes defective proliferation in Atm-/- NSCs as a result of p21 up-regulation. These observations strongly support the notion that down-regulation of Bmi-1 is associated with oxidative stress-dependent pathways in Atm-/NSCs with p38 activation, Akt deactivation, and p21 upregulation. Our findings have identified these players as potential targets for A-T treatment
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
