Calpain Determines the Propensity of Adult Hippocampal Neural Stem Cells to Autophagic Cell Death Following Insulin Withdrawal

Abstract

Programmed cell death (PCD) has significant effects on the function of neural stem cells (NSCs) during brain development and degeneration. We have previously reported that adult rat hippocampal neural stem (HCN) cells underwent autophagic cell death (ACD) rather than apoptosis following insulin withdrawal despite their intact apoptotic capabilities. Here, we report a switch in the mode of cell death in HCN cells with calpain as a critical determinant. In HCN cells, calpain 1 expression was barely detectable while calpain 2 was predominant. Inhibition of calpain in insulin-deprived HCN cells further augmented ACD. In contrast, expression of calpain 1 switched ACD to apoptosis. The proteasome inhibitor lactacystin blocked calpain 2 degradation and elevated the intracellular Ca 21 concentration. In combination, these effects potentiated calpain activity and converted the mode of cell death to apoptosis. Our results indicate that low calpain activity, due to absence of calpain 1 and degradation of calpain 2, results in a preference for ACD over apoptosis in insulin-deprived HCN cells. On the other hand, conditions leading to high calpain activity completely switch the mode of cell death to apoptosis. This is the first report on the PCD mode switching mechanism in NSCs. The dynamic change in calpain activity through the proteasome-mediated modulation of the calpain and intracellular Ca 21 levels may be the critical contributor to the demise of NSCs. Our findings provide a novel insight into the complex mechanisms interconnecting autophagy and apoptosis and their roles in the regulation of NSC death. STEM CELLS 2015;33:3052-3064 SIGNIFICANCE STATEMENTThe hostile environment of the aging or diseased brain greatly limits the proliferation and neurogenesis of neural stem cells, and our lack of knowledge regarding the molecular mechanisms governing the survival and death of neural stem cells has hindered their utilization for the treatment of brain diseases. This is the first report on the switch mechanism in the mode of cell death in neural stem cells. The results of our study provide a novel insight into the complex mechanisms interconnecting autophagy and apoptosis and their roles in the function of neural stem cells.

Autophagy in stem cells: repair, remodelling and metabolic reprogramming

“…Some authors have proposed very different roles for autophagy in the maintenance of adult rat hippocampal NSCs. Chung and coworkers found that rat hippocampal NSCs undergo autophagic cell death in response to insulin withdrawal despite the presence of intact apoptotic machinery, and that this effect is suppressed by the knockdown of Atg7 (Chung et al, 2015). Conversely, Yu and colleagues reported that the tight regulation of calpainsa family of calcium-dependent cytosolic proteasesby the proteasome and by Ca 2+ levels switches the mode of hippocampal NSC cell death from being autophagic, when calpain levels are low, to apoptotic, when calpains levels are high (Yu et al, 2008).…”

Less is more: Caloric regulation of neurogenesis and adult brain function

“…Autophagy is up‐regulated during cell differentiation in neuroblastoma cells and its inhibition prevents the differentiation of neurones, a process that is also regulated by mTOR . Several autophagy‐related proteins, including ambra1, beclin 1, ATG5, and calpain, have recently been implicated in regulating AHN . Because CR and acyl‐ghrelin regulate both autophagy and AHN, albeit in different brain regions, it is reasonable to suggest that CR and acyl‐ghrelin‐mediated regulation of AHN may be influenced by autophagy.…”

“…146 Several autophagy-related proteins, including ambra1, beclin 1, ATG5, and calpain, have recently been implicated in regulating AHN. [147][148][149] Because CR and acyl-ghrelin regulate both autophagy and AHN, albeit in different brain regions, it is reasonable to suggest that CR and acyl-ghrelin-mediated regulation of AHN may be influenced by autophagy.…”

Valosin-containing protein is a key mediator between autophagic cell death and apoptosis in adult hippocampal neural stem cells following insulin withdrawal

“…Importantly, cell death rate was significantly decreased with knockdown of Atg7 in insulin-deprived HCN cells. Of note, high calpain activity switched the cell death mode from ACD to apoptosis [ 14 ]. Interestingly, activation of glycogen synthase kinase-3β (GSK-3β), one of the key signaling molecules in regulation of neuronal apoptosis, also promoted ACD, not apoptosis, in insulin-deprived HCN cells [ 15 ].…”