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

Proteasome activation is critical for cell death induced by inhibitors of polo-like kinase 1 (PLK1) in multiple cancers

Inhibitors of polo-like kinase (PLK) are currently being evaluated as anticancer drugs. However, the molecular mechanism of PLK inhibitor-induced cell death is not fully understood. In this study, we found that GW843682X and BI2536, two inhibitors of PLK1, significantly induced cell death in multiple type cells. The induction of cell death was related to the preferring expression of PLK1. However, in human umbilical vascular endothelial cells (HUVEC) and human colorectal carcinoma cells, which expressed higher levels of both PLK1 and PLK2, PLK1 inhibitors induced very low levels of cell death. Clinical analysis reveals PLK1 presence in 26 of 30 NPC tumor tissues. In in vivo NPC lung metastasis nude mouse models, PLK1 inhibitors decreased NPC progress. Mechanistically, the PLK1 inhibitor did not activate p53, and the cell death was not reversed by p53 inhibition. Moreover, PLK1 inhibitor-induced cell death was PARP- and caspase-independent. Although PLK1 inhibitors induced down-regulation of calpain inhibitor calpastatin and calpain was activated by PLK1 inhibition, calpain blocking did not reverse cell death induced by PLK1 inhibitors, suggesting the non-involvement of calpain. Surprisingly, we found that PLK1 inhibitors induced the activation of proteasome, and the treatment of cells with PLK1 inhibitors reduced the levels of ubiquitinated proteins. And proteasome inhibitors reversed cell death induced by PLK1 inhibitors in various cell types in which PLK1 was preferentially expressed. Moreover, PLK1 inhibition reversed the degradation of proteins including p53, caspase 8, PARP and calpastatin. These results suggest that the activation of proteasome is critical for cell death induced by PLK1 inhibition.

Autophagy: Regulator of cell death

Autophagy is the process by which cells degrade and recycle proteins and organelles to maintain intracellular homeostasis. Generally, autophagy plays a protective role in cells, but disruption of autophagy mechanisms or excessive autophagic flux usually leads to cell death. Despite recent progress in the study of the regulation and underlying molecular mechanisms of autophagy, numerous questions remain to be answered. How does autophagy regulate cell death? What are the fine-tuned regulatory mechanisms underlying autophagy-dependent cell death (ADCD) and autophagy-mediated cell death (AMCD)? In this article, we highlight the different roles of autophagy in cell death and discuss six of the main autophagy-related cell death modalities, with a focus on the metabolic changes caused by excessive endoplasmic reticulum-phagy (ER-phagy)-induced cell death and the role of mitophagy in autophagy-mediated ferroptosis. Finally, we discuss autophagy enhancement in the treatment of diseases and offer a new perspective based on the use of autophagy for different functional conversions (including the conversion of autophagy and that of different autophagy-mediated cell death modalities) for the clinical treatment of tumors.

Distinct Signaling Pathways for Autophagy-Driven Cell Death and Survival in Adult Hippocampal Neural Stem Cells

Autophagy is a cellular catabolic process that degrades and recycles cellular materials. Autophagy is considered to be beneficial to the cell and organism by preventing the accumulation of toxic protein aggregates, removing damaged organelles, and providing bioenergetic substrates that are necessary for survival. However, autophagy can also cause cell death depending on cellular contexts. Yet, little is known about the signaling pathways that differentially regulate the opposite outcomes of autophagy. We have previously reported that insulin withdrawal (IW) or corticosterone (CORT) induces autophagic cell death (ACD) in adult hippocampal neural stem (HCN) cells. On the other hand, metabolic stresses caused by 2-deoxy-D-glucose (2DG) and glucose-low (GL) induce autophagy without death in HCN cells. Rather, we found that 2DG-induced autophagy was cytoprotective. By comparing IW and CORT conditions with 2DG treatment, we revealed that ERK and JNK are involved with 2DG-induced protective autophagy, whereas GSK-3β regulates death-inducing autophagy. These data suggest that cell death and survival-promoting autophagy undergo differential regulation with distinct signaling pathways in HCN cells.

Valosin-Containing Protein (VCP)/p97: A Prognostic Biomarker and Therapeutic Target in Cancer

Valosin-containing protein (VCP)/p97, a member of the AAA+ ATPase family, is a molecular chaperone recruited to the endoplasmic reticulum (ER) membrane by binding to membrane adapters (nuclear protein localization protein 4 (NPL4), p47 and ubiquitin regulatory X (UBX) domain-containing protein 1 (UBXD1)), where it is involved in ER-associated protein degradation (ERAD). However, VCP/p97 interacts with many cofactors to participate in different cellular processes that are critical for cancer cell survival and aggressiveness. Indeed, VCP/p97 is reported to be overexpressed in many cancer types and is considered a potential cancer biomarker and therapeutic target. This review summarizes the role of VCP/p97 in different cancers and the advances in the discovery of small-molecule inhibitors with therapeutic potential, focusing on the challenges associated with cancer-related VCP mutations in the mechanisms of resistance to inhibitors.

Role of Calpain-1 in Neurogenesis

While calpains have been implicated in neurogenesis for a long time, there is still little information regarding the specific contributions of various isoforms in this process. We took advantage of the availability of mutant mice with complete deletion of calpain-1 to analyze its contribution to neurogenesis. We first used the incorporation of BrdU in newly-generated cells in the subgranular zone of the dentate gyrus to determine the role of calpain-1 deletion in neuronal proliferation. Our results showed that the lack of calpain-1 decreased the rate of cell proliferation in adult hippocampus. As previously shown, it also decreased the long-term survival of newly-generated neurons. We also used data from previously reported RNA and miRNA sequencing analyses to identify differentially expressed genes in brain of calpain-1 knock-out mice related to cell division, cell migration, cell proliferation and cell survival. A number of differentially expressed genes were identified, which could play a significant role in the changes in neurogenesis in calpain-1 knock out mice. The results provide new information regarding the role of calpain-1 in neurogenesis and have implications for better understanding the pathologies associated with calpain-1 mutations in humans.

GSK3B induces autophagy by phosphorylating ULK1

Unc-51-like autophagy activating kinase 1 (ULK1), a mammalian homolog of the yeast kinase Atg1, has an essential role in autophagy induction. In nutrient and growth factor signaling, ULK1 activity is regulated by various posttranslational modifications, including phosphorylation, acetylation, and ubiquitination. We previously identified glycogen synthase kinase 3 beta (GSK3B) as an upstream regulator of insulin withdrawal-induced autophagy in adult hippocampal neural stem cells. Here, we report that following insulin withdrawal, GSK3B directly interacted with and activated ULK1 via phosphorylation of S405 and S415 within the GABARAP-interacting region. Phosphorylation of these residues facilitated the interaction of ULK1 with MAP1LC3B and GABARAPL1, while phosphorylation-defective mutants of ULK1 failed to do so and could not induce autophagy flux. Furthermore, high phosphorylation levels of ULK1 at S405 and S415 were observed in human pancreatic cancer cell lines, all of which are known to exhibit high levels of autophagy. Our results reveal the importance of GSK3B-mediated phosphorylation for ULK1 regulation and autophagy induction and potentially for tumorigenesis.

Similar to cellular starvation conditions, insulin withdrawal may trigger the modification of an enzyme involved in the induction of autophagy, a key cellular recycling process. The ULK1 enzyme has a critical role in autophagy induction. Seong-Woon Yu at the Daegu Gyeongbuk Institute of Science and Technology, South Korea, and co-workers investigated how ULK1 is activated under insulin withdrawal condition. They found that another enzyme called GSK3B modifies two specific ULK1 amino acids, activating ULK1 and triggering autophagy. Further, they found high levels of this type of ULK1 modification in human pancreatic cancer cell lines that exhibited increased autophagy, suggesting possible implications for the development of certain cancerous tumors.

Hypothyroidism Induces Interleukin-1-Dependent Autophagy Mechanism as a Key Mediator of Hippocampal Neuronal Apoptosis and Cognitive Decline in Postnatal Rats

Thyroid hormone (TH) is essential for brain development, and hypothyroidism induces cognitive deficits in children and young adults. However, the participating mechanisms remain less explored. Here, we examined the molecular mechanism, hypothesizing the involvement of a deregulated autophagy and apoptosis pathway in hippocampal neurons that regulate cognitive functions. Therefore, we used a rat model of developmental hypothyroidism, generated through methimazole treatment from gestation until young adulthood. We detected that methimazole stimulated the autophagy mechanism, characterized by increased LC3B-II, Beclin-1, ATG7, and ATG5-12 conjugate and decreased p-mTOR/mTOR and p-ULK1/ULK1 autophagy regulators in the hippocampus of developing and young adult rats. This methimazole-induced hippocampal autophagy could be inhibited by thyroxine treatment. Subsequently, probing the upstream mediators of autophagy revealed an increased hippocampal neuroinflammation, marked by upregulated interleukin (IL)-1alpha and beta and activated microglial marker, Iba1, promoting neuronal IL-1 receptor-1 expression. Hence, IL-1R-antagonist (IL-1Ra), which reduced hippocampal neuronal IL-1R1, also inhibited the enhanced autophagy in hypothyroid rats. We then linked these events with hypothyroidism-induced apoptosis and loss of hippocampal neurons, where we observed that like thyroxine, IL-1Ra and autophagy inhibitor, 3-methyladenine, reduced the cleaved caspase-3 and TUNEL-stained apoptotic neurons and enhanced Nissl-stained neuronal count in methimazole-treated rats. We further related these molecular results with cognition through Y-maze and passive avoidance tests, demonstrating an IL-1Ra and 3-methyladenine-mediated improvement in learning-memory performances of the hypothyroid rats. Taken together, our study enlightens the critical role of neuroinflammation-dependent autophagy mechanism in TH-regulated hippocampal functions, disrupted in developmental hypothyroidism.

Combined pre-conditioning with salidroside and hypoxia improves proliferation, migration and stress tolerance of adipose-derived stem cells

Oxidative stress after ischaemia impairs the function of transplanted stem cells. Increasing evidence has suggested that either salidroside (SAL) or hypoxia regulates growth of stem cells. However, the role of SAL in regulating function of hypoxia‐pre–conditioned stem cells remains elusive. Thus, this study aimed to determine the effect of SAL and hypoxia pre‐conditionings on the proliferation, migration and tolerance against oxidative stress in rat adipose‐derived stem cells (rASCs). rASCs treated with SAL under normoxia (20% O₂) or hypoxia (5% O₂) were analysed for the cell viability, proliferation, migration and resistance against H₂O₂‐induced oxidative stress. In addition, the activation of Akt, Erk1/2, LC3, NF‐κB and apoptosis‐associated pathways was assayed by Western blot. The results showed that SAL and hypoxia treatments synergistically enhanced the viability (fold) and proliferation of rASCs under non‐stressed conditions in association with increased autophagic flux and activation of Akt, Erk1/2 and LC3. H₂O₂‐induced oxidative stress, cytotoxicity, apoptosis, autophagic cell death and NF‐κB activation were inhibited by SAL or hypoxia, and further attenuated by the combined SAL and hypoxia pre‐treatment. The SAL and hypoxia pre‐treatment also enhanced the proliferation and migration of rASCs under oxidative stress in association with Akt and Erk1/2 activation; however, the combined pre‐treatment exhibited a more profound enhancement in the migration than proliferation. Our data suggest that SAL combined with hypoxia pre‐conditioning may enhance the therapeutic capacity of ASCs in post‐ischaemic repair.

Autophagy as a decisive process for cell death

Autophagy is an intracellular catabolic pathway in which cellular constituents are engulfed by autophagosomes and degraded upon autophagosome fusion with lysosomes. Autophagy serves as a major cytoprotective process by maintaining cellular homeostasis and recycling cytoplasmic contents. However, emerging evidence suggests that autophagy is a primary mechanism of cell death (autophagic cell death, ACD) and implicates ACD in several aspects of mammalian physiology, including tumor suppression and psychological disorders. However, little is known about the physiological roles and molecular mechanisms of ACD. In this review, we document examples of ACD and discuss recent progress in our understanding of its molecular mechanisms.

MicroRNA Alteration in Developing Rat Oligodendrocyte Precursor Cells Induced by Hypoxia-Ischemia

microRNAs (miRNAs) are involved in the pathogenesis of white matter injury (WMI). However, their roles in developing rat brains under hypoxia-ischemia (HI) insult remain unknown. Here, we examined the expression profiles of miRNAs in oligodendrocyte precursor cells using microarray analysis. We identified 162 miRNAs and only 6 were differentially regulated in HI compared with sham. Next, we used these 6 miRNAs and 525 extensively changed coding genes (fold change absolute: FC(abs) ≥2, p < 0.05) to establish the coexpression network, the result revealed that only 3 miRNAs (miR-142-3p, miR-466b-5p, and miR-146a-5p) have differentially expressed targeted mRNAs. RT-PCR analysis showed that the expression of the miRNAs was consistent with the microarray analysis. Further gene ontology and KEGG pathway analysis of the targets of these 3 miRNAs indicated that they were largely associated with neural activity. Furthermore, we found that 2 of the 3 miRNAs, miR-142-3p, and miR-466b-5p, have the same target gene, Capn6, an antiapoptotic gene that is tightly regulated in the pathogenesis of neurological diseases. Collectively, we have shown that a number of miRNAs change in oligodendrocyte precursor cells in response to HI insult in developing brains, and miR-142-3p/miR-466b-5p/Capn6 pathway might affect the pathogenesis of WMI, providing us new clues for the diagnosis and therapy for WMI.

Autophagy Mediates Astrogenesis in Adult Hippocampal Neural Stem Cells

Neural stem cells (NSCs) have the ability to self-renew and differentiate into neurons, oligodendrocytes, and astrocytes. Highly dynamic nature of NSC differentiation requires the intimate involvement of catabolic processes such as autophagy. Autophagy is a major intracellular degradation pathway necessary for cellular homeostasis and remodeling. Autophagy is important for mammalian development and its role in neurogenesis has recently drawn much attention. However, little is known about how autophagy is associated with differentiation of NSCs into other neural lineages. Here, we report that autophagy plays a critical role in differentiation of adult rat hippocampal neural stem (HCN) cells into astrocytes. During differentiation, autophagy flux peaked at early time points, and remained high. Pharmacological or genetic suppression of autophagy by stable knockdown of Atg7, LC3 or CRISPR-Cas9-mediated knockout (KO) of p62 impaired astrogenesis, while reintroduction of p62 recovered astrogenesis in p62 KO HCN cells. Taken together, our findings suggest that autophagy plays a key role in astrogenesis in adult NSCs.

Parkin Promotes Mitophagic Cell Death in Adult Hippocampal Neural Stem Cells Following Insulin Withdrawal

Regulated cell death (RCD) plays a fundamental role in human health and disease. Apoptosis is the best-studied mode of RCD, but the importance of other modes has recently been gaining attention. We have previously demonstrated that adult rat hippocampal neural stem (HCN) cells undergo autophagy-dependent cell death (ADCD) following insulin withdrawal. Here, we show that Parkin mediates mitophagy and ADCD in insulin-deprived HCN cells. Insulin withdrawal increased the amount of depolarized mitochondria and their colocalization with autophagosomes. Insulin withdrawal also upregulated both mRNA and protein levels of Parkin, gene knockout of which prevented mitophagy and ADCD. c-Jun is a transcriptional repressor of Parkin and is degraded by the proteasome following insulin withdrawal. In insulin-deprived HCN cells, Parkin is required for Ca2+ accumulation and depolarization of mitochondria at the early stages of mitophagy as well as for recognition and removal of depolarized mitochondria at later stages. In contrast to the pro-death role of Parkin during mitophagy, Parkin deletion rendered HCN cells susceptible to apoptosis, revealing distinct roles of Parkin depending on different modes of RCD. Taken together, these results indicate that Parkin is required for the induction of ADCD accompanying mitochondrial dysfunction in HCN cells following insulin withdrawal. Since impaired insulin signaling is implicated in hippocampal deficits in various neurodegenerative diseases and psychological disorders, these findings may help to understand the mechanisms underlying death of neural stem cells and develop novel therapeutic strategies aiming to improve neurogenesis and survival of neural stem cells.

Killing Two Angry Birds with One Stone: Autophagy Activation by Inhibiting Calpains in Neurodegenerative Diseases and Beyond

Proteolytic machineries execute vital cellular functions and their disturbances are implicated in diverse medical conditions, including neurodegenerative diseases. Interestingly, calpains, a class of Ca²⁺-dependent regulatory proteases, can modulate the degradational system of autophagy by cleaving proteins involved in this pathway. Moreover, both machineries are common players in many molecular pathomechanisms and have been targeted individually or together, as a therapeutic strategy in experimental setups. In this review, we briefly introduce calpains and autophagy, with their roles in health and disease, and focus on their direct pathologically relevant interplay in neurodegeneration and beyond. The modulation of calpain activity may comprise a promising treatment approach to attenuate the deregulation of these two essential mechanisms.

MicroRNA expression profiling of adult hippocampal neural stem cells upon cell death reveals an autophagic cell death-like pattern

Adult hippocampal neural (HCN) stem cells promptly undergo irreversible autophagic cell death (ACD) if deprived of insulin in culture. Small, non-coding microRNAs (miRNA) play an important role in regulating biological processes, including proliferation and cell death. However, there have been no reports thus far regarding miRNA involvement in the induction of adult HCN stem cell death under insulin-deprived conditions, for which we performed a microarray-based analysis to examine the expression signature of miRNAs in adult rat HCN stem cells. Three independent specimens per culture condition either with or without insulin were prepared and a miRNA microarray analysis carried out. A total of 12 exhibited significantly altered expression levels upon cell death due to the absence of insulin when compared to HCN stem cells cultured with insulin present (cut-off limit; p < 0.05 and fold-change >1.3) The resulting volcano plot showed that, among these miRNAs, seven were upregulated and five were downregulated. The upregulated miRNAs were capable of modulating HCN stem cell death. Caspase-3 activity analysis, LC3 conversion, and TEM of autophagosome formation consistently suggested that ACD, not apoptosis, was most likely the mechanism affecting HCN cell death. As such, we have come to term these miRNAs, "HCN stem cell-specific autophagic cell death regulators." Taken together, our data suggest that the miRNA expression profile of HCN stem cells is altered during ACD occurring due to insulin deprivation and that differentially expressed miRNAs are involved in HCN stem cell viability. Detailed explorations of the underlying mechanisms regarding HCN stem cell viability modulation by these miRNAs would be beneficial in further understanding the physiological features of adult HCN stem cells and are currently being investigated.

On the Role of Basal Autophagy in Adult Neural Stem Cells and Neurogenesis

Adult neurogenesis persists in the adult mammalian brain due to the existence of neural stem cell (NSC) reservoirs in defined niches, where they give rise to new neurons throughout life. Recent research has begun to address the implication of constitutive (basal) autophagy in the regulation of neurogenesis in the mature brain. This review summarizes the current knowledge on the role of autophagy-related genes in modulating adult NSCs, progenitor cells and their differentiation into neurons. The general function of autophagy in neurogenesis in several areas of the embryonic forebrain is also revisited. During development, basal autophagy regulates Wnt and Notch signaling and is mainly required for adequate neuronal differentiation. The available data in the adult indicate that the autophagy-lysosomal pathway regulates adult NSC maintenance, the activation of quiescent NSCs, the survival of the newly born neurons and the timing of their maturation. Future research is warranted to validate the results of these pioneering studies, refine the molecular mechanisms underlying the regulation of NSCs and newborn neurons by autophagy throughout the life-span of mammals and provide significance to the autophagic process in adult neurogenesis-dependent behavioral tasks, in physiological and pathological conditions. These lines of research may have important consequences for our understanding of stem cell dysfunction and neurogenic decline during healthy aging and neurodegeneration.

Chronic restraint stress induces hippocampal memory deficits by impairing insulin signaling

Chronic stress is a psychologically significant factor that impairs learning and memory in the hippocampus. Insulin signaling is important for the development and cognitive function of the hippocampus. However, the relation between chronic stress and insulin signaling at the molecular level is poorly understood. Here, we show that chronic stress impairs insulin signaling in vitro and in vivo, and thereby induces deficits in hippocampal spatial working memory and neurobehavior. Corticosterone treatment of mouse hippocampal neurons in vitro caused neurotoxicity with an increase in the markers of autophagy but not apoptosis. Corticosterone treatment impaired insulin signaling from early time points. As an in vivo model of stress, mice were subjected to chronic restraint stress. The chronic restraint stress group showed downregulated insulin signaling and suffered deficits in spatial working memory and nesting behavior. Intranasal insulin delivery restored insulin signaling and rescued hippocampal deficits. Our data suggest that psychological stress impairs insulin signaling and results in hippocampal deficits, and these effects can be prevented by intranasal insulin delivery.

Regulatory role of calpain in neuronal death

Calpains are a group of calcium-dependent proteases that are over activated by increased intracellular calcium levels under pathological conditions. A wide range of substrates that regulate necrotic, apoptotic and autophagic pathways are affected by calpain. Calpain plays a very important role in neuronal death and various neurological disorders. This review introduces recent research progress related to the regulatory mechanisms of calpain in neuronal death. Various neuronal programmed death pathways including apoptosis, autophagy and regulated necrosis can be divided into receptor interacting protein-dependent necroptosis, mitochondrial permeability transition-dependent necrosis, pyroptosis and poly (ADP-ribose) polymerase 1-mediated parthanatos. Calpains cleave series of key substrates that may lead to cell death or participate in cell death. Regarding the investigation of calpain-mediated programed cell death, it is necessary to identify specific inhibitors that inhibit calpain mediated neuronal death and nervous system diseases.

Autophagy in stem cells: repair, remodelling and metabolic reprogramming

Autophagy is a catabolic pathway by which cellular components are delivered to the lysosome for degradation and recycling. Autophagy serves as a crucial intracellular quality control and repair mechanism but is also involved in cell remodelling during development and cell differentiation. In addition, mitophagy, the process by which damaged mitochondria undergo autophagy, has emerged as key regulator of cell metabolism. In recent years, a number of studies have revealed roles for autophagy and mitophagy in the regulation of stem cells, which represent the origin for all tissues during embryonic and postnatal development, and contribute to tissue homeostasis and repair throughout adult life. Here, we review these studies, focussing on the latest evidence that supports the quality control, remodelling and metabolic functions of autophagy during the activation, self-renewal and differentiation of embryonic, adult and cancer stem cells.

Autophagy and Adult Neurogenesis: Discoveries Made Half a Century Ago Yet in their Infancy of being Connected

Within the brain, the physiological and pathological functions of autophagy in development and throughout the lifespan are being elucidated. This review summarizes recent in vitro and in vivo results that are defining the role of autophagy-related genes during the process of adult neurogenesis. We also discuss the need for future experiments to determine the molecular mechanism and functional significance of autophagy in the different neural stem cell populations and throughout the stages of adult neurogenesis.

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

Calorie intake is essential for regulating normal physiological processes and is fundamental to maintaining life. Indeed, both extremes of calorie intake result in increased morbidity and mortality. In this review, we discuss the effect of calorie intake on adult brain function, with an emphasis on the beneficial effects of mild calorie restriction. Recent findings relating to the regenerative and protective effects of the gastrointestinal hormone, ghrelin, suggest that it may underlie the beneficial effects of calorie restriction. We discuss the putative cellular mechanisms underlying the action of ghrelin and their possible role in supporting healthy brain ageing.

Targeting Adult Neurogenesis for Poststroke Therapy

Adult neurogenesis mainly occurs at the subventricular zone (SVZ) on the walls of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus (DG). However, the majority of newborn neurons undergo programmed cell death (PCD) during the period of proliferation, migration, and integration. Stroke activates neural stem cells (NSCs) in both SVZ and SGZ. This process is regulated by a wide variety of signaling pathways. However, the newborn neurons derived from adult neurogenesis are insufficient for tissue repair and function recovery. Thus, enhancing the endogenous neurogenesis driven by ischemia and promoting the survival of newborn neurons can be promising therapeutic interventions for stroke. Here, we present an overview of the process of adult neurogenesis and the potential of stroke-induced neurogenesis on brain repair.

Phosphorylation of p62 by AMP-activated protein kinase mediates autophagic cell death in adult hippocampal neural stem cells

In the adult brain, programmed death of neural stem cells is considered to be critical for tissue homeostasis and cognitive function and is dysregulated in neurodegeneration. Previously, we have reported that adult rat hippocampal neural (HCN) stem cells undergo autophagic cell death (ACD) following insulin withdrawal. Because the apoptotic capability of the HCN cells was intact, our findings suggested activation of unique molecular mechanisms linking insulin withdrawal to ACD rather than apoptosis. Here, we report that phosphorylation of autophagy-associated protein p62 by AMP-activated protein kinase (AMPK) drives ACD and mitophagy in HCN cells. Pharmacological inhibition of AMPK or genetic ablation of the AMPK α2 subunit by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 genome editing suppressed ACD, whereas AMPK activation promoted ACD in insulin-deprived HCN cells. We found that following insulin withdrawal AMPK phosphorylated p62 at a novel site, Ser-293/Ser-294 (in rat and human p62, respectively). Phosphorylated p62 translocated to mitochondria and induced mitophagy and ACD. Interestingly, p62 phosphorylation at Ser-293 was not required for staurosporine-induced apoptosis in HCN cells. To the best of our knowledge, this is the first report on the direct phosphorylation of p62 by AMPK. Our data suggest that AMPK-mediated p62 phosphorylation is an ACD-specific signaling event and provide novel mechanistic insight into the molecular mechanisms in ACD.

Mediation of Autophagic Cell Death by Type 3 Ryanodine Receptor (RyR3) in Adult Hippocampal Neural Stem Cells

Cytoplasmic Ca²⁺ actively engages in diverse intracellular processes from protein synthesis, folding and trafficking to cell survival and death. Dysregulation of intracellular Ca²⁺ levels is observed in various neuropathological states including Alzheimer’s and Parkinson’s diseases. Ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP₃Rs), the main Ca²⁺ release channels located in endoplasmic reticulum (ER) membranes, are known to direct various cellular events such as autophagy and apoptosis. Here we investigated the intracellular Ca²⁺-mediated regulation of survival and death of adult hippocampal neural stem (HCN) cells utilizing an insulin withdrawal model of autophagic cell death (ACD). Despite comparable expression levels of RyR and IP₃R transcripts in HCN cells at normal state, the expression levels of RyRs—especially RyR3—were markedly upregulated upon insulin withdrawal. While treatment with the RyR agonist caffeine significantly promoted the autophagic death of insulin-deficient HCN cells, treatment with its inhibitor dantrolene prevented the induction of autophagy following insulin withdrawal. Furthermore, CRISPR/Cas9-mediated knockout of the RyR3 gene abolished ACD of HCN cells. This study delineates a distinct, RyR3-mediated ER Ca²⁺ regulation of autophagy and programmed cell death in neural stem cells. Our findings provide novel insights into the critical, yet understudied mechanisms underlying the regulatory function of ER Ca²⁺ in neural stem cell biology.

Control of adult neurogenesis by programmed cell death in the mammalian brain

The presence of neural stem cells (NSCs) and the production of new neurons in the adult brain have received great attention from scientists and the public because of implications to brain plasticity and their potential use for treating currently incurable brain diseases. Adult neurogenesis is controlled at multiple levels, including proliferation, differentiation, migration, and programmed cell death (PCD). Among these, PCD is the last and most prominent process for regulating the final number of mature neurons integrated into neural circuits. PCD can be classified into apoptosis, necrosis, and autophagic cell death and emerging evidence suggests that all three may be important modes of cell death in neural stem/progenitor cells. However, the molecular mechanisms that regulate PCD and thereby impact the intricate balance between self-renewal, proliferation, and differentiation during adult neurogenesis are not well understood. In this comprehensive review, we focus on the extent, mechanism, and biological significance of PCD for the control of adult neurogenesis in the mammalian brain. The role of intrinsic and extrinsic factors in the regulation of PCD at the molecular and systems levels is also discussed. Adult neurogenesis is a dynamic process, and the signals for differentiation, proliferation, and death of neural progenitor/stem cells are closely interrelated. A better understanding of how adult neurogenesis is influenced by PCD will help lead to important insights relevant to brain health and diseases.

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

BACKGROUND

Programmed cell death (PCD) plays essential roles in the regulation of survival and function of neural stem cells (NSCs). Abnormal regulation of this process is associated with developmental and degenerative neuronal disorders. However, the mechanisms underlying the PCD of NSCs remain largely unknown. Understanding the mechanisms of PCD in NSCs is crucial for exploring therapeutic strategies for the treatment of neurodegenerative diseases.

RESULT

We have previously reported that adult rat hippocampal neural stem (HCN) cells undergo autophagic cell death (ACD) following insulin withdrawal without apoptotic signs despite their normal apoptotic capabilities. It is unknown how interconnection between ACD and apoptosis is mediated in HCN cells. Valosin-containing protein (VCP) is known to be essential for autophagosome maturation in mammalian cells. VCP is abundantly expressed in HCN cells compared to hippocampal tissue and neurons. Pharmacological and genetic inhibition of VCP at basal state in the presence of insulin modestly impaired autophagic flux, consistent with its known role in autophagosome maturation. Of note, VCP inaction in insulin-deprived HCN cells significantly decreased ACD and down-regulated autophagy initiation signals with robust induction of apoptosis. Overall autophagy level was also substantially reduced, suggesting the novel roles of VCP at initial step of autophagy.

CONCLUSION

Taken together, these data demonstrate that VCP may play an essential role in the initiation of autophagy and mediation of crosstalk between ACD and apoptosis in HCN cells when autophagy level is high upon insulin withdrawal. This is the first report on the role of VCP in regulation of NSC cell death. Elucidating the mechanism by which VCP regulates the crosstalk of ACD and apoptosis will contribute to understanding the molecular mechanism of PCD in NSCs.

Autophagy for the quality control of adult hippocampal neural stem cells

Autophagy plays an important role in neurodegeneration, as well as in normal brain development and function. Recent studies have also implicated autophagy in the regulation of stemness and neurogenesis in neural stem cells (NSCs). However, little is known regarding the roles of autophagy in NSC biology. It has been shown that in addition to cytoprotective roles of autophagy, pro-death autophagy, or ׳autophagic cell death (ACD),' regulates the quantity of adult NSCs. A tight regulation of survival and death of NSCs residing in the neurogenic niches through programmed cell death (PCD) is critical for maintenance of adult neurogenesis. ACD plays a primary role in the death of adult hippocampal neural stem (HCN) cells following insulin withdrawal. Despite the normal apoptotic capability of HCN cells, they are committed to death by autophagy following insulin withdrawal, suggesting the existence of a unique regulatory program that controls the mode of cell death. We propose that dual roles of autophagy for maintenance of NSC pluripotency, as well as for elimination of defective NSCs, may serve as a combined NSC quality control mechanism. This article is part of a Special Issue entitled SI:Autophagy.