Bcl2-Expressing Quiescent Type B Neural Stem Cells in the Ventricular–Subventricular Zone Are Resistant to Concurrent Temozolomide/X-Irradiation

Abstract

The ventricular–subventricular zone (V‐SVZ) of the mammalian brain is a site of adult neurogenesis. Within the V‐SVZ reside type B neural stem cells (NSCs) and type A neuroblasts. The V‐SVZ is also a primary site for very aggressive glioblastoma (GBM). Standard‐of‐care therapy for GBM consists of safe maximum resection, concurrent temozolomide (TMZ), and X‐irradiation (XRT), followed by adjuvant TMZ therapy. The question of how this therapy impacts neurogenesis is not well understood and is of fundamental importance as normal tissue tolerance is a limiting factor. Here, we studied the effects of concurrent TMZ/XRT followed by adjuvant TMZ on type B stem cells and type A neuroblasts of the V‐SVZ in C57BL/6 mice. We found that chemoradiation induced an apoptotic response in type A neuroblasts, as marked by cleavage of caspase 3, but not in NSCs, and that A cells within the V‐SVZ were repopulated given sufficient recovery time. 53BP1 foci formation and resolution was used to assess the repair of DNA double‐strand breaks. Remarkably, the repair was the same in type B and type A cells. While Bax expression was the same for type A or B cells, antiapoptotic Bcl2 and Mcl1 expression was significantly greater in NSCs. Thus, the resistance of type B NSCs to TMZ/XRT appears to be due, in part, to high basal expression of antiapoptotic proteins compared with type A cells. This preclinical research, demonstrating that murine NSCs residing in the V‐SVZ are tolerant of standard chemoradiation therapy, supports a dose escalation strategy for treatment of GBM. stem cells 2019;37:1629–1639

X-ray irradiated cultures of mouse cortical neural stem/progenitor cells recover cell viability and proliferation with dose-dependent kinetics

“…Taken together, the results of this study, along with previous reports [20][21][22][23][24][25][35][36][37][38][39][40] , suggest the following conclusions: i) apoptosis is an intrinsic response of cortical NSPCs to IR and is not solely due to alterations of the surrounding niche, although the latter can enhance IR-induced NSPC death; ii) the differential response of foetal www.nature.com/scientificreports www.nature.com/scientificreports/ and adult NSPCs to IR might be partially due to intrinsic differences in these NSPC populations and not solely to variations in the extrinsic environment of their niches, although the latter are also likely to play a role.…”

“…In contrast, NSPCs of the adult VZ/SVZ niche can exist in an actively proliferating or in a quiescent status 34 . The former subpopulation is radiosensitive and responds to IR by undergoing apoptosis or terminal differentiation, in either case depleting the proliferating NSPC pool; the latter subpopulation is radioresistant and responds to IR by entering the cell cycle and replenishing the proliferating NSPC pool, although the response was shown to depend on the total dose and the dose fractionation scheme that were applied 22,[35][36][37][38][39] . Irradiation of the NSPC niche in vivo, however, causes both cell-autonomous effects on NSPCs and non-cell-autonomous effects mediated by other, non-neurogenic, cell populations of the niche, and the direct and indirect influences of IR on NSPC fate are very difficult to distinguish.…”

“…The in vitro responses of cortical NSPC cultures are consistent with the effects observed following in vivo irradiation of the foetal mouse cortex and the neonatal VZ/SVZ [22][23][24][25]40 , but they are distinct from those described in the adult VZ/SVZ niche. In the latter case, the actively proliferating NSPC pool is wiped out due to the occurrence of both apoptosis and terminal differentiation, whereas the quiescent NSPC pool is insensitive to apoptosis and responds to IR by entering the cell cycle and repopulating the proliferating compartment 22,[35][36][37] . Of note, in vitro NSPC cultures derived from the adult VZ/SVZ 20,21 , differently from foetal cortical NSPC cultures (this study), undergo wider IR-induced cell cycle exit and differentiation, suggesting that the distinct responses of different NSPC niches to IR may be partially recapitulated in vitro.…”

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“…The standard of care for treatment of glioblastoma involves concurrent temozolomide (an alkylating agent that damages DNA and induces apoptosis) and radiation therapy followed by adjuvant temozolomide chemotherapy; however, the impact of this therapeutic regimen on SVZ‐resident NSCs and their neurogenic capacity remained to be fully evaluated. In their recent Stem Cells article, researchers led by Michael L. Freeman (Vanderbilt University Medical Center, Nashville, Tennessee, USA) discovered that while glioblastoma‐targeting chemotherapy and radiotherapy led to the induction of significant levels of apoptosis in neuroblasts in tumor‐bearing and non‐tumor‐bearing preclinical murine models, NSCs themselves displayed high levels of resistance. Interestingly, Cameron et al also established that the high resistance to apoptosis in NSCs did not derive from elevated levels of DNA repair; instead, NSCs expressed higher levels of the Bcl2 and Mcl1 antiapoptotic proteins than neuroblasts, and this characteristic permitted ongoing neurogenesis after exposure of NSCs to the stress of concurrent chemotherapy and radiotherapy.…”

“…In our second Featured Article published in Stem Cells Translational Medicine this month, Chen et al describe how prostaglandin E2 receptor 4 (EP4) antagonist‐induced MSCs secrete exosomes containing a myelin‐associated enzyme with the ability to promote neurogenesis in the damaged hippocampi . In a Related Article published in Stem Cells , Cameron et al established that neural stem cells (NSCs) resist apoptosis and ensure ongoing neurogenesis following exposure to concurrent chemotherapy and radiotherapy, thereby suggesting the relative safety of targeting neurogenic brain regions during the treatment of glioblastoma …”