Radial stem astrocytes (aka neural stem cells): Identity, development, physio-pathology, and therapeutic potential

Adult neurogenesis is a striking example of neuroplasticity, which enables adaptive network remodelling in response to all forms of environmental stimulation in physiological and pathological contexts. Dysregulation or cessation of adult neurogenesis contributes to neuropathology negatively affecting brain functions and hampering regeneration of the nervous tissue while targeting adult neurogenesis may provide the basis for potential therapeutic interventions. Neural stem cells in the adult mammalian brain are at the core and the entry point of adult neurogenesis. By their origin and properties, these cells belong to astroglia, and are represented by stem radial astrocytes (RSA) which exhibit multipotent "stemness". In the neurogenic niches, RSA interact with other cellular components, including protoplasmic astrocytes, which in turn regulate their neurogenic activity. In pathology, RSA become reactive, which affects their neurogenic capabilities, whereas reactive parenchymal astrocytes up-regulate stem cell hallmarks and are able to generate progeny that remain within astrocyte lineage. What makes RSA special is their multipotency, represented by self-renewing capacity capability to generate other cellular types as progeny. A broad understanding of the cellular features of RSA and parenchymal astrocytes provides an insight into the machinery that promotes/suppresses adult neurogenesis, clarifying principles of network remodelling. In this review, we discuss the cellular hallmarks, research tools, and models of RSA and astrocytes of the subventricular zone along the lateral ventricle and dentate gyrus of the hippocampus. We also discuss RSA in ageing, which has a great impact on the proliferative capacity of RSA, as well as the potential of RSA and astrocytes in therapeutic strategies aimed at cell replacement and regeneration.