Does the plasticity of neural stem cells and neurogenesis make them biosensors of disease and damage?

Autophagy in neural stem cells and glia for brain health and diseases

Autophagy is a multifaceted cellular process that not only maintains the homeostatic and adaptive responses of the brain but is also dynamically involved in the regulation of neural cell generation, maturation, and survival. Autophagy facilities the utilization of energy and the microenvironment for developing neural stem cells. Autophagy arbitrates structural and functional remodeling during the cell differentiation process. Autophagy also plays an indispensable role in the maintenance of stemness and homeostasis in neural stem cells during essential brain physiology and also in the instigation and progression of diseases. Only recently, studies have begun to shed light on autophagy regulation in glia (microglia, astrocyte, and oligodendrocyte) in the brain. Glial cells have attained relatively less consideration despite their unquestioned influence on various aspects of neural development, synaptic function, brain metabolism, cellular debris clearing, and restoration of damaged or injured tissues. Thus, this review composes pertinent information regarding the involvement of autophagy in neural stem cells and glial regulation and the role of this connexion in normal brain functions, neurodevelopmental disorders, and neurodegenerative diseases. This review will provide insight into establishing a concrete strategic approach for investigating pathological mechanisms and developing therapies for brain diseases.

Excitatory amino acid transporter 1 supports adult hippocampal neural stem cell self-renewal

Within the adult mammalian dentate gyrus (DG) of the hippocampus, glutamate stimulates neural stem cell (NSC) self-renewing proliferation, providing a link between adult neurogenesis and local circuit activity. Here, we show that glutamate-induced self-renewal of adult DG NSCs requires glutamate transport via excitatory amino acid transporter 1 (EAAT1) to stimulate lipogenesis. Loss of EAAT1 prevented glutamate-induced self-renewing proliferation of NSCs in vitro and in vivo, with little role evident for canonical glutamate receptors. Transcriptomics and further pathway manipulation revealed that glutamate simulation of NSCs relied on EAAT1 transport-stimulated lipogenesis. Our findings demonstrate a critical, direct role for EAAT1 in stimulating NSCs to support neurogenesis in adulthood, thereby providing insights into a non-canonical mechanism by which NSCs sense and respond to their niche.

The differential response to neuronal hyperexcitation and neuroinflammation of the hippocampal neurogenic niche

Hippocampal neurogenesis is a tightly regulated process in which neural stem cells (NSCs) get activated, enter in the cell cycle and give rise to neurons after a multistep process. Quiescent and activated NSCs, neural precursors, immature and mature neurons and newborn astrocytes coexist in the neurogenic niche in a strictly controlled environment which maintains the correct functioning of neurogenesis. NSCs are the first step in the neurogenic process and are a finite and, mostly, non-renewable resource, therefore any alteration of the intrinsic properties of NSCs will impact the total neurogenic output. Neuronal hyperexcitation is a strong activator of NSCs prompting them to divide and therefore increasing neurogenesis. However, neuronal hyperactivity is not an isolated process but often also involves excitotoxicity which is subsequently accompanied by neuroinflammation. Neuroinflammation normally reduces the activation of NSCs. It is technically difficult to isolate the effect of neuronal hyperexcitation alone, but neuroinflammation without neuronal hyperexcitation can be studied in a variety of models. In order to shed light on how the balance of neuronal hyperexcitation and neuroinflammation affect NSCs we analyzed proliferation and morphology of NSCs. We used two models of neuronal hyperactivity [an epilepsy model induced by KA, and a model of traumatic brain injury (TBI)] and different models of inflammation (LPS, Poly I:C, IFN-α and IL-6). We observed that only those models that induce neuronal hyperactivity induce NSCs activation but neuroinflammation causes the opposite effect. We also analyzed the response of other cell types in the neurogenic niche, focusing on astrocytes.