Hypoxia in the regulation of neural stem cells

Abstract

In aerobic organisms, oxygen is a critical factor in tissue and organ morphogenesis from embryonic development throughout post-natal life, as it regulates various intracellular pathways involved in cellular metabolism, proliferation, survival and fate. In the mammalian central nervous system, oxygen plays a critical role in regulating the growth and differentiation state of neural stem cells (NSCs), multipotent neuronal precursor cells that reside in a particular microenvironment called the neural stem cell niche and that, under certain physiological and pathological conditions, differentiate into fully functional mature neurons, even in adults. In both experimental and clinical settings, oxygen is one of the main factors influencing NSCs. In particular, the physiological condition of mild hypoxia (2.5-5.0% O(2)) typical of neural tissues promotes NSC self-renewal; it also favors the success of engraftment when in vitro-expanded NSCs are transplanted into brain of experimental animals. In this review, we analyze how O(2) and specifically hypoxia impact on NSC self-renewal, differentiation, maturation, and homing in various in vitro and in vivo settings, including cerebral ischemia, so as to define the O(2) conditions for successful cell replacement therapy in the treatment of brain injury and neurodegenerative diseases.

Fig. 1

Hypoxic conditioning of human NSC differentiation Precocious differentiation of human NSCs into neurons and oligodendrocytes in vitro after 17 days of severe hypoxia (1% O₂; a, c, e) and mild hypoxia (5% O₂; b, d, f), as shown by immunocytochemical detection of neuronal markers. a, b β-Tubulin III-positive neuronal cells; c, d O4-positive oligodendroglial cells; e, f Galactocerebroside C (GalC)-positive oligodendroglial cells (red) Scale bars 50 μm

Fig. 2

Homing of neural stem cells in the neurovascular niche. a The neurovascular niche. From the neural stem cell niche, NSC-derived progenitor cells (NPC) migrate towards blood vessels. Here, differentiated astrocytes come in contact with blood endothelial cells (BECs) and neuronal cells: cytokines and trophic factors are important mediators of these multiple interactions. VL ventricular lumen, EL ependymal layer b–e). Confocal microscopy analysis showing human NSCs [transduced with a lentiviral vector carrying green fluorescent protein (GFP) gene or immunolabeled with anti-human nuclei antibody, (HuN), green] 1 and 3 months after transplantation into the hippocampal fissure of an adult rat brain lesioned by transient global ischemia. b At 1 month, GFP+ human NSCs (green) have migrated from the injection site to a small blood vessel. c After 3 months, they have reached the endothelial wall and have integrated around the blood vessel, where they come in contact with astroglial cells (stained for glial fibrillary acidic protein, GFAP+, red). d In proximity to the blood vessel, human NSCs (HuN+, green) have differentiated into β-tubulin III+ neuronal cells (red). e Note the high density of microglial Iba1+ cells nested in the endothelial wall and the surrounding area, showing the persistence of an inflammatory environment. Scale bars (b) 70 μm, (c) 63 μm, (d) 25 μm, (e) 20 μm

Fig. 3

Mild hypoxia enhances self-renewal of human NSCs and their ability to generate neurons and oligodendrocytes. a Model of how changes of oxygen tension from atmospheric levels to severe hypoxia may regulate proliferation, self-renewal and differentiation of NSCs. Under conditions of atmospheric oxygen, human NSCs have a slow proliferation rate (SPSC, slowly proliferating stem cells) and a preferential commitment to the astroglial lineage. Mild hypoxia increases the self-renewal of human NSCs (FPSC, fast proliferating stem cells) and leads to an enrichment of neuronal and oligodendroglial progenitors. In contrast, severe hypoxia drives human NSCs toward a state of quiescence (QSC, quiescent stem cells), characterized by a very slow proliferation rate accompanied by tendency to exit from the cell cycle and to undergo precocious differentiation or apoptosis. b Confocal microscopy images of human NSCs at 17 days of differentiation under 20, 5 and 1% oxygen. Note proliferating cells (Ki67+, green) and the distribution of mitochondria (HuMi, red) at 1% oxygen, where they are aggregated and fixed in small clusters. Conversely, in mild hypoxia (5% O₂), mitochondria appear fused in long homogeneous chains running along the processes and nested in correspondence of the nuclei. In 20% O₂, mitochondria are still fused, but their chains appear shortened and more disorganized than in 5% O₂. Scale bars 30 μm