Toward an understanding of glucose metabolism in radial glial biology and brain development

Abstract

Decades of research have sought to determine the intrinsic and extrinsic mechanisms underpinning the regulation of neural progenitor maintenance and differentiation. A series of precise temporal transitions within progenitor cell populations generates all the appropriate neural cell types while maintaining a pool of self-renewing progenitors throughout embryogenesis. Recent technological advances have enabled us to gain new insights at the single-cell level, revealing an interplay between metabolic state and developmental progression that impacts the timing of proliferation and neurogenesis. This can have long-term consequences for the developing brain's neuronal specification, maturation state, and organization. Furthermore, these studies have highlighted the need to reassess the instructive role of glucose metabolism in determining progenitor cell division, differentiation, and fate. This review focuses on glucose metabolism (glycolysis) in cortical progenitor cells and the emerging focus on glycolysis during neurogenic transitions. Furthermore, we discuss how the field can learn from other biological systems to improve our understanding of the spatial and temporal changes in glycolysis in progenitors and evaluate functional neurological outcomes.

Figure 1.. Overview of cortical neurogenesis and angiogenesis during mouse and human corticogenesis.

(A) Early self-renewing neuroepithelial cells (NEC) proliferate in an avascular environment and contribute to the tangential expansion of the cortex. At embryonic day (E) 11.5, NECs transition into radial glia (RG), and endothelial cells from the ventral telencephalon migrate into the cortex in a ventral to dorsal fashion. From E12.5 onwards, the cortex becomes increasingly vascularized through angiogenic programs orchestrated partly by signals from RG. Dividing RG at the apical surface form close interactions with endothelial tip cell filopodia. The increased vascularization of the cortex coincides with the transition of RG from predominantly self-renewing symmetric divisions to asymmetric neurogenic divisions, generating neurons and basal progenitors, including intermediate progenitors (IPs) and outer RG in the subventricular zone (SVZ). From E16.5 onwards, RG transition to gliogenic programs, and RG basal processes form close associations with blood vessels. (B) At gestational week (GW) 14, the human fetal cortex consists of three discrete progenitor zones. The ventricular zone (VZ) consists of ventricular radial glia interacting with endothelial tip cells. The SVZ comprises intermediate progenitors, and neurons populate the cortical plate (CP). By GW16, the SVZ has two distinct zones: the inner SVZ (iSVZ) and outer SVZ (oSVZ) composed of IPs and outer radial glia (oRG). This region is significantly expanded in the human cortex. The onset of gliogenesis occurs from GW20 onwards. The iSVZ population of IP persists into these later stages of embryogenesis. b/oRG, basal/outer radial glia; CSF, cerebrospinal fluid; ctx, cortex; ECs, endothelial cells; GE, ganglionic eminence; IP, intermediate progenitor; iSVZ, inner subventricular zone; NEC, neuroepithelial cell; oSVZ, outer subventricular zone; RG, radial glia; SVZ, subventricular zone; VZ, ventricular zone.

Figure 2.. Glucose metabolism during anaerobic and aerobic glycolyses.

Membrane-associated glucose transporters transport environmental glucose into the cell. Through a series of enzymatic reactions, including hexokinase 2 (HK2) and 6-phosphofructo-2-kinase/fructose-2, 6-biphosphatase (Pfkfb3), glucose is converted into pyruvate. In anaerobic conditions, pyruvate is converted into lactate by lactate dehydrogenase A (Ldha) and transported out of the cell by monocarboxylate transporters. In aerobic conditions, pyruvate is transported into the mitochondria and converted into Acetyl CoA, which enters the tricarboxylic acid cycle. Oxidative phosphorylation (OxPhos) uses NADH produced by the tricarboxylic acid cycle via the electron transport chain. Anaerobic glycolysis produces a net gain of two ATP molecules, whereas aerobic glycolysis generates a net gain of 36. Seahorse metabolic flux assays measure oxygen consumption rate and extracellular acidification rate as readouts of OxPhos and lactate production, respectively. Förster resonance energy transfer-based reporters measure intracellular glucose, pyruvate, and lactate levels.

Figure 3.. Glycolytic programs used by cortical progenitors at early and mid-neurogenic stages of development.

(A) In the absence of vasculature, at E11.5 in mice, radial glia use anaerobic glycolysis. HIF1a target genes are up-regulated, including genes involved in glycolysis and angiogenesis. Lactate produced during anaerobic glycolysis is transported out of the cell, encouraging angiogenesis. Hypoxic conditions promote self-renewing symmetric RG divisions. (B) At E13.5 in a vascularized environment, RG divide asymmetrically to generate neurogenic radial glia that become intermediate progenitors and neurons. Intermediate progenitors depend on oxygen, generate energy through the tricarboxylic acid cycle/OxPhos, and exhibit decreased expression of glycolysis genes compared with self-renewing radial glia. Radial glial differentiation requires this transition to aerobic glycolysis. Self-renewing radial glia rely on anaerobic glycolysis and are acutely dependent on glucose. The mechanisms self-renewing radial glia use to repress OxPhos and withstand hypoxia are unknown.