Ablation of cholesterol biosynthesis in neural stem cells increases their VEGF expression and angiogenesis but causes neuron apoptosis

Abstract

Although sufficient cholesterol supply is known to be crucial for neurons in the developing mammalian brain, the cholesterol requirement of neural stem and progenitor cells in the embryonic central nervous system has not been addressed. Here we have conditionally ablated the activity of squalene synthase (SQS), a key enzyme for endogenous cholesterol production, in the neural stem and progenitor cells of the ventricular zone (VZ) of the embryonic mouse brain. Mutant embryos exhibited a reduced brain size due to the atrophy of the neuronal layers, and died at birth. Analyses of the E11.5-E15.5 dorsal telencephalon and diencephalon revealed that this atrophy was due to massive apoptosis of newborn neurons, implying that this progeny of the SQS-ablated neural stem and progenitor cells was dependent on endogenous cholesterol biosynthesis for survival. Interestingly, the neural stem and progenitor cells of the VZ, the primary target of SQS inactivation, did not undergo significant apoptosis. Instead, vascular endothelial growth factor (VEGF) expression in these cells was strongly upregulated via a hypoxia-inducible factor-1-independent pathway, and angiogenesis in the VZ was increased. Consistent with an increased supply of lipoproteins to these cells, the level of lipid droplets containing triacylglycerides with unsaturated fatty acyl chains was found to be elevated. Our study establishes a direct link between intracellular cholesterol levels, VEGF expression, and angiogenesis. Moreover, our data reveal a hitherto unknown compensatory process by which the neural stem and progenitor cells of the developing mammalian brain evade the detrimental consequences of impaired endogenous cholesterol biosynthesis.lipid droplets ͉ mass spectrometry ͉ neuroepithelial cells ͉ neurogenesis ͉ radial glia

Critical Time Window of Neuronal Cholesterol Synthesis during Neurite Outgrowth

“…A comparable survival period of 3 d (beginning of apoptosis after the onset of recombination) is found in vivo in SQS/Nestin-cre mutants targeting neuronal and glial progenitors (Saito et al, 2009). This is remarkable given the additional time required for SQS loss after Cre-mediated Fdft1 inactivation.…”

“…Newly postmitotic projection neurons (this study) are especially susceptible to cholesterol deprivation in contrast to neural progenitors (Saito et al, 2009), mature projection neurons (this study), or postmitotic postmigratory cerebellar granule cells (Fünfschilling et al, 2007). While neural progenitors stimulate angiogenesis for cholesterol uptake from the circulation (Saito et al, 2009), neurons in the cortical plate depend on the cholesterol supply from within the brain. Our data suggest that astrocytes respond with enhanced cholesterol biosynthesis to the lack of cholesterol in Figure 7.…”

“…Even during development, when the blood-brain barrier has not fully formed yet, the brain normally synthesizes the majority of its cholesterol locally (Tint et al, 2006). Cholesterol from the circulation can only be used by neuronal progenitors, when cholesterol biosynthesis has been inactivated in the ventricular zone (Saito et al, 2009). However, massive cell death in the cortical plate precludes further analysis of neuronal differentiation.…”

Lipid metabolism in myelinating glial cells: lessons from human inherited disorders and mouse models

“…Recent studies have also shed light on novel mechanisms of cholesterol exchange between the CNS and circulating lipoproteins under certain physiological or pathological conditions. Saito et al ( 157 ) recently showed that deletion of SQS in neural stem cells led to neuronal cell death, however, not of progenitor cells at the subventricular zone that protected themselves against cholesterol deprivation by promoting angiogenesis and consequently raising their lipoprotein cholesterol supply from the circulation. Furthermore, it was reported that specifi c loss of brain ABCA1, which is required for effl ux of cellular cholesterol, not only produced the expected reduction in brain cholesterol, but remarkably, also led to an apparently compensatory increase in the specifi c uptake of esterifi ed cholesterol from plasma HDL particles into the CNS ( 158 ).…”

Cholesterol metabolism changes under long-term dietary restrictions while the cholesterol homeostasis remains unaffected in the cortex and hippocampus of aging rats

“…Various mechanisms have been suggested to promote lipid transport across the BBB. They involve increased angiogenesis of brain capillaries (Saito et al 2009) and/or increased expression of lipoprotein receptors in brain capillary cells (Karasinska et al 2009), although the connection between cholesterol metabolism in the serum and brain needs to be investigated further.…”