Defective Brain Development in Mice Lacking the Hif-1α Gene in Neural Cells

Abstract

Hypoxia-inducible factor 1␣ (HIF-1␣) is essential for vascular development during embryogenesis and pathogenesis. However, little is known about its role in brain development. To investigate the function of HIF-1␣ in the central nervous system, a conditional knockout mouse was made with the Cre/LoxP system with a nestin promoter-driven Cre. Neural cell-specific HIF-1␣-deficient mice exhibit hydrocephalus accompanied by a reduction in neural cells and an impairment of spatial memory. Apoptosis of neural cells coincided with vascular regression in the telencephalon of mutant embryos, and these embryonic defects were successfully restored by in vivo gene delivery of HIF-1␣ to the embryos. These results showed that expression of HIF-1␣ in neural cells was essential for normal development of the brain and established a mouse model that would be useful for the evaluation of therapeutic strategies for ischemia, including hypoxia-mediated hydrocephalus.Oxygen deprivation initiates a wide range of responses to increase oxygen supply, including compensation for the loss of vital energy by alternating the expression of a variety of genes. Many of these hypoxia-inducible genes appear to have a common mode of regulation that involves activation of hypoxiainducible factor 1␣ (HIF-1␣), an oxygen-responsive subunit member of the basic helix-loop-helix PAS (PER-ARNT-SIM) family. HIF-1␣ heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT; HIF-1␤) and plays a key role in maintaining oxygen homeostasis by signaling hypoxic exposure to genes that are involved in angiogenesis, erythropoiesis, glycolysis, and cell survival (10,34,37). In addition to its roles in physiological oxygen homeostasis, HIF-1␣ has also been implicated in the pathogenesis of various diseases, including ischemic heart disease, stroke, and cancer (11,29,33,36).HIF-1␣ is expressed in the developing brain (16) and modulates gene activity in response to low oxygen in a hypoxic brain in vivo. HIF-1␣ has also been implicated as a critical factor in the pathogenesis of brain tumor vascularization and stroke by regulating local brain hypoxia (8,36,38). Although these results indicate that HIF-1␣ is involved in angiogenesis in the brain and neuroprotection, it has not been established whether HIF-1␣ contributes to brain development. Systemic disruption of the Hif-1␣ gene leads to embryonic lethality by day of embryonic development 11 (E11) that is accompanied by cardiovascular malformation and defective cephalic vascularization, indicating that HIF-1␣ is essential for embryonic vascularization (14,21,32). However, the significance of HIF-1␣ in the development of the central nervous system remains unclear.To investigate the function of HIF-1␣ in the central nervous system, a conditional knockout mouse was generated with the Cre/LoxP system with a nestin promoter-based neural precursor-specific Cre recombinase (12). The nestin promoter directs gene expression specifically in neural precursor cells, so that a loxP-flanked (floxed) gene can be disr...

Oxygen Tension Within the Neurogenic Niche Regulates Dopaminergic Neurogenesis in the Developing Midbrain

“…These data in conjunction with our results showing no direct involvement of HIF-1a in the development of the central dopaminergic system in all oxygen conditions in vivo strongly suggest a role of HIF-1a in mediating postdevelopmental regeneration and/ or survival of dopaminergic neurons [25]. However, other reasons for the discrepancies in the morphology of the dopaminergic system between the two HIF-1a CKO models could be the different genetic manipulation (knock out of exon 2 as used by Milosevic et al [25] and exon 13-15 in the present study [35]) or differences in the genetic background.…”

“…For HIF-1a studies, the Hif-1a gene was knocked out by crossing HIF-1a flox/flox mice [35] (generously provided by Shuhei Tomita, MD, PhD) with a conditional transgenic mouse strain expressing the Cre recombinase gene under the control of the promotor of the Nestin gene, which is typically upregulated in all neural precursors during embryonic development. Further details on the generation and genotyping of Hif-1a CKO embryos have been described previously [36].…”

IL-1β Regulates Blood-Brain Barrier Permeability via Reactivation of the Hypoxia-Angiogenesis Program

“…Two of the genes induced by IL-1␤ in our study, HIF-1␣ and VEGF-A, were of particular interest in the context of BBB breakdown. Both are involved in developmental angiogenesis, they are known to be functionally linked, and animal studies have shown the importance of the HIF-VEGF axis in vascular development (23,24,29,50).…”

“…In the developing embryo, the HIF-VEGF pathway is known to act as a critical mechanism in the induction of vascular plasticity that is associated with angiogenesis (23,24,50,51). Changes in vascular permeability are the first detectable signs of new lesion formation in MS (4), and we hypothesized that IL-1␤-induced reactivation of the HIF-VEGF axis in astrocytes might act as a significant contributing factor in the pathogenesis of these events.…”

Role of Hypoxia in the Evolution and Development of the Cardiovascular System

“…Although one could conclude that no major role for HIF exists in the embryonic ventricular myocardium, confounding factors in gene-inactivation studies may include family member redundancy, cell and physiologic compensations and adaptations within the embryo, issues in the timing of gene inactivation relative to morphogenesis, and efficiency of inactivation of the gene and its protein product. In a study using a different floxed HIF-1␣ allele, neural-specific inactivation of HIF-1␣ by using nes-Cre resulted in a much greater reduction in vessel density (80%) in the brain's ventral cortical plate associated with apoptosis of the neuronal cells (104). As a reduction in VEGF was found in the brains of these mice, this suggests that brain parenchymal hypoxic induction of HIF-1 and VEGF is required to generate a vascular supply to this organ.…”