Lack of hypoxia-inducible factor-1α impairs midbrain neural precursor cells involving vascular endothelial growth factor signaling

Oxygen tension is critical for proliferation of human and murine midbrain-derived neural precursor cells (mNPCs). Here, we conditionally inactivated the hypoxia-responsive transcription factor hypoxia-inducible factor-1 alpha (HIF-1 alpha) in murine NPCs to determine its role in proliferation, survival, and dopaminergic differentiation in vitro as well as survival of murine dopaminergic neurons in vivo. HIF-1 alpha conditional knock-out (HIF-1 alpha CKO) mNPCs showed midbrain-specific impairment of survival and proliferation. Dopaminergic differentiation of HIF-1 alpha CKO mNPCs in vitro was markedly reduced. Expression of vascular endothelial growth factor (VEGF) mRNA was reduced in HIF-1 alpha CKO mNPCs, whereas erythropoietin signaling was not affected. Treatment of HIF-1 alpha CKO mNPCs with 50 ng/ml VEGF partially recovered proliferation and dopaminergic differentiation in vitro. In substantia nigra (SN) of adult HIF-1 alpha CKO mice, protein levels of dopaminergic marker molecules such as tyrosine hydroxylase (TH) and aldehyde dehydrogenase were reduced by 41 and 61%, respectively. The cell survival marker Bcl-2 was reduced by 58% while caspase-3 was activated. Nonbiased stereological cell counts of TH-positive neurons in SN of young adult HIF-1 alpha CKO mice revealed a reduction of 31% compared with cre/wt mice (in which the wildtype Hif1a allele is expressed in parallel with the Cre recombinase allele). However, we found no impairment of striatal dopamine concentrations or locomotor behavior. In conclusion, HIF-1 alpha seems to be a transcription factor relevant to the development and survival of substantia nigra dopaminergic neurons involving VEGF signaling.