Lack of hypoxia-inducible factor-1 alpha 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-1alpha (HIF-1alpha) 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-1alpha conditional knock-out (HIF-1alpha CKO) mNPCs showed midbrain-specific impairment of survival and proliferation. Dopaminergic differentiation of HIF-1alpha CKO mNPCs in vitro was markedly reduced. Expression of vascular endothelial growth factor (VEGF) mRNA was reduced in HIF-1alpha CKO mNPCs, whereas erythropoietin signaling was not affected. Treatment of HIF-1alpha CKO mNPCs with 50 ng/ml VEGF partially recovered proliferation and dopaminergic differentiation in vitro. In substantia nigra (SN) of adult HIF-1alpha 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-1alpha CKO mice revealed a reduction of 31% compared with cre/wt mice (in which the wild-type 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-1alpha seems to be a transcription factor relevant to the development and survival of substantia nigra dopaminergic neurons involving VEGF signaling.