Korean Red Ginseng-induced astrocytic HIF-1α: A key regulator of neuroglobin derived from neural stem cell differentiation in physiologic retina and brain.

Background: Neuroglobin (Ngb) and growth-associated protein (GAP) 43 in neurons are associated with axonal regeneration. Korean Red Ginseng Extract (KRGE) enhances glial fibrillary acidic protein (GFAP)-positive astrocytes and hypoxia-inducible factor-1α (HIF-1α) protein activation in normoxic astrocytes. However, crosstalk between neural stem cell (NSC) differentiation and astrocytic HIF-1α in the KRGE-treated normoxic brain and retina remains unclear. We investigated whether KRGE-treated astrocytic HIF-1α can enhance NSC differentiation and increase the mature neurons expressing Ngb and GAP43.

Methods: Mature neuronal markers such as neuronal nuclei (NeuN) or microtubule-associated protein 2 (MAP2) were tested with Ngb in the mouse brain or retinal tissues post-KRGE administration. Direct KRGE treatment of NSCs or astrocytes was evaluated for Ngb levels. The KRGE-treated astrocyte conditioned media (ACM) were transferred to NSCs and HIF-1α levels were reduced using small interfering RNA transfection (si-HIF-1α) in astrocytes. si-HIF-1α-ACM with KRGE was tested for NSC differentiation.

Results: KRGE-administered mice showed significantly enhanced co-expression of Ngb with NeuN in the brain and MAP2 in the retina, along with the NSC marker Nestin, than water-administered mice. The KRGE treatment did not increase Ngb levels in NSCs, but stimulated astrocytes to secrete factors affecting NSCs' differentiate into mature neurons and astrocytes. The KRGE-treated mouse retinas showed GFAP- and HIF-1α double-positive cells. Co-treatment with si-HIF-1α-transfected KRGE-ACM blocked KRGE-ACM-induced NSC differentiation into astrocytes or Ngb-expressing neurons.

Conclusion: KRGE stimulates astrocytic HIF-1α, which regulates NSC differentiation into mature neurons expressing Ngb, thereby promoting regeneration by enhancing NSC-astrocyte crosstalk in the physiological retina and brain.