| Genome-wide expression studies from our lab have characterized the human brain transcriptome across the adult lifespan. Surprisingly, among the most highly age-upregulated genes are those in the hemoglobin family, including alpha- and beta-globin. These genes were long thought to be completely silenced outside the erythroid lineage, but the combined force of several recent studies has eroded that dogma. Here we describe the expression of hemoglobin in neurons and microvascular endothelial cells of the human brain and provide evidence for a novel cytoprotective role for hemoglobin.;Immunohistochemical analysis of postmortem human cortex indicates that alpha- and beta-globin are present in a subset of neurons and microvascular endothelial cells. No hemoglobin was detected in GFAP+ astrocytes or Iba1+ microglia. alpha- and beta-globin mRNA is also enriched in microvascular endothelial cells isolated from the postmortem human brain by laser capture microdissection, confirming that these genes are in fact expressed in brain endothelial cells in vivo..;In vitro, primary human brain microvascular endothelial cells (HBMEC) were found to express moderate levels of alpha-globin and low levels of beta-globin basally. alpha-globin was upregulated up to 10-fold by stress-related stimuli including hypoxia, nitric oxide, and heme. The stress-induced regulation of alpha-globin in HBMEC underscores that hemoglobin expression is not incidental, but a sensitive and robust stress response.;Survival assays suggest that hemoglobin may be cytoprotective in HBMEC. Lentiviral mediated knockdown of alpha-globin renders HBMEC vulnerable to cell death as measured by MTS, annexinV staining, and PARP1 cleavage. alpha-globin knockdown also resulted in higher basal levels of reactive oxygen species (ROS) as well as a higher mitochondrial membrane potential. Furthermore, alpha-globin deficient HBMEC failed to upregulate lactate production under hypoxia suggesting that alpha-globin has a role in the adaptive switch to anaerobic metabolism when oxygen is limiting.;Taken together, these results suggest a model whereby hemoglobin promotes cell survival by limiting ROS production perhaps by directly scavenging oxygen and sequestering it from mitochondria, thereby limiting respiration-associated ROS. The cytoprotective function of hemoglobin raises the possibility that expression may be reactivated in the aging brain as an adaptive response to age-associated stress. |
