HIF-1 Regulated Variant Splicing of CaMKIIγ in the Hypoxic Heart

Abstract

Hypoxia-inducible factor 1 (HIF-1) is an oxygen-labile transcription factor that plays a crucial role in the regulation of cellular processes associated with hypoxia in the heart. In this study, we evaluated the effects of HIF-1 on variant RNA splicing of calcium/calmodulindependent protein kinase II gamma (CaMKIIγ). CaMKIIγ is known to play an important role in calcium signaling and heart function. Previously collected data indicated that expression of an oxygen-stable HIF-1 altered the relative abundance of three CaMKIIγ splicing variants in the hearts of transgenic mice. Expression of the full-length variant appeared to be down-regulated whereas variants 2 and 3 increased in abundance following HIF-1 induction. Based on this preliminary data, we chose to examine the mechanism of HIF-mediated splicing using two different models of physiological hypoxia: cell culture and a mouse MI surgical model. My hypothesis was that the altered splicing pattern seen in the HIF transgenic mice would be recapitulated in the physiological settings. We have found that in cells, expression levels of all three transcript variants initially decreased when exposed to hypoxia but that the expression levels of variant 3 returned to normoxic levels, and thus greater relative abundance relative to the other isoforms by 24 hours. In our in vivo model of infarction, CaMKIIγ variant 1 expression decreased 3 days after surgery, as was seen in the transgenic model. These results support the hypothesis that HIF-1 is involved in the alternative splicing of CaMKIIγ and could contribute to the loss of cardiac contractility observed after hypoxic injury.