Expression and role of HIF-1α and HIF-2α in tissue regeneration: a study of hypoxia in house gecko tail regeneration

The house gecko (Hemidactylus platyurus) has evolved the ability to autotomize its tail when threatened. The lost part is then regrown via epimorphic regeneration in a process that requires high energy and oxygen levels. Oxygen demand is therefore likely to outstrip supply and this can result in relative hypoxia in the tissues of the regenerating tail. The hypoxic state is stabilized by the Hypoxia Inducible Factor-1 alpha (HIF-1 alpha) and HIF-2 alpha proteins. We induced tail autotomy in 30 mal H. platyurus adults using a standard procedure and then collected samples of the regenerated tail tissue on days 1, 3, 5, 8, 10, 13, 17, 21, 25, and 30 post autotomy. For each sample, mRNA expression was analyzed by qPCR, proteins were analyzed using Western Blot tests and immunohistochemistry, and the histological structure was analyzed using Hematoxylin and Eosin staining. On day 1, HIF-1 alpha mRNA expression increased and the tissue was dominated by leucocyte and erythrocyte cells. HIF-1 alpha mRNA expression peaked on day 3, at which time some cells were actively proliferating, migrating, and differentiating. At the same time as HIF-1 alpha expression decreased, HIF-2 alpha mRNA expression increased, as did overall cellular activity. HIF-2 alpha expression increased more gradually but was present over a longer period of time than HIF-1 alpha. We hypothesize that HIF-1 alpha helps to initially stimulate the tissue regeneration process while HIF-2 alpha functionally takes over the role of HIF-1 alpha after HIF-1 alpha succumbs to the oxygen conditions, but we suspect that both HIF-1 alpha and HIF-2 alpha play a role in overcoming the tissue's hypoxic state.