Slow Breathing

Science Signaling  15 Apr 2008:
Vol. 1, Issue 15, pp. ec132
DOI: 10.1126/stke.115ec132

The mitochondrial scaffolding protein A-kinase anchor protein 121 (AKAP121) assembles a signaling complex that includes components of the cAMP (adenosine 3',5'-monophosphate) and Src signaling pathways, facilitating their stimulation of mitochondrial metabolism. Carlucci et al. identified the E3-ubiquitin ligase Siah2 as an AKAP121 binding partner in a yeast two-hybrid screen and confirmed the interaction through in vitro assays, coimmunoprecipitation from human embryonic kidney (HEK) 293 cell lysates, and immunostaining of mouse fibroblasts. Cotransfection of HEK293 cells with AKAP121 increased association of a Siah2 mutant that lacked E3-ubiquitin ligase activity (Siah2rm) with mitochondria, and cotransfection with Siah2 (but not Siah2rm) markedly decreased AKAP121 abundance. Hypoxia promotes Siah2 transcription, and oxygen and glucose deprivation (OGD) stimulated a Siah2-dependent decrease in the abundance of AKAP121 transfected into HEK293 cells and of endogenous AKAP121 in cultured rat hippocampal neurons and mouse fibroblasts. AKAP121 was a substrate for Siah2 ubiquitination in vitro, and Siah2 overexpression promoted the ubiquitination and proteasomal degradation of AKAP121 in HEK293 cells, as did OGD. When transfected into HEK293 cells, AKAP121 increased mitochondrial membrane potential (ΔΨm) and metabolic activity and opposed decreases in ΔΨm and mitochondrial metabolic activity produced by OGD; Siah2 decreased basal ΔΨm and metabolic activity and opposed the effects of AKAP121. Further, siRNA directed against Siah2 increased metabolic activity in cells exposed to OGD. Occlusion of the middle cerebral artery in rats (which causes ischemic damage to the frontal cortex) led to a decrease in AKAP121 immunoreactivity in neurons in the ischemic region, which was blocked by siRNA directed against Siah2. The authors thus conclude that degradation of AKAP121 by Siah2 provides a mechanism whereby mitochondrial respiration can be slowed under hypoxic conditions.

A. Carlucci, A. Adornetto, A. Scorziello, D. Viggiano, M. Foca, O. Cuomo, L. Annunziato, M. Gottesman, A. Feliciello, Proteolysis of AKAP121 regulates mitochondrial activity during cellular hypoxia and brain ischaemia. EMBO J. 27, 1073-1084 (2008). [PubMed]