The present study establishes a previously undisclosed link between mitochondrial fission and mitochondrial adaptations to hypoxia. We demonstrate that hypoxia induces mitochondrial fragmentation, confirmed as fission, and that Siah2 functions in this process by regulation of AKAP121 levels under hypoxia. Our findings unveiled the function of AKAP121 in control of mitochondrial morphology by two complementary mechanisms—through inhibition of Drp1 and Fis1. While AKAP121 facilitates inhibitory phosphorylation of Drp1 by PKA signaling in the mitochondrial microenvironment, it also limits the formation of Drp1-Fis1-containing fission complex at the mitochondrial membrane by a PKA-independent mechanism (). Deregulation of AKAP121, as occurs upon increased Siah2 expression and activity under hypoxia (Nakayama et al., 2004
), alleviates Drp1 phosphorylation and enables interaction of Drp1 with Fis1 on mitochondria membrane, resulting in mitochondrial fission. Notably, the first detection of endogenous Drp1 and Fis1 complex in hypoxia-treated Siah1a/2WT
MEFs further substantiates regulatory roles in mitochondria fission under physiological stress condition.
Our studies also identified the minimal domain on AKAP121 that is required for the regulation of Drp1-Fis1 interaction. Notably, this central domain does not bind either of these fission factors directly, pointing to one or more additional factors, yet to be identified, that mediate AKAP121’s effect on Fis1 and Drp1 association. Notably, our studies revealed the proximal sites of three functional domains, the PKA binding (RII), the Siah2 degron, and the MRD (domain inhibiting Drp1-Fis1 interaction) (), pointing to possible coregulation among factors that bind to these neighboring domains.
As AKAP121 serves as a docking platform for multiple signaling pathways, diverse protein kinases are likely to modulate Drp1-Fis1 interactions, consistent with the extensive posttranslational modifications known to control Drp1 function. Accordingly, AKAP121 itself is also likely subject to tight regulation, which is yet to be identified. Siah control of AKAP121 is one mechanism, which limits its availability with a concomitant effect on Drp1/Fis1 complex formation and mitochondrial fission. Other pathways could control AKAP121 availability and its ability to associate with distinct mitochondrial components.
The induction of Siah2/AKAP121-mediated mitochondrial fission in response to hypoxia provides a conceptual framework for understanding mechanisms underlying mitochondrial adaptation to low oxygen. Since there is a bidirectional relationship between mitochondrial morphology and bioenergetics, remodeling of mitochondrial morphology by Siah2 adds a new layer to the complex regulation of cellular adaptations to low oxygen conditions by this ubiquitin ligase. Siah2 control of mitochondrial fission is independent of its previously characterized role in the regulation of PHD and consequently HIFα availability and transcriptional activity, and its role in inhibition of oxidative phosphorylation by AKAP121 degradation (Nakayama et al., 2004
; Carlucci et al., 2008a
). Importantly, the effect of Siah2 on mitochondrial fission was largely independent of HIF1α. This newly identified role for Siah2 illustrates three distinct mechanisms that synergize to govern cellular adaptation to hypoxia.
Mitochondria are critical in determining cell survival, and their morphology is closely associated with the susceptibility to cell death signals (Suen et al., 2008
). Our studies suggest a role of Siah2 in ischemia-induced cardiomyocyte cell death by regulating AKAP121 availability and subsequent mitochondria dynamics. Interestingly, in addition to previous mechanistic models of AKAP121 function on apoptosis, the newly identified role for AKAP121 in regulation of mitochondria dynamics and cell death explains the observation that delocalization of AKAP121 from mitochondria by competitor peptide increases apoptosis of cardiomyocytes (Perrino et al., 2010
Our study also identifies the role of mitochondrial fission on nematode life span. Notably, the effect of Siah2 or Drp1 on nematode life span required their inhibition during larval development, a stage during which mitochondrial fission and fusion events are maximal. Underscoring this is the observation that mutation and/or RNAi depletion of mitochondrial electron transport chain genes results in a wide spectrum of defects in life span. Consistent with the extensive degree of fission/fusion during larval development, these defects manifest themselves only when RNA-mediated depletion is initiated from hatch, and not when initiated during adulthood (Dillin et al., 2002
; Rea et al., 2007
). Overall, our study identifies a molecular mechanism underlying control of mitochondrial fission in a physiological condition, namely hypoxia, thereby linking control of mitochondrial morphology with corresponding physiological readouts.