Stress-Induced Phosphorylation and Proteasomal Degradation of Mitofusin 2 Facilitates Mitochondrial Fragmentation and Apoptosis

Abstract

SUMMARY Mitochondria play central roles in integrating pro- and anti-apoptotic stimuli and JNK is well-known to have roles in activating apoptotic pathways. We establish a critical link between stress-induced JNK activation, mitofusin 2, which is an essential component of the mitochondrial outer membrane fusion apparatus, and the ubiquitin-proteasome system (UPS). JNK phosphorylation of mitofusin 2 in response to cellular stress leads to recruitment of the ubiquitin ligase (E3) Huwe1/Mule/ARF-BP1/HectH9/E3Histone/Lasu1 to mitofusin 2, with the BH3 domain of Huwe1 implicated in this interaction. This results in ubiquitin-mediated proteasomal degradation of mitofusin 2, leading to mitochondrial fragmentation and enhanced apoptotic cell death. The stability of a non-phosphorylatable mitofusin 2 mutant is unaffected by stress and protective against apoptosis. Conversely, a mitofusin 2 phosphomimic is more rapidly degraded without cellular stress. These findings demonstrate how proximal signaling events can influence both mitochondrial dynamics and apoptosis through phosphorylation-stimulated degradation of the mitochondrial fusion machinery.

Quantitative Lys-ϵ-Gly-Gly (diGly) Proteomics Coupled with Inducible RNAi Reveals Ubiquitin-mediated Proteolysis of DNA Damage-inducible Transcript 4 (DDIT4) by the E3 Ligase HUWE1

“…Conditional loss of HUWE1 in the mouse brain leads to an aberrant increase in n-MYC protein levels, inhibition of neuronal progenitor cell differentiation to mature neurons and glial cells, and neonatal death (38 -40). HUWE1 also regulates proteins with important roles in cell stress response pathways such as MCL-1, TP53, c-MYC, HDAC2, and MFN2 (41)(42)(43)(44)(45). Not surprisingly, because of its involvement in regulating oncogenes and neurogenesis, various lines of evidence implicate HUWE1 in human disease.…”

Quality control of mitochondria during aging: Is there a good and a bad side of mitochondrial dynamics?

“…Alternatively, one could think about preventing the age-dependent deceleration of fusion-fission cycles. This may be achieved by stable (or inducible) overexpression in vivo of a variant of DRP1 that cannot be phosphorylated by protein kinase A (thus remaining fission competent) and/or of a variant of Mfn2 that cannot form disulphide bonds preventing fusion enhancement (see [40,44,57]). We would predict that both alterations result in increased cytotoxicity and a reduced lifespan.…”

“…Aging hypothesis: Deceleration of fusionfission cycles improves mitochondrial quality control Microscopic investigations revealed that mitochondria are organized within a highly dynamic network that is governed by mitochondria undergoing cycles of fusion and fission that result in the mixing of their molecular content [28][29][30][31][32]. Mitochondrial dynamics and consequently changes in mitochondrial morphology are regulated at multiple levels including limited proteolysis, ubiquitinylation, phosphorylation, sumoylation, and disulfide formation of critical fusion and/or fission factors [20,[33][34][35][36][37][38][39][40][41][42][43][44].…”

“…When mitochondrial function is impaired, the fusion machinery is inactivated, e.g. via stress-induced proteolytic processing of the fusion factor OPA1 and/or by ubiquitinylation and proteasome-dependent degradation of Mfn2 in mammals [41][42][43][44]. In this way dysfunctional mitochondria become spatially separated from the intact network and, importantly, distinguishable from intact mitochondria.…”

Mitofusins ‘bridge’ the gap between oxidative stress and mitochondrial hyperfusion

“…However, cells can also block fusion during stress-induced apoptosis. It was found that stress-activated c-Jun N-terminal kinase phosphorylates Mfn2, leading to its ubiquit ination and degradation [6]. How cells decide whether to turn on either survival mechanisms that activate mitofusins, or death mechanisms that inactivate them, requires additional studies.…”