Prohibitin depletion extends lifespan of a TORC2/SGK-1 mutant through autophagy

Abstract

13 14 -2 -Mitochondrial prohibitins (PHB) are highly conserved proteins with a peculiar 15 effect on lifespan. While PHB depletion shortens lifespan of wild type animals, it 16 enhances longevity of a plethora of metabolically compromised mutants, 17 including target of rapamycin complex 2 (TORC2) mutants sgk-1 and rict-1. Here, 18 we show that sgk-1 mutants have impaired mitochondrial homeostasis, 19 lipogenesis, yolk formation and autophagy flux due to alterations in membrane 20 lipid and sterol homeostasis. Remarkably, all these features are suppressed by 21 PHB depletion. Lifespan analysis shows that autophagy and the mitochondrial 22 unfolded protein response (UPR mt ), but not mitophagy, are required for the 23 enhanced longevity caused by PHB depletion in sgk-1 mutants. We hypothesize 24 that UPR mt induction upon PHB depletion extends lifespan of sgk-1 mutants 25 through autophagy. Our results strongly suggest that PHB depletion suppresses 26 the autophagy defects of sgk-1 mutants by altering membrane lipid composition 27 at ER-mitochondria contact sites, where TORC2 localizes. 28 29 93 mitochondria-associated endoplasmic reticulum (ER) membranes (MAM).94 95 -6 - Results 96Prohibitin depletion suppresses the altered mitochondrial structure and 97 function of sgk-1 mutants 98In C. elegans, loss of function of SGK-1 causes a delay in development (Jones et al., 992009) as well as reduced brood and body size (Soukas et al., 2009). These phenotypes 100 are consistent with phenotypes observed in mitochondrial mutants (Dillin et al., 2002). In 101 addition, worms lacking the TORC2 component RICT-1 or the downstream kinase SGK-102 1, have an induced mitochondrial unfolded protein response (UPR mt ) (Gatsi et al., 2014). 103We analyzed whether mitophagy, another mitochondrial quality control mechanism, 104 might be activated in sgk-1 deletion mutants using a mitophagy reporter based on PINK-105 1 protein (Kirienko et al., 2015). PINK-1 is a serine threonine protein kinase that, when 106 mitochondria are depolarized, accumulates in the outer mitochondrial membrane and 107 targets mitochondria for selective autophagy (Narendra and Youle, 2011; Palikaras et 108 al., 2015). We observed that depletion of sgk-1 increased PINK-1 protein levels ( Figure 109 1A). Similarly, PINK-1 protein levels increased upon depletion of phb-1 or atfs-1, the key 110 UPR mt regulator ( Figure 1A), confirming that sgk-1 mutants suffer from mitochondrial 111 stress. 112To better define the mitochondrial defect of sgk-1 deletion mutants we performed 113 transmission electron microscopy (TEM) analysis. Mitochondria in sgk-1 mutants were 114 swollen, and bigger compared to wild type worms in all tissues analyzed (hypodermis, 115 muscle and intestine) at both, day one and day five of adulthood ( Figure 1B and Figure 116 S1A, respectively). In contrast to previous observations (Zhou et al., 2019), no obvious 117 mitochondrial fragmentation or severe cristae defects in the intestine were observed in 118 the TEM sections analyzed. 119-7 -To ...