Abstract
Mitochondrial quality control is essential for maintaining cellular energy homeostasis, particularly in brown adipocytes where dynamic mitochondrial remodeling supports thermogenesis. Although the SEL1L-HRD1 endoplasmic reticulum (ER)-associated degradation (ERAD) pathway and autophagy are two major proteostatic systems, how these pathways intersect to regulate mitochondrial integrity in metabolically active tissues remains poorly understood. Here, using adipocyte-specific genetic mouse models combined with high-resolution 2D and 3D ultrastructural imaging technologies, we reveal an unexpected synergy between SEL1L-HRD1 ERAD and autophagy in maintaining mitochondrial structure and function in brown adipocytes. Loss of ERAD alone triggers compensatory autophagy, whereas combined deletion of both pathways (double knockout, DKO) results in severe mitochondrial abnormalities, including the accumulation of hyperfused megamitochondria penetrated by ER tubules, even under basal room temperature conditions. These phenotypes are absent in mice lacking either pathway individually or in SEL1L-IRE1α DKO, highlighting the pathway-specific coordination between ERAD and autophagy. Mechanistically, dual loss of ERAD and autophagy induces ER expansion, excessive ER-mitochondria contact, upregulation of mitochondria-associated membrane (MAM) tethering proteins, impaired calcium transfer, and defective mitochondrial turnover. As a result, DKO adipocytes accumulate dysfunctional mitochondria, exhibit respiratory deficits, and fail to sustain thermogenesis. Collectively, our study uncovers a cooperative and previously unrecognized mechanism of mitochondrial surveillance, emphasizing the critical role of ERAD-autophagy crosstalk in preserving mitochondrial integrity and thermogenic capacity in brown fat. ONE-SENTENCE SUMMARY: Our study uncovers a previously unrecognized synergy between SEL1L-HRD1 ERAD and autophagy that is essential for preserving mitochondrial integrity and thermogenic capacity in brown adipocytes, revealing new opportunities for targeting mitochondrial dysfunction in metabolic disease.