Journal article
ULK2 is essential for degradation of ubiquitinated protein aggregates and homeostasis in skeletal muscle
The FASEB Journal, Vol.33(11), pp.11735-11745
11/01/2019
DOI: 10.1096/fj.201900766R
PMCID: PMC6902739
PMID: 31361156
Abstract
Basal protein turnover, which largely relies on the degradation of ubiquitinated substrates, is instrumental for maintenance of muscle mass and function. However, the regulation of ubiquitinated protein degradation in healthy, nonatrophying skeletal muscle is still evolving, and potential tissue-specific modulators remain unknown. Using an unbiased expression analysis of 34 putative autophagy genes across mouse tissues, we identified unc-51 like autophagy activating kinase ( Ulk ) 2 , a homolog of the yeast autophagy related protein 1, as particularly enriched in skeletal muscle. Subsequent experiments revealed accumulations of insoluble ubiquitinated protein aggregates associated with the adaptors sequestosome 1 (SQSTM1, also known as p62) and next to breast cancer type 1 susceptibility protein gene 1 protein (NBR1) in adult muscles with ULK2 deficiency. ULK2 deficiency also led to impaired muscle force and caused myofiber atrophy and degeneration. These features were not observed in muscles with deficiency of the ULK2 paralog, ULK1. Furthermore, short-term ULK2 deficiency did not impair autophagy initiation, autophagosome to lysosome fusion, or protease activities of the lysosome and proteasome. Altogether, our results indicate that skeletal muscle ULK2 has a unique role in basal selective protein degradation by stimulating the recognition and proteolytic sequestration of insoluble ubiquitinated protein aggregates associated with p62 and NBR1. These findings have potential implications for conditions of poor protein homeostasis in muscles as observed in several myopathies and aging.—Fuqua, J. D., Mere, C. P., Kronemberger, A., Blomme, J., Bae, D., Turner, K. D., Harris, M. P., Scudese, E., Edwards, M., Ebert, S. M., de Sousa, L. G. O., Bodine, S. C., Yang, L., Adams, C. M., Lira, V. A. ULK2 is essential for degradation of ubiquitinated protein aggregates and homeostasis in skeletal muscle.
Skeletal muscle mass and contractile function are strong independent predictors of positive prognosis and rate of recovery in patients at intensive care units (1). Maintenance of skeletal muscle mass and function also preserves independence and reduces mortality in the elderly (2–5). The physiologic contributions of skeletal muscle to whole-body homeostasis are several. Besides its role in voluntary movement and in supporting respiration via diaphragm action, skeletal muscle helps preserve bone mass (6), clears most blood glucose in response to insulin (7, 8), and is the major source of amino acids for hepatic gluconeogenesis under conditions of energy stress such as in fasting or starvation (9). Therefore, a better understanding of the mechanisms involved in the maintenance of skeletal muscle mass and function is of paramount clinical importance.
Appropriate protein degradation and turnover is essential for maintaining skeletal muscle health throughout life. Because of its long-living nature and the expression of a high number of cytoskeletal proteins prone to misfolding and aggregation, skeletal muscle fibers are under a constant proteotoxic challenge (10, 11). Indeed, skeletal muscle is directly compromised by proteotoxic gene mutations such as in amyotrophic lateral sclerosis (12) and Huntington’s disease (13), or by defects in proteolysis such as in Danon disease (14), Pompe disease (15), and collagen VI muscular dystrophy (16). In addition, deficient protein degradation is a feature of age-related muscle weakness (17, 18). To this matter, skeletal muscle protein degradation is executed by several proteolytic systems, including calpains (19), caspases (20), the proteasomal system (21), and the autophagy-lysosomal system or macroautophagy (22). Importantly, the selective removal of cellular proteins in muscle largely relies on their ubiquitination followed by proteasomal or autophagy-lysosomal degradation (23–25), the latter hereafter referred to as autophagy. However, the molecular coordination of ubiquitinated protein degradation in skeletal muscle remains insufficiently understood.
Autophagy is a multistep process by which the largest variety of ubiquitinated cellular substrates, including long-lived proteins, insoluble protein aggregates, and organelles, are degraded (26). Still, the potential skeletal muscle–specific regulators of the autophagy pathway that may affect ubiquitinated protein degradation remain unknown. In the current study, we sought to identify new skeletal muscle–specific regulators of ubiquitinated protein degradation among putative or established autophagy genes. With the perspective that the skeletal muscle proteome requires tailored degradation and turnover, we hypothesized that essential factors modulating protein turnover would be enriched in skeletal muscle under normal, nonatrophying conditions. Our results revealed unc-51 like autophagy activating kinase (ULK)2 to be an essential protein for degradation of ubiquitinated proteins and homeostasis in skeletal muscle. Furthermore, our findings reveal that ULK2 does not directly regulate autophagy in skeletal muscle, thereby having a distinct function in relation to its better-studied paralog ULK1. These findings may have potential therapeutic implications for conditions of poor protein homeostasis in muscle such as in several myopathies and aging
Details
- Title: Subtitle
- ULK2 is essential for degradation of ubiquitinated protein aggregates and homeostasis in skeletal muscle
- Creators
- Jordan D Fuqua - University of IowaCaleb P Mere - University of IowaAna Kronemberger - University of IowaJay Blomme - University of IowaDam Bae - University of IowaKristen D Turner - University of IowaMatthew P Harris - University of IowaEstevão Scudese - University of IowaMitchell Edwards - University of IowaScott M Ebert - University of Iowa, Internal MedicineLuís G. O de Sousa - University of IowaSue C Bodine - University of Iowa, Internal MedicineLing Yang - University of Iowa, Anatomy and Cell BiologyChristopher M Adams - University of Iowa, Internal MedicineVitor A Lira - University of Iowa, Health, Sport, and Human Physiology
- Resource Type
- Journal article
- Publication Details
- The FASEB Journal, Vol.33(11), pp.11735-11745
- DOI
- 10.1096/fj.201900766R
- PMID
- 31361156
- PMCID
- PMC6902739
- NLM abbreviation
- FASEB J
- ISSN
- 0892-6638
- eISSN
- 1530-6860
- Publisher
- Federation of American Societies for Experimental Biology; Bethesda, MD, USA
- Grant note
- Funding: The authors acknowledge use of the University of Iowa Central Microscopy Research Facility, a core resource supported by the University of Iowa Vice President for Research, and the Carver College of Medicine. This study was supported by a Fraternal Order of Eagles Pilot Research Grant and American Heart Association Grant 16SDG30360001 (to V.A.L.). Conflicts of Interest: The authors declare no conflicts of interest.
- Language
- English
- Date published
- 11/01/2019
- Academic Unit
- Family and Community Medicine; Health, Sport, and Human Physiology ; Internal Medicine
- Record Identifier
- 9983763491302771
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