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Dissertation (Sonny)7.03 MB
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Dissertation
Sex-dependent resistance to atrophy in muscle-specific ULK1/2 deficient mice is associated with preserved protein synthesis
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2025
DOI: 10.25820/etd.008214
Abstract
Maintaining proteostasis, the balance between protein synthesis and degradation, is essential for skeletal muscle health. Among the degradative pathways, the autophagy–lysosome system is a major route of bulk and selective turnover of proteins and organelles. The Unc-51– like autophagy-activating kinases ULK1 and ULK2 (ULK1/2) function as the initiating kinases at the apex of control, but how they shape muscle phenotypes under physiological stress remains unclear. This dissertation tests (i) whether and how modulating ULK1/2 alters muscle phenotypes during disuse atrophy in vivo and (ii) how ULK1/2-dependent autophagy is recruited across diverse autophagy-activating conditions and muscle groups.
In Chapter 1, I provide a focused review of ULK1 and ULK2 in skeletal muscle, covering domain architecture and the upstream and downstream effectors that link the ULK axis to autophagy initiation and LC3 conjugation. I also synthesize our laboratory’s prior work showing that skeletal muscle–specific ULK1/2 deficiency impairs basal autophagic flux (p62/SQSTM1 accumulation and LC3-II build-up), increases centrally nucleated fibers, and reduces contractile performance, yet paradoxically produces hypertrophy driven by elevated protein synthesis and mTORC1 hyperactivity. These findings indicate that ULK1/2 are essential, yet partially redundant, regulators of autophagy in skeletal muscle and, beyond their canonical catabolic role, also modulate anabolic signaling, positioning them as strong controllers of myofiber size in vivo.
In Chapter 2, centered on this non-canonical, kinase-based role, I test whether modulating ULK1/2 during disuse preserves protein synthesis and mitigates atrophy in vivo. Using skeletal muscle–specific ULK1/2 double knockout (skmDKO) mice and fiber-localized microRNA (miR) ULK1/2 knockdown, I show that, in males, skmDKO attenuated immobilization-induced loss of tibialis anterior (TA) mass and fiber diameter and preserved protein synthesis during muscle atrophy, whereas postnatal fiber-level knockdown mitigated atrophy in both sexes. Protection aligned with sustained mTORC1 signaling (greater S6K/RPS6/4E-BP1 activation and reduced inhibitory inputs), while mitochondrial and ribosomal abundance were largely unchanged under this partial-unloading model.
In Chapter 3, building on ULK1/2’s canonical role in autophagy, I map how ULK-axis signaling is regulated across diverse physiological conditions (disuse, fasting, and aerobic exercise) and muscle groups (tibialis anterior, soleus, plantaris). Across these settings, condition- and muscle-specific patterns emerged in AMPK, ULK1/2, and LC3 (autophagy marker) readouts, yet a common theme was that ULK1/2 skmDKO blunted LC3 processing and downstream substrate phosphorylation when autophagy was activated. These data indicate that ULK1/2 are required to fully engage autophagy activation, but the specific ULK-axis nodes that respond are stimulus- and muscle-dependent, establishing ULK1/2 as contextsensitive hubs that determine when and how skeletal muscle activates autophagy.
In Chapter 4, I summarize the main conclusions of this dissertation and outline directions for future work. Overall, these studies show that ULK1 and ULK2 are central coordinators of skeletal muscle proteostasis, acting at the intersection of protein synthesis and autophagy. Lifelong or postnatal loss of ULK1/2 uncouples mTORC1-linked translational control from canonical autophagy, producing larger fibers with altered muscle quality at baseline and changing how muscle responds to disuse. Across multiple autophagy-inducing conditions and muscles, ULK1/2 are required to fully engage autophagy activation, while the responding nodes within the ULK axis are tuned by context. Future work should more precisely separate ULK1/2-dependent from ULK1/2-independent arms of the autophagy machinery in vivo and test whether short-term, temporally targeted ULK1/2 deficiency around periods of disuse or metabolic stress can preserve muscle mass and function—widening the scope of what ULK1/2 modulation can achieve without chronically impairing autophagy.
Details
- Title: Subtitle
- Sex-dependent resistance to atrophy in muscle-specific ULK1/2 deficient mice is associated with preserved protein synthesis
- Creators
- Wangkuk Son
- Contributors
- Vitor Lira (Advisor)Warren Darling (Committee Member)Renata Alambert (Committee Member)Erin Talbert (Committee Member)Ling Yang (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Health and Human Physiology
- Date degree season
- Autumn 2025
- DOI
- 10.25820/etd.008214
- Publisher
- University of Iowa
- Number of pages
- xiii, 88 pages
- Copyright
- Copyright 2025 Wangkuk Son
- Language
- English
- Date submitted
- 12/02/2025
- Description illustrations
- Illustrations, graphs, charts, tables
- Description bibliographic
- Includes bibliographical references (pages 84-88).
- Public Abstract (ETD)
Skeletal muscle stays healthy by continuously recycling worn-out parts through autophagy. Proteins called ULK1 and ULK2 help control when this recycling turns on. This dissertation asks two questions: (1) Can modulating ULK1/2 preserve muscles during disuse? (2) How does ULK1/2 regulate autophagy, across different conditions (disuse, fast, aerobic exercise) and across different muscle groups (tibialis anterior, soleus, plantaris), based on specific ULK-axis readouts (signals on Beclin-1, ATG14, and ATG16)?
We studied mice whose skeletal muscles lack ULK1/2 and also used a short-term, fiber-level reduction of these proteins. During disuse, muscles without ULK1/2 (in males) lost less size because protein-building signals remained higher; the same fiber-level protection appeared in both sexes with short-term knockdown. We then examined how ULK1/2 regulates autophagy during disuse, fasting and after an aerobic exercise bout in three different muscles (tibialis anterior, soleus, plantaris), showing that each condition engaged ULK1/2 in a distinct way. Exercise caused brief activation of partner proteins that start and build autophagosomes (Beclin-1, ATG14, ATG16), whereas disuse and fasting raised recycling markers without the same early-step signals. Removing ULK1/2 generally reduced these responses.
In short, our work shows that ULK1 and ULK2 are central controllers of autophagy in muscle, acting across multiple conditions and in different muscles. They are needed for a full autophagy response during limb immobilization, fasting, and endurance-type exercise, highlighting how broadly they help keep muscle cells healthy. At the same time, ULK1/2 also work as kinases that inhibit growth signals and protein-building (anabolic signaling), influencing how big and strong muscle fibers become. Together, these insights suggest that condition-specific control of ULK1/2, either to restore healthy autophagy or to support protein building, could be developed to improve treatment for autophagy-related diseases and to help protect against muscle loss.
- Academic Unit
- Health, Sport, and Human Physiology
- Record Identifier
- 9985135047902771
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