Finding. ULK1 and ULK2, the kinases that start autophagy, also decide how big a myofiber is allowed to get. Son, Lira and colleagues delete both in skeletal muscle. Autophagy flux dies. Fibers of every major myosin class swell 10 to 20 percent. Mitochondrial and sarcoplasmic proteins last longer. Myofibrillar synthesis rises 23 percent. mTORC1 climbs without AKT. Lifelong, that bargain is ugly: central nuclei explode, especially in male type 2b fibers (0.8% to 22.7%), and force falls about a fifth. Knock the kinases down for a few weeks in an adult and you get the extra size without the force loss or the nuclear mess.
Why this paper matters
Muscle mass is a synthesis-versus-degradation argument that usually gets filed as “mTOR versus proteasome.” Autophagy-initiating kinases sit on both sides. They keep the degradation program, including mitochondrial protein turnover, from falling asleep, and they keep mTORC1 from writing blank checks. Remove them and the fiber grows because it builds more myofibril and throws away less of the rest.
The time axis is the clinical sentence. Lifelong absence is a myopathy with hypertrophy. Short-term absence looks like a hypertrophic window. Anyone who wants ULK inhibitors for atrophy has to live inside that window or they will enlarge a weaker muscle.
The mitochondrial line
Deuterium-oxide labeling is why this is not just another LC3 blot. Mitochondrial protein degradation is 16% slower (p=0.09). Sarcoplasmic degradation is 14% slower. Synthesis of myofibrils is 23% faster. The organelle is on the ledger. Whether that is failed mitophagy or a jammed bulk autophagy queue is not settled in the abstract. The flux assay (LC3-II plus colchicine only in wild type) says autophagy itself is down.
How to read the score
High seventies. Clean genetic design, force, fiber quality, and a protein-turnover split that names mitochondria. Confidence is high for the hypertrophy-versus-quality phenomenology, a notch lower for the mitochondrial Kdeg p-value.
Caveats
Two kinases gone at once. Sex differences in force. No human dose. Do not advertise ULK blockade as a safe mass drug from this brief.
What to do with it
If you track muscle mitophagy or mTORC1, ULK1/2 is a node, not a generic ATG gene. If you score atrophy therapies, steal the short-term versus lifelong contrast. Pull the D2O Ksyn/Kdeg table before you write “autophagy equals wasting.”