TSKU

TSKU (Tsukushi) is a secreted, liver-derived small leucine-rich repeat proteoglycan (SLRP) that functions as a hepatokine coupling nutritional and hormonal signals to peripheral metabolism, musculoskeletal development, and tissue homeostasis (PMID:30271858, PMID:34183373, PMID:35025761). Hepatic TSKU expression and circulating TSKU are induced by feeding under control of melanocortin-4 receptor (MC4R) signaling, and TSKU acts downstream of this central circuit: its ablation in MC4R-deficient mice restores energy balance, relieves hyperphagia, augments brown-fat thermogenic gene expression, and protects against diet-induced steatohepatitis, while increasing feeding-induced c-Fos activation in the hypothalamic paraventricular nucleus (PMID:34183373). In the musculoskeletal system, TSKU is required for normal longitudinal bone growth, where it directs growth-plate chondrocyte differentiation by sustaining early and restraining mid-to-late chondrogenic gene programs (PMID:30271858), and for maintaining skeletal muscle mass, slow-twitch myofiber gene expression, neuromuscular junction integrity, and post-injury regeneration, with its action restricted to a developmental window since adult re-expression fails to rescue myofiber defects (PMID:35025761). In hepatocytes, TSKU lies downstream of HIF1α in the control of cellular lipid accumulation and additionally regulates expression of vitamin D-metabolizing enzymes [PMID:39832609, PMID:bio_10.1101_2024.07.24.604883]. The molecular partners and biochemical mechanism by which secreted TSKU transduces these effects have not been characterized in the available corpus.

1. 2017 Medium

   Established TSKU as a determinant of skeletal growth by showing it modulates the timing of chondrocyte differentiation in the growth plate, answering whether a secreted SLRP shapes endochondral bone elongation.

   Evidence TSK-knockout mouse skeletal phenotyping plus siRNA knockdown in a chondrogenic cell line with chondrogenic marker analysis

   PMID:30271858

   Open questions at the time

   • No molecular receptor or binding partner for TSKU in chondrocytes identified
   • Mechanism linking secreted TSKU to the chondrogenic transcriptional switch unknown
2. 2019 Medium

   Tested whether TSKU regulates brown adipose tissue thermogenesis directly and found that, in isolation, neither loss nor overexpression altered thermogenic gene programs or systemic metabolism, narrowing the conditions under which TSKU acts on energy balance.

   Evidence TSK-null and TSK-overexpressing mice with thermogenic gene expression, body weight, and glucose tolerance readouts

   PMID:31767170

   Open questions at the time

   • Negative result under baseline conditions does not exclude context-dependent action
   • Did not test TSKU in a sensitized metabolic background
3. 2021 High

   Positioned TSKU as a feeding-induced hepatokine acting downstream of MC4R signaling, reconciling the earlier negative thermogenesis result by showing TSKU's metabolic effects emerge within the central melanocortin context.

   Evidence TSK/MC4R double-knockout epistasis mice with thermogenic gene expression, hypothalamic c-Fos immunostaining, liver/adipose inflammation, and metabolic phenotyping

   PMID:34183373

   Open questions at the time

   • Receptor or target tissue mediating TSKU's peripheral and central effects not identified
   • Mechanism of feeding-induced hepatic TSKU transcription not defined
4. 2022 High

   Defined a developmental-stage-restricted role for liver-derived TSKU in establishing slow-twitch myofiber identity, neuromuscular junction integrity, and regenerative capacity, showing adult re-expression cannot rescue.

   Evidence TSK-knockout mouse grip strength and endurance testing, muscle transcriptomics, NMJ morphology, cardiotoxin regeneration assay, and AAV overexpression rescue

   PMID:35025761

   Open questions at the time

   • Molecular signal linking circulating TSKU to muscle fiber-type and NMJ programs unknown
   • Basis for the developmental window of action unresolved
5. 2024 Medium

   Identified a PXR-inducible enhancer controlling hepatic TSKU transcription and revealed an unexpected role for TSKU in supporting expression of vitamin D-metabolizing enzymes, linking xenobiotic/nuclear-receptor regulation to TSKU output.

   Evidence CAGE transcription initiation profiling, CRISPR/Cas9 enhancer knockout, and siRNA knockdown of TSKU with vitamin D enzyme readout (preprint)

   PMID:bio_10.1101_2024.07.24.604883

   Open questions at the time

   • Preprint, single lab, awaits peer review
   • Mechanism by which TSKU regulates vitamin D enzyme expression not defined
6. 2025 Medium

   Placed TSKU downstream of HIF1α in hepatocyte lipid handling, showing HIF1α inhibition raises TSKU and drives lipid accumulation reversible by TSKU knockdown.

   Evidence CDAHFD NASH mouse model with KC7F2 HIF1α inhibitor, in vitro hepatocyte culture, and siRNA knockdown rescue of lipid accumulation

   PMID:39832609

   Open questions at the time

   • Direct molecular effector by which TSKU promotes hepatocellular lipid accumulation unknown
   • Whether the HIF1α–TSKU axis intersects the MC4R feeding axis not tested

• The cell-surface receptor(s) and direct binding partners through which secreted TSKU transduces its diverse effects on adipose, muscle, bone, and hepatocyte biology remain unidentified.
• No TSKU receptor identified
• No structural or biochemical characterization of TSKU interactions
• Unclear whether one shared mechanism underlies effects across tissues