TAZ links exercise to mitochondrial biogenesis via mitochondrial transcription factor A

Mitochondrial biogenesis increases mitochondrial mass and genome copy number in response to extracellular cues such as nutrients, hormones, and exercise. Signals converge on transcriptional regulators, notably PGC1α, which coactivates transcription factors including NRF1 and NRF2 to induce genes required for mitochondrial biogenesis. NRF1/NRF2 promote expression of TFAM and mitochondrial transcription factor B proteins that regulate mitochondrial DNA transcription and replication. Exercise is a cornerstone therapy for metabolic disorders and promotes mitochondrial biogenesis, elevating metabolic rate and fat utilization. Repeated muscle contraction induces adaptation via changes in signaling, mitochondrial content and function, and metabolism. Despite clear benefits, mechanisms of exercise-induced mitochondrial biogenesis are incompletely understood. Hippo signaling, a regulator of energy-intensive processes of growth and differentiation, controls TAZ and YAP co-regulators that act with TEAD transcription factors and respond to Wnt, GPCR, and mechanotransduction cues. TAZ/YAP activity is modulated by exercise-associated signals, but roles in mitochondrial biogenesis have been unclear. The study tests the hypothesis that TAZ stimulates mitochondrial biogenesis via translational induction of Tfam and mediates exercise-induced muscle adaptation.

Background work establishes that PGC1α is a master regulator of mitochondrial biogenesis, acting with NRF1/NRF2 to induce TFAM and TFB proteins that control mitochondrial DNA transcription/replication. Exercise-induced adaptations in skeletal muscle include increased mitochondrial content and function and are regulated by specific signaling pathways affecting transcription and translation of exercise-responsive genes. Hippo pathway components TAZ/YAP integrate mechanical and signaling inputs via TEADs and can be modulated by exercise signals. Prior studies also show mTORC1 regulates mitochondrial biogenesis through 4E-BP–dependent translational control of mitochondrial genes. These bodies of work frame the investigation into whether TAZ links exercise signaling to mitochondrial biogenesis via translational mechanisms.

- Animal models: Muscle-specific TAZ knockout mice (MCK-Cre; Taz fl/fl, mKO) previously generated by deleting Taz exon 2; 8–12-week-old mice used. Institutional approvals and standard housing conditions described. - In vivo phenotyping: Transmission electron microscopy of gastrocnemius muscle to quantify mitochondrial number and area. Indirect calorimetry (Oxylet system) measured VO2, VCO2, energy expenditure, rearing, and movement under light/dark cycles. Rotarod performance over 3 days and treadmill endurance test assessed exercise capacity. Food and water intake monitored. - Viral manipulations: AAV6-shTAZ injected intramuscularly to deplete TAZ in adult muscle; rescue with AAV6 expressing WT TAZ or TEAD-binding-deficient TAZ S51A. AAV6-Rhebl1 introduced to restore Rhebl1 in mKO gastrocnemius. AAV production, titration, and local transduction details provided. - Pharmacology: mTOR activator MHY1485 administered intraperitoneally every 2 days for 2 weeks to mKO mice. - Exercise interventions: Endurance training over 4 weeks with progressive treadmill protocol (15–19 m/min, 60 min sessions, 5° incline); tissues collected 24 h post last bout. Acute exercise (15 m/min, 1 h) with sampling at 0, 3, 24 h for signaling assessments (Supplementary). - Cell culture studies: WT and TAZ−/− MEFs, and C2C12 myoblasts/myotubes with shRNA knockdown used to assess mitochondrial content (MitoTracker), mtDNA copy number, ATP production, ROS, oxygen consumption rate via Seahorse XF24. Polysome profiling with sucrose gradients assessed global translation and distribution of specific mRNAs (Tfam, Atp5d, Gapdh). - Molecular analyses: qRT-PCR for nuclear- and mitochondrial-encoded genes; immunoblotting for mitochondrial markers (Cytochrome C, Cox4, Sdha, Vdac), Tfam, signaling proteins (p-4E-BP1, 4E-BP1, p-p70S6K, p70S6K), TAZ/YAP, Rheb, Rhebl1, PGC1α. Immunofluorescence of COX2/COX4 in muscle sections. mtDNA quantification by qPCR (mt-Co2 normalized to β-globin). ChIP-qPCR for TAZ and TEAD4 recruitment to a TAZ-binding element in the Rhebl1 enhancer in muscle and C2C12 cells; FLAG-TAZ used for ChIP in knockdown/rescue settings. Luciferase reporter assays with Rhebl1 TBE and promoter constructs co-transfected with TAZ and/or TEAD4 in HEK293T; effects of TAZ KD assessed. - Mitochondrial function ex vivo: Isolation of muscle mitochondria and XF24 respiration assays with ADP, oligomycin, FCCP, and antimycin A. - Statistics: Student’s t-tests (one- or two-sided as appropriate), one-way or two-way ANOVA with Tukey’s/Sidak’s multiple comparison tests; data reported as mean ± SEM (in vivo) or mean ± SD (in vitro).

- TAZ deletion in skeletal muscle reduces mitochondrial content and function: TEM showed decreased mitochondrial area fraction (p=0.022) and mitochondrial number/µm² (p=0.0104) in mKO vs WT. Mitochondrial marker proteins (Cytochrome C, Cox4, Sdha, Vdac) were reduced in mKO muscle. - Metabolic and performance deficits in mKO: Indirect calorimetry revealed reduced VO2 (light p=0.0353; dark p=0.0089), VCO2 (p=0.0134), and energy expenditure (p≈0.0137) at night; decreased rearing (p=0.036) and activity (p=0.0267). Rotarod performance decreased across 3 days (day1 p=0.00789; day2 p=0.00158; day3 p=0.00022). Treadmill endurance distance reduced (p=0.0017). - TAZ regulates TFAM at the translational level: In mKO muscle and TAZ KD/KO cells, Tfam protein markedly decreased despite only marginal or no changes in Tfam mRNA and no significant changes in PGC1α, NRF1, or NRF2 proteins. Phosphorylation of 4E-BP1 and p70S6K was reduced in mKO muscle, TAZ KD C2C12 myotubes, and TAZ KO MEFs, and restored by TAZ reintroduction. - Polysome profiling indicated impaired translation initiation/elongation with TAZ loss: TAZ KO cells showed decreased polysome content and a shift of Tfam and Atp5d mRNAs toward lighter fractions; both were rescued by TAZ reintroduction. - TAZ induces Rheb/Rhebl1 to activate mTORC1: Rheb and Rhebl1 mRNA and protein levels were decreased in mKO muscle, TAZ KO MEFs, and TAZ KD myotubes; restored by TAZ expression. ChIP-qPCR demonstrated TAZ and TEAD4 binding at a TAZ-binding element ~4.6 kb upstream of Rhebl1; TEAD4 knockdown reduced TAZ recruitment. Rhebl1 TBE luciferase activity was increased by TAZ and further by TEAD4; reduced with TAZ KD. - Rhebl1 AAV rescue restores mitochondrial biogenesis in mKO muscle: Intramuscular AAV6-Rhebl1 increased Rhebl1 expression and p-4E-BP1, and partially restored Tfam and mitochondrial proteins (Cox4, CytC, Sdha). COX2 immunofluorescence increased; mtDNA-encoded transcripts (mt-Atp6, mt-Co2, mt-Cytb, mt-Nd5) upregulated; mtDNA copy number increased (WT vs mKO-Con p=0.0026; mKO-Con vs mKO-Rhebl1 p=0.039). - Pharmacologic mTOR activation (MHY1485) in mKO muscle increased p-4E-BP1/p-p70S6K, Tfam and mitochondrial proteins, mtDNA-encoded transcripts, and mtDNA content, supporting a TAZ–Rheb/Rhebl1–mTOR axis. - Exercise-induced mitochondrial biogenesis requires TAZ: Endurance training increased COX4 staining, mitochondrial proteins (Tfam, Sdha, Vdac, Cox4, CytC), and mtDNA copy number in WT but not in mKO muscle. mtDNA-encoded genes (mt-Atp6/Co2/Cytb/Nd5) increased with training in WT but not mKO. Nuclear-encoded mitochondrial genes regulated by PGC1α (Atp5a1, Idh3g, Ndufa10, Uqcrc1) were induced by training in both genotypes, consistent with preserved PGC1α activity (PGC1α protein increased in both). Training increased Rhebl1, p-4E-BP1, and p-p70S6K in WT but not mKO. - Exercise-related metabolic improvements depend on TAZ: Trained WT mice showed higher VO2, VCO2, and energy expenditure in dark cycles compared to untrained WT; these adaptations were blunted or absent in trained mKO mice (numerous significant p-values reported). Overall, data demonstrate that TAZ enhances mitochondrial biogenesis primarily by promoting TFAM translation via Rheb/Rhebl1–mTORC1 signaling and is necessary for exercise-induced mitochondrial adaptations.

The findings identify TAZ as a key mediator linking exercise-associated signals to mitochondrial biogenesis. TAZ promotes translation of TFAM by upregulating Rheb and Rhebl1 through TEAD-dependent transcriptional activation, which enhances mTORC1 activity and 4E-BP1/p70S6K signaling. This translational control complements PGC1α-driven transcriptional induction of Tfam, coordinating transcriptional and translational mechanisms to increase mitochondrial DNA replication and expression of mitochondrial-encoded genes. Loss of TAZ impairs mitochondrial content, respiratory metabolism, and exercise capacity, and specifically abrogates the exercise-induced increases in TFAM and mtDNA-encoded transcripts, while leaving PGC1α activity toward nuclear-encoded mitochondrial genes relatively intact. Rescue by Rhebl1 or pharmacologic mTOR activation restores translational signaling and mitochondrial biogenesis, underscoring the TAZ–Rheb/Rhebl1–mTOR axis. The data further suggest TAZ may broadly influence translation of nuclear-encoded mitochondrial genes, consistent with known mTORC1 control of the mitochondrial translatome, thereby contributing to comprehensive mitochondrial remodeling after endurance training.

This study establishes TAZ as a novel stimulator of mitochondrial biogenesis and a critical mediator of exercise-induced muscle adaptation. Mechanistically, TAZ activates TEAD-dependent transcription of Rheb/Rhebl1 to potentiate mTORC1 signaling, promoting TFAM translation and subsequent induction of mitochondrial DNA replication and mitochondrial gene expression. TAZ deficiency impairs mitochondrial content, respiratory metabolism, and exercise performance, and prevents exercise-induced mitochondrial biogenesis; Rhebl1 reintroduction or mTOR activation rescues these deficits. The work proposes an integrated model wherein PGC1α drives Tfam transcription during exercise while TAZ enhances Tfam translation during the post-exercise resting/feeding period to collectively promote mitochondrial biogenesis.