Muscle-specific TIF-1A deletion does not restrict basal rDNA transcription but impairs ribosome production and hypertrophy

Introduction: The transcription initiation factor-1 alpha (TIF-1A/Rrn3) recruits RNA polymerase I (Pol-I) to the ribosomal DNA protomer (rDNAp) to produce ribosomal (r)RNA. We developed a muscle specific conditional mouse model to investigate whether de-novo ribosome production is necessary for skeletal muscle hypertrophy. Methods: Adult (4 mo) TIF-1A f/f were crossed with Human Skeletal Actin Mer-Cre-Mer (HSA-MCM) mice to generate muscle-specific TIF-1A Δ /Δ deleted mice. Tamoxifen (TAM) was administered for 5 days and tissues were collected 8 weeks later either under basal conditions or following 3, 7, or 14 days of mechanical overload (OL). Tif-1a f/f x HSA-MCM littermates served as controls. Successful Tif-1a knockout was confirmed via qPCR and western blotting. The plantaris (PLA) muscles were excised and subjected to histological or biochemical analysis. Pol I enrichment at rDNAp was determined via chromatin immunoprecipitation (Ch-IP). Results: During post-natal development (48wks), absence of TIF-1A did not affect body weight (p=0.761), muscle mass (p=0.961), or fiber cross-sectional area (fCSA) (p=0.177). Puromycin incorporation did not differ between genotypes (p=0.780) neither was the ribosomal (r)RNA content of the plantaris muscle (p=0.469). We detected a minor reduction in 28S (19.0%, p=0.029), 18S (10.5%, p=0.017), and 5.8S (11.1%, p=0.031) rRNAs in Tif-1a Δ/Δ mice that was consistent with a lower rDNA ETS (29.8% p=0.004) and ITS transcription (34.8% p=0.002). Variant (v)rDNA I+II, IV, and VI were not affected in TIF-1A Δ/Δ mice (p>0.05) except for vrDNA III which was 49.9% lower (p=0.006) than in TIF-1A f/f mice. Pol I (RPA 194), UBTF, or Pol II enrichment at the rDNAp was similar between genotypes. Following OL, PLA muscle weight (33.3%) and fCSA (37.9%) were significantly lower in TIF-1A Δ/Δ mice than TIF-1A f/f (p<0.001). The failed hypertrophic response was consistent with reduced Pol I enrichment at the rDNAp (p=0.0085), while UBTF and Pol II enrichment at the rDNAp were similar between Tif-1a Δ/Δ and Tif-1a f/f mice. rDNA transcription was significantly lower (ETS 31.3% & 51.4% and ITS 43.2% & 44.5%) at both 3- and 14-days OL in Tif-1a Δ/Δ mice (p<0.05) with a consistent reduction in vrDNA expression (p<0.001), rRNA content at 3 (50.1%) and 14 (35.3%) days, 28S (32.9% & 36.8%), 18S (20.2% & 44.0%), and 5.8S (30.2% & 44.1%) rRNAs (p<0.05). The reduced anabolic capacity was reflected in depressed puromycin incorporation (32.2%, p<0.001) at the 7-day OL timepoint. Conclusion: TIF-1A is dispensable for basal Pol I transcription in skeletal muscle, but it is necessary for ribosome production and mechanical overload-induced hypertrophy. Thus, under rapid growth conditions, the dynamics of rDNA transcription and associated muscle growth response suggest a different requirement for Pol I associated factors. Funding was supported by grant AR078430 from the NIH to G.A.N. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.