NFATc1 controls skeletal muscle fiber type and is a negative regulator of MyoD activity

Abstract

Skeletal muscle comprises a heterogeneous population of fibers with important physiological differences. Fast fibers are glycolytic and fatigue rapidly. Slow fibers utilize oxidative metabolism and are fatigue resistant. Muscle diseases such as sarcopenia and atrophy selectively affect fast fibers, but the molecular mechanisms regulating fiber type-specific gene expression remain incompletely understood. Here, we show that the transcription factor NFATc1 controls fiber type composition and is required for fast-to-slow fiber type switching in response to exercise in vivo. Moreover, MyoD is a crucial transcriptional effector of the fast fiber phenotype, and we show that NFATc1 inhibits MyoD-dependent fast fiber gene promoters by physically interacting with the N-terminal activation domain of MyoD and blocking recruitment of the essential transcriptional coactivator p300. These studies establish a molecular mechanism for fiber type switching through direct inhibition of MyoD to control the opposing roles of MyoD and NFATc1 in fast versus slow fiber phenotypes.

Figure 1. NFATc1 inhibits MyoD activity

(A, C, D) MyoD transactivated the Mef2c (A), myogenin (C) and mrf4 (D) promoters (lane 2); NFATc1 (c1) significantly inhibited activation (lane 5). (B) A Mef2c skeletal muscle promoter lacking NFAT sites (Figure S1) was transactivated by MyoD (lane 3), and coexpression of NFATc1 inhibited activation (lane 4). Results are reported as the mean fold activation plus SEM from 4 independent transfections. (E–I) C3H10T1/2 cells were transfected, differentiated, and assessed for fast MyHC (MY32) by immunofluorescence (E–H) and western blot (I). Cells were transfected with the pRK5 vector (E), NFATc1 alone (F), MyoD alone (G), or MyoD plus NFATc1 (H). Coexpression of NFATc1 blocked MyoD-induced MyHC expression and myotube formation (white arrowheads). Nuclei were counterstained with DAPI in all panels. (I) Western blot analysis of myosin induction in C3H10T1/2 cells transfected with vector alone (lane 1), NFATc1 alone (lane 2), MyoD alone (lane 3), and NFATc1 plus MyoD (lane 4). α-tubulin was examined as a loading control on the same cell lysates. Nearly identical results were obtained in 6 independent experiments. See also Figure S1.

Figure 2. NFATc1 is required for normal fiber type composition, gene expression, and exercise-induced fiber type switching in vivo

(A) Strategy used to generate skeletal muscle specific knockout of nfatc1. (B) qPCR analysis of MyoD target gene expression in the soleus muscles of adult control (wt) and nfatc1^SkMKO (cko) mice. *, p < 0.05; **, p < 0.01. Data shown represent the mean ratio of expression in nfatc1^SkMKO compared to wild type muscle plus SEM for 6 mice of each genotype. (C–E) Metachromatic ATPase staining of soleus muscles showed a higher percentage of slow fibers (dark blue) in control (wt, panels C, D) than in nfatc1^SkMKO (cko, panel C′, D′) in the absence of exercise (no run; C, C′) or following 7 days of voluntary exercise (run; D, D′). Scale bars =100µm. (E) No run wt mice had a significantly higher percentage of slow fibers than unexercised nfatc1^SkMKO mice (compare lanes 1 and 2, p < 0.01). Following exercise, the percentage of slow fibers in wt soleus increased from 45% to 50% (compare lanes 1 and 3, p < 0.05); the percentage of slow fibers in the soleus of nfatc1^SkMKO mice showed no statistically significant difference (compare lanes 2 and 4). Data are presented as the mean percentage of slow fibers plus SEM for 6 mice in each group. (F) qPCR analysis of fast (light blue bars) and slow (dark blue bars) fiber gene expression in the soleus muscles of unexercised control (wt) and nfatc1^SkMKO (cko) male mice. Data are shown as the mean ratio of expression in nfatc1^SkMKO to wild type muscle plus SEM for 6 mice of each genotype. *, p < 0.05; n.s., not significant. (G) Western blot analysis of fast and slow myosin expression from wt (lanes 1, 2) and nfatc1^SkMKO (cko; lanes 3, 4) soleus muscles. α-tubulin was examined as a loading control on the same cell lysates. See also Figure S2 and Figure S3.

Figure 3. NFATc1 physically interacts with the N-terminus of MyoD

C3H10T1/2 cells were transfected with the indicated plasmids, and lysates were analyzed by immunoprecipitation (IP)-western blot. Sample inputs, IgG antibody controls, and beads only controls are indicated. MyoD physically interacted with full length NFATc1 (panel A, lane 8). IP, anti-MyoD; western, anti-NFATc1. MyoD interacts with the DNA binding domain (DBD) of NFATc1 (panel B, lane 6). Expression plasmids for full length MyoD and Flag-tagged truncation fragments of NFATc1, encoding the N-terminus (N), catalytic domain (cat) or DBD were cotransfected. IP, anti-MyoD; western, anti-Flag. (C–E) Expression plasmids for full length NFATc1 and Flag-tagged MyoD lacking the C-terminus (C), Flag-tagged MyoD lacking the N-terminus (D) or a Myc-tagged N-terminal fragment of MyoD (E) were cotransfected. IP, anti-NFATc1; western, anti-Flag (panels C and D), anti-Myc (panel E). MyoDΔC was efficiently co25 immunoprecipitated by anti-NFATc1 (panel C, lane 7). The N-terminus of MyoD alone [MyoD(N)] was co-immunoprecipitated with NFATc1 (panel E, lane 7). MyoDΔN was not co-immunoprecipitated by anti-NFATc1 (panel D, lane 7).

Figure 4. NFATc1 inhibits the MyoD N-terminal activation domain

C3H10T1/2 cells were transfected with the indicated GAL4(DBD) fusion protein expression plasmids and the UAS reporter plasmid pG₅E1b-luciferase. Full length MyoD, NFATc1 (c1), NFATc3 (c3), E12-VP16 or parental expression vector (dashes) were also co-transfected as indicated. (A) MyoD dimerization with GAL4- E12(bHLH) caused potent activation (lane 6), which was significantly inhibited by NFATc1 (lane 7). (B) E12-VP16-GAL4-MyoD(bHLH) dimers strongly activated the pG₅E1b-luciferase reporter (lane 6) and this was not inhibited by NFATc1 (lane 7). (C) Fusion of full length MyoD or the N terminal fragment of MyoD [MyoD(N)] to the GAL4 DBD resulted in potent activation of the UAS reporter due to the strong activation domains of MyoD. Co-expression of NFATc1 significantly inhibited the MyoD activation domain (compare lanes 3, 4 and lanes 5, 6). Results are reported as the mean fold activation plus SEM from 4 independent transfections; n.s., not significant. Models depicting the mammalian two hybrid (A, B) and one-hybrid (C) assays are shown to the right of their respective graphs. See also Figure S4 and Figure S5.

Figure 5. NFATc1 disrupts MyoD interaction with p300

C3H10T1/2 cells were co-transfected with MyoD and p300 plus or minus NFATc1, and lysates were analyzed by anti-p300 immunoprecipitation followed by anti-MyoD western blot (A), or cells were allowed to differentiate and were analyzed for myotube formation (C–E). (A) MyoD co-immunoprecipitates with p300 (lane 4, red asterisk); this interaction was inhibited by NFATc1 (lane 7, red asterisk). IgG isotype controls and sample inputs are indicated. (C–F) MyoD converted fibroblasts into multinucleated, MyHC+ (green staining) myotubes (white arrowheads) (C), which was potently inhibited by NFATc1 (C′). (D, E) Addition of p300 expression plasmid partially overcame the inhibitory effect of NFATc1 on MyoD (white arrowheads in D′, E′). The amount of p300 expression plasmid (µg) is indicated in parentheses. (F) Conversion to myotubes was quantified as the fraction of MY32+ cells in the MyoD alone control. MyHC+ cells were counted in 10 fields from 3 mice for each condition. Relative myogenic conversion plus standard deviation is indicated. The amount (µg) of each transfected expression plasmid is indicated. (B) ChIP-qPCR analyses for p300 occupancy of a negative control genomic region and the promoter regions of 7 genes from wt and cko soleus muscles, as indicated. Data from 3 independent experiments on different animals are shown as the percentage of input genomic DNA plus SEM. Enrichment for each promoter was calculated by the ΔCt method and normalized to input. Black bars, immunoprecipitation with isotype control IgG; white bars, immunoprecipitation with anti-p300 antibody. Data in B and F were analyzed by two-way ANOVA test with Bonferroni's multiple comparison post hoc analysis. n.s., not significant.