Validation of positional candidates Rps6ka6 and Pou3f4 for a locus associated with skeletal muscle mass variability

Genetic variability significantly contributes to individual differences in skeletal muscle mass; however, the specific genes involved in that process remain elusive. In this study, we examined the role of positional candidates, Rps6ka6 and Pou3f4, of a chromosome X locus, implicated in muscle mass variability in CFW laboratory mice. Histology of hindlimb muscles was studied in CFW male mice carrying the muscle "increasing" allele C (n = 15) or "decreasing" allele T (n = 15) at the peak marker of the locus, rs31308852, and in the Pou3f4y/- and their wild-type male littermates. To study the role of the Rps6ka6 gene, we deleted exon 7 (Rps6ka6-ΔE7) using clustered regularly interspaced palindromic repeats-Cas9 based method in H2Kb myogenic cells creating a severely truncated RSK4 protein. We then tested whether that mutation affected myoblast proliferation, migration, and/or differentiation. The extensor digitorum longus muscle was 7% larger (P < 0.0001) due to 10% more muscle fibers (P = 0.0176) in the carriers of the "increasing" compared with the "decreasing" CFW allele. The number of fibers was reduced by 15% (P = 0.0268) in the slow-twitch soleus but not in the fast-twitch extensor digitorum longus (P = 0.2947) of Pou3f4y/- mice. The proliferation and migration did not differ between the Rps6ka6-ΔE7 and wild-type H2Kb myoblasts. However, indices of differentiation (myosin expression, P < 0.0001; size of myosin-expressing cells, P < 0.0001; and fusion index, P = 0.0013) were significantly reduced in Rps6ka6-ΔE7 cells. This study suggests that the effect of the X chromosome locus on muscle fiber numbers in the fast-twitch extensor digitorum longus is mediated by the Rps6ka6 gene, whereas the Pou3f4 gene affects fiber number in slow-twitch soleus.

Stanniocalcin-2 inhibits skeletal muscle growth and is upregulated in functional overload-induced hypertrophy

AIMS

Stanniocalcin-2 (STC2) has recently been implicated in human muscle mass variability by genetic analysis. Biochemically, STC2 inhibits the proteolytic activity of the metalloproteinase PAPP-A, which promotes muscle growth by upregulating the insulin-like growth factor (IGF) axis. The aim was to examine if STC2 affects skeletal muscle mass and to assess how the IGF axis mediates muscle hypertrophy induced by functional overload.

METHODS

We compared muscle mass and muscle fiber morphology between Stc2-/- (n = 21) and wild-type (n = 15) mice. We then quantified IGF1, IGF2, IGF binding proteins -4 and -5 (IGFBP-4, IGFBP-5), PAPP-A and STC2 in plantaris muscles of wild-type mice subjected to 4-week unilateral overload (n = 14).

RESULTS

Stc2-/- mice showed up to 10% larger muscle mass compared with wild-type mice. This increase was mediated by greater cross-sectional area of muscle fibers. Overload increased plantaris mass and components of the IGF axis, including quantities of IGF1 (by 2.41-fold, p = 0.0117), IGF2 (1.70-fold, p = 0.0461), IGFBP-4 (1.48-fold, p = 0.0268), PAPP-A (1.30-fold, p = 0.0154) and STC2 (1.28-fold, p = 0.019).

CONCLUSION

Here we provide evidence that STC2 is an inhibitor of muscle growth upregulated, along with other components of the IGF axis, during overload-induced muscle hypertrophy.

Analysis of independent cohorts of outbred CFW mice reveals novel loci for behavioral and physiological traits and identifies factors determining reproducibility

Combining samples for genetic association is standard practice in human genetic analysis of complex traits, but is rarely undertaken in rodent genetics. Here, using 23 phenotypes and genotypes from two independent laboratories, we obtained a sample size of 3076 commercially available outbred mice and identified 70 loci, more than double the number of loci identified in the component studies. Fine-mapping in the combined sample reduced the number of likely causal variants, with a median reduction in set size of 51%, and indicated novel gene associations, including Pnpo, Ttll6, and GM11545 with bone mineral density, and Psmb9 with weight. However, replication at a nominal threshold of 0.05 between the two component studies was low, with less than one-third of loci identified in one study replicated in the second. In addition to overestimates in the effect size in the discovery sample (Winner's Curse), we also found that heterogeneity between studies explained the poor replication, but the contribution of these two factors varied among traits. Leveraging these observations, we integrated information about replication rates, study-specific heterogeneity, and Winner's Curse corrected estimates of power to assign variants to one of four confidence levels. Our approach addresses concerns about reproducibility and demonstrates how to obtain robust results from mapping complex traits in any genome-wide association study.

Genome-wide Associations Reveal Human-Mouse Genetic Convergence and Modifiers of Myogenesis, CPNE1 and STC2

Muscle bulk in adult healthy humans is highly variable even after height, age, and sex are accounted for. Low muscle mass, due to fewer and/or smaller constituent muscle fibers, would exacerbate the impact of muscle loss occurring in aging or disease. Genetic variability substantially influences muscle mass differences, but causative genes remain largely unknown. In a genome-wide association study (GWAS) on appendicular lean mass (ALM) in a population of 85,750 middle-aged (aged 38-49 years) individuals from the UK Biobank (UKB), we found 182 loci associated with ALM (p < 5 × 10-8). We replicated associations for 78% of these loci (p < 5 × 10-8) with ALM in a population of 181,862 elderly (aged 60-74 years) individuals from UKB. We also conducted a GWAS on hindlimb skeletal muscle mass of 1,867 mice from an advanced intercross between two inbred strains (LG/J and SM/J); this GWAS identified 23 quantitative trait loci. Thirty-eight positional candidates distributed across five loci overlapped between the two species. In vitro studies of positional candidates confirmed CPNE1 and STC2 as modifiers of myogenesis. Collectively, these findings shed light on the genetics of muscle mass variability in humans and identify targets for the development of interventions for treatment of muscle loss. The overlapping results between humans and the mouse model GWAS point to shared genetic mechanisms across species.

Replication and discovery of musculoskeletal QTLs in LG/J and SM/J advanced intercross lines

The genetics underlying variation in health‐related musculoskeletal phenotypes can be investigated in a mouse model. Quantitative trait loci (QTLs) affecting musculoskeletal traits in the LG/J and SM/J strain lineage remain to be refined and corroborated. The aim of this study was to map muscle and bone traits in males (n = 506) of the 50th filial generation of advanced intercross lines (LG/SM AIL) derived from the two strains. Genetic contribution to variation in all musculoskeletal traits was confirmed; the SNP heritability of muscle mass ranged between 0.46 and 0.56; and the SNP heritability of tibia length was 0.40. We used two analytical software, GEMMA and QTLRel, to map the underlying QTLs. GEMMA required substantially less computation and recovered all the QTLs identified by QTLRel. Seven significant QTLs were identified for muscle weight (Chr 1, 7, 11, 12, 13, 15, and 16), and two for tibia length, (Chr 1 and 13). Each QTL explained 4–5% of phenotypic variation. One muscle and both bone loci replicated previous findings; the remaining six were novel. Positional candidates for the replicated QTLs were prioritized based on in silico analyses and gene expression in muscle tissue. In summary, we replicated existing QTLs and identified novel QTLs affecting muscle weight, and replicated bone length QTLs in LG/SM AIL males. Heritability estimates substantially exceed the cumulative effect of the QTLs, hence a richer genetic architecture contributing to muscle and bone variability could be uncovered with a larger sample size.

Myostatin dysfunction does not protect from fasting-induced loss of muscle mass in mice

OBJECTIVES

The aim of the study was to investigate if myostatin dysfunction can ameliorate fasting-induced muscle wasting.

METHODS

18-week old males from Berlin high (BEH) strain with myostatin dysfunction and wild type myostatin (BEH+/+) strain were subjected to 48-h food deprivation (FD). Changes in body composition as well as contractile properties of soleus (SOL) and extensor digitorum longus (EDL) muscles were studied.

RESULTS

BEH mice were heavier than BEH+/+ mice (56.0±2.5 vs. 49.9±2.8 g, P<0.001, respectively). FD induced similar loss of body mass in BEH and BEH+/+ mice (16.6±2.4 vs. 17.4±2.2%, P>0.05), but only BEH mice experienced wasting of the gastrocnemius, tibialis anterior and plantaris muscles. FD induced a marked decrease in specific muscle force of SOL. EDL of BEH mice tended to be protected from this decline.

CONCLUSION

Myostatin dysfunction does not protect from loss of muscle mass during fasting.

Genome wide association analysis in a mouse advanced intercross line

The LG/J x SM/J advanced intercross line of mice (LG x SM AIL) is a multigenerational outbred population. High minor allele frequencies, a simple genetic background, and the fully sequenced LG and SM genomes make it a powerful population for genome-wide association studies. Here we use 1,063 AIL mice to identify 126 significant associations for 50 traits relevant to human health and disease. We also identify thousands of cis- and trans-eQTLs in the hippocampus, striatum, and prefrontal cortex of ~200 mice. We replicate an association between locomotor activity and Csmd1, which we identified in an earlier generation of this AIL, and show that Csmd1 mutant mice recapitulate the locomotor phenotype. Our results demonstrate the utility of the LG x SM AIL as a mapping population, identify numerous novel associations, and shed light on the genetic architecture of mammalian behavior.

H55N polymorphism is associated with low citrate synthase activity which regulates lipid metabolism in mouse muscle cells

The H55N polymorphism in the Cs gene of A/J mice has been linked to low activity of the enzyme in skeletal muscles. The aim of the study was to test this hypothesis and examine effects of low citrate synthase (CS) activity on palmitate metabolism in muscle cells. Results of the study showed that carriers of the wild type (WT) Cs (C57BL/6J and Balb/cByJ mouse strains) had higher CS activity (p < 0.01) than carriers of the A/J variant (B6.A-(rs3676616-D10Utsw1)/KjnB6 and A/J mouse strains) in the heart, liver and gastrocnemius muscle. Furthermore, the recombinant CS protein of WT showed higher CS activity than the A/J variant. In C2C12 muscle cells the shRNA mediated 47% knockdown of CS activity reduced the rate of fatty acid oxidation compared to the control cells. In summary, our results are consistent with the hypothesis that H55N substitution causes a reduction in CS activity. Furthermore, low CS activity interferes with metabolic flexibility of muscle cells.

Forced Running Endurance Is Influenced by Gene(s) on Mouse Chromosome 10

Phenotypic diversity between laboratory mouse strains provides a model for studying the underlying genetic mechanisms. The A/J strain performs poorly in various endurance exercise models. The aim of the study was to test if endurance capacity and contractility of the fast- and slow-twitch muscles are affected by the genes on mouse chromosome 10. The C57BL/6J (B6) strain and C57BL/6J-Chr 10^A/J/NaJ (B6.A10) consomic strain which carries the A/J chromosome 10 on a B6 strain background were compared. The B6.A10 mice compared to B6 were larger in body weight (p < 0.02): 27.2 ± 1.9 vs. 23.8 ± 2.7 and 23.4 ± 1.9 vs. 22.9 ± 2.3 g, for males and females, respectively, and in male soleus weight (p < 0.02): 9.7 ± 0.4 vs. 8.6 ± 0.9 mg. In the forced running test the B6.A10 mice completed only 64% of the B6 covered distance (p < 0.0001). However, there was no difference in voluntary wheel running (p = 0.6) or in fatigability of isolated soleus (p = 0.24) or extensor digitorum longus (EDL, p = 0.7) muscles. We conclude that chromosome 10 of the A/J strain contributes to reduced endurance performance. We also discuss physiological mechanisms and methodological aspects relevant to interpretation of these findings.

Fine-mapping of genes determining extrafusal fiber properties in murine soleus muscle

Muscle fiber cross-sectional area (CSA) and proportion of different fiber types are important determinants of muscle function and overall metabolism. Genetic variation plays a substantial role in phenotypic variation of these traits; however, the underlying genes remain poorly understood. This study aimed to map quantitative trait loci (QTL) affecting differences in soleus muscle fiber traits between the LG/J and SM/J mouse strains. Fiber number, CSA, and proportion of oxidative type I fibers were assessed in the soleus of 334 genotyped female and male mice of the F₃₄ generation of advanced intercross lines (AIL) derived from the LG/J and SM/J strains. To increase the QTL detection power, these data were combined with 94 soleus samples from the F₂ intercross of the same strains. Transcriptome of the soleus muscle of LG/J and SM/J females was analyzed by microarray. Genome-wide association analysis mapped four QTL (genome-wide P < 0.05) affecting the properties of muscle fibers to chromosome 2, 3, 4, and 11. A 1.5-LOD QTL support interval ranged between 2.36 and 4.67 Mb. On the basis of the genomic sequence information and functional and transcriptome data, we identified candidate genes for each of these QTL. The combination of analyses in F₂ and F₃₄ AIL populations with transcriptome and genomic sequence data in the parental strains is an effective strategy for refining QTL and nomination of the candidate genes.

Baseline Muscle Mass Is a Poor Predictor of Functional Overload-Induced Gain in the Mouse Model

Genetic background contributes substantially to individual variability in muscle mass. Muscle hypertrophy in response to resistance training can also vary extensively. However, it is less clear if muscle mass at baseline is predictive of the hypertrophic response. The aim of this study was to examine the effect of genetic background on variability in muscle mass at baseline and in the adaptive response of the mouse fast- and slow-twitch muscles to overload. Males of eight laboratory mouse strains: C57BL/6J (B6, n = 17), BALB/cByJ (n = 7), DBA/2J (D2, n = 12), B6.A-(rs3676616-D10Utsw1)/Kjn (B6.A, n = 9), C57BL/6J-Chr10^A/J/NaJ (B6.A10, n = 8), BEH+/+ (n = 11), BEH (n = 12), and DUHi (n = 12), were studied. Compensatory growth of soleus and plantaris muscles was triggered by a 4-week overload induced by synergist unilateral ablation. Muscle weight in the control leg (baseline) varied from 5.2 ± 07 mg soleus and 11.4 ± 1.3 mg plantaris in D2 mice to 18.0 ± 1.7 mg soleus in DUHi and 43.7 ± 2.6 mg plantaris in BEH (p < 0.001 for both muscles). In addition, soleus in the B6.A10 strain was ~40% larger (p < 0.001) compared to the B6. Functional overload increased muscle weight, however, the extent of gain was strain-dependent for both soleus (p < 0.01) and plantaris (p < 0.02) even after accounting for the baseline differences. For the soleus muscle, the BEH strain emerged as the least responsive, with a 1.3-fold increase, compared to a 1.7-fold gain in the most responsive D2 strain, and there was no difference in the gain between the B6.A10 and B6 strains. The BEH strain appeared the least responsive in the gain of plantaris as well, 1.3-fold, compared to ~1.5-fold gain in the remaining strains. We conclude that variation in muscle mass at baseline is not a reliable predictor of that in the overload-induced gain. This suggests that a different set of genes influence variability in muscle mass acquired in the process of normal development, growth, and maintenance, and in the process of adaptive growth of the muscle challenged by overload.

Genome-wide association of multiple complex traits in outbred mice by ultra-low-coverage sequencing

Two bottlenecks impeding the genetic analysis of complex traits in rodents are access to mapping populations able to deliver gene-level mapping resolution, and the need for population specific genotyping arrays and haplotype reference panels. Here we combine low coverage sequencing (0.15X) with a novel method to impute the ancestral haplotype space in 1,887 commercially available outbred mice. We mapped 156 unique quantitative trait loci for 92 phenotypes at 5% false discovery rate. Gene-level mapping resolution was achieved at about a fifth of loci, implicating Unc13c and Pgc1-alpha at loci for the quality of sleep, Adarb2 for home cage activity, Rtkn2 for intensity of reaction to startle, Bmp2 for wound healing, Il15 and Id2 for several T-cell measures and Prkca for bone mineral content. These findings have implications for diverse areas of mammalian biology and demonstrate how GWAS can be extended via low-coverage sequencing to species with highly recombinant outbred populations.

Genome-wide association study of behavioral, physiological and gene expression traits in outbred CFW mice

Although mice are the most widely used mammalian model organism, genetic studies have suffered from limited mapping resolution due to extensive linkage disequilibrium (LD) that is characteristic of crosses among inbred strains. Carworth Farms White (CFW) mice are a commercially available outbred mouse population that exhibit rapid LD decay in comparison to other available mouse populations. We performed a genome-wide association study (GWAS) of behavioral, physiological and gene expression phenotypes using 1,200 male CFW mice. We used genotyping by sequencing (GBS) to obtain genotypes at 92,734 SNPs. We also measured gene expression using RNA sequencing in three brain regions. Our study identified numerous behavioral, physiological and expression quantitative trait loci (QTLs). We integrated the behavioral QTL and eQTL results to implicate specific genes, including Azi2 in sensitivity to methamphetamine and Zmynd11 in anxiety-like behavior. The combination of CFW mice, GBS and RNA sequencing constitutes a powerful approach to GWAS in mice.

Deficiency of the zinc finger protein ZFP106 causes motor and sensory neurodegeneration

Zinc finger motifs are distributed amongst many eukaryotic protein families, directing nucleic acid–protein and protein–protein interactions. Zinc finger protein 106 (ZFP106) has previously been associated with roles in immune response, muscle differentiation, testes development and DNA damage, although little is known about its specific function. To further investigate the function of ZFP106, we performed an in-depth characterization of Zfp106 deficient mice (Zfp106^−/−), and we report a novel role for ZFP106 in motor and sensory neuronal maintenance and survival. Zfp106^−/− mice develop severe motor abnormalities, major deficits in muscle strength and histopathological changes in muscle. Intriguingly, despite being highly expressed throughout the central nervous system, Zfp106^−/− mice undergo selective motor and sensory neuronal and axonal degeneration specific to the spinal cord and peripheral nervous system. Neurodegeneration does not occur during development of Zfp106^−/− mice, suggesting that ZFP106 is likely required for the maintenance of mature peripheral motor and sensory neurons. Analysis of embryonic Zfp106^−/− motor neurons revealed deficits in mitochondrial function, with an inhibition of Complex I within the mitochondrial electron transport chain. Our results highlight a vital role for ZFP106 in sensory and motor neuron maintenance and reveal a novel player in mitochondrial dysfunction and neurodegeneration.

Myostatin dysfunction impairs force generation in extensor digitorum longus muscle and increases exercise-induced protein efflux from extensor digitorum longus and soleus muscles

Myostatin dysfunction promotes muscle hypertrophy, which can complicate assessment of muscle properties. We examined force generating capacity and creatine kinase (CK) efflux from skeletal muscles of young mice before they reach adult body and muscle size. Isolated soleus (SOL) and extensor digitorum longus (EDL) muscles of Berlin high (BEH) mice with dysfunctional myostatin, i.e., homozygous for inactivating myostatin mutation, and with a wild-type myostatin (BEH+/+) were studied. The muscles of BEH mice showed faster (P < 0.01) twitch and tetanus contraction times compared with BEH+/+ mice, but only EDL displayed lower (P < 0.05) specific force. SOL and EDL of age-matched but not younger BEH mice showed greater exercise-induced CK efflux compared with BEH+/+ mice. In summary, myostatin dysfunction leads to impairment in muscle force generating capacity in EDL and increases susceptibility of SOL and EDL to protein loss after exercise.

Regenerated soleus muscle shows reduced creatine kinase efflux after contractile activity in vitro

Regenerated skeletal muscles show less muscle damage after strenuous muscle exercise. The aim of the studies was to investigate if the regeneration is associated with reduced muscle creatine kinase (CK) efflux immediately after the exercise. Cryolesion was applied to the soleus muscle of 3-month-old C57BL/6J male mice. Then total CK efflux was assessed in vitro in the regenerated muscles without exercise or after 100 eccentric contractions. The same measurements were performed in the control muscles, which were not exposed to cryolesion. Regenerated muscles generated weaker (P < 0.05) twitches, but stronger (P < 0.05) 150-Hz and 300-Hz tetani with prolonged (P < 0.01) contraction times compared with the control muscles. There was no difference between regenerated and control muscles in the total CK efflux without exercise, but only control muscles showed an increase (P < 0.001) in the CK efflux after the exercise. Our results suggest that muscle regeneration is associated with modulation of contractile properties and improvement in muscle resistance to damage after eccentric exercise.

Novel mutations in human and mouse SCN4A implicate AMPK in myotonia and periodic paralysis

Mutations in the skeletal muscle channel (SCN4A), encoding the Nav1.4 voltage-gated sodium channel, are causative of a variety of muscle channelopathies, including non-dystrophic myotonias and periodic paralysis. The effects of many of these mutations on channel function have been characterized both in vitro and in vivo. However, little is known about the consequences of SCN4A mutations downstream from their impact on the electrophysiology of the Nav1.4 channel. Here we report the discovery of a novel SCN4A mutation (c.1762A>G; p.I588V) in a patient with myotonia and periodic paralysis, located within the S1 segment of the second domain of the Nav1.4 channel. Using N-ethyl-N-nitrosourea mutagenesis, we generated and characterized a mouse model (named draggen), carrying the equivalent point mutation (c.1744A>G; p.I582V) to that found in the patient with periodic paralysis and myotonia. Draggen mice have myotonia and suffer from intermittent hind-limb immobility attacks. In-depth characterization of draggen mice uncovered novel systemic metabolic abnormalities in Scn4a mouse models and provided novel insights into disease mechanisms. We discovered metabolic alterations leading to lean mice, as well as abnormal AMP-activated protein kinase activation, which were associated with the immobility attacks and may provide a novel potential therapeutic target.

Discovery and refinement of muscle weight QTLs in B6 × D2 advanced intercross mice

The genes underlying variation in skeletal muscle mass are poorly understood. Although many quantitative trait loci (QTLs) have been mapped in crosses of mouse strains, the limited resolution inherent in these conventional studies has made it difficult to reliably pinpoint the causal genetic variants. The accumulated recombination events in an advanced intercross line (AIL), in which mice from two inbred strains are mated at random for several generations, can improve mapping resolution. We demonstrate these advancements in mapping QTLs for hindlimb muscle weights in an AIL (n = 832) of the C57BL/6J (B6) and DBA/2J (D2) strains, generations F8-F13. We mapped muscle weight QTLs using the high-density MegaMUGA SNP panel. The QTLs highlight the shared genetic architecture of four hindlimb muscles and suggest that the genetic contributions to muscle variation are substantially different in males and females, at least in the B6D2 lineage. Out of the 15 muscle weight QTLs identified in the AIL, nine overlapped the genomic regions discovered in an earlier B6D2 F2 intercross. Mapping resolution, however, was substantially improved in our study to a median QTL interval of 12.5 Mb. Subsequent sequence analysis of the QTL regions revealed 20 genes with nonsense or potentially damaging missense mutations. Further refinement of the muscle weight QTLs using additional functional information, such as gene expression differences between alleles, will be important for discerning the causal genes.

Genetic and genomic analyses of musculoskeletal differences between BEH and BEL strains

Berlin high (BEH) and Berlin low (BEL) strains selected for divergent growth differ threefold in body weight. We aimed at examining muscle mass, which is a major contributor to body weight, by exploring morphological characteristics of the soleus muscle (fiber number and cross sectional area; CSA), by analyzing the transcriptome of the gastrocnemius and by initiating quantitative trait locus (QTL) mapping. BEH muscles were four to eight times larger than those of BEL. In substrain BEH+/+, mutant myostatin was replaced with a wild-type allele; however, BEH+/+muscles still were two to four times larger compared with BEL. BEH soleus muscle fibers were two times more numerous (P < 0.0001) and CSA was two times larger (P < 0.0001) compared with BEL. In addition, soleus femoral attachment anomaly (SFAA) was observed in all BEL mice. One significant (Chr 1) and four suggestive (Chr 3, 4, 6, and 9) muscle weight QTLs were mapped in a 21-day-old F2 intercross (n = 296) between BEH and BEL strains. The frequency of SFAA incidence in the F2 and in the backcross to BEL strain (BCL) suggested the presence of more than one causative gene. Two suggestive SFAA QTLs were mapped in BCL; however, their peak markers were not associated with the phenotype in F2. RNA-Seq analysis revealed 2,148 differentially expressed (P < 0.1) genes and 45,673 single nucleotide polymorphisms and >2,000 indels between BEH+/+ and BEL males. In conclusion, contrasting muscle traits and genomic and gene expression differences between BEH and BEL strains provide a promising model for the search for genes involved in muscle growth and musculoskeletal morphogenesis.

Analyses of muscle spindles in the soleus of six inbred mouse strains

Adult muscle size and fibre-type composition are heritable traits that vary substantially between individuals. We used inbred mouse strains in which soleus muscle mass varied by an order of magnitude to explore whether properties of muscle spindles can also be influenced by genetic factors. Skip-serial cross-sections of soleus muscles dissected from 15 male mice of BEH, BEL, C57BL/6J, DUH, LG/J and SM/J strains were analysed for number of muscle spindles and characteristics of intrafusal and extrafusal fibres following ATPase staining. The BEL and DUH strains determined the range of: soleus mean size, a 10-fold difference from 2.1 to 22.3 mg, respectively; the mean number of extrafusal fibres, a 2.5-fold difference from 497 to 1249; and mean fibre-cross-sectional area, three-fold difference, e.g. for type 1 fibres, from 678 to 1948 μm². The range of mean proportion of type 1 fibres was determined by C57BL/6J (31%) and DUH (64%) strains. The mean number of spindles per muscle ranged between nine (LG/J) and 13 (BEL) (strain effect P < 0.02). Genetic correlations between spindle count and muscle weight or properties of extrafusal fibres were weak and not statistically significant. However, there was a strong correlation between the proportion of spindles with more than one bag2 fibre and the proportion of extrafusal fibres that were of type 1, and strain-dependent variation in the numbers of such spindles was statistically significant. The numbers of intrafusal fibres per spindle ranged from 2 to 8, with the most common complement of four found in 75.6% of spindles. There were no significant differences between the strains in the mean numbers of intrafusal fibres; however, the variance of the number was significantly less for the C57BL/6J strain than for any of the others. We conclude that abundance of muscle spindles and their intrafusal-fibre composition are substantially determined by genetic factors, which are different from those affecting muscle size and properties of the extrafusal fibres.

Resolving candidate genes of mouse skeletal muscle QTL via RNA-Seq and expression network analyses

Background

We have recently identified a number of Quantitative Trait Loci (QTL) contributing to the 2-fold muscle weight difference between the LG/J and SM/J mouse strains and refined their confidence intervals. To facilitate nomination of the candidate genes responsible for these differences we examined the transcriptome of the tibialis anterior (TA) muscle of each strain by RNA-Seq.

Results

13,726 genes were expressed in mouse skeletal muscle. Intersection of a set of 1061 differentially expressed transcripts with a mouse muscle Bayesian Network identified a coherent set of differentially expressed genes that we term the LG/J and SM/J Regulatory Network (LSRN). The integration of the QTL, transcriptome and the network analyses identified eight key drivers of the LSRN (Kdr, Plbd1, Mgp, Fah, Prss23, 2310014F06Rik, Grtp1, Stk10) residing within five QTL regions, which were either polymorphic or differentially expressed between the two strains and are strong candidates for quantitative trait genes (QTGs) underlying muscle mass. The insight gained from network analysis including the ability to make testable predictions is illustrated by annotating the LSRN with knowledge-based signatures and showing that the SM/J state of the network corresponds to a more oxidative state. We validated this prediction by NADH tetrazolium reductase staining in the TA muscle revealing higher oxidative potential of the SM/J compared to the LG/J strain (p<0.03).

Conclusion

Thus, integration of fine resolution QTL mapping, RNA-Seq transcriptome information and mouse muscle Bayesian Network analysis provides a novel and unbiased strategy for nomination of muscle QTGs.

Divergent physiological characteristics and responses to endurance training among inbred mouse strains

Both baseline values and adaptive changes in mice can vary depending on the genetic background. We aimed to assess variation in a battery of variables and their adaptations to endurance training in six inbred mouse strains. Males, n = 184, from A/J, BALB/cByJ, C3H/HeJ, C57BL/6J, DBA/2J, and PWD/PhJ strains were assigned to a control or an endurance group (5 weeks swimming exercise). Enzyme activity, histology of soleus (SOL) muscle, swimming endurance, cardiac ventricular and hind limb muscle weight, and femur length were examined. Endurance capacity, morphological and histological variables, and enzyme activity substantially differed among strains. For example, SOL weight was twofold higher and cross-sectional area (CSA) of fibers was ≈ 30% greater in C57BL/6J than in PWD/PhJ strain. The CSA of type 1 fibers were larger than type 2A in PWD/PhJ (P < 0.01); however, the reverse was true in DBA/2J and BALB/cByJ strains (P < 0.05). Swimming endurance in DBA/2J strain was ≈ 9 times better than in BALB/cByJ. Endurance training increased the activity of citrate synthase in gastrocnemius across strains (P < 0.01), however, changes in endurance were strain-specific; the C57BL/6J and DBA/2J strains improved substantially, whereas A/J and BALB/cByJ strains did not. In conclusion, genetic background is a potent determinant of the physiological characteristics and adaptations to training in mice.

QTL Analysis of Type I and Type IIA Fibers in Soleus Muscle in a Cross between LG/J and SM/J Mouse Strains

Properties of muscle fibers, i.e., their type, number and size, are important determinants of functional characteristics of skeletal muscle, and of the quality of meat in livestock. Genetic factors play an important role in determining variation in fiber properties, however, specific genes remain largely elusive. We examined histological properties of soleus muscle fibers in two strains of mice exhibiting a twofold difference in muscle mass, LG/J and SM/J, and their F2 intercross. The total number of muscle fibers (555 ± 106; mean ± SD) did not differ between the strains or between males and females. A higher percentage of type I fibers was observed in the LG/J compared to the SM/J strain (P < 0.001) in both males (45 ± 3 vs. 37 ± 4%) and females (58 ± 4 vs. 41 ± 3%). Across strains, females had a higher percentage of type I fibers than males (P < 0.001), and the sex effect was greater in the LG/J strain (strain-by-sex interaction, P < 0.001). The cross-sectional area (CSA) did not differ between type I and type IIA fibers, but was greater in the LG/J than the SM/J strain (1365 ± 268 vs. 825 ± 229 μm², P < 0.001). Three significant quantitative trait locus (QTL) affecting CSA for type I and type IIA fibers mapped to chromosomes (Chr) 1, 6, and 11 and three suggestive QTL for percentage of type I fibers mapped to Chr 2, 3, and 4. Within each significant QTL, regions of conserved synteny were also implicated in variation of similar traits in an analogous study in pigs. Our results provide the evidence that the intercross between the SM/J and LG/J strains is a promising model to search for genes affecting muscle fiber properties.

H55N polymorphism as a likely cause of variation in citrate synthase activity of mouse skeletal muscle

Citrate synthase (CS) is an enzyme of the Krebs cycle that plays a key role in mitochondrial metabolism. The aim of this study was to investigate the mechanisms underlying low activity of citrate synthase (CS) in A/J mice compared with other inbred strains of mice. Enzyme activity, protein content, and mRNA levels of CS were studied in the quadriceps muscles of A/J, BALB/cByJ, C57BL/6J, C3H/HeJ, DBA/2J, and PWD/PhJ strains of mice. Cytochrome c protein content was also measured. The results of the study indicate that A/J mice have a 50–65% reduction in CS activity compared with other strains despite similar levels of Cs mRNA and lack of differences in CS and cytochrome c protein content. CS from A/J mice also showed lower Michaelis constant ( Km) for both acetyl CoA and oxaloacetate compared with the other strains of mice. In silico analysis of the genomic sequence identified a nonsynonymous single nucleotide polymorphism (SNP) (rs29358506, H55N) in Cs gene occurring near the site of the protein interacting with acetyl CoA. Allelic variants of the polymorphism segregated with the catalytic properties of CS enzyme among the strains. In summary, H55N polymorphism in Cs could be the underlying cause of low CS activity and its high affinity for substrates in A/J mice compared with other strains. This SNP might also play a role in resistance to obesity of A/J mice.

Fine-mapping of muscle weight QTL in LG/J and SM/J intercrosses

Genetic variation plays a substantial role in variation in strength, but the underlying mechanisms remain poorly understood. The objective of the present study was to examine the mechanisms underlying variation in muscle mass, a predictor of strength, between LG/J and SM/J strains, which are the inbred progeny of mice selected, respectively, for high and low body weight. We measured weight of five hindlimb muscles in LG/J and SM/J males and females, in F(1) and F(2) intercrosses, and in an advanced intercross (AI), F(34), between the two. F(2) mice were genotyped using 162 SNPs throughout the genome; F(34) mice were genotyped at 3,015 SNPs. A twofold difference in muscle mass between the LG/J and SM/J mouse strains was observed. Integrated genome-wide association analysis in the combined population of F(2) and AI identified 22 quantitative trait loci (QTL; genome-wide P < 0.05) affecting muscle weight on Chr 2 (2 QTL), 4, 5, 6 (7 QTL), 7 (4 QTL), 8 (4 QTL), and 11 (3 QTL). The LG/J allele conferred greater muscle weight in all cases. The 1.5-LOD QTL support intervals ranged between 0.3 and 13.4 Mb (median 3.7 Mb) restricting the list of candidates to between 5 and 97 genes. Selection for body weight segregated the alleles affecting skeletal muscle, the most abundant tissue in the body. Combination of analyses in an F(2) and AI was an effective strategy to detect and refine the QTL in a genome-wide manner. The achieved resolution facilitates further elucidation of the underlying genetic mechanisms affecting muscle mass.

Genomic analysis of variation in hindlimb musculature of mice from the C57BL/6J and DBA/2J lineage

The precise locations of attachment points of muscle to bone-the origin and insertion sites-are crucial anatomical and functional characteristics that influence locomotor performance. Mechanisms that control the development of these interactions between muscle, tendon, and bone are currently not well understood. In a subset of BXD recombinant inbred (RI) strains derived from the C57BL/6J and DBA/2J strains, we observed a soleus femoral attachment anomaly (SFAA) that was rare in both parental strains (Lionikas, Glover et al. 2006). The aim of the present study was to assess suitability of SFAA as a model to study the genetic mechanisms underlying variation in musculoskeletal anatomy. We scored the incidence of SFAA in 55 BXD strains (n = 9 to 136, median = 26, phenotyped animals per strain, for a total number of 2367). Seven strains (BXD1, 12, 38, 43, 48, 54, and 56) exhibited a high incidence of unilateral SFAA (47-89%), whereas 23 strains scored 0%. Exploration of the mechanisms underlying SFAA in 2 high incidence strains, BXD1 and BXD38, indicated that SFAA-relevant genes are to be found in both C57BL/6J and DBA/2J regions of the BXD1 genome. However, not all alleles relevant for the expression of the phenotype were shared between the 2 high-incidence BXD strains. In conclusion, the anatomical origin of the soleus muscle in mouse is controlled by a polygenic system. A panel of BXD RI strains is a useful tool in exploring the genetic mechanisms underlying SFAA and improving our understanding of musculoskeletal development.

QTL analyses of temporal and intensity components of home-cage activity in KJR and C57BL/6J strains

BACKGROUND

A variety of mouse strains exhibit diversity in spontaneous activity consistent with an important genetic contribution. To date, many studies have defined spontaneous home-cage activity as total distance or total counts of activity within a test period. However, spontaneous activity is, in fact, a composite of elements of 'temporal' and 'intensity' that is similar to 'velocity'. Here, we report on quantitative trait loci for different components of spontaneous activity, an important step towards dissection of the underlying genetic mechanisms.

RESULTS

In the analysis of total home-cage activity (THA) after habituation in female mice, KJR strain exhibit higher activity than C57BL/6J (B6). In this study, THA was partitioned into two components: active time (AT) was an index of the 'temporal element' of THA, average activity during active time (AA) was an index of 'intensity'. Correlation analysis using B6xKJR F2 female mice indicated that AA is a major component of THA, whereas AA and AT were associated to a lesser degree. To explore the genetic basis of the activity differences, we conducted quantitative trait loci (QTL) analysis on data of THA and its components, AT and AA. Three significant QTL affecting variation of different components of home cage activity were identified, two linked QTL Hylaq1 and Hylaq2 on Chr 2, and Hylaq3 on Chr 10. Chromosomal positions of these QTL were previously implicated in locomotor activity (Chr 2) or open-field ambulation (Chr 10). The results indicated that Hylaq1 influences AT, Hylaq2, AA, while Hylaq3 is associated with both AA and AT.

CONCLUSION

Through this study, we found that variation in total home cage activity over a 3 day period is affected by variation in active time and intensity of activity. The latter two variables are distinct components of home cage activity with only partially overlapping genetic architecture.

Genetic architecture for hole-board behaviors across substantial time intervals in young, middle-aged and old mice

A quantitative trait locus (QTL) analysis of behaviors across the life span was conducted in F(2) mice from a C57BL/6J x DBA/2J cross and 22 BXD recombinant inbred (RI) strains. Mice of three age groups were tested in a hole-board apparatus for 3 min on three occasions approximately 1 month apart (average age at test 150, 450 and 750 days, approximately 400 mice per group, divided equally by sex). Quantitative trait loci with small effect size were found on 11 chromosomes for hole-board activity (Hbact) and hole-board rearing (Hbrear). Analysis of 22 RI strains tested at 150 and 450 days of age found only suggestive linkage, with four QTL for Hbact overlapping with those from the F(2) analysis. There was a significant phenotypic correlation between Hbact and Hbrear (approximately 0.55-0.69) and substantial commonality among QTL for the two behaviors. QTL analyses of head-pokes (HP) and fecal boli (FB) only identified QTL at the suggestive level of significance. Age accounted for approximately 15% of the phenotypic variance (sex approximately 3%), and there were genotype by age interactions at approximately 25% of the Hbact and Hbrear QTL. Quantitative trait loci for Hbrear were relatively stable across the three measurement occasions (those for Hbact somewhat less so), although mean levels of each index declined markedly comparing the first to subsequent trials. Considered as a whole, the polygenic system influencing exploratory behaviors accounts for approximately the same amount of phenotypic variance as age (within the range studied), is stable across substantial periods of time, and acts, for the most part, independently of age and sex.

Blood pressure and heart rate QTL in mice of the B6/D2 lineage: sex differences and environmental influences

A quantitative trait locus (QTL) approach was used to define the genetic architecture underlying variation in systolic blood pressure (SBP) and heart rate (HR), measured indirectly on seven occasions by the tail cuff procedure. The tests were conducted in 395 F(2) adult mice (197 males, 198 females) derived from a cross of the C57BL/6J (B6) and DBA/2J (D2) strains and in 22 BXD recombinant-inbred (RI) strains. Interval mapping of F(2) data for the first 5 days of measurement nominated one statistically significant and one suggestive QTL for SBP on chromosomes (Chr) 4 and 14, respectively, and two statistically significant QTL for HR on Chr 1 (which was specific to female mice) and Chr 5. New suggestive QTL emerged for SBP on Chr 3 (female-specific) and 8 and for HR on Chr 11 for measurements recorded several weeks after mice had undergone stressful blood sampling procedures. The two statistically significant HR QTL were confirmed by analyses of BXD RI strain means. Male and female F(2) mice did not differ in SBP or HR but RI strain analyses showed pronounced strain-by-sex interactions and a negative genetic correlation between the two measures in both sexes. Evidence for a role for mitochondrial DNA was found for both HR and SBP. QTL for HR and SBP may differ in males and females and may be sensitive to different environmental contexts.

Echocardiographic parameters in athletes of different sports

Competitive athletics is often associated with moderate left ventricular (LV) hypertrophy, and it has been hypothesized that training mode and type of exercise modulates long-term cardiac adaptation. The purpose of the study was to compare cardiac structure and function among athletes of various sports and sedentary controls. Standard transthoracic two-dimensional M-mode and Doppler echocardiography was performed at rest in Caucasian male canoe/kayak paddlers (n = 9), long distance runners (LDR, n = 18), middle distance runners (MDR, n = 17), basketball players (BP, n = 31), road cyclists (n = 8), swimmers (n = 10), strength/power athletes (n = 9) of similar age (range, 15 to 31 yrs), training experience (4 to 9 years), and age-matched healthy male sedentary controls (n = 15). Absolute interventricular septum (IVS) thickness and LV wall thickness, but not LV diameter, were greater in athletes than sedentary controls. Left ventricular mass of all athletes but relative wall thickness of only BP, swimmers, cyclists, and strength/power athletes were higher as compared with controls (p < 0.05). Among athletes, smaller IVS thickness was observed in MDR than BP, cyclists, swimmers or strength/power athletes, while LDR had higher body size-adjusted LV diameter as compared to BP, cyclists and strength/power athletes. In conclusion, relative LV diameter was increased in long distance runners as compared with basketball players, cyclists, and strength/power athletes. Basketball, road cycling, strength/power, and swimming training were associated with increased LV concentricity as compared with paddling or distance running. Key pointsThe type of cardiac hypertrophy seems to be only moderately exercise-specific.Long-distance runners develop larger left ventricular dilation as compared with basketball players, cyclists, and strength/power athletes.Myocardial wall thickening is triggered by different sporting activities that involve large muscle groups.

Muscle cell and motor protein function in patients with a IIa myosin missense mutation (Glu-706 to Lys)

The pathogenic events leading to the progressive muscle weakness in patients with a E706K mutation in the head of the myosin heavy chain (MyHC) IIa were analyzed at the muscle cell and motor protein levels. Contractile properties were measured in single muscle fiber segments using the skinned fiber preparation and a single muscle fiber in vitro motility assay. A dramatic impairment in the function of the IIa MyHC isoform was observed at the motor protein level. At the single muscle fiber level, on the other hand, a general decrease was observed in the number of preparations where the specific criteria for acceptance were fulfilled irrespective of MyHC isoform expression. Our results provide evidence that the pathogenesis of the MyHC IIa E706K myopathy involves defective function of the mutated myosin as well as alterations in the structural integrity of all muscle cells irrespective of MyHC isoform expression.

QTL influencing baseline hematocrit in the C57BL/6J and DBA/2J lineage: age-related effects

Baseline serum hematocrit varies substantially in the population. While additive genetic factors account for a large part of this variability, little is known about the genetic architecture underlying the trait. Because hematocrit levels vary with age, it is plausible that quantitative trait loci (QTL) that influence the phenotype also show an age-specific profile. To investigate this possibility, hematocrit was measured in three different age cohorts of mice (150, 450, and 750 days) of the C57BL/6J (B6) and the DBA2/J (D2) lineage. QTL were searched in the B6D2F(2) intercross and the BXD recombinant inbred (RI) strains. The effects of these QTL were explored across the different age groups. On the phenotypic level, baseline serum hematocrit declines with age in a sex-specific manner. In the B6D2F(2) intercross, suggestive QTL that influence the phenotype were located on Chromosomes (Chr) 1, 2, 7, 11, 13, and 16. With the exception of the QTL on Chr 2, all of these QTL exerted their largest effect at 750 days. The QTL on Chr 1, 2, 7, 11 and 16 were confirmed in the BXD RIs in a sex- and age-specific manner. Linkage analysis in the BXD RIs revealed an additional significant QTL on Chr 19. Baseline serum hematocrit is influenced by several QTL that appear to vary with the age and sex of the animal. These QTL primarily overlap with QTL that have been shown to regulate hematopoietic stem cell phenotypes.

Genetic determinants of weight of fast- and slow-twitch skeletal muscles in old mice

The main goal of the study was to explore the genetic architecture underlying muscle weight in old mice. Weight of soleus, tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius muscles was measured in the C57BL/6J (B6) and DBA/2J (D2) strains and derivative generations: a panel of the BXD recombinant inbred (RI) strains and a B6D2 F(2) intercross at the age of 800 days. The between-strain difference in muscle weight (B6 > D2) ranged between 16% and 38%. Linkage analysis identified suggestive quantitative trait loci (QTL) on Chromosomes (Chr) 2, 6, 7, 8, 19, and X that influenced muscle weight in the 800-day-old group. Comparison of weights at 200, 500, and 800 days revealed a variable effect of age among the four muscles. Linkage analysis in the B6D2 F(2) population combined across the three different age groups identified muscle-, sex-, and age-specific QTL on Chr 1, 2, 3, 5, 6, 8, 9, 11, 13, 17, X, and Y. Genetic factors that influence the rate of weight change (within-strain weight difference at two ages) over the lifespan of BXD RIs were mapped to the markers D2Mit369 and D3Mit130 at the genome-wide p < 0.05 for TA muscle in males (between 200 and 800 days) and females (between 500 and 800 days), respectively. Analysis of all age groups supported previous findings that the genetic effects may be muscle-, age-, and sex-specific.

Anomaly of anatomical origin of soleus muscle: a mouse model

In the laboratory mouse, the soleus muscle arises at the head of the fibula and inserts via the Achilles tendon on the tuber calcanei together with the gastrocnemius muscle. During routine dissection of mice from the BXD recombinant inbred (RI) strains, we found that the soleus often originated from the lateral epicondyle of the femur instead of the head of the fibula. This soleus femoral attachment anomaly (SFAA) changes the soleus from being a single-joint to a two-joint muscle. The incidence of SFAA was 45% in the BXD38 RI strain. Bilateral inspection indicated that SFAA may be present unilaterally or bilaterally within an individual mouse. We explored the effect of SFAA on muscle weight in mice with unilateral expression. The weight of SFAA soleii was significantly less (P < 0.01) than that of the soleii with normal attachment by 6% (females) and 14% (males). Similar anatomical anomalies of the soleus muscle have been noted in humans. The mouse model will provide the means to explore the physiological consequences and genetic basis for such anomalies.

Human skeletal muscle myosin function at physiological and non-physiological temperatures

AIM

The aim of the study was to assess the function of human skeletal muscle myosin across a wide range of temperatures, including physiological.

METHODS

We used a single fibre in vitro motility assay. The in vitro motility speed of actin filaments propelled by myosin extracted from fibres expressing type I myosin heavy chain (MyHC; n = 9), IIa MyHC (n = 6), IIax MyHC (n = 4) and I/IIa MyHC (n = 1) was measured at 15, 20, 25, 30 and 35 degrees C.

RESULTS

The motility speed between groups of fibres expressing different MyHC differed significantly (P << 0.001). The increase in motility speed with an increase in temperature was statistically significant (P << 0.001) between all temperatures. The relative difference in motility speed between the slow type I and the fast IIax MyHC fibres decreased with increasing temperature, i.e. a 7.5-fold difference at 15 degrees C was reduced to twofold at 35 degrees C. Furthermore, the twofold difference in motility speed between type IIa and IIax MyHC at 15 degrees C disappeared completely at 35 degrees C. The activation energy, E(A), and temperature coefficient, Q(10), over the 15-35 degrees C temperature range was higher for type I MyHC, 54.47 +/- 4.37 kJ mol(-1) and 2.09 +/- 0.12, respectively, than for type IIa MyHC, 45.41 +/- 3.12 kJ mol(-1) (P < 0.001) and 1.85 +/- 0.08 (P < 0.001), or IIax MyHC, 34.71 +/- 1.75 kJ mol(-1) (P << 0.001) and 1.60 +/- 0.04 (P << 0.001).

CONCLUSION

The present results suggest a significantly reduced difference in shortening velocity between different human muscle fibre types at physiological temperature than previously reported at lower temperatures (12 or 15 degrees C) with implications for human in vivo muscle function.

Adjusting data to body size: a comparison of methods as applied to quantitative trait loci analysis of musculoskeletal phenotypes

The aim of this study was to compare three methods of adjusting skeletal data for body size and examine their use in QTL analyses. It was found that dividing skeletal phenotypes by body mass index induced erroneous QTL results. The preferred method of body size adjustment was multiple regression.

INTRODUCTION

Many skeletal studies have reported strong correlations between phenotypes for muscle, bone, and body size, and these correlations add to the difficulty in identifying genetic influence on skeletal traits that are not mediated through overall body size. Quantitative trait loci (QTL) identified for skeletal phenotypes often map to the same chromosome regions as QTLs for body size. The actions of a QTL identified as influencing BMD could therefore be mediated through the generalized actions of growth on body size or muscle mass.

MATERIALS AND METHODS

Three methods of adjusting skeletal phenotypes to body size were performed on morphologic, structural, and compositional measurements of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) second generation (F(2)) mice (n = 400). A common method of removing the size effect has been through the use of ratios. This technique and two alternative techniques using simple and multiple regression were performed on muscle and skeletal data before QTL analyses, and the differences in QTL results were examined.

RESULTS AND CONCLUSIONS

The use of ratios to remove the size effect was shown to increase the size effect by inducing spurious correlations, thereby leading to inaccurate QTL results. Adjustments for body size using multiple regression eliminated these problems. Multiple regression should be used to remove the variance of co-factors related to skeletal phenotypes to allow for the study of genetic influence independent of correlated phenotypes. However, to better understand the genetic influence, adjusted and unadjusted skeletal QTL results should be compared. Additional insight can be gained by observing the difference in LOD score between the adjusted and nonadjusted phenotypes. Identifying QTLs that exert their effects on skeletal phenotypes through body size-related pathways as well as those having a more direct and independent influence on bone are equally important in deciphering the complex physiologic pathways responsible for the maintenance of bone health.

Genetic determinants of weight of fast- and slow-twitch skeletal muscle in 500-day-old mice of the C57BL/6J and DBA/2J lineage

C57BL/6J (B6) and DBA/2J (D2) strains and two derivative populations, BXD recombinant inbred strains (BXD RIs) and B6D2F2, were used to explore genetic basis for variation in muscle weight at 500 days of age. In parallel with findings in 200-day-old mice (Lionikas A, Blizard DA, Vandenbergh DJ, Glover MG, Stout JT, Vogler GP, McClearn GE, and Larsson L. Physiol Genomics 16: 141-152, 2003), weight of slow-twitch soleus, mixed gastrocnemius, and fast-twitch tibialis anterior (TA) and extensor digitorum longus (EDL) muscles was 13-22% greater (P < 0.001) in B6 than in D2. Distribution of BXD RI strain means indicated that genetic influence on muscle weight (strain effect P < 0.001, all muscles) was of polygenic origin, and effect of genetic factors differed between males and females (strain-by-sex interaction: P < 0.01 for soleus, EDL; P < 0.05 for TA, gastrocnemius). Linkage analyses in B6D2F2 population identified QTL affecting muscle weight on Chr 1, 2, 6, and 9. Pleiotropic influences were observed for QTL on Chr 1 (soleus, TA), 2 (TA, EDL, gastrocnemius), and 9 (soleus, TA, EDL) and were not related to muscle type (fast/slow-twitch) or function (flexor/extensor). Effect of QTL on Chr 9 on soleus muscle was male specific. QTL on Chr 2 and 6 were previously observed at 200 days of age, whereas QTL on Chr 1 and 9 are novel muscle weight QTL. In summary, muscle weight in B6/D2 lineage is affected by a polygenic system that has variable influences at different ages, between males and females, and across muscles in a manner independent of muscle type.

Genetic architecture of fast- and slow-twitch skeletal muscle weight in 200-day-old mice of the C57BL/6J and DBA/2J lineage

The aim of the study was to explore the genetic architecture influencing weight of fast- and slow-twitch skeletal muscles. The weights of the slow-twitch soleus, the mixed gastrocnemius, the fast-twitch tibialis anterior (TA), and extensor digitorum longus (EDL) muscles were 11-34% greater (P < 0.001) in 200-day-old C57BL/6J (B6) than in DBA/2J (D2) mice. Male muscles were 13-28% larger than female (P < 1 x 10(-5), no strain by sex interaction). The sex-related difference in muscle weight, however, varied significantly among the 23 derivative BXD recombinant inbred (RI) strains (strain by sex interaction for soleus, P < 0.01; TA, P < 1 x 10(-4); EDL, not significant; and gastrocnemius, P < 0.001). Quantitative trait loci (QTL) affecting muscle weight were mapped in an F2 intercross of B6 and D2 mice (B6D2F2) and BXD RIs. A total of 10 autosomal, muscle-specific, but not muscle-type-specific, QTL, explaining a total of 5.4, 7.7, 22.9, and 8.6% of phenotypic variance for soleus, TA, EDL, and gastrocnemius muscles, respectively, were found across chromosomes 1 (Chr 1), 2, 3 (female-specific), 5 (two), 6, 7, 8, and 9 in B6D2F2 mice. The QTL on Chr 8 for EDL and the female-specific QTL on Chr 3 for gastrocnemius muscles were statistically significant, but the remaining QTL were at the suggestive level of statistical significance. Ten QTL on Chr 1, 2, 4, 5, 7, 8, 14, 17 (two), and 19 were identified in BXD RIs. Half of the QTL in BXD RIs had pleiotropic effects and were at the suggestive level of significance (except for the significant QTL for gastrocnemius muscle on Chr 17). The B6D2F2 nominated QTL on Chr 8 for EDL weight was validated in BXD RIs (P < 0.03). Support intervals for the QTL on Chr 1 and 5 overlapped between B6D2F2 and BXD RIs. An epistatic interaction between markers on Chr 1 and 17 affected gastrocnemius weight in BXD RIs. The interaction was not, however, validated in the B6D2F2 population. Our results indicate that the differences in muscle weight in the B6 and D2 segregating populations were the outcome of a polygenic system, with each factor contributing a small amount to the phenotypic variance and the genetic architecture affecting muscle weight was muscle specific, but not muscle-type specific, and in some instances sex specific.

Effects of concentric and eccentric contractions on phosphorylation of MAPK(erk1/2) and MAPK(p38) in isolated rat skeletal muscle

1. Exercise and contractions of isolated skeletal muscle induce phosphorylation of mitogen-activated protein kinases (MAPKs) by undefined mechanisms. The aim of the present study was to determine exercise-related triggering factors for the increased phosphorylation of MAPKs in isolated rat extensor digitorum longus (EDL) muscle. 2. Concentric or eccentric contractions, or mild or severe passive stretches were used to discriminate between effects of metabolic/ionic and mechanical alterations on phosphorylation of two MAPKs: extracellular signal-regulated kinase 1 and 2 (MAPK(erk1/2)) and stress-activated protein kinase p38 (MAPK(p38)). 3. Concentric contractions induced a 5-fold increase in MAPK(erk1/2) phosphorylation. Application of the antioxidants N-acetylcysteine (20 mM) or dithiothreitol (5 mM) suppressed concentric contraction-induced increase in MAPK(erk1/2) phosphorylation. Mild passive stretches of the muscle increased MAPK(erk1/2) phosphorylation by 1.8-fold, whereas the combination of acidosis and passive stretches resulted in a 2.8-fold increase. Neither concentric contractions, nor mild stretches nor acidosis significantly affected phosphorylation of MAPK(p38). 4. High force applied upon muscle by means of either eccentric contractions or severe passive stretches resulted in 5.7- and 9.5-fold increases of phosphorylated MAPK(erk1/2), respectively, whereas phosphorylation of MAPK(p38) increased by 7.6- and 1.9-fold (not significant), respectively. 5. We conclude that in isolated rat skeletal muscle an increase in phosphorylation of both MAPK(erk1/2) and MAPK(p38) is induced by mechanical alterations, whereas contraction-related metabolic/ionic changes (reactive oxygen species and acidosis) cause increased phosphorylation of MAPK(erk1/2) only. Thus, contraction-induced phosphorylation can be explained by the combined action of increased production of reactive oxygen species, acidification and mechanical perturbations for MAPK(erk1/2) and by high mechanical stress for MAPK(p38).

Influence of exercise intensity on ERK/MAP kinase signalling in human skeletal muscle

The mitogen-activated protein (MAP) kinase pathways have been highlighted as a possible link between exercise and adaptive changes in skeletal muscle. In this study, the effect of exercise intensity on the activation of the ERK/MAP kinase pathway was investigated in human skeletal muscle. One-leg exercise at low (40% maximal oxygen consumption, VO2max for 30 min) and high (75% VO2max for 30 min) intensity resulted in 11.5+8. I-fold and 39.7+/-6.3-fold (mean +/-SEM) increases in ERK1/2 phosphorylation (P<0.001), respectively. The phosphorylation of MEK1/2, the upstream kinase of ERK1/2, increased with exercise intensity (P<0.05) to 2.5+/-0.9 and 4.8+/-1.1 times the basal level at the low and high intensity, respectively. The statistical analysis revealed a systematic difference between basal, low and high intensity exercise levels for both kinases. There was no change in the phosphorylation of either kinase in the non-exercised leg. The phosphorylation of the transcription factor cyclic AMP response element binding protein (CREB), a possible downstream target of the ERK/MAP kinase signalling pathway, was unaffected by exercise. The phosphorylation of ERK1/2 was significantly higher in purified freeze-dried compared to crude wet muscle after exercise, whereas the opposite pattern was observed for CREB. In conclusion, phosphorylation of ERK1/2 and MEK1/2 increases in an exercise intensity-dependent manner in human skeletal muscle and this seems to originate in the muscle fibres themselves.

Differential activation of mitogen-activated protein kinase signalling pathways by isometric contractions in isolated slow- and fast-twitch rat skeletal muscle

Activation of mitogen-activated protein (MAP) kinases has been implicated in the signal transduction pathways linking exercise to adaptive changes of muscle protein expression. In the present study, we investigated whether contractions of isolated muscles induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and p38 MAPK in a fibre-type dependent manner. Slow-twitch (soleus) and fast-twitch (epitrochlearis, extensor digitorum longus) rat skeletal muscles were exposed to intermittent tetanic stimulation. Compared with the contralateral non-stimulated muscle, contractions increased ERK1/2 phosphorylation to the same extent in fast- and slow-twitch muscles. Significant increase in phosphorylation of p38 MAPK was observed in the fast-twitch muscles only. The total amount of ERK1/2 and p38 MAPK proteins was higher in the slow-twitch soleus muscle. In conclusion, MAP kinase signalling pathways are differentially activated and expressed in slow- and fast-twitch muscles. In addition, this activation is owing to muscle contraction per se and do not demand additional external influence.