Resistance exercise induces a hypertrophic response in skeletal muscle and recent studies have begun to shed light on the molecular mechanisms involved in this process. For example, several studies indicate that signalling by the mammalian target of rapamycin (mTOR) is necessary for a hypertrophic response. Furthermore, resistance exercise has been proposed to activate mTOR signalling through an upstream pathway involving the phosphoinositide 3-kinase (PI3K) and protein kinase B (PKB); however, this hypothesis has not been thoroughly tested. To test this hypothesis, we first evaluated the temporal pattern of signalling through PI3K-PKB and mTOR following a bout of resistance exercise with eccentric contractions (EC). Our results indicated that the activation of signalling through PI3K-PKB is a transient event (<15 min), while the activation of mTOR is sustained for a long duration (>12 h). Furthermore, inhibition of PI3K-PKB activity did not prevent the activation of mTOR signalling by ECs, indicating that PI3K-PKB is not part of the upstream regulatory pathway. These observations led us to investigate an alternative pathway for the activation of mTOR signalling involving the synthesis of phosphatidic acid (PA) by phospholipase D (PLD). Our results demonstrate that ECs induce a sustained elevation in [PA] and inhibiting the synthesis of PA by PLD prevented the activation of mTOR. Furthermore, we determined that similar to ECs, PA activates mTOR signalling through a PI3K-PKB-independent mechanism. Combined, the results of this study indicate that the activation of mTOR following eccentric contractions occurs through a PI3K-PKB-independent mechanism that requires PLD and PA.

Despite steady progress in identifying quantitative trait loci (QTLs) for bone phenotypes, relatively little progress has been made in moving from QTLs to identifying the relevant gene. We exploited the genetic structure of recombinant congenic mouse strains by performing a reciprocal intercross of the strains HcB-8 and HcB-23, phenotyped for body size, femoral biomechanical performance, and femoral diaphyseal geometry and mapped with R/qtl and QTL Cartographer. Significant QTLs are present on chromosomes 1, 2, 3, 4, 6, and 10. We found significant sex x QTL and cross-direction x QTL interactions. The chromosome 4 QTL affects multiple femoral anatomic features and biomechanical properties. The known segregating segment of chromosome 4 contains only 18 genes, among which Ece1, encoding endothelin-converting enzyme 1, stands out as a candidate. Endothelin signaling has been shown to promote the growth of osteoblastic metastases and to potentiate signaling via the Wnt pathway. The colocalizing chromosome 4 QTL Bmd7 (for bone mineral density 7) increases responsiveness to mechanical loading. By exploiting the short informative segment of chromosome 4 and the known biology, we propose that Ece1 is the gene responsible for Bmd7 and that it acts by increasing responsiveness to mechanical loading through modulation of Wnt signaling.

Various cell types can sense and convert mechanical forces into biochemical signaling events through a process called mechanotransduction, and this process is often highly specific to the types of mechanical forces applied. However, the mechanism(s) that allow for specificity in mechanotransduction remain undefined. Thus, the goal of this study was to gain insight into how cells distinguish among specific types of mechanical information. To accomplish this goal, we determined if skeletal myoblasts can distinguish among differences in strain, strain rate, and strain-time integral (STI). Our results demonstrate that mechanically induced signaling through the c-jun N-terminal kinase 2 [JNK2] is elicited via a mechanism that depends on an interaction between the magnitude of strain and strain rate and is independent of STI. In contrast to JNK2, mechanically induced signaling through the ribosomal S6 kinase [p70(389)] is not strain rate sensitive, but instead involves a magnitude of strain and STI dependent mechanisms. Mathematical modeling also indicated that mechanically induced signaling through JNK2 and p70(389) can be isolated to separate viscous and elastic mechanosensory elements, respectively. Based on these results, we propose that skeletal myoblasts contain multiple mechanosensory elements with distinct biomechanical properties and that these distinct biomechanical properties provide a mechanism for specificity in mechanotransduction.

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Dual energy X ray absorptiometry (DXA) has become a popular analytical technique in mice and other small animals. Comparative study of bone properties at different anatomical sites is an active area of study in model organisms. Such investigations require that site-specific data be generated and interpreted. There are no published data addressing the degree to which contralateral mouse bones resemble each other in the absence of an experimental intervention, nor are there data addressing the correlation of bone densitometry measurements between anatomically distant sites. To address these gaps in our knowledge, we used DXA to compare excised mouse femora and humeri. At the population level, left bones were slightly but significantly denser than right bones, with an overall adjusted bone mineral density (BMD) difference of 0.7 +/- 0.3 and 0.5 +/- 0.2 mg/cm2 at the femur and humerus, respectively. At the level of bone pairs from a single animal, absolute adjusted BMD disparities between the right and left sides were 2.3 +/- 1.9 mg/cm2 at the femur and 1.7 +/- 1.4 mg/cm2 at the humerus. Correlation coefficients between left and right sides were 0.78 for adjusted BMD at both sites. The correlation coefficient between side-averaged femoral and humeral BMD was 0.81, but ranged between 0.70 and 0.75 when limited to ipsilateral or contralateral femur-humerus pairs. Our findings suggest the desirability of randomizing limbs for treatment in studies using contralateral limb controls. These observations may represent the densitometric manifestation of behavioral and neuroanatomical lateralization in laboratory mice.