The regenerative capacity of skeletal muscle declines with age. Previous studies suggest that this process can be reversed by exposure to young circulation, but systemic age-specific factors responsible for this phenomenon are largely unknown. Here we report that oxytocin- a hormone best known for its role in lactation, parturition, and social behaviors - is required for proper muscle tissue regeneration and homeostasis, and that plasma levels of oxytocin decline with age. Inhibition of oxytocin signaling in young animals reduces muscle regeneration, whereas systemic administration of oxytocin rapidly improves muscle regeneration by enhancing aged muscle stem cell activation/proliferation throughactivation of the MAPK/ERK signalling pathway. We further show that the genetic lack of oxytocin does not cause a developmental defect in muscle, but instead leads to premature sarcopenia. Considering that oxytocin is an FDA approved drug, this work reveals a potential novel and safe way to combat or prevent skeletal muscle aging.

Terminal differentiation of skeletal myoblasts involves alignment of the mononucleated cells, fusion into multinucleated syncitia, and transcription of muscle-specific genes. Myogenesis in vivo is regulated partially by IGF-I initiated signaling that results in activation of an intracellular phosphatidylinositol 3 kinase (PI3K) signaling cascade. Downstream signaling through the Raf/MEK/ERK axis, a pathway initiated by IGF-I, also is implicated in the regulation of muscle formation. The involvement of ERK1 and ERK2 during myogenesis was examined in C2C12 myoblasts. C2C12 myoblasts stably expressing a small interfering RNA (siRNA) directed against ERK1 or ERK2 were created. Both of the kinases were reduced to trace levels as measured by Western for total ERK and retained the capacity to become phosphorylated. C2C12siERK2 knockdown myoblasts failed to fuse into multinucleated myofibers. By contrast, cells expressing a scrambled siRNA or ERK1 siRNA fused into large multinucleated structures. The block to muscle formation did not involve continued cell cycle progression or apoptosis. C2C12siERK1 myoblasts expressed an increased amount of ERK2 protein and formed larger myofibers in response to IGF-I treatment. Interestingly, IGF-I treatment of C2C12 ERK2 knockdown myoblasts did not reinstate the myogenic program arguing that ERK2 is required for differentiation. These results provide evidence for ERK2 as a positive regulator of myogenesis and suggest that ERK1 is dispensable for myoblast proliferation and differentiation.

The regenerative capacity of muscle dramatically decreases with age because old muscle stem cells fail to proliferate in response to tissue damage. Here we uncover key age-specific differences underlying this proliferative decline: namely, the genetic loci of CDK inhibitors (CDKI) p21 and p16 are more epigenetically silenced in young muscle stem cells, as compared to old, both in quiescent cells and those responding to tissue injury. Interestingly, phosphorylated ERK (pERK) induced in these cells by ectopic FGF-2 is found in association with p21 and p16 promoters, and moreover, only in the old cells. Importantly, in the old satellite cells FGF-2/pERK silences p21 epigenetically and transcriptionally, which leads to reduced p21 protein levels and enhanced cell proliferation. In agreement with the epigenetic silencing of the loci, young muscle stem cells do not depend as much as old on ectopic FGF/pERK for their myogenic proliferation. In addition, other CDKIs, such asp15INK4B and p27KIP1, become elevated in satellite cells with age, confirming and explaining the profound regenerative defect of old muscle. This work enhances our understanding of tissue aging, promoting strategies for combating age-imposed tissue degeneration.

Purpose: We present feasibility studies in support of a real‐time MRI guided external beam radiotherapy delivery system currently under commercial development. Method and Materials: The system, (ViewRay Inc., Renaissance™), combines a low field open MRI scanner and a multi‐headed 60Co γ‐ray IMRT unit equipped with multi‐leaf collimators. It is designed so that the center of the field of view of the MRI and the isocenter of the radiotherapy unit coincide. The inherent compatibility of the units allows for the acquisition of fast ciné MRI simultaneous to radiotherapy delivery to assess intra‐fraction organ motion. Computational feasibility studies were performed to investigate: the compatibility of the MRI and the 60Co γ‐ray IMRT unit; the impact of the MRI magnetic field on the dosimetry; and the feasibility of performing accurate heterogeneity dose computations with MRI data. Results: The 60Co γ‐ray IMRT unit was found not to significantly impact the operation of the MRI; the γ‐ray IMRT unit is capable of producing high quality IMRT treatment plans; the MRI magnetic field eliminates contamination electrons and does not significantly perturb the dose distribution in lung, soft tissue, and bone; and accurate heterogeneity dose computations are possible employing only MRI data. Conclusion: Performing IMRT allows for the seamless integration with, and simultaneous operation of, an open MRI unit. Conflict of Interest: Research sponsored by ViewRay, Inc., Gainesville, Florida USA.