Michael E. Llewellyn

Rejuvenation of the muscle stem cell population restores strength to injured aged muscles

The elderly often suffer from progressive muscle weakness and regenerative failure. We demonstrate that muscle regeneration is impaired with aging owing in part to a cell-autonomous functional decline in skeletal muscle stem cells (MuSCs). Two-thirds of MuSCs from aged mice are intrinsically defective relative to MuSCs from young mice, with reduced capacity to repair myofibers and repopulate the stem cell reservoir in vivo following transplantation. This deficiency is correlated with a higher incidence of cells that express senescence markers and is due to elevated activity of the p38α and p38β mitogen-activated kinase pathway. We show that these limitations cannot be overcome by transplantation into the microenvironment of young recipient muscles. In contrast, subjecting the MuSC population from aged mice to transient inhibition of p38α and p38β in conjunction with culture on soft hydrogel substrates rapidly expands the residual functional MuSC population from aged mice, rejuvenating its potential for regeneration and serial transplantation as well as strengthening of damaged muscles of aged mice. These findings reveal a synergy between biophysical and biochemical cues that provides a paradigm for a localized autologous muscle stem cell therapy for the elderly.

Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans

Sarcomeres are the basic contractile units of striated muscle. Our knowledge about sarcomere dynamics has primarily come from in vitro studies of muscle fibres and analysis of optical diffraction patterns obtained from living muscles. Both approaches involve highly invasive procedures and neither allows examination of individual sarcomeres in live subjects. Here we report direct visualization of individual sarcomeres and their dynamical length variations using minimally invasive optical microendoscopy to observe second-harmonic frequencies of light generated in the muscle fibres of live mice and humans. Using microendoscopes as small as 350 μm in diameter, we imaged individual sarcomeres in both passive and activated muscle. Our measurements permit in vivo characterization of sarcomere length changes that occur with alterations in body posture and visualization of local variations in sarcomere length not apparent in aggregate length determinations. High-speed data acquisition enabled observation of sarcomere contractile dynamics with millisecond-scale resolution. These experiments point the way to in vivo imaging studies demonstrating how sarcomere performance varies with physical conditioning and physiological state, as well as imaging diagnostics revealing how neuromuscular diseases affect contractile dynamics.

Orderly recruitment of motor units under optical control in vivo

A drawback of electrical stimulation for muscle control is that large, fatigable motor units are preferentially recruited before smaller motor units by the lowest-intensity electrical cuff stimulation. This phenomenon limits therapeutic applications because it is precisely the opposite of the normal physiological (orderly) recruitment pattern; therefore, a mechanism to achieve orderly recruitment has been a long-sought goal in physiology, medicine and engineering. Here we demonstrate a technology for reliable orderly recruitment in vivo. We find that under optical control with microbial opsins, recruitment of motor units proceeds in the physiological recruitment sequence, as indicated by multiple independent measures of motor unit recruitment including conduction latency, contraction and relaxation times, stimulation threshold and fatigue. As a result, we observed enhanced performance and reduced fatigue in vivo. These findings point to an unanticipated new modality of neural control with broad implications for nervous system and neuromuscular physiology, disease research and therapeutic innovation.

The Sonographic “Bright Band Sign” of Splenic Infarction

To evaluate the frequency of the “bright band sign” in patients with splenic infarcts as well as control patients and to thereby assess whether the bright band sign has potential utility as a sonographic sign of splenic infarction.