The influence of iron availability on the structural changes of skeletal muscle cells and cardiac muscle cells

In the presented study, the influence iron availability on the cell morphology and structure was examined in skeletal myocytes cultured under normoxic and hypoxic conditions. The study showed that iron deficiency in vitro (especially in combination with hypoxia) has a detrimental effect on skeletal myocytes. Cellular disfunction was manifested by atrophic changes in cell morphology, structural remodeling of cells and increased expression of muscle atrophy markers. Interestingly, increased iron availability appeared to have some protective properties in the context of aforementioned changes. Moreover, based on clinical data and recent transgenic models, structural and functional abnormalities in iron-deficient cardiac muscle were described and the potential pathomechanisms behind them have been discussed.

Sarcoplasmic reticular Ca2+-ATPase inhibition paradoxically upregulates murine skeletal muscle Nav1.4 function

Skeletal muscle Na+ channels possess Ca2+- and calmodulin-binding sites implicated in Nav1.4 current (INa) downregulation following ryanodine receptor (RyR1) activation produced by exchange protein directly activated by cyclic AMP or caffeine challenge, effects abrogated by the RyR1-antagonist dantrolene which itself increased INa. These findings were attributed to actions of consequently altered cytosolic Ca2+, [Ca2+]i, on Nav1.4. We extend the latter hypothesis employing cyclopiazonic acid (CPA) challenge, which similarly increases [Ca2+]i, but through contrastingly inhibiting sarcoplasmic reticular (SR) Ca2+-ATPase. Loose patch clamping determined Na+ current (INa) families in intact native murine gastrocnemius skeletal myocytes, minimising artefactual [Ca2+]i perturbations. A bespoke flow system permitted continuous INa comparisons through graded depolarizing steps in identical stable membrane patches before and following solution change. In contrast to the previous studies modifying RyR1 activity, and imposing control solution changes, CPA (0.1 and 1 µM) produced persistent increases in INa within 1–4 min of introduction. CPA pre-treatment additionally abrogated previously reported reductions in INa produced by 0.5 mM caffeine. Plots of peak current against voltage excursion demonstrated that 1 µM CPA increased maximum INa by ~ 30%. It only slightly decreased half-maximal activating voltages (V0.5) and steepness factors (k), by 2 mV and 0.7, in contrast to the V0.5 and k shifts reported with direct RyR1 modification. These paradoxical findings complement previously reported downregulatory effects on Nav1.4 of RyR1-agonist mediated increases in bulk cytosolic [Ca2+]. They implicate possible local tubule-sarcoplasmic triadic domains containing reduced [Ca2+]TSR in the observed upregulation of Nav1.4 function following CPA-induced SR Ca2+ depletion.

Lineage tracing reveals evidence of a popliteal lymphatic muscle progenitor cell that is distinct from skeletal and vascular muscle progenitors

Loss of popliteal lymphatic vessel (PLV) contractions, which is associated with damage to lymphatic muscle cells (LMCs), is a biomarker of disease progression in mice with inflammatory arthritis. Currently, the nature of LMC progenitors has yet to be formally described. Thus, we aimed to characterize the progenitors of PLV-LMCs during murine development, towards rational therapies that target their proliferation, recruitment, and differentiation onto PLVs. Since LMCs have been described as a hybrid phenotype of striated and vascular smooth muscle cells (VSMCs), we performed lineage tracing studies in mice to further clarify this enigma by investigating LMC progenitor contribution to PLVs in neonatal mice. PLVs from Cre-tdTomato reporter mice specific for progenitors of skeletal myocytes (Pax7+ and MyoD+) and VSMCs (Prrx1+ and NG2+) were analyzed via whole mount immunofluorescent microscopy. The results showed that PLV-LMCs do not derive from skeletal muscle progenitors. Rather, PLV-LMCs originate from Pax7−/MyoD−/Prrx1+/NG2+ progenitors similar to VSMCs prior to postnatal day 10 (P10), and from a previously unknown Pax7−/MyoD−/Prrx1+/NG2− muscle progenitor pathway during development after P10. Future studies of these LMC progenitors during maintenance and repair of PLVs, along with their function in other lymphatic beds, are warranted.

Biomarkers of Physical Frailty and Sarcopenia: Coming up to the Place?

Physical frailty and sarcopenia (PF&S) recapitulates all the hallmarks of aging and has become a focus in geroscience. Factors spanning muscle-specific processes (e.g., mitochondrial dysfunction in skeletal myocytes) to systemic changes (e.g., inflammation and amino acid dysmetabolism) have been pinpointed as possible contributors to PF&S pathophysiology. However, the search for PF&S biomarkers allowing the early identification and tracking of the condition over time is ongoing. This is mainly due to the phenotypic heterogeneity of PF&S, its unclear pathophysiology, and the frequent superimposition of other age-related conditions. Hence, presently, the identification of PF&S relies upon clinical, functional, and imaging parameters. The adoption of multi-marker approaches (combined with multivariate modeling) has shown great potential for addressing the complexity of PF&S pathophysiology and identifying candidate biological markers. Well-designed longitudinal studies are necessary for the incorporation of reliable biomarkers into clinical practice and for unveiling novel targets that are amenable to interventions.

Pre-Innervated Tissue Engineered Muscle Promotes a Pro-Regenerative Microenvironment Following Volumetric Muscle Loss

Volumetric Muscle Loss (VML) is defined as traumatic or surgical loss of skeletal muscle tissue beyond the inherent regenerative capacity of the body, generally leading to a severe functional deficit. Autologous muscle grafts remain the prevalent method of treatment whereas recent muscle repair techniques using biomaterials and tissue engineering are still at a nascent stage and have multiple challenges to address to ensure functional recovery of the injured muscle. Indeed, appropriate somato-motor innervations remain one of the biggest challenges for both autologous muscle grafts as well as tissue engineered muscle constructs. We aim to address this challenge by developing Pre-Innervated Tissue Engineered Muscle comprised of long aligned networks of spinal motor neurons and skeletal myocytes. Here, we developed methodology to biofabricate long fibrils of pre-innervated tissue engineered muscle using a co-culture of myocytes and motor neurons on aligned nanofibrous scaffolds. Motor neurons lead to enhanced differentiation and maturation of skeletal myocytes in vitro. These pre-innervated tissue engineered muscle constructs when implanted in vivo in a rat VML model significantly increase satellite cell migration, micro-vessel formation, and neuromuscular junction density in the host muscle near the injury area at an acute time point as compared to non-pre-innervated myocyte constructs and nanofiber scaffolds alone. These pro-regenerative effects can potentially lead to enhanced functional neuromuscular regeneration following VML, thereby improving the levels of functional recovery following these devastating injuries.