Summary by Jantima Tanboon, MD, PhD

Publication

Unraveling calcium dysregulation and autoimmunity in immune mediated rippling muscle disease

Nath SR, Dasgupta A, Dubey D, Kokesh E, Beecher G, Fadra N, Liewluck T, Pittock S, Doles JD, Litchy W, Milone M.

Acta Neuropathol Commun. 2025 Jan 16;13(1):11.

doi: 10.1186/s40478-025-01926-z. PMID: 39819455; PMCID: PMC11736958

https://actaneurocomms.biomedcentral.com/articles/10.1186/s40478-025-01926-z#Sec15

Summary

Rippling muscle disease (RMD) is characterised by rapid, wave-like muscle contractions that are triggered by mechanical stretch, leading to muscle pain, stiffness, and sometimes weakness. RMD can be divided into two types: hereditary (hRMD), linked to CAV3 (caveolin-3) or CAVIN1 (cavin-1) mutations, and immune-mediated (iRMD), associated with autoantibodies against cavin-4. Caveolin-3, cavin-1, and cavin-4 proteins are critical for the function of caveolae, which are structures in the cell membrane essential for muscle function, including the process of muscle contraction. Cavin-4 also plays a crucial role in maintaining muscle cell integrity and forming T-tubules necessary for regulating calcium during muscle contractions.

Recent findings of cavin-4 autoantibodies in iRMD, along with the patchy loss of cavin-4 on the muscle cell membrane similar to caveolin-3 loss, suggest that autoimmune responses targeting key proteins involved in muscle contraction and relaxation may drive the disease in iRMD.

To investigate this hypothesis, the authors conducted bulk RNA sequencing on muscle biopsies from eight patients (five males and three females, aged 26 to 80) with iRMD compared to non-disease controls and disease controls (dermatomyositis). The level of expression and localisation of proteins of interest was evaluated by immunofluorescence. The study highlighted alterations in RNA transcripts and associated protein expression which may contribute to the pathogenesis of iRMD.

In comparison to non-diseased controls, iRMD muscle showed differentially expressed CAV3 CAVIN4, and BIN1; the latter encoding a protein that interacts with cavin-4. The increased expression of CAV3, CAVIN4, and BIN1 was likely a compensatory response to the loss of caveolin-3 and cavin-4 proteins. Since pathways related to skeletal muscle development and differentiation involving CAV3 and CAVIN4 were enriched in iRMD, these genes were identified as key players in skeletal alterations in iRMD. CAVIN1 was differentially expressed when compared to non-diseased controls from a previous study, but no differences were observed when compared to a GTEX cohort. Although CACNA1S, which encodes the membrane calcium channel Cav1.1 responsible for initial calcium influx, showed differential expression, no changes were observed at the protein level. Similarly, RYR1, which encodes a calcium release channel on the sarcoplasmic reticulum essential for skeletal muscle contraction, did not show significant increases at either the mRNA or protein level. TOMM20, which encodes the translocase outer mitochondrial membrane 20 essential to transfer ATP to T-tubule, showed differential expression but its protein expression and RYR1 colocalisation remained unchanged.

Thus, the rippling phenomenon is unlikely caused by alteration in muscle excitation-contraction signaling; the findings observed in CACNA1S and TOMM20 mRNA and proteins reflected compensatory responses. iRMD muscle showed differential expression of genes involved in pumping calcium back into the extracellular space (PMCA, plasma membrane calcium ATPase), and into the sarcoplasmic reticulum (SERCA, sarcoendoplasmic reticulum calcium ATPase), as well as inhibition of SERCA pump activity (PLN, phospholamban). The increased expression of PMCA, SERCA and PLN was further confirmed at the protein level.

These findings suggest that alterations in muscle relaxation underly the pathomechanism of iRMD, with alterations in PLN at both mRNA and protein levels likely playing a crucial role. In contrast to inflammatory myopathies like dermatomyositis, interferon signaling pathways were not significantly induced. The IL-6 myokine signaling pathway was activated in iRMD; this pathway could be triggered by either continuous muscle activity or immune activation. Pathways related to immune response, immune activation, or immune activity signaling were significantly enriched among the overlapping genes between iRMD and DM. These findings likely reflect shared, nonspecific pathways involved in autoimmune myopathies, further supporting the role of immune activation in iRMD.

In conclusion, this study suggested that the depletion of cavin-4, caused by autoantibodies, leads to a disruption in T-tubule-SERCA interactions. This results in a reduced activity of calcium pumps, causing a failure in muscle relaxation, which the authors propose as the underlying cause of iRMD. Identifying IL-6 myokine signaling pathway activation, IL-6 receptor inhibitors and other drugs targeting the IL-6 pathway could offer a promising new therapeutic approach for iRMD.

About the author

Samir Nath, MD, PhD is a neurology resident at Mayo Clinic in Rochester, Minnesota, where he investigates the pathophysiology of neuromuscular disorders. He received his MD and PhD from the University of Michigan’s Medical Scientist Training Program, and completed his dissertation research under Dr. Andrew P. Lieberman, MD, PhD on the pathogenesis of spinal and bulbar muscular atrophy.

Dr. Nath has authored multiple peer‐reviewed publications and earned national recognition for his work, including awards from the Gordon Research Conference on CAG Triplet Repeat Diseases and the Protein Folding Disease Initiative Symposium. In addition to his clinical duties, he actively mentors aspiring physician‐scientists and participates in community outreach.

About the reviewer

Dr. Jantima Tanboon is an assistant professor of anatomical pathology at Siriraj Hospital, Mahidol University, Thailand. Dr.Tanboon was a research student and a postdoctoral research fellow at the Department of Neuromuscular Research at the National Center of Neurology and Psychiatry (NCNP), Japan under the supervision of Dr.Satoru Noguchi and Dr.Ichizo Nishino where she grew in depth passion in neuromuscular research. Dr.Tanboon is one of a few specialists in her country responsible for muscle biopsy diagnosis.