Circulating tumor cell-derived exosome-transmitted long non-coding RNA TTN-AS1 can promote the proliferation and migration of cholangiocarcinoma cells.

BACKGROUND: Exosomes assume a pivotal role as essential mediators of intercellular communication within tumor microenvironments. Within this context, long noncoding RNAs (LncRNAs) have been observed to be preferentially sorted into exosomes, thus exerting regulatory control over the initiation and progression of cancer through diverse mechanisms. RESULTS: Exosomes were successfully isolated from cholangiocarcinoma (CCA) CTCs organoid and healthy human serum. Notably, the LncRNA titin-antisense RNA1 (TTN-AS1) exhibited a conspicuous up-regulation within CCA CTCs organoid derived exosomes. Furthermore, a significant elevation of TTN-AS1 expression was observed in tumor tissues, as well as in blood and serum exosomes from patients afflicted with CCA. Importantly, this hightened TTN-AS1 expression in serum exosomes of CCA patients manifested a strong correlation with both lymph node metastasis and TNM staging. Remarkably, both CCA CTCs organoid-derived exosomes and CCA cells-derived exosomes featuring pronounced TTN-AS1 expression demonstrated the capability to the proliferation and migratory potential of CCA cells. Validation of these outcomes was conducted in vivo experiments. CONCLUSIONS: In conclusion, our study elucidating that CCA CTCs-derived exosomes possess the capacity to bolster the metastasis tendencies of CCA cells by transporting TTN-AS1. These observations underscore the potential of TTN-AS1 within CTCs-derived exosomes to serve as a promising biomarker for the diagnosis and therapeutic management of CCA.

Digenic inheritance involving a muscle-specific protein kinase and the giant titin protein causes a skeletal muscle myopathy.

In digenic inheritance, pathogenic variants in two genes must be inherited together to cause disease. Only very few examples of digenic inheritance have been described in the neuromuscular disease field. Here we show that predicted deleterious variants in SRPK3, encoding the X-linked serine/argenine protein kinase 3, lead to a progressive early onset skeletal muscle myopathy only when in combination with heterozygous variants in the TTN gene. The co-occurrence of predicted deleterious SRPK3/TTN variants was not seen among 76,702 healthy male individuals, and statistical modeling strongly supported digenic inheritance as the best-fitting model. Furthermore, double-mutant zebrafish (srpk3-/-; ttn.1+/-) replicated the myopathic phenotype and showed myofibrillar disorganization. Transcriptome data suggest that the interaction of srpk3 and ttn.1 in zebrafish occurs at a post-transcriptional level. We propose that digenic inheritance of deleterious changes impacting both the protein kinase SRPK3 and the giant muscle protein titin causes a skeletal myopathy and might serve as a model for other genetic diseases.

Protein Kinase D Plays a Crucial Role in Maintaining Cardiac Homeostasis by Regulating Post-Translational Modifications of Myofilament Proteins.

Protein kinase D (PKD) enzymes play important roles in regulating myocardial contraction, hypertrophy, and remodeling. One of the proteins phosphorylated by PKD is titin, which is involved in myofilament function. In this study, we aimed to investigate the role of PKD in cardiomyocyte function under conditions of oxidative stress. To do this, we used mice with a cardiomyocyte-specific knock-out of Prkd1, which encodes PKD1 (Prkd1loxP/loxP; αMHC-Cre; PKD1 cKO), as well as wild type littermate controls (Prkd1loxP/loxP; WT). We isolated permeabilized cardiomyocytes from PKD1 cKO mice and found that they exhibited increased passive stiffness (Fpassive), which was associated with increased oxidation of titin, but showed no change in titin ubiquitination. Additionally, the PKD1 cKO mice showed increased myofilament calcium (Ca2+) sensitivity (pCa50) and reduced maximum Ca2+-activated tension. These changes were accompanied by increased oxidation and reduced phosphorylation of the small myofilament protein cardiac myosin binding protein C (cMyBPC), as well as altered phosphorylation levels at different phosphosites in troponin I (TnI). The increased Fpassive and pCa50, and the reduced maximum Ca2+-activated tension were reversed when we treated the isolated permeabilized cardiomyocytes with reduced glutathione (GSH). This indicated that myofilament protein oxidation contributes to cardiomyocyte dysfunction. Furthermore, the PKD1 cKO mice exhibited increased oxidative stress and increased expression of pro-inflammatory markers interleukin (IL)-6, IL-18, and tumor necrosis factor alpha (TNF-α). Both oxidative stress and inflammation contributed to an increase in microtubule-associated protein 1 light chain 3 (LC3)-II levels and heat shock response by inhibiting the mammalian target of rapamycin (mTOR) in the PKD1 cKO mouse myocytes. These findings revealed a previously unknown role for PKD1 in regulating diastolic passive properties, myofilament Ca2+ sensitivity, and maximum Ca2+-activated tension under conditions of oxidative stress. Finally, we emphasized the importance of PKD1 in maintaining the balance of oxidative stress and inflammation in the context of autophagy, as well as cardiomyocyte function.

New mechanisms: From lactate to lactylation to rescue heart failure.

Lactylation of α-myosin heavy chain (α-MHC) has recently been reported to preserve sarcomeric structure and function and attenuate the development of heart failure. Specifically, lactylation enhanced the interaction of α-MHC with the sarcomeric protein Titin, thereby maintaining normal sarcomeric structure and myocardial contractile function. Furthermore, the administration of lactate or inhibition of lactate efflux potentially treats heart failure by restoring lactylation of α-MHC and the interaction of α-MHC with Titin. This finding highlights the significant role of α-MHC lactylation in myocardial diseases and presents a new therapeutic target for the treatment of heart failure.

TTN truncation variants produce sarcomere-integrating proteins of uncertain functional significance.

Titin (TTN) is one of the largest and most complex proteins expressed in humans, and truncation variants are the most prevalent genetic lesion identified in individuals with dilated cardiomyopathy (DCM) or other disorders of impaired cardiac contractility. Two reports in this issue of the JCI shed light on a potential mechanism involving truncated TTN sarcomere integration and the potential for disruption of sarcomere structural integrity. Kellermayer, Tordai, and colleagues confirmed the presence of truncated TTN protein in human DCM samples. McAfee and authors developed a patient-specific TTN antibody to study truncated TTN subcellular localization and to explore its functional consequences. A "poison peptide" mechanism emerges that inspires alternative therapeutic approaches while opening new lines for inquiry, such as the role of haploinsufficiency of full-length TTN protein, mechanisms explaining sarcomere dysfunction, and explanations for variable penetrance.

RBPMS regulates cardiomyocyte contraction and cardiac function through RNA alternative splicing.

AIMS: RNA binding proteins play essential roles in mediating RNA splicing and are key post-transcriptional regulators in the heart. Our recent study demonstrated that RBPMS (RNA binding protein with multiple splicing) is crucial for cardiac development through modulating mRNA splicing, but little is known about its functions in the adult heart. In this study, we aim to characterize the post-natal cardiac function of Rbpms and its mechanism of action. METHODS AND RESULTS: We generated a cardiac-specific knockout mouse line and found that cardiac-specific loss of Rbpms caused severe cardiomyocyte contractile defects, leading to dilated cardiomyopathy and early lethality in adult mice. We showed by proximity-dependent biotin identification assay and mass spectrometry that RBPMS associates with spliceosome factors and other RNA binding proteins, such as RBM20, that are important in cardiac function. We performed paired-end RNA sequencing and RT-PCR and found that RBPMS regulates mRNA alternative splicing of genes associated with sarcomere structure and function, such as Ttn, Pdlim5, and Nexn, generating new protein isoforms. Using a minigene splicing reporter assay, we determined that RBPMS regulates target gene splicing through recognizing tandem intronic CAC motifs. We also showed that RBPMS knockdown in human induced pluripotent stem cell-derived cardiomyocytes impaired cardiomyocyte contraction. CONCLUSION: This study identifies RBPMS as an important regulator of cardiomyocyte contraction and cardiac function by modulating sarcomeric gene alternative splicing.

A low-cost 2-D sarcomere model to demonstrate titin-related mechanisms for force production.

Given the recently proposed three-filament theory of muscle contraction, we present a low-cost physical sarcomere model aimed at illustrating the role of titin in the production of active force in skeletal muscle. With inexpensive materials, it is possible to illustrate actin-myosin cross-bridge interactions between the thick and thin filaments and demonstrate the two different mechanisms by which titin is thought to contribute to active and passive muscle force. Specifically, the model illustrates how titin, a molecule with springlike properties, may increase its stiffness by binding free calcium upon muscle activation and reducing its extensible length by attaching itself to actin, resulting in the greater force-generating capacity after an active than a passive elongation that has been observed experimentally. The model is simple to build and manipulate, and demonstration to high school students was shown to result in positive perception and improved understanding of the otherwise complex titin-related mechanisms of force production in skeletal and cardiac muscles.NEW & NOTEWORTHY Our physical sarcomere model illustrates not only the classic view of muscle contraction, the sliding filament and cross-bridge theories, but also the newly discovered role of titin in force regulation, called the three-filament theory. The model allows for easy visualization of the role of titin in muscle contraction and aids in explaining complex muscle properties that are not captured by the traditional cross-bridge theory.

Truncated titin is structurally integrated into the human dilated cardiomyopathic sarcomere.

Heterozygous (HET) truncating variant mutations in the TTN gene (TTNtvs), encoding the giant titin protein, are the most common genetic cause of dilated cardiomyopathy (DCM). However, the molecular mechanisms by which TTNtv mutations induce DCM are controversial. Here, we studied 127 clinically identified DCM human cardiac samples with next-generation sequencing (NGS), high-resolution gel electrophoresis, Western blot analysis, and super-resolution microscopy in order to dissect the structural and functional consequences of TTNtv mutations. The occurrence of TTNtv was found to be 15% in the DCM cohort. Truncated titin proteins matching, by molecular weight, the gene sequence predictions were detected in the majority of the TTNtv+ samples. Full-length titin was reduced in TTNtv+ compared with TTNtv- samples. Proteomics analysis of washed myofibrils and stimulated emission depletion (STED) super-resolution microscopy of myocardial sarcomeres labeled with sequence-specific anti-titin antibodies revealed that truncated titin was structurally integrated into the sarcomere. Sarcomere length-dependent anti-titin epitope position, shape, and intensity analyses pointed at possible structural defects in the I/A junction and the M-band of TTNtv+ sarcomeres, which probably contribute, possibly via faulty mechanosensor function, to the development of manifest DCM.

Structure of the native myosin filament in the relaxed cardiac sarcomere.

The thick filament is a key component of sarcomeres, the basic units of striated muscle1. Alterations in thick filament proteins are associated with familial hypertrophic cardiomyopathy and other heart and muscle diseases2. Despite the central importance of the thick filament, its molecular organization remains unclear. Here we present the molecular architecture of native cardiac sarcomeres in the relaxed state, determined by cryo-electron tomography. Our reconstruction of the thick filament reveals the three-dimensional organization of myosin, titin and myosin-binding protein C (MyBP-C). The arrangement of myosin molecules is dependent on their position along the filament, suggesting specialized capacities in terms of strain susceptibility and force generation. Three pairs of titin-α and titin-ß chains run axially along the filament, intertwining with myosin tails and probably orchestrating the length-dependent activation of the sarcomere. Notably, whereas the three titin-α chains run along the entire length of the thick filament, titin-ß chains do not. The structure also demonstrates that MyBP-C bridges thin and thick filaments, with its carboxy-terminal region binding to the myosin tails and directly stabilizing the OFF state of the myosin heads in an unforeseen manner. These results provide a foundation for future research investigating muscle disorders involving sarcomeric components.

Cryo-EM structure of the human cardiac myosin filament.

Pumping of the heart is powered by filaments of the motor protein myosin that pull on actin filaments to generate cardiac contraction. In addition to myosin, the filaments contain cardiac myosin-binding protein C (cMyBP-C), which modulates contractility in response to physiological stimuli, and titin, which functions as a scaffold for filament assembly1. Myosin, cMyBP-C and titin are all subject to mutation, which can lead to heart failure. Despite the central importance of cardiac myosin filaments to life, their molecular structure has remained a mystery for 60 years2. Here we solve the structure of the main (cMyBP-C-containing) region of the human cardiac filament using cryo-electron microscopy. The reconstruction reveals the architecture of titin and cMyBP-C and shows how myosin's motor domains (heads) form three different types of motif (providing functional flexibility), which interact with each other and with titin and cMyBP-C to dictate filament architecture and function. The packing of myosin tails in the filament backbone is also resolved. The structure suggests how cMyBP-C helps to generate the cardiac super-relaxed state3; how titin and cMyBP-C may contribute to length-dependent activation4; and how mutations in myosin and cMyBP-C might disturb interactions, causing disease5,6. The reconstruction resolves past uncertainties and integrates previous data on cardiac muscle structure and function. It provides a new paradigm for interpreting structural, physiological and clinical observations, and for the design of potential therapeutic drugs.

Geranylgeranylacetone reduces cardiomyocyte stiffness and attenuates diastolic dysfunction in a rat model of cardiometabolic syndrome.

Titin-dependent stiffening of cardiomyocytes is a significant contributor to left ventricular (LV) diastolic dysfunction in heart failure with preserved LV ejection fraction (HFpEF). Small heat shock proteins (HSPs), such as HSPB5 and HSPB1, protect titin and administration of HSPB5 in vitro lowers cardiomyocyte stiffness in pressure-overload hypertrophy. In humans, oral treatment with geranylgeranylacetone (GGA) increases myocardial HSP expression, but the functional implications are unknown. Our objective was to investigate whether oral GGA treatment lowers cardiomyocyte stiffness and attenuates LV diastolic dysfunction in a rat model of the cardiometabolic syndrome. Twenty-one-week-old male lean (n = 10) and obese (n = 20) ZSF1 rats were studied, and obese rats were randomized to receive GGA (200 mg/kg/day) or vehicle by oral gavage for 4 weeks. Echocardiography and cardiac catheterization were performed before sacrifice at 25 weeks of age. Titin-based stiffness (Fpassive ) was determined by force measurements in relaxing solution with 100 nM [Ca2+ ] in permeabilized cardiomyocytes at sarcomere lengths (SL) ranging from 1.8 to 2.4 µm. In obese ZSF1 rats, GGA reduced isovolumic relaxation time of the LV without affecting blood pressure, EF or LV weight. In cardiomyocytes, GGA increased myofilament-bound HSPB5 and HSPB1 expression. Vehicle-treated obese rats exhibited higher cardiomyocyte stiffness at all SLs compared to lean rats, while GGA reduced stiffness at SL 2.0 µm. In obese ZSF1 rats, oral GGA treatment improves cardiomyocyte stiffness by increasing myofilament-bound HSPB1 and HSPB5. GGA could represent a potential novel therapy for the early stage of diastolic dysfunction in the cardiometabolic syndrome.

Independent regulation of Z-lines and M-lines during sarcomere assembly in cardiac myocytes revealed by the automatic image analysis software sarcApp.

Sarcomeres are the basic contractile units within cardiac myocytes, and the collective shortening of sarcomeres aligned along myofibrils generates the force driving the heartbeat. The alignment of the individual sarcomeres is important for proper force generation, and misaligned sarcomeres are associated with diseases, including cardiomyopathies and COVID-19. The actin bundling protein, α-actinin-2, localizes to the 'Z-Bodies" of sarcomere precursors and the 'Z-Lines' of sarcomeres, and has been used previously to assess sarcomere assembly and maintenance. Previous measurements of α-actinin-2 organization have been largely accomplished manually, which is time-consuming and has hampered research progress. Here, we introduce sarcApp, an image analysis tool that quantifies several components of the cardiac sarcomere and their alignment in muscle cells and tissue. We first developed sarcApp to utilize deep learning-based segmentation and real space quantification to measure α-actinin-2 structures and determine the organization of both precursors and sarcomeres/myofibrils. We then expanded sarcApp to analyze 'M-Lines' using the localization of myomesin and a protein that connects the Z-Lines to the M-Line (titin). sarcApp produces 33 distinct measurements per cell and 24 per myofibril that allow for precise quantification of changes in sarcomeres, myofibrils, and their precursors. We validated this system with perturbations to sarcomere assembly. We found perturbations that affected Z-Lines and M-Lines differently, suggesting that they may be regulated independently during sarcomere assembly.

Disruption of Z-Disc Function Promotes Mechanical Dysfunction in Human Myocardium: Evidence for a Dual Myofilament Modulatory Role by Alpha-Actinin 2.

The ACTN2 gene encodes α-actinin 2, located in the Z-disc of the sarcomeres in striated muscle. In this study, we sought to investigate the effects of an ACTN2 missense variant of unknown significance (p.A868T) on cardiac muscle structure and function. Left ventricular free wall samples were obtained at the time of cardiac transplantation from a heart failure patient with the ACTN2 A868T heterozygous variant. This variant is in the EF 3-4 domain known to interact with titin and α-actinin. At the ultrastructural level, ACTN2 A868T cardiac samples presented small structural changes in cardiomyocytes when compared to healthy donor samples. However, contractile mechanics of permeabilized ACTN2 A868T variant cardiac tissue displayed higher myofilament Ca2+ sensitivity of isometric force, reduced sinusoidal stiffness, and faster rates of tension redevelopment at all Ca2+ levels. Small-angle X-ray diffraction indicated increased separation between thick and thin filaments, possibly contributing to changes in muscle kinetics. Molecular dynamics simulations indicated that while the mutation does not significantly impact the structure of α-actinin on its own, it likely alters the conformation associated with titin binding. Our results can be explained by two Z-disc mediated communication pathways: one pathway that involves α-actinin's interaction with actin, affecting thin filament regulation, and the other pathway that involves α-actinin's interaction with titin, affecting thick filament activation. This work establishes the role of α-actinin 2 in modulating cross-bridge kinetics and force development in the human myocardium as well as how it can be involved in the development of cardiac disease.

Urinary titin is not an early biomarker of skeletal muscle atrophy induced by muscle denervation in mice.

Early detection of skeletal muscle atrophy is important to prevent further muscle weakness. However, there are few non-invasive biomarkers for skeletal muscle atrophy. Recent studies have reported that the N-terminal fragment (N-titin) of titin, a giant sarcomeric protein, is detected in the urine of patients with muscle damage. In this study, we hypothesized that urinary N-titin would be a potential early biomarker of skeletal muscle atrophy in mice caused by sciatic nerve denervation. Male mice were randomly divided into control and denervation groups, and urinary N-titin levels were assessed daily for 9 days using an enzyme-linked immunosorbent assay system. Despite reduced titin protein levels in atrophic muscles 10 days after denervation, cleaved N-titin fragments were not increased in the urine of mice with denervation-induced muscle atrophy. Furthermore, we found no uptake of Evans blue dye from the extracellular space into the cytoplasm in atrophic muscles, suggesting that the sarcomeric membrane is intact in those muscles. The present results suggest that cleaved N-titin in the urine is not suitable as an early biomarker of skeletal muscle atrophy.

α-myosin heavy chain lactylation maintains sarcomeric structure and function and alleviates the development of heart failure.

The sarcomeric interaction of α-myosin heavy chain (α-MHC) with Titin is vital for cardiac structure and contraction. However, the mechanism regulating this interaction in normal and failing hearts remains unknown. Lactate is a crucial energy substrate of the heart. Here, we identify that α-MHC undergoes lactylation on lysine 1897 to regulate the interaction of α-MHC with Titin. We observed a reduction of α-MHC K1897 lactylation in mice and patients with heart failure. Loss of K1897 lactylation in α-MHC K1897R knock-in mice reduces α-MHC-Titin interaction and leads to impaired cardiac structure and function. Furthermore, we identified that p300 and Sirtuin 1 act as the acyltransferase and delactylase of α-MHC, respectively. Decreasing lactate production by chemical or genetic manipulation reduces α-MHC lactylation, impairs α-MHC-Titin interaction and worsens heart failure. By contrast, upregulation of the lactate concentration by administering sodium lactate or inhibiting the pivotal lactate transporter in cardiomyocytes can promote α-MHC K1897 lactylation and α-MHC-Titin interaction, thereby alleviating heart failure. In conclusion, α-MHC lactylation is dynamically regulated and an important determinant of overall cardiac structure and function. Excessive lactate efflux and consumption by cardiomyocytes may decrease the intracellular lactate level, which is the main cause of reduced α-MHC K1897 lactylation during myocardial injury. Our study reveals that cardiac metabolism directly modulates the sarcomeric structure and function through lactate-dependent modification of α-MHC.

Titin as a potential novel therapeutic target in colorectal cancer.

Colorectal cancer (CRC) is identified as a primary cause of death around the world. The current chemotherapies are not cost-effective. Therefore, finding novel potential therapeutic target is urgent. Titin (TTN) is a muscle protein that is critical in hypertrophic cardiomyopathy. However, its role in CRC is not well understood. The study focused on exploring the possible role of TTN in CRC carcinogenesis. TTN mRNA and protein expression levels presented an obvious downregulation in CRC tissue samples, relative to normal control (p < 0.05). TTN expression significantly correlated with the clinical stage (normal vs. Stage 1, p < 0.05; normal vs. Stage 4, p < 0.05), node metastasis (normal vs. N1, p < 0.05; N1 vs. N2, p < 0.05), histological type (normal vs. adenocarcinoma, p < 0.05), race (Caucasian vs. Asian, p < 0.05; African-American vs. Asian, p < 0.05) and TP53 mutation (normal vs. TP53 mutation, p < 0.05), considering The Cancer Genome Atlas database. However, for patients who had higher TTN expression, the overall survival was remarkably shorter than patients who had low TTN expression. Furthermore, TTN was lowly expressed in four CRC cell lines. TTN overexpression facilitated CRC cells in terms of the proliferation, metastasis and invasion. Based on gene set enrichment analysis, the ERB pathway might be responsible for TTN-related CRC. Besides, TTN was involved in the response to azacitidine. Overall, TTN might serve as a potential novel therapeutic target for treating and overcoming chemotherapy resistance in CRC.

Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte.

BACKGROUND: Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening. METHODS: Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch. RESULTS: We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening. CONCLUSIONS: Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

Molecular mechanisms of muscle contraction: A historical perspective.

Studies of muscle structure and function can be traced to at least 2,000 years ago. However, the modern era of muscle contraction mechanisms started in the 1950s with the classic works by AF Huxley and HE Huxley, both born in the United Kingdom, but not related and working independently. HE Huxley was the first to suggest that muscle contraction occurred through the sliding of two sets of filamentous structures (actin or thin filaments and myosin or thick filaments). AF Huxley then developed a biologically inspired mathematical model suggesting a possible molecular mechanism of how this sliding of actin and myosin might take place. This model then evolved from a two-state to a multi-state model of myosin-actin interactions, and from one that suggested a linear motor causing the sliding to a rotating motor. This model, the cross-bridge model of muscle contraction, is still widely used in biomechanics, and even the more sophisticated cross-bridge models of today still contain many of the features originally proposed by AF Huxley. In 2002, we discovered a hitherto unknown property of muscle contraction that suggested the involvement of passive structures in active force production, the so-called passive force enhancement. It was quickly revealed that this passive force enhancement was caused by the filamentous protein titin, and the three-filament (actin, myosin, and titin) sarcomere model of muscle contraction evolved. There are many suggestions of how these three proteins interact to cause contraction and produce active force, and one such suggestion is described here, but the molecular details of this proposed mechanism still need careful evaluation.

Calpain-3 Is Not a Sodium Dependent Protease and Simply Requires Calcium for Activation.

Calpain-3 (CAPN3) is a muscle-specific member of the calpain family of Ca2+-dependent proteases. It has been reported that CAPN3 can also be autolytically activated by Na+ ions in the absence of Ca2+, although this was only shown under non-physiological ionic conditions. Here we confirm that CAPN3 does undergo autolysis in the presence of high [Na+], but this only occurred if all K+ normally present in a muscle cell was absent, and it did not occur even in 36 mM Na+, higher than what would ever be reached in exercising muscle if normal [K+] was present. CAPN3 in human muscle homogenates was autolytically activated by Ca2+, with ~50% CAPN3 autolysing in 60 min in the presence of 2 µM Ca2+. In comparison, autolytic activation of CAPN1 required about 5-fold higher [Ca2+] in the same conditions and tissue. After it was autolysed, CAPN3 unbound from its tight binding on titin and became diffusible, but only if the autolysis led to complete removal of the IS1 inhibitory peptide within CAPN3, reducing the C-terminal fragment to 55 kDa. Contrary to a previous report, activation of CAPN3, either by raised [Ca2+] or Na+ treatment, did not cause proteolysis of the skeletal muscle Ca2+ release channel-ryanodine receptor, RyR1, in physiological ionic conditions. Treatment of human muscle homogenates with high [Ca2+] caused autolytic activation of CAPN1, accompanied by proteolysis of some titin and complete proteolysis of junctophilin (JP1, full length ~95 kDa), generating an equimolar amount of a diffusible ~75 kDa N-terminal JP1 fragment, but without any proteolysis of RyR1.