Titin fragment in urine: A noninvasive biomarker of muscle degradation

How Can Proteomics Help to Elucidate the Pathophysiological Crosstalk in Muscular Dystrophy and Associated Multi-System Dysfunction?

This perspective article is concerned with the question of how proteomics, which is a core technique of systems biology that is deeply embedded in the multi-omics field of modern bioresearch, can help us better understand the molecular pathogenesis of complex diseases. As an illustrative example of a monogenetic disorder that primarily affects the neuromuscular system but is characterized by a plethora of multi-system pathophysiological alterations, the muscle-wasting disease Duchenne muscular dystrophy was examined. Recent achievements in the field of dystrophinopathy research are described with special reference to the proteome-wide complexity of neuromuscular changes and body-wide alterations/adaptations. Based on a description of the current applications of top-down versus bottom-up proteomic approaches and their technical challenges, future systems biological approaches are outlined. The envisaged holistic and integromic bioanalysis would encompass the integration of diverse omics-type studies including inter- and intra-proteomics as the core disciplines for systematic protein evaluations, with sophisticated biomolecular analyses, including physiology, molecular biology, biochemistry and histochemistry. Integrated proteomic findings promise to be instrumental in improving our detailed knowledge of pathogenic mechanisms and multi-system dysfunction, widening the available biomarker signature of dystrophinopathy for improved diagnostic/prognostic procedures, and advancing the identification of novel therapeutic targets to treat Duchenne muscular dystrophy.

The diagnostic value of urinary N-terminal fragment of titin for skeletal muscle damage in idiopathic inflammatory myopathy

OBJECTIVES

N-terminal fragment of titin (N-titin) is a marker of sarcomere damage in striated muscles; however, its value in patients with IIM (idiopathic inflammatory myopathy) is unclear. This study aimed to investigate the diagnostic value of N-titin for skeletal muscle damage in patients with IIM.

METHODS

Urine samples from 62 patients with IIM, 59 patients with other CTD diseases, and 29 healthy controls were collected to detect N-titin by ELISA assays. Clinical features and laboratory data were all included in logistic regression analysis to obtain the independent predictive factor for skeletal muscle damage.

RESULTS

Urinary N-titin level of the IIM group [168.3 (19.0, 1279.0) pmol/mg cr] was significantly higher than that in CTD controls [2.80 (1.53, 3.60)] and healthy controls [1.83 (1.09, 2.95)] (P < 0.001). IIM patients with skeletal muscle injury had a significantly higher level of urinary N-titin [1001.0, (181.8, 1977.0)] than those without [9.3, (5.8, 23.9)] (P < 0.001). The N-titin level was strongly correlated with CK (r = 0.907, P < 0.001) and muscle disease activity assessment scores by Spearman correlation analysis. After adjusting for the anti-MDA5 antibody and cardiac troponin T, N-titin was shown to independently predict skeletal muscle damage in patients with IIM (odds ratio = 1.035, 95% CI: 1.002, 1.069, P = 0.039). The cut-off value of urinary N-titin to diagnose skeletal muscle damage was 89.9 pmol/mg Cr, with a sensitivity of 87.8% and a specificity of 100% (AUC = 0.971, 95% CI: 0.938, 1.000, P < 0.001).

CONCLUSION

Urinary N-titin is a non-invasive and independent predictive factor for determining skeletal muscle damage in patients with IIM.

Urinary titin N-fragment as a predictor of decreased skeletal muscle mass in patients with interstitial lung diseases

This study aimed to examine the validity of urinary N-terminal titin fragment/creatinine (urinary N-titin/Cr) reflecting muscle damage biomarker in patients with interstitial lung disease. This retrospective study enrolled patients with interstitial lung disease. We measured urinary N-titin/Cr. Furthermore, we measured the cross-sectional areas of the pectoralis muscles above the aortic arch (PMCSA) and erector spinae muscles of the 12th thoracic vertebra muscles (ESMCSA) to assess muscle mass until 1 year. We examined the correlation between urinary N-titin/Cr and the change in muscle mass. We plotted receiver operating characteristic curves to estimate the cut-off points for urinary N-titin/Cr for distinguishing the greater-than-median and smaller-than-median reduction of muscle mass after 1 year. We enrolled 68 patients with interstitial lung disease. The median urinary N-titin/Cr value was 7.0 pmol/mg/dL. We observed significant negative correlations between urinary N-titin/Cr and changes in the PMCSA after 1 year (p < 0.001) and changes in the ESMCSA after 6 months (p < 0.001) and 1 year (p < 0.001). The cut-off points for urinary N-titin/Cr were 5.2 pmol/mg/dL and 10.4 pmol/mg/dL in the PMCSA and ESMCSA, respectively. In summary, urinary N-titin/Cr may predict muscle loss in the long-term and act as a clinically useful biomarker reflecting muscle damage.

Urinary titin in myotonic dystrophy type 1

INTRODUCTION/AIMS

Urinary titin, an easy-to-obtain marker, has been investigated in muscular dystrophies, but not in myotonic dystrophy type 1 (DM1). We investigated the role of titin as a biomarker of muscle injury in DM1.

METHODS

We compared the urinary titin N-fragment/creatinine ratio in 29 patients with DM1 vs. 30 healthy controls. We also recorded clinical data such as muscle strength, serum creatine kinase, DM1-related outcome measures, and the 20-item DM1-activ questionnaire. The severity of the disease was graded using the Muscular Impairment Rating Scale (MIRS).

RESULTS

The titin/creatinine ratio was significantly higher in the urine samples of DM1 patients than of healthy controls (median ± mean absolute deviation [MAD]: 39.313 ± 26.546 vs. 6.768 ± 5.245 pmol/mg creatinine; P < .001), and was related to muscle impairment graded by MIRS (τ = 0.503, P = .038).

DISCUSSION

Urinary titin may be a biomarker for DM1. Long-term follow-up of DM1 patients is needed to investigate the potential role of titin as a biomarker for disease activity and progression.

Assessment of catabolic state in infants with the use of urinary titin N-fragment

BACKGROUND

Urinary titin N-fragment levels have been used to assess the catabolic state, and we used this biomarker to evaluate the catabolic state of infants.

METHODS

We retrospectively measured urinary titin N-fragment levels of urinary samples. The primary outcome was its changes according to postmenstrual age. The secondary outcomes included differences between gestational age, longitudinal change after birth, influence on growth, and relationship with blood tests.

RESULTS

This study included 219 patients with 414 measurements. Urinary titin N-fragment exponentially declined with postmenstrual age. These values were 12.5 (7.1-19.6), 8.1 (5.1-13.0), 12.8 (6.0-21.3), 26.4 (16.4-52.0), and 81.9 (63.3-106.4) pmol/mg creatinine in full, late, moderate, very, and extremely preterm infants, respectively (p < 0.01). After birth, urinary levels of titin N-fragment exponentially declined, and the maximum level within a week was associated with the time to return to birth weight in preterm infants (ρ = 0.39, p < 0.01). This was correlated with creatine kinase in full-term infants (ρ = 0.58, p < 0.01) and with blood urea nitrogen in preterm infants (ρ = 0.50, p < 0.01).

CONCLUSIONS

The catabolic state was increased during the early course of the postmenstrual age and early preterm infants.

IMPACT

Catabolic state in infants, especially in preterm infants, was expected to be increased, but no study has clearly verified this. In this retrospective study of 219 patients with 414 urinary titin measurements, the catabolic state was exponentially elevated during the early postmenstrual age. The use of the urinary titin N-fragment clarified catabolic state was prominently increased in very and extremely preterm infants.

A sandwich ELISA kit reveals marked elevation of titin N‐terminal fragment levels in the urine of mdx mice

The mdx mouse is a model of Duchenne muscular dystrophy (DMD), a fatal progressive muscle wasting disease caused by dystrophin deficiency, and is used most widely in preclinical studies. Mice with dystrophin deficiency, however, show milder muscle strength phenotypes than humans. In human, the introduction of a sandwich enzyme‐linked immunosorbent assay (ELISA) kit revealed a more than 700‐fold increase in titin N‐terminal fragment levels in the urine of pediatric patients with DMD. Notably, the urinary titin level declines with aging, reflecting progression of muscle wasting. In mouse, development of a highly sensitive ELISA kit has been awaited. Here, a sandwich ELISA kit to measure titin N‐terminal fragment levels in mouse urine was developed. The developed kit showed good linearity, recovery, and repeatability in measuring recombinant or natural mouse titin N‐terminal fragment levels. The titin N‐terminal fragment concentration in the urine of mdx mice was more than 500‐fold higher than that of normal mice. Urinary titin was further analyzed by extending the collection of urine samples to both young (3–11 weeks old) and aged (56–58 weeks old) mdx mice. The concentration in the young group was significantly higher than that in the aged group. It was concluded that muscle protein breakdown is active and persistent in mdx mice even though the muscle phenotype is mild. Our results provide an opportunity to develop DMD treatments that aim to alleviate muscle protein breakdown by monitoring urinary titin levels.

Novel protocol combining physical and nutrition therapies, Intensive Goal-directed REhabilitation with Electrical muscle stimulation and Nutrition (IGREEN) care bundle

Background

Although the combination of rehabilitation and nutrition may be important for the prevention of intensive care unit (ICU)-acquired weakness, a protocolized intervention of this combination has not yet been reported. We herein developed an original combined protocol and evaluated its efficacy.

Methods

In this single-center historical control study, we enrolled adult patients admitted to the ICU. Patients in the control group received standard care, while those in the intervention group received the protocol-based intervention. The ICU mobility scale was used to set goals for early mobilization and a neuromuscular electrical stimulation was employed when patients were unable to stand. The nutritional status was assessed for nutritional therapy, and target calorie delivery was set at 20 or 30 kcal/kg/day and target protein delivery at 1.8 g/kg/day in the intervention group. The primary endpoint was a decrease in femoral muscle volume in 10 days assessed by computed tomography.

Results

Forty-five patients in the control group and 56 in the intervention group were included in the analysis. Femoral muscle volume loss was significantly lower in the intervention group (11.6 vs 14.5%, p = 0.03). The absolute risk difference was 2.9% (95% CI 0.1–5.6%). Early mobilization to a sitting position by day 10 was achieved earlier (p = 0.03), and mean calorie delivery (20.1 vs. 16.8 kcal/kg/day, p = 0.01) and mean protein delivery (1.4 vs. 0.8 g/kg/day, p < 0.01) were higher in the intervention group.

Conclusion

The protocolized intervention, combining early mobilization and high-protein nutrition, contributed to the achievement of treatment goals and prevention of femoral muscle volume loss.

Trial registration number

The present study is registered at the University Hospital Medical Information Network-clinical trials registry (UMIN000040290, Registration date: May 7, 2020).

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03827-8.

Ratio of urinary N-terminal titin fragment to urinary creatinine is a novel biomarker for amyotrophic lateral sclerosis

OBJECTIVE

We aimed to investigate the validity of urinary N-terminal titin fragment as a biomarker for amyotrophic lateral sclerosis (ALS).

METHODS

We consecutively enrolled patients with ALS (n=70) and healthy controls (HC) (n=43). We assessed the urinary titin N-terminal fragment, urinary neurotrophin receptor p75 extracellular domain, serum neurofilament light chain (NfL), motor functional measurements and prognosis. We used urinary creatinine (Cr) levels to normalise the urinary levels of titin fragment.

RESULTS

Compared with HC, patients with ALS had significantly increased urinary levels of titin N-terminal fragment normalised with Cr (titin/Cr) (ALS, 27.2 pmol/mg/dL; HC, 5.8 pmol/mg/dL; p<0.001), which were correlated with the scores of the Revised Amyotrophic Lateral Sclerosis Functional Rating Scale (r=-0.422, p<0.001). A Cox proportional hazards model demonstrated that the high urinary level of titin/Cr was a survival predictor in patients with ALS. Multivariate analysis of prognostic factors showed that the urinary titin/Cr and serum NfL were independent factors for poor prognosis.

CONCLUSIONS

Our findings indicate that urinary N-terminal titin fragment is a non-invasive measure of muscle damage in ALS, which could be applied in disease monitoring and prediction of disease progression, in combination with serum NfL.

Complexity of skeletal muscle degeneration: multi-systems pathophysiology and organ crosstalk in dystrophinopathy

Duchenne muscular dystrophy is a highly progressive muscle wasting disorder due to primary abnormalities in one of the largest genes in the human genome, the DMD gene, which encodes various tissue-specific isoforms of the protein dystrophin. Although dystrophinopathies are classified as primary neuromuscular disorders, the body-wide abnormalities that are associated with this disorder and the occurrence of organ crosstalk suggest that a multi-systems pathophysiological view should be taken for a better overall understanding of the complex aetiology of X-linked muscular dystrophy. This article reviews the molecular and cellular effects of deficiency in dystrophin isoforms in relation to voluntary striated muscles, the cardio-respiratory system, the kidney, the liver, the gastrointestinal tract, the nervous system and the immune system. Based on the establishment of comprehensive biomarker signatures of X-linked muscular dystrophy using large-scale screening of both patient specimens and genetic animal models, this article also discusses the potential usefulness of novel disease markers for more inclusive approaches to differential diagnosis, prognosis and therapy monitoring that also take into account multi-systems aspects of dystrophinopathy. Current therapeutic approaches to combat muscular dystrophy are summarised.

Preventing and Treating Anthracycline Cardiotoxicity: New Insights

Anthracyclines are the cornerstone of many chemotherapy regimens for a variety of cancers. Unfortunately, their use is limited by a cumulative dose-dependent cardiotoxicity. Despite more than five decades of research, the biological mechanisms underlying anthracycline cardiotoxicity are not completely understood. In this review, we discuss the incidence, risk factors, types, and pathophysiology of anthracycline cardiotoxicity, as well as methods to prevent and treat this condition. We also summarize and discuss advances made in the last decade in the comprehension of the molecular mechanisms underlying the pathology.

Multiomic Approaches to Uncover the Complexities of Dystrophin-Associated Cardiomyopathy

Despite major progress in treating skeletal muscle disease associated with dystrophinopathies, cardiomyopathy is emerging as a major cause of death in people carrying dystrophin gene mutations that remain without a targeted cure even with new treatment directions and advances in modelling abilities. The reasons for the stunted progress in ameliorating dystrophin-associated cardiomyopathy (DAC) can be explained by the difficulties in detecting pathophysiological mechanisms which can also be efficiently targeted within the heart in the widest patient population. New perspectives are clearly required to effectively address the unanswered questions concerning the identification of authentic and effectual readouts of DAC occurrence and severity. A potential way forward to achieve further therapy breakthroughs lies in combining multiomic analysis with advanced preclinical precision models. This review presents the fundamental discoveries made using relevant models of DAC and how omics approaches have been incorporated to date.

Urinary Titin Is a Novel Biomarker for Muscle Atrophy in Nonsurgical Critically Ill Patients: A Two-Center, Prospective Observational Study

OBJECTIVES

Although skeletal muscle atrophy is common in critically ill patients, biomarkers associated with muscle atrophy have not been identified reliably. Titin is a spring-like protein found in muscles and has become a measurable biomarker for muscle breakdown. We hypothesized that urinary titin is useful for monitoring muscle atrophy in critically ill patients. Therefore, we investigated urinary titin level and its association with muscle atrophy in critically ill patients.

DESIGN

Two-center, prospective observational study.

SETTING

Mixed medical/surgical ICU in Japan.

PATIENTS

Nonsurgical adult patients who were expected to remain in ICU for greater than 5 days.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Urine samples were collected on days 1, 2, 3, 5, and 7 of ICU admission. To assess muscle atrophy, rectus femoris cross-sectional area and diaphragm thickness were measured with ultrasound on days 1, 3, 5, and 7. Secondary outcomes included its relationship with ICU-acquired weakness, ICU Mobility Scale, and ICU mortality. Fifty-six patients and 232 urinary titin measurements were included. Urinary titin (normal range: 1-3 pmol/mg creatinine) was 27.9 (16.8-59.6), 47.6 (23.5-82.4), 46.6 (24.4-97.6), 38.4 (23.6-83.0), and 49.3 (27.4-92.6) pmol/mg creatinine on days 1, 2, 3, 5, and 7, respectively. Cumulative urinary titin level was significantly associated with rectus femoris muscle atrophy on days 3-7 (p ≤ 0.03), although urinary titin level was not associated with change in diaphragm thickness (p = 0.31-0.45). Furthermore, cumulative urinary titin level was associated with occurrence of ICU-acquired weakness (p = 0.01) and ICU mortality (p = 0.02) but not with ICU Mobility Scale (p = 0.18).

CONCLUSIONS

In nonsurgical critically ill patients, urinary titin level increased 10-30 times compared with the normal level. The increased urinary titin level was associated with lower limb muscle atrophy, occurrence of ICU-acquired weakness, and ICU mortality.

Inherited Neuromuscular Disorders: Which Role for Serum Biomarkers?

Inherited neuromuscular disorders (INMD) are a heterogeneous group of rare diseases that involve muscles, motor neurons, peripheral nerves or the neuromuscular junction. Several different lab abnormalities have been linked to INMD: sometimes they are typical of the disorder, but they usually appear to be less specific. Sometimes serum biomarkers can point out abnormalities in presymtomatic or otherwise asymptomatic patients (e.g., carriers). More often a biomarker of INMD is evaluated by multiple clinicians other than expert in NMD before the diagnosis, because of the multisystemic involvement in INMD. The authors performed a literature search on biomarkers in inherited neuromuscular disorders to provide a practical approach to the diagnosis and the correct management of INMD. A considerable number of biomarkers have been reported that support the diagnosis of INMD, but the role of an expert clinician is crucial. Hence, the complete knowledge of such abnormalities can accelerate the diagnostic workup supporting the referral to specialists in neuromuscular disorders.

Urinary Titin N-Fragment Evaluation in a Randomized Controlled Trial of Beta-Hydroxy-Beta-Methylbutyrate for Acute Mild Trauma in Older Adults

The effects of beta-hydroxy-beta-methylbutyrate (HMB) complex administration and the significance of titin, a biomarker of muscle injury, in elderly minor trauma patients in acute phase has not been established. In this single-center, randomized controlled study, trauma patients aged ≥ 70 years with an injury severity score < 16 were included. Titin values on days 1 and 3 were measured and the intervention group received HMB complex (2.4 g of HMB + 14 g of glutamine + 14 g of arginine) and the control group received glutamine complex (7.2 g of protein including 6 g of glutamine). The cross-sectional area of the rectus femoris (RFCSA) on ultrasound, grip strength, and the Barthel Index were assessed on the first day of rehabilitation and after 2 weeks. We analyzed 24 HMB and 25 control participants. Titin values on day 3 correlated with grip strength (r = -0.34, p = 0.03) and the Barthel Index (r = -0.39, p = 0.01) at follow-up. HMB complex supplementation had no effect on the RFCSA (2.41 vs. 2.45 cm², p = 0.887), grip strength (13.3 vs. 13.1 kg, p = 0.946), or the Barthel Index (20.0 vs. 50.0, p = 0.404) at follow-up. Titin values might associate with subsequent physical function. Short-term HMB complex supplementation from acute phase did not ameliorate muscle injury.

Urinary Titin N-Fragment as a Biomarker of Muscle Atrophy, Intensive Care Unit-Acquired Weakness, and Possible Application for Post-Intensive Care Syndrome

Titin is a giant protein that functions as a molecular spring in sarcomeres. Titin interconnects the contraction of actin-containing thin filaments and myosin-containing thick filaments. Titin breaks down to form urinary titin N-fragments, which are measurable in urine. Urinary titin N-fragment was originally reported to be a useful biomarker in the diagnosis of muscle dystrophy. Recently, the urinary titin N-fragment has been increasingly gaining attention as a novel biomarker of muscle atrophy and intensive care unit-acquired weakness in critically ill patients, in whom titin loss is a possible pathophysiology. Furthermore, several studies have reported that the urinary titin N-fragment also reflected muscle atrophy and weakness in patients with chronic illnesses. It may be used to predict the risk of post-intensive care syndrome or to monitor patients' condition after hospital discharge for better nutritional and rehabilitation management. We provide several tips on the use of this promising biomarker in post-intensive care syndrome.

Elevated Urinary Titin and its Associated Clinical Outcomes after Acute Stroke

INTRODUCTION

Urinary titin is a biomarker of muscle atrophy, which is a serious complication after stroke. However, there are currently no clinical data regarding urinary titin in stroke patients.

METHODS

Consecutive stroke patients admitted to the stroke care unit were included. Spot urine samples were collected immediately after admission, and on days 3, 5, and 7. The primary outcome was the trend of urinary titin in patients after acute stroke. The secondary outcomes included the association between the peak urinary titin level and the modified Rankin Scale (mRS) score, the National Institutes of Health Stroke Scale (NIHSS) score, and the Barthel index (BI) upon hospital discharge. Multivariate analysis was adjusted for age, sex, NIHSS at admission, and the peak urinary titin to predict poor outcome (mRS 3-6).

RESULTS

Forty-one patients were included (29 male; age, 68 ± 15 years), 29 had ischemic stroke, 8 had intracerebral hemorrhage, and 4 had subarachnoid hemorrhage. The levels of urinary titin on days 1, 3, 5, and 7 were 9.9 (4.7-21.1), 16.2 (8.6-22.0), 8.9 (4.8-15.2), and 8.7 (3.6-16.2) pmol/mg Cr, respectively. The peak urinary titin level was associated with the mRS score (r = 0.55, p < 0.01), the NIHSS score (r = 0.72, p < 0.01), and the BI (r = -0.59, p < 0.01) upon hospital discharge. In multivariate analysis, the peak urinary titin was associated with poor outcome (p = 0.03).

CONCLUSIONS

Urinary titin rapidly increased after stroke and was associated with impaired functional outcomes at hospital discharge.

Cellular senescence-mediated exacerbation of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease characterised by chronic muscle degeneration and inflammation. Our previously established DMD model rats (DMD rats) have a more severe disease phenotype than the broadly used mouse model. We aimed to investigate the role of senescence in DMD using DMD rats and patients. Senescence was induced in satellite cells and mesenchymal progenitor cells, owing to the increased expression of CDKN2A, p16- and p19-encoding gene. Genetic ablation of p16 in DMD rats dramatically restored body weight and muscle strength. Histological analysis showed a reduction of fibrotic and adipose tissues invading skeletal muscle, with increased muscle regeneration. Senolytic drug ABT263 prevented loss of body weight and muscle strength, and increased muscle regeneration in rats even at 8 months—the late stage of DMD. Moreover, senescence markers were highly expressed in the skeletal muscle of DMD patients. In situ hybridization of CDKN2A confirmed the expression of it in satellite cells and mesenchymal progenitor cells in patients with DMD. Collectively, these data provide new insights into the integral role of senescence in DMD progression.

Pathological evaluation of rats carrying in-frame mutations in the dystrophin gene: a new model of Becker muscular dystrophy

Dystrophin, encoded by the DMD gene on the X chromosome, stabilizes the sarcolemma by linking the actin cytoskeleton with the dystrophin-glycoprotein complex (DGC). In-frame mutations in DMD cause a milder form of X-linked muscular dystrophy, called Becker muscular dystrophy (BMD), characterized by the reduced expression of truncated dystrophin. So far, no animal model with in-frame mutations in Dmd has been established. As a result, the effect of in-frame mutations on the dystrophin expression profile and disease progression of BMD remains unclear. In this study, we established a novel rat model carrying in-frame Dmd gene mutations (IF rats) and evaluated the pathology. We found that IF rats exhibited reduced expression of truncated dystrophin in a proteasome-independent manner. This abnormal dystrophin expression caused dystrophic changes in muscle tissues but did not lead to functional deficiency. We also found that the expression of additional dystrophin named dpX, which forms the DGC in the sarcolemma, was associated with the appearance of truncated dystrophin. In conclusion, the outcomes of this study contribute to the further understanding of BMD pathology and help elucidate the efficiency of dystrophin recovery treatments in Duchenne muscular dystrophy, a more severe form of X-linked muscular dystrophy.

Identification of marker proteins of muscular dystrophy in the urine proteome from the mdx-4cv model of dystrophinopathy

Since the protein constituents of urine present a dynamic proteome that can reflect a variety of disease-related alterations in the body, the mass spectrometric survey of proteome-wide changes in urine promises new insights into pathogenic mechanisms. Urine can be investigated in a completely non-invasive way and provides valuable biomedical information on body-wide changes. In this report, we have focused on the urine proteome in X-linked muscular dystrophy using the established mdx-4cv mouse model of dystrophinopathy. In order to avoid potential artefacts due to the manipulation of the biofluid proteome prior to mass spectrometry, crude urine specimens were analyzed without the prior usage of centrifugation steps or concentration procedures. Comparative proteomics revealed 21 increased and 8 decreased proteins out of 870 identified urinary proteoforms using 50 μl of biofluid per investigated sample, i.e. 14 wild type versus 14 mdx-4cv specimens. Promising marker proteins that were almost exclusively found in mdx-4cv urine included nidogen, parvalbumin and titin. Interestingly, the mass spectrometric identification of urine-associated titin revealed a wide spread of peptides over the sequence of this giant muscle protein. The newly established urinomic signature of dystrophinopathy might be helpful for the design of non-invasive assays to improve diagnosis, prognosis, therapy-monitoring and evaluation of potential harmful side effects of novel treatments in the field of muscular dystrophy research.

Myomesin is part of an integrity pathway that responds to sarcomere damage and disease

The structure and function of the sarcomere of striated muscle is well studied but the steps of sarcomere assembly and maintenance remain under-characterized. With the aid of chaperones and factors of the protein quality control system, muscle proteins can be folded and assembled into the contractile apparatus of the sarcomere. When sarcomere assembly is incomplete or the sarcomere becomes damaged, suites of chaperones and maintenance factors respond to repair the sarcomere. Here we show evidence of the importance of the M-line proteins, specifically myomesin, in the monitoring of sarcomere assembly and integrity in previously characterized zebrafish muscle mutants. We show that myomesin is one of the last proteins to be incorporated into the assembling sarcomere, and that in skeletal muscle, its incorporation requires connections with both titin and myosin. In diseased zebrafish sarcomeres, myomesin1a shows an early increase of gene expression, hours before chaperones respond to damaged muscle. We found that myomesin expression is also more specific to sarcomere damage than muscle creatine kinase, and our results and others support the use of myomesin assays as an early, specific, method of detecting muscle damage.