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Neuromuscular junction fragmentation and muscle wasting in heart failure: a sharp cut from a Sica sword?

2015; Elsevier BV; Volume: 17; Issue: 12 Linguagem: Inglês

10.1002/ejhf.437

1879-0844

Giorgio Vescovo,

Nutrition and Health in Aging

This article refers to ‘Detection of muscle wasting in patients with chronic heart failure using C-terminal agrin fragment: results from the Studies Investigating Co-morbidities Aggravating Heart Failure (SICA-HF)’‡, by L. Steinbeck et al., published in this issue on pages 1292–1302. Heart failure (HF) is frequently associated with muscle wasting, an ominous condition that can eventually lead to cardiac cachexia.1 The development of cachexia leads to a poor prognosis in both cardiac and non-cardiac diseases. To date, several studies have demonstrated that interventions made to prevent or fight the development of cachexia lead to a significant improvement both in prognosis and in quality of life.2 In order to achieve these targets there are three different steps to be undertaken: (i) make progress in understanding the pathophysiology of muscle wasting; (ii) identify the occurrence of the process at the very early stages using specific and sensitive tests including biomarkers; and (iii) develop targeted interventions. In the current issue of the journal, Steinbeck et al.3 present a substudy of the SICA-HF study where they looked at C-terminal agrin fragment (CAF) in patients with HF. Two groups of patients with or without muscle wasting were identified. This paper touches upon two different and important issues: (i) the role of neuromuscular junction (NMJ) fragmentation, with particular respect to agrin, in the pathophysiology of muscle wasting; and (ii) the possible role of circulating CAF, a neurotrypsin breakdown product of agrin either at the alpha- or the beta-level, as a biomarker of muscle wasting. This is a unique opportunity to shed further light on the pathophysiology of muscle wasting. It is in fact possible to look at the role of NMJs, but at the same time to offer to the medical community a novel biomarker of muscle wasting. It is known that NMJs play a paramount role in the maintenance of muscle trophism. Alterations of the regulation of the formation of acetylcholine receptors (AchRs) and breakdown may lead to NMJ dysfunction and consequent muscle wasting and atrophy.4 The AchR life cycle is summarized in Figure 1. Briefly, AchRs and rapsyn are synthesized in the endoplasmic reticulum and are transferred through a process of exocytosis to the cell membrane where they are clustered into an agrin/MuSK (muscle-specific kinase)/LRP4 (low-density lipoprotein receptor 4)/rapsyn complex. Neurotrypsin can cleave agrin at the alpha- or beta-site, with production of CAF and consequent dispersal of the NMJ. The AchR and rapsyn are endocytosed and either recycled or degraded trough activation of muscle ring finger 1 (MURF1)–E3 ubiquitin ligase. If the degradation of AchR is excessive, for instance because of denervation or exaggerated proteolytic cleavage of agrin, there may be an early onset of muscle wasting/sarcopenia. Therefore, destabilization of NMJs can be sufficient to promote a sarcopenic phenotype. In normal conditions, the half-life of the AchR is 13 days, and 25% of the receptors are recycled within 4 days. This helps to understand the importance of the process of damage and repair. Pathophysiological conditions may produce an imbalance between these two processes, leading to excessive receptor degradation. Notably MURF1 and E3 ubiquitin ligase are involved in the process of wasting, producing both ubiquitinization of contractile proteins5 and promoting the breakdown of the unassembled AchR. Alterations of the kinetics of NMJs may be the ‘primum movens’ of muscle wasting. In fact mice overexpressing neurotrypsin develop sarcopenia, suggesting that destabilization of NMJs is sufficient to produce muscle wasting. Fragmentation of NMJs offers a new insight into the understanding the pathophysiology of this process. In fact we can distinguish two different scenarios: the first is the degeneration of muscle fibre segments underlying synapsis. This occurs, for instance, during the development of cardiac cachexia secondary to skeletal muscle fibre apoptosis. The second is the occurrence of neuronal lesions leading to functional denervation typical of ageing sarcopenia (recently motoneuron apoptosis has been reported in this condition) and muscular dystrophy. In these two latter conditions, the deterioration of NMJ function is accompanied by alterations of primary signalling molecules such as nicotinic AchRs and agrin. It is clear that NMJ integrity is an indicator of motor health, while its degradation, with the appearance of molecules such as CAF, may indicate ongoing damage of the junction itself. There is still an unresolved issue: the difference between age-related sarcopenia [defined as loss of muscle mass (atrophy) and strength (dynapenia)] and non-sarcopenic atrophy, in other words muscle wasting secondary to chronic HF, COPD, cancer, chronic kidney disease (CKD), etc.6 These two conditions can be differentiated because they have well distinguished morpho-biochemical characteristics: (i) fibre size heterogeneity that is typical of elderly sarcopenia and absent in secondary cachexia; (ii) fibre type grouping, namely neighbouring fibres with the same fibre type, in sarcopenia; (iii) diffuse fibre atrophy without sparing in secondary cachexia; and (iv) co-expression of multiple MHC (major histocompatibility complex) isoforms in sarcopenia. It is obvious that muscle biopsy with immunohistochemical staining and morphometric analysis would be the gold standard for highlighting such differences. The use of fine-needle microbiopsies has been proposed. With this method, it is possible to measure fibre size, determine MHC composition and fibre type distribution, and count apoptotic cells.7, 8 This may represent an interesting and minimally invasive tool for differentiating sarcopenic from non-sarcopenic muscle wasting. Unfortunately muscle wasting is not a black and white phenomenon. HF, for instance, is a clinical syndrome characteristic of elderly patients with multiple co-morbidities. The process of muscle wasting can recognize different triggers such as age, CKD, and diabetes. Therefore, sarcopenic and non-sarcopenic wasting can co-exist. The final result at the muscle level is loss of lean mass and strength, but it is impossible to distinguish the contribution of each single element to the development of wasting. Clinically it would be important to differentiate patients with muscle wasting from those at risk of developing it or in the very early stages of this condition when available tests do not allow its detection. In the Steinbeck study,3 a new tool is proposed: the determination of circulating CAF levels. In patients with chronic HF, CAF is increased in those who have developed muscle wasting as well as in many who have not yet developed it. The sensitivity of total CAF and CAF22 is fairly good, being ∼80%, but the specificity is low. This is because of the high number of CAF-positive patients in whom muscle wasting was undetectable. These latter may be at risk of developing it. Alternatively they may have ongoing, but not clinically detectable, wasting. Patients with no signs of muscle wasting, diagnosed with global assessment of lean mass such as DEXA (dual-energy X-ray absorptiometry), may already have skeletal muscle alterations identifiable with more sophisticated methods such as biopsy or novel biomarkers. This kind of patient may already show increased levels of CAF. Their identification may allow specifically targeted and appropriate treatment. In fact we may envisage a more aggressive and early pharmacological treatment with newly developed drugs for HF, nutritional support, or exercise. In this way, we may prevent clinical worsening or ameliorate functional status. It is possible that measuring CAF in patients with chronic HF and no evidence of muscle wasting, may improve the diagnostic process. Of course the identification of muscle wasting, which is an independent prognostic factor for mortality in HF,9 deserves more refined methods in the future beyond the assessment of lean body mass. Moreover, this latter should be indexed for age so that the occurrence of age-related sarcopenia will be taken into account. Beside CAF, some other biomarkers of muscle wasting are under investigation.10 These include GDF15 (growth differentiation factor-15), ANP (atrial natriuretic peptide), P3NP (N-terminal propeptide of type III procollagen), myostatin, and type VI collagen N-terminal globular domain epitope. Very recently, molecules acting as a bridge between skeletal muscle and tissue metabolism, such as ghrelin and adiponectin, have been proposed as markers of muscle atrophy. The last one is irisin, a myokine, cleaved from FNDC5, a PGC1 gene product, that modulates the browning of adipocytes. It is unrealistic to think that a single biomarker may reflect all the features of this complex syndrome, but a multiple approach strategy could be pursued in the future. The use of more sophisticated scores or algorithms that will include novel biomarkers, co-morbidities and their severity, alterations in muscle mass and function, and even changes detectable at the muscle level may be developed. These latter could be integrated by the use of microbiopsies. This approach can provide information on the degree of fibre atrophy, prevalence and distribution of MHCs and fibre type, severity of muscle and vascular cell damage (apoptosis), and regenerative capability (MyoD and Pax 7 expression).11 This integrated approach may represent a possible future research avenue to be undertaken. Conflict of interest: none declared.