Decline in titin content in rat skeletal muscle after denervation

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.

The effect of NAMPT deletion in projection neurons on the function and structure of neuromuscular junction (NMJ) in mice

Nicotinamide adenine dinucleotide (NAD⁺) plays a critical role in energy metabolism and bioenergetic homeostasis. Most NAD⁺ in mammalian cells is synthesized via the NAD⁺ salvage pathway, where nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme, converting nicotinamide into nicotinamide mononucleotide (NMN). Using a Thy1-Nampt^−/− projection neuron conditional knockout (cKO) mouse, we studied the impact of NAMPT on synaptic vesicle cycling in the neuromuscular junction (NMJ), end-plate structure of NMJs and muscle contractility of semitendinosus muscles. Loss of NAMPT impaired synaptic vesicle endocytosis/exocytosis in the NMJs. The cKO mice also had motor endplates with significantly reduced area and thickness. When the cKO mice were treated with NMN, vesicle endocytosis/exocytosis was improved and endplate morphology was restored. Electrical stimulation induced muscle contraction was significantly impacted in the cKO mice in a frequency dependent manner. The cKO mice were unresponsive to high frequency stimulation (100 Hz), while the NMN-treated cKO mice responded similarly to the control mice. Transmission electron microscopy (TEM) revealed sarcomere misalignment and changes to mitochondrial morphology in the cKO mice, with NMN treatment restoring sarcomere alignment but not mitochondrial morphology. This study demonstrates that neuronal NAMPT is important for pre-/post-synaptic NMJ function, and maintaining skeletal muscular function and structure.

Weak by the machines: muscle motor protein dysfunction - a side effect of intensive care unit treatment

Intensive care interventions involve periods of mechanical ventilation, sedation and complete mechanical silencing of patients. Critical illness myopathy (CIM) is an ICU-acquired myopathy that is associated with limb muscle weakness, muscle atrophy, electrical silencing of muscle and motor proteinopathy. The hallmark of CIM is a preferential muscle myosin loss due to increased catabolic and reduced anabolic activity. The ubiquitin proteasome pathway plays an important role, apart from recently identified novel mechanisms affecting non-lysosomal protein degradation or autophagy. CIM is not reproduced by pure disuse atrophy, denervation atrophy, steroid-induced atrophy or septic myopathy, although combinations of high-dose steroids and denervation can mimic CIM. New animal models of critical illness and ICU treatment (i.e. mechanical ventilation and complete immobilization) provide novel insights regarding the time course of protein synthesis and degradation alterations, and the role of protective chaperone activities in the process of myosin loss. Altered mechano-signalling seems involved in triggering a major part of myosin loss in experimental CIM models, and passive loading of muscle potently ameliorates the CIM phenotype. We provide a systematic overview of similarities and distinct differences in the signalling pathways involved in triggering muscle atrophy in CIM and isolated trigger factors. As preferential myosin loss is mostly determined from biochemistry analyses providing no spatial resolution of myosin loss processes within myofibres, we also provide first results monitoring myosin signal intensities during experimental ICU intervention using multi-photon Second Harmonic Generation microscopy. Our results confirm that myosin loss is an evenly distributed process within myofibres rather than being confined to hot spots.

Comparative decline of the protein profiles of nebulin in response to denervation in skeletal muscle

The sliding filament model of the sarcomere was developed more than half a century ago. This model, consisting only of thin and thick filaments, has been efficacious in elucidating many, but not all, features of skeletal muscle. Work during the 1980s revealed the existence of two additional filaments: the giant filamentous proteins titin and nebulin. Nebulin, a giant myofibrillar protein, acts as a protein ruler to maintain the lattice arrays of thin filaments and plays a role in signal transduction and contractile regulation. However, the change of nebulin and its effect on thin filaments in denervation-induced atrophic muscle remains unclear. The purpose of this study is to examine the content and pattern of nebulin, myosin heavy chain (MHC), actin, and titin in innervated and denervated tibialis anterior (TA) muscles of rats using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), densitometry and electron microscopic (EM) analyses. The results revealed that denervation induced muscle atrophy is accompanied by decreased nebulin content in a time-dependent manner. For instant, the levels of nebulin in denervated muscles were markedly (P < 0.05) decreased, about 24.6% and 40.2% in comparison with innervated muscle after denervation of 28 and 56 days, respectively. The nebulin/MHC, nebulin/actin, and nebulin/titin ratios were decreased, suggesting a concomitant reduction of nebulin in denervated muscle. Moreover, a western blotting assay proved that nebulin declined faster than titin on 28 and 56 days of denervated muscle. In addition, EM study revealed that the disturbed arrangements of myofilaments and a disorganized contractile apparatus were also observed in denervated muscle. Overall, the present study provides evidence that nebulin is more sensitive to the effect of denervation than MHC, actin, and titin. Nebulin decline indeed resulted in disintegrate of thin filaments and shortening of sarcomeres.

Arpp/Ankrd2, a member of the muscle ankyrin repeat proteins (MARPs), translocates from the I-band to the nucleus after muscle injury

Ankyrin-repeat protein with a PEST motif and a proline-rich region (Arpp), also designated as Ankrd2, is a member of the muscle ankyrin repeat proteins (MARPs), which have been proposed to be involved in muscle stress response pathways. Arpp/Ankrd2 is localized mainly in the I-band of striated muscle. However, it has recently been reported that Arpp/Ankrd2 can interact with nuclear proteins, such as premyelocytic leukemia protein (PML), p53 and YB-1 in vitro. In this study, to determine whether nuclear accumulation of Arpp/Ankrd2 actually occurs, we performed an immunohistochemical investigation of gastrocnemius muscles that had been injured by injection of cardiotoxin or contact with dry ice. We found that Arpp/Ankrd2 accumulated in the nuclei of myofibers located adjacent to severely damaged myofibers after muscle injury. Double-labeled immunohistochemistry revealed that Arpp/Ankrd2 accumulated in the nuclei of sarcomere-damaged myofibers. Furthermore, we found that Arpp/Ankrd2 tended to be localized in euchromatin where genes are transcriptionally activated. Based on these findings, we suggest that Arpp/Ankrd2 may translocate from the I-band to the nucleus in response to muscle damage and may participate in the regulation of gene expression.

Effects of streptozotocin-induced diabetes and physical training on gene expression of titin-based stretch-sensing complexes in mouse striated muscle

In striated muscle, a sarcomeric noncontractile protein, titin, is proposed to form the backbone of the stress- and strain-sensing structures. We investigated the effects of diabetes, physical training, and their combination on the gene expression of proteins of putative titin stretch-sensing complexes in skeletal and cardiac muscle. Mice were divided into control (C), training (T), streptozotocin-induced diabetic (D), and diabetic training (DT) groups. Training groups performed for 1, 3, or 5 wk of endurance training on a motor-driven treadmill. Muscle samples from T and DT groups together with respective controls were collected 24 h after the last training session. Gene expression of calf muscles (soleus, gastrocnemius, and plantaris) and cardiac muscle were analyzed using microarray and quantitative PCR. Diabetes induced changes in mRNA expression of the proteins of titin stretch-sensing complexes in Z-disc (MLP, myostatin), I-band (CARP, Ankrd2), and M-line (titin kinase signaling). Training alleviated diabetes-induced changes in most affected mRNA levels in skeletal muscle but only one change in cardiac muscle. In conclusion, we showed diabetes-induced changes in mRNA levels of several fiber-type-biased proteins (MLP, myostatin, Ankrd2) in skeletal muscle. These results are consistent with previous observations of diabetes-induced atrophy leading to slower fiber type composition. The ability of exercise to alleviate diabetes-induced changes may indicate slower transition of fiber type.