In vivo and in vitro investigations of heterozygous nebulin knock-out mice disclose a mild skeletal muscle phenotype.

Nemaline myopathy is the most common congenital skeletal muscle disease, and mutations in the nebulin gene account for 50% of all cases. Recent studies suggest that the disease severity might be related to the nebulin expression levels. Considering that mutations in the nebulin gene are typically recessive, one would expect that a single functional nebulin allele would maintain nebulin protein expression which would result in preserved skeletal muscle function. We investigated skeletal muscle function of heterozygous nebulin knock-out (i.e., nebulin(+/-)) mice using a multidisciplinary approach including protein and gene expression analysis and combined in vivo and in vitro force measurements. Skeletal muscle anatomy and energy metabolism were studied strictly non-invasively using magnetic resonance imaging and 31P-magnetic resonance spectroscopy. Maximal force production was reduced by around 16% in isolated muscle of nebulin(+/-) mice while in vivo force generating capacity was preserved. Muscle weakness was associated with a shift toward a slower proteomic phenotype, but was not related to nebulin protein deficiency or to an impaired energy metabolism. Further studies would be warranted in order to determine the mechanisms leading to a mild skeletal muscle phenotype resulting from the expression of a single nebulin allele.

Diaphragm muscle fiber function and structure in humans with hemidiaphragm paralysis.

Recent studies proposed that mechanical inactivity of the human diaphragm during mechanical ventilation rapidly causes diaphragm atrophy and weakness. However, conclusive evidence for the notion that diaphragm weakness is a direct consequence of mechanical inactivity is lacking. To study the effect of hemidiaphragm paralysis on diaphragm muscle fiber function and structure in humans, biopsies were obtained from the paralyzed hemidiaphragm in eight patients with hemidiaphragm paralysis. All patients had unilateral paralysis of known duration, caused by en bloc resection of the phrenic nerve with a tumor. Furthermore, diaphragm biopsies were obtained from three control subjects. The contractile performance of demembranated muscle fibers was determined, as well as fiber ultrastructure and morphology. Finally, expression of E3 ligases and proteasome activity was determined to evaluate activation of the ubiquitin-proteasome pathway. The force-generating capacity, as well as myofibrillar ultrastructure, of diaphragm muscle fibers was preserved up to 8 wk of paralysis. The cross-sectional area of slow fibers was reduced after 2 wk of paralysis; that of fast fibers was preserved up to 8 wk. The expression of the E3 ligases MAFbx and MuRF-1 and proteasome activity was not significantly upregulated in diaphragm fibers following paralysis, not even after 72 and 88 wk of paralysis, at which time marked atrophy of slow and fast diaphragm fibers had occurred. Diaphragm muscle fiber atrophy and weakness following hemidiaphragm paralysis develops slowly and takes months to occur.

Heart failure decreases passive tension generation of rat diaphragm fibers.

BACKGROUND: Diaphragm dysfunction is well-known to limit quality of life and prognosis of patients with heart failure (HF), but its underlying mechanisms are not well understood. In an animal model for HF we recently showed that impaired diaphragm contractility arises at the single fiber level and is associated with sarcomeric injuries. For optimal muscle function and sarcomeric stability passive elastic structures, like titin, are indispensable. The current study aimed to investigate if impaired passive elasticity contributes to diaphragm dysfunction in rats with heart failure. METHODS: Skinned muscle fibers were isolated from the diaphragm and soleus of rats with chronic HF, induced by left coronary artery ligation and of sham-operated rats. Passive tension-length relationships were determined by applying segmental extension tests. Immunofluorescence was performed on muscle cryosections using antibodies (T12) against a titin epitope near the Z-line. Titin content was determined by SDS-agarose-gel electrophoresis. Titin's mobility on gel was studied to detect changes in titin size. RESULTS: Passive tension generation upon stretch was significantly reduced (>35%) in HF diaphragm fibers compared to sham. Immunostaining intensities against titin were reduced in diaphragm cryosections of HF rats compared to sham. Soleus fibers from HF and sham rats did not display differences, neither in passive tension nor in immunostaining. No differences in titin's size were detected in HF and sham diaphragm. Titin content, however, was significantly reduced ( approximately 25%) in HF diaphragm. DISCUSSION: We conclude that in the diaphragm of HF rats, passive elasticity is impaired, mainly resulting from titin loss.

Diaphragm dysfunction in chronic obstructive pulmonary disease: a role for heparan sulphate?

In the present study, phage display-derived antibodies were used to investigate the topology of glycosaminoglycan epitopes in the diaphragm of chronic obstructive pulmonary disease (COPD) and non-COPD patients. Furthermore, the potential physiological significance of changes in the occurrence of glycosaminoglycan epitopes in the diaphragm of COPD patients was studied by determining the overlap in epitope recognition of glycosaminoglycan antibodies and growth factors. Diaphragm cryosections from non-COPD (n = 5) and COPD patients (Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage I/II; n = 9) were incubated with antibodies directed against heparan sulphate, chondroitin sulphate and dermatan sulphate epitopes. Antibodies were visualised immunofluorescently. In addition, interference of antibody and growth factor binding with heparan sulphate epitopes was tested. Specific glycosaminoglycan epitopes showed increased expression in the diaphragm of COPD patients, whereas other epitopes were decreased or unaffected. Interestingly, the anti-heparan sulphate antibody HS4C3, which is directed against a downregulated epitope, interfered with the binding of hepatocyte growth factor. Three patients with the most severe airway obstruction also demonstrated interference of heparan sulphate antibody A04B08 with hepatocyte growth factor binding. Results indicate changes in glycosaminoglycan composition in the diaphragm of patients with chronic obstructive pulmonary disease. This may affect cellular physiology via alterations in growth factor handling and might be related to reduced levels of contractile protein in the diaphragm of these patients.