Desmin immunolocalisation in autosomal dominant Emery-Dreifuss muscular dystrophy

The tendon interfascicular basement membrane provides a vascular niche for CD146+ cell subpopulations

Introduction: The interfascicular matrix (IFM; also known as the endotenon) is critical to the mechanical adaptations and response to load in energy-storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT). We hypothesized that the IFM is a tendon progenitor cell niche housing an exclusive cell subpopulation. Methods: Immunolabelling of equine superficial digital flexor tendon was used to identify the interfascicular matrix niche, localising expression patterns of CD31 (endothelial cells), Desmin (smooth muscle cells and pericytes), CD146 (interfascicular matrix cells) and LAMA4 (interfascicular matrix basement membrane marker). Magnetic-activated cell sorting was employed to isolate and compare in vitro properties of CD146+ and CD146- subpopulations. Results: Labelling for CD146 using standard histological and 3D imaging of large intact 3D segments revealed an exclusive interfascicular cell subpopulation that resides in proximity to a basal lamina which forms extensive, interconnected vascular networks. Isolated CD146+ cells exhibited limited mineralisation (osteogenesis) and lipid production (adipogenesis). Discussion: This study demonstrates that the interfascicular matrix is a unique tendon cell niche, containing a vascular-rich network of basement membrane, CD31+ endothelial cells, Desmin+ mural cells, and CD146+ cell populations that are likely essential to tendon structure and/or function. Contrary to our hypothesis, interfascicular CD146+ subpopulations did not exhibit stem cell-like phenotypes. Instead, our results indicate CD146 as a pan-vascular marker within the tendon interfascicular matrix. Together with previous work demonstrating that endogenous tendon CD146+ cells migrate to sites of injury, our data suggest that their mobilisation to promote intrinsic repair involves changes in their relationships with local interfascicular matrix vascular and basement membrane constituents.

When lamins go bad: nuclear structure and disease

Mutations in nuclear lamins or other proteins of the nuclear envelope are the root cause of a group of phenotypically diverse genetic disorders known as laminopathies, which have symptoms that range from muscular dystrophy to neuropathy to premature aging syndromes. Although precise disease mechanisms remain unclear, there has been substantial progress in our understanding of not only laminopathies, but also the biological roles of nuclear structure. Nuclear envelope dysfunction is associated with altered nuclear activity, impaired structural dynamics, and aberrant cell signaling. Building on these findings, small molecules are being discovered that may become effective therapeutic agents.

Phosphorylation of connexin43 on S279/282 may contribute to laminopathy-associated conduction defects

An understanding of the molecular mechanism behind the arrhythmic phenotype associated with laminopathies has yet to emerge. A-type lamins have been shown to interact and sequester activated phospho-ERK1/2(pERK1/2) at the nucleus. The gap junction protein connexin43 (Cx43) can be phosphorylated by pERK1/2 on S279/282 (pS279/282), inhibiting intercellular communication. We hypothesized that without A-type lamins, pS279/282 Cx43 will increase due to inappropriate phosphorylation by pERK1/2, resulting in decreased gap junction function. We observed a 1.6-fold increase in pS279/282 Cx43 levels in Lmna(-/-) mouse embryonic fibroblasts (MEFs) compared to Lmna(+/+), and 1.8-fold more pERK1/2 co-precipitated from Lmna(-/-) MEFs with Cx43 antibodies. We found a 3-fold increase in the fraction of non-nuclear pERK1/2 and a concomitant 2-fold increase in the fraction of pS279/282 Cx43 in Lmna(-/-) MEFs by immunofluorescence. In an assay of gap junctional function, Lmna(-/-) MEFs transferred dye to 60% fewer partners compared to Lmna(+/+) controls. These results are mirrored in 5-6 week-old Lmna(-/-) mice compared to their Lmna(+/+) littermates as we detect increased pS279/282 Cx43 in gap junctions by immunofluorescence and 1.7-fold increased levels by immunoblot. We conclude that increased pS279/282 Cx43 in the Lmna(-/-) background results in decreased cell communication and may contribute to the arrhythmic pathology in vivo.

Defects in cell spreading and ERK1/2 activation in fibroblasts with lamin A/C mutations

In-frame mutations in nuclear lamin A/C lead to a multitude of tissue-specific degenerative diseases known as the 'laminopathies'. Previous studies have demonstrated that lamin A/C-null mouse fibroblasts have defects in cell polarisation, suggesting a role for lamin A/C in nucleo-cytoskeletal-cell surface cross-talk. However, this has not been examined in patient fibroblasts expressing modified forms of lamin A/C. Here, we analysed skin fibroblasts from 3 patients with Emery-Dreifuss muscular dystrophy and from 1 with dilated cardiomyopathy. The emerin-lamin A/C interaction was impaired in each mutant cell line. Mutant cells exhibited enhanced cell proliferation, collagen-dependent adhesion, larger numbers of filopodia and smaller cell spread size, compared with control cells. Furthermore, cell migration, speed and polarization were elevated. Mutant cells also showed an enhanced ability to contract collagen gels at early time points, compared with control cells. Phosphotyrosine measurements during cell spreading indicated an initial temporal lag in ERK1/2 activation in our mutant cells, followed by hyper-activation of ERK1/2 at 2 h post cell attachment. Deregulated ERK1/2 activation is linked with cardiomyopathy, cell spreading and proliferation defects. We conclude that a functional emerin-lamin A/C complex is required for cell spreading and proliferation, possibly acting through ERK1/2 signalling.

Investigating the pathology of Emery–Dreifuss muscular dystrophy

EDMD (Emery–Dreifuss muscular dystrophy) is caused by mutations in either the gene encoding for lamin A/C (LMNA) located at 1q21.2–q21.3 or emerin (EMD) located at Xq28. Autosomal dominant EDMD caused by LMNA mutations is more common than the X-linked form and often more severe, with an earlier onset. At the histological and histochemical levels, both X-linked and autosomal dominant EDMD appear similar. However, individuals with the same genetic disorder often show remarkable differences in clinical severity, a finding generally attributed to the genetic background. The clinical and pathological findings in EDMD patients found to have mutations in more than one gene are also discussed. There is now much interest in the phenotype of several animal models for EDMD which should lead to an increased insight into the pathogenesis of this disorder, particularly that relating to the heart phenotype.