Louise Helskov Jørgensen

Attenuated muscle regeneration is a key factor in dysferlin-deficient muscular dystrophy

Skeletal muscle requires an efficient and active membrane repair system to overcome the rigours of frequent contraction. Dysferlin is a component of that system and absence of dysferlin causes muscular dystrophy (dysferlinopathy) characterized by adult onset muscle weakness, high serum creatine kinase levels and a prominent inflammatory infiltrate. We have observed that dysferlinopathy patient biopsies show an excess of immature fibres and therefore investigated the role of dysferlin in muscle regeneration. Using notexin-induced muscle damage, we have shown that regeneration is attenuated in a mouse model of dysferlinopathy, with delayed removal of necrotic fibres, an extended inflammatory phase and delayed functional recovery. Satellite cell activation and myoblast fusion appear normal, but there is a reduction in early neutrophil recruitment in regenerating and also needle wounded muscle in dysferlin-deficient mice. Primary mouse dysferlinopathy myoblast cultures show reduced cytokine release upon stimulation, indicating that the secretion of chemotactic molecules is impaired. We suggest an extension to the muscle membrane repair model, where in addition to fusing patch repair vesicles with the sarcolemma dysferlin is also involved in the release of chemotactic agents. Reduced neutrophil recruitment results in incomplete cycles of regeneration in dysferlinopathy which combines with the membrane repair deficit to ultimately trigger dystrophic pathology. This study reveals a novel pathomechanism affecting muscle regeneration and maintenance in dysferlinopathy and highlights enhancement of the neutrophil response as a potential therapeutic avenue in these disorders.

Characterization of DLK1+ Cells Emerging During Skeletal Muscle Remodeling in Response to Myositis, Myopathies, and Acute Injury

Delta like 1 (DLK1) has been proposed to act as a regulator of cell fate determination and is linked to the development of various tissues including skeletal muscle. Herein we further investigated DLK1 expression during skeletal muscle remodeling. Although practically absent in normal adult muscle, DLK1 was upregulated in all human myopathies analyzed, including Duchenne‐ and Becker muscular dystrophies. Substantial numbers of DLK1+ satellite cells were observed in normal neonatal and Duchenne muscle, and furthermore, myogenic DLK1+ cells were identified during muscle regeneration in animal models in which the peak expression of Dlk1 mRNA and protein coincided with that of myoblast differentiation and fusion. In addition to perivascular DLK1+ cells, interstitial DLK1+ cells were numerous in regenerating muscle, and in agreement with colocalization studies of DLK1 and CD90/DDR2, qPCR of fluorescence‐activated cell sorting DLK1+ and DLK1− cells revealed that the majority of DLK1+ cells isolated at day 7 of regeneration had a fibroblast‐like phenotype. The existence of different DLK1+ populations was confirmed in cultures of primary derived myogenic cells, in which large flat nonmyogenic DLK1+ cells and small spindle‐shaped cells coexpressing DLK1 and muscle‐specific markers were observed. Myogenic differentiation was achieved when sorted DLK1+ cells were cocultured together with primary myoblasts revealing a myogenic potential that was 10% of the DLK1− population. Transplantation of DLK1+ cells into lacerated muscle did, however, not give rise to DLK1+ cell‐derived myofibers. We suggest that the DLK1+ subpopulations identified herein each may contribute at different levels/time points to the processes involved in muscle development and remodeling. STEM CELLS 2009;27:898–908

Secreted Protein Acidic and Rich in Cysteine (SPARC) in Human Skeletal Muscle

Secreted protein acidic and rich in cysteine (SPARC)/osteonectin is expressed in different tissues during remodeling and repair, suggesting a function in regeneration. Several gene expression studies indicated that SPARC was expressed in response to muscle damage. Studies on myoblasts further indicated a function of SPARC in skeletal muscle. We therefore found it of interest to study SPARC expression in human skeletal muscle during development and in biopsies from Duchenne and Becker muscular dystrophy and congenital muscular dystrophy, congenital myopathy, inclusion body myositis, and polymyositis patients to analyze SPARC expression in a selected range of inherited and idiopathic muscle wasting diseases. SPARC-positive cells were observed both in fetal and neonatal muscle, and in addition, fetal myofibers were observed to express SPARC at the age of 15–16 weeks. SPARC protein was detected in the majority of analyzed muscle biopsies (23 of 24), mainly in mono-nuclear cells of which few were pax7 positive. Myotubes and regenerating myofibers also expressed SPARC. The expression-degree seemed to reflect the severity of the lesion. In accordance with these in vivo findings, primary human-derived satellite cells were found to express SPARC both during proliferation and differentiation in vitro. In conclusion, this study shows SPARC expression both during muscle development and in regenerating muscle. The expression is detected both in satellite cells/myoblasts and in myotubes and muscle fibers, indicating a role for SPARC in the skeletal muscle compartment.

Long-Term Blocking of Calcium Channels in mdx Mice Results in Differential Effects on Heart and Skeletal Muscle

The disease mechanisms underlying dystrophin-deficient muscular dystrophy are complex, involving not only muscle membrane fragility, but also dysregulated calcium homeostasis. Specifically, it has been proposed that calcium channels directly initiate a cascade of pathological events by allowing calcium ions to enter the cell. The objective of this study was to investigate the effect of chronically blocking calcium channels with the aminoglycoside antibiotic streptomycin from onset of disease in the mdx mouse model of Duchenne muscular dystrophy (DMD). Treatment in utero onwards delayed onset of dystrophic symptoms in the limb muscle of young mdx mice, but did not prevent degeneration and regeneration events occurring later in the disease course. Long-term treatment had a positive effect on limb muscle pathology, reduced fibrosis, increased sarcolemmal stability, and promoted muscle regeneration in older mice. However, streptomycin treatment did not show positive effects in diaphragm or heart muscle, and heart pathology was worsened. Thus, blocking calcium channels even before disease onset does not prevent dystrophy, making this an unlikely treatment for DMD. These findings highlight the importance of analyzing several time points throughout the life of the treated mice, as well as analyzing many tissues, to get a complete picture of treatment efficacy.

Duplication in the Microtubule-Actin Cross-linking Factor 1 gene causes a novel neuromuscular condition

Spectrins and plakins are important communicators linking cytoskeletal components to each other and to cellular junctions. Microtubule-actin cross-linking factor 1 (MACF1) belongs to the spectraplakin family and is involved in control of microtubule dynamics. Complete knock out of MACF1 in mice is associated with developmental retardation and embryonic lethality. Here we present a family with a novel neuromuscular condition. Genetic analyses show a heterozygous duplication resulting in reduced MACF1 gene product. The functional consequence is affected motility observed as periodic hypotonia, lax muscles and diminished motor skills, with heterogeneous presentation among the affected family members. To corroborate these findings we used RNA interference to knock down the VAB-10 locus containing the MACF1 homologue in C. elegans, and we could show that this also causes movement disturbances. These findings suggest that changes in the MACF1 gene is implicated in this neuromuscular condition, which is an important observation since MACF1 has not previously been associated with any human disease and thus presents a key to understanding the essential nature of this gene.

Skeletal Muscle Remodelling as a Function of Disease Progression in Amyotrophic Lateral Sclerosis

Muscle weakness is considered the pivotal sign of amyotrophic lateral sclerosis (ALS). Knowledge about the skeletal muscle degeneration/regeneration process and the myogenic potential is limited in ALS patients. Therefore, we investigate these processes in a time course perspective by analysing skeletal muscle biopsies from ALS patients collected before and after a 12-week period of normal daily activities and compare these with healthy age-matched control tissue. We do this by evaluating mRNA and protein (immunohistochemical) markers of regeneration, neurodegeneration, myogenesis, cell cycle regulation, and inflammation. Our results show morphological changes indicative of active denervation and reinnervation and an increase in small atrophic fibres. We demonstrate differences between ALS and controls in pathways controlling skeletal muscle homeostasis, cytoskeletal and regenerative markers, neurodegenerative factors, myogenic factors, cell cycle determinants, and inflammatory markers. Our results on Pax7 and MyoD protein expression suggest that proliferation and differentiation of skeletal muscle stem cells are affected in ALS patients, and the myogenic processes cannot overcome the denervation-induced wasting.

Efficient and fast functional screening of microdystrophin constructs in vivo and in vitro for therapy of duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is an X-linked, lethal genetic disorder affecting the skeletal muscle compartment, and is caused by mutation(s) in the dystrophin gene. Gene delivery of microdystrophin constructs using adeno-associated virus (AAV) and antisense-mediated exon skipping restoring the genetic reading frame are two of the most promising therapeutic strategies for DMD. Both approaches use microdystrophin proteins either directly as a desired construct for gene delivery, using the capacity-limited AAV vectors, or as the therapeutic outcome of gene splicing. Although functionality of the resulting artificial dystrophin proteins can be predicted in silico, experimental evidence usually obtained in transgenic mice is required before human trials. However, the enormous number of potential constructs makes screening assays for dystrophin protein function in vitro and in vivo highly desirable. Here we present data showing that functionality of microdystrophins can be assessed using relatively simple and fast techniques.

SPARC is up-regulated during skeletal muscle regeneration and inhibits myoblast differentiation

Skeletal muscle repair is mediated primarily by the muscle stem cell, the satellite cell. Several factors, including extracellular matrix, are known to regulate satellite cell function and regeneration. One factor, the matricellular Secreted Protein Acidic and Rich in Cysteine (SPARC) is highly up-regulated during skeletal muscle disease, but its function remains elusive. In the present study, we demonstrate a prominent yet transient increase in SPARC mRNA and protein content during skeletal muscle regeneration that correlates with the expression profile of specific muscle factors like MyoD, Myf5, Myf6, Myogenin, NCAM, CD34, and M-Cadherin, all known to be implicated in satellite cell activation/proliferation following muscle damage. This up regulation was detected in more cell types. Ectopic expression of SPARC in the muscle progenitor cell line C2C12 was performed to mimic the high levels of SPARC seen in muscle disease. SPARC overexpression almost completely abolished myogenic differentiation in these cultures as determined by substantially reduced levels of myogenic factors (Pax7, Myf5, Myod, Mef2B, Myogenin, and Myostatin) and a lack of multinucleated myotubes. These results demonstrate that there is a delicate temporal regulation of SPARC to which more sources in the micro environment contribute, and that disturbances in this, such as extensive up regulation, may have an adverse effect on muscle regeneration.

Genetic ablation of soluble TNF does not affect lesion size and functional recovery after moderate spinal cord injury in mice

Traumatic spinal cord injury (SCI) is followed by an instant increase in expression of the microglial-derived proinflammatory cytokine tumor necrosis factor (TNF) within the lesioned cord. TNF exists both as membrane-anchored TNF (mTNF) and as cleaved soluble TNF (solTNF). We previously demonstrated that epidural administration of a dominant-negative inhibitor of solTNF, XPro1595, to the contused spinal cord resulted in changes in Iba1 protein expression in microglia/macrophages, decreased lesion volume, and improved locomotor function. Here, we extend our studies using mice expressing mTNF, but no solTNF (mTNFΔ/Δ), to study the effect of genetic ablation of solTNF on SCI. We demonstrate that TNF levels were significantly decreased within the lesioned spinal cord 3 days after SCI in mTNFΔ/Δ mice compared to littermates. This decrease did, however, not translate into significant changes in other pro- and anti-inflammatory cytokines (IL-10, IL-1β, IL-6, IL-5, IL-2, CXCL1, CCL2, or CCL5), despite a tendency towards increased IL-10 and decreased IL-1β, TNFR1, and TNFR2 levels in mTNFΔ/Δ mice. In addition, microglial and leukocyte infiltration, activation state (Iba1, CD11b, CD11c, CD45, and MHCII), lesion size, and functional outcome after moderate SCI were comparable between genotypes. Collectively, our data demonstrate that genetic ablation of solTNF does not significantly modulate postlesion outcome after SCI.

Delta-Like 1 Homolog (Dlk1): A Marker for Rhabdomyosarcomas Implicated in Skeletal Muscle Regeneration

Dlk1, a member of the Epidermal Growth Factor family, is expressed in multiple tissues during development, and has been detected in carcinomas and neuroendocrine tumors. Dlk1 is paternally expressed and belongs to a group of imprinted genes associated with rhabdomyosarcomas but not with other primitive childhood tumors to date. Here, we investigate the possible roles of Dlk1 in skeletal muscle tumor formation. We analyzed tumors of different mesenchymal origin for expression of Dlk1 and various myogenic markers and found that Dlk1 was present consistently in myogenic tumors. The coincident observation of Dlk1 with a highly proliferative state in myogenic tumors led us to subsequently investigate the involvement of Dlk1 in the control of the adult myogenic programme. We performed an injury study in Dlk1 transgenic mice, ectopically expressing ovine Dlk1 (membrane bound C2 variant) under control of the myosin light chain promotor, and detected an early, enhanced formation of myotubes in Dlk1 transgenic mice. We then stably transfected the mouse myoblast cell line, C2C12, with full-length Dlk1 (soluble A variant) and detected an inhibition of myotube formation, which could be reversed by adding Dlk1 antibody to the culture supernatant. These results suggest that Dlk1 is involved in controlling the myogenic programme and that the various splice forms may exert different effects. Interestingly, both in the Dlk1 transgenic mice and the DLK1-C2C12 cells, we detected reduced myostatin expression, suggesting that the effect of Dlk1 on the myogenic programme might involve the myostatin signaling pathway. In support of a relationship between Dlk1 and myostatin we detected reciprocal expression of these two transcripts during different cell cycle stages of human myoblasts. Together our results suggest that Dlk1 is a candidate marker for skeletal muscle tumors and might be involved directly in skeletal muscle tumor formation through a modulatory effect on the myogenic programme.