Dynamic equilibrium of skeletal muscle macrophage ontogeny in the diaphragm during homeostasis, injury, and recovery

The diaphragm is a unique skeletal muscle due to its continuous activation pattern during the act of breathing. The ontogeny of macrophages, pivotal cells for skeletal muscle maintenance and regeneration, is primarily based on two distinct origins: postnatal bone marrow-derived monocytes and prenatal embryonic progenitors. Here we employed chimeric mice to study the dynamics of these two macrophage populations under different conditions. Traditional chimeric mice generated through whole body irradiation showed virtually complete elimination of the original tissue-resident macrophage pool. We then developed a novel method which employs lead shielding to protect the diaphragm tissue niche from irradiation. This allowed us to determine that up to almost half of tissue-resident macrophages in the diaphragm can be maintained independently from bone marrow-derived monocytes under steady-state conditions. These findings were confirmed by long-term (5 months) parabiosis experiments. Acute diaphragm injury shifted the macrophage balance toward an overwhelming predominance of bone marrow (monocyte)-derived macrophages. However, there was a remarkable reversion to the pre-injury ontological landscape after diaphragm muscle recovery. This diaphragm shielding method permits analysis of the dynamics of macrophage origin and corresponding function under different physiological and pathological conditions. It may be especially useful for studying diseases which are characterized by acute or chronic injury of the diaphragm and accompanying inflammation.

Regenerative medicine: current research and perspective in pediatric surgery

The field of regenerative medicine, encompassing several disciplines including stem cell biology and tissue engineering, continues to advance with the accumulating research on cell manipulation technologies, gene therapy and new materials. Recent progress in preclinical and clinical studies may transcend the boundaries of regenerative medicine from laboratory research towards clinical reality. However, for the ultimate goal to construct bioengineered transplantable organs, a number of issues still need to be addressed. In particular, engineering of elaborate tissues and organs requires a fine combination of different relevant aspects; not only the repopulation of multiple cell phenotypes in an appropriate distribution but also the adjustment of the host environmental factors such as vascularisation, innervation and immunomodulation. The aim of this review article is to provide an overview of the recent discoveries and development in stem cells and tissue engineering, which are inseparably interconnected. The current status of research on tissue stem cells and bioengineering, and the possibilities for application in specific organs relevant to paediatric surgery have been specifically focused and outlined.

Macrophages escape Klotho gene silencing in the mdx mouse model of Duchenne muscular dystrophy and promote muscle growth and increase satellite cell numbers through a Klotho-mediated pathway

Duchenne muscular dystrophy (DMD) is a muscle wasting disease in which inflammation influences the severity of pathology. We found that the onset of muscle inflammation in the mdx mouse model of DMD coincides with large increases in expression of pro-inflammatory cytokines [tumor necrosis factor-α (TNFα); interferon gamma (IFNγ)] and dramatic reductions of the pro-myogenic protein Klotho in muscle cells and large increases of Klotho in pro-regenerative, CD206+ macrophages. Furthermore, TNFα and IFNγ treatments reduced Klotho in muscle cells and increased Klotho in macrophages. Because CD206+/Klotho+ macrophages were concentrated at sites of muscle regeneration, we tested whether macrophage-derived Klotho promotes myogenesis. Klotho transgenic macrophages had a pro-proliferative influence on muscle cells that was ablated by neutralizing antibodies to Klotho and conditioned media from Klotho mutant macrophages did not increase muscle cell proliferation in vitro. In addition, transplantation of bone marrow cells from Klotho transgenic mice into mdx recipients increased numbers of myogenic cells and increased the size of muscle fibers. Klotho also acted directly on macrophages, stimulating their secretion of TNFα. Because TNFα is a muscle mitogen, we tested whether the pro-proliferative effects of Klotho on muscle cells were mediated by TNFα and found that increased proliferation caused by Klotho was reduced by anti-TNFα. Collectively, these data show that pro-inflammatory cytokines contribute to silencing of Klotho in dystrophic muscle, but increase Klotho expression by macrophages. Our findings also show that macrophage-derived Klotho can promote muscle regeneration by expanding populations of muscle stem cells and increasing muscle fiber growth in dystrophic muscle.

The use of cell conditioned medium for musculoskeletal tissue regeneration

Tissue regenerative medicine combines the use of cells, scaffolds, and molecules to repair damaged tissues. Different cell types are employed for musculoskeletal diseases, both differentiated and mesenchymal stromal cells (MSCs). In recent years, the hypothesis that cell-based therapy is guided principally by cell-secreted factors has become increasingly popular. The aim of the present literature review was to evaluate preclinical and clinical studies that used conditioned medium (CM), rich in cell-factors, for musculoskeletal regeneration. Thirty-one were in vitro, 12 in vivo studies, 1 was a clinical study, and 2 regarded extracellular vesicles. Both differentiated cells and MSCs produce CM that induces reduction in inflammation and increases synthetic activity. MSC recruitment and differentiation, endothelial cell recruitment and angiogenesis have also been observed. In vivo studies were performed with CM in bone and periodontal defects, arthritis and muscle dystrophy pathologies. The only clinical study was performed with CM from MSCs in patients needing alveolar bone regeneration, showing bone formation and no systemic or local complications. Platelet derived growth factor receptor β, C3a, vascular endothelial growth factor, monocyte chemoattractant protein-1 and -3, interleukin 3 and 6, insulin-like growth factor-I were identified as responsible of cell migration, proliferation, osteogenic differentiation, and angiogenesis. The use of CM could represent a new regenerative treatment in several musculoskeletal pathologies because it overcomes problems associated with the use of cells and avoids the use of exogenous GFs or gene delivery systems. However, some issues remain to be clarified.

Biomaterials based strategies for skeletal muscle tissue engineering: existing technologies and future trends

Skeletal muscles have a robust capacity to regenerate, but under compromised conditions, such as severe trauma, the loss of muscle functionality is inevitable. Research carried out in the field of skeletal muscle tissue engineering has elucidated multiple intrinsic mechanisms of skeletal muscle repair, and has thus sought to identify various types of cells and bioactive factors which play an important role during regeneration. In order to maximize the potential therapeutic effects of cells and growth factors, several biomaterial based strategies have been developed and successfully implemented in animal muscle injury models. A suitable biomaterial can be utilized as a template to guide tissue reorganization, as a matrix that provides optimum micro-environmental conditions to cells, as a delivery vehicle to carry bioactive factors which can be released in a controlled manner, and as local niches to orchestrate in situ tissue regeneration. A myriad of biomaterials, varying in geometrical structure, physical form, chemical properties, and biofunctionality have been investigated for skeletal muscle tissue engineering applications. In the current review, we present a detailed summary of studies where the use of biomaterials favorably influenced muscle repair. Biomaterials in the form of porous three-dimensional scaffolds, hydrogels, fibrous meshes, and patterned substrates with defined topographies, have each displayed unique benefits, and are discussed herein. Additionally, several biomaterial based approaches aimed specifically at stimulating vascularization, innervation, and inducing contractility in regenerating muscle tissues are also discussed. Finally, we outline promising future trends in the field of muscle regeneration involving a deeper understanding of the endogenous healing cascades and utilization of this knowledge for the development of multifunctional, hybrid, biomaterials which support and enable muscle regeneration under compromised conditions.

Transplanted mesoangioblasts require macrophage IL-10 for survival in a mouse model of muscle injury

The aim of this study was to verify whether macrophages influence the fate of transplanted mesoangioblasts--vessel-associated myogenic precursors--in a model of sterile toxin-induced skeletal muscle injury. We have observed that in the absence of macrophages, transplanted mesoangioblasts do not yield novel fibers. Macrophages retrieved from skeletal muscles at various times after injury display features that resemble those of immunoregulatory macrophages. Indeed, they secrete IL-10 and express CD206 and CD163 membrane receptors and high amounts of arginase I. We have reconstituted the muscle-associated macrophage population by injecting polarized macrophages before mesoangioblast injection: alternatively activated, immunoregulatory macrophages only support mesoangioblast survival and function. This action depends on the secretion of IL-10 in the tissue. Our results reveal an unanticipated role for tissue macrophages in mesoangioblast function. Consequently, the treatment of muscle disorders with mesoangioblasts should take into consideration coexisting inflammatory pathways, whose activation may prove crucial for its success.

Biologic scaffold remodeling in a dog model of complex musculoskeletal injury

BACKGROUND

Current treatment principles for muscle injuries with volumetric loss have been largely derived from empirical observations. Differences in severity or anatomic location have determinant effects on the tissue remodeling outcome. Biologic scaffolds composed of extracellular matrix (ECM) have been successfully used to restore vascularized, innervated, and contractile skeletal muscle in animal models but limited anatomic locations have been evaluated. The aim of this study was to determine the ability of a xenogeneic ECM scaffold to restore functional skeletal muscle in a canine model of a complex quadriceps injury involving bone, tendon, and muscle.

MATERIALS AND METHODS

Sixteen dogs were subjected to unilateral resection of the distal third of the vastus lateralis and medial half of the distal third of the vastus medialis muscles including the proximal half of their associated quadriceps tendon. This defect was replaced with a biologic scaffold composed of small intestinal submucosa extracellular matrix (SIS-ECM) and the remodeling response was evaluated at 1, 2, 3, and 6 mo (N = 4 per group).

RESULTS

The initial remodeling process followed a similar pattern to other studies of ECM-mediated muscle repair with rapid vascularization and migration of myoblasts into the defect site. However, over time the remodeling response resulted in the formation of dense collagenous tissue with islands of muscle in the segments of the scaffold not in contact with bone, and foci of bone and cartilage in the segments that were adjacent to the underlying bone.

CONCLUSIONS

SIS-ECM was not successful at restoring functional muscle tissue in this model. However, the results also suggest that SIS-ECM may have potential to promote integration of soft and boney tissues when implanted in close apposition to bone.

Regeneration of skeletal muscle

Skeletal muscle has a robust capacity for regeneration following injury. However, few if any effective therapeutic options for volumetric muscle loss are available. Autologous muscle grafts or muscle transposition represent possible salvage procedures for the restoration of mass and function but these approaches have limited success and are plagued by associated donor site morbidity. Cell-based therapies are in their infancy and, to date, have largely focused on hereditary disorders such as Duchenne muscular dystrophy. An unequivocal need exists for regenerative medicine strategies that can enhance or induce de novo formation of functional skeletal muscle as a treatment for congenital absence or traumatic loss of tissue. In this review, the three stages of skeletal muscle regeneration and the potential pitfalls in the development of regenerative medicine strategies for the restoration of functional skeletal muscle in situ are discussed.