A mouse model for dominant collagen VI disorders: heterozygous deletion of Col6a3 Exon 16

Endotrophin, a Key Marker and Driver for Fibroinflammatory Disease

Our overview covers several key areas related to recent results obtained for collagen type VI and endotrophin (ETP). (1) An introduction to the history of ETP, including how it was identified, how it is released, and its function and potential receptors. (2) An introduction to the collagen family, with a focus on what differentiates collagen type VI from an evolutionary standpoint. (3) An overview of collagen type VI, the 6 individual chains (COL6A1, A2, A3, A4, A5, and A6), their differences and similarities, as well as their expression profiles and function. (4) A detailed analysis of COL6A3, including the cleaved product endotrophin, and what separates it from the other 5 collagen 6 molecules, including its suggested function based on insights gained from knockout and gain of function mouse models. (5) The pathology of ETP. What leads to its presence and release and what are the consequences thereof? (6) Functional implications of circulating ETP. Here we review the data with the functional roles of ETP in mind. (7) We propose that ETP is a mediator for fibrotic (or fibroinflammatory) disorders. Based on what we know about ETP, we have to consider it as a target for the treatment of fibrotic (or fibroinflammatory) disorders. What segment(s) of the patient population would most dramatically respond to an ETP-targeted intervention? How can we find the population that would profit most from an intervention? We aim to present a broad overview over the ETP field at large, providing an assessment of where the future research efforts need to be placed to tap into the vast potential of ETP, both as a marker and as a target in different diseases.

The coordinated activities of collagen VI and XII in maintenance of tissue structure, function and repair: evidence for a physical interaction

Collagen VI and collagen XII are structurally complex collagens of the extracellular matrix (ECM). Like all collagens, type VI and XII both possess triple-helical components that facilitate participation in the ECM network, but collagen VI and XII are distinct from the more abundant fibrillar collagens in that they also possess arrays of structurally globular modules with the capacity to propagate signaling to attached cells. Cell attachment to collagen VI and XII is known to regulate protective, proliferative or developmental processes through a variety of mechanisms, but a growing body of genetic and biochemical evidence suggests that at least some of these phenomena may be potentiated through mechanisms that require coordinated interaction between the two collagens. For example, genetic studies in humans have identified forms of myopathic Ehlers-Danlos syndrome with overlapping phenotypes that result from mutations in either collagen VI or XII, and biochemical and cell-based studies have identified accessory molecules that could form bridging interactions between the two collagens. However, the demonstration of a direct or ternary structural interaction between collagen VI or XII has not yet been reported. This Hypothesis and Theory review article examines the evidence that supports the existence of a functional complex between type VI and XII collagen in the ECM and discusses potential biological implications.

A humanized knock-in Col6a1 mouse recapitulates a deep-intronic splice-activating variant

Antisense therapeutics such as splice-modulating antisense oligonucleotides (ASOs) are promising tools to treat diseases caused by splice-altering intronic variants. However, their testing in animal models is hampered by the generally poor sequence conservation of the intervening sequences between human and other species. Here we aimed to model in the mouse a recurrent, deep-intronic, splice-activating, COL6A1 variant, associated with a severe form of Collagen VI-related muscular dystrophies (COL6-RDs), for the purpose of testing human-ready antisense therapeutics in vivo. The variant, c.930+189C>T, creates a donor splice site and inserts a 72-nt-long pseudoexon, which, when translated, acts in a dominant-negative manner, but which can be skipped with ASOs. We created a unique humanized mouse allele (designated as "h"), in which a 1.9 kb of the mouse genomic region encoding the amino-terminus (N-) of the triple helical (TH) domain of collagen a1(VI) was swapped for the human orthologous sequence. In addition, we also created an allele that carries the c.930+189C>T variant on the same humanized knock-in sequence (designated as "h+189T"). We show that in both models, the human exons are spliced seamlessly with the mouse exons to generate a chimeric mouse-human collagen a1(VI) protein. In homozygous Col6a1 h+189T/h+189T mice, the pseudoexon is expressed at levels comparable to those observed in heterozygous patients' muscle biopsies. While Col6a1h/h mice do not show any phenotype compared to wildtype animals, Col6a1 h/h+189T and Col6a1 h+189T/h+189T mice have smaller muscle masses and display grip strength deficits detectable as early as 4 weeks of age. The pathogenic h+189T humanized knock-in mouse allele thus recapitulates the pathogenic splicing defects seen in patients' biopsies and allows testing of human-ready precision antisense therapeutics aimed at skipping the pseudoexon. Given that the COL6A1 N-TH region is a hot-spot for COL6-RD variants, the humanized knock-in mouse model can be utilized as a template to introduce other COL6A1 pathogenic variants. This unique humanized mouse model thus represents a valuable tool for the development of antisense therapeutics for COL6-RDs.

Apelin stimulation of the vascular skeletal muscle stem cell niche enhances endogenous repair in dystrophic mice

Impaired skeletal muscle stem cell (MuSC) function has long been suspected to contribute to the pathogenesis of muscular dystrophy (MD). Here, we showed that defects in the endothelial cell (EC) compartment of the vascular stem cell niche in mouse models of Duchenne MD, laminin α2-related MD, and collagen VI-related myopathy were associated with inefficient mobilization of MuSCs after tissue damage. Using chemoinformatic analysis, we identified the 13-amino acid form of the peptide hormone apelin (AP-13) as a candidate for systemic stimulation of skeletal muscle ECs. Systemic administration of AP-13 using osmotic pumps generated a pro-proliferative EC-rich niche that supported MuSC function through angiocrine factors and markedly improved tissue regeneration and muscle strength in all three dystrophic mouse models. Moreover, EC-specific knockout of the apelin receptor led to regenerative defects that phenocopied key pathological features of MD, including vascular defects, fibrosis, muscle fiber necrosis, impaired MuSC function, and reduced force generation. Together, these studies provide in vivo proof of concept that enhancing endogenous skeletal muscle repair by targeting the vascular niche is a viable therapeutic avenue for MD and characterized AP-13 as a candidate for further study for the systemic treatment of MuSC dysfunction.

Mutational spectrum and phenotypic variability of Duchenne muscular dystrophy and related disorders in a Bangladeshi population

Duchenne muscular dystrophy (DMD) is a severe rare neuromuscular disorder caused by mutations in the X-linked dystrophin gene. Several mutations have been identified, yet the full mutational spectrum, and their phenotypic consequences, will require genotyping across different populations. To this end, we undertook the first detailed genotype and phenotype characterization of DMD in the Bangladeshi population. We investigated the rare mutational and phenotypic spectrum of the DMD gene in 36 DMD-suspected Bangladeshi participants using an economically affordable diagnostic strategy involving initial screening for exonic deletions in the DMD gene via multiplex PCR, followed by testing PCR-negative patients for mutations using whole exome sequencing. The deletion mapping identified two critical DMD gene hotspot regions (near proximal and distal ends, spanning exons 8-17 and exons 45-53, respectively) that comprised 95% (21/22) of the deletions for this population cohort. From our exome analysis, we detected two novel pathogenic hemizygous mutations in exons 21 and 42 of the DMD gene, and novel pathogenic recessive and loss of function variants in four additional genes: SGCD, DYSF, COL6A3, and DOK7. Our phenotypic analysis showed that DMD suspected participants presented diverse phenotypes according to the location of the mutation and which gene was impacted. Our study provides ethnicity specific new insights into both clinical and genetic aspects of DMD.

Features of Congenital Arthrogryposis Due to Abnormalities in Collagen Homeostasis, a Scoping Review

Congenital arthrogryposis (CA) refers to the presence of multiple contractures at birth. It is a feature of several inherited syndromes, notable amongst them are disorders of collagen formation. This review aims to characterize disorders that directly or indirectly impact collagen structure and function leading to CA in search for common phenotypic or pathophysiological features, possible genotype-phenotype correlation, and potential novel treatment approaches based on a better understanding of the underlying pathomechanism. Nine genes, corresponding to five clinical phenotypes, were identified after a literature search. The most notable trend was the extreme phenotype variability. Clinical features across all syndromes ranged from subtle with minimal congenital contractures, to severe with multiple congenital contractures and extra-articular features including skin, respiratory, or other manifestations. Five of the identified genes were involved in the function of the Lysyl Hydroxylase 2 or 3 enzymes, which enable the hydroxylation and/or glycosylation of lysyl residues to allow the formation of the collagen superstructure. Whilst current treatment approaches are post-natal surgical correction, there are also potential in-utero therapies being developed. Cyclosporin A showed promise in treating collagen VI disorders although there is an associated risk of immunosuppression. The treatments that could be in the clinical trials soon are the splice correction therapies in collagen VI-related disorders.

Collagen type VI regulates TGFβ bioavailability in skeletal muscle

Collagen VI-related disorders (COL6-RDs) are a group of rare muscular dystrophies caused by pathogenic variants in collagen VI genes (COL6A1, COL6A2, and COL6A3). Collagen type VI is a heterotrimeric, microfibrillar component of the muscle extracellular matrix (ECM), predominantly secreted by resident fibroadipogenic precursor cells in skeletal muscle. The absence or mislocalizatoion of collagen VI in the ECM underlies the non-cell autonomous dysfunction and dystrophic changes in skeletal muscle with an as of yet elusive direct mechanistic link between the ECM and myofiber dysfunction. Here, we conduct a comprehensive natural history and outcome study in a novel mouse model of COL6-RDs (Col6a2-/- mice) using standardized (Treat-NMD) functional, histological, and physiologic parameter. Notably, we identify a conspicuous dysregulation of the TGFβ pathway early in the disease process and propose that the collagen VI deficient matrix is not capable of regulating the dynamic TGFβ bioavailability at baseline and also in response to muscle injury. Thus, we propose a new mechanism for pathogenesis of the disease that links the ECM regulation of TGFβ with downstream skeletal muscle abnormalities, paving the way for developing and validating therapeutics that target this pathway.

Extracellular matrix: Brick and mortar in the skeletal muscle stem cell niche

The extracellular matrix (ECM) is an interconnected macromolecular scaffold occupying the space between cells. Amongst other functions, the ECM provides structural support to tissues and serves as a microenvironmental niche that conveys regulatory signals to cells. Cell-matrix adhesions, which link the ECM to the cytoskeleton, are dynamic multi-protein complexes containing surface receptors and intracellular effectors that control various downstream pathways. In skeletal muscle, the most abundant tissue of the body, each individual muscle fiber and its associated muscle stem cells (MuSCs) are surrounded by a layer of ECM referred to as the basal lamina. The core scaffold of the basal lamina consists of self-assembling polymeric laminins and a network of collagens that tether proteoglycans, which provide lateral crosslinking, establish collateral associations with cell surface receptors, and serve as a sink and reservoir for growth factors. Skeletal muscle also contains the fibrillar collagenous interstitial ECM that plays an important role in determining tissue elasticity, connects the basal laminae to each other, and contains matrix secreting mesenchymal fibroblast-like cell types and blood vessels. During skeletal muscle regeneration fibroblast-like cell populations expand and contribute to the transitional fibronectin-rich regenerative matrix that instructs angiogenesis and MuSC function. Here, we provide a comprehensive overview of the role of the skeletal muscle ECM in health and disease and outline its role in orchestrating tissue regeneration and MuSC function.

Autosomal dominant Ullrich congenital muscular dystrophy due to a de novo mutation in COL6A3 gene. A case report

Mutations in the genes encoding collagen VI cause Bethlem myopathy (MIM 158810), Ullrich congenital muscular dystrophy (MIM 254090), and myosclerosis myopathy (MIM #255600). BM is a dominantly inherited disorder, characterised by proximal muscle weakness and joint contractures mainly involving the elbows, ankles, and fingers, which usually follows a relatively mild course. By contrast, UCMD is a severe muscular dystrophy characterized by early onset, rapidly progressive muscle wasting and weakness, proximal joint contractures and distal joint hyperlaxity. Rapid progression usually leads to early death due to respiratory failure. UCMD is usually inherited as an autosomal recessive trait though dominant de novo heterozygous variants have recently been reported. We describe a further patient with UCMD classical presentation who showed, at the NGS analysis, the de novo variant c.6210+1G > A in the intron 16 of the gene COL6A3, known in the literature as pathogenic (VCV0000949S6.5).

Collagen misfolding mutations: the contribution of the unfolded protein response to the molecular pathology

Mutations in collagen genes cause a broad range of connective tissue pathologies. Structural mutations that impact procollagen assembly or triple helix formation and stability are a common and important mutation class. How misfolded procollagens engage with the cellular proteostasis machinery and whether they can elicit a cytotoxic unfolded protein response (UPR) is a topic of considerable research interest. Such interest is well justified since modulating the UPR could offer a new approach to treat collagenopathies for which there are no current disease mechanism-targeting therapies. This review scrutinizes the evidence underpinning the view that endoplasmic reticulum stress and chronic UPR activation contributes significantly to the pathophysiology of the collagenopathies. While there is strong evidence that the UPR contributes to the pathology for collagen X misfolding mutations, the evidence that misfolding mutations in other collagen types induce a canonical, cytotoxic UPR is incomplete. To gain a more comprehensive understanding about how the UPR amplifies to pathology, and thus what types of manipulations of the UPR might have therapeutic relevance, much more information is needed about how specific misfolding mutation types engage differentially with the UPR and downstream signaling responses. Most importantly, since the capacity of the proteostasis machinery to respond to collagen misfolding is likely to vary between cell types, reflecting their functional roles in collagen and extracellular matrix biosynthesis, detailed studies on the UPR should focus as much as possible on the actual target cells involved in the collagen pathologies.

Main and Minor Types of Collagens in the Articular Cartilage: The Role of Collagens in Repair Tissue Evaluation in Chondral Defects

Several collagen subtypes have been identified in hyaline articular cartilage. The main and most abundant collagens are type II, IX and XI collagens. The minor and less abundant collagens are type III, IV, V, VI, X, XII, XIV, XVI, XXII, and XXVII collagens. All these collagens have been found to play a key role in healthy cartilage, regardless of whether they are more or less abundant. Additionally, an exhaustive evaluation of collagen fibrils in a repaired cartilage tissue after a chondral lesion is necessary to determine the quality of the repaired tissue and even whether or not this repaired tissue is considered hyaline cartilage. Therefore, this review aims to describe in depth all the collagen types found in the normal articular cartilage structure, and based on this, establish the parameters that allow one to consider a repaired cartilage tissue as a hyaline cartilage.

Basic Structure, Physiology, and Biochemistry of Connective Tissues and Extracellular Matrix Collagens

The physiology of connective tissues like tendons and ligaments is highly dependent upon the collagens and other such extracellular matrix molecules hierarchically organized within the tissues. By dry weight, connective tissues are mostly composed of fibrillar collagens. However, several other forms of collagens play essential roles in the regulation of fibrillar collagen organization and assembly, in the establishment of basement membrane networks that provide support for vasculature for connective tissues, and in the formation of extensive filamentous networks that allow for cell-extracellular matrix interactions as well as maintain connective tissue integrity. The structures and functions of these collagens are discussed in this chapter. Furthermore, collagen synthesis is a multi-step process that includes gene transcription, translation, post-translational modifications within the cell, triple helix formation, extracellular secretion, extracellular modifications, and then fibril assembly, fibril modifications, and fiber formation. Each step of collagen synthesis and fibril assembly is highly dependent upon the biochemical structure of the collagen molecules created and how they are modified in the cases of development and maturation. Likewise, when the biochemical structures of collagens or are compromised or these molecules are deficient in the tissues - in developmental diseases, degenerative conditions, or injuries - then the ultimate form and function of the connective tissues are impaired. In this chapter, we also review how biochemistry plays a role in each of the processes involved in collagen synthesis and assembly, and we describe differences seen by anatomical location and region within tendons. Moreover, we discuss how the structures of the molecules, fibrils, and fibers contribute to connective tissue physiology in health, and in pathology with injury and repair.

Collagen-VI supplementation by cell transplantation improves muscle regeneration in Ullrich congenital muscular dystrophy model mice

Background

Mesenchymal stromal cells (MSCs) function as supportive cells on skeletal muscle homeostasis through several secretory factors including type 6 collagen (COL6). Several mutations of COL6A1, 2, and 3 genes cause Ullrich congenital muscular dystrophy (UCMD). Skeletal muscle regeneration deficiency has been reported as a characteristic phenotype in muscle biopsy samples of human UCMD patients and UCMD model mice. However, little is known about the COL6-dependent mechanism for the occurrence and progression of the deficiency. The purpose of this study was to clarify the pathological mechanism of UCMD by supplementing COL6 through cell transplantation.

Methods

To test whether COL6 supplementation has a therapeutic effect for UCMD, in vivo and in vitro experiments were conducted using four types of MSCs: (1) healthy donors derived-primary MSCs (pMSCs), (2) MSCs derived from healthy donor induced pluripotent stem cell (iMSCs), (3) COL6-knockout iMSCs (COL6KO-iMSCs), and (4) UCMD patient-derived iMSCs (UCMD-iMSCs).

Results

All four MSC types could engraft for at least 12 weeks when transplanted into the tibialis anterior muscles of immunodeficient UCMD model (Col6a1KO) mice. COL6 protein was restored by the MSC transplantation if the MSCs were not COL6-deficient (types 1 and 2). Moreover, muscle regeneration and maturation in Col6a1KO mice were promoted with the transplantation of the COL6-producing MSCs only in the region supplemented with COL6. Skeletal muscle satellite cells derived from UCMD model mice (Col6a1KO-MuSCs) co-cultured with type 1 or 2 MSCs showed improved proliferation, differentiation, and maturation, whereas those co-cultured with type 3 or 4 MSCs did not.

Conclusions

These findings indicate that COL6 supplementation improves muscle regeneration and maturation in UCMD model mice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02514-3.

Collagen VI as a driver and disease biomarker in human fibrosis

Fibrosis of visceral organs such as the lungs, heart, kidneys and liver remains a major cause of morbidity and mortality and is also associated with many other disorders, including cancer and metabolic disease. In this review, we focus upon the microfibrillar collagen VI, which is present in the extracellular matrix (ECM) of most tissues. However, expression is elevated in numerous fibrotic conditions, such as idiopathic pulmonary disease (IPF), and chronic liver and kidney diseases. Collagen VI is composed of three subunits α1, α2 and α3, which can be replaced with alternate chains of α4, α5 or α6. The C-terminal globular domain (C5) of collagen VI α3 can be proteolytically cleaved to form a biologically active fragment termed endotrophin, which has been shown to actively drive fibrosis, inflammation and insulin resistance. Tissue biopsies have long been considered the gold standard for diagnosis and monitoring of progression of fibrotic disease. The identification of neoantigens from enzymatically processed collagen chains have revolutionised the biomarker field, allowing rapid diagnosis and evaluation of prognosis of numerous fibrotic conditions, as well as providing valuable clinical trial endpoint determinants. Collagen VI chain fragments such as endotrophin (PRO-C6), C6M and C6Mα3 are emerging as important biomarkers for fibrotic conditions.

Tendon Extracellular Matrix Remodeling and Defective Cell Polarization in the Presence of Collagen VI Mutations

Mutations in collagen VI genes cause two major clinical myopathies, Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD), and the rarer myosclerosis myopathy. In addition to congenital muscle weakness, patients affected by collagen VI-related myopathies show axial and proximal joint contractures, and distal joint hypermobility, which suggest the involvement of tendon function. To gain further insight into the role of collagen VI in human tendon structure and function, we performed ultrastructural, biochemical, and RT-PCR analysis on tendon biopsies and on cell cultures derived from two patients affected with BM and UCMD. In vitro studies revealed striking alterations in the collagen VI network, associated with disruption of the collagen VI-NG2 (Collagen VI-neural/glial antigen 2) axis and defects in cell polarization and migration. The organization of extracellular matrix (ECM) components, as regards collagens I and XII, was also affected, along with an increase in the active form of metalloproteinase 2 (MMP2). In agreement with the in vitro alterations, tendon biopsies from collagen VI-related myopathy patients displayed striking changes in collagen fibril morphology and cell death. These data point to a critical role of collagen VI in tendon matrix organization and cell behavior. The remodeling of the tendon matrix may contribute to the muscle dysfunction observed in BM and UCMD patients.

Basement membrane collagens and disease mechanisms

Basement membranes (BMs) are specialised extracellular matrix (ECM) structures and collagens are a key component required for BM function. While collagen IV is the major BM collagen, collagens VI, VII, XV, XVII and XVIII are also present. Mutations in these collagens cause rare multi-systemic diseases but these collagens have also been associated with major common diseases including stroke. Developing treatments for these conditions will require a collective effort to increase our fundamental understanding of the biology of these collagens and the mechanisms by which mutations therein cause disease. Novel insights into pathomolecular disease mechanisms and cellular responses to these mutations has been exploited to develop proof-of-concept treatment strategies in animal models. Combined, these studies have also highlighted the complexity of the disease mechanisms and the need to obtain a more complete understanding of these mechanisms. The identification of pathomolecular mechanisms of collagen mutations shared between different disorders represent an attractive prospect for treatments that may be effective across phenotypically distinct disorders.

C-terminal proteolysis of the collagen VI α3 chain by BMP-1 and proprotein convertase(s) releases endotrophin in fragments of different sizes

The assembly of collagen VI microfibrils is a multistep process in which proteolytic processing within the C-terminal globular region of the collagen VI α3 chain plays a major role. However, the mechanisms involved remain elusive. Moreover, C5, the short and most C-terminal domain of the α3 chain, recently has been proposed to be released as an adipokine that enhances tumor progression, fibrosis, inflammation, and insulin resistance and has been named "endotrophin." Serum endotrophin could be a useful biomarker to monitor the progression of such disorders as chronic obstructive pulmonary disease, systemic sclerosis, and kidney diseases. Here, using biochemical and isotopic MS-based analyses, we found that the extracellular metalloproteinase bone morphogenetic protein 1 (BMP-1) is involved in endotrophin release and determined the exact BMP-1 cleavage site. Moreover, we provide evidence that several endotrophin-containing fragments are present in various tissues and body fluids. Among these, a large C2-C5 fragment, which contained endotrophin, was released by furin-like proprotein convertase cleavage. By using immunofluorescence microscopy and EM, we also demonstrate that these proteolytic maturations occur after secretion of collagen VI tetramers and during microfibril assembly. Differential localization of N- and C-terminal regions of the collagen VI α3 chain revealed that cleavage products are deposited in tissue and cell cultures. The detailed information on the processing of the collagen VI α3 chain reported here provides a basis for unraveling the function of endotrophin (C5) and larger endotrophin-containing fragments and for refining their use as biomarkers of disease progression.

Overexpression of miR-29 Leads to Myopathy that Resemble Pathology of Ullrich Congenital Muscular Dystrophy

Ullrich congenital muscular dystrophy (UCMD) bring heavy burden to patients’ families and society. Because the incidence of this disease is very low, studies in patients are extremely limited. Animal models of this disease are indispensable. UCMD belongs to extracellular matrix-related diseases. However, the disease models constructed by knocking out some pathogenic genes of human, such as the Col6a1, Col6a2, or Col6a3 gene, of mice could not mimic UCMD. The purpose of this study is to construct a mouse model which can resemble the pathology of UCMD. miR-29 is closely related to extracellular matrix deposition of tissues and organs. To address this issue, we developed a mouse model for overexpression miR-29 using Tet-on system. In the muscle-specific miR-29ab1 cluster transgenic mice model, we found that mice exhibited dyskinesia, dyspnea, and spinal anomaly. The skeletal muscle was damaged and regenerated. At the same time, we clarify the molecular mechanism of the role of miR-29 in this process. Different from human, Col4a1 and Col4a2, target genes of miR-29, are the key pathogenic genes associating with these phenotypes. This mouse model simulates the human clinical and pathological characteristics of UCMD patients and is helpful for the subsequent research and treatment of UCMD.

Molecular characterization of physis tissue by RNA sequencing

The physis is a well-established and anatomically distinct cartilaginous structure that is crucial for normal long-bone development and growth. Abnormalities in physis function are linked to growth plate disorders and other pediatric musculoskeletal diseases. Understanding the molecular pathways operative in the physis may permit development of regenerative therapies to complement surgically-based procedures that are the current standard of care for growth plate disorders. Here, we performed next generation RNA sequencing on mRNA isolated from human physis and other skeletal tissues (e.g., articular cartilage and bone; n = 7 for each tissue). We observed statistically significant enrichment of gene sets in the physis when compared to the other musculoskeletal tissues. Further analysis of these upregulated genes identified physis-specific networks of extracellular matrix proteins including collagens (COL2A1, COL6A1, COL9A1, COL14A1, COL16A1) and matrilins (MATN1, MATN2, MATN3), and signaling proteins in the WNT pathway (WNT10B, FZD1, FZD10, DKK2) or the FGF pathway (FGF10, FGFR4). Our results provide further insight into the gene expression networks that contribute to the physis' unique structural composition and regulatory signaling networks. Physis-specific expression profiles may guide ongoing initiatives in tissue engineering and cell-based therapies for treatment of growth plate disorders and growth modulation therapies. Furthermore, our findings provide new leads for therapeutic drug discovery that would permit future intervention through pharmacological rather than surgical strategies.

Extracellular matrix-driven congenital muscular dystrophies

Skeletal muscle function relies on the myofibrillar apparatus inside myofibers as well as an intact extracellular matrix surrounding each myofiber. Muscle extracellular matrix (ECM) plays several roles including but not limited to force transmission, regulation of growth factors and inflammatory responses, and influencing muscle stem cell (i.e. satellite cell) proliferation and differentiation. In most myopathies, muscle ECM undergoes remodeling and fibrotic changes that may be maladaptive for normal muscle function and recovery. In addition, mutations in skeletal muscle ECM and basement proteins can cause muscle disease. In this review, we summarize the clinical features of two of the most common congenital muscular dystrophies, COL6-related dystrophies and LAMA2-related dystrophies, which are caused by mutations in muscle ECM and basement membrane proteins. The study of clinical features of these diseases has helped to inform basic research and understanding of the biology of muscle ECM. In return, basic studies of muscle ECM have provided the conceptual framework to develop therapeutic interventions for these and other similar disorders of muscle.

Collagen VI in healthy and diseased nervous system

Collagen VI is a major extracellular matrix protein exerting a number of functions in different tissues, spanning from biomechanical to regulatory signals in the cell survival processes, and playing key roles in maintaining the stemness or determining the differentiation of several types of cells. In the last couple of years, emerging findings on collagen VI have led to increased interest in its role in the nervous system. The role of this protein in the peripheral nervous system was intensely studied and characterized in detail. Collagen VI acts as a regulator of Schwann cell differentiation and is required for preserving peripheral nerve myelination, function and structure, as well as for orchestrating nerve regeneration after injury. Although the role and distribution of collagen VI in the peripheral nervous system is now well established, the role of this distinctive extracellular matrix component in the central nervous system, along with its links to human neurological and neurodegenerative disorders, remains an open field of investigation. In this Review, we summarize and discuss a number of recent findings related to collagen VI in the central and peripheral nervous systems. We further link these findings to different aspects of the protein that are relevant to human diseases in these compartments in order to provide a comprehensive overview of the roles of this key matrix component in the nervous system.

Collagen VI disorders: Insights on form and function in the extracellular matrix and beyond

Mutations in the three canonical collagen VI genes, COL6A1, COL6A2 and COL6A3, cause a spectrum of muscle disease from Bethlem myopathy at the mild end to the severe Ullrich congenital muscular dystrophy. Mutations can be either dominant or recessive and the resulting clinical severity is influenced by the way mutations impact the complex collagen VI assembly process. Most mutations are found towards the N-terminus of the triple helical collagenous domain and compromise extracellular microfibril assembly. Outside the triple helix collagen VI is highly polymorphic and discriminating mutations from rare benign changes remains a major diagnostic challenge. Collagen VI deficiency alters extracellular matrix structure and biomechanical properties and leads to increased apoptosis and oxidative stress, decreased autophagy, and impaired muscle regeneration. Therapies that target these downstream consequences have been tested in a collagen VI null mouse and also in small human trials where they show modest clinical efficacy. An important role for collagen VI in obesity, cancer and diabetes is emerging. A major barrier to developing effective therapies is the paucity of information about how collagen VI deficiency in the extracellular matrix signals the final downstream consequences - the receptors involved and the intracellular messengers await further characterization.

Comparison of sitting and supine forced vital capacity in collagen VI-related dystrophy and laminin α2-related dystrophy

BACKGROUND

Progressive, restrictive, respiratory insufficiency is the major cause of morbidity and mortality in Congenital Muscular Dystrophy (CMD). Nocturnal hypoventilation precedes daytime alveolar hypoventilation, and if untreated, may lead to respiratory failure and cor pulmonale. CMD consensus care guidelines recommend screening for respiratory insufficiency by conventional and dynamic (sitting to supine) pulmonary function testing (PFT) and evaluating for sleep disordered breathing if there is more than 20% relative reduction from sitting to supine FVC(L) (ΔFVC).

OBJECTIVE

The objective of this retrospective study was to explore and characterize dynamic FVC measures in 51 individuals with two common subtypes of CMD, COL6-RD, and LAMA2-RD.

METHODS

We compared sitting and supine FVC in patients with confirmed mutation(s) in either COL6 or LAMA2. We investigated influences of age, CMD subtype, gender, race, ambulatory status, and non-invasive positive pressure ventilation (NIPPV) status on FVC percent predicted (FVCpp) and ΔFVC.

RESULTS

COL6-RD participants exhibited a significant difference between sitting and supine mean FVCpp (sitting 66.1, supine 55.1; P < 0.0001) and were 5.4 times more likely to have -ΔFVC >20% than those with LAMA2-RD when controlling for ambulant status. FVCpp sitting correlated inversely with age in individuals ≤18 years.

CONCLUSION

FVCpp sitting decreases progressively in childhood in both CMD subtypes. However, our results point to a difference in diaphragmatic involvement, with COL6-RD individuals having more disproportionate diaphragmatic weakness than LAMA2-RD. A ΔFVC of greater than -20% should continue to be used to prompt evaluation of sleep-disordered breathing. Timely initiation of NIPPV may be indicated to treat nocturnal hypoventilation. Pediatr Pulmonol. 2017;52:524-532. © 2017 Wiley Periodicals, Inc.

Non-myogenic Contribution to Muscle Development and Homeostasis: The Role of Connective Tissues

Skeletal muscles belong to the musculoskeletal system, which is composed of bone, tendon, ligament and irregular connective tissue, and closely associated with motor nerves and blood vessels. The intrinsic molecular signals regulating myogenesis have been extensively investigated. However, muscle development, homeostasis and regeneration require interactions with surrounding tissues and the cellular and molecular aspects of this dialogue have not been completely elucidated. During development and adult life, myogenic cells are closely associated with the different types of connective tissue. Connective tissues are defined as specialized (bone and cartilage), dense regular (tendon and ligament) and dense irregular connective tissue. The role of connective tissue in muscle morphogenesis has been investigated, thanks to the identification of transcription factors that characterize the different types of connective tissues. Here, we review the development of the various connective tissues in the context of the musculoskeletal system and highlight their important role in delivering information necessary for correct muscle morphogenesis, from the early step of myoblast differentiation to the late stage of muscle maturation. Interactions between muscle and connective tissue are also critical in the adult during muscle regeneration, as impairment of the regenerative potential after injury or in neuromuscular diseases results in the progressive replacement of the muscle mass by fibrotic tissue. We conclude that bi-directional communication between muscle and connective tissue is critical for a correct assembly of the musculoskeletal system during development as well as to maintain its homeostasis in the adult.

Tendon Extracellular Matrix Alterations in Ullrich Congenital Muscular Dystrophy

Collagen VI (COLVI) is a non-fibrillar collagen expressed in skeletal muscle and most connective tissues. Mutations in COLVI genes cause two major clinical forms, Bethlem myopathy and Ullrich congenital muscular dystrophy (UCMD). In addition to congenital muscle weakness, patients affected by COLVI myopathies show axial and proximal joint contractures and distal joint hypermobility, which suggest the involvement of the tendon function. We examined a peroneal tendon biopsy and tenocyte culture of a 15-year-old patient affected by UCMD with compound heterozygous COL6A2 mutations. In patient’s tendon biopsy, we found striking morphological alterations of tendon fibrils, consisting in irregular profiles and reduced mean diameter. The organization of the pericellular matrix of tenocytes, the primary site of collagen fibril assembly, was severely affected, as determined by immunoelectron microscopy, which showed an abnormal accumulation of COLVI and altered distribution of collagen I (COLI) and fibronectin (FBN). In patient’s tenocyte culture, COLVI web formation and cell surface association were severely impaired; large aggregates of COLVI, which matched with COLI labeling, were frequently detected in the extracellular matrix. In addition, metalloproteinase MMP-2, an extracellular matrix-regulating enzyme, was increased in the conditioned medium of patient’s tenocytes, as determined by gelatin zymography and western blot. Altogether, these data indicate that COLVI deficiency may influence the organization of UCMD tendon matrix, resulting in dysfunctional fibrillogenesis. The alterations of tendon matrix may contribute to the complex pathogenesis of COLVI related myopathies.

Collagen VI deficiency reduces muscle pathology, but does not improve muscle function, in the γ-sarcoglycan-null mouse

Muscular dystrophy is characterized by progressive skeletal muscle weakness and dystrophic muscle exhibits degeneration and regeneration of muscle cells, inflammation and fibrosis. Skeletal muscle fibrosis is an excessive deposition of components of the extracellular matrix including an accumulation of Collagen VI. We hypothesized that a reduction of Collagen VI in a muscular dystrophy model that presents with fibrosis would result in reduced muscle pathology and improved muscle function. To test this hypothesis, we crossed γ-sarcoglycan-null mice, a model of limb-girdle muscular dystrophy type 2C, with a Col6a2-deficient mouse model. We found that the resulting γ-sarcoglycan-null/Col6a2Δex5 mice indeed exhibit reduced muscle pathology compared with γ-sarcoglycan-null mice. Specifically, fewer muscle fibers are degenerating, fiber size varies less, Evans blue dye uptake is reduced and serum creatine kinase levels are lower. Surprisingly, in spite of this reduction in muscle pathology, muscle function is not significantly improved. In fact, grip strength and maximum isometric tetanic force are even lower in γ-sarcoglycan-null/Col6a2Δex5 mice than in γ-sarcoglycan-null mice. In conclusion, our results reveal that Collagen VI-mediated fibrosis contributes to skeletal muscle pathology in γ-sarcoglycan-null mice. Importantly, however, our data also demonstrate that a reduction in skeletal muscle pathology does not necessarily lead to an improvement of skeletal muscle function, and this should be considered in future translational studies.

A collagen VI-dependent pathogenic mechanism for Hirschsprung's disease

Hirschsprung's disease (HSCR) is a severe congenital anomaly of the enteric nervous system (ENS) characterized by functional intestinal obstruction due to a lack of intrinsic innervation in the distal bowel. Distal innervation deficiency results from incomplete colonization of the bowel by enteric neural crest cells (eNCCs), the ENS precursors. Here, we report the generation of a mouse model for HSCR--named Holstein--that contains an untargeted transgenic insertion upstream of the collagen-6α4 (Col6a4) gene. This insertion induces eNCC-specific upregulation of Col6a4 expression that increases total collagen VI protein levels in the extracellular matrix (ECM) surrounding both the developing and the postnatal ENS. Increased collagen VI levels during development mainly result in slower migration of eNCCs. This appears to be due to the fact that collagen VI is a poor substratum for supporting eNCC migration and can even interfere with the migration-promoting effects of fibronectin. Importantly, for a majority of patients in a HSCR cohort, the myenteric ganglia from the ganglionated region are also specifically surrounded by abundant collagen VI microfibrils, an outcome accentuated by Down syndrome. Collectively, our data thus unveil a clinically relevant pathogenic mechanism for HSCR that involves cell-autonomous changes in ECM composition surrounding eNCCs. Moreover, as COL6A1 and COL6A2 are on human Chr.21q, this mechanism is highly relevant to the predisposition of patients with Down syndrome to HSCR.

A TALEN-Exon Skipping Design for a Bethlem Myopathy Model in Zebrafish

Presently, human collagen VI-related diseases such as Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM) remain incurable, emphasizing the need to unravel their etiology and improve their treatments. In UCMD, symptom onset occurs early, and both diseases aggravate with ageing. In zebrafish fry, morpholinos reproduced early UCMD and BM symptoms but did not allow to study the late phenotype. Here, we produced the first zebrafish line with the human mutation frequently found in collagen VI-related disorders such as UCMD and BM. We used a transcription activator-like effector nuclease (TALEN) to design the col6a1^ama605003-line with a mutation within an essential splice donor site, in intron 14 of the col6a1 gene, which provoke an in-frame skipping of exon 14 in the processed mRNA. This mutation at a splice donor site is the first example of a template-independent modification of splicing induced in zebrafish using a targetable nuclease. This technique is readily expandable to other organisms and can be instrumental in other disease studies. Histological and ultrastructural analyzes of homozygous and heterozygous mutant fry and 3 months post-fertilization (mpf) fish revealed co-dominantly inherited abnormal myofibers with disorganized myofibrils, enlarged sarcoplasmic reticulum, altered mitochondria and misaligned sarcomeres. Locomotion analyzes showed hypoxia-response behavior in 9 mpf col6a1 mutant unseen in 3 mpf fish. These symptoms worsened with ageing as described in patients with collagen VI deficiency. Thus, the col6a1^ama605003-line is the first adult zebrafish model of collagen VI-related diseases; it will be instrumental both for basic research and drug discovery assays focusing on this type of disorders.

Defective collagen VI α6 chain expression in the skeletal muscle of patients with collagen VI-related myopathies

Collagen VI is a non-fibrillar collagen present in the extracellular matrix (ECM) as a complex polymer; the mainly expressed form is composed of α1, α2 and α3 chains; mutations in genes encoding these chains cause myopathies known as Ullrich congenital muscular dystrophy (UCMD), Bethlem myopathy (BM) and myosclerosis myopathy (MM). The collagen VI α6 chain is a recently identified component of the ECM of the human skeletal muscle. Here we report that the α6 chain was dramatically reduced in skeletal muscle and muscle cell cultures of genetically characterized UCMD, BM and MM patients, independently of the clinical phenotype, the gene involved and the effect of the mutation on the expression of the “classical” α1α2α3 heterotrimer. By contrast, the collagen VI α6 chain was normally expressed or increased in the muscle of patients affected by other forms of muscular dystrophy, the overexpression matching with areas of increased fibrosis. In vitro treatment with TGF-β1, a potent collagen inducer, promoted the collagen VI α6 chain deposition in the ECM of normal muscle cells, whereas, in cultures derived from collagen VI-related myopathy patients, the collagen VI α6 chain failed to develop a network outside the cells and accumulated in the endoplasmic reticulum. The defect of the α6 chain points to a contribution to the pathogenesis of collagen VI-related disorders.

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Collagen VI is an ECM component of the human skeletal muscle.

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We evaluated the α6 chain in collagen VI-related and other muscular dystrophies.

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The α6 chain was reduced in collagen VI-related diseases but not in other myopathies.

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A correlation between the α6 chain and fibrosis was demonstrated in MDC1A.

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The α6 chain is involved in the pathogenesis of collagen VI diseases and fibrosis.