Creation and characterization of novel rat model for recessive dystrophic epidermolysis bullosa: Frameshift mutation of the Col7a1 gene leads to severe blistered phenotype

Recessive dystrophic epidermolysis bullosa is a rare genodermatosis caused by a mutation of the Col7a1 gene. The Col7a1 gene codes for collagen type VII protein, a major component of anchoring fibrils. Mutations of the Col7a1 gene can cause aberrant collagen type VII formation, causing an associated lack or absence of anchoring fibrils. This presents clinically as chronic blistering, scarring, and fibrosis, often leading to the development of cutaneous squamous cell carcinoma. Patients also experience persistent pain and pruritus. Pain management and supportive bandaging remain the primary treatment options. The pathology of recessive dystrophic epidermolysis bullosa was first described in the 1980s, and there has since been a multitude of encouraging treatment options developed. However, in vivo research has been hindered by inadequate models of the disease. The various mouse models in existence possess longevity and surface area constraints, or do not adequately model a normal human disease state. In this paper, we describe a novel rat model of recessive dystrophic epidermolysis bullosa that offers an alternative to previous murine models. An 8-base pair deletion was induced in the Col7a1 gene of Lewis rats, which was subsequently found to cause a premature stop codon downstream. Homozygous mutants presented with a fragile and chronically blistered phenotype postnatally. Further histological analysis revealed subepidermal clefting and the absence of anchoring fibrils. The generation of this novel model offers researchers an easily maintained organism that possesses a larger surface area for experimental topical and transfused therapies to be tested, which may provide great utility in the future study of this debilitating disease.

Characterization of TGFβ1-induced tendon-like structure in the scaffold-free three-dimensional tendon cell culture system

The biological mechanisms regulating tenocyte differentiation and morphological maturation have not been well-established, partly due to the lack of reliable in vitro systems that produce highly aligned collagenous tissues. In this study, we developed a scaffold-free, three-dimensional (3D) tendon culture system using mouse tendon cells in a differentially adherent growth channel. Transforming Growth Factor-β (TGFβ) signaling is involved in various biological processes in the tendon, regulating tendon cell fate, recruitment and maintenance of tenocytes, and matrix organization. This known function of TGFβ signaling in tendon prompted us to utilize TGFβ1 to induce tendon-like structures in 3D tendon constructs. TGFβ1 treatment promoted a tendon-like structure in the peripheral layer of the constructs characterized by increased thickness with a gradual decrease in cell density and highly aligned collagen matrix. TGFβ1 also enhanced cell proliferation, matrix production, and morphological maturation of cells in the peripheral layer compared to vehicle treatment. TGFβ1 treatment also induced early tenogenic differentiation and resulted in sufficient mechanical integrity, allowing biomechanical testing. The current study suggests that this scaffold-free 3D tendon cell culture system could be an in vitro platform to investigate underlying biological mechanisms that regulate tenogenic cell differentiation and matrix organization.

Unraveling the role of TGFβ signaling in thoracic aortic aneurysm and dissection using Fbn1 mutant mouse models

Although abnormal TGFβ signaling is observed in several heritable forms of thoracic aortic aneurysms and dissections including Marfan syndrome, its precise role in aortic disease progression is still disputed. Using a mouse genetic approach and quantitative isobaric labeling proteomics, we sought to elucidate the role of TGFβ signaling in three Fbn1 mutant mouse models representing a range of aortic disease from microdissection (without aneurysm) to aneurysm (without rupture) to aneurysm and rupture. Results indicated that reduced TGFβ signaling and increased mast cell proteases were associated with microdissection. In contrast, increased abundance of extracellular matrix proteins, which could be reporters for positive TGFβ signaling, were associated with aneurysm. Marked reductions in collagens and fibrillins, and increased TGFβ signaling, were associated with aortic rupture. Our data indicate that TGFβ signaling performs context-dependent roles in the pathogenesis of thoracic aortic disease.

Lysyl hydroxylase 3-mediated post-translational modifications are required for proper biosynthesis of collagen α1α1α2(IV)

Collagens are the most abundant proteins in the body and among the most biosynthetically complex. A molecular ensemble of over 20 endoplasmic reticulum resident proteins participates in collagen biosynthesis and contributes to heterogeneous post-translational modifications. Pathogenic variants in genes encoding collagens cause connective tissue disorders, including osteogenesis imperfecta, Ehlers-Danlos syndrome, and Gould syndrome (caused by mutations in COL4A1 and COL4A2), and pathogenic variants in genes encoding proteins required for collagen biosynthesis can cause similar but overlapping clinical phenotypes. Notably, pathogenic variants in lysyl hydroxylase 3 (LH3) cause a multisystem connective tissue disorder that exhibits pathophysiological features of collagen-related disorders. LH3 is a multifunctional collagen-modifying enzyme; however, its precise role(s) and substrate specificity during collagen biosynthesis has not been defined. To address this critical gap in knowledge, we generated LH3 KO cells and performed detailed quantitative and molecular analyses of collagen substrates. We found that LH3 deficiency severely impaired secretion of collagen α1α1α2(IV) but not collagens α1α1α2(I) or α1α1α1(III). Amino acid analysis revealed that LH3 is a selective LH for collagen α1α1α2(IV) but a general glucosyltransferase for collagens α1α1α2(IV), α1α1α2(I), and α1α1α1(III). Importantly, we identified rare variants that are predicted to be pathogenic in the gene encoding LH3 in two of 113 fetuses with intracranial hemorrhage-a cardinal feature of Gould syndrome. Collectively, our findings highlight a critical role of LH3 in α1α1α2(IV) biosynthesis and suggest that LH3 pathogenic variants might contribute to Gould syndrome.

Elevated TGFβ signaling contributes to ocular anterior segment dysgenesis in Col4a1 mutant mice

Ocular anterior segment dysgenesis (ASD) refers to a collection of developmental disorders affecting the anterior structures of the eye. Although a number of genes have been implicated in the etiology of ASD, the underlying pathogenetic mechanisms remain unclear. Mutations in genes encoding collagen type IV alpha 1 (COL4A1) and alpha 2 (COL4A2) cause Gould syndrome, a multi-system disorder that often includes ocular manifestations such as ASD and glaucoma. COL4A1 and COL4A2 are abundant basement membrane proteins that provide structural support to tissues and modulate signaling through interactions with other extracellular matrix proteins, growth factors, and cell surface receptors. In this study, we used a combination of histological, molecular, genetic and pharmacological approaches to demonstrate that altered TGFβ signaling contributes to ASD in mouse models of Gould syndrome. We show that TGFβ signaling was elevated in anterior segments from Col4a1 mutant mice and that genetically reducing TGFβ signaling partially prevented ASD. Notably, we identified distinct roles for TGFβ1 and TGFβ2 in ocular defects observed in Col4a1 mutant mice. Importantly, we show that pharmacologically promoting type IV collagen secretion or reducing TGFβ signaling ameliorated ocular pathology in Col4a1 mutant mice. Overall, our findings demonstrate that altered TGFβ signaling contributes to COL4A1-related ocular dysgenesis and implicate this pathway as a potential therapeutic target for the treatment of Gould syndrome.

A new murine model of Barth syndrome neutropenia links TAFAZZIN deficiency to increased ER stress-induced apoptosis

Barth syndrome is an inherited X-linked disorder that leads to cardiomyopathy, skeletal myopathy, and neutropenia. These symptoms result from the loss of function of the enzyme TAFAZZIN, a transacylase located in the inner mitochondrial membrane that is responsible for the final steps of cardiolipin production. The link between defective cardiolipin maturation and neutropenia remains unclear. To address potential mechanisms of neutropenia, we examined myeloid progenitor development within the fetal liver of TAFAZZIN knockout (KO) animals as well as within the adult bone marrow of wild-type recipients transplanted with TAFAZZIN-KO hematopoietic stem cells. We also used the ER-Hoxb8 system (estrogen receptor fused to Hoxb8) of conditional immortalization to establish a new murine model system for the ex vivo study of TAFAZZIN-deficient neutrophils. The TAFAZZIN-KO cells demonstrated the expected dramatic differences in cardiolipin maturation that result from a lack of TAFAZZIN enzyme activity. Contrary to our hypothesis, we did not identify any significant differences in neutrophil development or neutrophil function across a variety of assays including phagocytosis and the production of cytokines or reactive oxygen species. However, transcriptomic analysis of the TAFAZZIN-deficient neutrophil progenitors demonstrated an upregulation of markers of endoplasmic reticulum stress and confirmatory testing demonstrated that the TAFAZZIN-deficient cells had increased sensitivity to certain ER stress-mediated and non-ER stress-mediated triggers of apoptosis. Although the link between increased sensitivity to apoptosis and the variably penetrant neutropenia phenotype seen in some patients with Barth syndrome remains to be clarified, our studies and new model system set a foundation for further investigation.

In vivo topical gene therapy for recessive dystrophic epidermolysis bullosa: a phase 1 and 2 trial

Recessive dystrophic epidermolysis bullosa (RDEB) is a lifelong genodermatosis associated with blistering, wounding, and scarring caused by mutations in COL7A1, the gene encoding the anchoring fibril component, collagen VII (C7). Here, we evaluated beremagene geperpavec (B-VEC), an engineered, non-replicating COL7A1 containing herpes simplex virus type 1 (HSV-1) vector, to treat RDEB skin. B-VEC restored C7 expression in RDEB keratinocytes, fibroblasts, RDEB mice and human RDEB xenografts. Subsequently, a randomized, placebo-controlled, phase 1 and 2 clinical trial (NCT03536143) evaluated matched wounds from nine RDEB patients receiving topical B-VEC or placebo repeatedly over 12 weeks. No grade 2 or above B-VEC-related adverse events or vector shedding or tissue-bound skin immunoreactants were noted. HSV-1 and C7 antibodies sometimes presented at baseline or increased after B-VEC treatment without an apparent impact on safety or efficacy. Primary and secondary objectives of C7 expression, anchoring fibril assembly, wound surface area reduction, duration of wound closure, and time to wound closure following B-VEC treatment were met. A patient-reported pain-severity secondary outcome was not assessed given the small proportion of wounds treated. A global assessment secondary endpoint was not pursued due to redundancy with regard to other endpoints. These studies show that B-VEC is an easily administered, safely tolerated, topical molecular corrective therapy promoting wound healing in patients with RDEB.

Identification of Missense Extracellular Matrix Gene Variants in a Large Glaucoma Pedigree and Investigation of the N700S Thrombospondin-1 Variant in Normal and Glaucomatous Trabecular Meshwork Cells

PURPOSE

Primary open-angle glaucoma (POAG) is a complex heterogeneous disease. While several POAG genes have been identified, a high proportion of estimated heritability remains unexplained. Elevated intraocular pressure (IOP) is a leading POAG risk factor and dysfunctional extracellular matrix (ECM) in the trabecular meshwork (TM) contributes to elevated IOP. In this study, we sought to identify missense variants in ECM genes that correlate with ocular hypertensive POAG.

METHODS

Whole-genome sequencing was used to identify genetic variants in five members of a large POAG family (n = 68) with elevated IOP. The remaining family members were screened by Sanger sequencing. Unrelated normal (NTM) and glaucomatous (GTM) cells were sequenced for the identified variants. The ECM protein levels were determined by Western immunoblotting and confocal and electron microscopy investigated ECM ultrastructural organization.

RESULTS

Three ECM gene variants were significantly associated with POAG or elevated IOP in a large POAG pedigree. These included rs2228262 (N700S; thrombospondin-1 (THBS1, TSP1)), rs112913396 (D563 G; collagen type VI, alpha 3 (COL6A3)) and rs34759087 (E987K; laminin subunit beta 2 (LAMB2)). Screening of unrelated TM cells (n = 27) showed higher prevalence of the THBS1 variant but not the LAMB2 variant, in GTM cells (39%) than NTM cells (11%). The rare COL6A3 variant was not detected. TSP1 protein was upregulated and COL6A3 was down-regulated in TM cells with N700S subject to mechanical stretch, an in vitro method that mimics elevated IOP. Immunofluorescence showed increased TSP1 immunostaining in cell strains with N700S compared to wild-type TM cells. Ultrastructural studies showed ECM disorganization and altered collagen type VI distribution in GTM versus NTM cells.

CONCLUSIONS

Our results suggest that missense variants in ECM genes may not cause catastrophic changes to the TM, but over many years, subtle changes in ECM may accumulate and cause structural disorganization of the outflow resistance leading to elevated IOP in POAG patients.

Cell autonomous TGFβ signaling is essential for stem/progenitor cell recruitment into degenerative tendons

Understanding cell recruitment in damaged tendons is critical for improvements in regenerative therapy. We recently reported that targeted disruption of transforming growth factor beta (TGFβ) type II receptor in the tendon cell lineage (Tgfbr2^ScxCre) resulted in resident tenocyte dedifferentiation and tendon deterioration in postnatal stages. Here we extend the analysis and identify direct recruitment of stem/progenitor cells into the degenerative mutant tendons. Cre-mediated lineage tracing indicates that these cells are not derived from tendon-ensheathing tissues or from a Scleraxis-expressing lineage, and they turned on tendon markers only upon entering the mutant tendons. Through immunohistochemistry and inducible gene deletion, we further find that the recruited cells originated from a Sox9-expressing lineage and their recruitment was dependent on cell autonomous TGFβ signaling. The cells identified in this study thus differ from previous reports of cell recruitment into injured tendons and suggest a critical role for TGFβ signaling in cell recruitment, providing insights that may support improvements in tendon repair.

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Targeted deletion of TGFβ signaling led to degenerative changes in mouse tendons

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Stem/progenitor cells were recruited into the degenerative mutant tendons

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The recruited cells are different from the ones so far reported in tendon injury

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Recruitment was dependent on cell autonomous TGFβ signaling in the recruited cells

Tendon and motor phenotypes in the Crtap-/- mouse model of recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by short stature, skeletal deformities, low bone mass, and motor deficits. A subset of OI patients also present with joint hypermobility; however, the role of tendon dysfunction in OI pathogenesis is largely unknown. Using the Crtap-/- mouse model of severe, recessive OI, we found that mutant Achilles and patellar tendons were thinner and weaker with increased collagen cross-links and reduced collagen fibril size at 1- and 4-months compared to wildtype. Patellar tendons from Crtap-/- mice also had altered numbers of CD146+CD200+ and CD146-CD200+ progenitor-like cells at skeletal maturity. RNA-seq analysis of Achilles and patellar tendons from 1-month Crtap-/- mice revealed dysregulation in matrix and tendon marker gene expression concomitant with predicted alterations in TGF-β, inflammatory, and metabolic signaling. At 4-months, Crtap-/- mice showed increased αSMA, MMP2, and phospho-NFκB staining in the patellar tendon consistent with excess matrix remodeling and tissue inflammation. Finally, a series of behavioral tests showed severe motor impairments and reduced grip strength in 4-month Crtap-/- mice - a phenotype that correlates with the tendon pathology.

Ezh2 Is Essential for Patterning of Multiple Musculoskeletal Tissues but Dispensable for Tendon Differentiation

An efficient musculoskeletal system depends on the precise assembly and coordinated growth and function of muscles, skeleton, and tendons. However, the mechanisms that drive integrated musculoskeletal development and coordinated growth and differentiation of each of these tissues are still being uncovered. Epigenetic modifiers have emerged as critical regulators of cell fate differentiation, but so far almost nothing is known about their roles in tendon biology. Previous studies have shown that epigenetic modifications driven by Enhancer of zeste homolog 2 (EZH2), a major histone methyltransferase, have significant roles in vertebrate development including skeletal patterning and bone formation. We now find that targeting Ezh2 through the limb mesenchyme also has significant effects on tendon and muscle patterning, likely reflecting the essential roles of early mesenchymal cues mediated by Ezh2 for coordinated patterning and development of all tissues of the musculoskeletal system. Conversely, loss of Ezh2 in the tendon cells did not disrupt overall tendon structure or collagen organization suggesting that tendon differentiation and maturation are independent of Ezh2 signaling.

Type I and type V procollagen triple helix uses different subsets of the molecular ensemble for lysine posttranslational modifications in the rER

Collagen is the most abundant protein in humans. It has a characteristic triple-helix structure and is heavily posttranslationally modified. The complex biosynthesis of collagen involves processing by many enzymes and chaperones in the rough endoplasmic reticulum. Lysyl hydroxylase 1 (LH1) is required to hydroxylate lysine for cross-linking and carbohydrate attachment within collagen triple helical sequences. Additionally, a recent study of prolyl 3-hydroxylase 3 (P3H3) demonstrated that this enzyme may be critical for LH1 activity; however, the details surrounding its involvement remain unclear. If P3H3 is an LH1 chaperone that is critical for LH1 activity, P3H3 and LH1 null mice should display a similar deficiency in lysyl hydroxylation. To test this hypothesis, we compared the amount and location of hydroxylysine in the triple helical domains of type V and I collagen from P3H3 null, LH1 null, and wild-type mice. The amount of hydroxylysine in type V collagen was reduced in P3H3 null mice, but surprisingly type V collagen from LH1 null mice contained as much hydroxylysine as type V collagen from wild-type mice. In type I collagen, our results indicate that LH1 plays a global enzymatic role in lysyl hydroxylation. P3H3 is also involved in lysyl hydroxylation, particularly at cross-link formation sites, but is not required for all lysyl hydroxylation sites. In summary, our study suggests that LH1 and P3H3 likely have two distinct mechanisms to recognize different collagen types and to distinguish cross-link formation sites from other sites in type I collagen.

Immune tolerance of allogeneic haematopoietic cell transplantation supports donor epidermal grafting of recessive dystrophic epidermolysis bullosa chronic wounds

BACKGROUND

Chronic wounds, a common morbidity in recessive dystrophic epidermolysis bullosa (RDEB), lack definitive therapies.

OBJECTIVES

To assess allogeneic epidermal skin grafts in terms of wound healing and durability over time.

METHODS

In a prospective, open-label clinical trial for postallogeneic haematopoietic cell transplantation (post-alloHCT) patients with RDEB, up to nine chronic wounds per patient were grafted over 1 year. Epidermal grafts measuring 5 cm² were obtained from related alloHCT donors in the outpatient setting using the CELLUTOME^TM Epidermal Harvesting System. Wounds were photographed and symptom inventories completed at baseline and 6, 12 and 52 weeks after grafting. The trial was registered at ClinicalTrials.gov (NCT02670837).

RESULTS

Between August 2016 and January 2019, eight patients with RDEB received a total of 35 epidermal allografts at a median of 1157 days (range 548-2884) post-alloHCT. The median (interquartile range) percentage reductions in wound surface area were 75% (52-94), 95% (72-100) and 100% (97-100) at 6, 12 and 52 weeks postgraft, respectively, each significantly reduced from baseline (P < 0·001). Donor harvest sites healed quickly without scarring. Biopsy evaluation at 1 year of an epidermal allograft site revealed wildtype type VII collagen (immunofluorescence), anchoring fibrils (electron microscopy), and full-thickness skin whole-DNA donor chimerism of 42% (compared with 16% in concurrently biopsied native skin). This strategy subsequently supported release of RDEB pseudosyndactyly.

CONCLUSIONS

The immune tolerance established by alloHCT supports successful adoptive transfer of donor epidermal grafts. Persistence of donor grafts in a single patient beyond 1 year and observed migration of donor-grafted cells into adjacent wound suggest that epidermal allografts include nonterminally differentiated cells and/or trigger recruitment of donor bone-marrow-derived cells to mediate wound healing.

Loss of Smad4 in the scleraxis cell lineage results in postnatal joint contracture

Growth of the musculoskeletal system requires precise coordination between bone, muscle, and tendon during development. Insufficient elongation of the muscle-tendon unit relative to bone growth results in joint contracture, a condition characterized by reduction or complete loss of joint range of motion. Here we establish a novel murine model of joint contracture by targeting Smad4 for deletion in the tendon cell lineage using Scleraxis-Cre (ScxCre). Smad4^ScxCre mutants develop a joint contracture shortly after birth. The contracture is stochastic in direction and increases in severity with age. Smad4^ScxCre mutant tendons exhibited a stable reduction in cellularity and a progressive reduction in extracellular matrix volume. Collagen fibril diameters were reduced in the Smad4^ScxCre mutants, suggesting a role for Smad4 signaling in the regulation of matrix accumulation. Although ScxCre also has sporadic activity in both cartilage and muscle, we demonstrate an essential role for Smad4 loss in tendons for the development of joint contractures. Disrupting the canonical TGFβ-pathway in Smad2;3^ScxCre mutants did not result in joint contractures. Conversely, disrupting the BMP pathway by targeting BMP receptors (Alk3^ScxCre/Alk6^null) recapitulated many features of the Smad4^ScxCre contracture phenotype, suggesting that joint contracture in Smad4^ScxCre mutants is caused by disruption of BMP signaling. Overall, these results establish a model of murine postnatal joint contracture and a role for BMP signaling in tendon elongation and extracellular matrix accumulation.

QR-313, an Antisense Oligonucleotide, Shows Therapeutic Efficacy for Treatment of Dominant and Recessive Dystrophic Epidermolysis Bullosa: A Preclinical Study

Dystrophic epidermolysis bullosa (DEB) is a blistering skin disease caused by mutations in the gene COL7A1 encoding collagen VII. DEB can be inherited as recessive DEB (RDEB) or dominant DEB (DDEB) and is associated with a high wound burden. Perpetual cycles of wounding and healing drive fibrosis in DDEB and RDEB, as well as the formation of a tumor-permissive microenvironment. Prolonging wound-free episodes by improving the quality of wound healing would therefore confer substantial benefit for individuals with DEB. The collagenous domain of collagen VII is encoded by 82 in-frame exons, which makes splice-modulation therapies attractive for DEB. Indeed, antisense oligonucleotide-based exon skipping has shown promise for RDEB. However, the suitability of antisense oligonucleotides for treatment of DDEB remains unexplored. Here, we developed QR-313, a clinically applicable, potent antisense oligonucleotide specifically targeting exon 73. We show the feasibility of topical delivery of QR-313 in a carbomer-composed gel for treatment of wounds to restore collagen VII abundance in human RDEB skin. Our data reveal that QR-313 also shows direct benefit for DDEB caused by exon 73 mutations. Thus, the same topically applied therapeutic could be used to improve the wound healing quality in RDEB and DDEB.

Tgfβ signaling is critical for maintenance of the tendon cell fate

Studies of cell fate focus on specification, but little is known about maintenance of the differentiated state. In this study, we find that the mouse tendon cell fate requires continuous maintenance in vivo and identify an essential role for TGFβ signaling in maintenance of the tendon cell fate. To examine the role of TGFβ signaling in tenocyte function the TGFβ type II receptor (Tgfbr2) was targeted in the Scleraxis-expressing cell lineage using the ScxCre deletor. Tendon development was not disrupted in mutant embryos, but shortly after birth tenocytes lost differentiation markers and reverted to a more stem/progenitor state. Viral reintroduction of Tgfbr2 to mutants prevented and even rescued tenocyte dedifferentiation suggesting a continuous and cell autonomous role for TGFβ signaling in cell fate maintenance. These results uncover the critical importance of molecular pathways that maintain the differentiated cell fate and a key role for TGFβ signaling in these processes.

Connective Tissue Ultrastructure: A Direct Comparison between Conventional Specimen Preparation and High-Pressure Freezing/Freeze-Substitution

It is generally agreed within the microscopy community that the quality of ultrastructure within the connective tissue matrix resulting from high-pressure freezing followed by freeze-substitution (HPF/FS) far exceeds that gained following the "conventional" preparation method, which includes aqueous fixation, dehydration, and embedding. Exposure to cryogen at high pressure is the only cryopreservation method capable of vitrifying tissue structure to a depth exceeding 200 μm. Cells within connective tissues prepared by HPF/FS are universally larger, filling the commonly seen void at the juncture between cell and matrix. Without significant shrinkage of cells and the coincident extraction of the cytosolic components, well-resolved organelles are less clustered within an expanded cytosol. Much of the artifact from "conventional" methods occurs as large space filling and also smaller fibril-associated proteoglycans are extracted during fixation. However, the visualization of some matrix features by electron microscopy is actually dependent on the collapse or extraction of these "masking" components. Herein, we argue that an impression of ultrastructure within commonly studied matrices, in particular skin, is best gained following the evaluation of both conventional preparations and tissue prepared by HPF/FS. Anat Rec, 2019. © 2019 American Association for Anatomy.

Optimizing a 3D model system for molecular manipulation of tenogenesis

PURPOSE

Tendon injuries are clinically challenging due to poor healing. A better understanding of the molecular events that regulate tendon differentiation would improve current strategies for repair. The mouse model system has been instrumental to tendon studies and several key molecules were initially established in mouse. However, the study of gene function has been limited by the absence of a standard in vitro tendon system for efficiently testing multiple mutations, physical manipulations, and mis-expression. The purpose of this study is therefore to establish such a system.

METHODS

We adapted an existing design for generating three-dimensional (3D) tendon constructs for use with mouse progenitor cells harboring the ScxGFP tendon reporter and the Rosa26-TdTomato Cre reporter. Using these cells, we optimized the parameters for construct formation, inducing tenogenesis via transforming growth factor-β2 (TGFβ2), and genetic recombination via an adenovirus encoding Cre recombinase. Finally, for proof of concept, we used Smad4 floxed cells and tested the robustness of the system for gene knockdown.

RESULTS

We found that TGFβ2 treatment induced a tenogenic phenotype depending on the timing of initiation. Addition of TGFβ2 after 3D "tensioning" enhanced tendon differentiation. Interestingly, while TGFβ2-induced proliferation depended on Smad4, tenogenic parameters such as ScxGFP expression and fibril diameter were independent of Smad4.

CONCLUSIONS

Our results demonstrate the feasibility of this optimized system for harnessing the power of mouse genetics for in vitro applications.

Novel Model of Tendon Regeneration Reveals Distinct Cell Mechanisms Underlying Regenerative and Fibrotic Tendon Healing

To date, the cell and molecular mechanisms regulating tendon healing are poorly understood. Here, we establish a novel model of tendon regeneration using neonatal mice and show that neonates heal via formation of a ‘neo-tendon’ that differentiates along the tendon specific lineage with functional restoration of gait and mechanical properties. In contrast, adults heal via fibrovascular scar, aberrant differentiation toward cartilage and bone, with persistently impaired function. Lineage tracing identified intrinsic recruitment of Scx-lineage cells as a key cellular mechanism of neonatal healing that is absent in adults. Instead, adult Scx-lineage tenocytes are not recruited into the defect but transdifferentiate into ectopic cartilage; in the absence of tenogenic cells, extrinsic αSMA-expressing cells persist to form a permanent scar. Collectively, these results establish an exciting model of tendon regeneration and uncover a novel cellular mechanism underlying regenerative vs non-regenerative tendon healing.

Musculoskeletal integration at the wrist underlies the modular development of limb tendons

The long tendons of the limb extend from muscles that reside in the zeugopod (arm/leg) to their skeletal insertions in the autopod (paw). How these connections are established along the length of the limb remains unknown. Here, we show that mouse limb tendons are formed in modular units that combine to form a functional contiguous structure; in muscle-less limbs, tendons develop in the autopod but do not extend into the zeugopod, and in the absence of limb cartilage the zeugopod segments of tendons develop despite the absence of tendons in the autopod. Analyses of cell lineage and proliferation indicate that distinct mechanisms govern the growth of autopod and zeugopod tendon segments. To elucidate the integration of these autopod and zeugopod developmental programs, we re-examined early tendon development. At E12.5, muscles extend across the full length of a very short zeugopod and connect through short anlagen of tendon progenitors at the presumptive wrist to their respective autopod tendon segment, thereby initiating musculoskeletal integration. Zeugopod tendon segments are subsequently generated by proximal elongation of the wrist tendon anlagen, in parallel with skeletal growth, underscoring the dependence of zeugopod tendon development on muscles for tendon anchoring. Moreover, a subset of extensor tendons initially form as fused structures due to initial attachment of their respective wrist tendon anlage to multiple muscles. Subsequent individuation of these tendons depends on muscle activity. These results establish an integrated model for limb tendon development that provides a framework for future analyses of tendon and musculoskeletal phenotypes.

Correlation of the same fields imaged in the TEM, confocal, LM, and microCT by image registration: from specimen preparation to displaying a final composite image

Correlated imaging is the process of imaging a specimen with two complementary modalities and then registering and overlaying the fields obtained in each modality to create a composite view. One of the images is made somewhat transparent, allowing detail in the underlying image to be visible and assisting in the registration of the two images. As an example, an image localizing a specific tissue component by fluorescence may be overlaid atop a TEM image of the same field. The resulting composite image would demonstrate specific ultrastructural features in the high-resolution TEM field, which are colorized in the overlay. Other examples include composites from MicroCT or soft X-ray images overlaid atop light microscopy or TEM images. Automated image registration may be facilitated by a variety of sophisticated computer programs utilized by high-throughput laboratories. This chapter is meant for the more occasional user wishing to align images manually. ImageJ is a public domain, image processing program developed at the National Institutes of Health and is available to anyone as a free download. ImageJ performs marvelously well for the purpose of image registration; therefore, step-by-step instructions are included here. Specimen handling, including fixation and choice of embedding media, is not straightforward for correlative imaging. A step-by-step description of the protocols which work in our laboratory is included for simultaneous localization in LM, EM and micro-CT, as well as maintaining GFP emission in tissue embedded for TEM.

A correlative method for imaging identical regions of samples by micro-CT, light microscopy, and electron microscopy: imaging adipose tissue in a model system

We present a method in which a precise region of interest within an intact organism is spatially mapped in three dimensions by non-invasive micro-computed X-ray tomography (micro-CT), then further evaluated by light microscopy (LM) and transmission electron microscopy (TEM). Tissues are prepared as if for TEM including osmium fixation, which imparts soft tissue contrast in the micro-CT due to its strong X-ray attenuation. This method may therefore be applied to embedded, archived TEM samples. Upon selection of a two-dimensional (2-D) projection from a region of interest (ROI) within the three-dimensional volume, the epoxy-embedded sample is oriented for microtomy so that the sectioning plane is aligned with the micro-CT projection. Registration is verified by overlaying LM images with 2-D micro-CT projections. Structures that are poorly resolved in the micro-CT may be evaluated at TEM resolution by observing the next serial ultrathin section, thereby accessing the same ROI by all three imaging techniques. We compare white adipose tissue within the forelimbs of mice harboring a lipid-altering mutation with their littermate controls. We demonstrate that individual osmium-stained lipid droplets as small as 15 µm and separated by as little as 35 µm may be discerned as separate entities in the micro-CT, validating this to be a high-resolution, non-destructive technique for evaluation of fat content.

Microenvironmental regulation by fibrillin-1

Fibrillin-1 is a ubiquitous extracellular matrix molecule that sequesters latent growth factor complexes. A role for fibrillin-1 in specifying tissue microenvironments has not been elucidated, even though the concept that fibrillin-1 provides extracellular control of growth factor signaling is currently appreciated. Mutations in FBN1 are mainly responsible for the Marfan syndrome (MFS), recognized by its pleiotropic clinical features including tall stature and arachnodactyly, aortic dilatation and dissection, and ectopia lentis. Each of the many different mutations in FBN1 known to cause MFS must lead to similar clinical features through common mechanisms, proceeding principally through the activation of TGFβ signaling. Here we show that a novel FBN1 mutation in a family with Weill-Marchesani syndrome (WMS) causes thick skin, short stature, and brachydactyly when replicated in mice. WMS mice confirm that this mutation does not cause MFS. The mutation deletes three domains in fibrillin-1, abolishing a binding site utilized by ADAMTSLIKE-2, -3, -6, and papilin. Our results place these ADAMTSLIKE proteins in a molecular pathway involving fibrillin-1 and ADAMTS-10. Investigations of microfibril ultrastructure in WMS humans and mice demonstrate that modulation of the fibrillin microfibril scaffold can influence local tissue microenvironments and link fibrillin-1 function to skin homeostasis and the regulation of dermal collagen production. Hence, pathogenetic mechanisms caused by dysregulated WMS microenvironments diverge from Marfan pathogenetic mechanisms, which lead to broad activation of TGFβ signaling in multiple tissues. We conclude that local tissue-specific microenvironments, affected in WMS, are maintained by a fibrillin-1 microfibril scaffold, modulated by ADAMTSLIKE proteins in concert with ADAMTS enzymes.

The microenvironment is specified by cell-surface molecules, growth factors, and the extracellular matrix. Here we report genetic evidence that implicates fibrillin-1, a ubiquitous extracellular matrix molecule that sequesters latent growth factor complexes, as a key determinant in the local control of musculoskeletal and skin microenvironments. A novel mutation in fibrillin-1 demonstrates that modulation of the fibrillin microfibril scaffold can influence tissue microenvironments and result in the clinical features of Weill-Marchesani syndrome (WMS), including thick skin, short stature, and brachydactyly. Dysregulated WMS microenvironments diverge from Marfan pathogenetic mechanisms, which lead to broad activation of TGFβ signaling in multiple tissues.

Transmission electron microscopy of cartilage and bone

This review of cartilage microscopy is restricted to a brief description of the major molecular components in cartilage and bone and an in-depth discussion of methods for the preparation of these tissues for examination by transmission electron microscopy. Included within are simple methods for the isolation of cartilage-specific macromolecules, methods for immunoidentification of tissue components, bone decalcification protocols, and fixative recipes designed to stabilize cells and proteoglycan within the cartilage matrix. A discussion of cartilage prepared by high-pressure freezing (HPF)/freeze substitution (FS) is also presented.

Confocal/TEM overlay microscopy: a simple method for correlating confocal and electron microscopy of cells expressing GFP/YFP fusion proteins

Genetic manipulation allows simultaneous expression of green fluorescent protein (GFP) and its derivatives with a wide variety of cellular proteins in a variety of living systems. Epifluorescent and confocal laser scanning microscopy (confocal) localization of GFP constructs within living tissue and cell cultures has become routine, but correlation of light microscopy and high resolution transmission electron microscopy (TEM) on components within identical cells has been problematic. In this study, we describe an approach that specifically localizes the position of GFP/yellow fluorescent protein (YFP) constructs within the same cultured cell imaged in the confocal and transmission electron microscopes. We present a simplified method for delivering cell cultures expressing fluorescent fusion proteins into LR White embedding media, which allows excellent GFP/YFP detection and also high-resolution imaging in the TEM. Confocal images from 0.5-microm-thick sections are overlaid atop TEM images of the same cells collected from the next serial ultrathin section. The overlay is achieved in Adobe Photoshop by making the confocal image somewhat transparent, then carefully aligning features within the confocal image over the same features visible in the TEM image. The method requires no specialized specimen preparation equipment; specimens are taken from live cultures to embedding within 8 h, and confocal transmission overlay microscopy can be completed within a few hours.

Developmental distribution of collagen type XII in cartilage: association with articular cartilage and the growth plate

Collagen type XII is a member of the fibril-associated collagens and is characterized by a short triple-helical domain with three extended noncollagenous NC3 domains. Previous studies suggested that collagen XII is a component of cartilage but little is known about its spatial-temporal distribution. This study uses a polyclonal antibody to the purified NC3 domain to investigate its developmental distribution in rat forelimb. Collagen XII was present at the joint interzone on embryonic day 16 (E16d) and restricted to the presumptive articular cartilage by E18d. Labeling of the articular surface intensified as development progressed postnatally (day 1 [1d] to 28d) and extended approximately six cell diameters deep. In juvenile rats, collagen XII antibodies also labeled the longitudinal and transverse septa of stacked chondrocytes in the growth plate. However, collagen XII was not associated at any developmental stage with the cartilaginous secondary ossification center and was only weakly expressed in epiphyseal cartilage. Ultrastructural localization of the NC3 domain epitope showed labeling of the surface of collagen II fibrils both in tissue and in isolated fibrils. The results presented provide further evidence that articular cartilage differs substantially from the underlying epiphyseal cartilage and that different chondrocytic developmental fates are reflected in the composition of their extracellular matrix starting early in development. In addition, collagen XII was distributed in areas of cartilage with more organized fibril orientation and may have a role in promoting alignment or stabilizing such an organization, thereby creating a matrix capable of withstanding load-bearing forces.