No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

<p> Musculoskeletal injuries are one of the leading causes of human disability. Tissue engineers aim to restore damaged musculoskeletal tissues by creating scaffolds that promote cellular adhesion, proliferation, and eventual differentiation into functional tissue. It is known that the extracellular matrix (ECM) regulates cellular behavior and is often used as a basis for biological scaffolds; however, current scaffolds often mimic the ECM of adult, homeostatic tissue and frequently lead to poor tissue restoration. What is rarely taken into consideration is that the ECM undergoes extensive remodeling during development to facilitate growth.</p> <p>In the musculoskeletal system, myogenic progenitors (<em>Pax3</em>+) and connective tissue cells (<em>Prx1</em>+) proliferate and differentiate into muscle, tendon, cartilage, and conjoining interfaces (<em>e.g.</em> myotendinous junction), while depositing and remodeling the ECM. As tissues mature, cells continue to refine ECM networks to withstand the functional demands to facilitate movement. The ECM composition and architecture of adult musculoskeletal tissues have been studied individually and are thought to be distinct; however, there has yet to be a comprehensive comparative analysis of the ECM in adult muscle, tendon, and the myotendinous junction (MTJ) in a single study. Additionally, how the matrisome of adult musculoskeletal system compares to the ECM dynamics during forelimb development, remain largely unknown due to lack of techniques to analyze embryonic matrisome composition and synthesis. </p> <p>To address these research gaps, we (1) used quantitative proteomics to map the matrisome composition in the mature murine MTJ, relative to the tendon and muscle; (2) adapted tissue fractionation and biorthogonal non-canonical amino acid tagging techniques to embryonic tissues as a method to quantify the global and nascent embryonic matrisome; and (3) subsequently used these techniques to establish a baseline of ECM dynamics during forelimb morphogenesis (embryonic day, E11.5-E14.5) and growth (postnatal day, P3 and P35). We hypothesized that proteomic evaluation of ECM composition and synthesis in developing and adolescent limbs would resolve differences between embryonic and growing tissues. Indeed, we saw significant differences in global and nascent matrisome composition between embryonic and adolescent forelimbs. Notably, the relative abundance and ratios of collagens associated with type I fibrillogenesis (I, III, and V) were significantly different as a function of development embryogenesis and across the adult muscle, MTJ, and tendon.</p> <p>Type I collagen fibrils are critical for tissue architecture and function. Using genetic mouse models, the regulatory roles of COL5A1 in the initiation of type I collagen fibrillogenesis, and organization of subsequent fibrils, have been well characterized in tendons and ligaments; however, is it unknown which cell types contribute COL5A1 to the ECM in the forelimb. To identify the functional contribution of COL5A1 by myogenic or connective tissue cell populations, we generated conditional (cre-flox) knock-out mouse models to inactivate <em>Col5a1</em> using <em>Pax3</em>- or <em>Prx1</em>-drivers, respectively. Haploinsufficiency of <em>COL5A1</em> in humans is associated classical Ehlers-Danlos syndrome, characterized by skin fragility and join instability; similar, albeit more severe, phenotypes were present in <em>Prx1Cre/+;Col5a1fl/fl</em> mutants, but not in <em>Pax3Cre/+;Col5a1fl/fl</em> mutants or controls. Interestingly, THBS4+ and COL22A1+ networks at the MTJ were morphologically affected in <em>Prx1Cre/+;Col5a1fl/fl</em> limbs. Additional work needs to be conducted to characterize the systematic phenotypes observed in <em>Prx1Cre/+;Col5a1fl/fl</em> limbs.</p> <p>Together, our results indicate that there are distinct, complex ECM dynamics, originating from distinct cell-types, that drive musculoskeletal morphogenesis in the forelimb. Further, the tools developed here will serve as a foundation for quantitative proteomic analyses of the matrisome composition in embryonic tissues. Collectively, this work provides a baseline of ECM protein dynamics during musculoskeletal morphogenesis, a helpful guide for tissue engineers in designing scaffolds to promote restoration of damaged tissues, with enhanced integration into the host tissue.</p>

Connective tissue extracellular matrix (ECM) consists of an interwoven network of contiguous collagen fibers that inform cell activity, direct biological function, and guide tissue homeostasis throughout life. Recently, ECM analogs have emerged as a unique ex vivo culture platform for studying healthy and diseased tissues and in the latter, enabling the screening for and development of therapeutic regimen. Unfortunately, current commonly used platforms, such as tissue-culture polystyrene (TCPS) or the basement membrane matrix, Matrigel, fail to fully recapitulate the physical and biochemical properties of the ECM. Tissue-culture polystyrene is significantly stiffer than typical ECM tissues and lacks the composition and 3-dimensional architecture that is critical for ECM function. Improving upon TCPS’s shortcomings, Matrigel retains a natural ECM structure and is comprised of native biopolymers. However, it is derived from mouse sarcomas, and thus, has significant batch-to-batch variability and often contains growth factors at non-physiologic concentrations. Moreover, despite being biopolymer based, Matrigel has relatively low amounts of type I collagen and high levels of type IV collagen, and as such, compositionally does not match the predominantly type I collagen matrix intrinsic to connective tissues. Thus, it is clear that new and improved models of the ECM are needed for in vitro culture. In pursuit of developing a highly biomimetic ECM analog, the objectives of this work were three-fold— first, to fabricate collagen-based ECM analogs with nanoscale mimicry, second, to systematically optimize crosslinking protocols in order to produce a stable substrate with continuous fibrous architecture, and third, to evaluate the substrate’s biocompatibility and utility as a platform for studying biomineralization. It was hypothesized that an architecturally and chemically relevant fibrous substrate could be prepared from gelatin and provide an optimal ex vivo platform for cell culture and new therapy screening and development. Thus, the ECM analog will be collagen-like, biocompatible, consist of continuous fibers, demonstrate both viscoelastic and elastic behavior, exhibit relevant mechanical properties, and remain stable for at least 14 days at cell culture conditions. To this end, first, a “green” electrospinning method was developed for preparing fibrous meshes from gelatin, which avoids typical electrospinning solvents that present significant health risks and barriers to large scale production. Next, crosslinking methods were developed using the reactive dialdehyde, glutaraldehyde (GTA), and the naturally derived enzyme, transglutaminase (TGase). These methods stabilized the meshes for over 28 days under cell culture conditions without disrupting its biomimetic architectures and chemical properties. In addition, a third approach to mesh fabrication using gelatin methacryloyl (gelMA) was developed to overcome the shortcomings of GTA and TGase crosslinking. With gelMA, the number of crosslinking sites were customized and, by taking advantage of its ability to undergo free radical polymerization, stable fibrous meshes were prepared with reproducible architecture, chemistry, and tunable mechanical properties. Following fabrication, the biocompatibility of the meshes was evaluated through macrophage, stem cell, and differentiated cell cultures. During culture, the macrophages maintained a naïve, non-polarized state, indicating they were not triggered towards an inflammatory response by the meshes. In addition, fibrochondrocytes, a cell critical for maintaining the collagen-based matrices where ligaments attach to bone, remained viable and maintained phenotypic expression on the meshes, as evident by their enhanced proteoglycan and collagen production relative to TCPS cultures. After demonstrating biocompatibility, the gelatin platform was coupled with a synthetic matrix vesicle (SMV) system and successfully acted as a mineralization platform in the presence of human osteoblast-like cells. Additionally, the platform supported mesenchymal stem cell expansion and mineralization when cultured with an alkaline phosphate conjugated SMV. In this work, three unique methods were developed for preparing ECM analogs. These efforts led to the production of a collagen-like mesh with nano- and micro-scale cues, fibrous continuity with little batch-to-batch variability, and proven stability in both dry and wet conditions. Importantly, these meshes did not instigate any inflammatory responses and supported fibrochondrocyte, osteoblast, and stem cell culture. Furthermore, the mesh successfully functioned as a template for biomineralization using both human osteoblast-like cells and stem cells. Collectively these findings demonstrate the potential of a collagen-like ECM analog with physiological relevance for ex vivo cell culture studies; and furthermore, its potential as a high-fidelity platform for studying cell-mediated biomineralization, cell-matrix interactions, and developing new therapeutic approaches for the treatment of connective tissue disorders.