The transcription factor scleraxis differentially regulates gene expression in tenocytes isolated at different developmental stages

Biology and physiology of tendon healing

Tendon disorders affect people of all ages, from elite and recreational athletes and workers to elderly patients. After an acute injury, 3 successive phases are described to achieve healing: an inflammatory phase followed by a proliferative phase, and finally by a remodeling phase. Despite this process, healed tendon fails to recover its original mechanical properties. In this review, we proposed to describe the key factors involved in the process such as cells, transcription factors, extracellular matrix components, cytokines and growth factors and vascularization among others. A better understanding of this healing process could help provide new therapeutic approaches to improve patients' recovery while tendon disorders management remains a medical challenge.

Decoding the transcriptome of calcified atherosclerotic plaque at single-cell resolution

Atherogenesis involves an interplay of inflammation, tissue remodeling and cellular transdifferentiation (CTD), making it especially difficult to precisely delineate its pathophysiology. Here we use single-cell RNA sequencing and systems-biology approaches to analyze the transcriptional profiles of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) in calcified atherosclerotic core (AC) plaques and patient-matched proximal adjacent (PA) portions of carotid artery tissue from patients undergoing carotid endarterectomy. Our results reveal an anatomic distinction whereby PA cells express inflammatory mediators, while cells expressing matrix-secreting genes occupy a majority of the AC region. Systems biology analysis indicates that inflammation in PA ECs and VSMCs may be driven by TNFa signaling. Furthermore, we identify POSTN, SPP1 and IBSP in AC VSMCs, and ITLN1, SCX and S100A4 in AC ECs as possible candidate drivers of CTD in the atherosclerotic core. These results establish an anatomic framework for atherogenesis which forms the basis for exploration of a site-specific strategy for disruption of disease progression.

Single-cell RNA sequencing and systems biology are used to profile the human vascular cell populations in calcified atherosclerotic core plaques from carotid endarterectomy samples, showing an anatomic distinction between gene expression of inflammatory versus matrix-secreting factors.

The loop of phenotype: Dynamic reciprocity links tenocyte morphology to tendon tissue homeostasis

Cells and their surrounding extracellular matrix (ECM) are engaged in dynamic reciprocity to maintain tissue homeostasis: cells deposit ECM, which in turn presents the signals that define cell identity. This loop of phenotype is obvious for biochemical signals, such as collagens, which are produced by and presented to cells, but the role of biomechanical signals is also increasingly recognised. In addition, cell shape goes hand in hand with cell function and tissue homeostasis. Aberrant cell shape and ECM is seen in pathological conditions, and control of cell shape in micro-fabricated platforms disclose the causal relationship between cell shape and cell function, often mediated by mechanotransduction. In this manuscript, we discuss the loop of phenotype for tendon tissue homeostasis. We describe cell shape and ECM organization in normal and diseased tissue, how ECM composition influences tenocyte shape, and how that leads to the activation of signal transduction pathways and ECM deposition. We further describe the use of technologies to control cell shape to elucidate the link between cell shape and its phenotypical markers and focus on the causal role of cell shape in the loop of phenotype. STATEMENT OF SIGNIFICANCE: The dynamic reciprocity between cells and their surrounding extracellular matrix (ECM) influences biomechanical and biochemical properties of ECM as well as cell function through activation of signal transduction pathways that regulate gene and protein expression. We refer to this reciprocity as Loop of Phenotype and it has been studied and demonstrated extensively by using micro-fabricated platforms to manipulate cell shape and cell fate. In this manuscript, we discuss this concept in tendon tissue homeostasis by giving examples in healthy and pathological tenson tissue. Furthermore, we elaborate this by showing how biomaterials are used to feed this reciprocity to manipulate cell shape and function. Finally, we elucidate the link between cell shape and its phenotypical markers and focus on the activation of signal transduction pathways and ECM deposition.

The Scleraxis Transcription Factor Directly Regulates Multiple Distinct Molecular and Cellular Processes During Early Tendon Cell Differentiation

Proper development of tendons is crucial for the integration and function of the musculoskeletal system. Currently little is known about the molecular mechanisms controlling tendon development and tendon cell differentiation. The transcription factor Scleraxis (Scx) is expressed throughout tendon development and plays essential roles in both embryonic tendon development and adult tendon healing, but few direct target genes of Scx in tendon development have been reported and genome-wide identification of Scx direct target genes in vivo has been lacking. In this study, we have generated a Scx^Flag knockin mouse strain, which produces fully functional endogenous Scx proteins containing a 2xFLAG epitope tag at the carboxy terminus. We mapped the genome-wide Scx binding sites in the developing limb tendon tissues, identifying 12,097 high quality Scx regulatory cis-elements in-around 7,520 genes. Comparative analysis with previously reported embryonic tendon cell RNA-seq data identified 490 candidate Scx direct target genes in early tendon development. Furthermore, we characterized a new Scx gene-knockout mouse line and performed whole transcriptome RNA sequencing analysis of E15.5 forelimb tendon cells from Scx^–/– embryos and control littermates, identifying 68 genes whose expression in the developing tendon tissues significantly depended on Scx function. Combined analysis of the ChIP-seq and RNA-seq data yielded 32 direct target genes that required Scx for activation and an additional 17 target genes whose expression was suppressed by Scx during early tendon development. We further analyzed and validated Scx-dependent tendon-specific expression patterns of a subset of the target genes, including Fmod, Kera, Htra3, Ssc5d, Tnmd, and Zfp185, by in situ hybridization and real-time quantitative polymerase chain reaction assays. These results provide novel insights into the molecular mechanisms mediating Scx function in tendon development and homeostasis. The ChIP-seq and RNA-seq data provide a rich resource for aiding design of further studies of the mechanisms regulating tendon cell differentiation and tendon tissue regeneration. The Scx^Flag mice provide a valuable new tool for unraveling the molecular mechanisms involving Scx in the protein interaction and gene-regulatory networks underlying many developmental and disease processes.

Scleraxis-lineage cell depletion improves tendon healing and disrupts adult tendon homeostasis

Despite the requirement for Scleraxis-lineage (Scx^Lin) cells during tendon development, the function of Scx^Lin cells during adult tendon repair, post-natal growth, and adult homeostasis have not been defined. Therefore, we inducibly depleted Scx^Lin cells (ScxLin^DTR) prior to tendon injury and repair surgery and hypothesized that ScxLin^DTR mice would exhibit functionally deficient healing compared to wild-type littermates. Surprisingly, depletion of Scx^Lin cells resulted in increased biomechanical properties without impairments in gliding function at 28 days post-repair, indicative of regeneration. RNA sequencing of day 28 post-repair tendons highlighted differences in matrix-related genes, cell motility, cytoskeletal organization, and metabolism. We also utilized ScxLin^DTR mice to define the effects on post-natal tendon growth and adult tendon homeostasis and discovered that adult Scx^Lin cell depletion resulted in altered tendon collagen fibril diameter, density, and dispersion. Collectively, these findings enhance our fundamental understanding of tendon cell localization, function, and fate during healing, growth, and homeostasis.