Single-cell RNA sequencing reveals cellular and molecular heterogeneity in fibrocartilaginous enthesis formation

Advances in spatial transcriptomics and its application in the musculoskeletal system

While bulk RNA sequencing and single-cell RNA sequencing have shed light on cellular heterogeneity and potential molecular mechanisms in the musculoskeletal system in both physiological and various pathological states, the spatial localization of cells and molecules and intercellular interactions within the tissue context require further elucidation. Spatial transcriptomics has revolutionized biological research by simultaneously capturing gene expression profiles and in situ spatial information of tissues, gradually finding applications in musculoskeletal research. This review provides a summary of recent advances in spatial transcriptomics and its application to the musculoskeletal system. The classification and characteristics of data acquisition techniques in spatial transcriptomics are briefly outlined, with an emphasis on widely-adopted representative technologies and the latest technological breakthroughs, accompanied by a concise workflow for incorporating spatial transcriptomics into musculoskeletal system research. The role of spatial transcriptomics in revealing physiological mechanisms of the musculoskeletal system, particularly during developmental processes, is thoroughly summarized. Furthermore, recent discoveries and achievements of this emerging omics tool in addressing inflammatory, traumatic, degenerative, and tumorous diseases of the musculoskeletal system are compiled. Finally, challenges and potential future directions for spatial transcriptomics, both as a field and in its applications in the musculoskeletal system, are discussed.

Spatial transcriptomic applications in orthopedics

PURPOSE

This review highlights the transformative impact of spatial transcriptomics on orthopedic research, focusing on its application in deciphering intricate gene expression patterns within musculoskeletal tissues.

METHODS

The paper reviews literature for spatial transcriptomic methods, specifically 10X Visium, 10X Xenium, seqFISH+, MERFISH, NanoString GeoMx DSP, used on musculoskeletal tissues (cartilage, joints, bone, tendon, ligament, and synovium).

RESULTS

Searches identified 29 published manuscripts reporting spatial transcriptomic data in cartilage, bone, tendon, synovium, and intervertebral disc. Most publications of spatial transcriptomic data are from tendon and synovium. 10X Visium has been used in 22 of the 29 papers identified. Spatial transcriptomics has been used to identify novel cell populations and cell signaling pathways that regulate development and disease.

CONCLUSIONS

Imaging-based spatial transcriptomic methods may be better for hypothesis testing as they generally have subcellular resolution but sequence fewer genes. Sequencing methods may be better for untargeted, shotgun approaches that can generate useful hypotheses from the spatial data from unimpaired tissue sections. Spatial transcriptomic methods could become useful for clinical diagnostics and precision medicine approaches.

Stem and progenitor cells in the synovial joint as targets for regenerative therapy

Damage to articular cartilage, tendons, ligaments and entheses as a result of trauma, degeneration or inflammation in rheumatic diseases is prevalent. Regenerative medicine offers promising strategies for repairing damaged tissues, with the aim of restoring both their structure and function. While these strategies have traditionally relied on tissue engineering approaches using exogenous cells, interventions based on the activation of endogenous repair mechanisms are an attractive alternative. Key to advancing such approaches is a comprehensive understanding of the diversity of the stem and progenitor cells that reside in the adult synovial joint and how they function to repair damaged tissues. Advances in developmental biology have provided a lens through which to understand the origins, identities and functions of these cells, and insights into the roles of stem and progenitor cells in joint tissue repair, as well as their complex relationship with fibroblasts, have emerged. Integration of knowledge obtained through studies using advanced single-cell technologies will be crucial to establishing unified models of cell populations, lineage hierarchies and their molecular regulation. Ultimately, a more complete understanding of how cells repair tissues in adult life will guide the development of innovative pro-regenerative drugs, which are poised to enter clinical practice in musculoskeletal medicine.

Single-cell RNA sequencing reveals the CRTAC1+ population actively contributes to the pathogenesis of spinal ligament degeneration by SPP1+ macrophage

Degenerative spinal stenosis is a chronic disease that affects the spinal ligaments and associated bones, resulting in back pain and disorders of the limbs among the elderly population. There are few preventive strategies for such ligament degeneration. We here aimed to establish a comprehensive transcriptomic atlas of ligament tissues to identify high-priority targets for pharmaceutical treatment of ligament degeneration. Here, single-cell RNA sequencing was performed on six degenerative ligaments and three traumatic ligaments to understand tissue heterogeneity. After stringent quality control, high-quality data were obtained from 32,014 cells. Distinct cell clusters comprising stromal and immune cells were identified in ligament tissues. Among them, we noted that collagen degradation associated with CTHRC1+ fibroblast-like cells and calcification linked to CRTAC1+ chondrocyte-like cells were key features of ligament degeneration. SCENIC analysis and further experiments identified ATF3 as a key transcription factor regulating the pathogenesis of CRTAC1+ chondrocyte-like cells. Typically, immune cells infiltrate localized organs, causing tissue damage. In our study, myeloid cells were found to be inflammatory-activated, and SPP1+ macrophages were notably enriched in degenerative ligaments. Further exploration via CellChat analysis demonstrated a robust interaction between SPP1+ macrophages and CRTAC1+ chondrocyte-like cells. Activated by SPP1, ATF3 propels the CRTAC1/MGP/CLU axis, fostering ligament calcification. Our unique resource provides novel insights into possible mechanisms underlying ligament degeneration, the target cell types, and molecules that are expected to mitigate degenerative spinal ligament. We also highlight the role of immune regulation in ligament degeneration and calcification, enhancing our understanding of this disease.

Achieving tendon enthesis regeneration across length scales

Surgical reattachment of tendon to bone is a clinical challenge, with unacceptably high retear rates in the early period after repair. A primary reason for these repeated tears is that the multiscale toughening mechanisms found at the healthy tendon enthesis are not regenerated during tendon-to-bone healing. The need for technologies to improve these outcomes is pressing, and the tissue engineering community has responded with many advances that hold promise for eventually regenerating the multiscale tissue interface that transfers loads between the two dissimilar materials, tendon, and bone. This review provides an assessment of the state of these approaches, with the aim of identifying a critical agenda for future progress.

Spatial gene expression in the adult rat patellar tendon

Tendons are dense connective tissues with relatively few cells which makes studying the molecular profile of the tissue challenging. There is not a consensus on the spatial location of various cell types within a tendon, nor the accompanying transcriptional profile. In the present study, we used two male rat patellar tendon samples for sequencing-based spatial transcriptomics to determine the gene expression profile. We integrated our data with a mouse Achilles single cell dataset to predict the cell type composition of the patellar tendon as a function of location within the tissue. The spatial location of the predicated cell types suggested that there were two populations of tendon fibroblasts, one located in the tendon midsubstance, while the other localized with red blood cells, pericytes, and immune cells to the tendon peripheral connective tissue. Of the highest expressed spatially variable genes, there were multiple genes with known function in tendon: Col1a1, Col1a2, Dcn, Fmod, Sparc, and Comp. Further, a novel spatially regulated gene (AABR07000398.1) with no known function was identified. The spatial gene expression of tendon associated genes (Scx, Thbs4, Tnmd, Can, Bgn, Lum, Adamts2, Lox, Ppib, Col2a1, Col3a1, Col6a2) was also visualized. Both patellar tendon samples had similar expression patterns for all these genes. This dataset provides new spatial insights into gene expression in a healthy tendon.