Single-cell transcriptomic analysis identifies extensive heterogeneity in the cellular composition of mouse Achilles tendons

Abstract

Tendon is a dense connective tissue that stores and transmits forces between muscles and bones. Cellular heterogeneity is increasingly recognized as an important factor in the biological basis of tissue homeostasis and disease, yet little is known about the diversity of cell types that populate tendon. To address this, we determined the heterogeneity of cell populations within mouse Achilles tendons using single-cell RNA sequencing. In assembling a transcriptomic atlas of Achilles tendons, we identified 11 distinct types of cells, including 3 previously undescribed populations of tendon fibroblasts. Prior studies have indicated that pericytes, which are found in the vasculature of tendons, could serve as a potential source of progenitor cells for adult tendon fibroblasts. Using trajectory inference analysis, we provide additional support for the notion that pericytes are likely to be at least one of the progenitor cell populations for the fibroblasts that compose adult tendons. We also modeled cell-cell interactions and identified previously undescribed ligand-receptor signaling interactions involved in tendon homeostasis. Our novel and interactive tendon atlas highlights previously underappreciated heterogeneity between and within tendon cell populations. The atlas also serves as a resource to further the understanding of tendon extracellular matrix assembly, maintenance, and in the design of therapies for tendinopathies.

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

“…While many studies have examined the potential of using various stem cell populations to promote healing, originating from both tendon intrinsic ( Walia and Huang, 2019 ) and extrinsic sources ( Costa-Almeida et al, 2019 ), little focus has been directed toward defining the functions and therapeutic potential of tendon cells during tendon healing following an acute injury. Tendon cells are increasingly recognized as a heterogenous population of cells where many, but not all, express the gene Scleraxis ( Scx ) ( Best and Loiselle, 2019 ; Kendal et al, 2020 ; De Micheli et al, 2020 ). Understanding the localization and function of tendon cell subpopulations during healing is likely to be instrumental in better defining the mechanisms that promote scar-mediated healing, which results in poor patient outcomes, and could therefore be used to develop pro-regenerative approaches to improve healing.…”

Altered TGFB1 regulated pathways promote accelerated tendon healing in the superhealer MRL/MpJ mouse

“…First, our study was limited in assessing transcriptional regulation in bulk tendon tissue in native tendon and 7 days after injury. Whole-genome transcriptional profiling from the injured tendon tissue site represents a heterogeneous cell population of tenocytes, fibroblasts, myofibroblasts and immune cells, which could be resolved by single-cell RNA-sequencing in the tendon as recently described (De Micheli et al, 2020, Kelly et al, 2020.…”

Tendon Stem/Progenitor Cell Subpopulations and Their Implications in Tendon Biology

“…CD146 is a commonly used marker to identify pericyte population ( Gumucio et al, 2020 ). By utilizing Monocle pseudotime analysis, recent research has confirmed that pericytes form a part of the TSPCs population ( De Micheli et al, 2020 ). Moreover, Xu et al, have reported a P75 (p75 neurotrophin receptor) expressing cell subpopulation with stem cell characteristics within the perivascular regions which could proliferate within the peritenon and migrate to interstitial space in response to injury ( Xu et al, 2015 ).…”

“…Besides Scx, tendon cells also express S100a4 which may help mark subsets of resident tendon stem cells, as S100a4 combined with Scx, label distinct but overlapping tendon cell subpopulations during homeostasis and healing ( Best and Loiselle, 2019 ). Recent single-cell sequencing and Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITE-seq) results unveiled other previously unidentified tendon cell populations ( De Micheli et al, 2020 ; Kendal et al, 2020 ). Canonical tenogenic markers Scx, Mkx and tenomodulin were only observed to be expressed in a subset of tenocytes and not necessarily co-expressed, which suggests great heterogenicity in tendon cells with different origins or functions ( De Micheli et al, 2020 ).…”