Scleraxis-lineage cells are required for tendon homeostasis and their depletion induces an accelerated extracellular matrix aging phenotype

  1. Antonion Korcari
  2. Anne EC Nichols
  3. Mark R Buckley
  4. Alayna E Loiselle

  • Curated by eLife

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    eLife assessment

    This fundamental work advances our understanding of the cellular and molecular changes of the aged tendon. The evidence supporting the conclusion is convincing, using a DTR-based ScxLin cell depletion model along with state-of-art proteomic and scRNA-seq analyses. This paper is of potential interest to scientists and physicians who study the mechanisms of the tendon aging process.

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Abstract

Aged tendons have disrupted homeostasis, increased injury risk, and impaired healing capacity. Understanding mechanisms of homeostatic disruption is crucial for developing therapeutics to retain tendon health through the lifespan. Here, we developed a novel model of accelerated tendon extracellular matrix (ECM) aging via depletion of Scleraxis-lineage cells in young mice (Scx-DTR). Scx-DTR recapitulates many aspects of tendon aging including comparable declines in cellularity, alterations in ECM structure, organization, and composition. Single-cell RNA sequencing demonstrated a conserved decline in tenocytes associated with ECM biosynthesis in aged and Scx-DTR tendons, identifying the requirement for Scleraxis-lineage cells during homeostasis. However, the remaining cells in aged and Scx-DTR tendons demonstrate functional divergence. Aged tenocytes become pro-inflammatory and lose proteostasis. In contrast, tenocytes from Scx-DTR tendons demonstrate enhanced remodeling capacity. Collectively, this study defines Scx-DTR as a novel model of accelerated tendon ECM aging and identifies novel biological intervention points to maintain tendon function through the lifespan.

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  1. Version published to 10.7554/elife.84194 on eLife
  2. eLife

    eLife assessment

    This fundamental work advances our understanding of the cellular and molecular changes of the aged tendon. The evidence supporting the conclusion is convincing, using a DTR-based ScxLin cell depletion model along with state-of-art proteomic and scRNA-seq analyses. This paper is of potential interest to scientists and physicians who study the mechanisms of the tendon aging process.

  3. eLife

    Reviewer #1 (Public Review):

    This study developed a novel model of accelerated tendon extracellular matrix (ECM) aging via depletion of Scleraxis-lineage (ScxLin) cells in young mice (DTR). The authors found the depletion reduced cell numbers to similar baselines as aged tendons, indicating that a minimum cell number threshold exists to maintain tendon. This cell loss coincided with disrupted ECM organization and reduced mechanical properties. The DTR and aged tendons had similar protein composition with the main difference compared to young healthy tendons being a loss of high turnover ECM proteins. Via scRNA-seq, DTR and aged tendon had fewer biosynthetic cells, correlating with loss of certain ECM proteins. Interestingly, the remaining cells in the DTR model differed from aged tendons. While somewhat artificial, this depletion model system is an interesting way to investigate mechanisms that lead to reduced ECM turnover and matrix degeneration, and may have inform the mechanisms by which aging affects the maintenance of dense connective tissues.

  4. eLife

    Reviewer #2 (Public Review):

    The molecular changes of the aged tendon are not well understood. Loiselle et al previously established a mouse model that mimics aging tendon, where they depleted Scleraxis lineage (Scxlin) cells from tendon by injecting diptheria toxin (DT) in mice expressing the DT receptor under the control of the Scx promoter (DTR mice). In this manuscript, the authors demonstrate that the tendons from DTR mice resemble tendons from aged WT mice, in that they both have decreased cellularity, altered collagen organization (via SHG imaging), and impaired biomechanical properties. Proteomic analysis of WT, DTR, and aged WT tendons show that both DTR and aged WT tendons have decreased expression of extracellular matrix proteins (ECM). Corresponding with this, single RNA seq analysis of tendons from these three groups of mice showed that while WT tendons are enriched for genes related to collagen and ECM synthesis and also inflammation, DTR tendons express genes associated with ECM organization and structure and aged tendons express genes that regulate inflammation. The authors point out that this supports designing therapies to prevent tendon cell death to prevent the changes seen in aging tendon.

    These data enhances the understanding of the protein and gene changes associated with aging in the tendon and in particular characterizes the importance of Scx+ cells to tendon organization and the aging process. The conclusions are supported by the data presented.

    The manuscript would be strengthened by:

    1. Improved clarity of figures presented
    2. More details on the methodology used for biomechanical testing
    3. Clarification if the decrease in ECM protein expression is due to decreased cellularity in the tendons of the DTR and aged mice, or decreased expression per cell
    4. Providing more details on genes that are downregulated in comparison between groups
  5. Version published to 10.1101/2022.01.20.477119v2 on bioRxiv
  6. Version published to 10.1101/2022.01.20.477119v1 on bioRxiv