The tendon interfascicular basement membrane provides a vascular niche for CD146⁺pericyte cell subpopulations

Abstract

The interfascicular matrix (IFM) is critical to the mechanical adaptations and response to load in energy-storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT). We hypothesized that the IFM is a tendon progenitor cell niche housing an exclusive cell subpopulation. Immunolabelling of equine SDFT was used to identify the IFM niche, localising expression patterns of CD31 (endothelial cells), CD146 (IFM cells) and LAMA4 (IFM basement membrane marker). Magnetic-activated cell sorting was employed to isolate and compare in vitro properties of CD146+ and CD146- subpopulations. CD146 demarcated an exclusive interfascicular cell subpopulation that resides in proximity to a basal lamina which forms interconnected vascular networks. Isolated CD146+ cells exhibited limited mineralization (osteogenesis) and lipid pro-duction (adipogenesis). This study demonstrates that the IFM is a unique tendon cell niche, con-taining a vascular-rich network of basement membrane, CD31+ endothelial cells and CD146+ cell populations that are likely essential to tendon structure- and/or function. Interfascicular CD146+ subpopulations did not exhibit stem cell-like phenotypes and are more likely to represent a per-icyte lineage. Previous work has shown that tendon CD146 cells migrate to sites of injury, therefore mobilisation of endogenous tendon IFM cell populations may promote intrinsic repair.

The interfascicular matrix of energy storing tendons houses heterogenous cell populations disproportionately affected by ageing

“…Our results demonstrate that the IFM houses a unique tenocyte population that can be distinguished from fascicular tenocytes due to higher expression of PRG4 and TNXB, and lower expression of COMP, LOX and THBS4. Tendon endothelial, mural and immune cell populations also localise to the IFM, supporting recent work that has identified the presence of an endothelial-like basement membrane in the IFM (Marr et al, 2022). Previous scRNAseq studies of mouse and human tendons have identified a range of similar cell populations to those identified in the current study, encompassing tenocytes, endothelial cells and immune cells (Kendal et al, 2020, De Micheli et al, 2020.…”

“…Results showed alterations in expression of matrisomal genes with ageing in tenocytes, with a general decrease in expression of collagens, with the exception of COL4A1&2 , which increased in IFM subclusters. These genes encode an integral component of the IFM basement membrane (Marr et al, 2022), and therefore these changes may result in alterations to IFM basement membrane structure. In addition, the decrease in LAMA4 expression with ageing in tenocytes may have important implications for healing, as it has previously been shown that LAMA4 may be important for the recruitment of IFM cell populations after injury (Marr et al, 2021).…”