Exploring cellular changes in ruptured human quadriceps tendons at single-cell resolution

Abstract

Tendon ruptures in humans have regularly been studied during the chronic phase of injury. However, the early response to injury remains less investigated. Quadriceps tendons, which require prompt surgical treatment, offer a model to investigate this early response. Therefore, this study aimed to explore the early cellular changes in ruptured compared to healthy human quadriceps tendons. Quadriceps tendon samples were collected from patients undergoing tibial shaft fracture repair (healthy) or tendon repair surgery for complete rupture (collected 7-8 days post-injury). Nuclei were isolated for single-nucleus RNA sequencing, and comprehensive transcriptomic analysis was conducted to profile cellular changes. The transcriptomes of 12,808 nuclei were profiled, including 7,268 nuclei from healthy and 5,540 nuclei from ruptured quadriceps tendons, revealing 12 major cell types and several cell subtypes and states. Rupture samples showed increased expression of genes related to extracellular matrix organisation and cell cycle signalling, and a decrease in expression of genes in lipid metabolism pathways. These changes were driven predominantly by gene expression changes in the fibroblast, vascular endothelial cells (VECs), mural cell, and macrophage populations: fibroblasts shift to an activated phenotype upon rupture and there is an increase in proportion of capillary and dividing VECs, suggesting an angiogenic response. A diverse immune environment was observed, with a shift from homeostatic to activated macrophages following rupture. Cell-cell interactions increased in rupture, both in their number and diversity, and primarily involving fibroblast and endothelial cell populations. Collectively, this transcriptomic analysis suggests that fibroblasts and endothelial cells are key orchestrators of the early injury response within ruptured quadriceps tendon.