Single-Cell RNA Sequencing Reveals the Cell Types Heterogenicity of Human Discoid Lateral Meniscus Cells

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Background: Discoid lateral meniscus (DLM) is more prone to injury than a normally shaped meniscus. A previous paper successfully identified the different cell types and their corresponding marker genes in meniscus tissues; however, no study has compared the gene expression and cell heterogeneity between discoid and normal menisci. Purpose: To identify specific cell clusters and their marker genes in discoid meniscus, thereby providing a theoretical basis for the treatment and etiology of DLM. Study Design: Controlled laboratory study. Methods: ScRNA-seq was used in DLM and osteoarthritis lateral meniscus (OAM) cells to identify cell subsets and their gene signatures, determine their differentiation relationships, as well as identify specific cell types. Pseudo-time analysis and immunohistochemical staining were used to investigate the temporal and spatial distribution of DLM-specific clusters. Results: ScRNA-seq identified nine clusters originating from DLM and OAM, composed of seven empirically defined populations and two novel populations specific to DLM, namely, the prehypertrophic chondrocyte 2 (PreHTC-2)26 and regulatory chondrocyte (RegC-2) populations. Single-cell trajectory showed that RegC-2 and PreHTC-2 were mainly distributed in a specific cell fate, with the PreHTC-2 marker gene HAPLN1 highly expressed at the end of this fate.Immunohistochemical staining showed that HAPLN1+ cells were mainly distributed in the white zone of DLM. Matrix metalloproteinase (MMP) variants were expressed in DLM and OAM, with MMP2 highly expressed in OAM-dominant cell clusters, while MMP3 was highly expressed in DLM-dominant cell clusters. Conclusion: Two novel cell clusters including PreHTC-2 were identified using single-cell sequencing, which were mainly distributed in the white areas of DLM. We also found differentiated MMP expression in the trajectory, which may be a possible mechanism of DLM formation. Clinical Relevance: Meniscectomy resected the white part of DLM by arthroscopy, which is the main surgical treatment for DLM. We found that DLM-specific cells were mainly distributed in the white areas, providing an important theoretical basis for the treatment of DLM. Funding Statement: This study was funded by the National Natural Science Foundation of China (No. 82002293), China Postdoctoral Science Foundation funded project (No. 2019M663265, 2019TQ0387), Natural Science Foundation of Guangdong Province (No. 2018A030310231), The Fundamental Research Funds for the Central Universities (No. 18zxxt61), and Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515011647). Declaration of Interests: None declared. Ethics Approval Statement: Protocol approved by the Ethical Committee of Sun Yat-sen Memorial Hospital of Sun Yat‐sen University.