Single-Cell RNA-Seq Analysis Reveals Macrophage Involved in the Progression of Human Intervertebral Disc Degeneration

Zemin Ling,Yulin Huang,Bolin Chen,Baozhu Zeng,Jiaming Yang,Yong Liu,Yu Gao,Fuxin Wei,Ziji Zhang,Xiuhui Wang,Chang Jiang,Xuenong Zou,Zhe Wang

doi:10.3389/fcell.2021.833420

Abstract

Intervertebral disc degeneration (IDD) has been considered as the primary pathological mechanism that underlies low back pain. Understanding the molecular mechanisms underlying human IDD is imperative for making strategies to treat IDD-related diseases. Herein, we report the molecular programs, lineage progression patterns, and paths of cellular communications during the progression of IDD using single-cell RNA sequencing (scRNA-seq) on nucleus pulposus (NP) cells from patients with different grades of IDD undergoing discectomy. New subtypes of cells and cell-type-specific gene signatures of the metabolic homeostatic NP cells (Met NPC), adhesive NP cells (Adh NPC), inflammatory response NP cells (IR NPC), endoplasmic reticulum stress NP cells (ERS NPC), fibrocartilaginous NP cells (Fc NPC), and CD70 and CD82+ progenitor NP cells (Pro NPC) were identified. In the late stage of IDD, the IR NPC and Fc NPC account for a large proportion of NPC. Importantly, immune cells including macrophages, T cells, myeloid progenitors, and neutrophils were also identified, and further analysis showed that significant intercellular interaction between macrophages and Pro NPC occurred via MIF (macrophage migration inhibitory factor) and NF-kB signaling pathways during the progression of IDD. In addition, dynamic polarization of macrophage M1 and M2 cell subtypes was found in the progression of IDD, and gene set functional enrichment analysis suggested a significant role of the macrophage polarization in regulating cell metabolism, especially the Pro NPC. Finally, we found that the NP cells in the late degenerative stage were mainly composed of the cell types related to inflammatory and endoplasmic reticulum (ER) response, and fibrocartilaginous activity. Our results provided new insights into the identification of NP cell populations at single-cell resolution and at the relatively whole-transcriptome scale, accompanied by cellular communications between immune cells and NP cells, and discriminative markers in relation to specific cell subsets. These new findings present clues for effective and functional manipulation of human IDD-related bioremediation and healthcare.

Highlights

Low back pain (LBP), as one of the most common disorders in the musculoskeletal system (Ferguson and Steffen, 2003; Paige et al, 2017; Tong et al, 2017), causes significant economic losses worldwide each year (Hoffman and Dow, 2016; Juch et al, 2017; Hall et al, 2019)
The heterogeneity of cells was classified by gene expression and the functional analysis, and scRNA sequencing of nucleus pulposus (NP) tissues identified 11 clear separations of cell types, which included six types of NP cells and five types of immune cells (Figure 1B)
The results showed that the numbers of inflammatory response NP cells increased in grade IV than in grade II, but the metabolic homeostasis NP cells were significantly decreased, suggesting that NP cells may be influenced by the inflammatory factors from keeping homeostasis during disc degeneration

Summary

Introduction

Low back pain (LBP), as one of the most common disorders in the musculoskeletal system (Ferguson and Steffen, 2003; Paige et al, 2017; Tong et al, 2017), causes significant economic losses worldwide each year (Hoffman and Dow, 2016; Juch et al, 2017; Hall et al, 2019). Intervertebral disc degeneration (IDD) has been considered as the primary pathological mechanism that underlies LBP, and receives increasing attention from precision-related regenerative medicine (Zingg and Kendall, 2017). It has been reported that the initial degeneration of intervertebral disc may be presented as early as in the adolescence, when 20% of youngsters have mild signs of the IDD (Boxberger et al, 2009). The disc is a physiological non-self-renewing avascular tissue and consists of annulus fibrosus and nucleus pulposus (NP), which undergoes cellular and extracellular matrix (ECM) changes during IDD at the early pathological stage (Mwale et al, 2014). The process of IDD encompasses the structural damage of the disc and the changes in number and composition of cells. It is still a challenge to repair or regenerate the structure and functions of the intervertebral disc by conservative or surgical therapies (Yim et al, 2014; Wang et al, 2015)

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