Characteristics of tendon derived stem cells according to different factors to induce the tendinopathy

NSUN2 facilitates tenogenic differentiation of rat tendon-derived stem cells via m5C methylation of KLF2

Introduction

Tendon-derived stem cells (TDSCs) play a critical role in tendon repair. N5-methylcytosine (m5C) is a key regulator of cellular processes such as differentiation. This study aimed to investigate the impact of m5C on TDSC differentiation and the underlying mechanism.

Methods

TDSCs were isolated from rats and identified, and a tendon injury rat model was generated. Tenogenic differentiation in vitro was evaluated using Sirius red staining and quantitative real-time polymerase chain reaction, while that in vivo was assessed using immunohistochemistry and hematoxylin‒eosin staining. m5C methylation was analyzed using methylated RNA immunoprecipitation, dual-luciferase reporter assay, and RNA stability assay.

Results

The results showed that m5C levels and NSUN2 expression were increased in TDSCs after tenogenic differentiation. Knockdown of NSUN2 inhibited m5C methylation of KLF2 and decreased its stability, which was recognized by YBX1. Moreover, interfering with KLF2 suppressed tenogenic differentiation of TDSCs, which could be abrogated by KLF2 overexpression. Additionally, TDSCs after NSUN2 overexpression contributed to ameliorating tendon injury in vivo. In conclusion, NSUN2 promotes tenogenic differentiation of TDSCs via m5C methylation of KLF2 and accelerates tendon repair.

Conclusions

The findings suggest that overexpression of NSUN2 can stimulate the differentiation ability of TDSCs, which can be used in the treatment of tendinopathy.

Integrative single-cell RNA and ATAC sequencing reveals that the FOXO1-PRDX2-TNF axis regulates tendinopathy

Introduction

Tendinopathy, the most common form of chronic tendon disorder, leads to persistent tendon pain and loss of function. Profiling the heterogeneous cellular composition in the tendon microenvironment helps to elucidate rational molecular mechanisms of tendinopathy.

Methods and results

In this study, through a multi-modal analysis, a single-cell RNA- and ATAC-seq integrated tendinopathy landscape was generated for the first time. We found that a specific cell subpopulation with low PRDX2 expression exhibited a higher level of inflammation, lower proliferation and migration ability, which not only promoted tendon injury but also led to microenvironment deterioration. Mechanistically, a motif enrichment analysis of chromatin accessibility showed that FOXO1 was an upstream regulator of PRDX2 transcription, and we confirmed that functional blockade of FOXO1 activity induced PRDX2 silencing. The TNF signaling pathway was significantly activated in the PRDX2-low group, and TNF inhibition effectively restored diseased cell degradation.

Discussion

We revealed an essential role of diseased cells in tendinopathy and proposed the FOXO1-PRDX2-TNF axis is a potential regulatory mechanism for the treatment of tendinopathy.

Targeting Senescent Tendon Stem/Progenitor Cells to Prevent or Treat Age-Related Tendon Disorders

Age-related tendon disorder, a primary motor system disease, is characterized by biological changes in the tendon tissue due to senescence and seriously affects the quality of life of the elderly. The pathogenesis of this disease is not well-understood. Tendon stem/progenitor cells (TSPCs) exhibit multi-differentiation capacity. These cells are important cellular components of the tendon because of their roles in tendon tissue homeostasis, remodeling, and repair. Previous studies revealed alterations in the biological characteristics and tenogenic differentiation potential of TSPCs in senescent tendon tissue, in turn contributing to insufficient differentiation of TSPCs into tenocytes. Poor tendon repair can result in age-related tendinopathies. Therefore, targeting of senescent TSPCs may restore the tenogenic differentiation potential of these cells and achieve homeostasis of the tendon tissue to prevent or treat age-related tendinopathy. In this review, we summarize the biological characteristics of TSPCs and histopathological changes in age-related tendinopathy, as well as the potential mechanisms through which TSPCs contribute to senescence. This information may promote further exploration of innovative treatment strategies to rescue TSPCs from senescence.

Deep Insight into the Role of MIF in Spondyloarthritis

PURPOSE OF REVIEW

Pathological roles of macrophage migration inhibitory factor (MIF) have recently been demonstrated in spondyloarthritis (SpA) preclinical models, identifying MIF as a new treatment target for SpA. However, the specific contribution of MIF and therapeutic potential of MIF-targeted therapies to various tissue types affected by SpA are not well delineated.

RECENT FINDINGS

MIF and its cognate receptor CD74 are extensively involved in the pathogenesis of SpA including inflammation in the spine, joint, eyes, skin, and gut. The majority of the current evidence has consistently shown that MIF drives the inflammation in these distinct anatomical sites. In preclinical models, genetic deletion or blockade of MIF reduces the severity of inflammation. Although MIF is generally an upstream cytokine which regulates downstream effector cytokines, MIF also intensifies type 3 immunity by promoting helper T 17 (Th17) plasticity. MIF- or CD74-targeted therapies have also reported to be well tolerated in clinical trials for other diseases. Recent findings suggest that MIF-CD74 axis is a new therapeutic target for SpA to improve various clinical features. Clinical trials for MIF- or CD74-targeted therapies for SpA patients are warranted.

Advanced glycation end productions and tendon stem/progenitor cells in pathogenesis of diabetic tendinopathy

Tendinopathy is a challenging complication observed in patients with diabetes mellitus. Tendinopathy usually leads to chronic pain, limited joint motion, and even ruptured tendons. Imaging and histological analyses have revealed pathological changes in various tendons of patients with diabetes, including disorganized arrangement of collagen fibers, microtears, calcium nodules, and advanced glycation end product (AGE) deposition. Tendon-derived stem/ progenitor cells (TSPCs) were found to maintain hemostasis and to participate in the reversal of tendinopathy. We also discovered the aberrant osteochondrogenesis of TSPCs in vitro. However, the relationship between AGEs and TSPCs in diabetic tendinopathy and the underlying mechanism remain unclear. In this review, we summarize the current findings in this field and hypothesize that AGEs could alter the properties of tendons in patients with diabetes by regulating the proliferation and differentiation of TSPCs in vivo.

Altered Differentiation of Tendon-Derived Stem Cells in Diabetic Conditions Mediated by Macrophage Migration Inhibitory Factor

The purpose of our study was to evaluate the role of macrophage migration inhibitory factor (MIF) in the differentiation of tendon-derived stem cells (TdSCs) under hyperglycemic conditions. In the in vivo experiment, rats were classified into diabetic (DM) and non-DM groups depending on the intraperitoneal streptozotocin (STZ) or saline injection. Twelve-week after STZ injection, the supraspinatus tendon was harvested and prepared for histological evaluation and real-time reverse transcription polymerase chain reaction for osteochondrogenic (aggrecan, BMP-2, and Sox9) and tenogenic (Egr1, Mkx, scleraxis, type 1 collagen, and Tnmd) markers. For the in vitro experiment, TdSCs were isolated from healthy rat Achilles tendons. Cultured TdSCs were treated with methylglyoxal and recombinant MIF or MIF gene knockdown to determine the effect of hyperglycemic conditions and MIF on the differentiation function of TdSCs. These conditions were classified into four groups: hyperglycemic-control group, hyperglycemic-recombinant-MIF group, hyperglycemic-knockdown-MIF group, and normal-control group. The mRNA expression of osteochondrogenic and tenogenic markers was compared among the groups. In the in vivo experiment, the mRNA expression of all osteochondrogenic and tenogenic differentiation markers in the DM group was significantly higher and lower than that in the non-DM group, respectively. Similarly, in the in vitro experiments, the expression of all osteochondrogenic and tenogenic differentiation markers was significantly upregulated and downregulated, respectively, in the hyperglycemic-control group compared to that in the normal-control group. The hyperglycemic-knockdown-MIF group demonstrated significantly decreased expression of all osteochondrogenic differentiation markers and increased expression of only some tenogenic differentiation markers compared with the hyperglycemic-control group. In contrast, the hyperglycemic-recombinant-MIF group showed significantly increased expression of all osteochondrogenic differentiation markers, but no significant difference in any tenogenic marker level, compared to the hyperglycemic-control group. These results suggest that tendon homeostasis could be affected by hyperglycemic conditions, and MIF appears to alter the differentiation of TdSCs via enhancement of the osteochondrogenic differentiation in hyperglycemic conditions. These are preliminary findings, and must be confirmed in a further study.

Tendon Stem/Progenitor Cell Subpopulations and Their Implications in Tendon Biology

Tendon harbors a cell population that possesses stem cell characteristics such as clonogenicity, multipotency and self-renewal capacity, commonly referred to as tendon stem/progenitor cells (TSPCs). Various techniques have been employed to study how TSPCs are implicated in tendon development, homeostasis and healing. Recent advances in single-cell analysis have enabled much progress in identifying and characterizing distinct subpopulations of TSPCs, which provides a more comprehensive view of TSPCs function in tendon biology. Understanding the mechanisms of physiological and pathological processes regulated by TSPCs, especially a particular subpopulation, would greatly benefit treatment of diseased tendons. Here, we summarize the current scientific literature on the various subpopulations of TSPCs, and discuss how TSPCs can contribute to tissue homeostasis and pathogenesis, as well as examine the key modulatory signaling pathways that determine stem/progenitor cell state. A better understanding of the roles that TSPCs play in tendon biology may facilitate the development of novel treatment strategies for tendon diseases.

Moderate and intensive mechanical loading differentially modulate the phenotype of tendon stem/progenitor cells in vivo

To examine the differential mechanobiological responses of specific resident tendon cells, we developed an in vivo model of whole-body irradiation followed by injection of either tendon stem/progenitor cells (TSCs) expressing green fluorescent protein (GFP-TSCs) or mature tenocytes expressing GFP (GFP-TNCs) into the patellar tendons of wild type C57 mice. Injected mice were subjected to short term (3 weeks) treadmill running, specifically moderate treadmill running (MTR) and intensive treadmill running (ITR). In MTR mice, both GFP-TSC and GFP-TNC injected tendons maintained normal cell morphology with elevated expression of tendon related markers collagen I and tenomodulin. In ITR mice injected with GFP-TNCs, cells also maintained an elongated shape similar to the shape found in normal/untreated control mice, as well as elevated expression of tendon related markers. However, ITR mice injected with GFP-TSCs showed abnormal changes, such as cell morphology transitioning to a round shape, elevated chondrogenic differentiation, and increased gene expression of non-tenocyte related genes LPL, Runx-2, and SOX-9. Increased gene expression data was supported by immunostaining showing elevated expression of SOX-9, Runx-2, and PPARγ. This study provides evidence that while MTR maintains tendon homeostasis by promoting the differentiation of TSCs into TNCs, ITR causes the onset of tendinopathy development by inducing non-tenocyte differentiation of TSCs, which may eventually lead to the formation of non-tendinous tissues in tendon tissue after long term mechanical overloading conditions on the tendon.

Recovery of Tendon Characteristics by Inhibition of Aberrant Differentiation of Tendon-Derived Stem Cells from Degenerative Tendinopathy

The inhibition of the aberrant differentiation of tendon-derived stem cells (TDSCs) is a major target for the regeneration of damaged tendon tissues, as tendinopathy can be caused by the aberrant differentiation of TDSCs. We investigated whether the possible aberrant differentiation of TDSCs can be prevented by using adequate inhibitors. TDSCs extracted from chemically induced tendinopathy and injury-with-overuse tendinopathy models were cultured with 18α-glycyrrhetinic acid (AGA) and T0070907 to block osteogenic differentiation and adipogenic differentiation, respectively. The optimal dose of AGA decreased the osteogenic-specific marker Runx2 (Runt-related transcription factor 2), and T0070907 blocked the adipogenic-specific marker peroxisome proliferator-activated receptor gamma (PPARγ) in mRNA levels. We also found that AGA induced tenogenic differentiation in mRNA levels. However, T0070907 did not affect the tenogenic differentiation and regenerative capacity of TDSCs. We expect that optimal doses of AGA and T0070907 can prevent tendinopathy by inhibiting osteogenic and adipogenic differentiation, respectively. In addition, AGA and T0070907 may play important roles in the treatment of tendinopathy.

High cholesterol inhibits tendon-related gene expressions in tendon-derived stem cells through reactive oxygen species-activated nuclear factor-κB signaling

Clinical studies have indicated that increased serum cholesterol levels raised the risk of tendinopathy in hypercholesterolemia, but the effect of cholesterol on tendon-derived stem cells (TDSCs) and its underlying mechanism have not been studied. The purpose of this study is to investigate the association between cholesterol and tendinopathy in vitro and in vivo, and its underlying molecular mechanism as well. In TDSCs, the effect of cholesterol was assessed by quantitative polymerase chain reaction, western blot analysis, and immunofluorescence staining. Intracellular levels of reactive oxygen species (ROS) was detected, using flow cytometry. The link between nuclear factor (NF)-κB signaling and the effect of cholesterol was evaluated using a representative IκB kinase (IKK) inhibitor, BAY 11-7082. In addition, Achilles tendons from apolipoprotein E mice fed with a high-fat diet were histologically assessed using hematoxylin and eosin staining and immunohistochemistry. We found that high cholesterol apparently lowered the expression of tendon cell markers (collagen 1, scleraxis, tenomodulin), and elevated ROS levels via the NF-κB pathway both in vitro and in vivo. The ROS scavenger N-acetylcysteine (NAC) and BAY 11-7082 reversed the inhibiting effect of cholesterol on the tendon-related gene expressions of TDSCs. Moreover, NAC blocked cholesterol-induced phosphorylation of IκBα and p65. Significant histological alternation in vivo was shown in Achilles tendon in the hypercholesterolemic group. These results indicated that high cholesterol may inhibit the tendon-related gene expressions in TDSCs via ROS-activated NF-кB signaling, implying pathogenesis of tendinopathy in hypercholesterolemia and suggesting a new mechanism underlying hypercholesterolemia-induced tendinopathy.

Nanotopographic cues and stiffness control of tendon-derived stem cells from diverse conditions

Background

Tendon-derived stem cells (TDSCs) are key factors associated with regeneration and healing in tendinopathy. The aim of this study was to investigate the effects of mechanical stiffness and topographic signals on the differentiation of TDSCs depending on age and pathological conditions.

Materials and methods

We compared TDSCs extracted from normal tendon tissues with TDSCs from tendinopathic Achilles tendon tissues of Sprague Dawley rats in vitro and TDSCs cultured on nanotopographic cues and substrate stiffness to determine how to control the TDSCs. The tendinopathy model was created using a chemical induction method, and the tendon injury model was created via an injury-and-overuse method. Norland Optical Adhesive 86 (NOA86) substrate with 2.48 GPa stiffness with and without 800 nm-wide nanogrooves and a polyurethane substrate with 800 nm-wide nanogrooves were used.

Results

TDSCs from 5-week-old normal tendon showed high expression of type III collagen on the flat NOA86 substrate. In the 15-week normal tendon model, expression of type III collagen was high in TDSCs cultured on the 800 nm NOA86 substrates. However, in the 15-week tendon injury model, expression of type III collagen was similar irrespective of nanotopographic cues or substrate stiffness. The expression of type I collagen was also independent of nanotopographic cues and substrate stiffness in the 15-week normal and tendon injury models. Gene expression of scleraxis was increased in TDSCs cultured on the flat NOA86 substrate in the 5-week normal tendon model (P=0.001). In the 15-week normal tendon model, scleraxis was highly expressed in TDSCs cultured on the 800 nm and flat NOA86 substrate (P=0.043). However, this gene expression was not significantly different between the substrates in the 5-week tendinopathy and 15-week tendon injury models.

Conclusion

Development and maturation of tendon are enhanced when TDSCs from normal tendons were cultured on stiff surface, but not when the TDSCs came from pathologic models. Therapeutic applications of TDSCs need to be flexible based on tendon age and tendinopathy.