The Behavior of Tendon Progenitor Cells from Tendinopathic Tendons: Implications for Treatment

Upregulation of FABP4 induced inflammation in the pathogenesis of chronic tendinopathy

Objectives

Excessive inflammation contributes to the pathogenesis of tendinopathy. Fatty acid binding protein 4 (FABP4) is a pro-inflammatory adipokine mediating various metabolic and inflammatory diseases. This study aimed to examine the expression of FABP4 and its association with the expressions of inflammatory cytokines in tendinopathy. The effects of a single injection of FABP4 on tendon pathology and inflammation were examined. The effect of FABP4 on the expressions of inflammatory cytokines and the effect of IL-1β on the expression of FABP4 in tendon-derived stem/progenitor cells (TDSCs) were also investigated.

Methods

1) Clinical patellar tendinopathy samples, healthy hamstring tendon samples, and healthy patellar tendon samples, 2) rotator cuff tendinopathy samples and healthy hamstring tendon samples; and 3) Achilles tendons of mice after saline or collagenase injection (CI) were stained for FABP4, IL-1β, IL-6, TNF-α and IL-10 by immunohistochemistry (IHC). For the rotator cuff tendinopathy samples, co-localization of FABP4 with IL-1β and TNF-α was done by immunofluorescent staining (IF). Mouse Achilles tendons injected with FABP4 or saline were collected for histology and IHC as well as microCT imaging post-injection. TDSCs were isolated from human and mouse tendons. The mRNA expressions of inflammatory cytokines in human and mouse TDSCs after the addition of FABP4 was quantified by qRT-PCR. The expression of FABP4 in TDSCs isolated from rotator cuff tendinopathy samples and healthy hamstring tendon samples was examined by IF. Mouse Achilles TDSCs were treated with IL-1β. The mRNA and protein expressions of FABP4 were examined by qRT-PCR and IF, respectively.

Results

There was significant upregulation of FABP4 in the patellar tendinopathy samples and rotator cuff tendinopathy samples compared to their corresponding controls. FABP4 was mainly expressed in the pathological areas including blood vessels, hypercellular and calcified regions. The expressions of IL-1β and TNF-α increased in human rotator cuff tendinopathy samples and co-localized with the expression of FABP4. Collagenase induced tendinopathic-like histopathological changes and ectopic calcification in the mouse Achilles tendinopathy model. The expressions of inflammatory cytokines (IL-1β, IL-6, TNF-α, IL-10) and FABP4 increased in hypercellular region, round cells chondrocyte-like cells and calcified regions in the mouse Achilles tendons post-collagenase injection. A single injection of FABP4 in mouse Achilles tendons induced histopathological changes resembling tendinopathy, with increased cell rounding, loss of collagen fiber alignment, and additionally presence of chondrocyte-like cells and calcification post-injection. The expressions of IL1-β, IL-6, TNF-α and IL-10 increased in mouse Achilles tendons post-FABP4 injection. FABP4 increased the expressions of IL10, IL6, and TNFa in human TDSCs as well as the expressions of Il1b, Il6, and Il10 in mouse TDSCs. Human tendinopathy TDSCs expressed higher level of FABP4 compared to healthy hamstring TDSCs. Besides, IL-1β increased the expression of FABP4 in mouse TDSCs.

Conclusion

In conclusion, an upregulation of FABP4 is involved in excessive inflammation and pathogenesis of tendinopathy. TDSCs is a potential source of FABP4 during tendon inflammation.

Translation potential of this article

FABP4 can be a potential treatment target of tendinopathy.

Establishment of a Mouse Degenerative Model of Patellar Tendinopathy with Upregulation of Inflammation

There is no mouse model of patellar tendinopathy. This study aimed to establish a mouse inflammatory and degenerative patellar tendon injury model, which will facilitate research on patellar tendinopathy using advanced molecular tools including transgenic models. Collagenase at different doses (low dose (LD), medium dose (MD), high dose (HD)) or saline was injected over the mouse patellar tendon. At weeks 1, 2, 4, and 8 post-injection, the tendons were harvested for histology and further examined by micro-computed tomography (microCT) imaging at week 8. The optimal dose group and the saline group were further evaluated by immunohistochemical staining, gait pattern, and biomechanical properties. The histopathological score increased dose-dependently post-collagenase injection. Ectopic mineralization was observed and increased with collagenase dose. The LD group was selected for further analysis. The expression of IL-10, TNF-α, and MMP-1 significantly increased post-injection. The changes of limb idleness index (ΔLII) compared to preinjury state were significantly higher, while the ultimate load, stiffness, ultimate stress, and maximum Young's modulus were significantly lower in the LD group compared to the saline group. A mouse inflammatory degenerative model of patellar tendon injury resembling tendinopathy was established as indicated by the dose-dependent increase in tendon histopathology, ectopic calcification, decrease in biomechanical properties, and pain-associated gait changes.

In vitro collagen biomarkers in mechanically stimulated human tendon cells: a systematic review

OBJECTIVE

The aim of this study was to comprehensively examine and summarize the available in vitro evidence regarding the relationship between mechanical stimulation and biomarkers of collagen synthesis in human-derived tendon cells.

METHODS

Systematic review with narrative analyses and risk of bias assessment guided by the Health Assessment and Translation tool. The electronic databases MEDLINE (Ovid), EMBASE (Ovid), CENTRAL (Ovid) and COMPENDEX (Engineering Village) were systematically searched from inception to 3 August 2023. Inclusion criteria encompassed English language, original experimental, or quasi-experimental in vitro publications that subjected human tendon cells to mechanical stimulation, with collagen synthesis (total collagen, type I, III, V, XI, XII, and XIV) and related biomarkers (matrix metalloproteinases, transforming growth factor β, scleraxis, basic fibroblast growth factor) as outcomes.

RESULTS

Twenty-one publications were included. A pervasive definite high risk of bias was evident in all included studies. Owing to incomplete outcome reporting and heterogeneity in mechanical stimulation protocols, planned meta-analyses were unfeasible. Reviewed data suggested that human tendon cells respond to mechanical stimulation with increased synthesis of collagen (e.g., COL1A1, procollagen, total soluble collagen, etc.), scleraxis and several matrix metalloproteinases. Results also indicate that mechanical stimulation dose magnitude may influence synthesis in several biomarkers.

CONCLUSIONS

A limited number of studies, unfortunately characterized by a definite high risk of bias, suggest that in vitro mechanical stimulation primarily increases type I collagen synthesis by human tendon cells. Findings from this systematic review provide researchers and clinicians with biological evidence concerning the possible beneficial influence of exercise and loading on cellular-level tendon adaptation.

Rat Plantar Fascia Stem/Progenitor Cells Showed Lower Expression of Ligament Markers and Higher Pro-Inflammatory Cytokines after Intensive Mechanical Loading or Interleukin-1β Treatment In Vitro

The pathogenesis of plantar fasciitis is unclear, which hampers the development of an effective treatment. The altered fate of plantar fascia stem/progenitor cells (PFSCs) under overuse-induced inflammation might contribute to the pathogenesis. This study aimed to isolate rat PFSCs and compared their stem cell-related properties with bone marrow stromal cells (BMSCs). The effects of inflammation and intensive mechanical loading on PFSCs' functions were also examined. We showed that plantar fascia-derived cells (PFCs) expressed common MSC surface markers and embryonic stemness markers. They expressed lower Nanog but higher Oct4 and Sox2, proliferated faster and formed more colonies compared to BMSCs. Although PFCs showed higher chondrogenic differentiation potential, they showed low osteogenic and adipogenic differentiation potential upon induction compared to BMSCs. The expression of ligament markers was higher in PFCs than in BMSCs. The isolated PFCs were hence PFSCs. Both IL-1β and intensive mechanical loading suppressed the mRNA expression of ligament markers but increased the expression of inflammatory cytokines and matrix-degrading enzymes in PFSCs. In summary, rat PFSCs were successfully isolated. They had poor multi-lineage differentiation potential compared to BMSCs. Inflammation after overuse altered the fate and inflammatory status of PFSCs, which might lead to poor ligament differentiation of PFSCs and extracellular matrix degeneration. Rat PFSCs can be used as an in vitro model for studying the effects of intensive mechanical loading-induced inflammation on matrix degeneration and erroneous stem/progenitor cell differentiation in plantar fasciitis.

Tendon-Derived Progenitor Cells With Multilineage Potential Are Present Within Human Patellar Tendon

Background:

Progenitor cells serve as a promising source of regenerative potential in a variety of tissue types yet remain underutilized in tendinopathy. Tendon-derived progenitor cells (TDPCs) have previously been isolated from hamstring tendon but only as part of a concomitant medical procedure. Determining the presence of TDPCs in patellar tendon may facilitate clinical utilization of these cells because of the relative accessibility of this location for tissue harvest.

Purpose:

To characterize TDPCs in human patellar tendon samples.

Study Design:

Descriptive laboratory study.

Methods:

Human patellar tendon samples were obtained during elective knee surgery. TDPCs were isolated and seeded at an optimal low cell density and subcultured to confluence for up to 2 passages. Flow cytometry was used to analyze for the expression of CD90+, CD105+, CD44+, and CD31–, CD34–, and CD45– markers. The multilineage differentiation potential of TDPCs was tested in vitro via adipogenic, osteogenic, and chondrogenic culture with subsequent cytochemical staining for Oil Red O, Alizarin Red, and Alcian Blue, respectively. Enzyme-linked immunosorbent assay was used to quantify the amount of adiponectin, alkaline phosphatase, and SRY-box transcription factor 9 secreted into cell culture supernatant for further confirmation of lineage differentiation. Results were analyzed statistically using the 2-tailed Student t test.

Results:

TDPCs demonstrated near-uniform expression of CD90, CD105, and CD44 with minimal expression of CD34, CD31, and CD45. Adipogenic, osteogenic, and chondrogenic differentiation of TDPCs was confirmed using qualitative analysis. The expression of adiponectin, alkaline phosphatase, and SRY-box transcription factor 9 were significantly increased in differentiated cells versus undifferentiated TDPCs (P < .05).

Conclusion:

TDPCs can be successfully isolated from human patellar tendon samples, and they exhibit characteristics of multipotent progenitor cells.

Clinical Relevance:

These data demonstrate the promise of patellar tendon tissue as a source of progenitor cells for use in biologic therapies for the treatment of tendinopathy.

Single-cell transcriptomic profiling reveals distinct mechanical responses between normal and diseased tendon progenitor cells

Regenerative medicine approaches utilizing stem cells offer a promising strategy to address tendinopathy, a class of common tendon disorders associated with pain and impaired function. Tendon progenitor cells (TPCs) are important in healing and maintaining tendon tissues. Here we provide a comprehensive single cell transcriptomic profiling of TPCs from three normal and three clinically classified tendinopathy samples in response to mechanical stimuli. Analysis reveals seven distinct TPC subpopulations including subsets that are responsive to the mechanical stress, highly clonogenic, and specialized in cytokine or growth factor expression. The single cell transcriptomic profiling of TPCs and their subsets serves as a foundation for further investigation into the pathology and molecular hallmarks of tendinopathy in mechanical stimulation conditions.

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.