Understanding cellular and molecular mechanisms of pathogenesis of diabetic tendinopathy

Table 1 Biomechanical property changes of diabetic tendinopathy

Species	Tendon(s)	Groups	Duration of DM	Biomechanical properties of diabetic tendons	Ref.
Human	Achilles tendon	CG and DG	An average of 14 yr	Significantly less tendon elongation, higher tendon stiffness and hysteresis, and lower tendon forces in DM group during walking compared with CG.	Petrovic et al[23], 2018
Human	Achilles tendon	CG and DG	An average of 13 yr	No significance in maximum force including max force, stiffness, stress, strain, and modulus between DG and CG, but a trend towards reduced tendon strain in DG; significantly higher tendon modulus in common force in DG than in CG.	Couppé et al[24], 2016
Human	Achilles tendon	CG and DG	Stage V type II diabetes patients	Significantly inferior biomechanical properties of diabetic tendons in DG including decreased elasticity (Young′s modulus), maximum load, stiffness, toughness, load at yield point, energy, strain, and elongation at break point, tenacity, and strain at automatic load drop.	Guney et al[17], 2015
Human	Achilles tendon	CG, DG I (with foot ulcer), and DG II (without foot ulcer).	An average of 15 yr in DG I and an average of 6 yr in DG II.	Significantly higher thickness of proximal, medial, and distal third tendon in DG I than in DG II and CG, higher tendon thickness in DG II than in CG but no significance; significantly reduced stiffness of medial and distal third tendon in DG I.	Evranos et al[22], 2015
Male C57Bl/6J mice	FDL tendon	CG (low fat diet) and DM (high fat diet)	High or low-fat diet for 48 wk; at 12, 24, and 48 d post-injury.	Significantly decreased tendon range of motion at 40 and 48 wk in high fat diet group relative to low fat diet group; reduced max load at failure at 48 wk and increased stiffness at 24 wk in high fat diet group.	Studentsova et al[30], 2018
Male C57Bl/6J mice	FDL tendon	CG (low fat diet) and DM (high fat diet)	High or low-fat diet for 12 wk; at 10, 14, 21, and 28 d post-diet initiation.	Significantly lower maximum load, yield load, and energy to maximum force of tendon in DM compared with CG at 28 d; no differences in stiffness between the two groups.	Ackerman et al[31], 2017
Male C57BL/KsJ (db/db) mice	Achilles tendon	CG and DG	16 wk of DM	Significantly decreased maximum load, elastic modulus, maximum stress, and stiffness of tendons in DG; no significance in tensile strain.	Boivin et al[16], 2014
db/db Diabetic mice and db/+ non-diabetic heterozygous control mice	Supraspinatus, Achilles, and patellar tendons.	CG and DG	60 days for DM	Significantly reduced stiffness at the insertion site of tendons in DG for all three tendons and reduced modulus at the insertion site of Achilles tendons in DG; no significance in stiffness or modulus of mid-substance in any tendon between DG and CG.	Connizzo et al[33], 2014
Male C57BL/6J mice	FDL tendon	CG (low fat diet) and DG (high fat diet)	High or low-fat diet for 12 wk for uninjured tendons; high or low-fat diet for 24 wk for injured tendons, at 7, 14, and 28 d post-injury.	No significance in biomechanical parameters including maximum force, work to maximum force, and stiffness of uninjured FDL tendon at 12 wk; reduced maximum force of uninjured FDL tendon at 24 wk; significantly decreased biomechanical parameters of injured tendons in DG at 28 d.	David et al[32], 2014
Wistar rats	Achilles tendon	CG and DG	4 wk post-induction; 3 wk post-operation.	No significance in ultimate load, ultimate elongation, stiffness, ultimate strength, ultimate strain, elastic modulus, and cross-sectional area.	de Oliveira et al[36], 2019
Wistar rats	Achilles tendon	CG and DG	5 wk post-induction	Significantly increased elastic modulus and maximum tension, reduced transverse area in DG; no significance in maximum strength between DG and CG.	Bezerra et al[27], 2016
SD rats	Achilles and tail tendon	CG, acute DG (1 wk), and chronic DG (10 wk)	10 wk post-induction	No significance in biomechanical properties of Achilles and tail tendons between groups, including maximum force, deformation, stiffness, stress, strain, and Young’s modulus.	Volper et al[12], 2015
Wistar rats	Achilles tendon	CG and DG	30 d post-induction; at days 10 post-surgery.	Significantly decreased stress tensile load and Young's modulus of stiffness of tendons in DG than in CG.	Mohsenifar et al[20], 2014
ZDSD and control rats (CD: SD-derived)	Tail tendon	CG and DG	High fat diet for 12 wk	Significantly higher nanoscale modulus at tendon fibrils level in DG and more variable compared with CG; at the fascicle level, no significance in mechanical properties between DG and CG; at the material level, significantly greater ultimate stress and modulus in DG tendon than in CG.	Gonzalez et al[28], 2014
SD rats	Supraspinatus tendon	Hyperglycemia group and control group	8 wk following hyperglycemia induction	No significance in stiffness and modulus at both the insertion site and mid-substance of tendon between hyperglycemia group and control group.	Thomas et al[35], 2014
Lewis rats	Achilles tendon	CG and DG	5 d post-induction	Significantly reduced maximum tensile load of tendon in DG.	Lehner et al[19], 2012
Male diabetic GK rats and control Wistar rats	Achilles tendon	CG and DG	1 year of DM; at 14 d post-rupture.	No significance in biomechanical properties as peak load, energy at peak load and stress, except for lower stiffness of intact tendons in DG; lower stiffness of injured tendons in DG compared with the injured tendons in CG.	Ahmed et al[21], 2012
Wistar rats	Achilles tendon	CG and DG	70 d post-induction	Significantly decreased elastic modulus of tendon in DG; increased specific deformation, deformation at maximum force and energy/tendon area of tendon in DG.	de Oliveira et al[26], 2012
Wistar rats	Achilles tendon	CG and DG	70 d post-induction	Significantly decreased elastic modulus of tendon in DG; increased specific strain, maximum strain and energy/tendon area of tendon in DG.	de Oliveira et al[25], 2011
Lewis rats	Patellar tendon	CG and DG	12- and 19-d post-induction	Significantly reduced Young′s modulus of tendon in DG at both time points.	Fox et al[15], 2011
Lewis rats	Supraspinatus tendon	CG and DG	1 and 2 wk post-operation	Significantly reduced mean load-to-failure and stiffness of tendon-bone complex in DG at both time points.	Bedi et al[34], 2010
New Zealand rabbits	Achilles tendon	Non-glycated group and glycated group	60 d following glycation	Significant increase in maximum load, Young′s modulus of elasticity, energy to yield, and toughness of glycated tendon.	Reddy et al[29], 2003

FDL: Flexor digitorum longus; CG: Control group; DG: Diabetes group; DM: Diabetes mellitus; SD: Sprague-Dawley; ZDSD: Zucker diabetic SD; GK: Goto-Kakizaki.

Table 2 Histopathological features of diabetic tendinopathy

Species model	Tendon(s)	Groups	Duration of DM	Histopathological feature of diabetic tendons	Ref.
Human	Achilles tendons	CG and DG	Stage V diabetes patients	Diabetic tendons had mild impairment of collagen organization and focal collagen degeneration.	Guney et al[17], 2015
Human	Stenosing flexor tenosynovitis	CG and DG	DM for an average of 17 yr	SFTS in diabetic patients had fibrocartilage metaplasia including fibrocartilage-like cells, and granulation tissue contained newly formed microvessels stromal cells, a small number of inflammatory cells, and extracellular matrix that showed myxomatous degeneration.	Kameyama et al[18], 2013
Human	Rotator cuff tendon	CG and DG	DM for at least 5 yr	IHC showed increased MMP-9 and IL-6 in the torn tendon of diabetic patients.	Chung et al[46], 2017
Male C57Bl/6J mice	FDL tendons	CG (low fat diet) and DG (high fat diet)	At 40 wk post-induction	Lipid deposits were observed in the mid-substance of high fat diet-induced diabetic tendons.	Studentsova et al[30], 2018
C57BL/6 mice	Achilles tendons	CG and DG	DM for one-year post-induction	Fiber disorganization, uneven glycoprotein deposition, and increased interfibrillar spaces in diabetic tendon.	Wu et al[39], 2017
Male db/db C57BL/KsJ mice and wild type control C57BL/6 mice.	Achilles tendons	CG and DG	DM for 11 wk	Mild neutrophil infiltration, mild disorganization of the collagen fibers, mild increased basophilia of the tenocytes, and mildly increased nuclear size/rounding were observed in diabetic tendon. These pathologic changes are consistent with degenerative tendon.	Boivin et al[16], 2014
C57BL/6J Ob mice and wild-type mice	Achilles tendons	CG and DG (obese group, leptin-deficient)	12 wk	In diabetic Achilles tendon, some collagen fibers separated and lost their parallel orientation, with a decrease in fiber diameter and in density of collagen. Unequal and irregular crimping, loosening, increased waviness, lots of degeneration of tendon cells and chondrocyte-like cells were observed. Otherwise, diabetic tendons also showed obvious ruptures in insertion area, degeneration of tendinocytes, collagen fibers microtears, and vascular proliferation.	Ji et al[38], 2010
Male C57Bl/6J mice	FDL tendon	CG (low fat diet) and DG (high fat diet)	Surgery at 12 wk post-induction; days 7-28 post-repair.	After surgical transection injury, the diabetic tendons induced by high fat diet exhibited excess and prolonged scar tissue formation.	Ackerman et al[31], 2017
Male C57BL/6 mice	FDL tendons	CG (low fat diet) and DG (high fat diet)	Surgery at 12 wk post-induction; days 14 and 28 post-surgery.	Smaller cellular and fibrous repair tissue was observed at the injury site of diabetic tendons relative to non-diabetic tendons. The degree of collagen remodeling and fiber alignment in the injured area was less in the diabetic tendons.	David et al[32], 2014
SD rats	Patellar tendons	CG and DG	At 1, 2, and 4 wk post-induction.	Disordered arrangement of collagen fibers, micro-tears, red blood cells and small blood vessels, and the rounding changed tendon cells surrounding the tear sites were observed in the diabetic tendons. IHC staining of diabetic patellar tendons showed increased expression of osteo-chondrogenic differentiation markers including OPN, OCN, SOX9, and Col II, and reduced expression of tenogenic markers including Col I and TNMD.	Shi et al[40], 2019
SD rats	Achilles tendons	CG and DG	At 6 wk post-induction	No significant difference was observed in fibre structure, fibre arrangement, rounding of the nuclei, and regional variations in cellularity between diabetic and non-diabetic Achilles tendons. Immunohistochemical staining of the diabetic Achilles tendon showed markedly increased NOX1 expression within the tenocytes compared with the non-diabetic tendons.	Ueda et al[42], 2018
SD rats	Achilles tendons	CG and DG	DM for over 1 year post-induction	IHC: Increased PPARγ-positive, rounded cells were found to reside in the diabetic tendons, aligning along the collagen fibrils.	Wu et al[41], 2017
SD rats	Achilles tendons	CG, acute DG and chronic DG	1 wk for acute DM, 10 wk for chronic DM.	Total cell density and Achilles tendon cell proliferation were greater in the chronic diabetic tendons compared with the non-diabetic and acute diabetic tendons.	Volper et al[12], 2015
SD rats	Supraspinatus tendons	CG and hyperglycemia group	8 wk following hyperglycemia induction	Cell shape at the insertion site and mid-substance of the hyperglycemic tendon did not alter, nor did cell density at the insertion site; however, the hyperglycemic tendon had a greater cell density at the mid-substance of the tendon compared to the non-hyperglycemic group. Immunohistochemistry staining of the tendon demonstrated significantly increased IL1-β and AGE staining localized to the insertion and mid-substance of the hyperglycemic tendon.	Thomas et al[35], 2014
Wistar rats	Achilles tendons	CG and DG	DM for 24 d post-induction	Tendon thickness, the density of fibrocytes and total cellularity, blood vessels and mast cells were significantly increased in diabetic tendons compared with non-diabetic tendons. IHC showed increased density of type I collagen, associated with the disorganization of the fibers in the diabetic tendons, and expression of VEGF and NF-κB.	de Oliveira et al[37], 2013
Male diabetic GK rats and Wistar control rats	Achilles tendons	CG and DG	DM for 1 yr	Diabetic tendons exhibited slightly lesser transverse area, but showed no apparent alteration in structural organization of collagen fibers.	Ahmed et al[21], 2012
Male white rats	Achilles tendons	CG and DG	Surgery at two weeks post-induction; four weeks post-surgery.	Fibroblasts, capillary and collagen were reduced during the healing process of diabetic tendons after transection injury.	Sananta et al[44], 2019
Wistar rats	Achilles tendons	CG and DG	Surgery at one-week post-induction; 21 d post-surgery.	The fibroblasts in injured diabetic tendons were significantly increased. IHC of the injured diabetic Achilles tendons showed nearly no Col I expression in comparison with injured non-diabetic tendons.	de Oliveira et al[36], 2019
Wistar rats	Achilles tendons	CG (low fat diet) and DG (high fat diet)	Surgery at 30 d post-induction; days 5, 10 and 15 post-surgery.	The diabetic tendons displayed a significant increase in inflammation and a significant decrease in fibrosis compared to the non-diabetic tendons.	Mohsenifar et al[20], 2014
Male GK rats and control Wistar rats	Achilles tendons	CG and DG	DM for one year; two weeks post-surgery.	After rupture, the diabetic tendons had a reduced reparative activity with decreased transverse area, poor structural organization and decreased vascularity. IHC of injured diabetic tendons showed weaker VEGF, Tβ-4, TGF-β1 and IGF-1immunoreactivity and fewer positively stained tenocytes, but strong COX-2, HIF-1α, iNOS and IL-1β at the injured site compared with injured non-diabetic tendons.	Ahmed et al[45], 2014
Male GK rats and control Wistar rats	Achilles tendons	CG and DG	DM for one year; two weeks post-surgery.	After rupture, the diabetic tendons had a reduced reparative activity illustrated by a much smaller transverse area, poor structural organization with fewer longitudinally oriented collagen fibers along the functional loading axis, and decreased vascularity, compared with injured non-diabetic tendons. Most fibers were yellowish and arranged irregularly, denoting ruptured Col I structures. IHC showed that less Col I, Collagen III and biglycan were observed, but increased MMP-13 around blood vessels and cells in the callus in the healing diabetic tendons.	Ahmed et al[21], 2012
Wistar Albino rats	Achilles tendons	CG and DG	Surgery at 3 d post-induction; 2-, 4- and 6-wk post-surgery.	Although similar collagen deposition and vessels proliferation were observed in both injured diabetic and non-diabetic tendons during healing, the injured diabetic tendons exhibited a significantly smaller amount of fibroblast proliferation and lymphocyte infiltration, and osteochondroid metaplasia of some tenocytes.	Egemen et al[43], 2012

SFTS: Stenosing flexor tenosynovitis; IHC: Immunohistochemical; FDL: Flexor digitorum longus; CG: Control group; DG: Diabetes group; DM: Diabetes mellitus; OPN: Osteopontin; OCN: Osteocalcin; Col II: Collagen II; Col I: Collagen I; TNMD: Tenomodulin; PPARγ: Peroxisome proliferator-activated receptor γ; SD: Sprague-Dawley; AGE: Advanced glycation end-products; VEGF: Vascular endothelial growth factor; GK: Goto-Kakizaki; COX-2: Cyclooxygenase-2; HIF-1α: Hypoxia-inducible factor-1α; iNOS: Inducible nitric oxide synthase; TGF-β: Transforming growth factor beta; IGF: Insulin-like growth factor; MMP: Matrix metalloproteinase; IL-6: Interleukin-6; Tβ: Thymosin β; NOX: NADPH oxidase.

Table 3 Cellular and molecular alterations in tenocytes and tendon stem/progenitor cells

Cell type	Cell source	Study type	Groups	Main results	Ref.
Tendon-derived fibroblasts	SD rat, Achilles tendon	In vitro	NG (5.5 mmol/L) and HG (25 mmol/L) with different concentrations of AGEs (0, 50, 100, and 200 μg/mL).	HG had no effect on cell proliferation and expressions of genes associated with extracellular matrix remodeling. AGEs impaired proliferative capacity, ATP production, and electron transport chain efficiency, coupled with alterations in mitochondrial DNA content and expression of genes associated with extracellular matrix remodeling, mitochondrial energy metabolism, and apoptosis. While HG condition did impact some mitochondrial parameters, AGEs appear to be the primary insult and may be responsible for the development of the diabetic tendon phenotype.	Patel et al[51], 2019
Tenocytes	SD rat, Achilles tendon	In vitro and in vivo	CG (12 mmol/L) and HG (33 mmol/L)	Significantly higher gene expressions of NOX1, NOX4, MMP-2, TIMP-1, TIMP-2, IL-6, Col III and ROS accumulation but lower cell proliferation and type I collagen expression in HG than those in CG.	Ueda et al[42], 2018
Tenocytes	SD rat, Achilles tendon	In vitro	LG (5.5 mmol/L) and HG (25 mmol/L)	High glucose-treated tenocytes expressed higher levels of the adipogenic transcription factors PPARγ and C/EBPs. Increased adipogenic trans-differentiation and decreased cell migration induced by high glucose.	Wu et al[41], 2017
Tenocytes	SD rat, Achilles tendon	In vitro	LG (5.5 mmol/L) and HG (25 mmol/L)	No significant effect on cell growth and apoptosis between LG and HG. Increased glucose uptake and consumption in HG condition. Significantly decreased expression of tendon-related genes, including Egr1, Mkx, TGF-β1, Col1a2, and Bgn, in HG culture.	Wu, et al[39], 2017
Tenocytes	Human, hamstring tendon	In vitro	LG (5 mmol/L) and HG (17.5 mmol/L)	Apoptosis level of tenocytes was 1.5 times greater in peroxide-treated cells cultured in HG compared with untreated controls, while apoptosis level of tenocytes was not increased in peroxide-treated cells cultured in low glucose. Peroxide-treated tenocytes cultured in low glucose expressed higher RNA levels of col1a1 and col1a2.	Poulsen, et al[57], 2014
Tenocytes	SD rat, Achilles tendon.	In vitro	LG (6 mmol/L) and HG (12 mmol/L and 25 mmol/L)	The glucose concentration did not affect tendon cell proliferation. The mRNA expression of MMP-9 and MMP-13 was up-regulated by treatment with 25 mmol/L glucose, whereas the mRNA expression of type I and III collagen was not affected. 25 mmol/L glucose increased the enzymatic activity of MMP-9.	Tsai et al[50], 2013
Tenocytes	Porcine, patellar tendon.	In vitro	LG (5.5 mmol/L) and HG (25 mmol/L)	Exposure to HG or AGEs did not affect cell viability. Significantly decreased PG levels in tendons exposed to HG. Relative mRNA levels of biglycan and veriscan were unchanged in HG. Levels of fibromodulin were modestly increased, whereas mRNA for decorin and lumican were significantly decreased. High glucose media decreased PG production by tenocytes whereas AGE-modified type I collagen and free radical scavengers had no effects. High glucose conditions increase TGFβ1 levels in tenocyte.	Burner et al[52], 2012
Tenocytes	Porcmine, patellar tendon	In vitro	NG (5.5 mmol/L) and HG (25 mmol/L)	Significantly higher Tgase activity in tenocytes incubated in HG. CML-Collagen stimulated Tgase activity in tenocytes in both normal and high glucose media but did not induce markers of apoptosis or alter cell viability. Antioxidants reduced the effect of CML-Collagen on tenocytes Tgase activity.	Rosenthal et al[58], 2009
TDSCs	Human, patellar tendon, rotator cuff and hamstring tendons.	In vitro	LG (5.5 mmol/L) and HG (11.1 mmol/L)	HG stimulated inflammation and weakened pro-resolving inflammation response in TDSCs.	Kwan et al[87], 2020
TDSCs	SD rats, Achilles tendon	In vitro and in vivo	CG and AGEs-treated group	AGEs decreased the cell viability, induced apoptosis and senescence of TDSCs, exacerbated osteogenic differentiation of TDSCs and led to more ectopic calcification in Achilles tendon.	Xu et al[83], 2020
TPCs	Horses, superficial digital flexor tendons	In vitro	CG and insulin-treated group (insulin concentrations: 0, 0.07, 0.7 nmol/L)	Insulin increased proliferation and osteogenic differentiation of TPCs in vitro, with the increased ALP activity and elevated expression of osteogenic genes including Runx2, ALP and osteonection.	Durgam et al[84] 2019
TDSCs	SD rats, patellar tendon	In vitro	Non-diabetic TDSCs and diabetic TDSCs	Significantly decreased colony-forming ability, cell proliferation and tenogenic differentiation ability and increased osteogenic and chondrogenic differentiation ability were demonstrated in diabetic TDSCs.	Shi et al[40], 2019
TDSCs	SD rats, patellar tendon	In vitro	LG (5.5 mmol/L) and HG (15 mmol/L, 25 mmol/L)	High glucose could inhibit proliferation, induce cell apoptosis and suppress the tendon-related markers expression of TDSCs.	Lin et al[82], 2017

SD: Sprague-Dawley; NG: Normal glucose; LG: Low glucose; HG: High glucose; AGEs: Advanced glycation end-products; CG: Control glucose; ROS: Reactive oxygen species; PPARγ: Peroxisome proliferator-activated receptor γ; Egr1: Early growth response factor 1; Mkx: Mohawk; TGF-β1: Transforming growth factor beta 1; Bgn: Biglycan; PG: Proteoglycans; Tgase: Transglutaminase; CML: Carboxymethyl-lysine; TDSCs: Tendon derived stem cells; ALP: Alkaline phosphatase; Runx2: Runt-related transcription factor 2; NOX: NADPH oxidase; TPCs: Tendon progenitor cells; MMP: Matrix metalloproteinase; IL-6: Interleukin-6; TIMP: Tissue inhibitor of metalloproteinases; Col III: Collagen III.