Proliferative responses to platelet-derived growth factor in young and old rat patellar tendon

Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing

Due to the increasing age of our society and a rise in engagement of young people in extreme and/or competitive sports, both tendinopathies and tendon ruptures present a clinical and financial challenge. Tendon has limited natural healing capacity and often responds poorly to treatments, hence it requires prolonged rehabilitation in most cases. Till today, none of the therapeutic options has provided successful long-term solutions, meaning that repaired tendons do not recover their complete strength and functionality. Our understanding of tendon biology and healing increases only slowly and the development of new treatment options is insufficient. In this review, following discussion on tendon structure, healing and the clinical relevance of tendon injury, we aim to elucidate the role of stem cells in tendon healing and discuss new possibilities to enhance stem cell treatment of injured tendon. To date, studies mainly apply stem cells, often in combination with scaffolds or growth factors, to surgically created tendon defects. Deeper understanding of how stem cells and vasculature in the healing tendon react to growth factors, common drugs used to treat injured tendons and promising cellular boosters could help to develop new and more efficient ways to manage tendon injuries.

Increase in tendon protein synthesis in response to insulin-like growth factor-I is preserved in elderly men

Insulin-like growth factor-I (IGF-I) is known to be an anabolic factor in tendon, and the systemic levels are reduced with aging. However, it is uncertain how tendon fibroblasts are involved in tendon aging and how aging cells respond to IGF-I. The purpose of this study was to investigate the in vivo IGF-I stimulation of tendon protein synthesis in elderly compared with young men. We injected IGF-I in the patellar tendons of young ( n = 11, 20–30 yr of age) and old ( n = 11, 66–75 yr of age) men, and the acute fractional synthesis rate (FSR) of tendon protein was measured with the stable isotope technique and compared with the contralateral side (injected with saline as control). We found that tendons injected with IGF-I had significantly higher protein FSR compared with controls (old group: 0.018 ± 0.015 vs. 0.008 ± 0.008, young group: 0.016 ± 0.009 vs. 0.009 ± 0.006%/h, mean ± SE, P < 0.01). This increase in protein synthesis was seen in both young and old men, with no differences between age groups. The old group had markedly lower serum IGF-I levels compared with young (165 ± 17 vs. 281 ± 27 ng/ml, P < 0.01). In conclusion, local IGF-I stimulated tendon protein synthesis in both young and old men, despite lower systemic IGF-I levels in the old group. This could indicate that the changed phenotype in aging tendon is not caused by decreased fibroblast function.

Intralesional injection of insulin-like growth factor-I for treatment of superficial digital flexor tendonitis in Thoroughbred racehorses: 40 cases (2000-2004)

OBJECTIVE-To evaluate outcome after intralesional injection of insulin-like growth factor-I (IGF-I) for treatment of superficial digital flexor (SDF) tendonitis in Thoroughbred racehorses. DESIGN-Retrospective case series. ANIMALS-40 Thoroughbred racehorses. PROCEDURES-Medical records of racehorses with SDF tendonitis treated within 13 weeks after injury by intralesional injection of IGF-I (25 or 50 μg every other day for 4 or 5 treatments) were reviewed. Outcome was determined via analysis of race records, owner follow-up, and examination. RESULTS-Mean age of the horses was 3.1 years (range, 2 to 7 years), and time from injury to treatment ranged from 8 to 90 days. Mean ± SD approximate lesion length on admission was 15.6 ± 6.0 cm, and mean percentage cross-sectional area of the tendon affected was 26 ± 18%. Twenty-six of the 40 horses underwent desmotomy of the accessory ligament of the SDF tendon. Echolucency was reduced in 23 of 26 horses by the end of the treatment period. Twenty-one of 34 (62%) horses for which race data were available raced at least once after treatment, including 10 (30%) horses that raced between 1 and 4 times and 11 horses (32%) that raced ≥ 5 times. Thirteen of 28 (46%) horses had a recurrence of tendonitis or developed tendonitis elsewhere. CONCLUSIONS AND CLINICAL RELEVANCE-Results suggested that in Thoroughbred racehorses with SDF tendonitis, intralesional injection of IGF-I led to a decrease in ultrasonographic lesion severity, but treated horses had only a moderate prognosis for return to racing.

Regenerative tendon and ligament healing: opportunities with recombinant human platelet-derived growth factor BB-homodimer

Intrinsic tendon healing in response to injury is a reparative process that often results in formation of scar tissue with functional and mechanical properties inferior to those of the native tendon. Development of therapies that can promote regenerative, rather than reparative, healing hold the promise of improving patient recovery from tendon and ligament injuries by producing tissue that is morphologically and functionally equivalent to the native tissue. One therapeutic approach that has been a frequent topic of investigation in the preclinical literature is the use of recombinant human platelet-derived growth factor-BB (rhPDGF-BB) to augment tendon and ligament repair. The chemotactic, mitogenic, and pro-angiogenic properties of rhPDGF-BB have been shown to result in recruitment and proliferation of tenogenic cells and a commensurate boost in extracellular matrix deposition and organization, improving the morphological and biomechanical properties of healing tendons and ligaments. The outcomes of the preclinical studies reviewed here strongly suggest that rhPDGF-BB will provide a new therapeutic opportunity to improve the treatment of injured tendons and ligaments.

Caveolin-1 as a novel indicator of wound-healing capacity in aged human corneal epithelium

Excess caveolin-1 has been reported to play a role in age-dependent hyporesponsiveness to growth factors in vitro. Therefore, we hypothesized that caveolin-1-dependent hyporesponsiveness to growth factors in aged corneal epithelial cells might be responsible for delayed wound healing in vivo. To test this hypothesis, we evaluated corneal wound-healing time by vital staining using fluorescein after laser epithelial keratomileusis (LASEK). We compared wound-healing times in young, middle-aged and elderly patients. We also examined caveolin-1 levels and other aging markers, such as p53 and p21, in the corneal epithelium. Elderly patients generally had higher caveolin-1 levels in the corneal epithelia than young patients. There were, however, variations among individuals with increased caveolin-1 in some young patients and decreased levels in some elderly patients. Wound-healing time after LASEK correlated well with the corneal caveolin-1 status. Therefore, we suggest that caveolin-1 status might be responsible for delayed wound healing in elderly patients after LASEK. Caveolin-1 status might be a regulator for wound-healing capacity and a novel target for in vivo adjustment.

Tendon: biology, biomechanics, repair, growth factors, and evolving treatment options

Surgical treatment of tendon ruptures and lacerations is currently the most common therapeutic modality. Tendon repair in the hand involves a slow repair process, which results in inferior repair tissue and often a failure to obtain full active range of motion. The initial stages of repair include the formation of functionally weak tissue that is not capable of supporting tensile forces that allow early active range of motion. Immobilization of the digit or limb will promote faster healing but inevitably results in the formation of adhesions between the tendon and tendon sheath, which leads to friction and reduced gliding. Loading during the healing phase is critical to avoid these adhesions but involves increased risk of rupture of the repaired tendon. Understanding the biology and organization of the native tendon and the process of morphogenesis of tendon tissue is necessary to improve current treatment modalities. Screening the genes expressed during tendon morphogenesis and determining the growth factors most crucial for tendon development will likely lead to treatment options that result in superior repair tissue and ultimately improved functional outcomes.

Insulin-like growth factor-I improves cellular and molecular aspects of healing in a collagenase-induced model of flexor tendinitis

Flexor tendinitis is a common and debilitating injury of elite and recreational athletes. Healing may be improved through intratendinous injection of insulin-like growth factor-I (IGF-I), which has been shown in vitro to stimulate mitogenesis and enhance tendon matrix production. This study investigated the effects of intratendinous injection of IGF-I on tendon healing in an equine model of flexor tendinitis. Collagenase-induced lesions were created in the tensile region of theflexor digitorum superficialis tendon of both forelimbs of eight horses. Treated tendons were injected with 2 microg rhlGF-I intralesionally every other day for 10 injections, while controls received 0.9% NaCl. Tendon fiber deposition and organization were evaluated serially using ultrasonography throughout the 8 week trial period. Following euthanasia, the tendons were harvested and DNA, hydroxyproline, and glycosaminoglycan content determined, mechanical strength and stiffness evaluated, gene expression and spatial arrangement of collagen types I and III assessed by northern blot and in situ hybridization, and tendon fiber architecture assessed by polarized light microscopy. Local soft tissue swelling was reduced in the IGF-I treated limbs. Similarly, lesion size in IGF-I treated tendons was smaller 3 and 4 weeks after initiation of treatment. Cell proliferation and collagen content of the IGF-I treated tendons were increased compared to controls. Mechanically, IGF-I treated tendons showed a trend toward increased stiffness compared to saline treated controls. Considered together with the decreased soft tissue swelling and improved sonographic healing, these data support the potential use of intralesional IGF-I for treatment of debilitating tendon injuries.

Cell outgrowth from the human ACL in vitro: regional variation and response to TGF-beta1

One of the new methods being developed to stimulate healing of the human anterior cruciate ligament (ACL) after rupture is the implantation of a biodegradable scaffold which the host cells invade, populate and remodel. One of the cellular behaviors critical to the success of this method is cell outgrowth from the ligament remnants onto an adjacent scaffold. As morphological differences have been previously reported in the proximal and distal human ACL, the primary aim of this study was to determine if the cells from the proximal and distal ACL had different outgrowth behaviors as well. A second aim was to determine whether TGF-beta1, reported to be a mitogen for fibroblasts, would affect the outgrowth behaviors from the proximal and distal ACL. To achieve these aims, explants from both the proximal and distal human ACL were placed into culture under various conditions and outgrowth measured for 42 days. In explants cultured with 10% fetal bovine serum (high serum group) the explants from the proximal ACL had an earlier start of outgrowth than the distal explants (p < 0.015), and outgrowth rates were similar in the two groups. In explants cultured with 2% fetal bovine serum (low serum group), the explants from the proximal ACL had an earlier start to outgrowth (p < 0.003) as well as a faster rate of outgrowth (p < 0.004) than the distal explants. The addition of TGF-beta1 to the low serum cultures significantly slowed the rate of outgrowth from both groups of ACL explants (p < 0.025). These results suggest that outgrowth behaviors are different in the proximal and distal human ACL, and that TGB-beta1 has an inhibitory effect on cell outgrowth from ACL explants. However, this study is only a first step, and additional experiments are needed to further optimize tissue engineering parameters for enhancement of the repair of the ACL.

Patellar tendon and anterior cruciate ligament have different mitogenic responses to platelet-derived growth factor and transforming growth factor beta

The healing responses of the anterior cruciate ligament and the patellar tendon differ markedly. The anterior cruciate ligament fails to heal, whereas the patellar tendon heals slowly. The basis of these differences is unknown. Since cellular proliferation is a critical element of healing, we investigated the response to explants of anterior cruciate ligament and patellar tendon from sheep knees to platelet-derived growth factor-AB and transforming growth factor beta 1 as a function of time and dose. Explants cultured for 48, 72, and 96 hours with transforming growth factor beta 1 (0-100 ng/ml) or platelet-derived growth factor-AB (0-200 ng/ml) were radiolabeled for the final 24 hours with [3H]thymidine, and DNA synthesis was quantified as trichloroacetic acid-precipitable radioactivity normalized to dry tissue weight. Statistical analyses (analysis of variance) showed that transforming growth factor beta 1 induced a significant proliferative response in the anterior cruciate ligament at 96 hours with equivalent responses at 10, 50, and 100 ng/ml, whereas the patellar tendon only responded to one condition, 10 ng/ml at 96 hours. Conversely, the patellar tendon had a significant dose-dependent response to platelet-derived growth factor-AB at 72 and 96 hours, whereas the anterior cruciate ligament showed no proliferative response to platelet-derived growth factor-AB. The minimal response of anterior cruciate ligament to platelet-derived growth factor-AB could explain, at least in part, the poor repair capacity of this tissue. The response of the anterior cruciate ligament to transforming growth factor beta suggests that exogenous transforming growth factor beta may promote initial healing. Although growth factors have the potential to modulate soft-tissue repair, tissue responses in tendons and ligaments may vary at different anatomic sites.