Amplifying Bone Marrow Progenitors Expressing α‐Smooth Muscle Actin Produce Zonal Insertion Sites During Tendon‐to‐Bone Repair

Abstract

Traditional tendon‐to‐bone repair where the tendon is reattached to bone via suture anchors often results in disorganized scar production rather than the formation of a zonal insertion. In contrast, ligament reconstructions where tendon grafts are passed through bone tunnels can yield zonal tendon‐to‐bone attachments between the graft and adjacent bone. Therefore, ligament reconstructions can be used to study mechanisms that regulate zonal tendon‐to‐bone repair in the adult. Anterior cruciate ligament (ACL) reconstructions are one of the most common reconstruction procedures and while we know that cells from outside the graft produce the attachments, we have not yet established specific cell populations that give rise to this tissue. To address this knowledge gap, we performed ACL reconstructions in lineage tracing mice where α‐smooth muscle actin (αSMACreERT2) was used to label αSMA‐expressing progenitors within the bone marrow that produced zonal attachments. Expression of αSMA was increased during early stages of the repair process such that the contribution of SMA‐labeled cells to the tunnel integration was highest when tamoxifen was delivered in the first week post‐surgery. The zonal attachments shared features with normal entheses, including tidemarks oriented perpendicularly to collagen fibers, Col1a1‐expressing cells, alkaline phosphatase activity, and proteoglycan‐rich staining. Finally, the integration strength increased with time, requiring 112% greater force to remove the graft from the tunnel at 28 days compared with 14 days post‐surgery. Future studies will target these progenitor cells to define the pathways that regulate zonal tendon‐to‐bone repair in the adult. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:105–116, 2020

Tmem100- and Acta2-Lineage Cells Contribute to Implant Osseointegration in a Mouse Model

“…Our data suggest that implant surgery leads to Acta2.creERT2 expression in Pdgfra + and Ly6a + cells. Our findings are consistent with reports that performed Acta2 lineage tracing in other models of bone repair, such as endochondral fracture healing ( 10,15 ) and anterior cruciate ligament reconstruction; (41 ) these studies have shown that tamoxifen treatment at the time of bone injury results in the highest level of cre‐recombination as opposed to tamoxifen treatment at other time points. Thus, Acta2 expression is highly dynamic in healing bone tissue and is likely induced in PαS cells immediately after injury.…”

Cells from a GDF5 origin produce zonal tendon‐to‐bone attachments following anterior cruciate ligament reconstruction

“…Finally, the radiographic‐based anterior drawer test was limited in that we did not control rotation of the femur during the test and viscoelastic properties could not be measured. However, our laboratory has recently developed an anterior−posterior drawer test on a mechanical testing machine, so viscoelastic properties can now be measured . Nevertheless, the radiographic method yielded percent changes in displacement that fell within previous reports for anterior drawer tests in ACL‐deficient human knees .…”

“…All mice were approved by the UConn Health Institutional Animal Care and Use Committee. Mice were anesthetized, sterilely prepped, and the surgery was conducted similarly to previous work . Three small incisions (3 mm each, spaced ∼30 mm apart) were made in the tail to access the tail tendons (Fig.…”

“…For instance, a rat femoral tunnel implant model using non‐green fluorescent protein (GFP) allografts into GFP recipients demonstrated clearly that cell populations from outside the graft drive tunnel integration, which was later confirmed in a rabbit GFP allograft model . ACL reconstruction procedures in mice have been established, which opens up the possibility of using powerful genetic tools found in various mouse strains to better define the cellular origin and molecular regulation of tendon‐to‐bone repair.…”

Discovery to delivery in tendon research

“…A series of original research papers delves into tendon pathogenesis and healing, focusing on a broad range of topics: the effect of age and fatigue on tendon biomechanics, transcriptomics analysis of tendon healing, and treatment strategies that include hyaluronidase, pulsed electromagnetic fields, and parathyroid hormone . A collection of stem cell papers describes methods to generate and optimize stem cells and uses them to enhance tendon healing . In vitro models are used to examine how dexamethasone can protect the tendon from degeneration, the roles of substrate stiffness and pulsed electromagnetic fields in tendon responses to inflammation, and tendon responses to mechanical loading .…”