A single cell transcriptional atlas of early synovial joint development.

Synovial joint development begins with the formation of the interzone, a region of condensed mesenchymal cells at the site of the prospective joint. Recently, lineage-tracing strategies have revealed that Gdf5-lineage cells native to and from outside the interzone contribute to most, if not all, of the major joint components. However, there is limited knowledge of the specific transcriptional and signaling programs that regulate interzone formation and fate diversification of synovial joint constituents. To address this, we have performed single cell RNA-Seq analysis of 7329 synovial joint progenitor cells from the developing murine knee joint from E12.5 to E15.5. By using a combination of computational analytics, in situ hybridization and in vitro characterization of prospectively isolated populations, we have identified the transcriptional profiles of the major developmental paths for joint progenitors. Our freely available single cell transcriptional atlas will serve as a resource for the community to uncover transcriptional programs and cell interactions that regulate synovial joint development.

Dynamic profile and adipogenic role of growth differentiation factor 5 (GDF5) in the differentiation of 3T3-L1 preadipocytes.

Adipocyte differentiation is key to determining the number of adipocytes during the development of obesity. Recent studies have shown that growth differentiation factor-5 (GDF5) promotes brown adipogenesis, however its role in white adipogenesis is still uncertain. The aim of the present study was to investigate the effect of GDF5 on white adipogenesis using 3T3-L1 preadipocyte model. In the present study, GDF5 was found to be differentially regulated during adipocyte differentiation. GDF5 protein increased the differentiation of 3T3-L1 preadipocytes, especially when these cells were exposed to hormone cocktails without insulin. During adipogenesis, GDF5 enhanced the expression of genes related to adipocyte differentiation and caused cells to enter the S phase. Short-hairpin-RNA knockdown of GDF5 in 3T3-L1 cells was found to prevent adipogenesis induced by a standard hormone cocktail and to downregulate the expression of adipocyte genes and proteins, this impairment could be partly rescued by GDF5 protein. Collectively, these results suggest that GDF5 can promote progression of the cell-cycle and increase numbers of cells in S phase, GDF5 might play a critical role in 3T3-L1 preadipocyte differentiation.

Delayed fracture healing in growth differentiation factor 5-deficient mice: a pilot study.

BACKGROUND: Growth differentiation factor-5 (GDF-5) is a key regulator of skeletogenesis and bone repair and induces bone formation in spinal fusions and nonunion applications by enhancing chondrocytic and osteocytic differentiation and stimulating angiogenesis. Elucidating the contribution of GDF-5 to fracture repair may support its clinical application in complex fractures. QUESTIONS/PURPOSE: We therefore asked whether the absence of GDF-5 during fracture repair impaired bone healing as assessed radiographically, histologically, and mechanically. METHODS: In this pilot study, we performed tibial osteotomies on 10-week-old male mice, stabilized by intramedullary and extramedullary nailing. Healing was assessed radiographically and histologically on Days 1 (n = 1 wild-type; n = 5 bp [brachopodism]), 5 (n = 3 wild-type; n = 3 bp), 10 (n = 6 wild-type; n = 3 bp), 14 (n = 6 wild-type; n = 6 bp), 21 (n = 6 wild-type; n = 6 bp), 28 (n = 7 wild-type; n = 6 bp), and 56 (n = 6 wild-type; n = 6 bp) after fracture. After 10 (n = 7 wild-type; n = 7 bp contralateral and n = 3 bp fractured tibiae), 14 (n = 6 wild-type; n = 6 bp), 21 (n = 6 wild-type; n = 6 bp), 28 (n = 6 wild-type; n = 3 bp), and 56 (n = 8 wild-type; n = 6 bp) days, the callus cross-sectional area was calculated. We characterized the mechanical integrity of the healing fracture by yield stress and Young's modulus at 28 (n = 6 wild-type; n = 3 bp) and 56 (n = 8 wild-type; n = 6 bp) days postfracture. RESULTS: The absence of GDF-5 impaired cartilaginous matrix deposition in the callus and reduced callus cross-sectional area. After 56 days, the repaired bp fracture was mechanically comparable to that of controls. CONCLUSIONS: Although GDF-5 deficiency did not compromise long-term fracture healing, a delay in cartilage formation and remodeling supports roles for GDF-5 in the early phase of bone repair. CLINICAL RELEVANCE: Local delivery of GDF-5 to clinically difficult fractures may simulate cartilage formation in the callus and support subsequent remodeling.

Nuclei pulposi formation from the embryonic notochord occurs normally in GDF-5-deficient mice.

STUDY DESIGN: The transition of the mouse embryonic notochord into nuclei pulposi was determined ("fate mapped") in vivo in growth and differentiating factor-5 (GDF-5)-null mice using the Shhcre and R26R alleles. OBJECTIVE: To determine whether abnormal nuclei pulposi formation from the embryonic notochord was responsible for defects present in adult nuclei pulposi of Gdf-5-null mice. SUMMARY OF BACKGROUND DATA: The development, maintenance, and degeneration of the intervertebral disc are not understood. Previously, we demonstrated that all cells in the adult nucleus pulposus of normal mice are derived from the embryonic notochord. Gdf-5-null mice have been reported to contain intervertebral discs in which the nucleus pulposus is abnormal. It is currently unclear if disc defects in Gdf-5-null mice arise during the formation of nuclei pulposi from the notochord during embryogenesis or result from progressive postnatal degeneration of nuclei pulposi. METHODS: Gdf-5 messenger RNA expression was examined in the discs of wild-type embryos by RNA in situ hybridization to determine when and where this gene was expressed. To examine nucleus pulposus formation in Gdf-5-null mice, intervertebral discs in which embryonic notochord cells were marked were analyzed in newborn and 24-week-old mice. RESULTS: Our Gdf-5 messenger RNA in situ experiments determined that this gene is localized to the annulus fibrosus and not the nucleus pulposus in mouse embryos. Notochord fate-mapping experiments revealed that notochord cells in Gdf-5-null mice correctly form nuclei pulposi. CONCLUSION: Our data suggest that the defects reported in the nucleus pulposus of adult Gdf-5-null mice do not result from abnormal patterning of the embryonic notochord. The use of mouse alleles to mark cells that produce all cell types that reside in the adult nucleus pulposus will allow for a detailed examination of disc formation in other mouse mutants that have been reported to contain disc defects.

GDF5 deficiency in mice is associated with instability-driven joint damage, gait and subchondral bone changes.

OBJECTIVES: A functional polymorphism leading to reduced levels of growth and differentiation factor 5 (GDF5) was recently identified as a susceptibility factor for osteoarthritis. The authors studied the potential mechanisms of GDF5 involvement in osteoarthritis using haploinsufficient Gdf5(Bp-J/+) mice. METHODS: Gdf5(Bp-J/+) mice were challenged in the collagenase-induced arthritis model, the medial meniscus destabilisation model, the papain-induced arthritis model and a treadmill running model. Bone density and subchondral bone parameters were determined using dual energy x-ray absorptiometry and peripheral quantitative CT. Additional in-vitro and ex-vivo analyses studied cartilage metabolism, gait and collagen characteristics. RESULTS: Gdf5(Bp-J/+) mice appeared phenotypically normal. No difference in osteoarthritis severity was found in the different models, with the exception of increased synovial hyperplasia in the joints of Gdf5(Bp-J/+) mice in the treadmill model. However, in the collagenase-induced model severe joint damage was found in the contralateral joints of Gdf5(Bp-J/+) mice. Gait analysis demonstrated an aberrant walking pattern in Gdf5(Bp-J/+) mice. In addition, Gdf5(Bp-J/+) mice have a decreased subchondral bone density and a distorted arrangement of collagen fibres in bone. CONCLUSIONS: These data suggest that decreased GDF5 levels in mice can contribute to osteoarthritis development by different mechanisms including altered loading and subchondral bone changes. This highlights the importance of the joint as an organ with different tissues involved in joint disease.