Meniscus injuries are among the most common orthopaedic injuries. Tears in the inner one-third of the meniscus heal poorly and present a significant clinical challenge. In this study, we hypothesized that progenitor cells from healthy human articular cartilage (C-PCs) may be more suitable than bone-marrow mesenchymal stem cells (BM-MSCs) to mediate bridging and reintegration of fibrocartilage tissue tears in meniscus. C-PCs were isolated from healthy human articular cartilage based on their expression of mesenchymal stem/progenitor marker ALCAM (CD166). Our findings revealed that healthy human C-PCs are CD166+, CD90+, CD54+, CD106- cells with multi-lineage differentiation potential and elevated basal expression of chondrogenesis marker SOX-9. We show that, similar to BM-MSCs, C-PCs are responsive to the chemokine stromal cell derived factor-1 (SDF-1) and they can successfully migrate to the area of meniscal tissue damage promoting collagen bridging across inner meniscal tears. In contrast to BM-MSCs, C-PCs maintaining reduced expression of cellular hypertrophy marker collagen X in monolayer culture and in an ex-plant organ culture model of meniscus repair. Treatment of C-PCs with SDF-1/CXCR4 pathway inhibitor AMD3100 disrupted cell localization to area of injury and prevented meniscus tissue bridging thereby indicating that the SDF-1/CXCR4 axis is an important mediator of this repair process. This study suggests that C-PCs from healthy human cartilage may potentially be a useful tool for fibrocartilage tissue repair/regeneration because they resist cellular hypertrophy and mobilize in response to chemokine signaling.

Aggrecan is an integral component of the extracellular matrix in cartilaginous tissues, including the growth plate. Heterozygous defects in the aggrecan gene have been identified as a cause of autosomal dominant short stature, bone age acceleration, and premature growth cessation. The mechanisms accounting for this phenotype remain unknown. We used ATDC5 cells, an established model of chondrogenesis, to evaluate the effects of aggrecan deficiency. ATDC5 aggrecan knockdown cell lines (AggKD) were generated using lentiviral shRNA transduction particles. Cells were stimulated with insulin/transferrin/selenium for up to 21 days to induce chondrogenesis. Control ATDC5 cells showed induction of Col2a1 starting at day 8 and induction of Col10a1 starting at day 12. AggKD cells had significantly reduced expression of Col2a1 and Col10a1 (p<0.0001) with only minimal increases in expression over time, indicating that chondrogenesis was markedly impaired. The induction of Col2a1 and Col10a1 was not rescued by culturing of AggKD cells in wells pre-conditioned with ATDC5 extracellular matrix or in co-culture with wild-type ATDC5 cells. We interpret our studies as indicating that aggrecan has an integral role in chondrogenesis that may be mediated through intracellular mechanisms.

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Chondrocyte hypertrophy is a hallmark of osteoarthritis (OA) pathology. In the present study, we elucidated the mechanism underlying the relationship between the hypertrophy/apoptotic phenotype and OA pathogenesis in bone marrow-derived mesenchymal stem cells (BM-MSCs) via gene targeting of distal-less homeobox 5 (DLX5). Our primary objectives were (1) to determine whether DLX5 is a predictive biomarker of cellular hypertrophy in human osteoarthritic tissues; (2) To determine whether modulating DLX5 activity can regulate cell hypertrophy in mesenchymal stem/progenitor cells from marrow and cartilage. Whole transcriptome sequencing was performed to identify differences in the RNA expression profile between human-cartilage-derived mesenchymal progenitors (C-PCs) and bone-marrow-derived mesenchymal progenitors (BM-MSCs). Ingenuity Pathway Analysis (IPA) software was used to compare molecular pathways known to regulate hypertrophic terminal cell differentiation. RT-qPCR was used to measure DLX5 and hypertrophy marker COL10 in healthy human chondrocytes and OA chondrocytes. DLX5 was knocked down or overexpressed in BM-MSCs and C-PCs and RT-qPCR were used to measure the expression of hypertrophy/terminal differentiation markers following DLX5 modulation. Apoptotic cell activity was characterized by immunostaining for cleaved caspase 3/7. We demonstrate that DLX5 and downstream hypertrophy markers were significantly upregulated in BM-MSCs, relative to C-PCs. DLX5 and COL10 were also significantly upregulated in cells from OA knee joint tissues, relative to normal non-arthritic joint tissues. Knocking down DLX5 in BM-MSCs inhibited cell hypertrophy and apoptotic activity without attenuating their chondrogenic potential. Overexpression of DLX5 in C-PCs stimulated hypertrophy markers and increased apoptotic cell activity. Modulating DLX5 activity regulates cell hypertrophy and apoptosis in BM-MSCs and C-PCs. These findings suggest that DLX5 is a biomarker of OA changes in human knee joint tissues and confirms the DLX5 mechanism contributes to hypertrophy and apoptosis in BM-MSCs.

Harnessing adult mesenchymal stem/progenitor cells to stimulate skeletal tissue repair is a strategy that is being actively investigated. While scientists continue to develop creative and thoughtful ways to utilize these cells for tissue repair, the vast majority of these methodologies can ultimately be categorized into two main approaches: (1) Facilitating the recruitment of endogenous host cells to the injury site; and (2) physically administering into the injury site cells themselves, exogenously, either by autologous or allogeneic implantation. The aim of this paper is to comprehensively review recent key literature on the use of these two approaches in stimulating healing and repair of different skeletal tissues. As expected, each of the two strategies have their own advantages and limitations (which we describe), especially when considering the diverse microenvironments of different skeletal tissues like bone, tendon/ligament, and cartilage/fibrocartilage. This paper also discusses stem/progenitor cells commonly used for repairing different skeletal tissues, and it lists ongoing clinical trials that have risen from the implementation of these cells and strategies. Lastly, we discuss our own thoughts on where the field is headed in the near future.

The epidermal keratinocytes express two major pairs of keratin polypeptides. One pair (K5/K14) expressed specifically in basal generative compartment and the other (K1/K10) expressed specifically in the differentiating suprabasal compartment. The switch in the expression of the keratins from proliferating to differentiating compartment indicates the changes that occur in the keratin filament organization which in turn influences the functional properties of the epidermis. Proper regulation of keratin gene expression and the filament organization are absolutely necessary for normal functioning of the skin. Keratin gene mutations can influence the filament integrity thereby causing several heritable blistering disorders of the skin such as epidermolysis bullosa, bullous icthyosiform erythroderma, etc. Changes in the keratin gene expression may lead to incomplete differentiation of the epidermal keratinocyte, causing hyperproliferative diseases of the skin such as psoriasis, carcinomas, etc. This review briefly describes the changes in keratin structure or gene expression that are known to result in various disorders of the skin.

Surgical repair of meniscus injury is intended to help alleviate pain, prevent further exacerbation of the injury, restore normal knee function, and inhibit the accelerated development of post-traumatic osteoarthritis (PTOA). Meniscus injuries that are treated poorly or left untreated are reported to significantly increase the risk of PTOA in patients. Current surgical approaches for the treatment of meniscus injuries do not eliminate the risk of accelerated PTOA development. Through recent efforts by scientists to develop innovative and more effective meniscus repair strategies, the use of biologics, allografts, and scaffolds have come into the forefront in pre-clinical investigations. However, gauging the extent to which these (and other) approaches inhibit the development of PTOA in the knee joint is often overlooked, yet an important consideration for determining the overall efficacy of potential treatments. In this review, we catalog recent advancements in pre-clinical therapies for meniscus injuries and discuss the assessment methodologies that are used for gauging the success of these treatments based on their effect on PTOA severity. Methodologies include histopathological evaluation of cartilage, radiographic evaluation of the knee, analysis of knee function, and quantification of OA predictive biomarkers. Lastly, we analyze the prevalence of these methodologies using a systemic PubMed® search for original scientific journal articles published in the last 3-years. We indexed 37 meniscus repair/replacement studies conducted in live animal models. Overall, our findings show that approximately 75% of these studies have performed at least one assessment for PTOA following meniscus injury repair. Out of this, 84% studies have reported an improvement in PTOA resulting from treatment.

Meniscal tearing in the knee increases the risk of post-traumatic osteoarthritis (OA) in patients. The therapeutic application of tissue-specific mesenchymal progenitor cells is currently being investigated as an emerging biologic strategy to help improve healing of musculoskeletal tissues like meniscal fibrocartilage and articular hyaline cartilage. However, many of these approaches involve isolating cells from healthy tissues, and the low yield of rare progenitor populations (&lt; 1% of total cells residing in tissues) can make finding a readily available cell source for therapeutic use a significant logistical challenge. In the present study, we investigated the therapeutic efficacy of using expanded cartilage-derived and bone marrow-derived progenitor cell lines, which were stabilized using retroviral SV40, for repair of meniscus injury in a rodent model. Our findings indicate that these cell lines express the same cell surface marker phenotype of primary cells (CD54+, CD90+, CD105+, CD166+), and that they exhibit improved proliferative capacity that is suitable for extensive expansion. Skeletally mature male athymic rats treated with 3.2 million cartilage-derived progenitor cell line exhibited approximately 79% greater meniscal tear reintegration/healing, compared to injured animals that left untreated, and 76% greater compared to animals treated with the same number of marrow-derived stromal cells. Histological analysis of articular surfaces also showed that cartilage-derived progenitor cell line treated animals exhibited reduced post-traumatic OA associated articular cartilage degeneration. Stable cell line treatment did not cause tumor formation or off-target engraftment in animals. Taken together, we present a proof-of-concept study demonstrating, for the first time, that intra-articular injection of a stable human cartilage-derived progenitor cell line stimulates meniscus tear healing and provide chondroprotection in an animal model. These outcomes suggest that the use of stable cell lines may help overcome cell source limitations for cell-based medicine.