Bone-forming perivascular cells: Cellular heterogeneity and use for tissue repair

Distinctiveness of Femoral and Acetabular Mesenchymal Stem and Progenitor Populations in Patients with Primary and Secondary Hip Osteoarthritis Due to Developmental Dysplasia

Primary hip osteoarthritis (pOA) develops without an apparent underlying reason, whereas secondary osteoarthritis arises due to a known cause, such as developmental dysplasia of the hips (DDH-OA). DDH-OA patients undergo total hip arthroplasty at a much younger age than pOA patients (50.58 vs. 65 years in this study). Recently, mesenchymal stem and progenitor cells (MSPCs) have been investigated for the treatment of osteoarthritis due to their immunomodulatory and regenerative potential. This study identified cells in subchondral bone expressing common MSPC markers (CD10, CD73, CD140b, CD146, CD164, CD271, GD2, PDPN) in vivo and compared the proportions of these populations in pOA vs. DDH-OA, further correlating them with clinical, demographic, and morphological characteristics. The differences in subchondral morphology and proportions of non-hematopoietic cells expressing MSPC markers were noted depending on OA type and skeletal location. Bone sclerosis was more prominent in the pOA acetabulum (Ac) in comparison to the DDH-OA Ac and in the pOA Ac compared to the pOA femoral head (Fh). Immunophenotyping indicated diagnosis-specific differences, such as a higher proportion of CD164+ cells and their subsets in DDH-OA, while pOA contained a significantly higher proportion of CD10+ and GD2+ cells and subsets, with CD271+ being marginally higher. Location-specific differences showed that CD271+ cells were more abundant in the Fh compared to the Ac in DDH-OA patients. Furthermore, immunohistochemical characterization of stromal bone-adjacent cells expressing MSPC markers (CD10, CD164, CD271, GD2) in the Ac and Fh compartments was performed. This research proved that immunophenotype profiles and morphological changes are both location- and disease-specific. Furthermore, it provided potentially effective targets for therapeutic strategies. Future research should analyze the differentiation potential of subsets identified in this study. After proper characterization, they can be selectively targeted, thus enhancing personalized medicine approaches in joint disease management.

ZIC1 Dictates Osteogenesis Versus Adipogenesis in Human Mesenchymal Progenitor Cells Via a Hedgehog Dependent Mechanism

Numerous intrinsic factors regulate mesenchymal progenitor commitment to a specific cell fate, such as osteogenic or adipogenic lineages. Identification and modulation of novel intrinsic regulatory factors represent an opportunity to harness the regenerative potential of mesenchymal progenitors. In the present study, the transcription factor (TF) ZIC1 was identified to be differentially expressed among adipose compared with skeletal-derived mesenchymal progenitor cells. We observed that ZIC1 overexpression in human mesenchymal progenitors promotes osteogenesis and prevents adipogenesis. ZIC1 knockdown demonstrated the converse effects on cell differentiation. ZIC1 misexpression was associated with altered Hedgehog signaling, and the Hedgehog antagonist cyclopamine reversed the osteo/adipogenic differentiation alterations associated with ZIC1 overexpression. Finally, human mesenchymal progenitor cells with or without ZIC1 overexpression were implanted in an ossicle assay in NOD-SCID gamma mice. ZIC1 overexpression led to significantly increased ossicle formation in comparison to the control, as assessed by radiographic and histologic measures. Together, these data suggest that ZIC1 represents a TF at the center of osteo/adipogenic cell fate determinations-findings that have relevance in the fields of stem cell biology and therapeutic regenerative medicine.

Aldehyde Dehydrogenase, a Marker of Normal and Malignant Stem Cells, Typifies Mesenchymal Progenitors in Perivascular Niches

Innate mesenchymal stem cells exhibiting multilineage differentiation and tissue (re)generative-or pathogenic-properties reside in perivascular niches. Subsets of these progenitors are committed to either osteo-, adipo-, or fibrogenesis, suggesting the existence of a developmental organization in blood vessel walls. We evaluated herein the activity of aldehyde dehydrogenase, a family of enzymes catalyzing the oxidation of aldehydes into carboxylic acids and a reported biomarker of normal and malignant stem cells, within human adipose tissue perivascular areas. A progression of ALDHLow to ALDHHigh CD34+ cells was identified in the tunica adventitia. Mesenchymal stem cell potential was confined to ALDHHigh cells, as assessed by proliferation and multilineage differentiation in vitro of cells sorted by flow cytometry with a fluorescent ALDH substrate. RNA sequencing confirmed and validated that ALDHHigh cells have a progenitor cell phenotype and provided evidence that the main isoform in this fraction is ALDH1A1, which was confirmed by immunohistochemistry. This demonstrates that ALDH activity, which marks hematopoietic progenitors and stem cells in diverse malignant tumors, also typifies native, blood vessel resident mesenchymal stem cells.

Skeletal stem/progenitor cells provide the niche for extramedullary hematopoiesis in spleen

In bone marrow, the niche which supports hematopoiesis and nurtures hematopoietic stem cells (HSCs) contains perivascular reticular cells representing a subset of skeletal stem/progenitor cells (SSPCs). These stromal cells which provide the niche are lost or become inadequate during stress, disease or ageing, such that HSCs leave bone marrow and enter spleen and other peripheral sites to initiate extramedullary hematopoiesis and particularly myelopoiesis. Spleen also maintains niches for HSCs under steady-state conditions, evident since neonatal and adult spleen contain HSCs in low number and provide low-level hematopoiesis. In spleen, HSCs are found in the sinusoidal-rich red pulp region also in the vicinity of perivascular reticular cells. These cells resemble to some extent the known stromal elements reflecting HSC niches in bone marrow, and are investigated here for their characteristics as a subset of SSPCs. The isolation of spleen stromal subsets and the generation of cell lines which support HSCs and myelopoiesis in vitro has led to the identification of perivascular reticular cells which are unique to spleen. Analysis of gene and marker expression, as well as differentiative potential, identifies an osteoprogenitor cell type, reflective of one of several subsets of SSPCs described previously in bone, bone marrow and adipose tissue. The combined information supports a model for HSC niches in spleen involving perivascular reticular cells as SSPCs having osteogenic, stroma-forming capacity. These associate with sinusoids in red pulp to form niches for HSCs and to support the differentiation of hematopoietic progenitors during extramedullary hematopoiesis.

The Characteristics and Survival Potential Under Sub-lethal Stress of Mesenchymal Stromal/Stem Cells Isolated from the Human Vascular Wall

Mesenchymal stromal/stem cells (MSCs) have been identified in multiple human tissues, including the vascular wall. High proliferative potential, multilineage, and immunomodulatory properties make vascular MSCs promising candidates for regenerative medicine. Indeed, their location is strategic for controlling vascular and extra-vascular tissue homeostasis. However, the clinical application of MSCs, and in particular vascular MSCs, is still challenging. Current studies are focused on developing strategies to improve MSC therapeutic applications, like priming MSCs with stress conditions (hypoxia, nutrient deprivation) to achieve a higher therapeutic potential. The goal of the present study is to review the main findings regarding the MSCs isolated from the human vascular wall. Further, the main priming strategies tested on MSCs from different sources are reported, together with the experience on vascular MSCs isolated from healthy cryopreserved and pathological arteries. Stress induction can be a priming approach able to improve MSC effectiveness through several mechanisms that are discussed in this review. Nevertheless, these issues have not been completely explored in vascular MSCs and potential side effects need to be investigated.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Bone-forming perivascular cells: Cellular heterogeneity and use for tissue repair

Mesenchymal progenitor cells are broadly distributed across perivascular niches-an observation conserved between species. One common histologic zone with a high frequency of mesenchymal progenitor cells within mammalian tissues is the tunica adventitia, the outer layer of blood vessel walls populated by cells with a fibroblastic morphology. The diversity and functions of (re)generative cells present in this outermost perivascular niche are under intense investigation; we have reviewed herein our current knowledge of adventitial cell potential with a somewhat narrow focus on bone formation. Antigens of interest to functionally segregate adventicytes are discussed, including CD10, CD107a, aldehyde dehydrogenase isoforms, and CD140a among others. Purified adventicytes (such as CD10⁺ , CD107a^low , and CD140a⁺ cells) have stronger osteogenic potential and promote bone formation in vivo. Recent bone tissue engineering applications of adventitial cells are also presented. A better understanding of perivascular progenitor cell subsets may represent a beneficial advance for future efforts in tissue repair and bioengineering.