Placental perivascular cells for human muscle regeneration

The therapeutic value of adipose-derived pericyte transplantation after intracerebral hemorrhage in rats

Intracerebral hemorrhage (ICH) is a devastating cerebrovascular associated with inflammation and BBB disruption. Pericytes plays a critical role in neurological diseases, while whether pericytes could be utilized to treat ICH remains to be elucidated. Here, we isolated CD146+CD34- pericytes from rat adipose tissues (ADPs). Fluorescence-activated cells maintained their cell morphology and differentiation potential and expressed pericytes markers (CD146, NG2, and PDGFRβ) but not endothelial markers (CD31, CD34, and CD45). ADPs transplantation improved the neuro-behavioral functions in ICH rats and resulted in decreased hematoma volume and neuron loss after ICH. Besides, ADPs graft restrained the infiltration of neutrophils and reactive microgliosis after ICH injury around the peri-hematoma area of rats, as evidenced by increased Iba1- and MPO immunoreactivity. The transplanted pericytes were covered on endothelial cells, and promoted angiogenesis and vascular basement membrane formation in the peri-hematoma area of ICH rats, as shown by double staining of PDGFRβ and CD31/CollagenIV. The decreased brain water content and Evans Blue leakage proved the protective role of ADPs graft on BBB permeability. Finally, transplanted ADPs increased the expression of VE-cadherin, ZO-1, and claudin-5, leading to stable endothelial cell-cell adhesion and tight junction. In conclusion, the transplantation of APDs improved neuronal after ICH, which involved different mechanisms including neuroinflammation regulation and BBB dysfunction recovery. Our results supported that ADPs might be the ideal cell type for ICH therapy and provided insights into the potential cell therapy for further ICH treatment.

The Role of Supporting Cell Populations in Satellite Cell Mediated Muscle Repair

Skeletal muscle has a high capacity to repair and remodel in response to damage, largely through the action of resident muscle stem cells, termed satellite cells. Satellite cells are required for the proper repair of skeletal muscle through a process known as myogenesis. Recent investigations have observed relationships between satellite cells and other cell types and structures within the muscle microenvironment. These findings suggest that the crosstalk between inflammatory cells, fibrogenic cells, bone-marrow-derived cells, satellite cells, and the vasculature is essential for the restoration of muscle homeostasis. This review will discuss the influence of the cells and structures within the muscle microenvironment on satellite cell function and muscle repair.

Protocol to generate endothelial cells, pericytes, and fibroblasts in one differentiation round from human-induced pluripotent stem cells

Here, we present a protocol for differentiating human-induced pluripotent stem cells into three distinct mesodermal cell types: vascular endothelial cells (ECs), pericytes, and fibroblasts. We describe steps for using monolayer serum-free differentiation and isolating ECs (CD31⁺) and mesenchymal pre-pericytes (CD31^-) from a single differentiation set. We then differentiate pericytes into fibroblasts using a commercial fibroblast culture medium. The three cell types differentiated in this protocol are useful for vasculogenesis, drug testing, and tissue engineering applications. For complete details on the use and execution of this protocol, please refer to Orlova et al. (2014).¹.

Transplantation-based screen identifies inducers of muscle progenitor cell engraftment across vertebrate species

Stem cell transplantation presents a potentially curative strategy for genetic disorders of skeletal muscle, but this approach is limited by the deleterious effects of cell expansion in vitro and consequent poor engraftment efficiency. In an effort to overcome this limitation, we sought to identify molecular signals that enhance the myogenic activity of cultured muscle progenitors. Here, we report the development and application of a cross-species small-molecule screening platform employing zebrafish and mice, which enables rapid, direct evaluation of the effects of chemical compounds on the engraftment of transplanted muscle precursor cells. Using this system, we screened a library of bioactive lipids to discriminate those that could increase myogenic engraftment in vivo in zebrafish and mice. This effort identified two lipids, lysophosphatidic acid and niflumic acid, both linked to the activation of intracellular calcium-ion flux, which showed conserved, dose-dependent, and synergistic effects in promoting muscle engraftment across these vertebrate species.

The role of the aging microenvironment on the fate of PDGFRβ lineage cells in skeletal muscle repair

Background

During aging, perturbation of muscle progenitor cell (MPC) constituents leads to progressive loss of muscle mass and accumulation of adipose and fibrotic tissue. Mesenchymal stem cells (MSCs) give rise to adipocytes and fibroblasts that accumulate in injured and pathological skeletal muscle through constitutive activation of platelet-derived growth factor receptors (PDGFRs). Although the role of the PDGFRα has been widely explored, there is a paucity of evidence demonstrating the role of PDGFRβ in aged skeletal muscle.

Methods

In this study, we investigated the role of PDGFRβ lineage cells in skeletal muscle during aging by using Cre/loxP lineage tracing technology. The PDGFR-Cre mice were crossed with global double-fluorescent Cre reporter mice (mTmG) that indelibly marks PDGFRβ lineage cells. Those cells were analyzed and compared at different ages in the skeletal muscle of the mice.

Results

Our results demonstrated that PDGFRβ lineage cells isolated from the muscles of young mice are MPC-like cells that exhibited satellite cell morphology, expressed Pax7, and undergo myogenic differentiation producing myosin heavy chain expressing myotubes. Conversely, the PDGFRβ lineage cells isolated from muscles of old mice displayed MSC morphology with a reduced myogenic differentiation potential while expressing adipogenic and fibrotic differentiation markers. PDGFRβ lineage cells also gave rise to newly regenerated muscle fibers in young mice after muscle injury, but their muscle regenerative process is reduced in old mice.

Conclusions

Our data suggest that PDGFRβ lineage cells function as MPCs in young mice, while the same PDGFRβ lineage cells from old mice undergo a fate switch participating in adipose and fibrotic tissue infiltration in aged muscle. The inhibition of fate-switching in PDGFRβ lineage cells may represent a potential approach to prevent fibrosis and fatty infiltration in skeletal muscle during the aging process.

On vasa vasorum: A history of advances in understanding the vessels of vessels

The vasa vasorum are a vital microvascular network supporting the outer wall of larger blood vessels. Although these dynamic microvessels have been studied for centuries, the importance and impact of their functions in vascular health and disease are not yet fully realized. There is now rich knowledge regarding what local progenitor cell populations comprise and cohabitate with the vasa vasorum and how they might contribute to physiological and pathological changes in the network or its expansion via angiogenesis or vasculogenesis. Evidence of whether vasa vasorum remodeling incites or governs disease progression or is a consequence of cardiovascular pathologies remains limited. Recent advances in vasa vasorum imaging for understanding cardiovascular disease severity and pathophysiology open the door for theranostic opportunities. Approaches that strive to control angiogenesis and vasculogenesis potentiate mitigation of vasa vasorum-mediated contributions to cardiovascular diseases and emerging diseases involving the microcirculation.

Engineering microparticles based on solidified stem cell secretome with an augmented pro-angiogenic factor portfolio for therapeutic angiogenesis

Tissue (re)vascularization strategies face various challenges, as therapeutic cells do not survive long enough in situ, while the administration of pro-angiogenic factors is hampered by fast clearance and insufficient ability to emulate complex spatiotemporal signaling. Here, we propose to address these limitations by engineering a functional biomaterial capable of capturing and concentrating the pro-angiogenic activities of mesenchymal stem cells (MSCs).

In particular, dextran sulfate, a high molecular weight sulfated glucose polymer, supplemented to MSC cultures, interacts with MSC-derived extracellular matrix (ECM) components and facilitates their co-assembly and accumulation in the pericellular space. Upon decellularization, the resulting dextran sulfate-ECM hybrid material can be processed into MIcroparticles of SOlidified Secretome (MIPSOS). The insoluble format of MIPSOS protects protein components from degradation, while facilitating their sustained release. Proteomic analysis demonstrates that MIPSOS are highly enriched in pro-angiogenic factors, resulting in an enhanced pro-angiogenic bioactivity when compared to naïve MSC-derived ECM (cECM). Consequently, intravital microscopy of full-thickness skin wounds treated with MIPSOS demonstrates accelerated revascularization and healing, far superior to the therapeutic potential of cECM. Hence, the microparticle-based solidified stem cell secretome provides a promising platform to address major limitations of current therapeutic angiogenesis approaches.

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Dextran sulfate assembles with mesenchymal stem cell secretome.

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As a result, microparticles of solidified stem cell secretome (MIPSOS) are formed.

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The insoluble MIPSOS format protects proteins from premature degradation.

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MIPSOS are enriched in pro-angiogenic factors and exhibit gradual release kinetics.

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MIPSOS demonstrate superior pro-angiogenic properties and thus therapeutic potential.

Cell Surface Proteins for Enrichment and In Vitro Characterization of Human Pluripotent Stem Cell-Derived Myogenic Progenitors

Human myogenic progenitors can be derived from pluripotent stem cells (PSCs) for use in modeling natural and pathological myogenesis, as well as treating muscle diseases. Transgene-free methods of deriving myogenic progenitors from different PSC lines often produce mixed populations that are heterogeneous in myogenic differentiation potential, yet detailed and accurate characterization of human PSC-derived myogenic progenitors remains elusive in the field. The isolation and purification of human PSC-derived myogenic progenitors is thus an important methodological consideration when we investigate the properties and behaviors of these cells in culture. We previously reported a transgene-free, serum-free floating sphere culture method for the derivation of myogenic progenitors from human PSCs. In this study, we first performed comprehensive cell surface protein profiling of the sphere culture cells through the screening of 255 antibodies. Next, we used magnetic activated cell sorting and enriched the cells according to the expression of specific surface markers. The ability of muscle differentiation in the resulting cells was characterized by immunofluorescent labeling and quantification of positively stained cells. Our results revealed that myotube-forming cells resided in the differentiated cultures of CD29+, CD56+, CD271+, and CD15– fractions, while thick and multinucleated myotubes were identified in the differentiated cultures from CD9+ and CD146+ fractions. We found that PAX7 localization to the nucleus correlates with myotube-forming ability in these sorted populations. We also demonstrated that cells in unsorted, CD271+, and CD15– fractions responded differently to cryopreservation and prolonged culture expansion. Lastly, we showed that CD271 expression is essential for terminal differentiation of human PSC-derived myogenic progenitors. Taken together, these cell surface proteins are not only useful markers to identify unique cellular populations in human PSC-derived myogenic progenitors but also functionally important molecules that can provide valuable insight into human myogenesis.

OUP accepted manuscript

The vascular wall is comprised of distinct layers controlling angiogenesis, blood flow, vessel anchorage within organs, and cell and molecule transit between blood and tissues. Moreover, some blood vessels are home to essential stem-like cells, a classic example being the existence in the embryo of hemogenic endothelial cells at the origin of definitive hematopoiesis. In recent years, microvascular pericytes and adventitial perivascular cells were observed to include multi-lineage progenitor cells involved not only in organ turnover and regeneration but also in pathologic remodeling, including fibrosis and atherosclerosis. These perivascular mesodermal elements were identified as native forerunners of mesenchymal stem cells. We have presented in this brief review our current knowledge on vessel wall-associated tissue remodeling cells with respect to discriminating phenotypes, functional diversity in health and disease, and potential therapeutic interest.

ELF3 mediates IL-1α induced differentiation of mesenchymal stem cells to inflammatory iCAFs

Stromal cells in the tumor microenvironment regulate the immune landscape and tumor progression. Yet, the ontogeny and heterogeneity of reactive stromal cells within tumors is not well understood. Carcinoma-associated fibroblasts exhibiting an inflammatory phenotype (iCAFs) have been identified within multiple cancers; however, mechanisms that lead to their recruitment and differentiation also remain undefined. Targeting these mechanisms therapeutically may be important in managing cancer progression. Here, we identify the ELF3 transcription factor as the canonical mediator of IL-1α-induced differentiation of prostate mesenchymal stem cells to an iCAF phenotype, typical of the tumor microenvironment. Furthermore, IL-1α-induced iCAFs were subsequently refractive to TGF-β1 induced trans-differentiation to a myofibroblast phenotype (myCAF), another key carcinoma-associated fibroblast subtype typical of reactive stroma in cancer. Restricted trans-differentiation was associated with phosphorylation of the YAP protein, indicating that interplay between ELF3 action and activation of the Hippo pathway are critical for restricting trans-differentiation of iCAFs. Together, these data show that the IL-1α/ELF3/YAP pathways are coordinate for regulating inflammatory carcinoma-associated fibroblast differentiation.

Pain relief and cartilage repair by Nanofat against osteoarthritis: preclinical and clinical evidence

Background

Osteoarthritis (OA) is the most common joint degenerative disorder, with little effective therapy to date. Nanofat is a cocktail of cells obtained from fat tissue, which possesses regenerative capacity and has a potential in treating OA. This study aimed to determine the anti-OA efficacy of Nanofat from basic and clinical aspects and explore its action mode.

Methods

Flow cytometry was performed to characterize Nanofat. A monoiodoacetate-induced OA rat model was employed for in vivo study. Cell viability and wound healing assays were conducted for in vitro study. Real-time PCR and Western blot assays were applied to explore the molecular action mode of Nanofat. Moreover, a retrospective analysis was conducted to determine the clinical efficacy and safety of Nanofat on knee OA patients.

Results

The in vivo results showed that Nanofat significantly attenuated pain symptoms and protected cartilage ECM (Col2) from damage, and its effects were not significantly differed with adipose tissue-derived stem cells (both P > 0.05). The in vitro results showed that Nanofat promoted the cell viability and migration of chondrocytes and significantly restored the IL-1β-induced abnormal gene expressions of Col2, Aggrecan, Sox9, Adamts5, Mmp3, Mmp9 Mmp13, IL-6 and Col10 and protein expressions of Col2, MMP9, MMP13, and Sox9 of chondrocytes. The regulatory actions of Nanofat on these anabolic, catabolic, and hypertrophic molecules of chondrocytes were similar between two treatment routes: co-culture and conditioned medium, suggesting a paracrine-based mode of action of Nanofat. Moreover, the clinical data showed that Nanofat relieved pain and repaired damaged cartilage of OA patients, with no adverse events.

Conclusion

In sum, this study demonstrated the anti-OA efficacy as well as a paracrine-based action mode of Nanofat, providing novel knowledge of Nanofat and suggesting it as a promising and practical cell therapy for clinical treatment of OA.

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.

Regenerative Potential of DPSCs and Revascularization: Direct, Paracrine or Autocrine Effect?

A new source of mesenchymal stem cells has recently been discovered, the so-called dental pulp derived stem cells (DPSCs) which therefore could represent potentially tools for regenerative medicine. DPSC originate from the neural crest and are physiologically involved in dentin homeostasis; moreover, they contribute to bone remodeling and differentiation into several tissues including cartilage, bone, adipose and nervous tissues. DPSCs have also been shown to influence the angiogenesis process, for example through the release of secretory factors or by differentiating into vascular and/or perivascular cells. Angiogenesis, that has a pivotal role in tissue regeneration and repair, is defined as the formation of new vessels from preexisting vessels and is mediated by mutual and reciprocal interactions between endothelial cells and perivascular cells. It is also known that co-cultures of perivascular and endothelial cells (ECs) can form a vascular network in vitro and also in vivo. Since DPSCs seem to have characteristics similar to pericytes, understanding the possible mechanism of interaction between DPSCs and ECs during neo-angiogenesis is dramatically important for the development of advanced clinical application in the field of regeneration.

Pericytes improve locomotor recovery after spinal cord injury in male and female neonatal rats

OBJECTIVE

It is not known how activation of the hypoxia-inducible factor (HIF) pathway in pericytes, cells of the microvascular wall, influences new capillary growth. We tested the hypothesis that HIF-activated pericytes promote angiogenesis in a neonatal model of spinal cord injury (SCI).

METHODS

Human placental pericytes stimulated with cobalt chloride and naïve pericytes were injected into the site of a thoracic hemi-section of the spinal cord in rat pups on postnatal day three (P3). Hindlimb motor recovery and Doppler blood flow perfusion at the site of transection were measured on P10. Immunohistochemistry was used to visualize vessel and neurofilament density for quantification.

RESULTS

Injection of HIF-activated pericytes resulted in greater vascular density in males but did not result in improved motor function for males or females. Injection of non-HIF-activated pericytes resulted improved motor function recovery in both sexes (males, 2.722 ± 0.31-fold score improvement; females, 3.824 ± 0.58-fold score improvement, P < .05) but produced no significant changes in vessel density.

CONCLUSIONS

HIF-activated pericytes promote vascular density in males post-SCI. Acute delivery of non-HIF-activated pericytes at the site of injury can improve motor recovery post-SCI.

The biology of vascular calcification

Vascular calcification (VC), characterized by different mineral deposits (i.e., carbonate apatite, whitlockite and hydroxyapatite) accumulating in blood vessels and valves, represents a relevant pathological process for the aging population and a life-threatening complication in acquired and in genetic diseases. Similarly to bone remodeling, VC is an actively regulated process in which many cells and molecules play a pivotal role. This review aims at: (i) describing the role of resident and circulating cells, of the extracellular environment and of positive and negative factors in driving the mineralization process; (ii) detailing the types of VC (i.e., intimal, medial and cardiac valve calcification); (iii) analyzing rare genetic diseases underlining the importance of altered pyrophosphate-dependent regulatory mechanisms; (iv) providing therapeutic options and perspectives.

Five Decades Later, Are Mesenchymal Stem Cells Still Relevant?

Mesenchymal stem cells are culture-derived mesodermal progenitors isolatable from all vascularized tissues. In spite of multiple fundamental, pre-clinical and clinical studies, the native identity and role in tissue repair of MSCs have long remained elusive, with MSC selection in vitro from total cell suspensions essentially unchanged as a mere primary culture for half a century. Recent investigations have helped understand the tissue origin of these progenitor cells, and uncover alternative effects of MSCs on tissue healing via growth factor secretion and interaction with the immune system. In this review, we describe current trends in MSC biology and discuss how these may improve the use of these therapeutic cells in tissue engineering and regenerative medicine.

Multipotency of mouse trophoblast stem cells

BACKGROUND

In a number of disease processes, the body is unable to repair injured tissue, promoting the need to develop strategies for tissue repair and regeneration, including the use of cellular therapeutics. Trophoblast stem cells (TSCs) are considered putative stem cells as they differentiate into other subtypes of trophoblast cells. To identify cells for future therapeutic strategies, we investigated whether TSCs have properties of stem/progenitor cells including self-renewal and the capacity to differentiate into parenchymal cells of fetal organs, in vitro and in vivo.

METHODS

TSCs were isolated using anti-CD117 micro-beads, from embryonic day 18.5 placentas. In vitro, CD117+ TSCs were cultured, at a limiting dilution in growth medium for the development of multicellular clones and in specialized medium for differentiation into lung epithelial cells, cardiomyocytes, and retinal photoreceptor cells. CD117+ TSCs were also injected in utero into lung, heart, and the sub-retinal space of embryonic day 13.5 fetuses, and the organs were harvested for histological assessment after a natural delivery.

RESULTS

We first identified CD117+ cells within the labyrinth zone and chorionic basal plate of murine placentas in late pregnancy, embryonic day 18.5. CD117+ TSCs formed multicellular clones that remained positive for CD117 in vitro, consistent with self-renewal properties. The clonal cells demonstrated multipotency, capable of differentiating into lung epithelial cells (endoderm), cardiomyocytes (mesoderm), and retinal photoreceptor cells (ectoderm). Finally, injection of CD117+ TSCs in utero into lungs, hearts, and the sub-retinal spaces of fetuses resulted in their engraftment on day 1 after birth, and the CD117+ TSCs differentiated into lung alveolar epithelial cells, heart cardiomyocytes, and retina photoreceptor cells, corresponding with the organs in which they were injected.

CONCLUSIONS

Our findings demonstrate that CD117+ TSCs have the properties of stem cells including clonogenicity, self-renewal, and multipotency. In utero administration of CD117+ TSCs engraft and differentiate into resident cells of the lung, heart, and retina during mouse development.

Coding Cell Identity of Human Skeletal Muscle Progenitor Cells Using Cell Surface Markers: Current Status and Remaining Challenges for Characterization and Isolation

Skeletal muscle progenitor cells (SMPCs), also called myogenic progenitors, have been studied extensively in recent years because of their promising therapeutic potential to preserve and recover skeletal muscle mass and function in patients with cachexia, sarcopenia, and neuromuscular diseases. SMPCs can be utilized to investigate the mechanisms of natural and pathological myogenesis via in vitro modeling and in vivo experimentation. While various types of SMPCs are currently available from several sources, human pluripotent stem cells (PSCs) offer an efficient and cost-effective method to derive SMPCs. As human PSC-derived cells often display varying heterogeneity in cell types, cell enrichment using cell surface markers remains a critical step in current procedures to establish a pure population of SMPCs. Here we summarize the cell surface markers currently being used to detect human SMPCs, describing their potential application for characterizing, identifying and isolating human PSC-derived SMPCs. To date, several positive and negative markers have been used to enrich human SMPCs from differentiated PSCs by cell sorting. A careful analysis of current findings can broaden our understanding and reveal potential uses for these surface markers with SMPCs.

Molecular Profiles of Cell-to-Cell Variation in the Regenerative Potential of Mesenchymal Stromal Cells

Cell-to-cell variation in the regenerative potential of mesenchymal stromal cells (MSCs) impedes the translation of MSC therapies into clinical practice. Cellular heterogeneity is ubiquitous across MSC cultures from different species and tissues. This review highlights advances to elucidate molecular profiles that identify cell subsets with specific regenerative properties in heterogeneous MSC cultures. Cell surface markers and global signatures are presented for proliferation and differentiation potential, as well as immunomodulation and trophic properties. Key knowledge gaps are discussed as potential areas of future research. Molecular profiles of MSC heterogeneity have the potential to enable unprecedented control over the regenerative potential of MSC therapies through the discovery of new molecular targets and as quality attributes to develop robust and reproducible biomanufacturing processes. These advances would have a positive impact on the nascent field of MSC therapeutics by accelerating the development of therapies with more consistent and effective treatment outcomes.

Pericytes for Therapeutic Bone Repair

Besides seminal functions in angiogenesis and blood pressure regulation, microvascular pericytes possess a latent tissue regenerative potential that can be revealed in culture following transition into mesenchymal stem cells. Endowed with robust osteogenic potential, pericytes and other related perivascular cells extracted from adipose tissue represent a potent and abundant cell source for refined bone tissue engineering and improved cell therapies of fractures and other bone defects. The use of diverse bone formation assays in vivo, which include mouse muscle pocket osteogenesis and calvaria replenishment, rat and dog spine fusion, and rat non-union fracture healing, has confirmed the superiority of purified perivascular cells for skeletal (re)generation. As a surprising observation though, despite strong endogenous bone-forming potential, perivascular cells drive bone regeneration essentially indirectly, via recruitment by secreted factors of local osteo-progenitors.

Pericytes as a Source of Osteogenic Cells in Bone Fracture Healing

Pericytes are mesenchymal cells that surround the endothelial cells of small vessels in various organs. These cells express several markers, such as NG2, CD146, and PDGFRβ, and play an important role in the stabilization and maturation of blood vessels. It was also recently revealed that like mesenchymal stem cells (MSCs), pericytes possess multilineage differentiation capacity, especially myogenic, adipogenic, and fibrogenic differentiation capacities. Although some previous studies have reported that pericytes also have osteogenic potential, the osteogenesis of pericytes can still be further elucidated. In the present study, we established novel methods for isolating and culturing primary murine pericytes. An immortalized pericyte line was also established. Multilineage induction of the pericyte line induced osteogenesis, adipogenesis, and chondrogenesis of the cells in vitro. In addition, pericytes that were injected into the fracture site of a bone fracture mouse model contributed to callus formation. Furthermore, in vivo pericyte-lineage-tracing studies demonstrated that endogenous pericytes also differentiate into osteoblasts and osteocytes and contribute to bone fracture healing as a cellular source of osteogenic cells. Pericytes can be a promising therapeutic candidate for treating bone fractures with a delayed union or nonunion as well as bone diseases causing bone defects.

Pericytes reduce inflammation and collagen deposition in acute wounds

BACKGROUND

Pericytes have been shown to have mesenchymal stromal cell-like properties and play a role in tissue regeneration. The goal of this study was to determine whether the addition of a pericyte sheet to a full-thickness dermal wound would enhance the healing of an acute wound.

METHODS

Human muscle-derived pericytes and human dermal fibroblasts were formed into cell sheets, then applied to full-thickness excisional wounds on the dorsum of nu/nu mice. Histology was performed to evaluate epidermal and dermal reformation, inflammation and fibrosis. In addition, real-time reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine cytokine response.

RESULTS

Pericytes were detected in the wounds until day 16 but not fibroblasts. Decrease in wound size was noted in pericyte sheet-treated wounds. Enhanced neo-vascularization and healthy granulation tissue formation were noted in the pericyte-treated wounds. Expression of type I collagen messenger RNA (mRNA) was significantly higher in the fibroblast-treated group, whereas Type III collagen mRNA showed significant increase in the pericyte group at days 3, 6 and 9 compared with the fibroblast and no-cell groups. Trichrome staining revealed thick unorganized collagen fibrils in the fibroblast-treated wounds, whereas pericyte-treated wounds contained thinner and more alligned collagen fibrils. Tumor necrosis factor (TNF)-α mRNA levels were increased in the fibroblast-treated wounds compared with pericyte-treated wounds.

DISCUSSION

The addition of pericytes may confer beneficial effects to wound healing resulting in reduced recruitment of inflammatory cells and collagen I deposition, potential to enhance wound closure and better collagen alignment promoting stronger tissue.

The controversial origin of pericytes during angiogenesis - Implications for cell-based therapeutic angiogenesis and cell-based therapies

Pericytes reside within the basement membrane of small vessels and are often in direct cellular contact with endothelial cells, fulfilling important functions during blood vessel formation and homeostasis. Recently, these pericytes have been also identified as mesenchymal stem cells. Mesenchymal stem cells, and especially their specialized subpopulation of pericytes, represent promising candidates for therapeutic angiogenesis applications, and have already been widely applied in pre-clinical and clinical trials. However, cell-based therapies of ischemic diseases (especially of myocardial infarction) have not resulted in significant long-term improvement. Interestingly, pericytes from a hematopoietic origin were observed in embryonic skin and a pericyte sub-population expressing leukocyte and monocyte markers was described during adult angiogenesis in vivo. Since mesenchymal stem cells do not express hematopoietic markers, the latter cell type might represent an alternative pericyte population relevant to angiogenesis. Therefore, we sourced blood-derived angiogenic cells (BDACs) from monocytes that closely resembled hematopoietic pericytes, which had only been observed in vivo thus far. BDACs displayed many pericytic features and exhibited enhanced revascularization and functional tissue regeneration in a pre-clinical model of critical limb ischemia. Comparison between BDACs and mesenchymal pericytes indicated that BDACs (while resembling hematopoietic pericytes) enhanced early stages of angiogenesis, such as endothelial cell sprouting. In contrast, mesenchymal pericytes were responsible for blood vessel maturation and homeostasis, while reducing endothelial sprouting.Since the formation of new blood vessels is crucial during therapeutic angiogenesis or during integration of implants into the host tissue, hematopoietic pericytes (and therefore BDACs) might offer an advantageous addition or even an alternative for cell-based therapies.

The ratio of ADSCs to HSC-progenitors in adipose tissue derived SVF may provide the key to predict the outcome of stem-cell therapy

BACKGROUND

Stromal vascular fraction (SVF) represents an attractive source of adult stem cells and progenitors, holding great promise for numerous cell therapy approaches. In 2017, it was reported that 1524 patients received autologous SVF following the enzymatic digestion of liposuction fat. The treatment was safe and effective and patients showed significant clinical improvement. In a collaborative study, we analyzed SVF obtained from 58 patients having degenerative, inflammatory, autoimmune diseases, and advanced stage cancer.

RESULTS

Flow analysis showed that freshly isolated SVF was very heterogeneous and harbored four major subsets specific to adipose tissue; CD34high CD45- CD31- CD146- adipose-derived stromal/stem cells (ADSCs), CD34low CD45+ CD206+CD31- CD146- hematopoietic stem cell-progenitors (HSC-progenitors), CD34high CD45- CD31+CD146+ adipose tissue-endothelial cells and CD45-CD34-CD31-CD146+ pericytes. Culturing and expanding of SVF revealed a homogenous population lacking hematopoietic lineage markers CD45 and CD34, but were positive for CD90, CD73, CD105, and CD44. Flow cytometry sorting of viable individual subpopulations revealed that ADSCs had the capacity to grow in adherent culture. The identity of the expanded cells as mesenchymal stem cells (MSCs) was further confirmed based on their differentiation into adipogenic and osteogenic lineages. To identify the potential factors, which may determine the beneficial outcome of treatment, we followed 44 patients post-SVF treatment. The gender, age, clinical condition, certain SVF-dose and route of injection, did not play a role on the clinical outcome. Interestingly, SVF yield seemed to be affected by patient's characteristic to various extents. Furthermore, the therapy with adipose-derived and expanded-mesenchymal stem cells (ADE-MSCs) on a limited number of patients, did not suggest increased efficacies compared to SVF treatment. Therefore, we tested the hypothesis that a certain combination, rather than individual subset of cells may play a role in determining the treatment efficacy and found that the combination of ADSCs to HSC-progenitor cells can be correlated with overall treatment efficacy.

CONCLUSIONS

We found that a 2:1 ratio of ADSCs to HSC-progenitors seems to be the key for a successful cell therapy. These findings open the way to future rational design of new treatment regimens for individuals by adjusting the cell ratio before the treatment.

Hepatic perivascular mesenchymal stem cells with myogenic properties

Pericytes are multipotent mesenchymal stem cells located on the walls of blood vessels in various organs and are characterized as CD146⁺ cells. In this study, we first immunohistochemically detected pericytes in the perivascular regions of liver from two mouse genotypes, namely wild-type (WT) and myostatin null (Mstn^-/- ). We further isolated pericytes using sorting as CD146⁺ CD34^- CD56^- CD45^- cells. The main finding of this study involves the contrasting fibrogenic vs. myogenic behaviour of liver pericytes from WT and Mstn^-/- mice, respectively. Sorted CD146⁺ liver pericytes (WT and Mstn^-/- ) expressed PDGFRβ, NG2, vimentin, adult stem cell markers CD73, CD105, CD44 and could be readily differentiated into adipogenic, osteogenic and chondrogenic lineages. Furthermore, these CD146⁺ cells from WT and Mstn^-/- livers did not express myostatin, in contrast to the total liver tissue of WT. The absence of αSMA and GFAP made these cells easily distinguishable from hepatic stellate cells. When subjected to standard myogenic differentiation with low serum the CD146⁺ cells from WT liver differentiated into myofibroblasts (fibrogenic) and the CD146⁺ cells from Mstn^-/- liver differentiated into multinucleated myotubes (myogenic). Finally, we transplanted CD146⁺ pericytes into tibialis anterior muscle of dystrophic mice and established the generation of novel myofibres, thereby proving their cell therapy potential. The liver tissue microenvironment with myostatin in WT and the absence of myostatin in Mstn^-/- conditions might exert a paracrine effect in determining the fate of pericyte-like cells in the liver.

Skeletal Muscle Regenerative Potential of Human MuStem Cells following Transplantation into Injured Mice Muscle

After intra-arterial delivery in the dystrophic dog, allogeneic muscle-derived stem cells, termed MuStem cells, contribute to long-term stabilization of the clinical status and preservation of the muscle regenerative process. However, it remains unknown whether the human counterpart could be identified, considering recent demonstrations of divergent features between species for several somatic stem cells. Here, we report that MuStem cells reside in human skeletal muscle and display a long-term ability to proliferate, allowing generation of a clinically relevant amount of cells. Cultured human MuStem (hMuStem) cells do not express hematopoietic, endothelial, or myo-endothelial cell markers and reproducibly correspond to a population of early myogenic-committed progenitors with a perivascular/mesenchymal phenotypic signature, revealing a blood vessel wall origin. Importantly, they exhibit both myogenesis in vitro and skeletal muscle regeneration after intramuscular delivery into immunodeficient host mice. Together, our findings provide new insights supporting the notion that hMuStem cells could represent an interesting therapeutic candidate for dystrophic patients.

Pericytes and their potential in regenerative medicine across species

The discovery that pericytes are in vivo counterparts of Mesenchymal Stem/Stromal Cells (MSCs) has placed these perivascular cells in the research spotlight, bringing up hope for a well-characterized cell source for clinical applications, alternative to poorly defined, heterogeneous MSCs preparations currently in use. Native pericytes express typical MSC markers and, after isolation by fluorescence-activated cell sorting, display an MSC phenotype in culture. These features have been demonstrated in different species, including humans and horses, the main targets of regenerative treatments. Significant clinical potential of pericytes has been shown by transplantation of human cells into rodent models of tissue injury, and it is hoped that future studies will demonstrate clinical potential in veterinary species. Here, we provide an overview of the current knowledge on pericytes across different species including humans, companion and large animal models, in relation to their identification in different body tissues, methodology for prospective isolation, characterization, and potential for tissue regeneration. © 2017 International Society for Advancement of Cytometry.

Classification and Functional Characterization of Vasa Vasorum-Associated Perivascular Progenitor Cells in Human Aorta

In the microcirculation, pericytes are believed to function as mesenchymal stromal cells (MSCs). We hypothesized that the vasa vasorum harbor progenitor cells within the adventitia of human aorta. Pericytes, endothelial progenitor cells, and other cell subpopulations were detected among freshly isolated adventitial cells using flow cytometry. Purified cultured pericytes were enriched for the MSC markers CD105 and CD73 and depleted of the endothelial markers von Willebrand factor and CD31. Cultured pericytes were capable of smooth muscle lineage progression including inducible expression of smooth muscle myosin heavy chain, calponin, and α-smooth muscle actin, and adopted a spindle shape. Pericytes formed spheroids when cultured on Matrigel substrates and peripherally localized with branching endothelial cells in vitro. Our results indicate that the vasa vasorum form a progenitor cell niche distinct from other previously described progenitor populations in human adventitia. These findings could have important implications for understanding the complex pathophysiology of human aortic disease.

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Perivascular progenitor cells were classified in human ascending aorta

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Adventitial vasa vasorum were identified as a progenitor cell niche

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Purified pericytes were functional in vitro as smooth muscle cell progenitors

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Branching endothelial cell networks were associated with pericytes in vitro

The Importance of Pericytes in Healing: Wounds and other Pathologies

Much of current research investigates the beneficial properties of mesenchymal stem cells (MSCs) as a treatment for wounds and other forms of injury. In this review, we bring attention to and discuss the role of the pericyte, a cell type which shares much of the differentiation potential and regenerative properties of the MSC as well as specific roles in the regulation of angiogenesis, inflammation and fibrosis. Pericytes have been identified as dysfunctional or depleted in many disease states, and observing the outcomes of pericyte perturbation in models of disease and wound healing informs our understanding of overall pericyte function and identifies these cells as an important target in the development of therapies to encourage healing.

Placental stem cells: The promise of curing diseases before birth

Regenerative medicine is a rapidly expanding and promising field for many diseases and injuries. Stem cells for regenerative therapies have originally been obtained from bone marrow, but are now readily extracted from a variety of adult tissues. Fetal tissue has recently garnered interest for its ease of differentiation into a variety of phenotypes and its relative abundance of pluripotent-linked transcription factors. However, much ethical concern surrounds the methods of obtaining fetal cells. The placenta has emerged as a potential source of fetal derived cells due to its favorable technical and ethical characteristics, as well as its promising therapeutic properties. This preview focuses on providing on overview on the derivation and characteristics of placental derived stem cells as well as delving into their various clinical applications and potential future directions.

Isolation and characterization of equine native MSC populations

Background

In contrast to humans in which mesenchymal stem/stromal cell (MSC) therapies are still largely in the clinical trial phase, MSCs have been used therapeutically in horses for over 15 years, thus constituting a valuable preclinical model for humans. In human tissues, MSCs have been shown to originate from perivascular cells, namely pericytes and adventitial cells, which are identified by the presence of the cell surface markers CD146 and CD34, respectively. In contrast, the origin of MSCs in equine tissues has not been established, preventing the isolation and culture of defined cell populations in that species. Moreover, a comparison between perivascular CD146⁺ and CD34⁺ cell populations has not been performed in any species.

Methods

Immunohistochemistry was used to identify adventitial cells (CD34⁺) and pericytes (CD146⁺) and to determine their localization in relation to MSCs in equine tissues. Isolation of CD34⁺ (CD34⁺/CD146^–/CD144^–/CD45^–) and CD146⁺ (CD146⁺/CD34^–/CD144^–/CD45^–) cell fractions from equine adipose tissue was achieved by fluorescence-activated cell sorting. The isolated cell fractions were cultured and analyzed for the expression of MSC markers, using qPCR and flow cytometry, and for the ability to undergo trilineage differentiation. Angiogenic properties were analyzed in vivo using a chorioallantoic membrane (CAM) assay.

Results

Both CD34⁺ and CD146⁺ cells displayed typical MSC features, namely growth in uncoated tissue culture dishes, clonal growth when seeded at low density, expression of typical MSC markers, and multipotency shown by the capacity for trilineage differentiation. Of note, CD146⁺ cells were distinctly angiogenic compared with CD34⁺ and non-sorted cells (conventional MSCs), demonstrated by the induction of blood vessels in a CAM assay, expression of elevated levels of VEGFA and ANGPT1, and association with vascular networks in cocultures with endothelial cells, indicating that CD146⁺ cells maintain a pericyte phenotype in culture.

Conclusion

This study reports for the first time the successful isolation and culture of CD146⁺ and CD34⁺ cell populations from equine tissues. Characterization of these cells evidenced their distinct properties and MSC-like phenotype, and identified CD146⁺ cells as distinctly angiogenic, which may provide a novel source for enhanced regenerative therapies.

Hypoxia Promotes Osteogenesis of Human Placental-Derived Mesenchymal Stem Cells

Placental-derived mesenchymal stem cells (pMSCs) are promising candidates for regenerative medicine because they possess high proliferative capacity and multi-differentiation potential. Human pMSCs are residing in an environment with low oxygen tension in the body. Heme oxygenase-1 (HO-1) is known to participate in the regulation of MSC differentiation. The present study aimed to investigate the impact of hypoxia on the osteogenic differentiation of human pMSCs, and to elucidate the role of HO-1 in the osteogenic differentiation of hypoxic pMSCs. Human pMSCs were cultured under normoxia (21% O2) or hypoxia (5% O2) for 3 days. We found that hypoxia maintained the morphology and immunophenotype of human pMSCs. The expression of stemness markers Oct4, Nanog, and Sox2 was increased under hypoxia. After a 5-day hypoxic culture, the proliferation ability of pMSCs was increased, which might be correlated with the increased expression of stem cell factor. During osteogenic induction, hypoxia increased the expression of osteogenic genes including osteopontin, osteocalcin, and alkaline phosphatase (ALP). Moreover, hypoxia increased the mineralization and ALP levels of human pMSCs as evidenced by Alizarin Red staining and ALP staining. Upregulation of HO-1 by cobalt-protoporphyrin treatment increased the osteogenic differentiation of pMSCs under hypoxia, while inhibition of HO-1 by Zn-protoporphyrin reduced the osteogenic differentiation of hypoxic pMSCs. Taken together, our data suggest that hypoxia can promote the osteogenic differentiation of human pMSCs. Upregulation of HO-1 can further increase the osteogenesis of human pMSCs under hypoxia. Our findings will highlight the therapeutic potential of MSCs in the tissue engineering of bones.

Perivascular Stem Cells Diminish Muscle Atrophy Following Massive Rotator Cuff Tears in a Small Animal Model

BACKGROUND

Rotator cuff tears are a common cause of shoulder pain and often necessitate operative repair. Muscle atrophy, fibrosis, and fatty infiltration can develop after rotator cuff tears, which may compromise surgical outcomes. This study investigated the regenerative potential of 2 human adipose-derived progenitor cell lineages in a murine model of massive rotator cuff tears.

METHODS

Ninety immunodeficient mice were used (15 groups of 6 mice). Mice were assigned to 1 of 3 surgical procedures: sham, supraspinatus and infraspinatus tendon transection (TT), or TT and denervation via suprascapular nerve transection (TT + DN). Perivascular stem cells (PSCs) were harvested from human lipoaspirate and sorted using fluorescence-activated cell sorting into pericytes (CD146 CD34 CD45 CD31) and adventitial cells (CD146 CD34 CD45 CD31). Mice received no injection, injection with saline solution, or injection with pericytes or adventitial cells either at the time of the index procedure ("prophylactic") or at 2 weeks following the index surgery ("therapeutic"). Muscles were harvested 6 weeks following the index procedure. Wet muscle weight, muscle fiber cross-sectional area, fibrosis, and fatty infiltration were analyzed.

RESULTS

PSC treatment after TT (prophylactic or therapeutic injections) and after TT + DN (therapeutic injections) resulted in less muscle weight loss and greater muscle fiber cross-sectional area than was demonstrated for controls (p < 0.05). The TT + DN groups treated with pericytes at either time point or with adventitial cells at 2 weeks postoperatively had less fibrosis than the TT + DN controls. There was less fatty infiltration in the TT groups treated with pericytes at either time point or with adventitial cells at the time of surgery compared with controls.

CONCLUSIONS

Our findings demonstrated significantly less muscle atrophy in the groups treated with PSCs compared with controls. This suggests that the use of PSCs may have a role in the prevention of muscle atrophy without leading to increased fibrosis or fatty infiltration.

CLINICAL RELEVANCE

Improved muscle quality in the setting of rotator cuff tears may increase the success rates of surgical repair and lead to superior clinical outcomes.

Pericytes: The Role of Multipotent Stem Cells in Vascular Maintenance and Regenerative Medicine

Blood vessels consist of an inner endothelial cell layer lining the vessel wall and perivascular pericytes, also known as mural cells, which envelop the vascular tube surface. Pericytes have recently been recognized for their central role in blood vessel formation. Pericytes are multipotent cells that are heterogeneous in their origin, function, morphology and surface markers. Similar to other types of stem cells, pericytes act as a repair system in response to injury by maintaining the structural integrity of blood vessels. Several studies have shown that blood vessels lacking pericytes become hyperdilated and haemorrhagic, leading to vascular complications ranging from diabetic retinopathy to embryonic death. The role of pericytes is not restricted to the formation and development of the vasculature: they have been shown to possess stem cell-like characteristics and may differentiate into cell types from different lineages. Recent discoveries regarding the contribution of pericytes to tumour metastasis and the maintenance of tumour vascular supply and angiogenesis have led researchers to propose targeting pericytes with anti-angiogenic therapies. In this review, we will examine the different physiological roles of pericytes, their differentiation potential, and how they interact with surrounding cells to ensure the integrity of blood vessel formation and maintenance.

Metformin Enhances Osteogenesis and Suppresses Adipogenesis of Human Chorionic Villous Mesenchymal Stem Cells

Metformin is the first-line anti-hyperglycemic drugs commonly used to treat type 2 diabetes. Recent studies have shown that metformin can enhance bone formation through induction of endothelial nitric oxide synthase (eNOS). Human chorionic villous mesenchymal stem cells (CV-MSCs) are promising candidates for regenerative medicine. The present study aimed to investigate the effects of metformin on the osteogenic and adipocytic differentiation of human CV-MSCs, and to elucidate the underlying mechanism. CV-MSCs, prepared from human term placentae, were cultured with different concentrations of metformin. Treatment for 72 hours with 0.05 mM metformin had no noticeable effect on the proliferation of CV-MSCs. Consequently, CV-MSCs were cultured for seven or 14 days in the osteogenic medium supplemented with 0.05 mM metformin. Treatment for seven days with metformin increased the expression levels of osteogenic protein mRNAs, including alkaline phosphatase, runt-related transcription factor 2, and osteopontin. Metformin also enhanced the mineralization of CV-MSCs. Furthermore, metformin induced the expression of eNOS in CV-MSCs during osteogenic differentiation. By contrast, when CV-MSCs were cultured for 14 days in the adipogenic medium, 0.05 mM metformin inhibited the expression of adipogenic protein mRNAs, including proliferators-activated receptor-γ and CCAAT/enhancer binding protein-α. The lipid droplet accumulation was also reduced on 28 days after metformin treatment. These findings indicate that metformin can enhance osteogenic differentiation of CV-MSCs and reduce adipocyte formation. The effect of metformin on osteogenic differentiation of CV-MSCs may be associated with eNOS expression. Our findings will highlight the therapeutic potential of metformin in osteoporosis and bone fracture.

Pericytes, an overlooked player in vascular pathobiology

Pericytes are a heterogeneous population of cells located in the blood vessel wall. They were first identified in the 19th century by Rouget, however their biological role and potential for drug targeting have taken time to be recognised. Isolation of pericytes from several different tissues has allowed a better phenotypic and functional characterization. These findings revealed a tissue-specific, multi-functional group of cells with multilineage potential. Given this emerging evidence, pericytes have acquired specific roles in pathobiological events in vascular diseases. In this review article, we will provide a compelling overview of the main diseases in which pericytes are involved, from well-established mechanisms to the latest findings. Pericyte involvement in diabetes and cancer will be discussed extensively. In the last part of the article we will review therapeutic approaches for these diseases in light of the recently acquired knowledge. To unravel pericyte-related vascular pathobiological events is pivotal not only for more tailored treatments of disease but also to establish pericytes as a therapeutic tool.

Pericytes, integral components of adult hematopoietic stem cell niches

The interest in perivascular cells as a niche for adult hematopoietic stem cells (HSCs) is significantly growing. In the adult bone marrow (BM), perivascular cells and HSCs cohabit. Among perivascular cells, pericytes are precursors of mesenchymal stem/stromal cells (MSCs) that are capable of differentiating into osteoblasts, adipocytes and chondrocytes. In situ, pericytes are recognised by their localisation to the abluminal side of the blood vessel wall and closely associated with endothelial cells, in combination with the expression of markers such as CD146, neural glial 2 (NG2), platelet derived growth factor receptor β (PDGFRβ), α-smooth muscle actin (α-SMA), nestin (Nes) and/or leptin receptor (LepR). However, not all pericytes share a common phenotype: different immunophenotypes can be associated with distinct mesenchymal features, including hematopoietic support. In adult BM, arteriolar and sinusoidal pericytes control HSC behaviour, maintenance, quiescence and trafficking through paracrine effects. Different groups identified and characterized hematopoietic supportive pericyte subpopulations using various markers and mouse models. In this review, we summarize recent work performed by others to understand the role of the perivascular niche in the biology of HSCs in adults, as well as their importance in the development of therapies.

Perivascular cells and tissue engineering: Current applications and untapped potential

The recent development of tissue engineering provides exciting new perspectives for the replacement of failing organs and the repair of damaged tissues. Perivascular cells, including vascular smooth muscle cells, pericytes and other tissue specific populations residing around blood vessels, have been isolated from many organs and are known to participate to the in situ repair process and angiogenesis. Their potential has been harnessed for cell therapy of numerous pathologies; however, in this Review we will discuss the potential of perivascular cells in the development of tissue engineering solutions for healthcare. We will examine their application in the engineering of vascular grafts, cardiac patches and bone substitutes as well as other tissue engineering applications and we will focus on their extensive use in the vascularization of engineered constructs. Additionally, we will discuss the emerging potential of human pericytes for the development of efficient, vascularized and non-immunogenic engineered constructs.

Perivascular Mesenchymal Stem Cells in Sheep: Characterization and Autologous Transplantation in a Model of Articular Cartilage Repair

Previous research has indicated that purified perivascular stem cells (PSCs) have increased chondrogenic potential compared to conventional mesenchymal stem cells (MSCs) derived in culture. This study aimed to develop an autologous large animal model for PSC transplantation and to specifically determine if implanted cells are retained in articular cartilage defects. Immunohistochemistry and fluorescence-activated cell sorting were used to ascertain the reactivity of anti-human and anti-ovine antibodies, which were combined and used to identify and isolate pericytes (CD34^-CD45^-CD146⁺) and adventitial cells (CD34⁺CD45^-CD146^-). The purified cells demonstrated osteogenic, adipogenic, and chondrogenic potential in culture. Autologous ovine PSCs (oPSCs) were isolated, cultured, and efficiently transfected using a green fluorescence protein (GFP) encoding lentivirus. The cells were implanted into articular cartilage defects on the medial femoral condyle using hydrogel and collagen membranes. Four weeks following implantation, the condyle was explanted and confocal laser scanning microscopy demonstrated the presence of oPSCs in the defect repaired with the hydrogel. These data suggest the testability in a large animal of native MSC autologous grafting, thus avoiding possible biases associated with xenotransplantation. Such a setting will be used in priority for indications in orthopedics, at first to model articular cartilage repair.

Human kidney pericytes produce renin

Pericytes, perivascular cells embedded in the microvascular wall, are crucial for vascular homeostasis. These cells also play diverse roles in tissue development and regeneration as multi-lineage progenitors, immunomodulatory cells and as sources of trophic factors. Here, we establish that pericytes are renin producing cells in the human kidney. Renin was localized by immunohistochemistry in CD146 and NG2 expressing pericytes, surrounding juxtaglomerular and afferent arterioles. Similar to pericytes from other organs, CD146⁺CD34^–CD45^–CD56^– renal fetal pericytes, sorted by flow cytometry, exhibited tri-lineage mesodermal differentiation potential in vitro. Additionally, renin expression was triggered in cultured kidney pericytes by cyclic AMP as confirmed by immuno-electron microscopy, and secretion of enzymatically functional renin, capable of generating angiotensin I. Pericytes derived from second trimester human placenta also expressed renin in an inducible fashion although the renin activity was much lower than in renal pericytes. Thus, our results confirm and extend the recently discovered developmental plasticity of microvascular pericytes, and may open new perspectives to the therapeutic regulation of the renin-angiotensin system.

Mesenchymal stem cell subpopulations: phenotype, property and therapeutic potential

Mesenchymal stem cells (MSC) are capable of differentiating into cells of multiple cell lineages and have potent paracrine effects. Due to their easy preparation and low immunogenicity, MSC have emerged as an extremely promising therapeutic agent in regenerative medicine for diverse diseases. However, MSC are heterogeneous with respect to phenotype and function in current isolation and cultivation regimes, which often lead to incomparable experimental results. In addition, there may be specific stem cell subpopulations with definite differentiation capacity toward certain lineages in addition to stem cells with multi-differentiation potential. Recent studies have identified several subsets of MSC which exhibit distinct features and biological activities, and enhanced therapeutic potentials for certain diseases. In this review, we give an overview of these subsets for their phenotypic, biological and functional properties.

Not All Pericytes Are Born Equal: Pericytes from Human Adult Tissues Present Different Differentiation Properties

Pericytes (PCs) have been recognized for a long time only as structural cells of the blood vessels. The identification of tight contacts with endothelial cells and the ability to interact with surrounding cells through paracrine signaling revealed additional functions of PCs in maintaining the homeostasis of the perivascular environment. PCs got the front page, in the late 1990s, after the identification and characterization of a new embryonic cell population, the mesoangioblasts, from which PCs present in the adult organism are thought to derive. From these studies, it was clear that PCs were also endowed with multipotent mesodermal abilities. Furthermore, their ability to cross the vascular wall and to reconstitute skeletal muscle tissue after systemic injection opened the way to a number of studies aimed to develop therapeutic protocols for a cell therapy of muscular dystrophy. This has resulted in a major effort to characterize pericytic cell populations from skeletal muscle and other adult tissues. Additional studies also addressed their relationship with other cells of the perivascular compartment and with mesenchymal stem cells. These data have provided initial evidence that PCs from different adult tissues might be endowed with distinctive differentiation abilities. This would suggest that the multipotent mesenchymal ability of PCs might be restrained within different tissues, likely depending on the specific cell renewal and repair requirements of each tissue. This review presents current knowledge on human PCs and highlights recent data on the differentiation properties of PCs isolated from different adult tissues.

Pericytes: Properties, Functions and Applications in Tissue Engineering

Mesenchymal stem cells (MSCs) are one of the most studied adult stem cells and in recent years. They have become attractive agents/cell source for cellular therapy and regenerative medicine applications. During investigations about their origin, researchers hypothesized that perivascular regions are the common anatomical regions where MSCs come from and perivascular cells like pericytes (PCs) (Rouget cells, mural cells) are in vivo counterparts of MSCs. Beside capillaries and microvessels as their most common locations, PCs are also found in large vessels (arteries and veins). They can be isolated from several tissues and organs particularly from retina and brain. There are different approaches about their isolation, characterization and culture but there has been no common protocol yet because of the lack of defined PC-specific marker. They make special contact with endothelial cells in the basement membrane and have very important functions in several tissues and organs. They participate in vascular development, stabilization, maturation, and remodeling, blood pressure control, endothelial cell proliferation and differentiation, contractility of vascular smooth muscle cells, wound healing, vasculogenesis and angiogenesis, long-term maintenance of hematopoietic stem cells in bone marrow niche. Their multipotential differentiation capacity and participation in many events in the body make PCs preferred cells in tissue engineering applications including 3D blood-brain barrier models, skeletal muscle constructs, bone tissue engineering and tissue-engineered vascular grafts.

The Placenta as an Organ and a Source of Stem Cells and Extracellular Matrix: A Review

The placenta is a temporal, dynamic and diverse organ with important immunological features that facilitate embryonic and fetal development and survival, notwithstanding the fact that several aspects of its formation and function closely resemble tumor progression. Placentation in mammals is commonly used to characterize the evolution of species, including insights into human evolution. Although most placentas are discarded after birth, they are a high-yield source for the isolation of stem/progenitor cells and are rich in extracellular matrix (ECM), representing an important resource for regenerative medicine purposes. Interactions among cells, ECM and bioactive molecules regulate tissue and organ generation and comprise the foundation of tissue engineering. In the present article, differences among several mammalian species regarding the placental types and classifications, phenotypes and potency of placenta-derived stem/progenitor cells, placental ECM components and current placental ECM applications were reviewed to highlight their potential clinical and biomedical relevance.

Prospective purification of perivascular presumptive mesenchymal stem cells from human adipose tissue: process optimization and cell population metrics across a large cohort of diverse demographics

BACKGROUND

Adipose tissue is an attractive source of mesenchymal stem cells (MSC) as it is largely dispensable and readily accessible through minimally invasive procedures such as liposuction. Until recently MSC could only be isolated in a process involving ex-vivo culture and their in-vivo identity, location and frequency remained elusive. We have documented that pericytes (CD45-, CD146+, and CD34-) and adventitial cells (CD45-, CD146-, CD34+) (collectively termed perivascular stem cells or PSC) represent native ancestors of the MSC, and can be prospectively purified using fluorescence activated cell sorting (FACS). In this study we describe an optimized protocol that aims to deliver pure, viable and consistent yields of PSC from adipose tissue. We analysed the frequency of PSC within adipose tissue, and the effect of patient and procedure based variables on this yield.

METHODS

Within this twin centre study we analysed the adipose tissue of n = 131 donors using flow cytometry to determine the frequency of PSC and correlate this with demographic and processing data such as age, sex, BMI and cold storage time of the tissue.

RESULTS

The mean number of stromal vascular fraction (SVF) cells from 100 ml of lipoaspirate was 34.4 million. Within the SVF, mean cell viability was 83 %, with 31.6 % of cells being haematopoietic (CD45+). Adventitial cells and pericytes represented 33.0 % and 8 % of SVF cells respectively. Therefore, a 200 ml lipoaspirate would theoretically yield 23.2 million viable prospectively purified PSC - sufficient for many reconstructive and regenerative applications. Minimal changes were observed in respect to age, sex and BMI suggesting universal potential application.

CONCLUSIONS

Adipose tissue contains two anatomically and phenotypically discreet populations of MSC precursors - adventitial cells and pericytes - together referred to as perivascular stem cells (PSC). More than 9 million PSC per 100 ml of lipoaspirate can be rapidly purified to homogeneity using flow cytometry in clinically relevant numbers potentially circumventing the need for purification and expansion by culture prior to clinical use. The number and viability of PSC are minimally affected by patient age, sex, BMI or the storage time of the tissue, but the quality and consistency of yield can be significantly influenced by procedure based variables.

Adipose derived pericytes rescue fractures from a failure of healing--non-union

Atrophic non-union is attributed to biological failure of the fracture repair process. It occurs in up to 10% of fractures, results in significant morbidity to patients, and treatment often requires complex reconstructive procedures. We tested the ability of human bone derived marrow mesenchymal stem cells (MSC), and human adipose derived pericytes (the native ancestor of the MSC) delivered percutaneously to the fracture gap to prevent the formation of atrophic non-union in a rat model. At eight weeks, 80% of animals in the cell treatment groups showed evidence of bone healing compared to only 14% of those in the control group. Radiographic parameters showed significant improvement over the eight-week period in the cell treatment groups, and histology confirmed bone bridges at the fracture gap in the both treatment groups. The quality of bone produced and its biomechanical properties were significantly enhanced in both treatment groups. The results from this study demonstrate that MSC and pericytes have significant bone regeneration potential in an atrophic non-union model. These cells may have a role in the prevention of atrophic non-union and could enable a paradigm shift in the treatment of fractures at high risk of failing to heal and developing non-union.

Accelerated Chondrogenic Differentiation of Human Perivascular Stem Cells with NELL-1

Osteoarthritis is the leading cause of disability in the US. Consequently, there is a pressing need for restoring the structural and functional properties of diseased articular cartilage. Yet the search for the right combination of proper target cells and growth factors for cartilage regeneration remains challenging. In this study, we first tested the intrinsic chondrogenic differentiation ability of human perivascular stem cells (hPSCs), a novel source of mesenchymal stem cells (MSCs) isolated by fluorescence-activated cell sorting (FACS) from human adipose tissue. A putative prochondrogenic growth factor, NEL-like molecule-1 (NELL-1), was added to the hPSC pellets to upregulate gene expression of chondrogenic markers, including AGGRECAN, COLLAGEN II, and COMP. Furthermore, the addition of NELL-1 to a transforming growth factor beta 3 (TGF-β3) + bone morphogenetic protein-6 (BMP-6) "cocktail" resulted in the best combinatorial stimulation in accelerating the chondrogenic differentiation of hPSCs, as evidenced by increased gene and protein expression of chondrogenic markers in a shortened induction time without elevating expression of hypertrophic, fibrotic, and osteogenic markers. Mechanistically, this acceleration rendered by NELL-1 may be partially attributed to NELL-1's upregulation of BMP receptors and TGF-β receptor type I in hPSCs for increased responsiveness to BMPs + TGF-βs. In conclusion, lipoaspirate-derived hPSCs present a novel and abundant cell source of MSCs for cartilage regeneration, and the combinatorial application of NELL-1, TGF-β3, and BMP-6 with hPSCs may remarkably enhance and accelerate cartilage repair.

Isolation, characterisation and comparative analysis of human umbilical cord vein perivascular cells and cord blood mesenchymal stem cells

Perivascular cells are known to be ancestors of mesenchymal stem cells (MSCs) and can be obtained from heart, skin, bone marrow, eye, placenta and umbilical cord (UC). However detailed characterization of perivascular cells around the human UC vein and comparative analysis of them with MSCs haven't been done yet. In this study, our aim is to isolate perivascular cells from human UC vein and characterize them versus UC blood MSCs (UCB-MSCs). For this purpose, perivascular cells around the UC vein were isolated enzymatically and then purified with magnetic activated cell sorting (MACS) method using CD146 Microbead Kit respectively. MSCs were isolated from UCB by Ficoll density gradient solution. Perivascular cells and UCB-MSCs were characterized by osteogenic and adipogenic differentiation procedures, flow cytometric analysis [CD146, CD105, CD31, CD34, CD45 and alpha-smooth muscle actin (α-SMA)], and immunofluorescent staining (MAP1B and Tenascin C). Alizarin red and Oil red O staining results showed that perivascular cells and MSCs had osteogenic and adipogenic differentiation capacity. However, osteogenic differentiation capacity of perivascular cells were found to be less than UCB-MSCs. According to flow cytometric analysis, CD146 expression of perivascular cells were appeared to be 4.8-fold higher than UCB-MSCs. Expression of α-SMA, MAP1B and Tenascin-C from perivascular cells was determined by flow cytometry analysis and immunfluorescent staining. The results appear to support the fact that perivascular cells are the ancestors of MSCs in vascular area. They may be used as alternative cells to MSCs in the field of vascular tissue engineering.