Fetal muscle contains different CD34+ cell subsets that distinctly differentiate into adipogenic, angiogenic and myogenic lineages

Single-cell analysis of mesenchymal cells in permeable neural vasculature reveals novel diverse subpopulations of fibroblasts

BACKGROUND

In the choroid plexus and pituitary gland, vasculature is known to have a permeable, fenestrated phenotype which allows for the free passage of molecules in contrast to the blood brain barrier observed in the rest of the CNS. The endothelium of these compartments, along with secretory, neural-lineage cells (choroid epithelium and pituitary endocrine cells) have been studied in detail, but less attention has been given to the perivascular mesenchymal cells of these compartments.

METHODS

The Hic1CreERT2 Rosa26LSL-TdTomato mouse model was used in conjunction with a PdgfraH2B-EGFP mouse model to examine mesenchymal cells, which can be subdivided into Pdgfra+ fibroblasts and Pdgfra- pericytes within the choroid plexus (CP) and pituitary gland (PG), by histological, immunofluorescence staining and single-cell RNA-sequencing analyses.

RESULTS

We found that both CP and PG possess substantial populations of distinct Hic1+ mesenchymal cells, including an abundance of Pdgfra+ fibroblasts. Within the pituitary, we identified distinct subpopulations of Hic1+ fibroblasts in the glandular anterior pituitary and the neurosecretory posterior pituitary. We also identified multiple distinct markers of CP, PG, and the meningeal mesenchymal compartment, including alkaline phosphatase, indole-n-methyltransferase and CD34.

CONCLUSIONS

Novel, distinct subpopulations of mesenchymal cells can be found in permeable vascular interfaces, including the CP, PG, and meninges, and make distinct contributions to both organs through the production of structural proteins, enzymes, transporters, and trophic molecules.

CD34 Protein: Its expression and function in inflammation

CD34 has spear-headed the field of basic research and clinical transplantation since the first reports of its expression on hematopoietic stem cells (HSCs). Expressed in mice, humans, rats and other species, CD34 has been used for more than 40 years as a hematopoietic stem and progenitor cell marker. It was later found that muscle satellite cells and epidermal precursors can also be identified with the aid of CD34. Despite the usefulness of CD34 as a marker of HSCs, its overall purpose in animal physiology has remained unclear. This review recaptures CD34 structure, evolutionary conservation, proposed functions, and role in lung inflammation, to describe current research findings and to provide guidance for future studies on CD34.

CD34-Structure, Functions and Relationship with Cancer Stem Cells

The CD34 protein was identified almost four decades ago as a biomarker for hematopoietic stem cell progenitors. CD34 expression of these stem cells has been exploited for therapeutic purposes in various hematological disorders. In the last few decades, studies have revealed the presence of CD34 expression on other types of cells with non-hematopoietic origins, such as interstitial cells, endothelial cells, fibrocytes, and muscle satellite cells. Furthermore, CD34 expression may also be found on a variety of cancer stem cells. Nowadays, the molecular functions of this protein have been involved in a variety of cellular functions, such as enhancing proliferation and blocking cell differentiation, enhanced lymphocyte adhesion, and cell morphogenesis. Although a complete understanding of this transmembrane protein, including its developmental origins, its stem cell connections, and other functions, is yet to be achieved. In this paper, we aimed to carry out a systematic analysis of the structure, functions, and relationship with cancer stem cells of CD34 based on the literature overview.

Tissue Engineering Applied to Skeletal Muscle: Strategies and Perspectives

Muscle tissue is formed by elongated and contractile cells with specific morphofunctional characteristics. Thus, it is divided into three basic types: smooth muscle tissue, cardiac striated muscle tissue and skeletal striated muscle tissue. The striated skeletal muscle tissue presents high plasticity, regeneration and growth capacity due to the presence of satellite cells, quiescent myoblasts that are activated in case of injury to the tissue and originate new muscle fibers when they differentiate. In more severe deficiencies or injuries there is a loss of their regenerative capacity, thus compromising the body's functionality at different levels. Tissue engineering studies the development of biomaterials capable of stimulating the recovery of cellular activity in injured body tissues, as well as the activity of cells with muscle differentiation potential in injury repair. However, the need for three-dimensional re-assembly in a complex organization makes it difficult to mimic this tissue and fully regenerate it for the sake of precise and effective movements. Thus, this article aims to provide a narrative review of tissue engineering strategies applied to the regeneration of skeletal muscle, in a critical evaluation of research, whether aimed at injury or atrophies such as spinal muscular atrophy.

Fibro-adipogenic progenitors in skeletal muscle homeostasis, regeneration and diseases

Skeletal muscle possesses a remarkable regenerative capacity that relies on the activity of muscle stem cells, also known as satellite cells. The presence of non-myogenic cells also plays a key role in the coordination of skeletal muscle regeneration. Particularly, fibro-adipogenic progenitors (FAPs) emerged as master regulators of muscle stem cell function and skeletal muscle regeneration. This population of muscle resident mesenchymal stromal cells has been initially characterized based on its bi-potent ability to differentiate into fibroblasts or adipocytes. New technologies such as single-cell RNAseq revealed the cellular heterogeneity of FAPs and their complex regulatory network during muscle regeneration. In acute injury, FAPs rapidly enter the cell cycle and secrete trophic factors that support the myogenic activity of muscle stem cells. Conversely, deregulation of FAP cell activity is associated with the accumulation of fibrofatty tissue in pathological conditions such as muscular dystrophies and ageing. Considering their central role in skeletal muscle pathophysiology, the regulatory mechanisms of FAPs and their cellular and molecular crosstalk with muscle stem cells are highly investigated in the field. In this review, we summarize the current knowledge on FAP cell characteristics, heterogeneity and the cellular crosstalk during skeletal muscle homeostasis and regeneration. We further describe their role in muscular disorders, as well as different therapeutic strategies targeting these cells to restore muscle regeneration.

Establishment and Characterization of a Sclerosing Spindle Cell Rhabdomyosarcoma Cell Line with a Complex Genomic Profile

Sclerosing spindle cell rhabdomyosarcoma (SSRMS) is a rare rhabdomyosarcomas (RMS) subtype. Especially cases bearing a myogenic differentiation 1 (MYOD1) mutation are characterized by a high recurrence and metastasis rate, often leading to a fatal outcome. SSRMS cell lines are valuable in vitro models for studying disease mechanisms and for the preclinical evaluation of new therapeutic approaches. In this study, a cell line established from a primary SSRMS tumor of a 24-year-old female after multimodal chemotherapeutic pretreatment has been characterized in detail, including immunohistochemistry, growth characteristics, cytogenetic analysis, mutation analysis, evaluation of stem cell marker expression, differentiation potential, and tumorigenicity in mice. The cell line which was designated SRH exhibited a complex genomic profile, including several translocations and deletions. Array-comparative genomic hybridization (CGH) revealed an overall predominating loss of gene loci. The mesenchymal tumor origin was underlined by the expression of mesenchymal markers and potential to undergo adipogenic and osteogenic differentiation. Despite myogenic marker expression, terminal myogenic differentiation was inhibited, which might be elicited by the MYOD1 hotspot mutation. In vivo tumorigenicity could be confirmed after subcutaneous injection into NOD/SCID/γ_c^null mice. Summarized, the SRH cell line is the first adult SSRMS cell line available for preclinical research on this rare RMS subtype.

Hypofractionated Radiotherapy With Volumetric Modulated Arc Therapy Decreases Postoperative Complications in Prosthetic Breast Reconstructions: A Clinicopathologic Study

Background

Emerging radiation technologies are expected to provide a positive impact on the reduction in postoperative complications in patients receiving prosthetic breast reconstruction. This study aimed to determine whether hypofractionated radiation therapy(RT) with volumetric modulated arc therapy(VMAT) is superior to conventional RT in the setting of prosthetic reconstruction.

Methods

From retrospective data collections, postoperative complications were compared for all patients with mastectomy and staged prosthetic reconstruction without RT, with hypofractionation using 40 Gy in 15 fractions with VMAT (Hypo-VMAT) or conventional RT (50 Gy over 5 weeks). After harvesting subpectoral capsules from patients with informed consents, histologic analysis including immunohistochemistry and immunofluorescence for collagen type I, α-smooth muscle actin, CD34 and CD31 expression was performed.

Results

A total of 288 reconstructions without RT, 55 reconstructions with Hypo-VMAT, and 29 reconstructions with conventional RT were examined. During average follow-up period of 34.8 months, rates of overall complications were 6.3% in the no-radiation group, 18.2% in Hypo-VMAT group and 44.8% in conventional-RT group with significant differences (no-RT vs Hypo-VMAT: p=0.006; Hypo-VMAT vs conventional-RT: p=0.012). Levels of myofibroblasts and tissue fibrosis were lower in the Hypo-VMAT group than in conventional-RT group (p=0.016 and p=0.040, respectively), while those of progenitor cells and microvessel density were higher in the Hypo-VMAT group than in conventional-RT group (p<0.001 and p<0.001, respectively).

Conclusion

We demonstrated that hypofractionated RT with VMAT served to reduce radiation-related morbidities in prosthetic reconstruction from a clinicopathologic perspective, compared to conventional RT. It may offer a practical strategy to mitigate radiation-related complications in clinical settings.

The Role of Integrin α6 (CD49f) in Stem Cells: More than a Conserved Biomarker

Stem cells have the capacity for self-renewal and differentiation into specialized cells that form and repopulated all tissues and organs, from conception to adult life. Depending on their capacity for differentiation, stem cells are classified as totipotent (ie, zygote), pluripotent (ie, embryonic stem cells), multipotent (ie, neuronal stem cells, hematopoietic stem cells, epithelial stem cells, etc.), and unipotent (ie, spermatogonial stem cells). Adult or tissue-specific stem cells reside in specific niches located in, or nearby, their organ or tissue of origin. There, they have microenvironmental support to remain quiescent, to proliferate as undifferentiated cells (self-renewal), and to differentiate into progenitors or terminally differentiated cells that migrate from the niche to perform specialized functions. The presence of proteins at the cell surface is often used to identify, classify, and isolate stem cells. Among the diverse groups of cell surface proteins used for these purposes, integrin α6, also known as CD49f, may be the only biomarker commonly found in more than 30 different populations of stem cells, including some cancer stem cells. This broad expression among stem cell populations indicates that integrin α6 may play an important and conserved role in stem cell biology, which is reaffirmed by recent demonstrations of its role maintaining self-renewal of pluripotent stem cells and breast and glioblastoma cancer stem cells. Therefore, this review intends to highlight and synthesize new findings on the importance of integrin α6 in stem cell biology.

Complement C3 of the innate immune system secreted by muscle adipogenic cells promotes myogenic differentiation

Myogenic differentiation results in different cell type cooperation, but the molecules involved in the myogenic cell activation remain elusive. Here, we show that muscle-resident pre-adipocytes promote myogenic differentiation through the secretion of factors. Using proteomic and transcriptomic analyses, we identified that proliferative adipogenic lineage cells produce and secrete a key factor of the innate immune system, the complement C3. Cell culture experiments revealed that C3 promotes the differentiation of myogenic progenitors following internalisation of the immune molecule. These data demonstrate that the third component of the complement system, which is a pivotal factor in the immune response to pathogens, is also involved in the differentiation of myogenic progenitor cells.

The Limbal Epithelial Progenitors in the Limbal Niche Environment

Limbal epithelial progenitors are stem cells located in limbal palisades of vogt. In this review, we present the audience with recent evidence that limbal epithelial progenitors may be a powerful stem cell resource for the cure of human corneal stem cell deficiency. Further understanding of their mechanism may shed lights to the future successful application of stem cell therapy not only to the eye tissue, but also to the other tissues in the human body.

Concise review: evidence for CD34 as a common marker for diverse progenitors

CD34 is a transmembrane phosphoglycoprotein, first identified on hematopoietic stem and progenitor cells. Clinically, it is associated with the selection and enrichment of hematopoietic stem cells for bone marrow transplants. Due to these historical and clinical associations, CD34 expression is almost ubiquitously related to hematopoietic cells, and it is a common misconception that CD34-positive (CD34⁺) cells in nonhematopoietic samples represent hematopoietic contamination. The prevailing school of thought states that multipotent mesenchymal stromal cells (MSC) do not express CD34. However, strong evidence demonstrates CD34 is expressed not only by MSC but by a multitude of other nonhematopoietic cell types including muscle satellite cells, corneal keratocytes, interstitial cells, epithelial progenitors, and vascular endothelial progenitors. In many cases, the CD34⁺ cells represent a small proportion of the total cell population and also indicate a distinct subset of cells with enhanced progenitor activity. Herein, we explore common traits between cells that express CD34, including associated markers, morphology and differentiation potential. We endeavor to highlight key similarities between CD34⁺ cells, with a focus on progenitor activity. A common function of CD34 has yet to be elucidated, but by analyzing and understanding links between CD34⁺ cells, we hope to be able to offer an insight into the overlapping properties of cells that express CD34. Stem Cells2014;32:1380–1389

Maternal obesity induces epigenetic modifications to facilitate Zfp423 expression and enhance adipogenic differentiation in fetal mice

Maternal obesity (MO) predisposes offspring to obesity and type 2 diabetes despite poorly defined mechanisms. Zfp423 is the key transcription factor committing cells to the adipogenic lineage, with exceptionally dense CpG sites in its promoter. We hypothesized that MO enhances adipogenic differentiation during fetal development through inducing epigenetic changes in the Zfp423 promoter and elevating its expression. Female mice were subjected to a control (Con) or obesogenic (OB) diet for 2 months, mated, and maintained on their diets during pregnancy. Fetal tissue was harvested at embryonic day 14.5 (E14.5), when the early adipogenic commitment is initiated. The Zfp423 expression was 3.6-fold higher and DNA methylation in the Zfp423 promoter was lower in OB compared with Con. Correspondingly, repressive histone methylation (H3K27me3) was lower in the Zfp423 promoter of OB fetal tissue, accompanied by reduced binding of enhancer of zeste 2 (EZH2). Gain- and loss-of-function analysis showed that Zfp423 regulates early adipogenic differentiation in fetal progenitor cells. In summary, MO enhanced Zfp423 expression and adipogenic differentiation during fetal development, at least partially through reducing DNA methylation in the Zfp423 promoter, which is expected to durably elevate adipogenic differentiation of progenitor cells in adult tissue, programming adiposity and metabolic dysfunction later in life.

Human Ng2+ adipose stem cells loaded in vivo on a new crosslinked hyaluronic acid-Lys scaffold fabricate a skeletal muscle tissue

Mesenchymal stem cell (MSC) therapy holds promise for treating diseases and tissue repair. Regeneration of skeletal muscle tissue that is lost during pathological muscle degeneration or after injuries is sustained by the production of new myofibers. Human Adipose stem cells (ASCs) have been reported to regenerate muscle fibers and reconstitute the pericytic cell pool after myogenic differentiation in vitro. Our aim was to evaluate the differentiation potential of constructs made from a new cross-linked hyaluronic acid (XHA) scaffold on which different sorted subpopulations of ASCs were loaded. Thirty days after engraftment in mice, we found that NG2(+) ASCs underwent a complete myogenic differentiation, fabricating a human skeletal muscle tissue, while NG2(-) ASCs merely formed a human adipose tissue. Myogenic differentiation was confirmed by the expression of MyoD, MF20, laminin, and lamin A/C by immunofluorescence and/or RT-PCR. In contrast, adipose differentiation was confirmed by the expression of adiponectin, Glut-4, and PPAR-γ. Both tissues formed expressed Class I HLA, confirming their human origin and excluding any contamination by murine cells. In conclusion, our study provides novel evidence that NG2(+) ASCs loaded on XHA scaffolds are able to fabricate a human skeletal muscle tissue in vivo without the need of a myogenic pre-differentiation step in vitro. We emphasize the translational significance of our findings for human skeletal muscle regeneration.

Inferring the in vivo cellular program of developing bovine skeletal muscle from expression data

We outline an in vivo cellular program of bovine longissimus muscle development inferred from expression data from 60 days post conception to 3months postnatal. Analytic challenges included changes in cellular composition, ambiguous 'diagnostic' markers of cell type and contrasts between cattle human and mouse myogenesis. Nevertheless, the expression profiles of the myosin isoforms support slow and fast muscle fibres emanating from primary and secondary myogenesis respectively, while expression of the prenatal myosin subunits is down regulated prior to birth. Of the canonical pro-myogenic transcription factors (TF), MYF6 and MYF5 are negatively co-expressed, with MYF6 displaying higher expression in the post-natal samples and MYF5, MYOG, HES6 and PAX7 displaying higher expression in early development. A set of TFs (SIX1, EYA2 and DACH2) considered important in undifferentiated murine cells were equally abundant in differentiated bovine cells. An examination of mammalian regulators of fibre composition, muscle mass and muscle metabolism, underscored the roles of PPARGC1A, TGFβ signalling and the NHR4 Nuclear Hormone Receptors on bovine muscle development. Enriched among the most variably expressed genes from the entire data set were molecules regulating mitochondrial metabolism of carbohydrate (PDK4), fat (UCP3), protein (AGXT2L1) and high energy phosphate (CKMT2). The dramatic increase in the expression of these transcripts, which may enable the peri-natal transition to metabolic independence critical for new-born herbivores, provides surprising evidence for substantial developmental remodelling of muscle mitochondria and reflects changes in nutrient availability. Overall, despite differences in size, metabolism and physiology, the muscle structural subunit expression program appears very similar in ruminants, rodents and humans.

Hypoxia increases mouse satellite cell clone proliferation maintaining both in vitro and in vivo heterogeneity and myogenic potential

Satellite cells (SCs) are essential for postnatal muscle growth and regeneration, however, their expansion potential in vitro is limited. Recently, hypoxia has been used to enhance proliferative abilities in vitro of various primary cultures. Here, by isolating SCs from single mouse hindlimb skeletal myofibers, we were able to distinguish two subpopulations of clonally cultured SCs (Low Proliferative Clones - LPC - and High Proliferative Clones - HPC), which, as shown in rat skeletal muscle, were present at a fixed proportion. In addition, culturing LPC and HPC at a low level of oxygen we observed a two fold increased proliferation both for LPC and HPC. LPC showed higher myogenic regulatory factor (MRF) expression than HPC, particularly under the hypoxic condition. Notably, a different myogenic potential between LPC and HPC was retained in vivo: green fluorescent protein (GFP)+LPC transplantation in cardiotoxin-injured Tibialis Anterior led to a higher number of new GFP+muscle fibers per transplanted cell than GFP+HPC. Interestingly, the in vivo myogenic potential of a single cell from an LPC is similar if cultured both in normoxia and hypoxia. Therefore, starting from a single satellite cell, hypoxia allows a larger expansion of LPC than normal O₂ conditions, obtaining a consistent amount of cells for transplantation, but maintaining their myogenic regeneration potential.