An information theoretic, microfluidic-based single cell analysis permits identification of subpopulations among putatively homogeneous stem cells

Endothelial-specific CXCL12 regulates neovascularization during tissue repair and tumor progression

C-X-C motif chemokine ligand 12 (CXCL12; Stromal Cell-Derived Factor 1 [SDF-1]), most notably known for its role in embryogenesis and hematopoiesis, has been implicated in tumor pathophysiology and neovascularization. However, its cell-specific role and mechanism of action have not been well characterized. Previous work by our group has demonstrated that hypoxia-inducible factor (HIF)-1 modulates downstream CXCL12 expression following ischemic tissue injury. By utilizing a conditional CXCL12 knockout murine model, we demonstrate that endothelial-specific deletion of CXCL12 (eKO) modulates ischemic tissue survival, altering tissue repair and tumor progression without affecting embryogenesis and morphogenesis. Loss of endothelial-specific CXCL12 disrupts critical endothelial-fibroblast crosstalk necessary for stromal growth and vascularization. Using murine parabiosis with novel transcriptomic technologies, we demonstrate that endothelial-specific CXCL12 signaling results in downstream recruitment of non-inflammatory circulating cells, defined by neovascularization modulating genes. These findings indicate an essential role for endothelial-specific CXCL12 expression during the neovascular response in tissue injury and tumor progression.

A top-down measure of gene-to-gene coordination for analyzing cell-to-cell variability

Recent technological advances, such as single-cell RNA sequencing (scRNA-seq), allow the measurement of gene expression profiles of individual cells. These expression profiles typically exhibit substantial variations even across seemingly homogeneous populations of cells. Two main different sources contribute to this measured variability: actual differences between the biological activity of the cells and technical measurement errors. Analysis of the biological variability may provide information about the underlying gene regulation of the cells, yet distinguishing it from the technical variability is a challenge. Here, we apply a recently developed computational method for measuring the global gene coordination level (GCL) to systematically study the cell-to-cell variability in numerical models of gene regulation. We simulate ‘biological variability’ by introducing heterogeneity in the underlying regulatory dynamic of different cells, while ‘technical variability’ is represented by stochastic measurement noise. We show that the GCL decreases for cohorts of cells with increased ‘biological variability’ only when it is originated from the interactions between the genes. Moreover, we find that the GCL can evaluate and compare—for cohorts with the same cell-to-cell variability—the ratio between the introduced biological and technical variability. Finally, we show that the GCL is robust against spurious correlations that originate from a small sample size or from the compositionality of the data. The presented methodology can be useful for future analysis of high-dimensional ecological and biochemical dynamics.

Characterization of Diabetic and Non-Diabetic Foot Ulcers Using Single-Cell RNA-Sequencing

Background: Recent advances in high-throughput single-cell sequencing technologies have led to their increasingly widespread adoption for clinical applications. However, challenges associated with tissue viability, cell yield, and delayed time-to-capture have created unique obstacles for data processing. Chronic wounds, in particular, represent some of the most difficult target specimens, due to the significant amount of fibrinous debris, extracellular matrix components, and non-viable cells inherent in tissue routinely obtained from debridement. Methods: Here, we examined the feasibility of single cell RNA sequencing (scRNA-seq) analysis to evaluate human chronic wound samples acquired in the clinic, subjected to prolonged cold ischemia time, and processed without FACS sorting. Wound tissue from human diabetic and non-diabetic plantar foot ulcers were evaluated using an optimized 10X Genomics scRNA-seq platform and analyzed using a modified data pipeline designed for low-yield specimens. Cell subtypes were identified informatically and their distributions and transcriptional programs were compared between diabetic and non-diabetic tissue. Results: 139,000 diabetic and non-diabetic wound cells were delivered for 10X capture after either 90 or 180 min of cold ischemia time. cDNA library concentrations were 858.7 and 364.7 pg/µL, respectively, prior to sequencing. Among all barcoded fragments, we found that 83.5% successfully aligned to the human transcriptome and 68% met the minimum cell viability threshold. The average mitochondrial mRNA fraction was 8.5% for diabetic cells and 6.6% for non-diabetic cells, correlating with differences in cold ischemia time. A total of 384 individual cells were of sufficient quality for subsequent analyses; from this cell pool, we identified transcriptionally-distinct cell clusters whose gene expression profiles corresponded to fibroblasts, keratinocytes, neutrophils, monocytes, and endothelial cells. Fibroblast subpopulations with differing fibrotic potentials were identified, and their distributions were found to be altered in diabetic vs. non-diabetic cells. Conclusions: scRNA-seq of clinical wound samples can be achieved using minor modifications to standard processing protocols and data analysis methods. This simple approach can capture widespread transcriptional differences between diabetic and non-diabetic tissue obtained from matched wound locations.

Small molecule inhibition of dipeptidyl peptidase-4 enhances bone marrow progenitor cell function and angiogenesis in diabetic wounds

In diabetes, stromal cell-derived factor-1 (SDF-1) expression and progenitor cell recruitment are reduced. Dipeptidyl peptidase-4 (DPP-4) inhibits SDF-1 expression and progenitor cell recruitment. Here we examined the impact of the DPP-4 inhibitor, MK0626, on progenitor cell kinetics in the context of wound healing. Wildtype (WT) murine fibroblasts cultured under high-glucose to reproduce a diabetic microenvironment were exposed to MK0626, glipizide, or no treatment, and SDF-1 expression was measured with ELISA. Diabetic mice received MK0626, glipizide, or no treatment for 6 weeks and then were wounded. Immunohistochemistry was used to quantify neovascularization and SDF-1 expression. Gene expression was measured at the RNA and protein level using quantitative polymerase chain reaction and ELISA, respectively. Flow cytometry was used to characterize bone marrow-derived mesenchymal progenitor cell (BM-MPC) population recruitment to wounds. BM-MPC gene expression was assayed using microfluidic single cell analysis. WT murine fibroblasts exposed to MK0626 demonstrated increased SDF-1 expression. MK0626 treatment significantly accelerated wound healing and increased wound vascularity, SDF-1 expression, and dermal thickness in diabetic wounds. MK0626 treatment increased the number of BM-MPCs present in bone marrow and in diabetic wounds. MK0626 had no effect on BM-MPC population dynamics. BM-MPCs harvested from MK0626-treated mice exhibited increased chemotaxis in response to SDF-1 when compared to diabetic controls. Treatment with a DPP-4 inhibitor significantly improved wound healing, angiogenesis, and endogenous progenitor cell recruitment in the setting of diabetes.

Single-Cell Transcriptomics of Human Mesenchymal Stem Cells Reveal Age-Related Cellular Subpopulation Depletion and Impaired Regenerative Function

Although bone marrow-derived mesenchymal stem cells (BM-MSCs) are widely recognized as promising therapeutic agents, the age-related impacts on cellular function remain largely uncharacterized. In this study, we found that BM-MSCs from young donors healed wounds in a xenograft model faster compared with their aged counterparts (p < .001). Given this significant healing advantage, we then used single-cell transcriptomic analysis to provide potential molecular insights into these observations. We found that the young cells contained a higher proportion of cells characterized by a higher expression of genes involved in tissue regeneration. In addition, we identified a unique, quiescent subpopulation that was exclusively present in young donor cells. Together, these findings may explain a novel mechanism for the enhanced healing capacity of young stem cells and may have implications for autologous cell therapy in the extremes of age. Stem Cells 2019;37:240-246.

Single-Cell Gene Expression Analysis and Evaluation of the Therapeutic Function of Murine Adipose-Derived Stromal Cells (ASCs) from the Subcutaneous and Visceral Compartment

Introduction

Adipose-derived stromal cells (ASCs) are a promising resource for wound healing and tissue regeneration because of their multipotent properties and cytokine secretion. ASCs are typically isolated from the subcutaneous fat compartment, but can also be obtained from visceral adipose tissue. The data on their equivalence diverges. The present study analyzes the cell-specific gene expression profiles and functional differences of ASCs derived from the subcutaneous (S-ASCs) and the visceral (V-ASCs) compartment.

Material and Methods

Subcutaneous and visceral ASCs were obtained from mouse inguinal fat and omentum. The transcriptional profiles of the ASCs were compared on single-cell level. S-ASCs and V-ASCs were then compared in a murine wound healing model to evaluate their regenerative functionality.

Results

On a single-cell level, S-ASCs and V-ASCs displayed distinct transcriptional profiles. Specifically, significant differences were detected in genes associated with neoangiogenesis and tissue remodeling (for example, Ccl2, Hif1α, Fgf7, and Igf). In addition, a different subpopulation ecology could be identified employing a cluster model. Nevertheless, both S-ASCs and V-ASCs induced accelerated healing rates and neoangiogenesis in a mouse wound healing model.

Conclusion

With similar therapeutic potential in vivo, the significantly different gene expression patterns of ASCs from the subcutaneous and visceral compartments suggest different signaling pathways underlying their efficacy. This study clearly demonstrates that review of transcriptional results in vivo is advisable to confirm the tentative effect of cell therapies.

Spatial Analysis of Single Fiber Cells of the Developing Ocular Lens Reveals Regulated Heterogeneity of Gene Expression

The developing eye lens presents an exceptional paradigm for spatial transcriptomics. It is composed of highly organized long, slender transparent fiber cells, which differentiate from the edges of the anterior epithelium of the lens (equator), attended by high expression of crystallins, which generates transparency. Every fiber cell, therefore, is an optical unit whose refractive properties derive from its gene activity. Here, we probe this tangible relationship between the gene activity and the phenotype by studying the expression of all known 17 crystallins and 77 other non-crystallin genes in single fiber cells isolated from three states/regions of differentiation, allowing us to follow molecular progression at the single-cell level. The data demonstrate highly variable gene activity in cortical fibers, interposed between the nascent and the terminally differentiated fiber cell transcription. These data suggest that the so-called stochastic, highly heterogeneous gene activity is a regulated intermediate in the realization of a functional phenotype.

Delivery of monocyte lineage cells in a biomimetic scaffold enhances tissue repair

The monocyte lineage is essential to normal wound healing. Macrophage inhibition or knockout in mice results in impaired wound healing through reduced neovascularization, granulation tissue formation, and reepithelialization. Numerous studies have either depleted macrophages or reduced their activity in the context of wound healing. Here, we demonstrate that by increasing the number of macrophages or monocytes in the wound site above physiologic levels via pullulan-collagen composite dermal hydrogel scaffold delivery, the rate of wound healing can be significantly accelerated in both wild-type and diabetic mice, with no adverse effect on the quality of repair. Macrophages transplanted onto wounds differentiate into M1 and M2 phenotypes of different proportions at various time points, ultimately increasing angiogenesis. Given that monocytes can be readily isolated from peripheral blood without in vitro manipulation, these findings hold promise for translational medicine aimed at accelerating wound healing across a broad spectrum of diseases.

The Role of Focal Adhesion Kinase in Keratinocyte Fibrogenic Gene Expression

Abnormal skin scarring causes functional impairment, psychological stress, and high socioeconomic cost. Evidence shows that altered mechanotransduction pathways have been linked to both inflammation and fibrosis, and that focal adhesion kinase (FAK) is a key mediator of these processes. We investigated the importance of keratinocyte FAK at the single cell level in key fibrogenic pathways critical for scar formation. Keratinocytes were isolated from wildtype and keratinocyte-specific FAK-deleted mice, cultured, and sorted into single cells. Keratinocytes were evaluated using a microfluidic-based platform for high-resolution transcriptional analysis. Partitive clustering, gene enrichment analysis, and network modeling were applied to characterize the significance of FAK on regulating keratinocyte subpopulations and fibrogenic pathways important for scar formation. Considerable transcriptional heterogeneity was observed within the keratinocyte populations. FAK-deleted keratinocytes demonstrated increased expression of genes integral to mechanotransduction and extracellular matrix production, including Igtbl, Mmpla, and Col4a1. Transcriptional activities upon FAK deletion were not identical across all single keratinocytes, resulting in higher frequency of a minor subpopulation characterized by a matrix-remodeling profile compared to wildtype keratinocyte population. The importance of keratinocyte FAK signaling gene expression was revealed. A minor subpopulation of keratinocytes characterized by a matrix-modulating profile may be a keratinocyte subset important for mechanotransduction and scar formation.

Isolation of CD248-expressing stromal vascular fraction for targeted improvement of wound healing

Wound healing remains a global issue of disability, cost, and health. Addition of cells from the stromal vascular fraction (SVF) of adipose tissue has been shown to increase the rate of full thickness wound closure. This study aimed to investigate the angiogenic mechanisms of CD248+ SVF cells in the context of full thickness excisional wounds. Single cell transcriptional analysis was used to identify and cluster angiogenic gene-expressing cells, which was then correlated with surface marker expression. SVF cells isolated from human lipoaspirate were FACS sorted based on the presence of CD248. Cells were analyzed for angiogenic gene expression and ability to promote microvascular tubule formation in vitro. Following this, 6mm full thickness dermal wounds were created on the dorsa of immunocompromised mice and then treated with CD248+, CD248-, or unsorted SVF cells delivered in a pullalan-collagen hydrogel or the hydrogel alone. Wounds were measured every other day photometrically until closure. Wounds were also evaluated histologically at 7 and 14 days post-wounding and when fully healed to assess for reepithelialization and development of neovasculature. Wounds treated with CD248+ cells healed significantly faster than other treatment groups, and at 7 days, had quantitatively more reepithelialization. Concurrently, immunohistochemistry of CD31 revealed a much higher presence of vascularity in the CD248+ SVF cells treated group at the time of healing and at 14 days post-op, consistent with a pro-angiogenic effect of CD248+ cells in vivo. Therefore, using CD248+ pro-angiogenic cells obtained from SVF presents a viable strategy in wound healing by promoting increased vessel growth in the wound.

Adipose-Derived Stem Cell-Seeded Hydrogels Increase Endogenous Progenitor Cell Recruitment and Neovascularization in Wounds

Adipose-derived mesenchymal stem cells (ASCs) are appealing for cell-based wound therapies because of their accessibility and ease of harvest, but their utility is limited by poor cell survival within the harsh wound microenvironment. In prior work, our laboratory has demonstrated that seeding ASCs within a soft pullulan-collagen hydrogel enhances ASC survival and improves wound healing. To more fully understand the mechanism of this therapy, we examined whether ASC-seeded hydrogels were able to modulate the recruitment and/or functionality of endogenous progenitor cells. Employing a parabiosis model and fluorescence-activated cell sorting analysis, we demonstrate that application of ASC-seeded hydrogels to wounds, when compared with injected ASCs or a noncell control, increased the recruitment of provascular circulating bone marrow-derived mesenchymal progenitor cells (BM-MPCs). BM-MPCs comprised 23.0% of recruited circulating progenitor cells in wounds treated with ASC-seeded hydrogels versus 8.4% and 2.1% in those treated with controls, p < 0.05. Exploring the potential for functional modulation of BM-MPCs, we demonstrate a statistically significant increase in BM-MPC migration, proliferation, and tubulization when exposed to hydrogel-seeded ASC-conditioned medium versus control ASC-conditioned medium (73.8% vs. 51.4% scratch assay closure; 9.1% vs. 1.4% proliferation rate; 10.2 vs. 5.5 tubules/HPF; p < 0.05 for all assays). BM-MPC expression of genes related to cell stemness and angiogenesis was also significantly increased following exposure to hydrogel-seeded ASC-conditioned medium (p < 0.05). These data suggest that ASC-seeded hydrogels improve both progenitor cell recruitment and functionality to effect greater neovascularization.

Dissecting stem cell differentiation using single cell expression profiling

Many assumptions about the way cells behave are based on analyses of populations. However, it is now widely recognized that even apparently pure populations can display a remarkable level of heterogeneity. This is particularly true in stem cell biology where it hinders our understanding of normal development and the development of strategies for regenerative medicine. Over the past decade technologies facilitating gene expression analysis at the single cell level have become widespread, providing access to rare cell populations and insights into population structure and function. Here we review the contributions of single cell biology to understanding stem cell differentiation so far, both as a new methodology for defining cell types and a tool for understanding the complexities of cellular decision-making.

Regenerative Medicine: Charting a New Course in Wound Healing

Significance: Chronic wounds are a prevalent and costly problem in the United States. Improved treatments are needed to heal these wounds and prevent serious complications such as infection and amputation. Recent Advances: In wound healing, as in other areas of medicine, technologies that have the potential to regenerate as opposed to repair tissue are gaining ground. These include customizable nanofiber matrices incorporating novel materials; a variety of autologous and allogeneic cell types at various stages of differentiation (e.g., pluripotent, terminally differentiated); peptides; proteins; small molecules; RNA inhibitors; and gene therapies. Critical Issues: Wound healing is a logical target for regenerative medicine due to the accessibility and structure of skin, the regenerative nature of healing, the lack of good limb salvage treatments, and the current use of cell therapies. However, more extensive knowledge of pathophysiologic targets is needed to inform regenerative strategies, and new technologies must demonstrate value in terms of outcomes and related health economic measures to achieve successful market access and penetration. Future Directions: Due to similarities in cell pathways and developmental mechanisms, regenerative technologies developed in one therapeutic area may be applicable to others. Approaches that proceed from human genomic or other big data sources to models are becoming increasingly common and will likely suggest novel therapeutic avenues. To fully capitalize on the advances in regenerative medicine, studies must demonstrate the value of new therapies in identified patient populations, and sponsors must work with regulatory agencies to develop appropriate dossiers supporting timely approval.

Microfluidic single-cell transcriptional analysis rationally identifies novel surface marker profiles to enhance cell-based therapies

Current progenitor cell therapies have only modest efficacy, which has limited their clinical adoption. This may be the result of a cellular heterogeneity that decreases the number of functional progenitors delivered to diseased tissue, and prevents correction of underlying pathologic cell population disruptions. Here, we develop a high-resolution method of identifying phenotypically distinct progenitor cell subpopulations via single-cell transcriptional analysis and advanced bioinformatics. When combined with high-throughput cell surface marker screening, this approach facilitates the rational selection of surface markers for prospective isolation of cell subpopulations with desired transcriptional profiles. We establish the usefulness of this platform in costly and highly morbid diabetic wounds by identifying a subpopulation of progenitor cells that is dysfunctional in the diabetic state, and normalizes diabetic wound healing rates following allogeneic application. We believe this work presents a logical framework for the development of targeted cell therapies that can be customized to any clinical application.

Unrecognized progenitor cell perturbations underlying a disease state may limit the efficacy of cell therapies. Here, the authors use high-throughput, single-cell transcriptional analysis to identify disease-specific cellular alterations and prospectively isolate restorative cell subpopulations.

Multiple Subsets of Brain Tumor Initiating Cells Coexist in Glioblastoma

Brain tumor-initiating cells (BTICs) are self-renewing multipotent cells critical for tumor maintenance and growth. Using single-cell microfluidic profiling, we identified multiple subpopulations of BTICs coexisting in human glioblastoma, characterized by distinct surface marker expression and single-cell molecular profiles relating to divergent bulk tissue molecular subtypes. These data suggest BTIC subpopulation heterogeneity as an underlying source of intra-tumoral bulk tissue molecular heterogeneity, and will support future studies into BTIC subpopulation-specific therapies. Stem Cells 2016;34:1702-1707.

High-Resolution Microfluidic Single-Cell Transcriptional Profiling Reveals Clinically Relevant Subtypes among Human Stem Cell Populations Commonly Utilized in Cell-Based Therapies

Stem cell therapies can promote neural repair and regeneration, yet controversy regarding optimal cell source and mechanism of action has slowed clinical translation, potentially due to undefined cellular heterogeneity. Single-cell resolution is needed to identify clinically relevant subpopulations with the highest therapeutic relevance. We combine single-cell microfluidic analysis with advanced computational modeling to study for the first time two common sources for cell-based therapies, human NSCs and MSCs. This methodology has the potential to logically inform cell source decisions for any clinical application.

Gene expression variability in clonal populations: Causes and consequences

During the last decade it has been shown that among cell variation in gene expression plays an important role within clonal populations. Here, we provide an overview of the different mechanisms contributing to gene expression variability in clonal populations. These are ranging from inherent variations in the biochemical process of gene expression itself, such as intrinsic noise, extrinsic noise and bistability to individual responses to variations in the local micro-environment, a phenomenon called phenotypic plasticity. Also genotypic variations caused by clonal evolution and phase variation can contribute to gene expression variability. Consequently, gene expression studies need to take these fluctuations in expression into account. However, frequently used techniques for expression quantification, such as microarrays, RNA sequencing, quantitative PCR and gene reporter fusions classically determine the population average of gene expression. Here, we discuss how these techniques can be adapted towards single cell analysis by integration with single cell isolation, RNA amplification and microscopy. Alternatively more qualitative selection-based techniques, such as mutant screenings, in vivo expression technology (IVET) and recombination-based IVET (RIVET) can be applied for detection of genes expressed only within a subpopulation. Finally, differential fluorescence induction (DFI), a protocol specially designed for single cell expression is discussed.

Evaluating the Effect of Cell Culture on Gene Expression in Primary Tissue Samples Using Microfluidic-Based Single Cell Transcriptional Analysis

Significant transcriptional heterogeneity is an inherent property of complex tissues such as tumors and healing wounds. Traditional methods of high-throughput analysis rely on pooling gene expression data from hundreds of thousands of cells and reporting a population-wide average that is unable to capture differences within distinct cell subsets. Recent advances in microfluidic technology have permitted the development of large-scale single cell analytic methods that overcome this limitation. The increased granularity afforded by such approaches allows us to answer the critical question of whether expansion in cell culture significantly alters the transcriptional characteristics of cells isolated from primary tissue. Here we examine an established population of human adipose-derived stem cells (ASCs) using a novel, microfluidic-based method for high-throughput transcriptional interrogation, coupled with advanced bioinformatic analysis, to evaluate the dynamics of single cell gene expression among primary, passage 0, and passage 1 stem cells. We find significant differences in the transcriptional profiles of cells from each group, as well as a considerable shift in subpopulation dynamics as those subgroups better able to adhere and proliferate under these culture conditions gradually emerge as dominant. Taken together, these findings reinforce the importance of using primary or very early passage cells in future studies.

Progenitor cell dysfunctions underlie some diabetic complications

Stem cells and progenitor cells are integral to tissue homeostasis and repair. They contribute to health through their ability to self-renew and commit to specialized effector cells. Recently, defects in a variety of progenitor cell populations have been described in both preclinical and human diabetes. These deficits affect multiple aspects of stem cell biology, including quiescence, renewal, and differentiation, as well as homing, cytokine production, and neovascularization, through mechanisms that are still unclear. More important, stem cell aberrations resulting from diabetes have direct implications on tissue function and seem to persist even after return to normoglycemia. Understanding how diabetes alters stem cell signaling and homeostasis is critical for understanding the complex pathophysiology of many diabetic complications. Moreover, the success of cell-based therapies will depend on a more comprehensive understanding of these deficiencies. This review has three goals: to analyze stem cell pathways dysregulated during diabetes, to highlight the effects of hyperglycemic memory on stem cells, and to define ways of using stem cell therapy to overcome diabetic complications.

Studies in fat grafting: Part IV. Adipose-derived stromal cell gene expression in cell-assisted lipotransfer

BACKGROUND

Fat graft volume retention remains highly unpredictable, but addition of adipose-derived stromal cells to fat grafts has been shown to improve retention. The present study aimed to investigate the mechanisms involved in adipose-derived stromal cell enhancement of fat grafting.

METHODS

Adipose-derived stromal cells isolated from human lipoaspirate were labeled with green fluorescent protein and luciferase. Fat grafts enhanced with adipose-derived stromal cells were injected into the scalp and bioluminescent imaging was performed to follow retention of adipose-derived stromal cells within the fat graft. Fat grafts were also explanted at days 1, 5, and 10 after grafting for adipose-derived stromal cell extraction and single-cell gene analysis. Finally, CD31 immunohistochemical staining was performed on fat grafts enriched with adipose-derived stromal cells.

RESULTS

Bioluminescent imaging demonstrated significant reduction in luciferase-positive adipose-derived stromal cells within fat grafts at 5 days after grafting. A similar reduction in viable green fluorescent protein-positive adipose-derived stromal cells retrieved from explanted grafts was also noted. Single-cell analysis revealed expression of multiple genes/markers related to cell survival and angiogenesis, including BMPR2, CD90, CD105, FGF2, CD248, TGFß1, and VEGFA. Genes involved in adipogenesis were not expressed by adipose-derived stromal cells. Finally, CD31 staining revealed significantly higher vascular density in fat grafts explanted at day 10 after grafting.

CONCLUSIONS

Although adipose-derived stromal cell survival in the hypoxic graft environment decreases significantly over time, these cells provide multiple angiogenic growth factors. Therefore, improved fat graft volume retention with adipose-derived stromal cell enrichment may be attributable to improved graft vascularization.

Mesenchymal Stem Cells and Cutaneous Wound Healing: Current Evidence and Future Potential

Human skin is a remarkable organ that sustains insult and injury throughout life. The ability of skin to expeditiously repair wounds is paramount to survival. With an aging global population, coupled with a rise in the prevalence of conditions such as diabetes, chronic wounds represent a significant biomedical burden. Mesenchymal stem cells (MSC), a progenitor cell population of the mesoderm lineage, have been shown to be significant mediators in inflammatory environments. Preclinical studies of MSC in various animal wound healing models point towards a putative therapy. This review examines the body of evidence suggesting that MSC accelerate wound healing in both clinical and preclinical studies and also the possible mechanisms controlling its efficacy. The delivery of a cellular therapy to the masses presents many challenges from a safety, ethical, and regulatory point of view. Some of the issues surrounding the introduction of MSC as a medicinal product are also delineated in this review.

Skin fibrosis. Identification and isolation of a dermal lineage with intrinsic fibrogenic potential

Dermal fibroblasts represent a heterogeneous population of cells with diverse features that remain largely undefined. We reveal the presence of at least two fibroblast lineages in murine dorsal skin. Lineage tracing and transplantation assays demonstrate that a single fibroblast lineage is responsible for the bulk of connective tissue deposition during embryonic development, cutaneous wound healing, radiation fibrosis, and cancer stroma formation. Lineage-specific cell ablation leads to diminished connective tissue deposition in wounds and reduces melanoma growth. Using flow cytometry, we identify CD26/DPP4 as a surface marker that allows isolation of this lineage. Small molecule-based inhibition of CD26/DPP4 enzymatic activity during wound healing results in diminished cutaneous scarring. Identification and isolation of these lineages hold promise for translational medicine aimed at in vivo modulation of fibrogenic behavior.

Regulatory network control of blood stem cells

Hematopoietic stem cells (HSCs) are characterized by their ability to execute a wide range of cell fate choices, including self-renewal, quiescence, and differentiation into the many different mature blood lineages. Cell fate decision making in HSCs, as indeed in other cell types, is driven by the interplay of external stimuli and intracellular regulatory programs. Given the pivotal nature of HSC decision making for both normal and aberrant hematopoiesis, substantial research efforts have been invested over the last few decades into deciphering some of the underlying mechanisms. Central to the intracellular decision making processes are transcription factor proteins and their interactions within gene regulatory networks. More than 50 transcription factors have been shown to affect the functionality of HSCs. However, much remains to be learned about the way in which individual factors are connected within wider regulatory networks, and how the topology of HSC regulatory networks might affect HSC function. Nevertheless, important progress has been made in recent years, and new emerging technologies suggest that the pace of progress is likely to accelerate. This review will introduce key concepts, provide an integrated view of selected recent studies, and conclude with an outlook on possible future directions for this field.

Rapid single-molecule imaging in cyclic olefin copolymer channels

Rapid preparation of high quality capture surfaces is a major challenge for surface-based single-molecule protein binding assays. Here we introduce a simple method to activate microfluidic chambers made from cyclic olefin copolymer for single-molecule imaging with total internal reflection fluorescence microscopy. We describe a surface coating protocol and demonstrate single-molecule imaging in off-the-shelf microfluidic parts that can be activated for binding assays within a few minutes. As the first example, biotinylated protein directly captured on the neutravidin-coated surface was detected using fluorescently labeled antibody. We then showed detection of a fusion construct containing green fluorescence protein and verified its single fluorophore behavior by observing stepwise photobleaching events. Finally, a target protein was identified in the crude cell lysate using antibody-sandwich complex formation. In all experiments, controls were completed to ensure that nonspecific binding to the surface was minimal. Based on our results, we conclude that the simple surface preparation described in this paper enables single-molecule imaging assays without time-consuming coating procedures.

Isolation and enrichment of human adipose-derived stromal cells for enhanced osteogenesis

Bone marrow-derived mesenchymal stromal cells (BM-MSCs) are considered the gold standard for stem cell-based tissue engineering applications. However, the process by which they must be harvested can be associated with significant donor site morbidity. In contrast, adipose-derived stromal cells (ASCs) are more readily abundant and more easily harvested, making them an appealing alternative to BM-MSCs. Like BM-MSCs, ASCs can differentiate into osteogenic lineage cells and can be used in tissue engineering applications, such as seeding onto scaffolds for use in craniofacial skeletal defects. ASCs are obtained from the stromal vascular fraction (SVF) of digested adipose tissue, which is a heterogeneous mixture of ASCs, vascular endothelial and mural cells, smooth muscle cells, pericytes, fibroblasts, and circulating cells. Flow cytometric analysis has shown that the surface marker profile for ASCs is similar to that for BM-MSCs. Despite several published reports establishing markers for the ASC phenotype, there is still a lack of consensus over profiles identifying osteoprogenitor cells in this heterogeneous population. This protocol describes how to isolate and use a subpopulation of ASCs with enhanced osteogenic capacity to repair critical-sized calvarial defects.

Aging disrupts cell subpopulation dynamics and diminishes the function of mesenchymal stem cells

Advanced age is associated with an increased risk of vascular morbidity, attributable in part to impairments in new blood vessel formation. Mesenchymal stem cells (MSCs) have previously been shown to play an important role in neovascularization and deficiencies in these cells have been described in aged patients. Here we utilize single cell transcriptional analysis to determine the effect of aging on MSC population dynamics. We identify an age-related depletion of a subpopulation of MSCs characterized by a pro-vascular transcriptional profile. Supporting this finding, we demonstrate that aged MSCs are also significantly compromised in their ability to support vascular network formation in vitro and in vivo. Finally, aged MSCs are unable to rescue age-associated impairments in cutaneous wound healing. Taken together, these data suggest that age-related changes in MSC population dynamics result in impaired therapeutic potential of aged progenitor cells. These findings have critical implications for therapeutic cell source decisions (autologous versus allogeneic) and indicate the necessity of strategies to improve functionality of aged MSCs.

Deterministic and stochastic approaches in the clinical application of mesenchymal stromal cells (MSCs)

Mesenchymal stromal cells (MSCs) have enormous intrinsic clinical value due to their multi-lineage differentiation capacity, support of hemopoiesis, immunoregulation and growth factors/cytokines secretion. MSCs have thus been the object of extensive research for decades. After completion of many pre-clinical and clinical trials, MSC-based therapy is now facing a challenging phase. Several clinical trials have reported moderate, non-durable benefits, which caused initial enthusiasm to wane, and indicated an urgent need to optimize the efficacy of therapeutic, platform-enhancing MSC-based treatment. Recent investigations suggest the presence of multiple in vivo MSC ancestors in a wide range of tissues, which contribute to the heterogeneity of the starting material for the expansion of MSCs. This variability in the MSC culture-initiating cell population, together with the different types of enrichment/isolation and cultivation protocols applied, are hampering progress in the definition of MSC-based therapies. International regulatory statements require a precise risk/benefit analysis, ensuring the safety and efficacy of treatments. GMP validation allows for quality certification, but the prediction of a clinical outcome after MSC-based therapy is correlated not only to the possible morbidity derived by cell production process, but also to the biology of the MSCs themselves, which is highly sensible to unpredictable fluctuation of isolating and culture conditions. Risk exposure and efficacy of MSC-based therapies should be evaluated by pre-clinical studies, but the batch-to-batch variability of the final medicinal product could significantly limit the predictability of these studies. The future success of MSC-based therapies could lie not only in rational optimization of therapeutic strategies, but also in a stochastic approach during the assessment of benefit and risk factors.

Diabetes irreversibly depletes bone marrow-derived mesenchymal progenitor cell subpopulations

Diabetic vascular pathology is largely attributable to impairments in tissue recovery from hypoxia. Circulating progenitor cells have been postulated to play a role in ischemic recovery, and deficiencies in these cells have been well described in diabetic patients. Here, we examine bone marrow-derived mesenchymal progenitor cells (BM-MPCs) that have previously been shown to be important for new blood vessel formation and demonstrate significant deficits in the context of diabetes. Further, we determine that this dysfunction is attributable to intrinsic defects in diabetic BM-MPCs that are not correctable by restoring glucose homeostasis. We identify two transcriptionally distinct subpopulations that are selectively depleted by both type 1 and type 2 diabetes, and these subpopulations have provasculogenic expression profiles, suggesting that they are vascular progenitor cells. These results suggest that the clinically observed deficits in progenitor cells may be attributable to selective and irreversible depletion of progenitor cell subsets in patients with diabetes.

Technology advancement for integrative stem cell analyses

Scientists have endeavored to use stem cells for a variety of applications ranging from basic science research to translational medicine. Population-based characterization of such stem cells, while providing an important foundation to further development, often disregard the heterogeneity inherent among individual constituents within a given population. The population-based analysis and characterization of stem cells and the problems associated with such a blanket approach only underscore the need for the development of new analytical technology. In this article, we review current stem cell analytical technologies, along with the advantages and disadvantages of each, followed by applications of these technologies in the field of stem cells. Furthermore, while recent advances in micro/nano technology have led to a growth in the stem cell analytical field, underlying architectural concepts allow only for a vertical analytical approach, in which different desirable parameters are obtained from multiple individual experiments and there are many technical challenges that limit vertically integrated analytical tools. Therefore, we propose--by introducing a concept of vertical and horizontal approach--that there is the need of adequate methods to the integration of information, such that multiple descriptive parameters from a stem cell can be obtained from a single experiment.

Positive selection for bone morphogenetic protein receptor type-IB promotes differentiation and specification of human adipose-derived stromal cells toward an osteogenic lineage

BACKGROUND

Adipose tissue represents an abundant and easily accessible source of multipotent cells that may serve as an excellent building block for tissue engineering. However, adipose-derived stromal cells (ASCs) are a heterogeneous group and subpopulations may be identified with enhanced osteogenic potential.

METHODS

Human ASC subpopulations were prospectively isolated based on expression of bone morphogenetic protein receptor type-IB (BMPR-IB). Unsorted, BMPR-IB(+), and BMPR-IB(-) cells were analyzed for their osteogenic capacity through histological staining and gene expression. To evaluate their in vivo osteogenic potential, critical-sized calvarial defects were created in immunocompromised mice and treated with unsorted, BMPR-IB(+), or BMPR-IB(-) cells. Healing was assessed using microcomputed tomography and pentachrome staining of specimens at 8 weeks.

RESULTS

Increased osteogenic differentiation was noted in the BMPR-IB(+) subpopulation, as demonstrated by alkaline phosphatase staining at day 7 and extracellular matrix mineralization with Alizarin red staining at day 14. This was also associated with increased expression for osteocalcin, a late marker of osteogenesis. Radiographic analysis demonstrated significantly enhanced healing of critical-sized calvarial defects treated with BMPR-IB(+) ASCs compared with unsorted or BMPR-IB(-) cells. This was confirmed through pentachrome staining, which revealed more robust bone regeneration in the BMPR-IB(+) group.

CONCLUSION

BMPR-IB(+) human ASCs have an enhanced ability to form bone both in vitro and in vivo. These data suggest that positive selection for BMPR-IB(+) and manipulation of the BMP pathway in these cells may yield a highly osteogenic subpopulation of cells for bone tissue engineering.

Diabetes impairs the angiogenic potential of adipose-derived stem cells by selectively depleting cellular subpopulations

Introduction

Pathophysiologic changes associated with diabetes impair new blood vessel formation and wound healing. Mesenchymal stem cells derived from adipose tissue (ASCs) have been used clinically to promote healing, although it remains unclear whether diabetes impairs their functional and therapeutic capacity.

Methods

In this study, we examined the impact of diabetes on the murine ASC niche as well as on the potential of isolated cells to promote neovascularization in vitro and in vivo. A novel single-cell analytical approach was used to interrogate ASC heterogeneity and subpopulation dynamics in this pathologic setting.

Results

Our results demonstrate that diabetes alters the ASC niche in situ and that diabetic ASCs are compromised in their ability to establish a vascular network both in vitro and in vivo. Moreover, these diabetic cells were ineffective in promoting soft tissue neovascularization and wound healing. Single-cell transcriptional analysis identified a subpopulation of cells which was diminished in both type 1 and type 2 models of diabetes. These cells were characterized by the high expression of genes known to be important for new blood vessel growth.

Conclusions

Perturbations in specific cellular subpopulations, visible only on a single-cell level, represent a previously unreported mechanism for the dysfunction of diabetic ASCs. These data suggest that the utility of autologous ASCs for cell-based therapies in patients with diabetes may be limited and that interventions to improve cell function before application are warranted.

Tracking the elusive fibrocyte: identification and characterization of collagen-producing hematopoietic lineage cells during murine wound healing

Fibrocytes are a unique population of circulating cells reported to exhibit characteristics of both hematopoietic and mesenchymal cells, and play an important role in wound healing. However, putative fibrocytes have been found to lose expression of hematopoietic surface markers such as CD45 during differentiation, making it difficult to track these cells in vivo with conventional methodologies. In this study, to distinguish hematopoietic and nonhematopoietic cells without surface markers, we took advantage of the gene vav 1, which is expressed solely on hematopoietic cells but not on other cell types, and established a novel transgenic mouse, in which hematopoietic cells are irreversibly labeled with green fluorescent protein and nonhematopoietic cells with red fluorescent protein. Use of single-cell transcriptional analysis in this mouse model revealed two discrete types of collagen I (Col I) expressing cells of hematopoietic lineage recruited into excisional skin wounds. We confirmed this finding on a protein level, with one subset of these Col I synthesizing cells being CD45+ and CD11b+, consistent with the traditional definition of a fibrocyte, while another was CD45- and Cd11b-, representing a previously unidentified population. Both cell types were found to initially peak, then reduce posthealing, consistent with a disappearance from the wound site and not a loss of identifying surface marker expression. Taken together, we have unambiguously identified two cells of hematopoietic origin that are recruited to the wound site and deposit collagen, definitively confirming the existence and natural time course of fibrocytes in cutaneous healing.

Tissue engineering and regenerative repair in wound healing

Wound healing is a highly evolved defense mechanism against infection and further injury. It is a complex process involving multiple cell types and biological pathways. Mammalian adult cutaneous wound healing is mediated by a fibroproliferative response leading to scar formation. In contrast, early to mid-gestational fetal cutaneous wound healing is more akin to regeneration and occurs without scar formation. This early observation has led to extensive research seeking to unlock the mechanism underlying fetal scarless regenerative repair. Building upon recent advances in biomaterials and stem cell applications, tissue engineering approaches are working towards a recapitulation of this phenomenon. In this review, we describe the elements that distinguish fetal scarless and adult scarring wound healing, and discuss current trends in tissue engineering aimed at achieving scarless tissue regeneration.

Single-cell analysis of embryoid body heterogeneity using microfluidic trapping array

The differentiation of pluripotent stem cells as embryoid bodies (EBs) remains a common method for inducing differentiation toward many lineages. However, differentiation via EBs typically yields a significant amount of heterogeneity in the cell population, as most cells differentiate simultaneously toward different lineages, while others remain undifferentiated. Moreover, physical parameters, such as the size of EBs, can modulate the heterogeneity of differentiated phenotypes due to the establishment of nutrient and oxygen gradients. One of the challenges in examining the cellular composition of EBs is the lack of analytical methods that are capable of determining the phenotype of all of the individual cells that comprise a single EB. Therefore, the objective of this work was to examine the ability of a microfluidic cell trapping array to analyze the heterogeneity of cells comprising EBs during the course of early differentiation. The heterogeneity of single cell phenotype on the basis of protein expression of the pluripotent transcription factor OCT-4 was examined for populations of EBs and single EBs of different sizes at distinct stages of differentiation. Results from the cell trap device were compared with flow cytometry and whole mount immunostaining. Additionally, single cells from dissociated pooled EBs or individual EBs were examined separately to discern potential differences in the value or variance of expression between the different methods of analysis. Overall, the analytical method described represents a novel approach for evaluating how heterogeneity is manifested in EB cultures and may be used in the future to assess the kinetics and patterns of differentiation in addition to the loss of pluripotency.

Transcriptional mechanisms of cell fate decisions revealed by single cell expression profiling

Transcriptional networks regulate cell fate decisions, which occur at the level of individual cells. However, much of what we know about their structure and function comes from studies averaging measurements over large populations of cells, many of which are functionally heterogeneous. Such studies conceal the variability between cells and so prevent us from determining the nature of heterogeneity at the molecular level. In recent years, many protocols and platforms have been developed that allow the high throughput analysis of gene expression in single cells, opening the door to a new era of biology. Here, we discuss the need for single cell gene expression analysis to gain deeper insights into the transcriptional control of cell fate decisions, and consider the insights it has provided so far into transcriptional regulatory networks in development.

Reduced BMPR2 expression induces GM-CSF translation and macrophage recruitment in humans and mice to exacerbate pulmonary hypertension

Idiopathic pulmonary arterial hypertension (PAH [IPAH]) is an insidious and potentially fatal disease linked to a mutation or reduced expression of bone morphogenetic protein receptor 2 (BMPR2). Because intravascular inflammatory cells are recruited in IPAH pathogenesis, we hypothesized that reduced BMPR2 enhances production of the potent chemokine granulocyte macrophage colony-stimulating factor (GM-CSF) in response to an inflammatory perturbation. When human pulmonary artery (PA) endothelial cells deficient in BMPR2 were stimulated with tumor necrosis factor (TNF), a twofold increase in GM-CSF was observed and related to enhanced messenger RNA (mRNA) translation. The mechanism was associated with disruption of stress granule formation. Specifically, loss of BMPR2 induced prolonged phospho-p38 mitogen-activated protein kinase (MAPK) in response to TNF, and this increased GADD34-PP1 phosphatase activity, dephosphorylating eukaryotic translation initiation factor (eIF2α), and derepressing GM-CSF mRNA translation. Lungs from IPAH patients versus unused donor controls revealed heightened PA expression of GM-CSF co-distributing with increased TNF and expanded populations of hematopoietic and endothelial GM-CSF receptor α (GM-CSFRα)-positive cells. Moreover, a 3-wk infusion of GM-CSF in mice increased hypoxia-induced PAH, in association with increased perivascular macrophages and muscularized distal arteries, whereas blockade of GM-CSF repressed these features. Thus, reduced BMPR2 can subvert a stress granule response, heighten GM-CSF mRNA translation, increase inflammatory cell recruitment, and exacerbate PAH.

Using microfluidics to investigate hematopoietic stem cell and microniche interactions at the single cell level

In recent years, microfluidic devices have become widely used in biology, and with the advantage of requiring low sample volumes, enables previously technologically infeasible experiments in hematopoietic stem cell (HSC) research. Here, we introduce a microfluidic device to investigate dynamic interactions between HSC and model niches in vitro. The device comprises a pneumatic valve which enables the culturing of different types of niche cells in different parts of the same device. Single HSCs can then be injected into the microfluidic device, manipulated, and placed onto different niches within the same device as controlled by the user. Here, we describe the device fabrication method, the HSC collection methodology, and the operational procedure for the device.

High-Throughput Single-Cell Analysis for Wound Healing Applications

SIGNIFICANCE

Wound repair is a complex biological process that integrates multiple physiologic pathways to restore skin homeostasis after a wide array of gross and anatomical insults. As such, a scientific examination of the wound typically requires broad sampling of numerous factors and is commonly achieved through DNA microarray analysis.

CRITICAL ISSUES

In the last several years, it has become increasingly evident that the granularity afforded by such traditional population-based assays may be insufficient to capture the complex relationships in heterogeneous processes such as those associated with wound healing and stem cell biology.

RECENT ADVANCES

Several emerging technologies have recently become available that permit high-throughput single-cell gene expression analysis in a manner which provides novel insights into the relationships of complex tissue. The most prominent among these employs microfluidic-based devices to achieve a high-resolution analysis of tissue samples.

FUTURE DIRECTIONS

The intrinsically heterogeneous nature of injured tissue, in conjunction with its temporal dynamics, makes wound repair and tissue regeneration an attractive target for high-throughput single-cell analysis. Given the staggering costs associated with chronic and non-healing wounds, the development of predictive and diagnostic tools using this technology would likely be attractive to healthcare providers.

Molecular analysis and differentiation capacity of adipose-derived stem cells from lymphedema tissue

BACKGROUND

Many breast cancer patients are plagued by the disabling complication of upper limb lymphedema after axillary surgery. Conservative treatments using massage and compression therapy do not offer a lasting relief, as they fail to address the chronic transformation of edema into excess adipose tissue. Liposuction to address the adipose nature of the lymphedema has provided an opportunity for a detailed analysis of the stromal fraction of lymphedema-associated fat to clarify the molecular mechanisms for this adipogenic transformation.

METHODS

Adipose-derived stem cells were harvested from human lipoaspirate of the upper extremity from age-matched patients with lymphedema (n = 3) or subcutaneous adipose tissue from control patients undergoing cosmetic procedures (n = 3). Immediately after harvest, adipose-derived stem cells were analyzed using single-cell transcriptional profiling techniques. Osteogenic, adipogenic, and vasculogenic gene expression and differentiation were assessed by quantitative real-time polymerase chain reaction and standard in vitro differentiation assays.

RESULTS

Differential transcriptional clusters of adipose-derived stem cells were found between lymphedema and subcutaneous fat. Interestingly, lymphedema-associated stem cells had a much higher adipogenic gene expression and enhanced ability to undergo adipogenic differentiation. Conversely, they had lower vasculogenic gene expression and diminished capability to form tubules in vitro, whereas the osteogenic differentiation capacity was not significantly altered.

CONCLUSIONS

Adipose-derived stem cells from extremities affected by lymphedema appear to exhibit transcriptional profiles similar to those of abdominal adipose-derived stem cells; however, their adipogenic differentiation potential is strongly increased and their vasculogenic capacity is compromised. These results suggest that the underlying pathophysiology of lymphedema drives adipose-derived stem cells toward adipogenic differentiation.

Transcriptional regulation of lineage commitment--a stochastic model of cell fate decisions

Molecular mechanisms employed by individual multipotent cells at the point of lineage commitment remain largely uncharacterized. Current paradigms span from instructive to noise-driven mechanisms. Of considerable interest is also whether commitment involves a limited set of genes or the entire transcriptional program, and to what extent gene expression configures multiple trajectories into commitment. Importantly, the transient nature of the commitment transition confounds the experimental capture of committing cells. We develop a computational framework that simulates stochastic commitment events, and affords mechanistic exploration of the fate transition. We use a combined modeling approach guided by gene expression classifier methods that infers a time-series of stochastic commitment events from experimental growth characteristics and gene expression profiling of individual hematopoietic cells captured immediately before and after commitment. We define putative regulators of commitment and probabilistic rules of transition through machine learning methods, and employ clustering and correlation analyses to interrogate gene regulatory interactions in multipotent cells. Against this background, we develop a Monte Carlo time-series stochastic model of transcription where the parameters governing promoter status, mRNA production and mRNA decay in multipotent cells are fitted to experimental static gene expression distributions. Monte Carlo time is converted to physical time using cell culture kinetic data. Probability of commitment in time is a function of gene expression as defined by a logistic regression model obtained from experimental single-cell expression data. Our approach should be applicable to similar differentiating systems where single cell data is available. Within our system, we identify robust model solutions for the multipotent population within physiologically reasonable values and explore model predictions with regard to molecular scenarios of entry into commitment. The model suggests distinct dependencies of different commitment-associated genes on mRNA dynamics and promoter activity, which globally influence the probability of lineage commitment.

Deciphering the single-cell omic: innovative application for translational medicine

Traditional technologies to investigate system biology are limited by the detection of parameters resulting from the averages of large populations of cells, missing cells produced in small numbers, and attempting to uniform the heterogeneity. The advent of proteomics and genomics at a single-cell level has set the basis for an outstanding improvement in analytical technology and data acquisition. It has been well demonstrated that cellular heterogeneity is closely related to numerous stochastic transcriptional events leading to variations in patterns of expression among single genetically identical cells. The new-generation technology of single-cell analysis is able to better characterize a cell's population, identifying and differentiating outlier cells, in order to provide both a single-cell experiment and a corresponding bulk measurement, through the identification, quantification and characterization of all system biology aspects (genomics, transcriptomics, proteomics, metabolomics, degradomics and fluxomics). The movement of omics into single-cell analysis represents a significant and outstanding shift.

Stem cell recruitment after injury: lessons for regenerative medicine

Tissue repair and regeneration are thought to involve resident cell proliferation as well as the selective recruitment of circulating stem and progenitor cell populations through complex signaling cascades. Many of these recruited cells originate from the bone marrow, and specific subpopulations of bone marrow cells have been isolated and used to augment adult tissue regeneration in preclinical models. Clinical studies of cell-based therapies have reported mixed results, however, and a variety of approaches to enhance the regenerative capacity of stem cell therapies are being developed based on emerging insights into the mechanisms of progenitor cell biology and recruitment following injury. This article discusses the function and mechanisms of recruitment of important bone marrow-derived stem and progenitor cell populations following injury, as well as the emerging therapeutic applications targeting these cells.

Characterization of transcriptional networks in blood stem and progenitor cells using high-throughput single-cell gene expression analysis

Cellular decision-making is mediated by a complex interplay of external stimuli with the intracellular environment, in particular transcription factor regulatory networks. Here we have determined the expression of a network of 18 key haematopoietic transcription factors in 597 single primary blood stem and progenitor cells isolated from mouse bone marrow. We demonstrate that different stem/progenitor populations are characterized by distinctive transcription factor expression states, and through comprehensive bioinformatic analysis reveal positively and negatively correlated transcription factor pairings, including previously unrecognized relationships between Gata2, Gfi1 and Gfi1b. Validation using transcriptional and transgenic assays confirmed direct regulatory interactions consistent with a regulatory triad in immature blood stem cells, where Gata2 may function to modulate cross-inhibition between Gfi1 and Gfi1b. Single-cell expression profiling therefore identifies network states and allows reconstruction of network hierarchies involved in controlling stem cell fate choices, and provides a blueprint for studying both normal development and human disease.

Analysis of multiple physical parameters for mechanical phenotyping of living cells

Since the cytoskeleton is known to regulate many cell functions, an increasing amount of effort to characterize cells by their mechanical properties has occured. Despite the structural complexity and dynamics of the multicomponent cytoskeleton, mechanical measurements on single cells are often fit to simple models with two to three parameters, and those parameters are recorded and reported. However, different simple models are likely needed to capture the distinct mechanical cell states, and additional parameters may be needed to capture the ability of cells to actively deform. Our new approach is to capture a much larger set of possibly redundant parameters from cells' mechanical measurement using multiple rheological models as well as dynamic deformation and image data. Principal component analysis and network-based approaches are used to group parameters to reduce redundancies and develop robust biomechanical phenotyping. Network representation of parameters allows for visual exploration of cells' complex mechanical system, and highlights unexpected connections between parameters. To demonstrate that our biomechanical phenotyping approach can detect subtle mechanical differences, we used a Microfluidic Optical Cell Stretcher to mechanically stretch circulating human breast tumor cells bearing genetically-engineered alterations in c-src tyrosine kinase activation, which is known to influence reattachment and invasion during metastasis.

Genome-scale technology driven advances to research into normal and malignant haematopoiesis

Haematopoiesis or blood development has long served as a model system for adult stem cell biology. Moreover, when combined, the various cancers of the blood represent one of the commonest human malignancies. Large numbers of researchers have therefore dedicated their scientific careers to studying haematopoiesis for more than a century. Throughout this period, many new technologies have first been applied towards the study of blood cells, and the research fields of normal and malignant haematopoiesis have also been some of the earliest adopters of genome-scale technologies. This has resulted in significant new insights with implications ranging from basic biological mechanisms to patient diagnosis and prognosis and also produced lessons likely to be relevant for many other areas of biomedical research. This paper discusses the current state of play for a range of genome-scale applications within haemopoiesis research, including gene expression profiling, ChIP-sequencing, genomewide association analysis, and cancer genome sequencing. A concluding outlook section explores likely future areas of progress as well as potential technological and educational bottlenecks.

Data exploration, quality control and testing in single-cell qPCR-based gene expression experiments

Motivation: Cell populations are never truly homogeneous; individual cells exist in biochemical states that define functional differences between them. New technology based on microfluidic arrays combined with multiplexed quantitative polymerase chain reactions now enables high-throughput single-cell gene expression measurement, allowing assessment of cellular heterogeneity. However, few analytic tools have been developed specifically for the statistical and analytical challenges of single-cell quantitative polymerase chain reactions data.

Results: We present a statistical framework for the exploration, quality control and analysis of single-cell gene expression data from microfluidic arrays. We assess accuracy and within-sample heterogeneity of single-cell expression and develop quality control criteria to filter unreliable cell measurements. We propose a statistical model accounting for the fact that genes at the single-cell level can be on (and a continuous expression measure is recorded) or dichotomously off (and the recorded expression is zero). Based on this model, we derive a combined likelihood ratio test for differential expression that incorporates both the discrete and continuous components. Using an experiment that examines treatment-specific changes in expression, we show that this combined test is more powerful than either the continuous or dichotomous component in isolation, or a t-test on the zero-inflated data. Although developed for measurements from a specific platform (Fluidigm), these tools are generalizable to other multi-parametric measures over large numbers of events.

Availability: All results presented here were obtained using the SingleCellAssay R package available on GitHub (http://github.com/RGLab/SingleCellAssay).

Contact:rgottard@fhcrc.org

Supplementary information:Supplementary data are available at Bioinformatics online.

Enabling stem cell therapies for tissue repair: current and future challenges

Stem cells embody the tremendous potential of the human body to develop, grow, and repair throughout life. Understanding the biologic mechanisms that underlie stem cell-mediated tissue regeneration is key to harnessing this potential. Recent advances in molecular biology, genetic engineering, and material science have broadened our understanding of stem cells and helped bring them closer to widespread clinical application. Specifically, innovative approaches to optimize how stem cells are identified, isolated, grown, and utilized will help translate these advances into effective clinical therapies. Although there is growing interest in stem cells worldwide, this enthusiasm must be tempered by the fact that these treatments remain for the most part clinically unproven. Future challenges include refining the therapeutic manipulation of stem cells, validating these technologies in randomized clinical trials, and regulating the global expansion of regenerative stem cell therapies.

Hematopoietic stem cell heterogeneity takes center stage

Over the past 10 years, increasing evidence has accumulated that heterogeneity is a feature of hematopoietic stem cell (HSC) proliferation, self-renewal, and differentiation based on examination of these properties at a clonal level. The heterogeneous behavior of HSCs reflects the operation of a complex interplay of intrinsic and extrinsic variables. In this review, we discuss key findings from the last 5 years that reveal new insights into the mechanisms involved.