The Dynamic Duo: Niche/Stem Cell Interdependency

Epigenetic Inheritance From Normal Origin Cells Can Determine the Aggressive Biology of Tumor-Initiating Cells and Tumor Heterogeneity

The acquisition of genetic- and epigenetic-abnormalities during transformation has been recognized as the two fundamental factors that lead to tumorigenesis and determine the aggressive biology of tumor cells. However, there is a regularity that tumors derived from less-differentiated normal origin cells (NOCs) usually have a higher risk of vascular involvement, lymphatic and distant metastasis, which can be observed in both lymphohematopoietic malignancies and somatic cancers. Obviously, the hypothesis of genetic- and epigenetic-abnormalities is not sufficient to explain how the linear relationship between the cellular origin and the biological behavior of tumors is formed, because the cell origin of tumor is an independent factor related to tumor biology. In a given system, tumors can originate from multiple cell types, and tumor-initiating cells (TICs) can be mapped to different differentiation hierarchies of normal stem cells, suggesting that the heterogeneity of the origin of TICs is not completely chaotic. TIC’s epigenome includes not only genetic- and epigenetic-abnormalities, but also established epigenetic status of genes inherited from NOCs. In reviewing previous studies, we found much evidence supporting that the status of many tumor-related “epigenetic abnormalities” in TICs is consistent with that of the corresponding NOC of the same differentiation hierarchy, suggesting that they may not be true epigenetic abnormalities. So, we speculate that the established statuses of genes that control NOC’s migration, adhesion and colonization capabilities, cell-cycle quiescence, expression of drug transporters, induction of mesenchymal formation, overexpression of telomerase, and preference for glycolysis can be inherited to TICs through epigenetic memory and be manifested as their aggressive biology. TICs of different origins can maintain different degrees of innate stemness from NOC, which may explain why malignancies with stem cell phenotypes are usually more aggressive.

Lgr6 marks epidermal stem cells with a nerve-dependent role in wound re-epithelialization

Stem cells support lifelong maintenance of adult organs, but their specific roles during injury are poorly understood. Here we demonstrate that Lgr6 marks a regionally restricted population of epidermal stem cells that interact with nerves and specialize in wound re-epithelialization. Diphtheria toxin-mediated ablation of Lgr6 stem cells delays wound healing, and skin denervation phenocopies this effect. Using intravital imaging to capture stem cell dynamics after injury, we show that wound re-epithelialization by Lgr6 stem cells is diminished following loss of nerves. This induces recruitment of other stem cell populations, including hair follicle stem cells, which partially compensate to mediate wound closure. Single-cell lineage tracing and gene expression analysis reveal that the fate of Lgr6 stem cells is shifted toward differentiation following loss of their niche. We conclude that Lgr6 epidermal stem cells are primed for injury response and interact with nerves to regulate their fate.

The anti-apoptotic Bcl-2 protein regulates hair follicle stem cell function

Maintaining the architecture, size and composition of an intact stem cell (SC) compartment is crucial for tissue homeostasis and regeneration throughout life. In mammalian skin, elevated expression of the anti‐apoptotic Bcl‐2 protein has been reported in hair follicle (HF) bulge SCs (BSCs), but its impact on SC function is unknown. Here, we show that systemic exposure of mice to the Bcl‐2 antagonist ABT‐199/venetoclax leads to the selective loss of suprabasal BSCs (sbBSCs), thereby disrupting cyclic HF regeneration. RNAseq analysis shows that the pro‐apoptotic BH3‐only proteins BIM and Bmf are upregulated in sbBSCs, explaining their addiction to Bcl‐2 and the marked susceptibility to Bcl‐2 antagonism. In line with these observations, conditional knockout of Bcl‐2 in mouse epidermis elevates apoptosis in BSCs. In contrast, ectopic Bcl‐2 expression blocks apoptosis during HF regression, resulting in the accumulation of quiescent SCs and delaying HF growth in mice. Strikingly, Bcl‐2‐induced changes in size and composition of the HF bulge accelerate tumour formation. Our study identifies a niche‐instructive mechanism of Bcl‐2‐regulated apoptosis response that is required for SC homeostasis and tissue regeneration, and may suppress carcinogenesis.

A Scarless Healing Tale: Comparing Homeostasis and Wound Healing of Oral Mucosa With Skin and Oesophagus

Epithelial tissues are the most rapidly dividing tissues in the body, holding a natural ability for renewal and regeneration. This ability is crucial for survival as epithelia are essential to provide the ultimate barrier against the external environment, protecting the underlying tissues. Tissue stem and progenitor cells are responsible for self-renewal and repair during homeostasis and following injury. Upon wounding, epithelial tissues undergo different phases of haemostasis, inflammation, proliferation and remodelling, often resulting in fibrosis and scarring. In this review, we explore the phenotypic differences between the skin, the oesophagus and the oral mucosa. We discuss the plasticity of these epithelial stem cells and contribution of different fibroblast subpopulations for tissue regeneration and wound healing. While these epithelial tissues share global mechanisms of stem cell behaviour for tissue renewal and regeneration, the oral mucosa is known for its outstanding healing potential with minimal scarring. We aim to provide an updated review of recent studies that combined cell therapy with bioengineering exporting the unique scarless properties of the oral mucosa to improve skin and oesophageal wound healing and to reduce fibrotic tissue formation. These advances open new avenues toward the ultimate goal of achieving scarless wound healing.

WTAP Function in Sertoli Cells Is Essential for Sustaining the Spermatogonial Stem Cell Niche

Sertoli cells are the major component of the spermatogonial stem cell (SSC) niche; however, regulatory mechanisms in Sertoli cells that dictate SSC fate decisions remain largely unknown. Here we revealed features of the N⁶-methyladenosine (m⁶A) mRNA modification in Sertoli cells and demonstrated the functions of WTAP, the key subunit of the m⁶A methyltransferase complex in spermatogenesis. m⁶A-sequencing analysis identified 21,909 m⁶A sites from 15,365 putative m⁶A-enriched transcripts within 6,122 genes, including many Sertoli cell-specific genes. Conditional deletion of Wtap in Sertoli cells resulted in sterility and the progressive loss of the SSC population. RNA sequencing and ribosome nascent-chain complex-bound mRNA sequencing analyses suggested that alternative splicing events of transcripts encoding SSC niche factors were sharply altered and translation of these transcripts were severely dysregulated by Wtap deletion. Collectively, this study uncovers a novel regulatory mechanism of the SSC niche and provide insights into molecular interactions between stem cells and their cognate niches in mammals.

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WTAP is highly expressed in Sertoli cell and is essential in spermatogenesis

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Wtap knockout in Sertoli cell causes defective spermatogonial stem cell maintenance

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WTAP regulates transcription and translation of m⁶A-enriched genes in Sertoli cell

Functional anatomy of the hair follicle: The Secondary Hair Germ

The secondary hair germ (SHG)-a transitory structure in the lower portion of the mouse telogen hair follicle (HF)-is directly involved in anagen induction and eventual HF regrowth. Some crucial aspects of SHG functioning and ontogenetic relations with other HF parts, however, remain undefined. According to recent evidence (in contrast to previous bulge-centric views), the SHG is the primary target of anagen-inducing signalling and a source of both the outer root sheath (ORS) and ascending HF layers during the initial (morphogenetic) anagen subphase. The SHG is comprised of two functionally distinct cell populations. Its lower portion (originating from lower HF cells that survived catagen) forms all ascending HF layers, while the upper SHG (formed by bulge-derived cells) builds up the ORS. The predetermination of SHG cells to a specific morphogenetic fate contradicts their attribution to the "stem cell" category and supports SHG designation as a "germinative" or a "founder" cell population. The mechanisms of this predetermination driving transition of the SHG from "refractory" to the "competent" state during the telogen remain unknown. Functionally, the SHG serves as a barrier, protecting the quiescent bulge stem cell niche from the extensive follicular papilla/SHG signalling milieu. The formation of the SHG is a prerequisite for efficient "precommitment" of these cells and provides for easier sensing and a faster response to anagen-inducing signals. In general, the formation of the SHG is an evolutionary adaptation, which allowed the ancestors of modern Muridae to acquire a specific, highly synchronized pattern of hair cycling.

Reduced SMAD2/3 activation independently predicts increased depth of human cutaneous squamous cell carcinoma

The incidence of cutaneous squamous cell carcinoma (cSCC) is rising. Whilst the majority are cured surgically, aggressive metastatic cSCC carry a poor prognosis. Inactivating mutations in transforming growth factor beta (TGF-β) receptors have been identified amongst genetic drivers of sporadic tumours and murine models of cSCC, suggesting a tumour suppressor function for TGF-β in normal skin. However, paradoxically, TGF-β acts as a tumour promoter in some murine model systems. Few studies have analysed the role of TGF-β/activin signalling in human normal skin, hyper-proliferative skin disorders and cSCC. Antibodies recognising phospho-SMAD proteins which are activated during canonical TGF-β/activin signalling were validated for use in immunohistochemistry. A tissue microarray comprising FFPE lesional and perilesional tissue from human primary invasive cSCC (n=238), cSCC in-situ (n=2) and keratocanthoma (n=9) were analysed in comparison with tissues from normal human scalp (n=10). Phosphorylated SMAD2 and SMAD3 were detected in normal interfollicular epidermal keratinocytes and were also highly localised to inner root sheath, matrix cells and Keratin 15 positive cells. Lesional cSCC tissue had significantly reduced activated SMAD2/3 compared to perilesional tissue, consistent with a tumour suppressor role for SMAD2/3 activators in cSCC. Increased cSCC tumour thickness inversely correlated with the presence of phospho-SMADs in tumour tissue suggesting that a reduction in canonical TGF-β/activin signalling may be associated with disease progression.

Current Understanding of the Pathways Involved in Adult Stem and Progenitor Cell Migration for Tissue Homeostasis and Repair

With the advancements in the field of adult stem and progenitor cells grows the recognition that the motility of primitive cells is a pivotal aspect of their functionality. There is accumulating evidence that the recruitment of tissue-resident and circulating cells is critical for organ homeostasis and effective injury responses, whereas the pathobiology of degenerative diseases, neoplasm and aging, might be rooted in the altered ability of immature cells to migrate. Furthermore, understanding the biological machinery determining the translocation patterns of tissue progenitors is of great relevance for the emerging methodologies for cell-based therapies and regenerative medicine. The present article provides an overview of studies addressing the physiological significance and diverse modes of stem and progenitor cell trafficking in adult mammalian organs, discusses the major microenvironmental cues regulating cell migration, and describes the implementation of live imaging approaches for the exploration of stem cell movement in tissues and the factors dictating the motility of endogenous and transplanted cells with regenerative potential.

Identification of regulators of germ stem cell enwrapment by its niche in C. elegans

Many stem cell niches contain support cells that increase contact with stem cells by enwrapping them in cellular processes. One example is the germ stem cell niche in C. elegans, which is composed of a single niche cell termed the distal tip cell (DTC) that extends cellular processes, constructing an elaborate plexus that enwraps germ stem cells. To identify genes required for plexus formation and to explore the function of this specialized enwrapping behavior, a series of targeted and tissue-specific RNAi screens were performed. Here we identify genes that promote stem cell enwrapment by the DTC plexus, including a set that specifically functions within the DTC, such as the chromatin modifier lin-40/MTA1, and others that act within the germline, such as the 14-3-3 signaling protein par-5. Analysis of genes that function within the germline to mediate plexus development reveal that they are required for expansion of the germ progenitor zone, supporting the emerging idea that germ stem cells signal to the niche to stimulate enwrapping behavior. Examination of wild-type animals with asymmetric plexus formation and animals with reduced DTC plexus elaboration via loss of two candidates including lin-40 indicate that cellular enwrapment promotes GLP-1/Notch signaling and germ stem cell fate. Together, our work identifies novel regulators of cellular enwrapment and suggests that reciprocal signaling between the DTC niche and the germ stem cells promotes enwrapment behavior and stem cell fate.

Intercellular crosstalk in human malignant melanoma

Incidence of malignant melanoma is increasing globally. While the initial stages of tumors can be easily treated by a simple surgery, the therapy of advanced stages is rather limited. Melanoma cells spread rapidly through the body of a patient to form multiple metastases. Consequently, the survival rate is poor. Therefore, emphasis in melanoma research is given on early diagnosis and development of novel and more potent therapeutic options. The malignant melanoma is arising from melanocytes, cells protecting mitotically active keratinocytes against damage caused by UV light irradiation. The melanocytes originate in the neural crest and consequently migrate to the epidermis. The relationship between the melanoma cells, the melanocytes, and neural crest stem cells manifests when the melanoma cells are implanted to an early embryo: they use similar migratory routes as the normal neural crest cells. Moreover, malignant potential of these melanoma cells is overdriven in this experimental model, probably due to microenvironmental reprogramming. This observation demonstrates the crucial role of the microenvironment in melanoma biology. Indeed, malignant tumors in general represent complex ecosystems, where multiple cell types influence the growth of genetically mutated cancer cells. This concept is directly applicable to the malignant melanoma. Our review article focuses on possible strategies to modify the intercellular crosstalk in melanoma that can be employed for therapeutic purposes.

Engineered stem cell niche matrices for rotator cuff tendon regenerative engineering

Rotator cuff (RC) tears represent a large proportion of musculoskeletal injuries attended to at the clinic and thereby make RC repair surgeries one of the most widely performed musculoskeletal procedures. Despite the high incidence rate of RC tears, operative treatments have provided minimal functional gains and suffer from high re-tear rates. The hypocellular nature of tendon tissue poses a limited capacity for regeneration. In recent years, great strides have been made in the area of tendonogenesis and differentiation towards tendon cells due to a greater understanding of the tendon stem cell niche, development of advanced materials, improved scaffold fabrication techniques, and delineation of the phenotype development process. Though in vitro models for tendonogenesis have shown promising results, in vivo models have been less successful. The present work investigates structured matrices mimicking the tendon microenvironment as cell delivery vehicles in a rat RC tear model. RC injuries augmented with a matrix delivering rat mesenchymal stem cells (rMSCs) showed enhanced regeneration over suture repair alone or repair with augmentation, at 6 and 12-weeks post-surgery. The local delivery of rMSCs led to increased mechanical properties and improved tissue morphology. We hypothesize that the mesenchymal stem cells function to modulate the local immune and bioactivity environment through autocrine/paracrine and/or cell homing mechanisms. This study provides evidence for improved tendon healing with biomimetic matrices and delivered MSCs with the potential for translation to larger, clinical animal models. The enhanced regenerative healing response with stem cell delivering biomimetic matrices may represent a new treatment paradigm for massive RC tendon tears.

Tissue-scale coordination of cellular behaviour promotes epidermal wound repair in live mice

Tissue repair is fundamental to our survival as tissues are challenged by recurrent damage. During mammalian skin repair, cells respond by migrating and proliferating to close the wound. However, the coordination of cellular repair behaviours and their effects on homeostatic functions in a live mammal remains unclear. Here we capture the spatiotemporal dynamics of individual epithelial behaviours by imaging wound re-epithelialization in live mice. Differentiated cells migrate while the rate of differentiation changes depending on local rate of migration and tissue architecture. Cells depart from a highly proliferative zone by directionally dividing towards the wound while collectively migrating. This regional coexistence of proliferation and migration leads to local expansion and elongation of the repairing epithelium. Finally, proliferation functions to pattern and restrict the recruitment of undamaged cells. This study elucidates the interplay of cellular repair behaviours and consequent changes in homeostatic behaviours that support tissue-scale organization of wound re-epithelialization.

Organoids: A historical perspective of thinking in three dimensions

In this perspective, Simian and Bissell discuss the evolution of the 3D culture and organoid research field up to now as well as its future directions.

In the last ten years, there has been a dramatic surge in the number of publications where single or groups of cells are grown in substrata that have elements of basement membrane leading to the formation of tissue-like structures referred to as organoids. However, this field of research began many decades ago, when the pioneers of cell culture began to ask questions we still ask today: How does organogenesis occur? How do signals integrate to make such vastly different tissues and organs given that the sequence of the genome in our trillions of cells is identical? Here, we summarize how work over the past century generated the conceptual framework that has allowed us to make progress in the understanding of tissue-specific morphogenetic programs. The development of cell culture systems that provide accurate and physiologically relevant models are proving to be key in establishing appropriate platforms for the development of new therapeutic strategies.

Molecular mechanisms of asymmetric divisions in mammary stem cells

Stem cells have the remarkable ability to undergo proliferative symmetric divisions and self-renewing asymmetric divisions. Balancing of the two modes of division sustains tissue morphogenesis and homeostasis. Asymmetric divisions of Drosophila neuroblasts (NBs) and sensory organ precursor (SOP) cells served as prototypes to learn what we consider now principles of asymmetric mitoses. They also provide initial evidence supporting the notion that aberrant symmetric divisions of stem cells could correlate with malignancy. However, transferring the molecular knowledge of circuits underlying asymmetry from flies to mammals has proven more challenging than expected. Several experimental approaches have been used to define asymmetry in mammalian systems, based on daughter cell fate, unequal partitioning of determinants and niche contacts, or proliferative potential. In this review, we aim to provide a critical evaluation of the assays used to establish the stem cell mode of division, with a particular focus on the mammary gland system. In this context, we will discuss the genetic alterations that impinge on the modality of stem cell division and their role in breast cancer development.

The thrill of scientific discovery and leadership with my group

My group and I feel tremendously honored to be recognized with the 2016 Early Career Life Scientist Award from the American Society for Cell Biology. In this essay I share the scientific questions that my lab has been excitedly pursuing since starting in August 2009 and the leadership behaviors we have adopted that enable our collective scientific productivity.

Primary scalp alopecia: new histopathological tools, new concepts and a practical guide to diagnosis

The diagnosis of primary scalp alopecia remains one of the most challenging fields in dermatopathology. In this review, we would like to connect the established classification of primary alopecia into scarring (cicatricial) and non-scarring (non-cicatricial) with current concepts. We introduce a simplified pathway for the diagnosis of the most common causes of alopecia, including a discussion of tissue processing techniques and use of immunohistochemistry.

sPLA2 -IIA Overexpression in Mice Epidermis Depletes Hair Follicle Stem Cells and Induces Differentiation Mediated Through Enhanced JNK/c-Jun Activation

Secretory phospholipase A2 Group-IIA (sPLA2 -IIA) catalyzes the hydrolysis of the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. sPLA2 -IIA is deregulated in various cancers; however, its role in hair follicle stem cell (HFSC) regulation is obscure. Here we report a transgenic mice overexpressing sPLA2 -IIA (K14-sPLA2 -IIA) showed depletion of HFSC pool. This was accompanied with increased differentiation, loss of ortho-parakeratotic organization and enlargement of sebaceous gland, infundibulum and junctional zone. The colony forming efficiency of keratinocytes was significantly reduced. Microarray profiling of HFSCs revealed enhanced level of epithelial mitogens and transcription factors, c-Jun and FosB that may be involved in proliferation and differentiation. Moreover, K14-sPLA2 -IIA keratinocytes showed enhanced activation of EGFR and JNK1/2 that led to c-Jun activation, which co-related with enhanced differentiation. Further, depletion of stem cells in bulge is associated with high levels of chromatin silencing mark, H3K27me3 and low levels of an activator mark, H3K9ac suggestive of alteration in gene expression contributing toward stem cells differentiation. Our results, first time uncovered that overexpression of sPLA2 -IIA lead to depletion of HFSCs and differentiation associated with altered histone modification. Thus involvement of sPLA2 -IIA in stem cells regulation and disease pathogenesis suggest its prospective clinical implications. Stem Cells 2016;34:2407-2417.

Organoids from adult liver and pancreas: Stem cell biology and biomedical utility

The liver and pancreas are critical organs maintaining whole body metabolism. Historically, the expansion of adult-derived cells from these organs in vitro has proven challenging and this in turn has hampered studies of liver and pancreas stem cell biology, as well as being a roadblock to disease modelling and cell replacement therapies for pathologies in these organs. Recently, defined culture conditions have been described which allow the in vitro culture and manipulation of adult-derived liver and pancreatic material. Here we review these systems and assess their physiological relevance, as well as their potential utility in biomedicine.

Comparative biology of tissue repair, regeneration and aging

The Symposium on the Comparative Biology of Tissue Repair, Regeneration and Aging, held 26 June to 28 June 2015 at the MDI Biological Laboratory in Salisbury Cove, Maine, brought together a diverse group of biologists with a common interest in understanding why regenerative capacity varies among animal species, why it is progressively lost in senescence, and how answers obtained from studies that address those questions might be applied in regenerative medicine.

Bioregulatory systems medicine: an innovative approach to integrating the science of molecular networks, inflammation, and systems biology with the patient's autoregulatory capacity?

Bioregulatory systems medicine (BrSM) is a paradigm that aims to advance current medical practices. The basic scientific and clinical tenets of this approach embrace an interconnected picture of human health, supported largely by recent advances in systems biology and genomics, and focus on the implications of multi-scale interconnectivity for improving therapeutic approaches to disease. This article introduces the formal incorporation of these scientific and clinical elements into a cohesive theoretical model of the BrSM approach. The authors review this integrated body of knowledge and discuss how the emergent conceptual model offers the medical field a new avenue for extending the armamentarium of current treatment and healthcare, with the ultimate goal of improving population health.