Stem cell heterogeneity and plasticity in epithelia

Recent advances in the role of neuroregulation in skin wound healing

Neuroregulation during skin wound healing involves complex interactions between the nervous system and intricate tissue repair processes. The skin, the largest organ, depends on a complex system of nerves to manage responses to injury. Recent research has emphasized the crucial role of neuroregulation in maximizing wound healing outcomes. Recently, researchers have also explained the interactive contact between the peripheral nervous system and skin cells during the different phases of wound healing. Neurotransmitters and neuropeptides, once observed as simple signalling molecules, have since been recognized as effective regulators of inflammation, angiogenesis, and cell proliferation. The significance of skin innervation and neuromodulators is underscored by the delayed wound healing observed in patients with diabetes and the regenerative capabilities of foetal skin. Foetal skin regeneration is influenced by the neuroregulatory environment, immature immune system, abundant growth factors, and increased pluripotency of cells. Foetal skin cells exhibit greater flexibility and specialized cell types, and the extracellular matrix composition promotes regeneration. The extracellular matrix composition of foetal skin promotes regeneration, making it more capable than adult skin because neuroregulatory signals affect skin regeneration. The understanding of these systems can facilitate the development of therapeutic strategies to alter the nerve supply to the skin to enhance the process of wound healing. Neuroregulation is being explored as a potential therapeutic strategy for enhancing skin wound repair. Bioelectronic strategies and neuromodulation techniques can manipulate neural signalling, optimize the neuroimmune axis, and modulate inflammation. This review describes the function of skin innervation in wound healing, emphasizing the importance of neuropeptides released by sensory and autonomic nerve fibres. This article discusses significant discoveries related to neuroregulation and its impact on skin wound healing.

Bridging tissue repair and epithelial carcinogenesis: epigenetic memory and field cancerization

The epigenome coordinates spatial-temporal specific gene expression during development and in adulthood, for the maintenance of homeostasis and upon tissue repair. The upheaval of the epigenetic landscape is a key event in the onset of many pathologies including tumours, where epigenetic changes cooperate with genetic aberrations to establish the neoplastic phenotype and to drive cell plasticity during its evolution. DNA methylation, histone modifiers and readers or other chromatin components are indeed often altered in cancers, such as carcinomas that develop in epithelia. Lining the surfaces and the cavities of our body and acting as a barrier from the environment, epithelia are frequently subjected to acute or chronic tissue damages, such as mechanical injuries or inflammatory episodes. These events can activate plasticity mechanisms, with a deep impact on cells' epigenome. Despite being very effective, tissue repair mechanisms are closely associated with tumour onset. Here we review the similarities between tissue repair and carcinogenesis, with a special focus on the epigenetic mechanisms activated by cells during repair and opted by carcinoma cells in multiple epithelia. Moreover, we discuss the recent findings on inflammatory and wound memory in epithelia and describe the epigenetic modifications that characterise them. Finally, as wound memory in epithelial cells promotes carcinogenesis, we highlight how it represents an early step for the establishment of field cancerization.

Expansion of Human Epidermal Keratinocytes in Co-culture with Human Feeder Cells

The co-culture of human epidermal keratinocytes with mouse 3T3-J2 feeder cells, a method developed by Green and colleagues, has been widely used since the early 1980s to generate skin autografts. Additionally, co-culture with 3T3-J2 cells has been a crucial tool in skin stem cell biology, as it facilitates the evaluation of self-renewal capacity and differentiation of epidermal stem cells. This chapter presents a recent enhancement of the Green method, aimed at further promoting the expansion of human epidermal keratinocytes by utilizing a small molecule inhibitor of TGF-β signaling. This new protocol also enables the rapid expansion of human epidermal keratinocytes in co-culture with human feeder cells, including human dermal fibroblasts and human preadipocytes-two key alternatives to 3T3-J2 cells-with the long-term goal of developing customized Xeno-free skin autografts.

The TMEM63B Channel Facilitates Intestinal Motility and Enhances Proliferation of Intestinal Stem Cells

The intestines are in a constant state of motion and self-renewal. The mechanical breakdown of food facilitates intestinal movement and aids digestion. It is believed that mechanical stimulation, triggered by changes in osmotic pressure within the intestines, plays a crucial role in regulating gastrointestinal motility. While TRPs and PIEZO1/2 have been identified as mechanosensitive ion channels involved in this process, there still exist numerous unidentified channels with similar properties. In this study, we demonstrate that the TMEM63B expressed in intestinal stem cells contributes to the regulation of intestinal motility and digestion. The deletion of TMEM63B in intestinal stem cells not only decelerates intestinal motility and impairs digestion but also attenuates the proliferation of intestinal stem cells and exacerbates DSS-induced colitis in mice. Collectively, our findings unveil the pivotal role of TMEM63B in governing optimal digestive function and modulating intestinal motility.

Cell death as an architect of adult skin stem cell niches

Our skin provides a physical and immunological barrier against dehydration and environmental insults ranging from microbial attacks, toxins and UV irradiation to wounding. Proper functioning of the skin barrier largely depends on the interplay between keratinocytes- the epithelial cells of the skin- and immune cells. Two spatially distinct populations of keratinocyte stem cells (SCs) maintain the epidermal barrier function and the hair follicle. These SCs are inherently long-lived, but cell death can occur within their niches and impacts their functionality. The default cell death programme in skin is apoptosis, an orderly and non-inflammatory suicide programme. However, recent findings are shedding light on the significance of various modes of regulated necrotic cell death, which are lytic and can provoke inflammation within the local skin environment. While the presence of dying cells was generally regarded as a mere consequence of inflammation, findings in various human dermatological conditions and experimental mouse models of aberrant cell death control demonstrated that cell death programmes in keratinocytes (KCs) can drive skin inflammation and even tumour initiation. When cells die, they need to be removed by phagocytosis and KCs can function as non-professional phagocytes of apoptotic cells with important implications for their SC capacities. It is becoming apparent that in conditions of heightened SC activity, distinct cell death modalities differentially impact the different skin SC populations in their local niches. Here, we describe how regulated cell death modalities functionally affect epidermal SC niches along with their relevance to injury repair, inflammatory skin disorders and cancer.

Engineered exosomes as a prospective therapy for diabetic foot ulcers

Diabetic foot ulcer (DFU), characterized by high recurrence rate, amputations and mortality, poses a significant challenge in diabetes management. The complex pathology involves dysregulated glucose homeostasis leading to systemic and local microenvironmental complications, including peripheral neuropathy, micro- and macro-angiopathy, recurrent infection, persistent inflammation and dysregulated re-epithelialization. Novel approaches to accelerate DFU healing are actively pursued, with a focus on utilizing exosomes. Exosomes are natural nanovesicles mediating cellular communication and containing diverse functional molecular cargos, including DNA, mRNA, microRNA (miRNA), lncRNA, proteins, lipids and metabolites. While some exosomes show promise in modulating cellular function and promoting ulcer healing, their efficacy is limited by low yield, impurities, low loading content and inadequate targeting. Engineering exosomes to enhance their curative activity represents a potentially more efficient approach for DFUs. This could facilitate focused repair and regeneration of nerves, blood vessels and soft tissue after ulcer development. This review provides an overview of DFU pathogenesis, strategies for exosome engineering and the targeted therapeutic application of engineered exosomes in addressing critical pathological changes associated with DFUs.

Multiplicity of stem cell memories of inflammation and tissue repair in epithelia

Adaptation enables cells to change their behavior in response to transient stimuli. While adaptive programs of immune cells have been widely described, it has recently emerged that epithelial cells also acquire memories in vivo. Here, we discuss and classify the adaptations identified in epithelia and describe the associated long-term consequences.

Defining the identity and the niches of epithelial stem cells with highly pleiotropic multilineage potency in the human thymus

Thymus is necessary for lifelong immunological tolerance and immunity. It displays a distinctive epithelial complexity and undergoes age-dependent atrophy. Nonetheless, it also retains regenerative capacity, which, if harnessed appropriately, might permit rejuvenation of adaptive immunity. By characterizing cortical and medullary compartments in the human thymus at single-cell resolution, in this study we have defined specific epithelial populations, including those that share properties with bona fide stem cells (SCs) of lifelong regenerating epidermis. Thymic epithelial SCs display a distinctive transcriptional profile and phenotypic traits, including pleiotropic multilineage potency, to give rise to several cell types that were not previously considered to have shared origin. Using here identified SC markers, we have defined their cortical and medullary niches and shown that, in vitro, the cells display long-term clonal expansion and self-organizing capacity. These data substantively broaden our knowledge of SC biology and set a stage for tackling thymic atrophy and related disorders.

Autophagy critically controls skin inflammation and apoptosis-induced stem cell activation

Macroautophagy/autophagy is a cellular recycling program regulating cell survival and controlling inflammatory responses in a context-dependent manner. Here, we demonstrate that keratinocyte-selective ablation of Atg16l1, an essential autophagy mediator, results in exacerbated inflammatory and neoplastic skin responses. In addition, mice lacking keratinocyte autophagy exhibit precocious onset of hair follicle growth, indicating altered activation kinetics of hair follicle stem cells (HFSCs). These HFSCs also exhibit expanded potencies in an autophagy-deficient context as shown by de novo hair follicle formation and improved healing of abrasion wounds. ATG16L1-deficient keratinocytes are markedly sensitized to apoptosis. Compound deletion of RIPK3-dependent necroptotic and CASP8-dependent apoptotic responses or of TNFRSF1A/TNFR1 reveals that the enhanced sensitivity of autophagy-deficient keratinocytes to TNF-dependent cell death is driving altered activation of HFSCs. Together, our data demonstrate that keratinocyte autophagy dampens skin inflammation and tumorigenesis but curtails HFSC activation by restraining apoptotic responses.Abbreviations: ATG16L1: autophagy related 16 like 1; DMBA: 2,4-dimethoxybenzaldehyde; DP: dermal papilla; EpdSCs: epidermal stem cells; Gas6: growth arrest specific 6; HF: hair follicle; HFSC: hair follicle stem cell; IFE: interfollicular epidermis; KRT5: keratin 5; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; PMK: primary mouse keratinocyte; RIPK3: receptor-interacting serine-threonine kinase 3; scRNAseq: single-cell RNA-sequencing; SG: sebaceous gland; TEWL: transepidermal water loss; TPA: 12-O-tetradecanoylphorbol-13-acetate; TNF: tumor necrosis factor; TNFRSF1A/TNFR1: tumor necrosis factor receptor superfamily, member 1a; UMAP: uniform manifold approximation and projection.

Identification of the regulatory circuit governing corneal epithelial fate determination and disease

The transparent corneal epithelium in the eye is maintained through the homeostasis regulated by limbal stem cells (LSCs), while the nontransparent epidermis relies on epidermal keratinocytes for renewal. Despite their cellular similarities, the precise cell fates of these two types of epithelial stem cells, which give rise to functionally distinct epithelia, remain unknown. We performed a multi-omics analysis of human LSCs from the cornea and keratinocytes from the epidermis and characterized their molecular signatures, highlighting their similarities and differences. Through gene regulatory network analyses, we identified shared and cell type-specific transcription factors (TFs) that define specific cell fates and established their regulatory hierarchy. Single-cell RNA-seq (scRNA-seq) analyses of the cornea and the epidermis confirmed these shared and cell type-specific TFs. Notably, the shared and LSC-specific TFs can cooperatively target genes associated with corneal opacity. Importantly, we discovered that FOSL2, a direct PAX6 target gene, is a novel candidate associated with corneal opacity, and it regulates genes implicated in corneal diseases. By characterizing molecular signatures, our study unveils the regulatory circuitry governing the LSC fate and its association with corneal opacity.

Myc-dependent dedifferentiation of Gata6+ epidermal cells resembles reversal of terminal differentiation

Dedifferentiation is the process by which terminally differentiated cells acquire the properties of stem cells. During mouse skin wound healing, the differentiated Gata6-lineage positive cells of the sebaceous duct are able to dedifferentiate. Here we have integrated lineage tracing and single-cell mRNA sequencing to uncover the underlying mechanism. Gata6-lineage positive and negative epidermal stem cells in wounds are transcriptionally indistinguishable. Furthermore, in contrast to reprogramming of induced pluripotent stem cells, the same genes are expressed in the epidermal dedifferentiation and differentiation trajectories, indicating that dedifferentiation does not involve adoption of a new cell state. We demonstrate that dedifferentiation is not only induced by wounding, but also by retinoic acid treatment or mechanical expansion of the epidermis. In all three cases, dedifferentiation is dependent on the master transcription factor c-Myc. Mechanotransduction and actin-cytoskeleton remodelling are key features of dedifferentiation. Our study elucidates the molecular basis of epidermal dedifferentiation, which may be generally applicable to adult tissues.

Matricellular Proteins in the Homeostasis, Regeneration, and Aging of Skin

Matricellular proteins are secreted extracellular proteins that bear no primary structural functions but play crucial roles in tissue remodeling during development, homeostasis, and aging. Despite their low expression after birth, matricellular proteins within skin compartments support the structural function of many extracellular matrix proteins, such as collagens. In this review, we summarize the function of matricellular proteins in skin stem cell niches that influence stem cells' fate and self-renewal ability. In the epidermal stem cell niche, fibulin 7 promotes epidermal stem cells' heterogeneity and fitness into old age, and the transforming growth factor-β-induced protein ig-h3 (TGFBI)-enhances epidermal stem cell growth and wound healing. In the hair follicle stem cell niche, matricellular proteins such as periostin, tenascin C, SPARC, fibulin 1, CCN2, and R-Spondin 2 and 3 modulate stem cell activity during the hair cycle and may stabilize arrector pili muscle attachment to the hair follicle during piloerections (goosebumps). In skin wound healing, matricellular proteins are upregulated, and their functions have been examined in various gain-and-loss-of-function studies. However, much remains unknown concerning whether these proteins modulate skin stem cell behavior, plasticity, or cell-cell communications during wound healing and aging, leaving a new avenue for future studies.

Modelling adult stem cells and their niche in health and disease with epithelial organoids

Adult stem cells are responsible for homoeostasis and regeneration of epithelial tissues. Stem cell function is regulated by both cell autonomous mechanisms as well as the niche. Deregulated stem cell function contributes to diseases such as cancer. Epithelial organoid cultures generated from tissue-resident adult stem cells have allowed unprecedented insights into the biology of epithelial tissues. The subsequent adaptation of organoid technology enabled the modelling of the communication of stem cells with their cellular and non-cellular niche as well as diseases. Starting from its first model described in 2009, the murine small intestinal organoid, we discuss here how epithelial organoid cultures have been become a prime in vitro research tool for cell and developmental biology, bioengineering, and biomedicine in the last decade.

Single-cell expression profile of Drosophila ovarian follicle stem cells illuminates spatial differentiation in the germarium

BACKGROUND

How stem cell populations are organized and regulated within adult tissues is important for understanding cancer origins and for developing cell replacement strategies. Paradigms such as mammalian gut stem cells and Drosophila ovarian follicle stem cells (FSC) are characterized by population asymmetry, in which stem cell division and differentiation are separately regulated processes. These stem cells behave stochastically regarding their contributions to derivative cells and also exhibit dynamic spatial heterogeneity. Drosophila FSCs provide an excellent model for understanding how a community of active stem cells maintained by population asymmetry is regulated. Here, we use single-cell RNA sequencing to profile the gene expression patterns of FSCs and their immediate derivatives to investigate heterogeneity within the stem cell population and changes associated with differentiation.

RESULTS

We describe single-cell RNA sequencing studies of a pre-sorted population of cells that include FSCs and the neighboring cell types, escort cells (ECs) and follicle cells (FCs), which they support. Cell-type assignment relies on anterior-posterior (AP) location within the germarium. We clarify the previously determined location of FSCs and use spatially targeted lineage studies as further confirmation. The scRNA profiles among four clusters are consistent with an AP progression from anterior ECs through posterior ECs and then FSCs, to early FCs. The relative proportion of EC and FSC clusters are in good agreement with the prevalence of those cell types in a germarium. Several genes with graded profiles from ECs to FCs are highlighted as candidate effectors of the inverse gradients of the two principal signaling pathways, Wnt and JAK-STAT, that guide FSC differentiation and division.

CONCLUSIONS

Our data establishes an important resource of scRNA-seq profiles for FSCs and their immediate derivatives that is based on precise spatial location and functionally established stem cell identity, and facilitates future genetic investigation of regulatory interactions guiding FSC behavior.

Tissue memory relies on stem cell priming in distal undamaged areas

Epithelial cells that participated in wound repair elicit a more efficient response to future injuries, which is believed to be locally restricted. Here we show that cell adaptation resulting from a localized tissue damage has a wide spatial impact at a scale not previously appreciated. We demonstrate that a specific stem cell population, distant from the original injury, originates long-lasting wound memory progenitors residing in their own niche. Notably, these distal memory cells have not taken part in the first healing but become intrinsically pre-activated through priming. This cell state, maintained at the chromatin and transcriptional level, leads to an enhanced wound repair that is partially recapitulated through epigenetic perturbation. Importantly wound memory has long-term harmful consequences, exacerbating tumourigenesis. Overall, we show that sub-organ-scale adaptation to injury relies on spatially organized memory-dedicated progenitors, characterized by an actionable cell state that establishes an epigenetic field cancerization and predisposes to tumour onset.

Bioengineered MSC-derived exosomes in skin wound repair and regeneration

Refractory skin defects such as pressure ulcers, diabetic ulcers, and vascular ulcers represent a challenge for clinicians and researchers in many aspects. The treatment strategies for wound healing have high cost and limited efficacy. To ease the financial and psychological burden on patients, a more effective therapeutic approach is needed to address the chronic wound. MSC-derived exosomes (MSC-exosomes), the main bioactive extracellular vesicles of the paracrine effect of MSCs, have been proposed as a new potential cell-free approach for wound healing and skin regeneration. The benefits of MSC-exosomes include their ability to promote angiogenesis and cell proliferation, increase collagen production, regulate inflammation, and finally improve tissue regenerative capacity. However, poor targeting and easy removability of MSC-exosomes from the wound are major obstacles to their use in clinical therapy. Thus, the concept of bioengineering technology has been introduced to modify exosomes, enabling higher concentrations and construction of particles of greater stability with specific therapeutic capability. The use of biomaterials to load MSC-exosomes may be a promising strategy to concentrate dose, create the desired therapeutic efficacy, and maintain a sustained release effect. The beneficial role of MSC-exosomes in wound healing is been widely accepted; however, the potential of bioengineering-modified MSC-exosomes remains unclear. In this review, we attempt to summarize the therapeutic applications of modified MSC-exosomes in wound healing and skin regeneration. The challenges and prospects of bioengineered MSC-exosomes are also discussed.

Sebaceous immunobiology - skin homeostasis, pathophysiology, coordination of innate immunity and inflammatory response and disease associations

This review presents several aspects of the innovative concept of sebaceous immunobiology, which summarizes the numerous activities of the sebaceous gland including its classical physiological and pathophysiological tasks, namely sebum production and the development of seborrhea and acne. Sebaceous lipids, which represent 90% of the skin surface lipids in adolescents and adults, are markedly involved in the skin barrier function and perifollicular and dermal innate immune processes, leading to inflammatory skin diseases. Innovative experimental techniques using stem cell and sebocyte models have clarified the roles of distinct stem cells in sebaceous gland physiology and sebocyte function control mechanisms. The sebaceous gland represents an integral part of the pilosebaceous unit and its status is connected to hair follicle morphogenesis. Interestingly, professional inflammatory cells contribute to sebocyte differentiation and homeostasis, whereas the regulation of sebaceous gland function by immune cells is antigen-independent. Inflammation is involved in the very earliest differentiation changes of the pilosebaceous unit in acne. Sebocytes behave as potent immune regulators, integrating into the innate immune responses of the skin. Expressing inflammatory mediators, sebocytes also contribute to the polarization of cutaneous T cells towards the Th17 phenotype. In addition, the immune response of the perifollicular infiltrate depends on factors produced by the sebaceous glands, mostly sebaceous lipids. Human sebocytes in vitro express functional pattern recognition receptors, which are likely to interact with bacteria in acne pathogenesis. Sex steroids, peroxisome proliferator-activated receptor ligands, neuropeptides, endocannabinoids and a selective apoptotic process contribute to a complex regulation of sebocyte-induced immunological reaction in numerous acquired and congenital skin diseases, including hair diseases and atopic dermatitis.

Homogenic Evaluation for Spatial Distribution in Osteoclast Differentiation

BACKGROUND

Cells have heterogeneous cellular diversity in size, morphology, cell cycle, metabolism, differentiation degree, and spatial distribution. The shift of specific cells towards the desired cells is crucial for maintaining uniform cellular function and can be represented by homogeneity and heterogeneity. Here, we developed a simple and direct method for evaluating the homogeneous distribution of desired cells in a constant region.

METHODS

We differentiated osteoclast progenitors into bone-resorbing multinucleated giant osteoclasts in a 2-dimensional culture plate under 2 conditions. Cells were stained with tartrate-resistant acid phosphatase to assess osteoclast differentiation, images were taken using a microscope and divided into sectors, and the number of osteoclasts (≥3 nuclei) in each sector was counted. To assess the homogeneity of the spatial distribution of osteoclasts, the standard deviation (SD) was calculated from the mean number of osteoclasts within each sector.

RESULTS

From the 2 groups, a value with a SD close to 0 indicates high spatial homogeneity while a relatively high SD represents low spatial homogeneity.

CONCLUSIONS

Our findings suggest that spatial homogeneity can be represented as SD.

Pgc-1α controls epidermal stem cell fate and skin repair by sustaining NAD+ homeostasis during aging

OBJECTIVE

The epidermal barrier is renewed by the activation, proliferation, and differentiation of keratinocyte stem cells after injury and aging impedes this repair process through undefined mechanisms. We previously identified a gene signature of metabolic dysfunction in aged murine epidermis, but the precise regulators of epidermal repair and age-related growth defects are not well established. Aged mouse models as well as mice with conditional epidermal loss of the metabolic regulator, peroxisome proliferator-activated receptor gamma coactivator-1 alpha (Pgc-1α) were used to explore the cellular pathways which control skin repair after injury and stress.

METHODS

Aged mice or those with epidermal Pgc-1α deletion (epiPgc-1α KO) and young or Pgc1a^fl/fl controls were subjected to wound injury, UVB exposure or the inflammatory agent TPA. In vivo and ex vivo analyses of wound closure, skin structure, cell growth and stem cell differentiation were used to understand changes in epidermal re-growth and repair resulting from aging or Pgc-1α loss.

RESULTS

Aging impairs epidermal re-growth during wound healing and results in lower expression of Pgc-1α. Mice with conditional deletion of epidermal Pgc-1α exhibit greater inflammation- and UVB-induced cell differentiation, reduced proliferation, and slower wound healing. epiPgc-1α KO mice also displayed reduced keratinocyte NAD⁺ levels, shorter telomeres, and greater poly ADP-ribosylation, resulting in enhanced stress-stimulated p53 and p21 signaling. When NAD⁺ was reduced by Pgc-1α loss or pharmacologic inhibition of NAD⁺ synthesis, there was reduced stress-induced proliferation, increased differentiation, and protection against DNA damage via enhanced epidermal shedding. Similarly, aged mice exhibit disrupted epidermal NAD⁺ homeostasis and enhanced p53 activation, resulting in p21 growth arrest after wounding. NAD⁺ precursor treatment restores epidermal growth from old skin to that of young.

CONCLUSIONS

Our studies identify a novel role for epidermal Pgc-1α in controlling epidermal repair via its regulation of cellular NAD⁺ and downstream effects on p53-driven growth arrest. We also establish that parallel mechanisms are evident in aged epidermis, showing that NAD⁺ signaling is an important controller of physiologic skin repair and that dysfunction of this pathway contributes to age-related wound repair defects.

Identification of Human Breast Adipose Tissue Progenitors Displaying Distinct Differentiation Potentials and Interactions with Cancer Cells

Breast adipose tissue (AT) participates in the physiological evolution and remodeling of the mammary gland due to its high plasticity. It is also a favorable microenvironment for breast cancer progression. However, information on the properties of human breast adipose progenitor cells (APCs) involved in breast physiology or pathology is scant. We performed differential enzymatic dissociation of human breast AT lobules. We isolated and characterized two populations of APCs. Here we report that these distinct breast APC populations selectively expressed markers suitable for characterization. The population preferentially expressing ALPL (MSCA1) showed higher adipogenic potential. The population expressing higher levels of INHBA and CD142 acquired myofibroblast characteristics upon TGF-β treatment and a myo-cancer-associated fibroblast profile in the presence of breast cancer cells. This population expressed the immune checkpoint CD274 (PD-L1) and facilitated the expansion of breast cancer mammospheres compared with the adipogenic population. Indeed, the breast, as with other fat depots, contains distinct types of APCs with differences in their ability to specialize. This indicates that they were differentially involved in breast remodeling. Their interactions with breast cancer cells revealed differences in the potential for tumor dissemination and estrogen receptor expression, and these differences might be relevant to improve therapies targeting the tumor microenvironment.

ARTS, an unusual septin, regulates tumorigenesis by promoting apoptosis

Apoptosis plays particularly important roles in tumorigenesis through various mechanisms. Apoptosis can be initiated by both extrinsic and intrinsic signals centered in and coming from the mitochondria. Antiapoptotic proteins promote tumor progression, and the occurrence and progression of tumors are closely related to antiapoptotic protein expression. As the only member of the septin gene family with proapoptotic function, apoptosis-related proteins in the TGF-β signaling pathway (ARTS) has received extensive attention for its unique structure. In contrast, unlike other known inhibitors of apoptosis protein (IAP) antagonists, ARTS exhibits a stronger tumor suppressor potential. Recent research has shown that ARTS can bind and inhibit XIAP and Bcl-2 directly or assist p53 in the degradation of Bcl-XL. Here, we review recent advances in the molecular mechanisms by which the proapoptotic protein ARTS, with its unique structure, inhibits tumorigenesis. We also discuss the possibility of mimicking ARTS to develop small-molecule drugs.

Cellular Heterogeneity and Plasticity of Skin Epithelial Cells in Wound Healing and Tumorigenesis

Cellular differentiation, the fundamental hallmark of cells, plays a critical role in homeostasis. And stem cells not only regulate the process where embryonic stem cells develop into a complete organism, but also replace ageing or damaged cells by proliferation, differentiation and migration. In characterizing distinct subpopulations of skin epithelial cells, stem cells show large heterogeneity and plasticity for homeostasis, wound healing and tumorigenesis. Epithelial stem cells and committed progenitors replenish each other or by themselves owing to the remarkable plasticity and heterogeneity of epidermal cells under certain circumstance. The development of new assay methods, including single-cell RNA sequence, lineage tracing assay, intravital microscopy systems and photon-ablation assay, highlight the plasticity of epidermal stem cells in response to injure and tumorigenesis. However, the critical mechanisms and key factors that regulate cellular plasticity still need for further exploration. In this review, we discuss the recent insights about the heterogeneity and plasticity of epithelial stem cells in homeostasis, wound healing and skin tumorigenesis. Understanding how stem cells collaborate together to repair injury and initiate tumor will offer new solutions for relevant diseases. Schematic abstract of cellular heterogeneity and plasticity of skin epithelial cells in wound healing and tumorigenesis.

Germ cells commit somatic stem cells to differentiation following priming by PI3K/Tor activity in the Drosophila testis

How and when potential becomes restricted in differentiating stem cell daughters is poorly understood. While it is thought that signals from the niche are actively required to prevent differentiation, another model proposes that stem cells can reversibly transit between multiple states, some of which are primed, but not committed, to differentiate. In the Drosophila testis, somatic cyst stem cells (CySCs) generate cyst cells, which encapsulate the germline to support its development. We find that CySCs are maintained independently of niche self-renewal signals if activity of the PI3K/Tor pathway is inhibited. Conversely, PI3K/Tor is not sufficient alone to drive differentiation, suggesting that it acts to license cells for differentiation. Indeed, we find that the germline is required for differentiation of CySCs in response to PI3K/Tor elevation, indicating that final commitment to differentiation involves several steps and intercellular communication. We propose that CySC daughter cells are plastic, that their fate depends on the availability of neighbouring germ cells, and that PI3K/Tor acts to induce a primed state for CySC daughters to enable coordinated differentiation with the germline.

Stem cells are unique in their ability to regenerate adult tissues by dividing to provide new stem cells, a process called self-renewal, and cells that will differentiate and maintain tissue function. How and when the daughters that differentiate lose the ability to self-renew is still poorly understood. Self-renewal depends on signals that are provided by the supportive micro-environment, or niche, in which the stem cells reside. It was assumed that simply losing access to this environment and the signals it provides was sufficient to direct differentiation. Here we use the Drosophila testis as a model to show that this is not the case. Instead, differentiation must be actively induced by signalling, and stem cells deprived of all signals can be maintained. Studying the relative timings of the various inputs into differentiation leads us to propose that a series of events ensure appropriate differentiation. First, stem cells receive differentiation-inducing signals that promote a permissive, or primed, state which is reversible and does not preclude self-renewal. The final commitment comes from interacting with other cells in the tissue, ensuring that differentiation always occurs in a coordinated manner among the different cell types composing this tissue.

Visualization of individual cell division history in complex tissues using iCOUNT

The division potential of individual stem cells and the molecular consequences of successive rounds of proliferation remain largely unknown. Here, we developed an inducible cell division counter (iCOUNT) that reports cell division events in human and mouse tissues in vitro and in vivo. Analyzing cell division histories of neural stem/progenitor cells (NSPCs) in the developing and adult brain, we show that iCOUNT can provide novel insights into stem cell behavior. Further, we use single-cell RNA sequencing (scRNA-seq) of iCOUNT-labeled NSPCs and their progenies from the developing mouse cortex and forebrain-regionalized human organoids to identify functionally relevant molecular pathways that are commonly regulated between mouse and human cells, depending on individual cell division histories. Thus, we developed a tool to characterize the molecular consequences of repeated cell divisions of stem cells that allows an analysis of the cellular principles underlying tissue formation, homeostasis, and repair.

Adult stem cells and niche cells segregate gradually from common precursors that build the adult Drosophila ovary during pupal development

Production of proliferative follicle cells (FCs) and quiescent escort cells (ECs) by follicle stem cells (FSCs) in adult Drosophila ovaries is regulated by niche signals from anterior (cap cells, ECs) and posterior (polar FCs) sources. Here we show that ECs, FSCs, and FCs develop from common pupal precursors, with different fates acquired by progressive separation of cells along the AP axis and a graded decline in anterior cell proliferation. ECs, FSCs, and most FCs derive from intermingled cell (IC) precursors interspersed with germline cells. Precursors also accumulate posterior to ICs before engulfing a naked germline cyst projected out of the germarium to form the first egg chamber and posterior polar FC signaling center. Thus, stem and niche cells develop in appropriate numbers and spatial organization through regulated proliferative expansion together with progressive establishment of spatial signaling cues that guide adult cell behavior, rather than through rigid early specification events.

Novel, Fully Characterised Bovine Taste Bud Cells of Fungiform Papillae

Current understanding of functional characteristics and biochemical pathways in taste bud cells have been hindered due the lack of long-term cultured cells. To address this, we developed a holistic approach to fully characterise long term cultured bovine taste bud cells (BTBCs). Initially, cultured BTBCs were characterised using RT-PCR gene expression profiling, immunocytochemistry, flowcytometry and calcium imaging, that confirmed the cells were mature TBCs that express taste receptor genes, taste specific protein markers and capable of responding to taste stimuli, i.e., denatonium (2 mM) and quinine (462.30 μM). Gene expression analysis of forty-two genes implicated in taste transduction pathway (map04742) using custom-made RT-qPCR array revealed high and low expressed genes in BTBCs. Preliminary datamining and bioinformatics demonstrated that the bovine α-gustducin, gustatory G-protein, have higher sequence similarity to the human orthologue compared to rodents. Therefore, results from this work will replace animal experimentation and provide surrogate cell-based throughput system to study human taste transduction.

Evolution of mechanisms controlling epithelial morphogenesis across animals: new insights from dissociation-reaggregation experiments in the sponge Oscarella lobularis

BACKGROUND

The ancestral presence of epithelia in Metazoa is no longer debated. Porifera seem to be one of the best candidates to be the sister group to all other Metazoa. This makes them a key taxon to explore cell-adhesion evolution on animals. For this reason, several transcriptomic, genomic, histological, physiological and biochemical studies focused on sponge epithelia. Nevertheless, the complete and precise protein composition of cell-cell junctions and mechanisms that regulate epithelial morphogenetic processes still remain at the center of attention.

RESULTS

To get insights into the early evolution of epithelial morphogenesis, we focused on morphogenic characteristics of the homoscleromorph sponge Oscarella lobularis. Homoscleromorpha are a sponge class with a typical basement membrane and adhaerens-like junctions unknown in other sponge classes. We took advantage of the dynamic context provided by cell dissociation-reaggregation experiments to explore morphogenetic processes in epithelial cells in a non-bilaterian lineage by combining fluorescent and electron microscopy observations and RNA sequencing approaches at key time-points of the dissociation and reaggregation processes.

CONCLUSIONS

Our results show that part of the molecular toolkit involved in the loss and restoration of epithelial features such as cell-cell and cell-matrix adhesion is conserved between Homoscleromorpha and Bilateria, suggesting their common role in the last common ancestor of animals. In addition, sponge-specific genes are differently expressed during the dissociation and reaggregation processes, calling for future functional characterization of these genes.

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.

Universal principles of lineage architecture and stem cell identity in renewing tissues

Adult tissues in multicellular organisms typically contain a variety of stem, progenitor and differentiated cell types arranged in a lineage hierarchy that regulates healthy tissue turnover. Lineage hierarchies in disparate tissues often exhibit common features, yet the general principles regulating their architecture are not known. Here, we provide a formal framework for understanding the relationship between cell molecular 'states' and cell 'types', based on the topology of admissible cell state trajectories. We show that a self-renewing cell type - if defined as suggested by this framework - must reside at the top of any homeostatic renewing lineage hierarchy, and only there. This architecture arises as a natural consequence of homeostasis, and indeed is the only possible way that lineage architectures can be constructed to support homeostasis in renewing tissues. Furthermore, under suitable feedback regulation, for example from the stem cell niche, we show that the property of 'stemness' is entirely determined by the cell environment, in accordance with the notion that stem cell identities are contextual and not determined by hard-wired, cell-intrinsic characteristics. This article has an associated 'The people behind the papers' interview.

Hair follicle stem cell progeny heal blisters while pausing skin development

Injury in adult tissue generally reactivates developmental programs to foster regeneration, but it is not known whether this paradigm applies to growing tissue. Here, by employing blisters, we show that epidermal wounds heal at the expense of skin development. The regenerated epidermis suppresses the expression of tissue morphogenesis genes accompanied by delayed hair follicle (HF) growth. Lineage tracing experiments, cell proliferation dynamics, and mathematical modeling reveal that the progeny of HF junctional zone stem cells, which undergo a morphological transformation, repair the blisters while not promoting HF development. In contrast, the contribution of interfollicular stem cell progeny to blister healing is small. These findings demonstrate that HF development can be sacrificed for the sake of epidermal wound regeneration. Our study elucidates the key cellular mechanism of wound healing in skin blistering diseases.

A stem cell population at the anorectal junction maintains homeostasis and participates in tissue regeneration

At numerous locations of the body, transition zones are localized at the crossroad between two types of epithelium and are frequently associated with neoplasia involving both type of tissues. These transition zones contain cells expressing markers of adult stem cells that can be the target of early transformation. The mere fact that transition zone cells can merge different architecture with separate functions implies for a unique plasticity that these cells must display in steady state. However, their roles during tissue regeneration in normal and injured state remain unknown. Here, by using in vivo lineage tracing, single-cell transcriptomics, computational modeling and a three-dimensional organoid culture system of transition zone cells, we identify a population of Krt17+ basal cells with multipotent properties at the squamo-columnar anorectal junction that maintain a squamous epithelium during normal homeostasis and can participate in the repair of a glandular epithelium following tissue injury.

Inflation-collapse dynamics drive patterning and morphogenesis in intestinal organoids

How stem cells self-organize to form structured tissues is an unsolved problem. Intestinal organoids offer a model of self-organization as they generate stem cell zones (SCZs) of typical size even without a spatially structured environment. Here we examine processes governing the size of SCZs. We improve the viability and homogeneity of intestinal organoid cultures to enable long-term time-lapse imaging of multiple organoids in parallel. We find that SCZs are shaped by fission events under strong control of ion channel-mediated inflation and mechanosensitive Piezo-family channels. Fission occurs through stereotyped modes of dynamic behavior that differ in their coordination of budding and differentiation. Imaging and single-cell transcriptomics show that inflation drives acute stem cell differentiation and induces a stretch-responsive cell state characterized by large transcriptional changes, including upregulation of Piezo1. Our results reveal an intrinsic capacity of the intestinal epithelium to self-organize by modulating and then responding to its mechanical state.

Nanoscale microenvironment engineering for expanding human hair follicle stem cell and revealing their plasticity

Background

Periodically regenerated hair follicles provide an excellent research model for studying tissue regeneration and stem cell homeostasis. Periodic activation and differentiation of hair follicle stem cells (HFSCs) fuel cyclical bouts of hair regeneration. HFSCs represent an excellent paradigm for studying tissue regeneration and somatic stem cell homeostasis. However, these crucial studies are hampered by the lack of a culture system able to stably expand human HFSCs and regulate their fate.

Results

Here, we use layer-by-layer (LbL) self-assembly with gelatin/alginate to construct a nanoscale biomimetic extracellular matrix (ECM) for an HFSC population. The LbL coating provides ECM and mechanical support for individual cells, which helps to maintain the CD200⁺α6⁺ HFSC population to a certain extent. Addition of key signal molecules (FGF-7 and VEGF-A) simulates the minimum essential components of the stem cell microenvironment, thereby effectively and stably expanding HFSCs and maintaining the CD200⁺α6⁺ HFSC population. Subsequently, BMP2 loaded to the nanocoated layer, as a slow-release signal molecule, activates BMP signaling to regulate HFSCs’ fate in order to obtain a purified CD200⁺α6⁺ HFSC population.

Conclusion

This system can minimize the microenvironment of HFSCs; thus, stably amplifying HFSCs and revealing their plasticity. Our study thus provides a new tool for studies of hair follicle reconstruction and stem cell homeostasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00840-5.

Stem and progenitor cells in sebaceous gland development, homeostasis and pathologies

Sebaceous glands (SGs), typically associated with hair follicles, are critical for the homeostasis and function of mammalian skin. The main physiological function of SGs is the production and holocrine secretion of sebum to lubricate and protect the skin. Defective SGs have been linked to a variety of skin disorders, including acne, seborrheic dermatitis and formation of sebaceous tumors. Thus, a better understanding how SGs are formed and maintained is important to unravel the underlying molecular and cellular mechanisms of SG pathologies and to find better and effective therapies. Over the last two decades, research has come a long way from the initial identification of skin epithelial stem cells to the isolation and functional characterization of multiple stem cell pools as well as a better understanding of their unique and complex activities that drive skin homeostasis and operate in skin pathologies. Here, we discuss recent progress in unravelling cellular mechanisms underlying SG development, homeostasis and sebaceous tumor formation and assess the role of stem and progenitor cells in controlling SG physiology and disease processes. The development of elegant in vivo imaging as well as various in vitro and ex vivo stem cell and SG tissue models will advance mechanistic studies on SG function and allow drug screening and testing for efficient and successful targeting SG pathologies.

TEM, SEM, and STEM-based immuno-CLEM workflows offer complementary advantages

Identifying endogenous tissue stem cells remains a key challenge in developmental and regenerative biology. To distinguish and molecularly characterise stem cell populations in large heterogeneous tissues, the combination of cytochemical cell markers with ultrastructural morphology is highly beneficial. Here, we realise this through workflows of multi-resolution immuno-correlative light and electron microscopy (iCLEM) methodologies. Taking advantage of the antigenicity preservation of the Tokuyasu technique, we have established robust protocols and workflows and provide a side-by-side comparison of iCLEM used in combination with scanning EM (SEM), scanning TEM (STEM), or transmission EM (TEM). Evaluation of the applications and advantages of each method highlights their practicality for the identification, quantification, and characterization of heterogeneous cell populations in small organisms, organs, or tissues in healthy and diseased states. The iCLEM techniques are broadly applicable and can use either genetically encoded or cytochemical markers on plant, animal and human tissues. We demonstrate how these protocols are particularly suited for investigating neural stem and progenitor cell populations of the vertebrate nervous system.

Interplay among p21Waf1/Cip1, MUSASHI-1 and Krüppel-like factor 4 in activation of Bmi1-CreER reserve intestinal stem cells after gamma radiation-induced injury

Gamma radiation is a commonly used adjuvant treatment for abdominally localized cancer. Since its therapeutic potential is limited due to gastrointestinal (GI) syndrome, elucidation of the regenerative response following radiation-induced gut injury is needed to develop a preventive treatment. Previously, we showed that Krüppel-like factor 4 (KLF4) activates certain quiescent intestinal stem cells (ISCs) marked by Bmi1-Cre^ER to give rise to regenerating crypts following γ irradiation. In the current study, we showed that γ radiation-induced expression of p21^Waf1/Cip1 in Bmi1-Cre^ER cells is likely mitigated by MUSASHI-1 (MSI1) acting as a negative regulator of p21^Waf1/Cip1 mRNA translation, which promotes exit of the Bmi1-Cre^ER cells from a quiescent state. Additionally, Bmi1-specific Klf4 deletion resulted in decreased numbers of MSI1⁺ cells in regenerating crypts compared to those of control mice. We showed that KLF4 binds to the Msi1 promoter and activates its expression in vitro. Since MSI1 has been shown to be crucial for crypt regeneration, this finding elucidates a pro-proliferative role of KLF4 during the postirradiation regenerative response. Taken together, our data suggest that the interplay among p21^Waf1/Cip1, MSI1 and KLF4 regulates Bmi1-Cre^ER cell survival, exit from quiescence and regenerative potential upon γ radiation-induced injury.

Single-Cell Chromatin Analysis of Mammary Gland Development Reveals Cell-State Transcriptional Regulators and Lineage Relationships

Technological improvements enable single-cell epigenetic analyses of organ development. We reasoned that high-resolution single-cell chromatin accessibility mapping would provide needed insight into the epigenetic reprogramming and transcriptional regulators involved in normal mammary gland development. Here, we provide a single-cell resource of chromatin accessibility for murine mammary development from the peak of fetal mammary stem cell (fMaSC) functional activity in late embryogenesis to the differentiation of adult basal and luminal cells. We find that the chromatin landscape within individual cells predicts both gene accessibility and transcription factor activity. The ability of single-cell chromatin profiling to separate E18 fetal mammary cells into clusters exhibiting basal-like and luminal-like chromatin features is noteworthy. Such distinctions were not evident in analyses of droplet-based single-cell transcriptomic data. We present a web application as a scientific resource for facilitating future analyses of the gene regulatory networks involved in mammary development.

Universality of clonal dynamics poses fundamental limits to identify stem cell self-renewal strategies

How adult stem cells maintain self-renewing tissues is commonly assessed by analysing clonal data from in vivo cell lineage-tracing assays. To identify strategies of stem cell self-renewal requires that different models of stem cell fate choice predict sufficiently different clonal statistics. Here, we show that models of cell fate choice can, in homeostatic tissues, be categorized by exactly two 'universality classes', whereby models of the same class predict, under asymptotic conditions, the same clonal statistics. Those classes relate to generalizations of the canonical asymmetric vs. symmetric stem cell self-renewal strategies and are distinguished by a conservation law. This poses both challenges and opportunities to identify stem cell self-renewal strategies: while under asymptotic conditions, self-renewal models of the same universality class cannot be distinguished by clonal data only, models of different classes can be distinguished by simple means.

Tracing the Dynamics of Stem Cell Fate

The mechanisms that regulate the balance between stem cell duplication and differentiation in adult tissues remain in debate. Using a combination of genetic lineage tracing and marker-based assays, the quantitative statistical analysis of clone size and cell composition has provided insights into the patterns of stem cell fate across a variety of tissue types and organisms. These studies have emphasized the role of niche factors and environmental cues in promoting stem cell competence, fate priming, and stochastic renewal programs. At the same time, evidence for injury-induced "cellular reprogramming" has revealed the remarkable flexibility of cell states, allowing progenitors to reacquire self-renewal potential during regeneration. Together, these findings have questioned the nature of stem cell identity and function. Here, focusing on a range of canonical tissue types, we review how quantitative modeling-based approaches have uncovered conserved patterns of stem cell fate and provided new insights into the mechanisms that regulate self-renewal.

Profiling Cell Signaling Networks at Single-cell Resolution

Signaling networks process intra- and extracellular information to modulate the functions of a cell. Deregulation of signaling networks results in abnormal cellular physiological states and often drives diseases. Network responses to a stimulus or a drug treatment can be highly heterogeneous across cells in a tissue because of many sources of cellular genetic and non-genetic variance. Signaling network heterogeneity is the key to many biological processes, such as cell differentiation and drug resistance. Only recently, the emergence of multiplexed single-cell measurement technologies has made it possible to evaluate this heterogeneity. In this review, we categorize currently established single-cell signaling network profiling approaches by their methodology, coverage, and application, and we discuss the advantages and limitations of each type of technology. We also describe the available computational tools for network characterization using single-cell data and discuss potential confounding factors that need to be considered in single-cell signaling network analyses.

Cellular Heterogeneity in Pressure and Growth Emerges from Tissue Topology and Geometry

Cell-to-cell heterogeneity prevails in many systems, as exemplified by cell growth, although the origin and function of such heterogeneity are often unclear. In plants, growth is physically controlled by cell wall mechanics and cell hydrostatic pressure, alias turgor pressure. Whereas cell wall heterogeneity has received extensive attention, the spatial variation of turgor pressure is often overlooked. Here, combining atomic force microscopy and a physical model of pressurized cells, we show that turgor pressure is heterogeneous in the Arabidopsis shoot apical meristem, a population of stem cells that generates all plant aerial organs. In contrast with cell wall mechanical properties that appear to vary stochastically between neighboring cells, turgor pressure anticorrelates with cell size and cell neighbor number (local topology), in agreement with the prediction by our model of tissue expansion, which couples cell wall mechanics and tissue hydraulics. Additionally, our model predicts two types of correlations between pressure and cellular growth rate, where high pressure may lead to faster- or slower-than-average growth, depending on cell wall extensibility, yield threshold, osmotic pressure, and hydraulic conductivity. The meristem exhibits one of these two regimes, depending on conditions, suggesting that, in this tissue, water conductivity may contribute to growth control. Our results unravel cell pressure as a source of patterned heterogeneity and illustrate links between local topology, cell mechanical state, and cell growth, with potential roles in tissue homeostasis.

Plasticity of Lgr5-Negative Cancer Cells Drives Metastasis in Colorectal Cancer

Colorectal cancer stem cells (CSCs) express Lgr5 and display extensive stem cell-like multipotency and self-renewal and are thought to seed metastatic disease. Here, we used a mouse model of colorectal cancer (CRC) and human tumor xenografts to investigate the cell of origin of metastases. We found that most disseminated CRC cells in circulation were Lgr5⁻ and formed distant metastases in which Lgr5⁺ CSCs appeared. This plasticity occurred independently of stemness-inducing microenvironmental factors and was indispensable for outgrowth, but not establishment, of metastases. Together, these findings show that most colorectal cancer metastases are seeded by Lgr5⁻ cells, which display intrinsic capacity to become CSCs in a niche-independent manner and can restore epithelial hierarchies in metastatic tumors.

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The majority of disseminating cells of colorectal cancer are Lgr5⁻

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Lgr5⁻ cancer cells are the main seeds of colorectal cancer metastatic lesions

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Long-term metastatic growth from Lgr5⁻ cells requires appearance of Lgr5⁺ cells

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Lgr5⁻ metastases have the intrinsic capacity to re-establish the cellular hierarchy

The Simplest Protocol for Rapid and Long-Term Culture of Primary Epidermal Keratinocytes from Human and Mouse

Although mouse models have been used as an essential tool for studying the physiology and diseases of the skin, propagation of mouse primary epidermal keratinocytes remains challenging. In this chapter, we introduce the simplest, at least to our knowledge, protocol that enables long-term expansion of p63⁺ mouse epidermal keratinocytes in low Ca²⁺ media without the need of progenitor cell-purification steps or support by a feeder cell layer. Pharmacological inhibition of TGF-β signaling in crude preparations of mouse epidermis robustly increases proliferative capacity of p63⁺ epidermal progenitor cells, while preserving their ability to differentiate. Suppression of TGF-β signaling also permits p63⁺ epidermal keratinocytes to form macroscopically large clones in 3T3-J2 feeder cell co-culture. Suppression of TGF-β signaling also enhances the clonal growth of human keratinocytes in co-culture with a variety of feeder cells. This simple and efficient approach will not only facilitate the use of mouse models by providing p63⁺ primary epidermal keratinocytes in quantity but also significantly reduce the time needed for preparing the customized skin grafts in Green method.

Long-Term In Vitro Expansion of Epithelial Stem Cells Enabled by Pharmacological Inhibition of PAK1-ROCK-Myosin II and TGF-β Signaling

Despite substantial self-renewal capability in vivo, epithelial stem and progenitor cells located in various tissues expand for a few passages in vitro in feeder-free condition before they succumb to growth arrest. Here, we describe the EpiX method, which utilizes small molecules that inhibit PAK1-ROCK-Myosin II and TGF-β signaling to achieve over one trillion-fold expansion of human epithelial stem and progenitor cells from skin, airway, mammary, and prostate glands in the absence of feeder cells. Transcriptomic and epigenomic studies show that this condition helps epithelial cells to overcome stresses for continuous proliferation. EpiX-expanded basal epithelial cells differentiate into mature epithelial cells consistent with their tissue origins. Whole-genome sequencing reveals that the cells retain remarkable genome integrity after extensive in vitro expansion without acquiring tumorigenicity. EpiX technology provides a solution to exploit the potential of tissue-resident epithelial stem and progenitor cells for regenerative medicine.

Differential expression of α6 and β1 integrins reveals epidermal heterogeneity at single-cell resolution

Skin epidermis is a constantly renewing epithelium that is composed of various cell types and provides an ideal model system for tissue regeneration and heterogeneity studies. Integrins are a family of transmembrane receptors that mediate cell adhesion in the epidermis, and integrin expression spatially reflects epidermal heterogeneity. It remains unclear whether differential expression of integrins can characterize cell types in skin epidermis. This study applied a fluorescence-activated cell sorting (FACS) strategy based on differential expression of α6 and β1 integrins, and used transcriptome analysis to explore epidermal heterogeneity. First, epidermal cells were acquired from C57BL/6 mice back skin. Nine bulk-cell populations were sorted with differential expressions of α6 and β1 integrins, and were successfully characterized as the main cell types in the epidermis through RNA-seq and transcriptome analysis. Then, tetOKrt14-H2BGFP mice were used to trace the cell proliferation rate during wound healing with GFP intensity. Epidermal cells were acquired from the re-epithelialized back skin wounds, and a total of 576 single cells were sorted, combining integrin expression and GFP tracing. FACS single-cell RNA-seq enabled high resolution in the classification of subtypes in both interfollicular epidermis and hair follicle, and both quiescent and intermediate cell states of the basal and infundibulum stem cell populations were distinguished. This study proposed a presorted method to investigate the relationship between integrin expression and epidermal heterogeneity. Multiple epidermal cell types and their expression profiles were identified, which provides data resources for dermatology research.

IRF2 is a master regulator of human keratinocyte stem cell fate

Resident adult epithelial stem cells maintain tissue homeostasis by balancing self-renewal and differentiation. The stem cell potential of human epidermal keratinocytes is retained in vitro but lost over time suggesting extrinsic and intrinsic regulation. Transcription factor-controlled regulatory circuitries govern cell identity, are sufficient to induce pluripotency and transdifferentiate cells. We investigate whether transcriptional circuitry also governs phenotypic changes within a given cell type by comparing human primary keratinocytes with intrinsically high versus low stem cell potential. Using integrated chromatin and transcriptional profiling, we implicate IRF2 as antagonistic to stemness and show that it binds and regulates active cis-regulatory elements at interferon response and antigen presentation genes. CRISPR-KD of IRF2 in keratinocytes with low stem cell potential increases self-renewal, migration and epidermis formation. These data demonstrate that transcription factor regulatory circuitries, in addition to maintaining cell identity, control plasticity within cell types and offer potential for therapeutic modulation of cell function.

Epidermal homeostasis requires long term stem cell function. Here, the authors apply transcriptional circuitry analysis based on integrated epigenomic profiling of primary human keratinocytes with high and low stem cell function to identify IRF2 as a negative regulator of stemness.

Peptide-Functionalized Hydrogels Modulate Integrin Expression and Stemness in Adult Human Epidermal Keratinocytes

The extracellular matrix (ECM) controls keratinocyte proliferation, migration, and differentiation through β-integrin signaling. Wound-healing research requires expanding cells in vitro while maintaining replicative capacity; however, early terminal differentiation under traditional culture conditions limits expansion. Here, a design of experiments approach identifies poly(ethylene glycol)-based hydrogel formulations with mechanical properties (elastic modulus, E = 20.9 ± 0.56 kPa) and bioactive peptide sequences that mimic the epidermal ECM. These hydrogels enable systematic investigation of the influence of cell-binding domains from fibronectin (RGDS), laminin (YIGSR), and collagen IV (HepIII) on keratinocyte stemness and β1 integrin expression. Quantification of 14-day keratin protein expression shows four hydrogels improve stemness compared to standard techniques. Three hydrogels increase β1 integrin expression, demonstrating a positive linear relationship between stemness and β1 integrin expression. Multifactorial statistical analysis predicts an optimal peptide combination ([RGDS] = 0.67 mm, [YIGSR] = 0.13 mm, and [HepIII] = 0.02 mm) for maintaining stemness in vitro. Best-performing hydrogels exhibit no decrease in Ki-67-positive cells compared to standards (15% decrease, day 7 to 14; p < 0.05, Tukey Test). These data demonstrate that precisely designed hydrogel biomaterials direct integrin expression and promote proliferation, improving the regenerative capability of cultured keratinocytes for basic science and translational work.

Developmental plasticity of epithelial stem cells in tooth and taste bud renewal

Nearly one-third of adults over the age of 65 have lost all their teeth. We set out to understand tooth renewal in animals that have replacement and regeneration capabilities. Using cichlid fishes and mouse models, we discovered plasticity between tooth and taste bud progenitor cell derivatives, mediated by BMP. Our results suggest that oral organs have surprising regenerative capabilities and can be manipulated to express characteristics of different tissue types.

In Lake Malawi cichlids, each tooth is replaced in one-for-one fashion every ∼20 to 50 d, and taste buds (TBs) are continuously renewed as in mammals. These structures are colocalized in the fish mouth and throat, from the point of initiation through adulthood. Here, we found that replacement teeth (RT) share a continuous band of epithelium with adjacent TBs and that both organs coexpress stem cell factors in subsets of label-retaining cells. We used RNA-seq to characterize transcriptomes of RT germs and TB-bearing oral epithelium. Analysis revealed differential usage of developmental pathways in RT compared to TB oral epithelia, as well as a repertoire of genome paralogues expressed complimentarily in each organ. Notably, BMP ligands were expressed in RT but excluded from TBs. Morphant fishes bathed in a BMP chemical antagonist exhibited RT with abrogated shh expression in the inner dental epithelium (IDE) and ectopic expression of calb2 (a TB marker) in these very cells. In the mouse, teeth are located on the jaw margin while TBs and other oral papillae are located on the tongue. Previous study reported that tongue intermolar eminence (IE) oral papillae of Follistatin (a BMP antagonist) mouse mutants exhibited dysmorphic invagination. We used these mutants to demonstrate altered transcriptomes and ectopic expression of dental markers in tongue IE. Our results suggest that vertebrate oral epithelium retains inherent plasticity to form tooth and taste-like cell types, mediated by BMP specification of progenitor cells. These findings indicate underappreciated epithelial cell populations with promising potential in bioengineering and dental therapeutics.