Organ-level quorum sensing directs regeneration in hair stem cell populations

Cellular Dialogues: Cell-Cell Communication through Diffusible Molecules Yields Dynamic Spatial Patterns

Cells form spatial patterns by coordinating their gene expressions. How a group of mesoscopic numbers (hundreds to thousands) of cells, without pre-existing morphogen gradients and spatial organization, self-organizes spatial patterns remains poorly understood. Of particular importance are dynamic spatial patterns such as spiral waves that perpetually move and transmit information. We developed an open-source software for simulating a field of cells that communicate by secreting any number of molecules. With this software and a theory, we identified all possible "cellular dialogues"-ways of communicating with two diffusing molecules-that yield diverse dynamic spatial patterns. These patterns emerge despite widely varying responses of cells to the molecules, gene-expression noise, spatial arrangements, and cell movements. A three-stage, "order-fluctuate-settle" process forms dynamic spatial patterns: cells form long-lived whirlpools of wavelets that, following erratic dynamics, settle into a dynamic spatial pattern. Our work helps in identifying gene-regulatory networks that underlie dynamic pattern formations.

Modeling cell-cell communication for immune systems across space and time

Communicating is crucial for cells to coordinate their behaviors. Immunological processes, involving diverse cytokines and cell types, are ideal for developing frameworks for modeling coordinated behaviors of cells. Here, we review recent studies that combine modeling and experiments to reveal how immune systems use autocrine, paracrine, and juxtacrine signals to achieve behaviors such as controlling population densities and hair regenerations. We explain that models are useful because one can computationally vary numerous parameters, in experimentally infeasible ways, to evaluate alternate immunological responses. For each model, we focus on the length-scales and time-scales involved and explain why integrating multiple length-scales and time-scales in a model remain challenging. We suggest promising modeling strategies for meeting this challenge and their practical consequences.

Pten loss in Lgr5+ hair follicle stem cells promotes SCC development

Accumulating data support that tissue stem cells give rise to cancer cells. Hair follicle stem cells (HFSCs) undergo cyclic quiescence and activation and may sever as the origin of cutaneous squamous cell carcinoma (SCC). Pten is a tumor suppressor gene that is frequently mutated in hereditary cancer syndromes such as Cowden disease, which is featured with papillomatosis in cutaneous tissues and hyperkeratosis in the acral region of the skin. Additionally, mice with keratinocyte-specific Pten deficiency (k5-Pten^-/- mice) show epidermal hyperplasia and spontaneous tumor formation. However, the impact of Pten mutation in HFSCs, such as in Lgr5⁺ HFSCs, on SCC formation is unclear.

Methods: We established experiments with wildtype and Lgr5-CreER; Pten^flox/flox mice, and used DMBA/TPA two-stage skin carcinogenesis model to explore the effect of Pten loss in Lgr5⁺ HFSCs of 3 weeks old mice in skin carcinogenesis. In vitro experiments (cell culture and protein expression analysis) are employed to investigate molecular mechanisms involved.

Results: Pten loss in Lgr5⁺ HFSCs promoted SCC formation, which was attenuated in TNF^-/- mice. Notably, β-catenin loss in Lgr5⁺ HFSCs decreased the formation of SCC. In addition, Pten loss in cultured epidermal stem cells upregulated the levels of both phospho-Akt and β-catenin.

Conclusion: Pten loss in Lgr5⁺ cells induced Akt/β-catenin signaling, and SCCs can subsequently be raised as progeny from these primed Lgr5⁺ stem cells.

Lymphatic vessels interact dynamically with the hair follicle stem cell niche during skin regeneration in vivo

Lymphatic vessels are essential for skin fluid homeostasis and immune cell trafficking. Whether the lymphatic vasculature is associated with hair follicle regeneration is, however, unknown. Here, using steady and live imaging approaches in mouse skin, we show that lymphatic vessels distribute to the anterior permanent region of individual hair follicles, starting from development through all cycle stages and interconnecting neighboring follicles at the bulge level, in a stem cell-dependent manner. Lymphatic vessels further connect hair follicles in triads and dynamically flow across the skin. At the onset of the physiological stem cell activation, or upon pharmacological or genetic induction of hair follicle growth, lymphatic vessels transiently expand their caliber suggesting an increased tissue drainage capacity. Interestingly, the physiological caliber increase is associated with a distinct gene expression correlated with lymphatic vessel reorganization. Using mouse genetics, we show that lymphatic vessel depletion blocks hair follicle growth. Our findings point toward the lymphatic vasculature being important for hair follicle development, cycling, and organization, and define lymphatic vessels as stem cell niche components, coordinating connections at tissue-level, thus provide insight into their functional contribution to skin regeneration.

Stem cells in tissues, organoids, and cancers

Stem cells give rise to all cells and build the tissue structures in our body, and heterogeneity and plasticity are the hallmarks of stem cells. Epigenetic modification, which is associated with niche signals, determines stem cell differentiation and somatic cell reprogramming. Stem cells play a critical role in the development of tumors and are capable of generating 3D organoids. Understanding the properties of stem cells will improve our capacity to maintain tissue homeostasis. Dissecting epigenetic regulation could be helpful for achieving efficient cell reprograming and for developing new drugs for cancer treatment. Stem cell-derived organoids open up new avenues for modeling human diseases and for regenerative medicine. Nevertheless, in addition to the achievements in stem cell research, many challenges still need to be overcome for stem cells to have versatile application in clinics.

Quorum Sensing by Monocyte-Derived Populations

Quorum sensing is a type of cellular communication that was first described in bacteria, consisting of gene expression regulation in response to changes in cell-population density. Bacteria synthesize and secrete diffusive molecules called autoinducers, which concentration varies accordingly with cell density and can be detected by the producing cells themselves. Once autoinducer concentration reaches a critical threshold, all bacteria within the autoinducer-rich environment react by modifying their genetic expression and adopt a coordinated behavior (e.g., biofilm formation, virulence factor expression, or swarming motility). Recent advances highlight the possibility that such type of communication is not restricted to bacteria, but can exist among other cell types, including immune cells and more specifically monocyte-derived cells (1). For such cells, quorum sensing mechanisms may not only regulate their population size and synchronize their behavior at homeostasis but also alter their activity and function in unexpected ways during immune reactions. Although the nature of immune autoinducers and cellular mechanisms remains to be fully characterized, quorum sensing mechanisms in the immune system challenge our traditional conception of immune cell interactions and likely represent an important mode of communication at homeostasis or during an immune response. In this mini-review, we briefly present the prototypic features of quorum sensing in bacteria and discuss the existing evidence for quorum sensing within the immune system. Mainly, we review quorum sensing mechanisms among monocyte-derived cells, such as the regulation of inflammation by the density of monocyte-derived cells that produce nitric oxide and discuss the relevance of such models in the context of immune-related pathologies.

Inflammatory Cytokine TNFα Promotes the Long-Term Expansion of Primary Hepatocytes in 3D Culture

In the healthy adult liver, most hepatocytes proliferate minimally. However, upon physical or chemical injury to the liver, hepatocytes proliferate extensively in vivo under the direction of multiple extracellular cues, including Wnt and pro-inflammatory signals. Currently, liver organoids can be generated readily in vitro from bile-duct epithelial cells, but not hepatocytes. Here, we show that TNFα, an injury-induced inflammatory cytokine, promotes the expansion of hepatocytes in 3D culture and enables serial passaging and long-term culture for more than 6 months. Single-cell RNA sequencing reveals broad expression of hepatocyte markers. Strikingly, in vitro-expanded hepatocytes engrafted, and significantly repopulated, the injured livers of Fah^-/- mice. We anticipate that tissue repair signals can be harnessed to promote the expansion of otherwise hard-to-culture cell-types, with broad implications.

Choreographing Immunity in the Skin Epithelial Barrier

The skin interfaces with the external environment and is home to a myriad of immune cells that patrol the barrier to ward off harmful agents and aid in tissue repair. The formation of the cutaneous immune arsenal begins before birth and evolves throughout our lifetime, incorporating exogenous cues from microbes and inflammatory encounters, to achieve optimal fitness and function. Here, we discuss the context-specific signals that drive productive immune responses in the skin epithelium, highlighting key modulators of these reactions, including hair follicles, neurons, and commensal microbes. We thus also discuss the causal and mechanistic underpinning of inflammatory skin diseases that have been revealed in recent years. Finally, we discuss the non-canonical functions of cutaneous immune cells including their burgeoning role in epithelial regeneration and repair. The rapidly growing field of cutaneous immunity is revealing immune mechanisms and functions that can be harnessed to boost skin health and treat disease.

The origins and non-canonical functions of macrophages in development and regeneration

The discovery of new non-canonical (i.e. non-innate immune) functions of macrophages has been a recurring theme over the past 20 years. Indeed, it has emerged that macrophages can influence the development, homeostasis, maintenance and regeneration of many tissues and organs, including skeletal muscle, cardiac muscle, the brain and the liver, in part by acting directly on tissue-resident stem cells. In addition, macrophages play crucial roles in diseases such as obesity-associated diabetes or cancers. Increased knowledge of their regulatory roles within each tissue will therefore help us to better understand the full extent of their functions and could highlight new mechanisms modulating disease pathogenesis. In this Review, we discuss recent studies that have elucidated the developmental origins of various macrophage populations and summarize our knowledge of the non-canonical functions of macrophages in development, regeneration and tissue repair.

Homeostasis, regeneration and tumour formation in the mammalian epidermis

The epidermis is the outer covering of the skin and provides a protective interface between the body and the environment. It is well established that the epidermis is maintained by stem cells that self-renew and generate differentiated cells. In this review, we discuss how recent technological advances, including single cell transcriptomics and in vivo imaging, have provided new insights into the nature and plasticity of the stem cell compartment and the differing roles of stem cells in homeostasis, wound repair and cancer.

Mechanical stretch induces hair regeneration through the alternative activation of macrophages

Tissues and cells in organism are continuously exposed to complex mechanical cues from the environment. Mechanical stimulations affect cell proliferation, differentiation, and migration, as well as determining tissue homeostasis and repair. By using a specially designed skin-stretching device, we discover that hair stem cells proliferate in response to stretch and hair regeneration occurs only when applying proper strain for an appropriate duration. A counterbalance between WNT and BMP-2 and the subsequent two-step mechanism are identified through molecular and genetic analyses. Macrophages are first recruited by chemokines produced by stretch and polarized to M2 phenotype. Growth factors such as HGF and IGF-1, released by M2 macrophages, then activate stem cells and facilitate hair regeneration. A hierarchical control system is revealed, from mechanical and chemical signals to cell behaviors and tissue responses, elucidating avenues of regenerative medicine and disease control by demonstrating the potential to manipulate cellular processes through simple mechanical stimulation.

Comparative regenerative biology of spiny (Acomys cahirinus) and laboratory (Mus musculus) mouse skin

Wound-induced hair follicle neogenesis (WIHN) has been demonstrated in laboratory mice (Mus musculus) after large (>1.5 × 1.5 cm2 ) full-thickness wounds. WIHN occurs more robustly in African spiny mice (Acomys cahirinus), which undergo autotomy to escape predation. Yet, the non-WIHN regenerative ability of the spiny mouse skin has not been explored. To understand the regenerative ability of the spiny mouse, we characterized skin features such as hair types, hair cycling, and the response to small and large wounds. We found that spiny mouse skin contains a large portion of adipose tissue. The spiny mouse hair bulge is larger and shows high expression of stem cell markers, K15 and CD34. All hair types cycle synchronously. To our surprise, the hair cycle is longer and less frequent than in laboratory mice. Newborn hair follicles in anagen are more mature than C57Bl/6 and demonstrate molecular features similar to C57Bl/6 adult hairs. The second hair cycling wave begins at week 4 and lasts for 5 weeks, then telogen lasts for 30 weeks. The third wave has a 6-week anagen, and even longer telogen. After plucking, spiny mouse hairs regenerate in about 5 days, similar to that of C57Bl/6. After large full-thickness excisional wounding, there is more de novo hair formation than C57Bl/6. Also, all hair types are present and pigmented, in contrast to the unpigmented zigzag hairs in C57Bl/6 WIHN. These findings shed new light on the regenerative biology of WIHN and may help us understand the control of skin repair vs regeneration.

Homeostatic Control of Sebaceous Glands by Innate Lymphoid Cells Regulates Commensal Bacteria Equilibrium

Immune cells and epithelium form sophisticated barrier systems in symbiotic relationships with microbiota. Evidence suggests that immune cells can sense microbes through intact barriers, but regulation of microbial commensalism remain largely unexplored. Here, we uncovered spatial compartmentalization of skin-resident innate lymphoid cells (ILCs) and modulation of sebaceous glands by a subset of RORγt⁺ ILCs residing within hair follicles in close proximity to sebaceous glands. Their persistence in skin required IL-7 and thymic stromal lymphopoietin, and localization was dependent on the chemokine receptor CCR6. ILC subsets expressed TNF receptor ligands, which limited sebocyte growth by repressing Notch signaling pathway. Consequently, loss of ILCs resulted in sebaceous hyperplasia with increased production of antimicrobial lipids and restricted commensalism of Gram-positive bacterial communities. Thus, epithelia-derived signals maintain skin-resident ILCs that regulate microbial commensalism through sebaceous gland-mediated tuning of the barrier surface, highlighting an immune-epithelia circuitry that facilitates host-microbe symbiosis.

Rethinking Communication in the Immune System: The Quorum Sensing Concept

Quorum sensing was first described as the communication process bacteria employ to coordinate changes in gene expression and therefore, their collective behavior in response to population density. Emerging new evidence suggests that quorum sensing can also contribute to the regulation of immune cell responses. Quorum sensing might be achieved by the ability of immune cells to perceive the density of their own populations or those of other cells in their environment; responses to alterations in cell density might then be coordinated via changes in gene expression and protein signaling. Quorum sensing mechanisms can regulate T and B cell as well as macrophage function. We posit that perturbations in quorum sensing may undermine the balance between diverse immune cell populations and predispose the host to immune abnormalities.

Stem cells repurpose proliferation to contain a breach in their niche barrier

Adult stem cells are responsible for life-long tissue maintenance. They reside in and interact with specialized tissue microenvironments (niches). Using murine hair follicle as a model, we show that when junctional perturbations in the niche disrupt barrier function, adjacent stem cells dramatically change their transcriptome independent of bacterial invasion and become capable of directly signaling to and recruiting immune cells. Additionally, these stem cells elevate cell cycle transcripts which reduce their quiescence threshold, enabling them to selectively proliferate within this microenvironment of immune distress cues. However, rather than mobilizing to fuel new tissue regeneration, these ectopically proliferative stem cells remain within their niche to contain the breach. Together, our findings expose a potential communication relay system that operates from the niche to the stem cells to the immune system and back. The repurposing of proliferation by these stem cells patch the breached barrier, stoke the immune response and restore niche integrity.

Two to Tango: Dialog between Immunity and Stem Cells in Health and Disease

Stem cells regenerate tissues in homeostasis and under stress. By taking cues from their microenvironment or "niche," they smoothly transition between these states. Immune cells have surfaced as prominent members of stem cell niches across the body. Here, we draw parallels between different stem cell niches to explore the context-specific interactions that stem cells have with tissue-resident and recruited immune cells. We also highlight stem cells' innate ability to sense and respond to stress and the enduring memory that forms from such encounters. This fascinating crosstalk holds great promise for novel therapies in inflammatory diseases and regenerative medicine.

Hair follicle epidermal stem cells define a niche for tactile sensation

The heterogeneity and compartmentalization of stem cells is a common principle in many epithelia, and is known to function in epithelial maintenance, but its other physiological roles remain elusive. Here we show transcriptional and anatomical contributions of compartmentalized epidermal stem cells in tactile sensory unit formation in the mouse hair follicle. Epidermal stem cells in the follicle upper-bulge, where mechanosensory lanceolate complexes innervate, express a unique set of extracellular matrix (ECM) and neurogenesis-related genes. These epidermal stem cells deposit an ECM protein called EGFL6 into the collar matrix, a novel ECM that tightly ensheathes lanceolate complexes. EGFL6 is required for the proper patterning, touch responses, and αv integrin-enrichment of lanceolate complexes. By maintaining a quiescent original epidermal stem cell niche, the old bulge, epidermal stem cells provide anatomically stable follicle-lanceolate complex interfaces, irrespective of the stage of follicle regeneration cycle. Thus, compartmentalized epidermal stem cells provide a niche linking the hair follicle and the nervous system throughout the hair cycle.

The Senescence-Stemness Alliance - A Cancer-Hijacked Regeneration Principle

Activated oncogenes or anticancer therapies evoke senescent cell-cycle arrest in (pre-)malignant cells, thereby interrupting tumor formation or progression. Physiologically, cellular senescence contributes to embryonic development and tissue regeneration. These observations and the overlap of numerous gene products in senescence and stem cell signaling prompted investigations into whether epigenetic establishment of the senescent state may concomitantly reprogram the cell into a latent stem-like condition, whose functional impact becomes evident when arrested cells resume proliferation. We review here recent discoveries underscoring the unexpected senescence-stemness alliance, elucidate underlying molecular mechanisms, and discuss its fundamentally different implications in normal tissue repair - to replenish the exhausted repopulation capacity - as compared to cancer biology, where usurpation of this natural principle accounts for particularly aggressive tumor behavior.

Quorum sensing in the immune system

Quorum sensing is the regulation of gene expression programmes in response to changes in population density. It is probably best recognized as a mechanism through which bacterial communities can synchronize behaviours, such as biofilm formation and bioluminescence. This Comment article highlights the emerging evidence suggesting that quorum sensing also contributes to the regulation of immune cell responses.

Advancing insights into stem cell niche complexities with next-generation technologies

Adult tissue-specific stem cells are essential for homeostatic tissue maintenance and key to regeneration during injury repair or disease. Many critical stem cell functions rely on the presence of well-timed cues from the microenvironment or niche, which includes a diverse range of components, including neuronal, circulating and extracellular matrix inputs as well as an array of neighboring niche cells directly interacting with the stem cells. However, studies of stem cells and their niche have been challenging due to the complexity of adult stem cell functions, their intrinsic controls and the multiple regulatory niche components. Here, we review recent major advances in our understanding of the complex interplay between stem cells and their niche that were enabled by the tremendous technological leaps in single-cell transcriptome analyses, 3D in vitro cultures and 4D in vivo microscopy of stem cell niches.

External light activates hair follicle stem cells through eyes via an ipRGC-SCN-sympathetic neural pathway

Changes in external light patterns can alter cell activities in peripheral tissues through slow entrainment of the central clock in suprachiasmatic nucleus (SCN). It remains unclear whether cells in otherwise photo-insensitive tissues can achieve rapid responses to changes in external light. Here we show that light stimulation of animals' eyes results in rapid activation of hair follicle stem cells with prominent hair regeneration. Mechanistically, light signals are interpreted by M1-type intrinsically photosensitive retinal ganglion cells (ipRGCs), which signal to the SCN via melanopsin. Subsequently, efferent sympathetic nerves are immediately activated. Increased norepinephrine release in skin promotes hedgehog signaling to activate hair follicle stem cells. Thus, external light can directly regulate tissue stem cells via an ipRGC-SCN autonomic nervous system circuit. Since activation of sympathetic nerves is not limited to skin, this circuit can also facilitate rapid adaptive responses to external light in other homeostatic tissues.

Mesenchymal Stem Cell-Macrophage Choreography Supporting Spinal Cord Repair

Spinal cord injury results in destructive events that lead to tissue loss and functional impairments. A hallmark of spinal cord injury is the robust and persistent presence of inflammatory macrophages. Mesenchymal stem cells (MSCs) are known to benefit repair of the damaged spinal cord often associated with improved functional recovery. Transplanted MSCs immediately encounter the abundance of inflammatory macrophages in the injury site. It is known that MSCs interact closely and reciprocally with macrophages during tissue healing. Here, we will review the roles of (transplanted) MSCs and macrophages in spinal cord injury and repair. Molecular interactions between MSCs and macrophages and the deficiencies in our knowledge about the underlying mechanisms will be reviewed. We will discuss possible ways to benefit from the MSC-macrophage choreography for developing repair strategies for the spinal cord.

Statistical Dynamics of Spatial-Order Formation by Communicating Cells

Communicating cells can coordinate their gene expressions to form spatial patterns, generating order from disorder. Ubiquitous "secrete-and-sense cells" secrete and sense the same molecule to do so. Here we present a modeling framework-based on cellular automata and mimicking approaches of statistical mechanics-for understanding how secrete-and-sense cells with bistable gene expression, from disordered beginnings, can become spatially ordered by communicating through rapidly diffusing molecules. Classifying lattices of cells by two "macrostate" variables-"spatial index," measuring degree of order, and average gene-expression level-reveals a conceptual picture: a group of cells behaves as a single particle, in an abstract space, that rolls down on an adhesive "pseudo-energy landscape" whose shape is determined by cell-cell communication and an intracellular gene-regulatory circuit. Particles rolling down the landscape represent cells becoming more spatially ordered. We show how to extend this framework to more complex forms of cellular communication.

Macrophages Promote Wound-Induced Hair Follicle Regeneration in a CX3CR1- and TGF-β1-Dependent Manner

Hair follicle stem cells are regulated by intrafollicular and extrafollicular niche signals. Appropriate hair follicle regeneration relies on the coordinated release and integration of these signals. How immune cells, particularly cutaneous macrophages, influence the hair follicle stem cell niche and regeneration is not well understood. We took advantage of wound-induced hair growth (WIHG) to explore the relationship between wound macrophages and hair follicle regeneration. First, we showed that WIHG is dependent on CD11b⁺F4/80⁺ macrophages at 7-11 days after injury. Next, using CX₃CR1^gfp/+:CCR2^rfp/+ mice to capture the dynamic spectrum of macrophage phenotypes during wound healing, we showed that wound macrophages transition from a CX₃CR1^lo/med to a CX₃CR1^hi phenotype at the onset of WIHG. Finally, WIHG is abolished in mice deficient for CX₃CR1, delayed with pharmacological inhibition of transforming growth factor-β receptor type 1, and rescued with exogenous transforming growth factor-β1. Overall, we propose a model in which transforming growth factor-β1 and CX₃CR1 are critical for recruiting and maintaining the CCR2⁺CX₃CR1^hiLy6C^loTNFα⁺ macrophages critical for stimulating WIHG.

Regulation of melanocyte stem cell behavior by the niche microenvironment

Somatic stem cells are regulated by their niches to maintain tissue homeostasis and repair throughout the lifetime of an organism. An excellent example to study stem cell/niche interactions is provided by the regeneration of melanocytes during the hair cycle and in response to various types of injury. These processes are regulated by neighboring stem cells and multiple signaling pathways, including WNT/β-catenin, KITL/KIT, EDNs/EDNRB, TGF-β/TGF-βR, α-MSH/MC1R, and Notch signaling. In this review, we highlight recent studies that have advanced our understanding of the molecular crosstalk between melanocyte stem cells and their neighboring cells, which collectively form the niche microenvironment, and we focus on the question of how McSCs/niche interactions shape the responses to genotoxic damages and mechanical injury.

Lineage tracking of origin and fate of smooth muscle cells in atherosclerosis

Advances in lineage-tracking techniques have provided new insights into the origins and fates of smooth muscle cells (SMCs) in atherosclerosis. Yet new tools present new challenges for data interpretation that require careful consideration of the strengths and weaknesses of the methods employed. At the same time, discoveries in other fields have introduced new perspectives on longstanding questions about steps in atherogenesis that remain poorly understood. In this article, we address both the challenges and opportunities for a better understanding of the mechanisms by which cells appearing as or deriving from SMCs accumulate in atherosclerosis.

Endocytosis as a stabilizing mechanism for tissue homeostasis

Cells in tissues communicate by secreted growth factors (GF) and other signals. An important function of cell circuits is tissue homeostasis: maintaining proper balance between the amounts of different cell types. Homeostasis requires negative feedback on the GFs, to avoid a runaway situation in which cells stimulate each other and grow without control. Feedback can be obtained in at least two ways: endocytosis in which a cell removes its cognate GF by internalization and cross-inhibition in which a GF down-regulates the production of another GF. Here we ask whether there are design principles for cell circuits to achieve tissue homeostasis. We develop an analytically solvable framework for circuits with multiple cell types and find that feedback by endocytosis is far more robust to parameter variation and has faster responses than cross-inhibition. Endocytosis, which is found ubiquitously across tissues, can even provide homeostasis to three and four communicating cell types. These design principles form a conceptual basis for how tissues maintain a healthy balance of cell types and how balance may be disrupted in diseases such as degeneration and fibrosis.

Anatomical, Physiological, and Functional Diversity of Adipose Tissue

Adipose tissue depots can exist in close association with other organs, where they assume diverse, often non-traditional functions. In stem cell-rich skin, bone marrow, and mammary glands, adipocytes signal to and modulate organ regeneration and remodeling. Skin adipocytes and their progenitors signal to hair follicles, promoting epithelial stem cell quiescence and activation, respectively. Hair follicles signal back to adipocyte progenitors, inducing their expansion and regeneration, as in skin scars. In mammary glands and heart, adipocytes supply lipids to neighboring cells for nutritional and metabolic functions, respectively. Adipose depots adjacent to skeletal structures function to absorb mechanical shock. Adipose tissue near the surface of skin and intestine senses and responds to bacterial invasion, contributing to the body's innate immune barrier. As the recognition of diverse adipose depot functions increases, novel therapeutic approaches centered on tissue-specific adipocytes are likely to emerge for a range of cancers and regenerative, infectious, and autoimmune disorders.

Stem cell therapy on skin: Mechanisms, recent advances and drug reviewing issues

Stem cell products and its clinical applications have been widely discussed in recent years, particularly when the Japanese “induced pluripotent stem cells” founder Dr. Yamanaka was awarded as Nobel Prize laureate in 2013. For decades, major progresses have been achieved in the stem cell biology field, and more and more evidence showed that skin stem cells are involved in the process of skin repair. Stem/progenitor cells of the epidermis are recognized to play the most essential role in the tissue regeneration of skin. In this review, we first illustrated basic stem cell characteristics and various stem cell subtypes resided in the skin. Second, we provided several literatures to elucidate how stem/progenitor cells collaborate in the process of skin repair with the evidence from animal model studies and in vitro experiments. Third, we also introduced several examples of skin cell products on the pharmaceutic market and the ongoing clinical trials aiming for unmet medical difficulties of skin. Last but not least, we summarized general reviewing concerns and some disputatious issues on dermatological cell products. With this concise review, we hope to provide further beneficial suggestions for the development of more effective and safer dermatological stem/progenitor cell products in the future.

Mesenchymal Stem Cells Secretory Responses: Senescence Messaging Secretome and Immunomodulation Perspective

Mesenchymal stem/stromal cells (MSC) have been tested in a significant number of clinical trials, where they exhibit regenerative and repair properties directly through their differentiation into the cells of the mesenchymal origin or by modulation of the tissue/organ microenvironment. Despite various clinical effects upon transplantation, the functional properties of these cells in natural settings and their role in tissue regeneration in vivo is not yet fully understood. The omnipresence of MSC throughout vascularized organs equates to a reservoir of potentially therapeutic regenerative depots throughout the body. However, these reservoirs could be subjected to cellular senescence. In this review, we will discuss current progress and challenges in the understanding of different biological pathways leading to senescence. We set out to highlight the seemingly paradoxical property of cellular senescence: its beneficial role in the development and tissue repair and detrimental impact of this process on tissue homeostasis in aging and disease. Taking into account the lessons from the different cell systems, this review elucidates how autocrine and paracrine properties of senescent MSC might impose an additional layer of complexity on the regulation of the immune system in development and disease. New findings that have emerged in the last few years could shed light on sometimes seemingly controversial results obtained from MSC therapeutic applications.

Stimulation of hair follicle stem cell proliferation through an IL-1 dependent activation of γδT-cells

The cutaneous wound-healing program is a product of a complex interplay among diverse cell types within the skin. One fundamental process that is mediated by these reciprocal interactions is the mobilization of local stem cell pools to promote tissue regeneration and repair. Using the ablation of epidermal caspase-8 as a model of wound healing in Mus musculus, we analyzed the signaling components responsible for epithelial stem cell proliferation. We found that IL-1α and IL-7 secreted from keratinocytes work in tandem to expand the activated population of resident epidermal γδT-cells. A downstream effect of activated γδT-cells is the preferential proliferation of hair follicle stem cells. By contrast, IL-1α-dependent stimulation of dermal fibroblasts optimally stimulates epidermal stem cell proliferation. These findings provide new mechanistic insights into the regulation and function of epidermal cell–immune cell interactions and into how components that are classically associated with inflammation can differentially influence distinct stem cell niches within a tissue.

The skin is a physical barrier that protects the body from the outside world. If the skin is injured, the body mounts a “wound healing” response to rapidly mend and restore this protective barrier. Wound healing is a complex process and relies on the different types of cells in the skin communicating with each other.

Stem cells provide tissues, like the skin, with new cells. Normally, stem cells are in a resting or inactive state. Yet, during wound healing, stem cells near the injured area are awakened and start producing more cells to repair the wound. Understanding how stem cells become activated in a wound has proved challenging because only a small number of cells near a damaged site will respond, and it is difficult to distinguish their response from that of other cells slightly further away.

Now, Lee et al. overcome this hurdle by analyzing a genetically engineered mouse in which the entire skin displays a wound healing response, even without any injury or trauma. In these mice, most of the stem cells in the skin are awakened from their normal resting state and behave as if there is a wound to heal.

It turns out that a protein called interleukin-1, which is released from damaged skin cells known as keratinocytes, can activate two different groups of stem cells in the skin to help repair the injured tissue. One group lives in the hair follicle and is normally responsible for replacing the hair that falls from the body. Lee et al. found that when the skin is wounded interleukin-1 activates certain immune cells (called γδT-cells). These immune cells then awaken the resting stem cells in the hair follicle to multiply and travel to the wound site to repair the injury. The other group of stem cells resides in the outermost layer of the skin. Interleukin-1 can also activate so-called fibroblast cells, which then stimulate this second group of stem cells to divide and cover the open wound.

Quickly healing wounds has many health benefits such as preventing infection and shortening the time to recover from an injury. These new findings may help to repair injured skin in diseases such as diabetes, where wounds can take months to heal and often leads to permanent tissue damage. The next challenge is to identify the cues that instruct the stem cells to travel to the wound site and turn into the specific cells that are required to replace the damaged cells.

NF-κB Participates in Mouse Hair Cycle Control and Plays Distinct Roles in the Various Pelage Hair Follicle Types

The transcription factor NF-κB controls key features of hair follicle (HF) development, but the role of NF-κB in adult HF cycle regulation remains obscure. Using NF-κB reporter mouse models, strong NF-κB activity was detected in the secondary hair germ of late telogen and early anagen HFs, suggesting a potential role for NF-κB in HF stem/progenitor cell activation during anagen induction. At mid-anagen, NF-κB activity was observed in the inner root sheath and unilaterally clustered in the HF matrix, which indicates that NF-κB activity is also involved in hair fiber morphogenesis during HF cycling. A mouse model with inducible NF-κB suppression in the epithelium revealed pelage hair-type-dependent functions of NF-κB in cycling HFs. NF-κB participates in telogen-anagen transition in awl and zigzag HFs, and is required for zigzag hair bending and guard HF cycling. Interestingly, zigzag hair shaft bending depends on noncanonical NF-κB signaling, which previously has only been associated with lymphoid cell biology. Furthermore, loss of guard HF cycling suggests that in this particular hair type, NF-κB is indispensable for stem cell activation, maintenance, and/or growth.

Epilation induces hair and skin pigmentation through an EDN3/EDNRB-dependent regenerative response of melanocyte stem cells

In response to various types of injury, melanocyte stem cells (McSCs) located in the bulge of hair follicles can regenerate mature melanocytes for hair and skin pigmentation. How McSCs respond to injury, however, remains largely unknown. Here we show that after epilation of mice, McSCs regenerate follicular and epidermal melanocytes, resulting in skin and hair hyperpigmentation. We further show that epilation leads to endogenous EDN3 upregulation in the dermal papilla, the secondary hair germ cells, and the epidermis. Genetic and pharmacological disruption of the EDN3 receptor EDNRB in vivo significantly blocks the effect of epilation on follicular and epidermal melanocyte regeneration as well as skin and hair hyperpigmentation. Taken together, these results indicate that epilation induces McSCs activation through EDN3/EDNRB signaling and in turn leads to skin and hair hyperpigmentation. The findings suggest that EDN/EDNRB signaling may serve as a potential therapeutic target to promote repigmentation in hypopigmentation disorders.

Effects of gamma rays on the regeneration of murine hair follicles in the natural hair cycle

PURPOSE

This review evaluates the effects of γ-rays on the regeneration of murine hair follicles in the natural hair cycle. A series of studies were performed to investigate this issue, in which the whole bodies of C57BL/10JHir mice in the 1st telogen phase of the hair cycle were irradiated with γ-rays.

RESULTS

The dermis of the irradiated skin showed a decrease in hair follicle density and induction of curved hair follicles along with the presence of white hairs and hypopigmented hair bulbs in the 2nd and 3rd anagen phases. An increased frequency of hypopigmented hair bulbs was still observed in the later hair cycle at postnatal day 200. There was no significant difference in the number of stem cells in the hair bulge region between control and irradiated skin.

CONCLUSIONS

These results show that the effects of γ-rays on the pigmentation of murine hair follicles are persistently carried over to later hair cycles, although those on the number and structure of hair follicles appear to be hidden by the effects of aging. Our findings may be important for understanding the mechanisms of the actions of stem cells on hair regeneration in connection with age-related phenotypes.

Conceptual and Experimental Tools to Understand Spatial Effects and Transport Phenomena in Nonlinear Biochemical Networks Illustrated with Patchy Switching

Many biochemical systems are spatially heterogeneous and exhibit nonlinear behaviors, such as state switching in response to small changes in the local concentration of diffusible molecules. Systems as varied as blood clotting, intracellular calcium signaling, and tissue inflammation are all heavily influenced by the balance of rates of reaction and mass transport phenomena including flow and diffusion. Transport of signaling molecules is also affected by geometry and chemoselective confinement via matrix binding. In this review, we use a phenomenon referred to as patchy switching to illustrate the interplay of nonlinearities, transport phenomena, and spatial effects. Patchy switching describes a change in the state of a network when the local concentration of a diffusible molecule surpasses a critical threshold. Using patchy switching as an example, we describe conceptual tools from nonlinear dynamics and chemical engineering that make testable predictions and provide a unifying description of the myriad possible experimental observations. We describe experimental microfluidic and biochemical tools emerging to test conceptual predictions by controlling transport phenomena and spatial distribution of diffusible signals, and we highlight the unmet need for in vivo tools.

A Pilot Study on the Evaluation of Physicians' Laser Delivery Performance Using a Laser Beam Detection Kit

BACKGROUND AND OBJECTIVE

Many laser devices have been developed over the past decades for various skin conditions. However, variations in the technical skill of physicians for laser skin treatment delivery have not yet been evaluated. This study evaluates the differences in omission and overlap percentages during simulated laser hair removal treatments among physicians at two clinics.

MATERIALS AND METHODS

A laser beam detection kit was developed to record and collect laser irradiation from a diode laser device. Eight physicians (primary private clinic 4, tertiary referral hospital 4) were recruited to perform 80 trials of laser delivery simulation. The simulation process was captured in video frames by a camera built inside of the detection kit. The laser distribution map was reconstructed, and each physician's performance result was determined by a computer calculation.

RESULTS

Various assumption tests showed that each physician had different laser delivery skills. Four physicians from clinic A had an average omission rate of 13.4%, and four physicians from clinic B had an average omission rate of 19.7%. Regarding the average overlap rate of the two clinics, clinic A had a higher rate than clinic B (26.1% vs. 14.6%).

CONCLUSIONS

The study's findings confirmed the differences of the technical skills among the physicians and between the two clinics. The proposed computer-assisted evaluation of technical skill is useful for assessing physicians' performance during laser skin treatments.

Macrophages and fibroblasts during inflammation and tissue repair in models of organ regeneration

This review provides a concise summary of the changing phenotypes of macrophages and fibroblastic cells during the local inflammatory response, the onset of tissue repair, and the resolution of inflammation which follow injury to an organ. Both cell populations respond directly to damage and present coordinated sequences of activation states which determine the reparative outcome, ranging from true regeneration of the organ to fibrosis and variable functional deficits. Recent work with mammalian models of organ regeneration, including regeneration of full‐thickness skin, hair follicles, ear punch tissues, and digit tips, is summarized and the roles of local immune cells in these systems are discussed. New investigations of the early phase of amphibian limb and tail regeneration, including the effects of pro‐inflammatory and anti‐inflammatory agents, are then briefly discussed, focusing on the transition from the normally covert inflammatory response to the initiation of the regeneration blastema by migrating fibroblasts and the expression of genes for limb patterning.

Quorum sensing signal-response systems in Gram-negative bacteria

Bacteria use quorum sensing to orchestrate gene expression programmes that underlie collective behaviours. Quorum sensing relies on the production, release, detection and group-level response to extracellular signalling molecules, which are called autoinducers. Recent work has discovered new autoinducers in Gram-negative bacteria, shown how these molecules are recognized by cognate receptors, revealed new regulatory components that are embedded in canonical signalling circuits and identified novel regulatory network designs. In this Review we examine how, together, these features of quorum sensing signal-response systems combine to control collective behaviours in Gram-negative bacteria and we discuss the implications for host-microbial associations and antibacterial therapy.

Macrophages induce AKT/β-catenin-dependent Lgr5+ stem cell activation and hair follicle regeneration through TNF

Skin stem cells can regenerate epidermal appendages; however, hair follicles (HF) lost as a result of injury are barely regenerated. Here we show that macrophages in wounds activate HF stem cells, leading to telogen-anagen transition (TAT) around the wound and de novo HF regeneration, mostly through TNF signalling. Both TNF knockout and overexpression attenuate HF neogenesis in wounds, suggesting dose-dependent induction of HF neogenesis by TNF, which is consistent with TNF-induced AKT signalling in epidermal stem cells in vitro. TNF-induced β-catenin accumulation is dependent on AKT but not Wnt signalling. Inhibition of PI3K/AKT blocks depilation-induced HF TAT. Notably, Pten loss in Lgr5+ HF stem cells results in HF TAT independent of injury and promotes HF neogenesis after wounding. Thus, our results suggest that macrophage-TNF-induced AKT/β-catenin signalling in Lgr5+ HF stem cells has a crucial role in promoting HF cycling and neogenesis after wounding.

The senescence-associated secretory phenotype induces cellular plasticity and tissue regeneration

Senescence is a form of cell cycle arrest induced by stress such as DNA damage and oncogenes. However, while arrested, senescent cells secrete a variety of proteins collectively known as the senescence-associated secretory phenotype (SASP), which can reinforce the arrest and induce senescence in a paracrine manner. However, the SASP has also been shown to favor embryonic development, wound healing, and even tumor growth, suggesting more complex physiological roles than currently understood. Here we uncover timely new functions of the SASP in promoting a proregenerative response through the induction of cell plasticity and stemness. We show that primary mouse keratinocytes transiently exposed to the SASP exhibit increased expression of stem cell markers and regenerative capacity in vivo. However, prolonged exposure to the SASP causes a subsequent cell-intrinsic senescence arrest to counter the continued regenerative stimuli. Finally, by inducing senescence in single cells in vivo in the liver, we demonstrate that this activates tissue-specific expression of stem cell markers. Together, this work uncovers a primary and beneficial role for the SASP in promoting cell plasticity and tissue regeneration and introduces the concept that transient therapeutic delivery of senescent cells could be harnessed to drive tissue regeneration.

A multi-scale model for hair follicles reveals heterogeneous domains driving rapid spatiotemporal hair growth patterning

The control principles behind robust cyclic regeneration of hair follicles (HFs) remain unclear. Using multi-scale modeling, we show that coupling inhibitors and activators with physical growth of HFs is sufficient to drive periodicity and excitability of hair regeneration. Model simulations and experimental data reveal that mouse skin behaves as a heterogeneous regenerative field, composed of anatomical domains where HFs have distinct cycling dynamics. Interactions between fast-cycling chin and ventral HFs and slow-cycling dorsal HFs produce bilaterally symmetric patterns. Ear skin behaves as a hyper-refractory domain with HFs in extended rest phase. Such hyper-refractivity relates to high levels of BMP ligands and WNT antagonists, in part expressed by ear-specific cartilage and muscle. Hair growth stops at the boundaries with hyper-refractory ears and anatomically discontinuous eyelids, generating wave-breaking effects. We posit that similar mechanisms for coupled regeneration with dominant activator, hyper-refractory, and wave-breaker regions can operate in other actively renewing organs.

AFM study shows prominent physical changes in elasticity and pericellular layer in human acute leukemic cells due to inadequate cell-cell communication

Biomechanical properties of single cells in vitro or ex vivo and their pericellular interfaces have recently attracted a lot of attention as a potential biophysical (and possibly prognostic) marker of various diseases and cell abnormalities. At the same time, the influence of the cell environment on the biomechanical properties of cells is not well studied. Here we use atomic force microscopy to demonstrate that cell-cell communication can have a profound effect on both cell elasticity and its pericellular coat. A human pre-B p190^BCR/ABL acute lymphoblastic leukemia cell line (ALL3) was used in this study. Assuming that cell-cell communication is inversely proportional to the distance between cells, we study ALL3 cells in vitro growing at different cell densities. ALL3 cells demonstrate a clear density dependent behavior. These cells grow very well if started at a relatively high cell density (HD, >2 × 10⁵ cells ml^-1) and are poised to grow at low cell density (LD, <1 × 10⁴ cells ml^-1). Here we observe ∼6× increase in the elastic (Young's) modulus of the cell body and ∼3.6× decrease in the pericellular brush length of LD cells compared to HD ALL3 cells. The difference observed in the elastic modulus is much larger than typically reported for pathologically transformed cells. Thus, cell-cell communication must be taken into account when studying biomechanics of cells, in particular, correlating cell phenotype and its biophysical properties.

Gata6 promotes hair follicle progenitor cell renewal by genome maintenance during proliferation

Cell proliferation is essential to rapid tissue growth and repair, but can result in replication-associated genome damage. Here, we implicate the transcription factor Gata6 in adult mouse hair follicle regeneration where it controls the renewal of rapidly proliferating epithelial (matrix) progenitors and hence the extent of production of terminally differentiated lineages. We find that Gata6 protects against DNA damage associated with proliferation, thus preventing cell cycle arrest and apoptosis. Furthermore, we show that in vivo Gata6 stimulates EDA-receptor signaling adaptor Edaradd level and NF-κB pathway activation, known to be important for DNA damage repair and stress response in general and for hair follicle growth in particular. In cultured keratinocytes, Edaradd rescues DNA damage, cell survival, and proliferation of Gata6 knockout cells and restores MCM10 expression. Our data add to recent evidence in embryonic stem and neural progenitor cells, suggesting a model whereby developmentally regulated transcription factors protect from DNA damage associated with proliferation at key stages of rapid tissue growth. Our data may add to understanding why Gata6 is a frequent target of amplification in cancers.