Clonal analysis reveals nerve-dependent and independent roles on mammalian hind limb tissue maintenance and regeneration

Hair-follicle-associated pluripotent (HAP) stem-cell-sheet implantation accelerates cutaneous wound closure and suppresses scar formation in a mouse model

Patients frequently experience physical, mental, and even financial distress because of acute or chronic skin wounds. In severe situations, scarring on the skin can be quite noticeable, cause persistent discomfort, restrict joint motion, or be mentally taxing. Hair-follicle-associated pluripotent (HAP) stem cells were discovered by our laboratory, in the bulge area of the hair follicle and can differentiate to neurons, glia, beating cardiomyocytes, keratinocytes and nascent vessels. In the present study, HAP stem cell sheets were formed by culturing the upper part of hair follicles and implanting into mice with skin ulcers. The HAP stem cell sheets contained keratinocytes, endothelial cells and neurons. Autologous HAP stem cell sheet implantation to the dorsal wound in C57BL/6J mice significantly accelerated wound closure compared with non-implanted control mice. HAP-stem-cell sheets expressing green fluorescent protein (GFP) implanted into nude mice differentiated into keratinocytes in the epidermis, and neurons and endothelial cells in the dermis. The thicknesses of the epidermis and dermis and M2 macrophage and myofibroblast infiltration into the wound were significantly decreased in HAP-stem cell-implanted mice compared with non-implanted control mice. Expression levels of TGF-β1, COL1A2 and COL3A1 mRNA in the wound were significantly decreased in HAP stem cell-implanted mice compared with non-implanted control mice. These results suggest that implanting HAP stem cell sheets accelerates cutaneous wound closure and suppresses scar formation. The HAP stem cells used in the present study thus have potential as a future clinical strategy for accelerating wound healing.

Simulation of adult limb regeneration with lizard tail spinal cord implants reveals distinct roles of radial glia and microglia populations

Lizards are the closest relatives of humans able to suppress fibrosis and regrow multiple tissue lineages following appendage regeneration. As amniotes capable of tail, but not limb regrowth, lizards are also distinguished as the only vertebrate group that include both regenerative and non-regenerative appendages in the same animal. Lizard tail stumps naturally form blastemas - heterogenous collections of fibroblasts, adult stem cells, and immune cells that suppress scar formation and potentiate new tissue growth. Conversely, amputated lizard limbs form scars similar to those observed in human patients. Lizard blastema formation is dependent upon tail spinal cord tissue, which contains distinct populations of radial glia and microglia. Using the parthenogenetic lizard Lepidodactylus lugubris as a platform for tail-to-limb spinal cord implantations, we developed an ectopic blastema model toward defining the roles of radial glial and microglia in appendage regeneration. Removal of either population inhibits fibroblast proliferation and blastema formation, but only microglia depletion leads to enhanced fibrosis. Similarly, effects of radial glia, but not microglia, depletion on fibroblast proliferation are reversed via Hedgehog agonism. Taken together, these results indicate that lizard limbs contain all the necessary cell types and biological responses necessary for blastema formation but lack the proliferative and anti-fibrotic signals provided by tail spinal cord radial glia and microglia, respectively. Radial glia contribute Hedgehog signals that cause fibroblast proliferation but do not affect fibrosis. Conversely, microglia enhance fibroblast sensitivity to Hedgehog signaling and inhibit differentiation into fibrocytes. In summary, this study demonstrates blastema stimulation in amputated limbs of adult amniotes with application of lizard spinal cord cells and holds promise as a blueprint for limiting painful scarring and supporting new tissue growth following amputation injuries in human patients.

Schwann cells in regeneration and cancer

Schwann cells are specific peripheral glial cells with remarkable plasticity following peripheral nerve injury. Injury responses stimulate c-Jun activation in Schwann cells, drive epithelial-mesenchymal transition and cellular phenotypic changes, and induce the generation of reprogrammed repair Schwann cells to orchestrate peripheral nerve regeneration process. Schwann cells and/or Schwann cell-derived molecules are commonly used as supporting cells and/or neurotrophic factors to construct Schwann cell-based tissue-engineered nerve grafts for repairing severe peripheral nerve injury with long defects. Transplantation of Schwann cells and/or Schwann cell-derived molecules also serves as a helpful approach for the treatment of other injured tissues, such as the spinal cord, skin, digit tip, and bone. Schwann cells are not only associated with tissue regeneration but also involved in tumorigenesis and tumor progression. Schwann cells are the major cellular component of neurofibromatosis type 1 and the sole cell type in neurofibromatosis type 2 and schwannomatosis. In addition, Schwann cells also function as an important player in the tumor microenvironment and aid in the growth and invasiveness of many other solid cancers. In the present review, we outline the physiological and pathological activities of Schwann cells and discuss the functional roles of Schwann cells in homeostasis, regeneration, and cancer.

Innervated Coculture Device to Model Peripheral Nerve-Mediated Fibroblast Activation

Cutaneous wound healing is a complex process involving various cellular and molecular interactions, resulting in the formation of a collagen-rich scar with imperfect function and morphology. Dermal fibroblasts are crucial to successful wound healing, migrating to the wound site where they are activated to provide extracellular matrix remodeling and wound closure. Peripheral nerves have been shown to play an important role in wound healing, with loss or damage to these nerves often leading to impaired healing and the formation of chronic nonhealing wounds. Previous research has suggested that sensory nerves secrete trophic factors that can regulate wound healing, including fibroblast activation; however, the direct cell-cell interaction between nerves and fibroblasts has not been extensively studied. To address this knowledge gap, we developed an in vitro co-culture model using a device called the IFlowPlate. This model supports the long-term viability of multiple cell types while allowing for direct contact between sensory nerve cells and dermal fibroblasts. Using the IFlowPlate, we demonstrate that co-culture of dorsal root ganglia with dermal fibroblasts increases fibroblast proliferation, collagen and α-smooth muscle actin expression, and secretion of pro-wound healing factors, suggesting that nerves can promote wound healing by modulating fibroblast activation. The IFlowPlate offers a user-friendly and high-throughput platform to study the in vitro interactions between nerves and a variety of cell types that can be applied to wound healing and other important biological processes.

CGRP sensory neurons promote tissue healing via neutrophils and macrophages

The immune system has a critical role in orchestrating tissue healing. As a result, regenerative strategies that control immune components have proved effective1,2. This is particularly relevant when immune dysregulation that results from conditions such as diabetes or advanced age impairs tissue healing following injury2,3. Nociceptive sensory neurons have a crucial role as immunoregulators and exert both protective and harmful effects depending on the context4-12. However, how neuro-immune interactions affect tissue repair and regeneration following acute injury is unclear. Here we show that ablation of the NaV1.8 nociceptor impairs skin wound repair and muscle regeneration after acute tissue injury. Nociceptor endings grow into injured skin and muscle tissues and signal to immune cells through the neuropeptide calcitonin gene-related peptide (CGRP) during the healing process. CGRP acts via receptor activity-modifying protein 1 (RAMP1) on neutrophils, monocytes and macrophages to inhibit recruitment, accelerate death, enhance efferocytosis and polarize macrophages towards a pro-repair phenotype. The effects of CGRP on neutrophils and macrophages are mediated via thrombospondin-1 release and its subsequent autocrine and/or paracrine effects. In mice without nociceptors and diabetic mice with peripheral neuropathies, delivery of an engineered version of CGRP accelerated wound healing and promoted muscle regeneration. Harnessing neuro-immune interactions has potential to treat non-healing tissues in which dysregulated neuro-immune interactions impair tissue healing.

Neural dependency in wound healing and regeneration

In response to injury, humans and many other mammals form a fibrous scar that lacks the structure and function of the original tissue, whereas other vertebrate species can spontaneously regenerate damaged tissues and structures. Peripheral nerves have been identified as essential mediators of wound healing and regeneration in both mammalian and nonmammalian systems, interacting with the milieu of cells and biochemical signals present in the post-injury microenvironment. This review examines the diverse functions of peripheral nerves in tissue repair and regeneration, specifically during the processes of wound healing, blastema formation, and organ repair. We compare available evidence in mammalian and nonmammalian models, identifying critical nerve-mediated mechanisms for regeneration and providing future perspectives toward integrating these mechanisms into a therapeutic framework to promote regeneration.

A Novel Perspective on Neuronal Control of Anatomical Patterning, Remodeling, and Maintenance

While the nervous system may be best known as the sensory communication center of an organism, recent research has revealed a myriad of multifaceted roles for both the CNS and PNS from early development to adult regeneration and remodeling. These systems work to orchestrate tissue pattern formation during embryonic development and continue shaping pattering through transitional periods such as metamorphosis and growth. During periods of injury or wounding, the nervous system has also been shown to influence remodeling and wound healing. The neuronal mechanisms responsible for these events are largely conserved across species, suggesting this evidence may be important in understanding and resolving many human defects and diseases. By unraveling these diverse roles, this paper highlights the necessity of broadening our perspective on the nervous system beyond its conventional functions. A comprehensive understanding of the complex interactions and contributions of the nervous system throughout development and adulthood has the potential to revolutionize therapeutic strategies and open new avenues for regenerative medicine and tissue engineering. This review highlights an important role for the nervous system during the patterning and maintenance of complex tissues and provides a potential avenue for advancing biomedical applications.

Deer antler renewal gives insights into mammalian epimorphic regeneration

Deer antlers are the only known mammalian organ that, once lost, can fully grow back naturally. Hence, the antler offers a unique opportunity to learn how nature has solved the problem of mammalian epimorphic regeneration (EpR). Comprehensive comparisons amongst different types of EpR reveal that antler renewal is fundamentally different from that in lower vertebrates such as regeneration of the newt limb. Surprisingly, antler renewal is comparable to wound healing over a stump of regeneration-incompetent digit/limb, bone fracture repair, and to a lesser extent to digit tip regeneration in mammals. Common to all these mammalian cases of reaction to the amputation/mechanical trauma is the response of the periosteal cells at the distal end/injury site with formation of a circumferential cartilaginous callus (CCC). Interestingly, whether the CCC can proceed to the next stage to transform to a blastema fully depends on the presence of an interactive partner. The actual form of the partner can vary in different cases with the nail organ in digit tip EpR, the opposing callus in bone fracture repair, and the closely associated enveloping skin in antler regeneration. Due to absence of such an interactive partner, the CCC of a mouse/rat digit/limb stump becomes involuted gradually. Based on these discoveries, we created an interactive partner for the rat digit/limb stump through surgically removal of the interposing layers of loose connective tissue and muscle between the resultant CCC and the enveloping skin after amputation and by forcefully bonding two tissue types tightly together. In so doing partial regeneration of the limb stump occurred. In summary, if EpR in humans is to be realized, then I envisage that it would be more likely in a manner akin to antler regeneration rather to that of lower vertebrates such as newt limbs.

Toeing the line between regeneration and fibrosis

Understanding the remarkable capacity of vertebrates to naturally regenerate injured body parts has great importance for potential translation into human therapeutic applications. As compared to other vertebrates, mammals have low regenerative capacity for composite tissues like the limb. However, some primates and rodents can regenerate the distal tips of their digits following amputation, indicating that at least very distal mammalian limb tissues are competent for innate regeneration. It follows that successful digit tip regenerative outcome is highly dependent on the location of the amputation; those proximal to the position of the nail organ do not regenerate and result in fibrosis. This distal regeneration versus proximal fibrosis duality of the mouse digit tip serves as a powerful model to investigate the driving factors in determining each process. In this review, we present the current understanding of distal digit tip regeneration in the context of cellular heterogeneity and the potential for different cell types to function as progenitor cells, in pro-regenerative signaling, or in moderating fibrosis. We then go on to discuss these themes in the context of what is known about proximal digit fibrosis, towards generating hypotheses for these distinct healing processes in the distal and proximal mouse digit.

Failure of digit tip regeneration in the absence of Lmx1b suggests Lmx1b functions disparate from dorsoventral polarity

Mammalian digit tip regeneration is linked to the presence of nail tissue, but a nail-explicit model is missing. Here, we report that nail-less double-ventral digits of ΔLARM1/2 mutants that lack limb-specific Lmx1b enhancers fail to regenerate. To separate the nail's effect from the lack of dorsoventral (DV) polarity, we also interrogate double-dorsal double-nail digits and show that they regenerate. Thus, DV polarity is not a prerequisite for regeneration, and the nail requirement is supported. Transcriptomic comparison between wild-type and non-regenerative ΔLARM1/2 mutant blastemas reveals differential upregulation of vascularization and connective tissue functional signatures in wild type versus upregulation of inflammation in the mutant. These results, together with the finding of Lmx1b expression in the postnatal dorsal dermis underneath the nail and uniformly in the regenerative blastema, open the possibility of additional Lmx1b roles in digit tip regeneration, in addition to the indirect effect of mediating the formation of the nail.

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Neural Regeneration in Regenerative Endodontic Treatment: An Overview and Current Trends

Pulpal and periapical diseases are the most common dental diseases. The traditional treatment is root canal therapy, which achieves satisfactory therapeutic outcomes-especially for mature permanent teeth. Apexification, pulpotomy, and pulp revascularization are common techniques used for immature permanent teeth to accelerate the development of the root. However, there are obstacles to achieving functional pulp regeneration. Recently, two methods have been proposed based on tissue engineering: stem cell transplantation, and cell homing. One of the goals of functional pulp regeneration is to achieve innervation. Nerves play a vital role in dentin formation, nutrition, sensation, and defense in the pulp. Successful neural regeneration faces tough challenges in both animal studies and clinical trials. Investigation of the regeneration and repair of the nerves in the pulp has become a serious undertaking. In this review, we summarize the current understanding of the key stem cells, signaling molecules, and biomaterials that could promote neural regeneration as part of pulp regeneration. We also discuss the challenges in preclinical or clinical neural regeneration applications to guide deep research in the future.

The Contribution of Innervation to Tissue Repair and Regeneration

Animals such as amphibians have an incredible capacity for regeneration with some being able to regrow their tail or appendages. Although some mammalian tissues like the skin and bones can repair following injury, there are only a few examples of true multilineage regeneration, including the distal portion of the digit tip. In both amphibians and mammals, however, to achieve successful repair or regeneration, it is now appreciated that intact nerve innervation is a necessity. Here, we review the current state of literature and discuss recent advances that identify axon-derived signals, Schwann cells, and nerve-derived mesenchymal cells as direct and indirect supporters of adult tissue homeostasis and repair. We posit that understanding how nerves positively influence repair and regeneration could lead to targeted regenerative medicine strategies to enhance tissue repair in humans.

Mouse Digit Tip Regeneration Is Mechanical Load Dependent

Amputation of the mouse digit tip results in blastema-mediated regeneration. In this model, new bone regenerates de novo to lengthen the amputated stump bone, resulting in a functional replacement of the terminal phalangeal element along with associated non-skeletal tissues. Physiological examples of bone repair, such as distraction osteogenesis and fracture repair, are well known to require mechanical loading. However, the role of mechanical loading during mammalian digit tip regeneration is unknown. In this study, we demonstrate that reducing mechanical loading inhibits blastema formation by attenuating bone resorption and wound closure, resulting in the complete inhibition of digit regeneration. Mechanical unloading effects on wound healing and regeneration are completely reversible when mechanical loading is restored. Mechanical unloading after blastema formation results in a reduced rate of de novo bone formation, demonstrating mechanical load dependence of the bone regenerative response. Moreover, enhancing the wound-healing response of mechanically unloaded digits with the cyanoacrylate tissue adhesive Dermabond improves wound closure and partially rescues digit tip regeneration. Taken together, these results demonstrate that mammalian digit tip regeneration is mechanical load-dependent. Given that human fingertip regeneration shares many characteristics with the mouse digit tip, these results identify mechanical load as a previously unappreciated requirement for de novo bone regeneration in humans. © 2021 American Society for Bone and Mineral Research (ASBMR).

Mammalian Digit Tip Regeneration: Moving from Phenomenon to Molecular Mechanism

In this review, we present the current state of knowledge surrounding mammalian digit tip regeneration. We discuss the origin and formation of the blastema, a structure integral to digit tip regeneration, as well as recent insights driven by single-cell RNA sequencing into the molecular markers and cellular composition of the blastema. The digit tip is a composite of many different tissue types and we address what is known about the role of these separate tissues in regeneration of the whole digit tip. Specifically, we discuss the most extensively studied tissues in the digit tip: bone, nail epithelium, and peripheral nerves. We also address how known molecular pathways in limb development can inform research into digit tip regeneration. Overall, the mouse digit tip is an excellent model of complex mammalian regeneration that can provide insight into inducing regeneration in human tissues.

Neural regulations of the tumor microenvironment

The identification of nerves in the tumor microenvironment has ushered in a new area of research in cancer biology. Numerous studies demonstrate the presence of various types of peripheral nerves (sympathetic, parasympathetic, sensory) within the tumor microenvironment; moreover, an increased density of nerves in the tumor microenvironment correlates with worse prognosis. In this review, we address the current understanding of nerve-mediated alterations of the tumor microenvironment and how they impact disease through a variety of processes, including direct nerve-cancer cell communication, alteration of the infiltrative immune population, and alteration of stromal components.

The nervous system: Orchestra conductor in cancer, regeneration, inflammation and immunity

Although the role of nerves in stimulating cellular growth and dissemination has long been described in tissue regeneration studies, until recently a similar trophic role of nerves in disease was not well recognized. However, recent studies in oncology have demonstrated that the growth and dissemination of cancers also requires the infiltration of nerves in the tumor microenvironment. Nerves generate various neurosignaling pathways, which orchestrate cancer initiation, progression, and metastases. Similarly, nerves are increasingly implicated for their regulatory functions in immunity and inflammation. This orchestrator role of nerves in cellular and molecular interactions during regeneration, cancer, immunity, and inflammation offers new possibilities for targeting or enhancing neurosignaling in human health and diseases.

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

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

NGF-TrkA signaling dictates neural ingrowth and aberrant osteochondral differentiation after soft tissue trauma

Pain is a central feature of soft tissue trauma, which under certain contexts, results in aberrant osteochondral differentiation of tissue-specific stem cells. Here, the role of sensory nerve fibers in this abnormal cell fate decision is investigated using a severe extremity injury model in mice. Soft tissue trauma results in NGF (Nerve growth factor) expression, particularly within perivascular cell types. Consequently, NGF-responsive axonal invasion occurs which precedes osteocartilaginous differentiation. Surgical denervation impedes axonal ingrowth, with significant delays in cartilage and bone formation. Likewise, either deletion of Ngf or two complementary methods to inhibit its receptor TrkA (Tropomyosin receptor kinase A) lead to similar delays in axonal invasion and osteochondral differentiation. Mechanistically, single-cell sequencing suggests a shift from TGFβ to FGF signaling activation among pre-chondrogenic cells after denervation. Finally, analysis of human pathologic specimens and databases confirms the relevance of NGF-TrkA signaling in human disease. In sum, NGF-mediated TrkA-expressing axonal ingrowth drives abnormal osteochondral differentiation after soft tissue trauma. NGF-TrkA signaling inhibition may have dual therapeutic use in soft tissue trauma, both as an analgesic and negative regulator of aberrant stem cell differentiation.

Nerve-mediated FGF-signaling in the early phase of various organ regeneration

Amphibians have a very high capacity for regeneration among tetrapods. This superior regeneration capability in amphibians can be observed in limbs, the tail, teeth, external gills, the heart, and some internal organs. The mechanisms underlying the superior organ regeneration capability have been studied for a long time. Limb regeneration has been investigated as the representative phenomenon for organ-level regeneration. In limb regeneration, a prominent difference between regenerative and nonregenerative animals after limb amputation is blastema formation. A regeneration blastema requires the presence of nerves in the stump region. Thus, nerve regulation is responsible for blastema induction, and it has received much attention. Nerve regulation in regeneration has been investigated using the limb regeneration model and newly established alternative experimental model called the accessory limb model. Previous studies have identified some candidate genes that act as neural factors in limb regeneration, and these studies also clarified related events in early limb regeneration. Consistent with the nervous regulation and related events in limb regeneration, similar regeneration mechanisms in other organs have been discovered. This review especially focuses on the role of nerve-mediated fibroblast growth factor in the initiation phase of organ regeneration. Comparison of the initiation mechanisms for regeneration in various amphibian organs allows speculation about a fundamental regenerative process.

Appendage regeneration is context dependent at the cellular level

Species that can regrow their lost appendages have been studied with the ultimate aim of developing methods to enable human limb regeneration. These examinations highlight that appendage regeneration progresses through shared tissue stages and gene activities, leading to the assumption that appendage regeneration paradigms (e.g. tails and limbs) are the same or similar. However, recent research suggests these paradigms operate differently at the cellular level, despite sharing tissue descriptions and gene expressions. Here, collecting the findings from disparate studies, I argue appendage regeneration is context dependent at the cellular level; nonetheless, it requires (i) signalling centres, (ii) stem/progenitor cell types and (iii) a regeneration-permissive environment, and these three common cellular principles could be more suitable for cross-species/paradigm/age comparisons.

A finger on the pulse of regeneration: insights into the cellular mechanisms of adult digit tip regeneration

In mammals, multi-tissue regeneration is largely restricted to the distal portion of the digit tip and involves the formation of a blastema, a transient, proliferating cell mass that reforms the diverse tissues of the digit. Historically little was known about the mammalian blastema but with recent advances in single cell transcriptomic approaches and genetic lineage tracing, a more precise understanding of this critical structure has begun to emerge. In this review we summarise the cellular mechanisms underlying adult mammalian digit tip regeneration. We posit that understanding how some mammals naturally regenerate complex tissues will lead to strategies for enhancing regenerative abilities in humans.

Activation of the Melanocortin-4 receptor signaling by α-MSH stimulates nerve-dependent mouse digit regeneration

BACKGROUND

Expression of Mc4r in peripheral organs indicates it has broader roles in organ homeostasis and regeneration. However, the expression and function of Mc4r in the mouse limb and digit has not been fully investigated. Our previous work showed that Mc4r-/- mice fail to regenerate the digit, but whether activation of MC4R signaling could rescue digit regeneration, or stimulate proximal digit regeneration is not clear.

RESULTS

We analyzed the expression dynamics of Mc4r in the embryonic and postnatal mouse limb and digit using the Mc4r-gfp mice. We found that Mc4r-GFP is mainly expressed in the limb nerves, and in the limb muscles that are undergoing secondary myogenesis. Expression of Mc4r-GFP in the adult mouse digit is restricted to the nail matrix. We also examined the effect of α-MSH on mouse digit regeneration. We found that administration of α-MSH in the Mc4r+/- mice rescue the delayed regeneration of distal digit tip. α-MSH could rescue distal digit regeneration in denervated hindlimbs. In addition, α-MSH could stimulate regeneration of the proximally amputated digit, which is non-regenerative.

CONCLUSIONS

Mc4r expression in the mouse limb and digit is closely related to nerve tissues, and α-MSH/MC4R signaling has a neurotrophic role in mouse digit tip regeneration.

Differential Expression of Insulin-Like Growth Factor 1 and Wnt Family Member 4 Correlates With Functional Heterogeneity of Human Dermal Fibroblasts

Although human dermis contains distinct fibroblast subpopulations, the functional heterogeneity of fibroblast lines from different donors is under-appreciated. We identified one commercially sourced fibroblast line (c64a) that failed to express α-smooth muscle actin (α-SMA), a marker linked to fibroblast contractility, even when treated with transforming growth factor-β1 (TGF-β1). Gene expression profiling identified insulin-like growth factor 1 (IGF1) as being expressed more highly, and Asporin (ASPN) and Wnt family member 4 (WNT4) expressed at lower levels, in c64a fibroblasts compared to three fibroblast lines that had been generated in-house, independent of TGF-β1 treatment. TGF-β1 increased expression of C-X-C motif chemokine ligand 1 (CXCL1) in c64a cells to a greater extent than in the other lines. The c64a gene expression profile did not correspond to any dermal fibroblast subpopulation identified by single-cell RNAseq of freshly isolated human skin cells. In skin reconstitution assays, c64a fibroblasts did not support epidermal stratification as effectively as other lines tested. In fibroblast lines generated in-house, shRNA-mediated knockdown of IGF1 increased α-SMA expression without affecting epidermal stratification. Conversely, WNT4 knockdown had no consistent effect on α-SMA expression, but increased the ability of fibroblasts to support epidermal stratification. Thus, by comparing the properties of different lines of cultured dermal fibroblasts, we have identified IGF1 and WNT4 as candidate mediators of two distinct dermal functions: myofibroblast formation and epidermal maintenance.

From the Basis of Epimorphic Regeneration to Enhanced Regenerative Therapies

Current cell-based therapies to treat degenerative diseases such as osteoarthritis (OA) fail to offer long-term beneficial effects. The therapeutic effects provided by mesenchymal stem cell (MSC) injection, characterized by reduced pain and an improved functional activity in patients with knee OA, are reported at short-term follow-up since the improved outcomes plateau or, even worse, decline several months after MSC administration. This review tackles the limitations of MSC-based therapy for degenerative diseases and highlights the lessons learned from regenerative species to comprehend the coordination of molecular and cellular events critical for complex regeneration processes. We discuss how MSC injection generates a positive cascade of events resulting in a long-lasting systemic immune regulation with limited beneficial effects on tissue regeneration while in regenerative species fine-tuned inflammation is required for progenitor cell proliferation, differentiation, and regeneration. Finally, we stress the direct or indirect involvement of neural crest derived cells (NCC) in most if not all adult regenerative models studied so far. This review underlines the regenerative potential of NCC and the limitations of MSC-based therapy to open new avenues for the treatment of degenerative diseases such as OA.

Cellular and molecular mechanisms that regulate mammalian digit tip regeneration

Digit tip regeneration is one of the few examples of true multi-tissue regeneration in an adult mammal. The key step in this process is the formation of the blastema, a transient proliferating cell mass that generates the different cell types of the digit to replicate the original structure. Failure to form the blastema results in a lack of regeneration and has been postulated to be the reason why mammalian limbs cannot regrow following amputation. Understanding how the blastema forms and functions will help us to determine what is required for mammalian regeneration to occur and will provide insights into potential therapies for mammalian tissue regeneration and repair. This review summarizes the cellular and molecular mechanisms that influence murine blastema formation and govern digit tip regeneration.

Skeletal Stem Cell-Schwann Cell Circuitry in Mandibular Repair

Regenerative paradigms exhibit nerve dependency, including regeneration of the mouse digit tip and salamander limb. Denervation impairs regeneration and produces morphological aberrancy in these contexts, but the direct effect of innervation on the stem and progenitor cells enacting these processes is unknown. We devised a model to examine nerve dependency of the mouse skeletal stem cell (mSSC), the progenitor responsible for skeletal development and repair. We show that after inferior alveolar denervation, mandibular bone repair is compromised because of functional defects in mSSCs. We present mSSC reliance on paracrine factors secreted by Schwann cells as the underlying mechanism, with partial rescue of the denervated phenotype by Schwann cell transplantation and by Schwann-derived growth factors. This work sheds light on the nerve dependency of mSSCs and has implications for clinical treatment of mandibular defects.

Nerve input to tumours: Pathophysiological consequences of a dynamic relationship

It is well known that tumours arising in different organs are innervated and that 'perineural invasion' (cancer cells escaping from the tumour by following the nerve trunk) is a negative prognostic factor. More surprisingly, increasing evidence suggests that the nerves can provide active inputs to tumours and there is two-way communication between nerves and cancer cells within the tumour microenvironment. Cells of the immune system also interact with the nerves and cancer cells. Thus, the nerve connections can exert significant control over cancer progression and modulating these (physically or chemically) can affect significantly the cancer process. Nerve inputs to tumours are derived mainly from the sympathetic (adrenergic) and the parasympathetic (cholinergic) systems, which are interactive. An important component of the latter is the vagus nerve, the largest of the cranial nerves. Here, we present a two-part review of the nerve inputs to tumours and their effects on tumorigenesis. First, we review briefly some relevant general issues including ultrastructural aspects, stemness, interactions between neurones and primary tumours, and communication between neurones and metastasizing tumour cells. Ultrastructural characteristics include synaptic vesicles, tumour microtubes and gap junctions enabling formation of cellular networks. Second, we evaluate the pathophysiology of the nerve input to five major carcinomas: cancers of prostate, stomach, colon, lung and pancreas. For each cancer, we present (i) the nerve inputs normally present in the cancer organ and (ii) how these interact and influence the cancer process. The best clinical evidence for the role of nerves in promoting tumorigenesis comes from prostate cancer patients where metastatic progression has been shown to be suppressed significantly in cases of spinal cord injury. The balance of the sympathetic and parasympathetic contributions to early versus late tumorigenesis varies amongst the different cancers. Different branches of the vagus provide functional inputs to several of the carcinomas and, in two-way interaction with the sympathetic nervous system, affect different stages of the cancer process. Overall, the impact of the vagus nerve can be 'direct' or 'indirect'. Directly, the effect of the vagus is primarily to promote tumorigenesis and this is mediated through cholinergic receptor mechanisms. Indirectly, pro- and anti-tumour effects can occur by stimulation or inhibition of the sympathetic nervous system, respectively. Less well understood are the 'indirect' anti-tumour effect of the vagus nerve via immunomodulation/inflammation, and the role of sensory innervation. A frequent occurrence in the nerve-tumour interactions is the presence of positive feedback driven by agents like nerve growth factor. We conclude that the nerve inputs to tumours can actively and dynamically impact upon cancer progression and are open to clinical exploitation.

Transcriptomic analysis of bone and fibrous tissue morphogenesis during digit tip regeneration in the adult mouse

Humans have limited regenerative potential of musculoskeletal tissues following limb or digit loss. The murine digit has been used to study mammalian regeneration, where stem/progenitor cells (the "blastema") completely regenerate the digit tip after distal, but not proximal, amputation. However, the molecular mechanisms responsible for this response remain to be determined. Here, we evaluated the spatiotemporal formation of bone and fibrous tissues after level-dependent amputation of the murine terminal phalanx and quantified the transcriptome of the repair tissue. Distal (regenerative) and proximal (non-regenerative) amputations showed significant differences in temporal gene expression and tissue regrowth over time. Genes that direct skeletal system development and limb morphogenesis are transiently upregulated during blastema formation and differentiation, including distal Hox genes. Overall, our results suggest that digit tip regeneration is controlled by a gene regulatory network that recapitulates aspects of limb development, and that failure to activate this developmental program results in fibrotic wound healing.

The Autonomic Regulation of Tumor Growth and the Missing Links

Accumulating evidence now indicates that peripheral nerves and solid tumors mutually support the growth of each other. Tumor-derived molecular cues guide nerve infiltration to the tumor milieu, while the tumor-infiltrating nerves provide molecular support to promote tumor growth and dissemination. In this mini-review, we discuss the unique roles of sympathetic and parasympathetic nerves in promoting tumor growth and metastasis. The contribution of adrenergic and cholinergic signals, the specific receptors involved, and the downstream molecular links in both cancer cells and stromal cells are discussed for their intrinsic capacity to modulate tumor growth. We identified unappreciated niche areas in the field, an investigation of which are critical to filling the knowledge gap in understanding the biology of neuromodulation of cancers.

Nerves in cancer

The contribution of nerves to the pathogenesis of malignancies has emerged as an important component of the tumour microenvironment. Recent studies have shown that peripheral nerves (sympathetic, parasympathetic and sensory) interact with tumour and stromal cells to promote the initiation and progression of a variety of solid and haematological malignancies. Furthermore, new evidence suggests that cancers may reactivate nerve-dependent developmental and regenerative processes to promote their growth and survival. Here we review emerging concepts and discuss the therapeutic implications of manipulating nerves and neural signalling for the prevention and treatment of cancer.

Nerves Do It Again: Donation of Mesenchymal Cells for Tissue Regeneration

Nerves are necessary for proper tissue repair and complex regeneration. Now in Cell Stem Cell, Carr et al. (2019) identify a specific population of mesenchymal cells that reside within nerves and represent a reservoir of precursors that expand in number and differentiate into bone and dermis during tissue regeneration and repair.

The multifaceted role of nerves in animal regeneration

The discovery that the nervous system plays a critical role in salamander limb regeneration, in 1823, provided the first mechanistic insights into regenerative phenomena and stimulated a long quest for molecular regulators. A role for nerves in the context of regeneration has been suggested for most vertebrate and invertebrate groups, thus offering a possible shared mechanism for the regulation of regenerative processes among animals. Methodological differences and technical limitations, especially in invertebrate groups, have so far hampered broad comparisons and the search for common principles on the role of nerves. This review considers both old and recent work in this topic and provides a broad perspective on the roles of nerves during regeneration. Nerves are found consistently to have important roles in regeneration, but their mode of action varies across species. The ongoing technological developments in a broad range of invertebrate models are now paving the way for the discovery of the shared and unique roles of nerves in animal regeneration.

Common themes in tetrapod appendage regeneration: a cellular perspective

Complete and perfect regeneration of appendages is a process that has fascinated and perplexed biologists for centuries. Some tetrapods possess amazing regenerative abilities, but the regenerative abilities of others are exceedingly limited. The reasons underlying these differences have largely remained mysterious. A great deal has been learned about the morphological events that accompany successful appendage regeneration, and a handful of experimental manipulations can be reliably applied to block the process. However, only in the last decade has the goal of attaining a thorough molecular and cellular biological understanding of appendage regeneration in tetrapods become within reach. Advances in molecular and genetic tools for interrogating these remarkable events are now allowing for unprecedented access to the fundamental biology at work in appendage regeneration in a variety of species. This information will be critical for integrating the large body of detailed observations from previous centuries with a modern understanding of how cells sense and respond to severe injury and loss of body parts. Understanding commonalities between regenerative modes across diverse species is likely to illuminate the most important aspects of complex tissue regeneration.

Digit Tip Injuries: Current Treatment and Future Regenerative Paradigms

Over the past several decades there has been a profound increase in the understanding of tissue regeneration, driven largely by the observance of the tremendous regenerative capacity in lower order life forms, such as hydra and urodeles. However, it is known that humans and other mammals retain the ability to regenerate the distal phalanges of the digits after amputation. Despite the increased knowledge base on model organisms regarding regenerative paradigms, there is a lack of application of regenerative medicine techniques in clinical practice in regard to digit tip injury. Here, we review the current understanding of digit tip regeneration and discuss gaps that remain in translating regenerative medicine into clinical treatment of digit amputation.

Mesenchymal Precursor Cells in Adult Nerves Contribute to Mammalian Tissue Repair and Regeneration

Peripheral innervation plays an important role in regulating tissue repair and regeneration. Here we provide evidence that injured peripheral nerves provide a reservoir of mesenchymal precursor cells that can directly contribute to murine digit tip regeneration and skin repair. In particular, using single-cell RNA sequencing and lineage tracing, we identify transcriptionally distinct mesenchymal cell populations within the control and injured adult nerve, including neural crest-derived cells in the endoneurium with characteristics of mesenchymal precursor cells. Culture and transplantation studies show that these nerve-derived mesenchymal cells have the potential to differentiate into non-nerve lineages. Moreover, following digit tip amputation, neural crest-derived nerve mesenchymal cells contribute to the regenerative blastema and, ultimately, to the regenerated bone. Similarly, neural crest-derived nerve mesenchymal cells contribute to the dermis during skin wound healing. These findings support a model where peripheral nerves directly contribute mesenchymal precursor cells to promote repair and regeneration of injured mammalian tissues.

Axonal regrowth is impaired during digit tip regeneration in mice

Mice are intrinsically capable of regenerating the tips of their digits after amputation. Mouse digit tip regeneration is reported to be a peripheral nerve-dependent event. However, it is presently unknown what types of nerves and Schwann cells innervate the digit tip, and to what extent these cells regenerate in association with the regenerative response. Given the necessity of peripheral nerves for mammalian regeneration, we investigated the neuroanatomy of the unamputated, regenerating, and regenerated mouse digit tip. Using immunohistochemistry for β-III-tubulin (β3T) or neurofilament H (NFH), substance P (SP), tyrosine hydroxylase (TH), myelin protein zero (P0), and glial fibrillary acidic protein (GFAP), we identified peripheral nerve axons (sensory and sympathetic), and myelinating- and non-myelinating-Schwann cells. Our findings show that the digit tip is innervated by two digital nerves that each bifurcate into a bone marrow (BM) and connective tissue (CT) branch. The BM branches are composed of sympathetic axons that are ensheathed by non-myelinating-Schwann cells whereas the CT branches are composed of sensory and sympathetic axons and are ensheathed by myelinating- and non-myelinating-Schwann cells. The regenerated digit neuroanatomy differs from unamputated digit in several key ways. First, there is 7.5 fold decrease in CT branch axons in the regenerated digit compared to the unampuated digit. Second, there is a 5.6 fold decrease in myelinating-Schwann cells in the regenerated digit compared to the unamputated digit that is consistent with the decrease in CT branch axons. Importantly, we also find that the central portion of the regenerating digit blastema is aneural, with axons and Schwann cells restricted to peripheral and distal blastema regions. Finally, we show that even with impaired innervation, digits maintain the ability to regenerate after re-amputation. Taken together, these data indicate that nerve regeneration is impaired in the context of mouse digit tip regeneration.

Adrenergic nerve degeneration in bone marrow drives aging of the hematopoietic stem cell niche

Aging of hematopoietic stem cells (HSCs) is associated with a decline in their regenerative capacity and multi-lineage differentiation potential, contributing to the development of blood disorders. The bone marrow microenvironment has recently been suggested to influence HSC aging, but the underlying mechanisms remain largely unknown. Here, we show that HSC aging critically depends on bone marrow innervation by the sympathetic nervous system (SNS), as loss of SNS nerves or adrenoreceptor β3 (ADRβ3) signaling in the bone marrow microenvironment of young mice led to premature HSC aging, as evidenced by appearance of HSC phenotypes reminiscent of physiological aging. Strikingly, supplementation of a sympathomimetic acting selectively on ADRβ3 to old mice significantly rejuvenated the in vivo function of aged HSCs, suggesting that the preservation or restitution of bone marrow SNS innervation during aging may hold the potential for new HSC rejuvenation strategies.

The Impact of Aging on Mechanisms of Mammalian Epimorphic Regeneration

Aging is associated with a significant decline of tissue repair and regeneration, ultimately resulting in tissue dysfunction, multimorbidity, and death. Salamanders possess remarkable regenerative abilities and have been studied with the prospect of inducing regeneration in humans and counteracting regenerative decline with aging. However, epimorphic regeneration, the full replacement of amputated structures, also occurs in mammals. One of the best studied models is digit tip regeneration, which is described for mice, and occurs in humans in a comparable manner. To accomplish regeneration, the amputated digit tip has to undergo three interdependent, overlapping steps: (i) wound healing without formation of a scar; (ii) formation of a blastema, a highly proliferative cell mass; and (iii) spatiotemporally regulated differentiation to generate a pattern similar to the original structure. Aging likely interferes with each of these steps. In this article, we provide an overview of the critical signaling pathways for regeneration, as revealed by investigating mammalian digit regeneration, the possible impact of aging on these pathways, and approaches to induce regeneration in the elderly. We hypothesize that with aging, increased Wnt signaling, NF-κB and tumor suppressor activity, and loss of positional information hampers regeneration. Knowledge about the impact of aging on regenerative mechanisms will enable us to safely activate endogenous regeneration in the elderly, and to generate a regeneration-permissive environment for cell therapies.

Neuronal delivery of Hedgehog directs spatial patterning of taste organ regeneration

How organs maintain and restore functional integrity during ordinary tissue turnover or following injury represents a central biological problem. The maintenance of taste sensory organs in the tongue was shown 140 years ago to depend on innervation from distant ganglion neurons, but the underlying mechanism has remained unknown. Here, we show that Sonic hedgehog (Shh), which encodes a secreted protein signal, is expressed in these sensory neurons, and that experimental ablation of neuronal Shh expression causes loss of taste receptor cells (TRCs). TRCs are also lost upon pharmacologic blockade of Hedgehog pathway response, accounting for the loss of taste sensation experienced by cancer patients undergoing Hedgehog inhibitor treatment. We find that TRC regeneration following such pharmacologic ablation requires neuronal expression of Shh and can be substantially enhanced by pharmacologic activation of Hedgehog response. Such pharmacologic enhancement of Hedgehog response, however, results in additional TRC formation at many ectopic sites, unlike the site-restricted regeneration specified by the projection pattern of Shh-expressing neurons. Stable regeneration of TRCs thus requires neuronal Shh, illustrating the principle that neuronal delivery of cues such as the Shh signal can pattern distant cellular responses to assure functional integrity during tissue maintenance and regeneration.

Generation of iPSC-derived limb progenitor-like cells for stimulating phalange regeneration in the adult mouse

The capacity of digit tip regeneration observed both in rodents and humans establishes a foundation for promoting robust regeneration in mammals. However, stimulating regeneration at more proximal levels, such as the middle phalanges (P2) of the adult mouse, remains challenging. Having shown the effectiveness of transplantation of limb progenitor cells in stimulating limb regeneration in Xenopus, we are now applying the cell transplantation approach to the adult mouse. Here we report that both embryonic and induced pluripotent stem cell (iPSC)-derived limb progenitor-like cells can promote adult mouse P2 regeneration. We have established a simple and efficient protocol for deriving limb progenitor-like cells from mouse iPSCs. iPSCs are cultured as three-dimensional fibrin bodies, followed by treatment with combinations of Fgf8, CHIR99021, Purmorphamine and SB43542 during differentiation. These iPSC-derived limb progenitor-like cells resemble embryonic limb mesenchyme cells in their expression of limb-related genes. After transplantation, the limb progenitor-like cells can promote adult mouse P2 regeneration, as embryonic limb bud cells do. Our results provide a basis for further developing progenitor cell-based approaches for improving regeneration in the adult mouse limbs.

First Insights into Human Fingertip Regeneration by Echo-Doppler Imaging and Wound Microenvironment Assessment

Fingertip response to trauma represents a fascinating example of tissue regeneration. Regeneration derives from proliferative mesenchymal cells (blastema) that subsequently differentiate into soft and skeletal tissues. Clinically, conservative treatment of the amputated fingertip under occlusive dressing can shift the response to tissue loss from a wound repair process towards regeneration. When analyzing by Immunoassay the wound exudate from occlusive dressings, the concentrations of brain-derived neurotrophic factor (BDNF) and leukemia inhibitory factor (LIF) were higher in fingertip exudates than in burn wounds (used as controls for wound repair versus regeneration). Vascular endothelial growth factor A (VEGF-A) and platelet-derived growth factor (PDGF) were highly expressed in both samples in comparable levels. In our study, pro-inflammatory cytokines were relatively higher expressed in regenerative fingertips than in the burn wound exudates while chemokines were present in lower levels. Functional, vascular and mechanical properties of the regenerated fingertips were analyzed three months after trauma and the data were compared to the corresponding fingertip on the collateral uninjured side. While sensory recovery and morphology (pulp thickness and texture) were similar to uninjured sides, mechanical parameters (elasticity, vascularization) were increased in the regenerated fingertips. Further studies should be done to clarify the importance of inflammatory cells, immunity and growth factors in determining the outcome of the regenerative process and its influence on the clinical outcome.

A brief history of the study of nerve dependent regeneration

Nerve dependence is a phenomenon observed across a stunning array of species and tissues. From zebrafish to fetal mice to humans, research across various animal models has shown that nerves are critical for the support of tissue repair and regeneration. Although the study of this phenomenon has persisted for centuries, largely through research conducted in salamanders, the cellular and molecular mechanisms of nerve dependence remain poorly-understood. Here we highlight the near-ubiquity and clinical relevance of vertebrate nerve dependence while providing a timeline of its study and an overview of recent advancements toward understanding the mechanisms behind this process. In presenting a brief history of the research of nerve dependence, we provide both historical and modern context to our recent work on nerve dependent limb regeneration in the Mexican axolotl.

Microtubule-Targeting Agents Eribulin and Paclitaxel Differentially Affect Neuronal Cell Bodies in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of anticancer treatment with microtubule-targeted agents (MTAs). The frequency of severe CIPN, which can be dose limiting and even life threatening, varies widely among different MTAs. For example, paclitaxel induces a higher frequency of severe CIPN than does eribulin. Different MTAs also possess distinct mechanisms of microtubule-targeted action. Recently, we demonstrated that paclitaxel and eribulin differentially affect sciatic nerve axons, with paclitaxel inducing more pronounced neurodegenerative effects and eribulin inducing greater microtubule stabilizing biochemical effects. Here, we complement and extend these axonal studies by assessing the effects of paclitaxel and eribulin in the cell bodies of sciatic nerve axons, housed in the dorsal root ganglia (DRG). Importantly, the microtubule network in cell bodies is known to be significantly more dynamic than in axons. Paclitaxel induced activating transcription factor 3 expression, a marker of neuronal stress/injury. Paclitaxel also increased expression levels of acetylated tubulin and end binding protein 1, markers of microtubule stability and growth, respectively. These effects are hypothesized to be detrimental to the dynamic microtubule network within the cell bodies. In contrast, eribulin had no significant effect on any of these parameters in the cell bodies. Taken together, DRG cell bodies and their axons, two distinct neuronal cell compartments, contain functionally distinct microtubule networks that exhibit unique biochemical responses to different MTA treatments. We hypothesize that these distinct mechanistic actions may underlie the variability seen in the initiation, progression, persistence, and recovery from CIPN.

Nerve Dependence: From Regeneration to Cancer

Nerve dependence has long been described in animal regeneration, where the outgrowth of axons is necessary to the reconstitution of lost body parts and tissue remodeling in various species. Recent discoveries have demonstrated that denervation can suppress tumor growth and metastasis, pointing to nerve dependence in cancer. Regeneration and cancer share similarities in regard to the stimulatory role of nerves, and there are indications that the stem cell compartment is a preferred target of innervation. Thus, the neurobiology of cancer is an emerging discipline that opens new perspectives in oncology.

A Comparative Perspective on Wnt/β-Catenin Signalling in Cell Fate Determination

The Wnt/β-catenin pathway is an ancient and highly conserved signalling pathway that plays fundamental roles in the regulation of embryonic development and adult homeostasis. This pathway has been implicated in numerous cellular processes, including cell proliferation, differentiation, migration, morphological changes and apoptosis. In this chapter, we aim to illustrate with specific examples the involvement of Wnt/β-catenin signalling in cell fate determination. We discuss the roles of the Wnt/β-catenin pathway in specifying cell fate throughout evolution, how its function in patterning during development is often reactivated during regeneration and how perturbation of this pathway has negative consequences for the control of cell fate.The origin of all life was a single cell that had the capacity to respond to cues from the environment. With evolution, multicellular organisms emerged, and as a result, subsets of cells arose to form tissues able to respond to specific instructive signals and perform specialised functions. This complexity and specialisation required two types of messages to direct cell fate: intra- and intercellular. A fundamental question in developmental biology is to understand the underlying mechanisms of cell fate choice. Amongst the numerous external cues involved in the generation of cellular diversity, a prominent pathway is the Wnt signalling pathway in all its forms.