YAP and TAZ control peripheral myelination and the expression of laminin receptors in Schwann cells

Endothelial cells promote the proliferation and migration of Schwann cells

Background

After peripheral nerve injury, Schwann cells proliferate and migrate to the injured site, thereby promoting peripheral nerve regeneration. The process is regulated by various factors. Endothelial cells participate in the process via angiogenesis. However, the effects of endothelial cells on Schwann cells are not yet known. The present study sought to evaluate whether endothelial cells accelerate Schwann cell proliferation and migration.

Methods

We established a co-culture model of rat Schwann cells (RSC96s) and rat aortic endothelial cells (RAOECs), and studied the effects of endothelial cells on Schwann cells by evaluating changes in Schwann cell proliferation and migration and related multiple genes and their protein expressions in the co-culture model.

Results

The results showed that increasing the proportion of endothelial cells in the co-culture model enhanced the proliferation. At days 1 and 3 following the co-culturing, the relative growth rates of the co-cultured cells were 122.87% and 127.37%, respectively, which showed a significant increase in the viability compared to that of the RSC96s (P<0.05). In this process, the expression of Ki67 increased. The migration ability of Schwann cells was also enhanced. The migration capacity of Schwann cells was detected by wound-healing and Transwell assays. The results of the group with 15% of endothelial cells was significantly higher than the results of the other groups (P<0.0001 and P<0.05, respectively). Further, neuregulin 1 and glial fibrillary acidic protein increased the process of Schwann cell migration.

Conclusions

The results showed that endothelial cells can promote the proliferation and migration of Schwann cells and participate in peripheral nerve regeneration.

Four Seasons for Schwann Cell Biology, Revisiting Key Periods: Development, Homeostasis, Repair, and Aging

Like the seasons of the year, all natural things happen in stages, going through adaptations when challenged, and Schwann cells are a great example of that. During maturation, these cells regulate several steps in peripheral nervous system development. The Spring of the cell means the rise and bloom through organized stages defined by time-dependent regulation of factors and microenvironmental influences. Once matured, the Summer of the cell begins: a high energy stage focused on maintaining adult homeostasis. The Schwann cell provides many neuron-glia communications resulting in the maintenance of synapses. In the peripheral nervous system, Schwann cells are pivotal after injuries, balancing degeneration and regeneration, similarly to when Autumn comes. Their ability to acquire a repair phenotype brings the potential to reconnect axons to targets and regain function. Finally, Schwann cells age, not only by growing old, but also by imposed environmental cues, like loss of function induced by pathologies. The Winter of the cell presents as reduced activity, especially regarding their role in repair; this reflects on the regenerative potential of older/less healthy individuals. This review gathers essential information about Schwann cells in different stages, summarizing important participation of this intriguing cell in many functions throughout its lifetime.

Extracellular vesicles derived from astrocytes facilitated neurite elongation by activating the Hippo pathway

Spinal cord injury (SCI) often causes severe neurological dysfunction, and facilitating neurite elongation is particularly important in its treatment. Astrocytes (AS) play an important role in the central nervous system (CNS), and their high plasticity and versatility provide a feasible entry point for relevant research. Our purpose was to explore whether extracellular vesicles (EVs) from astrocytes (AS-EVs) and lipopolysaccharide (LPS)-preactivated astrocytes (LPAS-EVs) facilitate neurite elongation, to explore the underlying mechanism, and to verify whether these EVs promote locomotor recovery in rats. We used LPS to preactivate astrocytes and cocultured them with PC12 cells to observe neurite changes, then extracted and identified AS-EVs and LPAS-EVs and the role and mechanism of these EVs in facilitating neurite elongation was examined in vivo and vitro. We demonstrated that AS-EVs and LPAS-EVs facilitated the elongation of neurites and the recovery of rats with SCI. LPAS-EVs had a stronger effect than AS-EVs, by activating the Hippo pathway, promoting monopole spindle binding protein 1 (MOB1) expression, and reducing Yes-associated protein (YAP) levels. The data also suggest a feedback regulation between MOB1 and p-YAP/YAP. In sum, AS-EVs and LPAS-EVs can play an active role in facilitating neurite elongation by activating the Hippo pathway. These findings provide a new strategy for treating SCI and other CNS-related injuries.

Distinct roles for the Charcot-Marie-tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings. However, adult Mtmr5-/- mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Consistently, Mtmr5 levels were highest during axon radial sorting and fell sharply after postnatal day seven. Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.

Low-intensity extracorporeal shock wave therapy promotes recovery of sciatic nerve injury and the role of mechanical sensitive YAP/TAZ signaling pathway for nerve regeneration

Background:

Histological and functional recovery after peripheral nerve injury (PNI) is of significant clinical value as delayed surgical repair and longer distances to innervate terminal organs may account for poor outcomes. Low-intensity extracorporeal shock wave therapy (LiESWT) has already been proven to be beneficial for injured tissue recovery on various pathological conditions. The objective of this study was to explore the potential effect and mechanism of LiESWT on PNI recovery.

Methods:

In this project, we explored LiESWT's role using an animal model of sciatic nerve injury (SNI). Shockwave was delivered to the region of the SNI site with a special probe at 3 Hz, 500 shocks each time, and 3 times a week for 3 weeks. Rat Schwann cells (SCs) and rat perineurial fibroblasts (PNFs) cells, the two main compositional cell types in peripheral nerve tissue, were cultured in vitro, and LiESWT was applied through the cultured dish to the adherent cells. Tissues and cell cultures were harvested at corresponding time points for a reverse transcription-polymerase chain reaction, Western blotting, and immunofluorescence staining. Multiple groups were compared by using one-way analysis of variance followed by the Tukey-Kramer test for post hoc comparisons.

Results:

LiESWT treatment promoted the functional recovery of lower extremities with SNI. More nerve fibers and myelin sheath were found after LiESWT treatment associated with local upregulation of mechanical sensitive yes-associated protein (YAP)/transcriptional co-activator with a PDZ-binding domain (TAZ) signaling pathway. In vitro results showed that SCs were more sensitive to LiESWT than PNFs. LiESWT promoted SCs activation with more expression of p75 (a SCs dedifferentiation marker) and Ki67 (a SCs proliferation marker). The SCs activation process was dependent on the intact YAP/TAZ signaling pathway as knockdown of TAZ by TAZ small interfering RNA significantly attenuated this process.

Conclusion:

The LiESWT mechanical signal perception and YAP/TAZ upregulation in SCs might be one of the underlying mechanisms for SCs activation and injured nerve axon regeneration.

Astrocytic YAP prevents the demyelination through promoting expression of cholesterol synthesis genes in experimental autoimmune encephalomyelitis

Cholesterols are the main components of myelin, and are mainly synthesized in astrocytes and transported to oligodendrocytes and neurons in the adult brain. It has been reported that Hippo/yes-associated protein (YAP) pathways are involved in cholesterol synthesis in the liver, however, it remains unknown whether YAP signaling can prevent the demyelination through promoting cholesterol synthesis in experimental autoimmune encephalomyelitis (EAE), a commonly used animal model of multiple sclerosis characterized by neuroinflammation and demyelination. Here, we found that YAP was upregulated and activated in astrocytes of spinal cords of EAE mice through suppression of the Hippo pathway. YAP deletion in astrocytes aggravated EAE with earlier onset, severer inflammatory infiltration, demyelination, and more loss of neurons. Furthermore, we found that the neuroinflammation was aggravated and the proliferation of astrocytes was decreased in YAPGFAP-CKO EAE mice. Mechanically, RNA-seq revealed that the expression of cholesterol-synthesis pathway genes such as HMGCS1 were decreased in YAP-/- astrocytes. qPCR, western blot, and immunostaining further confirmed the more significant reduction of HMGCS1 in spinal cord astrocytes of YAPGFAP-CKO EAE mice. Interestingly, upregulation of cholesterol-synthesis pathways by diarylpropionitrile (DPN) (an ERβ-ligand, to upregulate the expression of HMGCS1) treatment partially rescued the demyelination deficits in YAPGFAP-CKO EAE mice. Finally, activation of YAP by XMU-MP-1 treatment promoted the expression of HMGCS1 in astrocytes and partially rescued the demyelination and inflammatory infiltration deficits in EAE mice. These findings identify unrecognized functions of astrocytic YAP in the prevention of demyelination through promoting cholesterol synthesis in EAE, and reveal a novel pathway of YAP/HMGCS1 for cholesterol synthesis in EAE pathology.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

Peripheral Nerve Development and the Pathogenesis of Peripheral Neuropathy: the Sorting Point

Nerve development requires a coordinated sequence of events and steps to be accomplished for the generation of functional peripheral nerves to convey sensory and motor signals. Any abnormality during development may result in pathological structure and function of the nerve, which evolves in peripheral neuropathy. In this review, we will briefly describe different steps of nerve development while we will mostly focus on the molecular mechanisms involved in radial sorting of axons, one of these nerve developmental steps. We will summarize current knowledge of molecular pathways so far reported in radial sorting and their possible interactions. Finally, we will describe how disruption of these pathways may result in human neuropathies.

Myelin Biology

Myelin is a key evolutionary specialization and adaptation of vertebrates formed by the plasma membrane of glial cells, which insulate axons in the nervous system. Myelination not only allows rapid and efficient transmission of electric impulses in the axon by decreasing capacitance and increasing resistance but also influences axonal metabolism and the plasticity of neural circuits. In this review, we will focus on Schwann cells, the glial cells which form myelin in the peripheral nervous system. Here, we will describe the main extrinsic and intrinsic signals inducing Schwann cell differentiation and myelination and how myelin biogenesis is achieved. Finally, we will also discuss how the study of human disorders in which molecules and pathways relevant for myelination are altered has enormously contributed to the current knowledge on myelin biology.

Activation of mTORC1 and c-Jun by Prohibitin1 loss in Schwann cells may link mitochondrial dysfunction to demyelination

Schwann cell (SC) mitochondria are quickly emerging as an important regulator of myelin maintenance in the peripheral nervous system (PNS). However, the mechanisms underlying demyelination in the context of mitochondrial dysfunction in the PNS are incompletely understood. We recently showed that conditional ablation of the mitochondrial protein Prohibitin 1 (PHB1) in SCs causes a severe and fast progressing demyelinating peripheral neuropathy in mice, but the mechanism that causes failure of myelin maintenance remained unknown. Here, we report that mTORC1 and c-Jun are continuously activated in the absence of Phb1, likely as part of the SC response to mitochondrial damage. Moreover, we demonstrate that these pathways are involved in the demyelination process, and that inhibition of mTORC1 using rapamycin partially rescues the demyelinating pathology. Therefore, we propose that mTORC1 and c-Jun may play a critical role as executioners of demyelination in the context of perturbations to SC mitochondria.

Wrestling and Wrapping: A Perspective on SUMO Proteins in Schwann Cells

Schwann cell development and peripheral nerve myelination are finely orchestrated multistep processes; some of the underlying mechanisms are well described and others remain unknown. Many posttranslational modifications (PTMs) like phosphorylation and ubiquitination have been reported to play a role during the normal development of the peripheral nervous system (PNS) and in demyelinating neuropathies. However, a relatively novel PTM, SUMOylation, has not been studied in these contexts. SUMOylation involves the covalent attachment of one or more small ubiquitin-like modifier (SUMO) proteins to a substrate, which affects the function, cellular localization, and further PTMs of the conjugated protein. SUMOylation also regulates other proteins indirectly by facilitating non-covalent protein–protein interaction via SUMO interaction motifs (SIM). This pathway has important consequences on diverse cellular processes, and dysregulation of this pathway has been reported in several diseases including neurological and degenerative conditions. In this article, we revise the scarce literature on SUMOylation in Schwann cells and the PNS, we propose putative substrate proteins, and we speculate on potential mechanisms underlying the possible involvement of this PTM in peripheral myelination and neuropathies.

The Role of Hippo Signaling Pathway in the Development of the Nervous System

Hippo signaling pathway is a highly conserved and crucial signaling pathway that controls the size of tissues and organs by regulating the proliferation, differentiation, and apoptosis of cells. The nervous system is a complicated system that participates in information collection, integration, and procession. The balance of various aspects of the nervous system is vital for the normal regulation of physiological conditions of the body, like the population and distribution of nerve cells, nerve connections, and so on. Defects in these aspects may lead to cognitive, behavioral, and neurological dysfunction, resulting in various nervous system diseases. Recently, accumulating evidence proposes that Hippo pathway maintains numerous biological functions in the nervous system development, including modulating the proliferation and differentiation of nerve cells and promoting the development of synapse, corpus callosum, and cortex. In this review, we will summarize recent findings of Hippo pathway in the nervous system to improve our understanding on its function and to provide potential therapeutic strategies of nervous system diseases in the future.

Mechanotransduction assays for neural regeneration strategies: A focus on glial cells

Glial cells are mechanosensitive, and thus, engineered systems have taken a step forward to design mechanotransduction platforms in order to impart diverse mechanical stresses to cells. Mechanical strain encountered in the central nervous system can arise from diverse mechanisms, such as tissue reorganization, fluid flow, and axon growth, as well as pathological events including axon swelling or mechanical trauma. Biomechanical relevance of the in vitro mechanical testing requires to be placed in line with the physiological and mechanical changes in central nervous tissues that occur during the progression of neurodegenerative diseases. Mechanotransduction signaling utilized by glial cells and the recent approaches intended to model altered microenvironment adapted to pathological context are discussed in this review. New insights in systems merging substrate's stiffness and topography should be considered for further glial mechanotransduction studies, while testing platforms for drug discoveries promise great advancements in pharmacotherapy. Potential leads and strategies for clinical outcomes are expected to be developed following the exploration of these glial mechanosensitive signaling pathways.

Natural Biomaterials as Instructive Engineered Microenvironments That Direct Cellular Function in Peripheral Nerve Tissue Engineering

Nerve tissue function and regeneration depend on precise and well-synchronised spatial and temporal control of biological, physical, and chemotactic cues, which are provided by cellular components and the surrounding extracellular matrix. Therefore, natural biomaterials currently used in peripheral nerve tissue engineering are selected on the basis that they can act as instructive extracellular microenvironments. Despite emerging knowledge regarding cell-matrix interactions, the exact mechanisms through which these biomaterials alter the behaviour of the host and implanted cells, including neurons, Schwann cells and immune cells, remain largely unclear. Here, we review some of the physical processes by which natural biomaterials mimic the function of the extracellular matrix and regulate cellular behaviour. We also highlight some representative cases of controllable cell microenvironments developed by combining cell biology and tissue engineering principles.

Cell Shape and Matrix Stiffness Impact Schwann Cell Plasticity via YAP/TAZ and Rho GTPases

Schwann cells (SCs) are a highly plastic cell type capable of undergoing phenotypic changes following injury or disease. SCs are able to upregulate genes associated with nerve regeneration and ultimately achieve functional recovery. During the regeneration process, the extracellular matrix (ECM) and cell morphology play a cooperative, critical role in regulating SCs, and therefore highly impact nerve regeneration outcomes. However, the roles of the ECM and mechanotransduction relating to SC phenotype are largely unknown. Here, we describe the role that matrix stiffness and cell morphology play in SC phenotype specification via known mechanotransducers YAP/TAZ and RhoA. Using engineered microenvironments to precisely control ECM stiffness, cell shape, and cell spreading, we show that ECM stiffness and SC spreading downregulated SC regenerative associated proteins by the activation of RhoA and YAP/TAZ. Additionally, cell elongation promoted a distinct SC regenerative capacity by the upregulation of Rac1/MKK7/JNK, both necessary for the ECM and morphology changes found during nerve regeneration. These results confirm the role of ECM signaling in peripheral nerve regeneration as well as provide insight to the design of future biomaterials and cellular therapies for peripheral nerve regeneration.

Gene expression profiles of YAP1, TAZ, CRB3, and VDR in familial and sporadic multiple sclerosis among an Iranian population

Alterations in the regulatory mechanisms that control the process of myelination in the nervous system, may lead to the impaired myelination in the Multiple sclerosis. The Hippo pathway is an important mediator of myelination in the nervous system and might contribute to the pathophysiology of MS. This study examined via qPCR the RNA expression of YAP1, TAZ, and CRB3 as the key effectors of the Hippo pathway and also, VDR in the peripheral blood of 35 sporadic, 37 familial MS patients; and also 34 healthy first-degree relatives of the familial MS patients (HFR) and 40 healthy individuals without a family history of the disease (control). The results showed the increased expression of VDR in the sporadic group, as compared to other groups. There was also an increased expression of TAZ in the familial and HFR groups, as compared to the control group. The familial and sporadic patients displayed a significantly lower level of expression of YAP1 in comparison to the HFR group. The increased expression level in the sporadic patients and control group, as compared to the HFR group, was seen in CRB3. We also assessed different clinical parameters and MRI characteristics of the patients. Overall, these findings suggest that Hippo pathway effectors and also VDR gene may play a potential role in the pathophysiology of the sporadic and familial forms of MS. Confirmation of different gene expression patterns in sporadic and familial MS groups may have obvious implications for the personalization of therapies in the disease.

Human Platelet Lysate Acts Synergistically With Laminin to Improve the Neurotrophic Effect of Human Adipose-Derived Stem Cells on Primary Neurons in vitro

Background

Despite the advancements in microsurgical techniques and noteworthy research in the last decade, peripheral nerve lesions have still weak functional outcomes in current clinical practice. However, cell transplantation of human adipose-derived stem cells (hADSC) in a bioengineered conduit has shown promising results in animal studies. Human platelet lysate (hPL) has been adopted to avoid fetal bovine serum (FBS) in consideration of the biosafety concerns inherent with the use of animal-derived products in tissue processing and cell culture steps for translational purposes. In this work, we investigate how the interplay between hPL-expanded hADSC (hADSC^hPL) and extracellular matrix (ECM) proteins influences key elements of nerve regeneration.

Methods

hADSC were seeded on different ECM coatings (laminin, LN; fibronectin, FN) in hPL (or FBS)-supplemented medium and co-cultured with primary dorsal root ganglion (DRG) to establish the intrinsic effects of cell–ECM contact on neural outgrowth. Co-cultures were performed “direct,” where neural cells were seeded in contact with hADSC expanded on ECM-coated substrates (contact effect), or “indirect,” where DRG was treated with their conditioned medium (secretome effect). Brain-derived nerve factor (BDNF) levels were quantified. Tissue culture plastic (TCPS) was used as the control substrate in all the experiments.

Results

hPL as supplement alone did not promote higher neurite elongation than FBS when combined with DRG on ECM substrates. However, in the presence of hADSC, hPL could dramatically enhance the stem cell effect with increased DRG neurite outgrowth when compared with FBS conditions, regardless of the ECM coating (in both indirect and direct co-cultures). The role of ECM substrates in influencing neurite outgrowth was less evident in the FBS conditions, while it was significantly amplified in the presence of hPL, showing better neural elongation in LN conditions when compared with FN and TCPS. Concerning hADSC growth factor secretion, ELISA showed significantly higher concentrations of BDNF when cells were expanded in hPL compared with FBS-added medium, without significant differences between cells cultured on the different ECM substrates.

Conclusion

The data suggest how hADSC grown on LN and supplemented with hPL could be active and prone to support neuron–matrix interactions. hPL enhanced hADSC effects by increasing both proliferation and neurotrophic properties, including BDNF release.

Dynamic Environmental Physical Cues Activate Mechanosensitive Responses in the Repair Schwann Cell Phenotype

Schwann cells plastically change in response to nerve injury to become a newly reconfigured repair phenotype. This cell is equipped to sense and interact with the evolving and unusual physical conditions characterizing the injured nerve environment and activate intracellular adaptive reprogramming as a consequence of external stimuli. Summarizing the literature contributions on this matter, this review is aimed at highlighting the importance of the environmental cues of the regenerating nerve as key factors to induce morphological and functional changes in the Schwann cell population. We identified four different microenvironments characterized by physical cues the Schwann cells sense via interposition of the extracellular matrix. We discussed how the physical cues of the microenvironment initiate changes in Schwann cell behavior, from wrapping the axon to becoming a multifunctional denervated repair cell and back to reestablishing contact with regenerated axons.

Insights Into the Role and Potential of Schwann Cells for Peripheral Nerve Repair From Studies of Development and Injury

Peripheral nerve injuries arising from trauma or disease can lead to sensory and motor deficits and neuropathic pain. Despite the purported ability of the peripheral nerve to self-repair, lifelong disability is common. New molecular and cellular insights have begun to reveal why the peripheral nerve has limited repair capacity. The peripheral nerve is primarily comprised of axons and Schwann cells, the supporting glial cells that produce myelin to facilitate the rapid conduction of electrical impulses. Schwann cells are required for successful nerve regeneration; they partially “de-differentiate” in response to injury, re-initiating the expression of developmental genes that support nerve repair. However, Schwann cell dysfunction, which occurs in chronic nerve injury, disease, and aging, limits their capacity to support endogenous repair, worsening patient outcomes. Cell replacement-based therapeutic approaches using exogenous Schwann cells could be curative, but not all Schwann cells have a “repair” phenotype, defined as the ability to promote axonal growth, maintain a proliferative phenotype, and remyelinate axons. Two cell replacement strategies are being championed for peripheral nerve repair: prospective isolation of “repair” Schwann cells for autologous cell transplants, which is hampered by supply challenges, and directed differentiation of pluripotent stem cells or lineage conversion of accessible somatic cells to induced Schwann cells, with the potential of “unlimited” supply. All approaches require a solid understanding of the molecular mechanisms guiding Schwann cell development and the repair phenotype, which we review herein. Together these studies provide essential context for current efforts to design glial cell-based therapies for peripheral nerve regeneration.

Integration of Hippo-YAP Signaling with Metabolism

The Hippo-Yes-associated protein (YAP) signaling network plays a central role as an integrator of signals that control cellular proliferation and differentiation. The past several years have provided an increasing appreciation and understanding of the diverse mechanisms through which metabolites and metabolic signals influence Hippo-YAP signaling, and how Hippo-YAP signaling, in turn, controls genes that direct cellular and organismal metabolism. These connections enable Hippo-YAP signaling to coordinate organ growth and homeostasis with nutrition and metabolism. In this review, we discuss the current understanding of some of the many interconnections between Hippo-YAP signaling and metabolism and how they are affected in disease conditions.

The Hippo pathway: Horizons for innovative treatments of peripheral nerve diseases

Initially identified in Drosophila, the Hippo signaling pathway regulates how cells respond to their environment by controlling proliferation, migration and differentiation. Many recent studies have focused on characterizing Hippo pathway function and regulation in mammalian cells. Here, we present a brief overview of the major components of the Hippo pathway, as well as their regulation and function. We comprehensively review the studies that have contributed to our understanding of the Hippo pathway in the function of the peripheral nervous system and in peripheral nerve diseases. Finally, we discuss innovative approaches that aim to modulate Hippo pathway components in diseases of the peripheral nervous system.

Ferroptosis-Related Genes in Neurodevelopment and Central Nervous System

Ferroptosis, first introduced as a new form of regulated cell death induced by erastin, is accompanied by the accumulation of iron and lipid peroxides, thus it can be inhibited either by iron chelators or by lipophilic antioxidants. In the past decade, multiple studies have introduced the potential importance of ferroptosis in many human diseases, including cancer and neurodegenerative diseases. In this review, we will discuss the genetic association of ferroptosis with neurological disorders and development of the central nervous system.

αV integrins in Schwann cells promote attachment to axons, but are dispensable in vivo

In the developing peripheral nervous system, Schwann cells (SCs) extend their processes to contact, sort, and myelinate axons. The mechanisms that contribute to the interaction between SCs and axons are just beginning to be elucidated. Using a SC-neuron coculture system, we demonstrate that Arg-Gly-Asp (RGD) peptides that inhibit αV -containing integrins delay the extension of SCs elongating on axons. αV integrins in SC localize to sites of contact with axons and are expressed early in development during radial sorting and myelination. Short interfering RNA-mediated knockdown of the αV integrin subunit also delays SC extension along axons in vitro, suggesting that αV -containing integrins participate in axo-glial interactions. However, mice lacking the αV subunit in SCs, alone or in combination with the potentially compensating α5 subunit, or the αV partners β3 or β8 , myelinate normally during development and remyelinate normally after nerve crush, indicating that overlapping or compensatory mechanisms may hide the in vivo role of RGD-binding integrins.

YAP and TAZ regulate Schwann cell proliferation and differentiation during peripheral nerve regeneration

YAP and TAZ are effectors of the Hippo pathway that controls multicellular development by integrating chemical and mechanical signals. Peripheral nervous system development depends on the Hippo pathway. We previously showed that loss of YAP and TAZ impairs the development of peripheral nerve as well as Schwann cell myelination. The role of the Hippo pathway in peripheral nerve regeneration has just started to be explored. After injury, Schwann cells adopt new identities to promote regeneration by converting to a repair‐promoting phenotype. While the reprogramming of Schwann cells to repair cells has been well characterized, the maintenance of such repair phenotype cannot be sustained for a very long period, which limits nerve repair in human. First, we show that short or long‐term myelin maintenance is not affected by defect in YAP and TAZ expression. Using crush nerve injury and conditional mutagenesis in mice, we also show that YAP and TAZ are regulators of repair Schwann cell proliferation and differentiation. We found that YAP and TAZ are required in repair Schwann cells for their redifferentiation into myelinating Schwann cell following crush injury. In this present study, we describe how the Hippo pathway and YAP and TAZ regulate remyelination over time during peripheral nerve regeneration.

Biomimetic mineralized microenvironment stiffness regulated BMSCs osteogenic differentiation through cytoskeleton mediated mechanical signaling transduction

Construction of biomimetic microenvironment is vital to understand the relationship between matrix mechanical cues and cell fate, as well as to explore potential tissue engineering scaffolds for clinical application. In this study, through the enzymatic mineralizable collagen hydrogel system, we established the biomimetic bone matrix which was capable of realizing mechanical regulation independent of mineralization by incorporation of phosphorylated molecules (vinylphosphonic acid, VAP). Then, based on the biomimetic mineralized matrix with same composition but significantly different mechanical stiffness, we further investigated the effect of matrix stiffness on osteogenic differentiation of bone marrow stromal cells (BMSCs). The results clearly demonstrated that biomimetic mineralized microenvironment with higher mechanical strength promoted osteogenic differentiation of BMSCs. Further mechanism analysis demonstrated that the mineralized hydrogel with higher stiffness promoted cytoskeletal assembly, which enhanced the expression and nuclear colocalization of YAP and RUNX2, thereby promoted the osteogenic differentiation of stem cells. This study supplies a promising material platform not only for bone tissue engineering but also for exploring the mechanism of biomimetic bone matrix mechanics on osteogenesis.

Schwann cell development: From neural crest to myelin sheath

Vertebrate nervous system function requires glial cells, including myelinating glia that insulate axons and provide trophic support that allows for efficient signal propagation by neurons. In vertebrate peripheral nervous systems, neural crest-derived glial cells known as Schwann cells (SCs) generate myelin by encompassing and iteratively wrapping membrane around single axon segments. SC gliogenesis and neurogenesis are intimately linked and governed by a complex molecular environment that shapes their developmental trajectory. Changes in this external milieu drive developing SCs through a series of distinct morphological and transcriptional stages from the neural crest to a variety of glial derivatives, including the myelinating sublineage. Cues originate from the extracellular matrix, adjacent axons, and the developing SC basal lamina to trigger intracellular signaling cascades and gene expression changes that specify stages and transitions in SC development. Here, we integrate the findings from in vitro neuron-glia co-culture experiments with in vivo studies investigating SC development, particularly in zebrafish and mouse, to highlight critical factors that specify SC fate. Ultimately, we connect classic biochemical and mutant studies with modern genetic and visualization tools that have elucidated the dynamics of SC development. This article is categorized under: Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: Regional Development.

Synthesis and Evaluation of Cytocompatible Alkyne-Containing Poly(β-amino ester)-Based Hydrogels Functionalized via Click Reaction

Although poly(β-amino esters) (PAEs) have been widely applied in nonviral gene transfection, drug delivery systems, and regenerative medicine, the multifunctional modification of PAEs and bio-orthogonal strategies of PAE-based hydrogel functionalization is still a challenge. Herein, a strategy of poly(β-amino ester)-based hydrogel functionalization was developed via bio-orthogonal reactions in this study. Acrylate-terminated poly(β-amino esters) containing alkyne groups were synthesized by Michael addition reaction. Alkyne groups on poly(β-amino esters) could conjugate bioactive molecules with azide of K(N₃)RGD via copper-catalyzed azide-alkyne cycloaddition, and terminal acrylate groups could in situ polymerize to prepare a hydrogel. A biomimetic peptide K(N₃)RGD functionalized hydrogel was prepared by polymerization of acrylate-terminated poly(β-amino esters) containing conjugated peptide and polyethylene glycol diacrylate (PEGDA). The storage modulus and mechanical properties exhibited an increased trend with the increased concentration; nevertheless, swelling ratio and surface wetting properties demonstrated a decreased tendency by increased concentrations. Cell proliferation and live/dead staining showed that Schwann cells plated on the hydrogel with an elastic modulus of 25.39 KPa are more suitable for proliferation and function exertion of Schwann cells compared with that of 42.11 and 57.86 KPa, and KRGD-conjugated hydrogel could increase the elongation of Schwann cells relative to nonconjugated hydrogels. This azide-alkyne strategy may be a promising candidate for hydrogel functionalization in tissue engineering and other biomedical applications.

Biomimicking Fiber Platform with Tunable Stiffness to Study Mechanotransduction Reveals Stiffness Enhances Oligodendrocyte Differentiation but Impedes Myelination through YAP-Dependent Regulation

A key hallmark of many diseases, especially those in the central nervous system (CNS), is the change in tissue stiffness due to inflammation and scarring. However, how such changes in microenvironment affect the regenerative process remains poorly understood. Here, a biomimicking fiber platform that provides independent variation of fiber structural and intrinsic stiffness is reported. To demonstrate the functionality of these constructs as a mechanotransduction study platform, these substrates are utilized as artificial axons and the effects of axon structural versus intrinsic stiffness on CNS myelination are independently analyzed. While studies have shown that substrate stiffness affects oligodendrocyte differentiation, the effects of mechanical stiffness on the final functional state of oligodendrocyte (i.e., myelination) has not been shown prior to this. Here, it is demonstrated that a stiff mechanical microenvironment impedes oligodendrocyte myelination, independently and distinctively from oligodendrocyte differentiation. Yes-associated protein is identified to be involved in influencing oligodendrocyte myelination through mechanotransduction. The opposing effects on oligodendrocyte differentiation and myelination provide important implications for current work screening for promyelinating drugs, since these efforts have focused mainly on promoting oligodendrocyte differentiation. Thus, the platform may have considerable utility as part of a drug discovery program in identifying molecules that promote both differentiation and myelination.

CTCF-mediated chromatin looping in EGR2 regulation and SUZ12 recruitment critical for peripheral myelination and repair

Chromatin organization is critical for cell growth, differentiation, and disease development, however, its functions in peripheral myelination and myelin repair remain elusive. In this report, we demonstrate that the CCCTC-binding factor (CTCF), a crucial chromatin organizer, is essential for Schwann cell myelination and myelin regeneration after nerve injury. Inhibition of CTCF or its deletion blocks Schwann cell differentiation at the pro-myelinating stage, whereas overexpression of CTCF promotes the myelination program. We find that CTCF establishes chromatin interaction loops between enhancer and promoter regulatory elements and promotes expression of a key pro-myelinogenic factor EGR2. In addition, CTCF interacts with SUZ12, a component of polycomb-repressive-complex 2 (PRC2), to repress the transcriptional program associated with negative regulation of Schwann cell maturation. Together, our findings reveal a dual role of CTCF-dependent chromatin organization in promoting myelinogenic programs and recruiting chromatin-repressive complexes to block Schwann cell differentiation inhibitors to control peripheral myelination and repair.

Myelination by Schwann cells (SC) in the peripheral nervous system is essential for motor function, and dysregulation of SC myelination can lead to various neuropathies. Here the authors describe a critical role of CCCTC-binding factor (CTCF)-dependent chromatin reorganization in peripheral myelination and myelin regeneration after injury.

Effects of NP-1 on proliferation, migration, and apoptosis of Schwann cell line RSC96 through the NF-κB signaling pathway

Peripheral nerve injury is a common refractory disease in the clinic that often leads to dysfunction of movement and sensation. Different from other tissue injuries, peripheral nerve injury needs a longer time for regeneration. Therefore, effective drug therapy is needed to promote nerve regeneration in the treatment of peripheral nerve injury. Our preliminary studies have shown that continuous intramuscular injection of NP-1 promotes the regeneration of injured sciatic nerve in rats, but the mechanisms were still unknown. Schwann cells are very important cells in the formation of myelin sheath of peripheral nerves and participate in the repair and regeneration of peripheral nerve injury. To further investigate the effect of NP-1 on rat Schwann cells and the underlying mechanism, different concentrations of NP-1 were used to treat rat Schwann cell line RSC96. Light microscopy, CCK-8 assay, cell scratch assay, and special cell staining were performed to investigate RSC96 cell aging and apoptosis. mRNA and protein expression of NF-κB signaling pathway-related factors were determined using qPCR and immunohistochemistry respectively. Light microscopy, CCK-8 assay, cell scratch assay, and special cell staining showed NP-1 could improve the ability of proliferation, immigration of Schwann cells. QPCR and immunohistochemistry showed NP-1 influenced the expression of multiple factors associated with nerve regeneration which NF-κB signaling pathway played a key role. The results show that NP-1 promoted the proliferation and migration of RSC96 cells and inhibited cell aging and apoptosis possibly through the NF-κB signaling pathway. These findings provide a potential target for clinical treatment of peripheral neuropathy and experimental data support.

Role of YAP/TAZ in Cell Lineage Fate Determination and Related Signaling Pathways

The penultimate effectors of the Hippo signaling pathways YAP and TAZ, are transcriptional co-activator proteins that play key roles in many diverse biological processes, ranging from cell proliferation, tumorigenesis, mechanosensing and cell lineage fate determination, to wound healing and regeneration. In this review, we discuss the regulatory mechanisms by which YAP/TAZ control stem/progenitor cell differentiation into the various major lineages that are of interest to tissue engineering and regenerative medicine applications. Of particular interest is the key role of YAP/TAZ in maintaining the delicate balance between quiescence, self-renewal, proliferation and differentiation of endogenous adult stem cells within various tissues/organs during early development, normal homeostasis and regeneration/healing. Finally, we will consider how increasing knowledge of YAP/TAZ signaling might influence the trajectory of future progress in regenerative medicine.

Signals Orchestrating Peripheral Nerve Repair

The peripheral nervous system has retained through evolution the capacity to repair and regenerate after assault from a variety of physical, chemical, or biological pathogens. Regeneration relies on the intrinsic abilities of peripheral neurons and on a permissive environment, and it is driven by an intense interplay among neurons, the glia, muscles, the basal lamina, and the immune system. Indeed, extrinsic signals from the milieu of the injury site superimpose on genetic and epigenetic mechanisms to modulate cell intrinsic programs. Here, we will review the main intrinsic and extrinsic mechanisms allowing severed peripheral axons to re-grow, and discuss some alarm mediators and pro-regenerative molecules and pathways involved in the process, highlighting the role of Schwann cells as central hubs coordinating multiple signals. A particular focus will be provided on regeneration at the neuromuscular junction, an ideal model system whose manipulation can contribute to the identification of crucial mediators of nerve re-growth. A brief overview on regeneration at sensory terminals is also included.

Regulation and functions of the Hippo pathway in stemness and differentiation

The Hippo pathway plays important roles in organ development, tissue regeneration, and human diseases, such as cancer. In the canonical Hippo pathway, the MST1/2-LATS1/2 kinase cascade phosphorylates and inhibits transcription coactivators Yes-associated protein and transcription coactivator with PDZ-binding motif and thus regulates transcription of genes important for cell proliferation and apoptosis. However, recent studies have depicted a much more complicate picture of the Hippo pathway with many new components and regulatory stimuli involving both chemical and mechanical signals. Furthermore, accumulating evidence indicates that the Hippo pathway also plays important roles in the determination of cell fates, such as self-renewal and differentiation. Here, we review regulations of the Hippo pathway and its functions in stemness and differentiation emphasizing recent discoveries.

Axon-dependent expression of YAP/TAZ mediates Schwann cell remyelination but not proliferation after nerve injury

Previously we showed that YAP/TAZ promote not only proliferation but also differentiation of immature Schwann cells (SCs), thereby forming and maintaining the myelin sheath around peripheral axons (Grove et al., 2017). Here we show that YAP/TAZ are required for mature SCs to restore peripheral myelination, but not to proliferate, after nerve injury. We find that YAP/TAZ dramatically disappear from SCs of adult mice concurrent with axon degeneration after nerve injury. They reappear in SCs only if axons regenerate. YAP/TAZ ablation does not impair SC proliferation or transdifferentiation into growth promoting repair SCs. SCs lacking YAP/TAZ, however, fail to upregulate myelin-associated genes and completely fail to remyelinate regenerated axons. We also show that both YAP and TAZ are redundantly required for optimal remyelination. These findings suggest that axons regulate transcriptional activity of YAP/TAZ in adult SCs and that YAP/TAZ are essential for functional regeneration of peripheral nerve.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

Tead transcription factors differentially regulate cortical development

Neural stem cells (NSCs) generate neurons of the cerebral cortex with distinct morphologies and functions. How specific neuron production, differentiation and migration are orchestrated is unclear. Hippo signaling regulates gene expression through Tead transcription factors (TFs). We show that Hippo transcriptional coactivators Yap1/Taz and the Teads have distinct functions during cortical development. Yap1/Taz promote NSC maintenance and Satb2⁺ neuron production at the expense of Tbr1⁺ neuron generation. However, Teads have moderate effects on NSC maintenance and do not affect Satb2⁺ neuron differentiation. Conversely, whereas Tead2 blocks Tbr1⁺ neuron formation, Tead1 and Tead3 promote this early fate. In addition, we found that Hippo effectors regulate neuronal migration to the cortical plate (CP) in a reciprocal fashion, that ApoE, Dab2 and Cyr61 are Tead targets, and these contribute to neuronal fate determination and migration. Our results indicate that multifaceted Hippo signaling is pivotal in different aspects of cortical development.

Neuronal contact guidance and YAP signaling on ultra-small nanogratings

Contact interaction of neuronal cells with extracellular nanometric features can be exploited to investigate and modulate cellular responses. By exploiting nanogratings (NGs) with linewidth from 500 nm down to 100 nm, we here study neurite contact guidance along ultra-small directional topographies. The impact of NG lateral dimension on the neuronal morphotype, neurite alignment, focal adhesion (FA) development and YAP activation is investigated in nerve growth factor (NGF)-differentiating PC12 cells and in primary hippocampal neurons, by confocal and live-cell total internal reflection fluorescence (TIRF) microscopy, and at molecular level. We demonstrate that loss of neurite guidance occurs in NGs with periodicity below 400 nm and correlates with a loss of FA lateral constriction and spatial organization. We found that YAP intracellular localization is modulated by the presence of NGs, but it is not sensitive to their periodicity. Nocodazole, a drug that can increase cell contractility, is finally tested for rescuing neurite alignment showing mild ameliorative effects. Our results provide new indications for a rational design of biocompatible scaffolds for enhancing nerve-regeneration processes.

Functions of histone modifications and histone modifiers in Schwann cells

Schwann cells (SCs) are the main glial cells present in the peripheral nervous system (PNS). Their primary functions are to insulate peripheral axons to protect them from the environment and to enable fast conduction of electric signals along big caliber axons by enwrapping them in a thick myelin sheath rich in lipids. In addition, SCs have the peculiar ability to foster axonal regrowth after a lesion by demyelinating and converting into repair cells that secrete neurotrophic factors and guide axons back to their former target to finally remyelinate regenerated axons. The different steps of SC development and their role in the maintenance of PNS integrity and regeneration after lesion are controlled by various factors among which transcription factors and chromatin-remodeling enzymes hold major functions. In this review, we discussed how histone modifications and histone-modifying enzymes control SC development, maintenance of PNS integrity and response to injury. The functions of histone modifiers as part of chromatin-remodeling complexes are discussed in another review published in the same issue of Glia.

YAP/TAZ functions and their regulation at a glance

YAP and TAZ proteins are transcriptional coactivators encoded by paralogous genes, which shuttle between the cytoplasm and the nucleus in response to multiple inputs, including the Hippo pathway. In the nucleus, they pair with DNA-binding factors of the TEAD family to regulate gene expression. Nuclear YAP/TAZ promote cell proliferation, organ overgrowth, survival to stress and dedifferentiation of post-mitotic cells into their respective tissue progenitors. YAP/TAZ are required for growth of embryonic tissues, wound healing and organ regeneration, where they are activated by cell-intrinsic and extrinsic cues. Surprisingly, this activity is dispensable in many adult self-renewing tissues, where YAP/TAZ are constantly kept in check. YAP/TAZ lay at the center of a complex regulatory network including cell-autonomous factors but also cell- and tissue-level structural features such as the mechanical properties of the cell microenvironment, the establishment of cell-cell junctions and of basolateral tissue polarity. Enhanced levels and activity of YAP/TAZ are observed in many cancers, where they sustain tumor growth, drug resistance and malignancy. In this Cell Science at a Glance article and the accompanying poster, we review the biological functions of YAP/TAZ and their regulatory mechanisms, and highlight their position at the center of a complex signaling network.

Syringic acid promotes proliferation and migration of Schwann cells via down-regulating miR-451-5p

Schwann cells are the main force in spontaneous regeneration after peripheral nerve injury. The neurotrophic factors could promote the regeneration, but clinical applications of these factors are limited by some constraints. Hence, searching for new substances to elevate the function of Schwann cells and facilitate the regeneration of nerve is urgently needed. Syringic acid (SA) is a natural product with neuroprotective activity in vivo, but the role of SA on Schwann cells remains unclear. In this study, we for the first time found that SA was able to promote the proliferation and migration of Schwann cells, two important abilities in the process of regeneration. Then, microRNA (miRNA) microarray analysis was performed and 26 differentially expressed miRNAs (22 down-regulated and 4 up-regulated) were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) signaling pathway analyses found that the target genes of these miRNAs were mainly enriched in cellular response to chemical stimulus and cancer-related pathways, respectively. Subsequently, the levels of top 6 down-regulated miRNAs were validated by RT-qPCR and miR-451-5p was shown to be the most down-regulated one. Further experiments demonstrated that inhibition of miR-451-5p significantly promoted the proliferation and migration of Schwann cells. These results suggested that SA promoted the proliferation and migration of Schwann cells via down-regulation of miR-451-5p, and SA could be developed into a promising nutritional supplement to assist peripheral nerve regeneration.

Extracellular matrix cues modulate Schwann cell morphology, proliferation, and protein expression

Peripheral nerve injuries require a complex set of signals from cells, macrophages, and the extracellular matrix (ECM) to induce regeneration across injury sites and achieve functional recovery. Schwann cells (SCs), the major glial cell in the peripheral nervous system (PNS), are critical to nerve regeneration due to their inherent capacity for altering phenotype postinjury to facilitate wound healing. The ECM plays a vital role in wound healing as well as regulating cell phenotype during tissue repair. To examine the underlying mechanisms between the ECM and SCs, this work sought to determine how specific ECM cues regulate the phenotype of SCs. To address this, SCs were cultured on polydimethylsiloxane substrates of a variable Young's modulus coated with ECM proteins. Cells were analyzed for spreading area, proliferation, cell and nuclear shape, and c-Jun expression. It was found that substrates with a stiffness of 8.67 kPa coated with laminin promoted the highest expression of c-Jun, a marker signifying a "regenerative" SC. Microcontact printed, cell adhesive areas were then utilized to precisely control the geometry and spreading of SCs and by controlling spreading area and cellular elongation; expression of c-Jun was either promoted or downregulated. These results begin to address the significant interplay between ECM cues and phenotype of SCs, while offering a potential means to enhance PNS regeneration through cellular therapies.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

Yap/Taz are required for establishing the cerebellar radial glia scaffold and proper foliation

Yap/Taz are well-established downstream effectors of the Hippo pathway, known to regulate organ size by directing proliferation and apoptosis. Although the functions of Yap/Taz have been extensively studied, little is known about their role in brain development. Here, through genetic ablation, we show that Yap/Taz are required for cerebellar morphogenesis. Yap/Taz deletion in neural progenitors causes defects in secondary fissure formation, leading to abnormal folia development. Although they seemed very likely to serve an important function in the development of cerebellar granule cell precursors, Yap/Taz are dispensable for their proliferation. Furthermore, Yap/Taz loss does not rescue the medulloblastoma phenotype caused by constitutively active Smoothened. Importantly, Yap/Taz are highly expressed in radial glia and play a crucial role in establishing the radial scaffold and cellular polarity of neural progenitors during embryogenesis. We found that Yap/Taz are necessary to establish and maintain junctional integrity of cerebellar neuroepithelium as prominent junction proteins are not maintained at the apical junction in the absence of Yap/Taz. Our study identifies a novel function of Yap/Taz in cerebellar foliation and finds that they are required to establish the radial glia scaffold and junctional stability.

Regulating PMP22 expression as a dosage sensitive neuropathy gene

Structural variation in the human genome has emerged as a major cause of disease as genomic data have accumulated. One of the most common structural variants associated with human disease causes the heritable neuropathy known as Charcot-Marie-Tooth (CMT) disease type 1A. This 1.4 Mb duplication causes nearly half of the CMT cases that are genetically diagnosed. The PMP22 gene is highly induced in Schwann cells during development, although its precise role in myelin formation and homeostasis is still under active investigation. The PMP22 gene can be considered as a nucleoprotein complex with enzymatic activity to produce the PMP22 transcript, and the complex is allosterically regulated by transcription factors that respond to intracellular signals and epigenomic modifications. The control of PMP22 transcript levels has been one of the major therapeutic targets of therapy development, and this review summarizes those approaches as well as efforts to characterize the regulation of the PMP22 gene.

Mechanical changes of peripheral nerve tissue microenvironment and their structural basis during development

Peripheral nerves are constantly exposed to mechanical stresses associated with body growth and limb movements. Although some aspects of these nerves' biomechanical properties are known, the link between nerve biomechanics and tissue microstructures during development is poorly understood. Here, we used atomic force microscopy to comprehensively investigate the elastic modulus of living peripheral nerve tissue cross sections ex vivo at distinct stages of development and correlated these elastic moduli with various cellular and extracellular aspects of the underlying histological microstructure. We found that local nerve tissue stiffness is spatially heterogeneous and evolves biphasically during maturation. Furthermore, we found the intracellular microtubule network and the extracellular matrix collagens type I and type IV as major contributors to the nerves' biomechanical properties, but surprisingly not cellular density and myelin content as previously shown for the central nervous system. Overall, these findings characterize the mechanical microenvironment that surrounds Schwann cells and neurons and will further our understanding of their mechanosensing mechanisms during nerve development. These data also provide the design of artificial nerve scaffolds to promote biomedical nerve regeneration therapies by considering mechanical properties that better reflect the nerve microenvironment.

Laminin α2 controls mouse and human stem cell behaviour during midbrain dopaminergic neuron development

Development of the central nervous system requires coordination of the proliferation and differentiation of neural stem cells. Here, we show that laminin alpha 2 (lm-α2) is a component of the midbrain dopaminergic neuron (mDA) progenitor niche in the ventral midbrain (VM) and identify a concentration-dependent role for laminin α2β1γ1 (lm211) in regulating mDA progenitor proliferation and survival via a distinct set of receptors. At high concentrations, lm211-rich environments maintain mDA progenitors in a proliferative state via integrins α6β1 and α7β1, whereas low concentrations of lm211 support mDA lineage survival via dystroglycan receptors. We confirmed our findings in vivo, demonstrating that the VM was smaller in the absence of lm-α2, with increased apoptosis; furthermore, the progenitor pool was depleted through premature differentiation, resulting in fewer mDA neurons. Examination of mDA neuron subtype composition showed a reduction in later-born mDA neurons of the ventral tegmental area, which control a range of cognitive behaviours. Our results identify a novel role for laminin in neural development and provide a possible mechanism for autism-like behaviours and the brainstem hypoplasia seen in some individuals with mutations of LAMA2.

Somatic Mutations of lats2 Cause Peripheral Nerve Sheath Tumors in Zebrafish

The cellular signaling pathways underlying peripheral nerve sheath tumor (PNST) formation are poorly understood. Hippo signaling has been recently implicated in the biology of various cancers, and is thought to function downstream of mutations in the known PNST driver, NF2. Utilizing CRISPR-Cas9 gene editing, we targeted the canonical Hippo signaling kinase Lats2. We show that, while germline deletion leads to early lethality, targeted somatic mutations of zebrafish lats2 leads to peripheral nerve sheath tumor formation. These peripheral nerve sheath tumors exhibit high levels of Hippo effectors Yap and Taz, suggesting that dysregulation of these transcriptional co-factors drives PNST formation in this model. These data indicate that somatic lats2 deletion in zebrafish can serve as a powerful experimental platform to probe the mechanisms of PNST formation and progression.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.

YAP and TAZ Regulate Cc2d1b and Purβ in Schwann Cells

Schwann cells (SCs) are exquisitely sensitive to the elasticity of their environment and their differentiation and capacity to myelinate depend on the transduction of mechanical stimuli by YAP and TAZ. YAP/TAZ, in concert with other transcription factors, regulate several pathways including lipid and sterol biosynthesis as well as extracellular matrix receptor expressions such as integrins and G-proteins. Yet, the characterization of the signaling downstream YAP/TAZ in SCs is incomplete. Myelin sheath production by SC coincides with rapid up-regulation of numerous transcription factors. Here, we show that ablation of YAP/TAZ alters the expression of transcription regulators known to regulate SC myelin gene transcription and differentiation. Furthermore, we link YAP/TAZ to two DNA binding proteins, Cc2d1b and Purβ, which have no described roles in myelinating glial cells. We demonstrate that silencing of either Cc2d1b or Purβ limits the formation of myelin segments. These data provide a deeper insight into the myelin gene transcriptional network and the role of YAP/TAZ in myelinating glial cells.

YAP promotes neural crest emigration through interactions with BMP and Wnt activities

Background

Premigratory neural crest progenitors undergo an epithelial-to-mesenchymal transition and leave the neural tube as motile cells. Previously, we showed that BMP generates trunk neural crest emigration through canonical Wnt signaling which in turn stimulates G1/S transition. The molecular network underlying this process is, however, not yet completely deciphered.

Yes-associated-protein (YAP), an effector of the Hippo pathway, controls various aspects of development including cell proliferation, migration, survival and differentiation. In this study, we examined the possible involvement of YAP in neural crest emigration and its relationship with BMP and Wnt.

Methods

We implemented avian embryos in which levels of YAP gene activity were either reduced or upregulated by in ovo plasmid electroporation, and monitored effects on neural crest emigration, survival and proliferation. Neural crest-derived sensory neuron and melanocyte development were assessed upon gain of YAP function. Imunohistochemistry was used to assess YAP expression. In addition, the activity of specific signaling pathways including YAP, BMP and Wnt was monitored with specific reporters.

Results

We find that the Hippo pathway transcriptional co-activator YAP is expressed and is active in premigratory crest of avian embryos. Gain of YAP function stimulates neural crest emigration in vivo, and attenuating YAP inhibits cell exit. This is associated with an accumulation of FoxD3-expressing cells in the dorsal neural tube, with reduced proliferation, and enhanced apoptosis. Furthermore, gain of YAP function inhibits differentiation of Islet-1-positive sensory neurons and augments the number of EdnrB2-positive melanocytes. Using specific in vivo reporters, we show that loss of YAP function in the dorsal neural tube inhibits BMP and Wnt activities whereas gain of YAP function stimulates these pathways. Reciprocally, inhibition of BMP and Wnt signaling by noggin or Xdd1, respectively, downregulates YAP activity. In addition, YAP-dependent stimulation of neural crest emigration is compromised upon inhibition of either BMP or Wnt activities. Together, our results suggest a positive bidirectional cross talk between these pathways.

Conclusions

Our data show that YAP is necessary for emigration of neural crest progenitors. In addition, they incorporate YAP signaling into a BMP/Wnt-dependent molecular network responsible for emigration of trunk-level neural crest.