Akt regulates neurite growth by phosphorylation-dependent inhibition of radixin proteasomal degradation

Chondroitin Sulfate E Regulates Neuronal Adhesion by Promoting Actin Polymerization Through Activation of the VEGF Receptor/Akt/GSK3β Pathway

Neuronal adhesion is regulated by interactions between neurons and the extracellular matrix and plays a critical role in neural development. Chondroitin sulfate proteoglycans (CSPGs), key structural components of the extracellular matrix in the nervous system, are involved in various processes including neuronal migration, neurite outgrowth, and axonal regeneration. The diverse functions of CSPGs are controlled by the sulfated structures of chondroitin sulfate (CS) polysaccharides. In this study, we found that CS-E, a highly sulfated CS polysaccharide, but not CS-A, CS-B, CS-C, or CS -D, induced the formation of neuronal aggregates in primary cortical cells with features of immature neurons. This effect is likely due to the unique properties of CS-E, which altered cell adhesion to the coated surface of culture coverslips, leading to cell detachment and subsequent aggregate formation. Pharmacological and phosphorylation analyses aimed at elucidating signaling cascades revealed that the VEGF receptor is a cell surface target of CS-E, and subsequent activation of the Akt/GSK3β pathway plays a crucial role in aggregate formation. The c-Raf/GSK3β pathway is also involved in CS-E-induced aggregate formation. Furthermore, actin polymerization and the organization of the F-actin cytoskeleton, which are mediated through activation of the Akt/GSK3β pathway, were required for CS-E-induced aggregate formation. In summary, CS-E regulates neuronal adhesion by activating the VEGF receptor/Akt/GSK3β pathway. The identification of signaling cascades involving CS-E may provide insights into the mechanisms underlying neuronal development.

Radixin: Roles in the Nervous System and Beyond

BACKGROUND

Radixin is an ERM family protein that includes radixin, moesin, and ezrin. The importance of ERM family proteins has been attracting more attention, and studies on the roles of ERM in biological function and the pathogenesis of some diseases are accumulating. In particular, we have found that radixin is the most dramatically changed ERM protein in elevated glucose-treated Schwann cells.

METHOD

We systemically review the literature on ERM, radixin in focus, and update the roles of radixin in regulating cell morphology, interaction, and cell signaling pathways. The potential of radixin as a therapeutic target in neurodegenerative diseases and cancer was also discussed.

RESULTS

Radixin research has focused on its cell functions, activation, and pathogenic roles in some diseases. Radixin and other ERM proteins maintain cell shape, growth, and motility. In the nervous system, radixin has been shown to prevent neurodegeneration and axonal growth. The activation of radixin is through phosphorylation of its conserved threonine residues. Radixin functions in cell signaling pathways by binding to membrane proteins and relaying the cell signals into the cells. Deficiency of radixin has been involved in the pathogenic process of diseases in the central nervous system and diabetic peripheral nerve injury. Moreover, radixin also plays a role in cell growth and drug resistance in multiple cancers. The trials of therapeutic potential through radixin modulation have been accumulating. However, the exact mechanisms underlying the roles of radixin are far from clarification.

CONCLUSIONS

Radixin plays various roles in cells and is involved in developing neurodegenerative diseases and many types of cancers. Therefore, radixin may be considered a potential target for developing therapeutic strategies for its related diseases. Further elucidation of the function and the cell signaling pathways that are linked to radixin may open the avenue to finding novel therapeutic strategies for diseases in the nervous system and other body systems.

Non-integrin laminin receptor (LamR) plays a role in axonal outgrowth from chicken DRG via modulating the Akt and Erk signaling

37/67 kDa laminin receptor (LamR)/ribosomal protein SA exhibits dual function as both a ribosomal protein and cell surface receptor for laminin. LamR influences critical cellular processes such as invasion, adhesion, and migration when acting as a receptor. Despite the acknowledged importance of LamR/67LR in various cellular processes, its contribution to the peripheral nervous system development is obscure. Thus, this study investigated the biological activity of LamR in peripheral axonal outgrowth in the presence of laminin-1 or Ile-Lys-Val-Ala-Val (IKVAV) peptide, whose important role in dorsal root ganglia (DRG) axonal outgrowth we recently showed. Unexpectedly, we did not observe LamR on the surface of DRG cells or in a conditioned medium, suggesting its intracellular action in the negative regulation of DRG axonal outgrowth. Using C-terminus LamR-targeting IgG, we demonstrated the role of LamR in that process, which is independent of the presence of Schwann cell precursors (SCPs) and is mediated by extracellular signal-regulated kinase (Erk) and Protein kinase B (Akt1/2/3) signaling pathways. Additionally, we show that the action of LamR towards laminin-1-dependent axonal outgrowth is unmasked only when the activity of integrin β1 is perturbed. We believe that modulation of LamR activity provides the basis for its use for inhibiting axon growth as a potential therapeutic agent for regulating abnormal or excessive neurite growth during neurodevelopmental diseases or pathological nerve regeneration.

Novel insight into the role of A-kinase anchoring proteins (AKAPs) in ischemic stroke and therapeutic potentials

Ischemic stroke, a devastating disease associated with high mortality and disability worldwide, has emerged as an urgent public health issue. A-kinase anchoring proteins (AKAPs) are a group of signal-organizing molecules that compartmentalize and anchor a wide range of receptors and effector proteins and have a major role in stabilizing mitochondrial function and promoting neurodevelopmental development in the central nervous system (CNS). Growing evidence suggests that dysregulation of AKAPs expression and activity is closely associated with oxidative stress, ion disorder, mitochondrial dysfunction, and blood-brain barrier (BBB) impairment in ischemic stroke. However, the underlying mechanisms remain inadequately understood. This review provides a comprehensive overview of the composition and structure of A-kinase anchoring protein (AKAP) family members, emphasizing their physiological functions in the CNS. We explored in depth the molecular and cellular mechanisms of AKAP complexes in the pathological progression and risk factors of ischemic stroke, including hypertension, hyperglycemia, lipid metabolism disorders, and atrial fibrillation. Herein, we highlight the potential of AKAP complexes as a pharmacological target against ischemic stroke in the hope of inspiring translational research and innovative clinical approaches.

Multifunctional wet-adhesive chitosan/acrylic conduit for sutureless repair of peripheral nerve injuries

The incidence of peripheral nerve injury (PNI) is high worldwide, and a poor prognosis is common. Surgical closure and repair of the affected area are crucial to ensure the effective treatment of peripheral nerve injuries. Despite being the standard treatment approach, reliance on sutures to seal the severed nerve ends introduces several limitations and restrictions. This technique is intricate and time-consuming, and the application of threading and punctate sutures may lead to tissue damage and heightened tension concentrations, thus increasing the risk of fixation failure and local inflammation. This study aimed to develop easily implantable chitosan-based peripheral nerve repair conduits that combine acrylic acid and cleavable N-hydroxysuccinimide to reduce nerve damage during repair. In ex vivo tissue adhesion tests, the conduit achieved maximal interfacial toughness of 705 J m-2 ± 30 J m-2, allowing continuous bridging of the severed nerve ends. Adhesive repair significantly reduces local inflammation caused by conventional sutures, and the positive charge of chitosan disrupts the bacterial cell wall and reduces implant-related infections. This promises to open new avenues for sutureless nerve repair and reliable medical implants.

Heimdall, an alternative protein issued from a ncRNA related to kappa light chain variable region of immunoglobulins from astrocytes: a new player in neural proteome

The dogma "One gene, one protein" is clearly obsolete since cells use alternative splicing and generate multiple transcripts which are translated into protein isoforms, but also use alternative translation initiation sites (TISs) and termination sites on a given transcript. Alternative open reading frames for individual transcripts give proteins originate from the 5'- and 3'-UTR mRNA regions, frameshifts of mRNA ORFs or from non-coding RNAs. Longtime considered as non-coding, recent in-silico translation prediction methods enriched the protein databases allowing the identification of new target structures that have not been identified previously. To gain insight into the role of these newly identified alternative proteins in the regulation of cellular functions, it is crucial to assess their dynamic modulation within a framework of altered physiological modifications such as experimental spinal cord injury (SCI). Here, we carried out a longitudinal proteomic study on rat SCI from 12 h to 10 days. Based on the alternative protein predictions, it was possible to identify a plethora of newly predicted protein hits. Among these proteins, some presented a special interest due to high homology with variable chain regions of immunoglobulins. We focus our interest on the one related to Kappa variable light chains which is similarly highly produced by B cells in the Bence jones disease, but here expressed in astrocytes. This protein, name Heimdall is an Intrinsically disordered protein which is secreted under inflammatory conditions. Immunoprecipitation experiments showed that the Heimdall interactome contained proteins related to astrocyte fate keepers such as "NOTCH1, EPHA3, IPO13" as well as membrane receptor protein including "CHRNA9; TGFBR, EPHB6, and TRAM". However, when Heimdall protein was neutralized utilizing a specific antibody or its gene knocked out by CRISPR-Cas9, sprouting elongations were observed in the corresponding astrocytes. Interestingly, depolarization assays and intracellular calcium measurements in Heimdall KO, established a depolarization effect on astrocyte membranes KO cells were more likely that the one found in neuroprogenitors. Proteomic analyses performed under injury conditions or under lipopolysaccharides (LPS) stimulation, revealed the expression of neuronal factors, stem cell proteins, proliferation, and neurogenesis of astrocyte convertor factors such as EPHA4, NOTCH2, SLIT3, SEMA3F, suggesting a role of Heimdall could regulate astrocytic fate. Taken together, Heimdall could be a novel member of the gatekeeping astrocyte-to-neuroprogenitor conversion factors.

Ste20-like Kinase Is Critical for Inhibitory Synapse Maintenance and Its Deficiency Confers a Developmental Dendritopathy

The size and structure of the dendritic arbor play important roles in determining how synaptic inputs of neurons are converted to action potential output. The regulatory mechanisms governing the development of dendrites, however, are insufficiently understood. The evolutionary conserved Ste20/Hippo kinase pathway has been proposed to play an important role in regulating the formation and maintenance of dendritic architecture. A key element of this pathway, Ste20-like kinase (SLK), regulates cytoskeletal dynamics in non-neuronal cells and is strongly expressed throughout neuronal development. However, its function in neurons is unknown. We show that, during development of mouse cortical neurons, SLK has a surprisingly specific role for proper elaboration of higher, ≥ third-order dendrites both in male and in female mice. Moreover, we demonstrate that SLK is required to maintain excitation-inhibition balance. Specifically, SLK knockdown caused a selective loss of inhibitory synapses and functional inhibition after postnatal day 15, whereas excitatory neurotransmission was unaffected. Finally, we show that this mechanism may be relevant for human disease, as dysmorphic neurons within human cortical malformations revealed significant loss of SLK expression. Overall, the present data identify SLK as a key regulator of both dendritic complexity during development and inhibitory synapse maintenance.SIGNIFICANCE STATEMENT We show that dysmorphic neurons of human epileptogenic brain lesions have decreased levels of the Ste20-like kinase (SLK). Decreasing SLK expression in mouse neurons revealed that SLK has essential functions in forming the neuronal dendritic tree and in maintaining inhibitory connections with neighboring neurons.

Inhibitor of DNA binding 2 (Id2) mediates microtubule polymerization in the brain by regulating αK40 acetylation of α-tubulin

Acetylation of α-tubulin lysine 40 (αK40) contributes to microtubule (MT) stability and is essential for neuronal development and function, whereas excessive αK40 deacetylation is observed in neurodegenerative disorders including Alzheimer’s disease (AD). Here we identified inhibitor of DNA binding 2 (Id2) as a novel MT-binding partner that interacts with α-tubulin and enhances αK40 acetylation, leading to MT polymerization in the neurons. Commensurate with our finding that the low levels of Id2 expression along with a reduced αK40 acetylation in the postmortem human AD patient and 5X-FAD, AD model mice brain, Id2 upregulation in the hippocampus of 5X-FAD, which exhibit high levels of Sirt2 expression, increased αK40 acetylation and reconstitutes axon growth. Hence our study suggests that Id2 is critical for maintaining MT stability during neural development and the potential of Id2 to counteract pathogenic Sirt2 activity in AD.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

Akt and Src mediate the photocrosslinked fibroin-induced neural differentiation

Neural transplantation is a promising modality for treatment of neurodegenerative diseases, traumatic brain injury and stroke. Biocompatible scaffolds with optimized properties improve the survival of transplanted neural cells and differentiation of progenitor cells into the desired types of neurons. Silk fibroin is a biocompatible material for tissue engineering. Here, we describe thin-film scaffolds based on photocrosslinked methacrylated silk fibroin (FBMA). These scaffolds exhibit an increased mechanical stiffness and improved water stability. Photocrosslinking of fibroin increased its rigidity from 25 to 480 kPa and the contact angle from 59.7 to 70.8, the properties important for differentiation of neural cells. Differentiation of SH-SY5Y neuroblastoma cells on FBMA increased the length of neurites as well as the levels of neural differentiation markers MAP2 and βIII-tubulin. Growth of SH-SY5Y cells on the unmodified fibroin and FBMA substrates led to a spontaneous phosphorylation of Src and Akt protein kinases critical for neuronal differentiation; this effect was paralleled by neural cell adhesion molecule elevation. Thus, FBMA is an easily manufactured, cytocompatible material with improved and sustainable properties applicable for neural tissue engineering.

Nystatin Regulates Axonal Extension and Regeneration by Modifying the Levels of Nitric Oxide

Nystatin is a pharmacological agent commonly used for the treatment of oral, mucosal and cutaneous fungal infections. Nystatin has also been extensively applied to study the cellular function of cholesterol-enriched structures because of its ability to bind and extract cholesterol from mammalian membranes. In neurons, cholesterol level is tightly regulated, being essential for synapse and dendrite formation, and axonal guidance. However, the action of Nystatin on axon regeneration has been poorly evaluated. Here, we examine the effect of Nystatin on primary cultures of hippocampal neurons, showing how acute dose (minutes) of Nystatin increases the area of growth cones, and chronic treatment (days) enhances axon length, axon branching, and axon regeneration post-axotomy. We describe two alternative signaling pathways responsible for the observed effects and activated at different concentrations of Nystatin. At elevated concentrations, Nystatin promotes growth cone expansion through phosphorylation of Akt; whereas, at low concentrations, Nystatin enhances axon length and regrowth by increasing nitric oxide levels. Together, our findings indicate new signaling pathways of Nystatin and propose this compound as a novel regulator of axon regeneration.

Ezrin-radixin-moesin proteins are regulated by Akt-GSK3β signaling in the rat nucleus accumbens core

The ezrin-radixin-moesin (ERM) proteins are a family of membrane-associated proteins known to play roles in cell-shape determination as well as in signaling pathways. We have previously shown that amphetamine decreases phosphorylation levels of these proteins in the nucleus accumbens (NAcc), an important neuronal substrate mediating rewarding effects of drugs of abuse. In the present study, we further examined what molecular pathways may be involved in this process. By direct microinjection of LY294002, a PI3 kinase inhibitor, or of S9 peptide, a proposed GSK3β activator, into the NAcc core, we found that phosphorylation levels of ERM as well as of GSK3β in this site are simultaneously decreased. These results indicate that ERM proteins are under the regulation of Akt-GSK3β signaling pathway in the NAcc core. The present findings have a significant implication to a novel signal pathway possibly leading to structural plasticity in relation with drug addiction.

Two Sides of the Coin: Ezrin/Radixin/Moesin and Merlin Control Membrane Structure and Contact Inhibition

The merlin-ERM (ezrin, radixin, moesin) family of proteins plays a central role in linking the cellular membranes to the cortical actin cytoskeleton. Merlin regulates contact inhibition and is an integral part of cell–cell junctions, while ERM proteins, ezrin, radixin and moesin, assist in the formation and maintenance of specialized plasma membrane structures and membrane vesicle structures. These two protein families share a common evolutionary history, having arisen and separated via gene duplication near the origin of metazoa. During approximately 0.5 billion years of evolution, the merlin and ERM family proteins have maintained both sequence and structural conservation to an extraordinary level. Comparing crystal structures of merlin-ERM proteins and their complexes, a picture emerges of the merlin-ERM proteins acting as switchable interaction hubs, assembling protein complexes on cellular membranes and linking them to the actin cytoskeleton. Given the high level of structural conservation between the merlin and ERM family proteins we speculate that they may function together.