Extracellular inhibitors, repellents, and semaphorin/plexin/MICAL-mediated actin filament disassembly

Sema-3E/PlexinD1 axis modulates dendritic cell phenotypes and functions: Current status and future implications

This comprehensive research review explores the complex interplay between the Sema-3E/PlexinD1 axis and dendritic cells (DCs), highlighting its critical role in immune modulation with implications for clinical application Critical regulators of immune responses Dendritic cells are central to adaptive immunity, and the Sema-3E /PlexinD1 axis emerges as a key modulator affecting their phenotypes and functions Review delineates the impact of this signaling axis on DC maturation, migration, antigen presentation, and cytokine production, unravels its multifaceted role in shaping the immune response. Recognizing the limitations and gaps in current knowledge, the study highlights the need for further studies to condition downstream signaling events and related information experienced by the Sema-3E/PlexinD1 axis emphasizes the clarity of the immune system. The review concludes by identifying opportunities for translation, focusing on therapeutic and diagnostic potential. It highlights the importance of collaborative, interdisciplinary efforts to address the challenges and harness the therapeutic and pathological potential of targeting the Sema-3E/PlexinD1 axis, thus opening the way for transformative advances in immunology and clinical medicine.

Structure and function of Semaphorin-5A glycosaminoglycan interactions

Integration of extracellular signals by neurons is pivotal for brain development, plasticity, and repair. Axon guidance relies on receptor-ligand interactions crosstalking with extracellular matrix components. Semaphorin-5A (Sema5A) is a bifunctional guidance cue exerting attractive and inhibitory effects on neuronal growth through the interaction with heparan sulfate (HS) and chondroitin sulfate (CS) glycosaminoglycans (GAGs), respectively. Sema5A harbors seven thrombospondin type-1 repeats (TSR1-7) important for GAG binding, however the underlying molecular basis and functions in vivo remain enigmatic. Here we dissect the structural basis for Sema5A:GAG specificity and demonstrate the functional significance of this interaction in vivo. Using x-ray crystallography, we reveal a dimeric fold variation for TSR4 that accommodates GAG interactions. TSR4 co-crystal structures identify binding residues validated by site-directed mutagenesis. In vitro and cell-based assays uncover specific GAG epitopes necessary for TSR association. We demonstrate that HS-GAG binding is preferred over CS-GAG and mediates Sema5A oligomerization. In vivo, Sema5A:GAG interactions are necessary for Sema5A function and regulate Plexin-A2 dependent dentate progenitor cell migration. Our study rationalizes Sema5A associated developmental and neurological disorders and provides mechanistic insights into how multifaceted guidance functions of a single transmembrane cue are regulated by proteoglycans.

Fat2 polarizes Lar and Sema5c to coordinate the motility of collectively migrating epithelial cells

Migrating epithelial cells globally align their migration machinery to achieve tissue-level movement. Biochemical signaling across leading-trailing cell-cell interfaces can promote this alignment by partitioning migratory behaviors like protrusion and retraction to opposite sides of the interface. However, how the necessary signaling proteins become organized at this site is poorly understood. The follicular epithelial cells of Drosophila melanogaster have two signaling modules at their leading-trailing interfaces-one composed of the atypical cadherin Fat2 and the receptor tyrosine phosphatase Lar, and one composed of Semaphorin 5c and its receptor Plexin A. Here we show that these modules form one interface signaling system with Fat2 at its core. Trailing edge-enriched Fat2 concentrates both Lar and Sema5c at cells' leading edges, likely by slowing their turnover at this site. Once localized, Lar and Sema5c act in parallel to promote collective migration. Our data suggest a model in which Fat2 couples and polarizes the distributions of multiple effectors that work together to align the migration machinery of neighboring cells.

Actin Bundles Dynamics and Architecture

Cells use the actin cytoskeleton for many of their functions, including their division, adhesion, mechanosensing, endo- and phagocytosis, migration, and invasion. Actin bundles are the main constituent of actin-rich structures involved in these processes. An ever-increasing number of proteins that crosslink actin into bundles or regulate their morphology is being identified in cells. With recent advances in high-resolution microscopy and imaging techniques, the complex process of bundles formation and the multiple forms of physiological bundles are beginning to be better understood. Here, we review the physiochemical and biological properties of four families of highly conserved and abundant actin-bundling proteins, namely, α-actinin, fimbrin/plastin, fascin, and espin. We describe the similarities and differences between these proteins, their role in the formation of physiological actin bundles, and their properties-both related and unrelated to their bundling abilities. We also review some aspects of the general mechanism of actin bundles formation, which are known from the available information on the activity of the key actin partners involved in this process.

MICAL-mediated oxidation of actin and its effects on cytoskeletal and cellular dynamics

Actin and its dynamic structural remodelings are involved in multiple cellular functions, including maintaining cell shape and integrity, cytokinesis, motility, navigation, and muscle contraction. Many actin-binding proteins regulate the cytoskeleton to facilitate these functions. Recently, actin's post-translational modifications (PTMs) and their importance to actin functions have gained increasing recognition. The MICAL family of proteins has emerged as important actin regulatory oxidation-reduction (Redox) enzymes, influencing actin's properties both in vitro and in vivo. MICALs specifically bind to actin filaments and selectively oxidize actin's methionine residues 44 and 47, which perturbs filaments' structure and leads to their disassembly. This review provides an overview of the MICALs and the impact of MICAL-mediated oxidation on actin's properties, including its assembly and disassembly, effects on other actin-binding proteins, and on cells and tissue systems.

Quantitative modeling of regular retinal microglia distribution

Microglia are resident immune cells in the central nervous system, showing a regular distribution. Advancing microscopy and image processing techniques have contributed to elucidating microglia’s morphology, dynamics, and distribution. However, the mechanism underlying the regular distribution of microglia remains to be elucidated. First, we quantitatively confirmed the regularity of the distribution pattern of microglial soma in the retina. Second, we formulated a mathematical model that includes factors that may influence regular distribution. Next, we experimentally quantified the model parameters (cell movement, process formation, and ATP dynamics). The resulting model simulation from the measured parameters showed that direct cell–cell contact is most important in generating regular cell spacing. Finally, we tried to specify the molecular pathway responsible for the repulsion between neighboring microglia.

Profilin and Mical combine to impair F-actin assembly and promote disassembly and remodeling

Cellular events require the spatiotemporal interplay between actin assembly and actin disassembly. Yet, how different factors promote the integration of these two opposing processes is unclear. In particular, cellular monomeric (G)-actin is complexed with profilin, which inhibits spontaneous actin nucleation but fuels actin filament (F-actin) assembly by elongation-promoting factors (formins, Ena/VASP). In contrast, site-specific F-actin oxidation by Mical promotes F-actin disassembly and release of polymerization-impaired Mical-oxidized (Mox)-G-actin. Here we find that these two opposing processes connect with one another to orchestrate actin/cellular remodeling. Specifically, we find that profilin binds Mox-G-actin, yet these complexes do not fuel elongation factors’-mediated F-actin assembly, but instead inhibit polymerization and promote further Mox-F-actin disassembly. Using Drosophila as a model system, we show that similar profilin–Mical connections occur in vivo – where they underlie F-actin/cellular remodeling that accompanies Semaphorin–Plexin cellular/axon repulsion. Thus, profilin and Mical combine to impair F-actin assembly and promote F-actin disassembly, while concomitantly facilitating cellular remodeling and plasticity.

Actin-based structures in cells and tissues are built and maintained through a poorly understood balance between assembly and disassembly. Here, our findings provide insights into how factors known to promote these opposing effects dynamically integrate to shape cells and tissue systems.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Oxidative Stress and the Neurovascular Unit

The neurovascular unit (NVU) is a relatively recent concept that clearly describes the relationship between brain cells and their blood vessels. The components of the NVU, comprising different types of cells, are so interrelated and associated with each other that they are considered as a single functioning unit. For this reason, even slight disturbances in the NVU could severely affect brain homeostasis and health. In this review, we aim to describe the current state of knowledge concerning the role of oxidative stress on the neurovascular unit and the role of a single cell type in the NVU crosstalk.

Defective Mitochondrial Dynamics Underlie Manganese-Induced Neurotoxicity

Perturbations in mitochondrial dynamics have been observed in most neurodegenerative diseases. Here, we focus on manganese (Mn)-induced Parkinsonism-like neurodegeneration, a disorder associated with the preferential of Mn in the basal ganglia where the mitochondria are considered an early target. Despite the extensive characterization of the clinical presentation of manganism, the mechanism by which Mn mediated mitochondrial toxicity is unclear. In this study we hypothesized whether Mn exposure alters mitochondrial activity, including axonal transport of mitochondria and mitochondrial dynamics, morphology, and network. Using primary neuron cultures exposed to 100 μM Mn (which is considered the threshold of Mn toxicity in vitro) and intraperitoneal injections of MnCl2 (25mg/kg) in rat, we observed that Mn increased mitochondrial fission mediated by phosphorylation of dynamin-related protein-1 at serine 616 (p-s616-DRP1) and decreased mitochondrial fusion proteins (MFN1 and MFN2) leading to mitochondrial fragmentation, defects in mitochondrial respiratory capacity, and mitochondrial ultrastructural damage in vivo and in vitro. Furthermore, Mn exposure impaired mitochondrial trafficking by decreasing dynactin (DCTN1) and kinesin-1 (KIF5B) motor proteins and increasing destabilization of the cytoskeleton at protein and gene levels. In addition, mitochondrial communication may also be altered by Mn exposure, increasing the length of nanotunnels to reach out distal mitochondria. These findings revealed an unrecognized role of Mn in dysregulation of mitochondrial dynamics providing a potential explanation of early hallmarks of the disorder, as well as a possible common pathway with neurological disorders arising upon chronic Mn exposure.

Propagation of F-actin disassembly via Myosin15-Mical interactions

The F-actin cytoskeleton drives cellular form and function. However, how F-actin-based changes occur with spatiotemporal precision and specific directional orientation is poorly understood. Here, we identify that the unconventional class XV myosin [Myosin 15 (Myo15)] physically and functionally interacts with the F-actin disassembly enzyme Mical to spatiotemporally position cellular breakdown and reconstruction. Specifically, while unconventional myosins have been associated with transporting cargo along F-actin to spatially target cytoskeletal assembly, we now find they also target disassembly. Myo15 specifically positions this F-actin disassembly by associating with Mical and using its motor and MyTH4-FERM cargo-transporting functions to broaden Mical's distribution. Myo15's broadening of Mical's distribution also expands and directionally orients Mical-mediated F-actin disassembly and subsequent cellular remodeling, including in response to Semaphorin/Plexin cell surface activation signals. Thus, we identify a mechanism that spatiotemporally propagates F-actin disassembly while also proposing that other F-actin-trafficked-cargo is derailed by this disassembly to directionally orient rebuilding.

Semaphorin3E-PlexinD1 signaling in coronary artery and lymphatic vessel development with clinical implications in myocardial recovery

Blood and lymphatic vessels surrounding the heart develop through orchestrated processes from cells of different origins. In particular, cells around the outflow tract which constitute a primordial transient vasculature, referred to as aortic subepicardial vessels, are crucial for the establishment of coronary artery stems and cardiac lymphatic vessels. Here, we revealed that the epicardium and pericardium-derived Semaphorin 3E (Sema3E) and its receptor, PlexinD1, play a role in the development of the coronary stem, as well as cardiac lymphatic vessels. In vitro analyses demonstrated that Sema3E may demarcate areas to repel PlexinD1-expressing lymphatic endothelial cells, resulting in proper coronary and lymphatic vessel formation. Furthermore, inactivation of Sema3E-PlexinD1 signaling improved the recovery of cardiac function by increasing reactive lymphangiogenesis in an adult mouse model of myocardial infarction. These findings may lead to therapeutic strategies that target Sema3E-PlexinD1 signaling in coronary artery diseases.

Enhanced Production of the Mical Redox Domain for Enzymology and F-actin Disassembly Assays

To change their behaviors, cells require actin proteins to assemble together into long polymers/filaments—and so a critical goal is to understand the factors that control this actin filament (F-actin) assembly and stability. We have identified a family of unusual actin regulators, the MICALs, which are flavoprotein monooxygenase/hydroxylase enzymes that associate with flavin adenine dinucleotide (FAD) and use the co-enzyme nicotinamide adenine dinucleotide phosphate (NADPH) in Redox reactions. F-actin is a specific substrate for these MICAL Redox enzymes, which oxidize specific amino acids within actin to destabilize actin filaments. Furthermore, this MICAL-catalyzed reaction is reversed by another family of Redox enzymes (SelR/MsrB enzymes)—thereby revealing a reversible Redox signaling process and biochemical mechanism regulating actin dynamics. Interestingly, in addition to the MICALs’ Redox enzymatic portion through which MICALs covalently modify and affect actin, MICALs have multiple other domains. Less is known about the roles of these other MICAL domains. Here we provide approaches for obtaining high levels of recombinant protein for the Redox only portion of Mical and demonstrate its catalytic and F-actin disassembly activity. These results provide a ground state for future work aimed at defining the role of the other domains of Mical — including characterizing their effects on Mical’s Redox enzymatic and F-actin disassembly activity.

Semaphorin 3A potentiates the profibrotic effects of transforming growth factor-β1 in the cornea

Corneal scarring is a major cause of blindness worldwide with few effective therapeutic options. Finding a treatment would be of tremendous public health benefit, but requires a thorough understanding of the complex interactions that underlie this phenomenon. Here, we tested the hypothesis that the large increase in expression of Semaphorin 3A (SEMA3A) in corneal wounds contributes to the development of stromal fibrosis. We first verified this increased expression in vivo, in a cat model of photorefractive keratectomy-induced corneal wounding. We then examined the impact of adding exogenous SEMA3A to cultured corneal fibroblasts, and assessed how this affected the ability of transforming growth factor-beta1 (TGF-β1) to induce their differentiation into myofibroblasts. Finally, we examined how siRNA knockdown of endogenous SEMA3A affected these same phenomena. We found exogenous SEMA3A to significantly potentiate TGF-β1's profibrotic effects, with only a minimal contribution from cell-intrinsic SEMA3A. Our results suggest a previously unrecognized interaction between SEMA3A and TGF-β1 in the wounded cornea, and a possible contribution of SEMA3A to the regulation of tissue fibrosis and remodeling in this transparent organ.

Actin assembly and non-muscle myosin activity drive dendrite retraction in an UNC-6/Netrin dependent self-avoidance response

Dendrite growth is constrained by a self-avoidance response that induces retraction but the downstream pathways that balance these opposing mechanisms are unknown. We have proposed that the diffusible cue UNC-6(Netrin) is captured by UNC-40(DCC) for a short-range interaction with UNC-5 to trigger self-avoidance in the C. elegans PVD neuron. Here we report that the actin-polymerizing proteins UNC-34(Ena/VASP), WSP-1(WASP), UNC-73(Trio), MIG-10(Lamellipodin) and the Arp2/3 complex effect dendrite retraction in the self-avoidance response mediated by UNC-6(Netrin). The paradoxical idea that actin polymerization results in shorter rather than longer dendrites is explained by our finding that NMY-1 (non-muscle myosin II) is necessary for retraction and could therefore mediate this effect in a contractile mechanism. Our results also show that dendrite length is determined by the antagonistic effects on the actin cytoskeleton of separate sets of effectors for retraction mediated by UNC-6(Netrin) versus outgrowth promoted by the DMA-1 receptor. Thus, our findings suggest that the dendrite length depends on an intrinsic mechanism that balances distinct modes of actin assembly for growth versus retraction.

Neurons may extend highly branched dendrites to detect input over a broad receptive field. The formation of actin filaments may drive dendrite elongation. The architecture of the dendritic arbor also depends on mechanisms that limit expansion. For example, sister dendrites from a single neuron usually do not overlap due to self-avoidance. Although cell surface proteins are known to mediate self-avoidance, the downstream pathways that drive dendrite retraction in this phenomenon are largely unknown. Studies of the highly branched PVD sensory neuron in C. elegans have suggested a model of self-avoidance in which the UNC-40/DCC receptor captures the diffusible cue UNC-6/Netrin at the tips of PVD dendrites where it interacts with the UNC-5 receptor on an opposing sister dendrite to induce retraction. Here we report genetic evidence that UNC-5-dependent retraction requires downstream actin polymerization. This finding evokes a paradox: How might actin polymerization drive both dendrite growth and retraction? We propose two answers: (1) Distinct sets of effectors are involved in actin assembly for growth vs retraction; (2) Non-muscle myosin interacts with a nascent actin assemblage to trigger retraction. Our results show that dendrite length depends on the balanced effects of specific molecular components that induce growth vs retraction.

Planar-Polarized Semaphorin-5c and Plexin A Promote the Collective Migration of Epithelial Cells in Drosophila

Collective migration of epithelial cells is essential for morphogenesis, wound repair, and the spread of many cancers, yet how individual cells signal to one another to coordinate their movements is largely unknown. Here, we introduce a tissue-autonomous paradigm for semaphorin-based regulation of collective cell migration. Semaphorins typically regulate the motility of neuronal growth cones and other migrating cell types by acting as repulsive cues within the migratory environment. Studying the follicular epithelial cells of Drosophila, we discovered that the transmembrane semaphorin, Sema-5c, promotes collective cell migration by acting within the migrating cells themselves, not the surrounding environment. Sema-5c is planar polarized at the basal epithelial surface such that it is enriched at the leading edge of each cell. This location places it in a prime position to send a repulsive signal to the trailing edge of the cell ahead to communicate directional information between neighboring cells. Our data show that Sema-5c can signal across cell-cell boundaries to suppress protrusions in neighboring cells and that Plexin A is the receptor that transduces this signal. Finally, we present evidence that Sema-5c antagonizes the activity of Lar, another transmembrane guidance cue that operates along leading-trailing cell-cell interfaces in this tissue, via a mechanism that appears to be independent of Plexin A. Together, our results suggest that multiple transmembrane guidance cues can be deployed in a planar-polarized manner across an epithelium and work in concert to coordinate individual cell movements for collective migration.

Metabolism as an early predictor of DPSCs aging

Tissue resident adult stem cells are known to participate in tissue regeneration and repair that follows cell turnover, or injury. It has been well established that aging impedes the regeneration capabilities at the cellular level, but it is not clear if the different onset of stem cell aging between individuals can be predicted or prevented at an earlier stage. Here we studied the dental pulp stem cells (DPSCs), a population of adult stem cells that is known to participate in the repair of an injured tooth, and its properties can be affected by aging. The dental pulp from third molars of a diverse patient group were surgically extracted, generating cells that had a high percentage of mesenchymal stem cell markers CD29, CD44, CD146 and Stro1 and had the ability to differentiate into osteo/odontogenic and adipogenic lineages. Through RNA seq and qPCR analysis we identified homeobox protein, Barx1, as a marker for DPSCs. Furthermore, using high throughput transcriptomic and proteomic analysis we identified markers for DPSC populations with accelerated replicative senescence. In particular, we show that the transforming growth factor-beta (TGF-β) pathway and the cytoskeletal proteins are upregulated in rapid aging DPSCs, indicating a loss of stem cell characteristics and spontaneous initiation of terminal differentiation. Importantly, using metabolic flux analysis, we identified a metabolic signature for the rapid aging DPSCs, prior to manifestation of senescence phenotypes. This metabolic signature therefore can be used to predict the onset of replicative senescence. Hence, the present study identifies Barx1 as a DPSCs marker and dissects the first predictive metabolic signature for DPSCs aging.

Semaphorin signaling via MICAL3 induces symmetric cell division to expand breast cancer stem-like cells

Tumors are composed of both cancer stem-like cells (CSCs) and differentiated cancer cells. Each CSC can undergo either a symmetric cell division to produce two CSCs or an asymmetric cell division to produce one CSC and one differentiated cancer cell. It is believed that the rate of symmetric division increases as more CSCs become malignant; however, underlying molecular mechanisms remain elusive. Here we show that stimulation with a cytokine, semaphorin (Sema), activates monooxygenase of MICAL3, a cytoplasmic signal transducer, through the neuropilin (NP) receptor that is specifically expressed on the breast CSC plasma membrane. The activation of MICAL3 induces symmetric division of CSCs. Each molecule in this signaling pathway represents a promising therapeutic target for eliminating CSCs.

Cancer stem-like cells (CSCs) are expanded in the CSC niche by increased frequency of symmetric cell divisions at the expense of asymmetric cell divisions. The symmetric division of CSCs is important for the malignant properties of cancer; however, underlying molecular mechanisms remain largely elusive. Here, we show a cytokine, semaphorin 3 (Sema3), produced from the CSC niche, induces symmetric divisions of CSCs to expand the CSC population. Our findings indicate that stimulation with Sema3 induced sphere formation in breast cancer cells through neuropilin 1 (NP1) receptor that was specifically expressed in breast CSCs (BCSCs). Knockdown of MICAL3, a cytoplasmic Sema3 signal transducer, greatly decreased tumor sphere formation and tumor-initiating activity. Mechanistically, Sema3 induced interaction among MICAL3, collapsin response mediator protein 2 (CRMP2), and Numb. It appears that activity of MICAL3 monooxygenase (MO) stimulated by Sema3 is required for tumor sphere formation, interaction between CRMP2 and Numb, and accumulation of Numb protein. We found that knockdown of CRMP2 or Numb significantly decreased tumor sphere formation. Moreover, MICAL3 knockdown significantly decreased Sema3-induced symmetric divisions in NP1/Numb-positive BCSCs and increased asymmetric division that produces NP1/Numb negative cells without stem-like properties. In addition, breast cancer patients with NP1-positive cancer tissues show poor prognosis. Therefore, the niche factor Sema3-stimulated NP1/MICAL3/CRMP2/Numb axis appears to expand CSCs at least partly through increased frequency of MICAL3-mediated symmetric division of CSCs.

Axon formation, extension, and navigation: only a neuroscience phenomenon?

Understanding how neurons form, extend, and navigate their finger-like axonal and dendritic processes is crucial for developing therapeutics for the diseased and damaged brain. Although less well appreciated, many other types of cells also send out similar finger-like projections. Indeed, unlike neuronal specific phenomena such as synapse formation or synaptic transmission, an important issue for thought is that this critical long-standing question of how a cellular process like an axon or dendrite forms and extends is not primarily a neuroscience problem but a cell biological problem. In that case, the use of simple cellular processes - such as the bristle cell process of Drosophila - can aid in the fight to answer these critical questions. Specifically, determining how a model cellular process is generated can provide a framework for manipulations of all types of membranous process-containing cells, including different types of neurons.

Microgravity-Induced Alterations of Inflammation-Related Mechanotransduction in Endothelial Cells on Board SJ-10 Satellite

Endothelial cells (ECs) are mechanosensitive cells undergoing morphological and functional changes in space. Ground-based study has provided a body of evidences about how ECs can respond to the effect of simulated microgravity, however, these results need to be confirmed by spaceflight experiments in real microgravity. In this work, we cultured EA.hy926 ECs on board the SJ-10 Recoverable Scientific Satellite for 3 and 10 days, and analyzed the effects of space microgravity on the ECs. Space microgravity suppressed the glucose metabolism, modulated the expression of cellular adhesive molecules such as ICAM-1, VCAM-1, and CD44, and depressed the pro-angiogenesis and pro-inflammation cytokine secretion. Meanwhile, it also induced the depolymerization of actin filaments and microtubules, promoted the vimentin accumulation, restrained the collagen I and fibronectin deposition, regulated the mechanotransduction through focal adhesion kinase and Rho GTPases, and enhanced the exosome-mediated mRNA transfer. Unlike the effect of simulated microgravity, neither three-dimensional growth nor enhanced nitric oxide production was observed in our experimental settings. This work furthers the understandings in the effects and mechanisms of space microgravity on ECs, and provides useful information for future spaceflight experimental design.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

The MICALs are a Family of F-actin Dismantling Oxidoreductases Conserved from Drosophila to Humans

Cellular form and function – and thus normal development and physiology – are specified via proteins that control the organization and dynamic properties of the actin cytoskeleton. Using the Drosophila model, we have recently identified an unusual actin regulatory enzyme, Mical, which is directly activated by F-actin to selectively post-translationally oxidize and destabilize filaments – regulating numerous cellular behaviors. Mical proteins are also present in mammals, but their actin regulatory properties, including comparisons among different family members, remain poorly defined. We now find that each human MICAL family member, MICAL-1, MICAL-2, and MICAL-3, directly induces F-actin dismantling and controls F-actin-mediated cellular remodeling. Specifically, each human MICAL selectively associates with F-actin, which directly induces MICALs catalytic activity. We also find that each human MICAL uses an NADPH-dependent Redox activity to post-translationally oxidize actin’s methionine (M) M44/M47 residues, directly dismantling filaments and limiting new polymerization. Genetic experiments also demonstrate that each human MICAL drives F-actin disassembly in vivo, reshaping cells and their membranous extensions. Our results go on to reveal that MsrB/SelR reductase enzymes counteract each MICAL’s effect on F-actin in vitro and in vivo. Collectively, our results therefore define the MICALs as an important phylogenetically-conserved family of catalytically-acting F-actin disassembly factors.

Common effects of attractive and repulsive signaling: Further analysis of Mical-mediated F-actin disassembly and regulation by Abl

To change their size, shape, and connectivity, cells require actin and tubulin proteins to assemble together into long polymers – and numerous extracellular stimuli have now been identified that alter the assembly and organization of these cytoskeletal structures. Yet, there remains a lack of defined signaling pathways from the cell surface to the cytoskeleton for many of these extracellular signals, and so we still know little of how they exert their precise structural effects. These extracellular cues may be soluble or substrate-bound and have historically been classified into two independently acting and antagonistic groups: growth-promoting/attractants (inducing turning toward the source of the factor/positive chemotropism) or growth-preventing/repellents (turning away from the source of the factor/negative chemotropism). Paradoxically, our recent results directly link the action of growth factors/chemoattractants and their signaling pathways to the promotion of the disassembly of the F-actin cytoskeleton (a defined readout of repellents/repulsive signaling). Herein, we add to this by simply driving a constitutively active form of Mical, which strongly disassembles F-actin/remodels cells in vivo independent of repulsive cues – and find that loss of Abl, which mediates growth factor signaling in these cells, decreases Mical's F-actin disassembly/cellular remodeling effects. Thus, our results are consistent with a hypothesis that cues defined as positive effectors of movement (growth factors/chemoattractants) can at least in some contexts enhance the F-actin disassembly and remodeling activity of repellents.

Overexpression of MICAL2, a novel tumor-promoting factor, accelerates tumor progression through regulating cell proliferation and EMT

Molecule interacting with CasL 2 (MICAL2), a microtubule associated monooxygenase, is involved in cell growth, axon guidance, vesicle trafficking and apoptosis. Recent studies have demonstrated that MICAL2 is highly expressed in tumor and accelerates tumor progression and it is deemed to be a novel tumor-promoting factor. MICAL2 overexpression increases cell proliferation to accelerate tumor growth, and MICAL2 also promotes epithelial-mesenchymal transition (EMT)-related proteins to increase cancer cell metastasis. On mechanism, MICAL2 induces EMT by regulating SRF (serum response factor)/MRTF-A (myocardin related transcription factor A) signaling, Semaphorin/Plexin pathway and inducing ROS (Reactive oxygen species) production. In the present review, we introduced MICAL family, expatiated the structure and functions of MICALs, and summarized the mechanisms of MICAL2 involving tumor progression. The challenges and perspectives for MICAL2 in tumor are also discussed.

Flavin monooxygenases regulate Caenorhabditis elegans axon guidance and growth cone protrusion with UNC-6/Netrin signaling and Rac GTPases

The guidance cue UNC-6/Netrin regulates both attractive and repulsive axon guidance. Our previous work showed that in C. elegans, the attractive UNC-6/Netrin receptor UNC-40/DCC stimulates growth cone protrusion, and that the repulsive receptor, an UNC-5:UNC-40 heterodimer, inhibits growth cone protrusion. We have also shown that inhibition of growth cone protrusion downstream of the UNC-5:UNC-40 repulsive receptor involves Rac GTPases, the Rac GTP exchange factor UNC-73/Trio, and the cytoskeletal regulator UNC-33/CRMP, which mediates Semaphorin-induced growth cone collapse in other systems. The multidomain flavoprotein monooxygenase (FMO) MICAL (Molecule Interacting with CasL) also mediates growth cone collapse in response to Semaphorin by directly oxidizing F-actin, resulting in depolymerization. The C. elegans genome does not encode a multidomain MICAL-like molecule, but does encode five flavin monooxygenases (FMO-1, -2, -3, -4, and 5) and another molecule, EHBP-1, similar to the non-FMO portion of MICAL. Here we show that FMO-1, FMO-4, FMO-5, and EHBP-1 may play a role in UNC-6/Netrin directed repulsive guidance mediated through UNC-40 and UNC-5 receptors. Mutations in fmo-1, fmo-4, fmo-5, and ehbp-1 showed VD/DD axon guidance and branching defects, and variably enhanced unc-40 and unc-5 VD/DD axon guidance defects. Developing growth cones in vivo of fmo-1, fmo-4, fmo-5, and ehbp-1 mutants displayed excessive filopodial protrusion, and transgenic expression of FMO-5 inhibited growth cone protrusion. Mutations suppressed growth cone inhibition caused by activated UNC-40 and UNC-5 signaling, and activated Rac GTPase CED-10 and MIG-2, suggesting that these molecules are required downstream of UNC-6/Netrin receptors and Rac GTPases. From these studies we conclude that FMO-1, FMO-4, FMO-5, and EHBP-1 represent new players downstream of UNC-6/Netrin receptors and Rac GTPases that inhibit growth cone filopodial protrusion in repulsive axon guidance.

Mechanisms that guide axons to their targets in the developing nervous system have been elucidated, but how these pathways affect behavior of the growth cone of the axon during outgrowth remains poorly understood. We previously showed that the guidance cue UNC-6/Netrin and its receptors UNC-40/DCC and UNC-5 inhibit lamellipodial and filopodial growth cone protrusion to mediate repulsion from UNC-6/Netrin in C. elegans. Here we report a new mechanism downstream of UNC-6/Netrin involving flavin monooxygenase redox enzymes (FMOs). We show that FMOs are normally required for axon guidance and to inhibit growth cone protrusion. Furthermore, we show that they are required for the anti-protrusive effects of activated UNC-40 and UNC-5 receptors, and that they can partially compensate for loss of molecules in the pathway, indicating that they act downstream of UNC-6/Netrin signaling. Based on the function of the FMO-containing MICAL molecules in Drosophila and vertebrates, we speculate that the FMOs might directly oxidize actin, leading to filament disassembly and collapse, and/or lead to the phosphorylation of UNC-33/CRMP, which we show also genetically interacts with the FMOs downstream of UNC-6/Netrin. In conclusion, this is the first evidence that FMOs might act downstream of UNC-6/Netrin signaling in growth cone protrusion and axon repulsion.

Regulated methionine oxidation by monooxygenases

Protein function can be regulated via post-translational modifications by numerous enzymatic and non-enzymatic mechanisms, including oxidation of cysteine and methionine residues. Redox-dependent regulatory mechanisms have been identified for nearly every cellular process, but the major paradigm has been that cellular components are oxidized (damaged) by reactive oxygen species (ROS) in a relatively unspecific way, and then reduced (repaired) by designated reductases. While this scheme may work with cysteine, it cannot be ascribed to other residues, such as methionine, whose reaction with ROS is too slow to be biologically relevant. However, methionine is clearly oxidized in vivo and enzymes for its stereoselective reduction are present in all three domains of life. Here, we revisit the chemistry and biology of methionine oxidation, with emphasis on its generation by enzymes from the monooxygenase family. Particular attention is placed on MICALs, a recently discovered family of proteins that harbor an unusual flavin-monooxygenase domain with an NADPH-dependent methionine sulfoxidase activity. Based on structural and kinetic information we provide a rational framework to explain MICAL mechanism, inhibition, and regulation. Methionine residues that are targeted by MICALs are reduced back by methionine sulfoxide reductases, suggesting that reversible methionine oxidation may be a general mechanism analogous to the regulation by phosphorylation by kinases/phosphatases. The identification of new enzymes that catalyze the oxidation of methionine will open a new area of research at the forefront of redox signaling.

Amplification of F-Actin Disassembly and Cellular Repulsion by Growth Factor Signaling

Extracellular cues that regulate cellular shape, motility, and navigation are generally classified as growth promoting (i.e., growth factors/chemoattractants and attractive guidance cues) or growth preventing (i.e., repellents and inhibitors). Yet, these designations are often based on complex assays and undefined signaling pathways and thus may misrepresent direct roles of specific cues. Here, we find that a recognized growth-promoting signaling pathway amplifies the F-actin disassembly and repulsive effects of a growth-preventing pathway. Focusing on Semaphorin/Plexin repulsion, we identified an interaction between the F-actin-disassembly enzyme Mical and the Abl tyrosine kinase. Biochemical assays revealed Abl phosphorylates Mical to directly amplify Mical Redox-mediated F-actin disassembly. Genetic assays revealed that Abl allows growth factors and Semaphorin/Plexin repellents to combinatorially increase Mical-mediated F-actin disassembly, cellular remodeling, and repulsive axon guidance. Similar roles for Mical in growth factor/Abl-related cancer cell behaviors further revealed contexts in which characterized positive effectors of growth/guidance stimulate such negative cellular effects as F-actin disassembly/repulsion.

A Feed-Forward Mechanism Involving the NOX Complex and RyR-Mediated Ca2+ Release During Axonal Specification

Physiological levels of ROS support neurite outgrowth and axonal specification, but the mechanisms by which ROS are able to shape neurons remain unknown. Ca²⁺, a broad intracellular second messenger, promotes both Rac1 activation and neurite extension. Ca²⁺ release from the endoplasmic reticulum, mediated by both the IP3R1 and ryanodine receptor (RyR) channels, requires physiological ROS levels that are mainly sustained by the NADPH oxidase (NOX) complex. In this work, we explore the contribution of the link between NOX and RyR-mediated Ca²⁺ release toward axonal specification of rat hippocampal neurons. Using genetic approaches, we find that NOX activation promotes both axonal development and Rac1 activation through a RyR-mediated mechanism, which in turn activates NOX through Rac1, one of the NOX subunits. Collectively, these data suggest a feedforward mechanism that integrates both NOX activity and RyR-mediated Ca²⁺ release to support cellular mechanisms involved in axon development.

SIGNIFICANCE STATEMENT

High levels of ROS are frequently associated with oxidative stress and disease. In contrast, physiological levels of ROS, mainly sustained by the NADPH oxidase (NOX) complex, promote neuronal development and axonal growth. However, the mechanisms by which ROS shape neurons have not been described. Our work suggests that NOX-derived ROS promote axonal growth by regulating Rac1 activity, a molecular determinant of axonal growth, through a ryanodine receptor (RyR)-mediated Ca²⁺ release mechanism. In addition, Rac1, one of the NOX subunits, was activated after RyR-mediated Ca²⁺ release, suggesting a feedforward mechanism between NOX and RyR. Collectively, our data suggest a novel mechanism that is instrumental in sustaining physiological levels of ROS required for axonal growth of hippocampal neurons.

Actin-based growth cone motility and guidance

Nerve growth cones, the dilated tip of developing axons, are equipped with exquisite abilities to sense environmental cues and to move rapidly through complex terrains of developing brain, leading the axons to their specific targets for precise neuronal wiring. The actin cytoskeleton is the major component of the growth cone that powers its directional motility. Past research has provided significant insights into the mechanisms by which growth cones translate extracellular signals into directional migration. In this review, we summarize the actin-based mechanisms underlying directional growth cone motility, examine novel findings, and discuss the outstanding questions concerning the actin-based growth cone behaviors.

New aspects of the Seam4D-dependent control of lymphocyte activation

Novel targets for action of the class IV semaphorin Seam4D have been identified in the immune system. The low-affinity CD72 receptor for Seam4D was detected not only on B lymphocytes, but also in a proportion of T cells, whereas the high-affinity semaphorin receptor, plexin B1, originally considered to belong to non-immune cells, proved to be in a great proportion of intact T and B cells. Seam4D is constitutively expressed in B cells, which, along with T cells, can serve as a source of both membrane and soluble semaphorin. The results obtained make significant adjustments in understanding of Seam4D effects in lymphoid cells.

Characterizing F-actin Disassembly Induced by the Semaphorin-Signaling Component MICAL

The MICALs are a family of phylogenetically conserved cytoplasmic proteins that modulate numerous cellular behaviors and play critical roles in semaphorin-plexin signaling. Our recent results have revealed that the MICALs are an unusual family of actin regulatory proteins that use actin filaments (F-actin) as a direct substrate-controlling F-actin dynamics via stereospecific oxidation of conserved methionine (Met44 and Met47) residues within actin. In particular, the MICALs have a highly conserved flavoprotein monooxygenase (redox) enzymatic domain in their N-terminus that directly oxidizes and destabilizes F-actin. Here, we describe methods to characterize MICAL-mediated F-actin disassembly using in vitro assays with purified proteins.

Actin filaments-A target for redox regulation

Actin and its ability to polymerize into dynamic filaments is critical for the form and function of cells throughout the body. While multiple proteins have been characterized as affecting actin dynamics through noncovalent means, actin and its protein regulators are also susceptible to covalent modifications of their amino acid residues. In this regard, oxidation-reduction (Redox) intermediates have emerged as key modulators of the actin cytoskeleton with multiple different effects on cellular form and function. Here, we review work implicating Redox intermediates in post-translationally altering actin and discuss what is known regarding how these alterations affect the properties of actin. We also focus on two of the best characterized enzymatic sources of these Redox intermediates-the NADPH oxidase NOX and the flavoprotein monooxygenase MICAL-and detail how they have both been identified as altering actin, but share little similarity and employ different means to regulate actin dynamics. Finally, we discuss the role of these enzymes and redox signaling in regulating the actin cytoskeleton in vivo and highlight their importance for neuronal form and function in health and disease. © 2016 Wiley Periodicals, Inc.

F-actin dismantling through a redox-driven synergy between Mical and cofilin

Numerous cellular functions depend on actin filament (F-actin) disassembly. The best-characterized disassembly proteins, the ADF/cofilins/twinstar, sever filaments and recycle monomers to promote actin assembly. Cofilin is also a relatively weak actin disassembler, posing questions about mechanisms of cellular F-actin destabilization. Here we uncover a key link to targeted F-actin disassembly by finding that F-actin is efficiently dismantled through a post-translational-mediated synergism between cofilin and the actin-oxidizing enzyme Mical. We find that Mical-mediated oxidation of actin improves cofilin binding to filaments, where their combined effect dramatically accelerates F-actin disassembly compared to either effector alone. This synergism is also necessary and sufficient for F-actin disassembly in vivo, magnifying the effects of both Mical and cofilin on cellular remodeling, axon guidance, and Semaphorin/Plexin repulsion. Mical and cofilin, therefore, form a Redox-dependent synergistic pair that promotes F-actin instability by rapidly dismantling F-actin and generating post-translationally modified actin that has altered assembly properties.

A simple and efficient method for generating high-quality recombinant Mical enzyme for in vitro assays

We have recently identified a new family of multidomain oxidoreductase (redox) enzymes, the MICALs, that directly regulate the actin cytoskeletal elements necessary for the morphology, motility, and trajectory of cells. Our genetic assays reveal that Mical is both necessary and sufficient for actin organization and cellular effects in vivo and our biochemical assays with purified Mical protein reveal that Mical utilizes its redox activity to directly disassemble actin filaments. These results identify Mical proteins as novel actin disassembly factors and uncover a redox signaling mechanism that directly regulates the actin cytoskeleton. These results have also set the stage for in-depth characterization of the Mical enzyme. However, it has been difficult to obtain sufficient amounts of highly-pure Mical protein to conduct further biochemical, structural, imaging, catalytic, and other high-precision studies. Herein, we describe a means for expressing high levels of soluble recombinant Mical protein in bacteria. Likewise, we have designed a new purification strategy that enables the rapid and efficient purification of milligram quantities of highly-pure and >99% active Mical protein. This new strategy for generating large amounts of highly-pure and active Mical protein will aid research objectives designed to characterize the biochemical, enzymology, and structural biology of Mical and its effects on actin filament dynamics.

Modulation of MICAL Monooxygenase Activity by its Calponin Homology Domain: Structural and Mechanistic Insights

MICALs (Molecule Interacting with CasL) are conserved multidomain enzymes essential for cytoskeletal reorganization in nerve development, endocytosis, and apoptosis. In these enzymes, a type-2 calponin homology (CH) domain always follows an N-terminal monooxygenase (MO) domain. Although the CH domain is required for MICAL-1 cellular localization and actin-associated function, its contribution to the modulation of MICAL activity towards actin remains unclear. Here, we present the structure of a fragment of MICAL-1 containing the MO and the CH domains—determined by X-ray crystallography and small angle scattering—as well as kinetics experiments designed to probe the contribution of the CH domain to the actin-modification activity. Our results suggest that the CH domain, which is loosely connected to the MO domain by a flexible linker and is far away from the catalytic site, couples F-actin to the enhancement of redox activity of MICAL_MO-CH by a cooperative mechanism involving a trans interaction between adjacently bound molecules. Binding cooperativity is also observed in other proteins regulating actin assembly/disassembly dynamics, such as ADF/Cofilins.

Sema6A and Mical1 control cell growth and survival of BRAFV600E human melanoma cells

We used whole genome microarray analysis to identify potential candidate genes with differential expression in BRAFV600E vs NRASQ61R melanoma cells. We selected, for comparison, a peculiar model based on melanoma clones, isolated from a single tumor characterized by mutually exclusive expression of BRAFV600E and NRASQ61R in different cells. This effort led us to identify two genes, SEMA6A and MICAL1, highly expressed in BRAF-mutant vs NRAS-mutant clones. Real-time PCR, Western blot and immunohistochemistry confirmed preferential expression of Sema6A and Mical1 in BRAFV600E melanoma. Sema6A is a member of the semaphorin family, and it complexes with the plexins to regulate actin cytoskeleton, motility and cell proliferation. Silencing of Sema6A in BRAF-mutant cells caused cytoskeletal remodeling, and loss of stress fibers, that in turn induced cell death. Furthermore, Sema6A depletion caused loss of anchorage-independent growth, inhibition of chemotaxis and invasion. Forced Sema6A overexpression, in NRASQ61R clones, induced anchorage-independent growth, and a significant increase of invasiveness. Mical1, that links Sema/PlexinA signaling, is also a negative regulator of apoptosis. Indeed, Mical-1 depletion in BRAF mutant cells restored MST-1-dependent NDR phosphorylation and promoted a rapid and massive NDR-dependent apoptosis. Overall, our data suggest that Sema6A and Mical1 may represent new potential therapeutic targets in BRAFV600E melanoma.

Control of cellular motility by neuropilin-mediated physical interactions

The neuropilin (Nrp) family consists of multifunctional cell surface receptors with critical roles in a number of different cell and tissue types. A core aspect of Nrp function is in ligand-dependent cellular migration, where it controls the multistep process of cellular motility through integration of ligand binding and receptor signaling. At a molecular level, the role of Nrp in migration is intimately connected to the control of adhesive interactions and cytoskeletal reorganization. Here, we review the physiological role of Nrp in cellular adhesion and motility in the cardiovascular and nervous systems. We also discuss the emerging pathological role of Nrp in tumor cell migration and metastasis, providing motivation for continued efforts toward developing Nrp inhibitors.

Regulation of protein function by reversible methionine oxidation and the role of selenoprotein MsrB1

SIGNIFICANCE

Protein structure and function can be regulated via post-translational modifications by numerous enzymatic and nonenzymatic mechanisms. Regulation involving oxidation of sulfur-containing residues emerged as a key mechanism of redox control. Unraveling the participants and principles of such regulation is necessary for understanding the biological significance of redox control of cellular processes.

RECENT ADVANCES

Reversible oxidation of methionine residues by monooxygenases of the Mical family and subsequent reduction of methionine sulfoxides by a selenocysteine-containing methionine sulfoxide reductase B1 (MsrB1) was found to control the assembly and disassembly of actin in mammals, and the Mical/MsrB pair similarly regulates actin in fruit flies. This finding has opened up new avenues for understanding the use of stereospecific methionine oxidation in regulating cellular processes and the roles of MsrB1 and Micals in regulation of actin dynamics.

CRITICAL ISSUES

So far, Micals have been the only known partners of MsrB1, and actin is the only target. It is important to identify additional substrates of Micals and characterize other Mical-like enzymes.

FUTURE DIRECTIONS

Oxidation of methionine, reviewed here, is an emerging but not well-established mechanism. Studies suggest that methionine oxidation is a form of oxidative damage of proteins, a modification that alters protein structure or function, a tool in redox signaling, and a mechanism that controls protein function. Understanding the functional impact of reversible oxidation of methionine will require identification of targets, substrates, and regulators of Micals and Msrs. Linking the biological processes, in which these proteins participate, might also lead to insights into disease conditions, which involve regulation of actin by Micals and Msrs.

Regulation of cytoskeletal dynamics by redox signaling and oxidative stress: implications for neuronal development and trafficking

A proper balance between chemical reduction and oxidation (known as redox balance) is essential for normal cellular physiology. Deregulation in the production of oxidative species leads to DNA damage, lipid peroxidation and aberrant post-translational modification of proteins, which in most cases induces injury, cell death and disease. However, physiological concentrations of oxidative species are necessary to support important cell functions, such as chemotaxis, hormone synthesis, immune response, cytoskeletal remodeling, Ca²⁺ homeostasis and others. Recent evidence suggests that redox balance regulates actin and microtubule dynamics in both physiological and pathological contexts. Microtubules and actin microfilaments contain certain amino acid residues that are susceptible to oxidation, which reduces the ability of microtubules to polymerize and causes severing of actin microfilaments in neuronal and non-neuronal cells. In contrast, inhibited production of reactive oxygen species (ROS; e.g., due to NOXs) leads to aberrant actin polymerization, decreases neurite outgrowth and affects the normal development and polarization of neurons. In this review, we summarize emerging evidence suggesting that both general and specific enzymatic sources of redox species exert diverse effects on cytoskeletal dynamics. Considering the intimate relationship between cytoskeletal dynamics and trafficking, we also discuss the potential effects of redox balance on intracellular transport via regulation of the components of the microtubule and actin cytoskeleton as well as cytoskeleton-associated proteins, which may directly impact localization of proteins and vesicles across the soma, dendrites and axon of neurons.

Semaphorin3a promotes advanced diabetic nephropathy

The onset of diabetic nephropathy (DN) is highlighted by glomerular filtration barrier abnormalities. Identifying pathogenic factors and targetable pathways driving DN is crucial to developing novel therapies and improving the disease outcome. Semaphorin3a (sema3a) is a guidance protein secreted by podocytes. Excess sema3a disrupts the glomerular filtration barrier. Here, using immunohistochemistry, we show increased podocyte SEMA3A in renal biopsies from patients with advanced DN. Using inducible, podocyte-specific Sema3a gain-of-function (Sema3a(+)) mice made diabetic with streptozotocin, we demonstrate that sema3a is pathogenic in DN. Diabetic Sema3a(+) mice develop massive proteinuria, renal insufficiency, and extensive nodular glomerulosclerosis, mimicking advanced DN in humans. In diabetic mice, Sema3a(+) exacerbates laminin and collagen IV accumulation in Kimmelstiel-Wilson-like glomerular nodules and causes diffuse podocyte foot process effacement and F-actin collapse via nephrin, αvβ3 integrin, and MICAL1 interactions with plexinA1. MICAL1 knockdown and sema3a inhibition render podocytes not susceptible to sema3a-induced shape changes, indicating that MICAL1 mediates sema3a-induced podocyte F-actin collapse. Moreover, sema3a binding inhibition or podocyte-specific plexinA1 deletion markedly ameliorates albuminuria and abrogates renal insufficiency and the diabetic nodular glomerulosclerosis phenotype of diabetic Sema3a(+) mice. Collectively, these findings indicate that excess sema3a promotes severe diabetic nephropathy and identifies novel potential therapeutic targets for DN.

The emerging role of class-3 semaphorins and their neuropilin receptors in oncology

The semaphorins, discovered over 20 years ago, are a large family of secreted or transmembrane and glycophosphatidylinositol -anchored proteins initially identified as axon guidance molecules crucial for the development of the nervous system. It has now been established that they also play important roles in organ development and function, especially involving the immune, respiratory, and cardiovascular systems, and in pathological disorders, including cancer. During tumor progression, semaphorins can have both pro- and anti-tumor functions, and this has created complexities in our understanding of these systems. Semaphorins may affect tumor growth and metastases by directly targeting tumor cells, as well as indirectly by interacting with and influencing cells from the micro-environment and vasculature. Mechanistically, semaphorins, through binding to their receptors, neuropilins and plexins, affect pathways involved in cell adhesion, migration, invasion, proliferation, and survival. Importantly, neuropilins also act as co-receptors for several growth factors and enhance their signaling activities, while class 3 semaphorins may interfere with this. In this review, we focus on the secreted class 3 semaphorins and their neuropilin co-receptors in cancer, including aspects of their signaling that may be clinically relevant.

Semaphorin signalling during development

Semaphorins are secreted and membrane-associated proteins that regulate many different developmental processes, including neural circuit assembly, bone formation and angiogenesis. Trans and cis interactions between semaphorins and their multimeric receptors trigger intracellular signal transduction networks that regulate cytoskeletal dynamics and influence cell shape, differentiation, motility and survival. Here and in the accompanying poster we provide an overview of the molecular biology of semaphorin signalling within the context of specific cell and developmental processes, highlighting the mechanisms that act to fine-tune, diversify and spatiotemporally control the effects of semaphorins.

SelR reverses Mical-mediated oxidation of actin to regulate F-actin dynamics

Actin's polymerization properties are markedly altered by oxidation of its conserved Met 44 residue. Mediating this effect is a specific oxidation-reduction (redox) enzyme, Mical, that works with Semaphorin repulsive guidance cues and selectively oxidizes Met 44. We now find that this actin-regulatory process is reversible. Employing a genetic approach, we identified a specific methionine sulfoxide reductase (MsrB) enzyme SelR that opposes Mical redox activity and Semaphorin-Plexin repulsion to direct multiple actin-dependent cellular behaviours in vivo. SelR specifically catalyses the reduction of the R isomer of methionine sulfoxide (methionine-R-sulfoxide) to methionine, and we found that SelR directly reduced Mical-oxidized actin, restoring its normal polymerization properties. These results indicate that Mical oxidizes actin stereospecifically to generate actin Met-44-R-sulfoxide (actin(Met(R)O-44)), and also implicate the interconversion of specific Met/Met(R)O residues as a precise means to modulate protein function. Our results therefore uncover a specific reversible redox actin regulatory system that controls cell and developmental biology.

Excess podocyte semaphorin-3A leads to glomerular disease involving plexinA1-nephrin interaction

Semaphorin-3A (Sema3a), a guidance protein secreted by podocytes, is essential for normal kidney patterning and glomerular filtration barrier development. Here, we report that podocyte-specific Sema3a gain-of-function in adult mice leads to proteinuric glomerular disease involving the three layers of the glomerular filtration barrier. Reversibility of the glomerular phenotype upon removal of the transgene induction provided proof-of-principle of the cause-and-effect relationship between podocyte Sema3a excess and glomerular disease. Mechanistically, excess Sema3a induces dysregulation of nephrin, matrix metalloproteinase 9, and αvβ3 integrin in vivo. Sema3a cell-autonomously disrupts podocyte shape. We identified a novel direct interaction between the Sema3a signaling receptor plexinA1 and nephrin, linking extracellular Sema3a signals to the slit-diaphragm signaling complex. We conclude that Sema3a functions as an extracellular negative regulator of the structure and function of the glomerular filtration barrier in the adult kidney. Our findings demonstrate a crosstalk between Sema3a and nephrin signaling pathways that is functionally relevant both in vivo and in vitro.

MICAL, the flavoenzyme participating in cytoskeleton dynamics

MICAL (from the Molecule Interacting with CasL) indicates a family of recently discovered cytosolic, multidomain proteins, which uniquely couple an N-terminal FAD-containing monooxygenase-like domain to typical calponine homology, LIM and coiled-coil protein-interaction modules. Genetic and cell biology approaches have demonstrated an essential role of the catalytic activity of the monooxygenase-like domain in transducing the signal initiated by semaphorins interaction with their plexin receptors, which results in local actin cytoskeleton disassembly as part of fundamental processes that include differentiation, migration and cell-cell contacts in neuronal and non-neuronal cell types. This review focuses on the structure-function relations of the MICAL monooxygenase-like domain as they are emerging from the available in vitro studies on mouse, human and Drosophila MICAL forms that demonstrated a NADPH-dependent actin depolymerizing activity of MICAL. With Drosophila MICAL forms, actin depolymerization was demonstrated to be associated to conversion of Met44 to methionine sulfone through a postulated hydroxylating reaction. Arguments supporting the concept that MICAL effect on F-actin may be reversible will be discussed.

Regulating small G protein signaling to coordinate axon adhesion and repulsion

Small GTPases play critical roles in diverse biological events including regulating both the cytoskeletal and adhesive properties of cells. The importance of small GTPases to these events stems from their ability to be turned on and off, respectively, by specific GEFs and GAPs. In neurons, for example, regulation of small GTPase activity by extracellular guidance cues controls axonal and dendritic process shape, extension and navigation. Here, we discuss recent findings that indicate a specific regulator of small GTPase signaling, the Plexin transmembrane GAP, is differentially controlled by specific extracellular cues to guide growing axons. In particular, Plexins are receptors for one of the largest families of axon guidance cues, Semaphorins and negatively regulate cell morphology and motility by serving as GAPs for Ras/Rap family GTPases. Recent observations reveal that Plexin's GAP activity is controlled by the cAMP-dependent protein kinase (PKA), which phosphorylates Plexin and generates a binding site for the phospho-serine/threonine binding protein 14-3-3ε. This PKA-mediated Plexin-14-3-3ε interaction prevents Plexin from associating with its GTPase substrate, and thus antagonizes Semaphorin signaling. We now further examine these interactions and how they provide a new logic by which axon guidance signaling pathways over-ride one another to steer growing axons. We also further explore how Plexin interacting proteins, including Ras, PKA and 14-3-3 may interact with the Plexin GAP domain. Our observations also further indicate that 14-3-3 proteins may have conserved roles in the regulation of GTPase activity.

Post-translational modification and regulation of actin

Many of the best-studied actin regulatory proteins use non-covalent means to modulate the properties of actin. Yet, actin is also susceptible to covalent modifications of its amino acids. Recent work is increasingly revealing that actin processing and its covalent modifications regulate important cellular events. In addition, numerous pathogens express enzymes that specifically use actin as a substrate to regulate their hosts' cells. Actin post-translational alterations have been linked to different normal and disease processes and the effects associated with metabolic and environmental stressors. Herein, we highlight specific co-translational and post-translational modifications of actin and discuss the current understanding of the role that these modifications play in regulating actin.

14-3-3ε couples protein kinase A to semaphorin signaling and silences plexin RasGAP-mediated axonal repulsion

The biochemical means through which multiple signaling pathways are integrated in navigating axons is poorly understood. Semaphorins are among the largest families of axon guidance cues and utilize Plexin (Plex) receptors to exert repulsive effects on axon extension. However, Semaphorin repulsion can be silenced by other distinct cues and signaling cascades, raising questions of the logic underlying these events. We now uncover a simple biochemical switch that controls Semaphorin/Plexin repulsive guidance. Plexins are Ras/Rap family GTPase activating proteins (GAPs) and we find that the PlexA GAP domain is phosphorylated by the cAMP-dependent protein kinase (PKA). This PlexA phosphorylation generates a specific binding site for 14-3-3ε, a phospho-binding protein that we find to be necessary for axon guidance. These PKA-mediated Plexin-14-3-3ε interactions prevent PlexA from interacting with its Ras family GTPase substrate and antagonize Semaphorin repulsion. Our results indicate that these interactions switch repulsion to adhesion and identify a point of convergence for multiple guidance molecules.

Neuropilins: expression and roles in the epithelium

Initially found expressed in neuronal and then later in endothelial cells, it is well established that the transmembrane glycoproteins neuropilin-1 (NRP1) and neuropilin-2 (NRP2) play essential roles in axonal growth and guidance and in physiological and pathological angiogenesis. Neuropilin expression and function in epithelial cells has received little attention when compared with neuronal and endothelial cells. Overexpression of NRPs is shown to enhance growth, correlate with invasion and is associated with poor prognosis in various tumour types, especially those of epithelial origin. The contribution of NRP and its ligands to tumour growth and metastasis has spurred a strong interest in NRPs as novel chemotherapy drug targets. Given NRP's role as a multifunctional co-receptor with an ability to bind with disparate ligand families, this has sparked new areas of research implicating NRPs in diverse biological functions. Here, we review the growing body of research demonstrating NRP expression and role in the normal and neoplastic epithelium.

Differential regulation of actin microfilaments by human MICAL proteins

The Drosophila melanogaster MICAL protein is essential for the neuronal growth cone machinery that functions through plexin- and semaphorin-mediated axonal signaling. Drosophila MICAL is also involved in regulating myofilament organization and synaptic structures, and serves as an actin disassembly factor downstream of plexin-mediated axonal repulsion. In mammalian cells there are three known isoforms, MICAL1, MICAL2 and MICAL3, as well as the MICAL-like proteins MICAL-L1 and MICAL-L2, but little is known of their function, and information comes almost exclusively from neural cells. In this study we show that in non-neural cells human MICALs are required for normal actin organization, and all three MICALs regulate actin stress fibers. Moreover, we provide evidence that the generation of reactive oxygen species by MICAL proteins is crucial for their actin-regulatory function. However, although MICAL1 is auto-inhibited by its C-terminal coiled-coil region, MICAL2 remains constitutively active and affects stress fibers. These data suggest differential but complementary roles for MICAL1 and MICAL2 in actin microfilament regulation.