Plexin structures are coming: opportunities for multilevel investigations of semaphorin guidance receptors, their cell signaling mechanisms, and functions

Plexin-Bs enhance their GAP activity with a novel activation switch loop generating a cooperative enzyme

Plexins receive guidance cues from semaphorin ligands and transmit their signal through the plasma membrane. This family of proteins is unique amongst single-pass transmembrane receptors as their intracellular regions interact directly with several small GTPases, which regulate cytoskeletal dynamics and cell adhesion. Here, we characterize the GTPase Activating Protein (GAP) function of Plexin-B1 and find that a cooperative GAP activity towards the substrate GTPase, Rap1b, is associated with the N-terminal Juxtamembrane region of Plexin-B1. Importantly, we unveil an activation mechanism of Plexin-B1 by identifying a novel functional loop which partially blocks Rap1b entry into the plexin GAP domain. Consistent with the concept of allokairy developed for other systems, Plexin-B activity is increased by an apparent substrate-mediated cooperative effect. Simulations and mutagenesis suggest the repositioned JM conformation is stabilized by the new activation switch loop when the active site is occupied, giving rise to faster enzymatic turnover and cooperative behavior. The biological implications, essentially those of a threshold behavior for cell migration, are discussed.

Transcriptomic Changes Related to Cellular Processes with Particular Emphasis on Cell Activation in Lysosomal Storage Diseases from the Group of Mucopolysaccharidoses

Although mucopolysaccharidoses (MPS), inherited metabolic diseases from the group of lysosomal storage diseases (LSD), are monogenic disorders, recent studies indicated that their molecular mechanisms are complicated. Storage of glycosaminoglycans (GAGs), arising from a deficiency in one of the enzymes involved in the degradation of these compounds, is the primary cause of each MPS type. However, dysfunctions of various cellular organelles and disturbance of cellular processes have been reported which contribute considerably to pathomechanisms of the disease. Here, we present a complex transcriptomic analysis in which all types and subtypes of MPS were investigated, with special emphasis on genes related to cell activation processes. Complex changes in expression of these genes were found in fibroblasts of all MPS types, with number of transcripts revealing higher or lower levels (relative to control fibroblasts) between 19 and over 50, depending on MPS type. Genes in which expression was significantly affected in most MPS types code for proteins involved in following processes, classified according to Gene Ontology knowledge database: cell activation, cell growth, cell recognition, and cell division. Levels of some transcripts (including CD9, CLU, MME and others) were especially significantly changed (over five times relative to controls). Our results are discussed in the light of molecular pathomechanisms of MPS, indicating that secondary and/or tertiary changes, relative to GAG storage, might significantly modulate cellular dysfunctions and contribute to molecular mechanisms of the disease. This may influence the efficacy of various therapies and suggests why various treatments are not fully effective in improving the complex symptoms of MPS.

Cryo-EM structure of the PlexinC1/A39R complex reveals inter-domain interactions critical for ligand-induced activation

Plexins are receptors for semaphorins that transduce signals for regulating neuronal development and other processes. Plexins are single-pass transmembrane proteins with multiple domains in both the extracellular and intracellular regions. Semaphorin activates plexin by binding to its extracellular N-terminal Sema domain, inducing the active dimer of the plexin intracellular region. The mechanism underlying this activation process of plexin is incompletely understood. We present cryo-electron microscopic structure of full-length human PlexinC1 in complex with the viral semaphorin mimic A39R. The structure shows that A39R induces a specific dimer of PlexinC1 where the membrane-proximal domains from the two PlexinC1 protomers are placed close to each other, poised to promote the active dimer of the intracellular region. This configuration is imposed by a distinct conformation of the PlexinC1 extracellular region, stabilized by inter-domain interactions among the Sema and membrane-proximal domains. Our mutational analyses support the critical role of this conformation in PlexinC1 activation.

Plexins are the receptors for the guidance molecules semaphorins and regulate immunity and the development of the nervous and cardiovascular systems. Here authors present a structure of full-length human PlexinC1 in complex with its ligand A39R, which reveals how inter-domain interactions couple extracellular ligand binding to receptor activation and signaling.

Glial Cell-Axonal Growth Cone Interactions in Neurodevelopment and Regeneration

The developing nervous system is a complex yet organized system of neurons, glial support cells, and extracellular matrix that arranges into an elegant, highly structured network. The extracellular and intracellular events that guide axons to their target locations have been well characterized in many regions of the developing nervous system. However, despite extensive work, we have a poor understanding of how axonal growth cones interact with surrounding glial cells to regulate network assembly. Glia-to-growth cone communication is either direct through cellular contacts or indirect through modulation of the local microenvironment via the secretion of factors or signaling molecules. Microglia, oligodendrocytes, astrocytes, Schwann cells, neural progenitor cells, and olfactory ensheathing cells have all been demonstrated to directly impact axon growth and guidance. Expanding our understanding of how different glial cell types directly interact with growing axons throughout neurodevelopment will inform basic and clinical neuroscientists. For example, identifying the key cellular players beyond the axonal growth cone itself may provide translational clues to develop therapeutic interventions to modulate neuron growth during development or regeneration following injury. This review will provide an overview of the current knowledge about glial involvement in development of the nervous system, specifically focusing on how glia directly interact with growing and maturing axons to influence neuronal connectivity. This focus will be applied to the clinically-relevant field of regeneration following spinal cord injury, highlighting how a better understanding of the roles of glia in neurodevelopment can inform strategies to improve axon regeneration after injury.

High Expression Levels of Long Noncoding RNA Small Nucleolar RNA Host Gene 18 and Semaphorin 5A Indicate Poor Prognosis in Multiple Myeloma

BACKGROUND

The aim of this study was to detect the expression of long noncoding RNA small nucleolar RNA host gene 18 (SNHG18) andsemaphorin 5A (SEMA5A) genes in multiple myeloma (MM) patients and to explore the correlation of the expression of these genes with the clinical characteristics and prognosis of MM patients.

METHODS

Forty-seven newly diagnosed MM, 18 complete remission MM, 13 refractory/relapse MM, and 22 iron deficiency anemia (serving as control) samples were extracted at the Department of Hematology, Second Affiliated Hospital of Xian Jiaotong University between January 2015 and December 2016. The clinical features of the MM patients are summarized. Real-time quantitative PCR was performed to analyze the relative expression levels of the SNHG18 and SEMA5Agenes. The clinical characteristics and overall survival (OS) of the MM patients were statistically analyzed while measuring different levels of SNHG18 and SEMA5Agene expression. At the same time, the correlation between the expression of SNHG18 and SEMA5A was also analyzed.

RESULTS

The analysis confirmed that SNHG18 and its possible target gene SEMA5A were both highly expressed in newly diagnosed MM patients. After analyzing the clinical significance of SNHG18 and SEMA5A in MM patients, we found that the expression of SNHG18 and SEMA5A was related to the Durie-Salmon (DS), International Staging System (ISS), and Revised International Staging System (R-ISS) classification systems, and the Mayo Clinic Risk Stratification for Multiple Myeloma (mSMART; p < 0.05). Moreover, we observed a significant difference in OS between the SNHG18/SEMA5A high expression group and the low expression group. We found a positive correlation between SNHG18 and SEMA5A expression (r = 0.709, p < 0.01). Surprisingly, the expected median OS times of both the SNHG18 and SEMA5Ahigh expression groups were significantly decreased, which was in contrast to those of both the SNHG18 and SEMA5Alow expression groups and the single-gene high expression group (p < 0.05).

CONCLUSION

High expression of both SNHG18 and SEMA5A is associated with poor prognosis in patients with MM.

Local Ordering at the N-H Sites of the Rho GTPase Binding Domain of Plexin-B1: Impact of Dimerization

We have developed a new molecular dynamics (MD) based method for describing analytically local potentials at mobile N-H sites in proteins. Here we apply it to the monomer and dimer of the Rho GTPase binding domain (RBD) of the transmembrane receptor plexin-B1 to gain insight into dimerization, which can compete with Rho GTPase binding. In our method, the local potential is given by linear combinations, u^(D_L,K), of the real combinations of the Wigner rotation matrix elements, D_L,K, with L = 1-4 and appropriate symmetry. The combination that "fits best" the corresponding MD potential of mean force, u^(MD), is the potential we are seeking, u^{(D_L,K - BEST)}. For practical reasons the fitting process involves probability distributions, P_eq ∝ exp(-u), instead of potentials, u. The symmetry of the potential, u^(D_L,K), may be related to the irreducible representations of the D_2h point group. The monomer (dimer) potentials have mostly A_g and B_2u (B_1u and B_2u) symmetry. For the monomer, the associated probability distributions are generally dispersed in space, shallow, and centered at the "reference N-H orientation" (defined in section 3.1. below); for the dimer many are more concentrated, deep and centered away from the "reference N-H orientation". The u^(D_L,K) functions provide a consistent description of the potential energy landscape at protein N-H sites. The L1-loop of the plexin-B1 RBD is not seen in the crystal structure, and many resonances of the L4 loop are missing in the NMR ¹⁵N-¹H HSQC spectrum of the dimer; we suggest reasons for these features. An allosteric signal transmission pathway was reported previously for the monomer. We find that it has shallow N-H potentials at its ends, which become deeper as one proceeds toward the middle, complementing structurally the previously derived dynamic picture. Prospects of this study include correlating u^{(D_L,K - BEST)} with MD force-fields, and using them without further adjustment in NMR relaxation analysis schemes.

Genome-wide DNA methylation in suicide victims revealing impact on gene expression

BACKGROUND

Suicidal behavior is a multifactorial, polygenic state that affects millions worldwide. It is a result of interplay between hereditary and environmental factors, tied together by epigenetics. Despite vast knowledge on suicidality complete mechanism and factors leading to suicide are unknown. However there is an indication between changes in DNA methylation patterns and suicidal behavior.

METHODS

To identify differential methylation we formed a homogenous group of male suicide victims who died by hanging and control group. Altogether our study included 18 subjects in which two brain regions, Brodmann area 9 (9 suicide victims and 9 controls)) and hippocampus (6 suicide victims and 6 controls) were investigated using next-generation sequencing (NGS).

RESULTS

Our results have shown several differences in methylation level between suicide victims and controls in both brain regions (>25% difference in methylation and q-value < 0.01), with gene ontology pointing towards cell structural integrity and nervous system regulation. Additional gene expression analysis identified changes in two genes, ZNF714 (p-value = 0.002) and NRIP3 (p-value = 0.046).

LIMITATIONS

Major limitation is small sample size. Our analysis was conducted on brain tissue including different cell types so the results are a representation of a methylation pattern for the whole brain tissue sample.

CONCLUSIONS

We performed a preliminary methylation study with single base pair resolution using NGS on one of the world populations with a very high suicide risk. Obtained results offer novel insights into altered methylation patterns in suicide victims, which could provide a starting point for further studies on clinical samples with highly expressed suicide risk.

Semaphorin 4D promotes the proliferation and metastasis of bladder cancer by activating the PI3K/AKT pathway

The present study aimed to investigate the role of semaphorin 4D (Sema4D) in bladder cancer cell proliferation and metastasis in vivo and in vitro. Effects of Sema4D modulation on cancer cell viability and clonogenic abilities were assessed by MTT assay and colony formation assay. Cell apoptosis, cell cycle analysis, transwell assays, and wound-healing assays were also assayed. A mouse model of bladder cancer was established to observe the tumorigenesis in vivo. Our data showed that Sema4D was 4-fold upregulated in clinical bladder cancer tissues relative to noncancerous ones and differentially expressed in bladder cancer cell lines. Knockdown of Sema4D in bladder cancer T24 and 5637 cells significantly decreased cell proliferation, clonogenic potential, and motility. On the contrary, overexpression of Sema4D in bladder cancer SV-HUC-1 cells significantly increased cell viability and motility. Concordantly, knockdown of Sema4D impaired while overexpression of Sema4D promoted bladder cancer cell growth rates in xenotransplanted mice. Cell cycle was arrested by modulation of Sema4D. Cell apoptotic rates and the mitochondrial membrane potentials were consistently increased upon knockdown of Sema4D in T24 cells and 5637 cells. Western blotting revealed that epithelial-mesenchymal transition was promoted by Sema4D. The PI3K/AKT pathway was activated upon Sema4D overexpression in SV-HUC-1 cells, while it was inactivated by knockdown of Sema4D in T24 cells. All these data suggest that Sema4D promotes cell proliferation and metastasis in bladder cancer in vivo and in vitro. The oncogenic behavior of Sema4D is achieved by activating the PI3K/AKT pathway.

Divergent roles of Plexin D1 in cancer

Plexin D1 belongs to a family of transmembrane proteins called plexins. It was characterized as a receptor for semaphorins and is known to be essential for axonal guidance and vascular patterning. Mutations in Plexin D1 have been implicated in pathologic conditions such as truncus arteriosus and Möbius syndrome. Emerging data show that expression of Plexin D1 is deregulated in several cancers; it can support tumor development by aiding in tumor metastasis and EMT; and conversely, it can act as a dependence receptor and stimulate cell death in the absence of its canonical ligand, semaphorin 3E. The role of Plexin D1 in tumor development and progression is thereby garnering research interest for its potential as a biomarker and as a therapeutic target. In this review, we describe its discovery, structure, mutations, role(s) in cancer, and therapeutic potential.

Ras-Specific GTPase-Activating Proteins-Structures, Mechanisms, and Interactions

Ras-specific GTPase-activating proteins (RasGAPs) down-regulate the biological activity of Ras proteins by accelerating their intrinsic rate of GTP hydrolysis, basically by a transition state stabilizing mechanism. Oncogenic Ras is commonly not sensitive to RasGAPs caused by interference of mutants with the electronic or steric requirements of the transition state, resulting in up-regulation of activated Ras in respective cells. RasGAPs are modular proteins containing a helical catalytic RasGAP module surrounded by smaller domains that are frequently involved in the subcellular localization or contributing to regulatory features of their host proteins. In this review, we summarize current knowledge about RasGAP structure, mechanism, regulation, and dual-substrate specificity and discuss in some detail neurofibromin, one of the most important negative Ras regulators in cellular growth control and neuronal function.

Origin and evolution of plexins, semaphorins, and Met receptor tyrosine kinases

The transition from unicellular to multicellular organisms poses the question as to when genes that regulate cell-cell interactions emerged during evolution. The receptor and ligand pairing of plexins and semaphorins regulates cellular interactions in a wide range of developmental and physiological contexts. We surveyed here genomes of unicellular eukaryotes and of non-bilaterian and bilaterian Metazoa and performed phylogenetic analyses to gain insight into the evolution of plexin and semaphorin families. Remarkably, we detected plexins and semaphorins in unicellular choanoflagellates, indicating their evolutionary origin in a common ancestor of Choanoflagellida and Metazoa. The plexin domain structure is conserved throughout all clades; in contrast, semaphorins are structurally diverse. Choanoflagellate semaphorins are transmembrane proteins with multiple fibronectin type III domains following the N-terminal Sema domain (termed Sema-FN). Other previously not yet described semaphorin classes include semaphorins of Ctenophora with tandem immunoglobulin domains (Sema-IG) and secreted semaphorins of Echinoderamata (Sema-SP, Sema-SI). Our study also identified Met receptor tyrosine kinases (RTKs), which carry a truncated plexin extracellular domain, in several bilaterian clades, indicating evolutionary origin in a common ancestor of Bilateria. In addition, a novel type of Met-like RTK with a complete plexin extracellular domain was detected in Lophotrochozoa and Echinodermata (termed Met-LP RTK). Our findings are consistent with an ancient function of plexins and semaphorins in regulating cytoskeletal dynamics and cell adhesion that predates their role as axon guidance molecules.

Class-3 Semaphorins and Their Receptors: Potent Multifunctional Modulators of Tumor Progression

Semaphorins are the products of a large gene family containing 28 genes of which 21 are found in vertebrates. Class-3 semaphorins constitute a subfamily of seven vertebrate semaphorins which differ from the other vertebrate semaphorins in that they are the only secreted semaphorins and are distinguished from other semaphorins by the presence of a basic domain at their C termini. Class-3 semaphorins were initially characterized as axon guidance factors, but have subsequently been found to regulate immune responses, angiogenesis, lymphangiogenesis, and a variety of additional physiological and developmental functions. Most class-3 semaphorins transduce their signals by binding to receptors belonging to the neuropilin family which subsequently associate with receptors of the plexin family to form functional class-3 semaphorin receptors. Recent evidence suggests that class-3 semaphorins also fulfill important regulatory roles in multiple forms of cancer. Several class-3 semaphorins function as endogenous inhibitors of tumor angiogenesis. Others were found to inhibit tumor metastasis by inhibition of tumor lymphangiogenesis, by direct effects on the behavior of tumor cells, or by modulation of immune responses. Notably, some semaphorins such as sema3C and sema3E have also been found to potentiate tumor progression using various mechanisms. This review focuses on the roles of the different class-3 semaphorins in tumor progression.

Making Connections: Guidance Cues and Receptors at Nonneural Cell-Cell Junctions

The field of axon guidance was revolutionized over the past three decades by the identification of highly conserved families of guidance cues and receptors. These proteins are essential for normal neural development and function, directing cell and axon migration, neuron-glial interactions, and synapse formation and plasticity. Many of these genes are also expressed outside the nervous system in which they influence cell migration, adhesion and proliferation. Because the nervous system develops from neural epithelium, it is perhaps not surprising that these guidance cues have significant nonneural roles in governing the specialized junctional connections between cells in polarized epithelia. The following review addresses roles for ephrins, semaphorins, netrins, slits and their receptors in regulating adherens, tight, and gap junctions in nonneural epithelia and endothelia.

The Sema3A receptor Plexin-A1 suppresses supernumerary axons through Rap1 GTPases

The highly conserved Rap1 GTPases perform essential functions during neuronal development. They are required for the polarity of neuronal progenitors and neurons as well as for neuronal migration in the embryonic brain. Neuronal polarization and axon formation depend on the precise temporal and spatial regulation of Rap1 activity by guanine nucleotide exchange factors (GEFs) and GTPases-activating proteins (GAPs). Several Rap1 GEFs have been identified that direct the formation of axons during cortical and hippocampal development in vivo and in cultured neurons. However little is known about the GAPs that limit the activity of Rap1 GTPases during neuronal development. Here we investigate the function of Sema3A and Plexin-A1 as a regulator of Rap1 GTPases during the polarization of hippocampal neurons. Sema3A was shown to suppress axon formation when neurons are cultured on a patterned substrate. Plexin-A1 functions as the signal-transducing subunit of receptors for Sema3A and displays GAP activity for Rap1 GTPases. We show that Sema3A and Plexin-A1 suppress the formation of supernumerary axons in cultured neurons, which depends on Rap1 GTPases.

Rap2 and TNIK control Plexin-dependent tiled synaptic innervation in C. elegans

During development, neurons form synapses with their fate-determined targets. While we begin to elucidate the mechanisms by which extracellular ligand-receptor interactions enhance synapse specificity by inhibiting synaptogenesis, our knowledge about their intracellular mechanisms remains limited. Here we show that Rap2 GTPase (rap-2) and its effector, TNIK (mig-15), act genetically downstream of Plexin (plx-1) to restrict presynaptic assembly and to form tiled synaptic innervation in C. elegans. Both constitutively GTP- and GDP-forms of rap-2 mutants exhibit synaptic tiling defects as plx-1 mutants, suggesting that cycling of the RAP-2 nucleotide state is critical for synapse inhibition. Consistently, PLX-1 suppresses local RAP-2 activity. Excessive ectopic synapse formation in mig-15 mutants causes a severe synaptic tiling defect. Conversely, overexpression of mig-15 strongly inhibited synapse formation, suggesting that mig-15 is a negative regulator of synapse formation. These results reveal that subcellular regulation of small GTPase activity by Plexin shapes proper synapse patterning in vivo.

Genes do more than just direct the color of our hair or eyes. They produce proteins that are involved in almost every process in the body. In humans, the majority of active genes can be found in the brain, where they help it to develop and work properly – effectively controlling how we move and behave.

The brain’s functional units, the nerve cells or neurons, communicate with each other by releasing messenger molecules in the gap between them, the synapse. These molecules are then picked up from specific receptor proteins of the receiving neuron.

In the nervous system, neurons only form synapses with the cells they need to connect with, even though they are surrounded by many more cells. This implies that they use specific mechanisms to stop neurons from forming synapses with incorrect target cells. This is important, because if too many synapses were present or if synapses formed with incorrect target cells, it would compromise the information flow in the nervous system. This would ultimately lead to various neurological conditions, including Autism Spectrum Disorder.

In 2013, researchers found that in the roundworm Caenorhabditis elegans, a receptor protein called Plexin, is located at the surface of the neurons and can inhibit the formation of nearby synapses. Now, Chen et al. – including one author involved in the previous research – wanted to find out what genes Plexin manipulates when it stops synapses from growing. Knowing what each of those genes does can help us understand how neurons can inhibit synapses.

The results revealed that Plexin appears to regulate two genes, Rap2 and TNIK. Plexin reduced the activity of Rap2 in the neuron that released the messenger, which hindered the formation of synapses. The gene TNIK and its protein on the other hand, have the ability to modify other proteins and could so inhibit the growth of synapses. When TNIK was experimentally removed, the number of synapses increased, but when its activity was increased, the number of synapses was strongly reduced.

These findings could help scientists understand how mutations in Rap2 or TNIK can lead to various neurological conditions. A next step will be to test if these genes also affect the formation of synapses in other species such as mice, which have a more complex nervous system that is structurally and functionally more similar to that of humans.

Sema4C/PlexinB2 signaling controls breast cancer cell growth, hormonal dependence and tumorigenic potential

Semaphorin 4C (Sema4C) expression in human breast cancers correlates with poor disease outcome. Surprisingly, upon knock-down of Sema4C or its receptor PlexinB2 in diverse mammary carcinoma cells (but not their normal counterparts), we observed dramatic growth inhibition associated with impairment of G2/M phase transition, cytokinesis defects and the onset of cell senescence. Mechanistically, we demonstrated a Sema4C/PlexinB2/LARG-dependent signaling cascade that is required to maintain critical RhoA-GTP levels in cancer cells. Interestingly, we also found that Sema4C upregulation in luminal-type breast cancer cells drives a dramatic phenotypic change, with disassembly of polarity complexes, mitotic spindle misorientation, cell-cell dissociation and increased migration and invasiveness. We found that this signaling cascade is dependent on the PlexinB2 effectors ErbB2 and RhoA-dependent kinases. Moreover, Sema4C-overexpressing luminal breast cancer cells upregulated the transcription factors Snail, Slug and SOX-2, and formed estrogen-independent metastatic tumors in mice. In sum, our data indicate that Sema4C/PlexinB2 signaling is essential for the growth of breast carcinoma cells, featuring a novel potential therapeutic target. In addition, elevated Sema4C expression enables indolent luminal-type tumors to become resistant to estrogen deprivation, invasive and metastatic in vivo, which could account for its association with a subset of human breast cancers with poor prognosis.

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.

Semaphorin 3A promotes osteogenic differentiation in human alveolar bone marrow mesenchymal stem cells

The aim of the present study was to investigate the role of Semaphorin 3A (Sema3A) in the osteogenic differentiation of human alveolar bone marrow mesenchymal stem cells (hABMMSCs). To investigate whether Sema3A affects hABMMSC proliferation and osteogenic differentiation, a stable Sema3A-overexpression cell line was generated by infection with the pAdCMV-SEMA3A-MCS-EGFP vector. Cell counting kit-8 and clone formation assays were performed to determine the proliferation ability of hABMMSCs, while cell osteogenic differentiation was assayed using Alizarin Red S staining. In addition, reverse transcription-quantitative polymerase chain reaction was employed to detect the mRNA expression level of osteogenesis-associated genes, Runt-related transcription factor 2 (Runx2), osteopontin (Opn) and osteocalcin (Ocn), during the osteogenic differentiation. The results revealed that, compared with the normal control group, the cell morphology of the infected cells was stable and no significant alterations were observed. Overexpression of Sema3A in hABMMSCs significantly increased the cell proliferation ability compared with the control group. Furthermore, the Alizarin Red S staining assay results indicated that the ossification process of hABMMSCs overexpressing Sema3A was evidently faster in comparison with that of the control group cells. Overexpression of Sema3A by pAdCMV-SEMA3A-MCS-EGFP infection also significantly increased the mRNA expression levels of the osteogenic marker genes Runx2, Opn and Ocn. In conclusion, Sema3A was observed to be a key positive regulator in hABMMSC osteogenic differentiation.

Analysis of the interaction of Plexin-B1 and Plexin-B2 with Rnd family proteins

The Rnd family of proteins, Rnd1, Rnd2 and Rnd3, are atypical Rho family GTPases, which bind to but do not hydrolyse GTP. They interact with plexins, which are receptors for semaphorins, and are hypothesised to regulate plexin signalling. We recently showed that each Rnd protein has a distinct profile of interaction with three plexins, Plexin-B1, Plexin-B2 and Plexin-B3, in mammalian cells, although it is unclear which region(s) of these plexins contribute to this specificity. Here we characterise the binary interactions of the Rnd proteins with the Rho-binding domain (RBD) of Plexin-B1 and Plexin-B2 using biophysical approaches. Isothermal titration calorimetry (ITC) experiments for each of the Rnd proteins with Plexin-B1-RBD and Plexin-B2-RBD showed similar association constants for all six interactions, although Rnd1 displayed a small preference for Plexin-B1-RBD and Rnd3 for Plexin-B2-RBD. Furthermore, mutagenic analysis of Rnd3 suggested similarities in its interaction with both Plexin-B1-RBD and Plexin-B2-RBD. These results suggest that Rnd proteins do not have a clear-cut specificity for different Plexin-B-RBDs, possibly implying the contribution of additional regions of Plexin-B proteins in conferring functional substrate selection.

Optogenetic activation of Plexin-B1 reveals contact repulsion between osteoclasts and osteoblasts

During bone remodelling, osteoclasts induce chemotaxis of osteoblasts and yet maintain spatial segregation. We show that osteoclasts express the repulsive guidance factor Semaphorin 4D and induce contact inhibition of locomotion (CIL) in osteoblasts through its receptor Plexin-B1. To examine causality and elucidate how localized Plexin-B1 stimulation may spatiotemporally coordinate its downstream targets in guiding cell migration, we develop an optogenetic tool for Plexin-B1 designated optoPlexin. Precise optoPlexin activation at the leading edge of migrating osteoblasts readily induces local retraction and, unexpectedly, distal protrusions to steer cells away. These morphological changes are accompanied by reorganization of Myosin II, PIP₃, adhesion and active Cdc42. We attribute the resultant repolarization to RhoA/ROCK-mediated redistribution of β-Pix, which activates Cdc42 and promotes protrusion. Thus, our data demonstrate a causal role of Plexin-B1 for CIL in osteoblasts and reveals a previously unknown effect of Semaphorin signalling on spatial distribution of an activator of cell migration.

Safety/tolerability of the anti-semaphorin 4D Antibody VX15/2503 in a randomized phase 1 trial

Objective:

To evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of VX15/2503 in a randomized, single-dose, dose-escalation, double-blind, placebo-controlled study enrolling adult patients with MS.

Methods:

Single IV doses of VX15/2503 or placebo were administered. Ten patients each were randomized (4:1 randomization ratio) into 5 ascending dose cohorts of 1, 3, 6, 10, or 20 mg/kg. Safety, immunogenicity, PK/PD, MRI, ECG, and lymphocyte subset levels were evaluated. A Dose Escalation Safety Committee (DESC) approved each dose escalation.

Results:

VX15/2503 was well tolerated, and all participants completed the study. Antibody treatment–related adverse events were primarily grade 1 or 2 and included urinary tract infection (12.5%) and muscle weakness, contusion, and insomnia (each 7.5%). No dose-limiting toxicities were observed, and no maximum tolerated dose was determined. One subject (20 mg/kg) experienced disease relapse 3 months before study entry and exhibited a grade 3 (nonserious) increase in brain lesions by day 29, possibly related to VX15/2503. Twenty-nine patients exhibited human anti-humanized antibody responses; 5 with titer ≥100. No anti-VX15/2503 antibody responses were fully neutralizing. VX15/2503 C_max, area under the time-concentration curve, and mean half-life increased with dose level; at 20 mg/kg, the T_1/2 was 20 days. Cellular SEMA4D saturation occurred at serum antibody concentrations ≤0.3 μg/mL, resulting in decreased cSEMA4D expression. At 20 mg/kg, cSEMA4D saturation persisted for ≥155 days. Total sSEMA4D levels increased with dose level and declined with antibody clearance.

Conclusions:

These results support the continued investigation of VX15/2503 in neurodegenerative diseases.

ClinicalTrials.gov identifier:

NCT01764737.

Classification of evidence:

This study provides Class III evidence that anti-semaphorin 4D antibody VX15/2503 at various doses was safe and well tolerated vs placebo, although an increase in treatment-emergent adverse events in the treatment group could not be excluded (risk difference −0.7%, 95% CI −28.0% to 32.7%).

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

15N-H-Related Conformational Entropy Changes Entailed By Plexin-B1 RBD Dimerization: Combined Molecular Dynamics/NMR Relaxation Approach

We report on a new method for determining function-related conformational entropy changes in proteins. Plexin-B1 RBD dimerization serves as example, and internally mobile N-H bonds serve as probes. S_k (entropy in units of k_BT) is given by -∫(P_eqlnP_eq)dΩ, where P_eq = exp(-u) is the probability density for probe orientation, and u the local potential. Previous slowly relaxing local structure (SRLS) analyses of ¹⁵N-H relaxation in proteins determined linear combinations of D₀₀²(Ω) and (D₀₂²(Ω) + D_0-2²(Ω)) (D_0K^L(Ω) represents a Wigner rotation matrix element in uniaxial local medium) as "best-fit" form of u. SRLS also determined the "best-fit" orientation of the related ordering tensor. On the basis of this information the coefficients (in the linear combination) of the terms specified above are determined with molecular dynamics (MD) simulations. With the explicit expression for u thus in hand, S_k is calculated. We find that in general S_k decreases, i.e., the local order increases, upon plexin-B1 RBD dimerization. The largest decrease in S_k occurs in the helices α₁ and α₂, followed by the α₂/β₆ turn. Only the relatively small peripheral β₂ strand, β₂/α₁ turn, and L3 loop become more disordered. That α-helices dominate ΔS_k = S_k(dimer) - S_k(monomer), a few peripheral outliers partly counterbalance the overall decrease in S_k, and the probability density function, P_eq, has rhombic symmetry given that the underlying potential function, u, has rhombic symmetry, are interesting features. We also derive S² (the proxy of u in the simple "model-free (MF)" limit of SRLS) with MD. Its conversion into a potential requires assumptions and adopting a simple axial form of u. Ensuing ΔS_k(MF) profiles are u-dependent and differ from ΔS_k(SRLS). A method that provides consistent, general, and accurate S_k, atomistic/mesoscopic in nature, has been developed. Its ability to provide new insights in protein research has been illustrated.

Molecular Dynamics Simulations Reveal Isoform Specific Contact Dynamics between the Plexin Rho GTPase Binding Domain (RBD) and Small Rho GTPases Rac1 and Rnd1

The Plexin family of transmembrane receptors are unique in that their intracellular region interacts directly with small GTPases of the Rho family. The Rho GTPase binding domain of plexin (RBD)-which is responsible for these interactions-can bind with Rac1 as well as Rnd1 GTPases. GTPase complexes have been crystallized with the RBDs of plexinA1, -A2, and -B1. The protein association is thought to elicit different functional responses in a GTPase and plexin isoform specific manner, but the origin of this is unknown. In this project, we investigated complexes between several RBD and Rac1/Rnd1 GTPases using multimicrosecond length all atom molecular dynamics simulations, also with reference to the free forms of the RBDs and GTPases. In accord with the crystallographic data, the RBDs experience more structural changes than Rho-GTPases upon complex formation. Changes in protein dynamics and networks of correlated motions are revealed by analyzing dihedral angle fluctuations in the proteins. The extent of these changes differs between the different RBDs and also between the Rac1 and Rnd1 GTPases. While the RBDs in the free and bound states have similar-if not decreased-correlations, correlations within the GTPases are increased upon binding. Mapping highly correlated residues to the structures, it is found that the plexinA1, -B1, and -A2 RBDs all have similar communication pathways within the ubiquitin fold, but that different residues are involved. Dynamic network analyses indicate that plexinA1 and -B1 RBDs interact with small GTPases in a similar manner, whereas complexes with the plexinA2 RBD display different features. Importantly complexes with Rnd1 have a considerable number of dynamic correlations and network connections between the proteins, whereas such features are missing in the RBD-Rac1 complexes. Overall, the simulations suggest mechanisms that are consistent with the experimental data on plexinB1 and indicate RBD and GTPase isoform specific changes in protein dynamics upon complex formation.

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.

Characterizing PKA-Mediated Phosphorylation of Plexin Using Purified Proteins

Protein phosphorylation is one of the widely used posttranslational modifications that alter protein function in vivo. We recently showed phosphorylation of Drosophila Plexin A by cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and subsequent inhibition of plexin-mediated repulsive guidance. This phosphorylation occurs in the active site of the plexin GTPase-activating protein (GAP) domain, which in turn inhibits endogenous GAP activity toward Ras/Rap family small GTP-binding proteins by recruiting the phospho-serine/threonine-binding protein 14-3-3ε. Here we describe how phosphorylation of Plexin A can be detected and quantitated using an in vitro kinase assay and radioactive [γ-P32] adenosine 5'-triphosphate (ATP).

A reverse signaling pathway downstream of Sema4A controls cell migration via Scrib

Semaphorins comprise a large family of ligands that regulate key cellular functions through their receptors, plexins. In this study, we show that the transmembrane semaphorin 4A (Sema4A) can also function as a receptor, rather than a ligand, and transduce signals triggered by the binding of Plexin-B1 through reverse signaling. Functionally, reverse Sema4A signaling regulates the migration of various cancer cells as well as dendritic cells. By combining mass spectrometry analysis with small interfering RNA screening, we identify the polarity protein Scrib as a downstream effector of Sema4A. We further show that binding of Plexin-B1 to Sema4A promotes the interaction of Sema4A with Scrib, thereby removing Scrib from its complex with the Rac/Cdc42 exchange factor βPIX and decreasing the activity of the small guanosine triphosphatase Rac1 and Cdc42. Our data unravel a role for Plexin-B1 as a ligand and Sema4A as a receptor and characterize a reverse signaling pathway downstream of Sema4A, which controls cell migration.

IQGAP1 Interaction with RHO Family Proteins Revisited: KINETIC AND EQUILIBRIUM EVIDENCE FOR MULTIPLE DISTINCT BINDING SITES

IQ motif-containing GTPase activating protein 1 (IQGAP1) plays a central role in the physical assembly of relevant signaling networks that are responsible for various cellular processes, including cell adhesion, polarity, and transmigration. The RHO family proteins CDC42 and RAC1 have been shown to mainly interact with the GAP-related domain (GRD) of IQGAP1. However, the role of its RASGAP C-terminal (RGCT) and C-terminal domains in the interactions with RHO proteins has remained obscure. Here, we demonstrate that IQGAP1 interactions with RHO proteins underlie a multiple-step binding mechanism: (i) a high affinity, GTP-dependent binding of RGCT to the switch regions of CDC42 or RAC1 and (ii) a very low affinity binding of GRD and a C terminus adjacent to the switch regions. These data were confirmed by phosphomimetic mutation of serine 1443 to glutamate within RGCT, which led to a significant reduction of IQGAP1 affinity for CDC42 and RAC1, clearly disclosing the critical role of RGCT for these interactions. Unlike CDC42, an extremely low affinity was determined for the RAC1-GRD interaction, suggesting that the molecular nature of IQGAP1 interaction with CDC42 partially differs from that of RAC1. Our study provides new insights into the interaction characteristics of IQGAP1 with RHO family proteins and highlights the complementary importance of kinetic and equilibrium analyses. We propose that the ability of IQGAP1 to interact with RHO proteins is based on a multiple-step binding process, which is a prerequisite for the dynamic functions of IQGAP1 as a scaffolding protein and a critical mechanism in temporal regulation and integration of IQGAP1-mediated cellular responses.

Rnd3-induced cell rounding requires interaction with Plexin-B2

Rnd proteins are atypical members of the Rho GTPase family that induce actin cytoskeletal reorganization and cell rounding. Rnd proteins have been reported to bind to the intracellular domain of several plexin receptors, but whether plexins contribute to the Rnd-induced rounding response is not known. Here we show that Rnd3 interacts preferentially with plexin-B2 of the three plexin-B proteins, whereas Rnd2 interacts with all three B-type plexins, and Rnd1 shows only very weak interaction with plexin-B proteins in immunoprecipitations. Plexin-B1 has been reported to act as a GAP for R-Ras and/or Rap1 proteins. We show that all three plexin-B proteins interact with R-Ras and Rap1, but Rnd proteins do not alter this interaction or R-Ras or Rap1 activity. We demonstrate that plexin-B2 promotes Rnd3-induced cell rounding and loss of stress fibres, and enhances the inhibition of HeLa cell invasion by Rnd3. We identify the amino acids in Rnd3 that are required for plexin-B2 interaction, and show that mutation of these amino acids prevents Rnd3-induced morphological changes. These results indicate that plexin-B2 is a downstream target for Rnd3, which contributes to its cellular function.

The role of the semaphorins in cancer

The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression.

Characterizing Plexin GTPase Interactions Using Gel Filtration, Surface Plasmon Resonance Spectrometry, and Isothermal Titration Calorimetry

Plexins are unique, as they are the first example of a transmembrane receptor that interacts directly with small GTPases, a family of proteins that are essential for cell motility and proliferation/survival. We and other laboratories have determined the structure of the Rho GTPase-binding domain (RBD) of several plexins and also of the entire intracellular region of plexin-B1. Structures of plexin complexes with Rho GTPases, Rac1 and Rnd1, and a structure with a Ras GTPase, Rap1b, have also been solved. The relationship between plexin-Rho and plexin-Ras interactions is still unclear and in vitro biophysical experiments that characterize the protein interactions of purified components play an important role in advancing our understanding of the molecular mechanisms that underlie the function of plexin. This chapter describes the use of gel filtration (also known as size-exclusion chromatography or SEC), surface plasmon resonance (SPR), and isothermal titration calorimetry (ITC) in studies of plexin-small GTPase interactions with plexin-B1:Rac1 as an example. Together with other assays and manipulations (e.g., by mutagenesis or protein domain truncation/deletion), these in vitro measurements provide an important reference for the role and extent of the interactions.

Semaphorin signaling in bone

Semaphorin molecules regulate cell adhesion and motility in a wide variety of cell types and are therefore involved in numerous processes including axon guidance, angiogenesis, cardiogenesis, tumor growth, and immune response. Increasing evidence points to a role of transmembrane, membrane-associated and soluble semaphorins during bone development as well as in the control of normal bone homeostasis. Within bone, semaphorins are implicated in the communication between different cell types by relaying signals in an autocrine or paracrine way. Semaphorins are not only involved in bone resorption but also in bone formation. Therefore, targeting semaphorin-induced signaling in bone may constitute an interesting new therapeutic strategy in osteoporosis. However, all the pioneering research on semaphorins is performed in mice and it remains to be established to what extent semaphorin signaling pathways are conserved between mice and men. In addition, knowledge of semaphorin signaling in bone mostly arises from loss/gain of function studies of one single semaphorin and/or receptor. However, different semaphorin molecules are co-expressed in bone and their signaling pathways are likely to interact in a complex and coherent way that needs proper understanding before targeting semaphorin signaling can be therapeutically exploited.

Regulation of dendritic development by semaphorin 3A through novel intracellular remote signaling

Numerous cell adhesion molecules, extracellular matrix proteins and axon guidance molecules participate in neuronal network formation through local effects at axo-dendritic, axo-axonic or dendro-dendritic contact sites. In contrast, neurotrophins and their receptors play crucial roles in neural wiring by sending retrograde signals to remote cell bodies. Semaphorin 3A (Sema3A), a prototype of secreted type 3 semaphorins, is implicated in axon repulsion, dendritic branching and synapse formation via binding protein neuropilin-1 (NRP1) and the signal transducing protein PlexinAs (PlexAs) complex. This review focuses on Sema3A retrograde signaling that regulates dendritic localization of AMPA-type glutamate receptor GluA2 and dendritic patterning. This signaling is elicited by activation of NRP1 in growth cones and is propagated to cell bodies by dynein-dependent retrograde axonal transport of PlexAs. It also requires interaction between PlexAs and a high-affinity receptor for nerve growth factor, toropomyosin receptor kinase A. We propose a control mechanism by which retrograde Sema3A signaling regulates the glutamate receptor localization through trafficking of cis-interacting PlexAs with GluA2 along dendrites; this remote signaling may be an alternative mechanism to local adhesive contacts for neural network formation.

Structure of the Plexin Ectodomain Bound by Semaphorin-Mimicking Antibodies

Semaphorin family proteins act on cells to mediate both repulsive and attractive guidance via binding to plexin family receptors, thereby playing fundamental roles in the morphogenesis and homeostasis of various tissues. Although semaphorin-plexin signaling is implicated in various diseases and is thus a target of intensive research, our mechanistic understanding of how semaphorins activate plexins on the cell surface is limited. Here, we describe unique anti-plexin-A1 antibodies that can induce a collapsed morphology in mouse dendritic cells as efficiently as the semaphorin 3A (Sema3A) ligand. Precise epitope analysis indicates that these “semaphorin-mimicking” antibodies dimerize cell-surface plexin-A1 by binding to the N-terminal sema domain of the plexin at sites away from the interface used by the Sema3A ligand. Structural analysis of plexin-A1 fragments using negative stain electron microscopy further revealed that this agonistic capacity is closely linked to the location and orientation of antibody binding. In addition, the full-length plexin-A1 ectodomain exhibited a highly curved “C” shape, reinforcing the very unusual dimeric receptor conformation of this protein at the cell surface when engaged with Sema3A or agonistic antibodies.

Transmembrane semaphorins, forward and reverse signaling: have a look both ways

Semaphorins are signaling molecules playing pivotal roles not only as axon guidance cues, but are also involved in the regulation of a range of biological processes, such as immune response, angiogenesis and invasive tumor growth. The main functional receptors for semaphorins are plexins, which are large single-pass transmembrane molecules. Semaphorin signaling through plexins-the "classical" forward signaling-affects cytoskeletal remodeling and integrin-dependent adhesion, consequently influencing cell migration. Intriguingly, semaphorins and plexins can interact not only in trans, but also in cis, leading to differentiated and highly regulated signaling outputs. Moreover, transmembrane semaphorins can also mediate a so-called "reverse" signaling, by acting not as ligands but rather as receptors, and initiate a signaling cascade through their own cytoplasmic domains. Semaphorin reverse signaling has been clearly demonstrated in fruit fly Sema1a, which is required to control motor axon defasciculation and target recognition during neuromuscular development. Sema1a invertebrate semaphorin is most similar to vertebrate class-6 semaphorins, and examples of semaphorin reverse signaling in mammalians have been described for these family members. Reverse signaling is also reported for other vertebrate semaphorin subsets, e.g. class-4 semaphorins, which bear potential PDZ-domain interaction motifs in their cytoplasmic regions. Therefore, thanks to their various signaling abilities, transmembrane semaphorins can play multifaceted roles both in developmental processes and in physiological as well as pathological conditions in the adult.

Semaphorin 4D Promotes Skeletal Metastasis in Breast Cancer

Bone density is controlled by interactions between osteoclasts, which resorb bone, and osteoblasts, which deposit it. The semaphorins and their receptors, the plexins, originally shown to function in the immune system and to provide chemotactic cues for axon guidance, are now known to play a role in this process as well. Emerging data have identified Semaphorin 4D (Sema4D) as a product of osteoclasts acting through its receptor Plexin-B1 on osteoblasts to inhibit their function, tipping the balance of bone homeostasis in favor of resorption. Breast cancers and other epithelial malignancies overexpress Sema4D, so we theorized that tumor cells could be exploiting this pathway to establish lytic skeletal metastases. Here, we use measurements of osteoblast and osteoclast differentiation and function in vitro and a mouse model of skeletal metastasis to demonstrate that both soluble Sema4D and protein produced by the breast cancer cell line MDA-MB-231 inhibits differentiation of MC3T3 cells, an osteoblast cell line, and their ability to form mineralized tissues, while Sema4D-mediated induction of IL-8 and LIX/CXCL5, the murine homologue of IL-8, increases osteoclast numbers and activity. We also observe a decrease in the number of bone metastases in mice injected with MDA-MB-231 cells when Sema4D is silenced by RNA interference. These results are significant because treatments directed at suppression of skeletal metastases in bone-homing malignancies usually work by arresting bone remodeling, potentially leading to skeletal fragility, a significant problem in patient management. Targeting Sema4D in these cancers would not affect bone remodeling and therefore could elicit an improved therapeutic result without the debilitating side effects.

The Chemorepulsive Protein Semaphorin 3A and Perineuronal Net-Mediated Plasticity

During postnatal development, closure of critical periods coincides with the appearance of extracellular matrix structures, called perineuronal nets (PNN), around various neuronal populations throughout the brain. The absence or presence of PNN strongly correlates with neuronal plasticity. It is not clear how PNN regulate plasticity. The repulsive axon guidance proteins Semaphorin (Sema) 3A and Sema3B are also prominently expressed in the postnatal and adult brain. In the neocortex, Sema3A accumulates in the PNN that form around parvalbumin positive inhibitory interneurons during the closure of critical periods. Sema3A interacts with high-affinity with chondroitin sulfate E, a component of PNN. The localization of Sema3A in PNN and its inhibitory effects on developing neurites are intriguing features and may clarify how PNN mediate structural neural plasticity. In the cerebellum, enhanced neuronal plasticity as a result of an enriched environment correlates with reduced Sema3A expression in PNN. Here, we first review the distribution of Sema3A and Sema3B expression in the rat brain and the biochemical interaction of Sema3A with PNN. Subsequently, we review what is known so far about functional correlates of changes in Sema3A expression in PNN. Finally, we propose a model of how Semaphorins in the PNN may influence local connectivity.

Pathophysiological Functions of Rnd3/RhoE

Rnd3, also known as RhoE, belongs to the Rnd subclass of the Rho family of small guanosine triphosphate (GTP)-binding proteins. Rnd proteins are unique due to their inability to switch from a GTP-bound to GDP-bound conformation. Even though studies of the biological function of Rnd3 are far from being concluded, information is available regarding its expression pattern, cellular localization, and its activity, which can be altered depending on the conditions. The compiled data from these studies implies that Rnd3 may not be a traditional small GTPase. The basic role of Rnd3 is to report as an endogenous antagonist of RhoA signaling-mediated actin cytoskeleton dynamics, which specifically contributes to cell migration and neuron polarity. In addition, Rnd3 also plays a critical role in arresting cell cycle distribution, inhibiting cell growth, and inducing apoptosis and differentiation. Increasing data have shown that aberrant Rnd3 expression may be the leading cause of some systemic diseases; particularly highlighted in apoptotic cardiomyopathy, developmental arrhythmogenesis and heart failure, hydrocephalus, as well as tumor metastasis and chemotherapy resistance. Therefore, a better understanding of the function of Rnd3 under different physiological and pathological conditions, through the use of suitable models, would provide a novel insight into the origin and treatment of multiple human diseases.

An extracellular biochemical screen reveals that FLRTs and Unc5s mediate neuronal subtype recognition in the retina

In the inner plexiform layer (IPL) of the mouse retina, ~70 neuronal subtypes organize their neurites into an intricate laminar structure that underlies visual processing. To find recognition proteins involved in lamination, we utilized microarray data from 13 subtypes to identify differentially-expressed extracellular proteins and performed a high-throughput biochemical screen. We identified ~50 previously-unknown receptor-ligand pairs, including new interactions among members of the FLRT and Unc5 families. These proteins show laminar-restricted IPL localization and induce attraction and/or repulsion of retinal neurites in culture, placing them in an ideal position to mediate laminar targeting. Consistent with a repulsive role in arbor lamination, we observed complementary expression patterns for one interaction pair, FLRT2-Unc5C, in vivo. Starburst amacrine cells and their synaptic partners, ON-OFF direction-selective ganglion cells, express FLRT2 and are repelled by Unc5C. These data suggest a single molecular mechanism may have been co-opted by synaptic partners to ensure joint laminar restriction.

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.

Neuropilin Functions as an Essential Cell Surface Receptor

The Neuropilins (Nrps) are a family of essential cell surface receptors involved in multiple fundamental cellular signaling cascades. Nrp family members have key functions in VEGF-dependent angiogenesis and semaphorin-dependent axon guidance, controlling signaling and cross-talk between these fundamental physiological processes. More recently, Nrp function has been found in diverse signaling and adhesive functions, emphasizing their role as pleiotropic co-receptors. Pathological Nrp function has been shown to be important in aberrant activation of both canonical and alternative pathways. Here we review key recent insights into Nrp function in human health and disease.