Cell-cell signalling of semaphorin ligands through interaction with plexin receptors is important for the homeostasis and morphogenesis of many tissues and is widely studied for its role in neural connectivity, cancer, cell migration and immune responses. SEMA4D and Sema6A exemplify two diverse vertebrate, membrane-spanning semaphorin classes (4 and 6) that are capable of direct signalling through members of the two largest plexin classes, B and A, respectively. In the absence of any structural information on the plexin ectodomain or its interaction with semaphorins the extracellular specificity and mechanism controlling plexin signalling has remained unresolved. Here we present crystal structures of cognate complexes of the semaphorin-binding regions of plexins B1 and A2 with semaphorin ectodomains (human PLXNB1(1-2)-SEMA4D(ecto) and murine PlxnA2(1-4)-Sema6A(ecto)), plus unliganded structures of PlxnA2(1-4) and Sema6A(ecto). These structures, together with biophysical and cellular assays of wild-type and mutant proteins, reveal that semaphorin dimers independently bind two plexin molecules and that signalling is critically dependent on the avidity of the resulting bivalent 2:2 complex (monomeric semaphorin binds plexin but fails to trigger signalling). In combination, our data favour a cell-cell signalling mechanism involving semaphorin-stabilized plexin dimerization, possibly followed by clustering, which is consistent with previous functional data. Furthermore, the shared generic architecture of the complexes, formed through conserved contacts of the amino-terminal seven-bladed β-propeller (sema) domains of both semaphorin and plexin, suggests that a common mode of interaction triggers all semaphorin-plexin based signalling, while distinct insertions within or between blades of the sema domains determine binding specificity.
"Originally depicted as axon-guidance molecules, recent studies have demonstrated their involvement outside of the nervous system, where they play key roles in cell migration, tissue development, and angiogenesis (Tamagnone and Comoglio, 2000; Dickson, 2002; Huber et al., 2003; Tran et al., 2007; Suzuki et al., 2008; Larrivee et al., 2009). Genetic evidence from knockout animals has demonstrated a specific role for Sema4d derived from osteoclasts as a key regulator of bone formation through its receptor Plexin-B1 expressed by osteoblasts (Janssen et al., 2010; Negishi-Koga et al., 2011). It has previously been demonstrated that osteoclasts repel osteoblast activity through Sema4d, and it was demonstrated in a gene knockdown animal model that while Sema4d does not affect osteoclast behavior, it has a pronounced effect on osteoblast behavior by increasing osteoblast numbers on bone surfaces and "
[Show abstract][Hide abstract] ABSTRACT: Semaphorin 4d (Sema4d) has been proposed as a novel target gene for the treatment of osteoporosis. Recently, we fabricated a site-specific bone-targeting system from polymeric nanoparticles that demonstrates an ability to prevent bone loss in an osteoporotic model by interfering with Sema4d gene expression using small interference RNA (siRNA) molecules. The aim of the present investigation was to determine the effects of this targeting system on the periodontium, an area of high bone turnover. We demonstrated, by single photon emission computed tomography, that intravenous injection of this molecule in ovariectomized Balb/C mice is able to target alveolar bone peaking 4 hr post-injection. We then compared, by histological analysis, the bone volume/total volume (BV/TV), alveolar bone height loss, immunohistochemical expression of Sema4d, and total number of osteoclasts in mandibular alveolar bone. Four treatment modalities were compared as follows: (1) sham-operated, (2) OVX-operated, (3) OVX+estrogen replacement therapy, and (4) OVX+siRNA-Sema4d animals. The results from the present study demonstrate that an osteoporotic condition significantly increases alveolar bone height loss, and that the therapeutic effects via bone-targeting systems featuring interference of Sema4d are able to partly counteract alveolar bone loss caused by osteoporosis. While the future therapeutic demand for the large number of patients suffering from osteoporosis faces many challenges, we demonstrate within the present study an effective drug-delivery moiety with anabolic effects on the bone remodeling cycle able to locate and target alveolar bone regeneration.
Journal of Dental Research 09/2014; 93(11). DOI:10.1177/0022034514552676 · 4.14 Impact Factor
"Sema4D (also called CD100) is a class 4 transmembrane-type semaphorin that binds to Plexin-B1, which is a transmembrane receptor and a member of the plexin family, to induce repulsive cytoskeletal changes in growth cones of cultured neurons . Sema4D and Plexin-B1 interact via their conserved sema domains of ∼400 amino acids each; each sema domain forms a seven-blade β-propeller fold in the extracellular domain of the respective protein , , . Binding of Sema4D to Plexin-B1 causes Plexin-B1 molecules to cluster; this clustering facilitates GTPase activating protein (GAP) activities of the two GAP domains in the intracellular region of each Plexin-B1 molecule . "
[Show abstract][Hide abstract] ABSTRACT: Around the fifth week after birth, the vaginal cavity in female mouse pups opens to the overlaying skin. This postnatal tissue remodeling of the genital tract occurs during puberty, and it largely depends upon hormonally induced apoptosis that mainly occurs in the epithelium at the lower part of the mouse vaginal cavity. Previously, we showed that most BALB/c mice lacking the class IV Semaphorin (Sema4D) develop imperforate vagina and hydrometrocolpos; therefore, we reasoned that the absence of Sema4D-induced apoptosis in vaginal epithelial cells may cause the imperforate vagina. Sema4D signals via the Plexin-B1 receptor; nevertheless detailed mechanisms mediating this hormonally triggered apoptosis are not fully documented. To investigate the estrogen-dependent control of Sema4D signaling during the apoptosis responsible for mouse vaginal opening, we examined structural and functional modulation of Sema4D, Plexin-B1, and signaling molecules by analyzing both wild-type and Sema4D-/- mice with or without ovariectomy. Both the release of soluble Sema4D and the conversion of Plexin-B1 by proteolytic processing in vaginal tissue peaked 5 weeks after birth of wild-type BALB/c mice at the time of vaginal opening. Estrogen supplementation of ovariectomized wild-type mice revealed that both the release of soluble Sema4D and the conversion of Plexin-B1 into an active form were estrogen-dependent and concordant with apoptosis. Estrogen supplementation of ovariectomized Sema4D-/- mice did not induce massive vaginal apoptosis in 5-week-old mice; therefore, Sema4D may be an essential apoptosis-inducing ligand that acts downstream of estrogen action in vaginal epithelium during this postnatal tissue remodeling. Analysis of ovariectomized mice also indicated that Sema4D contributed to estrogen-dependent dephosphorylation of Akt and ERK at the time of vaginal opening. Based on our results, we propose that apoptosis in vaginal epithelium during postnatal vaginal opening is induced by enhanced Sema4D signaling that is caused by estrogen-dependent structural changes of Sema4D and Plexin-B1.
PLoS ONE 05/2014; 9(5):e97909. DOI:10.1371/journal.pone.0097909 · 3.23 Impact Factor
"All hiPSC lines used in this study were able to generate neuronal cultures with comparable percentages of neurons (∼70%) and glial cells (∼30%). We then analyzed the neuronal morphology by transfecting hiPSC-derived neurons with a pEF-1-dTomato construct to visualize single neurons within a dense culture as previously reported (11,12). We identified profound reductions in the neuritic complexity in neurons derived from SPG11 patients compared with controls (Fig. 3A and B). "
[Show abstract][Hide abstract] ABSTRACT: Hereditary spastic paraplegias are a group of inherited motor neuron diseases characterized by progressive paraparesis and spasticity. Mutations in the spastic paraplegia gene SPG11, encoding spatacsin, cause an autosomal recessive disease trait, however the precise knowledge about the role of spatacsin in neurons is very limited.We for the first time analyzed the expression and function of spatacsin in human forebrain neurons derived from human pluripotent stem cells including lines from two SPG11 patients and two controls. SPG11 patients' derived neurons exhibited downregulation of specific axonal-related genes, decreased neurite complexity, and accumulation of membranous bodies within axonal processes. Altogether, these data point towards axonal pathologies in human neurons with SPG11 mutations.To further corroborate spatacsin function, we investigated human pluripotent stem cell derived neurons and mouse cortical neurons. In these cells spatacsin was located in axons and dendrites. It colocalized with cytoskeletal and synaptic vesicle markers, and was present in synaptosomes. Knockdown of spatacsin in mouse cortical neurons evidenced that the loss of function of spatacsin leads to axonal instability by downregulation of acetylated tubulin. Finally, time-lapse assays performed in SPG11 patients' derived neurons and spatacsin-silenced mouse neurons highlighted a reduction in the anterograde vesicle trafficking indicative of impaired axonal transport.By employing SPG11 patient derived forebrain neurons and mouse cortical neurons, this study provides the first evidence that SPG11 is implicated in axonal maintenance and cargo trafficking. Understanding the cellular functions of spatacsin will allow deciphering mechanisms of motor cortex dysfunction in autosomal recessive hereditary spastic paraplegia.
Human Molecular Genetics 05/2014; 23(18). DOI:10.1093/hmg/ddu200 · 6.39 Impact Factor
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