Age-dependent effects of secreted Semaphorins 3A, 3F, and 3E on developing hippocampal axons: in vitro effects and phenotype of Semaphorin 3A (-/-) mice

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.

Gestational 1-nitropyrene exposure causes anxiety-like behavior partially by altering hippocampal epigenetic reprogramming of synaptic plasticity in male adult offspring

1-Nitropyrene (1-NP), a typical nitro-polycyclic aromatic hydrocarbon, is a developmental toxicant. This study was to evaluate gestational 1-NP-induced anxiety-like behavior in male adult offspring. Pregnant mice were orally administered to 1-NP daily throughout pregnancy. Anxiety-like behaviors, as determined by Elevated Plus-Maze (EPM) and Open-Field Test (OFT), were showed in male adult offspring whose mothers were exposed to 1-NP. Gestational 1-NP exposure reduced dendritic arborization, dendritic length and dendritic spine density in ventral hippocampus of male adult offspring. Additional experiments showed that gephyrin, an inhibitory synaptic marker, was reduced in fetal forebrain and hippocampus in male adult offspring. Nrg1 and Erbb4, two gephyrin-related genes, were reduced in 1-NP-exposed fetuses. Accordingly, 5hmC contents in two CpG sites (32008909 and 32009239) of Nrg1 gene and three CpG sites (69107743, 69107866 and 69107899) of Erbb4 gene were decreased in 1-NP-exposed fetuses. Mechanistically, ten-eleven translocation (TET) activity and alpha-ketoglutarate (α-KG) content were decreased in 1-NP-exposed fetal forebrain. Supplementation with α-KG alleviated 1-NP-induced downregulation of gephyrin-related genes, prevented hippocampal synaptic damage, and improved anxiety-like behavior in male adult offspring. These results indicate that early-life 1-NP exposure causes anxiety-like behavior in male adulthood partially by altering hippocampal epigenetic reprogramming of synaptic plasticity.

Phosphorylation of Collapsin Response Mediator Protein 1 (CRMP1) at Tyrosine 504 residue regulates Semaphorin 3A-induced cortical dendritic growth

Collapsin response mediator proteins (CRMPs) have been identified as mediating proteins of repulsive axon guidance cue Semaphorin-3A (Sema3A). Phosphorylation of CRMPs plays a crucial role in the Sema3A signaling cascade. It has been shown that Fyn phosphorylates CRMP1 at Tyrosine 504 residue (Tyr504); however, the physiological role of this phosphorylation has not been examined. We found that CRMP1 was the most strongly phosphorylated by Fyn among the five members of CRMPs. We confirmed Tyr504 phosphorylation of CRMP1 by Fyn. Immunocytochemistry of mouse dorsal root ganglion (DRG) neurons showed that phosphotyrosine signal in the growth cones was transiently increased in the growth cones upon Sema3A stimulation. Tyr504-phosphorylated CRMP1 also tended to increase after Sema3A simulation. Ectopic expression of a single amino acid mutant of CRMP1 replacing Tyr504 with phenylalanine (CRMP1-Tyr504Phe) suppressed Sema3A-induced growth cone collapse response in chick DRG neurons. CRMP1-Tyr504Phe expression in mouse hippocampal neurons also suppressed Sema3A but not Sema3F-induced growth cone collapse response. Immunohistochemistry showed that Tyr504-phosphorylated CRMP1 was present in the cell bodies and in the dendritic processes of mouse cortical neurons. CRMP1-Tyr504Phe suppressed Sema3A-induced dendritic growth of primary cultured mouse cortical neurons as well as the dendritic development of cortical pyramidal neurons in vivo. Fyn^± ; Crmp1^± double heterozygous mutant mice exhibited poor development of cortical layer V basal dendrites, which was the similar phenotype observed in Sema3a^-/- , Fyn^-/- , and Crmp1^-/- mice. These findings demonstrate that Tyr504 phosphorylation of CRMP1 by Fyn is an essential step of Sema3A-regulated dendritic development of cortical pyramidal neurons. (247 words).

CRMP2 mediates Sema3F-dependent axon pruning and dendritic spine remodeling

Regulation of axon guidance and pruning of inappropriate synapses by class 3 semaphorins are key to the development of neural circuits. Collapsin response mediator protein 2 (CRMP2) has been shown to regulate axon guidance by mediating semaphorin 3A (Sema3A) signaling; however, nothing is known about its role in synapse pruning. Here, using newly generated crmp2^-/- mice we demonstrate that CRMP2 has a moderate effect on Sema3A-dependent axon guidance in vivo, and its deficiency leads to a mild defect in axon guidance in peripheral nerves and the corpus callosum. Surprisingly, crmp2^-/- mice display prominent defects in stereotyped axon pruning in hippocampus and visual cortex and altered dendritic spine remodeling, which is consistent with impaired Sema3F signaling and with models of autism spectrum disorder (ASD). We demonstrate that CRMP2 mediates Sema3F signaling in primary neurons and that crmp2^-/- mice display ASD-related social behavior changes in the early postnatal period as well as in adults. Together, we demonstrate that CRMP2 mediates Sema3F-dependent synapse pruning and its dysfunction shares histological and behavioral features of ASD.

Functions of Plexins/Neuropilins and Their Ligands during Hippocampal Development and Neurodegeneration

There is emerging evidence that molecules, receptors, and signaling mechanisms involved in vascular development also play crucial roles during the development of the nervous system. Among others, specific semaphorins and their receptors (neuropilins and plexins) have, in recent years, attracted the attention of researchers due to their pleiotropy of functions. Their functions, mainly associated with control of the cellular cytoskeleton, include control of cell migration, cell morphology, and synapse remodeling. Here, we will focus on their roles in the hippocampal formation that plays a crucial role in memory and learning as it is a prime target during neurodegeneration.

The chemorepellent draxin is involved in hippocampal mossy fiber projection

Lamina-specific afferent innervation of the mammalian hippocampus is critical for its function. We investigated the relevance of the chemorepellent draxin to the laminar projections of three principal hippocampal afferents: mossy fibers, entorhinal, and associational/commissural fibers. We observed that draxin deficiency led to abnormal projection of mossy fibers but not other afferents. Immunohistochemical analysis indicated that draxin is expressed in the dentate gyrus and cornu ammonis (CA) 3 at postnatal day 0, when dentate granule cells begin to extend mossy fibers towards CA3. Furthermore, a neurite growth assay using dissociated cells of the neonatal dentate gyrus revealed that draxin inhibited the growth of calbindin-D28k-expressing mossy fibers in vitro. Taken together, we conclude that draxin is a key molecule in the regulation of mossy fiber projections.

New functions of Semaphorin 3E and its receptor PlexinD1 during developing and adult hippocampal formation

The development and maturation of cortical circuits relies on the coordinated actions of long and short range axonal guidance cues. In this regard, the class 3 semaphorins and their receptors have been seen to be involved in the development and maturation of the hippocampal connections. However, although the role of most of their family members have been described, very few data about the participation of Semaphorin 3E (Sema3E) and its receptor PlexinD1 during the development and maturation of the entorhino-hippocampal (EH) connection are available. In the present study, we focused on determining their roles both during development and in adulthood. We determined a relevant role for Sema3E/PlexinD1 in the layer-specific development of the EH connection. Indeed, mice lacking Sema3E/PlexinD1 signalling showed aberrant layering of entorhinal axons in the hippocampus during embryonic and perinatal stages. In addition, absence of Sema3E/PlexinD1 signalling results in further changes in postnatal and adult hippocampal formation, such as numerous misrouted ectopic mossy fibers. More relevantly, we describe how subgranular cells express PlexinD1 and how the absence of Sema3E induces a dysregulation of the proliferation of dentate gyrus progenitors leading to the presence of ectopic cells in the molecular layer. Lastly, Sema3E mutant mice displayed increased network excitability both in the dentate gyrus and the hippocampus proper.

Protein Tyrosine Phosphatase δ Mediates the Sema3A-Induced Cortical Basal Dendritic Arborization through the Activation of Fyn Tyrosine Kinase

Leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) are critical for axonal guidance; however, their relation to specific guidance cues is poorly defined. We here show that PTP-3, a LAR homolog in Caenorhabditis elegans, is involved in axon guidance regulated by Semaphorin-2A-signaling. PTPδ, one of the vertebrate LAR class PTPs, participates in the Semaphorin-3A (Sema3A)-induced growth cone collapse response of primary cultured dorsal root ganglion neurons from Mus musculus embryos. In vivo, however, the contribution of PTPδ in Sema3A-regualted axon guidance was minimal. Instead, PTPδ played a major role in Sema3A-dependent cortical dendritic growth. Ptpδ^-/- and Sema3a^-/- mutant mice exhibited poor arborization of basal dendrites of cortical layer V neurons. This phenotype was observed in both male and female mutants. The double-heterozygous mutants, Ptpδ^+/-; Sema3a^+/-, also showed a similar phenotype, indicating the genetic interaction. In Ptpδ^-/- brains, Fyn and Src kinases were hyperphosphorylated at their C-terminal Tyr527 residues. Sema3A-stimulation induced dephosphorylation of Tyr527 in the dendrites of wild-type cortical neurons but not of Ptpδ^-/- Arborization of cortical basal dendrites was reduced in Fyn^-/- as well as in Ptpδ^+/-; Fyn^+/- double-heterozygous mutants. Collectively, PTPδ mediates Sema3A-signaling through the activation of Fyn by C-terminal dephosphorylation.SIGNIFICANCE STATEMENT The relation of leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) and specific axon guidance cues is poorly defined. We show that PTP-3, a LAR homolog in Caenorhabditis elegans, participates in Sema2A-regulated axon guidance. PTPδ, a member of vertebrate LAR class PTPs, is involved in Sema3A-regulated cortical dendritic growth. In Sema3A signaling, PTPδ activates Fyn and Src kinases by dephosphorylating their C-terminal Tyr residues. This is the first evidence showing that LAR class PTPs participate in Semaphorin signaling in vivo.

In vitro models of axon regeneration

A variety of in vitro models have been developed to understand the mechanisms underlying the regenerative failure of central nervous system (CNS) axons, and to guide pre-clinical development of regeneration-promoting therapeutics. These range from single-cell based assays that typically focus on molecular mechanisms to organotypic assays that aim to recapitulate in vivo behavior. By utilizing a combination of models, researchers can balance the speed, convenience, and mechanistic resolution of simpler models with the biological relevance of more complex models. This review will discuss a number of models that have been used to build our understanding of the molecular mechanisms of CNS axon regeneration.

Prolyl Isomerase Pin1 Regulates Axon Guidance by Stabilizing CRMP2A Selectively in Distal Axons

Axon guidance relies on precise translation of extracellular signal gradients into local changes in cytoskeletal dynamics, but the molecular mechanisms regulating dose-dependent responses of growth cones are still poorly understood. Here, we show that during embryonic development in growing axons, a low level of Semaphorin3A stimulation is buffered by the prolyl isomerase Pin1. We demonstrate that Pin1 stabilizes CDK5-phosphorylated CRMP2A, the major isoform of CRMP2 in distal axons. Consequently, Pin1 knockdown or knockout reduces CRMP2A levels specifically in distal axons and inhibits axon growth, which can be fully rescued by Pin1 or CRMP2A expression. Moreover, Pin1 knockdown or knockout increases sensitivity to Sema3A-induced growth cone collapse in vitro and in vivo, leading to developmental abnormalities in axon guidance. These results identify an important isoform-specific function and regulation of CRMP2A in controlling axon growth and uncover Pin1-catalyzed prolyl isomerization as a regulatory mechanism in axon guidance.

Antagonistic Effects of BACE1 and APH1B-γ-Secretase Control Axonal Guidance by Regulating Growth Cone Collapse

ΒACE1 is the major drug target for Alzheimer's disease, but we know surprisingly little about its normal function in the CNS. Here, we show that this protease is critically involved in semaphorin 3A (Sema3A)-mediated axonal guidance processes in thalamic and hippocampal neurons. An active membrane-bound proteolytic CHL1 fragment is generated by BACE1 upon Sema3A binding. This fragment relays the Sema3A signal via ezrin-radixin-moesin (ERM) proteins to the neuronal cytoskeleton. APH1B-γ-secretase-mediated degradation of this fragment stops the Sema3A-induced collapse and sensitizes the growth cone for the next axonal guidance cue. Thus, we reveal a cycle of proteolytic activity underlying growth cone collapse and restoration used by axons to find their correct trajectory in the brain. Our data also suggest that BACE1 and γ-secretase inhibition have physiologically opposite effects in this process, supporting the idea that combination therapy might attenuate some of the side effects associated with these drugs.

Semaphorin 3A activates the guanosine triphosphatase Rab5 to promote growth cone collapse and organize callosal axon projections

Axon guidance (pathfinding) wires the brain during development and is regulated by various attractive and repulsive cues. Semaphorin 3A (Sema3A) is a repulsive cue, inducing the collapse of axon growth cones. In the mammalian forebrain, the corpus callosum is the major commissure that transmits information flow between the two hemispheres, and contralateral axons assemble into well-defined tracts. We found that the patterning of callosal axon projections in rodent layer II and III (L2/3) cortical neurons in response to Sema3A was mediated by the activation of Rab5, a small guanosine triphosphatase (GTPase) that mediates endocytosis, through the membrane fusion protein Rabaptin-5 and the Rab5 guanine nucleotide exchange factor (GEF) Rabex-5. Rabaptin-5 bound directly to Plexin-A1 in the Sema3A receptor complex [an obligate heterodimer formed by Plexin-A1 and neuropilin 1 (NP1)]; Sema3A enhanced this interaction in cultured neurons. Rabaptin-5 bridged the interaction between Rab5 and Plexin-A1. Sema3A stimulated endocytosis from the cell surface of callosal axon growth cones. In utero electroporation to reduce Rab5 or Rabaptin-5 impaired axon fasciculation or caused mistargeting of L2/3 callosal projections in rats. Overexpression of Rabaptin-5 or Rab5 rescued the defective callosal axon fasciculation or mistargeting of callosal axons caused by the loss of Sema3A-Plexin-A1 signaling in rats expressing dominant-negative Plexin-A1 or in NP1-deficient mice. Thus, our findings suggest that Rab5, its effector Rabaptin-5, and its regulator Rabex-5 mediate Sema3A-induced axon guidance during brain development.

N-cadherin promotes recruitment and migration of neural progenitor cells from the SVZ neural stem cell niche into demyelinated lesions

Discrete cellular microenvironments regulate stem cell pools and their development, as well as function in maintaining tissue homeostasis. Although the signaling elements modulating neural progenitor cells (NPCs) of the adult subventricular zone (SVZ) niche are fairly well understood, the pathways activated following injury and the resulting outcomes, are less clear. In the present study, we used mouse models of demyelination and proteomics analysis to identify molecular cues present in the adult SVZ niche during injury, and analyzed their role on NPCs in the context of promoting myelin repair. Proteomic analysis of SVZ tissue from mice with experimental demyelination identified several proteins that are known to play roles in NPC proliferation, adhesion, and migration. Among the proteins found to be upregulated were members of the N-cadherin signaling pathway. During the onset of demyelination in the subcortical white matter (SCWM), activation of epidermal growth factor receptor (EGFR) signaling in SVZ NPCs stimulates the interaction between N-cadherin and ADAM10. Upon cleavage and activation of N-cadherin signaling by ADAM10, NPCs undergo cytoskeletal rearrangement and polarization, leading to enhanced migration out of the SVZ into demyelinated lesions of the SCWM. Genetically disrupting either EGFR signaling or ADAM10 inhibits this pathway, preventing N-cadherin regulated NPC polarization and migration. Additionally, in vivo experiments using N-cadherin gain- and loss-of-function approaches demonstrated that N-cadherin enhances the recruitment of SVZ NPCs into demyelinated lesions. Our data revealed that EGFR-dependent N-cadherin signaling physically initiated by ADAM10 cleavage is the response of the SVZ niche to promote repair of the injured brain.

Sema3E/PlexinD1 regulates the migration of hem-derived Cajal-Retzius cells in developing cerebral cortex

During the development of the cerebral cortex, Cajal-Retzius (CR) cells settle in the preplate and coordinate the precise growth of the neocortex. Indeed, CR cells migrate tangentially from specific proliferative regions of the telencephalon (for example, the cortical hem (CH)) to populate the entire cortical surface. This is a very finely tuned process regulated by an emerging number of factors that has been sequentially revealed in recent years. However, the putative participation of one of the major families of axon guidance molecules in this process, the Semaphorins, was not explored. Here we show that Semaphorin-3E (Sema3E) is a natural negative regulator of the migration of PlexinD1-positive CR cells originating in the CH. Our results also indicate that Sema3E/PlexinD1 signalling controls the motogenic potential of CR cells in vitro and in vivo. Indeed, absence of Sema3E/PlexinD1 signalling increased the migratory properties of CR cells. This modulation implies negative effects on CXCL12/CXCR4 signalling and increased ADF/Cofilin activity.

The serine/threonine kinase Ndr2 controls integrin trafficking and integrin-dependent neurite growth

Integrins have been implicated in various processes of nervous system development, including proliferation, migration, and differentiation of neuronal cells. In this study, we show that the serine/threonine kinase Ndr2 controls integrin-dependent dendritic and axonal growth in mouse hippocampal neurons. We further demonstrate that Ndr2 is able to induce phosphorylation at the activity- and trafficking-relevant site Thr(788/789) of β1-integrin to stimulate the PKC- and CaMKII-dependent activation of β1-integrins, as well as their exocytosis. Accordingly, Ndr2 associates with integrin-positive early and recycling endosomes in primary hippocampal neurons and the surface expression of activated β1-integrins is reduced on dendrites of Ndr2-deficient neurons. The role of Ndr2 in dendritic differentiation is also evident in vivo, because Ndr2-null mutant mice show arbor-specific alterations of dendritic complexity in the hippocampus. This indicates a role of Ndr2 in the fine regulation of dendritic growth; in fact, treatment of primary neurons with Semaphorin 3A rescues Ndr2 knock-down-induced dendritic growth deficits but fails to enhance growth beyond control level. Correspondingly, Ndr2-null mutant mice show a Semaphorin 3A(-/-)-like phenotype of premature dendritic branching in the hippocampus. The results of this study show that Ndr2-mediated integrin trafficking and activation are crucial for neurite growth and guidance signals during neuronal development.

Increased smooth muscle contractility in mice deficient for neuropilin 2

Neuropilins (NRPs) are transmembrane receptors that bind class 3 semaphorins and VEGF family members to regulate axon guidance and angiogenesis. Although expression of NRP1 by vascular smooth muscle cells (SMCs) has been reported, NRP function in smooth muscle (SM) in vivo is unexplored. Using Nrp2(+/LacZ) and Nrp2(+/gfp) transgenic mice, we observed robust and sustained expression of Nrp2 in the SM compartments of the bladder and gut, but no expression in vascular SM, skeletal muscle, or cardiac muscle. This expression pattern was recapitulated in vitro using primary human SM cell lines. Alterations in cell morphology after treatment of primary visceral SMCs with the NRP2 ligand semaphorin-3F (SEMA3F) were accompanied by inhibition of RhoA activity and myosin light chain phosphorylation, as well as decreased cytoskeletal stiffness. Ex vivo contractility testing of bladder muscle strips exposed to electrical stimulation or soluble agonists revealed enhanced tension generation of tissues from mice with constitutive or SM-specific knockout of Nrp2, compared with controls. Mice lacking Nrp2 also displayed increased bladder filling pressures, as assessed by cystometry in conscious mice. Together, these findings identify Nrp2 as a mediator of prorelaxant stimuli in SMCs and suggest a novel function for Nrp2 as a regulator of visceral SM contractility.

Calpain cleaves and activates the TRPC5 channel to participate in semaphorin 3A-induced neuronal growth cone collapse

The nonselective cation channel transient receptor potential canonical (TRPC)5 is found predominantly in the brain and has been proposed to regulate neuronal processes and growth cones. Here, we demonstrate that semaphorin 3A-mediated growth cone collapse is reduced in hippocampal neurons from TRPC5 null mice. This reduction is reproduced by inhibition of the calcium-sensitive protease calpain in wild-type neurons but not in TRPC5(-/-) neurons. We show that calpain-1 and calpain-2 cleave and functionally activate TRPC5. Mutation of a critical threonine at position 857 inhibits calpain-2 cleavage of the channel. Finally, we show that the truncated TRPC5 predicted to result from calpain cleavage is functionally active. These results indicate that semaphorin 3A initiates growth cone collapse via activation of calpain that in turn potentiates TRPC5 activity. Thus, TRPC5 acts downstream of semaphorin signaling to cause changes in neuronal growth cone morphology and nervous system development.

A signaling mechanism coupling netrin-1/deleted in colorectal cancer chemoattraction to SNARE-mediated exocytosis in axonal growth cones

Directed cell migration and axonal guidance are essential steps in neural development. Both processes are controlled by specific guidance cues that activate the signaling cascades that ultimately control cytoskeletal dynamics. Another essential step in migration and axonal guidance is the regulation of plasmalemma turnover and exocytosis in leading edges and growth cones. However, the cross talk mechanisms linking guidance receptors and membrane exocytosis are not understood. Netrin-1 is a chemoattractive cue required for the formation of commissural pathways. Here, we show that the Netrin-1 receptor deleted in colorectal cancer (DCC) forms a protein complex with the t-SNARE (target SNARE) protein Syntaxin-1 (Sytx1). This interaction is Netrin-1 dependent both in vitro and in vivo, and requires specific Sytx1 and DCC domains. Blockade of Sytx1 function by using botulinum toxins abolished Netrin-1-dependent chemoattraction of axons in mouse neuronal cultures. Similar loss-of-function experiments in the chicken spinal cord in vivo using dominant-negative Sytx1 constructs or RNAi led to defects in commissural axon pathfinding reminiscent to those described in Netrin-1 and DCC loss-of-function models. We also show that Netrin-1 elicits exocytosis at growth cones in a Sytx1-dependent manner. Moreover, we demonstrate that the Sytx1/DCC complex associates with the v-SNARE (vesicle SNARE) tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP) and that knockdown of TI-VAMP in the commissural pathway in the spinal cord results in aberrant axonal guidance phenotypes. Our data provide evidence of a new signaling mechanism that couples chemotropic Netrin-1/DCC axonal guidance and Sytx1/TI-VAMP SNARE proteins regulating membrane turnover and exocytosis.

Rb/E2F regulates expression of neogenin during neuronal migration

The Rb/E2F pathway has long been appreciated for its role in regulating cell cycle progression. Emerging evidence indicates that it also influences physiological events beyond regulation of the cell cycle. We have previously described a requirement for Rb/E2F mediating neuronal migration; however, the molecular mechanisms remain unknown, making this an ideal system to identify Rb/E2F-mediated atypical gene regulation in vivo. Here, we report that Rb regulates the expression of neogenin, a gene encoding a receptor involved in cell migration and axon guidance. Rb is capable of repressing E2F-mediated neogenin expression while E2F3 occupies a region containing E2F consensus sites on the neogenin promoter in native chromatin. Absence of Rb results in aberrant neuronal migration and adhesion in response to netrin-1, a known ligand for neogenin. Increased expression of neogenin through ex vivo electroporation results in impaired neuronal migration similar to that detected in forebrain-specific Rb deficiency. These findings show direct regulation of neogenin by the Rb/E2F pathway and demonstrate that regulation of neogenin expression is required for neural precursor migration. These studies identify a novel mechanism through which Rb regulates transcription of a gene beyond the classical E2F targets to regulate events distinct from cell cycle progression.

Roles of semaphorin-6B and plexin-A2 in lamina-restricted projection of hippocampal mossy fibers

Hippocampal mossy fibers project preferentially to the proximal-most lamina of the suprapyramidal region of CA3, the stratum lucidum, and proximal-most parts of the infrapyrmidal region of CA3c. Molecular mechanisms that govern the lamina-restricted projection of mossy fibers, however, have not been fully understood. We previously studied functions of neural repellent Semaphorin-6A (Sema6A), a class 6 transmembrane semaphorin, and its receptors, plexin-A2 (PlxnA2) and PlxnA4, in mossy fiber projection and have proposed that PlxnA4-expressing mossy fibers are principally prevented from entering the Sema6A-expressing suprapyramidal and infrapyramidal regions of CA3 but are permitted to grow into proximal parts of the regions, where repulsive activity of Sema6A is competitively suppressed by PlxnA2 (Suto et al., 2007). In the present study we demonstrate that Sema6B, another class 6 transmembrane semaphorin, is expressed in CA3 and repels mossy fibers in a PlxnA4-dependent manner in vitro. In Sema6B-deficient mice several mossy fibers aberrantly project to the stratum radiatum and the stratum oriens. The number of aberrant mossy fibers is increased in Sema6A;Sema6B double knock-out mice, indicating that Sema6A and Sema6B function additively to regulate proper projection of mossy fibers. PlxnA2 does not suppress the Sema6B response, but itself promotes growth of mossy fibers. Based on these results, we propose that the balance between mossy fiber repulsion by Sema6A and Sema6B and attraction by PlxnA2 and unknown molecule(s) prescribes the areas permissive for mossy fibers to innervate.

Mechanical tension alters semaphorin expression in the periodontium

BACKGROUND

Periodontal remodeling requires coordinated cell movement. Semaphorins are cell-surface signals that regulate cell migration and may be differentially regulated by periodontal cells. Mechanical tension can regulate periodontal ligament (PDL) remodeling. We predicted that mechanical tension alters the expression of the subset of semaphorins in the periodontium likely to be most involved with regulating the remodeling of this tissue.

METHODS

PDL and gingival cells were exposed to mechanical tension, and their attachment and movement on collagen matrices were evaluated. Alterations in extracellular matrix and semaphorin transcript expression were monitored by semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

Mechanical tension induced osteoclast regulatory transcripts in the PDL cells to a greater extent than gingival fibroblasts, increasing the expression of osteoprotegerin and decreasing receptor activator of nuclear factor-kappa B ligand. These mechanical forces reduced PDL cell mingling, without altering cell attachment or motility. Concurrently, these forces induced dynamic changes in several semaphorin molecules in PDL cells, increasing semaphorin 3D and 5B and decreasing semaphorin 7A. In addition, plexin transcript expression was altered, decreasing plexin A1 and increasing plexin C1. These changes were different than those observed in gingival fibroblasts.

CONCLUSIONS

These data suggest that a subset of semaphorins and plexins are dynamically regulated in the PDL. Because these molecules may be involved in cell guidance, changes in semaphorins may play a pivotal role in periodontal remodeling, affecting angiogenesis or PDL cell invasion into sites of injury.

Endothelin-1 regulates oligodendrocyte development

In the postnatal brain, oligodendrocyte progenitor cells (OPCs) arise from the subventricular zone (SVZ) and migrate into the developing white matter, where they differentiate into oligodendrocytes and myelinate axons. The mechanisms regulating OPC migration and differentiation are not fully defined. The present study demonstrates that endothelin-1 (ET-1) is an astrocyte-derived signal that regulates OPC migration and differentiation. OPCs in vivo and in culture express functional ET(A) and ET(B) receptors, which mediate ET-1-induced ERK (extracellular signal-regulated kinase) and CREB (cAMP response element-binding protein) phosphorylation. ET-1 exerts both chemotactic and chemokinetic effects on OPCs to enhance cell migration; it also prevents lineage progression from the O4(+) to the O1(+) stage without affecting cell proliferation. Astrocyte-conditioned medium stimulates OPC migration in culture through ET receptor activation, whereas multiphoton time-lapse imaging shows that selective ET receptor antagonists or anti-ET-1 antibodies inhibit OPC migration from the SVZ. Inhibition of ET receptor activity also derepresses OPC differentiation in the corpus callosum in slice cultures. Our findings indicate that ET-1 is a soluble astrocyte-derived signal that regulates OPC migration and differentiation during development.

A semaphorin 3A inhibitor blocks axonal chemorepulsion and enhances axon regeneration

Secreted semaphorins are a large group of extracellular proteins involved in a variety of processes during development, including neuronal migration and axon guidance. We screened a peptoid combinatorial library to search for semaphorin 3A inhibitors, and identified a peptoid (SICHI: semaphorin Induced chemorepulsion inhibitor) that blocks semaphorin 3A-chemorepulsion and growth-cone collapse in axons at millimolar concentrations. SICHI inhibits the binding of semaphorin 3A to its receptor complex (neuropilin 1/plexin A1) and semaphorin 3A-induced phosphorylation of GSK3. Chemorepulsion induced by semaphorin 3F or netrin 1 is not blocked by SICHI. We also show that SICHI promotes neural regeneration of damaged axons. We suggest that SICHI, a selective inhibitor of semaphorin 3A, is of therapeutic interest for approaches aimed at promoting axonal regeneration and brain repair.

The stimulation of dendrite growth by Sema3A requires integrin engagement and focal adhesion kinase

The rate and direction of axon and dendrite growth depend on multiple guidance signals and growth factors. Semaphorin 3A (Sema3A) acts as a repellent for axons and attractant for dendrites. Here, we show that the requirement for integrin engagement distinguishes the response of axons and dendrites to Sema3A in hippocampal neurons. Sema3A promotes the extension of hippocampal dendrites by a pathway that requires focal adhesion kinase (FAK). The stimulation of dendrite growth and FAK phosphorylation by Sema3A depend on integrin engagement. Unlike their function as a target of Sema3A during the collapse of axonal growth cones, integrins facilitate the stimulation of dendrite extension. Conditional inactivation of the genes encoding beta1 integrin or FAK blocks the growth-promoting effect of Sema3A but not the collapse of axonal growth cones. Our results demonstrate that different pathways mediate the stimulation of dendrite growth and the collapse of axonal growth cones by Sema3A.

Antagonistic interactions among Plexins regulate the timing of intersegmental vessel formation

The angioblast is an embryonic endothelial cell precursor that migrates long distances to reach its final position, navigating by sensing attractive and repulsive cues from the environment. Members of the semaphorin family have been implicated in controlling the behaviour of angioblast tip cells through repulsive signalling in vitro, but their in vivo roles are less clear. Here we show that zebrafish semaphorin3e (sema3e) is expressed by endothelial cells of the dorsal aorta, primary motoneurons, and endodermal cells. Further, loss of Sema3e leads to delayed exit of angioblasts from the dorsal aorta in ISV formation. Through transplant analysis, we show that Sema3e acts autonomously and non-autonomously in angioblasts to modulate interactions among themselves. The semaphorin receptors, PlexinD1 and PlexinB2, are expressed by zebrafish angioblasts. Loss of plxnB2 results in delayed ISV sprouting identical to that seen in sema3e morphants, while loss of plexinD1 in out of bounds (obd) mutants results in precocious ISV sprouting. Loss of either sema3e or plxnB2 in obd mutants generates an intermediate phenotype, suggesting that PlxnD1 and Sema3e/PlxnB2 antagonize each other to control timing of ISV sprouting. Consistent with this observation, we show that PlxnB2 acts cell autonomously in endothelial cells. This suggests a model where multiple semaphorin-plexin interactions control angioblast sprouting behaviour.

Neuropilin 1-Sema signaling regulates crossing of cingulate pioneering axons during development of the corpus callosum

Pioneer axons from the cingulate cortex initiate corpus callosum (CC) development, yet nothing is known about the molecules that regulate their guidance. We demonstrate that neuropilin 1 (Npn1) plays an integral role in the development of the CC. Npn1 is localized to axons of cingulate neurons as they cross the midline, and multiple class 3 semaphorins (Semas) are expressed around the developing CC, implicating these guidance molecules in the regulation of Npn1-expressing axons emanating from the cingulate cortex. Furthermore, axons from the cingulate cortex display guidance errors in Npn1(Sema-) mice, a knockin mouse line in which Npn1 is unable to bind Semas. Analysis of mice deficient in the transcription factor Emx2 demonstrated that the cingulate cortex of these mice was significantly reduced in comparison to wild-type controls at E17 and that the CC was absent in rostral sections. Expression of Npn1 was absent in rostral sections of Emx2 mutants, suggesting that Npn1-expressing cingulate pioneers are required for CC formation. These data highlight a central role for Npn1 in the development of projections from the cingulate cortex and further illustrate the importance of these pioneer axons in the formation of the CC.

Decreased number of interneurons and increased seizures in neuropilin 2 deficient mice: implications for autism and epilepsy

PURPOSE

Clinically, perturbations in the semaphorin signaling system have been associated with autism and epilepsy. The semaphorins have been implicated in guidance, migration, differentiation, and synaptic plasticity of neurons. The semaphorin 3F (Sema3F) ligand and its receptor, neuropilin 2 (NPN2) are highly expressed within limbic areas. NPN2 signaling may intimately direct the apposition of presynaptic and postsynaptic locations, facilitating the development and maturity of hippocampal synaptic function. To further understand the role of NPN2 signaling in central nevous system (CNS) plasticity, structural and functional alterations were assessed in NPN2 deficient mice.

METHODS

In NPN2 deficient mice, we measured seizure susceptibility after kainic acid or pentylenetetrazol, neuronal excitability and synaptic throughput in slice preparations, principal and interneuron cell counts with immunocytochemical protocols, synaptosomal protein levels with immunoblots, and dendritic morphology with Golgi-staining.

RESULTS

NPN2 deficient mice had shorter seizure latencies, increased vulnerability to seizure-related death, were more likely to develop spontaneous recurrent seizure activity after chemical challenge, and had an increased slope on input/output curves. Principal cell counts were unchanged, but GABA, parvalbumin, and neuropeptide Y interneuron cell counts were significantly reduced. Synaptosomal NPN2 protein levels and total number of GABAergic synapses were decreased in a gene dose-dependent fashion. CA1 pyramidal cells showed reduced dendritic length and complexity, as well as an increased number of dendritic spines.

DISCUSSION

These data suggest the novel hypothesis that the Sema 3F signaling system's role in appropriate placement of subsets of hippocampal interneurons has critical downstream consequences for hippocampal function, resulting in a more seizure susceptible phenotype.

Semaphorins 3A, 3C, and 3F in mesencephalic dopaminergic axon pathfinding

By analyzing the mechanisms that govern dopaminergic axon pathfinding from the midbrain to the striatum in embryonic rat brains, we identified neuroepithelial regions that exert chemotropic effects on mesencephalic dopaminergic axons. Explants from the pretectum and the striatum showed an attractive effect, whereas those from the midhindbrain boundary, the dorsal thalamus, and the ventral thalamus had no effect. Expression of semaphorin (Sema) 3C and Sema3F in the pretectum and of Sema3A in the striatum suggested a role for these axon guidance molecules in dopaminergic axon pathfinding. When expressed in HEK293 cell aggregates, Sema3C had an attractive effect and enhanced axon growth, Sema3A enhanced axon growth, and Sema3F had a repulsive effect on dopaminergic axons. Antineuropilin-1 and antineuropilin-2 antibodies reduced attraction by the pretectum, whereas attraction by the striatum was not affected by the presence of antineuropilin-1 antibodies. Moreover, neuropilin-1- and neuropilin-2-soluble Fc chimeras reduced the attraction by the pretectum. These results suggest that semaphorins may help to establish the dopaminergic projection from the midbrain to the striatum during embryonic development.

Stripe assay to examine axonal guidance and cell migration

Stripe assays have been widely employed as in vitro test systems to study the responses of growing axons, as well as migrating cells, to established or novel guidance molecules. We provide detailed protocols for both the original and the modified version of this assay, as they allow the analysis of the 'guidance properties' of active components present in crude membrane fractions or as purified molecules. Silicon matrices are used to produce striped patterns of active molecules on a surface (referred to as 'carpet'), followed by culturing of neurons, or any other cell type, on these carpets. After 1-2 days in culture, striped outgrowth of extending neurites--indicative of guided migration of cell processes--can be observed. We also discuss potential other applications (e.g., in neuronal regeneration and development) and modifications of the assay. The preparation of 10-12 carpets takes approximately 4-5 h.

Estrogen effects on cognition and hippocampal transcription in middle-aged mice

Young and middle-aged female mice were ovariectomized and given cyclic injections of either estradiol or vehicle treatments. During the fifth week after surgery the Morris water maze was used to assess cognitive function. Age and treatment effects emerged over the course of spatial training such that middle-aged vehicle treated mice exhibited deficits in acquiring a spatial search strategy compared to younger vehicle treated mice and middle-age estradiol treated mice. Following behavioral characterization, mice were maintained on their injection schedule until week seven and hippocampi were collected 24h after the last injection. Hippocampal RNA was extracted and genes responsive to age and estrogen were identified using cDNA microarrays. Estradiol treatment in middle-aged mice altered the expression of genes related to transcriptional regulation, biosynthesis, growth, neuroprotection, and elements of cell signaling pathways. Expression profiles for representative genes were confirmed in a separate set of animals using oligonucleotide arrays and RT-PCR. Our results indicate that estrogen treatment in middle-aged animals may promote hippocampal health during the aging process.

Signaling of secreted semaphorins in growth cone steering

Despite a tremendous amount of progress in the identification and characterization of many new players as components of class 3 secreted semaphorin signaling in growth cone steering (Fig. 1), our understanding of the molecular mechanisms is far from complete. More questions remain to be answered: how are differential cytoskeletal changes within a growth cone achieved in response to semaphorins? What are the target(s) of cyclic nucleotide modulation? How does a growth cone make a reliable decision in response to a shallow gradient? And finally, how does a growth cone maintain its sensitivity to a decreasing concentration ofsemaphorins when it is growing away from the source? With a high degree of interest in the field with the development of novel technologies in analyzing growth cone steering, we expect to see a much more complete picture of semaphoring signaling in the near future.

Molecular markers distinguishing supragranular and infragranular layers in the human prefrontal cortex

The human neocortex is organized into six layers that are differentiated by the size and packing density of their constituent neurons. The gene products that guide the establishment of this lamination have been studied extensively, but the gene expression gradients present across the layers of the adult human neocortex are mostly unknown. As the supragranular (SG) and infragranular (IG) layers of the human prefrontal cortex (PFC) differ in their connectivity and developmental time course, we hypothesized that the SG and IG layers will show distinct differences in their transcriptomes. To test this prediction, we used laser capture microdissection coupled with DNA microarray transcriptome profiling. Sixty-nine genes exhibited robust and highly consistent expression differences between the SG and IG layers. For six selected markers, in addition to validating the microarray findings, in situ hybridization revealed a complex, subpopulation-specific neuronal distribution. The markers we identified are likely to be related to the functional differences between the SG and IG layers of the human PFC and can be used for assessing alterations in structure and function of this cortical region in human brain disorders.

Semaphorins in development and adult brain: Implication for neurological diseases

As a group, Semaphorins are expressed in most tissues and this distribution varies considerably with age. Semaphorins are dynamically expressed during embryonic development and their expression is often associated with growing axons. This expression decreases with maturity and several observations support the idea that in adult brain the expression of secreted Semaphorins is sensitive to electrical activity and experience. The functional role of Semaphorins in guiding axonal projections is well established and more recent evidence points to additional roles in the development, function and reorganization of synaptic complexes. Semaphorins exert the majority of their effects by binding to cognate receptor proteins through their extracellular domains. A common theme is that Semaphorin-triggered signalling induces the rearrangement of the actin and microtubule cytoskeleton. Mutations in Semaphorin genes are linked to several human diseases associated with neurological changes, but their actual influence in the pathogenesis of these diseases remains to be demonstrated. In addition, Semaphorins and their receptors are likely to mediate cross-talk between neurons and other cell types, including in pathological situations where their influence can be damaging or favourable depending on the context. We discuss how the manipulation of Semaphorin function might be crucial for future clinical studies.

Semaphorin-3F attracts the growth cone of cerebellar granule cells through cGMP signaling pathway

Growth cone extension is guided by extracellular factors during the brain development but the underlying cellular mechanisms remain largely unclear. Here, we examined the potential function of class-3 semaphorins in cultured cerebellar granule cells. We found neuropilin-2 (NP2), the high-affinity receptor for semaphorin-3F (Sema3F), is highly expressed in cerebellar granule cells. An extracellular gradient of Sema3F triggered an NP2-dependent attractive turning of the growth cone of cultured cerebellar granule cells. This Sema3F-triggered growth cone attraction was abolished by inhibition of the cGMP signaling pathway and reduced by elevating the intracellular cGMP level. Furthermore, Sema3F partially rescued the collapse induced by inhibition of basal cGMP in granule cells. Thus, Sema3F may act as a chemoattractant for the growth cone of cerebellar granule cells through cGMP signaling pathway.

Interactions between Plexin-A2, Plexin-A4, and Semaphorin 6A Control Lamina-Restricted Projection of Hippocampal Mossy Fibers

Hippocampal mossy fibers project preferentially to the stratum lucidum, the proximal-most lamina of the suprapyramidal region of CA3. The molecular mechanisms that govern this lamina-restricted projection are still unknown. We examined the projection pattern of mossy fibers in mutant mice for semaphorin receptors plexin-A2 and plexin-A4, and their ligand, the transmembrane semaphorin Sema6A. We found that plexin-A2 deficiency causes a shift of mossy fibers from the suprapyramidal region to the infra- and intrapyramidal regions, while plexin-A4 deficiency induces inappropriate spreading of mossy fibers within CA3. We also report that the plexin-A2 loss-of-function phenotype is genetically suppressed by Sema6A loss of function. Based on these results, we propose a model for the lamina-restricted projection of mossy fibers: the expression of plexin-A4 on mossy fibers prevents them from entering the Sema6A-expressing suprapyramidal region of CA3 and restricts them to the proximal-most part, where Sema6A repulsive activity is attenuated by plexin-A2.

Hippocampal CA3 pyramidal cells selectively innervate aspiny interneurons

The specific connectivity among principal cells and interneurons determines the flow of activity in neuronal networks. To elucidate the connections between hippocampal principal cells and various classes of interneurons, CA3 pyramidal cells were intracellularly labelled with biocytin in anaesthetized rats and the three-dimensional distribution of their axon collaterals was reconstructed. The sections were double-stained for substance P receptor (SPR)- or metabotropic glutamate receptor 1alpha (mGluR-1alpha)-immunoreactivity to investigate interneuron targets of the CA3 pyramidal cells. SPR-containing interneurons represent a large portion of the GABAergic population, including spiny and aspiny classes. Axon terminals of CA3 pyramidal cells contacted SPR-positive interneuron dendrites in the hilus and in all hippocampal strata in both CA3 and CA1 regions (7.16% of all boutons). The majority of axons formed single contacts (87.5%), but multiple contacts (up to six) on single target neurons were also found. CA3 pyramidal cell axon collaterals innervated several types of morphologically different aspiny SPR-positive interneurons. In contrast, spiny SPR-interneurons or mGluR-1alpha-positive interneurons in the hilus, CA3 and CA1 regions were rarely contacted by the filled pyramidal cells. These findings indicate a strong target selection of CA3 pyramidal cells favouring the activation of aspiny classes of interneurons.

Semaphorin 6C leads to GSK-3-dependent growth cone collapse and redistributes after entorhino-hippocampal axotomy

We studied the changes in the distribution of a specific variant of Semaphorin Y/6C (Sema6C) in mouse forebrain after axotomy of the entorhino-hippocampal perforant pathway. We found this isoform to be widely expressed during development, remaining in the adult and showing variations in distribution when the perforant pathway was axotomized. These changes were detected in both the hippocampal and entorhinal cortices. Sema6C1 immunoreactivity (IR) was high in the stratum radiatum of the hippocampus proper and the inner molecular layer of the dentate gyrus; the entorhinal cortex showed Sema6C1 IR in both cell bodies and in fibers of the II/III and V/VI layers. In axotomized animals, the IR of the ipsilateral, but not the contralateral, hemisphere showed that IR had moved into the stratum lacunosum-moleculare, the medial molecular layer of the dentate gyrus and the fibers, but not the cell bodies, of the entorhinal cortex. These results were not reproduced after lateral axotomy of the fimbria fornix, indicating a specific role for Sema6C variants in the generation and/or stability of entorhino-hippocampal synapses. Growth cone collapse of entorhinal and pyramidal neurons, as well as activation of glycogen synthase kinase-3 (GSK-3) through depletion of the inactive pool, induced by diffusible Sema6C1 further supports this view.

The role of neuropilins in cancer

Neuropilins are multifunctional non-tyrosine kinase receptors that bind to class 3 semaphorins and vascular endothelial growth factor. NRP-1 and NRP-2 were first identified for their key role in mediating axonal guidance in the developing nervous system through their interactions with class 3 semaphorins. Growing evidence supports a critical role for these receptors in tumor progression. Neuropilin expression is up-regulated in multiple tumor types, and correlates with tumor progression and prognosis in specific tumors. Neuropilins may indirectly mediate effects on tumor progression by affecting angiogenesis or directly through effects on tumor cells. This article reviews emerging evidence for the role of neuropilins in tumor biology. The therapeutic implications of these data are far-reaching and suggest that neuropilin-targeted interventions may be useful as a component of antineoplastic therapy.

The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Synaptic cell adhesion molecules (SCAMs) are mostly membrane-anchored molecules with extracellular domains that extend into the synaptic cleft. Prototypical SCAMs interact with homologous or heterologous molecules on the surface of adjacent cells, ensuring the precise apposition of pre- and postsynaptic elements. More recent definitions of SCAMs often include molecules involved in axon pathfinding, cell recognition and synaptic differentiation events, making SCAMs functionally and molecularly a highly diverse group. In this review, we summarize the proposed in vivo functions of a large variety of SCAMs. We mainly focus on results obtained from analyses of genetic model organisms, mostly mouse knockout mutants, lacking expression of the respective candidate genes. In contrast to the substantial effect yielded by some knockouts of molecules involved in synaptic vesicle release, no SCAM mutant has been reported thus far that shows a prominently altered structure of the majority of synapses or even lacks synapses altogether. This surprising resilience of synaptic structure might be explained by a high redundancy between different SCAMs, by the assumption that the crucial molecular players in synapse structure have yet to be discovered or by a grand variability in the mechanisms of synapse formation that underlies the diversity of synapses. Whatever the final answer turns out to be, the genetic dissection of the SCAM superfamilies has led to a much better understanding of the different steps required to form, differentiate and modify a synapse.

The semaphorins

Semaphorins are secreted, transmembrane, and GPI-linked proteins, defined by cysteine-rich semaphorin protein domains, that have important roles in a variety of tissues. Humans have 20 semaphorins, Drosophila has five, and two are known from DNA viruses; semaphorins are also found in nematodes and crustaceans but not in non-animals. They are grouped into eight classes on the basis of phylogenetic tree analyses and the presence of additional protein motifs. The expression of semaphorins has been described most fully in the nervous system, but they are also present in most, or perhaps all, other tissues. Functionally, semaphorins were initially characterized for their importance in the development of the nervous system and in axonal guidance. More recently, they have been found to be important for the formation and functioning of the cardiovascular, endocrine, gastrointestinal, hepatic, immune, musculoskeletal, renal, reproductive, and respiratory systems. A common theme in the mechanisms of semaphorin function is that they alter the cytoskeleton and the organization of actin filaments and the microtubule network. These effects occur primarily through binding of semaphorins to their receptors, although transmembrane semaphorins also serve as receptors themselves. The best characterized receptors for mediating semaphorin signaling are members of the neuropilin and plexin families of transmembrane proteins. Plexins, in particular, are thought to control many of the functional effects of semaphorins; the molecular mechanisms of semaphorin signaling are still poorly understood, however. Given the importance of semaphorins in a wide range of functions, including neural connectivity, angiogenesis, immunoregulation, and cancer, much remains to be learned about these proteins and their roles in pathology and human disease.

Overexpression of the epidermal growth factor receptor confers migratory properties to nonmigratory postnatal neural progenitors

Approaches to successful cell transplantation therapies for the injured brain involve selecting the appropriate neural progenitor type and optimizing the efficiency of the cell engraftment. Here we show that epidermal growth factor receptor (EGFR) expression enhances postnatal neural progenitor migration in vitro and in vivo. Migratory NG2-expressing (NG2+) progenitor cells of the postnatal subventricular zone (SVZ) express higher EGFR levels than nonmigratory, cortical NG2+ cells. The higher endogenous EGFR expression in SVZ NG2+ cells is causally related with their migratory potential in vitro as well as in vivo after cell engraftment. EGFR overexpression in cortical NG2+ cells by transient transfection converted these cells to a migratory phenotype in vitro and in vivo. Finally, cortical NG2+ cells purified from a transgenic mouse in which the EGFR is overexpressed under the CNP promoter exhibited enhanced migratory capability. These findings reveal a new role for EGFR in the postnatal brain and open new avenues to optimize cell engraftment for brain repair.

Ephrin-A5 modulates the topographic mapping and connectivity of commissural axons in murine hippocampus

Entorhinal and commissural/associational projections show a non-overlapping distribution in the hippocampus proper and the dentate gyrus. The expression of Ephrins and their Eph receptors in the developing hippocampus indicates that this family of axonal guidance molecules may modulate the formation of these connections. Here we focused on the role of the ephrin-A5 ligand in the development of the main hippocampal afferents. In situ hybridization showed that ephrin-A5 mRNA was detected mainly in the principal cells of the hippocampus proper and in the dentate gyrus throughout postnatal development. Immunocytochemical analyses revealed prominent expression of the EphA3 receptor, a putative receptor for ephrin-A5, in the main cells and the neuropil of the developing hippocampus. Tracing experiments in ephrin-A5(-/-) mice showed that commissural projections were transiently altered in the hippocampus proper at P5, but they were mistargeted throughout the postnatal development in the dentate gyrus. Immunocytochemistry with anti-calbindin antibodies revealed that the dentate mossy fiber projection was not altered in ephrin-A5(-/-) mice. Electron microscopy studies showed alterations in the density of synapses and spines in commissural/associational layers, but not in entorhinal layers, and in the mossy fibers in these animals. Taken together, these findings indicate that ephrin-A5 signaling is involved in the formation and maturation of synapses in the hippocampus.

Proteolytic processing converts the repelling signal Sema3E into an inducer of invasive growth and lung metastasis

We have previously shown that the expression of a semaphorin, known as a repelling cue in axon guidance, Sema3E, correlates with the ability to form lung metastasis in murine adenocarcinoma cell models. Now, besides providing evidence for the relevance of SEMA3E to human disease by showing that SEMA3E is frequently expressed in human cancer cell lines and solid tumors from breast cancer patients, we show biological activities of Sema3E, which support the implication of Sema3E in tumor progression and metastasis. In vivo, expression of Sema3E in mammary adenocarcinoma cells induces the ability to form experimental lung metastasis, and in vitro, the Sema3E protein exhibits both migration and growth promoting activity on endothelial cells and pheochromocytoma cells. This represents the first evidence of a metastasis-promoting function of a class 3 semaphorin, as this class of genes has hitherto been implicated in tumor biology only as tumor suppressors and negative regulators of growth. Moreover, we show that the full-size Sema3E protein is converted into a p61-Sema3E isoform due to furin-dependent processing, and by analyzing processing-deficient and truncated forms, we show that the generation of p61-Sema3E is required and sufficient for the function of Sema3E in lung metastasis, cell migration, invasive growth, and extracellular signal-regulated kinase 1/2 activation of endothelial cells. These findings suggest that certain breast cancer cells may increase their lung-colonizing ability by converting the growth repellent, Sema3E, into a growth attractant and point to a type of semaphorin signaling different from the conventional signaling induced by full-size dimeric class 3 semaphorins.

Semaphorin SEMA3F has a repulsing activity on breast cancer cells and inhibits E-cadherin-mediated cell adhesion

Previously, we demonstrated that loss of SEMA3F, a secreted semaphorin encoded in 3p21.3, is associated with higher stages in lung cancer and primary tumor cells studied with anti-vascular endothelial growth factor (VEGF) and SEMA3F antibodies. In vitro, SEMA3F inhibits cell spreading; this activity is opposed by VEGF. These results suggest that VEGF and SEMA3F compete for binding to their common neuropilin receptor. In the present report, we investigated the attractive/repulsive effects of SEMA3F on cell migration when cells were grown in a three-dimensional system and exposed to a SEMA3F gradient. In addition, we adapted the neurobiologic stripe assay to analyze the migration of tumor cells in response to SEMA3F. In the motile breast cancer cell line C100, which expresses both neuropilin-1 (NRP1) and neuropilin-2 (NRP2) receptors, SEMA3F had a repulsive effect, which was blocked by anti-NRP2 antibody. In less motile MCF7 cells, which express only NRP1, SEMA3F inhibited cell contacts with loss of membrane-associated E-cadherin and beta-catenin without motility induction. Cell spreading and proliferation were reduced. These results support the concept that in a first step during tumorigenesis, normal tissues expressing SEMA3F would try to prevent tumor cells from spreading and attaching to the stroma for further implantation.

Role of class 3 semaphorins in the development and maturation of the septohippocampal pathway

In examining the role of Class 3 secreted semaphorins in the prenatal and postnatal development of the septohippocampal pathway, we found that embryonic (E14-E16) septal axons were repelled by the cingulate cortex and the striatum. We also found that the hippocampus exerts chemorepulsion on dorsolateral septal fibers, but not on fibers arising in the medial septum/diagonal band complex, which is the source of septohippocampal axons. These data indicate that endogenous chemorepellents prevent the growth of septal axons in nonappropriate brain areas and direct septohippocampal fibers to the target hippocampus. The embryonic septum expressed np-1 and np-2 mRNAs, and the striatum and cerebral cortex expressed sema 3A and sema 3F. Experiments with recombinant semaphorins showed that Sema 3A and 3F, but not Sema 3C or 3E, induce chemorepulsion of septal axons. Sema 3A and 3F also induce growth cone collapse of septal axons. This indicates that these factors are endogenous cues for the early guidance of septohippocampal fibers, including cholinergic and gamma-aminobutyric acid (GABA)ergic axons, during the embryonic stages. During postnatal stages, when target cell selection and synaptogenesis take place, np-1 and np-2 were expressed by septohippocampal neurons at all ages tested. In the target hippocampus, pyramidal and granule cells expressed sema 3E and sema 3A, whereas most interneurons expressed sema 3C, but few expressed sema 3E or 3A. Combined tracing and expression studies showed that GABAergic septohippocampal fibers terminated preferentially onto sema 3C-positive interneurons. In contrast, cholinergic septohippocampal fibers terminated onto sema 3E and sema 3A-expressing pyramidal and granule cells. The data suggest that Class 3 secreted semaphorins are involved in postnatal development. Moreover, because GABAergic and cholinergic axons terminate onto neurons expressing distinct, but overlapping, patterns of semaphorin expression, semaphorin functions may be regulated by different signaling mechanisms at postnatal stages.

Chemorepulsion and cell adhesion molecules in patterning initial trajectories of sensory axons

Research in the past decade has advanced our knowledge of the key role that diffusible cues play in axonal guidance during development. In higher vertebrates, dorsal root ganglion (DRG) neurons extend axons centrally to the spinal cord through the dorsal root entry zone and peripherally to muscle and skin targets. In this review, we focus on the role of proximate "non-target" tissues in the initial stages of DRG axonal growth. In the early stages of development, "non-target" tissues including the dermamyotome, the notochord, and the ventral spinal cord exert chemorepulsion for DRG axons. We describe how semaphorin 3A, chondroitin sulfate proteogrycans, and cell adhesion molecules participate in chemorepulsion and the way they provide spatio-temporal specificity to chemorepulsion. Axon chemorepulsion may act not only to shape DRG axonal trajectories but it also affects a variety of other axonal projections in the peripheral and central nervous system.

GDNF and GFRalpha1 promote differentiation and tangential migration of cortical GABAergic neurons

Cortical GABAergic neurons are generated in the ventral telencephalon and migrate dorsally into the cortex following a tangential path. GDNF signaling via GFRalpha1 was found to promote the differentiation of ventral precursors into GABAergic cells, enhancing their neuronal morphology and motility. GDNF stimulated axonal growth in cortical GABAergic neurons and acted as a potent chemoattractant of GABAergic cells. These effects required GFRalpha1 but neither RET nor NCAM, the two transmembrane signaling receptors known for GDNF. Mutant mice lacking GDNF or GFRalpha1, but neither RET nor NCAM, showed reduced numbers of GABAergic cells in the cerebral cortex and hippocampus. We conclude that one of the normal functions of GDNF signaling via GFRalpha1 in the developing brain is to promote the differentiation and migration of cortical GABAergic neurons. The lack of involvement of RET or NCAM in these processes suggests the existence of additional transmembrane effectors for GDNF.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.