Kv3 Channels: Enablers of Rapid Firing, Neurotransmitter Release, and Neuronal Endurance

The intrinsic electrical characteristics of different types of neurons are shaped by the K+ channels they express. From among the more than 70 different K+ channel genes expressed in neurons, Kv3 family voltage-dependent K+ channels are uniquely associated with the ability of certain neurons to fire action potentials and to release neurotransmitter at high rates of up to 1,000 Hz. In general, the four Kv3 channels Kv3.1-Kv3.4 share the property of activating and deactivating rapidly at potentials more positive than other channels. Each Kv3 channel gene can generate multiple protein isoforms, which contribute to the high-frequency firing of neurons such as auditory brain stem neurons, fast-spiking GABAergic interneurons, and Purkinje cells of the cerebellum, and to regulation of neurotransmitter release at the terminals of many neurons. The different Kv3 channels have unique expression patterns and biophysical properties and are regulated in different ways by protein kinases. In this review, we cover the function, localization, and modulation of Kv3 channels and describe how levels and properties of the channels are altered by changes in ongoing neuronal activity. We also cover how the protein-protein interaction of these channels with other proteins affects neuronal functions, and how mutations or abnormal regulation of Kv3 channels are associated with neurological disorders such as ataxias, epilepsies, schizophrenia, and Alzheimer's disease.

Structural Perspectives on Axon Guidance

Axon guidance relies on a combinatorial code of receptor and ligand interactions that direct adhesive/attractive and repulsive cellular responses. Recent structural data have revealed many of the molecular mechanisms that govern these interactions and enabled the design of sophisticated mutant tools to dissect their biological functions. Here, we discuss the structure/function relationships of four major classes of guidance cues (ephrins, semaphorins, slits, netrins) and examples of morphogens (Wnt, Shh) and of cell adhesion molecules (FLRT). These cell signaling systems rely on specific modes of receptor-ligand binding that are determined by selective binding sites; however, defined structure-encoded receptor promiscuity also enables cross talk between different receptor/ligand families and can also involve extracellular matrix components. A picture emerges in which a multitude of highly context-dependent structural assemblies determines the finely tuned cellular behavior required for nervous system development.

Retracted: Macrophage-Derived Slit3 Controls Cell Migration and Axon Pathfinding in the Peripheral Nerve Bridge

Slit-Robo signaling has been characterized as a repulsive signal for precise axon pathfinding and cell migration during embryonic development. Here, we describe a role for Sox2 in the regulation of Robo1 in Schwann cells and for Slit3-Robo1 signaling in controlling axon guidance within the newly formed nerve bridge following peripheral nerve transection injury. In particular, we show that macrophages form the outermost layer of the nerve bridge and secrete high levels of Slit3, while migratory Schwann cells and fibroblasts inside the nerve bridge express the Robo1 receptor. In line with this pattern of Slit3 and Robo1 expression, we observed multiple axon regeneration and cell migration defects in the nerve bridge of Sox2-, Slit3-, and Robo1-mutant mice. Our findings have revealed important functions for macrophages in the peripheral nervous system, utilizing Slit3-Robo1 signaling to control correct peripheral nerve bridge formation and precise axon targeting to the distal nerve stump following injury.

New insights into the molecular mechanisms of axon guidance receptor regulation and signaling

As the nervous system develops, newly differentiated neurons need to extend their axons toward their synaptic targets to form functional neural circuits. During this highly dynamic process of axon pathfinding, guidance receptors expressed at the tips of motile axons interact with soluble guidance cues or membrane tethered molecules present in the environment to be either attracted toward or repelled away from the source of these cues. As competing cues are often present at the same location and during the same developmental period, guidance receptors need to be both spatially and temporally regulated in order for the navigating axons to make appropriate guidance decisions. This regulation is exerted by a diverse array of molecular mechanisms that have come into focus over the past several decades and these mechanisms ensure that the correct complement of surface receptors is present on the growth cone, a fan-shaped expansion at the tip of the axon. This dynamic, highly motile structure is defined by a lamellipodial network lining the periphery of the growth cone interspersed with finger-like filopodial projections that serve to explore the surrounding environment. Once axon guidance receptors are deployed at the right place and time at the growth cone surface, they respond to their respective ligands by initiating a complex set of signaling events that serve to rearrange the growth cone membrane and the actin and microtubule cytoskeleton to affect axon growth and guidance. In this review, we highlight recent advances that shed light on the rich complexity of mechanisms that regulate axon guidance receptor distribution, activation and downstream signaling.

The Ciliary Protein Arl13b Functions Outside of the Primary Cilium in Shh-Mediated Axon Guidance

The small GTPase Arl13b is enriched in primary cilia and regulates Sonic hedgehog (Shh) signaling. During neural development, Shh controls patterning and proliferation through a canonical, transcription-dependent pathway that requires the primary cilium. Additionally, Shh controls axon guidance through a non-canonical, transcription-independent pathway whose connection to the primary cilium is unknown. Here we show that inactivation of Arl13b results in defective commissural axon guidance in vivo. In vitro, we demonstrate that Arl13b functions autonomously in neurons for their Shh-dependent guidance response. We detect Arl13b protein in axons and growth cones, far from its well-established ciliary enrichment. To test whether Arl13b plays a non-ciliary function, we used an engineered, cilia-localization-deficient Arl13b variant and found that it was sufficient to mediate Shh axon guidance in vitro and in vivo. Together, these results indicate that, in addition to its ciliary role in canonical Shh signaling, Arl13b plays a cilia-independent role in Shh-mediated axon guidance.

Reciprocal repulsions instruct the precise assembly of parallel hippocampal networks

Mammalian medial and lateral hippocampal networks preferentially process spatial- and object-related information, respectively. However, the mechanisms underlying the assembly of such parallel networks during development remain largely unknown. Our study shows that, in mice, complementary expression of cell surface molecules teneurin-3 (Ten3) and latrophilin-2 (Lphn2) in the medial and lateral hippocampal networks, respectively, guides the precise assembly of CA1-to-subiculum connections in both networks. In the medial network, Ten3-expressing (Ten3+) CA1 axons are repelled by target-derived Lphn2, revealing that Lphn2- and Ten3-mediated heterophilic repulsion and Ten3-mediated homophilic attraction cooperate to control precise target selection of CA1 axons. In the lateral network, Lphn2-expressing (Lphn2+) CA1 axons are confined to Lphn2+ targets via repulsion from Ten3+ targets. Our findings demonstrate that assembly of parallel hippocampal networks follows a "Ten3→Ten3, Lphn2→Lphn2" rule instructed by reciprocal repulsions.

Protective role of mRNA demethylase FTO on axon guidance molecules of nigro-striatal projection system in manganese-induced parkinsonism

One of the major environmental factors that induce PD is Manganese (Mn). Cellular and molecular mechanism of parkinsonism caused by Mn has not been explored clearly. The results of in vivo and in vitro experiments showed that Mn exposure caused abnormal projection of dopaminergic neurons and decreased mRNA expression and protein levels of FTO. This is due to Mn-induced the upregulation of Foxo3a. Using the cell model of overexpression of FTO, we found that FTO could antagonize Mn-induced the down-regulation of axon guidance molecule ephrin-B2 through RNA-seq, MeRIP-qPCR, and RT-qPCR experiments. Through RIP-seq and actinomycin D experiments, it was found that FTO can up-regulate the mRNA m⁶A level of ephrin-B2, which can be recognized by YTHDF2 and degraded. Finally, it is proved that Mn induces dopaminergic neurons projection injury and motor dysfunction through Foxo3a/FTO/m⁶A/ephrin-B2/YTHDF2 signal pathway.

Regulators of Rho GTPases in the Nervous System: Molecular Implication in Axon Guidance and Neurological Disorders

One of the fundamental steps during development of the nervous system is the formation of proper connections between neurons and their target cells-a process called neural wiring, failure of which causes neurological disorders ranging from autism to Down's syndrome. Axons navigate through the complex environment of a developing embryo toward their targets, which can be far away from their cell bodies. Successful implementation of neuronal wiring, which is crucial for fulfillment of all behavioral functions, is achieved through an intimate interplay between axon guidance and neural activity. In this review, our focus will be on axon pathfinding and the implication of some of its downstream molecular components in neurological disorders. More precisely, we will talk about axon guidance and the molecules implicated in this process. After, we will briefly review the Rho family of small GTPases, their regulators, and their involvement in downstream signaling pathways of the axon guidance cues/receptor complexes. We will then proceed to the final and main part of this review, where we will thoroughly comment on the implication of the regulators for Rho GTPases-GEFs (Guanine nucleotide Exchange Factors) and GAPs (GTPase-activating Proteins)-in neurological diseases and disorders.

Coordination of Heparan Sulfate Proteoglycans with Wnt Signaling To Control Cellular Migrations and Positioning in Caenorhabditis elegans

Heparan sulfates (HS) are linear polysaccharides with complex modification patterns, which are covalently bound via conserved attachment sites to core proteins to form heparan sulfate proteoglycans (HSPGs). HSPGs regulate many aspects of the development and function of the nervous system, including cell migration, morphology, and network connectivity. HSPGs function as cofactors for multiple signaling pathways, including the Wnt-signaling molecules and their Frizzled receptors. To investigate the functional interactions among the HSPG and Wnt networks, we conducted genetic analyses of each, and also between these networks using five cellular migrations in the nematode Caenorhabditis elegans We find that HSPG core proteins act genetically in a combinatorial fashion dependent on the cellular contexts. Double mutant analyses reveal distinct redundancies among HSPGs for different migration events, and different cellular migrations require distinct heparan sulfate modification patterns. Our studies reveal that the transmembrane HSPG SDN-1/Syndecan functions within the migrating cell to promote cellular migrations, while the GPI-linked LON-2/Glypican functions cell nonautonomously to establish the final cellular position. Genetic analyses with the Wnt-signaling system show that (1) a given HSPG can act with different Wnts and Frizzled receptors, and that (2) a given Wnt/Frizzled pair acts with different HSPGs in a context-dependent manner. Lastly, we find that distinct HSPG and Wnt/Frizzled combinations serve separate functions to promote cellular migration and establish position of specific neurons. Our studies suggest that HSPGs use structurally diverse glycans in coordination with Wnt-signaling pathways to control multiple cellular behaviors, including cellular and axonal migrations and, cellular positioning.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Crosstalk between the peripheral nervous system and breast cancer influences tumor progression

Recent studies have shown that peripheral nerves play an important role in the progression of breast cancer. Breast cancer cells (BCCs) promote local peripheral nerve growth and branching by secreting neuroactive molecules, including neurotrophins and axon guidance molecules (AGMs). Sympathetic nerves promote breast cancer progression, while parasympathetic and sensory nerves mainly have anti-tumor effects in the progression of breast cancer. Specifically, peripheral nerves can influence the progression of breast cancer by secreting neurotransmitters not only directly binding to the corresponding receptors of BCCs, but also indirectly acting on immune cells to modulate anti-tumor immunity. In this review, we summarize the crosstalk between breast cancer and peripheral nerves and the roles of important neuroactive molecules in the progression of breast cancer. In addition, we summarize indicators, including nerve fiber density and perineural invasion (PNI), that may help determine the prognosis of breast cancer based on current research results, as well as potential therapeutic approaches, such as β-blockers and retroviral-mediated genetic neuroengineering techniques, that may enhance the prognosis of breast cancer. In addition, we propose suggestions for future research priorities based on a current lack of knowledge in this area.

Revisiting and refining roles of neural guidance cues in circuit assembly

Neural guidance mechanisms ensure the precise targeting and synaptogenesis events essential for normal circuit function. Neuronal growth cones encounter numerous attractive and repulsive cues as they navigate toward their intermediate and final targets; temporal and spatial regulation of these responses are critical for circuit assembly. Recent work highlights the complexity of these events throughout neural development and the multifaceted functions of a wide range of guidance cues. Here, we discuss recent studies that leverage advances in genetics, single cell tracing, transcriptomics and proteomics to further our understanding of the molecular mechanisms underlying neural guidance and overall circuit organization.

Different Isoforms of the Neuronal Guidance Molecule Slit2 Directly Cause Chemoattraction or Chemorepulsion of Human Neutrophils

The movement of neutrophils between blood and tissues appears to be regulated by chemoattractants and chemorepellents. Compared with neutrophil chemoattractants, relatively little is known about neutrophil chemorepellents. Slit proteins are endogenously cleaved into a variety of N- and C-terminal fragments, and these fragments are neuronal chemorepellents and inhibit chemoattraction of many cell types, including neutrophils. In this report, we show that the ∼140-kDa N-terminal Slit2 fragment (Slit2-N) is a chemoattractant and the ∼110-kDa N-terminal Slit2 fragment (Slit2-S) is a chemorepellent for human neutrophils. The effects of both Slit2 fragments were blocked by Abs to the Slit2 receptor Roundabout homolog 1 or the Slit2 coreceptor Syndecan-4. Slit2-N did not appear to activate Ras but increased phosphatidylinositol 3,4,5-triphosphate levels. Slit2-N-induced chemoattraction was unaffected by Ras inhibitors, reversed by PI3K inhibitors, and blocked by Cdc42 and Rac inhibitors. In contrast, Slit2-S activated Ras but did not increase phosphatidylinositol 3,4,5-triphosphate levels. Slit2-S-induced chemorepulsion was blocked by Ras and Rac inhibitors, not affected by PI3K inhibitors, and reversed by Cdc42 inhibitors. Slit2-N, but not Slit2-S, increased neutrophil adhesion, myosin L chain 2 phosphorylation, and polarized actin formation and single pseudopods at the leading edge of cells. Slit2-S induced multiple pseudopods. These data suggest that Slit2 isoforms use similar receptors but different intracellular signaling pathways and have different effects on the cytoskeleton and pseudopods to induce neutrophil chemoattraction or chemorepulsion.

Contralateral migration of oculomotor neurons is regulated by Slit/Robo signaling

BACKGROUND

Oculomotor neurons develop initially like typical motor neurons, projecting axons out of the ventral midbrain to their ipsilateral targets, the extraocular muscles. However, in all vertebrates, after the oculomotor nerve (nIII) has reached the extraocular muscle primordia, the cell bodies that innervate the superior rectus migrate to join the contralateral nucleus. This motor neuron migration represents a unique strategy to form a contralateral motor projection. Whether migration is guided by diffusible cues remains unknown.

METHODS

We examined the role of Slit chemorepellent signals in contralateral oculomotor migration by analyzing mutant mouse embryos.

RESULTS

We found that the ventral midbrain expresses high levels of both Slit1 and 2, and that oculomotor neurons express the repellent Slit receptors Robo1 and Robo2. Therefore, Slit signals are in a position to influence the migration of oculomotor neurons. In Slit 1/2 or Robo1/2 double mutant embryos, motor neuron cell bodies migrated into the ventral midbrain on E10.5, three days prior to normal migration. These early migrating neurons had leading projections into and across the floor plate. In contrast to the double mutants, embryos which were mutant for single Slit or Robo genes did not have premature migration or outgrowth on E10.5, demonstrating a cooperative requirement of Slit1 and 2, as well as Robo1 and 2. To test how Slit/Robo midline repulsion is modulated, we found that the normal migration did not require the receptors Robo3 and CXCR4, or the chemoattractant, Netrin 1. The signal to initiate contralateral migration is likely autonomous to the midbrain because oculomotor neurons migrate in embryos that lack either nerve outgrowth or extraocular muscles, or in cultured midbrains that lacked peripheral tissue.

CONCLUSION

Overall, our results demonstrate that a migratory subset of motor neurons respond to floor plate-derived Slit repulsion to properly control the timing of contralateral migration.

Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

During animal development, cells and neurons navigate long distances to reach their final target destinations. Migrating cells are guided by extracellular molecular cues, and cellular responses to these cues are regulated by heparan sulfate proteoglycans. Heparan sulfate proteoglycans are proteins with long heparan sulfate polysaccharide chains attached. Here we identify and study previously unavailable viable mutants that disrupt the elongation of the heparan sulfate chains in the nematode C. elegans. Our analysis shows that these HS-chain-elongation mutations affect the development of the nervous system as they result in misguided migrations of neurons and axons. Furthermore, we find that heparan sulfate chain elongation occurs in numerous cell types during development and that the coordinated production of heparan sulfate proteoglycans, in both the migrating cell and neighboring tissues, ensures proper migration. Our findings highlight the critical roles of heparan sulfate proteoglycans in nervous system development and the evolutionary conservation of the molecular mechanisms driving guided migrations.

Recurrent DCC gene losses during bird evolution

During development, midline crossing by axons brings into play highly conserved families of receptors and ligands. The interaction between the secreted ligand Netrin-1 and its receptor Deleted in Colorectal Carcinoma (DCC) is thought to control midline attraction of crossing axons. Here, we studied the evolution of this ligand/receptor couple in birds taking advantage of a wealth of newly sequenced genomes. From phylogeny and synteny analyses we can infer that the DCC gene has been conserved in most extant bird species, while two independent events have led to its loss in two avian groups, passeriformes and galliformes. These convergent accidental gene loss events are likely related to chromosome Z rearrangement. We show, using whole-mount immunostaining and 3Disco clearing, that in the nervous system of all birds that have a DCC gene, DCC protein expression pattern is similar to other vertebrates. Surprisingly, we show that the early developmental pattern of commissural tracts is comparable in all birds, whether or not they have a DCC receptor. Interestingly, only 4 of the 5 genes encoding secreted netrins, the DCC ligands in vertebrates, were found in birds, but Netrin-5 was absent. Together, these results support a remarkable plasticity of commissural axon guidance mechanisms in birds.

The RhoGAP HUM-7/Myo9 integrates signals to modulate RHO-1/RhoA during embryonic morphogenesis in Caenorhabditiselegans

During embryonic morphogenesis, cells and tissues undergo dramatic movements under the control of F-actin regulators. Our studies of epidermal cell migrations in developing Caenorhabditiselegans embryos have identified multiple plasma membrane signals that regulate the Rac GTPase, thus regulating WAVE and Arp2/3 complexes, to promote branched F-actin formation and polarized enrichment. Here, we describe a pathway that acts in parallel to Rac to transduce membrane signals to control epidermal F-actin through the GTPase RHO-1/RhoA. RHO-1 contributes to epidermal migration through effects on underlying neuroblasts. We identify signals to regulate RHO-1-dependent events in the epidermis. HUM-7, the C. elegans homolog of human MYO9A and MYO9B, regulates F-actin dynamics during epidermal migration. Genetics and biochemistry support that HUM-7 behaves as a GTPase-activating protein (GAP) for the RHO-1/RhoA and CDC-42 GTPases. Loss of HUM-7 enhances RHO-1-dependent epidermal cell behaviors. We identify SAX-3/ROBO as an upstream signal that contributes to attenuated RHO-1 activation through its regulation of HUM-7/Myo9. These studies identify a new role for RHO-1 during epidermal cell migration, and suggest that RHO-1 activity is regulated by SAX-3/ROBO acting on the RhoGAP HUM-7.

Neuronal cell adhesion molecule (NrCAM) is expressed by sensory cells in the cochlea and is necessary for proper cochlear innervation and sensory domain patterning during development

BACKGROUND

In the cochlea, auditory development depends on precise patterns of innervation by afferent and efferent nerve fibers, as well as a stereotyped arrangement of hair and supporting cells. Neuronal cell adhesion molecule (NrCAM) is a homophilic cell adhesion molecule that controls diverse aspects of nervous system development, but the function of NrCAM in cochlear development is not well understood.

RESULTS

Throughout cochlear innervation, NrCAM is detectable on spiral ganglion neuron (SGN) afferent and olivocochlear efferent fibers, and on the membranes of developing hair and supporting cells. Neonatal Nrcam-null cochleae show errors in type II SGN fasciculation, reduced efferent innervation, and defects in the stereotyped packing of hair and supporting cells. Nrcam loss also leads to dramatic changes in the profiles of presynaptic afferent and efferent synaptic markers at the time of hearing onset. Despite these numerous developmental defects, Nrcam-null adults do not show defects in auditory acuity, and by postnatal day 21, the developmental deficits in ribbon synapse distribution and sensory domain structure appear to have been corrected.

CONCLUSIONS

NrCAM is expressed by several neural and sensory epithelial subtypes within the developing cochlea, and the loss of Nrcam confers numerous, but nonpermanent, developmental defects in innervation and sensory domain patterning. Developmental Dynamics 247:934-950, 2018. © 2018 Wiley Periodicals, Inc.

Investigation of neuronal pathfinding and construction of artificial neuronal networks on 3D-arranged porous fibrillar scaffolds with controlled geometry

Herein, we investigated the neurite pathfinding on electrospun microfibers with various fiber densities, diameters, and microbead islands, and demonstrated the development of 3D connected artificial neuronal network within a nanofiber-microbead-based porous scaffold. The primary culture of rat hippocampal embryonic neurons was deposited on geometry-controlled polystyrene (PS) fiber scaffolds while growth cone morphology, neurite outgrowth patterns, and focal adhesion protein expression were cautiously examined by microscopic imaging of immunostained and live neuronal cells derived from actin-GFP transgenic mice. It was demonstrated that the neurite outgrowth was guided by the overall microfiber orientation, but the increase in fiber density induced the neurite path alteration, thus, the reduction in neurite linearity. Indeed, we experimentally confirmed that growth cone could migrate to a neighboring, but, spatially disconnected microfiber by spontaneous filopodium extrusion, which is possibly responsible for the observed neurite steering. Furthermore, thinner microfiber scaffolds showed more pronounced expression of focal adhesion proteins than thicker ones, suggesting that the neuron-microfiber interaction can be delicately modulated by the underlying microfiber geometry. Finally, 3D connected functional neuronal networks were successfully constructed using PS nanofiber-microbead scaffolds where enhanced porosity and vertical fiber orientation permitted cell body inclusion within the scaffold and substantial neurite outgrowth in a vertical direction, respectively.

Novel spiral structured nerve guidance conduits with multichannels and inner longitudinally aligned nanofibers for peripheral nerve regeneration

Nerve guidance conduits (NGCs) are artificial substitutes for autografts, which serve as the gold standard in treating peripheral nerve injury. A recurring challenge in tissue engineered NGCs is optimizing the cross-sectional surface area to achieve a balance between allowing nerve infiltration while supporting maximum axonal extension from the proximal to distal stump. In this study, we address this issue by investigating the efficacy of an NGC with a higher cross-sectional surface composed of spiral structures and multi-channels, coupled with inner longitudinally aligned nanofibers and protein on aiding nerve repair in critical sized nerve defect. The NGCs were implanted into 15-mm-long rat sciatic nerve injury gaps for 4 weeks. Nerve regeneration was assessed using an established set of assays, including the walking track analysis, electrophysiological testing, pinch reflex assessment, gastrocnemius muscle measurement, and histological assessment. The results indicated that the novel NGC design yielded promising data in encouraging nerve regeneration within a relatively short recovery time. The performance of the novel NGC for nerve regeneration was superior to that of the control nerve conduits with tubular structures. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1410-1419, 2019.

Additive manufacturing of Schwann cell-laden collagen/alginate nerve guidance conduits by freeform reversible embedding regulate neurogenesis via exosomes secretion towards peripheral nerve regeneration

Peripheral nerve injury is a common clinical problem that could be debilitating to one's quality of life. The complex nerve guidance conduits (NGCs) with cells in order to improve nerve regeneration. Therefore, we used freeform reversible embedding of suspended hydrogels to fabricate Schwann cells (SCs)-laden collagen/alginate (Col/Alg) NGCs. First, we evaluated Col influence on the characteristics of NGCs. After which, Wharton's jelly mesenchymal stem cells (WJMSC) are seeded onto the inner channel of NGCs and evaluated neural regeneration behaviors. Results indicated the SCs-laden NGCs with 2.5 % Col found the highest proliferation and secretion of neurotrophic protein. Furthermore, co-culture of SCs promoted differentiation of WJMSC as seen from the increased neurogenic-related protein in NGCs. To determine the molecular mechanism between SCs and WJMSC, we demonstrated the neurotrophic factors secreted by SCs act on tropomyosin receptor kinase A (TrkA) receptors of WJMSC to promote nerve regeneration. In addition, our study demonstrated SCs-derived exosomes had a critical role in regulating neural differentiation of WJMSC. Taken together, this study demonstrates the fabrication of SCs-laden Col/Alg NGCs for nerve regeneration and understanding regarding the synergistic regenerative mechanisms of different cells could bring us a step closer for clinical treatment of large nerve defects.

Neuronal contact guidance and YAP signaling on ultra-small nanogratings

Contact interaction of neuronal cells with extracellular nanometric features can be exploited to investigate and modulate cellular responses. By exploiting nanogratings (NGs) with linewidth from 500 nm down to 100 nm, we here study neurite contact guidance along ultra-small directional topographies. The impact of NG lateral dimension on the neuronal morphotype, neurite alignment, focal adhesion (FA) development and YAP activation is investigated in nerve growth factor (NGF)-differentiating PC12 cells and in primary hippocampal neurons, by confocal and live-cell total internal reflection fluorescence (TIRF) microscopy, and at molecular level. We demonstrate that loss of neurite guidance occurs in NGs with periodicity below 400 nm and correlates with a loss of FA lateral constriction and spatial organization. We found that YAP intracellular localization is modulated by the presence of NGs, but it is not sensitive to their periodicity. Nocodazole, a drug that can increase cell contractility, is finally tested for rescuing neurite alignment showing mild ameliorative effects. Our results provide new indications for a rational design of biocompatible scaffolds for enhancing nerve-regeneration processes.

Axon Guidance Molecules and Pain

Chronic pain is a debilitating condition that influences the social, economic, and psychological aspects of patients’ lives. Hence, the need for better treatment is drawing extensive interest from the research community. Developmental molecules such as Wnt, ephrins, and semaphorins are acknowledged as central players in the proper growth of a biological system. Their receptors and ligands are expressed in a wide variety in both neurons and glial cells, which are implicated in pain development, maintenance, and resolution. Thereby, it is not surprising that the impairment of those pathways affects the activities and functions of the entire cell. Evidence indicates aberrant activation of their pathways in the nervous system in rodent models of chronic pain. In those conditions, Wnt, ephrin, and semaphorin signaling participate in enhancing neuronal excitability, peripheral sensitization, synaptic plasticity, and the production and release of inflammatory cytokines. This review summarizes the current knowledge on three main developmental pathways and their mechanisms linked with the pathogenesis and progression of pain, considering their impacts on neuronal and glial cells in experimental animal models. Elucidations of the downstream pathways may provide a new mechanism for the involvement of Wnt, ephrin, and semaphorin pathways in pain chronicity.

Priming with FGF2 stimulates human dental pulp cells to promote axonal regeneration and locomotor function recovery after spinal cord injury

Human dental pulp cells (DPCs), adherent cells derived from dental pulp tissues, are potential tools for cell transplantation therapy. However, little work has been done to optimize such transplantation. In this study, DPCs were treated with fibroblast growth factor-2 (FGF2) for 5-6 consecutive serial passages and were transplanted into the injury site immediately after complete transection of the rat spinal cord. FGF2 priming facilitated the DPCs to promote axonal regeneration and to improve locomotor function in the rat with spinal cord injury (SCI). Additional analyses revealed that FGF2 priming protected cultured DPCs from hydrogen-peroxide-induced cell death and increased the number of DPCs in the SCI rat spinal cord even 7 weeks after transplantation. The production of major neurotrophic factors was equivalent in FGF2-treated and untreated DPCs. These observations suggest that FGF2 priming might protect DPCs from the post-trauma microenvironment in which DPCs infiltrate and resident immune cells generate cytotoxic reactive oxygen species. Surviving DPCs could increase the availability of neurotrophic factors in the lesion site, thereby promoting axonal regeneration and locomotor function recovery.

Multiomic Analysis of Neurons with Divergent Projection Patterns Identifies Novel Regulators of Axon Pathfinding

Axon pathfinding is a key step in neural circuits formation. However, the transcriptional mechanisms regulating its progression remain poorly understood. The binary decision of crossing or avoiding the midline taken by some neuronal axons during development represents a robust model to investigate the mechanisms that control the selection of axonal trajectories. Here, to identify novel regulators of axon guidance, this work compares the transcriptome and chromatin occupancy profiles of two neuronal subpopulations, ipsilateral (iRGC) and contralateral retinal ganglion cells (cRGC), with similar functions but divergent axon trajectories. These analyses retrieved a number of genes encoding for proteins not previously implicated in axon pathfinding. In vivo functional experiments confirm the implication of some of these candidates in axonal navigation. Among the candidate genes, γ-synuclein is identified as essential for inducing midline crossing. Footprint and luciferase assays demonstrate that this small-sized protein is regulated by the transcription factor (TF) Pou4f1 in cRGCs. It is also shown that Lhx2/9 are specifically expressed in iRGCs and control a program that partially overlaps with that regulated by Zic2, previously described as essential for iRGC specification. Overall, the analyses identify dozens of new molecules potentially involved in axon guidance and reveal the regulatory logic behind the selection of axonal trajectories.