Pursuing a 'turning point' in growth cone research

Axon guidance pathways and the control of gene expression

Axons need to be properly guided to their targets to form synaptic connections, and this requires interactions between highly conserved extracellular and transmembrane ligands and their cell surface receptors. The majority of studies on axon guidance signaling pathways have focused on the role of these pathways in rearranging the local cytoskeleton and plasma membrane in growth cones and axons. However, a smaller body of work has demonstrated that axon guidance signaling pathways also control gene expression via local translation and transcription. Recent studies on axon guidance ligands and receptors have begun to uncover the requirements for these alternative mechanisms in processes required for neural circuit formation: axon guidance, synaptogenesis, and cell migration. Understanding the mechanisms by which axon guidance signaling regulates local translation and transcription will create a more complete picture of neural circuit formation, and they may be applied more broadly to other tissues where axon guidance ligands and receptors are required for morphogenesis. Developmental Dynamics 247:571-580, 2018. © 2017 Wiley Periodicals, Inc.

The ENU-3 protein family members function in the Wnt pathway parallel to UNC-6/Netrin to promote motor neuron axon outgrowth in C. elegans

The axons of the DA and DB classes of motor neurons fail to reach the dorsal cord in the absence of the guidance cue UNC-6/Netrin or its receptor UNC-5 in C. elegans. However, the axonal processes usually exit their cell bodies in the ventral cord in the absence of both molecules. Strains lacking functional versions of UNC-6 or UNC-5 have a low level of DA and DB motor neuron axon outgrowth defects. We found that mutations in the genes for all six of the ENU-3 proteins function to enhance the outgrowth defects of the DA and DB axons in strains lacking either UNC-6 or UNC-5. A mutation in the gene for the MIG-14/Wntless protein also enhances defects in a strain lacking either UNC-5 or UNC-6, suggesting that the ENU-3 and Wnt pathways function parallel to the Netrin pathway in directing motor neuron axon outgrowth. Our evidence suggests that the ENU-3 proteins are novel members of the Wnt pathway in nematodes. Five of the six members of the ENU-3 family are predicted to be single-pass trans-membrane proteins. The expression pattern of ENU-3.1 was consistent with plasma membrane localization. One family member, ENU-3.6, lacks the predicted signal peptide and the membrane-spanning domain. In HeLa cells ENU-3.6 had a cytoplasmic localization and caused actin dependent processes to appear. We conclude that the ENU-3 family proteins function in a pathway parallel to the UNC-6/Netrin pathway for motor neuron axon outgrowth, most likely in the Wnt pathway.

Mmp25β facilitates elongation of sensory neurons during zebrafish development

Matrix metalloproteinases (MMPs) are a large and complex family of zinc-dependent endoproteinases widely recognized for their roles in remodeling the extracellular matrix (ECM) during embryonic development, wound healing, and tissue homeostasis. Their misregulation is central to many pathologies, and they have therefore been the focus of biomedical research for decades. These proteases have also recently emerged as mediators of neural development and synaptic plasticity in vertebrates, however, understanding of the mechanistic basis of these roles and the molecular identities of the MMPs involved remains far from complete. We have identified a zebrafish orthologue of mmp25 (a.k.a. leukolysin; MT6-MMP), a membrane-type, furin-activated MMP associated with leukocytes and invasive carcinomas, but which we find is expressed by a subset of the sensory neurons during normal embryonic development. We detect high levels of Mmp25β expression in the trigeminal, craniofacial, and posterior lateral line ganglia in the hindbrain, and in Rohon-Beard cells in the dorsal neural tube during the first 48 h of embryonic development. Knockdown of Mmp25β expression with morpholino oligonucleotides results in larvae that are uncoordinated and insensitive to touch, and which exhibit defects in the development of sensory neural structures. Using in vivo zymography, we observe that Mmp25β morphant embryos show reduced Type IV collagen degradation in regions of the head traversed by elongating axons emanating from the trigeminal ganglion, suggesting that Mmp25β may play a pivotal role in mediating ECM remodeling in the vicinity of these elongating axons.

Frizzled-5 receptor is involved in neuronal polarity and morphogenesis of hippocampal neurons

The Wnt signaling pathway plays important roles during different stages of neuronal development, including neuronal polarization and dendritic and axonal outgrowth. However, little is known about the identity of the Frizzled receptors mediating these processes. In the present study, we investigated the role of Frizzled-5 (Fzd5) on neuronal development in cultured Sprague-Dawley rat hippocampal neurons. We found that Fzd5 is expressed early in cultured neurons on actin-rich structures localized at minor neurites and axonal growth cones. At 4 DIV, Fzd5 polarizes towards the axon, where its expression is detected mainly at the peripheral zone of axonal growth cones, with no obvious staining at dendrites; suggesting a role of Fzd5 in neuronal polarization. Overexpression of Fzd5 during the acquisition of neuronal polarity induces mislocalization of the receptor and a loss of polarized axonal markers. Fzd5 knock-down leads to loss of axonal proteins, suggesting an impaired neuronal polarity. In contrast, overexpression of Fzd5 in neurons that are already polarized did not alter polarity, but decreased the total length of axons and increased total dendrite length and arborization. Fzd5 activated JNK in HEK293 cells and the effects triggered by Fzd5 overexpression in neurons were partially prevented by inhibition of JNK, suggesting that a non-canonical Wnt signaling mechanism might be involved. Our results suggest that, Fzd5 has a role in the establishment of neuronal polarity, and in the morphogenesis of neuronal processes, in part through the activation of the non-canonical Wnt mechanism involving JNK.

Acetylcholine elongates neuronal growth cone filopodia via activation of nicotinic acetylcholine receptors

In addition to acting as a classical neurotransmitter in synaptic transmission, acetylcholine (ACh) has been shown to play a role in axonal growth and growth cone guidance. What is not well understood is how ACh acts on growth cones to affect growth cone filopodia, structures known to be important for neuronal pathfinding. We addressed this question using an identified neuron (B5) from the buccal ganglion of the pond snail Helisoma trivolvis in cell culture. ACh treatment caused pronounced filopodial elongation within minutes, an effect that required calcium influx and resulted in the elevation of the intracellular calcium concentration ([Ca]i ). Whole-cell patch clamp recordings showed that ACh caused a reduction in input resistance, a depolarization of the membrane potential, and an increase in firing frequency in B5 neurons. These effects were mediated via the activation of nicotinic acetylcholine receptors (nAChRs), as the nAChR agonist dimethylphenylpiperazinium (DMPP) mimicked the effects of ACh on filopodial elongation, [Ca]i elevation, and changes in electrical activity. Moreover, the nAChR antagonist tubucurarine blocked all DMPP-induced effects. Lastly, ACh acted locally at the growth cone, because growth cones that were physically isolated from their parent neuron responded to ACh by filopodial elongation with a similar time course as growth cones that remained connected to their parent neuron. Our data revealed a critical role for ACh as a modulator of growth cone filopodial dynamics. ACh signaling was mediated via nAChRs and resulted in Ca influx, which, in turn, caused filopodial elongation.

Strength in the periphery: growth cone biomechanics and substrate rigidity response in peripheral and central nervous system neurons

There is now considerable evidence of the importance of mechanical cues in neuronal development and regeneration. Motivated by the difference in the mechanical properties of the tissue environment between the peripheral (PNS) and central (CNS) nervous systems, we compare substrate-stiffness-dependent outgrowth and traction forces from PNS (dorsal root ganglion (DRG)) and CNS (hippocampal) neurons. We show that neurites from DRG neurons display maximal outgrowth on substrates with a Young's modulus of ∼1000 Pa, whereas hippocampal neurite outgrowth is independent of substrate stiffness. Using traction force microscopy, we also find a substantial difference in growth cone traction force generation, with DRG growth cones exerting severalfold larger forces compared with hippocampal growth cones. The traction forces generated by DRG and hippocampal growth cones both increase with increasing stiffness, and DRG growth cones growing on substrates with a Young's modulus of 1000 Pa strengthen considerably after 18-30 h. Finally, we find that retrograde actin flow is almost three times faster in hippocampal growth cones than in DRG. Moreover, the density of paxillin puncta is significantly lower in hippocampal growth cones, suggesting that stronger substrate coupling of the DRG cytoskeleton is responsible for the remarkable difference in traction force generation. These findings reveal a differential adaptation of cytoskeletal dynamics to substrate stiffness in growth cones of different neuronal types, and highlight the potential importance of the mechanical properties of the cellular environment for neuronal navigation during embryonic development and nerve regeneration.

Neural regenerative strategies incorporating biomolecular axon guidance signals

There are currently no acceptable cures for central nervous system injuries, and damage induced large gaps in the peripheral nervous system have been challenging to bridge to restore neural functionality. Innervation by neurons is made possible by the growth cone. This dynamic structure is unique to neurons, and can directly sense physical and chemical activity in its environment, utilizing these cues to propel axons to precisely reach their targets. Guidance can occur through chemoattractive factors such as neurotrophins and netrins, chemorepulsive agents like semaphorins and slits, or contact-mediated molecules such as ephrins and those located in the extracellular matrix. The understanding of biomolecular activity during nervous system development and injury has generated new techniques and tactics for improving and restoring function to the nervous system after injury. This review will focus on the major neuronal guidance molecules and their utility in current tissue engineering and neural regenerative strategies.

Xenopus sonic hedgehog guides retinal axons along the optic tract

The role of classic morphogens such as Sonic hedgehog (Shh) as axon guidance cues has been reported in a variety of vertebrate organisms (Charron and Tessier-Lavigne [2005] Development 132:2251-2262). In this work, we provide the first evidence that Xenopus sonic hedgehog (Xshh) signaling is involved in guiding retinal ganglion cell (RGC) axons along the optic tract. Xshh is expressed in the brain during retinal axon extension, adjacent to these axons in the ventral diencephalon. Retinal axons themselves express Patched 1 and Smoothened co-receptors during RGC axon growth. Blocking Shh signaling causes abnormal ventral pathfinding, and targeting errors at the optic tectum. Misexpression of exogenous N-Shh peptide in vivo also causes pathfinding errors. Retinal axons grown in culture respond to N-Shh in a dose-dependent manner, either by decreasing extension at lower concentrations, or retracting axons in the presence of higher doses. These data suggest that Shh signaling is required for normal RGC axon pathfinding and tectal targeting in the developing visual system of Xenopus. We propose that Shh serves as a ventral optic tract repellent that helps to define the caudal boundary for retinal axons in the diencephalon, and that this signaling is also required for initial target recognition at the optic tectum.

ENU-3 is a novel motor axon outgrowth and guidance protein in C. elegans

During the development of the nervous system, the migration of many cells and axons is guided by extracellular molecules. These molecules bind to receptors at the tips of the growth cones of migrating axons and trigger intracellular signaling to steer the axons along the correct trajectories. We have identified a novel mutant, enu-3 (enhancer of Unc), that enhances the motor neuron axon outgrowth defects observed in strains of Caenorhabditis elegans that lack either the UNC-5 receptor or its ligand UNC-6/Netrin. Specifically, the double-mutant strains have enhanced axonal outgrowth defects mainly in DB4, DB5 and DB6 motor neurons. enu-3 single mutants have weak motor neuron axon migration defects. Both outgrowth defects of double mutants and axon migration defects of enu-3 mutants were rescued by expression of the H04D03.1 gene product. ENU-3/H04D03.1 encodes a novel predicted putative trans-membrane protein of 204 amino acids. It is a member of a family of highly homologous proteins of previously unknown function in the C. elegans genome. ENU-3 is expressed in the PVT interneuron and is weakly expressed in many cell bodies along the ventral cord, including those of the DA and DB motor neurons. We conclude that ENU-3 is a novel C. elegans protein that affects both motor axon outgrowth and guidance.

A novel approach reveals temporal patterns of synaptogenesis between the isolated growth cones of Lymnaea neurons

All brain functions, ranging from motor behaviour to cognition, depend on precise developmental patterns of synapse formation between the growth cones of both pre- and postsynaptic neurons. While the molecular evidence for the presence of 'pre-assembled' elements of synaptic machinery prior to physical contact is beginning to emerge, the precise timing of functional synaptogenesis between the growth cones has not yet been defined. Moreover, it is unclear whether an initial assembly of various synaptic molecules located at the extrasomal regions (e.g. growth cones) can indeed result in fully mature and consolidated synapses in the absence of somata signalling. Such evidence is difficult to obtain both in vivo and in vitro because the extrasomal sites are often challenging, if not impossible, to access for electrophysiological analysis. Here we demonstrate a novel approach to precisely define various steps underlying synapse formation between the isolated growth cones of individually identifiable pre- and postsynaptic neurons from the mollusc Lymnaea stagnalis. We show for the first time that isolated growth cones transformed into 'growth balls' have an innate propensity to develop specific and multiple synapses within minutes of physical contact. We also demonstrate that a prior 'synaptic history' primes the presynaptic growth ball to form synapses quicker with subsequent partners. This is the first demonstration that isolated Lymnaea growth cones have the necessary machinery to form functional synapses.

Sensory axon targeting is increased by NGF gene therapy within the lesioned adult femoral nerve

Even though peripheral nerves regenerate well, axons are often misrouted and reinnervate inappropriate distal pathways post-injury. Misrouting most likely occurs at branch points where regenerating axons make choices. Here, we show that the accuracy of sensory axon reinnervation is enhanced by overexpression of the guidance molecule nerve growth factor (NGF) distal to the bifurcation. We used the femoral nerve as a model, which contains both sensory and motor axons that intermingle in the parent trunk and distally segregate into the saphenous (SB) and motor branches (MB). Transection of the parent trunk resulted in misrouting of axon reinnervation to SB and MB. To enhance sensory axon targeting, recombinant adenovirus encoding NGF was injected along the SB close to the bifurcation 1 week post-injury. The accuracy of axon reinnervation was assessed by retrograde tracing at 3 or 8 weeks after nerve injury. NGF overexpression significantly increased the accuracy of SB axon reinnervation to the appropriate nerve branch, in a manner independent of enhancing axon regeneration. This novel finding provides in vivo evidence that gradient expression of neurotrophin can be used to enhance targeting of distal peripheral pathways to increase axon regeneration into the appropriate nerve branch.

Lithium and neuropsychiatric therapeutics: neuroplasticity via glycogen synthase kinase-3beta, beta-catenin, and neurotrophin cascades

Mood disorders are not merely attributed to the functional defect of neurotransmission, but also are due to the structural impairment of neuroplasticity. Chronic stress decreases neurotrophin levels, precipitating or exacerbating depression; conversely, antidepressants increase expression of various neurotrophins (e.g., brain-derived neurotrophic factor and vascular endothelial growth factor), thereby blocking or reversing structural and functional pathologies via promoting neurogenesis. Since the worldwide approval of lithium therapy in 1970, lithium has been used for its anti-manic, antidepressant, and anti-suicidal effects, yet the therapeutic mechanisms at the cellular level remain not-fully defined. During the last five years, multiple lines of evidence have shown that the mood stabilization and neurogenesis by lithium are due to the lithium-induced inhibition of glycogen synthase kinase-3beta (GSK-3beta), allowing accumulation of beta-catenin and beta-catenin-dependent gene transcriptional events. Altered levels of GSK-3beta and beta-catenin are associated with various neuropsychiatric and neurodegenerative diseases, while various classical neuropsychiatric drugs inhibit GSK-3beta and up-regulate beta-catenin expression. In addition, evidence has emerged that insulin-like growth factor-I enhances antidepression, anti-anxiety, memory, neurogenesis, and angiogenesis; antidepressants up-regulate expression of insulin-like growth factor-I, while insulin-like growth factor-I up-regulates brain-derived neurotrophic factor expression and its receptor TrkB level, as well as brain-derived neurotrophic factor-induced synaptic protein levels. More importantly, physical exercise and healthy diet raise transport of peripheral circulating insulin-like growth factor I into the brain, reinforcing the expression of neurotrophins (e.g., brain-derived neurotrophic factor) and the strength of cell survival signalings (e.g., phosphoinositide 3-kinase / Akt / GSK-3beta pathway). This review will focus on the rapidly advancing new trends in the last five years about lithium, GSK-3beta/beta-catenin, and neurotrophin cascades.

Protein synthesis in distal axons is not required for growth cone responses to guidance cues

Recent evidence suggests that growth cone responses to guidance cues require local protein synthesis. Using chick neurons, we investigated whether protein synthesis is required for growth cones of several types to respond to guidance cues. First, we found that global inhibition of protein synthesis stops axonal elongation after 2 h. When protein synthesis inhibitors were added 15 min before adding guidance cues, we found no changes in the typical responses of retinal, sensory, and sympathetic growth cones. In the presence of cycloheximide or anisomycin, ephrin-A2, slit-3, and semaphorin3A still induced growth cone collapse and loss of actin filaments, nerve growth factor (NGF) and neurotrophin-3 still induced growth cone protrusion and increased filamentous actin, and sensory growth cones turned toward an NGF source. In compartmented chambers that separated perikarya from axons, axons grew for 24-48 h in the presence of cycloheximide and responded to negative and positive cues. Our results indicate that protein synthesis is not strictly required in the mechanisms for growth cone responses to many guidance cues. Differences between our results and other studies may exist because of different cellular metabolic levels in in vitro conditions and a difference in when axonal functions become dependent on local protein synthesis.

Nav1.7 sodium channel-induced Ca2+ influx decreases tau phosphorylation via glycogen synthase kinase-3beta in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells expressing Na(v)1.7 sodium channel isoform, veratridine increased Ser(473)-phosphorylation of Akt and Ser(9)-phosphorylation of glycogen synthase kinase-3beta by approximately 217 and approximately 195%, while decreasing Ser(396)-phosphorylation of tau by approximately 36% in a concentration (EC(50)=2.1 microM)- and time (t(1/2)=2.7 min)-dependent manner. These effects of veratridine were abolished by tetrodotoxin or extracellular Ca(2+) removal. Veratridine (10 microM for 5 min) increased translocation of Ca(2+)-dependent conventional protein kinase C-alpha from cytoplasm to membranes by 47%; it was abolished by tetrodotoxin, extracellular Ca(2+) removal, or Gö6976 (an inhibitor of protein kinase C-alpha), and partially attenuated by LY294002 (an inhibitor of phosphatidylinositol 3-kinase). LY294002 (but not Gö6976) abrogated veratridine-induced Akt phosphorylation. In contrast, either LY294002 or Gö6976 alone attenuated veratridine-induced glycogen synthase kinase-3beta phosphorylation by 65 or 42%; however, LY294002 plus Gö6976 completely blocked it. Veratridine (10 microM for 5 min)-induced decrease of tau phosphorylation was partially attenuated by LY294002 or Gö6976, but completely blocked by LY294002 plus Gö6976; okadaic acid or cyclosporin A (inhibitors of protein phosphatases 1, 2A, and 2B) failed to alter tau phosphorylation. These results suggest that Na(+) influx via Na(v)1.7 sodium channel and the subsequent Ca(2+) influx via voltage-dependent calcium channel activated (1) Ca(2+)/protein kinase C-alpha pathway, as well as (2) Ca(2+)/phosphatidylinositol 3-kinase/Akt and (3) Ca(2+)/phosphatidylinositol 3-kinase/protein kinase C-alpha pathways; these parallel pathways converged on inhibitory phosphorylation of glycogen synthase kinase-3beta, decreasing tau phosphorylation.