Control of Growth Cone Polarity, Microtubule Accumulation, and Protrusion by UNC-6/Netrin and Its Receptors in Caenorhabditis elegans

SRC-1 controls growth cone polarity and protrusion with the UNC-6/Netrin receptor UNC-5 in Caenorhabditis elegans

The Polarity/Protusion model of UNC-6/Netrin function in axon repulsion does not rely on a gradient of UNC-6/Netrin. Instead, the UNC-5 receptor polarizes the VD growth cone such that filopodial protrusions are biased to the dorsal leading edge. UNC-5 then inhibits growth cone protrusion ventrally based upon this polarity, resulting in dorsally-biased protrusion and dorsal migration away from UNC-6/Netrin. While previous studies have shown that UNC-5 inhibits growth cone protrusion by destabilizing actin, preventing microtubule + end entry, and preventing vesicle fusion, the signaling pathways involved are unclear. The SRC-1 tyrosine kinase has been previously shown to physically interact with and phosphorylate UNC-5, and to act with UNC-5 in axon guidance and cell migration. Here, the role of SRC-1 in VD growth cone polarity and protrusion is investigated. A precise deletion of src-1 was generated, and mutants displayed unpolarized growth cones with increased size, similar to unc-5 mutants. Transgenic expression of src-1(+) in VD/DD neurons resulted in smaller growth cones, and rescued growth cone polarity defects of src-1 mutants, indicating cell-autonomous function. Transgenic expression of a putative kinase-dead src-1(D831A) mutant caused a phenotype similar to src-1 loss-of-function, suggesting that this is a dominant negative mutation. The D381A mutation was introduced into the endogenous src-1 gene by genome editing, which also had a dominant-negative effect. Genetic interactions of src-1 and unc-5 suggest they act in the same pathway on growth cone polarity and protrusion, but might have overlapping, parallel functions in other aspects of axon guidance. src-1 function was not required for the effects of activated myr::unc-5, suggesting that SRC-1 might be involved in UNC-5 dimerization and activation by UNC-6, of which myr::unc-5 is independent. In sum, these results show that SRC-1 acts with UNC-5 in growth cone polarity and inhibition of protrusion.

Short- and long-range roles of UNC-6/Netrin in dorsal-ventral axon guidance in vivo in Caenorhabditis elegans

Recent studies in vertebrates and Caenorhabditis elegans have reshaped models of how the axon guidance cue UNC-6/Netrin functions in dorsal-ventral axon guidance, which was traditionally thought to form a ventral-to-dorsal concentration gradient that was actively sensed by growing axons. In the vertebrate spinal cord, floorplate Netrin1 was shown to be largely dispensable for ventral commissural growth. Rather, short range interactions with Netrin1 on the ventricular zone radial glial stem cells was shown to guide ventral commissural axon growth. In C. elegans, analysis of dorsally-migrating growth cones during outgrowth has shown that growth cone polarity of filopodial extension is separable from the extent of growth cone protrusion. Growth cones are first polarized by UNC-6/Netrin, and subsequent regulation of protrusion by UNC-6/Netrin is based on this earlier-established polarity (the Polarity/Protrusion model). In both cases, short-range or even haptotactic mechanisms are invoked: in vertebrate spinal cord, interactions of growth cones with radial glia expressing Netrin-1; and in C. elegans, a potential close-range interaction that polarizes the growth cone. To explore potential short-range and long-range functions of UNC-6/Netrin, a potentially membrane-anchored transmembrane UNC-6 (UNC-6(TM)) was generated by genome editing. Unc-6(tm) was hypomorphic for dorsal VD/DD axon pathfinding, indicating that it retained some unc-6 function. Polarity of VD growth cone filopodial protrusion was initially established in unc-6(tm), but was lost as the growth cones migrated away from the unc-6(tm) source in the ventral nerve cord. In contrast, ventral guidance of the AVM and PVM axons was equally severe in unc-6(tm) and unc-6(null). Together, these results suggest that unc-6(tm) retains short-range functions but lacks long-range functions. Finally, ectopic unc-6(+) expression from non-ventral sources could rescue dorsal and ventral guidance defects in unc-6(tm) and unc-6(null). Thus, a ventral directional source of UNC-6 was not required for dorsal-ventral axon guidance, and UNC-6 can act as a permissive, not instructive, cue for dorsal-ventral axon guidance. Possibly, UNC-6 is a permissive signal that activates cell-intrinsic polarity; or UNC-6 acts with another signal that is required in a directional manner. In either case, the role of UNC-6 is to polarize the pro-protrusive activity of UNC-40/DCC in the direction of outgrowth.

A short isoform of the UNC-6/Netrin receptor UNC-5 is required for growth cone polarity and robust growth cone protrusion in Caenorhabditis elegans

Introduction: UNC-6/Netrin is a conserved bi-functional guidance cue which regulates dorsal-ventral axon guidance in C. elegans. In the Polarity/Protrusion model of UNC-6/Netrin mediated dorsal growth away from UNC-6/Netrin, The UNC-5 receptor first polarizes the VD growth cone such that filopodial protrusions are biased dorsally. Based on this polarity, the UNC-40/DCC receptor stimulates growth cone lamellipodial and filopodial protrusion dorsally. The UNC-5 receptor maintains dorsal polarity of protrusion, and inhibits growth cone protrusion ventrally, resulting in net dorsal growth cone advance. Methods: Growth cone imaging in mutants, combined with Cas9 genome editing and genetic analysis, were used to analyze the role of a novel short isoform on unc-5 in growth cone polarity and protrusion. Results: Work presented here demonstrates a novel role of a previously undescribed, conserved short isoform of UNC-5 (UNC-5B). UNC-5B lacks the cytoplasmic domains of UNC-5 long, including the DEATH domain, the UPA/DB domain, and most of the ZU5 domain. Mutations that specifically affect only the unc-5 long isoforms were hypomorphic, suggesting a role of unc-5B short. A mutation specifically affecting unc-5B caused loss of dorsal polarity of protrusion and reduced growth cone filopodial protrusion, the opposite of unc-5 long mutations. Transgenic expression of unc-5B partially rescued unc-5 axon guidance defects, and resulted in large growth cones. Tyrosine 482 (Y482) in the cytoplasmic juxtamembrane region has been shown to be important for UNC-5 function, and is present in both UNC-5 long and UNC-5B short. Results reported here show that Y482 is required for the function of UNC-5 long and for some functions of UNC-5B short. Finally, genetic interactions with unc-40 and unc-6 suggest that UNC-5B short acts in parallel to UNC-6/Netrin to ensure robust growth cone lamellipodial protrusion. Discussion: These results demonstrate a previously-undescribed role for the UNC-5B short isoform, which is required for dorsal polarity of growth cone filopodial protrusion and to stimulate growth cone protrusion, in contrast to the previously-described role of UNC-5 long in inhibiting growth cone protrusion.

A short isoform of the UNC-6/Netrin receptor UNC-5 is required for growth cone polarity and robust growth cone protrusion in Caenorhabditis elegans

UNC-6/Netrin is a conserved bi-functional guidance cue which regulates dorsal-ventral axon guidance in C. elegans . In the Polarity/Protrusion model of UNC-6/Netrin mediated dorsal growth away from UNC-6/Netrin, The UNC-5 receptor first polarizes the VD growth cone such that filopodial protrusions are biased dorsally. Based on this polarity, the UNC-40/DCC receptor stimulates growth cone lamellipodial and filopodial protrusion dorsally. The UNC-5 receptor maintains dorsal polarity of protrusion, and inhibits growth cone protrusion ventrally, resulting in net dorsal growth cone advance. Work presented here demonstrates a novel role of a previously undescribed, conserved short isoform of UNC-5 (UNC-5B). UNC-5B lacks the cytoplasmic domains of UNC-5 long, including the DEATH domain, the UPA/DB domain, and most of the ZU5 domain. Mutations that specifically affect only the unc-5 long isoforms were hypomorphic, suggesting a role of unc-5B short. A mutation specifically affecting unc-5B cause loss of dorsal polarity of protrusion and reduced growth cone filopodial protrusion, the opposite of unc-5 long mutations. Transgenic expression of unc-5B partially rescued unc-5 axon guidance defects, and resulted in large growth cones. Tyrosine 482 (Y482) in the cytoplasmic juxtamembrane region has been shown to be important for UNC-5 function, and is present in both UNC-5 long and UNC-5B short. Results reported here show that Y482 is required for the function of UNC-5 long and for some functions of UNC-5B short. Finally, genetic interactions with unc-40 and unc-6 suggest that UNC-5B short acts in parallel to UNC-6/Netrin to ensure robust growth cone lamellipodial protrusion. In sum, these results demonstrate a previously-undescribed role for the UNC-5B short isoform, which is required for dorsal polarity of growth cone filopodial protrusion and to stimulate growth cone protrusion, in contrast to the previously-described role of UNC-5 long in inhibiting growth cone protrusion.

The PH/MyTH4/FERM molecule MAX-1 inhibits UNC-5 activity in the regulation of VD growth cone protrusion in Caenorhabditis elegans

UNC-6/Netrin is a secreted conserved guidance cue that regulates dorsal-ventral axon guidance of Caenorhabditis elegans and in the vertebral spinal cord. In the polarity/protrusion model of VD growth cone guidance away from ventrally expressed UNC-6 (repulsion), UNC-6 first polarizes the growth cone via the UNC-5 receptor such that filopodial protrusions are biased dorsally. UNC-6 then regulates a balance of protrusion in the growth cone based upon this polarity. UNC-5 inhibits protrusion ventrally, and the UNC-6 receptor UNC-40/DCC stimulates protrusion dorsally, resulting in net dorsal growth cone outgrowth. UNC-5 inhibits protrusion through the flavin monooxygenases FMO-1, 4, and 5 and possible actin destabilization, and inhibits pro-protrusive microtubule entry into the growth cone utilizing UNC-33/CRMP. The PH/MyTH4/FERM myosin-like protein was previously shown to act with UNC-5 in VD axon guidance utilizing axon guidance endpoint analysis. Here, we analyzed the effects of MAX-1 on VD growth cone morphology during outgrowth. We found that max-1 mutant growth cones were smaller and less protrusive than wild type, the opposite of the unc-5 mutant phenotype. Furthermore, genetic interactions suggest that MAX-1 might normally inhibit UNC-5 activity, such that in a max-1 mutant growth cone, UNC-5 is overactive. Our results, combined with previous studies suggesting that MAX-1 might regulate UNC-5 levels in the cell or plasma membrane localization, suggest that MAX-1 attenuates UNC-5 signaling by regulating UNC-5 stability or trafficking. Alternately, MAX-1 might inhibit UNC-5 independent of this known mechanism. We also show that the effects of MAX-1 in growth cone protrusion are independent of UNC-40/DCC, UNC-33/CRMP, and UNC-34/Enabled. In summary, in the context of growth cone protrusion, MAX-1 inhibits UNC-5, demonstrating the mechanistic insight that can be gained by analyzing growth cones during outgrowth in addition to axon guidance endpoint analysis.

Of worms and men

Following the spectacular success of molecular genetics in deciphering the genetic code in the 1960s, several of its leading practitioners felt sufficiently emboldened to use their newly acquired skills to move on and study that most enigmatic of biological organs - the brain. Sydney Brenner's approach was to focus on Caenorhabditis elegans, a nematode that is genetically tractable, has a nervous system that generates a rich repertoire of behaviours yet is small enough to allow anatomical reconstructions with ultrastructural precision. Through force of personality and some inspired pioneering studies, Brenner managed to ignite a bonfire of enthusiasm for this organism, which has resulted in its nervous system becoming the best understood of that in any organism. Initially, many were skeptical that this rather strange structure with just a few hundred neurons would yield insights that were relevant to vertebrate nervous systems. However, fifty years on we know that the basic repertoire of molecular components of worm and human nervous systems are remarkably similar. Furthermore, worms have a similar diversity of these components rather than a primitive sub-set. It appears that the fundamental difference in a vertebrate nervous system is a huge expansion of the neural units that comprise a basic brain such as that exemplified in C. elegans.

A perspective on C. elegans neurodevelopment: from early visionaries to a booming neuroscience research

The formation of the nervous system and its striking complexity is a remarkable feat of development. C. elegans served as a unique model to dissect the molecular events in neurodevelopment, from its early visionaries to the current booming neuroscience community. Soon after being introduced as a model, C. elegans was mapped at the level of genes, cells, and synapses, providing the first metazoan with a complete cell lineage, sequenced genome, and connectome. Here, I summarize mechanisms underlying C. elegans neurodevelopment, from the generation and diversification of neural components to their navigation and connectivity. I point out recent noteworthy findings in the fields of glia biology, sex dimorphism and plasticity in neurodevelopment, highlighting how current research connects back to the pioneering studies by Brenner, Sulston and colleagues. Multifaceted investigations in model organisms, connecting genes to cell function and behavior, expand our mechanistic understanding of neurodevelopment while allowing us to formulate emerging questions for future discoveries.

The netrin receptor UNC-40/DCC assembles a postsynaptic scaffold and sets the synaptic content of GABAA receptors

Increasing evidence indicates that guidance molecules used during development for cellular and axonal navigation also play roles in synapse maturation and homeostasis. In C. elegans the netrin receptor UNC-40/DCC controls the growth of dendritic-like muscle cell extensions towards motoneurons and is required to recruit type A GABA receptors (GABA_ARs) at inhibitory neuromuscular junctions. Here we show that activation of UNC-40 assembles an intracellular synaptic scaffold by physically interacting with FRM-3, a FERM protein orthologous to FARP1/2. FRM-3 then recruits LIN-2, the ortholog of CASK, that binds the synaptic adhesion molecule NLG-1/Neuroligin and physically connects GABA_ARs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA_ARs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA_ARs through the UNC-40-dependent intracellular scaffold. Since DCC is detected at GABA synapses in mammals, DCC might also tune inhibitory neurotransmission in the mammalian brain.

The netrin receptor UNC-40/DCC is required to recruit GABA_AR at neuromuscular junctions in C. elegans. Here, the authors show that UNC-40/DCC assembles an intracellular synaptic scaffold, regulating the content of GABA_AR and inhibitory neurotransmission.

Partially overlapping guidance pathways focus the activity of UNC-40/DCC along the anteroposterior axis of polarizing neuroblasts

Directional migration of neurons and neuronal precursor cells is a central process in nervous system development. In the nematode Caenorhabditis elegans, the two Q neuroblasts polarize and migrate in opposite directions along the anteroposterior body axis. Several key regulators of Q cell polarization have been identified, including MIG-21, DPY-19/DPY19L1, the netrin receptor UNC-40/DCC, the Fat-like cadherin CDH-4 and CDH-3/Fat, which we describe in this study. How these different transmembrane proteins act together to direct Q neuroblast polarization and migration is still largely unknown. Here, we demonstrate that MIG-21 and DPY-19, CDH-3 and CDH-4, and UNC-40 define three distinct pathways that have partially redundant roles in protrusion formation, but also separate functions in regulating protrusion direction. Moreover, we show that the MIG-21, DPY-19 and Fat-like cadherin pathways control the localization and clustering of UNC-40 at the leading edge of the polarizing Q neuroblast, and that this is independent of the UNC-40 ligands UNC-6/netrin and MADD-4. Our results provide insight into a novel mechanism for ligand-independent localization of UNC-40 that directs the activity of UNC-40 along the anteroposterior axis.

RHO-1 and the Rho GEF RHGF-1 interact with UNC-6/Netrin signaling to regulate growth cone protrusion and microtubule organization in Caenorhabditis elegans

UNC-6/Netrin is a conserved axon guidance cue that directs growth cone migrations in the dorsal-ventral axis of C. elegans and in the vertebrate spinal cord. UNC-6/Netrin is expressed in ventral cells, and growth cones migrate ventrally toward or dorsally away from UNC-6/Netrin. Recent studies of growth cone behavior during outgrowth in vivo in C. elegans have led to a polarity/protrusion model in directed growth cone migration away from UNC-6/Netrin. In this model, UNC-6/Netrin first polarizes the growth cone via the UNC-5 receptor, leading to dorsally biased protrusion and F-actin accumulation. UNC-6/Netrin then regulates protrusion based on this polarity. The receptor UNC-40/DCC drives protrusion dorsally, away from the UNC-6/Netrin source, and the UNC-5 receptor inhibits protrusion ventrally, near the UNC-6/Netrin source, resulting in dorsal migration. UNC-5 inhibits protrusion in part by excluding microtubules from the growth cone, which are pro-protrusive. Here we report that the RHO-1/RhoA GTPase and its activator GEF RHGF-1 inhibit growth cone protrusion and MT accumulation in growth cones, similar to UNC-5. However, growth cone polarity of protrusion and F-actin were unaffected by RHO-1 and RHGF-1. Thus, RHO-1 signaling acts specifically as a negative regulator of protrusion and MT accumulation, and not polarity. Genetic interactions are consistent with RHO-1 and RHGF-1 acting with UNC-5, as well as with a parallel pathway, to regulate protrusion. The cytoskeletal interacting molecule UNC-33/CRMP was required for RHO-1 activity to inhibit MT accumulation, suggesting that UNC-33/CRMP might act downstream of RHO-1. In sum, these studies describe a new role of RHO-1 and RHGF-1 in regulation of growth cone protrusion by UNC-6/Netrin.

Fungal homologues of human Rac1 as emerging players in signal transduction and morphogenesis

A wealth of data is accumulating on the physiological functions of human Rac1, a member of the Rho GTPase family of molecular switches and substrate of botulinum toxin, which was first identified as a regulator of cell motility through its effect on the actin cytoskeleton. Later on, it was found to be involved in different diseases like cancers, cardiac function, neuronal disorders, and apoptotic cell death. Despite the presence of Rac1 homologues in most fungi investigated so far, including Rho5 in the genetically tractable model yeast Saccharomyces cerevisiae, knowledge on their physiological functions is still scarce, let alone the details of the molecular mechanisms of their actions and interactions. Nevertheless, all functions proposed for human Rac1 seem to be conserved in one or the other fungus. This includes the regulation of MAPK cascades, polarized growth, and actin dynamics. Moreover, both the production and response to reactive oxygen species, as well as the reaction to nutrient availability, can be affected. We here summarize the studies performed on fungal Rac1 homologues, with a special focus on S. cerevisiae Rho5, which may be of use in drug development in medicine and agriculture.

A perspective on SOAL, a stochastic model of neuronal outgrowth

A functional nervous system requires neuronal connections to be made in a highly detailed and stereotypic manner. During development, neurons extend processes that can branch, travel in different directions, and form elaborate patterns. These patterns are essential for forming proper connections. Patterns of outgrowth are produced by complex molecular events that cause a fluid membrane to move. The collective impact of dynamic fluctuating events at the microscale cause the patterns of outgrowth observed at the macroscale. Patterning is genetically controlled, but the effects genes have on membrane movement and patterning are not well understood. To better understand how genes control outgrowth patterns, I propose a statistically-oriented asymmetric localization (SOAL) model. This model is based on the theory that receptor-mediated outgrowth activity is stochastically oriented and when the system is at equilibrium there is an equal probability of outgrowth being oriented in any direction. This concept allows a statistical mechanics approach that can correlate the microscale events of outgrowth to the observed macroscale patterns. Proof-of-concept experiments suggest this approach can be used to study the effect genes have on outgrowth patterns. The SOAL model also provides a new theoretical framework for conceptualizing guidance. According to the model, outgrowth activity becomes asymmetrically localized to the neuron's surface in a statistically dependent manner. Extracellular cues regulate the probability of outgrowth along the surface and the orientation of outgrowth fluctuates across the surface over time. This creates a directional bias that allows the growth cone to navigate in reference to the composition of extracellular cues.