Tyrosine kinase inhibition produces specific alterations in axon guidance in the grasshopper embryo

The translational repressor Pumilio regulates presynaptic morphology and controls postsynaptic accumulation of translation factor eIF-4E

Translational repression by Drosophila Pumilio (Pum) protein controls posterior patterning during embryonic development. Here, we show that Pum is an important mediator of synaptic growth and plasticity at the neuromuscular junction (NMJ). Pum is localized to the postsynaptic side of the NMJ in third instar larvae and is also expressed in larval neurons. Neuronal Pum regulates synaptic growth. In its absence, NMJ boutons are larger and fewer in number, while Pum overexpression increases bouton number and decreases bouton size. Postsynaptic Pum negatively regulates expression of the translation factor eIF-4E at the NMJ, and Pum binds selectively to the 3'UTR of eIF-4E mRNA. The GluRIIa glutamate receptor is upregulated in pum mutants. These results, together with genetic epistasis studies, suggest that postsynaptic Pum modulates synaptic function via direct control of eIF-4E expression.

Drosophila spastin regulates synaptic microtubule networks and is required for normal motor function

The most common form of human autosomal dominant hereditary spastic paraplegia (AD-HSP) is caused by mutations in the SPG4 (spastin) gene, which encodes an AAA ATPase closely related in sequence to the microtubule-severing protein Katanin. Patients with AD-HSP exhibit degeneration of the distal regions of the longest axons in the spinal cord. Loss-of-function mutations in the Drosophila spastin gene produce larval neuromuscular junction (NMJ) phenotypes. NMJ synaptic boutons in spastin mutants are more numerous and more clustered than in wild-type, and transmitter release is impaired. spastin-null adult flies have severe movement defects. They do not fly or jump, they climb poorly, and they have short lifespans. spastin hypomorphs have weaker behavioral phenotypes. Overexpression of Spastin erases the muscle microtubule network. This gain-of-function phenotype is consistent with the hypothesis that Spastin has microtubule-severing activity, and implies that spastin loss-of-function mutants should have an increased number of microtubules. Surprisingly, however, we observed the opposite phenotype: in spastin-null mutants, there are fewer microtubule bundles within the NMJ, especially in its distal boutons. The Drosophila NMJ is a glutamatergic synapse that resembles excitatory synapses in the mammalian spinal cord, so the reduction of organized presynaptic microtubules that we observe in spastin mutants may be relevant to an understanding of human Spastin's role in maintenance of axon terminals in the spinal cord.

Inhibition of protein tyrosine kinase activity disrupts early retinal development

In the present study, we have investigated the role of tyrosine kinase activity during early retinal development in Xenopus laevis. The protein tyrosine kinase (PTK) inhibitors lavendustin A and genistein were used to determine the possible role of tyrosine kinase activity during retinal development in vivo and in vitro. Application of the inhibitors to early embryonic retina disrupted the pattern of lamination in the developing retina. The plexiform layers were severely disorganized or were no longer apparent, and photoreceptor morphogenesis was disrupted. Immunocytochemical analysis verified the presence of focal adhesions in dissociated retinal neuroepithelial cells isolated from St 25 embryos. Application of the PTK inhibitors blocked focal adhesion assembly in these primary cultured cells. To further investigate the regulation of focal adhesions by PTK activity, we examined the effect of lavendustin A on cultured XR1 glial cells. Lavendustin A produced a dose-dependent decrease in the proportion of XR1 cells displaying focal adhesions. Taken together, these results suggest that tyrosine kinase activity is essential for regulating neuroepithelial cell adhesion, migration and morphogenesis during retinal development. Furthermore, the disruption of retinal development may, in part, be due to the inhibition of integrin-mediated signaling.

Abelson tyrosine kinase is required to transduce midline repulsive cues

Tyrosine phosphorylation-dependent signaling cascades play key roles in determining the formation of an axon pathway. The cytoplasmic Abelson tyrosine kinase participate in several signaling pathways that orchestrate both growth cone advance and steering in response to guidance cues. Here, a genetic approach is used to evaluate the role for Abelson in growth cones during a decision to cross or not to cross the Drosophila embryonic midline. Our data indicate that both loss- and gain-of-function conditions for Abl cause neurons within the pCC/MP2 pathway to project across the midline incorrectly. The frequency of abnormal crossovers is enhanced by mutations in the genes encoding the midline repellent, Slit, or its receptor, Roundabout. In comm mutants, where repulsive signals remain elevated, increasing or decreasing Abl activity partially rescues commissure formation. Thus, both too much and too little Abl activity causes axons to cross the midline inappropriately, indicating that Abl plays a critical role in transducing midline repulsive cues. How Abl functions in this role is not yet clear, but we suggest that Abl may help regulate cytoskeletal dynamics underlying a growth cone's response to midline cues.

Filopodial calcium transients regulate growth cone motility and guidance through local activation of calpain

Spontaneous intracellular calcium ([Ca2+](i)) transients in growth cone filopodia reduce filopodial motility, slow neurite outgrowth, and promote turning when generated asymmetrically; however, the downstream effectors of these Ca2+ -dependent behaviors are unknown. We report that Ca2+ transients in filopodia activate the intracellular protease calpain, which slows neurite outgrowth and promotes repulsive growth cone turning upon local activation. Active calpain alters the balance between tyrosine kinase and phosphatase activities in filopodia, resulting in a net decrease in tyrosine phosphorylation, which mediates both filopodial stabilization and reduced lamellipodial protrusion. Our findings indicate that locally generated Ca2+ signals repel axon outgrowth through calpain-dependent regulation of phosphotyrosine signaling at integrin-mediated adhesion sites.

Association of LAR-like receptor protein tyrosine phosphatases with an enabled homolog in Hirudo medicinalis

Receptor protein tyrosine phosphatases (RPTPs) are thought to play important functions in pathfinding and target recognition by growing neuronal processes. The leech RPTPs HmLAR1 and HmLAR2 are expressed selectively by central neurons, Comb cells, and peripheral muscle tissues in the Hirudo medicinalis embryo. To explore the functions of HmLARs, we have sought to determine their physiological substrates. We report here the cloning and embryonic expression of Lena, the leech homolog of Enabled, a cytosolic protein implicated in actin-based cell motility. Lena is expressed in embryonic central neurons and in the Comb cell. We present experimental evidences indicating that Lena associates selectively with the intracellular domain of HmLAR1 and HmLAR2. Additionally, RNA interference (RNAi) of HmLAR1 in intact leech embryos leads to the hyperphosphorylation of Lena. We propose, therefore, that Lena is an in vivo substrate of HmLAR1 in neurons and perhaps of HmLAR2 in the Comb cells.

Juvenile hormone stimulated tyrosine kinase-mediated protein phosphorylation in the CNS of the silk worm, Bombyx mori

In vitro studies with the larval CNS of the silkworm, Bombyx mori revealed the phosphorylation of a 48-kDa protein, which was not dependent on cyclic nucleotides. Studies also revealed modest phosphorylation of this protein by a calcium-dependent but calmodulin-independent mechanism. However, phosphorylation of this protein was greatly enhanced in the presence of juvenile hormone (JH) I by a calcium-independent mechanism. This stimulatory effect of JH was seen in both homogenates as well as in intact CNS of Bombyx. Immunoblotting studies revealed the cross-reaction of this 48-kDa protein with phosphotyrosine monoclonal antibody and the phosphorylation of this protein was inhibited by genistein. This study suggests that the 48-kDa protein is a substrate for tyrosine kinase. The phosphorylation of this protein was also observed in other larval tissues such as salivary gland, fat body, and epidermis of Bombyx.

Transmission of growth cone traction force through apCAM-cytoskeletal linkages is regulated by Src family tyrosine kinase activity

Tyrosine kinase activity is known to be important in neuronal growth cone guidance. However, underlying cellular mechanisms are largely unclear. Here, we report how Src family tyrosine kinase activity controls apCAM-mediated growth cone steering by regulating the transmission of traction forces through receptor-cytoskeletal linkages. Increased levels of tyrosine phosphorylation were detected at sites where beads coated with apCAM ligands were physically restrained to induce growth cone steering, but not at unrestrained bead binding sites. Interestingly, the rate and level of phosphotyrosine buildup near restrained beads were decreased by the myosin inhibitor 2,3-butanedione-2-monoxime, suggesting that tension promotes tyrosine kinase activation. While not affecting retrograde F-actin flow rates, genistein and the Src family selective tyrosine kinase inhibitors PP1 and PP2 strongly reduced the growth cone's ability to apply traction forces through apCAM-cytoskeletal linkages, assessed using the restrained bead interaction assay. Furthermore, increased levels of an activated Src family kinase were detected at restrained bead sites during growth cone steering events. Our results suggest a mechanism by which growth cones select pathways by sampling both the molecular nature of the substrate and its ability to withstand the application of traction forces.

Axon guidance at the midline of the developing CNS

Bilaterally symmetric animals must be capable of transmitting information between the left and right sides of their body to integrate sensory input and to coordinate motor control. Thus, many neurons in the central nervous system (CNS) of a wide variety of higher organisms project so-called commissural axons across the midline. Interestingly, these axons are never observed to re-cross the midline. On the other hand, some neurons project axons that remain on their own (ipsilateral) side of the CNS, without ever crossing the midline. Recent studies demonstrate that specialized cells which reside at the ventral midline of the developing vertebrate spinal cord and Drosophila ventral nerve cord play critical roles in regulating the guidance of both crossing and non-crossing axons. For example, these cells secrete positively-acting guidance cues that attract commissural axons over long distances to the midline of the CNS. Furthermore, short-range interactions between guidance cues present on the surfaces of midline cells, and their receptors expressed on the surfaces of pathfinding axons, allow commissural axons to cross the midline and prevent ipsilaterally projecting axons from entering the midline. Remarkably, as commissural axons cross over to the opposite side of the CNS, the molecular composition of their surfaces is dynamically altered so that they become responsive to repulsive midline guidance cues that they had previously ignored. Thus, this exquisitely controlled guidance system prevents commissural axons from crossing the midline more than once. Strikingly, many of the molecular mechanisms that control midline guidance appear to be evolutionarily conserved.

Possible role of the receptor protein tyrosine phosphatase HmLAR2 in interbranch repulsion in a leech embryonic cell

Accumulating evidence indicates that receptor protein tyrosine phosphatases (rPTPs) play major roles in growth cone migration. We have previously shown that the growth cones of the multiple parallel processes of an identified leech embryonic cell, the Comb cell (CC), express high levels of a leukocyte antigen-related (LAR)-like rPTP, HmLAR2. Embryonic injection of a polyclonal antibody to the receptor's ectodomain resulted in reduced process outgrowth and in processes crossing over each other, a behavior that is seldom observed in normal or control animals. Here we present results of injecting a soluble Fc-HmLAR2 ectodomain fusion protein into embryos in order to bind the endogenous ligands of HmLAR2. Single injections of the Fc-chimeric protein into the developing embryo resulted, 12 to 24 h postinjection, in clear morphological abnormalities, ranging from abnormally directed CC processes and crossovers to apparent growth cone collapse. At later times, 2 to 5 days post injection, growth cones appeared to have recovered and processes had continued to extend, but effects of the earlier guidance errors remained, with the CCs displaying a relatively high incidence of proximal guidance errors. When injected into the germinal plate of developing embryos, the fusion protein was found to bind selectively to the processes of the CCs themselves, in contrast to control injections of Fc alone or closely related Fc-tagged proteins, which did not decorate the CCs. Double-labeling experiments revealed an early phase of Fc-HmLAR2 labeling (within 20 min after application), during which the growth cones and filopodia of the CC showed significant binding of the receptor ectodomain, and a later phase (1-2 h after injection), when most of the label was redistributed away from the growth cones and into the proximal processes of the CC. In culture, HmLAR2-transfected COS cells were found to selectively bind the Fc-recombinant protein, but not Fc-tagged proteins bearing other closely related receptor ectodomains, demonstrating that the HmLAR2 ectodomain is capable of interacting homophilically. Together, our observations demonstrate that the rPTP HmLAR2 is critically involved in CC process extension through its participation in the regulation of growth cone structure, migration, and navigation. Moreover, since our experiments also indicate that HmLAR2 can bind to itself, we hypothesize that HmLAR2 has a key role in the mechanism of mutual repulsion that maintains the parallel growth of adjacent CC projections.

A paradoxical gradient of a basal lamina-associated repellent is essential for pathfinding by the Ti1 pioneer axons in cockroach embryos

Perturbations of in situ axon growth with proteolytic enzymes and monoclonal antibodies were used to determine the role of gradient guidance cues in the formation of the Ti1 pioneer axon trajectory in cultured cockroach embryos. Treatment with enzymes that degrade the basal lamina indicated that this substrate contains both an elastase-sensitive proximal directing cue and a collagenase-sensitive distal directing cue. The latter is shown to be a repellent of axon growth and is identical to the PROD-2 antigen that is distributed in a gradient along the proximal-distal axis of the leg with high levels in proximal regions. This means that throughout the course of their growth the axons extend in the direction of increasing levels of repellent. At a critical decision point in the trajectory the axons change both the direction of growth and the substrate to which their growth cones adhere. PROD-2 plays an essential role in both of these processes.

Dosage-sensitive, reciprocal genetic interactions between the Abl tyrosine kinase and the putative GEF trio reveal trio's role in axon pathfinding

The Abelson tyrosine kinase (Abl) is integrated into signal transduction networks regulating axon outgrowth. We have identified the Drosophila trio gene through a mutation that exacerbates the Abl mutant phenotype. Drosophila Trio is an ortholog of mammalian Trio, a protein that contains multiple spectrin-like repeats and two Dbl homology (DH) domains that affect actin cytoskeletal dynamics via the small GTPases Rho and Rac. Phenotypic analysis demonstrates that trio and Abl cooperate in regulating axon outgrowth in the embryonic central nervous system (CNS). Dosage-sensitive interactions between trio and Abl, failed axon connections (fax), and enabled (ena) indicate that Trio is integrated into common signaling networks with these gene products. These observations suggest a mechanism by which Abl-mediated signaling networks influence the actin cytoskeleton in neuronal growth cones.

Receptor tyrosine phosphatases regulate axon guidance across the midline of the Drosophila embryo

Neural receptor-linked protein tyrosine phosphatases (RPTPs) are required for guidance of motoneuron and photoreceptor growth cones in Drosophila. These phosphatases have not been implicated in growth cone responses to specific guidance cues, however, so it is unknown which aspects of axonal pathfinding are controlled by their activities. Three RPTPs, known as DLAR, DPTP69D, and DPTP99A, have been genetically characterized thus far. Here we report the isolation of mutations in the fourth neural RPTP, DPTP10D. The analysis of double mutant phenotypes shows that DPTP10D and DPTP69D are necessary for repulsion of growth cones from the midline of the embryonic central nervous system. Repulsion is thought to be triggered by binding of the secreted protein Slit, which is expressed by midline glia, to Roundabout (Robo) receptors on growth cones. Robo repulsion is downregulated by the Commissureless (Comm) protein, allowing axons to cross the midline. Here we show that the Rptp mutations genetically interact with robo, slit and comm. The nature of these interactions suggests that DPTP10D and DPTP69D are positive regulators of Slit/Roundabout repulsive signaling. We also show that elimination of all four neural RPTPs converts most noncrossing longitudinal pathways into commissures that cross the midline, indicating that tyrosine phosphorylation controls the manner in which growth cones respond to midline signals.

The tyrosine kinase Abl and its substrate enabled collaborate with the receptor phosphatase Dlar to control motor axon guidance

Genetic analysis of growth cone guidance choice points in Drosophila identified neuronal receptor protein tyrosine phosphatases (RPTPs) as key determinants of axon pathfinding behavior. We now demonstrate that the Drosophila Abl tyrosine kinase functions in the intersegmental nerve b (ISNb) motor choice point pathway as an antagonist of the RPTP Dlar. The function of Abl in this pathway is dependent on an intact catalytic domain. We also show that the Abl phosphoprotein substrate Enabled (Ena) is required for choice point navigation. Both Abl and Ena proteins associate with the Dlar cytoplasmic domain and serve as substrates for Dlar in vitro, suggesting that they play a direct role in the Dlar pathway. These data suggest that Dlar, Abl, and Ena define a phosphorylation state-dependent switch that controls growth cone behavior by transmitting signals at the cell surface to the actin cytoskeleton.