Stop or go in the target zone.

The development of the multilayered cellular architecture found in many regions of the CNS requires axons originating in one layer or locale to terminate their growth with specificity in the layers that harbor their postsynaptic targets. In the developing chick visual system, for example, retinal ganglion cells of distinct subtypes send their axons to subtype-specific layers of the tectum (Yamagata and Sanes 1995xYamagata, M. and Sanes, J.R. Development. 1995; 121: 3763–3776PubMedSee all ReferencesYamagata and Sanes 1995). A similar pattern of connectivity is found in the visual system of Drosophila, where photoreceptor axons, according to their cell type, terminate in either of two ganglion layers within the brain’s optic lobe. The six outer photoreceptors, R1–R6, of a single ommatidial unit of the fly compound eye terminate their axonal ingrowth within the brain’s most superficial layer, known as the lamina. The remaining two photoreceptors of an ommatidial unit, R7 and R8, extend beyond the lamina into a deeper layer known as the medulla. The specificity of outer photoreceptor axon termination in the lamina provides an attractive opportunity to elucidate the local mechanisms controlling a growth cone’s decision to stop or go.The eight axons from an ommatidial unit in the eye travel into the brain together as part of an ommatidium-specific fascicle (see the schematic diagram). Studies of photoreceptor axon elongation in larger insects reveal that transit along an ommatidial fascicle follows the temporal pattern of photoreceptor cell differentiation within the ommatidial unit in the eye (reviewed by Meinertzhagen and Hanson 1993xSee all ReferencesMeinertzhagen and Hanson 1993). The growth cone from the initial R8 cell can be thought to pioneer the path to a retinotopic target destination, while the growth cones of R2 and R5, R3 and R4, R1 and R6, and R7 follow in sequence along the expanding ommatidial fiber. Whether the R8 axon is an essential pioneer for the fiber, and whether the other photoreceptor axonal types make the navigational decisions required of the R8 growth cone, is unclear. However, because the R8 axons grow into the deeper medulla layer, the R1–R6 growth cones must make the common decision to stop in the more superficial lamina layer. One could imagine a switch that terminates R1–R6 axonal extension, employing signals to which the R7 and R8 growth cones are blind. This is indeed an implication of the work reported in this issue of Neuron by Garrity et al. 1999xGarrity, P.A., Lee, C.-H., Salecker, I., Robertson, H., Desai, C., Zinn, K., and Zipursky, S.L. Neuron. 1999; 22: 707–717Abstract | Full Text | Full Text PDF | PubMed | Scopus (115)See all ReferencesGarrity et al. 1999.Garrity et al. examine the role of the receptor protein tyrosine phosphatase PTP69D in photoreceptor axon pathfinding in Drosophila. Loss of PTP69D function in the retina results in a specific defect. The R1–R6 growth cones sometimes continue beyond their lamina termination point, following the paths of R7 and R8 axons into the deeper medulla layer. Further experiments demonstrate that an extracellular region harboring fibronectin-like motifs and at least one of the two intracellular phosphatase domains must be intact for PTP69D to function in lamina-specific termination. Presumably, upon ligand binding, PTP69D dephosphorylates substrates in the R1–R6 growth cones in order to terminate growth. The substrates that are dephosphorylated and the ligand involved remain to be determined.PTP69D, like two other Drosophila receptor tyrosine phosphatases (RPTPs), Dlar and PTP99A, is in the collection of molecules with a role in the establishment of neuromuscular connectivity (reviewed by Desai et al. 1997bxDesai, C.J., Sun, Q., and Zinn, K. Curr. Opin. Neurobiol. 1997; 7: 70–74Crossref | PubMed | Scopus (65)See all ReferencesDesai et al. 1997b). These neural RPTPs are expressed on a subset of motor axons and growth cones. The motor neuron pathfinding defects in animals harboring one or multiple RPTP mutations are typically a stall or miscue at particular choice points. In Dlar mutants, the intersegmental nerve branch b (ISNb) motor axons sometimes fail to properly defasciculate from the intersegmental nerve (ISN) at the choice point leading to the ventrolateral muscle (VLM) field. The ISNb axons instead continue to grow dorsally, bypassing their VLM targets. This characteristic “bypass” phenotype is strongly enhanced by the additional loss of PTP69D function (Desai et al. 1997axDesai, C., Krueger, N.X., Saito, H., and Zinn, K. Development. 1997; 124: 1941–1952PubMedSee all ReferencesDesai et al. 1997a), suggesting an overlap in the functional roles of PTP69D and Dlar at the ISNb choice point. What is the relationship of the neuromuscular RPTP mutant phenotypes to the behavior of R1–R6 growth cones in a Ptp69D mutant? One might view the visual system defect as a “bypass” of the R1–R6 axons’ lamina termination point. Blind to a lamina signal, the R1–R6 growth cones may fail to defasciculate from the R8 pioneer fiber. The observation that the majority of R1–R6 growth cones do stop in the lamina in a strong Ptp69D mutant background is consistent with the notion that other RPTPs, such as Dlar, contribute to proper R1–R6 termination. The answer to this question awaits the analysis of animals bearing multiple RPTP mutations.A deeper connection between RPTP activity and growth cone motility is suggested by the genetic and physical interactions of Dlar, the Abl tyrosine kinase, and the Abl substrate Enabled (Ena; Wills et al. 1999xWills, Z., Bateman, J., Korey, C.A., Comer, A., and Van Vactor, D. Neuron. 1999; 22: 301–312Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesWills et al. 1999; reviewed by Hu and Reichardt 1999xHu, S. and Reichardt, L.F. Neuron. 1999; 22: 419–422Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesHu and Reichardt 1999). As with Dlar and Ptp69D, abl and ena loss of function yields defects in the dissociation of SNb motor axons from the ISN. Ena associates with the actin cytoskeleton at points of polymerization, where it may, via binding to profilin, modulate actin polymerization (see, for example, Gertler et al. 1996xGertler, F.B., Niebuhr, K., Reinhard, M., Wehland, J., and Soriano, P. Cell. 1996; 87: 227–239Abstract | Full Text | Full Text PDF | PubMed | Scopus (484)See all ReferencesGertler et al. 1996). The observation that Ena binds one of the intracellular phosphatase domains of PTP69D (Wills et al. 1999xWills, Z., Bateman, J., Korey, C.A., Comer, A., and Van Vactor, D. Neuron. 1999; 22: 301–312Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesWills et al. 1999) suggests one possible connection between the reception of a lamina stop signal and the cytoskeletal events that control the motility of the R1–R6 growth cones.While the experiments of Garrity et al. implicate PTP69D in the R1–R6 growth cone’s initial decision to stop in the lamina, nitric oxide (NO) signal reception is apparently required to keep them there (Gibbs and Truman 1998xGibbs, S.M. and Truman, J.W. Neuron. 1998; 20: 83–93Abstract | Full Text | Full Text PDF | PubMed | Scopus (168)See all ReferencesGibbs and Truman 1998). NO is synthesized by lamina cells and can induce the synthesis of cGMP by the photoreceptor axons. Under pharmacological conditions that interfere with NO signaling, the R1–R6 growth cones initially stop in the lamina but later abandon the lamina and continue on further into the brain. The relative roles of these two signaling pathways in modulating R1–R6 growth cone motility is unclear. For example, it is not known whether the continued activity of PTP69D is required once the R1–R6 growth cones become dependent on NO to remain in the lamina. A further elaboration of the intersection between these two pathways in the modulation of growth cone motility would be an exciting avenue for future studies.Figure 101Layer-Specific Targeting of Photoreceptor Axons in the Drosophila Visual System The eye and brain develop coordinately during the period of photoreceptor axon ingrowth in the larval stage. The spatiotemporal pattern of ommatidial differentiation in the eye (posterior earlier than anterior) results in a temporal pattern of photoreceptor axon arrival in the brain. In the eye, new ommatidia initiate differentiation at the anterior (left), while older ommatidia add additional cells (R1–R6 in blue). The initial R8 cell (red color) sends its axon into the brain first; its growth cone (colored red) transits the lamina en route to its medulla target (e.g., the most anterior axon shown). The outer (R1–R6) photoreceptors send their growth cones (blue color) along the expanding ommatidial fascicle. The R1–R6 growth cones travel along the R8 fiber into the lamina (the middle fascicle shown). As shown for the most posterior fascicle, the R1–R6 growth cones stop between the layers of epithelial (E-) and marginal (M-) glia (green color). The R8 growth cones meanwhile have traveled on, beyond the lamina to their deeper targets in the medulla layer.View Large Image | View Hi-Res Image | Download PowerPoint Slide