The vascular and the nervous system are responsible for oxygen, nutrient, and information transfer and thereby constitute highly important communication systems in higher organisms. These functional similarities are reflected at the anatomical, cellular, and molecular levels, where common developmental principles and mutual crosstalks have evolved to coordinate their action. This resemblance of the two systems at different levels of complexity has been termed the "neurovascular link." Most of the evidence demonstrating neurovascular interactions derives from studies outside the CNS and from the CNS tissue of the retina. However, little is known about the specific properties of the neurovascular link in the brain. Here, we focus on regulatory effects of molecules involved in the neurovascular link on angiogenesis in the periphery and in the brain and distinguish between general and CNS-specific cues for angiogenesis. Moreover, we discuss the emerging molecular interactions of these angiogenic cues with the VEGF-VEGFR-Delta-like ligand 4 (Dll4)-Jagged-Notch pathway.

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During blood vessel formation, endothelial cells (ECs) establish cell-cell junctions and rearrange to form multicellular tubes. Here, we show that during lumen formation, the actin nucleator and elongation factor, formin-like 3 (fmnl3), localizes to EC junctions, where filamentous actin (F-actin) cables assemble. Fluorescent actin reporters and fluorescence recovery after photobleaching experiments in zebrafish embryos identified a pool of dynamic F-actin with high turnover at EC junctions in vessels. Knockdown of fmnl3 expression, chemical inhibition of formin function, and expression of dominant-negative fmnl3 revealed that formin activity maintains a stable F-actin content at EC junctions by continual polymerization of F-actin cables. Reduced actin polymerization leads to destabilized endothelial junctions and consequently to failure in blood vessel lumenization and lumen instability. Our findings highlight the importance of formin activity in blood vessel morphogenesis.

BackgroundLong-distance axonal growth relies on the precise interplay of guidance cues and cell adhesion molecules. While guidance cues provide positional and directional information for the advancing growth cone, cell adhesion molecules are essential in enabling axonal advancement. Such a dependence on adhesion as well as guidance molecules can be well observed in dorsal commissural interneurons, which follow a highly stereotypical growth and guidance pattern. The mechanisms and molecules involved in the attraction and outgrowth towards the ventral midline, the axon crossing towards the contralateral side, the rostral turning after midline crossing as well as the guidance along the longitudinal axis have been intensely studied. However, little is known about molecules that provide the basis for commissural axon growth along the anterior-posterior axis.ResultsMDGA2, a recently discovered cell adhesion molecule of the IgCAM superfamily, is highly expressed in dorsolaterally located (dI1) spinal interneurons. Functional studies inactivating MDGA2 by RNA interference (RNAi) or function-blocking antibodies demonstrate that either treatment results in a lack of commissural axon growth along the longitudinal axis. Moreover, results from RNAi experiments targeting the contralateral side together with binding studies suggest that homophilic MDGA2 interactions between ipsilaterally projecting axons and post-crossing commissural axons may be the basis of axonal growth along the longitudinal axis.ConclusionsDirected axonal growth of dorsal commissural interneurons requires an elaborate mixture of instructive (guidance) and permissive (outgrowth supporting) molecules. While Wnt and Sonic hedgehog (Shh) signalling pathways have been shown to specify the growth direction of post-crossing commissural axons, our study now provides evidence that homophilic MDGA2 interactions are essential for axonal extension along the longitudinal axis. Interestingly, so far each part of the complex axonal trajectory of commissural axons uses its own set of guidance and growth-promoting molecules, possibly explaining why such a high number of molecules influencing the growth pattern of commissural interneurons has been identified.

Filopodia are highly dynamic, rod-like protrusions that are found in abundance at the leading edge of migrating cells such as endothelial tip cells and at axonal growth cones of developing neurons. One proposed function of filopodia is that of an environmental probe, which serves to sense guidance cues during neuronal pathfinding and blood vessel patterning. However, recent studies show that tissue guidance occurs unhindered in the absence of filopodia, suggesting a dispensability of filopodia in this process. Here, we discuss evidence that support as well as dispute the role of filopodia in guiding the formation of stereotypic neuronal and blood vessel patterns.

that members of the FGF receptor (FGFR) family are expressed in the diencephalon (bordering this turn site) throughout the period of optic tract development, suggesting that FGF signalling in the neuroepithelium could maintain the pattern of guidance factors for axons in the developing optic tract. Furthermore, we found that exposing the developing optic tract and neuroepithelium to an FGFR inhibitor (SU5402) just prior to when RGC axons reach the turn site in the diencephalon has two effects: (1) RGC axons stall at the turn in the mid-diencephalon, and (2) the patterning in the diencephalon, adjacent to the turn site, is specifically and rapidly disrupted (as evidenced through in situ hybridization of postmitotic transcription factors, xLhx9, xLhx2, xLhx1, xLhx5, and xDll3). This suggests that FGF signalling is required both for RGC axons to navigate this guidance decision and to maintain the pattern of expression in the surrounding tissue. Moreover, the rapidity of this effect argues that the patterned cues required to guide axons in the developing optic tract are not stably expressed in that they must be maintained with the continued presence of signalling molecules.

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