Slit Proteins Regulate Distinct Aspects of Retinal Ganglion Cell Axon Guidance within Dorsal and Ventral Retina

Abstract

An early step in the formation of the optic pathway is the directed extension of retinal ganglion cell (RGC) axons into the optic fiber layer (OFL) of the retina in which they project toward the optic disc. Using analysis of knock-out mice and in vitro assays, we found that, in the mammalian retina, Slit1 and Slit2, known chemorepellents for RGC axons, regulate distinct aspects of intraretinal pathfinding in different regions of the retina. In ventral and, to a much lesser extent, dorsal retina, Slits help restrict RGC axons to the OFL. Additionally, within dorsal retina exclusively, Slit2 also regulates the initial polarity of outgrowth from recently differentiated RGCs located in the retinal periphery. This regional specificity occurs despite the fact that Slits are expressed throughout the retina, and both dorsal and ventral RGCs are responsive to Slits. The gross morphology and layering of the retina of the slit-deficient retinas is normal, demonstrating that these distinct guidance defects are not the result of changes in the organization of the tissue. Although displaced or disorganized, the aberrant axons within both dorsal and ventral retina exit the eye. We also have found that the lens, which because of its peripheral location within the developing eye is ideally located to influence the initial direction of RGC axon outgrowth, secretes Slit2, suggesting this is the source of Slit regulating OFL development. These data demonstrate clearly that multiple mechanisms exist in the retina for axon guidance of which Slits are an important component.

Expression of multiple class three semaphorins in the retina and along the path of zebrafish retinal axons

“…There are multiple factors known to be involved in intraretinal axon pathfinding, including netrin-1, slit, and sonic hedgehog (Deiner et al, 1997;Kolpak et al, 2005;Thompson et al, 2006). These factors comprise an expanding group of molecular cues known to be responsible for the guidance of RGC axons along the optic fiber layer, toward the optic disc and out of the retina.…”

Differential Role of Slits in Dorsal versus Ventral Retinal Axon Guidance: Figure 1.

“…The results suggest that Slit1 and Slit2 cooperate to maintain RGC axons in the inner surface of the retina. Strikingly, ectopic RGC axon bundles appear to form preferentially (greater than a 2:1 ratio) in the ventral rather than the dorsal part of the retina [Thompson et al (2006a), their Fig. 3 A, Ϫ/Ϫ and slit1/2 Ϫ/Ϫ organs were intermingled.…”

“…In 1963, Roger Wolcott Sperry performed his eyeball rotation experiment, still taught to almost every student of neuroscience and developmental biology, which showed that growing axons are guided by chemical factors in their environment. Sperry's (1963) chemoaffinity hypothesis spurred a series of studies to investigate the role of axon guidance molecules in the development of the visual system, including the one that is the focus of a recent article in the Journal of Neuroscience by Thompson et al (2006a). The authors addressed the role of Slit proteins in intraretinal axon guidance, the mechanisms by which axons growing from retinal ganglion cells (RGCs) form a layer at the internal surface of the retina, converging toward the optic disc.…”

“…Previous studies using in situ hybridization, a technique that allows identification of Slit expression in cell bodies but not fibers (the vast majority of mRNAs being restricted to the soma), demonstrated that both genes are expressed in the RGC layer of the retina (Erskine et al, 2000), but the identity of the expressing cells was not clear. The observation that GFP immunoreactivity is evident all along the RGC axon paths [Thompson et al (2006a) performed GFP-staining on longitudinal and transversal sections of retinas obtained from slit1-and slit2-deficient animals and analyzed them by confocal microscopy. Interestingly, they identified two distinct defects in these sections (Fig.…”

“…jneurosci.org/cgi/content/full/26/31/ 8082/F2), 3 (http://www.jneurosci.org/ cgi/content/full/26/31/8082/F3)] consists of the occurrence of thick bundles of RGC axons mislocalized to the outer layers of the retina, opposite to the inner surface were RGC axons normally navigate. The defect is virtually absent from both wild-type and slit1 Ϫ/Ϫ retinas, but occurs with moderate (an average of five bundles per retina) and high frequency (over thirty bundles) in slit2 Ϫ/Ϫ and slit1/2 doublemutant retinas, respectively [Thompson et al (2006a), their Fig. 3C (http://www.…”