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Discovery

Slit mutations were first discovered in the Nuesslein-Volhard/Wieschaus patterning screen where they were seen to affect the external midline structures in the embryo of Drosophila. In this experiment, the researchers screened for different mutations in Drosophila embryos that affected the neural development of axons in the central nervous system. They found that the mutations in commissureless genes (Slit genes) lead to the growth cones that typically cross the midline remaining on their own side. The findings from this screening suggests that Slit genes are responsible for repulsive signaling along the midline.

Structure

Each of the three types of Slit protein have the same basic structure. A major identifying feature of the Slit protein is the four leucine-rich repeat (LRR) domains and the N-terminus. Slits are one of only two protein families that contain multiple LRR domains. These LRRs are followed by six epidermal growth factor-like (EGF) repeats and a laminin G-like β-sandwich domain. Attached to the this, invertebrates have one EGF repeat, whereas vertebrates have three EGF repeats. In each case, the EGF is followed by a C-terminal cystine knot (CT) domain. er, S. Hussain, and J.A. Howitt, 2006

Slit LRR domains assist in controlling neurite outgrowth. The domains consist of five to seven LRRs each with disulphite-rich cap segments. Each LRR contains a LXXLXLXXN sequence which is one strand to a parallel β-sheet on the concave face of the LRR domain, while the back side of the domain consists of irregular loops. Each of the four domains of Slit are connected by short "linkers" which attach to the domains via a disulphide bridge, allowing the LRR region of Slit to remain very compact.

It is possible for Slits to be cleaved into fragments of the N-terminal and C-terminal as a result of an assumed proteolytic site between the fifth and sixth EGFs in Drosophila SLIT, Caenorhabditis elegans SLIT, rat SLIT1, rat SLIT3 and human SLIT2.

Function

Slit proteins were originally thought to repel neural guidance during development, but further investigation has proved that they have additional important functions. Some of these functions are performed in the vasculature and immune system, and there are implications for the reproductive system as well. Steroid hormones have been found to regulate Slits and adult cell functions in reproductive tissues. Further, Slits are expressed in cancerous cells in the reproductive system.

Abnormalities or absences in the expression of Slit1, Slit2 and Slit3 are associated with reproductive and hormone dependent cancers, particularly in females. Along with their function in tissue morphogenesis, these genes act as tumor suppressors. Therefore, deletion or lack of expression of these genes is associated with the development of certain tumors, particularly tumors within the epithelium of the ovaries, endometrium, and cervix (all from Dickinson and Duncan). In addition, absence of these genes allows the migration of cancer cells and thus is associated with increased cancer progression and increased metastasis. The role of this gene in cancer is becoming increasingly unraveled but increasingly complex.

The Slit2 protein has recently been discovered to be associated with the development of new blood vessels from pre-existing vessels (angiogenesis). Recent research has debated on whether this gene inhibits or stimulates this process. There has been significant proof to conclude that both are true, depending on the context. Acting solely, Slit2 has been shown to promote angiogenesis in mice (both in vitro and in vevo). However, presence of the receptor tyrosine kinase, EphA2, and its primary ligand, ephrin-A1, inhibits Slit2 vascular remodeling.

Chemorepellents help to direct growing axons toward the correct regions by directing them away from inappropriate regions. Slit genes, as well as their roundabout receptors, help prevent the wrong types of axons from crossing the midline of the central nervous system during establishment or remodeling of the circuits. The binding of Slit to any of the Roundabout receptor family results in axon repelling through changes in the axon growth cone. Three slit genes, slit 1, slit 2, and slit 3, have been cloned in mammals. Slit1 and slit2 have both been seen to collapse and repel olfactory axons. Positive effects are also correlated with slits. Slit 2 begins the formation of axon branches through neural growth factor genes of the dorsal root ganglia.

Subtypes

Slit1, Slit2, and Slit3 are all a human homologs of the 'slit' gene found in Drosophila. Each of these genes secrets a protein containing protein-protein interaction regions with leucine-rich repeats and EFGs. Slit2 is mainly expressed in the spinal cord, where is repels motor axons. Slit1 functions in the brain, and Slit3 in the thyroid. Both Slit1 and Slit2 are found in the murine postnatal septum as well as in the neocortex. Further, Slit2 also participates in inhibiting leukoctye chemotaxis. In rats, Slit1 was found in the neurons of adult and fetal forebrains. This shows that Slit proteins in mammals most likely contribute in forming and maintaining the endocrine and nervous systems through interactions between proteins. Slit3 gene is primarily expressed in the thyroid, as well as in human umbilical vein endothelial cells as well as endotheliel cells from the lung and diaphragm of the mouse. Slit3 interacts with ROBO1 and ROBO4.

References