By David N. Leff
One perk in the life of a fruit fly geneticist is being able to name the genes you discover. This indulgence has led to a constant accretion of whimsical monikers adorning the genome of Drosophila melanogaster. Witness: diaphanous, indy, hedgehog, patched, notch, rover, sitter, slit, which all purport to describe the insect¿s genes.
Consider slit: It was reported in 1984 by the German drosophilosopher, Nobelist Christiane Nuesslein-Volhard.
¿A phenotype at the fine midline of Drosophila¿s nervous system defines the term slit,¿¿ observed neurobiologist Yi Rao at Washington University in St. Louis. ¿It¿s involved in the development of the insect¿s embryonic central nervous system. (See BioWorld Today, July 22, 1999, p. 1.)
Rao is senior author of a paper in today¿s issue of Nature, dated April 18, 2001. It¿s titled: ¿The neuronal repellant Slit inhibits leukocyte chemotaxis induced by chemotactic factors.¿
¿That title,¿ Rao told BioWorld Today, ¿tells the essence of the finding we report ¿ that Slit, a secreted protein previously known for its role of repulsion in axon guidance and neuronal migration, can also inhibit leukocyte migration induced by chemotactic factors.¿
After a cell is born, it navigates to its destination, guided by signals from other molecules already in place. The nervous system uses molecules that attract migrating cells, molecules that stop cell migration and molecules that push cells away. But so far, only attractant molecules have been identified in the immune system.
Clinical Therapies In Prospect
¿This similarity between the immune and nervous systems,¿ Rao continued, ¿implies new therapeutic approaches to treat diseases that involve leukocyte migration to various areas of the body. For example, most, if not all, inflammations, and some tissue damage, involve leukocyte trafficking ¿ chemotaxis ¿ to the inflamed sites. So if you can inhibit leukocyte chemotaxis, you might be able to treat some inflammatory and autoimmune disorders ¿ arthritis, kidney inflammation, asthma ¿ which invoke that chemotaxis. Now we can inhibit this white-cell migration, and speculate that one might treat these disorders.¿
Taking asthma as his example, Rao traced the route that the Slit protein ¿ like fire engines answering a fire alarm ¿ negotiate chemotactically.
¿It depends on the disease,¿ he explained. ¿In the inflammatory case of asthma, an asthmatic attack would have to get the leukocyte out of its normal residence in the blood vessel and proceed to the inflamed endothelial surface of the trachea and other respiratory airways. Its route depends on where it¿s headed.¿
Switching examples, Rao went on: ¿For atherosclerosis, there¿s also involvement of leukocytes attracted to the endothelial surface of the coronary arteries, in which chemokines are involved. So if we can inhibit the function of those chemotactic chemokines, the potential exists for treating ¿ preventing ¿ coronary heart disease.
¿What we have shown,¿ he went on, ¿is that the Slit protein also affects immune system cell types, including leukocytes, lymphocytes, monocytes and macrophages. The classical model is that these are attracted by attractant molecules, specifically the chemokines. And that¿s how they get there. So what we are showing is that we have something that inhibits leukocyte response to chemokines.
¿There are at least three Slit proteins in humans,¿ Rao pointed out. ¿Two of the three have been found in organs other than the brain. Their function is to regulate neuronal migration of axonal nerve fibers. What we are showing here is that they also regulate migration of immune system white blood cells, which are expressed in different regions of the body. Functionally, we don¿t know precisely what the differences among them are. We would like to know. The chemokines have to act through their receptors,¿ Rao went on. ¿Slit¿s receptor, Robo, is the same molecule for all three.¿
Rao and his co-authors revealed Slit¿s alter ego ¿ repelling immune cells as well as propelling neurons ¿ via a series of in vitro experiments.
Cell Culture Proved Point
¿In a two-chamber set-up,¿ he recounted, ¿we put leukocytes in the upper chamber, and chemotactic factor ¿ chemokine ¿ in the lower one. The cells in the upper chamber migrated to the lower chamber. But then, when we added Slit in the upper chamber, it inhibited that reaction. The leukocyte would not respond to the chemokines. So that¿s the essence of the in vitro experiment we described in the Nature paper.¿
Rao¿s unpublished in vivo data were cited only in passing. ¿We used a glomerular nephritis model of an autoimmune disease,¿ he recounted. ¿In a rat with that kidney disease, an injection of the human Slit protein attenuated the autoimmune inflammation.¿
Advancing this Slit protein from lab bench to bedside will require further animal studies. ¿These preclinical trials are now in progress,¿ Rao said. ¿Our collaborators are doing them here at Washington University¿s neurobiology lab and the immunology lab at Baylor College of Medicine in Houston. The final therapeutic would take the form of a drug,¿ he suggested, ¿say, to treat inflammation, for example. After it¿s seen to work in animals, trying it in humans will take some time.¿
Rao¿s paper in Nature ends with the observation, ¿Because the inhibition of receptor signaling can block HIV infection, Slit inhibition of chemokine receptors including CXCR4 [and CCR5; data not shown] suggests that Slit cannot be ruled out as a useful reagent in inhibiting HIV infection; thus, the therapeutic potentials for Slit and other negative factors are an attractive area of further research.¿
While characterizing this HIV suggestion as ¿speculation upon speculation,¿ Rao added, ¿There are a few papers by immunologists who say: If you block the signal, you may also block or inhibit HIV infection.¿ I¿m not sure how widely accepted that idea is,¿ he observed, ¿but if it is true then there is also a possibility for our Slit protein to treat HIV.¿
¿The university has applied to patent the functional role of Slit,¿ Rao noted, and concluded: ¿Inhibitors of chemokines have been of great interest to some biotech companies, which want to treat inflammation. We are not yet in touch with them; we hope they¿ll be in touch with us.¿