Diffusible molecules of the Slit family inhibit midline crossing by axons and neurons that express Robo receptors. For example, migrating inferior olive (IO) neurons extend a leading process across the midline, but the somata stop upon reaching the floor plate, which expresses Slits; the leading process forms the axon. Robo3 knock-out prevents midline crossing by the leading process, suggesting that Robo3 may interfere with repulsive signaling by other Slit–Robo pairs. To test this hypothesis, Di Meglio et al. knocked out Slits and Robos individually and in combination. As expected, IO somata crossed the midline in Slit1/2 and Robo1/2 knock-outs, confirming that these proteins normally repel neurons. Unexpectedly, however, axons failed to cross the midline in Robo1/2/3 triple knock-outs, indicating that Robo3 actively promotes crossing, rather than simply interfering with Robo1/2 signaling. In addition, the patterning of IO subnuclei was disrupted in knock-outs, suggesting an additional role for Slits and Robos.

Soon after an individual becomes infected with HIV the virus infects cells in the brain and spinal cord (the central nervous system [CNS]).  Although this causes no immediate problems, during the late-stages of disease it can cause dementia and encephalitis (acute inflammation of the brain that can cause death).  Monkeys infected with a relative of HIV (SIV) also sometimes develop CNS damage and provide a good model of CNS disease in individuals infected with HIV.  Insight into the mechanisms of CNS damage in SIV-infected monkeys has now been provided by a team of researchers at The Scripps Research Institute, La Jolla, who developed an approach to identify molecular changes in the fluid bathing the CNS (the CSF).  The researchers, who were led by Howard Fox and Gary Siuzdak, hope that similar approaches could be used to provide new information about other neurodegenerative and neuropsychiatric disorders.

In the study, an approach known as global metabolomics was used to assess the levels of molecules known as metabolites in the CSF before and after SIV-induced encephalitis was manifest.  The level of a number of metabolites, including some known as fatty acids and phospholipids, was observed to increase during infection.  Consistent with this, a protein known to be important in the generation of fatty acids was found to be increased in the brain of monkeys with SIV-induced encephalitis.  Further studies will be required to determine the precise role of the increased level of each metabolite, but it should be noted that many of them are known to induce receptor signaling and thereby might be able to further modulate CNS function.

A new mouse model for the study of Inclusion Body Myositis (IBM), a type of muscular dystrophy, has been developed by Dr. Ze’ev Ronai and a worldwide team of researchers.  The protein RNF5 is over-produced in the mice, resulting in extensive muscle damage similar to that seen in IBM patients.  The IBM mouse model will allow researchers to further study the mechanisms underlying development of the disease, as well as test potential new therapies.

A group of scientists at Yale have now provided a glimpse of the ancient mechanism that helped diversify our genomes; it illuminated a relationship between gene processing in humans and the most primitive organisms by creating the first crystal structure of a crucial self-splicing region of RNA.

The genes of higher organisms code for production of proteins through intermediary RNA molecules. But, after transcription from the DNA, these RNAs must be cut into pieces and patched together before they are ready for translation into protein. Stretches of the RNA sequence that code for protein are kept, and the intervening sequences, or introns, are spliced out of the transcript.

This work, published in Science, highlights a 16-year quest by Anna Marie Pyle, the William Edward Gilbert Professor of Molecular Biophysics & Biochemistry at Yale, and her research team into the nature of “group II” introns, a particular type of intron within gene transcripts that catalyzes its own removal during the maturation of RNA.

Group II introns are found throughout nature, in all forms of living organisms. Although much has been learned about their structure and how they work through biochemical and computational analysis, until now there have been no high-resolution crystal structures available. The resulting images have provided both confirmation of the earlier work and new information on the three-dimensional structure of RNA and the mechanism of splicing.

“One of the most exciting aspects of this work was that we did not need to do anything disruptive to these molecules to prepare them for structural analysis,” said Pyle. “The molecules showed us their structure, their active site and their activity — all in a natural state. We were even able to visualize their associated ions.”

According to Pyle, the crystal structure revealed some unexpected features — showing two sections that were most implicated as key elements of the active site and strengthening a theory that the process of splicing in humans “shares a close evolutionary heritage” with ancient forms of bacteria.

Looking to future applications of the work, Pyle said, “Group II introns hold promise in the future as agents of gene therapy. A free intron is an infectious element that is special because it targets DNA sites very specifically. We hope that further knowledge of these structures may lead to the development of new genetic tools and therapeutics.”

Citation: Science 320 , 77-82 (April 4, 2008). [DOI: 10.1126/science.1153803]