28 March 2009. If a keystone was to be found at the recent symposium “Neurodegenerative Diseases: New Molecular Mechanisms,” held 17-22 February at Keystone, Colorado, it may well have been microRNAs (miRNAs). These non-coding RNAs have been cropping up almost anywhere researchers care to look, and because they can simultaneously regulate a multitude of seemingly unconnected genes and pathways, scientists are wondering if they may be a common link that spans disparate aspects of complex neurodegenerative diseases. Whether that turns out to be the case remains to be seen, but from control of Aβ production to protection from excitotoxicity to control of adult neurogenesis, miRNAs were central to many of the Keystone talks at last month’s symposium.
Alzforum readers have become familiar with miRNAs in part because of work from Bart De Strooper’s lab in KU Leuven, Belgium. This group has shown that levels of some 20 miRNAs are low in AD patients and that several of these microRNAs can theoretically bind amyloid precursor protein (APP), presenilin, or β-secretase (BACE) genes (see ARF related news story). At Keystone, De Strooper reviewed some of the evidence linking these miRNAs to BACE, the protease that kicks off amyloidogenic processing of APP. MicroRNAs miR 29a and 29b-1 are downregulated in some AD patients, and this correlates with high BACE expression. De Strooper showed that when added to cells in culture, these miRNAs downregulate BACE and Aβ production, suggesting that they could play important regulatory roles in the pathophysiology of AD.
The human genome contains at least 450 miRNAs, said De Strooper, and they regulate thousands of target genes. He suggested that they act like mini-cellular rheostats, dampening gene expression in response to external stimuli. The role of miRNAs, because they have multiple targets, can be profound. “Loss of miRNAs sets the stage for the ‘multiple hit’ hypothesis,” said De Strooper, adding that it will be important to look for other genes that may be regulated by these miRNAs in AD and in other diseases, as well.
The talk by Valina Dawson, Johns Hopkins University, Baltimore, Maryland, was germane to both miRNAs and the multiple hit hypothesis. Dawson has been working on factors that help cells survive a non-lethal, stressful stimulus. The multiple hit hypothesis suggests that several such stimuli might combine to deal a lethal blow to cells. By identifying pathways activated during non-lethal “preconditioning,” Dawson, who co-organized this Keystone symposium, hopes to identify mechanisms that could be exploited to promote neuronal survival in neurodegenerative conditions, such as Parkinson disease. She reported that one such pathway involves the transcription factor NFI-A (nuclear factor I/A). NFI/A is upregulated in primary cortical neurons exposed to stressful levels of the glutamate receptor agonist N-methyl-D-aspartate (NMDA). Dawson showed that knocking out the transcription factor weakens the preconditioning protection afforded by non-lethal doses of NMDA, blunting cell survival in the process; conversely, overexpressing NFI/A increases survival.
The NFI/A protective effects appear intimately linked to the protein’s role as a transcription factor, since cells expressing NFI/A mutants without the ability to bind DNA derive no protection from the preconditioning. Dawson also showed that the transcription factor is protective in vivo, since NFI/A+/- heterozygote mice are less resistant to NMDA toxicity and develop bigger lesions in response to the glutamate agonist.
The microRNA connection comes in because NFI/A has a binding site for miR223. Would this miRNA affect survival of neurons challenged with NMDA? In fact, that’s just what Dawson showed. Not only does overexpressing miR223 block the preconditioning effect, but preconditioning alone alters miR223 in a biphasic manner and in diametrical opposition to changes in NFI/A expression. The result suggests that the preconditioning stress stimulus is acting through this microRNA to relieve suppression of NFI/A and protect cells. However, the scenario is slightly more complicated. Dawson, in collaboration with Fernando Camargo at the Whitehead Institute, Cambridge, Massachusetts, has generated miR223 knockout animals. They are, in fact, more—not less—sensitive to glutamate toxicity. “The knockout is likely having pleiotropic effects,” said Dawson, which would be in keeping with miRNAs having multiple targets. One of miR223’s targets is, for example, a death effector protein. Dawson is planning to use microRNA sponges, transcripts that recognize and soak up miRNAs, to mop up miR223 in cells and to tease out its pleiotropic effects.
In other presentations, researchers outlined roles for miRNAs in Parkinson disease, Huntington disease, and adult neurogenesis. (See Part 2 of this story.)—Tom Fagan.