mGlu5 metabotropic glutamate receptors play a key role in mechanisms of activity-dependent synaptic plasticity underlying learning and memory processes, and are candidate drug targets for neurological and psychiatric disorders. Abnormalities in the expression or function of mGlu5 receptors are associated with most CNS disorders including autism spectrum disorders. mGlu5 receptor signaling proceeds through the hydrolysis of polyphosphoinositides (PI), a transduction mechanism that stimulate intracellular Ca2+ release and activation of protein kinase C as a result of inositol-1,4,5-trisphosphate (InsP3) and diacyglycerol formation, respectively. So far, mGlu5 receptor signaling has been assessed in brain slice preparation or cultured neurons by measuring the accumulation of radioactive inositolmonophosphate (InsP, the final product of InsP3 metabolism) in response to receptor activation with orthosteric agonists. These method is informative but not accurate and cannot be applied to the assessment of mGlu5 receptor signaling in living animals under physiological and pathological conditions. We have preliminary data indicating that mGlu5 receptor-mediated PI hydrolysis can be stimulated in vivo by treating animals with a brain permeant mGlu5 receptor PAM (positive allosteric modulator) combined with lithium salts (to inhibit the conversion of InsP into free inositol). The accumulation of non-radioactive InsP can then be measured in all CNS regions of individual animals by an ELISA method. Using this method we will perform a functional mapping of mGlu5 receptors in the mouse CNS during development and in the adult life and will examine how memory formation affects mGlu5 receptor-mediated PI hydrolysis in living mice. In addition, we will assess mGlu5 receptor signaling in the CNS of mutant mice modeling Fragile-X and Angeman's syndrome, two forms of monogenic autism which are characterized by abnormalities in mGlu5-dependent synaptic plasticity.
Our current knowledge of mGlu5 receptors is largely based on electrophysiological studies and on studies of receptor expression or receptor interaction with scaffolding and adaptor proteins. So far, the study of receptor signaling (polyphospinositide hydrolysis and the resulting intracellular signaling cascade) has been carried out using cultured neurons or brain slice preparation. The present project offers for the first time the chance to assess mGlu5 receptor signaling in living organisms at different stages of development and during the execution of memory tasks, and in mouse models of monogenic autism. This will allow to establish for the first time how mGlu5 receptors precisely behave during memory formation and in CNS disorders characterized by severe cognitive dysfunction. The expiated results will be of fundamental importance in the mGlu receptor field, taking into account that drugs selectively targeting mGlu5 receptors are under clinical development for the treatment of a variety of CNS disorders, including monogenic autism. We can easily predict that scientists of the mGlu receptor community will benefit of the findings generated by the project and will receive all necessary informations to apply this novel method of assessment of mGlu5 receptor signaling to their research. The project represents an advancement with respect to the current status of the art.