Grant reports

2007

The role of group I mGluR activation in AMPAR trafficking and enhanced LTD in the Fmr1 knockout mouse

In the brain, certain changes in the strength of neurotransmission at synapses (the sites of neurotransmitter release) are referred to as ‘synaptic plasticity’, a process thought to be involved in learning and memory. The synthesis of specific proteins is considered an important component of synaptic plasticity and FMRP – a protein that is normally abundantly expressed in neurons of the brain, but is absent in the fragile X syndrome – appears to function as a regulator of protein synthesis. FMRP is therefore thought to play an important role in synaptic plasticity.

Certain forms of synaptic plasticity, such as long-term depression (LTD), have been observed in specific regions of the brain at synapses that contain specific types of receptors on the post-synaptic membrane (on the receiving neuron), such as the α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). AMPARs appear to be the components that primarily express the plastic changes. In particular, the trafficking of AMPARs toward and away from the post-synaptic membrane is altered during LTD. In the hippocampus, a part of the brain involved in learning and memory, a form of LTD that is dependent on protein synthesis can be induced by the activation of specific receptors named group I metabotropic glutamate receptors (mGluRs). It has recently been shown that the Fmr1 knockout mouse (an experimental model of the fragile X syndrome) displays enhanced hippocampal LTD induced selectively by the activation of group I mGluRs. Some labs (including ours) have shown that group I mGluR activation with a pharmacological agent named DHPG leads to internalization (endocytosis) of AMPARs in cultured hippocampal neurons (thus, reduced AMPAR function). Endocytosis refers to the uptake of material into a cell by the formation of a membrane bound sac. Interestingly, we have found that blocking AMPAR endocytosis prevented DHPG-induced LTD.

We therefore hypothesize that, in the absence of FMRP, group I mGluR activation causes enhanced LTD and may lead to enhanced AMPAR endocytosis. The purpose of this project is to test this hypothesis in the Fmr1 knockout mouse.

Our study will help improve the current understanding of the molecular mechanisms underlying the fragile X syndrome and will potentially suggest appropriate molecular targets towards which novel therapies can be designed to efficaciously treat patients affected with this syndrome.

The role of interactive protein relations and synaptic balance in Fragile X Syndrome

The Fragile X Syndrome (FXS) is the most common inherited form of mental impairment in all races and ethnic groups. Affected individuals display a variety of intellectual deficits from learning problems to autism. FXS is caused by a loss of the FX protein (FMRP), that functions in local protein synthesis. A key advance in FXS research was the generation of a mouse model and an exciting discovery is that synaptic contacts of adult FMRP knockout (KO) mice display characteristics of an early development, indicating a deficit in synaptic maturation. Accordingly, the activity-induced increase in PSD95 –a locally synthesized scaffolding protein important for synapse maturation- is found to require FMRP, suggesting that deficits in local synthesis of PSD95 may lead to abnormal synapse development. Importantly, our work indicates the relationship of scaffolding and adhesion proteins (e.g. NLGs) to regulate synapse development and specificity. Hence, we hypothesize that altered PSD95 expression leads to an altered balance of excitatory and inhibitory synapses (E/I ratio) and that a reconstitution of this relationship will be important to rescue synaptic balance. Thus, a comprehensive bio- and immunohistochemical analysis will be performed on KO mice to determine alterations in the expression or distribution of synaptic proteins. We will also quantify the E/I ratio and test if expression of PSD95 or other synaptic proteins in cultured neurons will rescue the E/I balance. NLG over expressing mice are in frame for comparative studies and an in-vivo rescue experiment by crossbreeding.

The proposed studies will test a novel mechanism by which appropriate amounts of molecules are critical for synapse development and control of the E/I ratio. Thereby, important insights into synapse development and the relevance of the E/I ratio in FXS will be obtained. Furthermore, fundamental models of synapse development will be tested in vivo to find novel strategies in the therapeutic treatment of FXS patients.