Molecular Genetics Of Early Eye Development

National Eye Institute

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Abstract

Olfactomedin 1 (Olfm1), Olfm2 and Olfm3 (also known as Noelin1-3) are secreted glycoproteins expressed throughout the brain and retina in a cell type-specific manner. Noelin1-3, together with Neuritin, Brorin (Vwc2) and Brorin-like (Vwc2l) proteins, are extracellular components of the AMPA receptor (AMPAR) complex. AMPARs are the major ionotropic glutamate receptors within both the brain and the retina that play a major role in synaptic formation and communication. We used Olfm1, Olfm2, Olfm3, and Olfm1-3 triple knockout (KO) mice as well as Vwc2 and Neuritin KO mice to study the impact which the removal of these extracellular proteins have on AMPAR kinetics and function. In collaboration with Dr. Faklers laboratory (University of Freiburg, Germany), we found that distribution and dynamics of AMPARs in the plasma membrane are controlled by Noelins. Noelin tetramers tightly assemble with the extracellular domains of AMPARs and interconnect them in a network-like configuration with a variety of secreted and membrane-anchored proteins including Neurexin1, Neuritin1, and Seizure 6-like. Moreover, elimination of Noelins led to complete removal of Brorin and Neuritin from the AMPAR complex. Noelin1 may directly interact with GluA1-4 core subunits of AMPARs. Knockout of Noelins1-3 profoundly reduced AMPARs in synapses onto excitatory and inhibitory (inter)neurons, decreased their density and clustering in dendrites and abolished activity-dependent synaptic plasticity. Our data suggest that in Noelin-deprived neurons additional AMPARs may be initially inserted into the post-synaptic membrane where they cannot be stably trapped and are subsequently removed likely by endocytic pathway(s). In such a model, the Noelins and their associated networks are required for robust (post-synaptic) long term potentiation and, thus, information storage in response to increased activity. This is essential for memory formation and learning. We also identified three additional Noelin1 interacting partners in the mouse brain: Protein Kinase C-Binding Protein NELL2, G-Protein Coupled Receptor 37-Like 1, and protocadherin FAT4. Vwc2 KO demonstrated similar but less dramatic effects than Noelin1 and Noelin1-3 KO in the mouse brain. The composition of Vwc2-containing AMPARs in the retina is different from those in the brain as demonstrated by quantitative proteomics in collaboration with Dr. Faklers laboratory. Several auxiliary proteins of AMPARs were immunoprecipitated from brain but not from retinal membrane-enriched fractions using Vwc2 antibodies. In summary, our results uncover an endogenous mechanism for extracellular anchoring of AMPARs and establish Noelin-organized networks as versatile determinants of constitutive and context-dependent neurotransmission. Our data suggest that changes in the AMPAR activity induced by the elimination or mutations in their extracellular component (Noelins, Brorin) can lead to retinal pathologies.