Mechanisms of MHCI Signaling in Neurons: Characterization of Protein Interaction Motifs and the Interaction with Insulin Receptors

Abstract

Despite their function as antigen-presenting molecules in the adaptive immune response, proteins of the major histocompatibility complex class I (MHCI) play a significant role in normal brain development. These cell surface proteins are expressed by healthy neurons and are localized at the synapse, where they are involved in synaptic transmission and plasticity, two fundamental features of learning and memory. MHCI molecules are required for maintaining synaptic transmission in the hippocampus, where they inhibit NMDA receptor function and AMPA receptor trafficking. In the visual system, MHCI regulates the balance between synapse formation and elimination during development that leads to precise neural circuitry in the mature brain. A variety of proteins occur at the synapse that mediate similar functions, and it is unclear how MHCI may interact with these other proteins to execute its neuronal functions. This thesis addresses two aspects of this question. First, it seeks to determine a mechanism of protein interaction for MHCI, due to putative protein interaction motifs recognized in other proteins to facilitate binding with the PDZ domains of scaffold proteins. These interaction motifs (PDZ ligands) are found among highly conserved sequences in the intracellular domain of MHCI, and mutating specific residues in these motifs will determine their functional importance for potential interactions with known regulators of NMDA receptor function and synaptic transmission. Secondly, this thesis seeks to determine the nature of interaction with neuronal insulin receptors (IRs), which are required for the maintenance of synaptic connections in the Xenopus visual system. MHCI associates with IRs in neurons and may affect synapse density through the regulation of insulin signaling, but the mechanism of regulation is unknown and may be determined by manipulating MHCI function in cortical neurons. This information is important for understanding the pathways undertaken by MHCI in promoting normal brain development.