Ionotropic Glutamate Receptors

Involvement in Autism Spectrum Disorders

      Autism Spectrum Disorders (ASDs) are developmental disorders. During development, glutamate regulates neuronal growth and synaptogenesis, effectively dictating the underlying cytoarchitecture of the brain.^[3] Significant research into ASDs has been devoted to understanding how glutamate receptors function and how their disruption might lead to disorders. IGluRs are concentrated in regions of the brain that have been implicated in ASDs including the cerebellum and hippocampus, areas responsible for motor control, spatial navigation and memory. Studies have revealed that glutamate receptor proteins are increased in the cerebellum of autistic individuals by nearly 250% and specific small nucleotide polymorphisms in IGluRs have been identified which correlate with the ASDs. Further, many people with autism have clearly visible disturbances in the anatomy of the inferior olive (IO), a small part of the brain responsible for movement coordination and maintenance of an underlying 12 Hz brain rhythm through careful regulation of glutamate signaling.^[4] A well-known mutation in glutamate receptors in the “Lurcher” mouse model has revealed that constitutively leaky glutamate receptor ion channels result in IO neuron degeneration and loss of purkinje cells, with the mice exhibiting Autism-like characteristics.^[5] Such relationships between overly active glutamate receptors leading to increased excitation/inhibition ratios and autism have led some to propose using glutamate receptor inhibitors as a means of pharmaceutical intervention for improving those with autistic symptoms.^[6] Many pharmacological agents that reduce neural excitation, such as benzodiazapines and anticonvulsants, are thought to potentially have therapeutic value in treating autistic symptoms.^[3]

GluA2 Structure

      AMPA IGluRs form **homotetramers**. Each subunit includes an extracellular **amino terminal domain** (ATD) which is responsible for receptor trafficking and modulation, a **ligand-binding domain** (LBD) which activates the receptor upon binding glutamate, and a **transmembrane domain** (TMD) which forms the membrane-spanning ion channel. Also present is a carboxy-terminal domain involved in receptor localization and regulation, although the structure of this domain has not been solved.^[2]The structure of AMPA IGluRs or in this case GluA2, is unique in that the symmetry of the receptor changes depending on the domain. The ATD has a local two-fold symmetry, the LBD has a two-fold symmetry, while the TMD has a four-fold symmetry. This symmetry mismatch has implications for function of the receptor with subunits behaving differently depending upon their orientation despite identical primary sequence.^[2] For an excellent analysis, see: WAYNES PAGE

The Amino Terminal Domain

      **The ATD** is responsible for receptor assembly, trafficking and localization. It has two unique sets of interactions which hold the tetramer together. The **first set of interactions** is present in each pair of dimers and includes residues >…>>>>>>>. The **second set**, which includes residues >>>>>>>>>>>>, effectively holds the pair of dimers together at an angle that is roughly 24 degrees off of the overall two-fold axis.^[2][7]

The Transmembrane Domain

      **The TMD** has a pore structure that is nearly identical to that of the Potassium Channel. With complete four-fold symmetry, 16 helices form a **precise pore** through which cations can flow through. In the current, inhibitor bound structure, the M3 helices cross at a highly conserved **SYTANLAAF motif**, with Thr 617, Ala 621, and Thr 625 **occluding the ion permeation pathway**.^[2] The **narrowest part** of the channel includes the residues Met 629, Thr 625, Ala 621, and Thr 617. Located next to this narrow region lies **Alanine 622**, which is replaced with a threonine in the Lurcher mouse model mentioned previously. This mutation, which introduces a significantly bulkier residue, destabilizes the tight helix crossing associated with the closed state of the receptor, resulting in a constitutively open ion channel. ^[2]

The Ligand Binding Domain

      **The LBD** is located just above the TMD. It has an overall two-fold axis of symmetry with two major groups of interactions which hold the homotetramer together. The **first set of interactions** which holds each dimer together includes residues >>>>>>>>>>>>>>. The **second set of interactions** includes residues >>>>>>>>>>>>>, holding the two dimers in their tetrameric position. Within each LBD lies the so-called **“clamshell”**. This structure is responsible for binding glutamate and “sensitizing” the receptor to allow passage of cations through the channel. Residues >>>>>>>>>>>>, which are responsible for **tightly binding glutamate** within the clamshell, are highly conserved. Glutamate binding causes a conformational change in the LBD which pulls the M3 helices in the TMD apart, opening the channel and allowing for cation passage. A morph of the conformational change in the LBD upon glutamate binding can be **seen here**. Uniquely, due to the varied importance of the homotetramer subunits due to symmetry mismatch, the interaction of glutamate with the **distal subunits** is predicted to result in a greater conformational change and thus plays a more critical role in channel sensitization and activation.^[2]

Pharmaceutical Relevance

      As mentioned previously, extensive investigation into the pharmaceutical potential of IGluRs as a target for treating various ailments including Autism Spectrum Disorders symptoms is ongoing. In addition to agents which reduce neural excitation such as benzodiazapines and anticonvulsants, small molecules that potentiate AMPA receptor currents have been proven to relieve cognitive deficits caused by neurodegenerative diseases such as Alzheimer’s Disease.^[3] Modulators such as aniracetam and CX614 **bind on the backside** of the ligand-binding core through interactions with a proline ceiling and a serine floor, stabilizing the closed-clamshell conformation. Although these compounds would likely be ineffective in the case of Autism patients because they slow the deactivation of the IGluR channels, this class of compounds has exciting therapeutic potential. ^[1]