Molecular mechanism identified for activation and desensitization of prominent neurotransmitter receptor in the brain
Scientists at the NIH have used a technique called cryo-electron microscopy to determine a molecular mechanism for the activation and desensitization of ionotropic glutamate receptors, a prominent class of neurotransmitter receptors in the brain and spinal cord. Cryo-electron microscopy is an imaging technique that enables researchers to determine protein structure and structural changes without the need for crystallization, which is necessary when structures are determined using X-ray crystallography. Glutamate receptors are located primarily in the outer membrane of nerve cells, and play a vital role in nearly all aspects of nervous system development, including learning and memory. Abnormal functioning of these receptors is associated with major neurodegenerative and psychiatric disorders, including Alzheimer's and Parkinson's diseases, stroke, epilepsy, schizophrenia and depression. Animal and laboratory studies have also found a link to aberrant function of glutamate receptors and cancer, in particular melanoma. Glutamate receptors function as channels, or pores, to allow the entry of certain ions (charged atoms or molecules) into nerve cells. When glutamate binds to these receptors, positively charged ions such as sodium and potassium—and sometimes calcium—flow into the cell. The coordinated flow of ions across nerve cell membranes is how nerve signals are generated and propagated. Understanding how to block or stimulate the activity of glutamate receptors may one day lead to the development of therapies for these and other diseases and conditions. The study by Sriram Subramaniam, Ph.D., NCI, and Mark Mayer, Ph.D., National Institute of Child Health and Development, and their colleagues at NIH, with support from FEI, Hillsboro, Oregon, was published in Nature, August 3, 2014.
A major challenge in membrane protein structural biology is the crystallization of proteins in different structural, or conformational, states. Many membrane proteins are resistant to crystallization, and trapping them in different conformational states is a challenge because crystals often contain just one of several possible states. This is especially true for glutamate receptors, which have three major conformational states: a resting state, an active state, and a desensitized state in which glutamate is still bound to the receptor but the ion channel is closed. Even more challenging, the activation and desensitization of glutamate receptors takes place on a millisecond time scale. To better understand how structural changes in glutamate receptors modulate the flow of ions across the nerve cell membrane, the investigators used drugs to trap ionotropic glutamate receptors subtypes (AMPA and kainate) in their major conformational states and then analyzed the resulting structures using cryo-electron microscopy. They found that activation of ionotropic glutamate receptors involved a corkscrew motion of the glutamate-binding region, or domain, of the receptor. This conformational change was driven by closure of the glutamate binding site. The investigators suggest that their approach to studying glutamate receptors provides a paradigm for studies of a variety of other membrane receptors and ion channels using cryo-electron microscopy.