The Department of Biomedical Engineering at Texas A&M University will host the Department of Biology’s Dr. Mark Harlow for a lecture on neurotransmitters Wednesday (Oct. 28) at 3:30 p.m. in Room 203 of the Zachry Engineering Center on campus.

Harlow’s talk, “The Structural Organization of Macromolecules Responsible for Neurotransmitter Secretion During
Synaptic Transmission,” is part of the biomedical engineering department’s seminar series.

Abstract
Research in the Harlow laboratory focuses on the structural organization of macromolecules responsible for neurotransmitter secretion during synaptic transmission. Synaptic transmission regulates virtually all aspects of animal behavior; consequently, there is considerable interest in the underlying mechanisms. My previous research concerned dense aggregates of macromolecules that play an important role in synaptic transmission at chemical synapses, and the relationship of these aggregates to synaptic vesicles (SV). These aggregates, referred to as active zone material (AZM), stud the cytoplasmic surface of the plasma membrane in the presynaptic neuron just opposite the postsynaptic cell, and are situated next to releasable SVs docked on the presynaptic membrane and calcium channels concentrated within the membrane. Because the components of AZM are composed of macromolecules, they can only be visualized in situ using the electron microscope. I employed a technique called electron tomography (ET) to study the 3D structure of AZM at the highest resolution possible.

Although AZM was first detected more than 50 years ago, its structure and function have not been elucidated. Hypotheses as to the role AZM plays in synaptic transmission have included: a site of adhesion for pre- and post‐synaptic cells; a synaptic vesicle adhesion site; and a macromolecular complex directly involved in vesicle fusion. As a step toward making direct tests of these hypotheses, I characterized the structure of a subset of the AZM and its relationship to docked vesicles and the adjacent region of the presynaptic membrane that contains calcium channels. Specifically, I used ET to generate 3D reconstructions of tissue sections to determine the structure of specific components of the AZM and their spatial relationships at the frog’s neuromuscular junction, a model synapse. My results provide compelling evidence that the active zone material at the frog’s neuromuscular junction helps dock SVs and anchor calcium channels and that the architecture of the material provides for both a
particular spatial relationship and a structural linkage between the vesicles and the channels.

My current research project involves the study of SVs and their relationship to the AZM. In samples prepared under classic conditions, the lumens of SVs appear empty. However, numerous filaments can be seen in vesicles prepared by rapid freezing and cryo‐staining, with the filaments occupying approximately 10 percent of the lumen’s volume. These filaments, many of which are likely luminal domains of SV proteins, may help tether synaptic vesicle proteins together during vesicle recycling, and could play a role in vesicle protein organization. Indeed, the arrangement of filaments inside each vesicle appears to be constant from vesicle to vesicle, with differing orientations for docked and undocked vesicles. This raises the possibility that, much like the AZM, each vesicle contains a highly organized arrangement of proteins. In order for synaptic transmission to occur, these luminal structures may need to be in a specific orientation so that vesicle proteins can precisely align with the AZM.

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