MITOCHONDRIA NEAR RIBBON SYNAPSES IN INNER EAR HAIR CELLS

Abstract

Mitochondria, generally referred to as the “powerhouses” of a cell, sustain life in eukaryotes by producing ATP. These structures, once thought to be bacterial symbionts, are now considered one of the most crucial organelles in the cell (van der Giezen, 2011). The mitochondrial inner compartment consists of two components, the inner membrane and the cristae. Recent EM tomography studies have shown us that these two membrane domains are connected by small tubular structures known as crista junctions (CJs). CJs “have been proposed to regulate the dynamic distribution of proteins, lipids, and soluble metabolites between mitochondrial subcompartments” (Zick et al. 2009). In our research, 3D models of mitochondria from vestibular hair cells were constructed from EM tomograms to understand the structural role of the cristae in neuronal activity at ribbon synapses. The cristae in the mitochondria serve as a site for a majority of the enzymes responsible for cellular respiration and ATP production, therefore playing an essential role in cellular activities, such as transmitting neuronal signals. A ribbon synapse is known to be a “‘high capacity docking site’ of synaptic vesicles that provide the fusion sites for ‘active zones’” (Schmitz et al. 1996). Synaptic ribbons are found in all hair cells. These synapses and the vesicles associated with them rapidly release neurotransmitters that carry messages between cells. These vesicles must be powered by an energy source such as ATP in order for them to transport and release neurotransmitters at the synaptic cleft. The objective of this study is to test the hypothesis that ribbon synapses require a large amount of energy to provide the power to transmit signals across synapses and that this energy is provided by mitochondria, which produce and contain large amounts of ATP. Cristae present in the mitochondrion, especially those in close proximity to synaptic ribbons, are oriented perpendicular to the ribbon and polarized towards the ribbon and its associated synaptic vesicles. By creating a 3D model of mitochondria near the synaptic ribbon, we have observed that multiple cristae in a single cell can provide the necessary energy. From our model, it is inconclusive about whether CJs are more prevalent on the side adjacent to the synaptic ribbon in vestibular hair cells.Support or Funding InformationSupported by NIH R21‐DC01381 (AL) and P41‐RR004050 (GP, ME).