Cristae in Inner Ear Hair Cells Mitochondria are Polarized Toward Cuticular Plate

Abstract

The goal of this research is to study the structure and the functionality of mitochondria located in inner ear hair cells. According to size, mitochondria can be divided into three different sub‐types: large, medium, and small. In this particular scenario, our focus rests upon a single large mitochondrion with lamellar cristae (the high energy form of cristae) located in very close proximity to the cuticular plate (CP) in a vestibular Type 1 hair cell. One criterion we have been analyzing is the number of cristae junctions (CJs) facing the CP in comparison with the amount of CJs located on the other side facing away from the CP. CJs are tubular connections promoting a direct interaction between the mitochondrial cristae and the mitochondrial inner membrane (Rabl et al. 2009). We hypothesized that the number of CJs facing the CP is higher compared to the opposite side. This would suggest a polarization of CJs on one side, such that ATP and Ca2+ can be fed to points of interest, such as stereociliar rootlets and the CP (Perkins et al. 2010). In order to identify the desired structures, we have been using IMOD, a 3d reconstruction program developed by the Univ. of Colorado. First, we examined the mitochondrion tomogram section by section and using the software, we traced individual cristae. Doing this for every crista along with the mitochondria membranes, we then meshed it and get a 3D representation of the entire mitochondria in the tomogram. We have taken these data and constructed a short video to enable us to visualize both sides. In addition, IMOD is a tool to obtain accurate topographical data, such as crista surface area, from which we can compute energy production output. This large mitochondrion can produce about 4500 zeptomoles of ATP/sec, much larger than the average amount we have observed in other mitochondria with lamellar cristae, which is about 1000 zeptomoles/sec. This ATP, in turn, produces about 140 femtojoules of energy. After this large mitochondrion was fully reconstructed, we proceeded to analyze the number of CJs. We observed twice as many CJs facing the CP as away from the CP. In conclusion, the data obtained support our hypothesis that CJs are observed in greater amount in sites requiring more ATP.Support or Funding InformationSupported by NIH R21‐DC013181 (AL) and P41‐RR004050 (GP, ME)