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Talia Kaden, Jennifer Du, Anurag Goel, David Lu, Wei Li; Structural and Functional Properties of Photoreceptor Mitochondria in Awake and Hibernating Ground Squirrels. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6082.
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Photoreceptors are high energy-consuming cells. Their functionality therefore relies heavily on mitochondrial activities. Failure of mitochondria to adapt to metabolically challenging conditions has been implicated in many photoreceptor diseases. We are interested in identifying strategies adopted by photoreceptor mitochondria in the hibernating ground squirrel retina which enable it to cope with drastic changes in metabolic conditions and yet emerge without any evident damage to their tissues or organ systems. Thus, we set out to investigate changes in mitochondrial structure, enzyme activity and fusion/fission dynamics in these animals.
Retinal tissues from both hibernating and awake animals were imaged using serial blockface scanning electron microscopy with an x-y resolution of 7nm and z resolution of 30nm. Individual mitochondria were traced manually and analyzed using Reconstruct software. BN-PAGE was used to measure Complex V activity in the retinal mitochondria, while spectrophotometric cytochrome quantitation was used to quantify Cytochrome A concentration.
We reconstruct every mitochondria in individual cone photoreceptors. Our results thus far demonstrate that the hibernating photoreceptors have a significant increase in the number of mitochondria. However, both the surface area and the volume of these mitochondria are markedly reduced. When the mitochondria are categorized by the number of obvious fusions, the “solitary” mitochondria, those with limited fusion with their neighbors, have the largest relative increase between the hibernating and awake animals. Preliminary studies of the mitochondrial enzymatic activity indicate that while kinetics of the Complex V activity in the hibernating animals remain the same, there appears to be a decrease in Cytochrome A content.
Our initial results suggest that the hibernating animals have either an increased amount of fission or a decreased rate of fusion in their mitochondria. This may be the result of a decrease in the expression of fusion proteins such as OPA1, MFN1/2 or other large GTPases. The overall mitochondria function is down regulated by changing the Cytochrome content rather than regulating enzymatic activities.
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