UBC Theses and Dissertations
The role of Pannexin 2 in mitochondrial functions and cell death Le Vasseur, Maxence
Pannexins constitute a small family of membrane channels homologous to the invertebrate gap junction proteins. Three distinct isoforms, called Panx1, Panx2, and Panx3, are expressed alongside connexins in chordates. Unlike connexins, pannexins do not connect the cytoplasm of adjacent cells but function as unitary channels regulating the exchange of ions and small molecules between the cytoplasm and extracellular milieu. The biochemical properties and functions of Panx1 and Panx3 channels have been investigated, but our understanding on Panx2 channels has progressed at a much slower pace. The first objective of this thesis was to comprehensively map the expression and distribution of Panx2 protein in mammalian tissues. Prior to this work, Panx2 distribution had been studied exclusively by analyzing the expression of its transcript which was found to be largely restricted to the central nervous system (CNS). In this thesis, Panx2 mRNA and protein levels were analyzed in different tissues and Panx2 transcriptional activity was found to poorly predict Panx2 protein abundance. Panx2 protein levels were lower in nervous tissues although transcriptional analysis showed disproportionately high Panx2 mRNA levels in the CNS. Furthermore, endogenous Panx2 channels were found to be sequestered within the endomembrane system of the cell. The second objective focused on characterizing the subcellular localization and biological function of Panx2 channels. Using subcellular fractionation, it was determined that Panx2 co-fractionates with mitochondrial and endoplasmic reticulum (ER) markers thereby suggesting that Panx2-containing compartments can associate, at least transiently, with the ER and mitochondria. The analysis of Panx2 dynamics in living cells combined with immunogold electron microscopy confirmed that Panx2 is not randomly distributed within the cytoplasm but preferentially localizes at membrane contact sites called mitochondria-associated membranes (MAMs) which physically and functionally tether the ER to mitochondria. Finally, the biological function of Panx2 was partially elucidated by showing that Panx2 expression is modulated by the energetic requirements of the cell and can regulate apoptosis.
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