Commonalities and differences in the Oxidative Phosphorylation of Mitochondria and Neuronal Membranes

ABSTRACT Mitochondria are considered the exclusive site of aerobic metabolism. However, in recent years, the functional expression of the oxidative phosphorylation (OxPhos) machinery has been reported in several other membranous structures, including the plasma membrane, endoplasmic reticulum, nucleus, myelin sheath and disks of rod outer segments. Thus, to underline commonalities and differences between extra-mitochondrial and mitochondrial aerobic metabolism, we characterized the aerobic ATP synthesis in isolated myelin sheath (IM) and rod outer segment (OS) disks, using mitochondria-enriched fractions, as a positive control. Oxygen consumption and ATP synthesis were evaluated in the presence of conventional (pyruvate + malate or succinate) and unconventional (NADH) substrates. ATP synthesis was also assayed in the presence of 10-100 µM ATP in the assay medium. Data show that IM and OS disks consumed oxygen and synthesized ATP both in the presence of conventional and unconventional respiratory substrates, while the mitochondria-enriched fraction did not utilize NADH. Only in mitochondria, ATP synthesis was progressively lost in the presence of increasing ATP concentrations. Conversely, only myelin sheath and rod OS disks produced ATP at a later time or after the removal of respiratory substrates, reflecting their ability to accumulate energy and this opens up exciting perspectives in the study of sleep. Thus, these data suggest that the extramitochondrial OxPhos in IM and rod OS displays a different behavior concerning the classic mitochondrial aerobic metabolism, representing a possible basic molecular process involved in the physiology of the nervous system. Significance Statement Mitochondria are considered the cell powerhouse, being the site of the oxidative phosphorylation (OxPhos), which produces the major part of cellular chemical energy by oxygen consumption. However, proteomics, microscopy, and biochemical analyses have described the ectopic functional expression of the OxPhos machinery also in other membranous structures, such as isolated myelin (IM) and rod outer segments (OS). The results reported in this work shows that, although the proteins involved in IM and rod OS OxPhos appear the same expressed in mitochondria, the comparison of mitochondrial and extramitochondrial OxPhos display some differences, opening a new scenario about the energy metabolism modulation. Graphical Abstract