Oxaloacetate regulates mitochondrial complex II respiration in brown fat: dependence on UCP1 expression
Authors: Sivitz William , Som Ritu , Fink Brian , Yu Liping 
Published: May 2023

Objective: Oxaloacetate (OAA) is a long-known potent inhibitor of succinate dehydrogenase (SDH), a phenomenon that has, however, received only limited attention. We previously found that skeletal muscle mitochondria incubated at low membrane potential (Δψ) or interscapular brown adipose tissue (IBAT) mitochondria, wherein Δψ is intrinsically low, accumulate OAA in amounts sufficient to inhibit SDH (complex II) respiration. We proposed a mechanism wherein low Δψ reduces reverse electron transport (RET) to complex I causing a low NADH/NAD+ ratio favoring malate conversion to OAA. To further assess the mechanism and its physiologic relevance we carried out studies of mice with inherently different levels of IBAT mitochodnrial inner membrane potential. Hypothesis: Mice with differential expression of UCP1 would manifest Δψ-dependent differences in OAA-regulated mitochondrial respiration. Methodology: We examined several parameters in complex II and complex I energized IBAT mitochondria isolated from obesity prone C57BL/6J compared to obesity resistant 129SVE mice and from UCP1 knock-out (UCP1KO) compared to wildtype (WT) mice. Data were analyzed by unpaired, 2-tailed t-test and expressed as mean ± SE. Data: Isolated complex II (succinate)-energized IBAT mitochondria from 129SVE mice versus C57BL/6J displayed greater UCP1 expression, similar O2 flux despite lower Δψ, similar OAA concentrations, and similar NADH/NAD+. When GDP was added to inhibit UCP1, 129SVE IBAT mitochondria, despite their lower Δψ, exhibited much lower respiration (AUC of respiration versus time, 19963 ± 1764 vs 36015 ± 2527, p = 0.0004, n = 6), greater OAA concentrations (22.9 ± 1.8 vs 12.1 ± 3.3, p = 0.016, n = 6), much lower RET (as marked by ROS), and much lower NADH and NADH/NAD+ ratios compared to the C57BL/6J IBAT mitochondria. UCP1 knock-out abolished OAA accumulation by succinate-energized mitochondria associated with markedly greater Δψ, ROS, and NADH, but equal or greater O2 flux compared to WT mitochondria. GDP addition, compared to no GDP, increased Δψ and complex II respiration in WT mice associated with much less OAA. Respiration on complex I substrates for mitochondria of both strains of mice and for WT and UCP1KO mice followed the more classical dynamics of greater O2 flux at lower Δψ. Summary: These findings support the above-mentioned mechanism for OAA- and Δψ-dependent complex II respiration and support its physiologically relevance. Conclusions: Oxaloacetate regulates mitochondrial complex II dynamics in a manner dependent on inner membrane potential as intrinsically determined by mouse strain or genetic manipulation. NIH award, 1 R01 DK123043-01A1, VA Merit Review Award 2 I01 BX000285-06, and the Iowa Fraternal Order of the Eagles This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.