Functional Properties of the Mitochondrial Carrier System

Abstract

The mitochondrial carrier system (MCS) transports small molecules between mitochondria and the cytoplasm. It is integral to the core mitochondrial function to regulate cellular chemistry by metabolism. The mammalian MCS comprises the transporters of the 53-member canonical SLC25A family and a lesser number of identified noncanonical transporters. The recent discovery and investigations of the mitochondrial pyruvate carrier (MPC) illustrate the diverse effects a single mitochondrial carrier may exert on cellular function. However, the transport selectivities of many carriers remain unknown, and most have not been functionally investigated in mammalian cells. The mechanisms coordinating their function as a unified system remain undefined. Increased accessibility to molecular genetic and metabolomic technologies now greatly enables investigation of the MCS. Continued investigation of the MCS may reveal how mitochondria encode complex regulatory information within chemical thermodynamic gradients. This understanding may enable precision modulation of cellular chemistry to counteract the dysmetabolism inherent in disease.

Keywords: carrier; mitochondria; mitochondrial pyruvate carrier; redox cycles; systems biology; transporter.

Figure 1. The mitochondria pyruvate carrier regulates cellular decisions

A) MPC expression changes during differentiation and development. Lower expression may facilitate glycolysis-dependent ATP production and biosynthesis when PO₂ is low. Higher expression may facilitate oxidative phosphorylation and TCA cycle-dependent biosynthesis during terminal differentiation, when PO₂ is increased. The upper right panel is a qualitative representation of relative changes in mouse MPC mRNA abundance from embryonic day 17.5 (e17.5) to 28 days postnatal (P28), based on quantitative changes in Mpc1 and Mpc2 mRNA abundance previously reported [51]. B) MPC activity promotes pyruvate entry into the TCA cycle and oxidative phosphorylation. Decreased MPC activity promotes the Warburg Effect, where glycolysis and lactate production are favored over oxidative phosphorylation [, –43]. Inhibition of MCTs impairs cellular lactate export, increases MPC activity, and promotes pyruvate oxidation [38]. C) The MPC facilitates glucose sensing and insulin release in pancreatic β-cells [44, 52, 55]. Increased glucose levels drive MPC-dependent pyruvate oxidation. Increased pyruvate oxidation increases ATP levels. This leads to closure of ATP-gated potassium channels (K_ATP) and membrane depolarization. Depolarization increases calcium uptake by the voltage-gated calcium channels, thereby stimulating exocytosis of insulin granules. D) Disruption of MPC activity promotes pyruvate-alanine cycling, fatty acid oxidation, and glutaminolysis [18, 20, 22, 23, 51]. These mechanisms sustain TCA cycle flux. In hepatocytes, they also support gluconeogenesis [18, 20].

Figure 2. The mitochondrial carrier system enables redundant substrate utilization

A)AGC1, AGC2, glutamate carrier 1 (GC1), and glutamate carrier 2 (GC2) each transport glutamate into mitochondria. Import through AGC1/2 is in exchange for aspartate export [61]. Import through GC1/2 is by proton (H⁺) symport [66]. In cells that express each of these carriers, there are at least four distinct portals for glutamate import. Because of differences in co-transported substrates, mechanisms of regulation, and kinetics, functional redundancy is partial rather than absolute. Thus, each mode of glutamate import exerts unique effects on cellular function. B) The presence of distinct mitochondrial carriers for immediate chemical neighbors endows the mitochondrial carrier system with systems-level redundancy. Two examples are illustrated here. In the first, mitochondrial import of glutamine, glutamate, and α-ketoglutarate may all be utilized to anaplerotically replenish the TCA cycle with α-ketoglutarate. However, whether glutamine and glutamate are deaminated in the cytosol or mitochondria changes intracellular nitrogen partitioning. In the second, the MPC and the currently unidentified mitochondrial alanine carrier are functionally redundant. In coordination with the cytosolic (Alt1) and mitochondrial (Alt2) alanine transaminase, either carrier may be utilized to generate mitochondrial pyruvate or alanine. This is exemplified by the pyruvate-alanine cycling employed to bypass disrupted MPC activity [18, 20, 51].

Figure 3. The mitochondrial carriers coordinate redox cycles

A) The aspartate-glutamate carrier (AGC) and oxoglutarate (α-ketoglutarate) carrier (OGC) coordinate the malate-aspartate shuttle. This cycle transfers NADH equivalents from the cytosol to the mitochondrial matrix. Cytosolic oxaloacetate is reduced to malate, coupled to oxidation of NAD⁺ to NADH, by cytosolic malate dehydrogenase. Malate is imported into mitochondria and then re-oxidized to oxaloacetate, coupled to reduction of NAD⁺ to NADH, by mitochondrial malate dehydrogenase. Malate import in exchange for α-ketoglutarate export, by the OGC, is coordinated with glutamate import in exchange for aspartate export, by the AGC. B) Pyruvate cycling through the mitochondrial pyruvate carrier (MPC) and dicarboxylate carrier (DIC) [75] or Citrate Carrier (CIC) [77] regenerates cytosolic NADPH. Pyruvate imported through the CIC may be converted to malate or citrate, exported, and re-converted to pyruvate by malic enzyme 1 (ME1), coupled with reduction of NADP⁺ to NADPH. C) A glutaminolysis-dependent, citrate-α-ketoglutarate cycle transfers NADPH equivalents from the cytosol to mitochondria [79]. Cytosolic α-ketoglutarate is reductively carboxylated to isocitrate, coupled to NADPH oxidation to NADP⁺, and isomerized to citrate. Citrate is imported into mitochondria and oxidized to α-ketoglutarate by isocitrate dehydrogenase 2 (IDH2), coupled to reduction of NADP⁺ to NADPH. NADPH may be utilized to reduce glutathione (GSSG to GSH). In cancer cells, α-ketoglutarate may be re-exported to the cytosol to supply lipogenesis [69]. A potential alternative fate of exported α-ketoglutarate is reconversion to citrate, to promote sustained α-ketoglutarate-citrate cycling as an NADPH shuttle.