Mitochondrial potassium transport: the role of the MitoK ATP WeiGuo

Mitochondrial potassium cycle Mitochondria are structurally complex. The inner membrane contains the essential components of the electron transport proteins and all of the exchange carriers

Mitochondrial potassium cycle The mitochondrial K + cycle consists of influx and efflux pathways for K +, H +, and anions These ions are exchanged between the matrix and the intermembrane space ( IMS ); however, the outer membrane (OM) does not present a barrier to further exchange of small ions with the cytosol The mitochondrial K + cycle consists of influx and efflux pathways for K +, H +, and anions These ions are exchanged between the matrix and the intermembrane space ( IMS ); however, the outer membrane (OM) does not present a barrier to further exchange of small ions with the cytosol

Influx pathway for potassium IMS matrix MitoK ATP K + leak K+K+ K+K+ H+H+ ETS ∆Ψ Matrix alkalinization Pi - OH - Pi-H + symporter Electron transport system (ETS) generates membrane potential (∆Ψ). ∆Ψ can drive K + influx by diffusion (‘‘K + leak’’) and via the mitoK ATP. This K + for H + exchange will alkalinize the matrix, causing phosphate to enter via the Pi-H + symporter.

Efflux pathway for potassium Net uptake of K + salts will be accompanied by osmotically obligated water, resulting in matrix swelling. Excess matrix K + is then ejected by the K + /H + antiporter K + -H + antiporter

Early work on the potassium cycle Diffusive K + influx would be sufficient to cause matrix water content to increase, with eventual lysis. This would be avoided by the K + /H + antiporter

synthesizing ATP at very high rates ∆Ψ decreasesmatrix contraction mito-K ATP Maintain matrix volume MitoK ATP meets a different need in volume regulation

MitoK ATP on matrix and IMS volumes MitoK ATP opening was shown to regulate matrix volume during ischemia and state 3 respiration Addition of antimycin A to simulate ischemia DE depolarization and decrease in diffusive K + influx addition of ADP to trigger state 3 respiration 10–15% contraction in matrix volume matrix volume return to original state 5-HD

MitoK ATP on matrix and IMS volumes Changes in IMS could be estimated by means of membrane surface areas (SA) Studies shown that mitoK ATP opening decreases IMS volume Physiological changes in matrix volume may have important effects on IMS structure–function Changes in IMS could be estimated by means of membrane surface areas (SA) Studies shown that mitoK ATP opening decreases IMS volume Physiological changes in matrix volume may have important effects on IMS structure–function

Two distinct consequences of opening mitoK ATP When ∆Ψ is high → opening mitoK ATP → matrix alkalinization → production of reactive oxygen species (ROS) ↑ When ∆Ψ is depressed → opening mitoK ATP → prevent contraction of the matrix and expansion of the IMS When ∆Ψ is high → opening mitoK ATP → matrix alkalinization → production of reactive oxygen species (ROS) ↑ When ∆Ψ is depressed → opening mitoK ATP → prevent contraction of the matrix and expansion of the IMS

Is mitoK ATP involved in all modes of cardioprotection ? Ischemic preconditioning √ Calcium preconditioning √ KCO preconditioning √ Delayed preconditioning √ Adaptive preconditioning √ Na + /H + exchange inhibition √ Ischemic post-conditioning ? Ischemic preconditioning √ Calcium preconditioning √ KCO preconditioning √ Delayed preconditioning √ Adaptive preconditioning √ Na + /H + exchange inhibition √ Ischemic post-conditioning ?

During which phase is mitoK ATP opening crucial for cardioprotection? MitoK ATP is proposed to play distinct roles in each phase of ischemia– reperfusion MitoK ATP is proposed to play distinct roles in each phase of ischemia– reperfusion Preconditioning phase Ischemic phase Reperfusion phase As a end-effector of cardioprotection As a end-effector of cardioprotection As a trigger of cardioprotection

During the preconditioning phase The role of mitoK ATP opening is to increase production of ROS Moderate increases in ROS play an important second messenger role in a variety of signaling pathways The role of mitoK ATP opening is to increase production of ROS Moderate increases in ROS play an important second messenger role in a variety of signaling pathways

A proposed mechanism for increased ROS ROS ↑ IMS Matrix Matrix alkalinization OH - Pi-H + symporter Pi - K+K+ K+K+ MitoK ATP K + leak Pi - uptake will be less than K + uptake K + uptake creating a gradient for uptake of Pi on the Pi–H + symporter, Pi uptake will be less than K + uptake, because Pi is present in much lower concentrations than K +. For this reason, matrix pH always increases when matrix volume increases due to uptake of K + and Pi.

During the ischemic phase mitochondrial permeability transition (MPT) The primary role of matrix Ca 2 + is to stimulate ROS production upon reperfusion Ca 2 + cannot open MPT unless ROS are present Cytosolic Ca 2 + may play an additional role in promoting ROS oxidation of adenine nucleotide translocase (ANT) mitochondrial permeability transition (MPT) The primary role of matrix Ca 2 + is to stimulate ROS production upon reperfusion Ca 2 + cannot open MPT unless ROS are present Cytosolic Ca 2 + may play an additional role in promoting ROS oxidation of adenine nucleotide translocase (ANT)

The mechanism by which mitoK ATP protects the heart during ischemia phase The opening of mitoK ATP preserves the structure– function of the IMS and maintains the low permeability of the outer membrane to adenine nucleotides, thereby preserving ADP for phosphorylation upon reperfusion

Outer mitochondrial membrane permeability to ADP and ATP was controlled by voltage-dependent anion channel (VDAC) In heart, VDAC is normally in a low-conductance state that is poorly permeable to nucleotides, and energy transfers are mediated instead by creatine and creatine phosphate. matrix IMS Outer Mem Inner Mem ATP ADP ANT VDAC CK Cr / PCr

During ischemia, ∆Ψ will decrease, resulting in reduced uptake of K +, contraction of the matrix, and expansion of the IMS MitoK ATP regulation of VDAC permeability to nucleotides during ischemia This means that all of cellular ATP is available for hydrolysis, and, ultimately, unavailability of ADP for rephosphorylation upon reperfusion IMS expansion will cause Mi-CK to dissociate from VDAC, leading to a high outer membrane conductance to ATP and ADP

During the reperfusion phase The opening of mitoK ATP facilitates rapid energy conversion to phosphocreatine (PCr). Under these conditions, mitochondria will not produce a burst of ROS upon reperfusion, and the irreversible opening of the MPT will not occur

Outer Mem Inner Mem ATP / ADP ANT VDAC CK Energy transfer from mitochondria to myofibrils is mediated by two parallel pathways—creatine/creatine phosphate (Cr/CrP) and ATP/ADP In the Cr/CrP system, myofibrillar creatine kinase converts ADP to creatine. Mi-CK bridge the IMS between outer membrane VDAC and inner membrane ANT. Cr / PCr Cr/CrP is more efficient About 67% of the energy production in heart has been found to arise from the CK system

During reperfusion, expansion of the IMS will cause Mi-CK to dissociate from VDAC, leading to a high outer membrane conductance to ATP and ADP MitoK ATP facilitates rapid energy conversion to phosphocreatine (PCr) during the reperfusion phase If mitoK ATP is open, the outer membrane will retain its low permeability to nucleotides, and the mitochondria can restore energy levels using the more efficient metabolic channeling via Mi-CK

Summary Mitochondria potassium cycle Two distinct consequences of Opening mitoK ATP mitoK ATP plays cardio-protective effect during all three phases of the ischemia–reperfusion injury Mitochondria potassium cycle Two distinct consequences of Opening mitoK ATP mitoK ATP plays cardio-protective effect during all three phases of the ischemia–reperfusion injury

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