Energetic Crosstalk Between Organelles

Abstract

Abstract-Cells with high and fluctuating energy demands such as cardiomyocytes need efficient systems to link energy production to energy utilization. This is achieved in part by compartmentalized energy transfer enzymes such as creatine kinase (CK). However, hearts from CK-deficient mice develop normal cardiac function under conditions of moderate workload. We have therefore investigated whether a direct functional interplay exists between mitochondria and sarcoplasmic reticulum or between mitochondria and myofilaments in cardiac cells that catalyzes direct energy and signal channeling between organelles. We used the selective permeabilization of sarcolemmal membranes with saponin to study the functional interactions between organelles within the cellular architecture. We measured contractile kinetics, oxygen consumption, and caffeine-induced tension transients. The results show that in hearts of normal mice, ATP produced by mitochondria (supplied with substrates, oxygen, and adenine nucleotides) was able to sustain calcium uptake and contractile speed. Moreover, direct mitochondrially supplied ATP was nearly as effective as CK-supplied ATP and much more effective than externally supplied ATP, suggesting that a direct ATP/ADP channeling exists between the sites of energy production (mitochondria) and energy utilization (sarcoplasmic reticulum and myofilaments). On the other hand, in cardiac cells of mice deficient in mitochondrial and cytosolic CK, marked cytoarchitectural modifications were observed, and direct adenine nucleotide channeling between mitochondria and organelles was still effective for sarcoplasmic reticulum and myofilaments. Such direct crosstalk between organelles may explain the preserved cardiac function of CK-deficient mice under moderate workloads. Key Words: mitochondria Ⅲ sarcoplasmic reticulum Ⅲ myofibrils Ⅲ creatine kinase Ⅲ knockout mice D ifferentiation and maturation of adult mammalian muscle cells lead to complex specialization and organization. In cardiac cells, specialized cellular functions are highly organized within structural and functional compartments. Energy-consuming processes are localized to the sarcoplasmic reticulum (SR) and myofibrillar compartments, while energy production occurs mainly within mitochondria. Muscle cells contain complex and specialized energy transfer systems, which efficiently link energy production and utilization. One such system is the family of creatine kinase (CK) isoenzymes that catalyze the reversible transfer of a phosphate moiety between creatine (Cr) and ATP. The mitochondrial sarcomeric isoenzyme (mi-CK) is bound to the outer surface of the inner mitochondrial membrane so that ATP generated by oxidative phosphorylation is transphosphorylated to phosphocreatine (PCr). 1-3 On the other hand, the cytosolic isoenzyme (MM-CK) that is structurally associated with myofibrils and SR membranes can use PCr to rephosphorylate all of the ADP produced by the ATPases and thus provide enough energy for normal contractile kinetics or SR calcium uptake. 4 ...

Calcium signalling-dependent mitochondrial dysfunction and bioenergetics regulation in respiratory chain Complex II deficiency

“…Accordingly, the existence of ATP micro-domains beneath the plasma membrane and in ER-mitochondria contact sites has been already shown in some cell models. 29,30 Proteasome-dependent degradation of SERCA2a isoform was reported in an earlier work by Ihara et al 31 Ubiquitination of PMCA1 isoform has also been detected after preconditioning ischaemia in rat cortical neuronal cultures. 32 As polyubiquitylated proteins are the preferred proteasome 26S substrates, we postulate that SERCA2b and PMCA2 undergo proteasome-dependent degradation after their ubiquitination.…”

Excitation-contraction coupling and mitochondrial energetics

“…Besides the PCr system, which buffers high energy phosphates in the cytosol and in close spatial association with ATPases [39], direct ATP/ADP channeling also exists between mitochondria and the main sites of energy consumption, i. e., SR Ca 2+ ATPase and myofilaments [104]. Furthermore, ATP generation from SR-associated glycolytic enzymes supports SR Ca 2+ -ATPase activity [219].…”

ATP-sensitive K channel channel/enzyme multimer: Metabolic gating in the heart

“…This indicates that the bulk cytosolic nucleotide composition cannot be the sole determinant of K ATP channel function. Rather, K ATP channels could sense local nucleotide concentrations set by ATPases in the submembrane space at a level distinct from that of the "bulk" cytosol [52,54,86], provided that significant diffusional limitations within the cell exist to establish distinct cellular compartments [15,[87][88][89]. However, such cellular compartmentalization would obstruct proper energetic sensing by K ATP channels, as channel gating would be distorted by local fluctuations of nucleotides, remaining weakly dependent on the cellular metabolic status.…”