The cytoplasm: no longer a well-mixed bag.

Until the last decade or so, many intracellular signaling pathways were viewed as processes in which second messengers diffused uniformly through a well-mixed milieu of the cell’s cytoplasm to reach their targets. Although it was recognized that the cell’s interior was compartmentalized, this compartmentalization was believed to be largely defined by internal membranes, such as the nuclear envelope, endoplasmic reticulum (ER), sarcoplasmic reticulum (SR), and mitochondria. But like the joke about the person who has lost his keys in the dark but looks for them under the street lamp because the light is better, this view of the cytoplasm as a well-mixed milieu was less of a proven fact than a simplifying assumption. Over the last decade, advances in subcellular imaging have dramatically upset this view, so that now a high degree of compartmentalization of signaling pathways within the cytoplasm is considered the norm rather than the exception. It is now clear that the cytoplasm has a highly organized cytoskeleton and sophisticated molecular trafficking mechanisms that direct and tether proteins into macroaggregates at specific locations to facilitate localized signaling. The cytoplasm is now viewed as a system of microdomains with restricted diffusion (eg, hierarchical Ca signaling) and direct channeling of substrates to enzymes (eg, protein kinases/phosphatases cascades). In the field of metabolism, subcellular compartmentation of energy production has been a well-accepted fact ever since mitochondria were identified as the engines driving aerobic high-energy phosphate production. In addition, glycolytic enzymes complexes are well-known to be associated with specific intracellular structures, such as the SR.1 On the flip side, however, energy consumption by the cell has traditionally been viewed as a fairly democratic process, with highenergy phosphates freely diffusing throughout the cytoplasm to be consumed wherever they are needed. In tissues with high energy requirements, such as muscle, the creatine kinase (CK) system has been viewed as the essential equalizer in this design, with phosphocreatine (PC) shuttling rapidly to regenerate ATP from ADP wherever CK is located, maintaining free ADP concentration at low levels to maximize the free energy of ATP hydrolysis.2,3 In heart, this role of PC in mediating crosstalk between ATP production and ATP utilization has been shown for both contractile function and sarcolemmal function.3–5 But if the norm for other signaling pathways is a high degree of compartmentalization, perhaps energy consumption is not as democratic a process as once assumed. In this issue of Circulation Research, Kaasik et al6 address the following important question: does ATP, once generated, diffuse rapidly to wherever it is needed, or is it channeled directly and preferentially to nearby energy-consuming processes? This is not a new question, but it has been difficult to answer convincingly for two reasons. First, imaging tools to track energy production/consumption directly at the subcellular level are still not as well developed as they are, for example, for imaging Ca microdomains or hot spots of protein kinase activity using fluorescence techniques. Second, both the traditional “grind-and-bind” biochemical methods and global measures of metabolic function, such as nuclear magnetic resonance, are based on averaged cytoplasmic values of various metabolites and do not easily lend themselves to investigating subcellular compartmentalization of metabolism. Thus, the evidence for metabolic compartmentalization has largely rested on studies of functional responses to metabolic interventions. There is a long history of functional studies suggesting that energy production and consumption are colocalized at the subcellular level. A general theme in both muscle and nonmuscle cells has been that glycolytically derived ATP is used preferentially to support surface membrane–related functions (such as ion transporters and channels), whereas mitochondrially generated ATP is used preferentially for supporting functions in the cytoplasm (such as muscle contraction) (Figure). Several of the major observations supporting this idea are the following. Selective inhibition of anaerobic glycolysis and selective inhibition of mitochondrial oxidative phosphorylation have different functional effects, which cannot be explained by changes in global tissue high-energy phosphate levels.7–9 Manipulations of anaerobic glycolysis markedly affect functional performance and recovery during ischemia/reperfusion or hypoxia/reoxygenation, again in a manner that does not correlate with changes in global-tissue high phosphate levels.10–14 Finally, substrates for glycolysis and substrates for mitochondrial oxidative phosphorylation or ATP regeneration via CK have differential efficacies at supporting specific membrane or contractile functions in heart and other tissues.2,7,15–17 In this issue of Circulation Research, Kaasik et al6 take the latter approach to examine the relationship between the source of ATP and the efficacy of Ca uptake by the SR. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Cardiovascular Research Laboratory and the Departments of Medicine (Cardiology) and Physiology, University of California Los Angeles School of Medicine, Los Angeles, Calif. Correspondence to James N. Weiss, MD, 3645 MRL Building, UCLA School of Medicine, Los Angeles, CA 90095-1760. E-mail [email protected] (Circ Res. 2001;89:108-110.) © 2001 American Heart Association, Inc.