The cytoplasm: no longer a well-mixed bag

JN Weiss, P Korge - Circulation research, 2001 - Am Heart Assoc
JN Weiss, P Korge
Circulation research, 2001Am Heart Assoc
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 …
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 Ca2+ 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 Ca2+ 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.
Am Heart Assoc