The specific pH values of cellular compartments affect virtually all biochemical processes, including enzyme activity, protein folding and redox state. Accurate, sensitive and compartment-specific measurements of intracellular pH (pH_i) dynamics in living cells are therefore crucial to the understanding of stress response and adaptation. We used the pH-sensitive GFP derivative ‘ratiometric pHluorin’ expressed in the cytosol and in the mitochondrial matrix of growingSaccharomyces cerevisiaeto assess the variation in cytosolic pH (pH_cyt) and mitochondrial pH (pH_mit) in response to nutrient availability, respiratory chain activity, shifts in environmental pH and stress induced by addition of sorbic acid. Thein vivomeasurement allowed accurate determination of organelle-specific pH, determining a constant pH_cytof 7.2 and a constant pH_mitof 7.5 in cells exponentially growing on glucose. We show that pH_cytand pH_mitare differentially regulated by carbon source and respiratory chain inhibitors. Upon glucose starvation or sorbic acid stress, pH_idecrease coincided with growth stasis. Additionally, pH_iand growth coincided similarly in recovery after addition of glucose to glucose-starved cultures or after recovery from a sorbic acid pulse. We suggest a relation between pH_iand cellular energy generation, and therefore a relation between pH_iand growth.

Pacemaker and conduction system myocytes play crucial roles in initiating and regulating the contraction of the cardiac chambers. Genetic defects, acquired diseases, and aging cause dysfunction of the pacemaker and conduction tissues, emphasizing the clinical necessity to understand the molecular and cellular mechanisms of their development and homeostasis. Although all cardiac myocytes of the developing heart initially possess pacemaker properties, the majority differentiates into working myocardium. Only small populations of embryonic myocytes will form the sinus node and the atrioventricular node and bundle. Recent efforts have revealed that the development of these nodal regions is achieved by highly localized suppression of working muscle differentiation, and have identified transcriptional repressors that mediate this process. This review will summarize and reflect new experimental findings on the cellular origin and the molecular control of differentiation and morphogenesis of the pacemaker tissues of the heart. It will also shed light on the etiology of inborn and acquired errors of nodal tissues.

Intracellular pH and Lipid Metabolism: Intracellular pH regulates metabolism by poorly understood mechanisms, but biosensors are likely to be important in this process. Young et al. (p. 1085 ) took a systems-biology approach in yeast to identify in excess of 200 genes that regulate phospholipid metabolism. They found that the signaling lipid, phosphatidic acid, appeared to act as a cytosolic biosensor via the pH-dependent binding of protein effectors to phosphatidic acid. This pH-dependent mechanism directly affects gene expression and is involved in a pathway in which nutrient availability regulates phospholipid metabolism to control production of membranes.

Background: Because protonation affects the properties of almost all molecules in cells, cytosolic pH (pH_c) is usually assumed to be constant. In the model organism yeast, however, pH_c changes in response to the presence of nutrients and varies during growth. Since small changes in pH_c can lead to major changes in metabolism, signal transduction, and phenotype, we decided to analyze pH_c control. Results: Introducing a pH-sensitive reporter protein into the yeast deletion collection allowed quantitative genome-wide analysis of pH_c in live, growing yeast cultures. pH_c is robust towards gene deletion; no single gene mutation led to a pH_c of more than 0.3 units lower than that of wild type. Correct pH_c control required not only vacuolar proton pumps, but also strongly relied on mitochondrial function. Additionally, we identified a striking relationship between pH_c and growth rate. Careful dissection of cause and consequence revealed that pH_c quantitatively controls growth rate. Detailed analysis of the genetic basis of this control revealed that the adequate signaling of pH_c depended on inositol polyphosphates, a set of relatively unknown signaling molecules with exquisitely pH sensitive properties. Conclusions: While pH_c is a very dynamic parameter in the normal life of yeast, genetically it is a tightly controlled cellular parameter. The coupling of pH_c to growth rate is even more robust to genetic alteration. Changes in pH_c control cell division rate in yeast, possibly as a signal. Such a signaling role of pH_c is probable, and may be central in development and tumorigenesis.