LAUSR

Remodeling of the size and thickness of blood vessels is essential for adequate tissue perfusion but can also be detrimental, as a consequence of pathological conditions such as atherosclerosis or hypertension. High flow and pressure results in an increased passive lumen diameter and hypertrophy of the arterial wall, whereas low flow and pressure results in the opposite effects/outcomes (1). An important early step in the flowand pressure-mediated vascular remodeling is an inflammatory response (2, 3). The mechanisms by which changes in pressure and flow are transduced to the recruitment of inflammatory cells and ultimately to vascular remodeling are not fully elucidated. In PNAS, Suzuki et al. (4) propose that excitation–transcription coupling via Ca influx and activation of Ca/calmodulindependent protein kinase (CaMK) within caveolae of smooth muscle cells are key steps in the recruitment of macrophages to the adventitia. Arterial smooth muscle cells respond autonomously to changes in sarcolemmal stretch caused by variations in intravascular pressure (5, 6). The overall acute effect is to control luminal diameter and blood flow to vital organs. Membrane depolarization activates voltage-dependent L-type Ca channels, CaV1.2, which is the principal source of Ca in smooth muscle (7). Tension increases and decreases in parallel with the activity of CaV1.2 channels. In most arterial smooth muscle, the ryanodine receptors (RyR1 and RyR2) in the sarcoplasmic reticulum (SR) and the voltageand Ca-dependent, large-conductance K channels (BK) in the plasma membrane are essential for signaling relaxation (8, 9). By hyperpolarizing the membrane potential, BK channels exert negative-feedback control on the activity of CaV1.2 channels. BK channels are known to be activated by RyR channels in a microdomain formed by the close apposition of the SR and the plasma membrane (10). Thus, Ca entry through CaV1.2 causes contraction, whereas Ca flux through RyR channels causes relaxation. Location and neighborhood matter. Calcium regulates important aspects of the functional activity of many cell types. Cells precisely control the free concentration of intracellular Ca so that Ca can act as an intracellular messenger. Tension is affected by many different signaling pathways, which converge on Ca–calmodulindependent myosin-light-chain (MLC) kinase and on MLC phosphatase. MLC kinase activity increases with cytoplasmic Ca concentration (11). Increased MLC regulatory subunit phosphorylation increases myosin head-group cycling and increases tension, while decreased MLC phosphorylation decreases tension. Contraction and relaxation are both highly regulated and ongoing competing processes, and, paradoxically, Ca serves as a signal for both processes (12). This is possible because signaling is compartmentalized by the assembly of components of connected pathways and by slow diffusion of Ca near its points of entry (11–13). In these microdomains, Ca concentrations can be much higher than its cytoplasmic average. As described in this paper, a microdomain formed by caveolae is essential for the depolarizationinduced excitation–transcription coupling, recruitment of macrophages, and vascular remodeling Fig. 1 (4). In smooth muscle cells, Ca–calmodulin-dependent enzymes are involved in both contraction and activation of transcription factors. In addition to the effects on MLC kinase, Ca binding to calmodulin also activates the phosphatase calcineurin, which regulates the transcription factor NFAT (nuclear factor of activator T-cells), and Ca–calmodulin-dependent kinase (CaMK), which regulates the transcription factor CREB (cAMP response element binding protein) (14). Phosphorylation of CREB at serine 133 allows CREB to modulate transcription through Ca/cAMP response elements (CRE) in the promoter of genes. Membrane depolarization, caused by either KCl or ryanodine, a RyR blocker, increases the fraction of nuclei staining for phosphorylated CREB and levels of c-fos messenger RNA in intact mouse cerebral arteries, dependent upon activation of CaV1.2 channels (15, 16). Inhibition of CaMK blocks the depolarization-induced increases in phosphorylated CREB and c-fos transcript levels, implying that CaMK is a critical intermediary for excitation–transcription coupling (15, 16). In PNAS, Suzuki et al. extend these observations, both from mechanistic and functional perspectives (4). The signal from excitation to transcription starts at the plasma membrane in vascular smooth muscle cells (Fig. 1). Depolarizationinduced intranuclear phosphorylation of CREB, which is dependent upon CaV1.2 activation, was shown to require the activation of CaMK and CaMK kinase (CaMKK). Pharmacological inhibitors, KN93 (an inhibitor of CaMK) or STO609 (an inhibitor of CaMKK), and knockdown of either CaMK1a or CaMKK2 prevented CREB phosphorylation. Thus, voltagedependent Ca channels activate a CaMKK2–CaMK1a–CREB