The phosphorylation state of any protein represents a balance of the actions of specific protein kinases and protein phosphatases. Many protein phosphatases are highly enriched in, or exclusive to, the nuclear compartment, where they dephosphorylate key substrates to regulate various nuclear processes. In this review we will discuss recent findings that define the role of nuclear protein phosphatases in controlling transforming growth factor-beta (TGFbeta) and bone-morphogenetic protein (BMP) signalling, the DNA-damage response, RNA processing, cell-cycle progression and gene transcription.

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Radiographic contrast agents cause acute kidney injury (AKI), yet the underlying pathogenesis is poorly understood. Nod-like receptor pyrin containing 3-deficient (Nlrp3-deficient) mice displayed reduced epithelial cell injury and inflammation in the kidney in a model of contrast-induced AKI (CI-AKI). Unexpectedly, contrast agents directly induced tubular epithelial cell death in vitro that was not dependent on Nlrp3. Rather, contrast agents activated the canonical Nlrp3 inflammasome in macrophages. Intravital microscopy revealed diatrizoate (DTA) uptake within minutes in perivascular CX3CR1+ resident phagocytes in the kidney. Following rapid filtration into the tubular luminal space, DTA was reabsorbed and concentrated in tubular epithelial cells via the brush border enzyme dipeptidase-1 in volume-depleted but not euvolemic mice. LysM-GFP+ macrophages recruited to the kidney interstitial space ingested contrast material transported from the urine via direct interactions with tubules. CI-AKI was dependent on resident renal phagocytes, IL-1, leukocyte recruitment, and dipeptidase-1. Levels of the inflammasome-related urinary biomarkers IL-18 and caspase-1 were increased immediately following contrast administration in patients undergoing coronary angiography, consistent with the acute renal effects observed in mice. Taken together, these data show that CI-AKI is a multistep process that involves immune surveillance by resident and infiltrating renal phagocytes, Nlrp3-dependent inflammation, and the tubular reabsorption of contrast via dipeptidase-1.

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Zipper-interacting protein kinase (ZIPK) has been implicated in Ca2؉ -independent smooth muscle contraction, although its specific role is unknown. The addition of ZIPK to demembranated rat caudal arterial strips induced an increase in force, which correlated with increases in LC 20 and MYPT1 phosphorylation. However, because of the number of kinases capable of phosphorylating LC 20 and MYPT1, it has proven difficult to identify the mechanism underlying ZIPK action. Therefore, we set out to identify bona fide ZIPK substrates using a chemical genetics method that takes advantage of ATP analogs with bulky substituents at the N 6 position and an engineered ZIPK capable of utilizing such substrates.32 P-Labeled 6-phenyl-ATP and ZIPK-L93G mutant protein were added to permeabilized rat caudal arterial strips, and substrate proteins were detected by autoradiography following SDS-PAGE. Mass spectrometry identified LC 20 as a direct target of ZIPK in situ for the first time. Tissues were also exposed to 6-phenyl-ATP and ZIPK-L93G in the absence of endogenous ATP, and putative ZIPK substrates were identified by Western blotting. LC 20 was thereby confirmed as a direct target of ZIPK; however, no phosphorylation of MYPT1 was detected. We conclude that ZIPK is involved in the regulation of smooth muscle contraction through direct phosphorylation of LC 20 .Smooth muscle plays an important role in the regulation of diverse physiological processes, including vascular tone, gastrointestinal motility, penile erection, bronchial diameter, and parturitional/postparturitional myometrial contraction. All smooth muscle tissues rely on the Ca 2ϩ /calmodulin-dependent activation of myosin light chain kinase (MLCK) 6 and subsequent phosphorylation of the 20-kDa myosin regulatory light chains (LC 20 ) at Ser-19 to initiate actomyosin cross-bridge cycling and force development (1). On the other hand, relaxation is induced by dephosphorylation of LC 20 by myosin light chain phosphatase (MLCP), a type 1 protein serine/threonine phosphatase (2, 3). Contraction of multiple smooth muscle tissues has frequently been observed in the absence of an increase in cytosolic free Ca 2ϩ concentration in response to a variety of stimuli (4). This phenomenon, commonly referred to as Ca 2ϩ sensitization, involves alteration of the MLCK:MLCP activity ratio in favor of the kinase, which can be achieved by the following mechanisms: (i) activation of MLCK by a mechanism not involving Ca 2ϩ /calmodulin, e.g. phosphorylation of MLCK by proline-directed kinases (5, 6); (ii) an increase in Ca 2ϩ -independent LC 20 kinase activity (7); and (iii) inhibition of MLCP either directly by phosphorylation of inhibitory residues (Thr-697 and/or Thr-855) in the myosin phosphatase targeting subunit 1 (MYPT1) of MLCP (8 -10) or indirectly by phosphorylation of the protein kinase C-potentiated inhibitory protein for heterotrimeric MLCP of 17 kDa (CPI-17) at Thr-38 (11, 12). The observation that intact and permeabilized smooth muscle tissues exhibit Ca 2ϩ -independent contracti...