Regional registration of [6-(14)C]glucose metabolism during brain activation of α-syntrophin knockout mice.

α-Syntrophin is a component of the dystrophin scaffold-protein complex that serves as an adaptor for recruitment of key proteins to the cytoplasmic side of plasma membranes. α-Syntrophin knockout (KO) causes loss of the polarized localization of aquaporin4 (AQP4) at astrocytic endfeet and interferes with water and K(+) homeostasis. During brain activation, release of ions and metabolites from endfeet is anticipated to increase perivascular fluid osmolarity, AQP4-mediated osmotic water flow from endfeet, and metabolite washout from brain. This study tests the hypothesis that reduced levels of endfoot AQP4 increase retention of [(14)C]metabolites during sensory stimulation. Conscious KO and wild-type mice were pulse-labeled with [6-(14)C] glucose during unilateral acoustic stimulation or bilateral acoustic plus whisker stimulation, and label retention was assayed by computer-assisted brain imaging or analysis of [(14)C]metabolites in extracts, respectively. High-resolution autoradiographic assays detected a 17% side-to-side difference (p < 0.05) in inferior colliculus of KO mice, not wild-type mice. However, there were no labeling differences between KO and wild-type mice for five major HPLC fractions from four dissected regions, presumably because of insufficient anatomical resolution. The results suggest a role for AQP4-mediated water flow in support of washout of metabolites, and underscore the need for greater understanding of astrocytic water and metabolite fluxes.

Structure of the split PH domain and distinct lipid-binding properties of the PH-PDZ supramodule of alpha-syntrophin.

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains 'hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PH(N)-PDZ-PH(C) tandem of alpha-syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PH(N)-PDZ-PH(C) tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

Protein trafficking and anchoring complexes revealed by proteomic analysis of inward rectifier potassium channel (Kir2.x)-associated proteins.

Inward rectifier potassium (Kir) channels play important roles in the maintenance and control of cell excitability. Both intracellular trafficking and modulation of Kir channel activity are regulated by protein-protein interactions. We adopted a proteomics approach to identify proteins associated with Kir2 channels via the channel C-terminal PDZ binding motif. Detergent-solubilized rat brain and heart extracts were subjected to affinity chromatography using a Kir2.2 C-terminal matrix to purify channel-interacting proteins. Proteins were identified with multidimensional high pressure liquid chromatography coupled with electrospray ionization tandem mass spectrometry, N-terminal microsequencing, and immunoblotting with specific antibodies. We identified eight members of the MAGUK family of proteins (SAP97, PSD-95, Chapsyn-110, SAP102, CASK, Dlg2, Dlg3, and Pals2), two isoforms of Veli (Veli-1 and Veli-3), Mint1, and actin-binding LIM protein (abLIM) as Kir2.2-associated brain proteins. From heart extract purifications, SAP97, CASK, Veli-3, and Mint1 also were found to associate with Kir2 channels. Furthermore, we demonstrate for the first time that components of the dystrophin-associated protein complex, including alpha1-, beta1-, and beta2-syntrophin, dystrophin, and dystrobrevin, interact with Kir2 channels, as demonstrated by immunoaffinity purification and affinity chromatography from skeletal and cardiac muscle and brain. Affinity pull-down experiments revealed that Kir2.1, Kir2.2, Kir2.3, and Kir4.1 all bind to scaffolding proteins but with different affinities for the dystrophin-associated protein complex and SAP97, CASK, and Veli. Immunofluorescent localization studies demonstrated that Kir2.2 co-localizes with syntrophin, dystrophin, and dystrobrevin at skeletal muscle neuromuscular junctions. These results suggest that Kir2 channels associate with protein complexes that may be important to target and traffic channels to specific subcellular locations, as well as anchor and stabilize channels in the plasma membrane.