Caveolae in ventricular myocytes are required for stretch-dependent conduction slowing

Mechanical stretch of cardiac muscle modulates action potential propagation velocity, causing potentially arrhythmogenic conduction slowing. The mechanisms by which stretch alters cardiac conduction remain unknown, but previous studies suggest that stretch can affect the conformation of caveolae in myocytes and other cell types. We tested the hypothesis that slowing of action potential conduction due to cardiac myocyte stretch is dependent on caveolae. Cardiac action potential propagation velocities, measured by optical mapping in isolated mouse hearts and in micropatterned mouse cardiomyocyte cultures, decreased reversibly with volume loading or stretch, respectively (by 19±5% and 26±4%). Stretch-dependent conduction slowing was not altered by stretch-activated channel blockade with gadolinium or by GsMTx-4 peptide, but was inhibited when caveolae were disrupted via genetic deletion of caveolin-3 (Cav3 KO) or membrane cholesterol depletion by methyl-β-cyclodextrin. In wild-type mouse hearts, stretch coincided with recruitment of caveolae to the sarcolemma, as observed by electron microscopy. In myocytes from wild-type but not Cav3 KO mice, stretch significantly increased cell membrane capacitance (by 98±64%), electrical time constant (by 285±149%), and lipid recruitment to the bilayer (by 84±39%). Recruitment of caveolae to the sarcolemma during physiologic cardiomyocyte stretch slows ventricular action potential propagation by increasing cell membrane capacitance.

Examination of potential mechanisms of amyloid-induced defects in neuronal transport

Microtubule-based neuronal transport pathways are impaired during the progression of Alzheimer's disease and other neurodegenerative conditions. However, mechanisms leading to defects in transport remain to be determined. We quantified morphological changes in neuronal cells following treatment with fibrils and unaggregated peptides of beta-amyloid (Abeta). Abeta fibrils induce axonal and dendritic swellings indicative of impaired transport. In contrast, Abeta peptides induce a necrotic phenotype in both neurons and non-neuronal cells. We tested several popular hypotheses by which aggregated Abeta could disrupt transport. Using fluorescent polystyrene beads, we developed experimental models of physical blockage and localized release of reactive oxygen species (ROS) that reliably induce swellings. Like the beads, Abeta fibrils localize in close proximity to swellings; however, fibril internalization is not required for disrupting transport. ROS and membrane permeability are also unlikely to be responsible for fibril-mediated toxicity. Collectively, our results indicate that multiple initiating factors converge upon pathways of defective transport.

GIPC is recruited by APPL to peripheral TrkA endosomes and regulates TrkA trafficking and signaling

GIPC is a PDZ protein located on peripheral endosomes that binds to the juxtamembrane region of the TrkA nerve growth factor (NGF) receptor and has been implicated in NGF signaling. We establish here that endogenous GIPC binds to the C terminus of APPL, a Rab5 binding protein, which is a marker for signaling endosomes. When PC12(615) cells are treated with either NGF or antibody agonists to activate TrkA, GIPC and APPL translocate from the cytoplasm and bind to incoming, endocytic vesicles carrying TrkA concentrated at the tips of the cell processes. GIPC, but not APPL, dissociates from these peripheral endosomes prior to or during their trafficking from the cell periphery to the juxtanuclear region, where they acquire EEA1. GIPC's interaction with APPL is essential for recruitment of GIPC to peripheral endosomes and for TrkA signaling, because a GIPC PDZ domain mutant that cannot bind APPL or APPL knockdown with small interfering RNA inhibits NGF-induced GIPC recruitment, mitogen-activated protein kinase activation, and neurite outgrowth. GIPC is also required for efficient endocytosis and trafficking of TrkA because depletion of GIPC slows down endocytosis and trafficking of TrkA and APPL to the early EEA1 endosomes in the juxtanuclear region. We conclude that GIPC, following its recruitment to TrkA by APPL, plays a key role in TrkA trafficking and signaling from endosomes.

Identification and Characterization of GIV, a Novel Gαi/s -interacting Protein Found on COPI, Endoplasmic Reticulum-Golgi Transport Vesicles

In this report, we characterize GIV (Galpha-interacting vesicle-associated protein), a novel protein that binds members of the Galpha(i) and Galpha subfamilies of heterotrimeric G proteins. The Galpha(s) interaction site was mapped to an 83-amino acid region of GIV that is enriched in highly charged amino acids. BLAST searches revealed two additional mammalian family members, Daple and an uncharacterized protein, FLJ00354. These family members share the highest homology at the Galpha binding domain, are homologous at the N terminus and central coiled coil domain but diverge at the C terminus. Using affinity-purified IgG made against two different regions of the protein, we localized GIV to COPI, endoplasmic reticulum (ER)-Golgi transport vesicles concentrated in the Golgi region in GH3 pituitary cells and COS7 cells. Identification as COPI vesicles was based on colocalization with beta-COP, a marker for these vesicles. GIV also codistributes in the Golgi region with endogenous calnuc and the KDEL receptor, which are cis Golgi markers and with Galpha(i3)-yellow fluorescent protein expressed in COS7 cells. By immunoelectron microscopy, GIV colocalizes with beta-COP and Galpha(i3) on vesicles found in close proximity to ER exit sites and to cis Golgi cisternae. In cell fractions prepared from rat liver, GIV is concentrated in a carrier vesicle fraction (CV2) enriched in ER-Golgi transport vesicles. beta-COP and several Galpha subunits (Galpha(i1-3), Galpha(s)) are also most enriched in CV2. Our results demonstrate the existence of a novel Galpha-interacting protein associated with COPI transport vesicles that may play a role in Galpha-mediated effects on vesicle trafficking within the Golgi and/or between the ER and the Golgi.

Spatial regulation of Galphai protein signaling in clathrin-coated membrane microdomains containing GAIP

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAPs) that bind to Galpha subunits and attenuate G protein signaling, but where these events occur in the cell is not yet established. Here we investigated, by immunofluorescence labeling and deconvolution analysis, the site at which endogenous Galpha-interacting protein (GAIP) (RGS19) binds to Galphai3-YFP and its fate after activation of delta-opioid receptor (DOR). In the absence of agonist, GAIP is spatially segregated from Galphai3 and DOR in clathrin-coated domains (CCPs) of the cell membrane (PM), whereas Galphai3-YPF and DOR are located in non-clathrin-coated microdomains of the PM. Upon addition of agonist, Galphai3 partially colocalizes with GAIP in CCPs at the PM. When endocytosis is blocked by expression of a dynamin mutant [dyn(K44A)], there is a striking overlap in the distribution of DOR and Galphai3-YFP with GAIP in CCPs. Moreover, Galphai3-YFP and GAIP form a coprecipitable complex. Our results support a model whereby, after agonist addition, DOR and Galphai3 move together into CCPs where Galphai3 and GAIP meet and turn off G protein signaling. Subsequently, Galphai3 returns to non-clathrin-coated microdomains of the PM, GAIP remains stably associated with CCPs, and DOR is internalized via clathrin-coated vesicles. This constitutes a novel mechanism for regulation of Galpha signaling through spatial segregation of a GAP in clathrin-coated pits.

Disruption ofGja8(α8 connexin) in mice leads to microphthalmia associated with retardation of lens growth and lens fiber maturation

The development of the vertebrate lens utilizes a sophisticated cell-cell communication network via gap junction channels, which are made up of at least three connexin isoforms, α8 (Cx50), α3 (Cx46) and α1 (Cx43), and which are encoded by three different genes. In a previous study, we reported that, with a disruption of Gja3 (α3 connexin), mice developed nuclear cataracts with a normal sized lens. We show that Gja8tm1 (α8–/–) mice develop microphthalmia with small lenses and nuclear cataracts, while the α8 heterozygous (+/–) mice have relatively normal eyes and lenses. A comparative study of these α3 and α8 knockout mice showed that the protein levels of both α3 and α8 were independently regulated and there was no compensation for either the α3 or α8 protein from the wild-type allele when the other allele was disrupted. More interestingly, western blotting data indicated that the presence of α8 in the lens nucleus is dependent on α3 connexin, but not vice versa. The staining of the knock-in lacZ reporter gene showed the promoter activity of α8 connexin is much higher than that of α3 connexin in embryonic lenses and in adult lens epithelium. More importantly, a delayed denucleation process was observed in the interior fibers of the α8–/– lenses. Therefore, α8 connexin is required for proper fiber cell maturation and control of lens size.

Development of cataractous macrophthalmia in mice expressing an active MEK1 in the lens

PURPOSE

To characterize the extracellular signal-regulated kinase (ERK) pathway in the lens and to try to understand how this pathway contributes to lens function and cataractogenesis.

METHODS

The members of the ERK pathway in the lens were examined by Western blotting, immunohistochemical staining, and kinase assay. A gain-of-function approach was used to perturb the ERK pathway in the lenses of transgenic mice via expression of a constitutively active mutant of the mitogen-activated protein kinase kinase 1 (MEK1(E)), the direct upstream kinase of the ERK1 and ERK2 kinases, under the alphaA-crystallin promoter.

RESULTS

The presence of an active ERK pathway was found in lens epithelial cells and in differentiating fibers. Transgenic mice that expressed MEK1(E) developed postnatal cataracts as well as macrophthalmia. Distinct morphologic alterations, such as lens enlargement, swelling fiber cells, enlarged extracellular space, and vacuole formation, were observed in the lenses of these transgenic mice. A significant increase in the glucose transporter 1 (GLUT1) level, as well as in the glucose level, was detected in the lens.

CONCLUSIONS

The MAP kinase pathway is involved in the regulation of glucose metabolism and balance in the mouse lens. Moreover, the alteration of MAP kinase activity in the lens is sufficient to cause cataract formation with enlarged extracellular space and vacuoles in the differentiating fibers. This transgenic mouse may provide a useful model for understanding the mechanism(s) for some aspects of human cataracts.

Toxoplasma gondii resides in a vacuole that avoids fusion with host cell endocytic and exocytic vesicular trafficking pathways

Toxoplasma gondii actively penetrates its vertebrate host cell to establish a nonfusigenic compartment called the parasitophorous vacuole (PV) that has previously been characterized primarily in phagocytic cells. To determine the fate of this unique compartment in nonphagocytic cells, we examined the trafficking of host cell proteins and lipids in Toxoplasma-infected fibroblasts using quantitative immunofluorescence and immunoelectron microscopy. Toxoplasma-containing vacuoles remained segregated from all levels of the endocytic pathway, as shown by the absence of delivery of transferrin receptors, mannose phosphate receptors, and the lysosomal-associated protein LAMP1 to the vacuole. The PV was also inaccessible to lipids (DiIC16, and GM1) that were internalized from the plasma membrane via the endocytic system. In contrast, vacuoles containing dead parasites or zymosan sequentially acquired both endosomal and lysosomal protein markers and host lipids, reflecting the competency of fibroblasts to process phagocytic vacuoles. The mature PV often lies adjacent to the host cell Golgi, suggesting that it may intersect with vesicles from the exocytic pathway. Despite this proximity, the PV was inaccessible to nitrobenzadiazole-labeled sphingolipids exported from the Golgi and did not contain the host protein markers AP1 or beta-COP. Our results demonstrate that Toxoplasma resides in a compartment that excludes delivery of protein and lipid components from the host cell endocytic and exocytic pathways.

Stable DNA transformation of Toxoplasma gondii using phleomycin selection

Toxoplasma gondii (Tg) is an obligate intracellular protozoan parasite that is an important opportunistic pathogen in humans. To provide additional tools for molecular genetic analysis, we have developed a set of vectors for DNA transformation in Tg based on selection with the antibiotic phleomycin (Ph). These new vectors rely on the flanking sequences from the parasite genes GRA1, GRA2 or SAG1 to drive expression of the Tn5 ble gene encoding resistance to the DNA intercalating drug Ph (PhR). Treatment of extracellular parasites was used to select stable PhR transformants by plaque formation on host cell monolayers. Transfection of linear or circular forms of the pGRA1/ble, pGRA2/ble or pSAG1/ble vectors by electroporation resulted in stable transformation with an efficiency of approx. 10(-4)/micrograms DNA. Stable transformants contained 1-5 copies of ble that were integrated at non-homologous sites in the parasite nuclear genome. Ble provides a new dominant selectable marker for safe, efficient and rapid isolation of stable DNA transformants in Tg.