Endocytosis of E-cadherin regulated by Rac and Cdc42 small G proteins through IQGAP1 and actin filaments.

E-cadherin is a key cell-cell adhesion molecule at adherens junctions (AJs) and undergoes endocytosis when AJs are disrupted by the action of extracellular signals. To elucidate the mechanism of this endocytosis, we developed here a new cell-free assay system for this reaction using the AJ-enriched fraction from rat liver. We found here that non-trans-interacting, but not trans-interacting, E-cadherin underwent endocytosis in a clathrin-dependent manner. The endocytosis of trans-interacting E-cadherin was inhibited by Rac and Cdc42 small G proteins, which were activated by trans-interacting E-cadherin or trans-interacting nectins, which are known to induce the formation of AJs in cooperation with E-cadherin. This inhibition was mediated by reorganization of the actin cytoskeleton by Rac and Cdc42 through IQGAP1, an actin filament-binding protein and a downstream target of Rac and Cdc42. These results indicate the important role of the Rac/Cdc42-IQGAP1 system in the dynamic organization and maintenance of the E-cadherin-based AJs.

Regulation of SNAREs by tomosyn and ROCK: implication in extension and retraction of neurites.

Extension of neurites requires the SNARE-dependent fusion of plasmalemmal precursor vesicles with the plasma membrane of growth cones. Here, we show that tomosyn localizes at the palm of growth cones and inhibits the fusion of the vesicles there, thus promoting transport of the vesicles to the plasma membrane of the leading edges of growth cones. Tomosyn localizes because ROCK activated by Rho small G protein phosphorylates syntaxin-1, which increases the affinity of syntaxin-1 for tomosyn and forms a stable complex with tomosyn, resulting in inhibition of the formation of the SNARE complex. In retraction of neurites, tomosyn localizes all over the edges of the neurites and inhibits fusion of the vesicles with the plasma membrane. Thus, tomosyn demarcates the plasma membrane by binding to syntaxin-1 phosphorylated by ROCK, and thereby regulates extension and retraction of neurites.

Septo-optic dysplasia with congenital hepatic fibrosis.

This article reports a 7-year-old female with septo-optic dysplasia and congenital hepatic fibrosis. She manifested nystagmus and severe hepatosplenomegaly. Brain magnetic resonance imaging revealed agenesis of the septum pellucidum, optic nerve hypoplasia, pituitary gland stalk hypoplasia, and absence of the posterior pituitary gland. She was diagnosed with growth hormone deficiency, hypothyroidism, diabetes insipidus, and adrenal insufficiency. Thus, this case was regarded as septo-optic dysplasia. No mutation was evident in the coding and boundary regions of the homeobox gene HESX1. Percutaneous biopsy of the liver demonstrated the presence of broad septa of fibrous tissue containing abundant bile ducts without inflammatory cell infiltrates, a finding compatible with congenital hepatic fibrosis. Although there is an association between septo-optic dysplasia and neonatal cholestasis, believed to be related to hypopituitarism, this case of septo-optic dysplasia with congenital hepatic fibrosis is apparently the first reported in the English literature.

Acute encephalopathy and rhabdomyolysis following rotavirus gastroenteritis.

Rotavirus is a common cause of severe gastroenteritis in children, and other unusual extraintestinal manifestations have also been attributed to the virus. We report a case of acute encephalopathy and rhabdomyolysis following rotavirus gastroenteritis in a 6-month-old infant.

Cerebrospinal fluid cytokines in Salmonella urbana encephalopathy.

We present a case report of encephalopathy associated with Salmonella urbana infection in a child. A 5-year-old boy was admitted to our clinic with convulsions and coma. Cerebrospinal fluid (CSF) interleukin-6 (IL-6) and IL-8 were elevated at onset and were decreased within normal limit on the fifth day. Residual neurological deficits included severe mental deficits and spastic tetraplegia. High levels of CSF proinflammatory cytokines might be related to central nervous system (CNS) disease activity. Although encephalopathy is a rare complication of non-typhi Salmonella infection, it should be borne in mind as an occasionally serious and potentially lethal disease.

A novel rabconnectin-3-binding protein that directly binds a GDP/GTP exchange protein for Rab3A small G protein implicated in Ca(2+)-dependent exocytosis of neurotransmitter.

BACKGROUND: Rab3A, a member of the Rab3 small G protein family, regulates Ca2+-dependent exocytosis of neurotransmitter. The cyclical activation and inactivation of Rab3A are essential for the Rab3A action in exocytosis. GDP-Rab3A is activated to GTP-Rab3A by Rab3 GDP/GTP exchange protein (Rab3 GEP) and GTP-Rab3A is inactivated to GDP-Rab3A by Rab3 GTPase-activating protein (Rab3 GAP). We have recently found a novel protein, named rabconnectin-3, which is co-immunoprecipitated with Rab3 GEP or GAP from the extract of the crude synaptic vesicle (CSV) fraction of rat brain. Rabconnectin-3 is abundantly expressed in the brain where it is associated with synaptic vesicles. We have found that two more proteins are co-immunoprecipitated with Rab3 GEP from the CSV fraction of rat brain. We attempted here to isolate and characterize one of them. RESULTS: We determined its partial amino acid sequence, cloned its cDNA from a human cDNA library, and determined its primary structure. The protein consisted of 1490 amino acids (aa) and showed a calculated molecular weight of 163808. The protein had 7 WD domains. The protein was abundantly expressed in the brain where it co-localized with rabconnectin-3 on synaptic vesicles. The protein formed a stable complex with rabconnectin-3. We named this protein rabconnectin-3beta and renamed rabconnectin-3 rabconnectin-3alpha. Rabconnectin-3beta, but not rabconnectin-3alpha, directly bound Rab3 GEP. Neither rabconnectin-3alpha nor -3beta directly bound Rab3 GAP. CONCLUSION: These results indicate that rabconnectin-3 consists of the alpha and beta subunits and binds directly Rab3 GEP through the beta subunit and indirectly Rab3 GAP through an unidentified molecule(s).