Antidiuretic effects of oxytocin in the Brattleboro rat

Summary The antidiuretic activity of oxytocin (OT) was measured in Brattleboro rats with congenital diabetes insipidus. A dose dependent antidiuretic response was found in animals receiving chronic infusions of 0.1 g/h, 1.0 g/h, and 5 g/h of OT. OT infused at the rate of 5 g/h over a 7-day period completely reversed the symptoms of diabetes insipidus. The results support the concept that OT serves as a weak agonist of vasopressin at the level of the kidney and at pharmacological levels exhibits antidiuretic activity.     

Viral journeys on the intracellular highways

Viruses are obligate intracellular pathogens that are dependent on cellular machineries for their replication. Recent technological breakthroughs have facilitated reliable identification of host factors required for viral infections and better characterization of the virus–host interplay. While these studies have revealed cellular machineries that are uniquely required by individual viruses, accumulating data also indicate the presence of broadly required mechanisms. Among these overlapping cellular functions are components of intracellular membrane trafficking pathways. Here, we review recent discoveries focused on how viruses exploit intracellular membrane trafficking pathways to promote various stages of their life cycle, with an emphasis on cellular factors that are usurped by a broad range of viruses. We describe broadly required components of the endocytic and secretory pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Identification of such overlapping host functions offers new opportunities to develop broad-spectrum host-targeted antiviral strategies.     

The molecular sociology of the cell

Proteomic studies have yielded detailed lists of the proteins present in a cell. Comparatively little is known, however, about how these proteins interact and are spatially arranged within the 'functional modules' of the cell: that is, the 'molecular sociology' of the cell. This gap is now being bridged by using emerging experimental techniques, such as mass spectrometry of complexes and single-particle cryo-electron microscopy, to complement traditional biochemical and biophysical methods. With the development of integrative computational methods to exploit the data obtained, such hybrid approaches will uncover the molecular architectures, and perhaps even atomic models, of many protein complexes. With these structures in hand, researchers will be poised to use cryo-electron tomography to view protein complexes in action within cells, providing unprecedented insights into protein-interaction networks.     

A SNARE-adaptor interaction is a new mode of cargo recognition in clathrin-coated vesicles

Soluble NSF attachment protein receptors (SNAREs) are type II transmembrane proteins that have critical roles in providing the specificity and energy for transport-vesicle fusion and must therefore be correctly partitioned between vesicle and organelle membranes. Like all other cargo, SNAREs need to be sorted into the forming vesicles by direct interaction with components of the vesicles' coats. Here we characterize the molecular details governing the sorting of a SNARE into clathrin-coated vesicles, namely the direct recognition of the three-helical bundle H(abc) domain of the mouse SNARE Vti1b by the human clathrin adaptor epsinR (EPNR, also known as CLINT1). Structures of each domain and of their complex show that this interaction (dissociation constant 22 muM) is mediated by surface patches composed of approximately 15 residues each, the topographies of which are dependent on each domain's overall fold. Disruption of the interface with point mutations abolishes the interaction in vitro and causes Vti1b to become relocalized to late endosomes and lysosomes. This new class of highly specific, surface-surface interaction between the clathrin coat component and the cargo is distinct from the widely observed binding of short, linear cargo motifs by the assembly polypeptide (AP) complex and GGA adaptors and is therefore not vulnerable to competition from standard motif-containing cargoes for incorporation into clathrin-coated vesicles. We propose that conceptually similar but mechanistically different interactions will direct the post-Golgi trafficking of many SNAREs.     

Functional diversification of closely related ARF-GEFs in protein secretion and recycling

Guanine-nucleotide exchange factors on ADP-ribosylation factor GTPases (ARF-GEFs) regulate vesicle formation in time and space by activating ARF substrates on distinct donor membranes. Mammalian GBF1 (ref. 2) and yeast Gea1/2 (ref. 3) ARF-GEFs act at Golgi membranes, regulating COPI-coated vesicle formation. In contrast, their Arabidopsis thaliana homologue GNOM (GN) is required for endosomal recycling, playing an important part in development. This difference indicates an evolutionary divergence of trafficking pathways between animals and plants, and raised the question of how endoplasmic reticulum-Golgi transport is regulated in plants. Here we demonstrate that the closest homologue of GNOM in Arabidopsis, GNOM-LIKE1 (GNL1; NM_123312; At5g39500), performs this ancestral function. GNL1 localizes to and acts primarily at Golgi stacks, regulating COPI-coated vesicle formation. Surprisingly, GNOM can functionally substitute for GNL1, but not vice versa. Our results suggest that large ARF-GEFs of the GBF1 class perform a conserved role in endoplasmic reticulum-Golgi trafficking and secretion, which is done by GNL1 and GNOM in Arabidopsis, whereas GNOM has evolved to perform an additional plant-specific function of recycling from endosomes to the plasma membrane. Duplication and diversification of ARF-GEFs in plants contrasts with the evolution of entirely new classes of ARF-GEFs for endosomal trafficking in animals, which illustrates the independent evolution of complex endosomal pathways in the two kingdoms.     

Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients

Calcium is transported across the surface membrane of both nerve and muscle by a Na+-dependent mechanism, usually termed the Na:Ca exchange. It is well established from experiments on rod outer segments that one net positive charge enters the cell for every Ca2+ ion extruded by the exchange, which is generally interpreted to imply an exchange stoichiometry of 3 Na+:1 Ca2+. We have measured the currents associated with the operation of the exchange in both forward and reversed modes in isolated rod outer segments and we find that the reversed mode, in which Ca2+ enters the cell in exchange for Na+, depends strongly on the presence of external K+. The ability of changes in external K+ concentration ([K+]o) to perturb the equilibrium level of [Ca2+]i indicates that K+ is co-transported with calcium. From an examination of the relative changes of [Ca2+]o, [Na+]o, [K+]o and membrane potential required to maintain the exchange at equilibrium, we conclude that the exchange stoichiometry is 4 Na+:1 Ca2+, 1 K+ and we propose that the exchange should be renamed the Na:Ca, K exchange. Harnessing the outward K+ gradient should allow the exchange to maintain a Ca2+ efflux down to levels of internal [Ca2+] that are considerably lower than would be possible with a 3 Na+:1 Ca2+ exchange.     

Small nuclear ribonucleoprotein polypeptide N (SNRPN), an expressed gene in the Prader-Willi syndrome critical region.

Prader-Willi syndrome (PWS) is associated with paternally derived chromosomal deletions in region 15q11-13 or with maternal disomy for chromosome 15. Therefore, loss of the expressed paternal alleles of maternally imprinted genes must be responsible for the PWS phenotype. We have mapped the gene encoding the small nuclear RNA associated polypeptide SmN (SNRPN) to human chromosome 15q12 and a processed pseudogene SNRPNP1 to chromosome region 6pter-p21. Furthermore, SNRPN was mapped to the minimal deletion interval that is critical for PWS. The fact that the mouse Snrpn gene is maternally imprinted in brain suggests that loss of the paternally derived SNRPN allele may be involved in the PWS phenotype.     

Effect of different bathing media on the short-circuit current across the intestine of the rat and guinea-pig

Summary The changes in short-circuit current occurring when one or both solutions bathing the intestine of rat or guinea-pig mounted in flux chambers were recorded. The results with the guinea-pig can be explained in terms of diffusion potentials arising from the ionic replacements, and an electrogenic sodium pump, sensitive to ouabain, in the contraluminal membrane of the cell. In the rat, the situation is more complicated, and the enterocyte probably possesses an electrogenic sodium pump in the brush-border membrane.