Requirement for glucose during in vitro culture of sheep preimplantation embryos.

Glucose utilization by sheep embryos was examined in 8-cell (N = 36) and blastocyst (N = 36) stages, by measuring conversion of [5-3H]glucose to 3H2O. Fifty percent glucose utilization occurred at 0.79 +/- 0.69 mM for 8-cell embryos and -0.06 +/- 0.15 mM for blastocysts. Development of 1- and 2-cell sheep embryos (N = 264) was examined under different glucose concentrations (0, 1.5, 3, or 6 mM) and in the presence or absence of 0.33 mM pyruvate and 3.3 mM lactate (PL). Overall, the presence of glucose was detrimental (P less than 0.001) to embryonic development. By contrast, the presence of pyruvate and lactate was beneficial (P less than 0.001) to development. An interaction was observed between the concentration of glucose and presence or absence of PL (P less than 0.05). An optimum level of glucose occurs at 0-3 mM in the presence of PL (P less than 0.1). Development to the blastocyst stage was observed in medium when supplemented with amino acids and albumin alone. Thus, glucose metabolism is not critical for embryonic development, but beneficial at low concentrations. High concentrations can inhibit development, possibly by inhibiting the tricarboxylic acid (TCA) cycle. Sheep embryos may also be using amino acids as an energy source for development.     

A photoreceptor-specific cadherin is essential for the structural integrity of the outer segment and for photoreceptor survival.

A cadherin family member, prCAD, was identified in retina cDNA by subtractive hybridization and high throughput sequencing. prCAD is expressed only in retinal photoreceptors, and the prCAD protein is localized to the base of the outer segment of both rods and cones. In prCAD(-/-) mice, outer segments are disorganized and fragmented, and there is progressive death of photoreceptor cells. prCAD is unlikely to be involved in protein trafficking between inner and outer segments, since phototransduction proteins appear to be correctly localized and the light responses of both rods and cones are only modestly compromised in prCAD(-/-) mice. These experiments imply a highly specialized cell biological function for prCAD and suggest that localized adhesion activity is essential for outer segment integrity.     

Probing luminal negative charge in the type 3 ryanodine receptor

In this study, we have investigated block of potassium (K(+)) current by neomycin, a large polycation, from the luminal face of the type 3 ryanodine receptor (RyR3). Previous studies have shown that neomycin is an open channel blocker of RyR2 that interacts with negatively charged residues in the mouth of the conduction pathway to partially occlude it. In the current study, we have used neomycin as a probe to investigate proposed negatively charged regions in the luminal pore mouth of RyR3. Luminal neomycin induces concentration- and voltage-dependent partial block to a subconductance state in RyR3. Blocking parameters calculated in this study show that neomycin has a higher affinity for RyR3 than RyR2, but block may occur at the same site within the pore mouth. The change in affinity may be due to altered negative charge density at the site of interaction.     

The HIF prolyl hydroxylase PHD3 is a potential substrate of the TRiC chaperonin

Hypoxia-inducible factor-1 (HIF) is regulated by oxygen-dependent prolyl hydroxylation. Of the three HIF prolyl hydroxylases (PHD1, 2 and 3) identified, PHD3 exhibits restricted substrate specificity in vitro and is induced in different cell types by diverse stimuli. PHD3 may therefore provide an interface between oxygen sensing and other signalling pathways. We have used co-purification and mass spectrometry to identify proteins that interact with PHD3. The cytosolic chaperonin TRiC was found to copurify with PHD3 in extracts from several cell types. Our results indicate that PHD3 is a TRiC substrate, providing another step at which PHD3 activity may be regulated.     

Inhibition of TATA binding protein dimerization by RNA polymerase III transcription initiation factor Brf1

The Brf1 subunit of TFIIIB plays an important role in recruiting the TATA-binding protein (TBP) to the up-stream region of genes transcribed by RNA polymerase III. When TBP is not bound to promoters, it sequesters its DNA binding domain through dimerization. Promoter assembly factors therefore might be required to dissociate TBP into productively binding monomers. Here we show that Saccharomyces cerevisiae Brf1 induces TBP dimers to dissociate. The high affinity TBP binding domain of Brf1 is not sufficient to promote TBP dimer dissociation but in addition requires the TFIIB homology domain of Brf1. A model is proposed to explain how two distinct functional domains of Brf1 work in concert to dissociate TBP into monomers.     

RGS expression level precisely regulates the duration of rod photoresponses

Regulators of G protein signaling (RGS) constitute a family of proteins that bind specifically to the activated alpha subunits of G proteins (Galpha-GTP), acting as GTPase activating proteins, or GAPs, for the rate of GTP hydrolysis. In this issue of Neuron, Krispel et al. resolve a long-standing puzzle in phototransduction, establishing that RGS9 "GAPping" of G(t)alpha-GTP is the molecular mechanism underlying the dominant recovery time constant of mouse rod photoreceptors and that a precise level of expression of RGS9 is required for normal photoresponse timing.     

Mapping of potent and specific binding motifs, GLOGEN and GVOGEA, for integrin α1β1 using collagen toolkits II and III

Integrins are well characterized cell surface receptors for extracellular matrix proteins. Mapping integrin-binding sites within the fibrillar collagens identified GFOGER as a high affinity site recognized by α2β1, but with lower affinity for α1β1. Here, to identify specific ligands for α1β1, we examined binding of the recombinant human α1 I domain, the rat pheochromocytoma cell line (PC12), and the rat glioma Rugli cell line to our collagen Toolkit II and III peptides using solid-phase and real-time label-free adhesion assays. We observed Mg(2+)-dependent binding of the α1 I domain to the peptides in the following rank order: III-7 (GLOGEN), II-28 (GFOGER), II-7 and II-8 (GLOGER), II-18 (GAOGER), III-4 (GROGER). PC12 cells showed a similar profile. Using antibody blockade, we confirmed that binding of PC12 cells to peptide III-7 was mediated by integrin α1β1. We also identified a new α1β1-binding activity within peptide II-27. The sequence GVOGEA bound weakly to PC12 cells and strongly to activated Rugli cells or to an activated α1 I domain, but not to the α2 I domain or to C2C12 cells expressing α2β1 or α11β1. Thus, GVOGEA is specific for α1β1. Although recognized by both α2β1 and α11β1, GLOGEN is a better ligand for α1β1 compared with GFOGER. Finally, using biosensor assays, we show that although GLOGEN is able to compete for the α1 I domain from collagen IV (IC(50) ～3 μm), GFOGER is much less potent (IC(50) ～90 μm), as shown previously. These data confirm the selectivity of GFOGER for α2β1 and establish GLOGEN as a high affinity site for α1β1.