Mark A. Sanders

A contradictory GLABRA3 allele helps define gene interactions controlling trichome development in Arabidopsis.

Previously characterized Arabidopsis gl3 mutants have trichomes that are smaller, less branched and undergo fewer rounds of endoreplication than wild-type trichomes. A new gl3 mutant, called gl3-sst, has oddly shaped trichomes that over expand during early development, undergo more endoreduplication and that have a striking nuclear morphology. The mutant nuclei consist of many interconnected lobes; however, only a single set of polytene-like chromosomes reside in the mutant nuclei. The predicted gl3-sst polypeptide has a Leu to Phe substitution (codon 78) within a region responsible for protein-protein interaction. Yeast interaction assays comparing GL3 with gl3-sst proteins show that the mutant protein interaction with GL1 and TTG1 is decreased by 75% and 50%, respectively, but there is no difference in its interaction with TRY. Furthermore, TRY has the ability to prevent the GL1 GL3 interaction and the GL1 gl3-sst interaction is even more sensitive to TRY. Analysis of plants expressing functional GFP-tagged versions of GL1, GL3 and TRY show that the proteins are localized in trichome nuclei. These results have been used to model trichome initiation in terms of protein interactions and threshold levels of activator complex.

Engineered Protein Nano-Compartments for Targeted Enzyme Localization

Compartmentalized co-localization of enzymes and their substrates represents an attractive approach for multi-enzymatic synthesis in engineered cells and biocatalysis. Sequestration of enzymes and substrates would greatly increase reaction efficiency while also protecting engineered host cells from potentially toxic reaction intermediates. Several bacteria form protein-based polyhedral microcompartments which sequester functionally related enzymes and regulate their access to substrates and other small metabolites. Such bacterial microcompartments may be engineered into protein-based nano-bioreactors, provided that they can be assembled in a non-native host cell, and that heterologous enzymes and substrates can be targeted into the engineered compartments. Here, we report that recombinant expression of Salmonella enterica ethanolamine utilization (eut) bacterial microcompartment shell proteins in E. coli results in the formation of polyhedral protein shells. Purified recombinant shells are morphologically similar to the native Eut microcompartments purified from S. enterica. Surprisingly, recombinant expression of only one of the shell proteins (EutS) is sufficient and necessary for creating properly delimited compartments. Co-expression with EutS also facilitates the encapsulation of EGFP fused with a putative Eut shell-targeting signal sequence. We also demonstrate the functional localization of a heterologous enzyme (β-galactosidase) targeted to the recombinant shells. Together our results provide proof-of-concept for the engineering of protein nano-compartments for biosynthesis and biocatalysis.

Interaction of the Adipocyte Fatty Acid-binding Protein with the Hormone-sensitive Lipase REGULATION BY FATTY ACIDS AND PHOSPHORYLATION

Adipocyte fatty acid-binding protein (AFABP/aP2) forms a physical complex with the hormone-sensitive lipase (HSL) and AFABP/aP2-null mice exhibit reduced basal and hormone-stimulated lipolysis. To identify the determinants affecting the interaction fluorescence resonance energy transfer (FRET) imaging was used in conjunction with a mutagenesis strategy to evaluate the roles AFABP/aP2 fatty acid binding and HSL phosphorylation have in complex formation as well as determine the HSL binding site on AFABP/aP2. The nonfatty acid binding mutant of AFABP/aP2 (R126Q) failed to form a FRET-competent complex with HSL either under basal or forskolin-stimulated conditions, indicating that lipid binding is required for association. Once bound to HSL and on the surface of the lipid droplet, YFP-AFABP/aP2 (but not YFP-HSL) exhibited energy transfer between the fusion protein and BODIPY-C12-labeled triacylglycerol. Serine to alanine mutations at the two PKA phosphorylation sites of HSL (659 and 660), or at the AMPK phosphorylation sites (565), blocked FRET between HSL and AFABP/aP2. Substitution of isoleucine for lysine at position 21 of AFABP/aP2 (K21I), but not 31 (K31I), resulted in a non-HSL-binding protein indicating that residues on helix αI of AFABP/aP2 define a component of the HSL binding site. These results indicate that the ligand-bound form of AFABP/aP2.interacts with the activated, phosphorylated HSL and that the association is likely to be regulatory; either delivering FA to inhibit HSL (facilitating feedback inhibition) or affecting multicomponent complex formation on the droplet surface.

Bovine monocyte TLR2 receptors differentially regulate the intracellular fate of Mycobacterium avium subsp. paratuberculosis and Mycobacterium avium subsp. avium.

Pathogenic mycobacterial organisms have the capacity to inhibit macrophage activation and phagosome maturation. Although the mechanism is complex, several studies have incriminated signaling through TLR2 receptors with subsequent activation of the MAPK pathway p38 (MAPKp38) and overproduction of IL‐10 in the survival of pathogenic mycobacterial organisms. In the present study, we compared the response of bovine monocytes with infection by Mycobacterium avium subspecies paratuberculosis (MAP), the cause of paratuberculosis in ruminants, with the closely related organism M. avium subspecies avium (Maa), which usually does not cause disease in ruminants. Both MAP and Maa induced phosphorylation of MAPKp38 by bovine monocytes; however, addition of a blocking anti‐TLR2 antibody partially prevented MAPKp38 phosphorylation of MAP‐infected monocytes but not Maa‐infected monocytes. Addition of anti‐TLR2 antibody enhanced phagosome acidification and phagosome‐lysosome fusion in MAP‐containing phagosomes and enabled monocytes to kill MAP organisms. These changes were not observed in Maa‐infected monocytes. The effect on phagosome maturation appears to occur independently from the previously described inhibitory effects of IL‐10 on phagosome acidification and organism killing, as IL‐10 production was not affected by addition of anti‐TLR2 antibody to monocyte cultures. Therefore, signaling through the TLR2 receptor appears to play a role in phagosome trafficking and antimicrobial responses in MAP‐infected bovine mononuclear phagocytes.

Identification and characterization of a small molecule inhibitor of Fatty Acid binding proteins.

Molecular disruption of the lipid carrier AFABP/aP2 in mice results in improved insulin sensitivity and protection from atherosclerosis. Because small molecule inhibitors may be efficacious in defining the mechanism(s) of AFABP/aP2 action, a chemical library was screened and identified 1 (HTS01037) as a pharmacologic ligand capable of displacing the fluorophore 1-anilinonaphthalene 8-sulfonic acid from the lipid binding cavity. The X-ray crystal structure of 1 bound to AFABP/aP2 revealed that the ligand binds at a structurally similar position to a long-chain fatty acid. Similar to AFABP/aP2 knockout mice, 1 inhibits lipolysis in 3T3-L1 adipocytes and reduces LPS-stimulated inflammation in cultured macrophages. 1 acts as an antagonist of the protein-protein interaction between AFABP/aP2 and hormone sensitive lipase but does not activate PPARgamma in macrophage or CV-1 cells. These results identify 1 as an inhibitor of fatty acid binding and a competitive antagonist of protein-protein interactions mediated by AFABP/aP2.

Asymmetric Distribution of Nuclear Pore Complexes and the Cytoplasmic Localization of β2-Tubulin mRNA in Chlamydomonas reinhardtii

Although it is generally accepted that nuclear architecture is an important determinant of nuclear activity, it is not clear whether cytoplasmic events, such as transcript localization and cell polarity, are affected by this architecture. Characterization of the nuclear architecture of the single-cell alga Chlamydomonas reinhardtii revealed a polarized nucleus, with nuclear pore complexes preferentially concentrated at the posterior side of the nucleus. Nuclear asymmetry was greatly exaggerated during the upregulation of genes encoding flagellar proteins, when nuclear pore complexes (NPCs) were observed to hyperpolarize to the posterior side of the nucleus while heterochromatin polarized to the anterior side. Interestingly, prior to deflagellation, the beta2-tubulin gene was preferentially located in the posterior region of the nucleus, and following deflagellation, beta2-tubulin transcripts accumulated posteriorly in polysome-rich cytoplasmic regions adjacent to the highest concentration of NPCs, suggesting a connection between nuclear architecture and cytoplasmic transcript localization.

Insulin degradation by acinar cell proteases creates a dysfunctional environment for human islets before/after transplantation: benefits of α-1 antitrypsin treatment.

Background. Pancreatic acinar cells are commonly cotransplanted along with islets during auto- and allotransplantations. The aims of this study were to identify how acinar cell proteases cause human islet cell loss before and after transplantation of impure islet preparations and to prevent islet loss and improve function with supplementation of &agr;-1 antitrypsin (A1AT). Methods. Acinar cell protease activity, insulin levels, and percent islet loss were measured after culture of pure and impure clinical islet preparations. The effect of proteases on ultrastructure of islets and &bgr;-cell insulin granules were examined by transmission electron microscopy. The number of insulin granules and insulin-labeled immunogold particles were counted. The in vivo effect of proteases on islet function was studied by transplanting acinar cells adjacent to islet grafts in diabetic mice. The effects of A1AT culture supplementation on protease activity, insulin levels, and islet function were assessed in pure and impure islets. Results. Islet loss after culture was significantly higher in impure relative to pure preparations (30% vs. 14%, P<0.04). Lower islet purity was associated with increased protease activity and decreased insulin levels in culture supernatants. Reduced &bgr;-cell insulin granules and insulin degradation by proteases were confirmed by transmission electron microscopy. Transplantations in mice showed delayed islet graft function when acinar cells were transplanted adjacent to the islets under the kidney capsule. Supplementation of A1AT to impure islet cultures maintained islet cell mass, restored insulin levels, and preserved islet functional integrity. Conclusion. Culture of impure human islet fractions in the presence of A1AT prevents insulin degradation and improves islet recovery.

Advanced laboratory techniques for sample processing and immunolabeling using microwave radiation

A better understanding of improved microwave technology has increased the benefits and versatility of the technique as it applies to all aspects of immunohistochemistry. The role of continuous magnetron power output (wattage) combined with precise control of sample heating demonstrated their significance to complex labeling protocols. Here, we present results for microwave-assisted formaldehyde fixation and its effect on GFP expression in transfected HeLa cells. Rat brain sections and cultured hippocampal cells were labeled with 11 different primary antibodies using a unified microwave protocol. Microwave-assisted immunohistochemistry made it possible to sequentially label tissues and cells with several primary antibodies in a very short period of time with excellent labeling characteristics.

Mapping of the Hormone-sensitive Lipase Binding Site on the Adipocyte Fatty Acid-binding Protein (AFABP) IDENTIFICATION OF THE CHARGE QUARTET ON THE AFABP/aP2 HELIX-TURN-HELIX DOMAIN

The hormone-sensitive lipase (HSL) and adipocyte fatty acid-binding protein (AFABP/aP2) form a physical complex that affects basal and hormone-stimulated adipocyte fatty acid efflux. Previous work has established that AFABP/aP2-HSL complex formation requires that HSL be in its activated, phosphorylated form and AFABP/aP2 have a bound fatty acid. To identify the HSL binding site of AFABP/aP2 a combination of alanine-scanning mutagenesis and fluorescence resonance energy transfer was used. Mutation of Asp17, Asp18, Lys21, or Arg30 (but not other amino acids in the helix-turn-helix region) to alanine inhibited interaction with HSL without affecting fatty acid binding. The cluster of residues on the helical domain of AFABP/aP2 form two ion pairs (Asp17-Arg30 and Asp18-Lys21) and identifies the region we have termed the charge quartet as the HSL interaction site. To demonstrate direct association, the non-interacting AFABP/aP2-D18K mutant was rescued by complementary mutation of HSL (K196E). The charge quartet is conserved on other FABPs that interact with HSL such as the heart and epithelial FABPs but not on non-interacting proteins from the liver or intestine and may be a general protein interaction domain utilized by fatty acid-binding proteins in regulatory control of lipid metabolism.

Culture of Impure Human Islet Fractions in the Presence of Alpha-1 Antitrypsin Prevents Insulin Cleavage and Improves Islet Recovery

BACKGROUND Exocrine tissue is commonly cotransplanted with islets in autografting and allotransplantation of impure preparations. Proteases and insulin are released by acinar cells and islets, respectively, during pretransplantation culture and also systemically after transplantation. We hypothesized that released proteases could cleave insulin molecules and that addition of alpha-1 antitrypsin (A1AT) to impure islet cultures would block this cleavage, improving islet recovery and function. METHODS Trypsin, chymotrypsin, and elastase (TCE) activity and insulin levels were measured in culture supernates of pure (n = 5) and impure (n = 5) islet fractions, which were isolated from deceased donors. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to detect insulin after incubation with proteases. We assessed the effects of A1AT supplementation (0.5 mg/mL; n = 4] on TCE activity, insulin levels, culture recovery, and islet quality. The ultrastructure of islets exposed to TCE versus control medium was examined using electron microscopy (EM). RESULTS Protease (TCE) activity in culture supernatants was indirectly proportional to the percentage purity of islets: pure, impure, or highly impure. Increasingly lower levels of insulin were detected in culture supernatants when higher protease activity levels were present. Insulin levels measured from supernatants of impure and highly impure islet preparations were 61 +/- 23.7% and 34 +/- 33% of that in pure preparations, respectively. Incubation with commercially available proteases (TCE) or exocrine acinar cell supernatant cleaved insulin molecules as assessed using SDS-PAGE. Addition of A1AT to impure islet preparations reduced protease activity and restored normal insulin levels as detected using enzyme-linked immunosorbent assay (ELISA) and SDS-PAGE of culture supernates. A1AT improved insulin levels to 98% +/- 1.3% in impure and 78% +/- 34.2% in highly impure fractions compared with pure islet fractions. A1AT supplementation improved postculture recovery of islets in impure preparations compared with nontreated controls (72% +/- 9% vs 47% +/- 15%). Islet viability as measured using membrane integrity assays was similar in both the control (98% +/- 2%) and the A1AT-treated groups (99% +/- 1%). EM results revealed a reduction or absence of secretory granules after exposure to proteases (TCE). CONCLUSION Culture of impure human islet fractions in the presence of A1AT prevented insulin cleavage and improved islet recovery. A1AT supplementation of islet culture media, therefore, may increase the proportion of human islet products that meet release criteria for transplantation.