Faculty Bibliography

Asymmetric cell division is of fundamental importance in biology as it allows for the establishment of separate cell lineages during the development of multicellular organisms. Although microbial systems, including the yeast Saccharomyces cerevisiae, are excellent models of asymmetric cell division, this phenotype occurs in all cell divisions; consequently, models of lineage-specific segregation patterns in these systems do not exist. Here, we report the first example of lineage-specific asymmetric division in yeast. We used fluorescent tags to show that components of the yeast kinetochore, the protein complex that anchors chromosomes to the mitotic spindle, divide asymmetrically in a single postmeiotic lineage. This phenotype is not seen in vegetatively dividing haploid or diploid cells. This kinetochore asymmetry suggests a mechanism for the selective segregation of sister centromeres to daughter cells to establish different cell lineages or fates. These results provide a mechanistic link between lineage-defining asymmetry of metazoa with unicellular eukaryotes.

A central focus of aging research is to determine how calorie restriction (CR) extends lifespan and delays diseases of aging. SIRT1, the mammalian ortholog of Sir2 in yeast, is a longevity factor which mediates dietary restriction in diverse species. In addition, SIRT1 plays a protective role in several models of neurodegenerative disease. We tested the role of SIRT1 in mediating the effects of CR in a mouse model of prion disease. Prion diseases are protein misfolding disorders of the central nervous system with many similarities to other neurodegenerative diseases, including deposition of aggregated protein, gliosis, and loss of synapses and neurons. We report that the onset of prion disease is delayed by CR and in the SIRT1 KO mice fed ad libitum. CR exerts no further effect on the SIRT1 KO strain, suggesting the effects of CR and SIRT1 deletion are mechanistically coupled. In conjunction, SIRT1 is downregulated in certain brain regions of CR mice. The expression of PrP mRNA and protein is reduced in the brains of CR mice and in SIRT1 knockout mice, suggesting a possible mechanism for the delayed onset of disease, as PrP levels are a critical determinant of how quickly mice succumb to prion disease. Surprisingly, CR greatly shortens the duration of clinical symptoms of prion disease and ultimately shortens lifespan of prion-inoculated mice in a manner that is independent of SIRT1. Taken together, our results suggest a more complex interplay between CR, SIRT1, and neurodegenerative diseases than previously appreciated.

Calorie restriction (CR) has been reported to increase SIRT1 protein levels in mice, rats, and humans, and elevated activity of SIRT1 orthologs extends life span in yeast, worms, and flies. In this study, we challenge the paradigm that CR induces SIRT1 activity in all tissues by showing that activity of this sirtuin in the liver is, in fact, reduced by CR and activated by a high-caloric diet. We demonstrate this change both by assaying levels of SIRT1 and its small molecule regulators, NAD and NADH, as well as assessing phenotypes of a liver-specific SIRT1 knockout mouse on various diets. Our findings suggest that designing CR mimetics that target SIRT1 to provide uniform systemic benefits may be more complex than currently imagined.

The optical phenomenon of surface plasmon resonance (SPR) used by Biacore systems enables the detection and measurement of protein-protein interactions in real time, without the use of labels. In this unit, the application of Biacore technology to measure a protein-protein interaction is described using an antibody and its antigen as an example. The affinity of the antibody for its antigen is determined by measuring the binding kinetics of the interaction. The protocols are divided into three major steps that are required for measuring binding kinetics using Biacore: (1) surface preparation, (2) assay development, and (3) kinetic analysis.

Surface plasmon resonance (SPR) allows for the investigation of the functional nature of binding interactions and provides detailed kinetic information across a wide range of molecular weights, including small molecules, all without the use of labels. Here the various Biacore instrument platforms and their primary uses, ranging from semi-automated systems designed for simple, flexible basic research to fully automated, high-throughput systems, and systems designed to function in regulated environments, are all highlighted. The available sensor chip surfaces and immobilization techniques are also discussed. Biacore SPR biosensors can be used for a wide variety of assays, including specificity, active concentration measurement, kinetics, and affinity and thermodynamic parameters. Biacore SPR biosensors, which measure real-time analysis of biospecific interactions without the use of labeled molecules, can be used for a wide variety of protein interaction assays. In this unit, examples and recommendations for studying protein interactions with a variety of molecules are provided. This unit also shows how the technology can be used to determine binding specificity, active concentration measurements, and the determination of kinetic and thermodynamic parameters.

Increasing evidence suggests that an effective AIDS vaccine will need to elicit both broadly reactive humoral and cellular immune responses. Potent and cross-reactive neutralization of simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) by polyclonal and monoclonal antibodies is well documented. However, the mechanisms of antibody-mediated neutralization have not been defined. The current study was designed to determine whether the specificity and quantitative properties of antibody binding to SIV envelope proteins correlate with neutralization. Using a panel of rhesus monoclonal antibodies previously characterized for their ability to bind and neutralize variant SIVs, we compared the kinetic rates and affinity of antibody binding to soluble envelope trimers by using surface plasmon resonance. We identified significant differences in the kinetic rates but not the affinity of monoclonal antibody binding to the neutralization-sensitive SIV/17E-CL and neutralization-resistant SIVmac239 envelope proteins that correlated with the neutralization sensitivities of the corresponding virus strains. These results suggest for the first time that neutralization resistance may be related to quantitative differences in the rates but not the affinity of the antibody-envelope interaction and may provide one mechanism for the inherent resistance of SIVmac239 to neutralization in vitro. Further, we provide evidence that factors in addition to antibody binding, such as epitope specificity, contribute to the mechanisms of neutralization of SIV/17E-CL in vitro. This study will impact the method by which HIV/SIV vaccines are evaluated and will influence the design of candidate AIDS vaccines capable of eliciting effective neutralizing antibody responses.

PURPOSE/OBJECTIVE:The phosphorylated carboxyl terminus of rhodopsin is required for the stable binding of visual arrestin to the full length rhodopsin molecule. Phosphorylation of the carboxyl terminus has been shown to induce conformational changes in arrestin, which promote its binding to the cytoplasmic loops of rhodopsin. However, it has not been determined whether phosphorylation is also responsible for the direct binding of the rhodopsin carboxyl terminus to arrestin. To further investigate the role of rhodopsin phosphorylation on arrestin binding, surface plasmon resonance was used to measure the interaction between a synthetic phosphopeptide corresponding to the carboxyl terminus of rhodopsin and visual arrestin in real time. METHODS:Synthetic peptides were generated that correspond to the phosphorylated and nonphosphorylated carboxyl terminus of bovine rhodopsin. These peptides were immobilized on a biosensor chip and their interaction with purified visual arrestin was monitored by surface plasmon resonance on a BIAcore 2000 or 3000. RESULTS:A synthetic peptide phosphorylated on residues corresponding to Ser-338, Thr-340, Thr-342 and Ser-343 of bovine rhodopsin was sufficient for direct binding to visual arrestin. In contrast, a second phosphopeptide phosphorylated on Thr-340 and Thr-342 and a nonphosphorylated synthetic peptide were not able to bind arrestin. A peptide fully substituted at all serine and threonine residues with glutamic acid was unable to substitute for phosphorylation. CONCLUSIONS:Surface plasmon resonance is a sensitive method for detecting small differences in affinity. We were successful in using this technique to detect differences in the affinity of phosphorylated and nonphosphorylated rhodopsin peptides for visual arrestin. The data suggest that these are low-affinity interactions and indicate that phosphorylation is responsible for the direct binding of the rhodopsin carboxyl terminus to visual arrestin. Four phosphorylated residues are sufficient for this interaction. Because the affinity of the synthetic phosphopeptide for arrestin is substantially lower than the full length rhodopsin molecule, the cytoplasmic loops and rhodopsin carboxyl terminus appear to interact in a cooperative manner to stably bind arrestin.