Kinetics of the mating-specific aggregation in Saccharomyces cerevisiae

A comprehensive review of protein misfolding disorders, underlying mechanism, clinical diagnosis, and therapeutic strategies

INTRODUCTION

Proteins are the most common biological macromolecules in living system and are building blocks of life. They are extremely dynamic in structure and functions. Due to several modifications, proteins undergo misfolding, leading to aggregation and thereby developing neurodegenerative and systemic diseases. Understanding the pathology of these diseases and the techniques used to diagnose them is therefore crucial for their effective management . There are several techniques, currently being in use to diagnose them and those will be discussed in this review.

AIM/OBJECTIVES

Current review aims to discuss an overview of protein aggregation and the underlying mechanisms linked to neurodegeneration and systemic diseases. Also, the review highlights protein misfolding disorders, their clinical diagnosis, and treatment strategies.

METHODOLOGY

Literature related to neurodegenerative and systemic diseases was explored through PubMed, Google Scholar, Scopus, and Medline databases. The keywords used for literature survey and analysis are protein aggregation, neurodegenerative disorders, Alzheimer's disease, Parkinson's disease, systemic diseases, protein aggregation mechanisms, etc. DISCUSSION /CONCLUSION: This review summarises the pathogenesis of neurodegenerative and systemic disorders caused by protein misfolding and aggregation. The clinical diagnosis and therapeutic strategies adopted for the management of these diseases are also discussed to aid in a better understanding of protein misfolding disorders. Many significant concerns about the role, characteristics, and consequences of protein aggregates in neurodegenerative and systemic diseases are not clearly understood to date. Regardless of technological advancements, there are still great difficulties in the management and cure of these diseases. Therefore, for better understanding, diagnosis, and treatment of neurodegenerative and systemic diseases, more studies to identify novel drugs that may aid in their treatment and management are required.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Sexual agglutination in budding yeasts: structure, function, and regulation of adhesion glycoproteins

The sexual agglutinins of the budding yeasts are cell adhesion proteins that promote aggregation of cells during mating. In each yeast species, complementary agglutinins are expressed by cells of opposite mating type that interact to mediate aggregation. Saccharomyces cerevisiae alpha-agglutinin and its analogs from other yeasts are single-subunit glycoproteins that contain N-linked and O-linked oligosaccharides. The N-glycosidase-sensitive carbohydrate is not necessary for activity. The proposed binding domain of alpha-agglutinin has features characteristic of the immunoglobulin fold structures of cell adhesion proteins of higher eukaryotes. The C-terminal region of alpha-agglutinin plays a role in anchoring the glycoprotein to the cell surface. The S. cerevisiae alpha-agglutinin and its analogs from other species contain multiple subunits; one or more binding subunits, which interact with the opposite agglutinin, are disulfide bonded to a core subunit, which mediates cell wall anchorage. The core subunits are composed of 80 to 95% O-linked carbohydrate. The binding subunits have less carbohydrate, and both carbohydrate and peptide play roles in binding. The alpha-agglutinin and alpha-agglutinin genes from S. cerevisiae have been cloned and shown to be regulated by the mating-type locus, MAT, and by pheromone induction. The agglutinins are necessary for mating under conditions that do not promote cell-cell contact. The role of the agglutinins therefore is to promote close interactions between cells of opposite mating type and possibly to facilitate the response to phermone, thus increasing the efficiency of mating. We speculate that they mediate enhanced response to sex pheromones by providing a synapse at the point of cell-cell contact, at which both pheromone secretion and cell fusion occur.

Pheromone induction of agglutination in Saccharomyces cerevisiae a cells

a-Agglutinin, the cell surface sexual agglutinin of yeast a cells, was assayed by its ability to bind its complementary agglutinin, alpha-agglutinin. The specific binding of 125I-alpha-agglutinin to a cells treated with the sex pheromone alpha-factor was 2 to 2.5 times that of binding to a cells not treated with alpha-factor. Competition with unlabeled alpha-agglutinin revealed that the increased binding was due to increased cell surface expression of a-agglutinin, with no apparent change in the binding constant. The increase in site number was similar to the increase in cellular agglutinability. Increased expression of a-agglutinin followed the same kinetics as the increase in cellular agglutinability, with a 10-min lag followed by a 15- to 20-min response time. Induction kinetics were similar in cells in phases G1 and G2 of the cell cycle. Maximal expression levels were similar in cells treated with excess pheromone and in cells exposed to pheromone after destruction of constitutively expressed a-agglutinin.

Sexual agglutination in Saccharomyces cerevisiae

Treatment of either mating type of Saccharomyces cerevisiae with the appropriate sex pheromone increased cell-cell binding in a modified cocentrifugation assay. Constitutive agglutination of haploids was qualitatively similar to pheromone-induced agglutination. Regardless of exposure to pheromone, agglutinable combinations of cells exhibited maximal binding across similar ranges of ionic strength, pH, and temperature. Binding of all combinations was inhibited by 8 M urea, 1 M pyridine, or 0.05% sodium dodecyl sulfate. From alpha-cells we solubilized and partially purified an inhibitor of a-cell agglutinability. This inhibitor reversibly masked all a-cell adhesion sites and inactivated pheromone-treated and control cells with similar kinetics. The inhibitor behaved as a homogeneous species in heat inactivation experiments. Based on these results, we proposed a model for pheromone effects on agglutination in S. cerevisiae.

Fidelity of conjugation in Saccharomyces cerevisiae

An efficient method for the production of synchronous zygotes in Saccharomyces cerevisiae is described. Cells were synchronised under defined conditions in either an a, alpha mixed culture or by incubation of each mating type in cell-free medium in which cells of the opposite mating type had been grown. Synchronised cells were allowed to fuse under defined conditions on filter membranes. This method was used to test the fidelity of conjugation in S. cerevisiae. Under conditions where cells of a or alpha mating type were in contact with up to 6 cells of each of two strains of opposite mating type, less than 1 multiple mating in 10(4) diploid matings occurred. It is concluded that is sexual conjugation in S. cerevisiae some process distinct from cell contact restricts cell fusion to paired combinations of conjugant cells.

Cell-cell recognition in Saccharomyces cerevisiae: regulation of mating-specific adhesion

Mating-specific adhesion between haploid yeast cells of opposite mating type (a and alpha) was studied by using a quantitative agar plate assay. Washed a and alpha cells that had not previously been exposed to their respective opposite mating type ("naive" cells) adhered relatively weakly. In water, only 5 to 10% of the a cells stuck tightly enough to alpha cells to give rise subsequently to diploid clones on the assay plates. Under optimum conditions (pH 6 to 7, at least 0.1 M Nacl or 0.01 M Mg(2+)), there was about 20% adhesion. Nevertheless, this weak binding defined a mating type-specific interaction because, even under optimum conditions, the homologous interactions (a with a and alpha with alpha) yielded only 3 to 5% cohesion. In contrast to these results, washed cells that had been preincubated in the cell-free culture medium of their opposite mating type ("preconditioned" cells) adhered quite strongly. The degree of adhesion between preconditioned cells (40 to 50%) was essentially unaffected by extremes of ionic strength, pH, and temperature and by the absence of divalent cation. This strong interaction was also mating type specific since cohesion between preconditioned cells of like mating type was only about 5%. The increase in agglutinability was obtained if only the a cells were preconditioned and could be induced by highly purified preparations of natural or synthetically prepared alpha-factor, an oligopeptide pheromone released by the alpha cells. The appearance of increased adhesiveness was blocked by an inhibitor of RNA synthesis and by an inhibitor of protein synthesis, but not by an inhibitor of polysaccharide synthesis. Adhesion between preconditioned cells could be inhibited by pretreatment with functionally univalent succinylated concanavalin A or with extracts from preconditioned cells of the opposite mating type. These results confirm in a quantitative manner that the recognition between conjugating cells of S. cerevisiae is a developmentally regulated event that is under the control of the mating type locus.

a-Factor from Saccharomyces cerevisiae: partial characterization of a mating hormone produced by cells of mating type a

Conjugation between haploid cells of Saccharomyces cerevisiae is mediated through the action of diffusible mating hormones, two of which have been designated as a-factor and alpha-factor. Partially purified fractions exhibiting a-factor activity have been obtained from culture filtrates of a cells by ultrafiltration, ion-exchange chromatography, and gel filtration. The a-factor preparations specifically caused both G1 arrest and morphological alterations in cells of alpha-mating type, whereas a cells, a/alpha diploids, and nonmating alpha mutants were not affected. The a-factor activity was found in the culture filtrates of all a strains tested, but not in filtrates of alpha or a/alpha cell cultures. The hormone is sensitive to various proteases, showing that it is associated with a peptide or protein. Gel filtration studies suggest an apparent molecular weight greater than 600,000; however, this result may be due to aggregation with carbohydrate present in the preparations. Although the biological activities of a-factor are analogous to those described previously for alpha-factor, the chemical properties of these two hormones appear to be quite different.