For citation purposes: Garcia MC, Lipke PN, Klotz SA. Pathogenic microbial amyloids: their function and the host response. OA Microbiology 2013 Dec 01;1(1):2.

Review

 
Infection & Immunity

Pathogenic microbial amyloids: their function and the host response

MC Garcia1, PN Lipke1, SA Klotz2*
 

Authors affiliations

(1) Department of Biology, Brooklyn College of City University of New York, Brooklyn, NY, USA

(2) Department of Medicine, University of Arizona, Tucson, AZ, USA

* Corresponding author Email: sklotz@u.arizona.edu

Abstract

Introduction

Functional microbial amyloids are ubiquitous in nature and some contribute to the pathogenesis of infectious diseases. Three pathogenic microbialamyloids are compared and their contribution to the disease process explained. The recent demonstration and visualisation of fungal amyloid in human invasive candidiasis are discussed. Moreover, the binding of host serum amyloid P component to Candida functional amyloid in invasive human disease is presented in light of its possible role of masking fungi from the host defences.

Conclusion

Pathogenic fungi possess functional cell surface amyloids that are demonstrated by amyloidophilic dyes and these proteins bind serum amyloid P compound which may mask the fungal surface from host defences.

Introduction

Amyloid is a term ordinarily used to describe protein formed from spontaneously self-propagating, insoluble, β-sheet rich fibrils. These fibrils are resistant to enzymatic digestion, have characteristic patterns observed through electron microscopy and special tinctorial properties such as staining with the dyes, Congo red and thioflavin-T. Although amyloid fibres have been associated with disease, it is now apparent that the presence of amyloid is not always pathological. Amyloidoses are diseases in which amyloid deposits accumulate extracellularly and disrupt the structure and function of tissues and organs. The most common is AL amyloidosis caused by monoclonal immunoglobulin light chain deposition in the glomeruli of kidneys of patients with various plasma cell dyscrasias. One amyloidosis, AA, occurs in response to chronic inflammatory disorders including infectious diseases such as tuberculosis and chronic bacterial osteomyelitis. Examples of pathological amyloid deposits, but not considered amyloidoses, include Aβ-amyloid found in the plaques of Alzheimer’s disease and prions found in the spongiform encephalopathies. These disease states are often characterised by protein misfolding that exposes amyloid-forming properties.

The structure of amyloid assemblies can lead to functional properties. Amyloid’s tensile strength and resistance to degradation are advantageous in nanotechnology where it is used to manufacture tissue scaffolding, nanowires and nanotubes[1]. Natural amyloids include fibrils in skin cell melanosomes that impart a characteristic ultrastructure to the organelle, as seen on electron microscopy and are necessary for proper assembly and deposition of melanin[2].

Microbes elaborate amyloids that are used to fasten the microorganisms to a substratum. They are ubiquitous in nature and are important components of microbial biofilms[3,4]. Since microbial amyloids perform a beneficial function for the microorganism they are referred to as ‘functional amyloids’[5]. These fibrils serve to attach a microbe to a substratum and thus, secure a survival advantagefor the microorganism. Some functional amyloids attach microbes to inanimate surfaces, others to host cells, still others attach microbes to one another and some amyloids serve to stabilise the biofilm during infection[6]. We will briefly discuss three microbial functional amyloid proteins presumed to be integral to the pathogenesis of disease in humans: the curli protein of Escherichia coli, merozoite surface protein 2 (MSP2) of Plasmodium falciparum merozoites and the Als cell surface adhesins of Candida albicans.

Discussion

The authors have referenced some of their own studies in this review. These referenced studies have been conducted in accordance with the Declaration of Helsinki (1964) and the protocols of these studies have been approved by the relevant ethics committees related to the institution in which they were performed. All human subjects, in these referenced studies, gave informed consent to participate in these studies.

Three pathogenic amyloid adhesins of microbes

Curli

Pathogenic E. coli strains responsible for acute diarrhoeal diseases express multiple adhesins or attachment proteins including fimbriae which are long, hair-like appendages (micrometres in length) and short, amyloid fibrils known as curli. Fimbriae are resilient fibrils composed of repeating units of amino acids and the length of the assembled fibre provides for long-range interactions with substrata. They can bend and resist torsion and stretch to five times their normal length. Fimbriae are important in colonisation of a surface, for example, the intestinal wall (Table 1). Curli proteins are highly hydrophobic, attached to the cell membrane and contribute not only to adherence to tissue but also attachment of bacterium to other bacteria, that is, cell-to-cell aggregation which is critical for biofilm formation. Curli are found throughout Enterobacteriaceae and in some species such as E. coli and Salmonella spp., which are important in the pathogenesis of disease. In murine models, enterohaemorrhagic E. coli express curli in order to ensure cell-to-cell aggregation and adherence to intestinal cells. Curli are the product of several proteins. One subunit attaches to the outer cell membrane and then nucleates another protein on top of itself, a process that continues one after another to form a fibril[3]. It is interesting to note that some secreted proteins of E. coli form amyloid ropes (> cm in length) that demonstrate characteristics of amyloid, that is, the ability to self-propagate[7].

Table 1

Characteristics of three microbial cell surface amyloids that mediate adherence of the microbe to human tissue and cells

Merozoite surface protein 2

The most lethal form of malaria is caused by P. falciparum, which stands out from other less virulent species of malaria by reason of its ability to parasitise a high percentage of red blood cells. The stage of the parasite that attaches and enters the red blood cell is the merozoite. It is released initially from the liver, enters red blood cells and ultimately ruptures the cell. The parasites attach and enter the red blood cell in order to obtain their food source, haemoglobin. The initial contact of a recently released merozoite and red blood cell is thought to occur by random collision. On the surface coat of P. falciparum merozoites are numerous proteins that extend out from the parasite surface like small knobs[8]. There are number of merozoite surface proteins believed to be important in the adherence of the parasite to the red blood cell (several of these proteins are being incorporated into vaccines). MSP2 is one of these surface proteins and is unique in that it contains a functional amyloid in the N-terminus and is likely important in attachment of the merozoite to the red blood cell[9] (Table 1). The amyloid fibrils of MSP2 are resistant to proteinase, stain with Congo red and are formed under physiologic conditions, all characteristics of amyloid fibres[10]. After attachment of the merozoite to the red blood cell, the parasite reorients on the cell surface and, using different adhesins, penetrates the cell. Once it is intracellular the merozoite is free to digest haemoglobin.

Als proteins of C. albicans

There are eight different Als proteins in C. albicans located on the cell surface of yeasts and hyphae of this opportunistic pathogen. Although curli and MSP2 proteins appear to be required for pathogenesis of disease in the human, the intended purpose of Als proteins in C. albicans is likely to serve a social function, that is, aggregating fungal cells to another as this is the preferred phenotype in nature. Candida albicans is considereda commensal of human mucous surfaces. However, if host defences are compromised the Als proteins can mediate adherence of the fungus to peptides and proteins, the fungus does not ordinarily encounter, for example, human endothelium or urothelial cells. The best studied of the Als proteins, Als5p when attached to the surface of non-pathogenic Saccharomyces cerevisiae (baker’s yeast) has been used to model the functions of the different regions of the Als proteins. The protein is connected to the cell wall by a glycosylphosphatidyl inositol anchor where a C-terminal stalk region raises the protein above the cell surface (Figure 1). The stalk is followed by multiple 36-amino acid tandem repeats (the number of repeats varies in each protein), which are instrumental in cell-to-cell aggregation. A threonine-rich domain forms a connection between the tandem repeats, and the N-terminal immunoglobulin region binds to various peptides exposed on host cells and fungal cells (Figure 1). The threonine-rich domain is formed of β-sheets that form amyloid fibrils. In addition to amyloid formation this region contributes to the adherence process by stabilising adherence. It is characteristic of C. albicans adherence and aggregation that it is very resistant to physical and chemical force, for example, 8 M urea and formamide are required to undue adherence (as they are to break apart amyloid). We have termed the Als proteins the ‘perfect adhesive’: binding to a wide variety of protein targets with almost unbreakable adhesion even in the face of shear forces[11]. These cell surface proteins likely contribute to the phenotypic characteristics of the fungus (Table 2).

Table 2

Cell surface functional amyloids of Candida albicans influence cellular phenotypic characteristics of the fungus

Schematic representation of Candida albicans Als cell surface protein.

Fungal amyloid in human infection

The recent observation that Candida spp. elaborate amyloid in human disease and that these fibrils are ‘recognised’ by the host resulting in the deposition of host serum amyloid P component (SAP) onto the fungi has opened a new chapter in candidiasis[12,13]. The study involved 25 autopsy patients with invasive candidiasis of the intestines. Many, but not all of the patients had neutropenia near the time of death and some had candidemia, detected either antemortem or at autopsy[14]. Multiple specimens from each patient demonstrated yeast cells and hyphae entering the intestines through a point of ulceration of the bowel epithelium into the muscularis layer. The amyloidophilic dyes Congo red and thioflavin-T demonstrated the presenceof amyloid on all morphologies of invading Candida (Figures 2 and 3).A monoclonal antibody directed against SAP bound to the same surfaces as dyes (Figure 4). Accompanying in vitro work established the fact that SAP has a specific interaction with the fungi once functional amyloid is expressed on the fungal surface.

Thioflavin-T binds to fungal cell surface amyloids.Tissue was stained with 100 nM thioflavin-T: (a) Candida cells in human intestinal tissue binding the fluorescent thioflavin-1 and (b) same view, light microscopy.

Congo red and thioflavin-S bind to yeast surface amyloidsin situ. Tissue sections were stained with (a) 0.1% Congo red and (b) 0.1% thioflavin-S.

Serum amyloid P component binds to Candida in human tissue. Tissue sections were incubated with anti-SAP, washed and probed with TRITC-conjugated anti-rabbit antibody. The centre panel is an overlay of the immunofluorescence image (left) and the bright field (right). Serum amyloid P component deposits co-localise with fungi in the tissue.

This interaction of SAP with Candida spp. was a surprising finding and not previously known to occur. The story is intriguing because the presence of SAP is a characteristic finding of all the amyloidoses. SAP is a pentraxin, a pentameric protein that circulates at relatively stable concentrations of 20–35 mg/L. (C-reactive protein, the acute phase reactant, has homologous structures to SAP and is also a pentraxin with a dynamic range of 0.05–500 mg/L.) Human SAP, however, is not an acute phase reactant. It binds to amyloid whether it is intracellular or extracellular and is universally present where amyloid isdeposited[15]. In amyloidoses, SAP contributes to the mass effect of the amyloid deposition that leads to tissue and organ dysfunction. Removal of SAP from amyloid deposits in patients results in improvement[16]. It is easy to envision how pentameric SAP might cover up amyloid fibrils and mask recognition of amyloid. Perhaps this is one of the reasons that amyloid in humans demonstrates little or no host response—it is seemingly inert. We found in autopsy patients that there was little or no inflammation in response to the fungi present in tissue irrespective of the peripheral white blood cell counts–perhaps the SAP masked the functional amyloid. This masking property of SAP has been proposed as the mechanism by which SAP bound to Gram-negative lipopolysaccharide and prevented classical pathway complement activation[17]. It also might be why SAP may ameliorate asthma exacerbations caused by Aspergillus. Conidia of the aforementioned fungus are armoured with functional amyloid fibres on their cell surface[18] and they penetrate deep into the airways where SAP could conceivably interact with the fungi[19]. Finally, amyloids with their avidity for binding to ‘self’ could serve as probes to identify pathogens in tissue[20] or perhaps as anti-microbials[21].

Conclusion

As discussed, there are multiple examples of cell surface amyloids on microbes. These functional amyloids are critical for the success of the microbe allowing attachment to non-biological surfaces, forming cellular aggregates and adhering to host tissue. They are also necessary for the success of biofilms and also in infection of the host. These unique structures are utilised for the benefit of the microbe but sometimes to the detriment of the host as seen in some infectious diseases.

Abbreviations list

GPI, glycosylphosphatidyl inositol anchor; SAP, serum amyloid Pcomponent.

Authors contribution

All authors contributed to the conception, design, and preparation of the manuscript, as well as read and approved the final manuscript.

Competing interests

None declared.

Conflict of interests

None declared.

A.M.E

All authors abide by the Association for Medical Ethics (AME) ethical rules of disclosure.

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Characteristics of three microbial cell surface amyloids that mediate adherence of the microbe to human tissue and cells

Characteristic Escherichia coli curli Plasmodium falciparum MSP2 Candida albicans Als proteins
Molecular weight Polymers of ~15 kDa protein 30 kDa Heavily glycosylated, >200 kD
GPI cell surface anchor? No Yes Yes
Location of amyloid Entire structure is attached to the outer cell membrane N-terminus of the protein A threonine-rich repeat regionadjacent to N-terminalimmunoglobulin region
Agglutination oraggregation of microbes? Yes Not known Yes
Amyloid fibresdemonstrated? Yes Yes Yes
Attachment targets Host proteins, other Escherichia coli Possibly glycophorin Peptides/proteins of other candida albicans or host tissue
Vaccine candidate Antibodies to curli can be detected in patients with disease (Bian et al. 2000) Yes, one of several surface proteins being included in potential vaccines Yes (Edwards 2012)

GPI, glycosylphosphatidyl inositol anchor. These amyloids are all expressed under physiological conditions.

Cell surface functional amyloids of Candida albicans influence cellular phenotypic characteristics of the fungus

Characteristics of amyloids Does this apply to Candida albicans?
Hydrophobic Yes (Klotz et al. 1985)
Surface active Yes (Klotz 1989)
Avidity of Candida albicans amyloid for self Yes (Garcia et al.20)
Aggregation of cells Yes (Klotz and Penn 1987)
Repetitive motif Yes (Lipke et al. 2011)
Keywords