News from the Fungal Front: Wall Proteome Dynamics and Host–Pathogen Interplay

Klis FM (2012) News from the Fungal Front: Wall Proteome Dynamics and Host-Pathogen Interplay. PLoS Pathog 8(12): e1003050. doi:10.1371/journal.ppat.1003050 News from the Fungal Front: Wall Proteome Dynamics and Host-Pathogen Interplay Clemens J. Heilmann 0 Alice G. Sorgo 0 Frans M. Klis 0 Joseph Heitman, Duke University Medical Center, United States of America 0 Swammerdam Institute for Life Sciences, University of Amsterdam , Amsterdam , The Netherlands - In Candida albicans, like in Saccharomyces cerevisiae, the basal layer of the mature cell wall consists of a network of b-1,3- and b-1,6glucans and chitin and functions as a skeletal layer. This basal layer is covered by an external layer of highly glycosylated, covalently anchored wall proteins radiating from the cell surface, which are directly involved in the first contacts between the fungal pathogen and host cells. The majority of the covalently bound wall proteins are modular glycosylphosphatidylinositol (GPI)-proteins. In their final form, wall-bound GPI-proteins usually consist of a Cterminal, truncated GPI-anchor that attaches them to the b-glucan layer, followed by a heavily glycosylated serine/threonine-rich spacer domain that often includes repeats, and an N-terminally located functional domain protruding from the cell surface [1]. At any given time-point .20 different covalently bound wall proteins can be identified [2,3] that are involved in processes such as adhesion, biofilm formation, wall remodeling, iron acquisition, and coping with immune responses. Importantly, the wall proteome is highly dynamic and continuously adapts to the specific conditions that C. albicans encounters in the host environment. In this review we examine the role of wall proteins in infection-related processes and assess their potential as targets for antifungal and vaccine development. Why Do Most Wall Proteins Form Families? C. albicans is able to thrive in many host niches, including the skin, mucosal surfaces, the bloodstream, and internal organs. Wall proteins are subject to the surrounding conditions and come into contact with highly diverse, niche-associated, extracellular matrix proteins from the host as well as with bacterial surface proteins. This probably explains the evolution of many wall protein families with individual members showing optimal functionality dependent on environmental conditions and infection sites [1]. For example, the environmental pH strongly affects the wall proteome, revealing the preferred usage of specific family members at acidic and neutral pH [4]. Interestingly, invasive growth is generally associated with hyphal growth, and comparison of the wall proteomes of yeast and hyphal cells revealed a core set of hypha-associated wall proteins under various hyphal growth-inducing conditions (Als3, Hwp1, Hwp2, Hyr1, Plb5, and Sod5) [2,5]. The two largest wall protein families are the Als family [6] and the Hyr/Iff family [7]. The family of agglutinin-like sequence (ALS) proteins consists of eight GPI-modified, elongated, broadspecificity adhesins with an immunoglobulin-like N-terminal domain that can interact with a wide variety of host proteins [8]. Some Als proteins possess amyloid-forming sequences, which could play a role in forming biofilms [9]. Fascinatingly, Als3 has multiple functions, including ferritin binding [10] as well as binding to E-cadherin, thereby facilitating iron uptake and active internalization of C. albicans by host cells, respectively [11]. This supports that proteins of a family may share a particular function, but might also have additional functions that are not conserved throughout the family. Intriguingly, Hyr1, one of the 12 GPI-proteins belonging to the Iff/Hyr family, is strongly hypha-associated and confers resistance to neutrophil killing [12] through its N-terminal domain. Although the domain structure within the family is variable, the N-terminal domain is strongly conserved in all family members (Figure 1) [7]. This hints at a more general, niche-specific role of the family in evading immune cells under different growth conditions. One of the most restricted nutrients in the human body is iron. Because of its reactive nature, but also in order to restrict growth of invading microorganisms, free iron is highly limited in the host and mainly found in association with proteins, either as a prosthetic group like in hemoglobin and myoglobin, stored inside ferritin, transported by transferrin, or liganded by lactoferrin. C. albicans has evolved a number of strategies to scavenge iron from these complexes. Of the five Rbt5 family proteins, which belong to the CFEM superfamily and are characterized by an internal domain containing eight invariantly spaced cysteines [13], Csa1, Pga7, Pga10, and Rbt5 are found attached both to the plasma membrane and the wall, while Csa2 is secreted [3,1416]. It has been shown that Csa1, Pga10, and Rbt5 are involved in heme binding [17]. As the expression of CSA1, CSA2, PGA7, PGA10, and RBT5 is coregulated under various conditions, including iron restriction, the question arises whether the Rbt5 family proteins might act as a relay system, similar to bacterial iron uptake systems [18]. As mentioned above, Als3 is also important for iron acquisition as a receptor for ferritin, an iron-storage host molecule that contains about 30% of the total human iron pool. Without Als3, C. albicans is unable to grow with ferritin as its sole iron source [10]. Which Wall Proteins Allow C. albicans to Cope with the Host Immune Response? C. albicans has evolved various mechanisms to avoid or counteract the immune response. The cell wall is the first line of defense, but also a target for the immune system due to its immunogenic epitopes. For example, the receptor dectin-1, which is mainly expressed on dendritic cells and macrophages, recognizes the b-glucan of the wall and leads to the activation of proinflammatory cytokines [19]. However, the mannoprotein coat largely prevents the detection of the underlying b-glucan layer. Additionally, the wall protein Hyr1 effectively reduces immune cell killing of C. albicans [12]. In support of its protective role, heterologous expression of Hyr1 in Candida glabrata also mitigates immune cell killing, suggesting a direct function of the protein. C. albicans also has two wall-bound, morphotype-associated superoxide dismutases (Sod4, Sod5) [14]. These cell wallresident superoxide dismutases (Sods) are essential for dealing with extracellular ROS (reactive oxygen species), resulting from the oxidative burst, a general mechanism of immune cells to kill invading pathogens. As expected, SOD4 and SOD5 knockout mutants are more susceptible to oxidative stress [20]. Sod6, another GPI-anchored member of the Sod family, has not been detected in proteomic screens, and gene deletion did not reveal a clear phenotype [20]. Antimicrobial peptides, like histatins, defensins, and cathelicidins, belong to the arsenal of h (...truncated)