life Review Unique Endomembrane Systems and Virulence in Pathogenic Protozoa Mark F. Wiser Department of Tropical Medicine, Tulane University School of Public Health and Tropical Medicine, New Orleans, LA 70112, USA; [email protected] Abstract: Virulence in pathogenic protozoa is often tied to secretory processes such as the expression of adhesins on parasite surfaces or the secretion of proteases to assisted in tissue invasion and other proteins to avoid the immune system. This review is a broad overview of the endomembrane systems of pathogenic protozoa with a focus on Giardia, Trichomonas, Entamoeba, kinetoplastids, and apicomplexans. The focus is on unique features of these protozoa and how these features relate to virulence. In general, the basic elements of the endocytic and exocytic pathways are present in all protozoa. Some of these elements, especially the endosomal compartments, have been repurposed by the various species and quite often the repurposing is associated with virulence. The Apicomplexa exhibit the most unique endomembrane systems. This includes unique secretory organelles that play a central role in interactions between parasite and host and are involved in the invasion of host cells. Furthermore, as intracellular parasites, the apicomplexans extensively modify their host cells through the secretion of proteins and other material into the host cell. This includes a unique targeting motif for proteins destined for the host cell. Most notable among the apicomplexans is the malaria parasite, which extensively modifies and exports numerous proteins into the host erythrocyte. These modifications of the host erythrocyte include the formation of unique membranes and structures in the host erythrocyte cytoplasm and on the erythrocyte membrane. The transport of parasite proteins to the host erythrocyte involves several unique mechanisms and components, as well as the Citation: Wiser, M.F. Unique generation of compartments within the erythrocyte that participate in extraparasite trafficking. Endomembrane Systems and Virulence in Pathogenic Protozoa. Life Keywords: protozoa; secretory pathway; endomembranes; pathogenesis; virulence; host-parasite 2021, 11, 822. https://doi.org/ 10.3390/life11080822 interaction; Giardia; kinetoplastids; Trypanosoma; Apicomplexa; Plasmodium; Toxoplasma Academic Editor: Stanislas Tomavo Received: 15 July 2021 1. Introduction Accepted: 10 August 2021 Secretion has long been recognized as a virulence factor in pathogen bacteria. Var- Published: 12 August 2021 ious secretion pathways have been defined and these bacterial secretory pathways are responsible for the secretion of adhesins, invasins, and toxins associated with bacterial Publisher’s Note: MDPI stays neutral pathogenesis [1]. This same concept can also be applied to pathogenic protozoa. For with regard to jurisdictional claims in example, pathogenic protozoa express proteins that participate in host-parasite interactions published maps and institutional affil- that can be classified as adhesins. Some of these adhesins may assist the pathogen in iations. invading tissues or cells. In addition, many pathogens secrete proteases and other factors that break down tissues and cause inflammation. Obviously, adhesins and other virulence factors must be expressed at the interface between the pathogen and the host, such as a cell surface location or secretion into the host. Proteins expressed on the surface of cells Copyright: © 2021 by the author. typically have signal sequences that direct proteins to the cell surface via the endoplasmic Licensee MDPI, Basel, Switzerland. reticulum (ER) and Golgi apparatus. Similarly, secreted virulence factors are also processed This article is an open access article by these same endomembrane systems. distributed under the terms and Most of our knowledge on endomembrane systems is derived from yeast and mam- conditions of the Creative Commons malian systems and in general the endomembranes are not well characterized in protozoa. Attribution (CC BY) license (https:// All protozoa have the basic elements of the eukaryotic secretory pathway and endomem- creativecommons.org/licenses/by/ brane system. However, variations in the endomembrane system of protozoa have been 4.0/). Life 2021, 11, 822. https://doi.org/10.3390/life11080822 https://www.mdpi.com/journal/life Life 2021, 11, 822 2 of 23 noted and include the loss of some organelles or the acquisition of unique organelles that are only found in specific phylogenic groups [2]. Often these variations in endomembranes play a role in disease pathogenesis. This review provides a broad overview of endomem- brane systems and secretion of virulence factors in pathogenic protozoa. In particular, the uniqueness of endocytic and exocytic processes in pathogenic protozoa and how these processes contribute to pathogenesis is highlighted. Protozoa exhibit an extreme range of diversity and do not represent a monophyletic group [3]. Organisms that are typically viewed as protozoa are found in all the major eukaryotic supergroups (TSAR—an acronym for telonemids, stramenopiles, alveolates, and rhizaria, Haptista, Cryptista, Archaeplastida, Amorphea, CRuMs—an acronym for collodictyonids, Rigifilida, and Mantamonas, Discoba, Metamonada, Hemimastigophora, and Excavata) [4]. Several species from these various supergroups are human pathogens and a few species are capable of causing severe human disease [5,6]. The most com- mon pathogenic protozoa are Giardia duodenalis (giardiasis), Entamoeba histolytica (amebic dysentery), Trichomonas vaginalis (trichomoniasis), Trypanosoma brucei gambiense (African sleeping sickness), Trypanosoma cruzi (Chagas disease), Leishmania species (leishmania- sis), Plasmodium species (malaria), Toxoplasma gondii (toxoplasmosis), and Cryptosporidium species (cryptosporidiosis). These species and diseases are the focus of this review due to their medical importance, and the fact that more work has been done on these species as compared to less virulent protozoa. 2. Basic Endomembrane Systems Phagocytosis and the development of endomembrane systems and membrane traf- ficking is paramount in the origin of eukaryotes [7,8], and therefore, it is expected that pathogenic protozoa would contain similar endomembranes as other eukaryotes. Further, and indeed, the key components of endocytic and exocytic pathways are present in all protozoa. Key components of the endomembrane system include the nuclear envelope, endoplasmic reticulum (ER), Golgi, lysosomes and related vacuoles, endosomes, micro- bodies, vesicles that transport material between compartments, and the plasma membrane (Figure1) . Proteins destined to be secreted or transported to organelles typically have a signal sequence and pass through the ER and Golgi via transport vesicles [9]. How- ever, the secretomes of pathogenic protozoa include many proteins being exported by unconventional protein secretion [10]. The ER is a membrane network that emanates from the outer membrane of the nuclear envelope. The function of the ER includes lipid synthesis, the first step in protein secretion and sorting, and quality control of protein folding. Membrane networks corresponding to the ER have been identified in all protozoa, and for the most part, the functions of the ER are preserved. Although, in regards to quality control and ER stress, the elements of ER-associated degradation and unfolded protein response do differ slightly between the pathogenic protozoa and other eukaryotes [11]. Proteins destined for the plasma membrane, other organelles, or to be secreted are packaged into vesicles at ER exit sites and are usually transported to the Golgi. The Golgi consists of flattened membrane disks, called cisternae. The cisternal face proximal to the ER is the cis-Golgi network and the face on the opposite side is the trans-Golgi network. Some pathogenic protozoa do not have the iconic stacks of Golgi cisternae (Table1). Evolutionarily, this is likely due to a reduction of the Golgi and its function, and this reduction has occurred independently several times during evolution [12]. Life 2021Life 2021, 11,, 11x ,FOR 822 PEER REVIEW 3 of 23 3 of 25 FigureFigure 1. 1.Major Major components components of eukaryoticof eukaryotic endomembrane endomembrane systems systems that are that present are pres in protozoa.ent in protozoa. The Theendoplasmic endoplasmic reticulum reticulum (ER) (ER) plays plays a centralcentral role role in in the the generation generation of the of endomembranesthe endomembranes of of eukar- eukaryoticyotic cells. cells. Vesicles Vesicles shuttle shuttle membranemembrane components componen andts proteins and proteins between between the ER and the the variousER and the various endomembranesendomembranes such such as Golgi as Golgi and microsomes.and microsomes. Endocytosed Endo materialcytosed is material transported is transported to lysosomes to lysosomes oror multivesicularmultivesicular bodies bodies (MVB) (MVB) via the via endosome. the endosome. MVB can MVB be incorporated can be incorporated into the lysosome into the and lysosome and degradeddegraded or or secreted secreted from from the cellthe as cell extracellular as extracellular vesicles vesicl calledes exosomes. called exosomes. Table 1. Endomembranes of Major Protozoan Pathogens of Humans The ER is a membrane network that emanates from the outer membrane of the nu- Pathogen clearGolgi envelope. The function Microbodies of the ER includes Unique lipid Compartmentssynthesis, the first step in protein (Super Group) secretion and