CelTOS, a novel malarial protein that mediates transmission to mosquito and vertebrate hosts

Abstract

SummaryThe malarial parasite has two hosts in its life cycle, a vertebrate and a mosquito. We report here that malarial invasion into these hosts is mediated by a protein, designated cell-traversal protein for ookinetes and sporozoites (CelTOS), which is localized to micronemes that are organelles for parasite invasive motility. Targeted disruption of the CelTOS gene in Plasmodium berghei reduced parasite infectivity in the mosquito host approximately 200-fold. The disruption also reduced the sporozoite infectivity in the liver and almost abolished its cell-passage ability. Liver infectivity was restored in Kupffer cell-depleted rats, indicating that CelTOS is necessary for sporozoite passage from the circulatory system to hepatocytes through the liver sinusoidal cell layer. Electron microscopic analysis revealed that celtos -disrupted ookinetes invade the midgut epithelial cell by rupturing the cell membrane, but then fail to cross the cell, indicating that CelTOS is necessary for migration through the cytoplasm. These results suggest that conserved cell-passage mechanisms are used by both sporozoites and ookinetes to breach host cellular barriers. Elucidation of these mechanisms might lead to novel antimalarial strategies to block parasite's transmission.

The development of malaria parasites in the mosquito midgut

“…Other microneme proteins assigned to midgut traversal of P . berghei ookinetes include SOAP, WARP and CelTOS (cell‐traversal protein for ookinetes and sporozoites) (Yuda et al ., 2001; Dessens et al ., 2003; Kadota et al ., 2004; Li et al ., 2004; Kariu et al ., 2006; Ecker et al ., 2007, 2008; Kaneko et al ., 2015; Wirth et al ., 2015) (Table 1). …”

A long and winding road: The Plasmodium sporozoite's journey in the mammalian host

“…In addition, op/op mice which have reduced numbers of Kupffer cells have somewhat lower parasite burdens when inoculated with sporozoites by mosquito bite [49]. However, treating mice with CL, which as stated earlier eliminates Kupffer cells and creates large gaps in the sinusoidal barrier [49], leads to enhanced liver infection with wild type sporozoites and restores the infectivity of SPECT 1, SPECT 2 and CelTOS knockout sporozoites [25,27,28] indicating that Kupffer cell passage is not obligatory for hepatocyte infection. In the normal hepatic sinusoid, however, the majority of sporozoites probably gain access to the hepatic parenchyma by crossing Kupffer cells.…”

“…Sporozoites in which one of the SPECT proteins or CelTOS was deleted using gene transfection technology show similar phenotypes: In vitro they lack the ability to traverse cells but when placed directly on hepatocytes they invade and develop normally. In vivo these mutants lack infectivity for the mammalian host when injected intravenously, a phenotype that can be reversed by pretreating mice with liposomeencapsulated dichloromethylene diphosponate (CL) which depletes the liver of Kupffer cells and leaves gaps in the sinusoidal barrier thus allowing sporozoites direct access to hepatocytes [25,27,28]. It is likely that these proteins are also required for sporozoite exit from the dermis and in the case of the SPECT 1 knockout this has been recently directly demonstrated by intravital microscopy [Amino and Menard, unpublished data].…”

“…Although we are still far from a molecular understanding of this process, four proteins involved in cell traversal recently have been identified. Three of these, SPECT 1 (sporozoite microneme protein essential for cell traversal) and SPECT 2 (also called perforin-like protein 1; PLP1) and CelTOS (cell traversal protein for ookinetes and sporozoites) were identified using EST databases [25][26][27][28] while the fourth, a phospholipase (PL) was found using an RNA subtraction screen that identified genes up-regulated in salivary gland sporozoites [29,30]. Expression of SPECT 1, SPECT 2 and PL are specific to salivary gland sporozoites whereas CelTOS is expressed by both ookinetes and salivary gland sporozoites.…”

Cellular and molecular interactions between the apicomplexan parasitesPlasmodiumandTheileriaand their host cells

“…The process of cell transmigration is similar to that displayed by sporozoites in traversing hepatocytes, again requiring the action of perforin-like proteins (Ecker et al, 2007;Ishino et al, 2005;Kadota et al, 2004). In both processes, a prerequisite of parasite transmigration is secretion of the parasite protein CelTOS into the cytoplasm of the traversed cell (Kariu et al, 2006). Epithelial cells traversed by ookinetes invariably die by apoptosis and are extruded into the mosquito midgut (Han et al, 2000).…”