The Alveolin IMC1h Is Required for Normal Ookinete and Sporozoite Motility Behaviour and Host Colonisation in Plasmodium berghei

Abstract

Alveolins, or inner membrane complex (IMC) proteins, are components of the subpellicular network that forms a structural part of the pellicle of malaria parasites. In Plasmodium berghei, deletions of three alveolins, IMC1a, b, and h, each resulted in reduced mechanical strength and gliding velocity of ookinetes or sporozoites. Using time lapse imaging, we show here that deletion of IMC1h (PBANKA_143660) also has an impact on the directionality and motility behaviour of both ookinetes and sporozoites. Despite their marked motility defects, sporozoites lacking IMC1h were able to invade mosquito salivary glands, allowing us to investigate the role of IMC1h in colonisation of the mammalian host. We show that IMC1h is essential for sporozoites to progress through the dermis in vivo but does not play a significant role in hepatoma cell transmigration and invasion in vitro. Colocalisation of IMC1h with the residual IMC in liver stages was detected up to 30 hours after infection and parasites lacking IMC1h showed developmental defects in vitro and a delayed onset of blood stage infection in vivo. Together, these results suggest that IMC1h is involved in maintaining the cellular architecture which supports normal motility behaviour, access of the sporozoites to the blood stream, and further colonisation of the mammalian host.

Active migration and passive transport of malaria parasites

“…The omission of the skin phase is unfortunate because the necessity of active parasite migration in the skin has opened up new opportunities for stopping the parasites before they enter the bloodstream, and skin migration may be an excellent drug and vaccine target. Indeed, sporozoite mutants with motility defects are significantly more attenuated after inoculation into the skin compared to after intravenous inoculation [16][17][18][19]. Similarly, parasite mutants defective in cell traversal activity, the sporozoite capacity to wound and transmigrate a host cell, also show impaired progression in the skin [20][21][22].…”

“…The capacity to actively migrate is clearly important for sporozoite entry into hepatocytes [19,32]. However, fast and robust motility is only needed in the skin because diminished motility still allows sporozoites to effectively enter the liver if they are injected by syringe directly into the bloodstream [16][17][18][19]21,33]. This suggests that the malaria parasite has evolved a high speed specifically to cross the dermis, a finding that might well be important in intervention considerations that target their motility.…”

Gliding motility in apicomplexan parasites

“…In addition to these scaffolding proteins and the anchoring components of the motility apparatus that reside in the IMC, many additional proteins are known to occupy the pellicle or IMC domain butmost have yet to be characterized. [20][21][22].The importance of pellicle architecture in structurally supporting normal parasite motility has been demonstrated by several studies in which disruption of pellicle components has led to altered gliding behavior [23][24][25][26].…”

“…A productive forward translocation of the parasite, at a rate of ~1 to 3µm/sec, can be accomplished with either a circular gliding motility or a rotation-driven helical gliding behavior, though the latter appears to be the more important mode of movement as the parasite approaches and invades a potential host cell [40] (Fig 2a). Parasitesmay also anchor themselves at their posterior poles and engage in a seemingly non-productive vertical twirling behavior [40].However, when motility is observed within amore appropriateenvironment such as intact host tissue or even artificial,three-dimensional matrices, the movement patterns exhibited by parasitesin vitro translate into more of an irregular corkscrew-like motility that propels the parasitesin a somewhat curvilinear trajectory through the tissue [24][25][26][42][43][44][45] (Fig. 2b).…”

The Plasmodium alveolin IMC1a is stabilised by its terminal cysteine motifs and facilitates sporozoite morphogenesis and infectivity in a dose-dependent manner

“…In the genus Plasmodium , 13 conserved and syntenic alveolin family members have been identified that are differentially expressed among the three different zoites stages of malaria parasites [7], [8]. It has been shown in the rodent malaria species P. berghei that disruption of alveolins gives rise to morphological aberrations that are accompanied by reduced tensile strength of the zoite stages in which they are found [5], [8], [9], [10], [11]. In Tetrahymena thermophila , knockdown of the alveolin Tt ALV2 was also reported to affect cell morphology [12], indicating that alveolin functions, like their structures, are evolutionary conserved.…”