A conserved Plasmodium structural integrity maintenance protein (SIMP) is associated with sporozoite membrane and is essential for maintaining shape and infectivity

Abstract

Plasmodium sporozoites are extracellular forms introduced during mosquito bite that selectively invade mammalian hepatocytes. Sporozoites are delimited by a cell membrane that is linked to the underlying acto‐myosin molecular motor. While membrane proteins with roles in motility and invasion have been well studied, very little is known about proteins that maintain the sporozoite shape. We demonstrate that in Plasmodium berghei (Pb) a conserved hypothetical gene, PBANKA_1422900 specifies sporozoite structural integrity maintenance protein (SIMP) required for maintaining the sporozoite shape and motility. Sporozoites lacking SIMP exhibited loss of regular shape, extensive membrane blebbing at multiple foci, and membrane detachment. The mutant sporozoites failed to infect hepatocytes, though the altered shape did not affect the organization of cytoskeleton or inner membrane complex (IMC). Interestingly, the components of IMC failed to extend under the membrane blebs likely suggesting that SIMP may assist in anchoring the membrane to IMC. Endogenous C‐terminal HA tagging localized SIMP to membrane and revealed the C‐terminus of the protein to be extracellular. Since SIMP is highly conserved among Plasmodium species, these findings have important implications for utilizing it as a novel sporozoite‐specific vaccine candidate.

A conserved malaria parasite protein required for maintenance of sporozoite cell shape and transmission

“…PIC5 immunostaining also revealed that the IMC does not underlie the plasma membrane blebs indicating that SIMP depletion leads to loss of contact between the plasma membrane and IMC (Figure 2). This conclusion is indeed supported by their transmission electron microscopic examination of SIMP-depleted salivary gland sporozoites (Singh et al, 2022).…”

“…The latter observations strongly indicate that the carboxy terminus of SIMP is in fact extracellular. This notion was corroborated by showing that immunostaining with antibodies raised against the carboxy‐terminal part of SIMP also does not require sporozoite permeabilisation (Singh et al, 2022).…”

“…In a study by Singh and colleagues reported in this issue (Singh et al, 2022), the protein PBANKA_1422900 is named Structural Integrity Maintenance Protein (SIMP). The authors demonstrate by gene disruption that SIMP is dispensable for asexual, sexual, ookinete, oocyst and sporozoite stage development.…”

“…However, SIMP‐depleted sporozoites, albeit able to colonise the salivary glands in numbers comparable to wild‐type parasites, are not infective to the mouse and unable to establish blood‐stage infections after intravenous injection of sporozoites. Singh et al (2022) go on to investigate why SIMP knockout sporozoites are non‐infectious. By using circumsporozoite protein (CSP) immunostaining, they show that salivary gland‐resident SIMP‐depleted sporozoites have an abnormal shape that is characterised by the presence of multiple plasma membrane ‘blebs’, and further show that these sporozoites display highly reduced gliding motility on glass slides, which is a likely cause of their infectivity loss.…”

“…By using circumsporozoite protein (CSP) immunostaining, they show that salivary gland‐resident SIMP‐depleted sporozoites have an abnormal shape that is characterised by the presence of multiple plasma membrane ‘blebs’, and further show that these sporozoites display highly reduced gliding motility on glass slides, which is a likely cause of their infectivity loss. These shape and motility phenotypes were reversed in parasites where SIMP function is restored by complementation (Singh et al, 2022). Further cellular investigation of SIMP‐depleted sporozoites indicates that they possess an intact cytoskeleton (using phalloidin/actin staining), as well as an intact IMC organisation using immunostaining of the IMC marker PIC5.…”