Aedes aegypti SGS1 is critical for Plasmodium gallinaceum infection of both the mosquito midgut and salivary glands

Abstract

Background The invasion of the mosquito salivary glands by Plasmodium sporozoites is a critical step that defines the success of malaria transmission and a detailed understanding of the molecules responsible for salivary gland invasion could be leveraged towards control of vector-borne pathogens. Antibodies directed against the mosquito salivary gland protein SGS1 have been shown to reduce Plasmodium gallinaceum sporozoite invasion of Aedes aegypti salivary glands, but the specific role of this protein in sporozoite invasion and in other stages of the Plasmodium life cycle remains unknown. Methods RNA interference and CRISPR/Cas9 were used to evaluate the role of A. aegypti SGS1 in the P. gallinaceum life cycle. Results Knockdown and knockout of SGS1 disrupted sporozoite invasion of the salivary gland. Interestingly, mosquitoes lacking SGS1 also displayed fewer oocysts. Proteomic analyses confirmed the abolishment of SGS1 in the salivary gland of SGS1 knockout mosquitoes and revealed that the C-terminus of the protein is absent in the salivary gland of control mosquitoes. In silico analyses indicated that SGS1 contains two potential internal cleavage sites and thus might generate three proteins. Conclusion SGS1 facilitates, but is not essential for, invasion of A. aegypti salivary glands by P. gallinaceum and has a dual role as a facilitator of parasite development in the mosquito midgut. SGS1 could, therefore, be part of a strategy to decrease malaria transmission by the mosquito vector, for example in a transgenic mosquito that blocks its interaction with the parasite.

Identification of Aedes aegypti salivary gland proteins interacting with human immune receptor proteins

“…SGS1 is a 380 kDa protein. The identified lower molecular weight bands might be due to cleaved products as this protein is known to have three predicted cleavage sites [ 40 ]. ( G ) TLR4-interacting proteins.…”

“…Although the molecular weight of the protein band containing the SGS1 was lower (50–55 kDa) than the theoretical full-length protein (380 kDa), the MS data suggests that this might contain a cleaved N-terminal and C-terminal region of the SGS1 protein. SGS proteins are major immunogenic proteins implicated in Plasmodium parasites infection of mosquito salivary gland tissues [ 40 ]. Other SGS families such as SGS3, SGS4 and SGS6 are malaria blocking targets in Anopheles mosquitoes [ 52 ].…”

The salivary protein Saglin facilitates efficient midgut colonization of Anopheles mosquitoes by malaria parasites

“…Interestingly, a similar phenotype was recently described for the knockout of the salivary protein SGS1 in A. aegypti . Knockout mosquitoes lacking SGS1 expression showed a 48-79% decrease in oocyst numbers upon infection with P. gallinaceum (Kojin et al ., 2021). Immunofluorescence stainings provided here as well as in a previous study performed in A. stephensi (O’Brochta et al ., 2019) demonstrate that Saglin does not localize towards the salivary gland distal lobes where the majority of sporozoites invade (Wells and Andrew, 2019).…”

“…Interestingly, a similar phenotype was recently described for the knockout of the salivary protein SGS1 in A. aegypti. Knockout mosquitoes lacking SGS1 expression showed a 48-79% decrease in oocyst numbers upon infection with P. gallinaceum (Kojin et al, 2021).…”

“…In accordance with these observations a member of the SGS family in Aedes aegypti , aaSGS1, was shown to play a role in the salivary gland invasion process of P. gallinaceum sporozoites (Korochkina et al ., 2006). However, a recent study on aaSGS1 using knockout mosquitoes has shown that this protein mainly affects the colonization of the midgut by P. gallinaceum rather than the invasion of sporozoites into the salivary glands (Kojin et al ., 2021). Another interesting protein potentially involved in sporozoite invasion into the salivary glands is CSP-BP, a one to one orthologue of Drosophila UPF3.…”