Extensive Transcriptional and Translational Regulation Occur During the Maturation of Malaria Parasite Sporozoites

Abstract

22 Introduction: 41Malaria remains one of the great global health problems today, taking a large toll on people in the 42 tropics and subtropics. This disease, caused by Plasmodium parasites, affects over 200 million people 43 annually and kills over 400,000 (WHO World Malaria Report 2018). While a protein-based subunit 44 vaccine (RTS,S) has recently been licensed and is being used for pilot implementation in three Saharan African countries, its protection has been limited and relatively short-lived in clinical trials 1 . 46 Identifying and fully testing an effective and long-lasting malaria vaccine remains a chief goal that has 47 yet to be achieved. Accomplishing this will require greater knowledge of the basic biology of the pre-48 erythrocytic sporozoite and liver stage parasites. Promising new whole-parasite vaccine candidates, 49 based upon the sporozoite form of the parasite, are on the horizon and hopefully will meet these needs 50 2 . 51Plasmodium parasites are transmitted between hosts by an infected female Anopheles mosquito 52 (reviewed in 3 ). Following uptake of male and female gametocytes by the mosquito during a blood meal 53 of an infected host, these parasites activate into male and female gametes in the midgut, fuse into a 54 zygote, and develop into a motile ookinete, which burrows through the midgut wall and establishes an 55 oocyst under the basal lamina. Within the oocyst, the parasite undergoes sporogony to produce up to 56 five thousand oocyst sporozoites 4 . These oocyst sporozoites are weakly infectious if injected directly 57 into a naïve mammalian host 5 , but become highly infectious following proteolytic rupture of the oocyst 58 wall and transit through the mosquito hemocoel. Sporozoites further gain infectivity after invasion of 59 the salivary glands 5,6 . Interestingly, the gain of infectivity for the mammalian host is concomitant with a 60 loss of infectivity for the salivary glands. Within the glands, the sporozoites await transmission as 61 salivary gland sporozoites, which occurs when the mosquito takes its next blood meal. Infection of a 62 host begins with the injection of salivary gland sporozoites into the skin. Following this, sporozoites exit 63 the bite site in the skin, locate and enter the vasculature, and passively travel to the liver, where 64 sporozoites can productively infect hepatocytes, which initiates the life cycle progression in the 65 mammalian host 7 . Moreover, because relatively few sporozoites are injected during a mosquito bite 8 , 66 this transmission event bottleneck has been the focus of intervention efforts using drugs, subunit 67 vaccines, and attenuated whole parasite vaccines 2 . 68Fundamental studies of sporozoite biology have informed efforts to inhibit and/or arrest the parasite 69 during development. To date, genetically-attenuated parasite (GAP) vaccine candidates were generated 70 by the deletion of genes whose transcripts are Up-regulated in Infective (Salivary Gland) Sporozoites 71 (UIS). The UIS gene family was originally determi...

Downregulation of the secreted protein with an altered thrombospondin repeat (SPATR) impacts the infectivity of malaria sporozoites

“…SPATR is a conserved protein with adhesive domains expressed in all invasive stages of Plasmodium. Despite the upregulation of transcription in infectious sporozoites, we found that SPATR is, as previously described, essential for the asexual blood stages (12,15,19).…”

“…Moreover, maximum excision of the target locus is only achieved upon a temperature switch, which is usually performed once sporozoites are already present in the salivary glands, as the rise in temperature leads to the blockage of salivary gland colonization (63). Since spatr starts being transcribed in oocysts and is then subjected to translational repression (15), this creates a temporal window for sporozoites to produce and store mRNA that can be translated even after the recombination event. Therefore, the study of the importance of genes for the mosquito phase of the life cycle of the parasite using the Flp-FRT system may have additional limitations.…”

“…2K). The detection of the protein in WT oocyst-derived sporozoites was unexpected, since previous proteomics studies performed in P. falciparum and P. yoelii had only detected SPATR protein in salivary gland sporozoites, whereas transcripts were present in sporozoites collected from both the oocysts and the salivary glands, which was suggestive of translational repression 12,15 . Whether this was due to differences in the parasite species, the technical approaches, or the age of oocyst sporozoites -these were collected from day 19 post-infection onwards in this work, as opposed to days 10-14 in the proteomics studies -remains to be elucidated.…”

“…Accomplishing this goal, however, will require greater knowledge of the biology of the parasite, including the identification of sporozoite surface proteins for targeted immune interventions that interfere with the timely completion of the journey of the parasite in the mammalian host. To this end, significant advances have been made to define the Plasmodium proteome, including the surface proteome of infectious sporozoites (12)(13)(14)(15).…”

Transcriptome-wide dynamics of extensive m6A mRNA methylation during Plasmodium falciparum blood-stage development

“…In contrast, Articles NATURe MICRObIOlOgY P. falciparum encodes only two YTH proteins, and while their roles are unknown, the divergence of these two proteins beyond their YTH domain and the distinct expression patterns during the parasite lifecycle suggest that they mediate two distinct functions. m 6 A has not been associated with translational repression in any eukaryote yet, but this post-transcriptional regulation is of particular importance for P. falciparum during the IDC 15 and especially during transmission between the human host and mosquito vector 47,48 . During the latent gametocyte and salivary gland sporozoite stages, mRNAs are kept in a translationally repressed state and protein synthesis is only initiated following transmission, readily ensuring rapid continuation of development 12,49 .…”