The SET1 protein ofLeishmania donovanimoderates the parasite’s response to a hostile oxidative environment
Authors: Pal Jyoti , Sharma Varshni , Khanna Arushi , Saha Swati 
Published: September 2023

SET domain proteins mediate their effects through the methylation of specific lysine residues on target substrates, resulting in either the stimulation or repression of downstream processes. Initially identified as histone lysine methyltransferases, they are now known to target a wide-ranging conglomeration of non-histone subtrates as well. Twenty-nine SET domain proteins have been identified inLeishmania donovanithrough genome sequence annotations. This study initiates the first investigation into these proteins. We find LdSET1 is predominantly cytosolic and constitutively expressed. Theset1gene is not essential, although its deletion slows promastigote growth and hypersensitizes the parasite to hydroxyurea-induced G1/S arrest. Intriguingly,set1-nulls proliferate more proficiently thanset1+/+parasites within host macrophages, suggesting that LdSET1 moderates the parasite’s response to the inhospitable intracellular oxidative environment.set1-null parasites are highly tolerant to H2O2-induced oxidative stress inin vitropromastigote cultures as well, reflected in their growth pattern as well as almost complete absence of DNA damage at the H2O2concentrations tested. This is linked to ROS levels remaining virtually unperturbed in response to H2O2treatment inset1-nulls, contrasting to increased ROS inset1+/+cells under similar conditions. In analyzing the cell’s ability to scavenge hydroperoxides we find that peroxidase activity is not upregulated in response to H2O2exposure inset1-nulls. Rather, the constitutive basal levels of peroxidase activity are significantly higher in these cells, implicating this to be a factor contributing to the parasite’s apparent resistance to H2O2. The higher levels of peroxidase activity inset1-nulls are coupled to upregulation of tryparedoxin peroxidase transcripts in these cells. Thus, LdSET1 tunes the parasite’s behavior within host cells, enabling establishment and persistence of infection, and maintaining the balance with host without eradicating the host cell population it needs for survival. These findings uncover a new dimension to the mechanisms controlling the interplay ofLeishmania-host interactions.

Author Summary

Leishmaniases are a group of diseases afflicting people across 88 countries. Manifested in three forms: cutaneous, subcutaneous and visceral, different species cause the different forms of the disease.Leishmania donovaniis one of the species causing Visceral Leishmaniasis, and its cellular processes are an area of intensive investigations. These protozoan parasites display an unusual pattern of transcription, and gene regulation occurs primarily through epigenetic mechanisms and post-transcriptional processes. SET proteins control gene expression by methylating histones as well as non-histone proteins. In this study we have examined the role of the LdSET1 protein. We find that while theset1gene is not essential to cell survival, deletion of theset1gene makes the parasite highly tolerant to the effects of an oxidizing environment, signifying that the LdSET1 protein plays a role in regulating the cell’s response to oxidative stress. LdSET1 appears to moderate the parasite’s survival in the oxidative intracellular milieu of host macrophages, controlling it such that infection is firmly established, and persistent, without wiping out the population of host cells it needs for survival.