The Spliced Leader RNA Silencing (SLS) Pathway in Trypanosoma brucei Is Induced by Perturbations of Endoplasmic Reticulum, Golgi Complex, or Mitochondrial Protein Factors: Functional Analysis of SLS-Inducing Kinase PK3

Abstract

In this study, we found that SLS is induced by depletion of the essential ER-resident chaperones BiP and calreticulin, ER oxidoreductin 1 (ERO1), and the Golgi complex-localized quiescin sulfhydryl oxidase (QSOX). Most strikingly, silencing of Rhomboid-like 1 (TIMRHOM1), involved in mitochondrial protein import, also induces SLS.

The endoplasmic reticulum of trypanosomatids: An unrevealed road for chemotherapy

“…Furthermore, these authors argued that DTT was lethal to parasites by causing a strong redox stress, at all concentrations tested, making it unsuitable for use as an ER-stressor (Tiengwe et al, 2015;Sandes et al, 2019). The discrepancy in the phenotypes found following DTT treatment may be attributed to differences in the drug concentrations used in these studies as well as the distinct susceptibility existing between the evolutive forms of this parasite (Goldshmidt et al, 2010;Goldshmidt and Michaeli, 2011;Okalang et al, 2021). In A. deanei TM treatment at higher concentrations caused a decrease in proliferation at higher concentrations.…”

“…PK3 kinase have demonstrated to be essential in this process and its activation during persisting ER stress induces PCD and consequently the parasite elimination. Thus, small molecules that selectively activate PK3 could be considered an attractive drug target ( Okalang et al, 2021 ). Finally, but not least, a better understanding of how the ERQC and UPR work, as well as the mechanisms underlying cell death caused by persistent ER stress in trypanosomatids, can reveal molecular peculiarities, opening new avenues for the development of more effective and less toxic drugs to combat the devastating illness caused by these protozoans.…”

“…Consequently, expression levels are primarily controlled by mRNA stability ( Tiengwe et al., 2015 ). Because UPR activation in higher eukaryotes is performed by increased transcription of specific mRNA, it is unlikely that trypanosomatids have a classical UPR machinery ( Hope et al., 2014 ; Tiengwe et al., 2015 ; Okalang et al., 2021 ). However, trypanosomatids are able to respond ER stress by upregulating BiP, triggering the splice leader silencing (SLS) that in turn, inhibits SL RNA transcription.…”

“…The ER-stress induced by SEC63 silencing or low pH results in the translocation of PK3 from the ER to the nucleus where it phosphorylates the TATA-binding TRF4, causing the shutoff of SL RNA transcription, followed by induction of programmed cell death. The loss of PK3 function attenuates the programmed cell death triggered by ER stress, indicating that SLS may contribute to programmed cell death activation in T. brucei ( Bindereif and Preußer, 2014 ; Hope et al., 2014 ; Okalang et al, 2021 ).…”

Disulfide bond formation and redox regulation in the Golgi apparatus

“…QSOX2, in contrast, is largely uncharacterized but, like QSOX1 [22][23][24][25], has been reported to be associated with cancer progression [26][27][28]. Golgi localization of QSOX1 was seen in multiple mammalian cell types [15] and in the unicellular parasite Trypanosoma brucei [29], suggesting a conserved and widespread function in this compartment. Correspondingly, QSOX1 has no detectable impact on oxidative protein folding in the ER [30].…”