Spliced Leader RNA Silencing: A stress inducible programmed cell death pathway in Trypanosoma brucei

Trypanosoma brucei is the causative parasite of African sleeping sickness. In these parasites all mRNA undergo trans-splicing involving the ligation of a spliced leader (SL) from SL RNA. Perturbations of protein translocation across the endoplasmic reticulum (ER), low pH, and inducers of the unfolded protein response (UPR) in eukaryotes, induce SL RNA silencing (SLS) pathway (Lustig et al., EMBO Rep. 2007). SLS induction leads to programmed cell death (PCD) (Goldshmidt et al., PLoS Pathogens 2010). SLS is initiated by the activation of a serine-threonine kinase, PK3, which translocates to the nucleus, where it phosphorylates the TATA binding protein, TRF4, leading to the failure of the pre-initiation transcription complex to bind the SL RNA promoter, leading to shut-off of SL RNA transcription and PCD (Hope et al, Sci. Signal. 2014). Here, we show that SLS is also induced under depletion of essential ER resident chaperones, BiP and CRT, ER oxidoreductin 1 (ERO1), Golgi-localized quiescin sulfhydryl oxidase 1 (QSOX1) and the mitochondrial protein, Rhomboid-like I, involved in mitochondrial protein translocation. Furthermore, PCD inducing anti-trypanosomal drugs, such as camptothecin, novobiocin, nifurtimox and suramin induce SLS. On the other hand, arsenite [As (III)] in addition to heat shock; induces a pathway that is the direct opposite of SLS and leads to increase in the level of SL RNA, suggesting that SL RNA is a key regulator of different pathways in trypanosomes under various stresses. PK3 is an essential kinase that may keep the homeostasis among different organelles in these parasites.