University of Calcutta, India
Title: Leishmania donovani negatively regulates TLR4 mediated host response by preventing ubiquitination dependent degradation of TRAF3
Purnima Gupta is pursuing her PhD in Biochemistry from Calcutta University. She has received the prestigious CSIR-fellowship for doctoral studies. Her research interest includes understanding how intramacrophage pathogens evade host cell defences utilizing host signaling molecules to aid their survival within macrophage hostile environment. To elucidate this they chose visceral leishmaniasis as the disease model and L. donovani as a model organism. Till date they have unveiled how the parasite modulates TLR signaling pathway by means of upregulating the negative regulatory molecule, TRAF3. Moreover, they observed how the parasite regulates host molecule SOCS to inhibit host cell apoptosis and thereby save its niche for replication and propagation.
Leishmania donovani, causative agent of fatal visceral leishmaniasis, is an intramacrophage pathogen. It escapes host immune response by subverting Toll like receptor (TLR) signaling, which is critically regulated by protein ubiquitination. Our work identified tumour necrosis associated factor (TRAF) 3 as a target used by Leishmania to deactivate LPS-mediated TLR4 signaling. TRAF3 is an E3 ubiquitin ligase of the membrane associated signaling complex that regulates TLR pathway through distinct protein ubiquitination at specific residues. We observed that TRAF3 which is ubiquitinated at lys 48 position and subsequently degraded following LPS treatment, persisted in L. donovani as well as L. donovani + LPS co-stimulated cells due to defective lys 48 ubiquitination. Unlike lys 48, lys 63-linked ubiquitinated proteins has been implicated in the signaling mediated activation of the molecules. Our results revealed lys 63-linked ubiquitination of upstream proteins in the cascade (cIAP1/2 and TRAF6), mandatory for lys 48 linked TRAF3 ubiquitination and subsequent degradation was significantly reduced during infection. The reason may be reduced association between ubiquitin conjugating enzyme Ubc13 and TRAF6 during infection. Persistence of TRAF3 resulted in stabilization of signalosome complex at the membrane resulting in inhibition of TAK-1 mediated pro-inflammatory responses. Inhibition of TRAF3 prior to infection by small hairpin RNA in Balb/c mice also showed enhanced production of IL-12 and TNF-α and significantly decreased spleen and liver parasite burden. Our findings identified TRAF3 as a novel molecular regulator exploited by Leishmania for successful infection.