RNF41 is a key regulator of cytokine receptor trafficking and signaling

Cytokine receptor signaling is essential for several vital biological processes like energy metabolism, hematopoiesis, somatic growth, reproduction, tissue repair and host defense. A direct link between excess or lack of cytokine signaling and various pathologies like blood anomalies, dwarfism, obesity, cancer and immunodeficiency states, underscores the importance of strict control mechanisms that ensure adequate, temporary receptor activation. The magnitude and duration of cytokine responses depends on the bio-availability of the cytokine and correct cell surface exposure and subsequent intracellular trafficking of the receptor. Negative-feedback mechanisms that limit the intracellular signal transduction and gene transcription initiated after binding of the cytokine to its receptor are well-characterized. However, the molecular mechanisms controlling steady-state cytokine receptor cell surface levels and consequently the cellular cytokine sensitivity remain poorly understood. The number of surface-exposed receptors is a dynamic balance of de novo synthesis, transport to the plasma membrane, internalization, recycling, degradation and ectodomain shedding. This thesis characterizes RNF41 as a key regulator of basal cytokine receptor trafficking, proteolytic processing and signaling. The E3 ubiquitin ligase RNF41 was identified in multiple FACS-based MAPPIT screens as a novel interaction partner of the leptin receptor complex. Functional analysis of the interaction reveals that RNF41 inhibits the basal surface expression and signaling of various JAK2-associated cytokine receptors. Moreover, we show that RNF41 controls the constitutive intracellular trafficking of cytokine receptors. It inhibits a newly identified cathepsin L-dependent cytokine receptor cleavage which precedes lysosomal receptor degradation, and concomitantly enhances receptor ectodomain shedding by the metalloproteases ADAM10 and ADAM17. We further demonstrate that this results from RNF41-dependent ubiquitination and suppression of the deubiquitinating enzyme USP8. As USP8 in turn can deubiquitinate and stabilize RNF41, this uncovers an interesting reciprocal regulation between RNF41 and USP8. We demonstrate that RNF41-mediated loss of USP8 abrogates ESCRT-0 functionality and accounts for the rerouting of cytokine receptors from the lysosomal degradation pathway (where cathepsin L cleavage occurs), back to the plasma membrane (where receptors are subject to ectodomain shedding), by stimulating slow and fast recycling. Finally, using an array MAPPIT screen, we identify ASB6 as a new RNF41 interaction partner with a marked molecular mimicry between the USP8 and ASB6 domains involved in RNF41 binding. Contrary to USP8, RNF41 stabilizes ASB6 and significantly prolongs its half-life. Moreover, we provide evidence that ASB6 differentially affects leptin receptor STAT3 and STAT5 signaling. Our findings provide novel insights into the regulatory mechanisms that fine-tune cytokine signaling and safeguard correct intracellular routing of cytokine receptors. It may help to understand the established role of RNF41 in the pathogenesis of breast cancer and Parkinson’s disease, and could uncover unexplored contributions of RNF41 in pathologies like Alzheimer’s disease and disorders associated with elevated soluble cytokine receptor levels