Socs36E limits STAT signaling via Cullin2 and a SOCS-box independent mechanism in the Drosophila egg chamber

Abstract

The Suppressor of Cytokine Signaling (SOCS) proteins are critical, highly conserved feedback inhibitors of signal transduction cascades. The family of SOCS proteins is divided into two groups: ancestral and vertebrate-specific SOCS proteins. Vertebrate-specific SOCS proteins have been heavily studied as a result of their strong mutant phenotypes. However, the ancestral clade remains less studied, a potential result of genetic redundancies in mammals. Use of the genetically tractable organism Drosophila melanogaster enables in vivo assessment of signaling components and mechanisms with less concern about the functional redundancy observed in mammals. In this study, we investigated how the SOCS family member Suppressor of Cytokine Signaling at 36E (Socs36E) attenuates Janus Kinase/Signal Transducer and Activator of Transcription (Jak/STAT) activation during specification of motile border cells in Drosophila oogenesis. We found that Socs36E genetically interacts with the Cullin2 (Cul2) scaffolding protein. Like Socs36E, Cul2 is required to limit the number of motile cells in egg chambers. We demonstrated that loss of Cul2 in the follicle cells significantly increased nuclear STAT protein levels, which resulted in additional cells acquiring invasive properties. Further, reduction of Cul2 suppressed border cell migration defects that occur in a Stat92E-sensitized genetic background. Our data incorporated Cul2 into a previously described Jak/STAT-directed genetic regulatory network that is required to generate a discrete boundary between cell fates. We also found that Socs36E is able to attenuate STAT activity in the egg chamber when it does not have a functional SOCS box. Collectively, this work contributes mechanistic insight to a Jak/STAT regulatory genetic circuit, and suggests that Socs36E regulates Jak/STAT signaling via a Cul2-dependent mechanism, as well as by a Cullin-independent manner, in vivo.

The structure and regulation of Cullin 2 based E3 ubiquitin ligases and their biological functions

“…In Drosophila melanogaster , loss of function of CUL2 resulted in defects at the larval neuromuscular junction and aberrations in the development of female germ line [ 72 ]. Cul2 was also required to limit the number of motile cells in egg chambers [ 73 ] and for germline enclosure in testes [ 74 ]. Among several CRL2 complexes that were associated with germline development, CRL2 LRR-1 complex was a critical one.…”

Development and Analysis of a Quantitative Mathematical Model of Bistability in the Cross Repression System Between APT and SLBO Within the JAK/STAT Signaling Pathway

“…We tested our models to confirm if they reproduced certain behaviors identified in various experiments. It has been shown that if STAT activity is blocked by stage 9 of cell migration, the level of APT protein drops by about half (Starz-Gaiano et al, 2008 , 2009 ; Yoon et al, 2011 ; Monahan and Starz-Gaiano, 2015 ). It is also known that increasing the initial condition of APT should decrease the level of STAT protein and activity (Starz-Gaiano et al, 2008 ).…”

“…One was the decision to ignore the c 2 equation, which allows APT to act as a buffer on STAT. Since the action of APT on STAT is known to function by preventing activation through miRNAs (Yoon et al, 2011 ) as well as limiting active protein present (Monahan and Starz-Gaiano, 2015 , 2016 ), this assumption might oversimplify the larger system. Additionally, the quasi-steady state assumptions made in reducing STAT dynamics may also oversimplify the model.…”

“…Experimenting with apt mutants could tell us more about how the bistability we show in cell specification ultimately affects cluster migration and the further development of the egg chamber. When STAT is abnormally high more intermediate border cells trail behind the polar cells (Silver and Montell, 2001 ; Silver et al, 2005 ; Monahan and Starz-Gaiano, 2015 ; Starz-Gaiano et al, 2008 ). Socs36E limits STAT signaling and higher Socs36E reduces motile cell number.…”

“…APT acts as a feedback inhibitor on the JAK/STAT pathway, and this process is mediated by APT's activation of a microRNA that reduces STAT protein and activity (Yoon et al, 2011 ). APT also activates expression of Socs36E , which downregulates STAT signaling via a degradation pathway (Monahan and Starz-Gaiano, 2015 ). APT and SLBO exhibit cross-repressional behavior (Starz-Gaiano et al, 2008 , 2009 ).…”