Wen-Chieh Hsieh

Lowe syndrome (LS) is an X-linked developmental disease characterized by cognitive deficiencies, bilateral congenital cataracts and renal dysfunction. Unfortunately, this disease leads to the early death of affected children often due to kidney failure. Although this condition was first described in the early 1950s and the affected gene (OCRL1) was identified in the early 1990s, its pathophysiological mechanism is not fully understood and there is no LS-specific cure available to patients. Here we report two important signaling pathways affected in LS patient cells. While RhoGTPase signaling abnormalities led to adhesion and spreading defects as compared to normal controls, PI3K/mTOR hyperactivation interfered with primary cilia assembly (scenario also observed in other ciliopathies with compromised kidney function). Importantly, we identified two FDA-approved drugs able to ameliorate these phenotypes. Specifically, statins mitigated adhesion and spreading abnormalities while rapamycin facilitated ciliogenesis in LS patient cells. However, no single drug was able to alleviate both phenotypes. Based on these and other observations, we speculate that Ocrl1 has dual, independent functions supporting proper RhoGTPase and PI3K/mTOR signaling. Therefore, this study suggest that Ocrl1-deficiency leads to signaling defects likely to require combinatorial drug treatment to suppress patient phenotypes and symptoms.

It is well-known that in addition to its classical role in protein turnover, ubiquitination is required for a variety of membrane protein sorting events. However, and despite substantial progress in the field, a long-standing question remains:given that all ubiquitin (Ub) units are identical, how do different elements of the sorting machinery recognize their specific cargoes?Here we provide an answer to this question as we discovered a mechanism based on the coincidence detection of lysine ubiquitination and Ser/Thr phosphorylation for the endocytic adaptor epsin to mediate the internalization of the yeast Na⁺pump Ena1.Internalization of Ena1-GFP was abolished in double epsin knock-out inS. cerevisiaeand was rescued by re-introducing either one of the 2 yeast epsins, Ent1 or Ent2 in an UIM (Ub InteractingMotif)-dependent manner. Further, our results indicate that ubiquitination of its C-terminal Lys¹⁰⁹⁰is needed for internalization of Ena1 and requires the arrestin-related-trafficking adaptor, Art3.We determined that in addition to ubiquitination of K¹⁰⁹⁰, the presence of a Ser/Thr-rich patch (S¹⁰⁷⁶TST¹⁰⁷⁹) within Ena1 was also essential for its internalization. Our results suggest that this ST motif is targeted for phosphorylation by casein kinases. Nevertheless, phosphorylation of this S/T patch was not required for ubiquitination. Instead, ubiquitination of K¹⁰⁹⁰and phosphorylation of the ST motif were independently needed for epsin-mediated internalization of Ena1.We propose a model in which a dual detection mechanism is used by Ub-binding elements of the sorting machinery to differentiate among multiple Ub-cargoes.