Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PL004

Keynote Lecture

The ins and outs of protein trafficking - a "complex" story?

R. A. Fenton1

1. Aarhus University, Aarhus, Denmark.

The urinary concentrating mechanism in the mammalian kidney is essential for maintenance of body water homeostasis and thus critical for life (1) . One of the major mechanisms in urine concentration is initiated by the binding of the antidiuretic hormone vasopressin (AVP) to its type-2 receptor (AVPR2), resulting in increased abundance of the water channel aquaporin 2 (AQP2) at the apical membrane of collecting duct principal cells. This rate-limiting step for controlling the renal reabsorption of water is highlighted by the disease nephrogenic diabetes insipidus (NDI), where there is a lack of responsiveness of the collecting duct to the antidiuretic actions of AVP, resulting in marked polyuria and polydipsia in patients (2). In recent years, application of modern technologies has accelerated our understanding of AQP2 trafficking, and uncovered that a complex interplay of regulated and constitutive AQP2 exocytosis and endocytosis determines its plasma membrane levels. In addition to AVP, these processes are modulated via various regulatory factors, such as AVP, prostaglandins, secretin or calcitonin. Together they modulate intracellular signalling cascades in the principal cells leading to post-translational modifications within AQP2 and alteration of unique protein:protein interactions (3). In particular, site-specific phosphorylation of AQP2 can determine the rate of its exocytosis and endocytosis. Regulated exocytosis is dependent on a single phosphorylated serine residue (Ser256), which destabilizes actin thin filaments to open the apical actin cortex and facilitates fusion of AQP2 vesicles with the apical membrane. Endocytosis of AQP2 is controlled by concerted effects of Ser256 and Ser269, which when phosphorylated inhibit interaction of AQP2 with various components of the endocytic machinery and may stabilize AQP2 within plasma membrane lipid-rich domains. In contrast, K63-linked polyubiquitylation of AQP2 at Lys270 can mediate its endocytic retrieval from the plasma membrane and degradation, but site specific phosphorylation of AQP2 is able to over-ride this internalization signal, providing a novel cell biological concept for membrane protein trafficking. AQP2 protein:protein interactions are also important for modulating the proteasomal or lysosomal degradation of AQP2 to maintain AQP2 cellular homeostasis. Recent work indicates an important role for the C terminus of Hsc70 Interacting Protein (CHIP) for both of these processes and thus renal water handling (4). Ultimately, our increased understanding of the intracellular molecular mechanisms of AQP2 trafficking has led to promising new strategies for bypassing defective AVPR2 signalling and restoring AQP2 function in NDI and other water balance disorders (5).

Where applicable, experiments conform with Society ethical requirements