Yapping at the autophagy door? The answer is flowing in the kidney proximal tubule

ABSTRACT Shear stress induced by urinary flow stimulates macroautophagy (hereafter referred to as autophagy) in kidney proximal tubule epithelial cells. Autophagy and selective degradation of lipid droplets by lipophagy contribute to tubule homeostasis by the production of ATP and control of epithelial cell size. Autophagy/lipophagy is controlled by a signaling cascade emanating from the primary cilium, localized at the apical side of epithelial cells. Downstream of the primary cilium, AMPK controls mitochondrial biogenesis on the one hand and autophagy/lipophagy on the other hand, which together increase fatty acid production that fuels oxidative phosphorylation to increase energy production. Recently, we reported that the co-transcriptional factors YAP1 and WWTR1/TAZ act downstream of AMPK to control autophagy. In fact, YAP1 and the transcription factor TEAD control the expression of RUBCN/rubicon. Under shear stress, YAP1 is excluded from the nucleus in a SIRT1-dependent manner to favor autophagic flux by downregulating the expression of RUBCN. When simulating in vitro a pathological urinary flow in murine proximal tubule kidney epithelial cells, we observe the nuclear retention of YAP1 and, consequently, high expression of RUBCN and inhibition of autophagic flux. Importantly, these findings were confirmed in biopsies of patients suffering from diabetic nephropathy, a major cause of chronic kidney disease.

Autophagy is intimately linked to the physiological function of tissues and organs in basal conditions and in response to hormonal, nutritional and mechanical stimuli and diverse stress situations.In the kidney, urinary fluid flow stimulates autophagy in proximal tubule epithelial cells (PTEC).At the apical cell surface, the primary cilium senses fluid flow to initiate a signaling cascade that activates AMPK and autophagy.This shear stress-dependent autophagy is important to support the production of fatty acids by lipophagy to fuel mitochondrial oxidative phosphorylation to produce ATP.In parallel, mitochondrial biogenesis is stimulated by a primary cilium-AMPK axis.Other important outcomes of shear stress-dependent autophagy in PTEC are the regulation of cell size, gluconeogenesis and glucose transport.Recently, we showed that the co-transcriptional factors YAP1 and WWTR1 act downstream of AMPK to regulate autophagy (Figure 1).The nuclear exclusion of YAP1 and WWTR1 is mandatory to stimulate autophagic flux.This exclusion depends on the activity of the histone acetylase SIRT1.Interestingly, YAP1 sequestration in the cytoplasm upon shear stress is independent of the canonical Hippo pathway (initiated by LATS1 and LATS2), but dependent on AMPK, which phosphorylates YAP1 at position S61 to inhibit its shuttling back to the nucleus.The shear stress-dependent exclusion of YAP1 reduces the expression of RUBCN, an inhibitor of the autophagic flux.The expression of RUBCN depends on the interaction of YAP1 with the transcription factor TEAD.However, the target(s) of WWTR1 that control autophagic flux, if any, remain(s) to be identified.
At this stage, one of the main conclusions that can be drawn from this [1] and our previous studies is that AMPK is a signaling hub downstream of the primary cilium, which controls two complementary segments in PTEC: on the one hand, mitochondrial biogenesis and on the other hand, the YAP1-dependent lipophagy/autophagy pathway to produce fatty acids (to sustain the production of ATP by mitochondria) and control cell size.Another noticeable fact is that the role of YAP1 in controlling shear stress-dependent autophagy in kidneys is evolutionarily conserved at least from fishes to mammals based on our findings in zebrafish, mice and humans.
An increase in fluid flow in proximal tubules is a hallmark of chronic kidney disease (CKD).We first observed that forcing shear stress to 4 dyn/cm 2 (versus the physiological shear stress of 1 dyn/cm 2 ) inhibits AMPK and retains YAP1 in the nucleus, a phenomenon not observed for WWTR1 in mouse kidney epithelial cells.A consequence of YAP1 retention in the nucleus is the inhibition of autophagic flux.The nuclear presence of YAP1 is also observed during long-term unilateral urinary obstruction (UUO) in mice, as well as in kidney samples from patients suffering from diabetic nephropathy, a major cause of CKD.In both cases, a blockade of the autophagic flux is observed.Using a yap1 knockout mouse model, we further showed that the loss of YAP1 in renal tubules not only partially rescues the autophagic flux but also protects against tubular injury and fibrosis in UUO conditions.
Most likely, the interplay between shear stress intensity, YAP1, WWTR1, RUBCN and autophagy is not limited to the kidney proximal tubule.In fact, it has been reported that YAP1 is more active in atheroprone regions (aortic arch) exposed to low shear stress and characterized by an inefficient autophagy compared to the atheroprotective region (descending aorta) exposed to high shear stress and high functional autophagy.Considering the modulation of mechanical forces during the progression of many diseases, we thus propose that the YAP1autophagy interplay controlled by physical forces could be instrumental in the development of many pathologies.

Figure 1 .
Figure 1.Fluid flow intensity regulates the AMPK-SIRT1-YAP1 axis to control the autophagy flux in kidney epithelial cells.Urinary flow is sensed by renal cells, but its intensity is dysregulated in renal diseases.Left panel: physiological flow inhibits YAP1 and WWTR1 to promote autophagy.This inhibition is driven by a SIRT1dependent YAP1 nuclear exclusion and an AMPK-dependent cytoplasmic YAP1 retention.Right panel: pathological flow leads to AMPK inhibition, leading to YAP1 activation and autophagy inhibition.