The cytoprotective role of autophagy in podocytes

Abstract

Podocytes are highly differentiated cells which play an important role in guarding the permeability of the tripartite renal filtration barrier. Many glomerular diseases attribute to podocyte damage including apoptosis, cytoskeleton rearrangement and detachment. The gene mutation of podocyte cytoskeleton proteins has also been acknowledged in the pathogenesis of renal diseases. Thus, podocytes emerge as the therapeutic target of glomerular diseases. However, the strategies for preventing podocyte damage remain insufficient. Recently autophagy has been described as a ubiquitous catabolic process involving degradation of damaged organelles and protein aggregates. It shows cytoprotective effects in many cell types and helps to maintain cell homeostasis. Autophagy is being kept at basal levels in cells for safeguarding and promoting cell survival. This basal autophagy is one of the major processes that allow cells to respond rapidly to the metabolic stress. Under certain circumstances, autophagy can be induced to a higher level to protect cells. Currently, the better-known potential stimuli inducing autophagy include nutrient starvation, oxidative stress, mitochondrial dysfunction, ischemia-reperfusion and infection. Puromycin aminonucleoside (PAN) which induces podocyte apoptosis in vitro and in vivo is widely used for studying the pathophysiology of glomerular diseases. It has been shown that PAN induces autophagy in podocytes. However, the relationship between autophagy and apoptosis in PAN-treated human podocytes is not known and the role of PAN-induced autophagy in podocyte survival remains unclear. In this thesis, we demonstrated that PAN induced autophagy in human podocytes prior to apoptosis which was featured with the activation of mTOR complex 1 (mTORC1). When the PAN-induced autophagy was inhibited by 3-methyladenine (3-MA) or chloroquine (CQ), podocyte apoptosis increased significantly along with the elevation of active caspase-3. Under such circumstance, the podocyte cytoskeleton was also disrupted. These results suggested that the observed induction of autophagy may be an early adaptive cytoprotective mechanism for podocyte survival after PAN treatment. Since autophagy induction is beneficial for podocyte survival, it is reasonable to utilize this mechanism to attenuate podocyte injury. Trehalose, a natural disaccharide, is an mTOR independent autophagy inducer. It is unclear whether trehalose alleviates podocyte injury. Therefore, we investigated the efficacy of trehalose in PAN-treated podocytes. Human conditional immortalized podocytes were treated with trehalose with or without PAN. It was shown that trehalose induced podocyte autophagy in an mTOR independent manner and without reactive oxygen species involvement. PAN-induced podocyte apoptosis significantly decreased after trehalose treatment, while the inhibition of trehalose-induced autophagy abolished its protective effect. Additionally, the disrupted actin cytoskeleton of podocytes was partially reversed by trehalose, accompanied by less lamellipodias and diminished motility. These results suggested that trehalose induced autophagy in human podocytes and showed cytoprotective effects in PAN-treated podocytes. For confirming the efficacy of trehalose in vivo, PAN nephrosis rat model was established and treated with trehalose. It was shown that trehalose induced autophagy in glomeruli with increased LC3-II expression. However, proteinuria and hypoalbuminemia was not alleviated as well as the altered renal ultrastructure. These results suggested PAN-induced podocyte injury in vivo was not alleviated by trehalose. Overall, this thesis showed the cytoprotective role of autophagy in vitro. Further investigation is warranted to determine the application of autophagy in the treatment of podocyte related renal diseases.