Analysis of a lung defect in autophagy-deficient mouse strains

Abstract

Yeast Atg1 initiates autophagy in response to nutrient limitation. The Ulk gene family encompasses the mammalian orthologs of yeast ATG1. We created mice deficient for both Ulk1 and Ulk2 and found that the mice die within 24 h of birth. When found alive, pups exhibited signs of respiratory distress. Histological sections of lungs of the Ulk1/2 DKO pups showed reduced airspaces with thickened septae. A similar defect was seen in Atg5-deficient pups as both Ulk1/2 DKO and Atg5 KO lungs show numerous glycogen-laden alveolar type II cells by electron microscopy, PAS staining, and increased levels of glycogen in lung homogenates. No abnormalities were noted in expression of genes encoding surfactant proteins but the ability to incorporate exogenous choline into phosphatidylcholine, the major phospholipid component of surfactant, was increased in comparison to controls. Despite this, there was a trend for total phospholipid levels in lung tissue to be lower in Ulk1/2 DKO and Atg5 KO compared with controls. Autophagy was abundant in lung epithelial cells from wild-type mice, but lacking in Atg5 KO and Ulk1/2 DKO mice at P1. Analysis of the autophagy signaling pathway showed the existence of a negative feedback loop between the ULK1 and 2 and MTORC1 and 2, in lung tissue. In the absence of autophagy, alveolar epithelial cells are unable to mobilize internal glycogen stores independently of surfactant maturation. Together, the data suggested that autophagy plays a vital role in lung structural maturation in support of perinatal adaptation to air breathing.

Methylation Silencing of ULK2, an Autophagy Gene, Is Essential for Astrocyte Transformation and Tumor Growth

“…The existence of a negative feedback loop between ULK1/2 and mTOR is further confirmed by the presence of activated mTOR in ULK1/2 double knock-out mice (50). Further, it has been shown that ULK1 and ULK2 inhibit mTORC1 by binding to and phosphorylating the raptor, the regulatory-associated protein of mTOR (51).…”

Phosphorylation of eIF2α triggered by mTORC1 inhibition and PP6C activation is required for autophagy and is aberrant in PP6C-mutated melanoma

“…Although the ULK1/2 complex is required for autophagy induction in response to amino acid deprivation, autophagy is potently induced by glucose deprivation and ammonia, and more mildly with rapamycin treatment in ULK1/2 deficient cells, suggesting that autophagy regulation and mTOR signaling include a ULK independent pathway (21, 23-25). We therefore investigated whether mTORC1 regulation of autophagy by eIF2α phosphorylation was independent of ULK1/2 activity.…”

Atg13 Is Essential for Autophagy and Cardiac Development in Mice

“…S6D in the supplemental material). Second, the phenotype of ULK1/2 DKO mice is milder than that of Atg13 KO and FIP200 KO mice: ULK1/2 DKO mice die after birth (28). These milder effects of deletion of ULK1 and ULK2 may indicate that they are not essential for the function of Atg13 and FIP200 in certain settings.…”

“…Although mice lacking either ULK1 or ULK2 are viable, ULK1/2 double-knockout (DKO) mice die during the neonatal period (28). In addition, FIP200 is involved in various signaling pathways, including those of p53, focal adhesion kinase (FAK), Pyk2, tumor necrosis factor alpha (TNF-␣)-Jun N-terminal protein kinase (JNK), tuberous sclerosis complex (TSC)-mTOR, transforming growth factor beta (TGF-␤), and ␤-catenin (29)(30)(31).…”