Atg16L1, an essential factor for canonical autophagy, participates in hormone secretion from PC12 cells independently of autophagic activity

Autophagy and cellular immune responses

Autophagy constitutes a mechanism for the sequestration and lysosomal degradation of various cytoplasmic structures, including damaged organelles and invading microorganisms. Autophagy not only represents an essential cell-intrinsic mechanism to protect against internal and external stress conditions but also shapes cellular immunity. Recent evidence indicates that autophagic responses in antigen-donor cells affect the release of several cytokines and "danger signals." Thus, especially when it precedes cell death, autophagy alerts innate immune effectors to elicit cognate immune responses. Autophagy is also important for the differentiation, survival, and activation of myeloid and lymphoid cells. Accordingly, inherited mutations in autophagy-relevant genes are associated with immune diseases, whereas oncogenesis-associated autophagic defects promote the escape of developing tumors from immunosurveillance. Here, we discuss the regulation of autophagy in the course of cellular immune responses and emphasize its impact on the immunogenicity of antigen-donor cells and on the activity of antigen-presenting cells and T lymphocytes.

Amphisomes: out of the autophagosome shadow?

Amphisomes are intermediate organelles, formed during autophagy through the fusion between autophagosomes and endosomes. Complex multivesicular vacuoles that resemble amphisomes have been observed in various cell types, but whether they have cellular roles other than being a precursor structure is still enigmatic. While autophagy-related (ATG) proteins interact with the endocytic pathways in other processes different from autophagy, Patel and colleagues now report that these factors come together to generate amphisome-like compartments that regulate mucin secretion in goblet cells.

The Crohn's disease: associated ATG16L1 variant and Salmonella invasion

OBJECTIVE

A common genetic coding variant in the core autophagy gene ATG16L1 is associated with increased susceptibility to Crohn's disease (CD). The variant encodes an amino acid change in ATG16L1 such that the threonine at position 300 is substituted with an alanine (ATG16L1 T300A). How this variant contributes to increased risk of CD is not known, but studies with transfected cell lines and gene-targeted mice have demonstrated that ATG16L1 is required for autophagy, control of interleukin-1-β and autophagic clearance of intracellular microbes. In addition, studies with human cells expressing ATG16L1 T300A indicate that this variant reduces the autophagic clearance of intracellular microbes.

DESIGN/RESULTS

We demonstrate, using somatically gene-targeted human cells that the ATG16L1 T300A variant confers protection from cellular invasion by Salmonella. In addition, we show that ATG16L1-deficient cells are resistant to bacterial invasion.

CONCLUSIONS

These results suggest that cellular expression of ATG16L1 facilitates bacterial invasion and that the CD-associated ATG16L1 T300A variant may confer protection from bacterial infection.

Functions of autophagy in normal and diseased liver

Autophagy has emerged as a critical lysosomal pathway that maintains cell function and survival through the degradation of cellular components such as organelles and proteins. Investigations specifically employing the liver or hepatocytes as experimental models have contributed significantly to our current knowledge of autophagic regulation and function. The diverse cellular functions of autophagy, along with unique features of the liver and its principal cell type the hepatocyte, suggest that the liver is highly dependent on autophagy for both normal function and to prevent the development of disease states. However, instances have also been identified in which autophagy promotes pathological changes such as the development of hepatic fibrosis. Considerable evidence has accumulated that alterations in autophagy are an underlying mechanism of a number of common hepatic diseases including toxin-, drug- and ischemia/reperfusion-induced liver injury, fatty liver, viral hepatitis and hepatocellular carcinoma. This review summarizes recent advances in understanding the roles that autophagy plays in normal hepatic physiology and pathophysiology with the intent of furthering the development of autophagy-based therapies for human liver diseases.

Three isoforms of the Atg16L1 protein contribute different autophagic properties

The mammalian Atg16L1 protein consists of a coiled-coil domain and a tryptophan-aspartic acid (WD) repeat domain and is involved in the process of autophagy. However, the mechanisms underlying the effect of the Atg16L1 isoforms on autophagy remain to be elucidated in humans. In the present study, we successfully cloned three isoforms: Atg16L1-1, which contains the complete sequence; Atg16L1-2, which lacks all of exon 8; and Atg16L1-3, which lacks the coiled-coil domain. Subsequent experiments showed that the three isoforms of Atg16L1 were colocalised with MDC within the cells. Quantitative analysis of fluorescence showed that the average number of dots of Atg16L1-1 that colocalised with MDC was higher than those of Atg16L1-2 and Atg16L1-3. The three isoforms of Atg16L1 also colocalised with the lysosome within the cells. The average number of dots of Atg16L1-1 that colocalised with the lysosome was higher than those of Atg16L1-2 and Atg16L1-3. However, although Atg16L1-1 and Atg16L1-3 colocalised with the mitochondria, Atg16L1-2 did not. Functional analysis showed that overexpression of the three isoforms of Atg16L1 had a stimulative effect on autophagy. Significant increase in the number of positive LC3-II dots per cell was observed in Atg16L1-1 (70.2 ± 2.39 dots); this number was greater than those of the other two isoforms. Atg16L1-2 appeared to have an average of 59.25 ± 2.22 LC3-II dots per cell. Atg16L1-3 appeared to have the least number of LC3-II dots per cell (48.25 ± 2.22 dots) (P < 0.001). Our results indicated that the degree of autophagy varied with different Atg16L1 isoforms. The different domains of Atg16L1 played different roles in the process of autophagy. The coiled-coil domain of Atg16L1 was involved in the process of autophagy.

Non-autophagic roles of autophagy-related proteins

Autophagy and autophagy-related processes are fundamentally important in human health and disease. These processes are viewed primarily as cellular degradative pathways that recycle macromolecules and dysfunctional or redundant organelles into amino acids, sugars and lipids, especially during starvation. However, the ubiquitin-like autophagy proteins and other components of the autophagic machinery additionally participate in cellular reprogramming. We highlight these non-autophagic roles of autophagy proteins with the aim of drawing attention to this growing, but unexplored, research topic. We focus on the non-autophagic functions of autophagy proteins in cell survival and apoptosis, modulation of cellular traffic, protein secretion, cell signalling, transcription, translation and membrane reorganization.