Hiromi Kirisako

Bio: Hiromi Kirisako is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Autophagy & ATG8. The author has an hindex of 13, co-authored 16 publications receiving 1045 citations.

Receptor-mediated selective autophagy degrades the endoplasmic reticulum and the nucleus

Abstract: Macroautophagy (hereafter referred to as autophagy) degrades various intracellular constituents to regulate a wide range of cellular functions, and is also closely linked to several human diseases. In selective autophagy, receptor proteins recognize degradation targets and direct their sequestration by double-membrane vesicles called autophagosomes, which transport them into lysosomes or vacuoles. Although recent studies have shown that selective autophagy is involved in quality/quantity control of some organelles, including mitochondria and peroxisomes, it remains unclear how extensively it contributes to cellular organelle homeostasis. Here we describe selective autophagy of the endoplasmic reticulum (ER) and nucleus in the yeast Saccharomyces cerevisiae. We identify two novel proteins, Atg39 and Atg40, as receptors specific to these pathways. Atg39 localizes to the perinuclear ER (or the nuclear envelope) and induces autophagic sequestration of part of the nucleus. Atg40 is enriched in the cortical and cytoplasmic ER, and loads these ER subdomains into autophagosomes. Atg39-dependent autophagy of the perinuclear ER/nucleus is required for cell survival under nitrogen-deprivation conditions. Atg40 is probably the functional counterpart of FAM134B, an autophagy receptor for the ER in mammals that has been implicated in sensory neuropathy. Our results provide fundamental insight into the pathophysiological roles and mechanisms of 'ER-phagy' and 'nucleophagy' in other organisms.

The Atg2-Atg18 complex tethers pre-autophagosomal membranes to the endoplasmic reticulum for autophagosome formation

Abstract: The biogenesis of double-membrane vesicles called autophagosomes, which sequester and transport intracellular material for degradation in lysosomes or vacuoles, is a central event in autophagy. This process requires a unique set of factors called autophagy-related (Atg) proteins. The Atg proteins assemble to organize the preautophagosomal structure (PAS), at which a cup-shaped membrane, the isolation membrane (or phagophore), forms and expands to become the autophagosome. The molecular mechanism of autophagosome biogenesis remains poorly understood. Previous studies have shown that Atg2 forms a complex with the phosphatidylinositol 3-phosphate (PI3P)-binding protein Atg18 and localizes to the PAS to initiate autophagosome biogenesis; however, the molecular function of Atg2 remains unknown. In this study, we show that Atg2 has two membrane-binding domains in the N- and C-terminal regions and acts as a membrane tether during autophagosome formation in the budding yeast Saccharomyces cerevisiae. An amphipathic helix in the C-terminal region binds to membranes and facilitates Atg18 binding to PI3P to target the Atg2-Atg18 complex to the PAS. The N-terminal region of Atg2 is also involved in the membrane binding of this protein but is dispensable for the PAS targeting of the Atg2-Atg18 complex. Our data suggest that this region associates with the endoplasmic reticulum (ER) and is responsible for the formation of the isolation membrane at the PAS. Based on these results, we propose that the Atg2-Atg18 complex tethers the PAS to the ER to initiate membrane expansion during autophagosome formation.

Atg12–Atg5 conjugate enhances E2 activity of Atg3 by rearranging its catalytic site

Abstract: In the yeast autophagy system, the Atg12–Atg5 conjugate acts as an E3 to promote the E2 activity of Atg3, which conjugates Atg8 to phosphatidylethanolamine. Now structural and biochemical analyses reveal that Atg12–Atg5 induces a rearrangement in the catalytic center of Atg3, which employs a threonine residue in addition to the active cysteine to catalyze the conjugation reaction.

Hrr25 triggers selective autophagy–related pathways by phosphorylating receptor proteins

Abstract: In selective autophagy, degradation targets are specifically recognized, sequestered by the autophagosome, and transported into the lysosome or vacuole. Previous studies delineated the molecular basis by which the autophagy machinery recognizes those targets, but the regulation of this process is still poorly understood. In this paper, we find that the highly conserved multifunctional kinase Hrr25 regulates two distinct selective autophagy–related pathways in Saccharomyces cerevisiae. Hrr25 is responsible for the phosphorylation of two receptor proteins: Atg19, which recognizes the assembly of vacuolar enzymes in the cytoplasm-to-vacuole targeting pathway, and Atg36, which recognizes superfluous peroxisomes in pexophagy. Hrr25-mediated phosphorylation enhances the interactions of these receptors with the common adaptor Atg11, which recruits the core autophagy-related proteins that mediate the formation of the autophagosomal membrane. Thus, this study introduces regulation of selective autophagy as a new role of Hrr25 and, together with other recent studies, reveals that different selective autophagy–related pathways are regulated by a uniform mechanism: phosphoregulation of the receptor–adaptor interaction.

Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)

Abstract: In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure flux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation, it is imperative to target by gene knockout or RNA interference more than one autophagy-related protein. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways implying that not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular assays, we hope to encourage technical innovation in the field.

The machinery of macroautophagy

Abstract: Autophagy is a primarily degradative pathway that takes place in all eukaryotic cells. It is used for recycling cytoplasm to generate macromolecular building blocks and energy under stress conditions, to remove superfluous and damaged organelles to adapt to changing nutrient conditions and to maintain cellular homeostasis. In addition, autophagy plays a critical role in cytoprotection by preventing the accumulation of toxic proteins and through its action in various aspects of immunity including the elimination of invasive microbes and its participation in antigen presentation. The most prevalent form of autophagy is macroautophagy, and during this process, the cell forms a double-membrane sequestering compartment termed the phagophore, which matures into an autophagosome. Following delivery to the vacuole or lysosome, the cargo is degraded and the resulting macromolecules are released back into the cytosol for reuse. The past two decades have resulted in a tremendous increase with regard to the molecular studies of autophagy being carried out in yeast and other eukaryotes. Part of the surge in interest in this topic is due to the connection of autophagy with a wide range of human pathophysiologies including cancer, myopathies, diabetes and neurodegenerative disease. However, there are still many aspects of autophagy that remain unclear, including the process of phagophore formation, the regulatory mechanisms that control its induction and the function of most of the autophagy-related proteins. In this review, we focus on macroautophagy, briefly describing the discovery of this process in mammalian cells, discussing the current views concerning the donor membrane that forms the phagophore, and characterizing the autophagy machinery including the available structural information.

Guidelines for the use and interpretation of assays for monitoring autophagy (4th edition)

Abstract: In 2008, we published the first set of guidelines for standardizing research in autophagy. Since then, this topic has received increasing attention, and many scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Thus, it is important to formulate on a regular basis updated guidelines for monitoring autophagy in different organisms. Despite numerous reviews, there continues to be confusion regarding acceptable methods to evaluate autophagy, especially in multicellular eukaryotes. Here, we present a set of guidelines for investigators to select and interpret methods to examine autophagy and related processes, and for reviewers to provide realistic and reasonable critiques of reports that are focused on these processes. These guidelines are not meant to be a dogmatic set of rules, because the appropriateness of any assay largely depends on the question being asked and the system being used. Moreover, no individual assay is perfect for every situation, calling for the use of multiple techniques to properly monitor autophagy in each experimental setting. Finally, several core components of the autophagy machinery have been implicated in distinct autophagic processes (canonical and noncanonical autophagy), implying that genetic approaches to block autophagy should rely on targeting two or more autophagy-related genes that ideally participate in distinct steps of the pathway. Along similar lines, because multiple proteins involved in autophagy also regulate other cellular pathways including apoptosis, not all of them can be used as a specific marker for bona fide autophagic responses. Here, we critically discuss current methods of assessing autophagy and the information they can, or cannot, provide. Our ultimate goal is to encourage intellectual and technical innovation in the field.

The autophagosome: origins unknown, biogenesis complex

Abstract: Autophagosome biogenesis starts at the isolation membrane (also called the phagophore). Our understanding of the molecular processes that initiate the isolation membrane, the membrane sources from which this membrane originates and how it is expanded to the autophagosome membrane by autophagy-related (ATG) proteins and the vesicular trafficking machinery, is increasing.

Molecular definitions of autophagy and related processes

Abstract: Over the past two decades, the molecular machinery that underlies autophagic responses has been characterized with ever increasing precision in multiple model organisms. Moreover, it has become clear that autophagy and autophagy-related processes have profound implications for human pathophysiology. However, considerable confusion persists about the use of appropriate terms to indicate specific types of autophagy and some components of the autophagy machinery, which may have detrimental effects on the expansion of the field. Driven by the overt recognition of such a potential obstacle, a panel of leading experts in the field attempts here to define several autophagy-related terms based on specific biochemical features. The ultimate objective of this collaborative exchange is to formulate recommendations that facilitate the dissemination of knowledge within and outside the field of autophagy research.