Biology subjects

Koyama-Honda, I.

Publications and source records attributed to Koyama-Honda, I..

8 recordsLinked to original sources

bioRxiv2025

ATG gene duplication in vertebrates: evolutionary divergence and its functional implications

Macroautophagy (hereafter referred to as autophagy) requires the coordinated action of approximately 20 autophagy-related (ATG) genes. Duplication of ATG genes has had a major impact on the evolution of the autophagy pathway among major lineages. One duplication hotspot is in vertebrates. However, the exact duplication timing, post-duplication evolutionary divergence patterns, and its relation to functional differences among paralogs have not been investigated in detail. Here, we demonstrate that most ATG genes were likely duplicated by whole-genome duplication events near the root of vertebrates. We compared the sequence and gene expression divergence between paralogs and categorized the evolutionary fates (i.e., how ancestral function is divided between paralogs). Within the paralog pairs that evolved most asymmetrically, namely BECN, WIPI (WIPI1 and WIPI2), and ATG16, one paralog likely retained the ancestral function, allowing the other to evolve under less constraint. While no obvious asymmetry was observed between ATG9A and ATG9B in non-mammalian vertebrates, ATG9B experienced marked sequence divergence and expression level reduction in mammals, suggesting a shift in balance. Expression patterns among the ULK-1 (ULK1 and ULK2), GABARAP (GABARAP and GABARAPL1), and LC3 (LC3A and LC3B) pairs were more consistent with hypofunctionalization/dosage sharing, such that ancestral function depends on both paralogs. We also demonstrate that both ULK1 and ULK2 can support autophagy, whereas only BECN1, but not BECN2, has autophagic function and discuss the relationship between autophagic function and evolutionary divergence between paralogs. The present detailed analysis of ATG gene duplication in vertebrates provides a critical time-line for interpreting functional differentiation between homologs.

evolutionary biology↗

bioRxiv2024

The lysosomal membrane protein LAMP2B mediates microlipophagy to target obesity-related disorders

Lifestyle diseases, such as obesity, diabetes, and metabolic syndrome, are leading health problems, most of which are related to abnormal lipid metabolism. Lysosomes can degrade lipid droplets (LDs) via microautophagy. Here, we report the molecular mechanism and pathophysiological roles of microlipophagy, regulated by the lysosomal membrane protein LAMP2B. Our study revealed that LAMP2B interacts with phosphatidic acid, facilitating lysosomal-LD interactions and enhancing lipid hydrolysis via microlipophagy depending on endosomal sorting complexes required for transport. Correlative light and electron microscopy demonstrated direct LDs uptake into lysosomes at contact sites. Moreover, LAMP2B overexpression in mice prevents high-fat diet-induced obesity, insulin resistance, and adipose tissue inflammation; liver lipidomics analysis suggested enhanced triacylglycerol hydrolysis. Overall, the findings of this study elucidated the mechanism of microlipophagy, which could be promising for the treatment of obesity and related disorders.

cell biology↗

bioRxiv2024

Optogenetic tools for inducing organelle membrane rupture

Disintegration of organelle membranes induces various cellular responses and has pathological consequences, including autoinflammatory diseases and neurodegeneration. Establishing methods to induce membrane rupture of organelles of interest is essential to analyze the downstream effects of membrane rupture; however, the spatiotemporal induction of rupture of specific membranes remains challenging. Here, we develop a series of optogenetic tools to induce organelle membrane rupture by using engineered Bcl-2-associated X protein (BAX), whose primary function is to form membrane pores in the outer mitochondrial membrane (OMM) during apoptosis. When BAX is forced to target mitochondria, lysosomes, or the endoplasmic reticulum (ER) by replacing its C-terminal transmembrane domain (TMD) with organelle-targeting sequences, the BAX mutants rupture their target membranes. To regulate the activity of organelle-targeted BAX, the photosensitive light-oxygen-voltage-sensing 2 (LOV2) domain is fused to the N-terminus of BAX. The resulting LOV2-BAX fusion protein exhibits blue light-dependent membrane-rupture activity on various organelles, including mitochondria, the ER, and lysosomes. Thus, LOV2-BAX enables spatiotemporal induction of membrane rupture across a broad range of organelles, expanding research opportunities on the consequences of organelle membrane disruption.

cell biology↗

bioRxiv2023

Organelle landscape analysis using a multi-parametric particle-based method

Organelles have unique structures and molecular compositions for their functions and have been classified accordingly. However, many organelles are heterogeneous and in the process of maturation and differentiation. Because traditional methods have a limited number of parameters and spatial resolution, they struggle to capture the heterogeneous landscapes of organelles. Here, we present a method for multi-parametric particle-based analysis of organelles. After disrupting cells, fluorescence microscopy images of organelle particles labeled with six to eight different organelle markers were obtained, and their multi-dimensional data were represented in intuitive two-dimensional UMAP (uniform manifold approximation and projection) spaces. This method enabled visualization of landscapes of seven major organelles as well as the transitional states of endocytic organelles directed to the recycling and degradation pathways. Furthermore, endoplasmic reticulum-mitochondria contact sites were detected in these maps. Our proposed method successfully detects a wide array of organelles simultaneously, enabling the analysis of heterogeneous organelle landscapes.

cell biology↗

bioRxiv2023

Syntaxin 17 recruitment to mature autophagosomes is temporally regulated by PI4P accumulation

During macroautophagy, cytoplasmic constituents are engulfed by autophagosomes. Lysosomes fuse with closed autophagosomes but not with unclosed intermediate structures. This is achieved in part by the late recruitment of the autophagosomal SNARE syntaxin 17 (STX17) to mature autophagosomes. However, how STX17 recognizes autophagosome maturation is not known. Here, we show that this temporally regulated recruitment of STX17 depends on the positively charged C-terminal region of STX17. Consistent with this finding, mature autophagosomes are more negatively charged compared with unclosed intermediate structures. This electrostatic maturation of autophagosomes is likely driven by the accumulation of phosphatidylinositol 4-phosphate (PI4P) in the autophagosomal membrane. Accordingly, dephosphorylation of autophagosomal PI4P prevents the association of STX17 to autophagosomes. Furthermore, molecular dynamics simulations support PI4P-dependent membrane insertion of the transmembrane helices of STX17. Based on these findings, we propose a model in which STX17 recruitment to mature autophagosomes is temporally regulated by a PI4P-driven change in the surface charge of autophagosomes.

cell biology↗

bioRxiv2022

Experimental determination and mathematical modeling of standard shapes of forming autophagosomes

The formation of autophagosomes involves dynamic morphological changes of a phagophore from a disk-shaped membrane cisterna into a cup-shaped intermediate and a spherical autophagosome. However, the physical mechanism behind these morphological changes remains elusive. Here, we determined the average shapes of phagophores by statistically investigating three-dimensional electron micrographs of more than 100 phagophores. The results showed that the cup-shaped structures adopted a characteristic morphology; they were longitudinally elongated, and the rim was catenoidal with an outwardly recurved shape. To understand these characteristic shapes, we established a theoretical model of the shape of entire phagophores. The model quantitatively reproduced the average morphology and revealed that the characteristic shape of phagophores (i.e., an elongated shape with a catenoidal rim) was primarily determined by the relative size of the open rim to the total surface area. These results suggest that autophagosomal membranes are highly flexible and that the morphological changes during autophagosome formation follow a stable path determined by elastic bending energy minimization. SummaryThe formation of autophagosomes involves dynamic morphological changes of membrane cisternae. Sakai et al. determined the average shapes of forming autophagosomes by statistically investigating three-dimensional electron micrographs and established a theoretical model that quantitatively reproduces the phagophore shapes.

cell biology↗

bioRxiv2020

Direct uptake mechanism in lysosome required for neuromuscular homeostasis

The degradation of cellular components plays an essential role in homeostasis. However, the known degradation pathways cannot account for the levels of proteolysis in cells. Here, we demonstrate that cytosolic proteins are imported into lysosomes in an ATP-dependent manner for degradation through a direct uptake mechanism distinct from any known pathway. SIDT2, a lysosomal membrane protein previously reported as an RNA transporter, translocates substrate proteins across the lysosomal membrane. Furthermore, we identify a dominant-negative mutation in SIDT2 that causes neuropathy and distal myopathy with rimmed vacuoles, a protein aggregation disease in humans. We generate Sidt2 knockout mice, recapitulating the characteristic features of this disease. Our results reveal a novel degradation pathway and illustrate its crucial role in cellular proteostasis, physiology, and pathophysiology. One Sentence SummaryDiscovery of a novel proteolytic pathway in cells, the dysfunction of which leads to protein aggregation disease in humans.

cell biology↗

bioRxiv2020

High-speed single-molecule imaging reveals signal transduction by induced transbilayer raft phases

Using single-molecule imaging with enhanced time resolutions down to 5 ms, we found that CD59-cluster rafts and GM1-cluster rafts stably induced in the outer leaflet of the plasma membrane (PM), which triggered the activation of Lyn, H-Ras, and ERK, continually recruited Lyn and H-Ras right beneath them in the inner leaflet, with dwell lifetimes of <0.1 s. The detection was possible due to the enhanced time resolutions employed here. The recruitment depended on the PM cholesterol and saturated alkyl chains of Lyn and H-Ras, whereas it was blocked by the non-raftophilic transmembrane protein moiety and unsaturated alkyl chains linked to the inner-leaflet molecules. Since GM1-cluster rafts recruited Lyn and H-Ras as efficiently as CD59-cluster rafts, and the protein moieties of Lyn and H-Ras were not required for the recruitment, we conclude that the transbilayer raft phases induced by the outer-leaflet stabilized rafts recruit lipid-anchored signaling molecules by lateral raft-lipid interactions, and thus serve as a key signal transduction platform. SummaryHigh-speed single-molecule imaging indicated that CD59-cluster rafts and GM1-cluster rafts stably induced in the plasma membrane outer leaflet generated nano-scale transbilayer raft phases, which continually and transiently recruited Lyn and H-Ras in the inner leaflet by cooperative raft-lipid interactions.

cell biology↗