Biology subjects

Sakai, R.

Publications and source records attributed to Sakai, R..

14 recordsLinked to original sources

bioRxiv2026

Endocytosis of ALK promotes glucose uptake in ALK-amplified neuroblastoma

Activated receptor tyrosine kinases (RTKs), such as epidermal growth factor receptor (EGFR) and anaplastic lymphoma kinase (ALK), trigger intracellular signaling while undergoing receptor endocytosis. We recently identified a noncanonical mechanism in which RTK-containing endocytic vesicles deliver extracellular glucose to hexokinases associated with the outer mitochondrial membrane, thereby promoting cellular glucose uptake. Whether this mechanism contributes to cancer metabolism, however, remains unknown. Here, using neuroblastoma cell lines with distinct ALK alterations, we investigated the role of ALK endocytosis in glucose uptake. ALK-amplified, but not ALK-mutant, neuroblastoma cells exhibited a [~]40- 50% reduction in glucose uptake following inhibition of ALK or receptor endocytosis. This process was independent of the ERK MAPK and PI3K-AKT pathways but required dynamin-dependent endocytosis, cytoplasmic dynein, and GLUT1. Overexpressed ALK constitutively co-endocytosed with GLUT1 into vesicles transported to mitochondria. Inhibition of ALK activity or endocytosis suppressed glucose uptake without producing an additive effect, indicating that both function within the same pathway. Furthermore, disruption of the endocytic machinery selectively impaired the growth of ALK-amplified neuroblastoma cells. These findings identify ALK endocytosis as a major regulator of glucose uptake in ALK-amplified neuroblastoma and suggest that RTK endocytosis represents a previously unrecognized metabolic vulnerability that may be therapeutically exploitable in RTK-driven cancers.

cancer biology↗

bioRxiv2026

Rab32/Rab38-positive Lysosome-Related Organelle degrades lipid droplet in hepatocytes by microautophagy

Rab32 and Rab38 are paralogous small GTPases involved in the biogenesis of lysosome-related organelles (LROs), yet their roles in hepatic lipid metabolism remain poorly defined. Here, Rab32 and Rab38 double-knockout (DKO) male mice exhibited an age-dependent increase in body weight accompanied by hepatic lipid accumulation, suggesting impaired hepatic lipid processing. In AML12 hepatocytes, Rab32 and Rab38 localized to ring-like, LAMP1-positive structures characteristic of LROs, whose size increased with cell confluence. Pharmacological inhibition of lysosomal acid lipase with orlistat led to the accumulation of lipid droplets (LDs) within Rab32/38-positive LROs, indicating that LD degradation occurs in these compartments. Additional treatment with bafilomycin A1 revealed invagination-like internal membrane structures within enlarged LROs. These processes were not affected by artificial inhibition of macroautophagy, highlighting the involvement of microautophagy. Ring-like signals positive for phosphatidylinositol 3-phosphate (PI3P) or phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) were detected within or adjacent to LRO membranes, and LDs were frequently associated with these structures, suggesting a role for PI3P and PI(3,5)P2 in internal membrane formation. Vps4B was also required for efficient LD incorporation. Consistently, Rab32/38 double-knockdown (DKD) AML12 cells exhibited increased lipid accumulation, indicating impaired LD engulfment. Together, these findings identify Rab32/38-positive LROs as a structural platform for microautophagy-mediated lipid droplet degradation in hepatocytes.

cell biology↗

bioRxiv2025

Modulation of translational elongation by montanine alkaloids preferentially inhibits RNA viruses

Montanine, an Amaryllidaceae alkaloid, has demonstrated superior antiviral activity against dengue virus (DENV), SARS-CoV-2, vesicular stomatitis Indiana virus (VSV-G), and vesicular stomatitis virus pseudotyped with Ebola virus glycoprotein (VSV-ZGP), outperforming other antiviral alkaloids tested, including lycorine, narciclasine, tetracetylnarciclasine, and pancracine. We further showed that montanine inhibits translation in mammalian cells, as evidenced by a puromycin incorporation assay. Cryo-EM analysis revealed that montanine binds to the peptidyl transferase center of the human ribosome. A chemical genomics survey indicated that the knockdown of the GCN1 complex, which senses ribosome collisions and triggers the translation quality control process, increased the cytotoxicity of montanine while reducing viral infectivity. Together, these results suggest that the antiviral activity of montanine is closely linked to its impact on translational elongation and GCN1-related stress responses, underscoring the potential of translation control as a targeted mode for RNA virus therapeutics. Significance StatementMontanine, an alkaloid from the Amaryllidaceae family known for its diverse bioactivities, was identified as a potent antiviral compound from a library of natural plant- and fungus-derived substances. Through a combination of biochemical assays and cryo-EM analysis, we discovered that montanine binds to the peptidyl transfer center of the human ribosome, effectively inhibiting protein synthesis. Based on these findings and a chemical genomics approach, we hypothesize that the translational control of montanine triggers a cellular response that impedes viral replication. We propose that molecules capable of modulating translation efficacy could serve as unique lead compounds in the development of broad-spectrum antiviral agents.

biochemistry↗

bioRxiv2025

Unilateral damage to the entopeduncular nucleus causes forelimb motor dysfunction in rats

BackgroundThe entopeduncular nucleus (EP), corresponding to the human globus pallidus internal segment, is a basal ganglia output nucleus, and plays a critical role in motor control. However, the impact of EP damage on skilled motor function and the relationship between its damage in stroke, such as internal capsule hemorrhage (ICH), and motor dysfunction remains unclear. This study aimed to clarify whether EP damage causes motor dysfunction in two disease models. MethodsEP-related motor dysfunction was investigated by inducing localized unilateral EP damage in Long-Evans rats using a stereotactic kainic acid (KA) injection. Motor function was assessed using a single-pellet reaching task pre-injection and on postoperative days 2, 7, 14, 21, and 28. Immunohistochemical staining for NeuN, somatostatin (SST), and parvalbumin was conducted to quantify damage and its correlation with motor outcomes. In addition, unilateral ICH was induced via stereotactic injection of collagenase type IV, which dissolves the vascular basement membrane, into the internal capsule (IC) of Long-Evans rats. Injury sites were classified into the IC, dorsomedial region from the IC, ventral lateral region from the IC, and EP, and their volumes were measured. Measured volumes were analyzed for correlations with motor function assessments. ResultsKA-induced EP damage significantly reduced reaching success rates on postoperative day 2 compared to those in the control group (p<0.05). Immunohistochemical analysis showed that reaching success rates on day 28 positively correlated with the numbers of remaining NeuN-positive and SST-positive neurons (p<0.05). In the ICH experiment, all rats significantly reduced the success rate of the reaching task to 0% on day 2, and the success rate on day 28 correlated positively with the remaining EP volume, but not with total lesion volume. ConclusionsEP damage was strongly associated with motor impairments, highlighting its critical role in motor control and recovery.

neuroscience↗

bioRxiv2024

Symbiotic Bacterial Partners for Chemical Defense are Vertically Transmitted in a Marine Sponge

Marine sponges host diverse symbiotic bacteria, some of which are essential for chemical defense. However, the mechanisms governing the inheritance of these defensive symbionts remain elusive. Here we report the first case of a transmission mechanism for symbionts producing cytotoxins in the sponge Mycale nullarosette, for which we established the full life-cycle cultivation method. The identification of Candidatus Synechomycale tutelaris as the cytotoxins producer enabled us to unveil their vertical transmission to offspring via bacteriocyte-like structures from the embryonic stage. This study demonstrates that sponges have evolved a sophisticated system to ensure the inheritance of significant bacterial partners crucial for host defense, which is likely one of the reasons that sponges have survived throughout the long history of multicellular organisms.

microbiology↗

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↗

bioRxiv2023

Endocytic vesicles act as vehicles for glucose uptake in response to growth factor stimulation

Glycolysis is a fundamental cellular process, yet its regulatory mechanisms remain incompletely understood. Here, we show that a subset of glucose transporter 1 (GLUT1/SLC2A1) co-endocytoses with platelet-derived growth factor (PDGF) receptor (PDGFR) upon PDGF-stimulation. Furthermore, multiple glycolytic enzymes localize to these endocytosed PDGFR/GLUT1-containing vesicles adjacent to mitochondria. Contrary to current models, which emphasize the importance of glucose transporters on the cell surface, we find that PDGF-stimulated glucose uptake depends on receptor/transporter endocytosis. Our results suggest that growth factors generate glucose-loaded endocytic vesicles that deliver glucose to the glycolytic machinery in proximity to mitochondria, and argue for a new layer of regulation for glycolytic control governed by cellular membrane dynamics.

cell biology↗

bioRxiv2023

Smart utilization of betaine lipids in giant clam Tridacna crocea

The giant clam Tridacna crocea inhabits shallow tropical seas with poorly nourished water and severe sun irradiation. They harbor symbiotic algae "zooxanthellae" (dinoflagellate family Symbiodiniaceae) in the mantle tissue and are thought to thrive in this extreme environment by utilizing photosynthetic products from the algae. However, there is no measure of the detailed metabolic flow between the host and symbiont to evaluate one of the most successful symbiotic relationships in nature. Here, we employed liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based lipidomics and Fourier-transform ion cyclotron resonance MS imaging on T. crocea tissues, revealing a unique lipid composition and localization with their symbiont algae. We discovered that the non-phosphorous microalgal betaine lipid diacylglycerylcarboxy-hydroxymethylcholine (DGCC) was present in all tissues and organs of T. crocea to approximately the same degree as phosphatidylcholine (PC). The fatty acid composition of DGCC was similar to that of PC, which is thought to have physiological roles similar to that of DGCC. MS imaging showed co-localization of these lipids throughout the clam tissues. Glycerylcarboxy-hydroxymethylcholine (GCC), the deacylated derivative of DGCC, was found to be a free form of DGCC in the clams and was isolated and characterized from cultured Symbiodiniaceae strains that were isolated from giant clams. These results strongly suggest that giant clams have evolved to smartly utilize DGCCs, phosphorus-free polar lipids of symbiont algae, as essential membrane components to enable them to thrive in oligotrophic coral reef milieu.

zoology↗

bioRxiv2022

Tetraploidy-linked sensitization to CENP-E inhibition in human cells

Tetraploidy caused by whole-genome duplication is a hallmark of cancer cells, and tetraploidy-selective cell growth suppression is a potential strategy for targeted cancer therapy. However, how tetraploid cells differ from normal diploids in their sensitivity to anti-proliferative treatments remains largely unknown. In this study, we found that tetraploid cells are significantly more susceptible to inhibitors of a mitotic kinesin CENP-E than diploids. CENP-E inhibitor preferentially diminished the tetraploid cell population in diploid-tetraploid co-culture at optimum conditions. Live imaging revealed that tetraploidy-linked increase in unsolvable polar chromosome misalignment caused substantially longer mitotic delay in tetraploids than in diploids upon moderate CENP-E inhibition. This time gap of mitotic arrest resulted in cohesion fatigue and subsequent cell death, specifically in tetraploids, leading to tetraploidy-selective cell growth suppression. In contrast, the microtubule-stabilizing compound paclitaxel caused tetraploidy-selective growth suppression through the aggravation of spindle multipolarization. We also found that CENP-E inhibitor had superior generality to paclitaxel in its tetraploidy selectivity across a broader spectrum of cell lines. Our results highlight the unique properties of CENP-E inhibitors in tetraploidy-selective suppression, giving us clues on the further development of tetraploidy-targeting interventions in cancer.

cell biology↗

bioRxiv2021

Hidden pathway for cytokine receptor activation: Structural insights into a marine sponge-derived lectin that activates the thrombopoietin receptor via recognition of the fucose moiety

N-glycan-mediated activation of the thrombopoietin receptor (MPL) under pathological conditions has been implicated in myeloproliferative neoplasms induced by mutant calreticulin, which forms an endogenous receptor-agonist complex that constitutively activates the receptor. However, the molecular basis for this mechanism has not been studied because no external agonists existed. We describe the structure and function of a marine sponge-derived MPL agonist, thrombocorticin (ThC), a homodimerized lectin with calcium-dependent fucose-binding properties. ThC-induced activation persists due to limited receptor internalization. The strong synergy between ThC and thrombopoietin suggests that ThC catalyzes the formation of receptor dimers on the cell surface. MPL is subject to sugar-mediated activation, where the kinetics differ from those of cytokines. This result suggests the presence of diverse receptor activation pathways in human thrombopoiesis. One-sentence summaryA marine sponge lectin catalyzes thrombopoietin receptor dimerization and activation, exhibiting strong synergy with thrombopoietin, and modulates internalization of the receptor.

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

Elucidation of the antiviral mechanism of cystine and theanine through transcriptome analysis of mice and comparison with COVID-19 gene set data

We previously showed that oral administration of cystine and theanine (CT) to mice confers resistance to influenza virus infection. In human studies, CT prevented colds in healthy subjects and enhanced antibody production after influenza vaccination in elderly individuals with a poor nutritional status. The mechanism of action of CT is thought to be glutathione (GSH)-mediated regulation of intracellular redox, which might affect innate immune systems such as macrophages to exert physiological effects. The effect of CT on influenza is independent of viral type, and this treatment has a broad range of antiviral activities. To explore the mechanisms of CT in viral infection, we performed transcriptome profiling of spleen tissues isolated from influenza A virus (IAV)-infected mice. We identified unique gene signatures in response to CT in the IAV-infected mice. Genes upregulated by CT included redox-regulated genes such as GCLC/GCLM (subunits of glutamate cysteine ligase, a rate-limiting enzyme of GSH biosynthesis), TXN1, TXN2, TXNRD2, and SOD1, suggesting that the intracellular redox environment is substantially altered by CT. However, genes downregulated in response to CT included chemokine/chemokine receptor genes (CCL5, CCL19, CXCL9, CXCL12, CXCR3, CXCR4, and ACKR3), some of which are related to cytokine storm. A comparison with public COVID-19-related gene set data showed that the upregulated gene signature was highly similar to the downregulated gene sets of SARS-CoV/SARS-CoV-2-infected cells and the upregulated gene set of attenuated SARS-CoV-infected cells. In conclusion, the unique gene signatures observed in response to orally administered CT in IAV-infected mouse spleen tissues suggested that CT may attenuate viral infection, replication and associated symptoms such as cytokine storm.

molecular biology↗

bioRxiv2020

Cyclo olefin polymer-based solvent-free mass-productive microphysiological systems

A microphysiological system (MPS) holds a great promise for drug screening and toxicological testing as an alternative to animal models. However, this platform has several issues in terms of the materials used (e.g., polydimethylsiloxane), such as the absorbance of tested drug candidates and fluorescent dyes by the material, as well as the effect on cultured cellular status, thus misleading the results obtained from cell assays and fabrication processes. Hence, to eliminate the issues mentioned above, we developed a cyclo olefin polymer (COP)-based MPS via photobonding process using vacuum ultraviolet (VUV), named COP-VUV-MPS. COP-VUV-MPS showed better chemical resistance and avoided molecule absorption. COP-VUV-MPS could maintain the stemness of environmentally sensitive human-induced pluripotent stem cells without causing undesired cellular phenotypes and gene expression. These results suggested that COP-VUV-MPS might be broadly used for the advancement of MPS and applications in drug development and in vitro toxicological testing.Competing Interest StatementM.Y. is an employee of Ushio Inc. The part of this project was financially supported by Ushio Inc. Kyoto University (K.K.) and Ushio INC. (M.Y.) filed a Japanese patent application based on the research presented herein. The remaining authors declare no competing interests.View Full Text

bioengineering↗

bioRxiv2020

Integrated gut-liver-on-a-chip platform as an in vitro human model of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) afflicts a large percentage of the population, but no effective treatments have been established so far because of the unsuitability of in vitro assays and experimental models using animals. By co-culturing human gut and liver cell lines interconnected via microfluidics for a closed circulation loop, we created a gut-liver-on-a-chip (iGLC) platform as an in vitro human model of the gut-liver axis (GLA) for the initiation and progression of NAFLD. Microscopic high-content analysis followed by mRNA sequencing showed that co-culturing the gut and liver cells significantly affected each cell type compared to culturing them separately. NAFLD-inducing free fatty acids (FFAs) accumulated in the gut cells and elevated gene expressions associated with retinol metabolism and glucuronidation. The FFA-treated liver cells accumulated intracellular lipid droplets and showed an increase in gene expressions associated with a cellular response to copper ions and endoplasmic reticulum stress. As an in vitro human GLA model, the iGLC platform may serve as an alternative to animal experiments for investigating NAFLD mechanisms.

bioengineering↗