Researchers at Japan's renowned RIKEN institute have uncovered a groundbreaking mechanism involving the enzyme METTL18 methyltransferase that safeguards pancreatic function. This discovery reveals how METTL18 regulates protein synthesis to maintain pancreatic homeostasis, offering new insights into preventing diabetes and related disorders.

The pancreas plays a crucial role in regulating blood glucose levels through insulin production by beta cells and digestive enzyme secretion by acinar cells. Disruptions in this delicate balance contribute to diabetes, a major health challenge in Japan where approximately 11 million people are affected, representing a prevalence of around 20% among men over 40. Beta-cell dysfunction is particularly prominent in East Asian populations, including Japanese individuals, making research into pancreatic proteostasis— the maintenance of proper protein folding and quality control— vital.

Understanding METTL18: A Novel Histidine Methyltransferase

METTL18, or methyltransferase like 18, is an enzyme that specifically adds a methyl group to the N3 position of histidine residues in proteins. In this case, it targets histidine 245 (H245) on ribosomal protein L3 (RPL3, also known as uL3), a component of the ribosome—the cellular machinery responsible for translating messenger RNA (mRNA) into proteins. This post-translational modification (PTM) fine-tunes translation dynamics, slowing ribosome movement at certain codons to allow proper protein folding and prevent aggregation.

Prior studies from RIKEN had established METTL18's role in modulating translation elongation, particularly at tyrosine codons, to uphold proteostasis in cell lines. The new work extends this to an organ-specific context, demonstrating near-complete (100%) methylation of RPL3-H245 in wild-type mouse pancreas, which is abolished in METTL18-deficient models.

This modification ensures that proteins, especially those prone to misfolding like pancreatitis-associated Reg1, are synthesized correctly, avoiding toxic aggregates that trigger stress responses.

Experimental Breakthrough: Insights from Knockout Mouse Models

Led by Tadahiro Shimazu in Yo-ichi Shinkai's Cellular Memory Laboratory at RIKEN's Pioneering Research Institute (PRI) in Wako, Saitama, the team generated Mettl18 knockout (KO) mice using CRISPR-Cas9. These mice displayed partial preweaning lethality, with only about 20% surviving to adulthood, underscoring METTL18's essential role.

Surviving KO mice exhibited reduced body weight, elevated blood glucose levels, and significantly lower serum insulin (p=0.00034). Oral glucose tolerance tests (OGTT) revealed impaired glucose handling, mimicking type 2 diabetes phenotypes, while insulin sensitivity remained intact. Proteomic analysis identified 13 downregulated proteins, including insulin genes Ins1 and Ins2, and 19 upregulated ones enriched in proteases and lipases like Reg1.

In pancreatic acinar cell lines (266-6), METTL18 loss caused Reg1 protein accumulation without mRNA changes, leading to insoluble aggregates visualized by immunofluorescence. Remarkably, overexpressing wild-type METTL18 rescued this, but not methylation-deficient mutants, confirming the modification's necessity.

Translation Dysregulation: Codon-Specific Speed Control

Ribosome profiling in KO acinar cells showed global translational shifts: 1,615 transcripts upregulated and 1,159 downregulated in occupancy. Notably, proline (Pro) codon occupancy decreased, indicating faster elongation. Polysome profiling revealed reduced 40S ribosomal subunits, hinting at biogenesis issues.

Pro-rich proteins like Prh1 aggregated more in KO cells, linking rapid translation to folding defects. Unlike prior Tyr codon effects in other cells, pancreas-specific Pro acceleration drives proteotoxicity. This step-by-step process—methylation slows ribosome at Pro, aids folding, prevents ER stress—highlights tissue-specific regulation.

Activated unfolded protein response (UPR) markers (PERK, eIF2α, ATF4, IRE1) and inflammatory cytokines (IL-1β, IL-6) confirmed downstream consequences.

RIKEN's Pioneering Role in Japanese Biomedical Research

RIKEN, Japan's flagship research organization founded in 1917, exemplifies the nation's commitment to cutting-edge science. The Center for Biosystems Dynamics Research (BDR) in Yokohama and PRI in Saitama foster interdisciplinary work on life processes from development to aging. Collaborators like Shintaro Iwasaki (also University of Tokyo) bridge institutes and academia.

This METTL18 study, published in Molecular Metabolism, builds on RIKEN's diabetes legacy, including iPS cell therapies and beta-cell survival enhancements. For aspiring researchers, opportunities abound at RIKEN and partner universities like Tsukuba and Tokyo Tech, advancing regenerative medicine for diabetes.

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Broader Context: Diabetes Burden and Beta-Cell Research in Japan

Japan faces a diabetes epidemic, with beta-cell dysfunction contributing over 24% to type 2 diabetes incidence in community studies like Hisayama. Unlike insulin resistance-dominant Western cases, Asian profiles emphasize early beta-cell failure.

Pancreatic fat accumulation and reduced beta-cell mass (30-65% loss in T2D) are key. METTL18's role in suppressing aggregation aligns with iPS-derived beta-cell regeneration efforts at CiRA (Kyoto University) and Tokyo Tech, where Shoen Kume's team engineers glucose-responsive cells.

Statistics show 9.6-10.9% prevalence among adults, urging proteostasis-targeted therapies.

Mechanistic Insights: From Ribosome to ER Stress

Step-by-step: 1) METTL18 methylates RPL3-H245. 2) Slows ribosome at Pro codons. 3) Enables co-translational folding. 4) Prevents Reg1/Prh1 aggregation. 5) Averts UPR (PERK-eIF2-ATF4, IRE1-XBP1) and inflammation.

LC-MS/MS confirmed 100% stoichiometry in pancreas. No nutrient sensitivity under high-fat diet, suggesting constitutive role. This contrasts cell-line Tyr effects, emphasizing context-dependency.

Therapeutic Potential and Future Directions

METTL18 variants may underpin diabetes susceptibility; human studies needed. Beta-cell-specific KO models will clarify insulin links. Targeting histidine methylation could enhance proteostasis, complementing GLP-1 agonists or stem-cell transplants.

RIKEN's press release highlights: "Future studies... elucidate METTL18's contribution to beta-cell failure."RIKEN announcement. Broader proteome impacts await exploration.

For researchers eyeing Japan, higher-ed jobs in biotech proliferate amid national diabetes initiatives.

Collaborative Ecosystem: RIKEN and Japanese Universities

RIKEN partners with universities like Tsukuba (Shichino), Tokyo (Iwasaki), fostering talent. Similar efforts at Kumamoto University (IRCMS) and Osaka University target beta-cell regeneration. This ecosystem drives Japan's leadership in regenerative medicine, supported by MEXT grants.

Implications for Global Diabetes Management

With 592 million projected diabetes cases by 2035, METTL18 insights transcend Japan. Proteostasis enhancers could preserve beta/acinar function, reducing aggregation-driven decline. Actionable: Screen RPL3 methylation in cohorts; develop small-molecule mimetics.

Check career advice for biotech paths. Explore professor ratings at leading Japanese institutions.

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Conclusion: A Milestone in Pancreatic Research

RIKEN's METTL18 discovery illuminates proteostasis as a diabetes linchpin, paving ways for novel therapies. As Japan invests in research jobs and iPS innovation, global collaboration beckons. Stay updated via university jobs, higher ed jobs, and career advice at AcademicJobs.com.