METRNL represses beta-to-alpha cell trans-differentiation to maintain beta cell function under diabetic metabolic stress in mice.

Aims/hypothesis: In type 2 diabetes mellitus, beta cell failure is associated with pancreatic beta cell dedifferentiation and trans-differentiation into other types of islet cells. However, the mechanisms underlying this process remain unclear. Recently, meteorin-like (METRNL) protein, a newly discovered secretory protein, has demonstrated beneficial effects in obesity and insulin resistance. However, its role in islet cell function, particularly in differentiated beta cells, remains to be elucidated. This study aims to investigate the effects of Metrnl gene deletion in beta cells on islet function and determine whether METRNL-mediated maintenance of islet cell identity is necessary for beta cell compensation in diabetes.

Methods: Mice with a specific deletion of Metrnl in beta cells were studied under both normal (chow diet) and metabolic stress (high-fat diet [HFD]) conditions. The investigation focused on their glucose tolerance, insulin secretion, islet gene expression and glucose-stimulated insulin secretion (GSIS). Additionally, cell developmental trajectory and cell-cell interaction analyses of the isolated islets were conducted using single-cell RNA-seq. Furthermore, the impact of METRNL replenishment on the regulation of beta cells in response to HFD feeding or in db/db mice was also examined.

Results: METRNL was predominantly expressed in islet beta cells. However, its expression was reduced in the islets of db/db or HFD/streptozocin-induced mice, which positively correlated with insulin expression in these diabetic mice. Furthermore, the deletion of Metrnl in beta cells disrupted insulin secretion in mice fed with HFD, resulting in worsened diabetes and glucose intolerance. Pancreatic islets isolated from METRNL-deficient mice also exhibited reduced insulin secretion in GSIS assays in vitro. Additionally, single-cell RNA-seq analysis of isolated islets demonstrated that METRNL deficiency in beta cells was associated with a potential evolutionary differentiation relationship, indicating a trajectory toward alpha cells. This beta-to-alpha cell trans-differentiation was further evidenced by the upregulation of alpha cell genes (e.g. Gcg, Arx and Irx2) and downregulation of beta cell identity genes (e.g. Ins1, Ins2, Pdx1, and Mafa). Furthermore, METRNL deficiency was found to promote beta-to-alpha cell trans-differentiation during metabolic stress by impairing beta cell capacity, partially due to increased c-Jun levels. On the other hand, as a crucial executor of Kruppel-like transcription factor 6 (KLF6), METRNL may play an important role in maintaining beta cell integrity and function under metabolic stress. Moreover, recombinant METRNL administration significantly improved glucose uptake, lessened the severity of insulin resistance and increased plasma insulin levels in both HFD-fed and db/db mice.

Conclusions/interpretation: METRNL helps to maintain beta cell integrity, preventing beta-to-alpha cell trans-differentiation, and is necessary for beta cell compensation under metabolic stress, thereby inhibiting the progression of diabetes.

Data availability: scRNA-seq data are accessible via the NCBI Sequence Read Archive ( http://www.ncbi.nlm.nih.gov/bioproject/ ) under accession no. PRJNA1224190. Original western blotting analysis is publicly available in the figshare repository, as part of this record: https://doi.org/10.6084/m9.figshare.28379006 .