Longevity of Beta Cells and Protein Complexes Revealed by Isotope Imaging

Abstract

We discovered recently that long-lived cells (LLCs), such as neurons, contain long-lived proteins (LLPs) in the nucleus that can last a lifetime, and their decline during aging impairs nuclear function. Beta cells are LLCs with a slow cell replication process, yet it is unknown whether individual beta cells have different lifespans. We tested this hypothesis by using a hybrid microscopy and mapping platform (MIMS-EM) to identify beta cells and quantify cell and protein turnover in the islets of Langerhans. This is achieved by pulse-labelling mice with an 15N-rich diet followed by a chase period with a control 14N-rich diet for up to 2 years. Here, old cells and protein structures retain 15N while those that turnover incorporate 14N. The levels of 15N and 14N were measured with multi-isotope mass spectroscopy (MIMS) and the 15N-to-14N ratio reports on cell and protein turnover. Importantly, the MIMS data is correlated with scanning electron microscopy (EM) to create high-resolution maps of cells and intra-cellular structures overlaid with age information. Here we report the use of MIMS-EM on mouse islets to discover that beta cells have vastly different ages. In fact, while approximately 75% of beta cells replicate between 2-4 times, almost 25% of all beta cells does not replicate at all after weaning and remain quiescent for an entire lifetime, and therefore, are as old as cortical neurons. Such longevity was also found in other islet cell-types and at the protein super-complex level, where 15N accumulation on the primary cilium of beta cells strongly indicates that this important signaling structure is exceptionally long-lived. Strikingly, and similar to what is found in old rat neurons, beta cells from humans aged >60-years-old lose specific nuclear LLPs and show signs of impaired nuclear function. Together, our data shows that beta cell longevity is heterogeneous and identifies the loss of nuclear LLPs in human beta cells as a potential contributing factor to age-dependent loss of beta cell function. Disclosure R. Arrojo e Drigo: Stock/Shareholder; Self; VTV Therapeutics, Novo Nordisk A/S. B.H. Toyama: None. S. Tyagi: None. V. Lev-Ram: None. T.J.J. Deerinck: None. E.A. Bushong: None. R. Ramachandra: None. C. Lechene: None. M.H. Ellisman: None. M. Hetzer: None.