A mammalian model reveals inorganic polyphosphate channeling into the nucleolus and induction of a hyper-condensate state

Abstract

Summary: Inorganic polyphosphate (polyP) is a ubiquitous polymer that controls fundamental processes. To overcome the absence of a genetically tractable mammalian model, we developed an inducible mammalian cell line expressing Escherichia coli polyphosphate kinase 1 (EcPPK1). Inducing EcPPK1 expression prompted polyP synthesis, enabling validation of polyP analytical methods. Virtually all newly synthesized polyP accumulates within the nucleus, mainly in the nucleolus. The channeled polyP within the nucleolus results in the redistribution of its markers, leading to altered rRNA processing. Ultrastructural analysis reveals electron-dense polyP structures associated with a hyper-condensed nucleolus resulting from an exacerbation of the liquid-liquid phase separation (LLPS) phenomena controlling this membraneless organelle. The selective accumulation of polyP in the nucleoli could be interpreted as an amplification of polyP channeling to where its physiological function takes place. Indeed, quantitative analysis of several mammalian cell lines confirms that endogenous polyP accumulates within the nucleolus. Motivation: The processes regulated by polyP are numerous and varied, and some are directly linked to the pathophysiology of human diseases. However, a reliable model for manipulating polyP in mammalian cells is currently lacking. To provide researchers with such a model, we have developed an inducible cell line expressing Escherichia coli polyphosphate kinase 1 (EcPPK1). These cells allowed the validation of polyP detection methods, the development of new assays, and the dissection of polyP-regulated physiology.

Keywords

• CP: Molecular biology