The inositol pyrophosphate 5-InsP₇ regulates mitochondrial polyphosphate synthesis and bioenergetic function

Area of Science:

• Cell Biology
• Biochemistry
• Mitochondrial Biology

Background:

• Inorganic polyphosphate (polyP) is poorly understood in mammals due to low abundance and limited metabolic data.
• Mitochondria are increasingly recognized as key regulators of cellular energy and homeostasis.

Purpose of the Study:

• To develop a method for quantifying mammalian polyP.
• To investigate the role of polyP in mitochondrial function and regulation.

Main Methods:

• Developed a DAPI fluorescence-based assay for polyP quantification in mammalian cells and tissues.
• Investigated mitochondrial polyP synthesis using isolated mitochondria and cell lines with varying IP6K1 expression.
• Assessed mitochondrial respiration and ATP synthase activity.

Main Results:

• PolyP is enriched in mitochondria, with synthesis dependent on ATP synthase, proton gradient, and orthophosphate.
• Mitochondrial polyP levels are regulated by 5-InsP7, synthesized by IP6K1.
• IP6K1 deficiency reduces mitochondrial polyP and impairs respiration, with rescue effects varying by IP6K1 activity.

Conclusions:

• IP6K1 regulates mitochondrial polyP production and function through both 5-InsP7 synthesis and a catalytic-independent mechanism.
• A novel link between the energy sensor 5-InsP7 and the energy store polyP in mammalian mitochondrial homeostasis is uncovered.