Pearling drives mitochondrial DNA nucleoid homeostasis

The mitochondrial network carries a small multi-copy genome in the form of regularly distributed nucleoids, surrounded by intricate and restrictive membrane invaginations. As a result, the integrity of the local nucleoid affects the fitness of the surrounding region, making mitochondrial DNA distribution and proliferation critical for mitochondrial function and inheritance, as well as cellular health. Despite its fundamental importance, how seemingly static nucleoids are segregated and distributed in such a constrained system remains unknown, both for the proliferation of newly replicated nucleoids and for the isolation of damaged regions.

Our data reveal that mitochondria frequently undergo reversible pearling, a biophysical instability that undulates tubules into regularly spaced beads. Using a variety of super-resolution and adaptive microscopy approaches, we identify pearling as a major driver behind the puzzling disaggregation and regular distribution of nucleoids along the mitochondrial network with near-maximally achievable precision. Through the pharmacological and genetic modulation of its occurrence, we demonstrate that mitochondrial genome distribution is governed by the interplay between ER-derived calcium influx triggering rapidly reversible pearling, and lamellar cristae integrity mediating its duration, frequency, and preserving nucleoid separation following recovery. Additionally, we demonstrate the relationship between pearling, cristae microdomains, and the integrity of nucleoids in cells carrying deleted mitochondrial DNA populations, uncovering its potential role in enabling mitochondrial genome selection and quality control.