Thesis title: Role of the nuclear envelope in physiological and pathological conditions
Background: During “open” mitosis, the nuclear envelope disassembles to allow chromosome segregation and reassembles around daughter nuclei. The Endosomal Sorting Complex Required for Transport (ESCRT) mediates this process. While ESCRT-III proteins drive this process and are recruited by upstream factors such as BAF, the involvement of other ESCRT proteins remains unclear. AKTIP is a novel ESCRT-I-associated protein. It shows similarities with the ESCRT-I TSG101, it localizes at the midbody during abscission together with the ESCRT-I and ESCRT-III components. Intriguingly, AKTIP also localizes at the nuclear rim in interphase cells.
Aim: We therefore asked whether AKTIP contributes to nuclear envelope reformation by coordinating ESCRT-III organization and maintaining post-mitotic nuclear integrity.
Methods: To investigate the role of AKTIP in nuclear envelope dynamics, HeLa cells were depleted of AKTIP using lentiviral transduction. Immunofluorescence microscopy was used to examine AKTIP localization during mitosis, its co-distribution with BAF and IST1. The effects of AKTIP loss on the distribution of ESCRT-III components IST1 and CHMP2A was assessed. The role of AKTIP in nuclear integrity was also evaluated by analyzing cGAS foci, laminB1 discontinuities, and micronuclei formation.
Results: Data obtained during the PhD show that AKTIP localizes at chromatin discs during anaphase, where it colocalizes with the ESCRT-III-associated protein IST1. AKTIP depletion impaired proper ESCRT-III localization at this stage. During pro-metaphase, AKTIP assembled into a ring-like structure on nascent microtubule spindles, colocalizing with BAF; notably, BAF depletion disrupted AKTIP localization during mitosis. Functional analyses revealed that loss of AKTIP led to increased nuclear abnormalities, including laminB1 discontinuities, cGAS-positive foci, and micronuclei formation.
Conclusions: Our findings identify AKTIP as a crucial regulator of ESCRT-III organization during nuclear envelope reformation. Its recruitment begins as early as pro-metaphase, likely through a cooperative mechanism with BAF that links chromatin anchoring to membrane remodeling. AKTIP dysfunction compromises nuclear architecture, highlighting its potential relevance in aging and cancer biology.