Mechanisms of A-type lamin targeting to nuclear ruptures are disrupted in LMNA- and BANF1-associated progerias

Abstract

Mutations in the genes LMNA and BANF1 can lead to accelerated aging syndromes called progeria. The protein products of these genes, A-type lamins and BAF, respectively, are nuclear envelope (NE) proteins that interact and participate in various cellular processes, including nuclear envelope rupture and repair. BAF localizes to sites of nuclear rupture and recruits NE-repair machinery, including the LEM-domain proteins, ESCRT-III complex, A-type lamins, and membranes. Here, we show that it is a mobile, nucleoplasmic population of A-type lamins that is rapidly recruited to ruptures in a BAF-dependent manner via BAF′s association with the Ig-like β fold domain of A-type lamins. These initially mobile lamins become progressively stabilized at the site of rupture. Farnesylated prelamin A and lamin B1 fail to localize to nuclear ruptures, unless that farnesylation is inhibited. Progeria-associated LMNA mutations inhibit the recruitment affected A-type lamin to nuclear ruptures, due to either permanent farnesylation or inhibition of BAF binding. A progeria-associated BAF mutant targets to nuclear ruptures but is unable to recruit A-type lamins. Together, these data reveal the mechanisms that determine how lamins respond to nuclear ruptures and how progeric mutations of LMNA and BANF1 impair recruitment of A-type lamins to nuclear ruptures.

The BAF A12T mutation associated with premature aging impedes lamin A/C recruitment to sites of nuclear rupture, contributing to nuclear envelope fragility

“…Around 30% of all blebs showed FLAG-BAF enrichment for both the WT and the A12T protein (Figure 6c). Together, these results suggest that the recruitment of FLAG-BAF to DNA at nuclear rupture sites is not affected by the A12T mutation, consistent with our in vitro data (Marcelot et al, 2021) and recent reports by others in cells (Sears and Roux, 2022). Emerin nuclear localization was rescued by overexpression of FLAG-BAF WT in NGPS patient cells (Figure S7a, b).…”

“…Upon reversing the BAF A12T homozygous mutation using CRISPR-Cas9, we confirmed that this was indeed a specific effect of the mutation in BAF. This is consistent with recent reports showing that recruitment of A-type lamins to laser-induced nuclear ruptures is impaired in fibroblasts expressing BAF A12T (Sears and Roux, 2022). Although the function of the lamin A/C recruitment to sites of nuclear envelope ruptures remains unclear, it is tempting to speculate that the accumulation of lamins allows the stabilization of the nuclear envelope after it has ruptured, thereby protecting its integrity.…”

“…Furthermore, we identified that while not affecting BAF structure, the BAF A12T mutation reduces the affinity of BAF for lamin A/C by about 10 times in vitro . As a consequence, the recruitment of lamin A/C to NE rupture sites is greatly affected in NGPS patient cells, in accordance with recent reports using different cell models (Sears and Roux, 2022). This leads to the persistence of lamina gaps at sites of ruptures, and absence of lamin “scars” that could contribute to nuclear fragility in NGPS patient cells.…”

“…One hypothesis is that it might cause repetitive ruptures of the nuclear envelope at the same site due to the persistence of a lamina gap. Alternatively, as the presence of lamin A has been shown to be important for the accumulation of BAF at the sites of rupture (Kone et al, 2022), the repair process could be delayed or impaired although we and others (Sears and Roux, 2022) did not find an obvious defect in BAF A12T recruitment to sites of nuclear ruptures. It was also shown that cells depleted of A-type lamins are still able to repair the NE (Halfmann et al, 2019).…”

“…Nuclear blebs can be an early step in nuclear envelope rupture and recruitment of lamin A/C by BAF at sites of ruptures has recently been shown to be an early event in the repair of these ruptures (Kone et al, 2022; Sears and Roux, 2022). Initially, a mobile A-type lamin population is recruited to rupture sites which becomes stabilized, leaving behind a lamin “scar”.…”