Study finds transient nuclear softening in two-cell mouse embryos

Nuclear mechanics at the two-cell stage

A study published in Nature on 11 November 2026 reports that nuclei in early mouse embryos attain profound softness and plasticity during the two-cell embryonic stage, facilitating major zygotic gene activation. The research was conducted by Masahito Tanaka, Rin Sakanoue, Yasuki Miyagawa, Kai Otsuka, Yoshiaki Kobayashi, Atsushi Takasu, Ryunosuke Mori, Naoko Watanabe, Akihiro Inoue, Koyo Ikeda, Yu-Chia Chen, Aussie Suzuki, Yasuyuki Ohkawa, So Maezawa, Hiroshi Ochiai, Kei Miyamoto and Yuta Shimamoto.

Using high-resolution confocal scanning, the researchers observed that two-cell nuclei underwent slow, incremental deformation starting at around 21 hours post-insemination (h.p.i.), peaking at 24–26 h.p.i., and then gradually regressing by 32–36 h.p.i. At the one-cell and four-cell stages, nuclei were predominantly spherical.

A pressure-controlled nanocapillary setup measured nuclear stiffness. The study found that two-cell nuclei were significantly more deformable than one-cell and four-cell nuclei. Nuclear stiffness transiently lowered at the two-cell stage, with values of one-cell, 5.0 ± 1.3 nN µm−1; two-cell, 2.0 ± 0.5 nN µm−1; and four-cell, 4.6 ± 1.9 nN µm−1 (n = 18, 38 and 13 embryos, respectively). Two-cell nuclei also exhibited significant plasticity, with shape recovery of 66.9 ± 8.0% for two-cell, compared with 91.0 ± 4.3% for one-cell and 87.9 ± 4.6% for four-cell.

Lamin B1 clearance and LC3

The researchers examined four major lamins, A/C, B1 and B2, and found that lamin B1 transiently cleared from the two-cell nuclear envelope, nearly paralleling nuclear softening. The clearance was rapid and substantial, with an approximately 70% decrease in peripheral signal during the early-to-mid two-cell stage (21 to 24 h.p.i.). Live-embryo imaging confirmed the clearance of lamin B1 from the two-cell nuclear envelope with a decay time constant of 86 ± 44 minutes (n = 5 embryos).

The autophagy factor LC3 was diffusive at the one-cell stage and accumulated in the two-cell nucleus. An antibody microinjection targeting LC3 suppressed lamin B1 clearance. The researchers designed a construct, sfLB1, that interferes with the endogenous interaction between lamin B1 and LC3. When expressed in embryos, this fragment suppressed the clearance of lamin B1, whereas lamin A/C and B2 remained largely unperturbed. The development of embryos was impaired by sfLB1, but not by a variant, sfLB1Δ, which had abrogated LC3 binding.

The study suggests that lamin B1 is degraded through an autophagy–lysosome pathway. Tracking of fluorescent reporters showed predominant lysosomal localization and formation of foci for the wild-type construct, while mutations that abrogated LC3 interactions suppressed lamin B1 clearance.

Developmental consequences

Nanocapillary measurements indicated that two-cell nuclei with the envelope enriched with endogenous lamin B1 were stiffer and less plastic than controls, at 4.2 ± 1.3 nN µm−1 (n = 17 embryos). These nuclei were as stiff as normal one-cell and four-cell nuclei. Overexpression of lamin B1 constructs that resisted clearance profoundly increased nuclear elasticity and stiffness.

The study found that all fertilized embryos that timely reached the blastocyst stage had soft nuclei (2.0 ± 0.6 nN µm−1; n = 66 of 66 embryos), while nearly half of the embryos that impaired development had stiff nuclei (n = 7 of 15 embryos; defined as more than twice the mean nuclear stiffness of normal development). Detecting embryos with stiff nuclei was infrequent (8.6%; n = 7 out of 81 embryos).

High-resolution microscopy and sequential three-dimensional fluorescence in situ hybridization analyses revealed an accumulation of active RNA polymerase II and coordinated positioning of key genomic loci near the deformable nuclear envelope. An engineered molecular tether that stiffens the nuclei restricts these nuclear activities and suppresses expression of key zygotic gene activation genes.

Why it matters

The study identifies a distinct physical nuclear state that embryos acquire at the beginning of life that supports the initiation of the zygotic transcription program. Nuclear stiffening by lamin B1 retention or its overloading results in incomplete activation of zygotic genes and impaired development.

Sources

• nature.com: Transient nuclear softening for zygotic gene activation in mice | Nature

This article was drafted with AI assistance and checked against the sources above. Company claims are reported as claims. Cover image is AI-generated.