Extreme nuclear branching in healthy epidermal cells of the Xenopus tail fin

Abstract Changes in nuclear morphology contribute to regulation of complex cell properties, including differentiation and tissue elasticity. Perturbations of nuclear morphology are associated with pathologies that include, progeria, cancer, and muscular dystrophy. The mechanisms governing nuclear shape changes in healthy cells remain poorly understood, partially because there are few healthy models of nuclear shape variation. Here, we introduce nuclear branching in epidermal fin cells of Xenopus tropicalis as a model for extreme variation of nuclear morphology in a diverse population of healthy cells. We find that nuclear branching arises and elaborates during embryonic development. They contain broadly distributed marks of transcriptionally active chromatin and heterochromatin and have active cell cycles. We find that nuclear branches are disrupted by loss of filamentous actin and depend on epidermal expression of the nuclear lamina protein Lamin B1. Inhibition of nuclear branching disrupts fin morphology, suggesting that nuclear branching may be involved in fin development. This study introduces the nuclei of the fin as a powerful new model for extreme nuclear morphology in healthy cells to complement studies of nuclear shape variation in pathological contexts. List of abbreviations and symbolsLINC-Linker of Nucleoskeleton and Cytoskeleton, HGPS – Hutchinson-Gilford Progeria Syndrome, TEM – Transmission electron microscopy, PH3 – Phosphorylated Histone 3, Lat B – Latrunculin B, WT-Wild type, Cyto D-Cytochalasin D, Lmnb1 CRISPR – lmnb1 mutants generated by CRISPR/Cas9, E-Lmnb1 CRISPR – epidermal specific lmnb1 mutants generated by CRISPR/Cas9, Scrmbl-tadpoles injected with a scrambled version of the lmnb1 targeted sgRNA, Lmnb1-rod – dominant-negative Lamin B1 containing only the rod domain Summary StatementNuclei are highly branched throughout the heterogeneous population of healthy epidermal cells that comprise the Xenopus tail fin periphery, and disruption of nuclear branching mechanisms results in improper fin morphology.