Dynamic control of Cajal body number during zebrafish embryogenesis

Abstract

The Cajal body (CB) is an evolutionarily conserved nuclearsubcompartment, enriched in components of the RNA processingmachinery. The composition and dynamics of CBs in cells of livingorganisms is not well understood. Here we establish the zebrafishembryo as a model system to investigate the properties of CBs duringrapid growth and cell division, taking advantage of the ease of live-cellimaging. We show that zebrafish embryo CBs contain coilin andmultiple components of the pre-mRNA splicing machinery. HistonemRNA 3’ end processing factors, present in CBs in some systems, wereinstead concentrated in a distinct nuclear body. CBs were present inembryos before and after activation of zygotic gene expression,indicating a maternal contribution of CB components. During the first24 hours of development, embryonic cells displayed up to 30 CBs pernucleus; these dispersed prior to mitosis and reassembled withinminutes upon daughter cell nucleus formation. Following zygoticgenome activation, snRNP biogenesis was required for CB assemblyand maintenance, suggesting a self-assembly process that determinesCB numbers in embryos. Differentiation into muscle, neurons andepidermis was associated with the achievement of a steady statenumber of 2 CBs per nucleus. We propose that CB number is regulatedduring development to respond to the demands of gene expression ina rapidly growing embryo.