La-related protein 6 (Larp6) is a conserved RNA-binding protein found across eukaryotes that has been suggested to regulate collagen biogenesis, muscle development, ciliogenesis, and various aspects of cell proliferation and migration. Zebrafish have two Larp6 family genes: larp6a and larp6b. Viable and fertile single and double homozygous larp6a and larp6b zygotic mutants revealed no defects in muscle structure, and were indistinguishable from heterozygous or wild-type siblings. However, larp6a mutant females produced eggs with chorions that failed to elevate fully and were fragile. Eggs from larp6b single mutant females showed minor chorion defects, but chorions from eggs laid by larp6a;larp6b double mutant females were more defective than those from larp6a single mutants. Electron microscopy revealed defective chorionogenesis during oocyte development. Despite this, maternal zygotic single and double mutants were viable and fertile. Mass spectrometry analysis provided a description of chorion protein composition and revealed significant reductions in a subset of zona pellucida and lectin-type proteins between wild-type and mutant chorions that paralleled the severity of the phenotype. We conclude that Larp6 proteins are required for normal oocyte development, chorion formation and egg activation.

Graphical abstract

Differentiation of mammalian pluripotent cells involves large-scale changes in transcription and, among the molecules that orchestrate these changes, chromatin remodellers are essential to initiate, establish and maintain a new gene regulatory network. The NuRD complex is a highly conserved chromatin remodeller which fine-tunes gene expression in embryonic stem cells. While the function of NuRD in mouse pluripotent cells has been well defined, no study yet has defined NuRD function in human pluripotent cells. We investigated the structure and function of NuRD in human induced pluripotent stem cells (hiPSCs). Using immunoprecipitation followed by mass-spectrometry in hiPSCs and in naive or primed mouse pluripotent stem cells, we find that NuRD structure and biochemical interactors are generally conserved. Using RNA sequencing, we find that, whereas in mouse primed stem cells and in mouse naïve ES cells, NuRD is required for an appropriate level of transcriptional response to differentiation signals, hiPSCs require NuRD to initiate these responses. This difference indicates that mouse and human cells interpret and respond to induction of differentiation differently.Graphical AbstractNuRD acts like a conductor in an orchestra.A. In the presence of NuRD (pink blob figure, centre) differentiation occurs in an ordered fashion in both mouse (left) and human (right) ES cells. Gene expression changes in both cell types are tightly controlled with down-regulation of pluripotency genes and up-regulation of lineage appropriate genes. This is akin to a group of musicians producing musical notes in the right order and at the right amplitude to create a coherent piece of music. B. Loss of “the conductor” NuRD results in increased transcriptional noise in both systems, indicated here as a low-level blanket of sound in both systems. Consequences of MBD3/NuRD loss differs between human and mouse ES cells. In mouse ES cells, differentiation cues lead to some down-regulation of pluripotency genes and incomplete progression along a lineage appropriate pathway. This is like musicians who know that they should be making music but who lose their way without a conductor’s influence. In human iPS cells the background level of noise without NuRD results in a lack of order to gene expression changes in response to differentiation. The noise from these “musicians” would be truly awful.