Aims Vertebrate heart development requires the complex morphogenesis of a linear tube to form the mature organ, a process essential for correct cardiac form and function requiring coordination of embryonic laterality, cardiac growth, and regionalised cellular changes. While previous studies have demonstrated broad requirements for extracellular matrix (ECM) components in cardiac morphogenesis, we hypothesised that ECM regionalisation may fine tune cardiac shape during heart development. Methods and Results Using live in vivo light sheet imaging of zebrafish embryos we describe a left-sided expansion of the ECM between the myocardium and endocardium prior to the onset of heart looping and chamber ballooning. Analysis using an ECM sensor revealed the cardiac ECM is further regionalised along the atrioventricular axis. Spatial transcriptomic analysis of gene expression in the heart tube identified candidate genes that may drive ECM expansion. This approach identified regionalised expression of hapln1a, encoding an ECM cross-linking protein. Validation of transcriptomic data by in situ hybridisation confirmed regionalised hapln1a expression in the heart, with highest levels of expression in the future atrium and on the left side of the tube, overlapping with the observed ECM expansion. Analysis of CRISPR-Cas9-generated hapln1a mutants revealed a reduction in atrial size and reduced chamber ballooning. Loss-of-function analysis demonstrated that ECM expansion is dependent upon Hapln1a, together supporting a role for Hapln1a in regionalised ECM modulation and cardiac morphogenesis. Analysis of hapln1a expression in zebrafish mutants with randomised or absent embryonic left-right asymmetry revealed that laterality cues position hapln1a-expressing cells asymmetrically in the left side of the heart tube. Conclusions We identify a regionalised ECM expansion in the heart tube which promotes correct heart development, and propose a novel model whereby embryonic laterality cues orient the axis of ECM asymmetry in the heart, suggesting these two pathways interact to promote robust cardiac morphogenesis. Translational Perspective This study reveals that the cardiac ECM exhibits regional specialisation required for heart morphogenesis, and sheds light on how embryonic left-right asymmetry acts in concert with ECM regionalisation to fine tune heart shape. This work can help us understand the origins of congenital heart defects, and in particular the nature of morphological heart abnormalities in patients with heterotaxia-associated heart malformations. Furthermore, recent studies suggest the ECM is a key regulator of regenerative potential in the heart, thus defining how distinct ECM composition impacts upon heart form and function has implications for developing regenerative therapies in the future.

The developing heart is formed of two tissue layers separated by an extracellular matrix (ECM) that provides chemical and physical signals to cardiac cells. While deposition of specific ECM components creates matrix diversity, the cardiac ECM is also dynamic, with modification and degradation playing important roles in ECM maturation and function. In this Review, we discuss the spatiotemporal changes in ECM composition during cardiac development that support distinct aspects of heart morphogenesis. We highlight conserved requirements for specific ECM components in human cardiac development, and discuss emerging evidence of a central role for the ECM in promoting heart regeneration.

Localized mRNA translation is a widespread mechanism for targeting protein synthesis, important for cell fate, motility and pathogenesis. In Drosophila, the spatiotemporal control of gurken/TGF-a mRNA translation is required for establishing the embryonic body axes. A number of recent studies have highlighted key aspects of the mechanism of gurken mRNA translational control at the dorsoanterior corner of the mid-stage oocyte. Orb/CPEB and Wispy/GLD-2 are required for polyadenylation of gurken mRNA, but unlocalized gurken mRNA in the oocyte is not fully polyadenylated.1 At the dorsoanterior corner, Orb and gurken mRNA have been shown to be enriched at the edge of Processing bodies, where translation occurs.2 Over-expression of Orb in the adjacent nurse cells, where gurken mRNA is transcribed, is sufficient to cause mis-expression of Gurken protein.3 In orb mutant egg chambers, reducing the activity of CK2, a Serine/Threonine protein kinase, enhances the ventralized phenotype, consistent with perturbation of gurken translation. 4 Here we show that sites phosphorylated by CK2 overlap with active Orb and with Gurken protein expression. Together with our new findings we consolidate the literature into a working model for gurken mRNA translational control and review the role of kinases, cell cycle factors and polyadenylation machinery highlighting a multitude of conserved factors and mechanisms in the Drosophila egg chamber.

Egg activation is a universal process that includes a series of events to allow the fertilized egg to complete meiosis and initiate embryonic development. One aspect of egg activation, conserved across all organisms examined, is a change in the intracellular concentration of calcium (Ca 2+ ) often termed a 'Ca 2+ wave'. While the speed and number of oscillations of the Ca 2+ wave varies between species, the change in intracellular Ca 2+ is key in bringing about essential events for embryonic development. These changes include resumption of the cell cycle, mRNA regulation, cortical granule exocytosis, and rearrangement of the cytoskeleton.In the mature Drosophila egg, activation occurs in the female oviduct prior to fertilization, initiating a series of Ca 2+ -dependent events. Here we present a protocol for imaging the Ca 2+ wave in Drosophila. This approach provides a manipulable model system to interrogate the mechanism of the Ca 2+ wave and the downstream changes associated with it.

During early vertebrate heart development the heart transitions from a linear tube to a complex asymmetric structure, a morphogenetic process which occurs simultaneously with growth of the heart. Cardiac growth during early heart morphogenesis is driven by deployment of cells from the Second Heart Field (SHF) into both poles of the heart. Laminin is a core component of the extracellular matrix (ECM), and although mutations in laminin subunits are linked with cardiac abnormalities, no role for laminin has been identified in early vertebrate heart morphogenesis. We identified tissue-specific expression of laminin genes in the developing zebrafish heart, supporting a role for laminins in heart morphogenesis. Analysis of heart development in lamb1a zebrafish mutant embryos reveals mild morphogenetic defects and progressive cardiomegaly, and that Lamb1a functions to limit heart size during cardiac development by restricting SHF addition. lamb1a mutants exhibit hallmarks of altered haemodynamics, and blocking cardiac contractility in lamb1a mutants rescues heart size and atrial SHF addition. Together this suggests that laminin mediates interactions between SHF deployment and cardiac biomechanics during heart development and growth in the developing embryo.