Atrial natriuretic peptide () and brain natriuretic peptide () form a gene cluster with expression in the chambers of the developing heart. Despite restricted expression, a function in cardiac development has not been demonstrated by mutant analysis. This is attributed to functional redundancy; however, their genomic location has impeded formal analysis. Using genome editing, we have generated mutants for and , and found that single mutants were indistinguishable from wild type, whereas/ double mutants displayed heart morphogenesis defects and pericardial oedema. Analysis of atrioventricular canal (AVC) markers show expansion of ,, and expression into the atrium of double mutants. This expanded expression correlates with increased extracellular matrix in the atrium. Using a biosensor for hyaluronic acid to measure the cardiac jelly (cardiac extracellular matrix), we confirmed cardiac jelly expansion in / double mutants. Finally, knockdown rescued the expansion of expression and cardiac jelly in double mutants. This definitively shows that and function redundantly during cardiac development to restrict gene expression to the AVC, preventing excessive cardiac jelly synthesis in the atrial chamber.

The establishment of cardiac function in the developing embryo is essential to ensure blood flow and, therefore, growth and survival of the animal. The molecular mechanisms controlling normal cardiac rhythm remain to be fully elucidated. From a forward genetic screen, we identified a unique mutant, grime, that displayed a specific cardiac arrhythmia phenotype. We show that loss-of-function mutations in tmem161b are responsible for the phenotype, identifying Tmem161b as a regulator of cardiac rhythm in zebrafish. To examine the evolutionary conservation of this function, we generated knockout mice for Tmem161b. Tmem161b knockout mice are neonatal lethal and cardiomyocytes exhibit arrhythmic calcium oscillations. Mechanistically, we find that Tmem161b is expressed at the cell membrane of excitable cells and live imaging shows it is required for action potential repolarization in the developing heart. Electrophysiology on isolated cardiomyocytes demonstrates that Tmem161b is essential to inhibit Ca2+ and K+ currents in cardiomyocytes. Importantly, Tmem161b haploinsufficiency leads to cardiac rhythm phenotypes, implicating it as a candidate gene in heritable cardiac arrhythmia. Overall, these data describe Tmem161b as a highly conserved regulator of cardiac rhythm that functions to modulate ion channel activity in zebrafish and mice.

Background During heart morphogenesis, the cardiac chambers undergo ballooning: a process involving regionalized elongation of cardiomyocytes. Cardiomyocyte shape changes require reorganization of the actin cytoskeleton; however, the genetic regulation of this process is not well understood. Results From a forward genetic screen, we identified the zebrafish uq 23ks mutant which manifests chamber ballooning defects. Whole‐genome sequencing‐mapping identified a truncating mutation in the gene, myo5b. myo5b encodes an atypical myosin required for endosome recycling and, consistent with this, increased vesicles were observed in myo5b mutant cardiomyocytes. Expression of RFP‐Rab11a (a recycling endosome marker) confirmed increased recycling endosomes in cardiomyocytes of myo5b mutants. To investigate potential cargo of MyoVb‐associated vesicles, we examined the adherens junction protein, N‐cadherin. N‐cadherin appeared mispatterned at cell junctions, and an increase in the number of intracellular particles was also apparent. Co‐localization with RFP‐Rab11a confirmed increased N‐cadherin‐positive recycling endosomes, demonstrating N‐cadherin trafficking is perturbed in myo5b mutants. Finally, phalloidin staining showed disorganized F‐actin in myo5b cardiomyocytes, suggesting the cytoskeleton fails to remodel, obstructing chamber ballooning. Conclusions MyoVb is required for cardiomyocyte endosomal recycling and appropriate N‐cadherin localization during the onset of chamber ballooning. Cardiomyocytes lacking MyoVb are unable to reorganize their actin cytoskeleton, resulting in failed chamber ballooning. Developmental Dynamics 248:284–295, 2019. © 2019 Wiley Periodicals, Inc.

Medulloblastoma (MB) is an embryonal-derived lesion arising in the cerebellum, contributing to 20% of childhood brain tumours and 63% of intracranial embryonal tumours. Currently, standard of care includes surgical resection and chemotherapy, used in conjunction with radiation. While these treatments have significantly improved survival rates, the therapy side effects are detrimental to survivors’ quality of life. Furthermore, relapse occurs in 30% of patients and unfortunately these are untreatable and therefore fatal for 95% of patients. It is presumed that in patients that present with a recurrent tumour, initial treatment has been ineffective. A major cause of this can be the heterogenous vasculature in and around the tumour. Recent studies have identified that the blood brain barrier (BBB), in the context of MB, is highly heterogenous with differences in BBB cellular composition and integrity between MB sub-types and within single tumours. It remains unclear however, how these vascular differences emerge and what the primary defects are. This gap in our knowledge is mainly due to the lack of models that allow long-term live imaging. We have established a xenograft approach to examine human MB tumour cells in zebrafish embryonal brains. We have validated that human Medulloblastoma cells of the Group 3 (Gp3) MB subgroup, are viable in zebrafish brains. This work has also identified that in the presence of these human Gp3 MB tumour cells, the local vasculature becomes dysmorphic and tortuous over time. Timelapse imaging of these tumour and vasculature interactions demonstrates that there is a rapid and long ranging angiogenic response with new vessel sprouts growing towards the tumour cells. By utilising our large range of transgenic zebrafish lines, we will continue investigating how MB cells interact and alter distinctive BBB cell types and how these changes impact vessel function.