Muscarinic receptors promote pacemaker fate at the expense of secondary conduction system tissue in zebrafish

Deterioration or inborn malformations of the cardiac conduction system (CCS) interfere with proper impulse propagation in the heart and may lead to sudden cardiac death or heart failure. Patients afflicted with arrhythmia depend on antiarrhythmic medication or invasive therapy, such as pacemaker implantation. An ideal way to treat these patients would be CCS tissue restoration. This, however, requires precise knowledge regarding the molecular mechanisms underlying CCS development. Here, we aimed to identify regulators of CCS development. We performed a compound screen in zebrafish embryos and identified tolterodine, a muscarinic receptor antagonist, as a modifier of CCS development. Tolterodine provoked a lower heart rate, pericardiac edema, and arrhythmia. Blockade of muscarinic M3, but not M2, receptors induced transcriptional changes leading to amplification of sinoatrial cells and loss of atrioventricular identity. Transcriptome data from an engineered human heart muscle model provided additional evidence for the contribution of muscarinic M3 receptors during cardiac progenitor specification and differentiation. Taken together, we found that muscarinic M3 receptors control the CCS already before the heart becomes innervated. Our data indicate that muscarinic receptors maintain a delicate balance between the developing sinoatrial node and the atrioventricular canal, which is probably required to prevent the development of arrhythmia.

Resting cells rely on the DNA helicase component MCM2 to build cilia

Minichromosome maintenance (MCM) proteins facilitate replication by licensing origins and unwinding the DNA double strand. Interestingly, the number of MCM hexamers greatly exceeds the number of firing origins suggesting additional roles of MCMs. Here we show a hitherto unanticipated function of MCM2 in cilia formation in human cells and zebrafish that is uncoupled from replication. Zebrafish depleted of MCM2 develop ciliopathy-phenotypes including microcephaly and aberrant heart looping due to malformed cilia. In non-cycling human fibroblasts, loss of MCM2 promotes transcription of a subset of genes, which cause cilia shortening and centriole overduplication. Chromatin immunoprecipitation experiments show that MCM2 binds to transcription start sites of cilia inhibiting genes. We propose that such binding may block RNA polymerase II-mediated transcription. Depletion of a second MCM (MCM7), which functions in complex with MCM2 during its canonical functions, reveals an overlapping cilia-deficiency phenotype likely unconnected to replication, although MCM7 appears to regulate a distinct subset of genes and pathways. Our data suggests that MCM2 and 7 exert a role in ciliogenesis in post-mitotic tissues.

Analysis of cilia dysfunction phenotypes in zebrafish embryos depleted of Origin recognition complex factors

Meier-Gorlin syndrome (MGS) is a rare, congenital primordial microcephalic dwarfism disorder. MGS is caused by genetic variants of components of the origin recognition complex (ORC) consisting of ORC1-6 and the pre-replication complex, which together enable origin firing and hence genome replication. In addition, ORC1 has previously been shown to play a role in ciliogenesis. Here, we extend this work and investigate the function of ORC1 and two other members of the complex on cilia at an organismal level. Knockdown experiments in zebrafish confirmed the impact of ORC1 on cilia. ORC1-deficiency confers defects anticipated to arise from impaired cilia function such as formation of oedema, kidney cysts, curved bodies and left-right asymmetry defects. We found ORC1 furthermore required for cilium formation in zebrafish and demonstrate that ciliopathy phenotypes in ORC1-depleted zebrafish could not be rescued by reconstitution with ORC1 bearing a genetic variant previously identified in MGS patients. Loss-of-function of Orc4 and Orc6, respectively, conferred similar ciliopathy phenotypes and cilium shortening in zebrafish, suggesting that several, if not all, components of the ORC regulate ciliogenesis downstream to or in addition to their canonical function in replication initiation. This study presents the first in vivo evidence of an influence of the MGS genes of the ORC family on cilia, and consolidates the possibility that cilia dysfunction could contribute to the clinical manifestation of ORC-deficient MGS.

Centrobin controls primary ciliogenesis in vertebrates

The BRCA2 interactor, centrobin, is a centrosomal protein that has been implicated in centriole duplication and microtubule stability. We used genome editing to ablate CNTROB in hTERT-RPE1 cells and observed an increased frequency of monocentriolar and acentriolar cells. Using a novel monoclonal antibody, we found that centrobin primarily localizes to daughter centrioles but also associates with mother centrioles upon serum starvation. Strikingly, centrobin loss abrogated primary ciliation upon serum starvation. Ultrastructural analysis of centrobin nulls revealed defective axonemal extension after mother centriole docking. Ciliogenesis required a C-terminal portion of centrobin that interacts with CP110 and tubulin. We also depleted centrobin in zebrafish embryos to explore its roles in an entire organism. Centrobin-depleted embryos showed microcephaly, with curved and shorter bodies, along with marked defects in laterality control, morphological features that indicate ciliary dysfunction. Our data identify new roles for centrobin as a positive regulator of vertebrate ciliogenesis.

ATR promotes cilia signalling: links to developmental impacts

Mutations in ATR(ataxia telangiectasia and RAD3-related) cause Seckel syndrome (ATR-SS), a microcephalic primordial dwarfism disorder. Hitherto, the clinical manifestation of ATR deficiency has been attributed to its canonical role in DNA damage response signalling following replication fork stalling/collapse. Here, we show that ATR regulates cilia-dependent signalling in a manner that can be uncoupled from its function during replication. ATR-depleted or patient-derived ATR-SS cells form cilia of slightly reduced length but are dramatically impaired in cilia-dependent signalling functions, including growth factor and Sonic hedgehog signalling. To better understand the developmental impact of ATR loss of function, we also used zebrafish as a model. Zebrafish embryos depleted of Atr resembled ATR-SS morphology, showed a modest but statistically significant reduction in cilia length and other morphological features indicative of cilia dysfunction. Additionally, they displayed defects in left-right asymmetry including ambiguous expression of southpaw, incorrectly looped hearts and randomized localization of internal organs including the pancreas, features typically conferred by cilia dysfunction. Our findings reveal a novel role for ATR in cilia signalling distinct from its canonical function during replication and strengthen emerging links between cilia function and development.

Assessing Smoothened-mediated Hedgehog signaling in zebrafish

Smoothened belongs to the class of atypical G protein-coupled receptors and serves as the transducing molecule in Hedgehog (Hh) signaling. Hh proteins comprise a family of secreted, cholesterol-modified ligands, which act both as morphogens and as signaling molecules. Binding of Hh proteins to their direct receptor, the transmembrane protein Patched-1, relieves Smoothened from tonal inhibition by Patched-1 and causes the translocation of Smoothened into the cilium. Here, the Hh signaling cascade is initiated and results in transcriptional activation of Hh target genes such as gli1 or patched-1. This induces a plethora of physiological outcomes including normal embryonic development, but also cancer, which is the reason why scientists aim to develop strategies to manipulate as well as monitor Smoothened-mediated Hh signaling. The zebrafish has emerged as a valuable tool for the assessment of Smoothened-mediated Hh signaling. In this chapter we thus describe how Smoothened-mediated Hh signaling can be monitored and also quantified using zebrafish embryos.

Left-right asymmetry in the light of TOR: An update on what we know so far

The internal left‐right (LR) asymmetry is a characteristic that exists throughout the animal kingdom from roundworms over flies and fish to mammals. Cilia, which are antenna‐like structures protruding into the extracellular space, are involved in establishing LR asymmetry during early development. Humans who suffer from dysfunctional cilia often develop conditions such as heterotaxy, where internal organs appear to be placed randomly. As a consequence to this failure in asymmetry development, serious complications such as congenital heart defects (CHD) occur. The mammalian (or mechanistic) target of rapamycin (mTOR) pathway has recently emerged as an important regulator regarding symmetry breaking. The mTOR pathway governs fundamental processes such as protein translation or metabolism. Its activity can be transduced by two complexes, which are called TORC1 and TORC2, respectively. So far, only TORC1 has been implicated with asymmetry development and appears to require very precise regulation. A number of recent papers provided evidence that dysregulated TORC1 results in alterations of motile cilia and asymmetry defects. In here, we give an update on what we know so far of mTORC1 in LR asymmetry development.