No abstract

Neurons send out axons and dendrites over large distances into target areas where they eventually form synapses with selected target cells. Axonal navigation is controlled by a variety of extracellular signals and neurons express receptors only for that subset of signals they need to navigate to their own target area. How the expression of axon guidance receptors is regulated is not understood. In genetic screens for mutants with axon guidance defects, we identified an ETS-domain transcription factor, AST-1, specifically required for axon navigation in certain classes of interneurons. In addition, ast-1 has a role in the differentiation of the ventral cord pioneer neuron AVG. Outside the nervous system, ast-1 is essential for morphogenesis of the pharynx. Ast-1 is transiently expressed in several classes of neurons (including AVG) during neuronal differentiation with a peak expression during late stages of neuronal differentiation and axon outgrowth. Ast-1 genetically interacts with other transcription factors controlling neuronal differentiation like lin-11 and zag-1 as well as components of the netrin pathway suggesting that ast-1 might control the expression of components of the netrin signal transduction machinery.

The ventral cord in C. elegans is the major longitudinal axon tract containing essential components of the motor circuit. In genetic screens using transgenic animals expressing neuron specific GFP reporters, we identified twelve genes required for the correct outgrowth of interneuron axons of the motor circuit. In mutant animals, axons fail to navigate correctly towards the ventral cord or fail to fasciculate correctly within the ventral cord. Several of those mutants define previously uncharacterized genes. Two of the genes, ast-4 and ast-7, are involved in the generation of left-right asymmetry of the two ventral cord axon tracts. Three other genes specifically affect pioneer-follower relationships between early and late outgrowing axons, controlling either differentiation of a pioneer neuron (lin-11) or the ability of axons to follow a pioneer (ast-2, unc-130). Navigation of the ventral cord pioneer neuron AVG itself is defective in ast-4, ast-6 and unc-130 mutants. Correlation of these defects with navigation defects in different classes of follower axons revealed a true pioneer role for AVG in the guidance of interneurons in the ventral cord. Taken together, these genes provide a basis to address different aspects of axon navigation within the ventral cord of C. elegans.

Subtype-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are promising tools, e.g., to assess the potential of drugs to cause chronotropic effects (nodal hiPSC-CMs), atrial fibrillation (atrial hiPSC-CMs), or ventricular arrhythmias (ventricular hiPSC-CMs). We used single-cell patch-clamp reverse transcriptase-quantitative polymerase chain reaction to clarify the composition of the iCell cardiomyocyte population (Fujifilm Cellular Dynamics, Madison, WI, USA) and to compare it with atrial and ventricular Pluricytes (Ncardia, Charleroi, Belgium) and primary human atrial and ventricular cardiomyocytes. The comparison of beating and non-beating iCell cardiomyocytes did not support the presence of true nodal, atrial, and ventricular cells in this hiPSC-CM population. The comparison of atrial and ventricular Pluricytes with primary human cardiomyocytes showed trends, indicating the potential to derive more subtype-specific hiPSC-CM models using appropriate differentiation protocols. Nevertheless, the single-cell phenotypes of the majority of the hiPSC-CMs showed a combination of attributes which may be interpreted as a mixture of traits of adult cardiomyocyte subtypes: (i) nodal: spontaneous action potentials and high HCN4 expression and (ii) non-nodal: prominent INa-driven fast inward current and high expression of SCN5A. This may hamper the interpretation of the drug effects on parameters depending on a combination of ionic currents, such as beat rate. However, the proven expression of specific ion channels supports the evaluation of the drug effects on ionic currents in a more realistic cardiomyocyte environment than in recombinant non-cardiomyocyte systems.