Getting to the Heart of Left–Right Asymmetry: Contributions from the Zebrafish Model

Abstract

The heart is laterally asymmetric. Not only is it positioned on the left side of the body but the organ itself is asymmetric. This patterning occurs across scales: at the organism level, through left–right axis patterning; at the organ level, where the heart itself exhibits left–right asymmetry; at the cellular level, where gene expression, deposition of matrix and proteins and cell behaviour are asymmetric; and at the molecular level, with chirality of molecules. Defective left–right patterning has dire consequences on multiple organs; however, mortality and morbidity arising from disrupted laterality is usually attributed to complex cardiac defects, bringing into focus the particulars of left–right patterning of the heart. Laterality defects impact how the heart integrates and connects with neighbouring organs, but the anatomy of the heart is also affected because of its asymmetry. Genetic studies have demonstrated that cardiac asymmetry is influenced by left–right axis patterning and yet the heart also possesses intrinsic laterality, reinforcing the patterning of this organ. These inputs into cardiac patterning are established at the very onset of left–right patterning (formation of the left–right organiser) and continue through propagation of left–right signals across animal axes, asymmetric differentiation of the cardiac fields, lateralised tube formation and asymmetric looping morphogenesis. In this review, we will discuss how left–right asymmetry is established and how that influences subsequent asymmetric development of the early embryonic heart. In keeping with the theme of this issue, we will focus on advancements made through studies using the zebrafish model and describe how its use has contributed considerable knowledge to our understanding of the patterning of the heart.

Next-generation plasmids for transgenesis in zebrafish and beyond

“…The initial Tol2kit contained the destination vectors pDestTol2pA2 as “empty” destination vector and pDestTol2CG2 that included the myl7 (formerly cmlc2) promoter driving cytoplasmic EGFP in the developing myocardium from late somitogenesis stages on, easily screenable by 24 hpf (Huang et al, 2003; Kwan et al, 2007). However, myl7 -based transgenesis markers are not desirable for transgenes focused on heart development, a significant interest in the field (Bakkers, 2011; Cao and Poss, 2018; Kemmler et al, 2021; Potts et al, 2021; Smith et al, 2021; Yao et al, 2021). Further, the heart becomes increasingly obscured by skin pigmentation in standard zebrafish strains, rendering transgene detection in adults challenging (White et al, 2008).…”

Natural reversal of cavefish heart asymmetry is controlled by Sonic Hedgehog effects on the left-right organizer

“…Although the mechanisms responsible for establishing L-R visceral asymmetry in vertebrates are fairly well known (Smith and Uribe, 2021), those controlling natural changes in asymmetric patterning are not understood. One of the reasons for this gap in knowledge is that abnormal asymmetry is rare and difficult to study in most vertebrates.…”

Sequential action of JNK genes establishes the embryonic left-right axis

“…Zebrafish abdominal organs are asymmetrically positioned, with left-sided positioning of the liver and right-sided positioning of the stomach, pancreas and spleen (Horne-Badovinac et al, 2003). In the heart, the first evidence of morphological left-right identity is manifested as jogging: the extension of the linear heart tube under the left eye (Chen et al, 1997;Smith and Uribe, 2021). Dextral heart looping establishes the asymmetric placement of the single atrium and ventricle.…”

“…Using zebrafish to investigate the link between node function and organ asymmetry (Smith and Uribe, 2021), we set out to address whether a role exists for members of the JNK family in regulating left-right axis development and whether genetic compensation between JNK family members may obscure PCP functions. Generating stable mutants, we characterised the impact of loss of the four zebrafish JNK genes on nodal cilia development and the subsequent result on organ asymmetry.…”