Laminin β1a controls distinct steps during the establishment of digestive organ laterality

Abstract

SUMMARYVisceral organs, including the liver and pancreas, adopt asymmetric positions to ensure proper function. Yet the molecular and cellular mechanisms controlling organ laterality are not well understood. We identified a mutation affecting zebrafish laminin β1a (lamb1a) that disrupts left-right asymmetry of the liver and pancreas. In these mutants, the liver spans the midline and the ventral pancreatic bud remains split into bilateral structures. We show that lamb1a regulates asymmetric left-right gene expression in the lateral plate mesoderm (LPM). In particular, lamb1a functions in Kupffer's vesicle (KV), a ciliated organ analogous to the mouse node, to control the length and function of the KV cilia. Later during gut-looping stages, dynamic expression of Lamb1a is required for the bilayered organization and asymmetric migration of the LPM. Loss of Lamb1a function also results in aberrant protrusion of LPM cells into the gut. Collectively, our results provide cellular and molecular mechanisms by which extracellular matrix proteins regulate left-right organ morphogenesis. KEY WORDS: Asymmetry, Laminin, OrganogenesisLaminin β1a controls distinct steps during the establishment of digestive organ laterality Tatiana Here, we report that laminin plays an essential role in the establishment of L-R asymmetry of the zebrafish liver and pancreas. Laminins are large heterotrimeric glycoproteins, comprising of α, β and γ chains assembled into a cross-shaped molecule with a long arm and three short globular arms (Engel et al., 1981;Miner, 2008). We present a novel mutant allele of the laminin β1a (lamb1a) gene, which encodes a subunit of the ECM protein laminin 1. In this mutant, the liver spans the midline, and the ventral pancreatic bud remains split into bilateral structures. We find that laminin is necessary for controlling cilia length and fluid flow in the KV, and for restricting southpaw expression to the left LPM. Subsequently, lamb1a deficiency disrupts the dynamic deposition of laminin 1 in the LPM epithelium, resulting in a severely disorganized epithelium with abnormal protrusions into the gut, and a failure of asymmetric gut looping. Thus, laminin 1 participates in sequential events that are important for the establishment of L-R asymmetry of visceral organs, including an early role in the KV and a later role in the LPM.

EphrinB1/EphB3b Coordinate Bidirectional Epithelial-Mesenchymal Interactions Controlling Liver Morphogenesis and Laterality

“…Cell-shape analysis in transverse sections (encompassing dorsoventral and mediolateral axes) revealed no difference in hepatoblast elongation at 26 and 32 hpf (Figure S1), suggesting cell polarization along the anteroposterior axis. In contrast to previous models, in which gut looping and liver positioning are solely the result of asymmetric LPM migration and passive hepatoblast displacement (Hochgreb-Hagele et al., 2013, Horne-Badovinac, 2003, Yin et al., 2010), these shape changes indicate oriented hepatoblast movement during budding.…”

“…Concomitant with leftward gut looping and liver positioning, the left LPM moves dorsal to the endoderm, while the right LPM moves ventrolaterally toward the endoderm (Figures 4A‴–4B‴). Mutants with disrupted LPM epithelial morphology or impaired ECM degradation show defective LPM movement and midline-positioned gut and liver, which led to the model that active LPM movements, in particular of the right LPM, exert a motive force on the passive endodermal progenitors directing leftward gut looping and liver outgrowth (Hochgreb-Hagele et al., 2013, Horne-Badovinac, 2003, Yin et al., 2010). How exactly the mesoderm controls this complex morphogenetic rearrangement of the liver progenitors into the liver bud is unclear.…”

Left–Right Patterning: Breaking Symmetry to Asymmetric Morphogenesis

“…Upstream, MMP activity itself is regulated by the basic helix-loop-helix (bHLH) transcription factor HAND2, such that hand2 mutants fail to undergo looping morphogenesis [22]. Interestingly, both Laminin and HAND2 play a dual role in L-R patterning, being required for earlier steps in setting up the Nodal-Pitx2 pathway in addition to LPM migrations during gut looping [22, 23]. This demonstrates that regulators of asymmetry can have multiple roles at different stages of L-R patterning, something that can complicate analysis of some mutants and should thus be kept in mind.…”

Making It New Again

“…In heart and soul mutants (Stainier et al, 1996), where protein kinase C, iota ( prkci ) is inactivated, the epithelial structure of the LPM and its asymmetric movement are disrupted, resulting in a midline liver and gut (Sakaguchi et al, 2008). Likewise, mutants with impaired extracellular matrix (ECM) remodeling, specifically, laminin degradation, display defective LPM migration and midline livers (Hochgreb-Hägele et al, 2013; Yin et al, 2010). Together, these findings suggest that as the LPM migrates right, it displaces the midline endoderm to the left by a motive force.…”