Lacrimal gland budding requires PI3K-dependent suppression of EGF signaling

Phospholipase Cγ regulates lacrimal gland branching by competing with PI3K in phosphoinositide metabolism

Although the regulation of branching morphogenesis by spatially distributed cues is well established, the role of intracellular signaling in determining the branching pattern remains poorly understood. In this study, we investigated the regulation and function of phospholipase C gamma (PLCγ) in Fibroblast Growth Factor (FGF) signaling in lacrimal gland development. We showed that deletion of PLCγ1 in the lacrimal gland epithelium leads to ectopic branching and acinar hyperplasia, which was phenocopied by either mutating the PLCγ1 binding site on Fgfr2 or disabling any of its SH2 domains. PLCγ1 inactivation did not change the level of Fgfr2 or affect MAPK signaling, but instead led to sustained AKT phosphorylation due to increased PIP3 production. Consistent with this, PLCγ1 mutant phenotype can be reproduced by elevation of PI3K signaling in Pten knockout and attenuated by blocking AKT signaling. This study demonstrated that PLCγ modulates PI3K signaling by shifting phosphoinositide metabolism, revealing an important role of signaling dynamics in conjunction with spatial cues in shaping branching morphogenesis.

Crk mediates Csk-Hippo signaling independently of Yap tyrosine phosphorylation to induce cell extrusion

Src family kinases (SFKs), including Src, Fyn and Yes, play important roles in development and cancer. Despite being first discovered as the Yes-associated protein, the regulation of Yap by SFKs remains poorly understood. Here, through single-cell analysis and genetic lineage tracing, we show that the pan-epithelial ablation of C-terminal Src kinase (Csk) in the lacrimal gland unleashes broad Src signaling but specifically causes extrusion and apoptosis of acinar progenitors at a time when they are shielded by myoepithelial cells from the basement membrane. Csk mutants can be phenocopied by constitutively active Yap and rescued by deleting Yap or Taz, indicating a significant functional overlap between Src and Yap signaling. Although Src-induced tyrosine phosphorylation has long been believed to regulate Yap activity, we find that mutating these tyrosine residues in both Yap and Taz fails to perturb mouse development or alleviate the Csk lacrimal gland phenotype. In contrast, Yap loses Hippo signaling-dependent serine phosphorylation and translocates into the nucleus in Csk mutants. Further chemical genetics studies demonstrate that acute inhibition of Csk enhances Crk/CrkL phosphorylation and Rac1 activity, whereas removing Crk/CrkL or Rac1/Rap1 ameliorates the Csk mutant phenotype. These results show that Src controls Hippo-Yap signaling through the Crk/CrkL-Rac/Rap axis to promote cell extrusion.

Duct ligation/de-ligation model: exploring mechanisms for salivary gland injury and regeneration

Sialadenitis and sialadenitis-induced sialopathy are typically caused by obstruction of the salivary gland ducts. Atrophy of the salivary glands in experimental animals caused by duct ligation exhibits a histopathology similar to that of salivary gland sialadenitis. Therefore, a variety of duct ligation/de-ligation models have been commonly employed to study salivary gland injury and regeneration. Duct ligation is mainly characterised by apoptosis and activation of different signaling pathways in parenchymal cells, which eventually leads to gland atrophy and progressive dysfunction. By contrast, duct de-ligation can initiate the recovery of gland structure and function by regenerating the secretory tissue. This review summarizes the animal duct ligation/de-ligation models that have been used for the examination of pathological fundamentals in salivary disorders, in order to unravel the pathological changes and underlying mechanisms involved in salivary gland injury and regeneration. These experimental models have contributed to developing effective and curative strategies for gland dysfunction and providing plausible solutions for overcoming salivary disorders.

Jack of all trades, master of each: the diversity of fibroblast growth factor signalling in eye development

A central question in development biology is how a limited set of signalling pathways can instruct unlimited diversity of multicellular organisms. In this review, we use three ocular tissues as models of increasing complexity to present the astounding versatility of fibroblast growth factor (FGF) signalling. In the lacrimal gland, we highlight the specificity of FGF signalling in a one-dimensional model of budding morphogenesis. In the lens, we showcase the dynamics of FGF signalling in altering functional outcomes in a two-dimensional space. In the retina, we present the prolific utilization of FGF signalling from three-dimensional development to homeostasis. These examples not only shed light on the cellular basis for the perfection and complexity of ocular development, but also serve as paradigms for the diversity of FGF signalling.