The control of growth is a fundamental, yet poorly understood, aspect of development. What dictates the size of a particular organ (e.g., how does a hand or a heart "know" how large it should be) or a particular organism (e.g., why is a mouse small and an elephant large)? Decades ago, regeneration experiments revealed an intimate relationship between organ patterning and organ growth, but the molecular basis for this relationship has remained elusive. More recently, molecular insights into how growth is controlled have come from the identification and characterization in model systems of intercellular signaling pathways that are required for the normal control of organ growth. Many of these pathways are highly conserved among different phyla. We are engaged in projects whose long-term goals are to define relationships between patterning and growth in developing and regenerating organs and to determine how these patterning inputs are integrated with other factors that influence organ growth, such as nutrition and the physical connections between cells. Much of our research takes advantage of the powerful genetic, molecular, and cellular techniques available in Drosophila melanogaster, which facilitate both gene discovery and the analysis of gene function.
Our current research focuses on two intersecting signaling pathways, the Hippo pathway and the Dachsous-Fat pathway. These pathways control the growth and shape of developing organs. We study both the molecular mechanisms of signal transduction and the roles of these pathways in different developmental and physiological contexts. The Hippo signaling pathway has emerged over the past decade as one of the most important growth regulatory pathways in animals.
In certain contexts, the Hippo pathway is regulated by the Fat pathway. The fat gene encodes a large transmembrane protein of the cadherin family. In addition to its influence on Hippo signaling, Fat also influences planar cell polarity (PCP), which is a polarization of cell structures and cell behaviors within the plane of a tissue. In this way, Fat modulates not only organ size but also organ shape (e.g., by influencing the orientation of cell divisions). Fat is regulated by two proteins expressed in gradients: Dachsous (Ds), which like Fat is a large cadherin family protein and can bind to Fat, and Four-jointed (Fj), which we found is a novel Golgi-localized kinase that phosphorylates cadherin domains of Fat and Ds to modulate binding between them. One remarkable feature of Fat signaling is that rather than responding solely to the level of Ds and Fj, Fat is also regulated by the slope and vector of their expression gradients: the slope influences Hippo signaling and the vector influences PCP.
Clues to how this novel regulatory mechanism operates have come from the identification and characterization of downstream signaling components. Several years ago we identified Dachs as a key player in both Fat-Hippo and Fat-PCP signal transduction. Dachs, which is a myosin family protein, exhibits a polarized membrane localization that is regulated by Fat activity. Our studies imply that the direction in which Dachs is polarized is governed by the vector of the Fj and Ds gradients and controls PCP signaling, whereas the extent to which Dachs is polarized is influenced by the slope of the Fj and Ds gradients and controls Hippo signaling. The normal polarization of Dachs reflects a polarization of Fat activity within these gradients.
We have combined genetic, biochemical, and cell biological experiments to investigate how Fat influences Hippo signaling. Our current understanding is that when Dachs is at the membrane, it cooperates with another protein that we placed in the Fat-Hippo pathway, called Zyxin. Zyxin and Dachs interact with each other and promote degradation and inactivation of the Warts kinase, which is a central component of the Hippo signaling pathway. Zyxin has been characterized in mammalian cells as a protein involved in mechanotransduction; a current focus of interest in our lab extending from this involves investigating the influence of mechanical tension on Hippo signaling.
We have also investigated how other signaling pathways that modulate organ growth intersect with the Hippo signaling pathway. We recently identified a molecular crosstalk between epidermal growth factor receptor (EGFR) signaling and Hippo signaling that promotes growth, which is of particular interest because activation of EGFR or some of its downstream effectors, like Ras, is observed in many human cancers. EGFR can activate Yorkie through an influence of the Ras–mitogen-activated protein kinase branch of EGFR signaling, which results in phosphorylation of the Hippo pathway protein Jub. This phosphorylation enhances Jub binding to Warts, which could inhibit Warts activity. We have also identified a link between Hippo signaling and JNK signaling, which is particularly important for promoting regenerative growth after tissue damage.
Initial studies of Hippo signaling focused on its roles in controlling growth during development of the wing and eye discs of Drosophila. Our studies have now revealed important roles for Hippo signaling in controlling growth in many other contexts, including during development of neuroepithelial cells within the optic lobe, development of glial cells within the brain and eye, and regenerative growth after tissue damage. Each of these studies has revealed unique aspects of Hippo pathway regulation and function.
Homologs of many genes in Fat and Hippo signaling are conserved in mammals, but it was not initially clear whether mammals had a Fat signaling pathway equivalent to that in Drosophila, nor what the roles of this pathway were. To investigate this, we created a mutation in a murine ds homolog, Dchs1, and we and our collaborators have characterized it, together with mutations in a murine fat homolog, Fat4. Our analysis implies that Dchs1 and Fat4 function as a ligand-receptor pair during mouse development, and we have identified novel requirements for Dchs1-Fat4 signaling in multiple organs, including the ear, kidney, skeleton, intestine, heart, and lung. Dchs1 and Fat4 mutant phenotypes are suggestive of influences of this pathway on PCP, but not on Hippo signaling, which is consistent with our observation that the human FAT4 gene lacks the ability to transduce Hippo signaling in Drosophila, but can transduce Drosophila PCP signaling.
This research is also supported by a grant from the National Institutes of Health.
As of July 25, 2013