The breaking of symmetry – or the establishment of polarity - is one of the ancient and still unresolved mysteries of biology. Polarity is a prerequisite for spatial diversity, growth, development and homeostasis of all multicellular organisms. Having identified thousands of genes and molecular interactions in recent years, we now have the ability to ask which molecules generate and maintain an asymmetric state and how they accomplish this.
My laboratory studies polarity, organ morphogenesis and growth regulation in Caenorhabditis elegans. We investigate how polarity is regulated during development, and how it determines the formation of complex three-dimensional structures (particularly tubes) during growth. Tubes are building blocks of all internal organs, whose shape and function depend on their intact polarity. They are composed of polarized epithelial cells with their apical domains generating the lumen (the functional interface with the external environment), and their basolateral domains contacting adjacent cells and the extracellular matrix.
C. elegans is a small transparent roundworm with simple single-layered epithelia that allows the microscopic observation of polarized membrane biogenesis at single-cell resolution in a three-dimensional in vivo setting during organ morphogenesis (Figure). C. elegans is also a classical genetic model organism with a track record of discovering novel basic biological mechanisms that also operate in humans. In C. elegans, morphogenesis can be observed during embryonic development and animals can be engineered such that distinct organ surfaces fluoresce in different colors - all things that cannot be done in humans. Importantly, genetic screens can be performed, one of the few ways to determine how biological processes work on a molecular level, without any preexisting bias as to what the answer will be.
Our C. elegans screens therefore have the potential to identify, among the many thousands of genes, those that are indispensable to the process being examined (here, organ morphogenesis). Understanding organ development will advance our understanding of the pathogenesis of human diseases related to these organs, which in turn should lead to novel approaches for their diagnosis and treatment. Specifically, we hope that this work will translate into a better understanding of: (1) the still enigmatic link between the development of cancer and the disruption of polarity, cell shape and developmental genes, and (2) developmental diseases of internal organs and the vasculature.
We have conducted genome-wide RNA interference screens on multicellular (intestinal) and unicellular (excretory system) tubulogenesis by knocking down single genes and examining the consequences of the loss of each on polarity and organ formation in worms with fluorescing tubes and lumenal membranes. These screens have identified novel polarity and “tube” phenotypes, some of which remarkably resemble those seen in human diseases.
For instance, an intriguing intestinal polarity and multiple-ectopic-lumen phenotype (Figure) closely mimics the intestinal defects observed in Microvillus Inclusion Disease (MVID), an inherited neonatal intestinal failure syndrome with multiple intracytoplasmic lumens in the intestine. The characterization of the underlying gene products have identified unexpected roles in tubular polarity of well-known molecules that were previously recognized for either related or distinct functions in mammalian cell lines.
An intestinal tubulogenesis screen, for example, identified glycosphingolipids (proposed lipid raft components) as apicobasal membrane domain determinants in multicellular tubulogenesis;1 and it discovered an unexpected role for the classical vesicle-coat clathrin and its AP-1 adaptor in apical polarity and intestinal lumen morphogenesis.2 Loss of these genes generates an MVID-like phenotype in worms, demonstrating the compatibility of the invertebrate and vertebrate systems and identifying these genes as novel candidate MVID (or related intestinal development failure) genes.
An excretory canal tubulogenesis screens, on the other hand, identified a water channel (aquaporin) as being required to propel the extension of a unicellular tube by translumenal flux, a novel morphogenetic process that had not yet been demonstrated to directly shape tissues during animal development3 (Figure). This study also revealed that the membrane-cytoskeleton linker ERM-1, the ortholog of a mammalian regulator of cortical membrane dynamics, is strictly required for the de novo expansion of a lumenal membrane. These findings have implications for the development of human capillary-type tubes that generate part of the vascular system.
We are currently investigating additional intriguing genes that were also identified in these screens. We hope to assemble them into a coherent pathway that will molecularly characterize a novel mechanism for the establishment and maintenance of membrane polarity and also reveal how these genes are coordinated during the process of tube, lumen and organ morphogenesis. The equivalent human genes of some of these molecules have already been implicated in tumor development, suggesting that this analysis will also identify novel cancer-related genes and their respective roles in polarity and morphogenesis.