The role of ion channels in developmental patterning and growth

Life as a multicellular organism begins simply, as a single fertilized egg. However, simplicity begets complexity as cells divide, differentiate, and organize themselves into the tissues and organs that comprise complex lifeforms. The process by which this complexity is generated and constrained during development has held the fascination of scientists for centuries./nSeminal genetic screens carried out in the vinegar fly Drosophila melanogaster identified genes required for proper developmental growth and patterning. These genes were found to encode molecules with similar properties; many were secreted in specific locations in the animal, and influenced the gene expression of surrounding cells. It was proposed that these molecules, termed “morphogens”, directed development by imparting patterning information across a tissue./nAlthough a century of developmental genetics has identified many evolutionarily conserved factors with morphogen-like properties, we are still without a full understanding of the processes that shape development. Recently, non-morphogen-encoded signals, including physical forces and electrical signaling, have been shown to be instructive in development, suggesting that the process of development is an emergent property of many signaling modalities and cell interactions./nIn Chapter 1, I present an overview of our current understanding of developmental morphogenesis, and describe the biology of the Drosophila wing imaginal disc, a model epithelial structure which undergoes a stereotyped program of growth and patterning. I highlight signaling modalities that have been shown to be important for development, including electrochemical signaling, the focus of my dissertation research./nIon channels, proteins that mediate ionic flux across cell membranes, are increasingly understood to be critical regulators of cell behavior, but their role in development has only begun to be explored. I found that expression of ion channels is patterned in the Drosophila wing imaginal disc, giving rise to localized depolarization in the tissue. This physiological pattern promotes signaling through the conserved Hedgehog (Hh) pathway. This work, described in Chapter 2, adds to our understanding of critical signaling modalities in the development of the wing disc, and suggests that the Hh pathway is regulated in part by cell physiology./nDuring development, patterning occurs concomitantly with growth. The regulation of growth, which occurs both cell-intrinsically and -extrinsically, is an area of immense research interest with profound implications for cancer biology. I found that the mechanosensitive ion channel Piezo is involved in the growth regulation of the wing disc. Piezo is expressed in a sparse population of cells scattered throughout the disc, and impacts organ size by modulating cell size. Chapter 3 describes this work, which implicates mechanosensitive ion channels in growth regulation./nIn the course of exploring effects of modulating membrane potential, I made the surprising discovery that cells of altered membrane potential are recovered preferentially between the anterior and posterior regions of the wing disc. This process is mediated by a mechanism reminiscent of a cell-cell interaction termed “cell competition”. This work, described in Chapter 4, is the first report of compartment-specific cell competition in Drosophila, and suggests that membrane potential is a component of cell fitness./nA previous graduate student in the laboratory, Jo Bairzin, discovered that clones of cells which express a constitutively active allele of the Hippo pathway co-effector Yorkie overgrow, and misexpress developmental selector genes. I contributed to this work by defining the requirement of Scalloped in Yki-mediated misexpression of Ci, and validating other results. This work, presented in Chapter 5, highlights transcriptional plasticity in tumor models, as well as identifies the tumor boundary as an important signaling center./nTaken together, my work characterizes the role of ion channels in developmental growth and patterning, and presents new applications of tools used to study cell physiology in development.

Advisor: Hariharan, Iswar K.