Piotrowski Lab

In our laboratory we are studying the molecular mechanisms underlying morphogenesis and collective cell migration, as well as sensory hair cell regeneration.

Cell migration is a fundamental, tightly coordinated process during development and in adult animals. The mechanisms integrating migration and morphogenesis of groups of cells in vivo are among the least understood processes in developmental biology. During development, groups of cells self-organize into three-dimensional morphologies that are crucial for the function of tissues, organs and organisms (Aman and Piotrowski, 2010). Although it is widely appreciated that morphogenesis emerges from the coordinated behavior of many cells, understanding morphogenesis in terms of the molecular regulation of basic cell behaviors such as proliferation, cell migration and cell shape changes has proven to be a formidable challenge. Important questions awaiting satisfactory mechanistic explanations include how cluster polarity is maintained and how tip cells communicate with cells in the back to ensure coordinated, directed migration.

Over the past few years my lab has made important contributions to develop the zebrafish posterior lateral line system into a powerful system with which to interrogate the complex cell-cell signaling interactions and cell behaviors that occur in vertebrate morphogenesis. This is due to the relatively simple morphology and accessibility of the lateral line and the amenability of the zebrafish model to detailed genetic experimentation. The cell behaviors that drive morphogenesis can be observed directly in the living embryo and correlated with specific molecular perturbations afforded by the wealth of tools available for zebrafish research.

The sensory lateral line system of aquatic vertebrates consists of hair cells, which functionally and morphologically are similar to the hair cells of the inner ear of higher vertebrates. Lateral line hair cells sense water motion and initiate the appropriate behavioral response for capturing prey, avoiding predators and schooling. The lateral line develops from a cluster of cells (called primordium) that migrate from the head to the tail tip periodically depositing hair cell containing sensory organs (neuromasts).

Work in our laboratory has uncovered how the b-Catenin and Fgf signaling pathways interact to maintain polarity of a migrating group of cells by generating distinct gene expression domains in leading and trailing cells. These interactions, in turn inform the directional migration of the primordium via differential regulation of chemokine receptors. These findings are important because they uncover an molecular mechanism whereby a dynamic tissue maintains stable, polarized gene expression domains despite periodic loss of whole groups of cells and while migrating along the axis of the embryo. Our findings also bear significance for our understanding of cancer biology as several human cancers, including breast and prostate cancer, invade tissues as groups of cells. Although the Fgf, Wnt/b-catenin and chemokine signaling pathways are well known to be involved in cancer progression, we provide with these studies the first in vivo evidence that these pathways are functionally linked.

Our recent work focuses on elucidating the molecular mechanisms underlying the regulation of diffusion and function of signaling molecules in the primordium. The precise distribution of signaling molecules is crucial for cell fate specification and patterning and, therefore morphogenesis. Even though the establishment of morphogen gradients has been intensively studied, it is still not a very well understood process.

Another focus in the lab is the identification of signaling interactions between glia and lateral line stem cells. The analysis of zebrafish mutations that possess precociously developing sensory organs allowed us to make two fundamentally new observations: 1) the discovery that postembryonic sensory organs arise from latent multipotent sensory precursor cells (lateral line stem cells), and 2) the identification of a key role for glia in controlling their differentiation. Our findings attracted substantial attention because they raised the question of whether glia play similar roles in the regulation of stem cells in the CNS.  We are currently investigating the role of the Erbb signaling pathway in glia/lateral line stem cell interactions.

A second main avenue of research in our laboratory is the elucidation of the molecular mechanisms underlying sensory hair cell regeneration. In vertebrates sensory hair cells in the inner ear enable the animals to hear. Despite the unusual location of the lateral line hair cells on the trunk, lateral line and ear hair cells develop and differentiate by similar stereotyped developmental mechanisms. The similarity between lateral line and inner ear hair cells is exemplified by the finding that mutations that cause defects in lateral line hair cell function cause deafness if mutated in humans. Deafness is one of the most widespread disabilities in the world. A prominent cause of deafness is loss of hair cells due to age, noise or antibiotic treatments. In contrast to mammalian hair cells, fish, bird and amphibian lateral line and ear hair cells turn over frequently during the normal life cycle and regenerate following hair cell death. Little is known why lower vertebrates are able to regenerate hair cells but humans do not. This is partly due to the relative inaccessibility of inner ear hair cells to direct observation and manipulation. We are taking advantage of the lateral line of zebrafish to define and functionally characterize the molecular and cellular interactions occurring during hair cell regeneration. To characterize lateral line stem cells and the cellular basis of hair cell regeneration we are using high resolution time-lapse analysis, proliferation assays and cell fate analyses. We are also generating transgenic reporter lines, as well as transgenic lines that enable us to manipulate gene expression in subsets of neuromast cells. To gain an understanding of the expression profile of regenerating sensory organs, we are taking a two-pronged approach. On the one hand we are performing microarray analyses and next-generation sequencing of regenerating sensory organs and on the other hand we are undertaking an unbiased mutagenesis screen to identify novel genes crucial for hair cell regeneration. Results from these experiments will set the stage for mechanistic studies aimed at developing methods to regenerate hair cells in mammals.