Cellular and molecular mechanisms driving developmental and regenerative morphogenesis.

The zebrafish sensory lateral line as a model to study morphogenetic processes and hair cell regeneration

The lateral line is a sensory system for the detection of water movements, which initiates the appropriate behavioral responses for capturing prey, avoiding predators and schooling. The lateral line consists of mechanosensory organs (neuromasts) distributed in lines on the head and along the flanks of the animal. Neuromasts contain hair cells that are very similar to the hair cells of the inner ear of vertebrates. Despite the unusual location of the hair cells in the skin, lateral line and ear hair cells develop by similar stereotyped mechanisms and are derived from cephalic placodes. However, in contrast to the otic placode, the lateral line placode (also called primordium) undergoes a remarkable posterior migration towards the tail tip. This migration is a dynamic event that involves the primordium depositing neuromasts every 3-5 somites until it reaches the tail tip, patterning the future lateral line (see schematic). We have recently discovered that the primordium also deposits a trail of interneuromast cells in between these neuromasts which represent stem cells that give rise to postembryonic neuromasts. The complex cellular interactions resulting in the integration of stem cell regulation, migration and neuromast deposition in the placodes remain a mystery and form a central focus for our research.

We chose the zebrafish sensory lateral line system as a model to study fundamental developmental mechanisms, such as cell migration, cell-cell adhesion, stem cell regulation and regeneration based on 1) the accessibility of the sensory organs to direct observation and manipulation; 2) the simplicity of the system, which allows to study cross-talk between single cells in a vertebrate; 3) the similarity between lateral line hair cells and hair cells in the inner ear; 4) the fact that zebrafish hair cells regenerate and 5) the genetic tools available in zebrafish to molecularly dissect the process of sensory organ formation.

Stem cell regulation:
We have been performing mutagenesis screens, in which, up to now, we have identified 20 mutants affecting different aspects of lateral line development.  Five of these are particularly interesting because, when compared to their wild type siblings, they possess ‘extra’ sensory organs, and lack glia along the posterior lateral line nerve.  Analyses of these mutants allowed us to make two fundamentally new observations: 1) the discovery that postembryonic sensory organs arise from latent sensory precursor cells, and 2) the identification of a key role for glia in controlling their differentiation. Importantly, our findings raise the question of whether glia play similar roles in the regulation of stem cells in the CNS.  This project, consequently, encompasses three lines of investigation: 1) to identify the inhibitory signals from glia to neurons; 2) to determine if glia also influence proliferation of neurons in the CNS; and 3), to build from our discovery of lateral line stem cells and better define their development and behavior. Combined, these studies will help define how glia and neurons interact with each other to produce, for example, hair cells in the sensory organs during development, and how stem cells drive hair cell regeneration in the adult.

Cell Migration:
We also aim to identify cell-cell interactions during primordium migration and neuromast deposition. Cloning of some of the lateral line mutations has demonstrated that genes that affect primordium migration are also implicated to play a role in human cancers. In particular, we are characterizing one mutant, in which the primordium looses its compact organization and cells are migrating aberrantly, thus mimicking behavior of cancerous cells. Thus, the zebrafish lateral line presents an excellent model to study cellular functions of vertebrate oncogenes and tumor-suppressor genes in vivo. In order to identify additional mutations affecting cell migration we are currently performing a mutagenesis screen.

Hair cell regeneration:
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 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. Our aim is to take advantage of the lateral line of zebrafish to define and functionally characterize the molecular and cellular interactions occurring during hair cell regeneration. To identify genes involved in hair cell regeneration, we are performing mutagenesis screens and microarray analyses. Results from these experiments will set the stage for mechanistic studies aimed at developing methods to regenerate hair cells in mammals.

Research Funding

8/05-7/08              National Science Foundation (NSF)
                             ‘Zebrafish as a Model to Study Neuron and Glia Development’

7/07-6/08              Deafness Research Foundation (DRF)
                             ‘Molecular Analysis of Hair Cell Regeneration in the Zebrafish Lateral Line’

1/08-12/11            American Cancer Society (ACS)
                             ‘A vertabrate model to study the molecular basis of tumor supression’

1/08-12/10            National Organization for Hearing Research Foundation (NOHR)
                             ‘Identification of Hair Cell Regeneration Genes’