The zebrafish posterior lateral line (pLL) enables the fish to sense movement in the aqueous environment. At 3 days post fertilization (dpf), the pLL is composed of a series of neuromasts arranged linearly along the horizontal myoseptum. Neuromasts possess a central core of hair cells that are surrounded by supporting cells. Hair cells are the functional mechanosensory cells having apical stereocilia that project from the surface of the fish into the water. Movement results in deflection of the stereocilia, opening of mechanically gated ion channels and generation of an action potential. The supporting cells maintain hair cells and are essential for regeneration of damaged hair cells. pLL development is a highly dynamic process involving tissue patterning, migration and neuromast deposition. These processes are coordinated through delicately balanced interaction of multiple signaling pathways. After development, the pLL remains highly dynamic. Damage to the hair cells results in an immediate response that culminates in regeneration of hair cells. Understanding the pLL developmental process along with hair cell regeneration make the pLL highly relevant to a number of human physiological processes. Tissue patterning and migration are essential to the immune response and are involved in tumor metastasis. In addition, hair cell regeneration is interesting because human hair cells do not regenerate, and damage to these cells results in permanent hearing loss. Thus, study of this mechanosensory organ in zebrafish has the potential to provide clinical benefits in humans with hearing loss, metastatic cancers and immune deficiencies.

T-box transcription factors have previously been implicated in a number of developmental processes. Two in particular have emerged as targets for investigation due to expression profiling: tbx5 and tbx15. The roles of tbx5 and tbx15 in the pLL developmental process were examined, and it was found that both are involved in this dynamic process. Specifically, it is shown that Tbx5 is an upstream activator of a number of genes expressed within the primordium and additionally functions in establishing gene expression patterns in the primordium. One of these downstream targets, camk2b2, is also important in pLL development and impacts expression of signaling molecules as well. Furthermore, Camk2b2 is an activator of tbx15, which modulates tbx5 expression. Thus, tbx5, tbx15 and camk2b2 set up a negative feedback loop that results in carefully balanced Tbx5 and Camk2b2 activity. Methodologies for establishing RNA interference (RNAi) systems in zebrafish that will enable spatio-temporal control of gene expression have also been investigated. These systems are especially important in studies of hair cell regeneration, as they enable study of just the regenerative process after normal development has taken place. This goal is important because regeneration and development are regulated by a number of the same genes. Thus, novel roles for tbx5 and tbx15 in pLL development have been identified, and efforts are underway to develop mechanisms for study of the hair cell regenerative process.