Research

Larval Behavior and Neurobiology

A comprehensive understanding of nervous system function requires integration of multiple types of data, from behavioral to anatomical. We use an organism with unique attributes for multimodal nervous system study: the simple chordate, Ciona robusta. The Ciona larval tadpole CNS shows strong conservation with those of vertebrates, including homologs of the forebrain, midbrain/hindbrain, and spinal cord. Despite these conserved vertebrate-like features the Ciona larval tadpole is barely 1 mm long, and the CNS has only 177 neurons. While the development of the Ciona CNS has been the subject of investigation for many years, new research opportunities have opened with the recent completion of the Ciona tadpole CNS connectome. Our goal is to build from the connectome data to identify correlates of behavior at the level of single, identifiable neurons. The larvae rely on visual, geotactic, tactile and possibly chemosensory inputs to navigate, detect substrates, and perhaps avoid predation. Our studies have focused on the negative phototaxy behavior of Ciona larvae. Larvae orient themselves in a light field using short bursts of spontaneous helical swimming, which then may be followed by sustained swim away from the light source. We have developed models for negative phototaxy taking into account the several photoreceptor groups and their connectivity. In addition, the transparency and small size of Ciona tadpoles make them ideal for live imaging of brain activity using genetically encoded Ca2+ and voltage indicators. We have collected GCaMP recordings from agarose-immobilized tadpoles showing both spontaneous and dimming-induced behaviors using a sheet plane illumination microscope. From the dataset we have been able identified individual cells activated.  In summary the connectome makes specific and novel predictions about patterns of neural activity, and the relative simplicity of the Ciona CNS will provide an innovative system for validating such predictions.

Isolation of Developmental Mutants in Ciona

In addition its the embryonic and genomic simplicity, Ciona has one additional feature that makes it ideal for forward genetics: they are hermaphrodites with a capacity for self-fertilization, which allows for rapid screening for recessive mutations. 

CNS Development

One of our primary research areas is the development of the tunicate CNS. The tunicate CNS follows two distinct developmental pathways.  The spinal cord and hindbrain are specified autonomously by maternally-determined regional expression of transcription factors, while the anterior parts of the CNS (corresponding to the vertebrate fore- and midbrains) require FGF signaling from neighboring cells to be specified.  We have isolated several mutant lines that disrupt the development of the anterior parts of the Ciona CNS (Deschet 2004; Tresser et al, 2010; Hackely et al, 2013).  Most recently we published a study on one particularly informative mutant line, frimousse (frm), that demonstrated a requirement for gap junction communication and Ca2+ transients in the induction of the anterior CNS.  In the frm mutant the entire anterior CNS is transfated to epidermis .  The frm mutation mapped to a connexin gene (cx-11) that is expressed transiently in the very early developing CNS.

Connexin proteins are the constituent subunits of gap junctions.  We have found that cx-11 is essential for mediating Ca2+ transients in the neural plate. In the absence of the Ca2+ transients, the anterior CNS fails to maintain its induced state and mis-fates to epidermis (Hackely et al., 2013).  Most recently we have made an second anterior neural plate mutant, bugeye, to a T-type Ca2+ channel.  In bugeye mutants the anterior neural plate fails to close, giving an exencephalic phenotype.  The mechanism by which the Ca2+ signaling acts in neural plate induction and morphogenesis is currently under investigation.