Box 248, Rockefeller University, 1230 York Ave, New York, NY 10021, USA,
The major interest of the lab is the study of the molecular mechanisms that are involved in the establishment of neural connectivity. We are using two visual systems of the fly, the larval visual system for its simplicity, and the adult visual system for its complexity, to study the molecular basis of axon pathfinding and target recognition in vivo.
The larval photoreceptive organ, called the Bolwig Organ, consists of 12 photosensitive neurons which fasciculate to form a single nerve projecting into the central brain. The nerve has two characteristic checkpoints (P1, P2) in its path of growth, at which it alters its direction. In a screen for mutations that disrupt the Bolwig nerve projection (Schmucker et al., in prep.), we have isolated a gene, called abstract (abs), whose detailed genetic and molecular characterization is currently underway. The abstract phenotype is characterized by very early aberrant growth behavior of the nerve: Instead of fasciculating in a single bundle, the nerve splits into two or three branches, which mostly grow ectopically and never reach P2. We have isolated the abs genomic region and have identified the abstract gene, which by Northern analysis gives rise to a complex pattern of transcripts ranging from 2 - 6 kb in length. The sequence analysis of the gene is not complete, but the information we have so far, indicates that abs likely encodes a novel transmembrane protein. Our current model for the function of abstract in the Bolwig nerve is that it is involved in axon-axon interactions that are required to form a single nerve, to navigate in a coordinate fashion and to be able to recognize the target.
In the developing adult visual system, photoreceptor axons from the same ommatidium fasciculate and grow through the optic stalk toward the optic lobe. After exiting the optic stalk, the retinal axons separate from each other and form a fan-like structure with strict sorting according to their positions in the retina. While growth of the R1-R6 axons stalls in the developing lamina, R7 and R8 axons penetrate further into the developing medulla.
We have conducted a large scale mutagenesis screen to isolate recessive mutations on the third chromosome that cause retinal projection defects in third instar larval brains. We screened 5, 000 mutagenized lines and isolated 76 mutations with projection defects. Phenotypically, these mutations can be subdivided into several classes: 1) mutations that affect eye development (abnormal disc size, abnormal progression of differentiation, abnormal number/cell fate of differentiating R- cells), 2) mutations that affect the morphogenesis of the optic stalk (no optic stalk; widened optic stalk), 3) mutations that affect the retinotopic fanning of axons either directly after exiting the optic stalk (no fanning; reduced fanning; asymmetric fanning) or in the target region, and 4) mutations that affect the stalling of the R1-R6 projection in the lamina. The genetic analysis of these mutations (complementation; mapping) and the phenotypic characterization of the most interesting mutations, including a careful examination of the cytostructures of retina and optic lobe, and mosaic analysis, are underway.