Academic Background

Molecular Dissection of Drosophila Vision

We are using the Drosophila visual system as a genetic model to study molecular and cellular mechanisms underlying sensory functions.  By generating new fly mutants and characterizing visual phenotypes, we are investigating how photoreceptor neurons maintain their sensitivity to light and achieve rapid signaling, how visual signals are processed by interneurons, how visual glia support and modulate neuronal signaling, and how derailed signaling activities lead to neuronal degeneration in the eye.  Study on these topics will contribute to our understandings of sensory functions and neuron-glia interactions in general.  In addition, it will provide important insight to the pathology of retinitis pigmentosa and age-related macular degeneration, two human retinal disorders due to loss of photoreceptor neurons.

Using a combination of genetic, biochemical, electrophysiological, and imaging approaches, in addition to behavioral assays, we are currently focusing on the following two research directions.

Multifaceted regulation of rhodopsin in fly photoreceptors

The Drosophila visual transduction occurs in rhabdomere, a highly packed microvillar organelle in photoreceptor neurons.  Upon light-stimulated isomerization, rhodopsin triggers a trimeric G_q protein to activate phospholipase C, which leads to the open of transient-receptor-potential (TRP) channels and the depolarization of photoreceptor.  For response to repetitive stimuli, the visual transduction needs to be terminated promptly after each stimulus, which depends on deactivation of rhodopsin as well as closing of TRP channels.  Using new fly mutants, we have demonstrated that photoreceptors employ multiple mechanisms to regulate the signaling of rhodopsin, and that these regulations are important both for the speed of visual response and for the photoreceptor sensitivity to light. 

In addition to the classic, arrestin-mediated regulation, fly rhodopsin is deactivated through a mechanism that depends on a Ca²⁺/calmodulin-stimulated transcription factor dCAMTA.  We have identified several target genes of dCAMTA, and found that overexpression of an F-box gene dFbxl4 rescued the mutant phenotype of dCAMTA.  We will continue to investigate whether Rh1 undergoes dFbxl4-dependent ubiquitination, and whether the ubiquitination is important for Rh1 deactivation.  We have also obtained a knockout (KO) mouse line for CAMTA1, the mammalian homologue of dCAMTA, and observed similar visual defects in homozygous KO mice based on electroretinogram recordings.  Importantly, the level of mouse Fbxl4 was reduced in CAMTA1 KO mice.  Thus, the visual function of dCAMTA could be conserved from fly to mammals. 

Intriguingly, prompt deactivation of rhodopsin is also important for the maintenance of photoreceptor sensitivity.  In both dCAMTA mutant flies and a mutant lacking a visual arrestin Arr2, prolonged activation of G_q by rhodopsin triggered excessive endocytosis of the major rhodopsin Rh1 during light exposure, leading to reduced photoreceptor sensitivity to light.  In addition to the Rh1 deactivation, a CUB and LDLa domain protein (CULD) antagonizes Arr1-mediated Rh1 endocytosis for the maintenance of visual sensitivity.

As in human retinitis pigmentosa, mutations of rhodopsin and its regulatory molecules are major causes of retinal degeneration in the fly.  We have identified multiple types of retinal degeneration in fly mutants:  in older dCAMTA and arr2 mutants, excessive endocytosis of rhodopsin led to vacuolar degeneration of photoreceptor in a Ca²⁺-dependent manner; in a tadr (torn and diminished rhabdomeres) mutant, stimulated G_q protein failed to dissociate from the membrane and caused rhabdomere-initiated photoreceptor degeneration. These degeneration processes could also occur in human retinal disorders.

The importance of glia to visual signal transmission 

In the first visual neuropil region (lamina), fly photoreceptor axons release histamine upon light stimulation to hyperpolarize projective secondary sensory neurons and laminar local neurons. We recent found that a gap junction-dependent multicellular glial network mediates a long-distance recycling pathway of histamine for sustained visual transmission and normal visual alert response.  More importantly, we found that two ion channels in glia, Irk2 and GluCl, are essential for visual transmission in the lamina. We are currently investigating how glia receive neuronal signals through these channels and how these glial ion channels facilitate the visual transmission between photoreceptors and laminar neurons.

Rotation projects

Rotation students in the lab will have the following choices of research projects:

1) Further characterize the dCAMTA/dFbxl4-mediate deactivation of rhodopsin and the morphological defects due to loss of this pathway.

2) Participate in the characterization of additional mutant flies that have defective rhodopsin regulation, and investigate why abnormal rhodopsin signaling leads to retinal degeneration.

3) Explore the functions of rhodopsin regulatory molecules in other receptor signaling cascades.

4) Isolate new fly mutants with impaired GPCR signaling.

A postdoctoral position is available to study in this laboratory. Contact Dr. Li for additional details.