Current Investigations

THE ESSENTIALS

Synaptic transmission is a fundamental feature of nervous system function. In the Heidelberger laboratory, we study synaptic transmission from the presynaptic side, with a particular emphasis on synaptic vesicle dynamics and the modulation of neurotransmitter release in ribbon-style synapses of the vertebrate retina.

The overarching hypothesis of much of the current work in the laboratory is that syntaxin 3B, a retinal-specific SNARE protein, has both essential and modulatory roles in vision. We address our questions using powerful combinations of electrophysiological, molecular, optical and computational approaches.

A LITTLE BACKGROUND

Retinal photoreceptors and bipolar cells are neurons that are highly specialized for the continuous transmission of visual information in a manner that is typically graded with stimulus intensity. Not surprisingly, in addition to forming a specialized type of synapse called a “ribbon-style” synapse, they also express unusual isoforms of the presynaptic proteins found at more conventional synapses.

Syntaxin 3B is one of these unusual proteins. It is a SNARE protein, typically found on the plasma membrane, but also to some extent on vesicles, that is involved in membrane trafficking, membrane fusion, and neurotransmitter release. Syntaxin 3B is enriched in the synaptic terminals of photoreceptors and bipolar cells.

SOME OF OUR CURRENT PROJECTS

Project 1: We have identified syntaxin 3 as a phosphoprotein whose phosphorylation at T14 in the retina is regulated by light in a Ca²⁺-dependent manner. We postulate that the activity-dependent modification at T14 modulates SNARE complex formation and the delivery of new synaptic vesicles to the fusion-competent state. Furthermore, we suggest that this activity-dependent mechanism of syntaxin 3B activation helps restrict exocytosis to areas of elevated Ca²⁺. To test our hypotheses, we employ powerful combinations of in vitro membrane fusion assays and electrophysiological approaches with molecular, pharmacologic and genetic tools, and high-resolution microscopy.

Project 2: To better understand the essential roles of syntaxin 3B in the transmission of visual information across the mammalian retina, we employ a multi-pronged approach that includes genetic, molecular, biophysical and anatomical approaches to study the roles of syntaxin 3B in synapse structure and function and the throughput of visual information across the mammalian retina. In addition, we use genetically-modified mouse lines to dissect out the contributions of different visual information streams to downstream neurons, such as the AII amacrine cell.

Project 3: We have shown that the acute interference of syntaxin 3B function results in the reduction of neurotransmitter release at both photoreceptor and bipolar cell synaptic terminals and of multi-vesicular release. In addition, genetic deletion of syntaxin 3 in the mouse retina leads to the degeneration and death of photoreceptors. We have further shown that biallelic loss of function mutations in syntaxin 3 in human subjects gives rise to an early-onset retinal dystrophy. We want to understand why syntaxin 3B is required for photoreceptor survival.