Research

FORM AND FUNCTION are interdependent at synapses as synaptic architecture and neurotransmission must be tightly coordinated to ensure efficiency of communication. Drosophila offers a number of advantages for the study of the structure and function of synapses.

Click on the titles below to read more or view some sample NMJ and MUTANT images.

NEUROMUSCULAR JUNCTION SYNAPSES

Every abdominal hemisegment of a Drosophila larvae has exactly 30 muscle fibers which are stereotypically innervated by 35 motoneurons, each of which is uniquely identifiable allowing single cell experimental resolution. Drosophila larva grow at an extraordinary rate in the five days between embryo hatching and pupation increasing in body size by over 1000-fold. During this period, muscle surface area undergoes a 100–fold expansion and this increase in muscle size is accompanied by a concomitant increase in the size, complexity and neurotransmitter output of neuromuscular synapses. This dynamic morphological growth, coupled with the ability to carry out electrophysiological and ultrastructural studies makes this an ideal synapse to study plastic changes during development. Furthermore, altering motor activity can also influence the growth and function of these synapses allowing experience dependent aspects of plasticity to be studied. We have utilized transgenic fluorescent protein synaptic markers to screen for novel mutants that perturb synaptic assembly, growth, structure and stability. Characterization of these mutants continues but has revealed that the formation, growth and plasticity of synapses can be genetically dissected into discrete molecular and cellular events.

SYNAPTIC SIGNALING PATHWAYS

From our forward genetic mutant screens, we have characterized a number of novel signaling pathways that essential for synaptic growth and function. For example, several mutants with defective synaptic growth were identified as having defects in a Bone Morphogenetic Protein (BMP) signaling pathway. BMPs are members of the TGF-β superfamily of growth factors, a highly conserved signaling pathway that regulates many aspects of body plan and tissue development in both vertebrates and invertebrates. We, together with others, have shown that BMPs act in a retrograde signaling pathway that is essential for the normal developmental plasticity of the larval neuromuscular junction. Mutants of the BMP ligand Gbb, neuron specific type II receptor Wit, type I BMP receptors (Sax and Tkv) and downstream effectors including the Smad intracellular transcription factors Mad and Med have a dramatic reduction in NMJ size as well as neurotransmitter release compared to wild-type animals. We are continuing to investigate how retrograde BMP signaling is is induced, transmitted and regulated. We have identified additional signaling pathways that regulate other aspects of synaptic structural plasticity such as terminal arborization and synaptic bouton maturation.

POST-TRANSCRIPTIONAL REGULATION

By characterizing the molecular defects in synaptic mutants with dramatic structural over-expansion, we have found an important role for post-transcriptional regulation in presynaptic neurons. We have identified a cascade of RNA binding proteins that act as key regulators of synapse growth and function. These proteins are conserved in humans and are associated with neurological disease. We have also found important roles for regulation of neuronal messenger RNAs by post-transcriptional processing, stability and expression through neuronal proteins that alter alternative splicing or target mRNAs for degradation and microRNA's that inhibit translation.

CNS SYNAPTIC PLASTICITY AND CIRCUIT FUNCTION

While the neuromuscular junction has many advantages as a model synapse, it is not clear that the molecular mechanisms required for plasticity at the NMJ will also be utilized in an identical manner at central nervous system (CNS) synapses. To address this question, we are developing a drosophila model for CNS synaptic plasticity by labeling and making electrophysiological recordings from identified CNS synapses in identified micro-circuits in vivo.

MODELS OF MOTOR NEURON AND NEUROLOGICAL DISEASE

Historically, invertebrate genetic models such as Drosophila have provided insights into human physiology and disease through basic research into fundamental biological mechanisms. However, in the post-genomic era, realization of the extraordinary degree of genetic conservation between humans and invertebrates has prompted the development of Drosophila models of human disease genes in order to conscript the sophisticated genetic and cellular tools available in this system. We are developing and analyzing Drosophila models for the motor neuron diseases Amyotrophic Lateral Sclerosis and Spinal Muscular Atrophy. Together with our collaborators with are also involved in determining the molecular function of genes implicated in Parkinson's disease, Schizophrenia and Substance Abuse. We hope the molecular pathways and interacting genes discovered in Drosophila can serve as the basis for new drug or therapeutic targets to treat human disease.

FUNDING

We thank the following agencies for their generous support

ALSA