Loss of UNC-55 function in the nematode Caenorhabditis elegans causes one motor neuron class, the ventral D (VD) motor neurons, to adopt the synaptic pattern of another motor neuron class, the dorsal D (DD) motor neurons. Here we show that unc-55 encodes a member of the nuclear hormone receptor gene family that is similar to the vertebrate chicken ovalbumin upstream promoter transcription factors. Although the VD and DD motor neuron classes arise from different lineages at different developmental stages, they share a number of structural and functional features that appear to be the product of identical genetic programs. UNC-55 is expressed in the VD but not the DD motor neurons to modify this genetic program and to create the synaptic pattern that distinguishes the two motor neuron classes from one another.Key words: synaptic specificity; C. elegans; motor neurons; nuclear hormone receptor; neuromuscular junction; unc-55The complexity of synaptic patterns present in mature nervous systems raises a f undamental question: how does a finite amount of genetic material create the synaptic diversity present in even the simplest nervous systems? Several lines of evidence suggest that the genetic programs are less diverse than would be predicted by the multitude of synaptic patterns. For example, in the development of layer 5 neurons in the mammalian cortex, visual and motor cortical neurons initially extend projections to the same subcortical regions, including the spinal cord. Subsequently, the structural and f unctional differences that distinguish these neurons emerge because of differential pruning of collateral branches (O'Leary and Terashima, 1988). A conceptually similar example has been reported in the nematode Caenorhabditis elegans: two sets of mechanosensory neurons initiate either forward or backward locomotion by activating different neural circuits based on whether the animal is touched on the anterior or posterior part of the body (Chalfie and Au, 1989). However, the same genetic program regulates the differentiation and f unction of the two circuits (Way and Chalfie, 1988;Xue et al., 1993). In both examples a common genetic program plus features associated with the relative positions of the neurons cause them to form distinctive synaptic patterns (Chalfie et al., 1983;Walthall and Chalfie, 1988). Our goal is to investigate the mechanisms responsible for modifying the common genetic programs of neurons to create different synaptic specificities.The precisely defined sets of nerve and muscle cells responsible for locomotion in C. elegans allow genetic and cellular manipulations that provide a unique opportunity for investigating the generation of specific synaptic patterns. The sinuous forward and backward locomotion exhibited by C. elegans is produced by two neural circuits; one dedicated to forward movement and the other dedicated to backward movement (Chalfie et al., 1985). These two circuits converge on the dorsal and ventral body wall muscles and two classes of inhibitory motor neurons: 6 dorsal D (...
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