Worm Breeder's Gazette 14(2): 81 (February 1, 1996)
These abstracts should not be cited in bibliographies. Material
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unc-4 regulates neurotransmitter (FMRF-amide-like)expression in VC motor neurons
||Department of Biology, Boston University, Boston, MA
||Department of Anatomy and Neurobiology, Washington
University School of Medicine, St. Louis, MO
||Department of Cell Biology, Vanderbilt University Medical
Center, Nashville, TN
unc-4 encodes a homeodomain protein  that determines the pattern of synaptic input to VA motor neurons in the ventral cord . unc-4 mutations lead to an uncoordinated phenotype whereby animals can crawl forward but not backward. Electron microscopic examination of unc-4(e120) revealed that VA motor neurons, which are necessary for backward movement, receive synaptic input normally reserved for their sister cells, the VB motor neurons . Experiments with unc-4-lacZ reporter genes detected expression in all A-type motor neurons (SAB, DA, VA) and demonstrated that unc-4 activity in the VAs in L1/L2 larvae is sufficient to establish connections with the appropriate interneurons . Thus, one unc-4 function is to distinguish the fates of VA and VB sisters cells with respect to presynaptic input.
Here we describe new evidence that unc-4 is also required for specifying the fates of VC motor neurons. The VCs express FMRFamide (Phe-Met-Arg-Phe-NH2)-related neuropeptides in N2 animals and innervate vulval muscles. Although the VCs are generated in the L1, they do not become FMRFamide-like immunoreactive (FLI) until L4/early adulthood . While screening for altered patterns of FMRFamide-like immunoreactivity, we observed that the VCs are not FLI in unc-4(e120, wd1, and e2322ts) mutants. Transgenic rescue of unc-4(wd1) animals with the unc-4(+) cosmid, C07E2, restores immunoreactivity to the VCs, suggesting that unc-4 activity is necessary for FMRFamide-like expression in VC cells. Transgenic lines carrying unc-4-GFP constructs show GFP in the VCs as well as in A-type motor neurons which may indicate that endogenous UNC-4 is also expressed in the VCs. Interestingly, unc-4-GFP is not detected in the VCs until late L3/!
early L4 whereas VA motor neurons are GFP-positive in L1/L2 larvae (see figure below). A reexamination of staining of the nuclear-localized unc-4-lacZ reporter (pNC4-23Lz) confirms VC expression in late larvae/adults; this expression was previously not reported .
unc-4 could exert its actions in the VC cells in one of several ways. For instance, UNC-4 could directly activate transcription of a FMRFamide-related peptide coding gene; at least five flp (FMRFamide-like peptide) genes are present in C. elegans [5; Worm Sequencing Project; A. Stretton, pers. comm.]. Alternatively, unc-4 could regulate a broader range of downstream genes such that VC motor neurons are transformed to a different cell type in unc-4 mutants. Unfortunately, the EM reconstruction of unc-4(e120) did not include the midventral region occupied by the VC cells . It should be possible, however, to use our unc-4-GFP constructs to determine if VC axonal morphologies or synapses onto vulval muscles are affected by unc-4 mutations.
 Miller et al. (1992) Nature 355: 841.  White et al., (1992) Nature 355: 838.  Miller and Niemeyer (1995) Development 121: 2877.  Schinkmann and Li (1992) J. Comp. Neurol. 316: 251.  Rosoff et al. (1992) J. Neurosci. 12: 2356.
Lefthand panels: VC4 projecting single unbranched axon into L4 vulva
Righthand panel: Elaborate branching pattern of VC4 and VC5 axons in adult vulva