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The developmental stage at which a neuron becomes committed to a neurotransmitter phenotype is an important time in its ontogenetic history. The present study examines when choline acetyltransferase (ChAT) is first detected within each of four different subsets of cholinergic neurons previously identified in the cervical enlargement of the spinal cord: namely, motor neurons, partition cells, central canal cluster cells, and dorsal horn neurons. By examining the temporal sequence of embryonic development of these cholinergic neurons, we can infer the relationships between ChAT expression and other important developmental events. ChAT was first detected reliably on embryonic day 13 (E13) by both biochemical and immunocytochemical methods, and it was localized predominantly within motor neurons. A second group of primitive-appearing ChAT-positive cells was detected adjacent to the ventricular zone on E14. These neurons seemed to disperse laterally into the intermediate zone by E15, and, on the basis of their location, were tentatively identified as partition cells. A third group of primitive ChAT-immunoreactive cells was detected on E16, both within and around the ventral half of the ventricular zone. By E17, some members of this "U"-shaped group appeared to have dispersed dorsally and laterally, probably giving rise to dorsal horn neurons as well as dorsal central canal cluster cells. Other members of this group remained near the ventral ventricular zone, most likely differentiating into ventral central canal cluster cells. Combined findings from the present study and a previous investigation of neurogenesis (Phelps et al.: J. Comp. Neurol. 273:459-472, '88), suggest that premitotic precursor cells have not yet acquired the cholinergic phenotype because ChAT is not detectable until after the onset of neuronal generation for each of the respective subsets of cholinergic neurons. However, ChAT is expressed in primitive bipolar neurons located within or adjacent to the germinal epithelium. Transitional stages of embryonic development suggest that these primitive ChAT-positive cells migrate to different locations within the intermediate zone to differentiate into the various subsets of mature cholinergic neurons. Therefore, it seems likely that spinal cholinergic neurons are committed to the cholinergic phenotype at pre- or early migratory stages of their development. Our results also hint that the subsets of cholinergic cells may follow different migration routes. For example, presumptive partition cells may use radial glial processes for guidance, whereas dorsal horn neurons may migrate along nerve fibers of the commissural pathway. Cell-cell interactions along such diverse migratory pathways could play a role in determining the different morphological, and presumably functional, phenotypes expressed by spinal cholinergic neurons.

No abstract

The gypsy moth (Lymantria dispar) is nonpermissive forAutographa californica nucleopolyhedrovirus (AcNPV) infection. We previously isolated a gene, host range factor 1 (hrf-1), from L. dispar nucleopolyhedrovirus that promotes AcNPV replication in Ld652Y cells, a nonpermissiveL. dispar cell line (S. M. Thiem, X. Du, M. E. Quentin, and M. M. Berner, J. Virol. 70:2221–2229, 1996). In the present study, we investigated the ability of hrf-1 to alter the larval host range of AcNPV. Bioassays using recombinant AcNPV bearing hrf-1 were conducted with insect larvae by use of oral infection. AcNPV bearing hrf-1 was infectious for neonate L. dispar larvae, with a 50% lethal concentration of 1.2 × 105 polyhedral inclusion bodies/ml of diet, which is similar to that of wild-type AcNPV for permissive hosts. AcNPV can kill neonate L. dispar larvae at high doses, but it does not kill third-instar larvae. However, electron microscopy studies of AcNPV-inoculated third-instar larvae revealed virus replication in the midgut cells. PCR analyses indicated that the virus was AcNPV. These results suggest that the block for AcNPV infection of L. dispar larvae is its inability to spread systematically from primary infection sites in the midgut epithelium and that this barrier is leaky in neonates. hrf-1 allows AcNPV to overcome this barrier. AcNPV recombinants bearing hrf-1 were also significantly more infectious for Helicoverpa zea, a resistant species, suggesting that the blocks for AcNPV infection ofL. dispar and H. zea larvae may be similar.

The covalent binding of benzo[a]pyrene (B[a]P) to acid extractable chromosomal proteins and the subsequent effect on histone 1-DNA interaction have been characterized in a model system by utilizing calf thymus nuclei as targets and rat liver microsomes as an exogenous source of enzymes for the metabolic activation of B[a]P. A two-step ion-exchange chromatography and desalting procedure was employed for removing noncovalently bound B[a]P and other contaminants. Fluorography of acetic acid-urea and Triton-acetic acid-urea-polyacrylamide gels indicated that H1 and H3 were the only principal histone targets in [3H]B[a]P-modified calf thymus nuclei. The validity of this assignment was confirmed by comparison of the chromatographic distributions of [3H]B[a]P cpm among peptides derived from the HClO4- soluble (H1) and HClO4-insoluble (core histones) protein fractions to the distributions obtained for authentic individual histone fractions. Comparison of amino acid compositions in individual peptide fractions which bound [3H]B[a]P differentially yielded some insight into the probable target amino acid residues for B[a]P binding. On the basis of electrophoresis in polyacrylamide gels, it appeared as if B[a]P had bound to multiple subfractions of H1 and H3. The equivalent distribution of covalently attached [3H]B[a]P among the major peptides of H1 and H3 modified either in intact nuclei or while free in solution implied that the relative accessibility of major portions of the H1 and H3 molecules for covalent B[a]P binding is not affected by interactions with DNA or other chromosomal proteins. Covalent attachment of [3H]B[a]P to purified H1 reduced the affinity of this histone for DNA-cellulose.