Gabriel O. Aisemberg

New electrical properties of neurons induced by a homeoprotein.

To explore possible neurogenic functions of the genes of the Hox/HOM complexes, we injected the mRNA from the leech homeobox genes Lox1 and Lox4 into adult neurons that normally do not express them. The ectopic expression of Lox1 induced a specific transformation in the electrical properties of certain identified neurons: action potential amplitude increased about threefold after the injections. This effect of Lox1 expression was restricted, among cell types examined, to the anterior pagoda neurons (APs) and the nut neurons. This effect was also restricted to Lox1 ectopic expression; the action potentials of APs and nut neurons were not enlarged when the mRNAs of either Lox4, another leech Hox/HOM gene, or beta-galactosidase were injected. Lox1 mRNA injection did not affect the resting potential, input resistance, or axonal morphology of the transformed APs, raising the possibility that it acts via the modification of voltage-dependent ion channels. Thus, a specific homeobox gene can transform key neuronal characteristics in a cell-specific manner. We may thus add electrophysiologic properties to other aspects of neuronal identity determined by homeobox gene expression.

The cDNA sequence and expression of an ubiquitin-tail gene fusion in Neurospora crassa

The genome of Neurospora crassa contains at least one natural fusion gene encoding a single ubiquitin (UBI) unit with a 78-amino acid C-terminal extension. The predominantly basic tail sequence corresponds to a highly conserved ribosomal protein identified in other organisms. The 0.7-kb UBI fusion transcript is mainly expressed in germinating conidia and other stages of active cell replication. Under starvation conditions attained by nutrient depletion, or after polyamine depletion, the UBI fusion gene is shut off while the polyUBI transcript is preserved. Cycloheximide addition promotes polyUBI, but not UBI fusion transcript accumulation in N. crassa.

Genesis of segmental identity in the leech nervous system

The Antennapedia-class homeobox genes are likely to play a role in the specification of neuronal identities in invertebrates. The leech Hirudo medicinalis, a species well-suited for the analysis of these genes at the level of identified neurons, contains homologs of many Antennapedia-class and related genes. The expression pattern in the central nervous system of four leech homebox genes was examined in detail. Lox1 is expressed during early gangliogenesis in one pair of transient neurons present in every segment and, at later stages, in 15-20 pairs of neurons per segment. Lox2 is expressed in 25-30 pairs of neurons repeated in the posterior two-thirds of the midbody. Lox4 is present in 20-30 pairs of iterated neurons in the posterior half of the midbody, and in smaller subset of them in more anterior ganglia. Lox6 is expressed in 15-20 pairs of neurons of the third subesophageal neuromere and in fewer cells of more posterior ganglia. The subsets of neurons that express these homeobox genes are different but overlapping. Combinations of Lox genes could in theory generate enough variability to specify all central neurons in a leech ganglion.

Genes responsive to the alteration of polyamine biosynthesis in Neurospora crassa

Wild-type Neurospora crassa grown in minimal medium was exposed to -difluormethyl ornithine (DFMO), a specific inhibitor of ornithine-decarboxylase (ODC-ase) activity. Protein-synthesis rates impaired by DFMO were restored by the addition of spermidine. The pattern on SDS-acrylamide gels displayed three newly synthesized polypeptides, p27, p31 and p99 after DFMO action in the absence of exogenous polyamine. The ODC-ase mutant (spe-1) grown in spermidine-supplemented medium did not show an induced polypeptide pattern. The lack of ODC-ase activity promotes the expression of p27- and p31-coding genes in both strains but transcription of p31 gene is shut-off after spermidine addition. Both transcripts are also accumulated after exposure to low cycloheximide doses or nutrient starvation. Another cycloheximide-inducible gene coding for p70 is also expressed under DFMO-treatment.

Early response and induced tolerance to cycloheximide in Neurospora crassa

SummaryIncubation of Neurospora crassa mycelia with low doses of cycloheximide induces the expression of several genes. After 6 h in the presence of cycloheximide, mycelia become tolerant to further additions of the drug and the rate of protein synthesis exhibits a lower sensitivity to it. The polypeptide pattern is indicative of a stress situation.