A C. elegans Hox gene switches on, off, on and off again to regulate proliferation, differentiation and morphogenesis.

Hox genes establish body pattern throughout the animal kingdom, but the role these genes play at the cellular level to modify and shape parts of the body remains a mystery. We find that the C. elegans Antennapedia homolog, mab-5, sequentially programs many independent events within individual cell lineages. In one body region, mab-5 first switches ON in a lineage to stimulate proliferation, then OFF to specify epidermal structures, then ON in just one branch of the lineage to promote neuroblast formation, and finally OFF to permit proper sense organ morphology. In a neighboring lineage, continuous mab-5 expression leads to a different pattern of development. Thus, this Hox gene achieves much of its power to diversify the anteroposterior axis through fine spatiotemporal differences in expression coupled with a changing pattern of cellular response.

Patterning C. elegans: homeotic cluster genes, cell fates and cell migrations.

Despite its simple body form, the nematode C. elegans expresses homeotic cluster genes similar to those of insects and vertebrates in the patterning of many cell types and tissues along the anteroposterior axis. In the ventral nerve cord, these genes program spatial patterns of cell death, fusion, division and neurotransmitter production; in migrating cells they regulate the direction and extent of movement. Nematode development permits an analysis at the cellular level of how homeotic cluster genes interact to specify cell fates, and how cell behavior can be regulated to assemble an organism.

Multiple HOM-C gene interactions specify cell fates in the nematode central nervous system.

Intricate patterns of overlapping HOM-C gene expression along the A/P axis have been observed in many organisms; however, the significance of these patterns in establishing the ultimate fates of individual cells is not well understood. We have examined the expression of the Caenorhabditis elegans Antennapedia homolog mab-5 and its role in specifying cell fates in the posterior of the ventral nerve cord. We find that the pattern of fates specified by mab-5 not only depends on mab-5 expression but also on post-translational interactions with the neighboring HOM-C gene lin-39 and a second, inferred gene activity. Where mab-5 expression overlaps with lin-39 activity, they can interact in two different ways depending on the cell type: They can either effectively neutralize one another where they are both expressed or lin-39 can predominate over mab-5. As observed for Antennapedia in Drosophila, expression of mab-5 itself is repressed by the next most posterior HOM-C gene, egl-5. Thus, a surprising diversity in HOM-C regulatory mechanisms exists within a small set of cells even in a simple organism.

Role of GTP hydrolysis in microtubule dynamics: information from a slowly hydrolyzable analogue, GMPCPP.

The role of GTP hydrolysis in microtubule dynamics has been reinvestigated using an analogue of GTP, guanylyl-(alpha, beta)-methylene-diphosphonate (GMPCPP). This analogue binds to the tubulin exchangeable nucleotide binding site (E-site) with an affinity four to eightfold lower than GTP and promotes the polymerization of normal microtubules. The polymerization rate of microtubules with GMPCPP-tubulin is very similar to that of GTP-tubulin. However, in contrast to microtubules polymerized with GTP, GMPCPP-microtubules do not depolymerize rapidly after isothermal dilution. The depolymerization rate of GMPCPP-microtubules is 0.1 s-1 compared with 500 s-1 for GDP-microtubules. GMPCPP also completely suppresses dynamic instability. Contrary to previous work, we find that the beta--gamma bond of GMPCPP is hydrolyzed extremely slowly after incorporation into the microtubule lattice, with a rate constant of 4 x 10(-7) s-1. Because GMPCPP hydrolysis is negligible over the course of a polymerization experiment, it can be used to test the role of hydrolysis in microtubule dynamics. Our results provide strong new evidence for the idea that GTP hydrolysis by tubulin is not required for normal polymerization but is essential for depolymerization and thus for dynamic instability. Because GMPCPP strongly promotes spontaneous nucleation of microtubules, we propose that GTP hydrolysis by tubulin also plays the important biological role of inhibiting spontaneous microtubule nucleation.

Activation of a C. elegans Antennapedia homologue in migrating cells controls their direction of migration.

Anterior-posterior patterning in insects, vertebrates and nematodes involves members of conserved Antennapedia-class homeobox gene clusters (HOM-C) that are thought to give specific body regions their identities. The effects of these genes on region-specific body structures have been described extensively, particularly in Drosophila, but little is known about how HOM-C genes affect the behaviours of cells that migrate into their domains of function. In Caenorhabditis elegans, the Antennapedia-like HOM-C gene mab-5 not only specifies postembryonic fates of cells in a posterior body region, but also influences the migration of mesodermal and neural cells that move through this region. Here we show that as one neuroblast migrates into this posterior region, it switches on mab-5 gene expression; mab-5 then acts as a developmental switch to control the migratory behaviour of the neuroblast descendants. HOM-C genes can therefore not only direct region-specific patterns of cell division and differentiation, but can also act within migrating cells to programme region-specific migratory behaviour.

A movable and regulable inactivation function within the steroid binding domain of the glucocorticoid receptor.

The glucocorticoid receptor is a signal transducer that interacts both with the signal and with the genes it regulates. We showed previously that nuclear localization of the receptor requires hormone binding. We have now constructed recombinant receptors that relieve hormonal control of nuclear localization, and we demonstrate that the DNA binding/transcriptional regulatory functions of the receptor are also regulated directly by hormone. Surprisingly, regulation by the steroid binding domain appears to be relatively independent of protein structure. For example, regulation is maintained when the steroid binding region is repositioned from the C-terminus to the N-terminus of the receptor. Furthermore, the activity of an unrelated protein, the adenovirus E1A gene product, becomes hormone regulated upon fusion to the steroid binding domain. We speculate that the inhibitory effect of the unliganded steroid binding domain may be mediated by heat shock protein hsp90, which binds selectively to the unliganded receptor.

Constitutive and metal-inducible protein:DNA interactions at the mouse metallothionein I promoter examined by in vivo and in vitro footprinting.

A method of high resolution in vivo footprinting has been developed and used to survey the mouse metallothionein I (MT-I) promoter for protein : DNA interactions associated with basal-level transcription and with high-level metal-induced transcription. This promoter and its associated regulatory region is structurally complex. It contains multiple potential binding sites for metal regulatory factors and for other transcription factors, including SP1 and MLTF. In several cases potential recognition sites overlap, and the experiments reported here provide a view of which sites are utilized in vivo. These data also show how the pattern of protein : DNA contacts changes when cells are shifted from basal-level expression to metal-induced expression. The noninduced footprint pattern consists of interactions at basal elements that are thought to be responsible for the moderate transcription of this gene in the absence of added metals. These interactions remain unchanged upon metal induction. When MT-I expression is increased by exposing cells to zinc or cadmium, a new footprint pattern is observed. It includes the basal interactions and a new set of metal-dependent footprints that are positioned over all five genetically defined metal responsive elements (MREs), MRE-A--MRE-E. In addition, these data identify a sixth probable MRE, MRE-F, which displays a dimethylsulfate (DMS) footprint similar to that at other MREs.