BIOLOGICAL OVERVIEW

Runt is a novel protein, unrelated to homeodomain, zinc finger or other transcription factors. It has a mammalian homolog that binds to enhancers of retroviruses and polyoma virus, and is involved in T-cell maturation. Like hairy and even-skipped, runt is termed a primary pair rule gene, as opposed to a secondary pair rule gene. Transcription of primary pair rule genes is regulated directly by maternal genes and gap genes, while secondary pair rule genes are regulated by the primary pair rule genes. Arguments can be made as to the validity and ultimate usefulness of primary/secondary pair rule distinctions; the notion is discussed in more detail the fushi tarazu overview.

Runt's effects are felt throughout the developmental hierarchy. Runt can modulate the activity of other pair rule genes, including hairy, even-skipped and ftz. runt possesses gap gene properties as well, altering the expression of giant and hunchback when transiently overexpressed (Tsai, 1994). Runt acts early in sex determination. Its activity is necessary to activate Sex lethal in the soma, but not in the germ line (Garandino, 1993). Runt also interacts with bicoid, restricting bicoid expression in the trunk, although the mechanism for this regulation is not completely understood (Tsai,1994). runt is also involved in neurogenesis, in the specification of neuroblasts (Kania, 1990).

runt is expressed by a subset of the 30 neuroblasts that give rise to each neuromere of the Drosophila embryo. Runt is also expressed in a subset of ganglion mother cells and neurons and its activity has been shown to be necessary for the formation of a subset of even-skipped (eve)-expressing lateral neurons, the EL neurons. There are 8-10 EL neurons per abdominal hemisegment, which originate from neuroblast 3-3. The EL neurons are interneurons that express the zinc-finger transcription factor encoded by eagle. The EL neurons project axons through the anterior commissure across the midline, then turn anteriorly into the longitudinal fascicles. Inactivation of runt during neuroblast delamination, using a temperature-sensitive allele of runt, leads to a loss of eve expression in the EL neurons. Eve expression in the EL neurons is not affected when Runt is inactivated after the neuroblasts have delaminated, suggesting that Runt activity is necessary only at the time of neuroblast delamination for the development of the EL neurons (Dormand, 1998).

To determine which neuroblasts express Runt, embryos were triple labelled with anti-Runt, anti-En and anti-Gsb-d. En is expressed by neuroblasts in row 6 and 7 and neuroblast 1-2; Gsb-d is expressed by neuroblasts in row 5 and 6 and neuroblast 7-1. Contrary to the previously published expression pattern, which showed expression of Runt in the neuroblasts only up to stage 10, it was found that Runt is expressed in neuroblasts throughout neurogenesis. Neuroblasts 2-2, 2-5, 3-1, 3-2, 5-2 and 5-3 express Runt from the time of their delamination (stage 10). By stage 11, Runt is also expressed in neuroblasts 2-3 and 3-3, and expression is lost from neuroblast 2-5. Runt was verified to be expressed in neuroblast 3-3 by double staining for Eagle, which is expressed by neuroblasts 2-4, 3-3, 6-4 and 7-3. A previously published expression pattern of Runt had not shown Runt expression in neuroblast 3-3, which gives rise to the EL neurons. It has been reported that neuroblasts 1-1 and 4-1 also express Runt, but this is not seen in the current study. Therefore, Runt is expressed in five neuroblasts in rows 2 to 3 (neuroblast 2-2, 2-3, 3-1, 3-2 and 3-3) and two neuroblasts in row 5 (neuroblast 5-2 and 5-3) (Dormand, 1998).

Runt is expressed by a large number of GMCs and neurons including the EL neurons. Runt is also expressed in a cluster of two to four cells on the midline and in a pair of neurons one on each side of the midline. By double labelling with anti-Odd, which labels MP1 and dMP2, it was found that the neurons on each side of the midline are the MP1 neurons. Double labelling with antibody to Slit, which labels the midline glia, identifies the cluster of Runt-expressing cells on the midline as the midline glia (Dormand, 1998).

runt is a good candidate for a gene that specifies neuroblast identities. To test this, Runt was ectopically expressed in restricted subsets of neuroblasts. Runt is sufficient to activate even-skipped expression in the progeny of specific neuroblasts. Eve is ectopically induced when runt is mis-expressed in all neuroblasts, using the pan neural driver scabrous-GAL4. The average of 9 EL neurons per hemisegment is increased to an average of 16 eve-expressing lateral cells per hemisegment. Ectopic Runt expression causes a severe disruption of the nerve cord, as shown by the abnormal medial eve expression and severe disorganization of the axons. However, Runt is not sufficient to induce eve expression in the progeny of all the neuroblasts. Neuroblast 6-1 and/or neuroblast 6-2 must express another protein that is essential for Runt to activate eve expression. Using the marker Tau-green fluorescent protein to highlight the axons, it was found that the extra Even-skipped-expressing neurons project axons along the same pathway as the EL neurons. Runt is expressed in neuroblast 3-3, supporting an autonomous role for runt during neuroblast specification (Dormand, 1998).

Proteins expressed both by neuroblast 3-3 and by neuroblasts 6-1 or 6-2 are possible candidates for cofactors acting with Runt to induce EL neurons. Neuroblast 6-1 expresses the steroid receptor superfamily member Seven-up and neuroblast 6-2 expresses the zinc-finger transcription factor Ming (Castor) in common with neuroblast 3-3. Although Eve expression is not affected in castor mutants, it would be interesting to investigate whether either Ming or Seven-up contribute to other aspects of the EL neuron fate (Dormand, 1998).

GENE STRUCTURE

Bases in 5' UTR -251

Exons - two

Bases in 3' UTR - 621

PROTEIN STRUCTURE

Amino Acids - 509

Structural Domains

The absence of an identifiable transcription factor motif (e.g., homeo box, zinc finger, leucine zipper, or helix-loop-helix) makes Runt different from the other early-acting segmentation proteins. The subcellular location of the protein is in the nucleus (Kania, 1990).

A highly conserved region in the Runt protein is termed the Runt domain. The functional properties of the Runt domain from the D. melanogaster gene and the human AML1 (acute myeloid leukemia 1) gene were compared. The different DNA binding properties of Runt and AML1 are due to differences within their respective Runt domains. Proteins containing the AML1 Runt domain function in Drosophila embryos, but sequences outside of the runt domain are important in vivo (Pepling, 1995).

date revised:  12 December 98

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