Interactions between enhancer of rudimentary and Notch and deltex reveal a regulatory function of enhancer of rudimentary in the Notch signaling pathway in Drosophila melanogaster.

Enhancer of rudimentary, e(r), encodes a small nuclear protein, ER, that has been implicated in the regulation of pyrimidine metabolism, DNA replication, and cell proliferation. In Drosophila melanogaster, a new recessive Notch allele, N (nd-p) , was isolated as a lethal in combination with an e(r) allele, e(r) (p2) . Both mutants are viable as single mutants. N (nd-p) is caused by a P-element insertion in the 5' UTR, 378-bp upstream of the start of translation. Together the molecular and genetic data argue that N (nd-p) is a hypomorphic allele of N. The three viable notchoid alleles, N (nd-p) , N (nd-1) , and N (nd-3) , are lethal in combination with e(r) (-) alleles. Our present hypothesis is that e(r) is a positive regulator of the Notch signaling pathway and that the lethality of the N e(r) double mutants is caused by a reduction in the expression of the pathway. This is supported by the rescue of the lethality by a mutation in Hairless, a negative regulator of N, and by the synthetic lethality of dx e(r) double mutants. Further support for the hypothesis is a reduction in E(spl) expression in an e(r) (-) mutant. Immunostaining localizes ER to the nucleus, suggesting a nuclear function for ER. A role in the Notch signaling pathway, suggests that e(r) may be expressed in the nervous system. This turns out to be the case, as immunostaining of ER shows that ER is localized to the developing CNS.

The cell adhesion molecules Echinoid and Friend of Echinoid coordinate cell adhesion and cell signaling to regulate the fidelity of ommatidial rotation in the Drosophila eye.

Directed cellular movements are a universal feature of morphogenesis in multicellular organisms. Differential adhesion between the stationary and motile cells promotes these cellular movements to effect spatial patterning of cells. A prominent feature of Drosophila eye development is the 90 degrees rotational movement of the multicellular ommatidial precursors within a matrix of stationary cells. We demonstrate that the cell adhesion molecules Echinoid (Ed) and Friend of Echinoid (Fred) act throughout ommatidial rotation to modulate the degree of ommatidial precursor movement. We propose that differential levels of Ed and Fred between stationary and rotating cells at the initiation of rotation create a permissive environment for cell movement, and that uniform levels in these two populations later contribute to stopping the movement. Based on genetic data, we propose that ed and fred impart a second, independent, ;brake-like' contribution to this process via Egfr signaling. Ed and Fred are localized in largely distinct and dynamic patterns throughout rotation. However, ed and fred are required in only a subset of cells - photoreceptors R1, R7 and R6 - for normal rotation, cells that have only recently been linked to a role in planar cell polarity (PCP). This work also provides the first demonstration of a requirement for cone cells in the ommatidial rotation aspect of PCP. ed and fred also genetically interact with the PCP genes, but affect only the degree-of-rotation aspect of the PCP phenotype. Significantly, we demonstrate that at least one PCP protein, Stbm, is required in R7 to control the degree of ommatidial rotation.

Potentiation of insulin-stimulated glucose transport by the AMP-activated protein kinase.

Data from the use of activators and inhibitors of the AMP-activated protein kinase (AMPK) suggest that AMPK increases sensitivity of glucose transport to stimulation by insulin in muscle cells. We assayed insulin action after adenoviral (Ad) transduction of constitutively active (CA; a truncated form of AMPKalpha(1)) and dominant-negative (DN; which depletes endogenous AMPKalpha) forms of AMPKalpha (Ad-AMPKalpha-CA and Ad-AMPKalpha-DN, respectively) into C(2)C(12) myotubes. Compared with control (Ad-green fluorescent protein), Ad-AMPK-CA increased the ability of insulin to stimulate glucose transport. The increased insulin action in cells expressing AMPK-CA was suppressed by compound C (an AMPK inhibitor). Exposure of cells to 5-aminoimidazole-4-carboxamide-1beta-D-ribofuranoside (an AMPK activator) increased insulin action in uninfected myotubes and myotubes transduced with green fluorescent protein but not in Ad-AMPK-DN-infected myotubes. In Ad-AMPK-CA-transduced cells, serine phosphorylation of insulin receptor substrate 1 was decreased at a mammalian target of rapamycin (or p70 S6 kinase) target site that has been reported to be associated with insulin resistance. These data suggest that, in myotubes, activated AMPKalpha(1) is sufficient to increase insulin action and that the presence of functional AMPKalpha is required for 5-aminoimidazole-4-carboxamide-1beta,D-ribofuranoside-related increases in insulin action.

Echinoid is essential for regulation of Egfr signaling and R8 formation during Drosophila eye development.

Precisely regulated Egfr activity is essential for normal development and cell differentiation. We demonstrate that the transmembrane protein Echinoid is required to downregulate Egfr activity in the developing Drosophila eye, ensuring a normal array of R8 photoreceptor neurons. Echinoid is an L1-type transmembrane molecule that is expressed in all cells of the eye imaginal discs and, unlike many other Egfr inhibitors, does not appear to be regulated transcriptionally. Echinoid co-precipitates with Egfr from cultured cells and eye imaginal discs, and Egfr activity promotes tyrosine phosphorylation of Echinoid. These observations suggest that Echinoid inhibits Egfr through direct interactions.