Inhibition of phosphatidylinositol 3-kinase activity blocks cellular differentiation mediated by glial cell line-derived neurotrophic factor in dopaminergic neurons.

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival factor for midbrain dopaminergic neurons. To begin to understand the intracellular signaling pathways used by GDNF, we investigated the role of phosphatidylinositol 3-kinase activity in GDNF-stimulated cellular function and differentiation of dopaminergic neurons. We found that treatment of dopaminergic neuron cultures with 10 ng/ml GDNF induced maximal levels of Ret phosphorylation and produced a profound increase in phosphatidylinositol 3-kinase activity, as measured by western blot analysis and lipid kinase assays. Treatment with 1 microM 2-(4-morpholinyl)-8-phenylchromone (LY294002) or 100 nM wortmannin, two distinct and potent inhibitors of phosphatidylinositol 3-kinase activity, completely inhibited GDNF-induced phosphatidylinositol 3-kinase activation, but did not affect Ret phosphorylation. Furthermore, we examined specific biological functions of dopaminergic neurons: dopamine uptake activity and morphological differentiation of tyrosine hydroxylase-immunoreactive neurons. GDNF significantly increased dopamine uptake activity and promoted robust morphological differentiation. Treatment with LY294002 completely abolished the GDNF-induced increases of dopamine uptake and morphological differentiation of tyrosine hydroxylase-immunoreactive neurons. Our findings show that GDNF-induced differentiation of dopaminergic neurons requires phosphatidylinositol 3-kinase activation.

Molecular cloning and characterization of human and murine DNase II.

We have cloned and sequenced novel cDNAs that encode human and murine DNase II, the acidic deoxyribonuclease. Sequence analysis predicts that huDNase II contains an N-terminal signal sequence and that mature DNase II has 344 residues with a calculated molecular mass of 38 032 Da. DNase II is a novel enzyme with no homologies to proteins of known function. Surprisingly, C. elegans appears to possess a family of DNase II homologs. Unlike DNase I-like enzymes that have tissue-specific expression patterns, huDNase II is ubiquituously expressed at low levels. When huDNase II is expressed in human 293 cells, we observe secretion of a novel 42-44 kDa glycoprotein; approximately 20-30% of recombinant human DNase II activity is secreted in this system. The secreted enzyme possesses DNA hydrolytic activity and shares biochemical properties with purified DNase II obtained from other species. We also show that the mechanism by which DNase II cuts DNA is similar to DNase I in that the enzyme produces nicks rather than double-strand cuts.

Cloning and characterization of an actin-resistant DNase I-like endonuclease secreted by macrophages.

We have cloned human and murine DNase I-like cDNAs, termed LS-DNase, which are expressed at high levels in liver and spleen tissues. LS-DNase expression is highly specific to macrophage populations within these and other tissues. Mature LS-DNase from both species is a secreted, non-glycosylated protein containing 285 residues, with a calculated molecular mass of 33 kDa and a basic isoelectric point. Human and murine LS-DNase are highly conserved and share 83% identity. Sequence analysis reveals that LS-DNase shares 46% amino acid sequence identity with DNase I. However, several residues identified as important for interaction of human DNase I with actin are not conserved in both human and murine LS-DNase. Consistent with this observation, recombinant human LS-DNase possesses a DNA hydrolytic activity which, unlike DNase I, is not inhibited by G-actin. The existence of a family of DNase I-like molecules that have tissue-specific expression patterns and the possible role of a macrophage specific DNase are discussed.

Sources and evolution of human Alu repeated sequences.

Alu repeated sequences arising in DNA of the human lineage during about the last 30 million years are closely similar to a modern consensus. Alu repeats arising at earlier times share correlated blocks of differences from the current consensus at diagnostic positions in the sequence. Using these 26 positions, we can recognize four subfamilies and the older ones are each successively closer to the 7SL sequence. It appears that there has existed a series of conserved genes that are the primary sources of the Alu repeat family, presumably through retroposition. These genes have probably replaced each other in overlapping relays during the evolution of primates.