A highly conserved enhancer in the Dlx5/Dlx6 intergenic region is the site of cross-regulatory interactions between Dlx genes in the embryonic forebrain

Four Dlx homeobox genes, Dlx1, Dlx2, Dlx5, and Dlx6 are expressed in the same primordia of the mouse forebrain with temporally overlapping patterns. The four genes are organized as two tail-to-tail pairs, Dlx1/Dlx2 and Dlx5/Dlx6, a genomic arrangement conserved in distantly related vertebrates like zebrafish. The Dlx5/Dlx6 intergenic region contains two sequences of a few hundred base pairs, remarkably well conserved between mouse and zebrafish. Reporter transgenes containing these two sequences are expressed in the forebrain of transgenic mice and zebrafish with patterns highly similar to endogenous Dlx5 and Dlx6 expression. The activity of the transgene is drastically reduced in mouse mutants lacking both Dlx1 and Dlx2, consistent with the decrease in endogenous Dlx5 and Dlx6 expression. These results suggest that cross-regulation by Dlx proteins, mediated by the intergenic sequences, is essential for Dlx5 and Dlx6 expression in the forebrain. This hypothesis is supported by cotransfection and DNA-protein binding experiments. We propose that the Dlx genes are part of a highly conserved developmental pathway that regulates forebrain development.

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.

Adrenergic signaling and second messenger production in hepatocytes of two fish species

The activation of cAMP and inositol 1,4,5-trisphosphate (IP3) transduction pathways by epinephrine (EPI) has been studied in hepatocytes and hepatic membranes of two teleost species, the American eel (Anguilla rostrata) and the black bullhead (Ictalurus melas). EPI at 10 microM increased both cAMP and IP3 in a dose- and species-dependent manner. The activation of both systems was the greatest in the eel, and the IP3 system was activated at lower EPI concentrations than the cAMP system. The individual transduction pathways were identified by the EPI-induced increase in cAMP being blocked by propranolol (PROP) but not phentolamine (PHT) and the increase in IP3 by PHT but not PROP. alpha1-Adrenoceptors were characterized by the specific binding of [3H]prazosin (PRZ) to purified hepatic membranes: Kd and Bmax values were 3.0 and 2.2 nM and 108 and 178 fmol x mg-1 protein for eel and bullhead, respectively. PRZ binding was displaced by nonradioactive PRZ and PHT but not by PROP. [3H]IP3 bound specifically to microsomal membranes from hepatic tissues of both species and this binding could best be explained by a two-site binding model; significant differences in binding characteristics were seen between eel and bullhead membranes. This study demonstrates that EPI may act on hepatocytes isolated from the American eel and black bullhead through both signal transduction pathways, inducing changes in glucose production and Ca2+ mobilization previously reported in these species. The quantitative differences between the two species in aspects of this system may reflect differences at the level of the coupling between receptor and messenger production, or at the degradation of the messenger, aspects of the transduction pathways which may be either true species differences or related to the conditions under which the species were held.