Systematic status of the large squirrels (subgenus urosciurus) of the western Amazon basin

Patterns of geographic variation within and between regional samples of the large rufous western Amazon squirrels (Sciurus) are described based on univariate and multivariate analyses. Two species are clearly recognizable by independent patterns of geographic variation in measured characters, non‐overlapping qualitative cranial features, and certain pelage color characteristics. These species are a long‐muzzled, narrow‐skulled 5. spadiceus Olfers and a more normal‐muzzled, broader‐skulled S. igniventris Wagner. The two are sympatric over all of eastern Ecuador and most of Perú; only spadiceus is known from Bolivia.     

Histo- and cytophysiology of the lactating mammary gland of the African elephant (Loxodonta africana)

The lactating mammary gland of the African elephant (Loxodonta africana) has been studied with a panel of morphological techniques focusing on (1) the functional changes during the secretory process, (2) proliferative process [by application of proliferating cell nuclear antigen (PCNA) immunohistochemistry] and apoptotic phenomena [by use of the TUNEL technique] in the individual lobules, and (3) components of milk and milk-fat-globule membrane. In the lactating gland, the lobules are variably differentiated; within a lobule, however, the alveoli are usually similarly differentiated. The morphology of their alveoli suggests a classification of the lobules into types 1–3. Lobules of type 1 are composed of immature tubular alveoli with mitotic figures and numerous PCNA-positive nuclei; advanced type 1 alveoli contain abundant glycogen and specific secretory granules. Lobules of type 2 are further subdivided. In type 2a lobules, the epithelial cells of the alveoli form tall apical protrusions, which in part are occupied by small lipid droplets and which are pinched off in an apocrine fashion. The number of lysosomes varies considerably. Type 2b is the most common type, with striking basal membrane foldings, abundant rough endoplasmic reticulum cisterns, large Golgi apparatus, numerous mitochondria, lipid droplets, and protein vesicles with 30- to 90-nm-wide casein micelles. The lipid droplets are pinched off with minimal amounts of cytoplasm. Type 2c is composed of alveoli with a cuboidal epithelium and few signs of secretory activity. Increasing expression of peanut-agglutinin-binding sites parallels the maturation and differentiation of the glandular cells. Type 3 lobules are marked by numerous TUNEL-positive nuclei and large lipid droplets and are apparently degenerating structures. Cytokeratin (CK) 14 is usually present in the myoepithelial cells; CK 19 and CK 7 mark ductal and immature alveolar epithelia. Milk protein content varies between 2.6% and 6.3%, and casein micelles range from 35 to 90 nm in diameter. The diameter of intra-alveolar milk fat globules ranges from 5 to 25 µm and the membranes bear a filamentous surface coat composed of membrane-anchored mucins; gel-electrophoretic analysis of these mucins from different individuals demonstrates the presence of mucin MUC 1, which is expressed with considerable genetic heterogeneity.     

Phosphatidylinositol phosphate, phosphatidylinositol bisphosphate, and the phosphoinositol sphingolipids are found in the plasma membrane and stimulate the plasma membrane H(+)-ATPase of Saccharomyces cerevisiae.

Several plasma membrane phospholipids have been studied for their ability to modulate the activity of the plasma membrane H(+)-ATPase of Saccharomyces cerevisiae. We show here that phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate (PIP2), and/or the phosphatidylinositol and PIP kinases are localized primarily in the plasma membrane. Previous in vivo studies with S. cerevisiae have shown that large, rapid, and reversible changes occur in the levels of PIP and PIP2 congruent with changes in cellular ATP levels. We demonstrate here that isolated plasma membranes exhibit the same changes in PIP and PIP2 content when they are supplied with or washed free of ATP. Using a mixed micellar assay we systematically studied the efficacy of the plasma membrane lipids in sustaining the activity of the plasma membrane H(+)-ATPase. We demonstrate for the first time that a number of plasma membrane glycerophospholipids effectively stimulate the ATPase, including PIP, PIP2, and cardiolipin. Phosphoinositol-containing sphingolipids, major components of the plasma membrane, are also shown to stimulate the ATPase at significantly lower levels than the glycerophospholipids and must also be considered as important effectors in vivo.     

Effect of different molecular species of phosphatidylcholine on the clearance of emulsion particle lipids

Studies were performed to assess the effect of changes in the molecular species composition of phosphatidylcholine (PC) on the clearance of emulsion particles that were made to approximate chylomicrons in size and lipid composition. Emulsions were prepared with free [14C]cholesterol, [3H]cholesteryl oleate, triolein, and one of four single PCs that differed in hydrophilic strength (as assessed by the relative rate of elution of these PCs from a reverse phase column). Emulsions were injected as an intravenous bolus into unanesthetized rats and the clearance of lipids was determined at 2-min intervals for 10 min. All emulsion lipids were cleared from the serum in parallel and in an order that closely corresponded to the relative hydrophilic strength of the particular PC that was administered. Fractional rates of clearance, calculated from log-linear plots, were 2- to 10-fold greater for all lipids for the emulsion that was made with the most hydrophilic PC compared to the least hydrophilic PC. Although hepatectomy, performed in acutely anesthetized animals, generally slowed the clearance of lipids, hepatectomy did not abolish differences in the clearance of triolein or specific PCs from emulsions prepared with the most and least hydrophilic PCs. These results indicate that a change in the composition of emulsion particle PCs, independent of any other change in the lipid composition of these particles, can significantly change the metabolism of the whole emulsion particle in the live animal. More specifically, these studies show that the rate of clearance of all emulsion lipids closely corresponds to the hydrophilic strength of the PCs that occupy the emulsion particle surface.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional crosstalk between exon enhancers, polypyrimidine tracts and branchpoint sequences.

We recently identified enhancer elements that activate the weak 3' splice site of alpha-tropomyosin exon 2 as well as a variety of heterologous weak 3' splice sites. To understand their mechanism of action, we devised an iterative selection strategy to identify functional pyrimidine tracts and branchpoint sequences in the presence or absence of enhancer elements. Surprisingly, we found that strong pyrimidine tracts were selected regardless of the presence of enhancer elements. However, the presence of enhancer elements resulted in the selection of multiple, non-consensus branchpoint sequences. Thus, enhancer elements apparently activate weak 3' splice sites primarily by increasing the efficiency of splicing of introns containing branchpoint sequences with less than optimal U2-branchpoint pairing arrangements. Comparison of consensus sequences from both our selection strategy and compilations of published intron sequences suggests that exon enhancer elements could be widespread and play an important role in the selection of 3' splice sites.