Mammary gland neoplasia: insights from transgenic mouse models

Current theories of breast cancer progression have been greatly influenced by the development and refinement of mouse transgenic and gene targeting technologies. Early transgenic mouse models confirmed the involvement of oncogenes, previously implicated in human breast cancer, by establishing a causal relationship between overexpression or activation of these genes and mammary tumorigenesis. More recently, the importance of genes located at sites of loss of heterozygosity in human breast cancer have been examined in mice by their targeted disruption via homologous recombination. The union of these two approaches allows the generation of complex animal models that more accurately reflect the multistep nature of human breast cancer. This review will examine how the study of transgenic mice has increased our understanding of the molecular events responsible for oncogenic transformation of the mammary gland. BioEssays 22:554-563, 2000.     

Recent insights into poliovirus pathogenesis

The development of a mouse model for poliomyelitis that is transgenic for the human poliovirus receptor (hPVR) has made it much easier to investigate the efficiency of the viral dissemination process in a whole organism. These studies have given an insight into the mechanisms of blood-brain barrier permeation and neural transport. Strain-specific neurovirulence levels, however, appear to depend mainly on the replicating capacity of the virus in the central nervous system rather than the dissemination efficiency. Studies of the poliovirus-induced cytopathic effects on neural cells and specific subcellular localization of hPVR isoforms might determine a new course of investigation of poliovirus pathogenesis.     

New insights into maitotoxin action

Maitotoxin (3 ng/mol) induced a massive uptake of 45Ca2+ into BC3H1 cells. This effect exhibits a lag phase of 3 min. Inositol diphosphate formation occurred concomittantly with the 45Ca2+ uptake but inositol monophosphate formation was found only after a 5-min delay following toxin addition. Maitotoxin-induced 45Ca2+ influxes could not be blocked by either 1 microM verapamil, 1 microM nifedipine or 1 mM La3+ but was blocked by Zn2+ (IC50 = 41 microM). In addition to inositol phosphate formation and 45Ca2+ uptake, maitotoxin stimulated a large uptake of Na+ and a great loss of K+ in BC3H1 cells. In the absence of Ca2+ (1 mM EGTA) none of the four maitotoxin effects could be detected. After restoration of Ca2+, the maitotoxin effects reappeared even when the toxin itself was no longer present. The divalent cation, Co2+ (1 mM), inhibited ion movements induced by maitotoxin and also digitonin (8.1 microM). The toxin action showed a very pronounced pH dependence. At low pH, maitotoxin was inactive. The dose-response curves for H+ ion inhibition of maitotoxin-induced Ca2+ uptake showed a shift to the right when determined in the absence of HCO3- and HCO3-/Cl- ions. It was concluded that the primary action of maitotoxin in BC3H1 cells was a pore-forming or channel-forming activity of a non-classical type. Some properties of maitotoxin resemble those of alpha-latrotoxin, others those of pore-forming agents such as melittin or alpha-toxin of Staphylococcus aureus.     

Genomic insights into positive selection

The traditional way of identifying targets of adaptive evolution has been to study a few loci that one hypothesizes a priori to have been under selection. This approach is complicated because of the confounding effects that population demographic history and selection have on patterns of DNA sequence variation. In principle, multilocus analyses can facilitate robust inferences of selection at individual loci. The deluge of large-scale catalogs of genetic variation has stimulated many genome-wide scans for positive selection in several species. Here, we review some of the salient observations of these studies, identify important challenges ahead, consider the limitations of genome-wide scans for selection and discuss the potential significance of a comprehensive understanding of genomic patterns of selection for disease-related research.     

Mechanistic insights into glycosidase chemistry

The enzymatic hydrolysis of the glycosidic bond continues to gain importance, reflecting the critically important roles complex glycans play in health and disease as well as the rekindled interest in enzymatic biomass conversion. Recent advances include the broadening of our understanding of enzyme reaction coordinates, through both computational and structural studies, improved understanding of enzyme inhibition through transition state mimicry and fascinating insights into mechanism yielded by physical organic chemistry approaches.     

NMR insights into protein allostery

Allosterism is one of nature's principal methods for regulating protein function. Allosterism utilizes ligand binding at one site to regulate the function of the protein by modulating the structure and dynamics of a distant binding site. In this review, we first survey solution NMR techniques and how they may be applied to the study of allostery. Subsequently, we describe several examples of application of NMR to protein allostery and highlight the unique insight provided by this experimental technique.     

Genomic insights into mediator lipidomics

G protein-coupled receptors (GPCR) are used ubiquitously and widely for signal transduction across the plasma membrane. The ligands for GPCRs are structurally diverse and include peptides, odorants, photon, ions and lipids. It is thought that GPCRs evolved by gene duplication and mutational events that diversified the ligand binding and signaling properties, thereby resulting in paralogues in various organisms. Genomic sequencing efforts of various organisms indicate that GPCRs evolved very early in evolution; for example, unicellular eukaryotes use GPCRs for mating, differentiation and sporulation responses and prokarotes utilize these receptors for phototransduction, as exemplified by the bacteriorhodopsin, a photon sensor. Many GPCRs fall into subfamilies, usually determined by structural similarity to their ligands. Bioactive lipids such as lysophospholipids, eicosanoids, ether lipids and endocannabinoids, which are produced widely in evolution, also signal through GPCRs. Thus, distinct subfamilies of bioactive lipid GPCRs, such as prostanoid receptors, lysophosphatidic, sphingosine 1-phosphate, leukotrienes, hydroxy fatty acids, endocannabinoids and ether lipids exist in the mammalian genome. With the increasing availability of genomic information throughout the phylogenetic tree, orthologues of bioactive lipid receptors are found in the genomes of vertebrates and chordates but not in worms, flies or other lower organisms. This is in contrast to GPCRs for biogenic amines and polypeptide growth factors, which are conserved in invertebrates as well. Thus, it appears that with the evolution of chordates, lipids may have acquired novel roles in cell-cell communication events via GPCRs. This hypothesis will be discussed using the prostanoid and lysophospholipid signaling systems. Since such bioactive lipids play critical roles in immune, vascular and nervous systems, this suggests that lipid metabolite signaling via the GPCRs co-evolved with the development of sophisticated vascular, immune and nervous systems in chordates and vertebrates.     

New insights into plant transaldolase

The oxidative pentose phosphate pathway (OPPP) provides plants with important substrates for both primary and secondary metabolism via the oxidation of glucose-6-phosphate. The OPPP is also thought to generate large amounts of reducing power to drive various anabolic processes. In animals this major pathway is located within the cytoplasm of cells, but in plants its subcellular compartmentation is far from clear. Although several enzymes of the OPPP were demonstrated to have both cytosolic and plastidic counterparts, there is yet no evidence for a full set of functional enzymes in each compartment. We report here the isolation of two coding sequences from tomato (Lycopersicon esculentum L.) which encode phylogenetically distant sequences (ToTal1 and ToTal2) that putatively encode distinct plastidic TA isoforms. The kinetic characterization of ToTal1 revealed that, unlike other enzymes of the non-oxidative branch of the OPPP, ToTal1 does not follow a Michaelis-Menten mode of catalysis which has implications for its role in regulating carbon flux between primary and secondary metabolism. TA genes appear to be differentially regulated at the level of gene expression in plant tissues and in response to environmental factors which suggests that TA isoforms have a non-overlapping role for plant metabolism.     

Structural insights into clathrin-mediated endocytosis

The process of clathrin-mediated endocytosis from the plasma membrane has been the subject of many biological and biochemical investigations. Recent atomic resolution structures determined by X-ray crystallography now enable the molecular basis for the interactions of some components of the endocytic machinery to be understood in detail.