Structure of the receptor-binding protein of bacteriophage det7: a podoviral tail spike in a myovirus.

A new Salmonella enterica phage, Det7, was isolated from sewage and shown by electron microscopy to belong to the Myoviridae morphogroup of bacteriophages. Det7 contains a 75-kDa protein with 50% overall sequence identity to the tail spike endorhamnosidase of podovirus P22. Adsorption of myoviruses to their bacterial hosts is normally mediated by long and short tail fibers attached to a contractile tail, whereas podoviruses do not contain fibers but attach to host cells through stubby tail spikes attached to a very short, noncontractile tail. The amino-terminal 150 residues of the Det7 protein lack homology to the P22 tail spike and are probably responsible for binding to the base plate of the myoviral tail. Det7 tail spike lacking this putative particle-binding domain was purified from Escherichia coli, and well-diffracting crystals of the protein were obtained. The structure, determined by molecular replacement and refined at a 1.6-A resolution, is very similar to that of bacteriophage P22 tail spike. Fluorescence titrations with an octasaccharide suggest Det7 tail spike to bind its receptor lipopolysaccharide somewhat less tightly than the P22 tail spike. The Det7 tail spike is even more resistant to thermal unfolding than the already exceptionally stable homologue from P22. Folding and assembly of both trimeric proteins are equally temperature sensitive and equally slow. Despite the close structural, biochemical, and sequence similarities between both proteins, the Det7 tail spike lacks both carboxy-terminal cysteines previously proposed to form a transient disulfide during P22 tail spike assembly. Our data suggest receptor-binding module exchange between podoviruses and myoviruses in the course of bacteriophage evolution.

Increased generation of granule cells in adult Bcl-2-overexpressing mice: a role for cell death during continued hippocampal neurogenesis.

Programmed cell death is an important mechanism during brain development in order to control neuronal cell numbers and to correctly form neuronal circuitries. Programmed cell death is also present in neurogenic regions of the adult brain, and a significant portion of the adult-born cells is eliminated during the first months of maturation. We here address the question whether overexpression of the anti-apoptotic protein Bcl-2 would improve the survival of neural progenitor cells and, as a consequence, increase neurogenesis in the adult hippocampus. Transgenic animals, which express human Bcl-2 under the neuron-specific enolase promoter (NSE-huBcl-2), show a significant reduction of apoptotic cells in the hippocampal granule cell layer to about half of the wild-type level. These apoptotic cells are almost exclusively found in the zone of hippocampal progenitor activity and frequently co-label with the neuronal progenitor marker doublecortin (DCX). The rate of adult neurogenesis is doubled in the dentate gyrus of Bcl-2-overexpressing mice as demonstrated by quantification of progenitor cells using DCX and new neurons using bromodeoxyuridine (BrdU)/neuronal nuclei antigen (NeuN) double-labelling. The effect of Bcl-2 is limited to the late phase of progenitor maturation, as proliferation and early-phase progenitor cells were not affected. The increased level of neurogenesis leads to a significantly higher total number of granule cells in the dentate gyrus. These results underline the importance of developmental cell death during neurogenesis in the adult brain.

Caspase inhibition decreases cell death in regions of adult neurogenesis.

The generation of new neurons in adult neurogenic regions is paralleled by a high rate of cell death. To further characterize the interplay between generation and removal of new cells, we studied the role of caspase 2 (Nedd 2) and 3 (CPP 32) on the basis of the high expression of these cysteine proteases in neurogenic regions. By injecting the broad spectrum caspase inhibitor BOC-Asp(OMe)-fluoromethyl ketone into the lateral ventricle of adult rats, a 60% ;reduction of terminal deoxynucleotidyl transferase dUTP Nick End Labeling (TUNEL) profiles was observed in all neurogenic regions without changing the number of newly generated cells. These data suggest that inhibiting the caspase activity in vivo decreases the rate of cell death, but has no influence on the generation of new neurons.