Presenilin is required for proper morphology and function of neurons in C. elegans

Mutations in the human presenilin genes cause the most frequent and aggressive forms of familial Alzheimer's disease (FAD). Here we show that in addition to its role in cell fate decisions in non-neuronal tissues, presenilin activity is required in terminally differentiated neurons in vivo. Mutations in the Caenorhabditis elegans presenilin genes sel-12 and hop-1 result in a defect in the temperature memory of the animals. This defect is caused by the loss of presenilin function in two cholinergic interneurons that display neurite morphology defects in presenilin mutants. The morphology and function of the affected neurons in sel-12 mutant animals can be restored by expressing sel-12 only in these cells. The wild-type human presenilin PS1, but not the FAD mutant PS1 A246E, can also rescue these morphological defects. As lin-12 mutant animals display similar morphological and functional defects to presenilin mutants, we suggest that presenilins mediate their activity in postmitotic neurons by facilitating Notch signalling. These data indicate cell-autonomous and evolutionarily conserved control of neural morphology and function by presenilins.     

The functions of animal microRNAs

MicroRNAs (miRNAs) are small RNAs that regulate the expression of complementary messenger RNAs. Hundreds of miRNA genes have been found in diverse animals, and many of these are phylogenetically conserved. With miRNA roles identified in developmental timing, cell death, cell proliferation, haematopoiesis and patterning of the nervous system, evidence is mounting that animal miRNAs are more numerous, and their regulatory impact more pervasive, than was previously suspected.     

SPG20 is mutated in Troyer syndrome,an hereditary spastic paraplegia

Troyer syndrome (TRS) is an autosomal recessive complicated hereditary spastic paraplegia (HSP) that occurs with high frequency in the Old Order Amish. We report mapping of the TRS locus to chromosome 13q12.3 and identify a frameshift mutation in SPG20, encoding spartin. Comparative sequence analysis indicates that spartin shares similarity with molecules involved in endosomal trafficking and with spastin, a molecule implicated in microtubule interaction that is commonly mutated in HSP.     

Structure of the cell-puncturing device of bacteriophage T4

Bacteriophage T4 has a very efficient mechanism for infecting cells. The key component of this process is the baseplate, located at the end of the phage tail, which regulates the interaction of the tail fibres and the DNA ejection machine. A complex of gene product (gp) 5 (63K) and gp27 (44K), the central part of the baseplate, is required to penetrate the outer cell membrane of Escherichia coli and to disrupt the intermembrane peptidoglycan layer, promoting subsequent entry of phage DNA into the host. We present here a crystal structure of the (gp5-gp27)3 321K complex, determined to 2.9 A resolution and fitted into a cryo-electron microscopy map at 17 A resolution of the baseplate-tail tube assembly. The carboxy-terminal domain of gp5 is a triple-stranded beta-helix that forms an equilateral triangular prism, which acts as a membrane-puncturing needle. The middle lysozyme domain of gp5, situated on the periphery of the prism, serves to digest the peptidoglycan layer. The amino-terminal, antiparallel beta-barrel domain of gp5 is inserted into a cylinder formed by three gp27 monomers, which may serve as a channel for DNA ejection.     

Self-assembling phospholipid filaments

Aqueous dispersions of double-chain phospholipids spontaneously assemble into closed bilayers called vesicles (or liposomes). Although the vesicles are in general topologically spherical, cylindrical and helical liposomes have sometimes been observed. We present here video-enhanced microscopic studies of a diacetylenic phospholipid dispersed in ethanol/water, which reveal the existence of unusual bilayer morphologies. On cooling the dispersion from the isotropic phase, we have observed the formation of long (of the order of hundreds of micrometres), thin (0.2-2 microns) filaments, which fluctuate strongly. When the temperature is decreased further, the filaments rapidly retract into a mass of lipid. At constant temperature, on the other hand, the filaments transform into torus or ring-like vesicles. Such non-spherical structures have been predicted theoretically but not previously observed experimentally.