Mutations in Rab3a alter circadian period and homeostatic response to sleep loss in the mouse

Rab3a is the most abundant Rab (ras-associated binding) protein in the brain and has a regulatory role in synaptic vesicle trafficking. Mice with a targeted loss-of-function mutation in Rab3a have defects in Ca(2+)-dependent synaptic transmission: the number of vesicles released in response to an action potential is greater than in wildtype mice, resulting in greater synaptic depression and the abolishment of CA3 mossy-fiber long term potentiation. The effect of these changes on behavior is unknown. In a screen for mouse mutants with abnormal rest-activity and sleep patterns, we identified a semidominant mutation, called earlybird, that shortens the circadian period of locomotor activity. Sequence analysis of Rab3a identified a point mutation in the conserved amino acid (Asp77Gly) within the GTP-binding domain of this protein in earlybird mutants, resulting in significantly reduced levels of Rab3a protein. Phenotypic assessment of earlybird mice and a null allele of Rab3a revealed anomalies in circadian period and sleep homeostasis, providing evidence that Rab3a-mediated synaptic transmission is involved in these behaviors.     

High-throughput retroviral tagging to identify components of specific signaling pathways in cancer

Genetic screens carried out in lower organisms such as yeast, Drosophila melanogaster and Caenorhabditis elegans have revealed many signaling pathways. For example, components of the RAS signaling cascade were identified using a mutant eye phenotype in D. melanogaster as a readout. Screening is usually based on enhancing or suppressing a phenotype by way of a known mutation in a particular signaling pathway. Such in vivo screens have been difficult to carry out in mammals, however, owing to their relatively long generation times and the limited number of animals that can be screened. Here we describe an in vivo mammalian genetic screen used to identify components of pathways contributing to oncogenic transformation. We applied retroviral insertional mutagenesis in Myc transgenic (E mu Myc) mice lacking expression of Pim1 and Pim2 to search for genes that can substitute for Pim1 and Pim2 in lymphomagenesis. We determined the chromosomal positions of 477 retroviral insertion sites (RISs) derived from 38 tumors from E mu Myc Pim1(-/-) Pim2(-/-) mice and 27 tumors from E mu Myc control mice using the Ensembl and Celera annotated mouse genome databases. There were 52 sites occupied by proviruses in more than one tumor. These common insertion sites (CISs) are likely to contain genes contributing to tumorigenesis. Comparison of the RISs in tumors of Pim-null mice with the RISs in tumors of E mu Myc control mice indicated that 10 of the 52 CISs belong to the Pim complementation group. In addition, we found that Pim3 is selectively activated in Pim-null tumor cells, which supports the validity of our approach.     

Evidence of cis-acting factors in replication-mediated trinucleotide repeat instability in primate cells

The mechanism of disease-associated trinucleotide repeat instability involves cis-acting factors (cis-elements) in the vicinity of the repeat, but the nature of these elements is unknown. One cis-element may be the location of the replication origin relative to the repeat. We have used an SV40 DNA replication system to investigate the effect of the location of replication initiation on (CTG)(n)*(CAG)(n) stability in primate cells. Depending on the distance between the SV40 replication origin and the repeat tract, templates with 79 repeats yield predominantly expansions or predominantly deletions or remain intact. All templates with 17 repeats are stable. Thus, cis-elements that affect the sites of Okazaki fragment initiation relative to the repeat are crucial determinants of instability. This model system recapitulates the bias for expansions observed in many of the diseases associated with trinucleotide repeats. Our results might explain the variable amounts of CTG/CAG instability that are observed in different chromosomal contexts.     

Paxillin binds schwannomin and regulates its density-dependent localization and effect on cell morphology

Neurofibromatosis type 2 is an autosomal dominant disorder characterized by tumors, predominantly schwannomas, in the nervous system. It is caused by mutations in the gene NF2, encoding the growth regulator schwannomin (also known as merlin). Mutations occur throughout the 17-exon gene, with most resulting in protein truncation and undetectable amounts of schwannomin protein. Pathogenic mutations that result in production of defective schwannomin include in-frame deletions of exon 2 and three independent missense mutations within this same exon. Mice with conditional deletion of exon 2 in Schwann cells develop schwannomas, which confirms the crucial nature of exon 2 for growth control. Here we report that the molecular adaptor paxillin binds directly to schwannomin at residues 50-70, which are encoded by exon 2. This interaction mediates the membrane localization of schwannomin to the plasma membrane, where it associates with beta 1 integrin and erbB2. It defines a pathogenic mechanism for the development of NF2 in humans with mutations in exon 2 of NF2.     

Identification of genes involved in Drosophila melanogaster geotaxis,a complex behavioral trait

Identifying the genes involved in polygenic traits has been difficult. In the 1950s and 1960s, laboratory selection experiments for extreme geotaxic behavior in fruit flies established for the first time that a complex behavioral trait has a genetic basis. But the specific genes responsible for the behavior have never been identified using this classical model. To identify the individual genes involved in geotaxic response, we used cDNA microarrays to identify candidate genes and assessed fly lines mutant in these genes for behavioral confirmation. We have thus determined the identities of several genes that contribute to the complex, polygenic behavior of geotaxis.     

A mouse model of human L1 retrotransposition

The L1 retrotransposon has had an immense impact on the size and structure of the human genome through a variety of mechanisms, including insertional mutagenesis. To study retrotransposition in a living organism, we created a mouse model of human L1 retrotransposition. Here we show that L1 elements can retrotranspose in male germ cells, and that expression of a human L1 element under the control of its endogenous promoter is restricted to testis and ovary. In the mouse line with the highest level of L1 expression, we found two de novo L1 insertions in 135 offspring. Both insertions were structurally indistinguishable from natural endogenous insertions. This suggests that an individual L1 element can have substantial mutagenic potential. In addition to providing a valuable in vivo model of retrotransposition in mammals, these mice are an important step in the development of a new random mutagenesis system.     

Instant neural control of a movement signal

The activity of motor cortex (MI) neurons conveys movement intent sufficiently well to be used as a control signal to operate artificial devices, but until now this has called for extensive training or has been confined to a limited movement repertoire. Here we show how activity from a few (7-30) MI neurons can be decoded into a signal that a monkey is able to use immediately to move a computer cursor to any new position in its workspace (14 degrees x 14 degrees visual angle). Our results, which are based on recordings made by an electrode array that is suitable for human use, indicate that neurally based control of movement may eventually be feasible in paralysed humans.