The role of clonal selection and somatic mutation in autoimmunity

Polyclonal activation has been proposed as the reason that autoantibodies are produced during autoimmune disease. This model denies a role for specific antigen selection of B cells and predicts instead a multiclonal population of unmutated or randomly mutated autoantibodies. We have found that the genetic features and clonal composition of spontaneously derived immunoglobulin G (IgG) antiself-IgG (rheumatoid factor (RF] autoantibodies derived from the autoimmune MRL/lpr mouse strain are inconsistent with both the predictions of this model and the actual outcome of experimental polyclonal activation. Instead we have found that MRL/lpr RFs are oligoclonal or even monoclonal in origin. They harbour numerous somatic mutations which are distributed in a way that suggests immunoglobulin-receptor-dependent selection of these mutations. In this sense, the MRL/lpr RFs resemble antibodies elicited by exogenous antigens after secondary immunization. The parallels suggest that, like secondary immune responses, antigen stimulation is important in the generation of MRL/lpr RFs.     

Failure of familial Alzheimer's disease to segregate with the A4-amyloid gene in several European families

The gene coding for the amyloid protein, a component of neuritic plaques found in brain tissue from patients with Alzheimer's disease, has been localized to chromosome 21, and neighbouring polymorphic DNA markers segregate with Alzheimer's disease in several large families. These data, and the association of Alzheimer's disease with Down's syndrome, suggest that overproduction of the amyloid protein, or production of an abnormal variant of the protein, may be the underlying pathological change causing Alzheimer's disease. We have identified a restriction fragment length polymorphism of the A4-amyloid gene, and find recombinants in two Alzheimer's disease families between Alzheimer's disease and the A4-amyloid locus. This demonstrates that the gene for plaque core A4-amyloid cannot be the locus of a defect causing Alzheimer's disease in these families. These data indicate that alterations in the plaque core amyloid gene cannot explain the molecular pathology for all cases of Alzheimer's disease.     

Synapsin I bundles F-actin in a phosphorylation-dependent manner

Synapsin I is a neuron-specific phosphoprotein localized to the cytoplasmic surface of synaptic vesicles. This phosphoprotein is a major substrate for cyclic AMP-dependent and calcium/calmodulin-dependent protein kinases. Its state of phosphorylation can be altered both in vivo and in vitro by a variety of physiological and pharmacological manipulations known to affect synaptic function. Recent direct evidence suggests that it may be involved in the regulation of neurotransmitter release from the nerve terminal. In the nerve terminal, synaptic vesicles are embedded in a cytoskeletal network, consisting in part of actin. We report here the ability of the dephospho-form of synapsin I to bundle F-actin. This bundling activity is reduced when synapsin I is phosphorylated by cAMP-dependent protein kinase and virtually abolished when it is phosphorylated by calcium/calmodulin-dependent protein kinase II or by both kinases. These results, demonstrating an interaction of synapsin I with actin in vitro, support the possibility that synapsin I is involved in clustering of synaptic vesicles at the presynaptic terminal and that the phosphorylation of synapsin I may be involved in regulating the translocation of synaptic vesicles to their sites of release.     

Genomic imprinting determines methylation of parental alleles in transgenic mice

Mouse embryogenesis relies on the presence of both the maternal and the paternal genome for development to term. It has been proposed that specific modifications are imprinted onto the chromosomes during gametogenesis; these modifications are stably propagated, and their expression results in distinct and complementary contributions of the two parental genomes to the development of the embryo and the extraembryonic membranes. Genetic data further suggest that a substantial proportion of the genome could be subject to chromosomal imprinting, the molecular nature of which is unknown. We used random DNA insertions in transgenic mice to probe the genome for modified regions. The DNA methylation patterns of transgenic alleles were compared after transmission from mother or father in seven mouse strains carrying autosomal insertions of the same transgenic marker. One of these loci showed a clear difference in DNA methylation specific for its parental origin, with the paternally inherited copy being relatively undermethylated. This difference was observed in embryos on day 10 of gestation, but not in their extraembryonic membranes. Moreover, the methylation pattern was faithfully reversed upon each germline transmission to the opposite sex. Our findings provide evidence for heritable molecular differences between maternally and paternally derived alleles on mouse chromosomes.     

Endstopped neurons in the visual cortex as a substrate for calculating curvature

Neurons in the visual cortex typically respond selectively to the orientation, and velocity and direction of movement, of moving-bar stimuli. These responses are generally thought to provide information about the orientation and position of lines and edges in the visual field. Some cells are also endstopped, that is selective for bars of specific lengths. Hubel and Wiesel first observed that endstopped hypercomplex cells could respond to curved stimuli and suggested they might be involved in detection of curvature, but the exact relationship between endstopping and curvature has never been determined. We present here a mathematical model relating endstopping to curvature in which the difference in response of two simple cells gives rise to endstopping and varies in proportion to curvature. We also provide physiological evidence that endstopped cells in area 17 of the cat visual cortex are selective for curvature, whereas non-endstopped cells are not, and that some are selective for the sign of curvature. The prevailing view of edge and curve determination is that orientations are selected locally by the class of simple cortical cells and then integrated to form global curves. We have developed a computational theory of orientation selection which shows that measurements of orientation obtained by simple cells are not sufficient because there will be strong, incorrect responses from cells whose receptive fields (RFs) span distinct curves (Fig. 1). If estimates of curvature are available, however, these inappropriate responses can be eliminated. Curvature provides the key to structuring the network that underlies our theory and distinguishes it from previous lateral inhibition schemes.     

Inhibition of mitosis by an antibody to the mitotic calcium transport system

Calcium ions are important in the regulation of mitotic apparatus assembly and in the control of chromosome movement. Changes in intracellular free calcium concentration, [Ca2+]i are achieved by an intracellular calcium-transport system which is highly conserved in different cell types. A membrane-bound protein of relative molecular mass (Mr) 46,000 (46K) is part of this transport system and has been implicated in the regulation of the [Ca2+]i changes associated with the course of mitosis. A monoclonal antibody against this 46K protein inhibits Ca2+-uptake into isolated Ca2+-sequestering membranes and specifically labels membranes associated with the mitotic apparatus of sea urchin embryos. Here we investigate the relationship between the intracellular calcium transport system and mitosis by injection of this monoclonal antibody into living mitotic sea urchin embryos. We find that after injection the intracellular free calcium increases up to 10(-6) M, the mitotic apparatus is rapidly destroyed and the cell is irreversibly blocked in its development.     

Astrocytes induce blood-brain barrier properties in endothelial cells

The highly impermeable tight junctions between endothelial cells forming the capillaries and venules in the central nervous system (CNS) of higher vertebrates are thought to be responsible for the blood-brain barrier that impedes the passive diffusion of solutes from the blood into the extracellular space of the CNS. The ability of CNS endothelial cells to form a blood-brain barrier is not intrinsic to these cells but instead is induced by the CNS environment: Stewart and Wiley demonstrated that when avascular tissue from 3-day-old quail brain is transplanted into the coelomic cavity of chick embryos, the chick endothelial cells that vascularize the quail brain grafts form a competent blood-brain barrier; on the other hand, when avascular embryonic quail coelomic grafts are transplanted into embryonic chick brain, the chick endothelial cells that invade the mesenchymal tissue grafts form leaky capillaries and venules. It is, however, not known which cells in the CNS are responsible for inducing endothelial cells to form the tight junctions characteristic of the blood-brain barrier. Astrocytes are the most likely candidates since their processes form endfeet that collectively surround CNS microvessels. In this report we provide direct evidence that astrocytes are capable of inducing blood-brain barrier properties in non-neural endothelial cells in vivo.     

Active sliding between cytoplasmic microtubules

Microtubules are versatile cellular polymers that play a role in cell shape determination and mediate various motile processes such as ciliary and flagellar bending, chromosome movements and organelle transport. That a sliding microtubule mechanism can generate force has been demonstrated in highly ordered structures such as axonemes, and microtubule-based force generation almost certainly contributes to the function of mitotic and meiotic spindles. Most cytoplasmic microtubule arrays, however, do not exhibit the structural regularity of axonemes and some spindles, and often appear disorganized. Yet many cellular activities (such as shape changes during morphogenesis, axonal extension and spindle assembly) involve highly coordinated microtubule behaviour and possibly require force generated by an intermicrotubule sliding mechanism, or perhaps use sliding to move microtubules rapidly into a protrusion for stabilization. Here we show that active sliding between cytoplasmic microtubules can occur in microtubule bundles of the amoeba Reticulomyxa. A force-producing mechanism of this sort could be used by this organism to facilitate the extension of cell processes and to generate the dynamic movements of the cytoplasmic network.