Molecular genetic evidence for heterogeneity in manic depression

Manic depression is a severe cyclic mental illness that can be unipolar or bipolar and has a lifetime risk of approximately 7 per 1,000 in most populations. Families with multiple cases of manic depression have been described that are compatible with both autosomal dominant and X-linked modes of genetic transmission. Psychoactive antidepressant and stimulant drugs that help to ameliorate depression and mania are thought to act by affecting catecholamine neurotransmitter systems such as adrenaline, noradrenaline and dopamine, amongst others. Mutations affecting the tyrosine hydroxylase (TH) gene, which encodes the rate-limiting enzyme for the synthesis of these three neurotransmitters, might therefore be responsible for causing the manic depressive phenotype. We have studied three Icelandic kindreds amongst whom it appears that a single autosomal dominant disease allele is segregating. In these families there were 44 cases amongst 73 individuals at risk. Genetic linkage studies were carried out using clones encoding tyrosine hydroxylase the variable portion of the Harvey-ras-1 (HRAS1) locus and the variable region of the insulin gene (INS). All three markers are closely linked on chromosome 11 and were used to observe the segregation of restriction fragment length polymorphisms (RFLPs) in the three affected kindreds. We found no evidence for linkage to these markers in any of the three families. In contrast, Gerhard et al. found linkage between manic depression and HRAS1 in a single large Amish kindred. We conclude that there is genetic heterogeneity of linkage in manic depression. Therefore mutations at different loci are responsible for the manic depressive phenotype in the Amish and in Iceland.     

The genetic defect in familial Alzheimer's disease is not tightly linked to the amyloid beta-protein gene

Amyloid beta-protein (AP) is a peptide of relative molecular mass (Mr) 42,000 found in the senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles of patients with Alzheimer's disease and Down's syndrome (trisomy 21). Recent molecular genetic evidence has indicated that AP is encoded as part of a larger protein by a gene on chromosome 21 (refs 5-7). The defect in the inherited autosomal dominant form of Alzheimer's disease, familial Alzheimer's disease (FAD), has been mapped to the same approximate region of chromosome 21 by genetic linkage to anonymous DNA markers, raising the possibility that this gene product, which could be important in the pathogenesis of Alzheimer's disease, is also the site of the inherited defect in FAD (ref. 5). We have determined the pattern of segregation of the AP gene in FAD pedigrees using restriction fragment length polymorphisms. The detection of several recombination events with FAD suggests that the AP gene is not the site of the inherited defect underlying this disorder.     

Myosin subfragment-1 is sufficient to move actin filaments in vitro

The rotating crossbridge model for muscle contraction proposes that force is produced by a change in angle of the crossbridge between the overlapping thick and thin filaments. Myosin, the major component of the thick filament, is comprised of two heavy chains and two pairs of light chains. Together they form two globular heads, which give rise to the crossbridge in muscle, and a coiled-coil rod, which forms the shaft of the thick filament. The isolated head fragment, subfragment-1 (S1), contains the ATPase and actin-binding activities of myosin (Fig. 1). Although S1 seems to have the requisite enzymatic activity, direct evidence that S1 is sufficient to drive actin movement has been lacking. It has long been recognized that in vitro movement assays are an important approach for identifying the elements in muscle responsible for force generation. Hynes et al. showed that beads coated with heavy meromyosin (HMM), a soluble proteolytic fragment of myosin consisting of a part of the rod and the two heads, can move on Nitella actin filaments. Using the myosin-coated surface assay of Kron and Spudich, Harada et al. showed that single-headed myosin filaments bound to glass support movement of actin at nearly the same speed as intact myosin filaments. These studies show that the terminal portion of the rod and the two-headed nature of myosin are not required for movement. To restrict the region responsible for movement further, we have modified the myosin-coated surface assay by replacing the glass surface with a nitrocellulose film. Here we report that myosin filaments, soluble myosin, HMM or S1, when bound to a nitrocellulose film, support actin sliding movement (Fig. 2). That S1 is sufficient to cause sliding movement of actin filaments in vitro gives strong support to models of contraction that place the site of active movement in muscle within the myosin head.     

Expression and functional characterization of artificial mutants of interleukin-2 receptor

The physiological proliferation of T lymphocytes (T cells) requires interaction between the humoral growth factor, interleukin 2 (IL-2) and its cell-surface receptor. Studies of IL-2 binding to the IL-2 receptor (IL-2R) on T cells have revealed that there are two distinct species of IL-2R, one with high and one with low affinity. Isolation and characterization of cDNA for the human IL-2R made it possible to deduce the complete primary sequence (251 residues) of the receptor protein. However, expression of IL-2R alone is not sufficient for either growth signal transduction or high-affinity site formation: another lymphocyte-specific molecule called converter seems to be required for the biological activity of IL-2R. We found that the converter did not form a stable complex with IL-2R unless the receptor bound the ligand (the 'affinity conversion' model). To discover which are the functionally important parts of the human IL-2R we have constructed artificial mutant cDNAs encoding the receptor. The mutant receptors produced from them had deletions or substitutions in the cytoplasmic region (13 residues), the transmembrane region (19 residues) or the carboxy-terminal portion of the extracellular region (219 residues). All were active in growth signal transduction, efficient internalization and high-affinity site formation in two mouse T-cell lines, suggesting that the extracellular region of IL-2R and the converter may be responsible for growth signal transduction.     

Localization of the gene for familial adenomatous polyposis on chromosome 5

Colorectal cancer is the second most common cancer in the United Kingdom and other developed countries in the West. Although it is usually not familial, there is a rare dominantly inherited susceptibility to colon cancer, familial adenomatous polyposis (FAP; also often previously called familial polyposis coli). During adolescence affected individuals develop from a few hundred to over a thousand adenomatous polyps in their large bowel. These are sufficiently likely to give rise to adenocarcinomas to make prophylactic removal of the colon usual in diagnosed FAP individuals. Adenomas may occur elsewhere in the gastrointestinal tract and the condition is often associated with other extracolonic lesions, such as epidermoid cysts, jaw osteomata and fibrous desmoid tumours. Adenomata have been suggested to be precancerous states for most colorectal tumours. Knudson has suggested that the mutation for a dominantly inherited cancer susceptibility may be the first step in a recessive change in the tumour cells, and that the same gene may be involved in both familial and non-familial cases of a given tumour. Following up a case report of an interstitial deletion of chromosome 5 in a mentally retarded individual with multiple developmental abnormalities and FAP, we have now shown that the FAP gene is on chromosome 5, most probably near bands 5q21-q22.     

Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells

Glutamate is taken up avidly by glial cells in the central nervous system. Glutamate uptake may terminate the transmitter action of glutamate released from neurons, and keep extracellular glutamate at concentrations below those which are neurotoxic. We report here that glutamate evokes a large inward current in retinal glial cells which have their membrane potential and intracellular ion concentrations controlled by the whole-cell patch-clamp technique. This current seems to be due to an electrogenic glutamate uptake carrier, which transports at least two sodium ions with every glutamate anion carried into the cell. Glutamate uptake is strongly voltage-dependent, decreasing at depolarized potentials: when fully activated, it contributes almost half of the conductance in the part of the glial cell membrane facing the retinal neurons. The spatial localization, glutamate affinity and magnitude of the uptake are appropriate for terminating the synaptic action of glutamate released from photoreceptors and bipolar cells. These data challenge present explanations of how the b-wave of the electroretinogram is generated, and suggest a mechanism for non-vesicular voltage-dependent release of glutamate from neurons.     

Identification of receptor contact site involved in receptor-G protein coupling

The mammalian G proteins transduce information from extracellular signals, including neurotransmitters, hormones and sensory stimuli, into regulation of effector enzymes or ion channels within cells. Triggered by appropriate extracellular signals, receptor proteins specifically activate members of the G protein family by catalysing replacement of GDP by GTP at the guanine nucleotide binding site. Like the receptor proteins, the heterotrimeric G proteins exhibit impressive structural similarities, suggesting that all receptor-G protein interactions use homologous structural elements and a single molecular mechanism. Topologically equivalent portions of each G protein may therefore interact with the appropriate receptor. We recently predicted the secondary structure of a composite G protein alpha-chain and proposed that a predicted amphipathic alpha-helix at the extreme carboxy-terminus of the polypeptide directly contacts receptors. This proposal has now been confirmed by sequencing complementary DNAs of the gene that encodes the alpha-chain (alpha s) of the stimulatory regulator (Gs) of adenylyl cyclase in wild-type cells and in a mutant mouse S49 lymphoma cell line, unc, in which Gs cannot be activated by hormone receptors. The sequences reveal a point mutation in the unc gene that substitutes a proline residue for an arginine near the carboxy-terminus of the alpha s-polypeptide. Expression of recombinant alpha s-unc in genetically alpha s-deficient S49 cells reproduces the unc phenotype.     

HIV F/3' orf encodes a phosphorylated GTP-binding protein resembling an oncogene product

Apart from the retroviral gag, pol and env the HIV genome contains the F (3' orf) gene which encodes a polypeptide of 206 amino acids which is myristylated at the N-terminal and whose function is unknown. We have expressed the F gene in Escherichia coli and from a recombinant vaccinia virus, VVTGfHIV. The F-protein produced in VVTGfHIV-infected mammalian cells is myristilated, and is phosphorylated by protein kinase C at a residue close to the N-terminus like pp60-src (ref. 5). Purified bacterial F-protein also shows the GTPase, autophosphorylation and GTP-binding activities reported for the ras gene product. Furthermore, we show that expression of F in a CD4+ cell line down-regulates the CD4(T4) antigen. These results suggest that F is important in the pathophysiology of AIDS (acquired immune deficiency syndrome).     

Prevention of vaccinia virus infection in immunodeficient mice by vector-directed IL-2 expression

Recombinant vaccinia viruses have been proposed as live vaccines against a variety of infectious diseases, including AIDS (acquired immune deficiency syndrome). Objections have been concerned primarily with side effects of the vaccinia virus vector itself. Recently it has been shown that inactivation of the vaccinia virus thymidine kinase gene or deletion of certain other non-essential genes is associated with a marked reduction in pathogenicity. Nevertheless, the ability of vaccinia virus to produce a progressive infection in immunodeficient individuals remains a most serious problem. Indeed, an incident of this type in a vaccinated man seropositive for human immunodeficiency virus was recently reported. We have used immunodeficient athymic nude mice to establish a model of disseminated vaccinia virus infection, and to demonstrate a novel approach to virus attenuation which involves insertion of a gene encoding human interleukin-2 into the genome of vaccinia virus vectors.