Mutations in the channel domain alter desensitization of a neuronal nicotinic receptor.

A variety of ligand-gated ion channels undergo a fast activation process after the rapid application of agonist and also a slower transition towards desensitized or inactivated closed channel states when exposure to agonist is prolonged. Desensitization involves at least two distinct closed states in the acetylcholine receptor, each with an affinity for agonists higher than those of the resting or active conformations. Here we investigate how structural elements could be involved in the desensitization of the acetylcholine-gated ion channel from the chick brain alpha-bungarotoxin sensitive homo-oligomeric alpha 7 receptor, using site-directed mutagenesis and expression in Xenopus oocytes. Mutations of the highly conserved leucine 247 residue from the uncharged MII segment of alpha 7 suppress inhibition by the open-channel blocker QX-222, indicating that this residue, like others from MII, faces the lumen of the channel. But, unexpectedly, the same mutations decrease the rate of desensitization of the response, increase the apparent affinity for acetylcholine and abolish current rectification. Moreover, unlike wild-type alpha 7, which has channels with a single conductance level, the leucine-to-threonine mutant has an additional conducting state active at low acetylcholine concentrations. It is possible that mutation of Leu 247 renders conductive one of the high-affinity desensitized states of the receptor.     

A three-base-pair deletion in the peripherin-RDS gene in one form of retinitis pigmentosa.

The group of retinopathies termed retinitis pigmentosa (RP) greatly contribute to visual dysfunction in man with a frequency of roughly 1 in 4,000. We mapped the first autosomal dominant RP (adRP) gene to chromosome 3q, close to the gene encoding rhodopsin, a rod photoreceptor pigment protein. Subsequently, mutations in this gene have been implicated as responsible for some forms of adRP. Another adRP gene has been mapped to chromosome 8p. A third adRP gene in a large Irish pedigree has been mapped to chromosome 6p, showing tight linkage with the gene for peripherin, a photoreceptor cell-specific glycoprotein, which is thus a strong candidate for the defective gene. We have now identified a three-base-pair deletion which results in the loss of one of a pair of highly conserved cysteine residues in the predicted third transmembrane domain of peripherin. This deletion segregates with the disease phenotype but is not present in unaffected controls, and suggests that mutant peripherin gives rise to retinitis pigmentosa.     

Membrane protein association by potential intramembrane charge pairs.

The transmembrane domain of the alpha chain of the T-cell receptor is responsible both for its assembly with the CD3 delta chain and for rapid degradation of the unassembled chain within the endoplasmic reticulum. The determinant for both assembly and degradation is located in a segment of eight residues containing two basic amino acids. We show here that placement of a single basic residue in the transmembrane domain of the Tac antigen can induce interaction with the CD3 chain, through its transmembrane acidic residue. This interaction is most favoured when the interacting residues are located at the same level in the membrane. The ability to induce protein-protein interaction by placing charge pairs within transmembrane domains suggests an approach to producing artificial dimers.     

CD4+ and CD8+ T cells acquire specific lymphokine secretion potentials during thymic maturation.

Peripheral CD4+ and CD8+ T lymphocytes carry out different functions during immune reactions, partly as a result of the distinct patterns of lymphokines that they secrete upon stimulation. Using thymic cells from adult and newborn mice as well as from fetal organ cultures, we show here that this functional differentiation occurs inside the thymus and is completed during the single positive stage by the time the T-cell receptor becomes fully coupled to the intracellular activation pathways leading to lymphokine secretion. Surprisingly, CD4+8- thymocytes differ from their immediate progeny, naive peripheral CD4+ cells, in that they secrete a broader range of lymphokines, including interleukins 4, 5 and 10 and gamma-interferon, and more closely resemble immunologically experienced (activated or memory) CD4+ lymphocytes.     

Type 1 diabetes in mice is linked to the interleukin-1 receptor and Lsh/Ity/Bcg genes on chromosome 1.

Human type 1 (insulin-dependent) diabetes is a common auto-immune disease of the insulin-producing beta cells of the pancreas which is caused by both genetic and environmental factors. Several features of the genetics and immunopathology of diabetes in nonobese diabetic (NOD) mice are shared with the human disease. Of the three diabetes-susceptibility genes, Idd-1 -3 and -4 that have been mapped in mice to date, only in the case of Idd-1 is there any evidence for the identity of the gene product: allelic variation within the murine immune response I-A beta gene and its human homologue HLA-DQB1 correlates with susceptibility, implying that I-A beta is a component of Idd-1. We report here the mapping of Idd-5 to the proximal region of mouse chromosome 1. This region contains at least two candidate susceptibility genes, the interleukin-1 receptor gene and Lsh/Ity/Bcg, which encodes resistance to bacterial and parasitic infections and affects the function of macrophages.     

High-affinity NGF binding requires coexpression of the trk proto-oncogene and the low-affinity NGF receptor.

Nerve growth factor (NGF) interacts with two different low-affinity receptors that can be distinguished by affinity crosslinking. Reconstitution experiments by membrane fusion and transient transfection into heterologous cells indicate that high-affinity NGF binding requires coexpression and binding to both the low-affinity NGF receptor and the tyrosine kinase trk gene product. These studies reveal a new growth factor receptor-mediated mechanism of cellular differentiation involving trk and the low-affinity NGF receptor.     

Preferential DNA secondary structure mutagenesis in the lagging strand of replication in E. coli

When present in single-stranded DNA, palindromic or quasi-palindromic sequences have the potential to form complex secondary structures, including hairpins, which may facilitate interstrand misalignment of direct repeats and be responsible for diverse types of replication-based mutations, including deletions, additions, frameshifts and duplications. In regions of palindromic symmetry, specific deletion events may involve the formation of a hairpin or other DNA secondary structures which can stabilize the misalignment of direct repeats. One model suggests that these deletions occur during DNA replication by slippage of the template strand and misalignment with the progeny strand. The concurrent DNA replication model, involving an asymmetric dimeric DNA polymerase III complex which replicates the leading and lagging strands, has significant implications for mutagenesis. The intermittent looping of the lagging strand template, and the fact that the lagging strand template may contain a region of single-stranded DNA the length of an Okazaki fragment, provides an opportunity for DNA secondary-structure formation and misalignment. Here we report our design of a palindromic fragment to create an 'asymmetric palindromic insert' in the chloramphenicol acetyltransferase gene of plasmid pBR325. The frequency with which the insert was deleted in Escherichia coli depends on the orientation of the gene in the plasmid. Our results suggest that replication-dependent deletion between direct repeats may occur preferentially in the lagging strand.     

Humoral and cell-mediated immune responses to live recombinant BCG-HIV vaccines

Several viral and bacterial live recombinant vaccine vehicles are being developed to produce a new generation of vaccines against a broad spectrum of infectious diseases. The human tuberculosis vaccine Mycobacterium bovis bacillus Calmette-Guerin (BCG) has features that make it a particularly attractive live recombinant vaccine vehicle. BCG and other mycobacteria are highly effective adjuvants, and the immune response to mycobacteria has been studied extensively. With nearly two billion immunizations, BCG has a long record of safe use in man. It is one of the few vaccines that can be given at birth, it engenders long-lived immune responses with only a single dose, and there is a worldwide distribution network with experience in BCG vaccination. Recently developed molecular genetic tools and methods for mycobacteria have provided the means to introduce foreign genes into BCG. Here we report that a variety of human immunodeficiency virus type 1 polypeptides can be expressed in BCG recombinants under the control of the mycobacterial hsp70 promoter and that the foreign polypeptides produced in BCG can induce antibody and T-cell responses. These results demonstrate that BCG can be used as a live recombinant vaccine vehicle to induce immune responses to pathogen proteins produced by the bacillus.     

A selective deficit for writing vowels in acquired dysgraphia

Brain-damaged patients with acquired writing disorders provide important information about the normal processes of spelling and writing. Current models indicate that to produce a letter string, its 'abstract' representation is computed and stored in a temporary orthographic buffer, from which it is converted to a verbal code (if the word is to be spelled aloud) or to a physical letter code (if the word is to be written). The stored graphemic representations specify the identity and order of the component letters and their consonant/vowel status. Here I describe the spelling performance of two patients with a selective deficit in writing vowels. When writing words, the first patient omitted all vowels, leaving a blank space between consonants or consonant clusters, whereas the second produced errors that almost exclusively involved vowels. This pattern of performance supports the hypothesis that the consonant/vowel status of graphemes is differentially specified in the spelling process and may be selectively affected after brain damage.