Conductance Ratios and Cellular Identity

Recent experimental evidence suggests that coordinated expression of ion channels plays a role in constraining neuronal electrical activity. In particular, each neuronal cell type of the crustacean stomatogastric ganglion exhibits a unique set of positive linear correlations between ionic membrane conductances. These data suggest a causal relationship between expressed conductance correlations and features of cellular identity, namely electrical activity type. To test this idea, we used an existing database of conductance-based model neurons. We partitioned this database based on various measures of intrinsic activity, to approximate distinctions between biological cell types. We then tested individual conductance pairs for linear dependence to identify correlations. Contrary to experimental evidence, in which all conductance correlations are positive, 32% of correlations seen in this database were negative relationships. In addition, 80% of correlations seen here involved at least one calcium conductance, which have been difficult to measure experimentally. Similar to experimental results, each activity type investigated had a unique combination of correlated conductances. Finally, we found that populations of models that conform to a specific conductance correlation have a higher likelihood of exhibiting a particular feature of electrical activity. We conclude that regulating conductance ratios can support proper electrical activity of a wide range of cell types, particularly when the identity of the cell is well-defined by one or two features of its activity. Furthermore, we predict that previously unseen negative correlations and correlations involving calcium conductances are biologically plausible.     

Immune deficiency in the X-linked lymphoproliferative syndrome. I. Epstein-Barr virus-specific defects

Eleven males with XLP were evaluated for EBV-specific antibodies during periods of 2 to 7 yr. Variable responses to EBV-specific antigens were found. All 11 patients had subnormal anti-EBNA titers, which probably reflected a T cell deficiency. The patients showed four different patterns in their anti-VCA response: 1) two boys who had experienced malignant lymphoma mounted no antibodies at all; 2) two patients showed intermittent anti-VCA titers; 3) four males had persistently elevated anti-VCA titers; and 4) three patients showed normal anti-VCA titers. ADCC against EBV-infected cells was abnormally low in six patients and was elevated in two patients given gamma-globulin. ADCC titers did not correlate with anti-VCA titers. However, most patients with XLP failed to effect regression of autologous EBV-infected lymphoblastoid cell lines, indicating a deficiency in long-lived T cell-mediated immunity to EBV.     

Detection of femtomolar quantities of the ganglioside GM1 on thin-layer chromatography plates by native choleratoxin and labeled antisera

After separation of gangliosides by thin-layer chromatography, femtomolar quantities of G_M1 were detected by incubating the plate with native choleratoxin, followed by anticholeratoxin and species-specific labeled antiserum. Only stable reagents were involved when antiserum labeled with horseradish peroxidase was used. Native choleratoxin rather than iodinelabeled toxin ensured good reproducibility of the method.     

Ribonuclease P: the diversity of a ubiquitous RNA processing enzyme

Ribonuclease P is the endonuclease required for generating the mature tRNA 5'-end. The ribonucleoprotein character of this enzyme has now been proven in most organisms and organelles. Exceptions, however, are still the chloroplasts, plant nuclei and animal mitochondria where no associated RNAs have been detected to date. In contrast to the known RNA subunits, which are fairly well-conserved in size and structure among diverse phylogenetic groups, the protein contribution to the holoenzyme is highly variable in size and number of the individual components. The structure of the bacterial protein component has recently been solved. In contrast, the spatial arrangement of the multiple subunits in eukaryotic enzymes is still enigmatic. Substrate requirements of the enzymes or their catalytic RNA subunits are equally diverse, ranging from simple single domain mimics to an almost intact three-dimensional structure of the pre-tRNA substrate. As an example for an intermediate in the enzyme evolution, ribonuclease P from the Cyanophora paradoxa cyanelle will be discussed in more detail. This enzyme is unique, as it combines cyanobacterial and eukaryotic features in its function, subunit composition and holoenzyme topology.     

Glucose and amino acid metabolism in chick telencephalon slices: Changes with incubation conditions and animals' development

Glucose and amino acid metabolism in 1- and 30-day-old chick telencephalon slices was studied in two incubation media in the presence or in the absence of a continuous oxygenation. Medium 1 has a composition and a tonicity similar to cerebrospinal fluid, medium 2 is hypertonic and does not contain any K⁺ ions. The incorporation of glucose carbon into amino acids and the distribution of radioactivity between the different amino acids are close to the ones observed in the chick brain in vivo only when the slices are incubated in medium 1, with oxygen at 30 days and without oxygen for the 1-day-old chick. It also appears that if oxygenation is necessary for incubation of mature brain tissue in vitro, the absence of the medium oxygenation is more suitable for the study of glucose metabolism in 1-day-old chick brain slices.     

The nucleotide sequences of barley cytoplasmic glutamate transfer RNAs and structural features essential for formation of δ-aminolevulinic acid

In chloroplasts and a number of prokaryotes, -aminolevulinic acid (ALA), the universal precursor of porphyrins, is synthesized by a multistep enzymatic pathway with glutamyl-tRNA^Glu as an intermediate. The ALA synthesizing system from barley chloroplasts is highly specific in its tRNA requirement for chloroplast tRNA^Glu; a number of other Glu-tRNAs are inactive in ALA formation although they can be glutamylated by chloroplast aminoacyl-tRNA synthetases. In order to obtain more information about the structural features defining the ability of a tRNA to be recognized by the ALA synthesizing enzymes, we purified and sequenced two cytoplasmic tRNA^Glu species from barley embryos which are inactive in ALA synthesis. By using glutamylated tRNAs as a substrate for the overall reaction, we showed that Glu-tRNA reductase is the enzyme responsible for tRNA discrimination.     

Quantification and partial characterization of the hamster sperm proacrosin-acrosin system

The proacrosin-acrosin proteinase system was measured and partially characterized in unpurified extracts of washed hamster epididymal sperm. Autoactivation experiments demonstrated that proacrosin accounted for greater than 98% of the acrosin activity in the sperm extracts from individual animals. Several bands of proteinase activity were observed on gelatin-sodium dodecyl sulfate-polyacrylamide gel electrophoretic (gelatin-SDS-PAGE) zymography. The major proteinase activities in the nonactivated extracts corresponded to relative molecular masses (Mr) of 51,000 to 56,000, while less distinct digestion occurred with relative molecular masses of 37,000 to 49,000. It was demonstrated that after a serial dilution of the sperm extract, the proteinase activity in as few as 6,000 sperm could readily be detected by the gelatin-SDS-PAGE methods. Time-course activation studies showed that the zymogen was completely converted to active proteinase in 45-60 min at pH 8.0 and 25 degrees C. This autoconversion process was markedly inhibited by calcium, sodium, and heparin. However, each of these compounds stimulated the proteolytic activity of acrosin. These studies demonstrate that the proacrosin-acrosin system can be investigated in extracts of nonpurified hamster epididymal sperm.