Excitation-contraction coupling in isolated locomotor muscle fibres from the pelagic tunicate Doliolum which lack both sarcoplasmic reticulum and transverse tubular system

In the locomotor muscle of the pelagic tunicate Doliolum, both the sarcoplasmic reticulum (SR) and the transverse-tubular (T-tubular) system are absent. The mechanism of excitation-contraction (E-C) coupling was studied in single muscle fibres enzymatically dissociated from Doliolum denticulatum. Whole cell voltage clamp experiments demonstrated an inward ionic current associated with membrane depolarisation. This current was blocked by 5 mmol.l(-1)Co(2+), a calcium current blocker, and suppressed by nifedipine, a specific L-type calcium channel blocker. An increase in the external K(+) concentration to 200 mmol.l(-1) (K(+)-depolarisation) induced a rise in the intracellular Ca(2+) level detected with fluo-3, a Ca(2+)-sensitive dye. However, when 5-10 mmol.l(-1) Co(2+) or 10-15 micro mol.l(-1) nifedipine was present in the external solution, K(+)-depolarisation did not induce a rise in the intracellular Ca(2+) level. Externally applied 5-10 mmol.l(-1) caffeine or 20 micro mol.l(-1) ryanodine had no effect on the intracellular Ca(2+) level. K(+)-depolarisation induced a rise in the intracellular Ca(2+) level in the presence of caffeine or ryanodine. Replacement of external Na(+) with Li(+) increased intracellular Ca(2+) levels. Our results show that contraction of the locomotor muscle in Doliolum is solely due to the influx of Ca(2+) through L-type calcium channels, and that relaxation is due to extrusion of Ca(2+) by Na(+)/Ca(2+) exchange across the sarcolemma.     

Regulation of MTK1/MEKK4 kinase activity by its N-terminal autoinhibitory domain and GADD45 binding

A variety of cellular stresses activate the stress-responsive mitogen-activated protein (MAP) kinases p38 and JNK. In this study, we studied the activation mechanism of a human MAP kinase kinase kinase, MTK1 (also known as MEKK4), which mediates activation of both p38 and JNK. MTK1 has an extensive N-terminal noncatalytic domain composed of approximately 1,300 amino acids. Full-length or near full-length MTK1 is catalytically inactive when expressed in Saccharomyces cerevisiae cells, as it is in mammalian cells. Deletion of a segment including positions 253 to 553 activates kinase, indicating that this segment contains the autoinhibitory domain. In the autoinhibited conformation, the MTK1 kinase domain cannot interact with its substrate, MKK6. By a functional complementation screening with yeast cells, GADD45 proteins (GADD45alpha, beta, and gamma) were identified as MTK1 activators. GADD45 proteins bind a site in MTK1 near the inhibitory domain and relieve autoinhibition. Mutants of full-length MTK1 were isolated that can interact with MKK6 in the absence of the activator GADD45 proteins. These MTK1 mutants are constitutively active, in both yeast and mammalian cells. A model of MTK1 autoinhibition by the N-terminal inhibitory domain and activation by GADD45 binding is presented.     

Effect of the dose and composition of an autologous hapten-modified melanoma vaccine on the development of delayed-type hypersensitivity responses

We have reported that treatment of melanoma patients with a vaccine consisting of autologous tumor cells modified with the hapten, dinitrophenyl (DNP) and preceded by low-dose cyclophosphamide induces delayed-type hypersensitivity (DTH) to autologous, unmodified tumor cells and that this response is a significant predictor of survival. We analyzed the vaccines prepared for 284 patients who were treated following resection of regional or distant metastases to find out whether the dose and composition determined the immunological response. A positive DTH response (> or =5 mm induration) to unmodified autologous tumor cells was induced in 57% of the patients (median: 5 mm; range: 0-22 mm). Regression analysis showed no significant association between the magnitude of DTH and the number of live (trypan blue exclusion) melanoma cells per dose over a dosage range of 0.5-25.0 x 10(6). Surprisingly, there was a small but significant positive relationship between the mean number of dead cells in the vaccines of a given patient and that patient's maximum DTH to unmodified melanoma cells. Only 37% of patients whose vaccines contained >50% live cells developed DTH, as compared with 69% and 65% of patients whose vaccines contained 26% to 50% or < or =25% live cells, respectively. Thus, it appears that dead tumor cells contribute to the immunogenicity of the DNP vaccine, but other factors such as the administration schedule may be more important determinants of immunological and clinical outcome.     

A simple computational model of the right coronary artery on the beating heart--effects of the temporal change of curvature and torsion on the blood flow

A computational fluid dynamics study was conducted using a simplified model of the right coronary artery, which deforms with contraction of the heart. The right coronary artery was modeled using an ordinary helix, whose torsion and curvature changed in time with the contraction and dilatation of the heart which was modeled as a cylinder. In the computational result, the flow in the model right coronary artery was thought to be more affected by the change of the curvature compared to that of the torsion.     

Molecular diversity of skin mucus lectins in fish

Among lectins in the skin mucus of fish, primary structures of four different types of lectin have been determined. Congerin from the conger eel Conger myriaster and AJL-1 from the Japanese eel Anguilla japonica were identified as galectin, characterized by its specific binding to β-galactoside. Eel has additionally a unique lectin, AJL-2, which has a highly conserved sequence of C-type lectins but displays Ca²⁺-independent activity. This is rational because the lectin exerts its function on the cutaneous surface, which is exposed to a Ca²⁺ scarce environment when the eel is in fresh water. The third type lectin is pufflectin, a mannose specific lectin in the skin mucus of pufferfish Takifugu rubripes. This lectin showed no sequence similarity with any known animal lectins but, surprisingly, shares sequence homology with mannose-binding lectins of monocotyledonous plants. The fourth lectin was found in the ponyfish Leiognathus nuchalis and exhibits homology with rhamnose-binding lectins known in eggs of some fish species. These lectins, except ponyfish lectin, showed agglutination of certain bacteria. In addition, pufflectin was found to bind to a parasitic trematode, Heterobothrium okamotoi. Taken together, these results demonstrate that skin mucus lectins in fish have wide molecular diversity.     

Biosynthesis and action of neurosteroids in the cerebellar Purkinje neuron

The brain is considered to be a target site of peripheral steroid hormones. In contrast to this classical concept, new findings over the past decade have established that the brain itself also synthesizes steroids de novo from cholesterol through mechanisms at least partly independent of peripheral steroidogenic glands. Such steroids synthesized de novo in the brain, as well as other areas of the nervous system, are called neurosteroids. To understand neurosteroid actions in the brain, we need data on the specific synthesis in particular sites of the brain at particular times. Therefore, our studies for this exciting area of brain research have focused on the biosynthesis and action of neurosteroids in the identified neurosteroidogenic cells underlying important brain functions. We have demonstrated that the Purkinje cell, a typical cerebellar neuron, is a major site for neurosteroid formation in the brain. This is the first observation of neuronal neurosteroidogenesis in the brain. Subsequently, genomic and nongenomic actions of neurosteroids have become clear by a series of our studies using an excellent Purkinje cellular model. On the basis of these findings, we summarize the advances made in our understanding of biosynthesis and action of neurosteroids in the cerebellar Purkinje cell.     

Leucine zipper motif of chicken histone acetyltransferase-1 is essential for in vivo and in vitro interactions with the p48 subunit of chicken chromatin assembly factor-1

We cloned cDNA encoding chicken cytoplasmic histone acetyltransferase-1, chHAT-1, comprising 408 amino acids including a putative initiation Met. It exhibits 80.4% identity to the human homolog and possesses a typical leucine zipper motif. The glutathione S-transferase (GST) pull-down assay, involving truncated and missense mutants of the chicken chromatin assembly factor-1 (chCAF-1)p48, revealed not only that a region (comprising amino acids 376–405 of chCAF-1p48 and containing the seventh WD dipeptide motif) binds to chHAT-1 in vitro, but also that mutation of the motif has no influence on the in vitro interaction. The GST pull-down assay, involving truncated and missense chHAT-1 mutants, established that a region, comprising amino acids 380–408 of chHAT-1 and containing the leucine zipper motif, is required for its in vitro interaction with chCAF-1p48. In addition, mutation of each of four Leu residues in the leucine zipper motif prevents the in vitro interaction. The yeast two-hybrid assay revealed that all four Leu residues within the leucine zipper motif of chHAT-1 are necessary for its in vivo interaction with chCAF-1p48. These results indicate not only that the proper leucine zipper motif of chHAT-1 is essential for its interaction with chCAF-1p48, but also that the propeller structure of chCAF-1p48 expected to act as a platform for protein–protein interactions may not be necessary for this interaction of chHAT-1.