Michael Tal

The pancreatic β-cell glucose sensor

Abstract Pancreatic β cells secrete insulin in response to an increase in the level of blood glucose above 5mm, which is characteristic of the fasting state. Glucose metabolism is essential for glucose sensing, and both the high- K m glucose transporter GLUT2 and the high- K m glucose phosphorylating enzyme glucokinase have been implicated in coupling insulin secretion to extracellular glucose levels. Experiments in isolated islets, immortalized β-cell lines and transgenic animals, together with findings in humans with maturity-onset diabetes of the young, indicate that the primary β-cell glucose sensor is glucokinase. Although the level of GLUT2 is frequently reduced in animal models of type II diabetes, GLUT2 does not limit glucose metabolism in β cells and does not appear to regulate glucose induction of insulin secretion.

Murine Insulinoma Cell Line With Normal Glucose-Regulated Insulin Secretion

Pancreatic βTC lines derived from insulinomas arising in transgenic mice expressing SV40 Tag under control of the insulin promoter manifest a differentiated β-cell phenotype and secrete insulin in response to glucose. Previously reported βTC lines respond to subphysiological extracellular glucose levels compared with normal β-cells. Recently, several βTC lines were developed with normal glucose-regulated insulin secretion from insulinomas obtained by breeding of the RIP-Tag transgene from the original C57BI/6 mouse strain into the C3HeB/FeJ strain. One of these βTC lines, βTC7, was characterized in detail. βTC7 cells express GLUT2 and have levels of glucokinase and hexokinase activity similar to those of normal islets. As a result these cells exhibit a normal glucose concentration dependency for glycolysis and insulin secretion, thus representing an accurate model of β-cell function. On continuous propagation in culture, βTC7 cells acquired a response to lower extracellular glucose levels. This change was associated with a fourfold increase in hexokinase activity, without significant changes in glucokinase activity and glucose uptake rates. These findings suggest an important role for glucose phosphorylation rates in regulation of the β-cell insulin secretory response to glucose.

A new member of the C-type lectin family is a modulator of the mast cell secretory response.

A glycoprotein identified on RBL-2H3 cells as capable of inhibiting the secretory response induced by the type I Fc epsilon receptor was named mast-cell-function-associated antigen (MAFA). The amino acid sequence deduced from the cloned full-length cDNA has now shown that the MAFA has marked sequence homology with several members of the C-type (calcium-dependent) animal lectin family. The high conservation of cysteinyl residues suggests an important role for intrachain disulfide bonds in attaining its structure and biological activity. We further show that MAFA clustering by monoclonal antibody G63 also inhibits the de novo synthesis and secretion of interleukin-6 induced by the Fc epsilon RI stimulus. Though no ligand has yet been identified for the MAFA, experiments using antisense oligonucleotides suggest that this novel lectin may have a role in cell adhesion in addition to its immunomodulatory capacity.

Dp71, the nonmuscle product of the Duchenne muscular dystrophy gene is associated with the cell membrane

The 70.8 kDa protein, Dp71, is the major Duchenne muscular dystrophy (DMD) gene product in many nonmuscle tissues including the brain. Dp71 shares most of the C‐terminal and cysteine‐rich domains with the dystrophins but lacks the entire large rod shaped domain of spectrin‐like repeats, and the N‐terminal actin‐binding domain. The function of Dp71 is unknown. Using subcellular fractionation and immunostaining we show that Dp71 is associated with the plasma membrane. Dp71 is also associated with the plasma membrane in mdx myogenic cells transfected with a vector expressing Dp71.

DNA unwinding in the CYC1 and DED1 yeast promoters

The capacity of promoter DNA of two yeast genes to be unwound was studied. Both promoters, those of the CYC1 and DED1 genes, contain long oligopurine.oligopyrimidine (R.Y) tracts. The two promoters were cloned into negatively supercoiled plasmids, and their sensitivity to single-strand specific nuclease P1 was examined. Extensive P1 cleavage was located within the R.Y tracts, and cleavage sites were mapped. The extent of cleavage was only slightly dependent on P1 concentration, indicating a slow conversion of an intermediate form of DNA into the P1 reactive state. The cleavage required negative supercoiling and was suppressed by NaCl, MgCl2 and spermine. Two-dimensional topoisomer analysis showed that six superhelical turns were opened in the plasmids examined. The results indicate that at sufficient torsional stress, the R.Y tracts can intermittently undergo a transition into an unwound, ready-to-separate state. The oligopurine.oligopyrimidine tracts may thus serve as DNA unwinding centers in the gene promoters where they reside.

Streptococcus pneumoniae fructose-1,6-bisphosphate aldolase, a protein vaccine candidate, elicits Th1/Th2/Th17-type cytokine responses in mice

Streptococcus pneumoniae (S. pneumoniae) is a major pathogen worldwide. The currently available polysaccharide-based vaccines significantly reduce morbidity and mortality. However, the inherent disadvantages of the currently available polysaccharide-based vaccines have motivated the search for other bacterial immunogens capable of eliciting a protective immune response against S. pneumoniae. Fructose-1,6-bisphosphate aldolase (FBA) is a glycolytic enzyme, which was found to localize to the bacterial surface, where it functions as an adhesin. Previously, immunizing mice with recombinant FBA (rFBA) in the presence of alum elicited a protective immune response against a lethal challenge with S. pneumoniae. Thus, the aim of the present study was to determine the cytokine responses that are indicative of protective immunity following immunization with rFBA. The protective effects against pneumococcal challenge in mice immunized with rFBA with complete Freunds adjuvant (CFA) in the initial immunization and with incomplete Freunds adjuvant (IFA) in booster immunizations surpassed the protective effects observed following immunization with either rFBA + alum or pVACfba. CD4+ T-cells obtained from the rFBA/CFA/IFA/IFA-immunized mice co-cultured with rFBA-pulsed antigen-presenting cells (APCs), exhibited a significantly greater proliferative ability than CD4+ T-cells obtained from the adjuvant-immunized mice co-cultured with rFBA‑pulsed APCs. The levels of the Th1-type cytokines, interferon (IFN)-γ, interleukin (IL)-2, tumor necrosis factor (TNF)-α and IL-12, the Th2-type cytokines, IL-4, IL-5 and IL-10, and the Th17-type cytokine, IL-17A, significantly increased within 72 h of the initiation of co-culture with CD4+ T-cells obtained from the rFBA‑immunized mice, in comparison with the co-cultures with CD4+ T-cells obtained from the adjuvant-immunized mice. Immunizing mice with rFBA resulted in an IgG1/IgG2 ratio of 41, indicating a Th2 response with substantial Th1 involvement. In addition, rabbit and mouse anti-rFBA antisera significantly protected the mice against a lethal S. pneumoniae challenge in comparison with preimmune sera. Our results emphasize the mixed involvement of the Th1, Th2 and Th17 arms of the immune system in response to immunization with pneumococcal rFBA, a potential vaccine candidate.