Distinct and opposing roles for the phosphatidylinositol 3-OH kinase catalytic subunits p110α and p110β in the regulation of insulin secretion from rodent and human beta cells

AIMS/HYPOTHESIS

Phosphatidylinositol 3-OH kinases (PI3Ks) regulate beta cell mass, gene transcription, and function, although the contribution of the specific isoforms is unknown. As reduced type 1A PI3K signalling is thought to contribute to impaired insulin secretion, we investigated the role of the type 1A PI3K catalytic subunits α and β (p110α and -β) in insulin granule recruitment and exocytosis in rodent and human islets.

METHODS

The p110α and p110β subunits were inhibited pharmacologically or by small hairpin (sh)RNA-mediated knockdown, and were directly infused or overexpressed in mouse and human islets, beta cells and INS-1 832/13 cells. Glucose-stimulated insulin secretion (GSIS), single-cell exocytosis, Ca(2+) signalling, plasma membrane granule localisation, and actin density were monitored.

RESULTS

Inhibition or knockdown of p110α increased GSIS. This was not due to altered Ca(2+) responses, depolymerisation of cortical actin or increased cortical granule density, but to enhanced Ca(2+)-dependent exocytosis. Intracellular infusion of recombinant PI3Kα (p110α/p85β) blocked exocytosis. Conversely, knockdown (but not pharmacological inhibition) of p110β blunted GSIS, reduced cortical granule density and impaired exocytosis. Exocytosis was rescued by direct intracellular infusion of recombinant PI3Kβ (p110β/p85β) even when p110β catalytic activity was inhibited. Conversely, both the wild-type p110β and a catalytically inactive mutant directly facilitated exocytosis.

CONCLUSIONS/INTERPRETATION

Type 1A PI3K isoforms have distinct and opposing roles in the acute regulation of insulin secretion. While p110α acts as a negative regulator of beta cell exocytosis and insulin secretion, p110β is a positive regulator of insulin secretion through a mechanism separate from its catalytic activity.

The voltage-dependent potassium channel subunit Kv2.1 regulates insulin secretion from rodent and human islets independently of its electrical function

AIMS/HYPOTHESIS

It is thought that the voltage-dependent potassium channel subunit Kv2.1 (Kv2.1) regulates insulin secretion by controlling beta cell electrical excitability. However, this role of Kv2.1 in human insulin secretion has been questioned. Interestingly, Kv2.1 can also regulate exocytosis through direct interaction of its C-terminus with the soluble NSF attachment receptor (SNARE) protein, syntaxin 1A. We hypothesised that this interaction mediates insulin secretion independently of Kv2.1 electrical function.

METHODS

Wild-type Kv2.1 or mutants lacking electrical function and syntaxin 1A binding were studied in rodent and human beta cells, and in INS-1 cells. Small intracellular fragments of the channel were used to disrupt native Kv2.1-syntaxin 1A complexes. Single-cell exocytosis and ion channel currents were monitored by patch-clamp electrophysiology. Interaction between Kv2.1, syntaxin 1A and other SNARE proteins was probed by immunoprecipitation. Whole-islet Ca(2+)-responses were monitored by ratiometric Fura red fluorescence and insulin secretion was measured.

RESULTS

Upregulation of Kv2.1 directly augmented beta cell exocytosis. This happened independently of channel electrical function, but was dependent on the Kv2.1 C-terminal syntaxin 1A-binding domain. Intracellular fragments of the Kv2.1 C-terminus disrupted native Kv2.1-syntaxin 1A interaction and impaired glucose-stimulated insulin secretion. This was not due to altered ion channel activity or impaired Ca(2+)-responses to glucose, but to reduced SNARE complex formation and Ca(2+)-dependent exocytosis.

CONCLUSIONS/INTERPRETATION

Direct interaction between syntaxin 1A and the Kv2.1 C-terminus is required for efficient insulin exocytosis and glucose-stimulated insulin secretion. This demonstrates that native Kv2.1-syntaxin 1A interaction plays a key role in human insulin secretion, which is separate from the channel's electrical function.

A method for measurement of fibrin monomer with the use of an immune precipitate of fibrinogen

A semiquantitative test for measuring fibrin monomer in human plasma is described. The test is based upon the ability of fibrin monomer to form a complex with an immune precipitate of fibrinogen. The test is not sensitive to the plasmin digestion products of fibrinogen and relatively insensitive to plasmin digestion products of fibrin. The test is easily performed on small quantities of plasma in approximately 2 hours.

Growth of murine cytomegalovirus in a heterologous cell system and its enhancement by 5-iodo-2'-deoxyuridine

Mouse cytomegalovirus replicated in rabbit kidney cultures, a cell system of nonrodent origin. However, the sensitivity of these cultures, and the yields of virus therefrom, were lower than those of mouse cultures. Although a cytopathic effect developed in rabbit kidney cultures inoculated with sufficient amounts of the virus, such cultures were unsatisfactory for plaque assay. This was also true when rabbit fibroblast cultures were used, even though the murine cytomegalovirus replicated much better in mouse fibroblasts than in mouse kidney cultures, the latter of which contained extensive areas of epithelial cells. Viral growth in rabbit kidney cells was considerably enhanced when those cells had been initiated and grown in the presence of 5-iodo-2'-deoxyuridine; not only were the viral titers increased, but also the clarity and distinctness of the inclusion bodies.

Sulfasymazine

Equine herpesvirus 1 replicated in the brains of 2-week-old mice but did not produce fatal encephalitis; it thus simulated the majority of cases of herpes simplex encephalitis in man. This replication was not inhibited by iododeoxyuridine, although in tissue cultures the equine and human viruses were equally susceptible. The continued use of iododeoxyuridine for human encephalitis should be seriously questioned.

Equine herpesviruses: antigenic relationships and deoxyribonucleic acid densities

Equine herpesviruses with a deoxyribonucleic acid density of 1.716 to 1.717 g/cm(3) were compared with one another by the plaque-reduction test and by the rate of development of cytopathic effect as indicated by plaque size in rabbit kidney cultures. Of the 19 isolates studied, the 9 which had already been tentatively labeled equine abortion viruses were serologically similar to one another; each of them grew more quickly than did any of the other 10 isolates although the mean plaque sizes formed a series of gradations with no clear hiatus which would permit the unequivocal delineation of the abortion viruses from the slowly growing strains. The 10 slowly growing isolates showed antigenic heterogeneity even though complement was present; the neutralizing capacity of an antiserum against the heterologous strains was, in most instances, markedly less than against the homologous strains, the range of the 50% endpoints being much greater than that observed among the equine abortion viruses, or among isolates of herpes simplex type 1. There was no cross neutralization between the equine abortion viruses and any of the 10 slowly growing isolates. An extra band of deoxyribonucleic acid, at 1.723 to 1.725 g/cm(3), was present in two of the slowly growing strains when originally grown in rabbit cells, but was no longer present after passage in cat cells. This band occupied the same position as one reported in the hamster-passaged strain of equine abortion virus, and had a density similar to that of the equine genital herpesvirus. Although the taxonomic demarcation of the equine abortion viruses and the slowly growing herpesviruses from one another is still open to question, they can be conveniently labeled equine herpesviruses 1 and 2, respectively; the genital virus would be termed equine herpesvirus 3.

Antigenic relationships among four herpesviruses

Common viral antigens were detected, by fluorescent-antibody studies, in cells infected with herpes simplex virus 1, squirrel monkey herpesvirus 1, bovine rhinotracheitis, and equine abortion viruses. The two primate viruses showed slight cross-neutralization.

Chronic infection of the rabbit central nervous system by a slowly growing equine herpesvirus

The spinal cords of rabbits were chronically infected by a slowly growing horse herpesvirus (a "cytomegalovirus") inoculated directly therein. Virus was recovered from the central nervous systems of some of such animals after more than 1 year. The virus could be reisolated from all the animals killed during the first few weeks after its injection; acute focal meningomyelitis was present with involvement of gray and white matter of the cervical, thoracic, and lumbar levels of the spinal cords of these rabbits, though the nerve cells themselves remained undamaged. Thereafter, reisolation of the virus became sporadic, and no damage to the spinal cord could be histologically discerned even in animals from which the virus was recovered. No paralytic or other clinical effects could be attributed to the infection.

Chronic Infections by Herpes Simplex Viruses and by the Horse and Cat Herpesviruses

Rabbit eyes experimentally infected with either type 1 or type 2 herpes simplex viruses occasionally released virus spontaneously. Injection of adrenalin was not highly effective for stimulating virus release but did seem to have a slight and erratic activating capacity. No spontaneous virus release were detected from the eyes of six cats infected with cat herpesvirus, but, when adrenalin was administered, an episode of virus release did ensue in one animal. Rabbit spinal cords could be chronically infected with either herpes simplex virus type 2 or equine herpesvirus type 2. The viruses could be reisolated over subsequent months from about half the animals without prior stimulation; the interval between inoculation of trypsinized spinal tissue into tissue cultures and the development of cytopathic effect was often long—more than 4 weeks in some cases.

Type 1 and type 2 herpes simplex viruses: serological and biological differences

Forty isolates of herpes simplex virus were compared by means of cross-neutralization curves. The 11 oral isolates were serotype 1, and all 29 genital/anal isolates were serotype 2. The cytopathic effects of the two serotypes were consistently different. Passage of strains of type 1 and type 2 in mice and in rabbits yielded two variants, although the majority of the strains remained unchanged serologically and in their cytopathic effects. The two variants were derived from type 1 strains and differed from the parent strains in their cytopathic effects, each of them producing syncytia and enlarged plaques. They had, however, retained the serotypic properties and the deoxyribonucleic acid (DNA) densities of their parent strains. The Roizman syncytial/macroplaque strain of herpes simplex virus was also included in the study; the density of its DNA (1.727 g/ml) was typical of type 1 strains, and serologically it seemed to be basically a type 1 strain, although it was neutralized by type 2 antiserum slightly better than were other type 1 strains. Growth curves were performed of the two serotypes in rabbit kidney, human fibroblast, and mouse embryo tissue cultures. The type 2 strains attained lower titers of infectivity in these three cell systems; the levels of infectivity of type 2 virus in the culture fluid decreased much more rapidly after the maximum had been attained than did the levels of infectivity of the type 1 strains, due to the greater instability of the type 2 virus. Parallel titrations of different strains in tissue cultures and intracerebrally in mice indicated that the latter assay system was usually more sensitive for type 2 strains than it was for type 1 strains. The paralytic sequelae and inflammatory changes of lumbar ganglia and spinal cord in young rabbits inoculated extraneurally with strains of the two serotypes also indicate that the type 2 virus is more virulent in laboratory animals than is type 1 virus.

Comparison of four horse herpesviruses

Four equine herpesviruses (equine abortion virus, equine herpesvirus types 2 and 3, and equine cytomegalovirus) were compared. The equine abortion virus did not cross-neutralize with any of the other viruses, but the other three did show varying degrees of cross-neutralization among themselves. Equine abortion virus grew more quickly in tissue cultures than did the others, and attained higher titers of infectivity in the culture fluid; it also formed plaques in a wider range of tissue culture species, although the other three were not specific for one tissue culture system only, in that they would multiply in rabbit and cat kidney cultures. The densities of the deoxyribonucleic acids of all four viruses were in the range 1.716 to 1.717 g/ml (a guanine plus cytosine content of 57 to 58%). Taxonomic separation, as a distinct serotype, of equine abortion virus from the other herpesviruses seems to be justified. The other three are closely related to one another. They should perhaps be regarded as separate viruses and termed horse herpesviruses types 2, 3, and 4, although an alternative view would be to regard them as variants of a single virus type. The question of whether types 2, 3, and 4, or any other herpesviruses, should be placed in a phylogenetically distinct subgroup, known as cytomegaloviruses, is a moot point.