Brain protein phosphorylation in vitro: selective substrate action of insulin

Intrahippocampal insulin improves memory in a passive-avoidance task in male wistar rats

The main impacts of insulin favor the peripheral organs. Although it functions as a neuropeptide, insulin possesses also some central effects. The aim of this study was to determine the effect of intrahippocampal infusion of insulin on passive avoidance learning in healthy male rats. Thirty male wistar rats were divided into three groups (n=10 each). The experimental group had posttraining insulin infusion into the CA1 region of dorsal hippocampus, after which they were compared with sham (saline) and control (intact) groups. Insulin treated animals had greater latency to enter the dark compartment in compare with saline treated (p=0.023) or control groups (p=0.017). Upon our results, we concluded that intrahippocampal injections of insulin may enhance memory for a simple learning task which supports the concept that insulin possibly plays an endogenous role in memory formation.

Cognitive effects of insulin in the central nervous system

Evidence has been accumulating recently that the hormone insulin may modulate cognitive activity by acting in the central nervous system. Initially derived from the observation that insulin and insulin receptors are found in specific brain areas, this evidence also includes cognitive assessments of humans in insulin-deficient and insulin-resistant disease states and experimental manipulation of rodent models. Additional support is derived from in vivo and in vitro systems that are used to investigate the neurophysiological basis of learning and memory. This article is a brief review of the literature that suggests a connection between insulin and memory and draws together some of the findings relevant to possible physiological mechanisms for this cognitive effect.

Insulin-like growth factor I modulates voltage-dependent Ca2+ channels in neuronal cells

Insulin and insulin-like growth factors are neuroactive peptides. We investigated the effect of insulin-like growth factor I (IGF-I) on Ca2+ channel currents in 108CC15 neuroblastoma x glioma (N x G) cells and a possible role of protein kinase C (PKC). Whereas the native IGF-I enhanced the Ca2+ channel current density in N x G cells, the boiled IGF-I had no effect. The effect of IGF-I occurred after 1-2 h incubation and reversed within 24 h. Ca2+ channel currents recorded in control cells were mainly of a low-threshold fast inactivating type and showed a mean density of 5.9 +/- 0.3 pA/pF. Current density in cells incubated with IGF-I (0.2 micrograms/ml) for 2 h increased to 9.2 +/- 0.8 pA/pF. Ca2+ channel currents in cells treated with IGF-I showed an enhanced amount of a high-threshold slowly inactivating Ca2+ current type sensitive to the dihydropyridine isradipine and the snail toxin omega-conotoxin. The effect of IGF-I was suppressed by coincubation with the PKC inhibitors 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H-7) and staurosporin which were both without effect on current density in control cells. Whereas the inactive phorbol ester phorbol 12-myristate 13-acetate (PMA) failed to modulate Ca2+ channels in N x G cells, stimulation of PKC by the active phorbol ester PMA mimicked the effect of IGF-I. The effects of IGF-I and phorbol ester were not additive. Our data suggest an intracellular mechanism dependent on PKC and we propose a physiological relevance of the observed Ca2+ channel modulation by IGF-I in the neuroactivity of the peptide.

Inhibitory effect of insulin and cytoplasmic factor(s) on brain (Na(+) + K+) ATPase

(Na+ + K+)ATPase activity in cerebral cortex was modulated by insulin action depending on the Mg2+ concentration. Thus, in homogenates in the presence of 1-3 mM Mg2+, insulin stimulated the enzyme, whereas in the presence of 4-6 mM Mg2+ inhibition was observed. Exposure of synaptosomal membranes to the soluble fraction resulted in inhibition of ATPase activity in a dose-dependent manner. The inhibitory effect of insulin was regulated by a cytoplasmic factor in a dose-dependent manner. Similar variations to those obtained with a crude synaptosomal fraction were obtained by using a partially purified ATPase. These results indicated the importance of soluble factors in the modulation of ATPase by insulin and add more evidence in support for a role of insulin as a neuromodulator.

Insulin stimulates membrane phospholipid metabolism by enhancing endogenous alpha 1-adrenergic activity in the rat hippocampus

Insulin stimulates membrane phospholipid metabolism and activates protein kinase C (PKC) in its peripheral target tissues. Additionally, insulin can stimulate PKC activity in cultured fetal chick neurons. In the present study, we tested whether insulin can stimulate membrane phospholipid metabolism in the rat hippocampus, a CNS region in which insulin has been reported to stimulate the phosphorylation of a PKC substrate protein and to suppress spontaneous electrical activity of pyramidal cells. Concentrations of 1, 10 and 100 nM insulin significantly stimulated the accumulation of [3H]inositol phosphate ([3H]IP1) and [3H]IP2 in hippocampal slices labelled with [3H]myoinositol. Significant (P less than 0.05) increases of hippocampal diacylglycerol (a product of phosphoinositol hydrolysis) content were observed at 1, 5 or 10 min of incubation with 50 or 100 nM insulin. Addition of tetrodotoxin resulted in a suppression of insulin stimulation of [3H]IP1 release, suggesting that insulin effects may be indirect and mediated via release of an endogenous neuronal transmitter within the hippocampus. Norepinephrine has been shown to both stimulate PI turnover and suppress the spontaneous electrical activity of pyramidal cells via alpha 1-adrenergic receptors. Therefore, we tested whether the effects of insulin were mediated by norepinephrine. We measured [3H]IP1 release in the presence or absence of the alpha 1-adrenergic antagonists prazobind and prazosin. These compounds blocked insulin stimulation of IP1 accumulation, suggesting that the action of insulin to stimulate PI turnover is secondary to enhancement of endogenous noradrenergic activity within the hippocampus.

Selective time-dependent effects of insulin on brain phosphoinositide metabolism

The effect of insulin on phosphoinositide metabolism in the cerebral cortex was examined using 32P as precursor. A maximal increase was detected as early as 15 s; phospholipid labeling declined after this initial peak but then increased to another maximum at 30 min. The levels of these phospholipids were unchanged at the earliest time examined, but at 30 min insulin caused an increase in the content of all phospholipids tested. In pulse-chase experiments, insulin stimulated depletion of 32P-labeled phosphoinositides only at 15 s. On the other hand, insulin treatment caused a biphasic diacyglycerol (DAG) production. We conclude that in cerebral cortex, insulin has a dual mechanism of action on phosphoinositide metabolism. First, insulin causes a rapid but transient hydrolysis of phosphoinositides by a phospholipase C-dependent mechanism, followed by subsequent resynthesis; thereafter, insulin increases de novo phospholipid synthesis.

Insulin stimulates inositol incorporation in hippocampus of lean but not obese Zucker rats

We have previously reported that intraventricular insulin is ineffective in decreasing the body weight of obese (fa/fa) Zucker rats. In the present study, we report that insulin at concentrations of 1 and 5 nM significantly stimulates incorporation of 3H-myoinositol into inositol phosphates (29 +/- 5% and 20 +/- 6% stimulation over control levels) and membrane inositol lipids (29 +/- 4% and 20 +/- 6% stimulation over control levels) within hippocampal slices from lean (Fa/Fa) Zucker rats (n = 3 preparations). Smaller but significant stimulations were observed in the Fa/fa Zucker rat (n = 5) hippocampus [10 +/- 3 and 13 +/- 4% stimulation over control levels (inositol phosphates) and 10 +/- 4 and 14 +/- 3% stimulation over control levels (inositol lipids) with 1 and 5 nM insulin]. Insulin had no effect on 3H-inositol incorporation into hippocampal slices from obese (fa/fa) Zucker rats (n = 5). No difference in insulin binding to hippocampal membranes was observed among the three genotypes. These findings confirm the behavioral observation of insulin insensitivity within the fa/fa Zucker CNS and suggest that this insensitivity may be gene-dose-related.

Apparent involvement of protein kinase C in the central glucoregulatory action of insulin

We have studied the possible involvement of the calcium- and phospholipid/diacylglycerol-dependent enzyme, protein kinase C (PKC) in mediating insulin action in the central nervous system (CNS) by testing the effect of direct activation or blockade of the CNS PKC system on the plasma glucose responses to central insulin injection in mice. Insulin (0.1-1 microgram), injected into the CNS, produced rapid transient hypoglycemia. This effect appeared to involve interaction of insulin with specific receptors, since insulin analogs exhibiting diminished receptor binding affinity and peripheral bioactivity compared to the native hormone were much less active (i.e., insulin much greater than acetyl 3 insulin greater than proinsulin greater than IGF-I) or not active at all (i.e., insulin chain A and chain B). Central injection of the specific PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) (0.01-0.5 microgram), but not the inactive TPA analog, 4-alpha-phorbol or the unstable synthetic diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), significantly enhanced the hypoglycemic response to co-administered insulin (0.5 microgram) or the insulin derivative, acetyl 3 insulin (2.5 micrograms). Central TPA had no effect on basal glucose levels. Furthermore, central administration of the selective PKC blockers, polymyxin B (PMB, 1-25 micrograms) or 1-beta-galactosylsphingosine (psychosine, 0.5-10 micrograms) but not their respective inactive analogs, polymyxin E and sphingomyelin, strongly inhibited the hypoglycemic response to insulin (1 microgram) or acetyl 3 insulin (5 micrograms). PMB and psychosine, injected alone had no effect on basal glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Human Y-79 retinoblastoma cells exhibit specific insulin receptors

The presence of insulin receptors was investigated in human Y-79 retinoblastoma cells grown in suspension culture. The binding of [125I] insulin to these cells was time, temperature, and pH dependent, was competed for by insulin and proinsulin but not other peptides, and was inhibited by antibodies against the insulin receptor. The Scatchard plot of insulin competition data was curvilinear and was resolved into a high-affinity (KD approximately 0.5 X 10(-9) M)/low-capacity (approximately 3,000 sites/cell) and a low-affinity (KD approximately 1 X 10(-7) M)/high-capacity (approximately 155,000 sites/cell) component. Negative cooperativity was not found, in agreement with other studies in rodent neural cells. However, in contrast to studies with rodent cells, insulin specifically down-regulated its receptor on human Y-79 cells after prolonged exposure. In conclusion, these data show for the first time the presence of specific insulin receptors in human Y-79 retinoblastoma cells. Because these cells were previously shown to have several characteristics typical of neural cells, we propose their use as a model to study the effects of insulin on neural and retinal tissues of human origin.

Protein kinase C activation leading to protein F1 phosphorylation may regulate synaptic plasticity by presynaptic terminal growth

It has recently been proposed by the author that protein kinase C regulates the expression of synaptic plasticity. In the present review it is suggested that this regulation involves a growth of presynaptic terminals. This proposal was based on the discovery that one of the substrates of protein kinase C, protein F1 (molecular mass = 47 kDa, pI = 4.5) is increased in its phosphorylation 5 min, 1 hr, and 3 days following long-term potentiation (LTP) in the intact hippocampal formation. No other phosphoprotein studied was altered by LTP. The amplitude or persistence of synaptic plasticity was directly related to the extent of protein F1 phosphorylation. As a critical control, it was shown that protein F1 was unaltered following synaptic activation that did not alter synaptic strength. Protein F1 in the hippocampus was also altered in its phosphorylation after an experience involving memory of a spatial environment. Phosphorylation F1 may thus participate in both neurophysiological and behavioral events that evoke plasticity. The identification of the F1 substrate has recently been sought. The physical characteristics of protein F1 (mol wt., isoelectric point) indicate that it is the same as the B-50 protein and the growth protein, GAP-43. Protein F1 is then a brain-specific, synaptically enriched phosphoprotein. Recent evidence indicates that protein F1 is present in high concentration in growth cones of late embryonic rat brain in which postsynaptic specializations are not detected, suggesting a presynaptic locus. With respect to the identity of the F1 kinase, we have shown that protein F1, like B-50, is a substrate for protein kinase C, a Ca2+/phospholipid-dependent kinase. Activation of this enzyme by tumor-promoting phorbol esters can trigger cell growth and neurite extension. Recent evidence indicates a presynaptic localization of the enzyme. On the basis of the colocalization of enzyme and substrate in the presynaptic terminal it is proposed that protein kinase C control of the phosphorylation state of protein F1 may regulate the expression of synaptic plasticity via presynaptic terminal growth.

Protein kinase C phosphorylates a 47 Mr protein (F1) directly related to synaptic plasticity

Ca2+-phospholipid-dependent protein kinase C, and activators of protein kinase C (phosphatidylserine, phorbol esters) stimulate the in vitro phosphorylation of a 47 kdalton phosphoprotein (protein F1) previously shown (Routtenberg, Lovinger and Steward, Behav. neural Biol., 43 (1985) 3-11) to be directly related to the plasticity of long-term potentiation. These data indicate that protein F1 serves as a protein kinase C substrate, and suggest the hypothesis that protein kinase C is involved in processes of long-term potentiation.