Insulin-degrading enzyme regulates extracellular levels of amyloid beta-protein by degradation

The effect of insulin and glucose on the plasma concentration of Alzheimer's amyloid precursor protein

The deposition of beta amyloid is a critical event in the pathogenesis of Alzheimer's disease. This peptide is a metabolite of the amyloid precursor protein. Recent research suggests that there is a correlation between plasma insulin and glucose concentrations and memory performance in Alzheimer's disease sufferers. Additionally, in vitro evidence suggests that both insulin and glucose may affect the metabolism of amyloid precursor protein and therefore the production of beta amyloid--however, to our knowledge no in vivo data have yet been published. We investigated the effect of elevated plasma levels of glucose and insulin on the plasma concentration of amyloid precursor protein in non-Alzheimer's disease subjects. As would be expected following ingestion of a glucose drink, blood insulin and glucose levels significantly increased. Interestingly, however, plasma amyloid precursor protein concentration decreased. Whilst no correlation was observed between insulin or glucose levels and plasma amyloid precursor protein concentration, the decrease in plasma amyloid precursor protein concentration was affected by the apolipoprotein E genotype of the subject. Possession of an epsilon4 allele resulted in a reduced decrease in plasma amyloid precursor protein in response to glucose ingestion when compared to non-epsilon4 subjects. We conclude that glucose ingestion, and the subsequent elevation of plasma levels of glucose and insulin leads to a decrease in plasma amyloid precursor protein concentration. Further studies are required to determine the clinical significance of these physiological changes in plasma amyloid precursor protein and the implications for Alzheimer's disease pathogenesis.

Identification of the major Abeta1-42-degrading catabolic pathway in brain parenchyma: suppression leads to biochemical and pathological deposition

Alzheimer amyloid beta-peptide (Abeta) is a physiological peptide constantly anabolized and catabolized under normal conditions. We investigated the mechanism of catabolism by tracing multiple-radiolabeled synthetic peptide injected into rat hippocampus. The Abeta1-42 peptide underwent full degradation through limited proteolysis conducted by neutral endopeptidase (NEP) similar or identical to neprilysin as biochemically analyzed. Consistently, NEP inhibitor infusion resulted in both biochemical and pathological deposition of endogenous Abeta42 in brain. This NEP-catalyzed proteolysis therefore limits the rate of Abeta42 catabolism, up-regulation of which could reduce the risk of developing Alzheimer's disease by preventing Abeta accumulation.

Degradation of soluble amyloid beta-peptides 1-40, 1-42, and the Dutch variant 1-40Q by insulin degrading enzyme from Alzheimer disease and control brains

Insulin degrading enzyme (IDE) is a metalloprotease that has been involved in amyloid beta peptide (A(beta)) degradation in the brain. We analyzed the ability of human brain soluble fraction to degrade A(beta) analogs 1-40, 1-42 and the Dutch variant 1-40Q at physiological concentrations (1 nM). The rate of synthetic 125I-A(beta) degradation was similar among the A(beta) analogs, as demonstrated by trichloroacetic acid precipitation and SDS-PAGE. A 110 kDa protein, corresponding to the molecular mass of IDE, was affinity labeled with either 125I-insulin, 125I-Abeta 1-40 or 125I-A(beta) 1-42 and both A(beta) degradation and cross-linking were specifically inhibited by an excess of each peptide. Sensitivity to inhibitors was consistent with the reported inhibitor profile of IDE. Taken together, these results suggested that the degradation of A(beta) analogs was due to IDE or a closely related protease. The apparent Km, as determined using partially purified IDE from rat liver, were 2.2 +/- 0.4, 2.0 +/- 0.1 and 2.3 +/- 0.3 microM for A(beta) 1-40, A(beta) 1-42 and A(beta) 1-40Q, respectively. Comparison of IDE activity from seven AD brain cytosolic fractions and six age-matched controls revealed a significant decrease in A(beta) degrading activity in the first group, supporting the hypothesis that a reduced IDE activity may contribute to A(beta) accumulation in the brain.

Subcommissural organ-Reissner's fiber complex of the teleost Clarias batrachus responds to GABA treatment

Subcommissural organ (SCO) is a highly specialized ependymal gland located in the roof of the third ventricle. The secretory products of the SCO, which condense to form Reissner's fiber (RF), were recently found to cross-react with the anti-calcitonin antibody. To understand the mechanisms regulating the formation of the RF and the possible function of these discrete structures, we studied the response of the SCO-RF complex to intracranially administered GABA, using immunocytochemical labeling with anti-calcitonin antibody. Although the SCO-RF complex of control fish was intensely immunostained, 1 h after GABA treatment, the ependymal cells revealed partial loss of immunoreactivity; the RF showed occasional loss of immunoreactivity with its diameter increased by about 56% of the control value. Following 2 h of GABA treatment, the SCO revealed dramatic loss of calcitonin-like immunoreactivity from the ependymal cells. The RF showed a dual response in this group, while in some segments the RF appeared conspicuously thick, elsewhere it appeared thin. The mean diameter was, however, not significantly different from the normal. Following 4 h of GABA treatment, while calcitonin-like immunoreactive material made its reappearance in the SCO, the RF diameter was uniformly reduced to about 35% of the control value. The responses by the RF as well as the SCO to intracranially administered GABA were blocked by pretreatment with bicuculline, a GABA(A) receptor antagonist. The results suggest that GABA, acting via GABA(A) receptors, may trigger the release of secretory material from the SCO and induce histomorphological changes in the RF indicative of discharge of stored material.

The amyloid beta-protein precursor of Alzheimer's disease is degraded extracellularly by a Kunitz protease inhibitor domain-sensitive trypsin-like serine protease in cultures of chick sympathetic neurons

The amyloid beta-protein precursor (APP) of Alzheimer's disease (AD) is cleaved either by alpha-secretase to generate an N-terminally secreted fragment, or by beta- and gamma-secretases to generate the beta-amyloid protein (Abeta). The accumulation of Abeta in the brain is an important step in the pathogenesis of AD. Alternative mRNA splicing can generate isoforms of APP which contain a Kunitz protease inhibitor (KPI) domain. However, little is known about the physiological function of this domain. In the present study, the metabolic turnover of APP was examined in cultured chick sympathetic neurons. APP was labelled by incubating neurons for 5 h with [35S]methionine and [35S]cysteine. Intracellular labelled APP decayed in a biphasic pattern suggesting that trafficking occurs through two metabolic compartments. The half-lives for APP in each compartment were 1.5 and 5.7 h, respectively. A small fraction (10%) of the total APP was secreted into the culture medium where it was degraded with a half-life of 9 h. Studies using specific protease inhibitors demonstrated that this extracellular breakdown was due to cleavage by a trypsin-like serine protease that was secreted into the culture medium. Significantly, this protease was inhibited by a recombinant isoform of APP (sAPP751), which contains a region homologous to the Kunitz protease inhibitor (KPI) domain. These results suggest that KPI forms of APP regulate extracellular cleavage of secreted APP by inhibiting the activity of a secreted APP-degrading protease.

The insulin receptor tyrosine kinase substrate p58/53 and the insulin receptor are components of CNS synapses

The synapse is the primary locus of cell-cell communication in the nervous system. It is now clear that the synapse incorporates diverse cell signaling modalities in addition to classical neurotransmission. Here we show that two components of the insulin pathway are localized at CNS synapses, where they are components of the postsynaptic density (PSD). An immunochemical screen revealed that polypeptides of 58 and 53 kDa (p58/53) were highly enriched in PSD fractions from rat cerebral cortex, hippocampus, and cerebellum. These polypeptides were purified and microsequenced, revealing that p58/53 is identical to the insulin receptor tyrosine kinase substrate p58/53 (IRSp53). Our analysis of IRSp58/53 mRNA suggests that within rat brain there is one coding region for IRSp58 and IRSp53; we find no evidence of alternative splicing. We demonstrate that IRSp58/53 is expressed in the synapse-rich molecular layer of the cerebellum and is highly concentrated at the synapses of cultured hippocampal neurons, where it co-localizes with the insulin receptor. Together, these data suggest that insulin signaling may play a role at CNS synapses.

Insulin metabolism in Alzheimer's disease differs according to apolipoprotein E genotype and gender

Higher fasting plasma insulin levels and reduced CSF-to-plasma insulin ratios, suggestive of insulin resistance, have been observed in patients with Alzheimer's disease (AD) who do not possess an apolipoprotein E (APOE)-epsilon4 allele. We examined the relationship of APOE and gender to peripheral insulin action and hyperinsulinemic memory facilitation in patients with AD using a sensitive measure of insulin-mediated glucose disposal. Participants were 32 patients with AD (9 without an epsilon4 allele, 23 with an epsilon4 allele) and 25 healthy age-matched adults (16 without an epsilon4 allele, 9 with an epsilon4 allele). AD subjects without an epsilon4 allele had significantly lower insulin-mediated glucose disposal rates than AD patients with an epsilon4 allele (p < 0.03), or than normal adults without an epsilon4 allele (p < 0.02). Female AD subjects showed lower insulin-mediated glucose disposal rates than did male AD subjects (p < 0.02). No significant interaction was observed between APOE group and gender, suggesting that these effects are independent. AD subjects without an epsilon4 allele also showed significant memory facilitation in the hyperinsulinemic condition (p < 0.04), whereas the AD-epsilon4 group did not. Also in the hyperinsulinemic condition, AD patients without an epsilon4 allele had lower insulin levels than patients with an epsilon4 allele (p < 0.02), and women with AD had lower insulin levels than did men with AD despite similar insulin infusion rates and body mass (p < 0.004). No gender or genotype effects were observed in either condition for normal subjects. These results provide in vivo evidence of differences in insulin-mediated energy metabolism between epsilon4 and non-epsilon4 AD, and suggest that defective insulin action may be of particular pathophysiologic significance for patients without an epsilon-4 allele.

Metalloendopeptidase EC 3.4.24.15 is necessary for Alzheimer's amyloid-beta peptide degradation

We have investigated the functional relationship between metalloendopeptidase EC 3.4.24.15 (MP24.15) and the amyloid precursor protein involved in Alzheimer's disease (AD) and discovered that the enzyme promotes Abeta degradation. We show here that conditioned medium (CM) of MP24.15 antisense-transfected SKNMC neuroblastoma has significantly higher levels of Abeta. Furthermore, synthetic-Abeta degradation was increased or decreased following incubation with CM of sense or antisense-transfected cells, respectively. Soluble Abeta1-42 was degraded more slowly than soluble Abeta1-40, while aggregated Abeta1-42 showed almost no degradation. Pretreatment of CM with serine proteinase inhibitors 4-(2-aminoethyl)benzenesulfonyl fluoride and diisopropyl fluorophosphate completely inhibited Abeta degradation. Additionally, alpha1-antichymotrypsin (ACT), a serpin family inhibitor tightly associated with plaques and elevated in AD brain, blocked up to 60% of Abeta degradation. Interestingly, incubation of CM of MP24. 15-overexpressing cells with ACT formed an SDS-resistant ACT complex, suggesting an ACT-serine proteinase interaction. Recombinant MP24. 15 alone did not degrade Abeta. 14C-Diisopropyl fluorophosphate-radiolabeled CM from MP24.15-overexpressing cells contained increased levels of several active serine proteinases, suggesting that MP24.15 activates one or more Abeta-degrading serine proteases. Thus, ACT may cause Abeta accumulation by inhibiting an Abeta-degrading enzyme or by direct binding to Abeta, rendering it degradation-resistant. Identification of the Abeta-degrading enzyme and MP24.15's role in its activation is underway. Pharmacological modulation of either enzyme may provide a means of regulating Abeta in the brain.

Insulin-degrading enzyme in the Alzheimer's disease brain: prominent localization in neurons and senile plaques

The anatomical distribution of insulin-degrading enzyme (IDE) was studied in normal and Alzheimer's disease (AD) human brains. By use of a monospecific, polyclonal antiserum against the enzyme we identified IDE antigen in multiple cortical and subcortical neurons. Glia did not show IDE immunoreactivity. In AD brains immunostaining appeared stronger than in controls and appeared not only in neurons but also in senile plaques. In a probable case of Lewy body variant of AD Lewy bodies in neurons of the Nuc. basalis of Meynert were immunopositive for IDE. Our anatomical data suggest that the enzyme is associated with typical neuropathologic hallmarks of AD and its expression appears up-regulated in some brain areas.