The SAMP8 mouse: a model to develop therapeutic interventions for Alzheimer's disease

The senescence accelerate mouse P8 (SAMP8) is an excellent model of early learning and memory problems. A number of studies have shown that it has cholinergic deficits, oxidative damage, alterations in membrane lipids and circadian rhythm disturbances. The brains of the SAMP8 overproduce amyloid precursor protein (APP), amyloid-beta protein and have an increased physphorylation of tau. An antisense to APP has been developed that reverses the cognitive deficits and oxidative damage. This antisense represents a potential treatment for Alzheimer's disease.

Somatostatin receptor subtype-4 agonist NNC 26-9100 decreases extracellular and intracellular Aβ₁₋₄₂ trimers

Soluble amyloid β-protein (Aβ) oligomers are primary mediators of synaptic dysfunction associated with the progression of Alzheimer's disease. Such Aβ oligomers exist dependent on their rates of aggregation and metabolism. Use of selective somatostatin receptor-subtype agonists have been identified as a potential means to mitigate Aβ accumulation in the brain, via regulation of the enzyme neprilysin. Herein, we first evaluated the impact of the somatostatin receptor subtype-4 agonist 1-[3-[N-(5-Bromopyridin-2-yl)-N-(3,4-dichlorobenzyl)amino]propyl]-3-[3-(1H-imidazol-4-yl)propyl]thiourea (NNC 26-9100) on learning and memory in 12-month SAMP8 mice (i.c.v. injection). NNC 26-9100 (0.2 μg-dose) was shown to enhance both learning (T-maze) and memory (object recognition) compared to vehicle controls. Cortical and hippocampal tissues were evaluated subsequent to NNC 26-9100 (0.2 μg) or vehicle administration for changes in neprilysin activity, along with protein expression of amyloid-precursor protein (APP), neprilysin, and Aβ₁₋₄₂ oligomers within respective cellular fractions (extracellular, intracellular and membrane). NNC 26-9100 increased neprilysin activity in cortical tissue, with an associated protein expression increase in the extracellular fraction and decreased in the intracellular fraction. A decrease in intracellular APP expression was found with treatment in both cortical and hippocampal tissues. NNC 26-9100 also significantly decreased expression of Aβ₁₋₄₂ trimers within both the extracellular and intracellular cortical fractions. No expression changes were found in membrane fractions for any protein. These finding suggest the potential use of selective SSTR4 agonists to mitigate toxic oligomeric forms of Aβ₁₋₄₂ in critical regions of the brain identified with learning and memory decline.

Impairments in brain-to-blood transport of amyloid-β and reabsorption of cerebrospinal fluid in an animal model of Alzheimer's disease are reversed by antisense directed against amyloid-β protein precursor

The blood-brain barrier (BBB) influences brain levels of amyloid-β (Aβ) by transporting Aβ out of the brain (efflux) and by the reabsorption of cerebrospinal fluid (CSF) into the blood stream (bulk flow). In Alzheimer's disease (AD) and normal aging, unknown factors impair Aβ efflux and bulk flow in aging and in AD. These impairments have been proposed as mechanisms by which the Aβ burden in brain can increase. Impairment in Aβ efflux occurs in animal models of AD, including the aged SAMP8 mouse. Here, we show that CSF reabsorption is also reduced by about 50% in SAMP8 mice (p < 0.05). We then determined whether an antisense directed at the Aβ region of the amyloid-β protein precursor (AβPP) and previously shown to decrease brain levels of AβPP and to reverse the cognitive impairments of the SAMP8 mouse was able to reverse these impairments. We found that the antisense restored both the CSF reabsorption, more than doubling the rate of efflux, and the saturable efflux of Aβ. These findings suggest that AβPP/Aβ itself contributes to the impairments in bulk flow and saturable efflux of Aβ and that reduction of AβPP/Aβ levels can restore normal function of the BBB.

Nitric oxide is a central component in neuropeptide regulation of appetite

In recent years, there have been a large number of neuropeptides discovered that regulate food intake. Many of these peptides regulate food intake by increasing or decreasing nitric oxide (NO). In the current study, we compared the effect of the food modulators ghrelin, NPY and CCK in NOS KO mice. Satiated homozygous and heterozygous NOS KO mice and their wild type controls were administered ghrelin ICV. Food intake was measured for 2h post injection. Ghrelin did not increase food intake in the homozygous NOS KO mice compared to vehicle treated NOS KO mice, whereas food intake was increased in the wild type controls compared to vehicle treated wild type controls. NPY was administered ICV and food intake measured for 2h. Homozygous NOS KO mice showed no increase in food intake after NPY administration, whereas the wild type controls did. In our final study, we administered CCK intraperitoneally to homozygous and heterozygous NOS KO mice and their wild type controls after overnight food deprivation. Food intake was measured for 1h after injection. CCK inhibited food intake in wild type mice after overnight food deprivation, however, CCK failed to inhibit food intake in the NOS KO mice. The heterozygous mice showed partial food inhibition after the CCK. The current results add further support to the theory that NO is a central mediator in food intake.

Chronic peripheral administration of somatostatin receptor subtype-4 agonist NNC 26-9100 enhances learning and memory in SAMP8 mice

Selective somatostatin receptor subtype agonists have been proposed as a means to mitigate learning and memory loss associated with Alzheimer's disease. The first aim of this study evaluated blood-to-brain transport and regional brain distribution of NNC 26-9100, a selective somatostatin subtype-4 (sst4) receptor agonist. The entry rate of (131)I-NNC 26-9100 was K(i)=0.25 μl/g min, with an ~93% association with the parenchymal component. The second goal of this study was to evaluate the effect of chronic NNC 26-9100 administration (i.p.) on learning and memory, brain Aβ(x-42) levels, and protein expression of sst4 receptor and amyloid precursor protein (APP) in the senescence-accelerated mouse p8 (SAMP8) model of Alzheimer's disease. Mice chronically treated with NNC 26-9100 showed improved learning (day 21) and memory (day 28) using the T-maze paradigm (20 and 200 μg). Ex vivo tissue analyses showed a decline in Aβ(x-42) levels at the 20 μg dose, while no alterations were observed in sst4 receptor or APP protein expression compared to vehicle controls. These findings indicate NNC 26-9100 is taken up into key brain regions associated with learning and memory. Furthermore, chronic administration of NNC 26-9100 improved learning and memory and decreased Aβ(x-42) brain levels. These results suggest sst4 receptor agonists may provide a viable therapy in the treatment of Alzheimer's disease and other forms of cognitive impairment.

Brain distribution and behavioral effects of progesterone and pregnenolone after intranasal or intravenous administration

Neurosteroids hold great promise for the treatment of diseases of the central nervous system (CNS). We compared the uptake by 11 brain regions and appearance in blood of tritium-labeled pregnenolone and progesterone after intranasal and intravenous (IV) injection. Both neurosteroids appeared in blood and brain after either method of administration, but with important differences in uptake. Bioavailability based on appearance in arterial serum showed that about 23% and 14% of the intranasal administered doses of pregnenolone and progesterone, respectively, entered the blood. Brain levels were about two fold lower after intranasal administration for the two neurosteroids. With intranasal administration, brain levels of the two steroids did not vary over time (2-120 min), whereas brain levels were higher early (10 min or less) after i.v. administration. With i.v. administration, uptake by brain regions did not vary, whereas the olfactory bulb, hippocampus, and hypothalamus had high uptake rates after intranasal administration. Intranasal administration of prenenolone improved memory, whereas progesterone decreased anxiety, thus demonstrating that therapeutic levels of neurosteroids can be delivered to the brain by intranasal administration. The neurosteroids were rapidly degraded after i.v. or intranasal delivery, but pregnenolone was more resistant to degradation in the brain after intranasal administration and in serum after i.v. administration. These results show that either the i.v. or intranasal routes of administration can deliver neurosteroids to blood and brain, but that the two routes have significant differences with intranasal administration favoring some brain regions.

A physiological role for amyloid-beta protein:enhancement of learning and memory

Amyloid-beta protein (Abeta) is well recognized as having a significant role in the pathogenesis of Alzheimer's disease (AD). The reason for the presence of Abeta and its physiological role in non-disease states is not clear. In these studies, low doses of Abeta enhanced memory retention in two memory tasks and enhanced acetylecholine production in the hippocampus in vivo. We then tested whether endogenous Abeta has a role in learning and memory in young, cognitively intact mice by blocking endogenous Abeta in healthy 2-month-old CD-1 mice. Blocking Abeta with antibody to Abeta or DFFVG (which blocks Abeta binding) or decreasing Abeta expression with antisense directed at the Abeta precursor, AbetaPP, all resulted in impaired learning in T-maze foot-shock avoidance. Finally, Abeta 1-42 facilitated induction and maintenance of long term potentiation in hippocampal slices, whereas antibodies to Abeta inhibited hippocampal LTP. In conclusion, these results indicate that in normal healthy young animals the presence of Abeta is important for learning and memory.

Transport across the blood-brain barrier of pluronic leptin

Leptin is a peptide hormone produced primarily by adipose tissue that acts as a major regulator of food intake and energy homeostasis. Impaired transport of leptin across the blood-brain barrier (BBB) contributes to leptin resistance, which is a cause of obesity. Leptin as a candidate for the treatment of this obesity is limited because of the short half-life in circulation and the decreased BBB transport that arises in obesity. Chemical modification of polypeptides with amphiphilic poly(ethylene oxide)-poly(propylene oxide) block copolymers (Pluronic) is a promising technology to improve efficiency of delivery of polypeptides to the brain. In the present study, we determined the effects of Pluronic P85 (P85) with intermediate hydrophilic-lipophilic balance conjugated with leptin via a degradable SS bond [leptin(ss)-P85] on food intake, clearance, stability, and BBB uptake. The leptin(ss)-P85 exhibited biological activity when injected intracerebroventricularly after overnight food deprivation and 125I-leptin(ss)-P85 was stable in blood, with a half-time clearance of 32.3 min (versus 5.46 min for leptin). 125I-Leptin(ss)-P85 crossed the BBB [blood-to-brain unidirectional influx rate (K(i)) = 0.272 +/- 0.037 microl/g x min] by a nonsaturable mechanism unrelated to the leptin transporter. Capillary depletion showed that most of the 125I-leptin(ss)-P85 taken up by the brain reached the brain parenchyma. Food intake was reduced when 3 mg of leptin(ss)-P85 was administered via tail vein in normal body weight mice [0-30 min, p < 0.0005; 0-2 h, p < 0.001]. These studies show that the structure based Pluronic modification of leptin increased metabolic stability, reduced food intake, and allowed BBB penetration by a mechanism-independent BBB leptin transporter.

Testing the neurovascular hypothesis of Alzheimer's disease: LRP-1 antisense reduces blood-brain barrier clearance, increases brain levels of amyloid-beta protein, and impairs cognition

Decreased clearance is the main reason amyloid-beta protein (Abeta) is increased in the brains of patients with Alzheimer's disease (AD). The neurovascular hypothesis states that this decreased clearance is caused by impairment of low density lipoprotein receptor related protein-1 (LRP-1), the major brain-to-blood transporter of Abeta at the blood-brain barrier (BBB). As deletion of the LRP-1 gene is a lethal mutation, we tested the neurovascular hypothesis by developing a cocktail of phosphorothioate antisenses directed against LRP-1 mRNA. We found these antisenses in comparison to random antisense selectively decreased LRP-1 expression, reduced BBB clearance of Abeta42, increased brain levels of Abeta42, and impaired learning ability and recognition memory in mice. These results support dysfunction of LRP-1 at the BBB as a mechanism by which brain levels of Abeta could increase and AD would be promoted.

Nitric oxide activity and isoenzyme expression in the senescence-accelerated mouse p8 model of Alzheimer's disease: effects of anti-amyloid antibody and antisense treatments

Amyloid beta protein (Abeta) in Alzheimer's disease induces oxidative stress through several mechanisms, including stimulation of nitric oxide synthase (NOS) activity. We examined NOS activity and expression in the senescence-accelerated mouse P8 (SAMP8) line. The SAMP8 strain develops with aging cognitive impairments, increases in Abeta, and oxidative stress, all reversed by amyloid precursor protein antisense or Abeta antibody treatment. We found here that hippocampal NOS activity in 12-month-old SAMP8 mice was nearly double that of 2-month-old SAMP8 or CD-1 mice, but with no change in NOS isoenzyme mRNA and protein levels. Antisense or antibody treatment further increased NOS activity in aged SAMP8 mice. Antisense treatment increased inducible NOS (iNOS) mRNA levels, decreased neuronal NOS mRNA and protein levels, but did not affect endothelial NOS (eNOS) or iNOS protein or eNOS mRNA levels. These results suggest a complex relation between Abeta and NOS in the SAMP8 that is largely mediated through posttranslational mechanisms.

Increase in presenilin 1 (PS1) levels in senescence-accelerated mice (SAMP8) may indirectly impair memory by affecting amyloid precursor protein (APP) processing

Senescence-accelerated mice (SAMP8) serve as a model for Alzheimer's disease (AD) as they exhibit early loss of memory and increased amyloid precursor protein (APP) expression. APP is a ubiquitous membrane protein that is physiologically processed by site-specific proteolysis firstly by alpha- or beta-secretases, releasing a large fragment called APP(S) that contains most of the extracellular sequences of APP, a small extracellular stub, the transmembrane region and the cytoplasmic tail of APP (;AICD'-APP intracellular domain). These are subsequently cleaved by gamma-secretase at multiple sites in the transmembrane region, releasing small peptides, Abeta(1-40) and Abeta(1-42), the major components of AD-associated amyloid fibrils. gamma-secretase is a high-molecular-mass complex composed of presenilin-1 (PS1), nicastrin, APH-1 and Pen-2. As PS1 has been shown to play a critical role in facilitating gamma-secretase activity, and mutations in this protein are associated with familial AD (FAD), we have cloned it from SAMP8 mouse hippocampus and compared its sequence with those of other species. Furthermore, changes in the expression of PS1 with age in the hippocampal tissue of SAMP8 were studied. The results showed that the SAMP8 PS1 cDNA sequence is identical to that of normal mice. However, its expression in the hippocampus of SAMP8 exhibited an increase, while CD-1 mice, a strain that does not exhibit premature memory loss, showed no change with age. An increased amount or mutation(s) in PS1, which alters the stoichiometric balance of the gamma-secretase complex, may be the cause of aberrant or increased processing of APP, resulting in Abeta accumulation leading to loss of memory.

Isolation of peptide transport system-6 from brain endothelial cells: therapeutic effects with antisense inhibition in Alzheimer and stroke models

By isolating for the first time ever a peptide transporter from the blood-brain barrier (BBB) and developing an antisense that selectively targets the brain-to-blood efflux component, we were able to deliver a therapeutic concentration of the neurotrophic peptide pituitary adenylate cyclase-activating polypeptide (PACAP) 27 to brain in animal models of Alzheimer's and stroke. Efflux pumps at the BBB are major causes of BBB impermeability to peptides. PACAP is neuroprotective in vitro in femtomole amounts, but brain uptake of PACAP27 is limited by an efflux component of peptide transport system-6 (PTS-6). Here, we characterized, isolated, and sequenced this component of PTS-6, identifying it as beta-F1 ATPase, and colocalized it with PACAP27 on BBB endothelial cells. Antisenses targeting the BBB inhibited PACAP27 efflux, thus increasing brain uptake of PACAP27. Treatment with antisense+PACAP27 improved cognition in a mouse model of Alzheimer's disease and reduced infarct size after cerebral ischemia. This represents the first isolation from BBB tissue of a peptide transporter and shows that inhibition of peptide efflux pumps is a potential strategy for drug delivery to brain.

Effect of dietary n-3 polyunsaturated fatty acids on brain lipid fatty acid composition, learning ability, and memory of senescence-accelerated mouse

Animal studies have shown that a deficiency in brain of the n-3 polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA) is associated with memory loss and diminished cognitive function. The senescence-accelerated prone 8 (SAMP8) mouse develops impairments in learning and memory at 8-12 months of age. The effect of diet supplemented with n-3 PUFA on brain phospholipid DHA status, learning, and memory ability in aged SAMP8 mice was investigated. At the age of 10 months, SAMP8 mice were fed either a low-DHA or a high-DHA diet for 8 weeks. In comparison to SAMP8 mice fed the low-DHA diet, those fed a high-DHA diet had improved acquisition and retention in a T-maze foot shock avoidance test and a higher proportion of DHA in hippocampal and amygdala phospholipids. This study demonstrates that, in mature animals, DHA is incorporated into brain phospholipids and that dietary n-3 PUFA is associated with delay in cognitive decline.

Obesity and hypertriglyceridemia produce cognitive impairment

Obesity is associated with cognitive impairments. Long-term mechanisms for this association include consequences of hyperglycemia, dyslipidemia, or other factors comprising metabolic syndrome X. We found that hypertriglyceridemia, the main dyslipidemia of metabolic syndrome X, is in part responsible for the leptin resistance seen in obesity. Here we determined whether triglycerides have an immediate and direct effect on cognition. Obese mice showed impaired acquisition in three different cognitive paradigms: the active avoidance T-maze, the Morris water maze, and a food reward lever press. These impairments were not attributable to differences in foot shock sensitivity, swim speed, swimming distance, or voluntary milk consumption. Impaired cognition in obese mice was improved by selectively lowering triglycerides with gemfibrozil. Injection into the brain of the triglyceride triolein, but not of the free fatty acid palmitate, impaired acquisition in normal body weight mice. Triolein or milk (97% of fats are triglycerides), but not skim milk (no triglycerides), impaired maintenance of the N-methyl-d-aspartate component of the hippocampal long-term synaptic potential. Measures of oxidative stress in whole brain were reduced by gemfibrozil. We conclude that triglycerides mediate cognitive impairment as seen in obesity, possibly by impairing maintenance of the N-methyl-d-aspartate component of hippocampal long-term potentiation, and that lowering triglycerides can reverse the cognitive impairment and improve oxidative stress in the brain.

Delivery of galanin-like peptide to the brain: targeting with intranasal delivery and cyclodextrins

Galanin-like peptide (GALP) shows potential as a therapeutic in the treatment of obesity and related conditions. In this study, we compared the uptake by brain regions and peripheral tissues of radioactively iodinated GALP (I-GALP) after intranasal (i.n.), i.v., and i.c.v. administration. I-GALP was stable in blood and brain during the 10-min study time regardless of route of administration, and similar levels were achieved in cerebrospinal fluid after i.v. and i.n. administration. However, levels in most brain regions were approximately 4 to 10 times higher and uptake by spleen, representative of peripheral tissues, approximately 10% as high after i.n. than i.v. administration. Thus, i.n. administration provided about a 40- to 100 fold improvement in targeting brain versus peripheral tissues compared with i.v. administration. Uptake of I-GALP by whole brain after i.n. administration was inhibited by approximately 50% by 1 mug/mouse of unlabeled GALP, thus demonstrating a saturable component to uptake. Combining I-GALP with cyclodextrins increased brain uptake approximately 3-fold. Selectivity for brain region uptake was also seen with route of administration and with use of cyclodextrins. The hippocampus had the greatest uptake after i.c.v. administration, the cerebellum after i.v. administration, the hypothalamus with i.n. administration without cyclodextrins, the hypothalamus and olfactory bulb (OB) after i.n. administration with alpha-cyclodextrin, and the OB after i.n. administration with dimethyl-beta cyclodextrin. These studies show that intranasal administration is an effective route of administration for the delivery of GALP to the brain and that targeting among brain regions may be possible with the use of various cyclodextrins.

Anti-amyloid beta protein antibody passage across the blood-brain barrier in the SAMP8 mouse model of Alzheimer's disease: an age-related selective uptake with reversal of learning impairment

Amyloid beta protein (Abeta) levels are elevated in the brain of Alzheimer's disease patients. Anti-Abeta antibodies can reverse the histologic and cognitive impairments in mice which overexpress Abeta. Passive immunization appears safer than vaccination and treatment of patients will likely require human rather than xenogenic antibodies. Effective treatment will likely require antibody to cross the blood-brain barrier (BBB). Unfortunately, antibodies typically cross the BBB very poorly and accumulate less well in brain than even albumin, a substance nearly totally excluded from the brain. We compared the ability of two anti-Abeta human monoclonal IgM antibodies, L11.3 and HyL5, to cross the BBB of young CD-1 mice to that of young and aged SAMP8 mice. The SAMP8 mouse has a spontaneous mutation that induces an age-related, Abeta-dependent cognitive deficit. There was preferential uptake of intravenously administered L11.3 in comparison to HyL5, albumin, and a control human monoclonal IgM (RF), especially by hippocampus and olfactory bulb in aged SAMP8 mice. Injection of L11.3 into the brains of aged SAMP8 mice reversed both learning and memory impairments in aged SAMP8 mice, whereas IgG and IgM controls were ineffective. Pharmacokinetic analysis predicted that an intravenous dose 1000 times higher than the brain injection dose would reverse cognitive impairments. This predicted intravenous dose reversed the impairment in learning, but not memory, in aged SAMP8 mice. In conclusion, an IgM antibody was produced that crosses the BBB to reverse cognitive impairment in a murine model of Alzheimer's disease.

Effects of chronic ethanol administration on brain interstitial fluid levels of Methionine-enkephalin as measured by microdialysis in vivo

The level of Met-enkephalin in the brain is inversely correlated with ethanol consumption and is controlled partially through efflux activity of peptide transport system-1 (PTS-1) at the blood-brain barrier (BBB). Prolonged alcohol drinking can perturb aspects of this system, including a loss of control of Met-enkephalin levels at the transcriptional and translational levels, and impaired release of Met-enkephalin from tissue sources. Met-enkephalin levels in whole brain homogenates often first paradoxically increase after a few days of ethanol drinking and then decrease with the development of physical dependence. Which of those various changes drives the others is unclear. To clarify these interactions, we here determined the levels of Met-enkephalin in striatal interstitial fluid (ISF) by microdialysis, striatal tissue homogenates, and serum after chronic ethanol treatment and alcohol withdrawal. Mice received ethanol (5%) in liquid diet for 7 days (ethanol-treated) and others withdrawn for a day following 7-day treatment (withdrawal). There was a significant (P<0.05) difference in the levels of Met-enkephalin in striatal microdialysate between the control (79.1+/-5.9 pg/ml) and ethanol-treated group (94.9+/-4.3 pg/ml), which was lost by withdrawing ethanol (83.9+/-3.8 pg/ml). In contrast, ethanol treatment did not affect Met-enkephalin levels in the striatal tissue. In the ethanol-treated group, there was a significant (P<0.05) reduction of the levels of Met-enkephalin in serum to 70.5% of control levels. This decrease was restored to the level of control by withdrawing ethanol. These reversible changes in ISF and serum are readily explained by the known changes in the efflux activity of PTS-1 at the BBB.