Peripheral anti-Aβ antibody alters CNS and plasma Aβ clearance and decreases brain Aβ burden in a mouse model of Alzheimer's disease

Active immunization with the amyloid beta (A beta) peptide has been shown to decrease brain A beta deposition in transgenic mouse models of Alzheimer's disease and certain peripherally administered anti-A beta antibodies were shown to mimic this effect. In exploring factors that alter A beta metabolism and clearance, we found that a monoclonal antibody (m266) directed against the central domain of A beta was able to bind and completely sequester plasma A beta. Peripheral administration of m266 to PDAPP transgenic mice, in which A beta is generated specifically within the central nervous system (CNS), results in a rapid 1,000-fold increase in plasma A beta, due, in part, to a change in A beta equilibrium between the CNS and plasma. Although peripheral administration of m266 to PDAPP mice markedly reduces A beta deposition, m266 did not bind to A beta deposits in the brain. Thus, m266 appears to reduce brain A beta burden by altering CNS and plasma A beta clearance.     

Cerebral amyloid-beta protein accumulation with aging in cotton-top tamarins: a model of early Alzheimer's disease?

Alzheimer's disease (AD) is the most common progressive form of dementia in the elderly. Two major neuropathological hallmarks of AD include cerebral deposition of amyloid-beta protein (Abeta) into plaques and blood vessels, and the presence of neurofibrillary tangles in brain. In addition, activated microglia and reactive astrocytes are often associated with plaques and tangles. Numerous other proteins are associated with plaques in human AD brain, including Apo E and ubiquitin. The amyloid precursor protein and its shorter fragment, Abeta, are homologous between humans and non-human primates. Cerebral Abeta deposition has been reported previously for rhesus monkeys, vervets, squirrel monkeys, marmosets, lemurs, cynomologous monkeys, chimpanzees, and orangutans. Here we report, for the first time, age-related neuropathological changes in cotton-top tamarins (CTT, Saguinus oedipus), an endangered non-human primate native to the rainforests of Colombia and Costa Rica. Typical lifespan is 13-14 years of age in the wild and 15-20+ years in captivity. We performed detailed immunohistochemical analyses of Abeta deposition and associated pathogenesis in archived brain sections from 36 tamarins ranging in age from 6-21 years. Abeta plaque deposition was observed in 16 of the 20 oldest tamarins (>12 years). Plaques contained mainly Abeta42, and in the oldest animals, were associated with reactive astrocytes, activated microglia, Apo E, and ubiquitin-positive dystrophic neurites, similar to human plaques. Vascular Abeta was detected in 14 of the 20 aged tamarins; Abeta42 preceded Abeta40 deposition. Phospho-tau labeled dystrophic neurites and tangles, typically present in human AD, were absent in the tamarins. In conclusion, tamarins may represent a model of early AD pathology.     

Critical role for IL-1β in DNA damage-induced mucositis

β-TrCP, the substrate recognition subunit of SCF-type ubiquitin ligases, is ubiquitously expressed from two distinct paralogs, targeting for degradation many regulatory proteins, among which is the NF-κB inhibitor IκB. To appreciate tissue-specific roles of β-TrCP, we studied the consequences of inducible ablation of three or all four alleles of the E3 in the mouse gut. The ablation resulted in mucositis, a destructive gut mucosal inflammation, which is a common complication of different cancer therapies and represents a major obstacle to successful chemoradiation therapy. We identified epithelial-derived IL-1β as the culprit of mucositis onset, inducing mucosal barrier breach. Surprisingly, epithelial IL-1β is induced by DNA damage via an NF-κB–independent mechanism. Tissue damage caused by gut barrier disruption is exacerbated in the absence of NF-κB, with failure to express the endogenous IL-1β receptor antagonist IL-1Ra upon four-allele loss. Antibody neutralization of IL-1β prevents epithelial tight junction dysfunction and alleviates mucositis in β-TrCP–deficient mice. IL-1β antagonists should thus be considered for prevention and treatment of severe morbidity associated with mucositis.Only a few E3 ubiquitin ligases have been studied in vivo by gene disruption for the purpose of elucidating their functions and suitability as potential drug targets (1–3). Beta-transducin repeat containing protein (β-TrCP) is an E3 ubiquitin ligase that affects numerous major cell regulators (1, 4, 5), such as the effector of the Wnt pathway, β-catenin (6); the inhibitor of the NF-κB pathway, IκB (7); the cell cycle regulators CDC25A (8), WEE1 (9), Emi1 (10), and Bora (11); and DNA damage responsive proteins CDC25A and Claspin (12). β-TrCP is encoded by two paralog genes that are thought to be identical in their known biochemical features considering their high degree of sequence homology (77%) and virtually identical substrate recognition “pocket” (13). This redundancy may explain the mild phenotype observed upon in vivo ablation of β-TrCP1; the two best characterized substrates of β-TrCP, β-catenin and IκBα, do not accumulate in β-TrCP1–deficient MEFs (MEF^{β-TrCP1−/−}) (10). Furthermore, residual expression of ∼20% β-TrCP2 activity on a β-TrCP1–null background is sufficient for preserving homeostasis in most tissues (2). To gain a better understanding of in vivo β-TrCP functions, we created a floxed β-TrCP2 allele and crossed it to the β-TrCP1–null mouse (10).Here we show that the primary cellular assault following β-TrCP KO is DNA damage, which, when occurring in the gut, ignites a fatal colitis process that is IL-1β–dependent. This pathological process resembles the one that occurs in human patients following chemotherapy or radiation therapy-induced intestinal mucositis, and will therefore be referred to here as mucositis. We suggest that IL-1β is expressed and secreted by epithelial cells following DNA damage via an unknown mechanism. Expression of IL-1β is NF-κB–independent as observed in different experimental systems following DNA damaging treatments. Our study may have implications for human mucositis; we propose the possibility of using anti–IL-1β treatments [e.g., anakinra, an IL-1 receptor antagonist (IL-1Ra) (14)] as a preventive means for chemoradiation therapy-induced mucositis.     

p75 reduces β-amyloid-induced sympathetic innervation deficits in an Alzheimer's disease mouse model

β-Amyloid (Aβ) has adverse effects on brain cells, but little is known about its effects on the peripheral nervous system in Alzheimer''s disease (AD). Several lines of in vitro evidence suggest that the neurotrophin receptor p75 mediates or exacerbates Aβ-induced neurotoxicity. Here, we show that p75-deficient sympathetic neurons are more sensitive to Aβ-induced neurite growth inhibition. To investigate the role of p75 in the sympathetic nervous system of AD, p75 mutant mice were crossed with a mouse line of AD model. The majority of p75-deficient AD mice died by 3 weeks of age. The lethality is associated with severe defects in sympathetic innervation to multiple organs. When 1 copy of the BACE1 gene encoding a protein essential in Aβ production was deleted in p75-deficient AD mice, sympathetic innervation was significantly restored. These results suggest that p75 is neuroprotective for the sympathetic nervous system in a mouse model of AD.Although Alzheimer''s disease (AD) is generally considered a neurodegenerative disease that primarily affects the brain because of the presence of intracellular neurofibrillary tangles and plaques within the neocortex and hippocampus (1), deficits in other nervous systems, including the sympathetic nervous system, are observed and may contribute to the pathogenesis and mortality of AD (2, 3). β-Amyloid (Aβ) peptide is the product of stepwise processing of the amyloid protein precursor and is the major component of the plaques in the brain of AD (3). A large body of studies has established that Aβ oligomers or aggregates are toxic to brain cells, yet little is known about their effects on neurons of the peripheral nervous system. Identifying genetic factors that modify the neurotoxicity of Aβ in both the brain and the peripheral nervous system will increase our understanding of the biology of AD and may provide insights into development of treatments centered on Aβ (4).The neurotrophin receptor p75 is a member of the TNF receptor superfamily. p75 has been shown to interact with different partners to mediate diverse functions, including cell survival, cell death, and axon guidance, depending on the partner with which it complexes (5–7). Several lines of evidence suggest that p75 plays a role in neurotoxicity associated with Aβ; however, the exact role of p75 remains controversial. Aβ has been shown to bind p75 (8–10) and activate downstream signaling pathways, such as JNK, NF-κB, and PI3K. Thus, p75 has been suggested to be a receptor for Aβ and mediate Aβ-induced neurotoxicity. Several studies suggest that overexpression of p75 in a variety of cell lines could confer sensitivity to Aβ-induced toxicity (9, 11, 12), whereas p75-deficient mouse hippocampal neurons are resistant to Aβ-induced toxicity (13). Aβ-induced cell death of PC12 cells requires JNK activation, and p75 plays a role in JNK activation (14, 15). In contrast, a neuroprotective role for p75 is suggested in human hippocampal neurons (16). It is not clear whether these disparate observations are due to experimental and physiological differences in different culture systems. Additionally, the discrepancy from in vitro experiments underscores the importance of studying the role of p75 in Aβ-induced neurotoxicity in vivo.Here, we show that p75-deficient sympathetic neurons are more sensitive to Aβ peptide-induced neurite growth inhibition in vitro. In addition, when p75 mutant mice are crossed with a mouse line of AD model, sympathetic innervation to multiple organs is severely compromised in p75-deficient AD mice. When p75-deficient AD mice are crossed with BACE1 mutant mice, sympathetic innervation is markedly restored. Our results suggest that p75 is neuroprotective for the sympathetic nervous system in an AD mouse model.     

Liver X receptor β protects dopaminergic neurons in a mouse model of Parkinson disease

Parkinson disease (PD) is a progressive neurodegenerative disease whose progression may be slowed, but at present there is no pharmacological intervention that would stop or reverse the disease. Liver X receptor β (LXRβ) is a member of the nuclear receptor super gene family expressed in the central nervous system, where it is important for cortical layering during development and survival of dopaminergic neurons throughout life. In the present study we have used the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD to investigate the possible use of LXRβ as a target for prevention or treatment of PD. The dopaminergic neurons of the substantia nigra of LXRβ(-/-) mice were much more severely affected by MPTP than were those of their WT littermates. In addition, the number of activated microglia and GFAP-positive astrocytes was higher in the substantia nigra of LXRβ(-/-) mice than in WT littermates. Administration of the LXR agonist GW3965 to MPTP-treated WT mice protected against loss of dopaminergic neurons and of dopaminergic fibers projecting to the striatum, and resulted in fewer activated microglia and astroglia. Surprisingly, LXRβ was not expressed in the neurons of the substantia nigra but in the microglia and astroglia. We conclude that LXR agonists may have beneficial effects in treatment of PD by modulating the cytotoxic functions of microglia.     

The SAMP8 mouse: a model of Alzheimer disease?

The SAMP8 mice develops early abnormalities in learning and memory. These are related to abnormalities in septo-hippocampal function with a decrease in serotonin leading to an increase in GABA and a decrease in acetylcholine. The cognitive defects in these animals are due to overproduction of β-amyloidand can be reversed by antibodies toβ-amyloid or specific antisenseoligo nucleotides. The major defect produced byβ-amyloid in these mice appears to be reduction in Δ⁹ desaturase activity leading to altered membrane phospholipid content. The SAMP8 mouse appears to be an excellent model to examine the pathophysiology of early defects seen in Alzheimer disease. This revised version was published online in July 2006 with corrections to the Cover Date.     

Macrovascular disease in diabetes: is the mouse a suitable model?

To elucidate the pathogenesis of macrovascular disease in diabetes, animal models are widely used. Diabetic mice are of particular interest because they can be crossed to knockout mice lacking specific genes that are under consideration to contribute to diabetic vascular complications. However, the mouse is relative resistant to develop atherosclerosis. Therefore, we review some commonly used mouse models and discuss their advantages and disadvantages.     

In vivo characterization of endothelial cell activation in a transgenic mouse model of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common cause of dementia worldwide. AD is characterized by an excessive cerebral amyloid deposition leading to degeneration of neurons and eventually to dementia. It has been shown by epidemiological studies that cardiovascular drugs with an anti-angiogenic effect can influence the outcome of AD patients. Therefore, it has been speculated that in AD angiogenesis in the brain vasculature may play an important role. Here we report that in the brain of APP23 mice – a transgenic model of AD – after deposition of amyloid in blood vessels endothelial cell activation occurs in an age-dependent manner. Amyloid deposition is followed by the expression of β3-integrin, a specific marker molecule of activated endothelium. The β3-integrin expression is restricted to amyloid-positive vessels. Moreover, homogenates of the brains of APP23 mice induced the formation of new vessels in an in vivo angiogenesis assay. Vessel formation could be blocked by the VEGF antagonist SU 4312 as well as by statins, suggesting that these drugs may interfere with endothelial cell activation in AD. In conclusion our results indicate that amyloid deposition in the vasculature leads to endothelial cell apoptosis and endothelial cell activation, which can be modulated by anti-angiogenic drugs.     

Plasma β-amyloid peptides in canine aging and cognitive dysfunction as a model of Alzheimer's disease

Aging dogs naturally demonstrate cognitive impairment and neuropathology that model early Alzheimer's disease (AD). In particular, there is evidence that canine cognitive dysfunction syndrome (CDS) in aged dogs is accompanied by cortical deposition of Aβ peptides and neurodegeneration. Plasma Aβ levels have been examined in humans as putative biomarkers for AD, but to date, no similar studies have been conducted for canine dementia. The aim of the present study was to assess plasma Aβ1-42 and Aβ1-40 levels in a blind study using pet dogs that were either successfully aging or exhibiting CDS. The severity of cognitive impairment was assessed using an owner-based questionnaire. On average, young dogs presented significantly higher plasma levels of Aβ1-42 and Aβ1-40 than aged, cognitively unimpaired dogs. Notably, among aged dogs, the levels of Aβ1-42 and the Aβ42/40 ratio were significantly higher in those showing mild cognitive impairment than in either cognitively unimpaired or severely affected dogs. These results suggest that increased plasma Aβ1-42 levels and Aβ42/40 ratio could be a biomarker for canine cognitive dysfunction, which is considered an excellent natural model of early AD.     

Low-density lipoprotein receptor overexpression enhances the rate of brain-to-blood Aβ clearance in a mouse model of β-amyloidosis

The apolipoprotein E (APOE)-ε4 allele is the strongest genetic risk factor for late-onset, sporadic Alzheimer''s disease, likely increasing risk by altering amyloid-β (Aβ) accumulation. We recently demonstrated that the low-density lipoprotein receptor (LDLR) is a major apoE receptor in the brain that strongly regulates amyloid plaque deposition. In the current study, we sought to understand the mechanism by which LDLR regulates Aβ accumulation by altering Aβ clearance from brain interstitial fluid. We hypothesized that increasing LDLR levels enhances blood–brain barrier-mediated Aβ clearance, thus leading to reduced Aβ accumulation. Using the brain Aβ efflux index method, we found that blood–brain barrier-mediated clearance of exogenously administered Aβ is enhanced with LDLR overexpression. We next developed a method to directly assess the elimination of centrally derived, endogenous Aβ into the plasma of mice using an anti-Aβ antibody that prevents degradation of plasma Aβ, allowing its rate of appearance from the brain to be measured. Using this plasma Aβ accumulation technique, we found that LDLR overexpression enhances brain-to-blood Aβ transport. Together, our results suggest a unique mechanism by which LDLR regulates brain-to-blood Aβ clearance, which may serve as a useful therapeutic avenue in targeting Aβ clearance from the brain.