Takahiro Suemoto

Quinoline and Benzimidazole Derivatives: Candidate Probes for In Vivo Imaging of Tau Pathology in Alzheimer's Disease

Neurofibrillary tangles (NFTs), neuropil threads, and neuritic elements of senile plaques predominantly comprise hyperphosphorylated tau protein and represent pathological characteristics of Alzheimers disease (AD). These lesions occur before the presentation of clinical symptoms and correlate with the severity of dementia. In vivo detection of these lesions would thus prove useful for preclinical diagnosis of AD and for tracking disease progression. The present study introduces three novel compounds, 4-[2-(2-benzoimidazolyl)ethenyl]-N,N-diethylbenzenamine (BF-126), 2-[(4-methylamino)phenyl]quinoline (BF-158), and 2-(4-aminophenyl)quinoline (BF-170), as candidate probes for in vivo imaging of tau pathology in the AD brain. When solutions of these compounds are injected intravenously into normal mice, these agents exhibit excellent brain uptake and rapid clearance from normal brain tissue. These compounds display relatively lower binding affinity to β-amyloid fibrils and higher binding affinity to tau fibrils, compared with previously reported probe BF-168. In neuropathological examination using AD brain sections, BF-126, BF-158, and BF-170 clearly visualize NFTs, neuropil threads, and paired helical filament-type neuritis. Autoradiography using 11C-labeled BF-158 further demonstrated labeling of NFTs in AD brain sections. These findings suggest the potential usefulness of quinoline and benzimidazole derivatives for in vivo imaging of tau pathology in AD.

Neprilysin-sensitive Synapse-associated Amyloid-β Peptide Oligomers Impair Neuronal Plasticity and Cognitive Function

A subtle but chronic alteration in metabolic balance between amyloid-β peptide (Aβ) anabolic and catabolic activities is thought to cause Aβ accumulation, leading to a decade-long pathological cascade of Alzheimer disease. However, it is still unclear whether a reduction of the catabolic activity of Aβ in the brain causes neuronal dysfunction in vivo. In the present study, to clarify a possible connection between a reduction in neprilysin activity and impairment of synaptic and cognitive functions, we cross-bred amyloid precursor protein (APP) transgenic mice (APP23) with neprilysin-deficient mice and biochemically and immunoelectron-microscopically analyzed Aβ accumulation in the brain. We also examined hippocampal synaptic plasticity using an in vivo recording technique and cognitive function using a battery of learning and memory behavior tests, including Y-maze, novel-object recognition, Morris water maze, and contextual fear conditioning tests at the age of 13–16 weeks. We present direct experimental evidence that reduced activity of neprilysin, the major Aβ-degrading enzyme, in the brain elevates oligomeric forms of Aβ at the synapses and leads to impaired hippocampal synaptic plasticity and cognitive function before the appearance of amyloid plaque load. Thus, reduced neprilysin activity appears to be a causative event that is at least partly responsible for the memory-associated symptoms of Alzheimer disease. This supports the idea that a strategy to reduce Aβ oligomers in the brain by up-regulating neprilysin activity would contribute to alleviation of these symptoms.

Styrylbenzoxazole Derivatives for In Vivo Imaging of Amyloid Plaques in the Brain

Progressive deposition of senile plaques (SPs) is one of the major neuropathological features of Alzheimers disease (AD) that precedes cognitive decline. Noninvasive detection of SPs could, therefore, be a potential diagnostic test for early detection of AD patients. For imaging SPs in the living brain, we have developed a series of styrylbenzoxazole derivatives that achieve high binding affinity for amyloid-β (Aβ) fibrils. One of these compounds, 6-(2-Fluoroethoxy)-2-[2-(4-methylaminophenil) ethenyl]benzoxazole (BF-168), selectively binds SPs in AD brain sections and recognizes Aβ1-42-positive diffuse plaques as well as neuritic plaques in AD brain sections. Intravenous injection of BF-168 in PS1/APP and APP23 transgenic mice resulted in specific in vivo labeling to both compact and diffuse amyloid deposits in the brain. In addition, 18F-radiolabeled BF-168 demonstrated abundant initial brain uptake (3.9% injected dose/gm at 2 min after injection) and fast clearance (t1/2 = 24.7 min) after intravenous administration in normal mice. Furthermore, autoradiograms of brain sections from APP23 transgenic mice at 180 min after intravenous injection of [18F]BF-168 showed selective labeling of brain amyloid deposits with little nonspecific binding. These findings strongly suggest that styrylbenzoxazole derivatives are promising candidate probes for positron emission tomography and single-photon emission computed tomography imaging for early detection of amyloid plaque formation.

Neprilysin-sensitive synapse-associated Aβ oligomers impair neuronal plasticity and cognitive function

A subtle but chronic alteration in metabolic balance between amyloid-β peptide (Aβ) anabolic and catabolic activities is thought to cause Aβ accumulation, leading to a decade-long pathological cascade of Alzheimer disease. However, it is still unclear whether a reduction of the catabolic activity of Aβ in the brain causes neuronal dysfunction in vivo. In the present study, to clarify a possible connection between a reduction in neprilysin activity and impairment of synaptic and cognitive functions, we cross-bred amyloid precursor protein (APP) transgenic mice (APP23) with neprilysin-deficient mice and biochemically and immunoelectron-microscopically analyzed Aβ accumulation in the brain. We also examined hippocampal synaptic plasticity using an in vivo recording technique and cognitive function using a battery of learning and memory behavior tests, including Y-maze, novel-object recognition, Morris water maze, and contextual fear conditioning tests at the age of 13–16 weeks. We present direct experimental evidence that reduced activity of neprilysin, the major Aβ-degrading enzyme, in the brain elevates oligomeric forms of Aβ at the synapses and leads to impaired hippocampal synaptic plasticity and cognitive function before the appearance of amyloid plaque load. Thus, reduced neprilysin activity appears to be a causative event that is at least partly responsible for the memory-associated symptoms of Alzheimer disease. This supports the idea that a strategy to reduce Aβ oligomers in the brain by up-regulating neprilysin activity would contribute to alleviation of these symptoms.

Efhc1 deficiency causes spontaneous myoclonus and increased seizure susceptibility

Mutations in EFHC1 gene have been previously reported in patients with epilepsies, including those with juvenile myoclonic epilepsy. Myoclonin1, also known as mRib72-1, is encoded by the mouse Efhc1 gene. Myoclonin1 is dominantly expressed in embryonic choroid plexus, post-natal ependymal cilia, tracheal cilia and sperm flagella. In this study, we generated viable Efhc1-deficient mice. Most of the mice were normal in outward appearance, and both sexes were found to be fertile. However, the ventricles of the brains were significantly enlarged in the null mutants, but not in the heterozygotes. Although the ciliary structure was found intact, the ciliary beating frequency was significantly reduced in null mutants. In adult stages, both the heterozygous and null mutants developed frequent spontaneous myoclonus. Furthermore, the threshold of seizures induced by pentylenetetrazol was significantly reduced in both heterozygous and null mutants. These observations seem to further suggest that decrease or loss of function of myoclonin1 may be the molecular basis for epilepsies caused by EFHC1 mutations.

In vivo labeling of amyloid with BF-108.

Detection of aggregated amyloid-beta (Abeta) with a non-invasive imaging modality such as positron emission tomography (PET) was suggested to be ideal for the diagnosis of Alzheimers disease (AD) prior to the onset of clinical symptoms. We have been searching for imaging probe candidates with a high affinity for aggregated Abeta in vitro and in vivo and high lipophilicity, a characteristic that allows for the permeation of the blood-brain barrier (BBB). As analyzed by Thioflavin T (ThT) assay and octanol/water partition coefficient test (PC), 3-diethylamino-6-(2-fluoroethyl)ethylaminoacridine (BF-108) were found to have high affinity for Abeta aggregates in vitro and high lipophilicity. Intravenously administrated BF-108 labeled Abeta aggregates injected into the amygdala as observed under a fluorescence microscope, showing this compounds permeability of BBB and an ability to label Abeta in vivo. BF-108 also labeled neuritic senile plaques (SPs), neurofibrillary tangles, and amyloid-laden vessels in temporal and hippocampal sections from AD patients. Following intravenous administration of BF-108 to an APP23 transgenic (TG) mouse, in vivo labeling of endogenous plaques was seen in brain sections by fluorescence microscopy. These properties suggest the potential utility of BF-108 for in vivo imaging of AD pathology.

A novel imaging probe for in vivo detection of neuritic and diffuse amyloid plaques in the brain.

Extensive deposition of neuritic and diffuse amyloid plaques in the brain is a critical event for the pathogenesis of Alzheimer’s disease (AD) and considered to start before the appearance of clinical symptoms. In vivo detection of these brain β-amyloid (Aβ) deposits using positron emission tomography (PET), therefore, would be a useful marker for presymptomatic detection of AD. To develop a new agent for PET probe of imaging neuritic and diffuse amyloid deposits, novel fluorescent compounds, including styryl-fluorobenzoxazole derivatives, were examined. These compounds showed a high binding affinity for both synthetic Aβ1-40 and Aβ1-42 aggregates. Some of these compounds also displayed distinct staining of neuritic and diffuse amyloid plaques in AD brain sections. A biodistribution study of styryl-fluorobenzoxazole derivatives in normal mice exhibited excellent brain uptakes (4.5–5.5% injected dose/g at 2 min postinjection). Furthermore, iv administration of BF-145, a styryl-fluorobenzoxazole derivative, demonstrated specific in vivo labeling of compact and diffuse amyloid deposits in an APP23 transgenic mouse brain, in contrast to no accumulation in a wild-type mouse brain. These findings suggest that BF-145 is a potential candidate as a probe for imaging early brain pathology in AD patients.

Styrylbenzoazole derivatives for imaging of prion plaques and treatment of transmissible spongiform encephalopathies.

Recent prevalence of acquired forms of transmissible spongiform encephalopathies (TSEs) has urged the development of early diagnostic measures as well as therapeutic interventions. To extend our previous findings on the value of amyloid imaging probes for these purposes, styrylbenzoazole derivatives with better permeability of blood–brain barrier (BBB) were developed and analyzed in this study. The new styrylbenzoazole compounds clearly labeled prion protein (PrP) plaques in brain specimens from human TSE in a manner irrespective of pathogen strain, and a representative compound BF‐168 detected abnormal PrP aggregates in the brain of TSE‐infected mice when the probe was injected intravenously. On the other hand, most of the compounds inhibited abnormal PrP formation in TSE‐infected cells with IC50 values in the nanomolar range, indicating that they represent one of the most potent classes of inhibitor ever reported. BF‐168 prolonged the lives of mice infected intracerebrally with TSE when the compound was given intravenously at the preclinical stage. The new compounds, however, failed to detect synaptic PrP deposition and to show pathogen‐independent therapeutic efficacy, similar to the amyloid imaging probes we previously reported. The compounds were BBB permeable and non‐toxic at doses for imaging and treatment; therefore, they are expected to be of practical use in human TSE.

cDNA cloning and expression of a novel serine protease in the mouse brain.

A cDNA for a novel serine protease, termed brain type granzyme K (B-GRK) was cloned from the mouse brain. The cDNA codes a protein similar to granzyme K (GRK) but completely different at the N-terminus. Genomic Southern and PCR analysis of the gene suggests B-GRK is the alternative transcription form of GRK. B-GRK and GRK have a different organ-specific expression pattern: B-GRK is expressed in the brain, while GRK is expressed in the spleen. The recombinant fusion protein was detected in the neuro2a cells transfected with a plasmid containing B-GRK sequence. The mRNA for B-GRK/GRK was detected in cerebral cortex, hippocampus and diencephalon of the mouse brain. In situ hybridization for B-GRK/GRK revealed that several regions in the forebrain and hypothalamus express the mRNA. Developmental analysis showed that in the prenatal stage, the mRNA was expressed also in pituitary and pineal body in addition to the brain.

High expression of alternative transcript of granzyme M in the mouse retina.

We have isolated cDNAs to two transcripts, granzyme M and alternative granzyme M mRNA from the mouse eye. Analysis of genomic DNA revealed these transcripts were derived from alternative transcription initiations. Northern blot analysis and reverse transcription-polymerase chain reactions revealed that both transcripts were expressed in the eye, though the alternative form was the major type. In situ hybridization studies demonstrated that alternative granzyme M mRNA localized exclusively in the photoreceptor cells in the retina and expressed only after the opening of the eye, suggesting that these transcripts are related to the maintenance of the retinal structure or functions of matured photoreceptor cells rather than the development or differentiation of retinal cells.