Faculty Bibliography

Accumulating evidence shows that brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) significantly decrease early in Alzheimer's disease (AD). However, it remains unclear whether BDNF/TrkB reductions may be mechanistically involved in the pathogenesis of AD. To address this question, we generated 5XFAD transgenic mice with heterozygous TrkB knockout (TrkB(+/-).5XFAD), and tested the effects of TrkB reduction on AD-like features in this mouse model during an incipient stage that shows only modest amyloid-beta (Abeta) pathology and retains normal mnemonic function. TrkB(+/-) reduction exacerbated memory declines in 5XFAD mice at 4-5 months of age as assessed by the hippocampus-dependent spontaneous alternation Y-maze task, while the memory performance was not affected in TrkB(+/-) mice. Meanwhile, TrkB(+/-).5XFAD mice were normal in nest building, a widely used measure for social behavior, suggesting the memory-specific aggravation of AD-associated behavioral impairments. We found no difference between TrkB(+/-).5XFAD and 5XFAD control mice in cerebral plaque loads, Abeta concentrations including total Abeta42 and soluble oligomers and beta-amyloidogenic processing of amyloid precursor protein. Interestingly, reductions in hippocampal expression of AMPA/NMDA glutamate receptor subunits as well as impaired signaling pathways downstream to TrkB such as CREB (cAMP response element-binding protein) and Akt/GSK-3beta (glycogen synthase kinase-3beta) were observed in TrkB(+/-).5XFAD mice but not in 5XFAD mice. Among these signaling aberrations, only Akt/GSK-3beta dysfunction occurred in TrkB(+/-) mice, while others were synergistic consequences between TrkB reduction and subthreshold levels of Abeta in TrkB(+/-).5XFAD mice. Collectively, our results indicate that reduced TrkB does not affect beta-amyloidosis but exacerbates the manifestation of hippocampal mnemonic and signaling dysfunctions in early AD.

Endosomal-lysosomal and autophagic dysregulation occurs in the hippocampus in prodromal Alzheimer disease (AD), but its relationship with beta-amyloid (Abeta) and tau pathology remains unclear. To investigate this issue, we performed immunoblot analysis of hippocampal homogenates from cases with an antemortem clinical diagnosis of no cognitive impairment, mild cognitive impairment (MCI), and AD. Western blot analysis revealed significant increases in the acid hydrolase cathepsin D and early endosome marker rabaptin5 in the MCI group compared with AD, whereas levels of phosphorylated mammalian target of rapamycin proteins (pmTOR), total mammalian target of rapamycin (mTOR), p62, traf6, and LilrB2 were comparable across clinical groups. Hippocampal Abeta1-40 and Abeta1-42 concentrations and AT8-immunopositive neurofibrillary tangle density were not significantly different across the clinical groups. Greater cathepsin D expression was associated with global cognitive score and episodic memory score but not with mini mental state examination or advanced neuropathology criteria. These results indicate that alterations in hippocampal endosomal-lysosomal proteins in MCI are independent of tau or Abeta pathology.

Acute seizures after a severe brain insult can often lead to epilepsy and cognitive impairment. Aberrant hippocampal neurogenesis follows the insult but the role of adult-generated neurons in the development of chronic seizures or associated cognitive deficits remains to be determined. Here we show that the ablation of adult neurogenesis before pilocarpine-induced acute seizures in mice leads to a reduction in chronic seizure frequency. We also show that ablation of neurogenesis normalizes epilepsy-associated cognitive deficits. Remarkably, the effect of ablating adult neurogenesis before acute seizures is long lasting as it suppresses chronic seizure frequency for nearly 1 year. These findings establish a key role of neurogenesis in chronic seizure development and associated memory impairment and suggest that targeting aberrant hippocampal neurogenesis may reduce recurrent seizures and restore cognitive function following a pro-epileptic brain insult.

BACKGROUND: Accumulating evidence indicates that partial inhibition of beta-site APP-cleaving enzyme 1 (BACE1), which initiates amyloid-beta (Abeta) production, mitigates Alzheimer's disease (AD)-like pathologies and memory deficits in a battery of transgenic mouse models. However, our previous investigations suggest that therapeutic BACE1 suppression may be beneficial only if targeted on earlier stages of AD and encounter dramatic reductions in efficacy during disease progression. This study was designed to test the possibility that a combination approach, aimed at inhibiting BACE1 and boosting neprilysin (a major Abeta-degrading enzyme) activities, may be able to mechanistically overcome the limited efficacy of anti-Abeta therapy in advanced AD. RESULTS: After crossbreeding between BACE1 heterozygous knockout (BACE1(+/-)), neprilysin transgenic (NEP) and 5XFAD mice, we analyzed the resultant mice at 12 months of age when 5XFAD controls showed robust amyloid-beta (Abeta) accumulation and elevation of BACE1 expression (~2 folds). Although haploinsufficiency lowered BACE1 expression by ~50% in concordance with reduction in gene copy number, profound beta-amyloidosis, memory deficits and cholinergic neuron death were no longer rescued in BACE1(+/-) . 5XFAD mice concomitant with their persistently upregulated BACE1 (i.e., equivalent to wild-type control levels). Notably, neprilysin overexpression not only prevented Abeta accumulation but also suppressed the translation initiation factor eIF2alpha-associated elevation of BACE1 and lowered levels of the beta-secretase-cleaved C-terminal fragment of APP (C99) in NEP . 5XFAD mice. Interestingly, these markers for beta-amyloidogenesis in BACE1(+/-) . NEP . 5XFAD mice were further reduced to the levels reflecting a combination of single BACE1 allele ablation and the abolishment of translational BACE1 upregulation. However, since neprilysin overexpression was striking (~8-fold relative to wild-type controls), memory impairments, cholinergic neuronal loss and beta-amyloidosis were similarly prevented in NEP . 5XFAD and BACE1(+/-) . NEP . 5XFAD mice. CONCLUSIONS: Our findings indicate that robust overexpression of neprilysin is sufficient to ameliorate AD-like phenotypes in aged 5XFAD mice. We also found that Abeta-degrading effects of overexpressed neprilysin can block deleterious BACE1-elevating mechanisms that accelerate Abeta production, warranting further study to test whether interventions moderately activating neprilysin may be useful for boosting the limited efficacy of therapeutic BACE1 inhibition in treating AD with established Abeta pathology.

Research indicates that female risk of developing Alzheimer's disease (AD) is greater than that of males. Moderate reduction of calorie intake, known as calorie restriction (CR), reduces pathology in AD mouse models and is a potentially translatable prevention measure for individuals at-risk for AD, as well as an important tool for understanding how the brain endogenously attenuates age-related pathology. Whether sex influences the response to CR remains unknown. In this study, we assessed the effect of CR on beta-amyloid peptide (Abeta) pathology and hippocampal CA1 neuron specific gene expression in the Tg2576 mouse model of cerebral amyloidosis. Relative to ad libitum (AL) feeding, CR feeding significantly reduced hippocampal Abeta burden in 15-month-old female, but not age-matched male, Tg2576 mice. Sustained CR also significantly reduced expression of presenilin enhancer 2 (Psenen) and presenilin 1, components of the gamma-secretase complex, in Tg2576 females. These results indicate that long-term CR significantly reduces age-dependent female Tg2576 Abeta pathology, which is likely to involve CR-mediated reductions in gamma-secretase-dependent amyloid precursor protein (APP) metabolism.

Objectives: Given that epidemiologic studies have shown that diabetes mellitus increases the risk of Alzheimer's disease (AD), our objective was to examine the mechanistic links between the two diseases in a non-human primate. Methods: Tissue from multiple brain regions of a vervet monkey model of streptozotocin-induced type 1 diabetes (n=10 control; n=7 diabetic) was examined by Western blot analysis, sandwich ELISA, and qPCR for biochemical changes in tau protein and Abeta peptide, as well as changes in key enzymes that contribute to their processing and posttranslational modification. Results: Regional brain analyses showed a global increase in tau phosphorylation in areas vulnerable to AD pathology as well as in spared structures such as the cerebellum. An examination of tau phosphatases and kinases showed a brain-wide increase in active ERK1/2. A diabetes-induced increase in Abeta levels, however, was specific to brain regions affected during the early stages of AD pathogenesis, with the greatest increase observed in the hippocampus. Examination of the amyloid precursor protein, its metabolites, and proteins involved in the clearance and degradation of brain Abeta indicated that a hippocampal-specific decrease in the Abeta-degrading enzyme neprilysin is a major contributor to this localized Abeta increase. Conclusions: Our study suggests protein changes in the brain that link diabetes to AD risk: decreased neprilysin expression leads to an increase in Abeta in the temporal lobe structures that are at the earliest risk in AD while increased ERK1/2 activity appears to contribute to a brain-wide increase in tau phosphorylation

Over recent years, accumulated evidence suggests that autophagy induction is protective in animal models of a number of neurodegenerative diseases. Intense research in the field has elucidated different pathways through which autophagy can be upregulated and it is important to establish how modulation of these pathways impacts upon disease progression in vivo and therefore which, if any, may have further therapeutic relevance. In addition, it is important to understand how alterations in these target pathways may affect normal physiology when constitutively modulated over a long time period, as would be required for treatment of neurodegenerative diseases. Here we evaluate the potential protective effect of downregulation of calpains. We demonstrate, in Drosophila, that calpain knockdown protects against the aggregation and toxicity of proteins, like mutant huntingtin, in an autophagy-dependent fashion. Furthermore, we demonstrate that, overexpression of the calpain inhibitor, calpastatin, increases autophagosome levels and is protective in a mouse model of Huntington's disease, improving motor signs and delaying the onset of tremors. Importantly, long-term inhibition of calpains did not result in any overt deleterious phenotypes in mice. Thus, calpain inhibition, or activation of autophagy pathways downstream of calpains, may be suitable therapeutic targets for diseases like Huntington's disease.

Alzheimer's disease (AD) is a neurodegenerative disorder which is the most common cause of dementia in the elderly today. One of the earliest symptoms of AD is olfactory dysfunction. The present study investigated the effects of amyloid beta precursor protein (AbetaPP) metabolites, including amyloid-beta (Abeta) and AbetaPP C-terminal fragments (CTF), on olfactory processing in the lateral entorhinal cortex (LEC) using the Tg2576 mouse model of human AbetaPP over-expression. The entorhinal cortex is an early target of AD related neuropathology, and the LEC plays an important role in fine odor discrimination and memory. Cohorts of transgenic and age-matched wild-type (WT) mice at 3, 6, and 16months of age (MO) were anesthetized and acute, single-unit electrophysiology was performed in the LEC. Results showed that Tg2576 exhibited early LEC hyperactivity at 3 and 6MO compared to WT mice in both local field potential and single-unit spontaneous activity. However, LEC single-unit odor responses and odor receptive fields showed no detectable difference compared to WT at any age. Finally, the very early emergence of olfactory system hyper-excitability corresponded not to detectable Abeta deposition in the olfactory system, but rather to high levels of intracellular AbetaPP-CTF and soluble Abeta in the anterior piriform cortex (aPCX), a major afferent input to the LEC, by 3MO. The present results add to the growing evidence of AbetaPP-related hyper-excitability, and further implicate both soluble Abeta and non-Abeta AbetaPP metabolites in its early emergence.

Adult neurogenesis, the generation of new neurons in the adult brain, occurs in the hippocampal dentate gyrus (DG) and the olfactory bulb (OB) of all mammals, but the functions of these new neurons are not entirely clear. Originally, adult-born neurons were considered to have excitatory effects on the DG network, but recent studies suggest a net inhibitory effect. Therefore, we hypothesized that selective removal of newborn neurons would lead to increased susceptibility to the effects of a convulsant. This hypothesis was tested by evaluating the response to the chemoconvulsant kainic acid (KA) in mice with reduced adult neurogenesis, produced either by focal X-irradiation of the DG, or by pharmacogenetic deletion of dividing radial glial precursors. In the first 4 hrs after KA administration, when mice have the most robust seizures, mice with reduced adult neurogenesis had more severe convulsive seizures, exhibited either as a decreased latency to the first convulsive seizure, greater number of convulsive seizures, or longer convulsive seizures. Nonconvulsive seizures did not appear to change or they decreased. Four-21 hrs after KA injection, mice with reduced adult neurogenesis showed more interictal spikes (IIS) and delayed seizures than controls. Effects were greater when the anticonvulsant ethosuximide was injected 30 min prior to KA administration; ethosuximide allows forebrain seizure activity to be more easily examined in mice by suppressing seizures dominated by the brainstem. These data support the hypothesis that reduction of adult-born neurons increases the susceptibility of the brain to effects of KA.