Daniel W. Wesson

Reversal of autophagy dysfunction in the TgCRND8 mouse model of Alzheimer's disease ameliorates amyloid pathologies and memory deficits

Autophagy, a major degradative pathway for proteins and organelles, is essential for survival of mature neurons. Extensive autophagic-lysosomal pathology in Alzheimers disease brain contributes to Alzheimers disease pathogenesis, although the underlying mechanisms are not well understood. Here, we identified and characterized marked intraneuronal amyloid-β peptide/amyloid and lysosomal system pathology in the Alzheimers disease mouse model TgCRND8 similar to that previously described in Alzheimers disease brains. We further establish that the basis for these pathologies involves defective proteolytic clearance of neuronal autophagic substrates including amyloid-β peptide. To establish the pathogenic significance of these abnormalities, we enhanced lysosomal cathepsin activities and rates of autophagic protein turnover in TgCRND8 mice by genetically deleting cystatin B, an endogenous inhibitor of lysosomal cysteine proteases. Cystatin B deletion rescued autophagic-lysosomal pathology, reduced abnormal accumulations of amyloid-β peptide, ubiquitinated proteins and other autophagic substrates within autolysosomes/lysosomes and reduced intraneuronal amyloid-β peptide. The amelioration of lysosomal function in TgCRND8 markedly decreased extracellular amyloid deposition and total brain amyloid-β peptide 40 and 42 levels, and prevented the development of deficits of learning and memory in fear conditioning and olfactory habituation tests. Our findings support the pathogenic significance of autophagic-lysosomal dysfunction in Alzheimers disease and indicate the potential value of restoring normal autophagy as an innovative therapeutic strategy for Alzheimers disease.

Olfactory Dysfunction Correlates with Amyloid-β Burden in an Alzheimer's Disease Mouse Model

Alzheimers disease often results in impaired olfactory perceptual acuity—a potential biomarker of the disorder. However, the usefulness of olfactory screens to serve as informative indicators of Alzheimers is precluded by a lack of knowledge regarding why the disease impacts olfaction. We addressed this question by assaying olfactory perception and amyloid-β (Aβ) deposition throughout the olfactory system in mice that overexpress a mutated form of the human amyloid-β precursor protein. Such mice displayed progressive olfactory deficits that mimic those observed clinically—some evident at 3 months of age. Also, at 3 months of age, we observed nonfibrillar Aβ deposition within the olfactory bulb—earlier than deposition within any other brain region. There was also a correlation between olfactory deficits and the spatial-temporal pattern of Aβ deposition. Therefore, nonfibrillar, versus fibrillar, Aβ-related mechanisms likely contribute to early olfactory perceptual loss in Alzheimers disease. Furthermore, these results present the odor cross-habituation test as a powerful behavioral assay, which reflects Aβ deposition and thus may serve to monitor the efficacy of therapies aimed at reducing Aβ.

Sniffing out the contributions of the olfactory tubercle to the sense of smell: Hedonics, sensory integration, and more?

Since its designation in 1896 as a putative olfactory structure, the olfactory tubercle has received little attention in terms of elucidating its role in the processing and perception of odors. Instead, research on the olfactory tubercle has mostly focused on its relationship with the reward system. Here we provide a comprehensive review of research on the olfactory tubercle-with an emphasis on the likely role of this region in olfactory processing and its contributions to perception. Further, we propose several testable hypotheses regarding the likely involvement of the olfactory tubercle in both basic (odor detection, discrimination, parallel processing of olfactory information) and higher-order (social odor processing, hedonics, multi-modal integration) functions. Together, the information within this review highlights an understudied yet potentially critical component in central odor processing.

Smelling Sounds: Olfactory–Auditory Sensory Convergence in the Olfactory Tubercle

Historical and psychophysical literature has demonstrated a perceptual interplay between olfactory and auditory stimuli—the neural mechanisms of which are not understood. Here, we report novel findings revealing that the early olfactory code is subjected to auditory cross-modal influences. In vivo extracellular recordings from the olfactory tubercle, a trilaminar structure within the basal forebrain, of anesthetized mice revealed that olfactory tubercle single units selectively respond to odors—with 65% of units showing significant odor-evoked activity. Remarkably, 19% of olfactory tubercle single units also showed robust responses to an auditory tone. Furthermore, 29% of single units tested displayed supraadditive or suppressive responses to the simultaneous presentation of odor and tone, suggesting cross-modal modulation. In contrast, olfactory bulb units did not show significant responses to tone presentation nor modulation of odor-evoked activity by tone—suggesting a lack of olfactory–auditory convergence upstream from the olfactory tubercle. Thus, the tubercle presents itself as a source for direct multimodal convergence within an early stage of odor processing and may serve as a seat for psychophysical interactions between smells and sounds.

Why Sniff Fast? The Relationship Between Sniff Frequency, Odor Discrimination, and Receptor Neuron Activation in the Rat

Many mammals display brief bouts of high-frequency (4-10 Hz) sniffing when sampling odors. Given this, high-frequency sniffing is thought to play an important role in odor information processing. Here, we asked what role rapid sampling behavior plays in odor coding and odor discrimination by monitoring sniffing during performance of discrimination tasks under different paradigms and across different levels of difficulty and by imaging olfactory receptor neuron (ORN) input to the olfactory bulb (OB) during behavior. To eliminate confounds of locomotion and object approach, all experiments were performed in head-fixed rats. Rats showed individual differences in sniffing strategies that emerged during discrimination learning, with some rats showing brief bouts of rapid sniffing on odorant onset and others showing little or no change in sniff frequency. All rats performed with high accuracy, indicating that rapid sniffing is not necessary for odor discrimination. Sniffing strategies remained unchanged even when task difficulty was increased. In the imaging experiments, rapid sniff bouts did not alter the magnitude of odorant-evoked inputs compared with trials in which rapid sniffing was not expressed. Furthermore, rapid sniff bouts typically began before detectable activation of ORNs and ended immediately afterward. Thus rapid sniffing did not enable multiple samples of an odorant before decision-making. These results suggest that the major functional contribution of rapid sniffing to odor discrimination performance is to enable the animal to acquire the stimulus more quickly once it is available rather than to directly influence the low-level neural processes underlying odor perception.

Therapeutic effects of remediating autophagy failure in a mouse model of Alzheimer disease by enhancing lysosomal proteolysis

The extensive autophagic-lysosomal pathology in Alzheimer disease (AD) brain has revealed a major defect in the proteolytic clearance of autophagy substrates. Autophagy failure contributes on several levels to AD pathogenesis and has become an important therapeutic target for AD and other neurodegenerative diseases. We recently observed broad therapeutic effects of stimulating autophagic-lysosomal proteolysis in the TgCRND8 mouse model of AD that exhibits defective proteolytic clearance of autophagic substrates, robust intralysosomal amyloid-β peptide (Aβ) accumulation, extracellular β-amyloid deposition and cognitive deficits. By genetically deleting the lysosomal cysteine protease inhibitor, cystatin B (CstB), to selectively restore depressed cathepsin activities, we substantially cleared Aβ, ubiquitinated proteins and other autophagic substrates from autolysosomes/lysosomes and rescued autophagic-lysosomal pathology, as well as reduced total Aβ40/42 levels and extracellular amyloid deposition, highlighting the underappreciated importance of the lysosomal system for Aβ clearance. Most importantly, lysosomal remediation prevented the marked learning and memory deficits in TgCRND8 mice. Our findings underscore the pathogenic significance of autophagic-lysosomal dysfunction in AD and demonstrate the value of reversing this dysfunction as an innovative therapeautic strategy for AD.

Should olfactory dysfunction be used as a biomarker of Alzheimer's disease?

A major effort in Alzheimer’s disease (AD) research is driven by the need to identify biomarkers of the disease. Such bio markers would ideally predict a prognosis of AD prior to the development of significant neuro pathology and subsequent loss of cognitive function. Early indicators of disease are especially important for implementing interventions while brain systems are still functioning relatively normally. Thus, determination of an accurate and robust biomarker model may be pivotal in reducing the global impact of AD. Currently, several biomarkers of AD are being explored. Concentrations of amyloid-b and tau in the cerebrospinal fluid, F-f luorodeoxyglucose, amyloid-b and hippocampal volume imaging, and neuropsychological testing are all being explored for their utility in predicting the onset and stage of AD [1]. A recent overview of the typical progression of these biomarkers has resulted in a timeline model of AD progression wherein abnormalities are first detected in amyloid-b biomarkers, followed by neurodegenerative and cognitive biomarkers [2].

Chronic anti-murine Aβ immunization preserves odor guided behaviors in an Alzheimer's β-amyloidosis model.

Olfaction is often impaired in Alzheimers disease (AD) and is also dysfunctional in mouse models of the disease. We recently demonstrated that short-term passive anti-murine-Aβ immunization can rescue olfactory behavior in the Tg2576 mouse model overexpressing a human mutation of the amyloid precursor protein (APP) after β-amyloid deposition. Here we tested the ability to preserve normal olfactory behaviors by means of long-term passive anti-murine-Aβ immunization. Seven-month-old Tg2576 and non-transgenic littermate (NTg) mice were IP-injected biweekly with the m3.2 murine-Aβ-specific antibody until 16 mo of age when mice were tested in the odor habituation test. While Tg2576 mice treated with a control antibody showed elevations in odor investigation times and impaired odor habituation compared to NTg, olfactory behavior was preserved to NTg levels in m3.2-immunized Tg2576 mice. Immunized Tg2576 mice had significantly less β-amyloid immunolabeling in the olfactory bulb and entorhinal cortex, yet showed elevations in Thioflavin-S labeled plaques in the piriform cortex. No detectable changes in APP metabolite levels other than Aβ were found following m3.2 immunization. These results demonstrate efficacy of chronic, long-term anti-murine-Aβ m3.2 immunization in preserving normal odor-guided behaviors in a human APP Tg model. Further, these results provide mechanistic insights into olfactory dysfunction as a biomarker for AD by yielding evidence that focal reductions of Aβ may be sufficient to preserve olfaction.

Mechanisms of Neural and Behavioral Dysfunction in Alzheimer's Disease

This review critically examines progress in understanding the link between Alzheimer’s disease (AD) molecular pathogenesis and behavior, with an emphasis on the impact of amyloid-β. We present the argument that the AD research field requires more multifaceted analyses into the impacts of Alzheimer’s pathogenesis which combine simultaneous molecular-, circuit-, and behavior-level approaches. Supporting this argument is a review of particular research utilizing similar, “systems-level” methods in mouse models of AD. Related to this, a critique of common physiological and behavioral models is made—highlighting the likely usefulness of more refined and specific tools in understanding the relationship between candidate molecular pathologies and behavioral dysfunction. Finally, we propose challenges for future research which, if met, may greatly extend our current understanding of how AD molecular pathology impacts neural network function and behavior and possibly may lead to refinements in disease therapeutics.

Respiratory and sniffing behaviors throughout adulthood and aging in mice

Orienting responses are physiological and active behavioral reactions evoked by novel stimulus perception and are critical for survival. We explored whether odor orienting responses are impacted throughout both adulthood and normal and pathological aging in mice. Novel odor investigation (including duration and bout numbers) and its subsequent habituation as assayed in the odor habituation task were preserved in adult C57BL/6J mice up to 12 mo of age with <6% variability between age groups in investigation time. Separately, using whole-body plethysmography we found that both spontaneous respiration and odor-evoked sniffing behaviors were strikingly preserved in wildtype (WT) mice up to 26 mo of age. In contrast, mice accumulating amyloid-β protein in the brain by means of overexpressing mutations in the human amyloid precursor protein gene (APP) showed preserved spontaneous respiration up to 12 mo, but starting at 14 mo showed significant differences from WT. Similar to WTs, odor-evoked sniffing was not impacted in APP mice up to 26 mo. These results show that odor-orienting responses are minimally impacted throughout aging in mice, and suggest that the olfactomotor network is mostly spared of insults due to aging.