Olfactory impairment in men and mice related to aging and amyloid-induced pathology

Aging and chronic inflammation: impacts on olfactory dysfunction-a comprehensive review

Olfactory dysfunction (OD) is a common nasal disease, particularly prevalent among the elderly population, significantly impacting the affected individuals' quality of life. This review focuses on the influence of aging and chronic inflammation on olfactory dysfunction, presenting insights from both the peripheral and central olfactory systems. By exploring the molecular mechanisms and pathological changes underlying the occurrence of olfactory dysfunction in relation to age-related diseases and chronic inflammation conditions, we aim to provide a comprehensive theoretical foundation for further research and offer valuable insights for more effective treatment of olfactory dysfunction.

Contributions and future potential of animal models for geroscience research on sensory systems

Sensory systems mediate our social interactions, food intake, livelihoods, and other essential daily functions. Age-related decline and disease in sensory systems pose a significant challenge to healthy aging. Research on sensory decline in humans is informative but can often be difficult, subject to sampling bias, and influenced by environmental variation. Study of animal models, including mice, rats, rabbits, pigs, cats, dogs, and non-human primates, plays a complementary role in biomedical research, offering advantages such as controlled conditions and shorter lifespans for longitudinal study. Various species offer different advantages and limitations but have provided key insights in geroscience research. Here we review research on age-related decline and disease in vision, hearing, olfaction, taste, and touch. For each sense, we provide an epidemiological overview of impairment in humans, describing the physiological processes and diseases for each sense. We then discuss contributions made by research on animal models and ideas for future research. We additionally highlight the need for integrative, multimodal research across the senses as well as across disciplines. Long-term studies spanning multiple generations, including on species with longer life spans, are also highly valuable. Overall, integrative studies of appropriate animal models have high translational potential for clinical applications, the development of novel diagnostics, therapies, and medical interventions and future research will continue to close gaps in these areas. Research on animal models to improve understanding of the biology of the aging senses and improve the healthspan and additional research on sensory systems hold special promise for new breakthroughs.

Olfactory deficits in aging and Alzheimer's-spotlight on inhibitory interneurons

Cognitive function in healthy aging and neurodegenerative diseases like Alzheimer's disease (AD) correlates to olfactory performance. Aging and disease progression both show marked olfactory deficits in humans and rodents. As a clear understanding of what causes olfactory deficits is still missing, research on this topic is paramount to diagnostics and early intervention therapy. A recent development of this research is focusing on GABAergic interneurons. Both aging and AD show a change in excitation/inhibition balance, indicating reduced inhibitory network functions. In the olfactory system, inhibition has an especially prominent role in processing information, as the olfactory bulb (OB), the first relay station of olfactory information in the brain, contains an unusually high number of inhibitory interneurons. This review summarizes the current knowledge on inhibitory interneurons at the level of the OB and the primary olfactory cortices to gain an overview of how these neurons might influence olfactory behavior. We also compare changes in interneuron composition in different olfactory brain areas between healthy aging and AD as the most common neurodegenerative disease. We find that pathophysiological changes in olfactory areas mirror findings from hippocampal and cortical regions that describe a marked cell loss for GABAergic interneurons in AD but not aging. Rather than differences in brain areas, differences in vulnerability were shown for different interneuron populations through all olfactory regions, with somatostatin-positive cells most strongly affected.

Genomic signatures of sensory adaptation and evolution in pangolins

BACKGROUND

Pangolin is one of the most endangered mammals with many peculiar characteristics, yet the understanding of its sensory systems is still superficial. Studying the genomic basis of adaptation and evolution of pangolin's sensory system is expected to provide further potential assistance for their conservation in the future.

RESULTS

In this study, we performed a comprehensive comparative genomic analysis to explore the signature of sensory adaptation and evolution in pangolins. By comparing with the aardvark, Cape golden mole, and short-beaked echidna, 124 and 152 expanded gene families were detected in the genome of the Chinese and Malayan pangolins, respectively. The enrichment analyses showed olfactory-related genomic convergence among five concerned mammals. We found 769 and 733 intact OR genes, and 704 and 475 OR pseudogenes in the Chinese and Malayan pangolin species, respectively. Compared to other mammals, far more intact members of OR6 and OR14 were identified in pangolins, particularly for four genes with large copy numbers (OR6C2, OR14A2, OR14C36, and OR14L1). On the genome-wide scale, 1,523, 1,887, 1,110, and 2,732 genes were detected under positive selection (PSGs), intensified selection (ISGs), rapid evolution (REGs), and relaxed selection (RSGs) in pangolins. GO terms associated with visual perception were enriched in PSGs, ISGs, and REGs. Those related to rhythm and sound perception were enriched in both ISGs and REGs, ear development and morphogenesis were enriched in ISGs, and mechanical stimulus and temperature adaptation were enriched in RSGs. The convergence of two vision-related PSGs (OPN4 and ATXN7), with more than one parallel substituted site, was detected among five concerned mammals. Additionally, the absence of intact genes of PKD1L3, PKD2L1, and TAS1R2 and just six single-copy TAS2Rs (TAS2R1, TAS2R4, TAS2R7, TAS2R38, TAS2R40, and TAS2R46) were found in pangolins. Interestingly, we found two large insertions in TAS1R3, distributed in the N-terminal ectodomain, just in pangolins.

CONCLUSIONS

We found new features related to the adaptation and evolution of pangolin-specific sensory characteristics across the genome. These are expected to provide valuable and useful genome-wide genetic information for the future breeding and conservation of pangolins.

Differential functional change in olfactory bulb and olfactory eloquent areas in Parkinson's disease

Olfactory dysfunction, or hyposmia, frequently occurs as a prodromal symptom and ongoing sign of Parkinson's disease. Functional MRI is a powerful tool for studying functional changes in the olfactory brain regions in patients with Parkinson's disease. However, existing studies show inconsistent results and no study has measured olfactory functional MRI abnormalities in the human olfactory bulb directly. This is mainly due to the well-known susceptibility artefacts in conventional functional MRI images that affect several key olfactory-eloquent brain regions, and especially the olfactory bulb. In this study, olfactory functional MRI was performed using a recently developed functional MRI approach that can minimize susceptibility artefacts and measure robust functional MRI signals in the human olfactory bulb during olfactory stimulation. Experiments were performed on high magnetic field (7 T) in 24 early (<5 years of parkinsonian symptoms) Parkinson's disease patients and 31 matched healthy controls. Our data showed increased functional MRI signal changes (ΔS/S) in the olfactory bulb in patients with early Parkinson's disease, which correlated with behavioural olfactory measures. Temporally, functional MRI signals in the olfactory bulb returned to the pre-stimulus state earlier after reaching peak amplitude in patients with early Parkinson's disease, implicating a faster olfactory habituation effect. The piriform cortex showed reduced numbers of activated voxels in patients with early Parkinson's disease, which correlated with behavioural olfactory assessment. Several secondary olfactory regions including the orbitofrontal cortex, temporal pole and amygdala exhibited reduced numbers of activated voxels and increased functional MRI signal changes in patients with early Parkinson's disease. Our data also showed that functional MRI results are highly dependent on voxel selection in the functional analysis. In summary, we demonstrate differential spatial and temporal characteristics of olfactory functional MRI signals between the primary and secondary olfactory regions in patients with early Parkinson's disease. These results may assist the development of novel quantitative biomarkers (especially in the early stages of Parkinson's disease) to track and predict disease progression, as well as potential treatment targets for early intervention.

Reduction in the olfactory ability in aging Mitf mutant mice without evidence of neurodegeneration

Age-related decline occurs in most brain structures and sensory systems. An illustrative case is olfaction. The olfactory bulb (OB) undergoes deterioration with age, resulting in reduced olfactory ability. A decline in olfaction is also associated with early symptoms of neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). However, the underlying reasons are unclear. The microphthalmia-associated transcription factor (MITF) is expressed in the projection neurons (PNs) of the OB-the mitral and tufted (M/T) cells. Primary M/T cells from Mitf mutant mice show hyperactivity, potentially attributed to the reduced expression of a key potassium channel subunit, Kcnd3/Kv4.3. This influences intrinsic plasticity, an essential mechanism involving the non-synaptic regulation of neuronal activity. As neuronal hyperactivity often precedes neurodegenerative conditions, the current study aimed to determine whether the absence of Mitf causes degenerative effects during aging. Aged Mitf mutant mice showed reduced olfactory ability without inflammation. However, an increase in the expression of potassium channel subunit genes in the OBs of aged Mitf mi-vga9/mi-vga9 mice suggests that during aging, compensatory mechanisms lead to stabilization.

Aging is associated with a modality-specific decline in taste

Deficits in chemosensory processing are associated with healthy aging, as well as numerous neurodegenerative disorders, including Alzheimer's disease (AD). The fruit fly, Drosophila melanogaster, is a powerful model for studying chemosensation, aging, and aging-related pathologies, yet the effects of aging and neurodegeneration on taste function remain largely unexplored. Aging impaired response to sugars, but not medium-chain fatty acids that are sensed by a shared population of neurons. Selective expression of the human amyloid beta (Aβ) peptide phenocopied the effects of aging. Functional imaging of gustatory axon terminals revealed reduced response to sugar, but not fatty acids. Axonal innervation of the fly taste center was largely intact in aged flies; however, axonal innervation was reduced upon expression of Aβ. A comparison of transcript expression within the sugar-sensing taste neurons revealed age-related changes in 66 genes. Together, these findings suggest that different mechanisms underly taste deficits in aged and AD model flies.

Global research Activity on olfactory marker protein (OMP): A bibliometric and visualized analysis

Olfactory marker protein (OMP) is extensively studied in mature olfactory receptor neurons (ORNs) for understanding olfaction physiology. However, no bibliometric analysis on this topic exists. We conducted a bibliometric analysis of OMP research articles, wherein the publication count was assessed by year, country, journal, and author, collaboration by country, and productivity of the authors. Additionally, key terms and research themes were identified. Using the search phrase "olfactory marker protein" in Scopus, we retrieved 691 original research articles by 2487 authors since 1974. Publications showed an increasing trend, with the United States leading in quantity and collaboration. Our thematic map highlights "Olfactory bulb, regeneration, olfactory" as the primary research domain, while "olfaction, olfactory sensory neuron, glomerulus" and "olfactory receptor neurons, apoptosis, olfactory dysfunction" emerge as essential future research topics. These bibliometric findings offer insights into the global OMP research landscape, guiding researchers in potential collaborations and intriguing future research fields.

Aging is associated with a modality-specific decline in taste

Deficits in chemosensory processing are associated with healthy aging, as well as numerous neurodegenerative disorders, including Alzheimer's Disease (AD). In many cases, chemosensory deficits are harbingers of neurodegenerative disease, and understanding the mechanistic basis for these changes may provide insight into the fundamental dysfunction associated with aging and neurodegeneration. The fruit fly, Drosophila melanogaster , is a powerful model for studying chemosensation, aging, and aging-related pathologies, yet the effects of aging and neurodegeneration on chemosensation remain largely unexplored in this model, particularly with respect to taste. To determine whether the effects of aging on taste are conserved in flies, we compared the response of flies to different appetitive tastants. Aging impaired response to sugars, but not medium-chain fatty acids that are sensed by a shared population of neurons, revealing modality-specific deficits in taste. Selective expression of the human amyloid beta (Aβ) 1-42 peptide bearing the Arctic mutation (E693E) associated with early onset AD in the neurons that sense sugars and fatty acids phenocopies the effects of aging, suggesting that the age-related decline in response is localized to gustatory neurons. Functional imaging of gustatory axon terminals revealed reduced response to sugar, but not fatty acids. Axonal innervation of the fly taste center was largely intact in aged flies, suggesting that reduced sucrose response does not derive from neurodegeneration. Conversely, expression of the amyloid peptide in sweet-sensing taste neurons resulted in reduced innervation of the primary fly taste center. A comparison of transcript expression within the sugar-sensing taste neurons revealed age-related changes in 66 genes, including a reduction in odorant-binding protein class genes that are also expressed in taste sensilla. Together, these findings suggest that deficits in taste detection may result from signaling pathway-specific changes, while different mechanisms underly taste deficits in aged and AD model flies. Overall, this work provides a model to examine cellular deficits in neural function associated with aging and AD.

Serial reversal learning in an olfactory discrimination task in 3xTg-AD mice

Male and female 3xTg-AD mice between 5 and 24 mo of age and their B6129F2/J wild-type controls were tested on a series of 18 olfactory discrimination and reversal tasks in an operant olfactometer. All mice learned the odor discriminations and reversals to a criterion of 85% correct, but the 3xTg-AD mice made fewer errors than the B6129F2/J mice in the odor discriminations and in the first six reversal learning tasks. Many mice showed evidence of near errorless learning, and on the reversal tasks the 3xTg-AD mice showed more instances of near errorless learning than the B6129F2/J mice. There was no evidence of an age effect on odor discrimination, but there was a decrease in errorless reversal learning in aged B6129F2/J mice. In long-term memory tests, there was an increase in the number of errors made but no genotype difference. The high level of performance indicates that the mice were able to develop a "learning to learn" strategy. The finding that the 3xTg-AD mice outperformed their littermate controls provides an example of paradoxical functional facilitation in these mice.

Loss of smelling is an early marker of aging and is associated with inflammation and DNA damage in C57BL/6J mice

Olfactory dysfunction is a prevalent symptom and an early marker of age-related neurodegenerative diseases in humans, including Alzheimer's and Parkinson's Diseases. However, as olfactory dysfunction is also a common symptom of normal aging, it is important to identify associated behavioral and mechanistic changes that underlie olfactory dysfunction in nonpathological aging. In the present study, we systematically investigated age-related behavioral changes in four specific domains of olfaction and the molecular basis in C57BL/6J mice. Our results showed that selective loss of odor discrimination was the earliest smelling behavioral change with aging, followed by a decline in odor sensitivity and detection while odor habituation remained in old mice. Compared to behavioral changes related with cognitive and motor functions, smelling loss was among the earliest biomarkers of aging. During aging, metabolites related with oxidative stress, osmolytes, and infection became dysregulated in the olfactory bulb, and G protein coupled receptor-related signaling was significantly down regulated in olfactory bulbs of aged mice. Poly ADP-ribosylation levels, protein expression of DNA damage markers, and inflammation increased significantly in the olfactory bulb of older mice. Lower NAD⁺ levels were also detected. Supplementation of NAD⁺ through NR in water improved longevity and partially enhanced olfaction in aged mice. Our studies provide mechanistic and biological insights into the olfaction decline during aging and highlight the role of NAD⁺ for preserving smelling function and general health.

Variation in Group Composition Alters an Early-Stage Social Phenotype in hAPP-Transgenic J20 Mice

BACKGROUND

Altered social behavior is one of the symptoms of Alzheimer's disease (AD) that results in social withdrawal and loneliness and provides a major burden on patients and their relatives. Furthermore, loneliness is associated with an increased risk to develop AD and related dementias.

OBJECTIVE

We aimed to investigate if altered social behavior is an early indicator of amyloid-β (Aβ) pathology in J20 mice, and if co-housing with wild type (WT) mice can positively influence this social phenotype.

METHODS

The social phenotype of group-housed mice was assessed using an automated behavioral scoring system for longitudinal recordings. Female mice were housed in a same-genotype (4 J20 or WT mice per colony) or mixed-genotype (2 J20 mice + 2 WT mice) colony. At 10 weeks of age, their behavior was assessed for five consecutive days.

RESULTS

J20 mice showed increased locomotor activity and social sniffing, and reduced social contact compared to WT mice housed in same-genotype colonies. Mixed-genotype housing reduced the social sniffing duration of J20 mice, increased social contact frequency of J20 mice, and increased nest hide by WT mice.

CONCLUSION

Thus, altered social behavior can be used as an early indicator of Aβ-pathology in female J20 mice. Additionally, when co-housed with WT mice, their social sniffing phenotype is not expressed and their social contact phenotype is reduced. Our findings highlight the presence of a social phenotype in the early stages of AD and indicate a role for social environment variation in the expression of social behavior of WT and J20 mice.

Neurofilament light chain plasma levels are associated with area of brain damage in experimental cerebral malaria

Neurofilament light chain (NfL), released during central nervous injury, has evolved as a powerful serum marker of disease severity in many neurological disorders, including infectious diseases. So far NfL has not been assessed in cerebral malaria in human or its rodent model experimental cerebral malaria (ECM), a disease that can lead to fatal brain edema or reversible brain edema. In this study we assessed if NfL serum levels can also grade disease severity in an ECM mouse model with reversible (n = 11) and irreversible edema (n = 10). Blood–brain-barrier disruption and brain volume were determined by magnetic resonance imaging. Neurofilament density volume as well as structural integrity were examined by electron microscopy in regions of most severe brain damage (olfactory bulb (OB), cortex and brainstem). NfL plasma levels in mice with irreversible edema (317.0 ± 45.01 pg/ml) or reversible edema (528.3 ± 125.4 pg/ml) were significantly increased compared to controls (103.4 ± 25.78 pg/ml) by three to five fold, but did not differ significantly in mice with reversible or irreversible edema. In both reversible and irreversible edema, the brain region most affected was the OB with highest level of blood–brain-barrier disruption and most pronounced decrease in neurofilament density volume, which correlated with NfL plasma levels (r = − 0.68, p = 0.045). In cortical and brainstem regions neurofilament density was only decreased in mice with irreversible edema and strongest in the brainstem. In reversible edema NfL plasma levels, MRI findings and neurofilament volume density normalized at 3 months’ follow-up. In conclusion, NfL plasma levels are elevated during ECM confirming brain damage. However, NfL plasma levels fail short on reliably indicating on the final outcomes in the acute disease stage that could be either fatal or reversible. Increased levels of plasma NfL during the acute disease stage are thus likely driven by the anatomical location of brain damage, the olfactory bulb, a region that serves as cerebral draining pathway into the nasal lymphatics.

A Signal Detection Analysis of Olfactory Learning in 12-Month-Old 5xFAD Mice

Although Alzheimer’s disease is most often studied in terms of memory impairments, olfactory dysfunction begins in the early stages. We tested olfactory learning, sensitivity, and response bias using signal detection methods in 12-month-old male and female 5xFAD mice and their wildtype controls in the operant olfactometer. Odor detection was not reduced in the 5xFAD mice, but learning was, which was worse in female 5xFAD mice than in males. Female mice were more conservative in their response strategy. Signal detection analysis allows us to discriminate between cognitive and sensory deficits of male and female mouse models of AD.

Olfaction, Cognitive Impairment, and PET Biomarkers in Community-Dwelling Older Adults

BACKGROUND

Olfactory deficits are early features of preclinical Alzheimer's disease (AD). Whether olfaction is associated with PET biomarkers among community-dwelling older adults is less clear.

OBJECTIVE

Investigate cross-sectional and longitudinal associations of olfaction with mild cognitive impairment (MCI) and amyloid-β (Aβ) and tau deposition.

METHODS

We analyzed 364 initially cognitively normal participants (58% women, 24% black) who had baseline olfaction data and subsequent cognitive assessments during an average 2.4-year. A subset of 129 had PET-PiB (Aβ) (n = 72 repeated) and 105 had 18F-flortaucipir (FTP)-PET (tau) (n = 44 repeated). Olfaction was measured using a 16-item Sniffin' Sticks Odor Identification Test. The association of olfaction with incident MCI was examined using Cox regression. Associations with PiB-distribution volume ratio (DVR) and FTP-standardized uptake value ratio (SUVR) were examined using partial correlation. We tested whether PiB+/-status modified these associations. Analyses were adjusted for demographics and olfactory test version.

RESULTS

17 (5%) participants developed MCI. Each unit lower odor identification score was associated with 22% higher risk of developing MCI (p = 0.04). In the PET subset, lower scores were associated with higher mean cortical DVR and DVR in orbitofrontal cortex (OFC), precuneus, and middle temporal gyrus (p≤0.04). The "olfaction*PiB+/-" interaction in OFC DVR was significant (p = 0.03), indicating the association was limited to PiB positive individuals. Greater decline in odor identification score was associated with greater increase in anterior OFC DVR and entorhinal tau SUVR (p≤0.03).

CONCLUSION

Among community-dwelling older adults, poorer olfaction predicts incident MCI and is associated with overall and regional Aβ. Greater olfaction decline is associated with faster Aβ and tau accumulation in olfaction-related regions. Whether olfaction predicts AD-related neurodegenerative changes warrants further investigations.

The Olfactory System as Marker of Neurodegeneration in Aging, Neurological and Neuropsychiatric Disorders

Research studies that focus on understanding the onset of neurodegenerative pathology and therapeutic interventions to inhibit its causative factors, have shown a crucial role of olfactory bulb neurons as they transmit and propagate nerve impulses to higher cortical and limbic structures. In rodent models, removal of the olfactory bulb results in pathology of the frontal cortex that shows striking similarity with frontal cortex features of patients diagnosed with neurodegenerative disorders. Widely different approaches involving behavioral symptom analysis, histopathological and molecular alterations, genetic and environmental influences, along with age-related alterations in cellular pathways, indicate a strong correlation of olfactory dysfunction and neurodegeneration. Indeed, declining olfactory acuity and olfactory deficits emerge either as the very first symptoms or as prodromal symptoms of progressing neurodegeneration of classical conditions. Olfactory dysfunction has been associated with most neurodegenerative, neuropsychiatric, and communication disorders. Evidence revealing the dual molecular function of the olfactory receptor neurons at dendritic and axonal ends indicates the significance of olfactory processing pathways that come under environmental pressure right from the onset. Here, we review findings that olfactory bulb neuronal processing serves as a marker of neuropsychiatric and neurodegenerative disorders.