Study reveals how aging accelerates molecular changes in the brain, offering new hope for tackling cognitive decline and mental illness.
Resource: Single-nucleus transcriptomic profiling of human orbitofrontal cortex reveals convergent effects of aging and psychiatric disease. Image Credit: Atthapon Raksthaput / Shutterstock
In a recent study published in the journal Nature Neuroscience, researchers used next-gen single-nucleus RNA sequencing (snRNA-seq) techniques to elucidate age-associated gene expression changes that occur in orbitofrontal cortex (OFC) cells. They further investigated transcriptomic changes across different cell types that occur in OFC cells due to various common psychiatric disorders such as schizophrenia (SCZ) and Alzheimer's disease (AD).
Their findings reveal that the biological mechanisms (specifically the alterations in gene expression) underpinning cognitive dysfunction and memory loss due to aging show a notable convergence with those seen in psychiatric patients, particularly those diagnosed with AD. They identified LAMP5+LHX6+ interneurons as the cells experiencing the most significant degree of age-associated alteration. Notably, aging-associated changes appear to be to be accelerated in patients with preexisting psychiatric conditions.
These findings present substantial advancements in our understanding of cognitive aging and may form the basis for developing novel therapeutic interventions against age-related pathologies.
Background
Aging is a natural and complex process characterized by the deterioration of physiological (physical and mental) functions necessary for life. Unfortunately, the mechanisms underpinning aging remain poorly understood, particularly those associated with the brain. Extensive research on mice, nonhuman primates, and, in rare cases, human postmortem tissue has revealed that aging brains are structurally and functionally distinct from their younger counterparts.
The starkest distinctions between young and old brains can be observed in the white matter tracts and the prefrontal cortex. Interestingly, neuroimaging investigations of the brains of younger psychiatric patients reveal similar changes as those found in older neurotypical brains. Conversely, most psychiatric conditions are known to worsen with progressing age. Unfortunately, the molecular mechanisms and gene expression changes underpinning these observations remain elusive.
Medical advances ensure that human life expectancy continues to increase, resulting in a larger proportion of elderly individuals and, therefore, age-associated diseases than ever before. A simultaneous increase in the incidence and prevalence of psychiatric disorders makes understanding the cell-level biological changes occurring in both aging and neurodegenerative disorders a crucial first step in the future development of therapeutic interventions against these often debilitating conditions.
About the study
The present study aimed to address current knowledge gaps through transcriptomic analysis of nuclei extracted from the orbitofrontal cortex (OFC) of postmortem human brains (both neurotypical and with psychiatric disorders) across age groups (26-84 years), thereby elucidating the gene expression alterations associated with the two pathologies.
Samples for the study (n = 87) were obtained from the New South Wales Brain Tissue Resource Centre with written consent from donors or their immediate families. Individuals with a psychiatric diagnosis (bipolar disorder, major depressive disorder [MDD], schizophrenia [SCZ]) were classified as the psychiatric cohort (n = 54), while those without were included in the neurotypical cohort (n = 33). Dounce homogenization of samples immersed in nucleus extraction buffer was used to extract nuclei for downstream analysis.
The Chromium Single Cell 3′ Reagents kit v3.1, in tandem with the Illumina NovaSeq 6000 System, was used for single-nucleus RNA sequencing (snRNA-seq) library preparation with a targeted recovery of 10,000 for each sample. The resulting sequences were then aligned and demultiplexed using the Cell Ranger v6.0.1 tool. These sequences were labeled with known marker genes from the Allen Brain Atlas.
Age-associated cellular composition was estimated by comparing the proportions of observed cell types with the corresponding donor's age at death. A similar approach using snRNA-seq data instead of absolute cell proportions was used to elucidate transcriptomic differences ('differential expression' [DE]) across age groups and to identify cells with the highest degree of age-related gene expression alterations.
Comparisons between DE results from neurotypical and psychiatric brains were used to elucidate signatures (shared and unique) across the two pathologies (age and disease). To identify if observed transcriptomic alterations could result in cell-type-specific contributions to cognitive decline and other neurodegenerative outcomes, an over-representation analysis was carried out.
"To validate our cell-type-specific findings, we compared our identified DE genes in microglia and astrocytes (major cell-type cluster) to datasets that have identified gene expression changes over the course of aging in purified microglia and astrocytes from the cerebral cortex, respectively."
Study findings
Successfully extracted nuclei from donor OFCs totalled ~800,000. Demographic and neuropathological evaluations between neurotypical and psychiatric patient brains revealed statistical similarities between their ages, sex, postmortem interval (PMI), and RNA integrity number (RIN), validating biologically meaningful comparisons between these cohorts.
Cell composition analysis revealed that the abundances of most cell types did not decrease with age. Oligodendrocyte precursor cells (OPCs) were the only exception, presenting significant age-associated decreases in their relative proportions. Interestingly, while OPCs decreased, there was a trend of increase in oligodendrocytes, highlighting the complex nature of cellular changes with aging. In contrast, all investigated cell types presented substantial alterations (DE n = 3,299) in their age-associated transcriptome profiles. Upper-layer excitatory neurons were the most affected by advancing age.
DE from both age- and psychiatric-associated pathologies were observed to overlap/converge, particularly in oligodendrocytes and astrocytes. Notably, psychiatric conditions were found to accelerate age-associated DE, highlighting the additive effects of their molecular pathways.
"Differential gene expression analysis within the identified 21 cell types indicated that all cell types are affected by aging and that the majority of age-associated transcriptional changes are cell-type specific. However, a specific cell type (inhibitory LAMP5+LHX6+ neurons (In_LAMP5_2)) seems to be most strongly affected by aging. Interestingly, this LAMP5+LHX6+ subtype has been reported to increase in abundance in the primate cortex and to most closely resemble ivy cells of the mouse hippocampus."
Conclusions
This study highlights the overlap in cell-type-specific differential gene expression accompanying natural aging and psychiatric disease. It characterizes these changes, thereby comprehensively describing the biological pathways associated with loss of neural function and cognitive decline in the human OFC. Together, these data represent a crucial first step in discovering therapeutic interventions against both conditions by identifying their shared molecular underpinnings.
Journal reference:
- Fröhlich, A.S., Gerstner, N., Gagliardi, M. et al. Single-nucleus transcriptomic profiling of human orbitofrontal cortex reveals convergent effects of aging and psychiatric disease. Nat Neurosci (2024), DOI – 10.1038/s41593-024-01742-z, https://www.nature.com/articles/s41593-024-01742-z