Anticipated brain molecular aging in major depression

Accelerated biological brain aging in major depressive disorder

Major depressive disorder (MDD) patients commonly encounter multiple types of functional disabilities, such as social, physical, and role functioning. MDD is related to an accreted risk of brain atrophy, aging-associated brain diseases, and mortality. Based on recently available studies, there are correlations between notable biological brain aging and MDD in adulthood. Despite several clinical and epidemiological studies that associate MDD with aging phenotypes, the underlying mechanisms in the brain remain unknown. The key areas in the study of biological brain aging in MDD are structural brain aging, impairment in functional connectivity, and the impact on cognitive function and age-related disorders. Various measurements have been used to determine the severity of brain aging, such as the brain age gap estimate (BrainAGE) or brain-predicted age difference (BrainPAD). This review summarized the current results of brain imaging data on the similarities between the manifestation of brain structural changes and the age-associated processes in MDD. This review also provided recent evidence of BrainPAD or BrainAGE scores in MDD, brain structural abnormalities, and functional connectivity, which are commonly observed between MDD and age-associated processes. It serves as a basis of current reference for future research on the potential areas of investigation for diagnostic, preventive, and potentially therapeutic purposes for brain aging in MDD.

Plasma neurofilament light chain in association to late-life depression in the general population

AIM

Investigating what is underlying late-life depression is becoming increasingly important with the rapidly growing elderly population. Yet, the associations between plasma biomarkers of neuroaxonal damage and late-life depression remain largely unclear. Therefore, we determined cross-sectional and longitudinal associations of neurofilament light chain (NfL) with depression in middle-aged and elderly individuals, and total tau, β-amyloid 40 and 42 for comparison.

METHODS

We included 3,895 participants (71.78 years [SD = 7.37], 53.4% women) from the population-based Rotterdam Study. Between 2002 and 2005, NfL, total tau, β-amyloid 40 and β-amyloid 42 were determined in blood and depressive symptoms were measured with the Center for Epidemiologic Studies Depression scale (CES-D). Incident depressive events (clinically relevant depressive symptoms, depressive syndromes, major depressive disorders) were measured prospectively with the Center for Epidemiologic Studies Depression, a clinical interview and follow-up of medical records over a median follow-up of 7.0 years (interquartile range 1.80). We used linear and Cox proportional hazard regression models.

RESULTS

Each log2 pg./mL increase in NfL was cross-sectionally associated with more depressive symptoms (adjusted mean difference: 0.32, 95% CI 0.05-0.58), as well as with an increased risk of any incident depressive event over time (hazard ratio: 1.22, 95% CI 1.01-1.47). Further, more amyloid-β 40 was cross-sectionally associated with more depressive symptoms (adjusted mean difference: 0.70, 95% CI 0.15-1.25).

CONCLUSION

Higher levels of NfL are cross-sectionally associated with more depressive symptoms and a higher risk of incident depressive events longitudinally. The association was stronger for NfL compared to other plasma biomarkers, suggesting a potential role of neuroaxonal damage in developing late-life depression.

The physio-affective phenome of major depression is strongly associated with biomarkers of astroglial and neuronal projection toxicity which in turn are associated with peripheral inflammation, insulin resistance and lowered calcium

BACKGROUND

Major depressive disorder (MDD) is characterized by elevated activity of peripheral neuro-immune and neuro-oxidative pathways, which may cause neuro-affective toxicity by disrupting neuronal circuits in the brain. No study has explored peripheral indicators of neuroaxis damage in MDD in relation to serum inflammatory and insulin resistance (IR) biomarkers, calcium, and the physio-affective phenome consisting of depressive, anxious, chronic fatigue, and physiosomatic symptoms.

METHODS

Serum levels of phosphorylated tau protein 217 (P-tau217), platelet-derived growth factor receptor beta (PDGFR), neurofilament light chain (NF-L), glial fibrillary acidic protein (GFAP), C-reactive protein (CRP), calcium and the HOMA2-insulin resistance (IR) index were measured in 94 MDD patients and 47 controls.

RESULTS

61.1 % of the variance in the physio-affective phenome (conceptualized as a factor extracted from depression, anxiety, fatigue and physiosomatic symptoms) is explained by the regression on GFAP, NF-L, P-tau2017, PDGFRβ and HOMA2-IR (all positively associated), and decreased calcium. In addition, CRP and HOMA2-IR predicted 28.9 % of the variance in the neuroaxis index. We observed significant indirect effects of CRP and calcium on the physio-affective phenome which were partly mediated by the four neuroaxis biomarkers. Annotation and enrichment analysis revealed that the enlarged GFAP, P-tau217, PDGFR, and NF-L network was enriched in glial cell and neuronal projections, the cytoskeleton and axonal transport, including a mitochondrion.

CONCLUSIONS

Peripheral inflammation and IR may damage the astroglial and neuronal projections thereby interfering with mitochondrial transport. This neurotoxicity, combined with inflammation, IR and lowered calcium, may, at least in part, induce the phenome of MDD.

Neurofilaments light: Possible biomarker of brain modifications in bipolar disorder

INTRODUCTION

Brain white matter (WM) abnormalities are biomarkers that seem to be involved in bipolar disorder (BD) aetiology and maintenance. Evidences suggest a possible association between neurodegeneration, neuroaxonal alterations and BD. A biomarker that is recently drawing attention is neurofilaments light (NfL) chain, a cytoskeletal intermediate filament protein expressed in neurons. To investigate neuroimaging alterations associated with BD, we studied the association between NfL levels and WM microstructure.

METHODS

NfL plasma quantification was performed in a sample of 45 depressed BD patients compared with 29 healthy controls (HC) using Quanterix SIMOA assay. Statistical analysis were conducted to evaluate NfL levels differences between BD patients and controls. Analyses of the diffusion data were performed using Tract Based Spatial Statistics (TBSS) on Diffusion Tensor images acquired using a 3.0 Tesla MR scanner.

RESULTS

Patients had higher NfL levels than HC (9.13 ± 4.78 vs 4.28 ± 2.39 pg/ml; p < 0.001). The separate-slopes analysis of variance showed a significant interaction of age with diagnosis (Likelihood-ratio test: χ2 = 27.52, p < 0.0001) with significant effects only in the BD sample (p = 0.023). The TBSS analysis, performed within the BD sample, showed a significant positive correlation between NfL levels and axial diffusivity (AD) in a wide single cluster encompassing several tracts.

DISCUSSION

Our results suggest that the physiological age-dependent increment of NfL level is augmented in BD, possibly because of increased remodelling and plasticity processes related to an accelerated ageing condition. The positive association between NfL levels and AD, may reflect a condition of remyelination and axonal regeneration.

Ayurvedic formulations amalaki rasayana and rasa sindoor improve age-associated memory deficits in mice by modulating dendritic spine densities

BACKGROUND

Emerging reports indicate that age-associated cognitive decline begins with the transition from young to middle-aged, and this neurological condition manifests mainly due to the progressive impairment in the adaptive homeostasis process. Moreover, cognitive decline is associated with neurodegenerative changes in older adults.

OBJECTIVE

Previous studies have shown that the administration of Ayurvedic formulations restores the homeostatic pathways and ameliorates neurodegeneration in animal models of neurodegenerative diseases. Therefore, we wanted to check whether Ayurvedic formulations can rescue or delay the age-associated cognitive decline in middle-aged mice.

MATERIAL AND METHODS

We fed two-month-old mice with amalaki aasayana (AR, 1025 mg/kg per day) or rasa sindoor (RS, 41 mg/kg per day) mixed in a gelatin-based jelly for six months. Mice eating regular chow or blank jelly served as control. Subsequently, we looked at the improvements in the cognitive and behavioural traits of the treated animals. We have also analysed the effect of these formulations on the dendritic processes of neurons, glial activation, and the formation of corpora amylacea.

RESULTS

We found a significant improvement in episodic, working- and reference-spatiotemporal memory in animals fed on AR or RS. Microscopic analyses revealed a significant increase in the dendritic spine density in the apical dendrites of the hippocampal pyramidal neurons. The treatment, however, did not significantly affect gliosis and corpora amylacea in the brains.

CONCLUSIONS

Both AR and RS showed beneficial effects on memory functions of the middle-aged mice, possibly due to their effect on the dendritic spine densities. Our findings provide strong evidence to conclude that formulations AR and RS can prevent or delay the onset of age-associated cognitive decline.

Altered neuroaxonal integrity in schizophrenia and major depressive disorder assessed with neurofilament light chain in serum

BACKGROUND

Schizophrenia (SZ) and major depressive disorders (MDD) have been frequently linked to anatomical brain alterations. However, the relationship between brain pathology, inflammation and clinical symptoms in these disorders is still unclear. Thus, by applying novel blood markers of neuroaxonal integrity such as neurofilament light chain (NfL), we can now address main issues in psychiatric research and potentially offer innovative diagnostic tools toward better clinical characterizations and monitoring in both SZ and MDD.

METHODS

NfL levels were measured in serum of 44 patients with SZ and in 41 patients with MDD applying single molecule array technology and compared to a healthy norm population. Main inflammatory markers (C- reactive protein, interleukins IL-6 and IL-10) were measured to define patients with inflammatory phenotype. The Digit Symbol Substitution Task (DSST) and the Letter-Number-Sequencing Task were performed to estimate cognitive function in both groups.

RESULTS

NfL levels in MDD group (but not in SZ group) were significantly higher than reference values of healthy norm population. A higher than expected proportion of patients with NfL levels above age-specific cut-off values was observed in both SZ and MDD groups. No correlation was observed between NfL and inflammatory markers. A negative correlation between DSST and NfL-values was observed in patients with MDD.

CONCLUSIONS

Both SZ and MDD showed elevated serum levels of NfL, which were independent from inflammatory markers but associated with cognitive performance.

TERT rs2736100 and TERC rs16847897 genotypes moderate the association between internalizing mental disorders and accelerated telomere length attrition among HIV+ children and adolescents in Uganda

BACKGROUND

Internalizing mental disorders (IMDs) (depression, anxiety and post-traumatic stress disorder) have been associated with accelerated telomere length (TL) attrition; however, this association has not been investigated in the context of genetic variation that has been found to influence TL. We have previously reported an association between IMDs and accelerated TL attrition among Ugandan HIV+ children and adolescents. This study investigated the moderating effects of selected single nucleotide polymorphisms in the telomerase reverse transcriptase gene (TERT) (rs2736100, rs7726159, rs10069690 and rs2853669) and the telomerase RNA component gene (TERC) (rs12696304, rs16847897 and rs10936599) on the association between IMDs and TL, among Ugandan HIV+ children (aged 5-11 years) and adolescents (aged 12-17 years).

RESULTS

We found no significant interaction between IMDs as a group and any of the selected SNPs on TL at baseline. We observed significant interactions of IMDs with TERT rs2736100 (p = 0.007) and TERC rs16847897 (p = 0.012), respectively, on TL at 12 months.

CONCLUSIONS

TERT rs2736100 and TERC rs16847897 moderate the association between IMDs and TL among Ugandan HIV+ children and adolescents at 12 months. Understanding the nature of this association may shed light on the pathophysiological mechanisms underlying advanced cellular aging in IMDs.

Associations Between Smoking, Alcohol Consumption, Physical Activity and Depression in Middle-Aged Premenopausal and Postmenopausal Women

Background: Changes in lifestyle factors are known to affect mood. However, there is insufficient evidence supporting the association between smoking, alcohol consumption, physical activity and depression in middle-aged women who are likely to experience rapid hormonal changes. Methods: We used a nationwide database of medical records in South Korea. 901,721 premenopausal and 943,710 postmenopausal women aged 40 years or older included in this study. Information on smoking, alcohol consumption, physical activity was identified from health examination data and followed up for the occurrence of depression using claims data. Results: Compared with never-smokers, ex-smokers and current smokers among premenopausal and postmenopausal women showed an increased risk of depression in a dose-dependent manner (aHR 1.13 for ex-smokers; aHR 1.23 for current smokers). Compared with non-drinkers, mild drinkers showed a decreased risk of depression (aHR 0.98 for premenopausal women; aHR 0.95 for postmenopausal women), and heavy drinkers showed an increased risk of depression both among premenopausal (aHR 1.20) and postmenopausal women (aHR 1.05). The risk of depression due to smoking and heavy alcohol consumption was higher in premenopausal women than in postmenopausal women. Compared with those who had not engaged in regular physical activity, those who had engaged showed a decreased risk of depression both among premenopausal (aHR 0.96) and postmenopausal women (aHR 0.95). Conclusions: Smoking and heavy alcohol consumption increased the risk of depression, and the increased risk was prominent in premenopausal than in postmenopausal women. Regular physical activity decreased the risk of depression both in premenopausal and postmenopausal women.

Novel polygenic risk score as a translational tool linking depression-related changes in the corticolimbic transcriptome with neural face processing and anhedonic symptoms

Convergent data from imaging and postmortem brain transcriptome studies implicate corticolimbic circuit (CLC) dysregulation in the pathophysiology of depression. To more directly bridge these lines of work, we generated a novel transcriptome-based polygenic risk score (T-PRS), capturing subtle shifts toward depression-like gene expression patterns in key CLC regions, and mapped this T-PRS onto brain function and related depressive symptoms in a nonclinical sample of 478 young adults (225 men; age 19.79 +/- 1.24) from the Duke Neurogenetics Study. First, T-PRS was generated based on common functional SNPs shifting CLC gene expression toward a depression-like state. Next, we used multivariate partial least squares regression to map T-PRS onto whole-brain activity patterns during perceptual processing of social stimuli (i.e., human faces). For validation, we conducted a comparative analysis with a PRS summarizing depression risk variants identified by the Psychiatric Genomics Consortium (PGC-PRS). Sex was modeled as moderating factor. We showed that T-PRS was associated with widespread reductions in neural response to neutral faces in women and to emotional faces and shapes in men (multivariate p < 0.01). This female-specific reductions in neural response to neutral faces was also associated with PGC-PRS (multivariate p < 0.03). Reduced reactivity to neutral faces was further associated with increased self-reported anhedonia. We conclude that women with functional alleles mimicking the postmortem transcriptomic CLC signature of depression have blunted neural activity to social stimuli, which may be expressed as higher anhedonia.

Maternal Depressive Symptoms During Pregnancy and Brain Age in Young Adult Offspring: Findings from a Prenatal Birth Cohort

Maternal depression during pregnancy is associated with elevated risk of anxiety and depression in offspring, but the mechanisms are incompletely understood. Here we conducted a neuroimaging follow-up of a prenatal birth cohort from the European Longitudinal Study of Pregnancy and Childhood (n = 131; 53% women, age 23–24) to test whether deviations from age-normative structural brain development in young adulthood may partially underlie this link. Structural brain age was calculated based on previously published neuroanatomical age prediction models using cortical thickness maps from healthy controls aged 6–89. Brain age gap was computed as the difference between chronological and structural brain age. Participants also completed self-report measures of anxiety and mood dysregulation. Further, mothers of a subset of participants (n = 103, 54% women) answered a self-report questionnaire in 1990–1992 about depressive symptoms during pregnancy. Higher exposure to maternal depressive symptoms in utero showed a linear relationship with elevated brain age gap, which showed a quadratic relationship with anxiety and mood dysregulation in the young adult offspring. Our findings suggest that exposure to maternal depressive symptoms in utero may be associated with accelerated brain maturation and that deviations from age-normative structural brain development in either direction predict more anxiety and dysregulated mood in young adulthood.

Molecular and Cellular Evidence for Age by Disease Interactions: Updates and Path Forward

Characterization of age-associated gene expression changes shows that the brain engages a specific set of genes and biologic pathways along a continuous life-long trajectory and that these genes and pathways overlap with those associated with brain-related disorders. Based on this correlative observation, we have suggested a model of age-by-disease interaction by which brain ageing promotes biologic changes associated with diseases and where deviations from expected age-related trajectories, due to biologic and environmental factors, contribute to defining disease risk or resiliency. In this review, we first evaluate various biomarkers that can be used to study age-by-disease interactions and then focus on transcriptome analysis (i.e., the set of all expressed genes) as a useful tool to explore this interaction. Using the specific example of brain-derived neurotrophic factor and brain-derived neurotrophic factor-associated genes, we then describe molecular events and mechanisms potentially contributing to age-by-disease interactions. Finally, we suggest that long-term biologic adaptations within distinct cellular components of cortical microcircuits, as determined by transcriptome analysis, may integrate and mediate the effects of ageing and diseases. Moving forward, we suggest that analysis of transcriptome similarities between ageing and small molecule-induced system perturbations may lead to novel therapeutics discovery.

Telomere length in psychiatric disorders: Is it more than an ageing marker?

OBJECTIVES

Psychiatric and substance-use disorders have been associated with premature biological ageing. Telomere length (TL), considered an ageing marker, has been analysed in psychiatric disorders, and to a lesser extent in substance-use disorders, with recent findings suggesting TL may be related to disease pathology.

METHODS

We conducted a critical and non-systematic literature search of TL studies published up to June 2016 in psychiatric and substance-use disorders, focussing on studies describing mechanisms, including studies linking telomere biology with genetic factors, stress and mitochondrial alterations (104 studies selected).

RESULTS

Patients with major depressive disorder and anxiety appear to have shorter leukocyte telomeres compared to controls. Inconclusive results are found for other psychiatric disorders and for substance-use disorders. This may be due in part to differences in medication treatment and response, as studies suggest that some psychotropic medications may modulate TL. Importantly, some studies establish a relationship between telomere machinery, stress and mitochondria function in psychiatric and substance-use disorders.

CONCLUSIONS

While further longitudinal studies considering telomere genetics are needed to clarify the cause-effect link between telomeres and mitochondria function in psychiatric and substance-use disorders, the recent findings linking these biological processes suggest that telomeres may be more than ageing markers.

Low brain-derived neurotrophic factor levels in post-mortem brains of older adults with depression and dementia in a large clinicopathological sample

BACKGROUND

Disturbances in peripheral brain-derived neurotrophic factor (BDNF) have been reported in major depressive disorder (MDD). However, there are no studies measuring BDNF levels directly in post-mortem brains of older subjects with MDD and dementia. We aimed to verify if brain BDNF levels were lower in older adults with lifetime history of MDD with and without dementia.

METHODS

BDNF levels of post-mortem brains from 80 community-dwelling older individuals with lifetime MDD with and without dementia were compared with levels from 80 controls without lifetime MDD. Participants with no reliable close informant, or with prolonged agonal state were excluded. Lifetime MDD was defined as at least one previous episode according to the Structured Clinical Interview for DSM (SCID).

RESULTS

BDNF levels were lower in the MDD group with dementia than in participants with dementia and without MDD as confirmed by multivariate analysis adjusted for clinical and cardiovascular risk factors (ß = -0.106, 95%CI = -0.204; -0.009, p = 0.034). No difference was found in the group with MDD without dementia compared with their controls.

LIMITATIONS

The retrospective assessment of a lifetime history of depression may be subject to information bias and this study only establishes a cross-sectional association between lifetime history of MDD and lower levels of BDNF in patients with dementia.

CONCLUSIONS

In this community sample of older individuals, lower brain BDNF levels were found in cases with both lifetime MDD and dementia. Low BDNF levels could be a moderator to accelerated brain aging observed in MDD with dementia.

The Molecular Intersection Between Senescence and Major Depression in the Elderly

Major depressive disorder in the elderly-or late-life depression (LLD)-is one of the most common mental illnesses in the aging population. LLD has several negative effects on health and well-being. Individuals with LLD have an elevated risk of chronic and persistent depressive symptoms as well as high rates of treatment resistance. They also have a higher risk of developing cognitive impairment with progression to dementia and higher rates of medical comorbidity, frailty, and mortality. The mechanisms linking LLD to these adverse health outcomes are not well understood. In this article, we review the evidence that individuals with LLD present with, i.e., enhanced molecular and cellular senescent changes, focusing on the senescence-associated secretory phenotype (SASP). We then propose a mechanistic model linking SASP to the greater risk of negative health outcomes in this population. We finally provide evidence that SASP-and cellular senescence in general-can be a therapeutic target for mitigating the risk of these negative outcomes in LLD.

Distinct Expression Pattern of Epigenetic Machinery Genes in Blood Leucocytes and Brain Cortex of Depressive Patients

In major depressive disorder (MDD), altered gene expression in brain cortex and blood leucocytes may be due to aberrant expression of epigenetic machinery coding genes. Here, we explore the expression of these genes both at the central and peripheral levels. Using real-time quantitative PCR technique, we first measured expression levels of genes encoding DNA and histone modifying enzymes in the dorsolateral prefrontal cortex (DLPFC) and cingulate cortex (CC) of MDD patients (n = 24) and healthy controls (n = 12). For each brain structure, transcripts levels were compared between subject groups. In an exploratory analysis, we then compared the candidate gene expressions between a subgroup of MDD patients with psychotic characteristics (n = 13) and the group of healthy subjects (n = 12). Finally, we compared transcript levels of the candidate genes in blood leucocytes between separate samples of MDD patients (n = 17) and healthy controls (n = 16). In brain and blood leucocytes of MDD patients, we identified an overexpression of genes encoding enzymes which transfer repressive transcriptional marks: HDAC4-5-6-8 and DNMT3B in the DLPFC, HDAC2 in the CC and blood leucocytes. In the DLPFC of patients with psychotic characteristics, two genes (KAT2A and UBE2A) were additionally overexpressed suggesting a shift to a more transcriptionally permissive conformation of chromatin. Aberrant activation of epigenetic repressive systems may be involved in MDD pathogenesis both in brain tissue and blood leucocytes.

Reduced GABAergic cortical inhibition in aging and depression

The neurobiology underlying depression in older adults is less extensively evaluated than in younger adults, despite the putative influence of aging on depression neuropathology. Studies using transcranial magnetic stimulation (TMS), a neurophysiological tool capable of probing inhibitory and excitatory cortical neurotransmission, have identified dysfunctional GABAergic inhibitory activity in younger adults with depression. However, GABAergic and glutamatergic cortical neurotransmission have not yet been studied in late-life depression (LLD). Here, we used single- and paired-pulse TMS to measure cortical inhibition and excitation in 92 LLD patients and 41 age-matched healthy controls. To differentiate the influence of age and depression, we also compared these TMS indices to those of 30 younger depressed adults and 30 age- and sex-matched younger healthy adults. LLD patients, older healthy adults, and younger depressed adults demonstrated significantly lower GABA_A receptor-mediated cortical inhibition than younger healthy controls. By contrast, no significant differences in cortical inhibition were observed between older adults with and without depression. No significant differences in GABA_B receptor-mediated inhibition or cortical excitation were found between the groups. Altogether, these findings suggest that reduced cortical inhibition may be associated with both advancing age and depression, which (i) supports the model of depression as a disease of accelerated aging, and (ii) prompts future investigation into diminished GABAergic neurotransmission in late-life as a biological predisposing factor to the development of depression. Given that cortical neurophysiology was similar in depressed and healthy older adults, future prospective studies need to establish the relative influence of age and depression on cortical inhibition deficits.

The influence of aging on the methylation status of brain-derived neurotrophic factor gene in blood

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) is involved in the pathophysiology of psychiatric disorders in adults and elderly individuals, and as a result, the DNA methylation (DNAm) of the BDNF gene in peripheral tissues including blood has been extensively examined to develop a useful biomarker for psychiatric disorders. However, studies to date have not previously investigated the effect of age on DNAm of the BDNF gene in blood. In this context, we measured DNAm of 39 CpG units in the CpG island at the promoter of exon I of the BDNF gene.

METHODS

We analyzed genomic DNA from peripheral blood of 105 health Japanese women 20 to 80 years of age to identify aging-associated change in DNAm of the BDNF gene. In addition, we examined the relationship between total MMSE scores, numbers of stressful life events, and serum BDNF levels on DNAm of the BDNF gene. The DNAm rate at each CpG unit was measured using a MassArray^® system (Agena Bioscience), and serum BDNF levels were measured by ELISA.

RESULTS

There was a significant correlation between DNAm and age in 13 CpGs. However, there was no significant correlation between DNAm and total MMSE scores, numbers of life events, or serum BDNF levels.

CONCLUSION

Despite the small number of subjects and the inclusion of only female subjects, our results suggest that DNAm of 13 CpGs of the BDNF gene may be an appropriate biomarker for aging and useful for predicting increased susceptibility to age-related psychiatric disorders.

Lactobacillus paracasei PS23 Delays Progression of Age-Related Cognitive Decline in Senescence Accelerated Mouse Prone 8 (SAMP8) Mice

Probiotic supplements are potential therapeutic agents for age-related disorders due to their antioxidant and anti-inflammatory properties. However, the effect of probiotics on age-related brain dysfunction remains unclear. To investigate the effects of Lactobacillus paracasei PS23 (LPPS23) on the progression of age-related cognitive decline, male and female senescence-accelerated mouse prone 8 (SAMP8) mice were divided into two groups (n = 6 each): the control and PS23 groups. From the age of 16 weeks, these groups were given saline and LPPS23, respectively, because SAMP8 mice start aging rapidly after four months of age. After 12 weeks of treatment, we evaluated the effect of LPPS23 by analyzing their appearance, behavior, neural monoamines, anti-oxidative enzymes, and inflammatory cytokines. The PS23 group showed lower scores of senescence and less serious anxiety-like behaviors and memory impairment compared to the control group. The control mice also showed lower levels of neural monoamines in the striatum, hippocampus, and serum. Moreover, LPPS23 induced the anti-oxidative enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPx). Higher levels of tumor necrosis factor (TNF)-α and monocyte chemotactic protein-1 (MCP1) and lower levels of interleukin (IL)-10 indicated that LPPS23 modulated the inflammation. Our results suggest that LPPS23 supplements could delay age-related cognitive decline, possibly by preventing oxidation and inflammation and modulating gut–brain axis communication.

Somatostatin-Positive Gamma-Aminobutyric Acid Interneuron Deficits in Depression: Cortical Microcircuit and Therapeutic Perspectives

The functional integration of external and internal signals forms the basis of information processing and is essential for higher cognitive functions. This occurs in finely tuned cortical microcircuits whose functions are balanced at the cellular level by excitatory glutamatergic pyramidal neurons and inhibitory gamma-aminobutyric acidergic (GABAergic) interneurons. The balance of excitation and inhibition, from cellular processes to neural network activity, is characteristically disrupted in multiple neuropsychiatric disorders, including major depressive disorder (MDD), bipolar disorder, anxiety disorders, and schizophrenia. Specifically, nearly 3 decades of research demonstrate a role for reduced inhibitory GABA level and function across disorders. In MDD, recent evidence from human postmortem and animal studies suggests a selective vulnerability of GABAergic interneurons that coexpress the neuropeptide somatostatin (SST). Advances in cell type-specific molecular genetics have now helped to elucidate several important roles for SST interneurons in cortical processing (regulation of pyramidal cell excitatory input) and behavioral control (mood and cognition). Here, we review evidence for altered inhibitory function arising from GABAergic deficits across disorders and specifically in MDD. We then focus on properties of the cortical microcircuit, where SST-positive GABAergic interneuron deficits may disrupt functioning in several ways. Finally, we discuss the putative origins of SST cell deficits, as informed by recent research, and implications for therapeutic approaches. We conclude that deficits in SST interneurons represent a contributing cellular pathology and therefore a promising target for normalizing altered inhibitory function in MDD and other disorders with reduced SST cell and GABA functions.

Early-life decline in neurogenesis markers and age-related changes of TrkB splice variant expression in the human subependymal zone

Neurogenesis in the subependymal zone (SEZ) declines across the human lifespan, and reduced local neurotrophic support is speculated to be a contributing factor. While tyrosine receptor kinase B (TrkB) signalling is critical for neuronal differentiation, maturation and survival, little is known about subependymal TrkB expression changes during postnatal human life. In this study, we used quantitative PCR and in situ hybridisation to determine expression of the cell proliferation marker Ki67, the immature neuron marker doublecortin (DCX) and both full-length (TrkB-TK+) and truncated TrkB receptors (TrkB-TK-) in the human SEZ from infancy to middle age (n = 26-35, 41 days to 43 years). We further measured TrkB-TK+ and TrkB-TK- mRNAs in the SEZ from young adulthood into ageing (n = 50, 21-103 years), and related their transcript levels to neurogenic and glial cell markers. Ki67, DCX and both TrkB splice variant mRNAs significantly decreased in the SEZ from infancy to middle age. In contrast, TrkB-TK- mRNA increased in the SEZ from young adulthood into ageing, whereas TrkB-TK+ mRNA remained stable. TrkB-TK- mRNA positively correlated with expression of neural precursor (glial fibrillary acidic protein delta and achaete-scute homolog 1) and glial cell markers (vimentin and pan glial fibrillary acidic protein). TrkB-TK+ mRNA positively correlated with expression of neuronal cell markers (DCX and tubulin beta 3 class III). Our results indicate that cells residing in the human SEZ maintain their responsiveness to neurotrophins; however, this capability may change across postnatal life. We suggest that TrkB splice variants may differentially influence neuronal and glial differentiation in the human SEZ.

Resilient protein co-expression network in male orbitofrontal cortex layer 2/3 during human aging

The orbitofrontal cortex (OFC) is vulnerable to normal and pathologic aging. Currently, layer resolution large-scale proteomic studies describing "normal" age-related alterations at OFC are not available. Here, we performed a large-scale exploratory high-throughput mass spectrometry-based protein analysis on OFC layer 2/3 from 15 "young" (15-43 years) and 18 "old" (62-88 years) human male subjects. We detected 4193 proteins and identified 127 differentially expressed (DE) proteins (p-value ≤0.05; effect size >20%), including 65 up- and 62 downregulated proteins (e.g., GFAP, CALB1). Using a previously described categorization of biological aging based on somatic tissues, that is, peripheral "hallmarks of aging," and considering overlap in protein function, we show the highest representation of altered cell-cell communication (54%), deregulated nutrient sensing (39%), and loss of proteostasis (35%) in the set of OFC layer 2/3 DE proteins. DE proteins also showed a significant association with several neurologic disorders; for example, Alzheimer's disease and schizophrenia. Notably, despite age-related changes in individual protein levels, protein co-expression modules were remarkably conserved across age groups, suggesting robust functional homeostasis. Collectively, these results provide biological insight into aging and associated homeostatic mechanisms that maintain normal brain function with advancing age.

Enhanced Molecular Aging in Late-Life Depression: the Senescent-Associated Secretory Phenotype

OBJECTIVE

This study aims to investigate whether a systemic molecular pattern associated with aging (senescent-associated secretory phenotype [SASP]) is elevated in adults with late-life depression (LLD), compared with never-depressed elderly comparison participants.

DESIGN

Cross-sectional study.

PARTICIPANTS

We included 111 older adults (80 with LLD and 31 comparison participants) in this study.

MEASUREMENT

A panel of 22 SASP-related proteins was extracted from a previous multiplex protein panel performed in these participants. We conducted a principal component analysis to create the SASP index based on individual weights of each of protein.

RESULTS

Participants with LLD showed a significantly increased SASP index compared with comparison participants, after controlling for age, depressive symptoms, medical comorbidity (CIRS-G) scores, sex, and cognitive performance (F(1,98) = 7.3, p = 0.008). Correlation analyses revealed that the SASP index was positively correlated with age (r = 0.2, p = 0.03) and CIRS score (r = 0.27, p = 0.005), and negatively correlated with information processing speed (r = -0.34, p = 0.001), executive function (r = -0.27, p = 0.004) and global cognitive performance (r = -0.28, p = 0.007).

CONCLUSIONS

To the best of our knowledge, this is the first study to show that a set of proteins (i.e., SASP index) primarily associated with cellular aging is abnormally regulated and elevated in LLD. These results suggest that individuals with LLD display enhanced aging-related molecular patterns that are associated with higher medical comorbidity and worse cognitive function. Finally, we provide a set of proteins that can serve as potential therapeutic targets and biomarkers to monitor the effects of therapeutic or preventative interventions in LLD.

The Role of BDNF in Age-Dependent Changes of Excitatory and Inhibitory Synaptic Markers in the Human Prefrontal Cortex

Reduced brain-derived neurotrophic factor (BDNF) may underlie age-related synaptic loss, in turn contributing to cerebral atrophy, cognitive decline, and increased risk for psychiatric disorders. However, the specific contribution of BDNF to the age-related expression changes in synaptic markers and their temporal trajectories remain uncharacterized. Using microarray data from orbitofrontal cortex of control subjects (n=209; 16-96 years), we identified genes whose expression positively correlates with BDNF (r>0.575; n=200 genes) and analyzed them for enriched biological pathways. qPCR was performed to measure the expression level of transcript variants of BDNF, NTRK2, and selected BDNF-coexpressed genes in younger and older subjects. We confirmed age-related downregulation of BDNF and show 78 of the top 200 BDNF-coexpressed genes are associated with synaptic function. Both excitatory and inhibitory synaptic genes show decreased expression with age and are positively correlated with BDNF and NTRK2 expression and negatively correlated with dominant-negative truncated NTRK2 level. Results were validated at the RNA level in an independent cohort and at the protein level for selected findings. We next tested the causal link between the correlative human findings using mice with conditional blockade of BDNF/NTRK2 signaling. Blockade of NTRK2 activity in adult mice recapitulate the age-like pattern in the expression of markers for inhibitory presynaptic but notably not for excitatory synaptic genes. Together, these findings suggest that age-dependent decrease in BDNF signaling may cause synaptic alterations through an initial and preferential effect on GABA presynaptic genes. These results have implications for neuropsychiatric disorders characterized by accelerated aging molecular profiles, such as major depression.

Gene expression in the aging human brain: an overview

PURPOSE OF REVIEW

The review aims to provide a summary of recent developments in the study of gene expression in the aging human brain.

RECENT FINDINGS

Profiling differentially expressed genes or 'transcripts' in the human brain over the course of normal aging has provided valuable insights into the biological pathways that appear activated or suppressed in late life. Genes mediating neuroinflammation and immune system activation in particular, show significant age-related upregulation creating a state of vulnerability to neurodegenerative and neuropsychiatric disease in the aging brain. Cellular ionic dyshomeostasis and age-related decline in a host of molecular influences on synaptic efficacy may underlie neurocognitive decline in later life. Critically, these investigations have also shed light on the mobilization of protective genetic responses within the aging human brain that help determine health and disease trajectories in older age. There is growing interest in the study of pre and posttranscriptional regulators of gene expression, and the role of noncoding RNAs in particular, as mediators of the phenotypic diversity that characterizes human brain aging.

SUMMARY

Gene expression studies in healthy brain aging offer an opportunity to unravel the intricately regulated cellular underpinnings of neurocognitive aging as well as disease risk and resiliency in late life. In doing so, new avenues for early intervention in age-related neurodegenerative disease could be investigated with potentially significant implications for the development of disease-modifying therapies.

FRAS1-related extracellular matrix 3 (FREM3) single-nucleotide polymorphism effects on gene expression, amygdala reactivity and perceptual processing speed: An accelerated aging pathway of depression risk

The A allele of the FRAS1-related extracellular matrix protein 3 (FREM3) rs7676614 single nucleotide polymorphism (SNP) was linked to major depressive disorder (MDD) in an early genome-wide association study (GWAS), and to symptoms of psychomotor retardation in a follow-up investigation. In line with significant overlap between age- and depression-related molecular pathways, parallel work has shown that FREM3 expression in postmortem human brain decreases with age. Here, we probe the effect of rs7676614 on amygdala reactivity and perceptual processing speed, both of which are altered in depression and aging. Amygdala reactivity was assessed using a face-matching BOLD fMRI paradigm in 365 Caucasian participants in the Duke Neurogenetics Study (DNS) (192 women, mean age 19.7 ± 1.2). Perceptual processing speed was indexed by reaction times in the same task and the Trail Making Test (TMT). The effect of rs7676614 on FREM3 mRNA brain expression levels was probed in a postmortem cohort of 169 Caucasian individuals (44 women, mean age 50.8 ± 14.9). The A allele of rs7676614 was associated with blunted amygdala reactivity to faces, slower reaction times in the face-matching condition (p < 0.04), as well as marginally slower performance on TMT Part B (p = 0.056). In the postmortem cohort, the T allele of rs6537170 (proxy for the rs7676614 A allele), was associated with trend-level reductions in gene expression in Brodmann areas 11 and 47 (p = 0.066), reminiscent of patterns characteristic of older age. The low-expressing allele of another FREM3 SNP (rs1391187) was similarly associated with reduced amygdala reactivity and slower TMT Part B speed, in addition to reduced BA47 activity and extraversion (p < 0.05). Together, these results suggest common genetic variation associated with reduced FREM3 expression may confer risk for a subtype of depression characterized by reduced reactivity to environmental stimuli and slower perceptual processing speed, possibly suggestive of accelerated aging.

Psychiatric disorders and leukocyte telomere length: Underlying mechanisms linking mental illness with cellular aging

Many psychiatric illnesses are associated with early mortality and with an increased risk of developing physical diseases that are more typically seen in the elderly. Moreover, certain psychiatric illnesses may be associated with accelerated cellular aging, evidenced by shortened leukocyte telomere length (LTL), which could underlie this association. Shortened LTL reflects a cell's mitotic history and cumulative exposure to inflammation and oxidation as well as the availability of telomerase, a telomere-lengthening enzyme. Critically short telomeres can cause cells to undergo senescence, apoptosis or genomic instability, and shorter LTL correlates with poorer health and predicts mortality. Emerging data suggest that LTL may be reduced in certain psychiatric illnesses, perhaps in proportion to exposure to the psychiatric illnesses, although conflicting data exist. Telomerase has been less well characterized in psychiatric illnesses, but a role in depression and in antidepressant and neurotrophic effects has been suggested by preclinical and clinical studies. In this article, studies on LTL and telomerase activity in psychiatric illnesses are critically reviewed, potential mediators are discussed, and future directions are suggested. A deeper understanding of cellular aging in psychiatric illnesses could lead to re-conceptualizing them as systemic illnesses with manifestations inside and outside the brain and could identify new treatment targets.

Lower Cerebrospinal Fluid Concentration of Brain-Derived Neurotrophic Factor Predicts Progression from Mild Cognitive Impairment to Alzheimer's Disease

There is little information on the dynamics of BDNF in the CSF in the continuum between healthy aging, MCI and AD. We included 128 older adults (77 with amnestic MCI, 26 with AD and 25 healthy controls). CSF BDNF level was measured by ELISA assay, and AD biomarkers (Aβ42, T-Tau and P-Tau181) were measured using a Luminex xMAP assay. CSF BDNF levels were significantly reduced in AD subjects compared to MCI and healthy controls (p = 0.009). Logistic regression models showed that lower CSF BDNF levels (p = 0.008), lower CSF Aβ42 (p = 0.005) and lower MMSE scores (p = 0.007) are significantly associated with progression from MCI to AD. The present study adds strong evidence of the involvement of BDNF in the pathophysiology of neurodegenerative changes in AD. Interventions aiming to restore central neurotrophic support may represent future therapeutic targets to prevent or delay the progression from MCI to AD.

Plasma biosignature and brain pathology related to persistent cognitive impairment in late-life depression

Cognitive impairment is highly prevalent among individuals with late-life depression (LLD) and tends to persist even after successful treatment. The biological mechanisms underlying cognitive impairment in LLD are complex and likely involve abnormalities in multiple pathways, or 'cascades,' reflected in specific biomarkers. Our aim was to evaluate peripheral (blood-based) evidence for biological pathways associated with cognitive impairment in older adults with LLD. To this end, we used a data-driven comprehensive proteomic analysis (multiplex immunoassay including 242 proteins), along with measures of structural brain abnormalities (gray matter atrophy and white matter hyperintensity volume via magnetic resonance imaging), and brain amyloid-β (Aβ) deposition (PiB-positron emission tomography). We analyzed data from 80 older adults with remitted major depression (36 with mild cognitive impairment (LLD+MCI) and 44 with normal cognitive (LLD+NC)) function. LLD+MCI was associated with differential expression of 24 proteins (P<0.05 and q-value <0.30) related mainly to the regulation of immune-inflammatory activity, intracellular signaling, cell survival and protein and lipid homeostasis. Individuals with LLD+MCI also showed greater white matter hyperintensity burden compared with LLD+NC (P=0.015). We observed no differences in gray matter volume or brain Aβ deposition between groups. Machine learning analysis showed that a group of three proteins (Apo AI, IL-12 and stem cell factor) yielded accuracy of 81.3%, sensitivity of 75% and specificity of 86.4% in discriminating participants with MCI from those with NC function (with an averaged cross-validation accuracy of 76.3%, sensitivity of 69.4% and specificity of 81.8% with nested cross-validation considering the model selection bias). Cognitive impairment in LLD seems to be related to greater cerebrovascular disease along with abnormalities in immune-inflammatory control, cell survival, intracellular signaling, protein and lipid homeostasis, and clotting processes. These results suggest that individuals with LLD and cognitive impairment may be more vulnerable to accelerated brain aging and shed light on possible mediators of their elevated risk for progression to dementia.

Molecular and Genetic Characterization of Depression: Overlap with other Psychiatric Disorders and Aging

Genome-wide expression and genotyping technologies have uncovered the genetic bases of complex diseases at unprecedented rates; However despite its heavy burden and high prevalence, the molecular characterization of major depressive disorder (MDD) has lagged behind. Transcriptome studies report multiple brain disturbances but are limited by small sample sizes. Genome-wide association studies (GWAS) report weak results but suggest overlapping genetic risk with other neuropsychiatric disorders. We performed systematic molecular characterization of altered brain function in MDD, using meta-analysis of differential expression in eight gene array studies in three corticolimbic brain regions in 101 subjects. The identified "metaA-MDD" genes suggest altered neurotrophic support, brain plasticity and neuronal signaling in MDD. Notably, metaA-MDD genes display low connectivity and hubness in coexpression networks, and uniform genomic distribution, consistent with diffuse polygenic mechanisms. We next integrated these findings with results from over 1800 published GWAS and show that genetic variations nearby metaA-MDD genes predict greater risk for neuropsychiatric disorders and notably for age-related phenotypes, but not for other medical illnesses, including those frequently co-morbid with depression, or body characteristics. Collectively, the intersection of unbiased investigations of gene function (transcriptome) and structure (GWAS) provides novel leads to investigate molecular mechanisms of MDD and suggest common biological pathways between depression, other neuropsychiatric diseases, and brain aging.

Is schizophrenia a neurodegenerative disease? Evidence from age-related decline of brain-derived neurotrophic factor in the brains of schizophrenia patients and matched nonpsychiatric controls

BACKGROUND

Brain-derived neurotrophic factor (BDNF) protein levels decline in the brain during senescence and are also shown to be reduced in schizophrenia patients. BDNF is present in both the gray and white matters of the brain. It is unclear whether BDNF abnormalities in schizophrenia are specific to gray and/or white matter.

OBJECTIVE

We hypothesized that the age-related BDNF decline is abnormal and contributes to the reduced BDNF in schizophrenia.

METHODS

We tested this hypothesis by measuring BDNF protein levels in postmortem gray and white matter, using the prefrontal cortex (PFC) and the genu of the corpus callosum as regions of interests, from 20 schizophrenia patients and 20 matched nonpsychiatric controls. Samples were selected across the adult lifespan--from 20 to 80 years of age.

RESULTS

PFC gray matter BDNF protein levels were significantly lower in older age in both nonpsychiatric comparisons and patients, while BDNF in white matter did not decrease significantly with age in either group. PFC BDNF was linearly lower from 20 to 80 years of age in nonpsychiatric comparisons. In schizophrenia, the age effect was similarly linear in younger patients but a decline did not occur in older patients.

CONCLUSION

PFC BDNF does not follow a normative linear age effect in schizophrenia patients as they grow older, which may represent a 'floor effect' due to earlier decline or a survivor cohort of older patient donors who are less susceptible to a schizophrenia-related pathological aging process.

Shortened telomere length in white matter oligodendrocytes in major depression: potential role of oxidative stress

Telomere shortening is observed in peripheral mononuclear cells from patients with major depressive disorder (MDD). Whether this finding and its biological causes impact the health of the brain in MDD is unknown. Brain cells have differing vulnerabilities to biological mechanisms known to play a role in accelerating telomere shortening. Here, two glia cell populations (oligodendrocytes and astrocytes) known to have different vulnerabilities to a key mediator of telomere shortening, oxidative stress, were studied. The two cell populations were separately collected by laser capture micro-dissection of two white matter regions shown previously to demonstrate pathology in MDD patients. Cells were collected from brain donors with MDD at the time of death and age-matched psychiatrically normal control donors (N = 12 donor pairs). Relative telomere lengths in white matter oligodendrocytes, but not astrocytes, from both brain regions were significantly shorter for MDD donors as compared to matched control donors. Gene expression levels of telomerase reverse transcriptase were significantly lower in white matter oligodendrocytes from MDD as compared to control donors. Likewise, the gene expression of oxidative defence enzymes superoxide dismutases (SOD1 and SOD2), catalase (CAT) and glutathione peroxidase (GPX1) were significantly lower in oligodendrocytes from MDD as compared to control donors. No such gene expression changes were observed in astrocytes from MDD donors. These findings suggest that attenuated oxidative stress defence and deficient telomerase contribute to telomere shortening in oligodendrocytes in MDD, and suggest an aetiological link between telomere shortening and white matter abnormalities previously described in MDD.

The influence of aging on poststroke depression using a rat model via middle cerebral artery occlusion

Poststroke depression (PSD) is the most frequent psychological sequela following stroke. While previous studies describe the impact of age on brain infarct volume, brain edema, and blood-brain barrier (BBB) breakdown following ischemia, the role of age on PSD has yet to be described. Here, we examine the influence of age on PSD progression in a rat model of PSD by middle cerebral artery occlusion (MCAO). One hundred forty-three rats were divided into three groups. 48 rats 20 weeks of age underwent a sham procedure, 51 rats 20 weeks of age had MCAO, and 44 rats 22-26 months of age had MCAO. Groups were further divided into two subgroups. The first subgroup was used to measure infarct lesion volume, brain edema, and BBB breakdown at 24 h. In the second subgroup at 3 weeks after MCAO, rats were subjected to a sucrose preference test, two-way shuttle avoidance task, forced swimming test, and a brain-derived neurotrophic factor (BDNF) protein level measurement. Total and striatal infarct volume, brain edema, and BBB breakdown in the striatum were increased in older rats, as compared with younger rats. While both old and young rats exhibited depressive-like behaviors on each of the behavioral tests and lower BDNF levels post-MCAO, as compared with control rats, there were no differences between old and young rats. Although older rats suffered from larger infarct volumes, increased brain edema and more BBB disruption following MCAO, the lack of behavioral differences between young and old rats suggests that there was no effect of rat age on the incidence of PSD.

Brain-derived neurotrophic factor levels in late-life depression and comorbid mild cognitive impairment: a longitudinal study

Changes in brain-derived neurotrophic factor (BDNF) level are implicated in the pathophysiology of cognitive decline in depression and neurodegenerative disorders in older adults. We aimed to evaluate the longitudinal association over two years between BDNF and persistent cognitive decline in individuals with remitted late-life depression and Mild Cognitive Impairment (LLD + MCI) compared to either individuals with remitted LLD and no cognitive decline (LLD + NCD) or never-depressed, cognitively normal, elderly control participants. We additionally evaluated the effect of double-blind, placebo-controlled donepezil treatment on BDNF levels in all of the remitted LLD participants (across the levels of cognitive function). We included 160 elderly participants in this study (72 LLD + NCD, 55 LLD + MCI and 33 never-depressed cognitively normal elderly participants). At the same visits, cognitive assessments were conducted and blood sampling to determine serum BDNF levels were collected at baseline assessment and after one and two years of follow-up. We utilized repeated measure, mixed effect models to assess: (1) the effects of diagnosis (LLD + MCI, LLD + NCD, and controls), time, and their interaction on BDNF levels; and (2) the effects of donepezil treatment (donepezil vs. placebo), time, baseline diagnosis (LLD + MCI vs. LLD + NCD), and interactions between these contrasts on BDNF levels. We found a significant effect of time on BDNF level (p = 0.02) and a significant decline in BDNF levels over 2 years of follow-up in participants with LLD + MCI (p = 0.004) and controls (p = 0.04). We found no effect of donepezil treatment on BDNF level. The present results suggest that aging is an important factor related to decline in BDNF level. Clinicaltrials.gov Identifier: NCT00177671.

Cellular morphometry in late-life depression: a review of postmortem studies

The impact of major depression in late life is considerable and set to intensify with a worldwide shift in demographic profile toward an elderly population. Although the precise neurobiological mechanisms are not fully understood, a significant body of clinical, epidemiological, and imaging data have suggested divergent pathophysiological pathways underlie depression in late life, when compared with younger patients. Neuroimaging studies have demonstrated significant increases in white matter hyperintensities in late-life depression in several key areas involved in affective circuitry. Postmortem cellular morphometry studies have played a vital role in the identification of discrete changes in the brain microstructure in depression. This review draws together such postmortem studies, which have utilized tissue from younger/mixed age and late-life depressed patients. These findings have suggested varying neuronal and glial cell pathology in depression between different age cohorts. This age-related disparity may suggest different pathophysiological basis for depression, with vascular factors playing a potentially greater role in late life.

Accelerated aging in major depression: the role of nitro-oxidative stress

Nitro-oxidative stress (NOS) plays a fundamental role in aging, as well as in the pathogenesis of neurodegenerative disorders, and major depression (MD). The latter is a very frequent psychiatric illness characterized by accelerated aging, neurodegeneration, high comorbidity with age-related disorders, and premature mortality; all of these conditions find an explanation in an altered redox homeostasis. If aging, neurodegeneration, and major depression share a common biological base in their pathophysiology, common therapeutic tools could be investigated for the prevention and treatment of these disorders. As an example, antidepressants have been demonstrated to present neuroprotective and anti-inflammatory properties and to stimulate neurogenesis. In parallel, antioxidants that stimulate the antioxidant defense systems and interact with the monoaminergic system show an antidepressant-like activity. Further research on this topic could lead, in the near future, to the expansion of the therapeutic possibilities for the treatment of NOS-related disorders.

Molecular aging of the brain, neuroplasticity, and vulnerability to depression and other brain-related disorders

The increased risk for neurodegenerative and neuropsychiatric disorders associated with extended lifespan has long suggested mechanistic links between chronological age and brain-related disorders, including depression, Recent characterizations of age-dependent gene expression changes now show that aging of the human brain engages a specific set of biological pathways along a continuous lifelong trajectory, and that the same genes that are associated with normal brain aging are also frequently and similarly implicated in depression and other brain-related disorders. These correlative observations suggest a model of age-by-disease molecular interactions, in which brain aging promotes biological changes associated with diseases, and additional environmental factors and genetic variability contribute to defining disease risk or resiliency trajectories. Here we review the characteristic features of brain aging in terms of changes in gene function over time, and then focus on evidence supporting accelerated molecular aging in depression. This proposed age-by-disease biological interaction model addresses the current gap in research between “normal” brain aging and its connection to late-life diseases. The implications of this model are profound, as it provides an investigational framework for identifying critical moderating factors, outlines opportunities for early interventions or preventions, and may form the basis for a dimensional definition of diseases that goes beyond the current categorical system.

Reduced brain somatostatin in mood disorders: a common pathophysiological substrate and drug target?

Our knowledge of the pathophysiology of affect dysregulation has progressively increased, but the pharmacological treatments remain inadequate. Here, we summarize the current literature on deficits in somatostatin, an inhibitory modulatory neuropeptide, in major depression and other neurological disorders that also include mood disturbances. We focus on direct evidence in the human postmortem brain, and review rodent genetic and pharmacological studies probing the role of the somatostatin system in relation to mood. We also briefly go over pharmacological developments targeting the somatostatin system in peripheral organs and discuss the challenges of targeting the brain somatostatin system. Finally, the fact that somatostatin deficits are frequently observed across neurological disorders suggests a selective cellular vulnerability of somatostatin-expressing neurons. Potential cell intrinsic factors mediating those changes are discussed, including nitric oxide induced oxidative stress, mitochondrial dysfunction, high inflammatory response, high demand for neurotrophic environment, and overall aging processes. Together, based on the co-localization of somatostatin with gamma-aminobutyric acid (GABA), its presence in dendritic-targeting GABA neuron subtypes, and its temporal-specific function, we discuss the possibility that deficits in somatostatin play a central role in cortical local inhibitory circuit deficits leading to abnormal corticolimbic network activity and clinical mood symptoms across neurological disorders.

The age-by-disease interaction hypothesis of late-life depression

The phenomenologic diagnosis of depression is successful in increasing diagnostic reliability, but it is a classification scheme without biologic bases. One subtype of depression for which evidence suggests a unique biologic basis is late-life depression (LLD), with first onset of symptoms after the age of 65. LLD is common and poses a significant burden on affected individuals, caretakers, and society. The pathophysiology of LLD includes disruptions of the neural network underlying mood, which can be conceptualized as the result of dysfunction in multiple underlying biologic processes. Here, we briefly review current LLD hypotheses and then describe the characteristics of molecular brain aging and their overlap with disease processes. Furthermore, we propose a new hypothesis for LLD, the age-by-disease interaction hypothesis, which posits that the clinical presentation of LLD is the integrated output of specific biologic processes that are pushed in LLD-promoting directions by changes in gene expression naturally occurring in the brain during aging. Hence, the brain is led to a physiological state that is more susceptible to LLD, because additional pushes by genetic, environmental, and biochemical factors may now be sufficient to generate dysfunctional states that produce depressive symptoms. We put our propositions together into a decanalization model to aid in illustrating how age-related biologic changes of the brain can shift the repertoire of available functional states in a prodepression direction, and how additional factors can readily lead the system into distinct and stable maladaptive phenotypes, including LLD. This model brings together basic research on neuropsychiatric and neurodegenerative diseases more closely with the investigation of normal aging. Specifically, identifying biologic processes affected during normal aging may inform the development of new interventions for the prevention and treatment of LLD.

Age-by-disease biological interactions: implications for late-life depression

Onset of depressive symptoms after the age of 65, or late-life depression (LLD), is common and poses a significant burden on affected individuals, caretakers, and society. Evidence suggests a unique biological basis for LLD, but current hypotheses do not account for its pathophysiological complexity. Here we propose a novel etiological framework for LLD, the age-by-disease biological interaction hypothesis, based on the observations that the subset of genes that undergoes lifelong progressive changes in expression is restricted to a specific set of biological processes, and that a disproportionate number of these age-dependent genes have been previously and similarly implicated in neurodegenerative and neuropsychiatric disorders, including depression. The age-by-disease biological interaction hypothesis posits that age-dependent biological processes (i) are “pushed” in LLD-promoting directions by changes in gene expression naturally occurring during brain aging, which (ii) directly contribute to pathophysiological mechanisms of LLD, and (iii) that individual variability in rates of age-dependent changes determines risk or resiliency to develop age-related disorders, including LLD. We review observations supporting this hypothesis, including consistent and specific age-dependent changes in brain gene expression and their overlap with neuropsychiatric and neurodegenerative disease pathways. We then review preliminary reports supporting the genetic component of this hypothesis. Other potential biological mediators of age-dependent gene changes are proposed. We speculate that studies examining the relative contribution of these mechanisms to age-dependent changes and related disease mechanisms will not only provide critical information on the biology of normal aging of the human brain, but will inform our understanding of age-dependent diseases, in time fostering the development of new interventions for prevention and treatment of age-dependent diseases, including LLD.