DNA damage response, bioenergetics, and neurological disease: the challenge of maintaining brain health in an aging human population

Identification of Circulating hsa-miR-324-3p and hsa-miR-331-3p Exchanges in The Serum of Alzheimer's Patients and Insights into The Pathophysiological Pathways

Objective

Alzheimer's disease (AD) is a type of dementia. Currently, there are not any existing and reliable methods for the prognosis or diagnosis of AD. Hence, finding a diagnostic/prognostic biomarker for AD helps physicians to prescribe the treatments and methods preventing disease progression. Circulating microRNAs (miRNAs) are the most promising biomarkers due to their non-invasive and easily accessible for diagnosis and prognosis of AD. The aim of current study is to evaluate expression levels of two unwell-known circulating miRNAs including hsa-miR-324-3p and hsa-miR-331-3p in serums of AD patients and to understand their roles in AD physiopathogenesis by in silico analysis.

Materials and Methods

In this case and control study, to get the gene targets related to these two miRNAs, TargetScan, miRTargetLink Human and mirDIP web servers were applied. In addition, gene networks and gene ontology enrichment analysis were performed by STRING 10.5, KEGG and ShinyGO v0.41. Experimentally, expression levels of these two miRNAs in the serum of 21 patients with AD and 23 healthy individuals were compared using the quantitative reverse transcription polymerase chain reaction (qRT-PCR) method.

Results

The pathophysiological pathways associated with these two miRNAs were nucleotide metabolism and cellular response to stress pathway. Furthermore, the upregulated expression levels of hsa-miR-324-3p and hsa-miR-331-3p in comparison with the healthy control serums were not statistically significant (P>0.05).

Conclusion

Non-significant results were obtained from the expression levels of AD patients and two significant pathways were obtained by networks and gene enrichment analysis.

POLD1 deficiency is involved in cognitive function impairment in AD patients and SAMP8 mice

Age-related changes such as increased oxidative stress and DNA damage are important risk factors for Alzheimer's disease (AD). This study aimed to clarify the role of POLD1, the catalytic subunit of DNA polymerase δ, in neurodegeneration symptoms of AD. POLD1 expression levels were evaluated in patients with different neurodegenerative diseases by ELISA, RT-PCR and Western blot analysis. The impairment of cognitive ability in AD patients and senescence-accelerated mouse prone 8 (SAMP8) mice were evaluated by MMSE/MoCA score and Morris water maze (MWM) test. We found that serum concentration and expression levels of POLD1 in lymphocytes were reduced in AD patients. The cognitive impairment in AD patients and SAMP8 mice was associated with reduced POLD1 expression. In addition, POLD1 knockdown led to premature senescence and increased DNA damage in primary neuronal cells of SAMP8 mice. In conclusion, this is the first study suggesting that the deficiency of POLD1 may aggravate AD progression, and POLD1 is a potential diagnostic marker and therapeutic target for AD.

The Role of Mitochondrial Dysfunction in the Progression of Alzheimer's Disease

The current molecular understanding of Alzheimer's disease (AD) has still not resulted in successful interventions. Mitochondrial dysfunction of the AD brain is currently emerging as a hallmark of this disease. One mitochondrial function often affected in AD is oxidative phosphorylation responsible for ATP production, but also for production of reactive oxygen species (ROS) and for the de novo synthesis of pyrimidines. This paper reviews the role of mitochondrial produced ROS and pyrimidines in the aetiology of AD and their proposed role in oxidative degeneration of macromolecules, synthesis of essential phospholipids and maintenance of mitochondrial viability in the AD brain.

DNA-PK and P38 MAPK: A Kinase Collusion in Alzheimer's Disease?

The pathogenesis of Alzheimer's disease (AD), characterized by prevalent neuronal death and extracellular deposit of amyloid plaques, is poorly understood. DNA lesions downstream of reduced DNA repair ability have been reported in AD brains. Neurons predominantly use a mechanism to repair double-strand DNA breaks (DSB), which is non-homologous end joining (NHEJ). NHEJ requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kD catalytic subunit (DNA-PKcs) and its activator Ku, a heterodimer of p86 and p70 subunits. Ku first binds and then recruits DNA-PKcs to double-stranded DNA ends before NHEJ process begins. Studies have shown reduced NHEJ activity as well as DNA-PKcs and Ku protein levels in AD brains suggesting possible contribution of unrepaired DSB to AD development. However, normal aging brains also show reduced DNA-PKcs and Ku levels thus challenging the notion of any direct link between NHEJ and AD. Another kinase, p38 MAPK is induced by various DNA damaging agents and DSB itself. Increased DNA damage with aging could induce p38 MAPK and its induction may be sustained when DNA repair is compromised in the brain with reduced DNA-PK activity. Combined, these two events may potentially set the stage for an awry nervous system approaching AD.

Altered DNA base excision repair profile in brain tissue and blood in Alzheimer's disease

Background

Alzheimer’s disease (AD) is a progressive, multifactorial neurodegenerative disorder that is the main cause of dementia globally. AD is associated with increased oxidative stress, resulting from imbalance in production and clearance of reactive oxygen species (ROS). ROS can damage DNA and other macromolecules, leading to genome instability and disrupted cellular functions. Base excision repair (BER) plays a major role in repairing oxidative DNA lesions. Here, we compared the expression of BER components APE1, OGG1, PARP1 and Polβ in blood and postmortem brain tissue from patients with AD, mild cognitive impairment (MCI) and healthy controls (HC).

Results

BER mRNA levels were correlated to clinical signs and cerebrospinal fluid biomarkers for AD. Notably, the expression of BER genes was higher in brain tissue than in blood samples. Polβ mRNA and protein levels were significantly higher in the cerebellum than in the other brain regions, more so in AD patients than in HC. Blood mRNA levels of OGG1 was low and PARP1 high in MCI and AD.

Conclusions

These findings reflect the oxidative stress-generating energy-consumption in the brain and the importance of BER in repairing these damage events. The data suggest that alteration in BER gene expression is an event preceding AD. The results link DNA repair in brain and blood to the etiology of AD at the molecular level and can potentially serve in establishing novel biomarkers, particularly in the AD prodromal phase.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0237-z) contains supplementary material, which is available to authorized users.