Introduction
The global prevalence of dementia has exponentially increased due to prolonged life expectancy and an aging population. Currently, >55 million people are suffering from dementia globally (1). The syndrome of dementia is caused by a various diseases that eventually destroy nerve cells, thereby damaging the brain, and causing cognitive ability deterioration compared with that expected from the normal biological aging course (1,2). Mild cognitive impairment (MCI), can be considered the ‘transitional zone’ between normal cognition and dementia and has become a novel topic in clinical research (3).
MCI is an early stage that can result in dementia. MCI can be reversed and diagnosis at an early stage is crucial to control the progression to dementia. Currently, neuropsychological testing (4), transcranial ultrasonography (5), near-infrared spectroscopy that enables measurement of hemoglobin concentration changes in the brain (6), magnetic resonance imaging (7), detection of cerebrospinal fluid biomarkers, detection of such as amyloid beta and phosphorylated tau protein in spinal fluid (8,9), electroencephalography (10), and olfaction tests (11) are the diagnostic modalities used to diagnose dementia. However, all of these testing methods are useful in diagnosing dementia, but they cannot detect MCI with sensitivity.
MicroRNAs, which are small RNA molecules that regulate gene expression in cells, are found in blood and are mainly encapsulated in extracellular vesicles such as exosomes (12,13). In recent years, many researchers have revealed serum microRNAs as biomarkers for early disease detection (14,15). This study aimed to clarify changes in serum microRNAs in cognitively impaired mice.
Discussion
This study revealed microRNAs whose expression was altered in the serum of middle-aged 58W mice, which demonstrated reduced cognitive ability compared with young 8W mice with normal cognitive ability.
In general, women are at higher risk of developing dementia than men, but we used male mice in this study to examine changes in blood composition during aging because of the article by Hahm et al (20). Although not included in the present data, we conducted experiments using mice older than 58W, but because we could not obtain sufficient samples of older mice due to spontaneous mouse death and other factors, we used mice up to 58W in this study for this study.
Changes in body weight, serum biochemistry, and cognitive performance with aging were examined in 4W, 8W, 36W and 58W mice. A statistically significant increase in body weight was found among each mouse as they aged (Fig. 1A). Previous reports revealed that the body weight of mice increased with age (21). Further, serum ALP is higher in young age, and serum total cholesterol is higher in old age (21,22). Hence, serum ALP and serum total cholesterol were measured to infer the age of the mice used. Serum ALP was statistically significantly lower in the 58W group than in the 4W or 8W group (Fig. 1B). Furthermore, serum total cholesterol was significantly higher in the 58W group than in the 8W group (Fig. 1C). These results are consistent with those of previous studies. A decrease in cognitive ability was observed in 58W mice compared with 8W mice when the cognitive abilities of 8W and 58W mice with statistically significant differences in serum ALP and serum total cholesterol were confirmed (Fig. 1D). We then examined the difference in serum microRNA expression between 8W and 58W mice with significant differences in cognitive ability (Fig. 2C). We identified miR-21a-5p and miR-92a-3p as microRNAs that were upregulated and miR-6769b-5p and miR-709 as microRNAs that were downregulated in the serum of 58W mice compared with 8W mice. In this study, swimming time was assessed in the Morris water maze test. In order to fully evaluate cognitive function, it is necessary to measure and evaluate swim path, swim speed, and crossing number. In the future, we would like to evaluate cognitive function using mice aged more than 58W as well as various measurement methods in the Morris water maze test.
This study revealed elevated serum miR-21a-5p expression in mice with impaired cognitive ability (Fig. 2C). Very few studies have revealed the association between cognitive ability and serum miR-21a-5p levels. Yuan et al revealed an increased expression of serum miR-21 in patients with cognitive impairment following the incidence of stroke compared with patients with normal cognition following stroke (23). Other previously conducted studies have revealed that miR-21 plays a crucial role in cognitive ability regulation because an increase in miR-21 in the brains of Alzheimer's model mice was observed to restore cognitive ability (24,25). This result indicates that the loss of miR-21 in the brain and its leakage into the serum is a factor associated with cognitive decline. Further studies that explore the relationship between cognitive ability and miR-21 expression should be conducted in the future.
Several studies reported on the association between serum miR-92a-3p levels and dementia, and several investigators have reported an association between miR-92a-3p levels and cognitive ability. Siedlecki-Wullich et al demonstrated a statistically significant increase in serum miR-92a-3p in the patient group with Alzheimer's disease compared with the group with normal cognitive abilities and indicated an increasing trend in the incidence of this serum in the MCI group (26). However, Peña-Bautista et al contradicted this claim by revealing a decrease in plasma miR-92a-3p levels in patients with early Alzheimer's disease (27).
This study revealed that a decrease in serum miR-709 levels reduces in mice with cognitive decline. This is a novel result because previous reports of an association between serum miR-709 levels and cognitive ability are limited. Further studies are warranted to confirm and elaborate the association between serum miR-709 levels and cognitive ability. In the future, we would like to further analyze aged mice to reveal novel MCI biomarkers.