A research group of the Tokyo Metropolitan Institute of Medical Science and Nara Medical University has proposed that minocycline may prevent age-related cognitive impairment, using a mouse model of intellectual disability caused by heterozygous deletion
image: Left: In the dentate gyrus of the hippocampus of 4-5 month-old Rp58 heterozygous-KO mice, DNA damage accumulates in the mossy cells (yellow-green) (ssDNA; red), and the surrounding microglia (blue) are activated, showing a similar tissue image to that of old mice. ① Damaged mossy cells activate microglia (red arrow), and ② activated microglia further damage mossy cells (blue arrow) (hypothesis). As a result, signals from mossy cells to dentate gyrus granule cells, CA3 pyramidal cells, and CA1 pyramidal neurons are impaired, and ③ spatial cognitive ability declines.
Right: Chronic minocycline administration prevents age-associated changes in the hippocampal dentate gyrus and prevents a decline in spatial cognitive ability of 4-5 month-old Rp58 heterozygous-KO mice.
Credit: TMIMS
We found that Rp58 heterozygous- knockout (KO) mice, which are a model for intellectual disability associated with ZBTB18/RP58 haploinsufficiency, show early deterioration of spatial cognitive function with age, and that the cause is accumulation of DNA damage in the mossy cells of the hippocampal dentate gyrus, activation of microglia, and chronic inflammation, which can be prevented by minocycline. Specifically, we used Zbtb18/Rp58 (hereafter, Rp58) heterozygous-KO mice to clarify the following six points (Image).
(1) Rp58 heterozygous-KO mice show early age-related cognitive decline
The spatial recognition ability of wild-type mice at 4-5 months of age was almost the same as that of mice at 2 months of age, but at 12-18 months of age it was significantly lower, indicating age-dependent impairment of spatial learning or memory. On the other hand, Rp58 heterozygous-KO mice showed spatial recognition ability equivalent to that of wild-type mice at 2 months of age, but this ability significantly decreased at 4-5 months of age, indicating early-onset age-related cognitive dysfunction.
(2) In Rp58 heterozygous-KO mice, DNA damage accumulates early in the mossy cells of the hippocampal dentate gyrus.
The mossy cells of the hippocampal dentate gyrus receive convergent excitation from granule cells and play an important role in spatial memory by returning bilateral, widespread, and divergent excitation. At 2 months of age, Rp58 heterozygous-KO mice show almost no mossy cells that are positive for DNA damage accumulation markers (ssDNA, γ-H2AX), as in wild-type mice, but at 4-5 months of age, there is a significant increase compared to wild-type mice (Image).
(3) Increased activated microglia and chronic inflammation in Rp58 heterozygous-KO Rp58 heterozygous-KO mice
Since it is known that activated microglia accumulate around neurons that have suffered DNA damage, we next focused on the microglia in the dentate gyrus region where the mossy cells are located. In Rp58 heterozygous-KO mice, the proportion of activated microglia was the same as that in wild-type mice at 2 months of age months of age, the proportion of activated microglia was similar to that of wild-type mice, but at 4-5 months of age, a significant increase in activated microglia was observed when evaluated using CD68 as an indicator (Image).
(4) Ultrastructural changes in mitochondria in Rp58 heterozygous mice
At 4-5-month-old Rp58 heterozygous-KO mice, electron microscopy analysis revealed that the majority of mitochondria in hippocampal dentate gyrus neurons were morphologically the type seen in aging.
(5) DNA repair defects were detected in Rp58 heterozygous KO mice
The accelerated accumulation of DNA damage in Rp58 heterozygous mice may be due to impaired DNA repair. The observation of recovery from radiation-induced DNA damage in Rp58 heterozygous-KO mice suggests that the accelerated accumulation of DNA damage in Rp58 heterozygous mice is due to impaired DNA repair.
(6) Chronic treatment with minocycline prevents early-impaired aging phenotypes in Rp58 hetero-KO mice
Minocycline is a tetracycline antibiotic that has recently been shown to have neuroprotective and anti-inflammatory effects on the progression of several neurodegenerative diseases. Chronic administration of minocycline prevented the increase in DNA-damaged-accumulating mossy cells in the dentate gyrus of 4-5 month-old Rp58 heterozygous-KO mice. Furthermore, it suppressed the early increase in activated microglia observed in the dentate gyrus of Rp58-KO heterozygous mice (Image).
Behavioral analysis also showed that cognitive dysfunction in 4-5 month-old Rp58 heterozygous-KO mice was suppressed.
These results are expected to contribute to the elucidation of the pathological mechanisms of RP58/ZBTB18 haploinsufficiency, the development of preventive therapies, and the prevention of age-related cognitive decline.
Social Significance and Future Prospects
It was demonstrated that a decrease in the expression level of RP58 causes early age-related spatial cognitive decline, and the causes of this were suggested to be DNA damage accumulation due to defective DNA repair and microglial activation. This phenotype is seen in wild-type mice during old age. Therefore, it is thought that the lack of RP58 expression accelerates age-related changes in the hippocampus and causes a decline in spatial cognitive function. Further research is needed to elucidate the mechanism by which RP58 regulates DNA damage repair.
In this study, we succeeded in improving the above symptoms by administering minocycline. Since it has been reported that the expression of RP58 decreases with age, age-related cognitive decline may be due to the age-related decrease in the expression of RP58. Therefore, in addition to the fact that minocycline administration prevents cognitive decline in Rp58 haploinsufficiency, it is hoped that it will also lead to research on the prevention of age-related cognitive decline in the future.
The research results were published online in the English scientific journal Journal of Neuroinflammation on Saturday, October 12, 2024 at 11:30 p.m. (Japan Standard Time).