Calcium channel blockers show potential to restore cerebral blood flow in Alzheimer's disease

Blocking calcium channels in brain capillaries may enhance blood flow and alleviate early-stage Alzheimer's damage, new study suggests.

In a recent study published in Nature Neuroscience, researchers used murine Alzheimer's disease models and human brain tissue to investigate whether blocking the voltage-gated calcium channels (CaV) could prevent pericytes, which are cells that line the walls of small blood vessels, from constricting the capillaries and decreasing blood flow to the brain.

Background

Research on Alzheimer's disease therapies typically focuses on preventing the formation of amyloid β plaques or hyperphosphorylated tau proteins. However, these therapies have not been successful in preventing the cognitive decline that occurs in the disease, perhaps because significant brain damage has occurred by the time these treatments are administered.

Current Alzheimer's disease research has shifted focus to early-stage targets for treatment, which includes cerebral blood flow.

Cerebral blood flow is reduced by 45% in the early stages of Alzheimer's disease before the accumulation of amyloid or tau proteins occurs. This reduced blood flow to the brain causes attention and memory deficits and damages the neuronal connections within the brain.

The decreased blood flow is linked to capillary constrictions believed to be caused by the pericytes, which constrict in response to the reactive oxygen species triggered by amyloid β protein.

The constriction of pericytes in only the capillaries and not the larger blood vessels has been observed in humans and murine models.

About the study

In the present study, the researchers used nimodipine, a blood-brain barrier-permeable CaV blocker, to decrease the contraction of capillaries induced by pericytes to increase cerebral blood flow, dilate capillaries, and lower the viscosity of blood.

The study used several transgenic mouse lines, including those carrying Alzheimer's disease mutations, with fluorescent markers introduced for imaging.

The mice were also treated with tamoxifen to induce the expression of Cre recombinase-mutated estrogen receptor (Cre^ERT2), which allowed specific genes in the brain cells to be controlled. Nimodipine treatment was administered through drinking water for one and a half months before conducting the various analytical tests.

Additionally, human brain tissue samples were collected from patients undergoing neurosurgery and treated with amyloid β and nimodipine to study the vascular and cellular responses. Pericytes in the capillaries were identified based on morphology and fluorescent labeling.

Advanced microscopic techniques such as two-photon microscopy were used on anesthetized and awake mice for in vivo imaging.

Additionally, methods involving fluorescent dyes, laser doppler flowmetry, and fluorescein isothiocyanate (FITC)-dextran were used to measure pericyte activity, cerebral blood flow, and the integrity of the blood-brain barrier.

Blood flow changes were also monitored using magnetic resonance imaging and laser speckle flowmetry. Furthermore, the oxygen-deprived regions in the brain tissue were labeled by injecting hypoxia markers.

Laser scanning microscopy was employed to image reactive oxygen species and the microglia. Two-photon microscopy was also used for calcium ion (Ca²⁺) imaging of brain slices obtained from euthanized mice.

Additionally, immunohistochemical methods using various antibodies and fluorescent secondary antibodies were conducted for vessel segment tracing and for quantifying the pericyte coverage.

The antibody targets included the platelet endothelial cell adhesion molecule-1 CD31 and intercellular adhesion molecule 1 (ICAM-1), which play a role in cellular immune responses.

Results

The researchers found that pericyte contraction in capillaries is controlled by CaVs and transmembrane member 16A (TMEM16A), a calcium-activated chloride channel.

These two channels work in unison to increase the Ca²⁺ levels in pericytes, causing capillaries to constrict. Furthermore, the use of nimodipine lowered the Ca²⁺ levels in the pericytes, resulting in vessel dilation and increased cerebral blood flow.

Nimodipine was found to dilate capillaries and arteries in both brain slices and live mice, and the effect was more pronounced in the Alzheimer's disease mouse models. The cerebral blood flow increase was 74% greater in the Alzheimer's disease mouse models than that in normal mice, indicating that vascular constriction was also greater in Alzheimer's disease.

The study also found that the pericytes in even the most distal capillaries played a significant role in regulating blood flow, and blocking the calcium channels even in the more advanced stages of the disease helped relieve the capillary constrictions and improve cerebral blood flow.

The researchers also reported that reactive oxygen species and oxidative stress were important factors in the mechanisms behind decreased cerebral blood flow in Alzheimer's disease.

The reactive oxygen species generated by amyloid β oligomers and immune cells such as perivascular macrophages and microglia increased the Ca²⁺ levels in pericytes, which caused constrictions in the capillaries and impaired blood flow in the brain.

Using antioxidants such as N-acetyl cysteine or inhibiting the enzyme NOX2 (nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2), which is involved in the production of reactive oxygen species, was also found to significantly lower the Ca²⁺ levels in pericytes and increase blood flow.

Conclusions

Overall, the study suggested that targeting the calcium ion channels could help lower the Ca²⁺ levels in the pericytes and mitigate the cerebral blood flow reductions observed in Alzheimer's disease by dilating the capillaries.

The findings also highlighted the role of reactive oxygen species in vascular function in Alzheimer's disease and indicated that targeting the production of reactive oxygen species could help improve blood flow in the patients.

Journal reference:

• Korte, N., Barkaway, A., Wells, J., Freitas, F., Sethi, H., Andrews, S. P., Skidmore, J., Stevens, B., & Attwell, D. (2024). Inhibiting Ca2+ channels in Alzheimer’s disease model mice relaxes pericytes, improves cerebral blood flow and reduces immune cell stalling and hypoxia. Nature Neuroscience. doi:10.1038/s4159302401753w.https://www.nature .com/articles/s41593-024-01753-w