Key Information
Abstract
The progressive number of old adults with cognitive impairment worldwide and the lack of effective pharmacologic therapies require the development of non-pharmacologic strategies. The photobiomodulation (PBM) is a promising method in prevention of early or mild age-related cognitive impairments. However, it remains unclear the efficacy of PBM for old patients with significant age-related cognitive dysfunction. In our study on male mice, we show a gradual increase in the brain amyloid beta (Aβ) levels and a decrease in brain drainage with age, which, however, is associated with a decline in cognitive function only in old (24 months of age) mice but not in middle-aged (12 months of age) and young (3 month of age) animals. These age-related features are accompanied by the development of hyperplasia of the meningeal lymphatic vessels (MLVs) in old mice underlying the decrease in brain drainage. PBM improves cognitive training exercises and Aβ clearance only in young and middle-aged mice, while old animals are not sensitive to PBM. These results clearly demonstrate that the PBM effects on cognitive function are correlated with age-mediated changes in the MLV network and may be effective if the MLV function is preserved. These findings expand fundamental knowledge about age differences in the effectiveness of PBM for improvement of cognitive functions and Aβ clearance as well as about the lymphatic mechanisms responsible for age decline in sensitivity to the therapeutic PBM effects.
Introduction
Age is a biological factor triggering irreversible changes in brain tissue and functions [1external link, opens in a new tab–3external link, opens in a new tab]. The most common age-related manifestations are memory and cognitive decline [4external link, opens in a new tab, 5external link, opens in a new tab]. Age-related changes in brain functions are a long-term process and in the early and even middle stages can be improved by transcranial photobiomodulation (PBM) [6external link, opens in a new tab–12external link, opens in a new tab]. However, despite promising results in PBM-mediated cognitive improvement in middle-aged and elderly people, when only the first signs of cognitive dysfunction are evident, the efficacy of PBM in old people, when cognitive impairment and brain aging are progressive, remains poorly studied.
Meningeal lymphatic vessels (MLVs) are one of the targets for PBM [13external link, opens in a new tab–18external link, opens in a new tab]. MLVs have an important function in regulation of brain drainage and the excretion of metabolites and toxins, such as amyloid beta (Aβ), through the fluid stream [19external link, opens in a new tab]. A number of studies have found that the function of MLVs decreases with age, which may lead to Aβ accumulation in brain tissue as well as cognitive decline [18external link, opens in a new tab–20external link, opens in a new tab]. Indeed, aged mice demonstrated significant regression in branches of dorsal MLVs leading to suppression of drainage of the cerebral spinal fluid [20external link, opens in a new tab]. Aging results in morphological (decreased lymphatic vessel density) and functional (reduced transport capacity) alterations in the lymphatic vessels including thoracic duct, skin, meningeal and mesenteric lymphatic network [19external link, opens in a new tab–21external link, opens in a new tab]. The main reason of age-related alteration of the lymphatic vessels is lower production of lymphangiogenic factors and diminished regenerative capacity of the lymphatic endothelium [19external link, opens in a new tab, 20external link, opens in a new tab]. Aging also reduces the lymphatic smooth muscle actin coverage around valves leading to decreasing contractility of the lymphatic vessels [22external link, opens in a new tab–26external link, opens in a new tab]. There is evidence that the reduced MLV function is an important factor associated with the development of Alzheimer’s disease [16external link, opens in a new tab, 19external link, opens in a new tab, 27external link, opens in a new tab]. So, transgenic 5xFAD mice demonstrate diminished meningeal lymphatic coverage of the main venous sinuses that are accompanied with a significant Aβ deposits in brain tissue and cognitive decline [19external link, opens in a new tab]. The damage of MLV results in significant Aβ accumulation in the brain [15external link, opens in a new tab, 16external link, opens in a new tab, 19external link, opens in a new tab]. Thus, age-related changes in MLVs can impair brain drainage leading to accumulation of metabolites and toxins.
PBM has stimulatory effects on the MLV functions, improving lymphatic excretion of various toxins from the brain, such as Aβ in mice with Alzheimer’s disease [15external link, opens in a new tab, 16external link, opens in a new tab, 18external link, opens in a new tab, 28external link, opens in a new tab] and blood products in mice with a model of intraventricular hemorrhage [13external link, opens in a new tab].
PBM helps to increase the contractility of the lymphatic vessels thereby improving the lymph flow [13external link, opens in a new tab]. Two factors, such as nitric oxide (NO) and calcium (Ca2+)-channels, regulate peristaltic contractions of the lymphatics vessels and the functions of the lymphatic valves, which ensures unidirectional lymph flow [29external link, opens in a new tab–32external link, opens in a new tab]. There is a hypothesis based on experimental data that PBM-effect on the endothelium of basal MLVs leads to the generation of 1O2 in the mitochondria, which is accompanied by the NO formation, mainly in the valves because 50% of the endothelial NO-synthase is localized there [29external link, opens in a new tab, 33external link, opens in a new tab–35external link, opens in a new tab]. The described effects can be related to a PBM-mediated increase in the activity of endothelial synthase (eNOS) [36external link, opens in a new tab]. The NO is a vasodilator that acts via stimulation of soluble guanylate cyclase to form cyclic-GMP, which activates protein kinase G causing the opening of calcium-activated potassium channels and reuptake of Ca2+. The decrease in the concentration of Ca2+ prevents myosin light-chain kinase from phosphorylating the myosin molecule, leading to the relaxation of lymphatic vessels [37external link, opens in a new tab].
Traditionally, NO is considered to only relax the vascular endothelium. However, it is also a factor that triggers vascular contractility [29external link, opens in a new tab–32external link, opens in a new tab]. Liao et al. revealed that eNOS in lymphatic endothelial cells is required for robust lymphatic contractions under physiologic conditions [38external link, opens in a new tab]. PBM-mediated release of NO stimulates dilation of MLVs and increases their permeability, which leads to an increase in their volume due to the influx of fluid into them [13external link, opens in a new tab]. In this moment, the upstream valve is open, and the downstream valve is closed. When MLVs are filled, share stress decreases and NO is degraded. Afterward, a subsequent contraction of MLVs is initiated through Ca2+ influx both via stretch-, voltage-, or ion-activated channels and from the depot. The contraction of MLVs closes the upstream valves and opens the downstream valves leading to an increase in a wall shear stress and the NO production locally, thus starting the cycle again. This way is the peristaltic process in MLVs, which is the basis of their drainage and cleansing functions [33external link, opens in a new tab].
In recent studies on young mice, improved brain drainage and the MLV functions after a course of PBM has been shown to promote better learning [16external link, opens in a new tab, 17external link, opens in a new tab]. PBM is also effective in reducing the brain Aβ levels in adult healthy mice and animals with Alzheimer’s disease [15external link, opens in a new tab, 16external link, opens in a new tab, 18external link, opens in a new tab].