Impacts of systemic milieu on cerebrovascular and brain aging: insights from heterochronic parabiosis, blood exchange, and plasma transfer experiments

Abstract

Aging is a complex biological process that detrimentally affects the brain and cerebrovascular system, contributing to the pathogenesis of age-related diseases like vascular cognitive impairment and dementia (VCID) and Alzheimer's disease (AD). While cell-autonomous mechanisms that occur within cells, independent of external signals from neighboring cells or systemic factors, account for some aspects of aging, they cannot explain the entire aging process. Non-autonomous, paracrine and endocrine, pathways also play a crucial role in orchestrating brain and vascular aging. The systemic milieu modulates aging through pro-geronic and anti-geronic circulating factors that mediate age-related decline or confer rejuvenative effects. This review explores the impact of systemic factors on cerebrovascular and brain aging, with a particular focus on findings from heterochronic parabiosis, blood exchange, and plasma transfer experiments. We discuss how these factors influence fundamental cellular and molecular processes of aging and impact cerebrovascular endothelial function, neurovascular coupling mechanisms, blood-brain barrier integrity, neuroinflammation, capillary density, and amyloid pathologies, with significant consequences for cognitive function. Additionally, we address the translational potential and challenges of modifying the systemic milieu to promote brain health and prevent age-related cognitive impairment.

Keywords: Ageing; Cerebral blood flow; Cerebral circulation; Cerebral microcirculation; Cerebrovascular aging; Endocrine; Endothelial; Endothelial dysfunction; Geroscience; Humoral; Neurodegeneration; Neuroendocrine theory; Neuroinflammation; Regulation; Senescence; Systemic; Systemic determinants; Vascular aging.

Fig. 1

The systemic milieu and its influence on cerebrovascular health. The systemic milieu encompasses a diverse array of bioactive components, including acellular elements (e.g., hormones, cytokines, antibodies, lipids, metabolites, RNA species), cells (e.g., leukocytes, thrombocytes, circulating endothelial progenitor cells), and extracellular vesicles (EVs). These circulating factors collectively modulate age-related changes in the structure, function, and phenotype of the cerebral vasculature. Endothelial cells, as the first line of contact, are profoundly influenced by these circulating factors, which regulate critical functions such as vasomotor control, barrier integrity, molecular clearance, angiogenesis, and secretory activities. Age-related alterations in these systemic factors play a pivotal role in the progression of vascular dysfunction and the development of age-associated chronic diseases. This figure underscores the dynamic interplay between the systemic environment and cerebrovascular health, highlighting the multifaceted contributions of cellular and acellular components to vascular aging and disease

Fig. 2

Experimental approaches for investigating cell non-autonomous mechanisms of organismal aging. This figure summarizes six widely utilized methodologies for studying the influence of systemic factors and cell non-autonomous mechanisms on organismal aging. The approaches include heterochronic parabiosis, heterochronic blood and plasma exchange, bone marrow transplantation, plasma dilution, targeted modulation of aging-related circulating factors, and in vitro models using heterochronic sera. The table provides an overview of each method’s core concept, advantages, limitations, and key findings, offering insights into their applications in aging research. These experimental strategies highlight the systemic milieu’s pivotal role in regulating age-related changes in vasculature, cellular function, and overall organismal health

Fig. 3

Heterochronic parabiosis and cerebrovascular aging. Heterochronic parabiosis involves surgically connecting two animals of different ages (young and aged, center) to share a common circulatory system, enabling researchers to study the influence of systemic factors on aging processes across tissues and organs. This model has identified several pro-aging (“progeronic”) factors, such as eotaxin- 1 (CCL11), pro-inflammatory cytokines, and SASP components, which contribute to vascular and neuronal aging. Conversely, youthful systemic factors, including growth factors (IGF- 1, BDNF), metabolites (e.g., NAD⁺), and anti-inflammatory mediators, have demonstrated rejuvenating effects, improving endothelial function, neurogenesis, and cognitive performance. The interplay between these factors highlights their critical roles in cerebrovascular and brain aging, presenting novel opportunities for therapeutic intervention to counteract neurovascular decline and age-related cognitive impairments

Fig. 4

Systemic factors in cerebrovascular and brain aging. The systemic milieu undergoes profound age-related changes that significantly influence cerebrovascular and brain aging. Studies on cell non-autonomous mechanisms have highlighted circulating factors with either anti-geronic or pro-geronic properties. Young blood-enriched factors such as IGF- 1, NAD⁺, VEGF, and GDF11 enhance endothelial function, maintain BBB integrity, and support neurovascular coupling and vascularization. These factors promote cerebrovascular health, improve brain perfusion, and reduce neuroinflammation, collectively preserving synaptic density, white matter integrity, and cognitive function. Additional anti-geronic factors, including TIMP2 and PF4, exert neuroprotective effects critical for cognitive resilience and healthy aging. Conversely, aging is marked by elevated levels of pro-inflammatory cytokines, SASP factors, and harmful microRNAs, alongside other pro-geronic mediators such as CCL11, B2M, and GDF15. These factors drive endothelial dysfunction, BBB breakdown, microvascular rarefaction, and brain hypoperfusion. These cerebrovascular impairments contribute to neuroinflammation, synaptic loss, white matter degradation, and progressive cognitive decline, underpinning age-related neurodegenerative diseases. Abbreviations: BBB, blood–brain barrier; SASP, senescence-associated secretory phenotype; NAD⁺, nicotinamide adenine dinucleotide; IGF- 1, insulin-like growth factor 1; GDF11, growth differentiation factor 11; VEGF, vascular endothelial growth factor; TIMP2, tissue inhibitor of metalloproteinases 2; PF4, platelet factor 4

Fig. 5

Comparative effects of young and aged systemic milieu on hallmarks of aging. This figure illustrates the differential impact of young (left) and aged (right) systemic milieus on cellular and molecular hallmarks of aging. The young systemic milieu is enriched with anti-geronic factors such as IGF-1, VEGF, GDF11, NAD⁺, and beneficial microRNAs. These factors promote cellular stress resilience, support genomic stability, enhance mitochondrial function, and counteract inflammation and oxidative stress, thereby maintaining tissue homeostasis and delaying aging phenotypes. In contrast, the aged systemic milieu is characterized by pro-geronic factors, including SASP factors, pro-inflammatory cytokines (PICs) and harmful microRNAs (e.g., miR-29, miR-185). These factors exacerbate chronic inflammation, oxidative stress, and mitochondrial dysfunction, leading to genomic instability, cellular senescence, and telomere attrition. Collectively, these age-associated changes drive tissue dysfunction and accelerate aging. The outer ring highlights the core hallmarks of aging affected by these systemic environments. The figure underscores the importance of systemic factors in modulating organismal aging and highlights potential targets for therapeutic interventions aimed at reversing age-related pathologies

Fig. 6

Role of mitochondrial function in brain endothelial cells during aging and young blood-mediated rejuvenation. A Schematic representation of the brain microvasculature and its endothelial cells, highlighting the critical role of mitochondrial health in maintaining blood–brain barrier integrity and endothelial vasodilatory function. SIRT1 activation serves as a key regulator of mitochondrial function, cellular energetics, and the modulation of reactive oxygen species (ROS) production in brain endothelial cells. These mitochondrial processes are significantly altered with aging but can be restored by rejuvenating factors present in young blood, which enhance mitochondrial function and cerebromicrovascular health. B Gene set enrichment analysis (GSEA) depicting the enrichment profiles of oxidative phosphorylation (OXPHOS)-related genes in brain endothelial cells during aging (top) and following young blood exposure (bottom). Aging leads to the downregulation of OXPHOS-related genes (negative enrichment score), while young blood exposure reverses this trend by upregulating these genes (positive enrichment score), demonstrating the restorative influence of young systemic factors on mitochondrial pathways. C Detailed illustration of the oxidative phosphorylation pathway, showing the structural organization of mitochondrial ETC complexes (complexes I–V). Heatmaps represent mitochondrial subunits affected by aging (downregulated: red) and those restored by young blood (upregulated: green). These findings underscore the central role of mitochondrial function in endothelial cell aging and the rejuvenating effects of young systemic factors on mitochondrial health and cerebromicrovascular integrity. The plots showing the mitochondrion-related gene expression changes in heterochronic parabionts used in this figure are reproduced with permission from the publisher. [51]

Fig. 7

Mechanisms of neurovascular coupling and the impact of aging and young systemic milieu. A Schematic representation of the neurovascular unit and neurovascular coupling (NVC) mechanisms. Neuronal activation stimulates astrocytes and endothelial cells, leading to increased production of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS). This promotes vasodilation, enhances regional cerebral blood flow (CBF), and supports neuronal metabolic demands by delivering oxygen and nutrients while removing metabolic byproducts. The panels on the right compare the effects of young and aged systemic milieus. In young conditions, circulating anti-geronic factors (e.g., IGF- 1, NAD⁺) enhance eNOS activity, promote NO production, activate sirtuins, reduce oxidative stress, and improve vasodilation. Conversely, the aged systemic milieu is enriched with pro-geronic factors (e.g., TNFα), which impair eNOS function, increase reactive oxygen species (ROS) production, decrease NO bioavailability, and compromise vasodilation. B Experimental data from heterochronic parabiosis studies showing the influence of systemic milieus on NVC. NVC responses were assessed using laser speckle contrast imaging (LSCI) over the somatosensory cortex during contralateral whisker stimulation. Representative pseudocolor images depict baseline and stimulation-induced CBF changes, with differences highlighted in red within the whisker barrel cortex. Quantified changes in CBF, representing NVC responses, are displayed in bar graphs. Results show that exposure to young blood rejuvenates NVC responses in aged heterochronic parabionts, while exposure to old blood impairs NVC in young parabionts, mimicking aging-related phenotypes. Data are presented as mean ± S.D., analyzed using one-way ANOVA with post hoc Tukey’s tests (**p < 0.01, ****p < 0.0001, n = 8 per group). The representative images and quantitative data shown here are reproduced with permission from the publisher [163]

Fig. 8

Age-related blood–brain barrier disruption and the effects of systemic milieu. A Schematic representation of the BBB under young and aged systemic conditions, illustrating age-related mechanisms of BBB dysfunction. In young conditions, the BBB is intact, supported by tight junction integrity and selective transcytosis, which maintain neuronal homeostasis and cognitive health. Circulating anti-geronic factors, such as GDF11, IGF- 1, and FGF21, play critical roles in maintaining BBB function. In aging, the BBB becomes increasingly permeable due to reduced tight junction expression and enhanced unselective transcytosis. The aged systemic milieu, enriched with pro-geronic factors (e.g., SASP factors), exacerbates BBB disruption, leading to neuroinflammation, white matter (WM) damage, reduced synaptic density, neurodegeneration, and cognitive decline. B Experimental evidence demonstrating BBB permeability changes in heterochronic parabionts, assessed by the permeability of 3 kDa FITC-dextran. Representative two-photon microscopy images depict tracer extravasation in the brain microvasculature. Minimal extravasation is observed in young isochronic parabionts, shown by the predominantly dark blue extravascular space. Isochronic aged parabionts display significant BBB permeability, highlighted by increased yellow and white fluorescence in the brain parenchyma, indicating FITC-dextran leakage. Heterochronic aged parabionts exhibit reduced tracer extravasation, suggesting that young blood restores BBB integrity. Conversely, aged blood exposure increases BBB permeability in heterochronic young parabionts, as evidenced by heightened tracer leakage. Quantitative analysis of FITC-dextran extravasation is presented in the bar graph, with data shown as mean ± SD (n = 7–9 per group). Statistical significance was analyzed using one-way ANOVA with Sidak’s post hoc tests (***p < 0.001, ****p < 0.0001). The representative images and quantitative data are reproduced with permission from the publisher [164]. Scale bar, 100 µm

Fig. 9

Role of circulating factors in microvascular rarefaction. A Microvascular rarefaction, characterized by a reduction in capillary density, is a hallmark of cerebrovascular aging and is driven by impaired angiogenesis and increased vascular regression. Systemic anti-geronic factors, such as IGF- 1, GDF11, and VEGF, positively regulate angiogenesis and inhibit vascular regression, preserving capillary density. Conversely, the aged systemic milieu contains elevated levels of pro-geronic factors, including proinflammatory cytokines, which drive vascular regression and contribute to microvascular rarefaction. These changes compromise cerebral blood flow and capillarization, exacerbating age-related cognitive decline. B Representative images from two-photon microscopy show cortical blood vessels labeled with WGA-AF594 (red) from each experimental group, including isochronic young (Y-Y), isochronic aged (A-A), and heterochronic parabionts (Y-A, A-Y). These images highlight the effects of systemic milieu on microvascular health. Isochronic aged parabionts exhibited the lowest capillary density, significantly reduced compared to isochronic young parabionts (p < 0.01). In aged heterochronic parabionts, exposure to young blood preserved capillary density, demonstrating the rejuvenating effects of youthful systemic factors. In contrast, young heterochronic parabionts exposed to aged blood showed a significant reduction in capillary density, mirroring the vascular regression observed in aging. The bar graph quantifies capillary density differences across groups, showing mean ± SD (n = 7–9 per group, one-way ANOVA with post hoc Sidak’s tests; *p < 0.05, **p < 0.01). These findings provide evidence for the critical role of circulating factors in microvascular rarefaction, with young systemic milieu enhancing vascularization and aged systemic milieu accelerating vascular regression. Abbreviations: IGF- 1, insulin-like growth factor 1; GDF11, growth differentiation factor 11; VEGF, vascular endothelial growth factor; IL- 6, interleukin 6; TNFα, tumor necrosis factor-alpha; MMPs, matrix metalloproteinases. Representative images and quantitative data are reproduced with permission from the original authors and publisher [164]

Fig. 10

Cerebrovascular consequences of age-related deficiency in IGF- 1 signaling. This schematic highlights the multifaceted effects of age-related decline in insulin-like growth factor 1 (IGF- 1) signaling on cerebrovascular and neural health. IGF- 1, a pleiotropic vasculoprotective factor, is critical for maintaining neurovascular coupling (NVC), endothelial function, myogenic autoregulatory function, blood–brain barrier (BBB) integrity, capillarization, and structural integrity of the vascular wall. Age-related IGF- 1 deficiency impairs microvascular dilation, diminishes the conduction of vasodilatory signals, resulting in reduced cerebral blood flow (CBF). Declining IGF- 1 levels also promote endothelial senescence, increasing BBB permeability, which facilitates neuroinflammation, white matter damage, and neurodegeneration. Furthermore, IGF- 1 deficiency contributes to microvascular injury, rarefaction, pathological vascular remodeling and compromised myogenic autoregulatory protection, exacerbating ischemic vulnerability and the risk of cerebral microhemorrhages (CMHs). Collectively, these IGF- 1 deficiency-driven cerebromicrovascular pathologies culminate in cognitive impairment and highlight the critical role of IGF- 1 in maintaining cerebromicrovascular and brain health during aging

Fig. 11

Lifestyle and exposome influence on cerebrovascular and cognitive health via cell non-autonomous mechanisms. This schematic outlines how key environmental and lifestyle factors shape cerebrovascular and cognitive health through systemic, cell non-autonomous mechanisms. Five major categories are depicted, with their respective positive (green) and negative (red) impacts on vascular and brain health. Arrows illustrate the systemic mediators through which these exposures exert their effects, such as circulating anti-inflammatory factors, pro-inflammatory cytokines, growth factors, metabolites, and oxidative stress markers. For example, physical activity enhances levels of IGF- 1, BDNF, and VEGF, promoting cerebrovascular and cognitive resilience. Conversely, chronic stress and environmental toxins elevate SASP factors, inflammatory cytokines, and reactive oxygen species, contributing to vascular dysfunction and cognitive decline. This figure emphasizes the interplay between lifestyle and systemic factors, providing insights into modifiable risk and protective mechanisms for preserving brain health