Blood-Brain Barrier Dysfunction and Astrocyte Senescence as Reciprocal Drivers of Neuropathology in Aging

Abstract

As the most abundant cell types in the brain, astrocytes form a tissue-wide signaling network that is responsible for maintaining brain homeostasis and regulating various brain activities. Here, we review some of the essential functions that astrocytes perform in supporting neurons, modulating the immune response, and regulating and maintaining the blood-brain barrier (BBB). Given their importance in brain health, it follows that astrocyte dysfunction has detrimental effects. Indeed, dysfunctional astrocytes are implicated in age-related neuropathology and participate in the onset and progression of neurodegenerative diseases. Here, we review two mechanisms by which astrocytes mediate neuropathology in the aging brain. First, age-associated blood-brain barrier dysfunction (BBBD) causes the hyperactivation of TGFβ signaling in astrocytes, which elicits a pro-inflammatory and epileptogenic phenotype. Over time, BBBD-associated astrocyte dysfunction results in hippocampal and cortical neural hyperexcitability and cognitive deficits. Second, senescent astrocytes accumulate in the brain with age and exhibit a decreased functional capacity and the secretion of senescent-associated secretory phenotype (SASP) factors, which contribute to neuroinflammation and neurotoxicity. Both BBBD and senescence progressively increase during aging and are associated with increased risk of neurodegenerative disease, but the relationship between the two has not yet been established. Thus, we discuss the potential relationship between BBBD, TGFβ hyperactivation, and senescence with respect to astrocytes in the context of aging and disease and identify future areas of investigation in the field.

Keywords: TGF beta 1; albumin; astrocytes; blood–brain barrier; neuroinflammation; senescence.

Figure 1

Mechanisms of astrocyte-mediated neuropathology in the aging brain. Blood–brain barrier dysfunction (BBBD) and cellular senescence progressively increase during aging and are associated with an increased risk of neurodegenerative disease. Two processes that result in dysfunctional astrocyte phenotypes and subsequent neuropathology are BBBD-associated astrocyte activation and astrocyte senescence. BBBD causes albumin to leak into the brain, where it activates TGFβ and p38MAPK signaling in astrocytes, which elicits an epileptogenic and pro-inflammatory astrocyte phenotype. The epileptogenic phenotype is characterized by downregulation of K+ transporters (Kir4.1), inhibitory GABA_A receptors (GABAAR), and NMDA receptors (NMDAR) and upregulation of excitatory glutamate receptors (GluR). Second, certain types of cellular stress cause expression of tumor suppressors p16 and p21, which elicit a senescent phenotype characterized by decreased functional capacity and SASP expression. This decreased capacity involves downregulation of aquaporin 4 (AQP4), K+ transporters (Kir4.1), and glutamate transporters (EAAT1/2). Factors such as cytokines, chemokines, and proteases secreted by activated and senescent astrocytes may act on the endothelium to further exacerbate BBBD via downregulation of tight junction (TJ) proteins, glucose transporters (GLUT1), and waste efflux transporters (Pgp). Dysfunctional astrocytes contribute to neuronal hyperexcitability, neuroinflammation, neurotoxicity, and neurodegeneration, which manifest as cognitive deficits in the aging brain. Questions remain regarding the mechanistic links and relationships between BBBD-associated astrocyte activation and senescence. This figure was created with cell images from BioRender.