Effects of senescence on astrocyte to brain endothelial cell signaling

Aging affects almost all aspects of central nervous system (CNS) function, including the blood-brain barrier (BBB). Here, we use cell culture models to ask whether senescence, a cellular feature of aging, alters the BBB by modifying interactions between astrocytes and brain endothelial cells.

Human astrocyte and Human brain microvascular endothelial cells were subcultured and maintained for cells at low and high passages, then confirmed with senescence-associated β-galactosidase staining and gene expression of cyclin-dependent kinase inhibitor 2A (Cdkn2a). After coculturing with astrocyte, the Alexa Fluor 488-labeled bovine serum albumin (Alexa 488-BSA) was used as a tracer to measure the permeability of brain endothelial cells; the expression of related proteins was measured by quantitative real-time polymerase chain reaction. Reducing the angiotensinogen (AGT) by small interfering RNA (siRNA) in senescent astrocyte to test the effect of angiotensin signals on endothelial permeability.

Young astrocytes (cumulative population doublings [CPD] ≤ 4) modified the expression of barrier genes and decreased brain endothelial permeability in coculture, whereas aged senescent astrocytes (CPD ≥ 9) had no effects (45.5% ± 18.0% vs. 122.8% ± 28.6%, p = 0.0016). Angiotensin is known to alter the BBB. Its precursor, AGT, is highly expressed in astrocytes in the brain. Therefore, we asked whether angiotensin signaling may mediate the loss of endothelial barrier-promoting properties in senescent astrocytes. Both protein and messenger RNA (mRNA) levels of AGT were increased in high-passage senescent astrocytes. Reducing AGT levels through siRNA restored the endothelial barrier-promoting effects of high-passage senescent astrocytes (F _(2,15) = 6.508, p = 0.0092). By contrast, brain endothelial cells at different passages did not change the expression of AGT in astrocytes.

Taken together, these findings suggest that increased angiotensin signaling from astrocytes to brain endothelium may partly mediate the decrease of BBB function in the aging CNS.