Sex‐differences and cell type‐specific alterations in brain Bcl2 expression after transient focal cerebral ischemia in aged mice

Abstract

IntroductionStroke remains the second leading cause of death worldwide and remains the primary cause of long‐term disability in the US. To date, the only pharmacological treatment for stroke remains tissue plasminogen activator. A major factor for translational failure is likely overwhelming use of young male rodents despite the fact that stroke occurs predominantly in aged populations and is a sexually dimorphic disease with sex differences in incidence, prevalence, and outcomes. The Bcl2 family is known to play an important role in the evolution of injury following cerebral ischemia. Overexpression of pro‐survival Bcl2 protects against cerebral ischemia in vivo and in vitro. In the present study we explored whether Bcl2 could play a role in known age‐ and sex‐related differences in stroke.MethodsAll studies were conducted in accordance with National Institutes of Health guidelines for the use of experimental animals, with protocols approved by the Stanford Animal Care and Use Committee. Transient focal cerebral ischemia was induced by suture occlusion of the middle cerebral artery (MCAO) for 1h in 20 month‐old male and female C57BL/6 mice. Sham surgery consisted of anesthesia and internal carotid exposure without suture occlusion (n=6 per group). Histological assessment: Animals were sacrificed at post‐MCAO days 1, 3, and 30 (n=6 per group, per time point), and brains fixed with 4% phosphate‐buffered paraformaldehyde for stereological analysis. Fixed sections were stained for Bcl2, the astrocyte marker glial fibrillary acidic protein (GFAP), and the mature neuronal marker NeuN. An observer blinded to conditions quantified the cell‐type specific relative intensity of Bcl2 from maximum projection Z‐stack images.ResultsIn sham animals Bcl2 expression was significantly (p<0.05) higher in both female astrocytes and neurons relative to male. After MCAO male neuronal Bcl2 expression was significantly depressed from days 1 until 30, while female neuronal Bcl2 remained unchanged. Conversely in astrocytes, female Bcl2 expression decreased significantly at post‐injury days 1 and 3, but recovered by day 30, while male astrocyte Bcl2 decreased by day 3 and remained significantly repressed at day 30.ConclusionsBaseline differences in Bcl2 expression between aged males and females, and cell‐type specific differences in post‐injury Bcl2 expression may account for age‐ and sex‐related differences in injury outcome from stroke. Bcl2 may represent a therapeutic target for stroke for both sexes in clinically‐relevant aged populations.