Angiotensin II Receptor 1 Blockage Limits Brain Damage and Improves Functional Outcome After Brain Injury in Aged Animals Despite Age-Dependent Reduction in AT1 Expression

Abstract

Traumatic brain injury (TBI) is a frequent pathology associated with poor neurological outcome in the aged population. We recently observed accelerated cerebral inflammation in aged mice in response to TBI. Candesartan is a potent specific inhibitor of angiotensin II receptor type 1 (AT1) which limits cerebral inflammation and brain damage in juvenile animals after experimental TBI. In the present study, we show significantly lower posttraumatic AT1 mRNA levels in aged (21 months) compared to young (2 months) mice. Despite low cerebral At1 expression, pharmacologic blockade by treatment with candesartan [daily, beginning 30 min after experimental TBI by controlled cortical impact (CCI)] was highly effective in both young and aged animals and reduced histological brain damage by −20% after 5 days. In young mice, neurological improvement was enhanced by AT1 inhibition 5 days after CCI. In older animals, candesartan treatment reduced functional impairment already on day 3 after TBI and post-traumatic body weight (BW) loss was attenuated. Candesartan reduced microglia activation (−40%) in young and aged animals, and neutrophil infiltration (−40% to 50%) in aged mice, whereas T-cell infiltration was not changed in either age group. In young animals, markers of anti-inflammatory microglia M2a polarization [arginase 1 ( Arg1 ), chitinase3-like 3 ( Ym1 )] were increased by candesartan at days 1 and 5 after insult. In older mice 5 days after insult, expression of Arg1 was significantly higher independently of the treatment, whereas Ym1 gene expression was further enhanced by AT1 inhibition. Despite age-dependent posttraumatic differences in At1 expression levels, inhibition of AT1 was highly effective in a posttreatment paradigm. Targeting inflammation with candesartan is, therefore, a promising therapeutic strategy to limit secondary brain damage independent of the age.

Depletion of regulatory T cells increases T cell brain infiltration, reactive astrogliosis, and interferon-γ gene expression in acute experimental traumatic brain injury

“…Coronal brain tissue slices were cut at the level of the cortical impact and separated between the left and right hemispheres. Upper right quadrants containing the lesioned and the perilesional brain tissue were snap-frozen in liquid nitrogen, stored at − 80°C, and further processed for quantitative real-time PCR (qRT-PCR)-based gene expression analyses as described before [47,49]. Briefly, all assays were carried out in our laboratory by an investigator blind to group allocation.…”

“…Briefly, all assays were carried out in our laboratory by an investigator blind to group allocation. Using specific oligonucleotide primer pairs and optimized temperature conditions for qPCR, values were normalized to the reference gene PPIA (cyclophilin A) and absolute quantification was performed using a target-specific standard curve of mRNA copies [49]. The expression data analysis was performed with the LightCycler Software,…”

Traumatic Brain Injury: An Age-Dependent View of Post-Traumatic Neuroinflammation and Its Treatment

“…The different microglial phenotypes have not been deeply studied, but at 1 dpi, the M1 phenotype and most of the M2a phenotype genes are highly upregulated compared with young adults, in contrast with the M2c phenotype genes, which are downregulated [ 210 ]. Another study reported the reduced expression of M2a genes in aged TBI at 1 dpi compared with young adults, but this ratio was reversed at 5 dpi [ 205 ].…”

“…Qualitatively, more infiltration by neutrophils and fewer by monocytes was observed [ 200 ]. Quantitatively, the recruitment process in aged mice is emphasized and subchronically exacerbated as more newly recruited leukocytes are present in their brains than in those of young adults from 1 to 7 dpi [ 200 , 203 , 205 , 209 ]. However, a study reported altered neuroimmune response with fewer leukocytes infiltrating myeloid cells and microglia at 1 and 7 dpi in aged TBI rats, compared with young adults [ 211 ].…”

“…Cytokine production is also different in aged mice. The baseline expression of some pro-inflammatory cytokines, such as IL-1β, TNFα, iNOS and IL-6, and chemokines, like MCP-1, are already higher in aged animals compared with young adults [ 205 ], indicating a subclinical chronic inflammatory process in the elderly, referred to as “inflamm-aging” [ 204 ]. The overall pattern of inflammatory protein expression differs between aged and young adults.…”