Astrocytic ß2 Adrenergic Receptor Gene Deletion Affects Memory in Aged Mice.

In vitro and in vivo studies suggest that the astrocytic adrenergic signalling enhances glycogenolysis which provides energy to be transported to nearby cells and in the form of lactate. This energy source is important for motor and cognitive functioning. While it is suspected that the ß2-adrenergic receptor on astrocytes might contribute to this energy balance, it has not yet been shown conclusively in vivo. Inducible astrocyte specific ß2-adrenergic receptor knock-out mice were generated by crossing homozygous ß2-adrenergic receptor floxed mice (Adrb2flox) and mice with heterozygous tamoxifen-inducible Cre recombinase-expression driven by the astrocyte specific L-glutamate/L-aspartate transporter promoter (GLAST-CreERT2). Assessments using the modified SHIRPA (SmithKline/Harwell/Imperial College/Royal Hospital/Phenotype Assessment) test battery, swimming ability test, and accelerating rotarod test, performed at 1, 2 and 4 weeks, 6 and 12 months after tamoxifen (or vehicle) administration did not reveal any differences in physical health or motor functions between the knock-out mice and controls. However deficits were found in the cognitive ability of aged, but not young adult mice, reflected in impaired learning in the Morris Water Maze. Similarly, long-term potentiation (LTP) was impaired in hippocampal brain slices of aged knock-out mice maintained in low glucose media. Using microdialysis in cerebellar white matter we found no significant differences in extracellular lactate or glucose between the young adult knock-out mice and controls, although trends were detected. Our results suggest that ß2-adrenergic receptor expression on astrocytes in mice may be important for maintaining cognitive health at advanced age, but is dispensable for motor function.

ß2-adrenergic agonists modulate TNF-α induced astrocytic inflammatory gene expression and brain inflammatory cell populations.

BACKGROUND: The NF-κB signaling pathway orchestrates many of the intricate aspects of neuroinflammation. Astrocytic ß2-adrenergic receptors have emerged as potential regulators in central nervous system inflammation and are potential targets for pharmacological modulation. The aim of this study was to elucidate the crosstalk between astrocytic ß2-adrenergic receptors and the TNF-α induced inflammatory gene program. METHODS: Proinflammatory conditions were generated by the administration of TNF-α. Genes that are susceptible to astrocytic crosstalk between ß2-adrenergic receptors (stimulated by clenbuterol) and TNF-α were identified by qPCR-macroarray-based gene expression analysis in a human 1321 N1 astrocytoma cell line. Transcriptional patterns of the identified genes in vitro were validated by RT-PCR on the 1321 N1 cell line as well as on primary rat astrocytes. In vivo expression patterns were examined by intracerebroventricular administration of clenbuterol and/or TNF-α in rats. To examine the impact on the inflammatory cell content of the brain we performed extensive FACS analysis of rat brain immune cells after intracerebroventricular clenbuterol and/or TNF-α administration. RESULTS: Parallel transcriptional patterns in vivo and in vitro confirmed the relevance of astrocytic ß2-adrenergic receptors as modulators of brain inflammatory responses. Importantly, we observed pronounced effects of ß2-adrenergic receptor agonists and TNF-α on IL-6, CXCL2, CXCL3, VCAM1, and ICAM1 expression, suggesting a role in inflammatory brain cell homeostasis. Extensive FACS-analysis of inflammatory cell content in the brain demonstrated that clenbuterol/TNF-α co-administration skewed the T cell population towards a double negative phenotype and induced a shift in the myeloid brain cell population towards a neutrophilic predominance. CONCLUSIONS: Our results show that astrocytic ß2-adrenergic receptors are potent regulators of astrocytic TNF-α-activated genes in vitro and in vivo, and ultimately modulate the molecular network involved in the homeostasis of inflammatory cells in the central nervous system. Astrocytic ß2-adrenergic receptors and their downstream signaling pathway may serve as potential targets to modulate neuroinflammatory responses.

Astrocytes as potential targets to suppress inflammatory demyelinating lesions in multiple sclerosis.

A hallmark of multiple sclerosis (MS) is the occurrence of focal inflammatory demyelinating lesions in the central nervous system. The prevailing view that activated anti-myelin T cells inherently mediate these lesions has been challenged after observations that these T cells, which are part of the normal immune repertoire, can also intermittently become activated in healthy people and subjects with other diseases. Astrocytes in the white matter of subjects with MS are deficient in beta(2) adrenergic receptors. Stimulation of beta(2) adrenergic receptors increases cAMP, leading to activation of protein kinase A (PKA). beta(2) adrenergic receptor deficiency will reduce the suppressive action of PKA on coactivator class II transactivator (CIITA), which is a key regulator of interferon gamma-induced major histocompatibility (MHC) class II molecule transcription. The expression of MHC class II may deviate astrocytes to function as facultative antigen presenting cells, which can then initiate the inflammatory cascade. In a proof of concept study in MS subjects it was shown that fluoxetine, which activates PKA in astrocytes, reduced the development of focal inflammatory lesions. If confirmed and extended by additional studies, suppressing the antigen presenting capacity of astrocytes could be a novel therapeutic option for the treatment of MS.