PMC

Comparison of tamoxifen-induced DNA recombination in GLAST-CreERT2 and GFAP-CreERT2 mice. Comparison of DNA recombination in transgenic GFAP-CreERT2 and GLAST-CreERT2 knockin mice. (A,B) Sagittal overview of tdTomato reporter expression (Madisen et al., ) in the brain of GLAST-CreERT2 and GFAP-CreERT2 mice. (C–H) Magnified views of selected brain regions (ctx, cortex; hc, hippocampus; th, thalamus; hy, hypothalamus; col, superior colliculus; bs, brainstem). The upper panel shows reporter activation in GFAP-CreERT2 mice, the lower panel in GLAST-CreERT2. (I–K) In the cerebellum, tdTomato reporter activation of BG and other astrocytes is comparable in GFAP-CreERT2 and GLAST-CreERT2 mice (I) however, gene deletion (here GluA1) is more efficient (K) in BG of GLAST-CreERT2 mice (lower panel) than in GFAP-CreERT2 mice (upper panel) when compared to control mice (J).

Genetic mouse models reveal a diversity of astrocyte functions affecting mouse behavior. (A) Astroglial cannabinoid receptors are involved in spatial memory formation (Han et al., ). (B) The ionotropic glutamate receptors on BG contribute to fine motor coordination (Saab et al., ). (C) Although the lack of the widely expressed transcriptional repressor MECP2 results in synapse loss, severe mental retardation and premature death, the astrocyte-specific re-expression restores several vital functions like motor activity (Lioy et al., ). (D) Impairment of astroglial ATP release perturbs sleep behavior and induces memory loss (Pascual et al., ; Halassa et al., ).

Astrocyte heterogeneity and gene targeting strategies to influence astrocyte behavior. Throughout development and in different brain regions, the heterogeneity of astrocytes becomes rather obvious when looking at the different morphologies. It is more the cell volume with cytosol and cell membrane that helps to visualize astrocyte function rather than the cytoskeletal structure (A–F). Only few genetic strategies have been used to modify astrocyte function in vivo (G–J). (A) glial fibrillary acidic protein (GFAP)-stained acutely isolated astrocyte. (B) Cortical astrocytes expressing tdTomato in close contact to a blood vessel with their end feet, (C) Hippocampal astrocytes (CA1) expressing GFAP. (D) Single Bergmann glia (BG) cell with CreERT2/loxP controlled reporter expression (EGFP). (E) Cortical astrocyte expressing EGFP and surrounding a blood vessel. Scale bars: A,D,E = 10 μm, B = 20 μm, C = 50 μm. (F) Electron micrograph depicting the intimate enwrapping of pre- and postsynaptic terminals by astroglial processes (BG: Bergmann glial processes, scale bar: 1 μm). (G) Knock-in of CreERT2 into the GLAST locus leads to tamoxifen-sensitive recombination in all astrocytes with endogenous GLAST promoter activity (Mori et al., ). The DNA recombinase variant CreERT2 is trapped in the cytosol by heat shock proteins (HSP), after tamoxifen application the protein is released and translocated into the nucleus. (H,I) Transgenic GFAP-CreERT2 mice generated by non-homologous recombination can also be used to target astrocytes (Hirrlinger et al., ). The Cre/loxP system can either be used to selective excise gene alleles of interest (G,H; knockout) or to express genes of interest (e.g., reporter proteins such as GFP or genetically encoded Ca²⁺ indicators), but also to restore gene function (I) (Lioy et al., ). (J) Alternatively, the binary tTA/tetO system composed of (1) promoter-controlled expression of a tetracyclin transactivator protein; and (2) tetracycline/doxycycline-responsive elements driving the expression of proteins-of-interest (Pascual et al., ). This system allows for a certain degree of reversible gene regulation.