Layer-specific molecular expression in neocortical astrocytes

Protoplasmic astrocytes in layers II to VI of the mammalian neocortex have historically been thought to comprise a homogeneous population. Given that layer-specific neuronal subtypes play essential roles in cortical circuitry, astrocytes might also be expected to support and modify this circuitry in a layer-specific manner. In order to investigate whether protoplasmic astrocytes exhibit layer-specific heterogeneity, we compared the gene expression profiles of astrocytes between upper layers (layers II to IV) and deep layers (layers V and VI). Although most genes known to be preferentially expressed in astrocytes (astrocyte-enriched genes) were equally expressed between upper-layer astrocytes and deep-layer astrocytes, some such genes (astrocyte-enriched genes or genes with known function in astrocytes) were significantly enriched in upper-layer astrocytes or deep-layer astrocytes. With the use of fluorescence-activated cell sorting (FACS), we prepared upper-layer astrocytes and deep-layer astrocytes from the corresponding dissected layers of the somatosensory cortex of Aldh1l1-eGFP mice, in which all astrocytes are expected to be labeled with GFP. The meninges, layer I, and the corpus callosum were removed from upper- and deep-layer tissue samples. In addition, parts of layers IV and V were lost during separation of these layers in such a way as to prevent cross-contamination between the upper- and deep-layer samples. Total RNA from upper-layer astrocytes and deep-layer astrocytes (n = 3 brains from 4-week-old male mice) was isolated from sorted cells with TRIzol (Invitrogen) or RNAiso Plus (Takara) and was then subjected to reverse transcription with the use of a SMART-Seq v4 Ultra Low Input RNA Kit for Sequencing (Clontech). Bar-coded libraries were prepared with a Nextera XT DNA Library Preparation Kit (Illumina), and single-end 36-bp sequencing was performed with a HiSeq 2500 instrument (Illumina).