Laser Capture Microdissection and RT- PCR Analyses of Specific Cell Types in Locus Coeruleus From Postmortem Human Brain

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Morphological studies have shown pathology of neurons and glia in many brain disorders, including psychiatric disorders such as major depression. However, most biochemical characterizations of postmortem human brain tissue have not made a distinction between neurons and glia. Laser capture microdissection (LCM) to isolate specific cell types has the potential to advance our understanding of human brain pathologies. Here, RT-PCR was used to evaluate the utility of LCM in the capture of noradrenergic neurons, astrocytes and oligodendrocytes from the locus coeruleus (LC) of postmortem human brain. The 3 LC cell types were individually identified using modifications of established histological and morphological methods. LCM settings were optimized for each cell type and captured cell bodies were those having no nearby cell body of a different phenotype. LC neurons (200), astrocytes (500), and oligodendrocytes (500) were captured within the LC from 3 postmortem brains. RNA was isolated, reversed transcribed, and markers for neurons (tyrosine hydroxylase [TH], dopamine beta-hydroxylase [DBH]), astrocytes (glial fibrillary acidic protein [GFAP]), and oligodendrocytes (myelin oligodendrocyte glycoprotein [MOG]), along with 3 references (actin, GAPDH, ubiquitin C) were PCR amplified and quantified by standardized end-point PCR. RNA quality as assessed by RIN was not altered by LCM as compared to RNA isolated from homogenized tissue. TH gene expression was found only in neurons in 2 of the 3 brains. DBH gene expression was ~5-fold greater in neurons than in astrocytes and oligodendrocytes. GFAP gene expression in astrocytes was 7- and 5-fold greater than that in neurons and oligodendrocytes, respectively. MOG gene expression was only detected in oligodendrocytes. Different expression ratios of marker genes between neurons and glia suggest that simple cross contamination of mRNA is unlikely. Glial cells may contain DBH mRNA. Alternatively, DBH, but not TH, mRNA may occur in neuronal dendrites or axons in close association with glial cells that become captured with glia during LCM. GFAP may be expressed in low levels in neurons and oligodendrocytes, or alternatively, GFAP mRNA may be located in astrocytic processes in close association with neuronal and oligodendrocyte cell bodies. Use of a single marker to identify a cell type may be insufficient; other cell types for comparison or additional markers may be required. Multiple well-characterized markers can be used to evaluate clarity of cell capture for each sample. With due regard for specific limitations, LCM can be used to evaluate the molecular pathology of specific cell types in postmortem human brain.

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