Holger Hiemisch

Global Transcriptome Analysis of Genetically Identified Neurons in the Adult Cortex

The enormous cellular complexity of the brain is a major obstacle for gene expression profiling of neurological disease models, because physiologically relevant changes of transcription in a specific neuronal subset are likely to be lost in the presence of other neurons and glia. We solved this problem in transgenic mice by labeling genetically defined cells with a nuclear variant of GFP. When combined with laser-directed microdissection, intact RNA from unfixed, freeze-dried sections can be isolated, which is a prerequisite for high-quality global transcriptome analysis. Here, we compared gene expression profiles between pyramidal motor neurons and pyramidal somatosensory neurons captured from layer V of the adult neocortex. One striking feature of motor neurons is the elevated expression of ribosomal genes and genes involved in ATP synthesis. This suggests a molecular adaptation of the upper motor neurons to longer axonal projections and higher electrical activity. These molecular signatures were not detected when cortical layers and microareas were analyzed in toto. Additionally, we used microarrays to determine the global mRNA expression profiles of microdissected Purkinje cells and cellularly complex cerebellar cortex microregions. In summary, our analysis shows that cellularly complex targets lead to averaged gene expression profiles that lack substantial amounts of cell type-specific information. Thus, cell type-restricted sampling strategies are mandatory in the CNS. The combined use of a genetic label with laser-microdissection offers an unbiased approach to map patterns of gene expression onto practically any cell type of the brain.

Identification of regulated genes during permanent focal cerebral ischaemia: characterization of the protein kinase 9b5/MARKL1/MARK4

Cerebral ischaemia induces transcriptional changes in a number of pathophysiologically important genes. Here we have systematically studied gene expression changes after 90 min and 24 h of permanent focal ischaemia in the mouse by an advanced fragment display technique (restriction‐mediated differential display). We identified 56 transcriptionally altered genes, many of which provide novel hints to ischaemic pathophysiology. Particularly interesting were two pro‐apoptotic genes (Grim19 and Tdag51), whose role in cerebral ischaemia and neuronal cell death has not been recognized so far. Among the unknown sequences, we identified a gene that was rapidly and transiently up‐regulated. The encoded protein displayed high homology to the MARK family of serine–threonine protein kinases and has recently been described as MARKL1/MARK4. Here we demonstrate that this protein is a functional protein kinase with the ability to specifically phosphorylate a cognate peptide substrate for the AMP‐kinase family. Upon overexpression in heterologous cells, the functional wild‐type protein, but not its kinase‐dead mutant, led to decreased cell viability. We conclude that the up‐regulation of this kinase during focal ischaemia may represent an interesting new target for pharmacological intervention.

The functional genome of CA1 and CA3 neurons under native conditions and in response to ischemia

BackgroundThe different physiological repertoire of CA3 and CA1 neurons in the hippocampus, as well as their differing behaviour after noxious stimuli are ultimately based upon differences in the expressed genome. We have compared CA3 and CA1 gene expression in the uninjured brain, and after cerebral ischemia using laser microdissection (LMD), RNA amplification, and array hybridization.ResultsProfiling in CA1 vs. CA3 under normoxic conditions detected more than 1000 differentially expressed genes that belong to different, physiologically relevant gene ontology groups in both cell types. The comparison of each region under normoxic and ischemic conditions revealed more than 5000 ischemia-regulated genes for each individual cell type. Surprisingly, there was a high co-regulation in both regions. In the ischemic state, only about 100 genes were found to be differentially expressed in CA3 and CA1. The majority of these genes were also different in the native state. A minority of interesting genes (e.g. inhibinbetaA) displayed divergent expression preference under native and ischemic conditions with partially opposing directions of regulation in both cell types.ConclusionThe differences found in two morphologically very similar cell types situated next to each other in the CNS are large providing a rational basis for physiological differences. Unexpectedly, the genomic response to ischemia is highly similar in these two neuron types, leading to a substantial attenuation of functional genomic differences in these two cell types. Also, the majority of changes that exist in the ischemic state are not generated de novo by the ischemic stimulus, but are preexistant from the genomic repertoire in the native situation. This unexpected influence of a strong noxious stimulus on cell-specific gene expression differences can be explained by the activation of a cell-type independent conserved gene-expression program. Our data generate both novel insights into the relation of the quiescent and stimulus-induced transcriptome in different cells, and provide a large dataset to the research community, both for mapping purposes, as well as for physiological and pathophysiological research.

Verge: A Novel Vascular Early Response Gene

Vascular endothelium forms a continuous, semipermeable barrier that regulates the transvascular movement of hormones, macromolecules, and other solutes. Here, we describe a novel immediate early gene that is expressed selectively in vascular endothelial cells, verge (vascular early response gene). Verge protein includes an N-terminal region of ∼70 amino acids with modest homology (∼30% identity) to Apolipoprotein L but is otherwise unique. Verge mRNA and protein are induced selectively in the endothelium of adult vasculature by electrical or chemical seizures. Verge expression appears to be responsive to local tissue conditions, because it is induced in the hemisphere ipsilateral to transient focal cerebral ischemia. In contrast to the transient expression in adult, Verge mRNA and protein are constitutively expressed at high levels in the endothelium of developing tissues (particularly heart) in association with angiogenesis. Verge mRNA is induced in cultured endothelial cells by defined growth factors and hypoxia. Verge protein is dramatically increased by cysteine proteinase inhibitors, suggesting rapid turnover, and is localized to focal regions near the periphery of the cells. Endothelial cell lines that stably express Verge form monolayers that show enhanced permeability in response to activation of protein kinase C by phorbol esters. This response is accompanied by reorganization of the actin cytoskeleton and the formation of paracellular gaps. These studies suggest that Verge functions as a dynamic regulator of endothelial cell signaling and vascular function.