We investigated the regulation of the rat neuron-specific enolase gene using a transient transfection approach. Recent transgenic
mouse studies have shown that a 1.8-kb segment of the rat
NSE gene 5′ flanking region, including the first (noncoding) exon but not the first intron, is able to drive expression of a
reporter gene in parallel with endogenous
NSE. These data suggest that
cis-acting elements responsible for the spatial and temporal pattern of
NSE gene expression are located within the proximal 1.8 kb of the 5′ flanking sequence. To further investigate this region, we
joined the 1.8-kb regulatory cassette to the
cat reporter gene and generated a number of constructs in which the flanking sequence was progressively deleted from the 5′ end.
These constructs were tested by transient transfection into neuronal and nonneuronal cells, followed by an assay for CAT activity.
We found that as little as 255 bp of 5′ flanking sequence was able to confer cell type-specificity on the reporter gene. Further
truncation to 120 bp of 5′ sequence resulted in a sharp downregulation of reporter activity in PC12 cells but a significant
rise in both Neuro-2A neuroblastoma cells and nonneuronal Ltk- cells, indicating that
cis-acting elements controlling the regulation of
NSE in Ltk-, Neuro-2A, and PC12 cells may lie within the 135 bp region covered by this deletion. This region contains an AP-2
site and an element similar in sequence and position to a motif identified in the proximal promoter region of the neuron-specific
peripherin gene. Reduction to 95 bp of 5′ sequence resulted in a slight downregulation of CAT activity in all cell lines tested,
and further truncation to 65 bp of 5′ sequence caused a universal reduction to background levels of CAT activity, concomitant
with the disruption of the basal
NSE promoter. Our results show that the 5′ flanking region of the
NSE gene is capable of conferring cell type-specificity on a heterologous gene in transfected cells and that elements responsible
for this are located within the proximal 255 bp.
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