Megakaryoblastic
leukemia 2 (MKL2)/
myocardin-related
transcription factor-B (MRTFB) is a
serum response factor (SRF) cofactor that is enriched in the brain and controls SRF target genes and neuronal morphology. There are at least four
isoforms of MKL2/MRTFB. Among these, MKL2/MRTFB
isoform 1 and spliced neuronal long
isoform of SRF transcriptional coactivator (SOLOIST)/MRTFB
isoform 4 (MRTFB i4) are highly expressed in neurons. Although, when overexpressed in neurons,
isoform 1 and SOLOIST/MRTFB i4 have opposing effects on dendritic morphology and differentially regulate SRF target genes, it is unknown how endogenous SOLOIST/MRTFB i4 regulates gene expression. Using
isoform-specific knockdown, we investigated the role of endogenous SOLOST/MRTFB i4 in regulating the expression of other MKL2/MRTFB
isoforms and SRF-target genes in Neuro-2a cells. Knockdown of SOLOIST/MRTFB i4 downregulated SOLOIST/MRTFB i4, while it upregulated
isoform 1 without affecting
isoform 3. Knockdown of SOLOIST/MRTFB i4 downregulated the SRF target immediate early genes egr1 and
Arc, while it upregulated c-fos. Double knockdown of
isoform 1 and SOLOIST/MRTFB i4 inhibited c-fos expression. Taken together, our findings in Neuro-2a cells suggest that endogenous SOLOIST/MRTFB i4 positively regulates egr1 and
Arc expression. In addition, endogenous SOLOIST/MRTFB i4 may negatively regulate c-fos expression, possibly by downregulating
isoform 1 in Neuro-2a cells.