Long interspersed element-1 (L1) is the only autonomously mobile human retrotransposon. The L1 5′ untranslated region (5′UTR) contains an internal sense promoter that drives the expression of a bicistronic mRNA encoding the L1 retrotransposition machinery proteins ORF1p and ORF2p. The L1 5′UTR also possesses two SOX binding sites. Previous studies have described SOX proteins as L1 modulators. To date, L1 transcriptional activity has been identified in various somatic tissues, with the brain being an apparent hotspot for L1 mRNA expression and retrotransposition. However, the mechanisms by which SOX factors modulate L1 in the brain remain unclear.

To address this question, we generated novel L1-EGFP retrotransposition reporter and L1 5′UTR-EGFP promoter reporter assays. We found that SOX6 overexpression significantly increases endogenous L1 expression and ORF1p abundance, as well as L1 reporter mobility and transcriptional activity in HeLa. Mutations in the first SOX binding site of the L1 5′UTR significantly reduced these effects. When deleting the DNA binding domain of the SOX6 protein, we observe a reduction in L1 promoter reporter transcriptional activation. Lastly, we translated these findings into a recently established model of human induced pluripotent stem cell (hiPSCs)-derived neurons (i3Neurons). We demonstrate that i3Neurons express high endogenous L1 mRNA and ORF1p. Overexpression of SOX6 in fully mature and functional i3Neurons enhances ORF1p expression and L1 5′UTR reporter activity, compared to a SOX6 DNA binding domain mutant. In addition, we confirmed in our neuronal model previous reports of SOX2-mediated L1 transcriptional repression.

These findings together provide a deeper insight into SOX-mediated L1 regulation, presenting SOX6 as a novel L1 transcriptional modulator in human neurons.