Long interspersed element-1 (L1) is the only autonomously mobile human retrotransposon. The L1 sequence comprises a 5′ untranslated region (5′UTR) containing a promoter that drives the expression of the L1 retrotransposition machinery proteins ORF1p and ORF2p. The brain has been identified as a hotspot for somatic L1 expression and retrotransposition but the neuronal factors regulating L1 activity remain unclear. The L1 promoter contains two SOX transcription factor binding sites and thus, SOX proteins have been proposed as L1 modulators. We detected a high L1 signature in Parvalbumin interneurons, which are characterized by high SOX6 expression. Hence, we focused on studying the potential role of SOX6 as an L1 transcriptional activator in neurons.

To tackle this, we generated novel L1 retrotransposition and L1 transcriptional reporter assays. We found that SOX6 overexpression significantly enhanced endogenous L1 expression, as well as L1 retrotransposition and promoter reporter activity in HeLa cells. Mutating the first SOX binding site of the L1 5′UTR significantly reduced these effects. Moreover, deletion of the DNA binding domain of SOX6 decreased L1 transcriptional reporter activation. Lastly, we translated these findings into an in vitro human neuronal model (i3Neurons). We demonstrated that i3Neurons express endogenous L1 mRNA and ORF1p, and can accommodate L1 retrotransposition. Overexpression of SOX6 in mature i3Neurons increased ORF1p expression and L1 reporter activity.

These results present SOX6 as a novel transcription factor modulating L1 activity in the brain. Understanding L1 regulation in the brain is essential to comprehend its role in physiological neuronal function and neurological disease.