Tet1isoforms differentially regulate gene expression, synaptic transmission and memory in the mammalian brain

Abstract

The dynamic regulation of DNA methylation in post-mitotic neurons is necessary for memory formation and other adaptive behaviors. Ten-eleven translocation 1 (TET1) plays a part in these processes by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), thereby initiating active DNA demethylation. However, attempts to pinpoint its exact role in the nervous system have been hindered by contradictory findings, perhaps due in part, to a recent discovery that two isoforms of the Tet1 gene are differentially expressed from early development into adulthood. Here, we demonstrate that both the shorter transcript (Tet1S) encoding an N-terminally truncated TET1 protein and a full-length Tet1 (Tet1FL) transcript encoding canonical TET1 are co-expressed in the adult brain. We show that Tet1S is the predominantly expressed isoform, and is highly enriched in neurons, whereas Tet1FL is generally expressed at lower levels and more abundant in glia, suggesting their roles are at least partially cell-type specific. Using viral-mediated, isoform- and neuron-specific molecular tools, we find that Tet1S repression enhances, while Tet1FL impairs, hippocampal-dependent memory. In addition, the individual disruption of the two isoforms leads to contrasting changes in basal synaptic transmission and the dysregulation of unique gene ensembles in hippocampal neurons. Together, our findings demonstrate that each Tet1 isoform serves a distinct role in the mammalian brain.

The role of TET proteins in stress-induced neuroepigenetic and behavioural adaptations

“…Interestingly, TET1FL also appears to be the predominant isoform within placental tissue ( Yosefzon et al, 2017 ) suggesting likely CXXC-dependent functions in tissues sensitive to prenatal stress. The truncated TET1 short (TET1s) isoform lacking the N-terminus and CXXC domain, is the predominantly expressed TET1 isoform within most somatic tissues as well as within the brain ( Zhang et al, 2016 ; Yosefzon et al, 2017 ) with Tet1s mRNA enriched within neuronal populations ( Greer et al, 2020 ). Cell-type specific expression of TET1 isoforms is mediated via alternate promoter usage, with the TetFL promoter located upstream of the canonical TSS, and the Tet1s promoter located upstream of the canonical exon 2 ( Zhang et al, 2016 ; Yosefzon et al, 2017 ; Greer et al, 2020 ).…”

“…The truncated TET1 short (TET1s) isoform lacking the N-terminus and CXXC domain, is the predominantly expressed TET1 isoform within most somatic tissues as well as within the brain ( Zhang et al, 2016 ; Yosefzon et al, 2017 ) with Tet1s mRNA enriched within neuronal populations ( Greer et al, 2020 ). Cell-type specific expression of TET1 isoforms is mediated via alternate promoter usage, with the TetFL promoter located upstream of the canonical TSS, and the Tet1s promoter located upstream of the canonical exon 2 ( Zhang et al, 2016 ; Yosefzon et al, 2017 ; Greer et al, 2020 ). Within neurons, the Tet1FL promoter is marked by the repressive histone modification H3K27me3, which is absent from the Tet1s promoter resulting in the abundance of Tet1s mRNA within neuronal populations confirmed via quantitative real-time PCR (qRT-PCR) and RNA-seq ( Zhang et al, 2016 ; Greer et al, 2020 ).…”

“…Cell-type specific expression of TET1 isoforms is mediated via alternate promoter usage, with the TetFL promoter located upstream of the canonical TSS, and the Tet1s promoter located upstream of the canonical exon 2 ( Zhang et al, 2016 ; Yosefzon et al, 2017 ; Greer et al, 2020 ). Within neurons, the Tet1FL promoter is marked by the repressive histone modification H3K27me3, which is absent from the Tet1s promoter resulting in the abundance of Tet1s mRNA within neuronal populations confirmed via quantitative real-time PCR (qRT-PCR) and RNA-seq ( Zhang et al, 2016 ; Greer et al, 2020 ). Work from our own lab supports these findings as permissive chromatin accessibility (i.e., abundant ATAC-seq peak) is observed primarily at the Tet1s promoter within both excitatory ( Neurod6 positive) and inhibitory forebrain neurons ( Dlx5/6 positive) of the adult mouse brain (unpublished observations).…”

“…Functionally, TET1 isoforms appear to mediate distinct functions throughout development as well as in pathological states, with dysregulation of TET1s specifically associated with poor prognosis in several types of cancer ( Zhang et al, 2016 ; Yosefzon et al, 2017 ; Good et al, 2017 ). Within the brain, a recent study also emphasizes the unique functions of TET1 isoforms ( Greer et al, 2020 ). Neuronal activity and synaptic scaling induce regulation specifically of Tet1s mRNA with no changes in Tet1FL in primary hippocampal (HPC) neurons.…”

“…Neuronal activity and synaptic scaling induce regulation specifically of Tet1s mRNA with no changes in Tet1FL in primary hippocampal (HPC) neurons. Moreover, Transcription activator-like effector (TALE)-mediated knock-down (KD) of either Tet1s or Tet1FL results in disparate transcriptome changes and opposing modulation of glutamatergic transmission in HPC neurons in vitro ( Greer et al, 2020 ). These changes likely mediate the divergent impairments in contextual fear memory observed following either Tet1FL or Tet1s KD within the dorsal HPC (dHPC) in this study.…”