An epigenetic switch repressing Tet1 in gonadotropes activates the reproductive axis

Abstract

The TET enzymes catalyze conversion of 5-methyl cytosine (5mC) to 5-hydroxymethyl cytosine (5hmC) and play important roles during development. TET1 has been particularly well-studied in pluripotent stem cells, but -KO mice are viable, and the most marked defect is abnormal ovarian follicle development, resulting in impaired fertility. We hypothesized that TET1 might play a role in the central control of reproduction by regulating expression of the gonadotropin hormones, which are responsible for follicle development and maturation and ovarian function. We find that all three TET enzymes are expressed in gonadotrope-precursor cells, but mRNA levels decrease markedly with completion of cell differentiation, corresponding with an increase in expression of the luteinizing hormone gene, We demonstrate that poorly differentiated gonadotropes express a TET1 isoform lacking the N-terminal CXXC-domain, which represses gene expression directly and does not catalyze 5hmC at the gene promoter. We show that this isoform is also expressed in other differentiated tissues, and that it is regulated by an alternative promoter whose activity is repressed by the liganded estrogen and androgen receptors, and by the hypothalamic gonadotropin-releasing hormone through activation of PKA. Its expression is also regulated by DNA methylation, including at an upstream enhancer that is protected by TET2, to allow expression. The down-regulation of TET1 relieves its repression of the methylated gene promoter, which is then hydroxymethylated and activated by TET2 for full reproductive competence.

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

“…In the first study to describe Tet1 S , it was reported to be the only transcript expressed past early developmental stages (Zhang et al, 2016). Yet, others have recently provided evidence that in some adult tissues and/or cell types, Tet1 FL and Tet1 S are co-expressed (Good et al, 2017;Yosefzon et al, 2017). We found that while Tet1 S is the predominantly expressed isoform in the brain, Tet1 FL is also actively transcribed.…”

“…A potential explanation for some of these inconsistences comes from a recent report that the Tet1 gene undergoes an isoform switch from the full-length, canonical transcript (hereafter Tet1 FL ) in embryonic stem cells to a shorter, truncated variant (hereafter Tet1 S ) exclusive to somatic tissues (Zhang et al, 2016). In addition, evidence suggests that in some tissues both transcripts might be co-expressed (Good et al, 2017;Yosefzon et al, 2017). Whether this is the case in the adult brain, and if so, what functions these Tet1 isoforms might serve, has not been explored.…”

Cytosine Modifications and Distinct Functions of TET1 on Tumorigenesis

“…Highly expressed in ESCs, PGCs, and inner cell mass of blastocyst, TET1 protein has been proven to be mainly responsible for the initial oxidation of 5-mC to 5-hmC, and to establish the paradoxically dual distinct epigenetic patterns in transcriptional activation and repression in accordance with life processes of growth and development. Alternative splicing mechanism leads to several TET1 isoforms, including the full-length canonical and the short transcripts [69][70][71][72][73]. TET1 expression is regulated by very complicated factors including the reprogramming factors such as Oct3/4, Nanog, and Myc [68,70] in early embryos, ESCs and PGCs [69], the transcription factors in the differentiated cells, and STAT3/STAT5 in acute myeloid leukemia (AML) [74].…”

“…Differentiation with according increase in the expression of the luteinizing hormone gene (Lhb). The short isoform of TET1 with deletion of the N-terminal CXXC-domain binds the H3K27me2/3 enriched region located at the upstream promoter of the Lhb gene, downregulating its expression and leading to differentiation deficiency [73].…”

Sensitivity of pituitary gonadotropes to hyperglycemia leads to epigenetic aberrations and reduced follicle‐stimulating hormone levels

“…RNA was isolated using Trizol, DNase I digested and cDNA synthesized using the High Capacity cDNA Reverse Transcription Kit (Thermo Fisher Scientific, Waltham, MA, USA), and real-time quantitative PCR (qPCR) was conducted using the Perfecta SYBR Green FastMix (Quantabio, Beverly, MA, USA) both as previously reported (45); the primers listed in Table 1 were used for the analyses. Amplicon levels were normalized to levels of Rplp0.…”