In-vivo regulation of Krüppel-like factor 9 by corticosteroids and their receptors across tissues in tadpoles of Xenopus tropicalis

Krüppel-like Factor-9 and Krüppel-like Factor-13: Highly Related, Multi-Functional, Transcriptional Repressors and Activators of Oncogenesis

Specificity Proteins/Krüppel-like Factors (SP/KLF family) are a conserved family of transcriptional regulators. These proteins share three highly conserved, contiguous zinc fingers in their carboxy-terminus, requisite for binding to cis elements in DNA. Each SP/KLF protein has unique primary sequence within its amino-terminal and carboxy-terminal regions, and it is these regions which interact with co-activators, co-repressors, and chromatin-modifying proteins to support the transcriptional activation and repression of target genes. Krüppel-like Factor 9 (KLF9) and Krüppel-like Factor 13 (KLF13) are two of the smallest members of the SP/KLF family, are paralogous, emerged early in metazoan evolution, and are highly conserved. Paradoxically, while most similar in primary sequence, KLF9 and KLF13 display many distinct roles in target cells. In this article, we summarize the work that has identified the roles of KLF9 (and to a lesser degree KLF13) in tumor suppression or promotion via unique effects on differentiation, pro- and anti-inflammatory pathways, oxidative stress, and tumor immune cell infiltration. We also highlight the great diversity of miRNAs, lncRNAs, and circular RNAs which provide mechanisms for the ubiquitous tumor-specific suppression of KLF9 mRNA and protein. Elucidation of KLF9 and KLF13 in cancer biology is likely to provide new inroads to the understanding of oncogenesis and its prevention and treatments.

Characterization of a novel corticosterone response gene in Xenopus tropicalis tadpole tails

Corticosteroids are critical for development and for mediating stress responses across diverse vertebrate taxa. Study of frog metamorphosis has made significant breakthroughs in our understanding of corticosteroid signaling during development in non-mammalian vertebrate species. However, lack of adequate corticosterone (CORT) response genes in tadpoles make identification and quantification of CORT responses challenging. Here, we characterized a CORT-response gene frzb (frizzled related protein) previously identified in Xenopus tropicalis tadpole tail skin by an RNA-seq study. We validated the RNA-seq results that CORT and not thyroid hormone induces frzb in the tails using quantitative PCR. Further, maximum frzb expression was achieved by 100-250 nM CORT within 12-24 hours. frzb is not significantly induced in the liver and brain in response to 100 nM CORT. We also found no change in frzb expression across natural metamorphosis when endogenous CORT levels peak. Surprisingly, frzb is only induced by CORT in X. tropicalis tails and not in Xenopus laevis tails. The exact downstream function of increased frzb expression in tails in response to CORT is not known, but the specificity of hormone response and its high mRNA expression levels in the tail render frzb a useful marker of exogenous CORT-response independent of thyroid hormone for exogenous hormone treatments and in-vivo endocrine disruption studies.

cyp21a2 Knockout Tadpoles Survive Metamorphosis Despite Low Corticosterone

Corticosteroids are so vital for organ maturation that reduced corticosteroid signaling during postembryonic development causes death in terrestrial vertebrates. Indeed, death occurs at metamorphosis in frogs lacking proopiomelanocortin (pomc) or the glucocorticoid receptor (GR; nr3c1). Some residual corticosteroids exist in pomc mutants to activate the wild-type (WT) GR and mineralocorticoid receptor (MR), and the elevated corticosteroids in GR mutants may activate MR. Thus, we expected a more severe developmental phenotype in tadpoles with inactivation of 21-hydroxylase, which should eliminate all interrenal corticosteroid biosynthesis. Using CRISPR/Cas9 in Xenopus tropicalis, we produced an 11-base pair deletion in cyp21a2, the gene encoding 21-hydroxylase. Growth and development were delayed in cyp21a2 mutant tadpoles, but unlike the other frog models, they survived metamorphosis. Consistent with an absence of 21-hydroxylase, mutant tadpoles had a 95% reduction of aldosterone in tail tissue, but they retained some corticosterone (∼40% of WT siblings), an amount, however, too low for survival in pomc mutants. Decreased corticosteroid signaling was evidenced by reduced expression of corticosteroid-response gene, klf9, and by impaired negative feedback in the hypothalamus-pituitary-interrenal axis with higher messenger RNA expression levels of crh, pomc, star, and cyp11b2 and an approximately 30-fold increase in tail content of progesterone. In vitro tail-tip culture showed that progesterone can transactivate the frog GR. The inadequate activation of GR by corticosterone in cyp21a2 mutants was likely compensated for by sufficient corticosteroid signaling from other GR ligands to allow survival through the developmental transition from aquatic to terrestrial life.

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Glucocorticoid receptor mediates corticosterone-thyroid hormone synergy essential for metamorphosis in Xenopus tropicalis tadpoles

In all vertebrates, thyroid hormone (TH) is critical for normal growth and development. In amphibians, corticosterone (CORT) has no action to advance development by itself but can accelerate development induced by TH. CORT accomplishes this acceleration by increasing tissue sensitivity and responsivity to TH. However, the receptor through which CORT acts to affect TH signaling is not known. To examine the role of the glucocorticoid receptor (GR), GR knockout tadpoles and wild-type tadpoles treated with the GR antagonist, RU486, were exposed to exogenous TH and/or CORT then assayed for gene expression and morphology. We found that levels of the response genes klf9 and thrb induced by TH and associated changes in morphology were decreased in GR knockout tadpoles compared to wild-type tadpoles, suggesting that GR signaling contributes to tissue responsivity to TH. To directly examine the role of GR in TH signaling, we co-treated tadpoles with TH and CORT and found that the TH response gene, thrb, was induced significantly beyond the level induced by TH alone in wild-type tadpoles but not in GR knockout tadpoles or wild-type tadpoles treated with RU486. Similarly, tail and gill resorption was greater in tadpoles treated with CORT plus TH compared to TH alone in wild-type tadpoles but not in tadpoles with impaired GR signaling. Surprisingly, even though GR knockout tadpoles die at metamorphosis, treatment with TH alone enabled their survival. These results demonstrate that signaling through GR is responsible for enhancing TH signaling and is essential for the completion of metamorphosis.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

Chronic cortisol exposure in early development leads to neuroendocrine dysregulation in adulthood

Objective

Chronic early life stress can affect development of the neuroendocrine stress system, leading to its persistent dysregulation and consequently increased disease risk in adulthood. One contributing factor is thought to be epigenetic programming in response to chronic cortisol exposure during early development. We have previously shown that zebrafish embryos treated chronically with cortisol develop into adults with constitutively elevated whole-body cortisol and aberrant immune gene expression. Here we further characterize that phenotype by assessing persistent effects of the treatment on cortisol tissue distribution and dynamics, chromatin accessibility, and activities of glucocorticoid-responsive regulatory genes klf9 and fkbp5. To that end cortisol levels in different tissues of fed and fasted adults were measured using ELISA, open chromatin in adult blood cells was mapped using ATAC-seq, and gene activity in adult blood and brain cells was measured using qRT-PCR.

Results

Adults derived from cortisol-treated embryos have elevated whole-body cortisol with aberrantly regulated tissue distribution and dynamics that correlate with differential activity of klf9 and fkbp5 in blood and brain.

Klf9 is a key feedforward regulator of the transcriptomic response to glucocorticoid receptor activity

The zebrafish has recently emerged as a model system for investigating the developmental roles of glucocorticoid signaling and the mechanisms underlying glucocorticoid-induced developmental programming. To assess the role of the Glucocorticoid Receptor (GR) in such programming, we used CRISPR-Cas9 to produce a new frameshift mutation, GR^369-, which eliminates all potential in-frame initiation codons upstream of the DNA binding domain. Using RNA-seq to ask how this mutation affects the larval transcriptome under both normal conditions and with chronic cortisol treatment, we find that GR mediates most of the effects of the treatment, and paradoxically, that the transcriptome of cortisol-treated larvae is more like that of larvae lacking a GR than that of larvae with a GR, suggesting that the cortisol-treated larvae develop GR resistance. The one transcriptional regulator that was both underexpressed in GR^369- larvae and consistently overexpressed in cortisol-treated larvae was klf9. We therefore used CRISPR-Cas9-mediated mutation of klf9 and RNA-seq to assess Klf9-dependent gene expression in both normal and cortisol-treated larvae. Our results indicate that Klf9 contributes significantly to the transcriptomic response to chronic cortisol exposure, mediating the upregulation of proinflammatory genes that we reported previously.

Glucocorticoid receptor is required for survival through metamorphosis in the frog Xenopus tropicalis

Stress hormones, also known as glucocorticoids, are critical for survival at birth in mammals due at least in part to their importance in lung maturation. However, because air breathing is not always required for amphibian survival and because stress hormones have no known developmental impact except to modulate the developmental actions of thyroid hormone (TH), the requirement for stress hormone signaling during metamorphosis is not well understoodi. Here, we produced a glucocorticoid receptor knockout (GRKO) Xenopus line with a frameshift mutation in the first exon of the glucocorticoid receptor. Induction by exogenous corticosterone (CORT, the frog stress hormone) of the CORT response genes, klf9 (Krüppel-like factor 9, also regulated by TH) and ush1g (Usher's syndrome 1G), was completely abrogated in GRKO tadpoles. Surprisingly, GRKO tadpoles developed faster than wild-type tadpoles until forelimb emergence and then developed more slowly until their death at the climax of metamorphosis. Growth rate was not affected in GRKO tadpoles, but they achieved a smaller maximum size. Gene expression analysis of the TH response genes, thrb (TH receptor beta) and klf9 showed reduced expression in the tail at metamorphic climax consistent with the reduced development rate. These results indicate that glucocorticoid receptor is required for survival through metamorphosis and support dual roles for GR signaling in control of developmental rate.

Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids

Thyroid hormone (TH) is the most important hormone in frog metamorphosis, a developmental process which will not occur in the absence of TH but can be induced precociously by exogenous TH. However, such treatments including in-vitro TH treatments often do not replicate the events of natural metamorphosis in many organs, including lung, brain, blood, intestine, pancreas, tail, and skin. A potential explanation for the discrepancy between natural and TH-induced metamorphosis is the involvement of glucocorticoids (GCs). GCs are not able to advance development by themselves but can modulate the rate of developmental progress induced by TH via increased tissue sensitivity to TH. Global gene expression analyses and endocrine experiments suggest that GCs may also have direct actions required for completion of metamorphosis independent of their effects on TH signaling. Here, we provide a new review and analysis of the requirement and necessity of TH signaling in light of recent insights from gene knockout frogs. We also examine the independent and interactive roles GCs play in regulating morphological and molecular metamorphic events dependent upon TH.

A novel stress hormone response gene in tadpoles of Xenopus tropicalis

Previous work identified a transcribed locus, Str. 34945, induced by the frog stress hormone corticosterone (CORT) in Xenopus tropicalis tails. Because thyroid hormone had no influence on its expression, Str. 34945 was dubbed the first "CORT-only" gene known from tadpoles. Here, we examine the genomic annotation for this transcript, hormone specificity, time course of induction, tissue distribution, and developmental expression profile. The location of Str. 34945 on the X. tropicalis genome lies between the genes ush1g (Usher syndrome 1G) and fads6 (fatty acid desaturase 6). A blast search showed that it maps to the same region on the X. laevis genome, but no hits were found in the human genome. Using RNA-seq data and conventional reverse transcriptase PCR and sequencing, we show that Str. 34945 is part of the 3' untranslated region of ush1g. We find that CORT but not aldosterone or thyroid hormone treatment induces Str. 34945 in tadpole tails and that expression of Str. 34945 achieves maximal expression within 12-24 h of CORT treatment. Among tissues, Str. 34945 is induced to the highest degree in tail, with lesser induction in lungs, liver, and heart, and no induction in the brain or kidney. During natural metamorphosis, Str. 34945 expression in tails peaks at metamorphic climax. The role of ush1g in metamorphosis is not understood, but the specificity of its hormone response and its expression in tail make ush1g valuable as a marker of CORT-response gene induction independent of thyroid hormone.

Functional Characterization of Xenopus Thyroid Hormone Transporters mct8 and oatp1c1

Xenopus is an excellent model for studying thyroid hormone signaling as it undergoes thyroid hormone-dependent metamorphosis. Despite the fact that receptors and deiodinases have been described in Xenopus, membrane transporters for these hormones are yet to be characterized. We cloned Xenopus monocarboxylate transporter 8 (mct8) and organic anion-transporting polypeptide 1C1 (oatpc1c1), focusing on these two transporters given their importance for vertebrate brain development. Protein alignment and bootstrap analysis showed that Xenopus mct8 and oatp1c1 are closer to their mammalian orthologs than their teleost counterparts. We functionally characterized the two transporters using a radiolabeled hormones in vitro uptake assay in COS-1 cells. Xenopus mct8 was found to actively transport both T3 and T4 bidirectionally. As to the thyroid precursor molecules, diiodotyrosine (DIT) and monoiodotyrosine (MIT), both human and Xenopus mct8, showed active efflux, but no influx. Again similar to humans, Xenopus oatp1c1 transported T4 but not T3, MIT, or DIT. We used reverse transcription quantitative polymerase chain reaction and in situ hybridization to characterize the temporal and spatial expression of mct8 and oatp1c1 in Xenopus. Specific expression of the transporter was observed in the brain, with increasingly strong expression as development progressed. In conclusion, these results show that Xenopus thyroid hormone transporters are functional and display marked spatiotemporal expression patterns. These features make them interesting targets to elucidate their roles in determining thyroid hormone availability during embryonic development.