A neural circuit for male sexual behavior and reward

Male sexual behavior is innate and rewarding. Despite its centrality to reproduction, a molecularly specified neural circuit governing innate male sexual behavior and reward remains to be characterized. We have discovered a developmentally wired neural circuit necessary and sufficient for male mating. This circuit connects chemosensory input to BNSTprTac1 neurons, which innervate POATacr1 neurons that project to centers regulating motor output and reward. Epistasis studies demonstrate that BNSTprTac1 neurons are upstream of POATacr1 neurons, and BNSTprTac1-released substance P following mate recognition potentiates activation of POATacr1 neurons through Tacr1 to initiate mating. Experimental activation of POATacr1 neurons triggers mating, even in sexually satiated males, and it is rewarding, eliciting dopamine release and self-stimulation of these cells. Together, we have uncovered a neural circuit that governs the key aspects of innate male sexual behavior: motor displays, drive, and reward.

Oxytocin receptor is not required for social attachment in prairie voles

Prairie voles are among a small group of mammals that display long-term social attachment between mating partners. Many pharmacological studies show that signaling via the oxytocin receptor (Oxtr) is critical for the display of social monogamy in these animals. We used CRISPR mutagenesis to generate three different Oxtr-null mutant prairie vole lines. Oxtr mutants displayed social attachment such that males and females showed a behavioral preference for their mating partners over a stranger of the opposite sex, even when assayed using different experimental setups. Mothers lacking Oxtr delivered viable pups, and parents displayed care for their young and raised them to the weanling stage. Together, our studies unexpectedly reveal that social attachment, parturition, and parental behavior can occur in the absence of Oxtr signaling in prairie voles.

A functional cellular framework for sex and estrous cycle-dependent gene expression and behavior

Sex hormones exert a profound influence on gendered behaviors. How individual sex hormone-responsive neuronal populations regulate diverse sex-typical behaviors is unclear. We performed orthogonal, genetically targeted sequencing of four estrogen receptor 1-expressing (Esr1⁺) populations and identified 1,415 genes expressed differentially between sexes or estrous states. Unique subsets of these genes were distributed across all 137 transcriptomically defined Esr1⁺ cell types, including estrous stage-specific ones, that comprise the four populations. We used differentially expressed genes labeling single Esr1⁺ cell types as entry points to functionally characterize two such cell types, BNSTpr^Tac1/Esr1 and VMHvl^Cckar/Esr1. We observed that these two cell types, but not the other Esr1⁺ cell types in these populations, are essential for sex recognition in males and mating in females, respectively. Furthermore, VMHvl^Cckar/Esr1 cell type projections are distinct from those of other VMHvl^Esr1 cell types. Together, projection and functional specialization of dimorphic cell types enables sex hormone-responsive populations to regulate diverse social behaviors.

An Intact Krüppel-like factor 9 Gene Is Required for Acute Liver Period 1 mRNA Response to Restraint Stress

The clock protein period 1 (PER1) is a central component of the core transcription-translation feedback loop governing cell-autonomous circadian rhythms in animals. Transcription of Per1 is directly regulated by the glucocorticoid (GC) receptor (GR), and Per1 mRNA is induced by stressors or injection of GC. Circulating GCs may synchronize peripheral clocks with the central pacemaker located in the suprachiasmatic nucleus of the brain. Krüppel-like factor 9 (KLF9) is a zinc finger transcription factor that, like Per1, is directly regulated by liganded GR, and it associates in chromatin at clock and clock-output genes, including at Per1. We hypothesized that KLF9 modulates stressor-dependent Per1 transcription. We exposed wild-type (WT) and Klf9 null mice (Klf9-/-) of both sexes to 1 hour restraint stress, which caused similar 2- to 2.5-fold increases in plasma corticosterone (B) in each genotype and sex. Although WT mice of both sexes showed a 2-fold increase in liver Per1 mRNA level after restraint stress, this response was absent in Klf9-/- mice. However, injection of B in WT and Klf9-/- mice induced similar increases in Per1 mRNA. Our findings support that an intact Klf9 gene is required for liver Per1 mRNA responses to an acute stressor, but a possible role for GCs in this response requires further investigation.

The Paralogous Krüppel-like Factors 9 and 13 Regulate the Mammalian Cellular Circadian Clock Output Gene Dbp

An intricate transcription-translation feedback loop (TTFL) governs cellular circadian rhythms in mammals. Here, we report that the zinc finger transcription factor Krüppel-like factor 9 (KLF9) is regulated by this TTFL, it associates in chromatin at the core circadian clock and clock-output genes, and it acts to modulate transcription of the clock-output gene Dbp. Our earlier genome-wide analysis of the mouse hippocampus-derived cell line HT22 showed that KLF9 associates in chromatin with Per1, Per3, Dbp, Tef, Bhlhe40, Bhlhe41, Nr1d1, and Nr1d2. Of the 3514 KLF9 peaks identified in HT22 cells, 1028 contain E-box sequences to which the transcriptional activators CLOCK and BMAL1 may bind, a frequency significantly greater than expected by chance. Klf9 mRNA showed circadian oscillation in synchronized HT22 cells, mouse hippocampus, and liver. At the clock-output gene Dbp, KLF9 exhibited circadian rhythmicity in its association in chromatin in HT22 cells and hippocampus. Forced expression of KLF9 in HT22 cells repressed basal Dbp transcription and strongly inhibited CLOCK+BMAL1-dependent transcriptional activation of a transfected Dbp reporter. Mutational analysis showed that this action of KLF9 depended on 2 intact KLF9-binding motifs within the Dbp locus that are in close proximity to E-boxes. Knockout of Klf9 or the paralogous gene Klf13 using CRISPR/Cas9 genome editing in HT22 cells had no effect on Dbp expression, but combined knockout of both genes strongly impaired circadian Dbp mRNA oscillation. Like KLF9, KLF13 also showed association in chromatin with clock- and clock-output genes, and forced expression of KLF13 inhibited the actions of CLOCK+BMAL1 on Dbp transcription. Our results suggest novel and partly overlapping roles for KLF9 and KLF13 in modulating cellular circadian clock output by a mechanism involving direct interaction with the core TTFL.

Periodic Remodeling in a Neural Circuit Governs Timing of Female Sexual Behavior

Behaviors are inextricably linked to internal state. We have identified a neural mechanism that links female sexual behavior with the estrus, the ovulatory phase of the estrous cycle. We find that progesterone-receptor (PR)-expressing neurons in the ventromedial hypothalamus (VMH) are active and required during this behavior. Activating these neurons, however, does not elicit sexual behavior in non-estrus females. We show that projections of PR+ VMH neurons to the anteroventral periventricular (AVPV) nucleus change across the 5-day mouse estrous cycle, with ∼3-fold more termini and functional connections during estrus. This cyclic increase in connectivity is found in adult females, but not males, and regulated by estrogen signaling in PR+ VMH neurons. We further show that these connections are essential for sexual behavior in receptive females. Thus, estrogen-regulated structural plasticity of behaviorally salient connections in the adult female brain links sexual behavior to the estrus phase of the estrous cycle.

Molecular mechanisms underlying sexual differentiation of the nervous system

A long-standing goal in developmental neuroscience is to understand the mechanisms by which steroid sex hormones pattern the mammalian central nervous system along male or female pathways to enable subsequent displays of sexually dimorphic behaviors. In this article, we review recent advances in understanding the epigenetic and transcriptional mechanisms mediating sexual differentiation of the brain in mammals, flies, and worms. These studies suggest a model of sexual differentiation wherein master regulators of sex determination initiate a cascade of sexually dimorphic gene expression that controls development of neural pathways and behavioral displays in a strikingly modular manner. With these advances in molecular genetics, it is now feasible to disassemble different components of sexually dimorphic social behaviors without disrupting other behavioral interactions. Such experimental tractability promises rapid advances in this exciting field.

The Krüppel-like factor 9 cistrome in mouse hippocampal neurons reveals predominant transcriptional repression via proximal promoter binding

Background

Krüppel-like factor 9 (Klf9) is a zinc finger transcription factor that functions in neural cell differentiation, but little is known about its genomic targets or mechanism of action in neurons.

Results

We used the mouse hippocampus-derived neuronal cell line HT22 to identify genes regulated by Klf9, and we validated our findings in mouse hippocampus. We engineered HT22 cells to express a Klf9 transgene under control of the tetracycline repressor, and used RNA sequencing to identify genes modulated by Klf9. We found 217 genes repressed and 21 induced by Klf9. We also engineered HT22 cells to co-express biotin ligase and a Klf9 fusion protein containing an N-terminal biotin ligase recognition peptide. Using chromatin-streptavidin precipitation (ChSP) sequencing we identified 3,514 genomic regions where Klf9 associated. Seventy-five percent of these were within 1 kb of transcription start sites, and Klf9 associated in chromatin with 60% of the repressed genes. We analyzed the promoters of several repressed genes containing Klf9 ChSP peaks using transient transfection reporter assays and found that Klf9 repressed promoter activity, which was abolished after mutation of Sp/Klf-like motifs. Knockdown or knockout of Klf9 in HT22 cells caused dysregulation of Klf9 target genes. Chromatin immunoprecipitation assays showed that Klf9 associated in chromatin from mouse hippocampus with genes identified by ChSP sequencing on HT22 cells, and expression of Klf9 target genes was dysregulated in the hippocampus of neonatal Klf9-null mice. Gene ontology analysis revealed that Klf9 genomic targets include genes involved in cystokeletal remodeling, Wnt signaling and inflammation.

Conclusions

We have identified genomic targets of Klf9 in hippocampal neurons and created a foundation for future studies on how it functions in chromatin, and regulates neuronal morphology and survival across the lifespan.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3640-7) contains supplementary material, which is available to authorized users.

A Mechanism to Enhance Cellular Responsivity to Hormone Action: Krüppel-Like Factor 9 Promotes Thyroid Hormone Receptor-β Autoinduction During Postembryonic Brain Development

Thyroid hormone (TH) receptor (TR)-β (trb) is induced by TH (autoinduced) in Xenopus tadpoles during metamorphosis. We previously showed that Krüppel-like factor 9 (Klf9) is rapidly induced by TH in the tadpole brain, associates in chromatin with the trb upstream region in a developmental stage and TH-dependent manner, and forced expression of Klf9 in the Xenopus laevis cell line XTC-2 accelerates and enhances trb autoinduction. Here we investigated whether Klf9 can promote trb autoinduction in tadpole brain in vivo. Using electroporation-mediated gene transfer, we transfected plasmids into premetamorphic tadpole brain to express wild-type or mutant forms of Klf9. Forced expression of Klf9 increased baseline trb mRNA levels in thyroid-intact but not in goitrogen-treated tadpoles, supporting that Klf9 enhances liganded TR action. As in XTC-2 cells, forced expression of Klf9 enhanced trb autoinduction in tadpole brain in vivo and also increased TH-dependent induction of the TR target genes klf9 and thbzip. Consistent with our previous mutagenesis experiments conducted in XTC-2 cells, the actions of Klf9 in vivo required an intact N-terminal region but not a functional DNA binding domain. Forced expression of TRβ in tadpole brain by electroporation-mediated gene transfer increased baseline and TH-induced TR target gene transcription, supporting a role for trb autoinduction during metamorphosis. Our findings support that Klf9 acts as an accessory transcription factor for TR at the trb locus during tadpole metamorphosis, enhancing trb autoinduction and transcription of other TR target genes, which increases cellular responsivity to further TH action on developmental gene regulation programs.

Deciphering the regulatory logic of an ancient, ultraconserved nuclear receptor enhancer module

Cooperative, synergistic gene regulation by nuclear hormone receptors can increase sensitivity and amplify cellular responses to hormones. We investigated thyroid hormone (TH) and glucocorticoid (GC) synergy on the Krüppel-like factor 9 (Klf9) gene, which codes for a zinc finger transcription factor involved in development and homeostasis of diverse tissues. We identified regions of the Xenopus and mouse Klf9 genes 5-6 kb upstream of the transcription start sites that supported synergistic transactivation by TH plus GC. Within these regions, we found an orthologous sequence of approximately 180 bp that is highly conserved among tetrapods, but absent in other chordates, and possesses chromatin marks characteristic of an enhancer element. The Xenopus and mouse approximately 180-bp DNA element conferred synergistic transactivation by hormones in transient transfection assays, so we designate this the Klf9 synergy module (KSM). We identified binding sites within the mouse KSM for TH receptor, GC receptor, and nuclear factor κB. TH strongly increased recruitment of liganded GC receptor and serine 5 phosphorylated (initiating) RNA polymerase II to chromatin at the KSM, suggesting a mechanism for transcriptional synergy. The KSM is transcribed to generate long noncoding RNAs, which are also synergistically induced by combined hormone treatment, and the KSM interacts with the Klf9 promoter and a far upstream region through chromosomal looping. Our findings support that the KSM plays a central role in hormone regulation of vertebrate Klf9 genes, it evolved in the tetrapod lineage, and has been maintained by strong stabilizing selection.

Krüppel-like factors are effectors of nuclear receptor signaling

Binding of steroid and thyroid hormones to their cognate nuclear receptors (NRs) impacts virtually every aspect of postembryonic development, physiology and behavior, and inappropriate signaling by NRs may contribute to disease. While NRs regulate genes by direct binding to hormone response elements in the genome, their actions may depend on the activity of other transcription factors (TFs) that may or may not bind DNA. The Krüppel-like family of transcription factors (KLF) is an evolutionarily conserved class of DNA-binding proteins that influence many aspects of development and physiology. Several members of this family have been shown to play diverse roles in NR signaling. For example, KLFs (1) act as accessory transcription factors for NR actions, (2) regulate expression of NR genes, and (3) as gene products of primary NR response genes function as key players in NR-dependent transcriptional networks. In mouse models, deletion of different KLFs leads to aberrant transcriptional and physiological responses to hormones, underscoring the importance of these proteins in the regulation of hormonal signaling. Understanding the functional relationships between NRs and KLFs will yield important insights into mechanisms of NR signaling. In this review we present a conceptual framework for understanding how KLFs participate in NR signaling, and we provide examples of how these proteins function to effect hormone action.