The impact of thyroid hormone in seasonal breeding has a restricted transcriptional signature

Seasonal regulation of Tsh-β, Dio2, Dio3, and GnRH-I mRNA expressions in Eurasian tree sparrow (Passer montanus) under natural conditions

Seasonal transitions in avian reproductive cycles are governed by neuroendocrine adaptability. The molecular mechanisms behind seasonal regulation are still not fully understood in many species and remain an important area of ongoing research. Despite recognizing the importance of regulatory genes, gaps persist in comprehending the exact molecular processes that control the transitions between different reproductive phases. We investigated the expression patterns of Tsh-β (Thyroid-stimulating hormone subunit beta), Dio2 (Iodothyronine deiodinase 2), Dio3 (Iodothyronine deiodinase 3), and GnRH-I (Gonadotropin-releasing hormone I) mRNAs in regulating reproduction in both sexes of the Eurasian tree sparrow (Passer montanus). Adult tree sparrows (n = 4 for each sex) were procured from the wild, and mRNA expression of Tsh-β, Dio2, Dio3, and GnRH-I, along with gonadal size and body weight, was examined on a monthly basis for 1 year. Results revealed distinct annual cycles of Tsh-β, Dio2, Dio3, and GnRH-I mRNA expression and gonadal size in both sexes. Significantly higher expressions of Tsh-β, Dio2, and GnRH-I mRNA were observed during the breeding phase (April-May), while Dio3 expression was reported higher during the non-breeding phase. The synchronization in the pattern of increase in the Tsh-β, Dio2, and GnRH-I during the breeding phase is associated with gonadal growth, suggesting their potential involvement in regulating seasonal reproduction in tree sparrows. Thus, the control of the reproductive cycle in tree sparrows involves the actions of Tsh-β and Dio2/Dio3 leading to the activation and deactivation of GnRH-I via the HPG (hypothalamic-pituitary-gonadal) axis.

Tanycytes from a bird's eye view: gene expression profiling of the tanycytic region under different seasonal states in the Svalbard ptarmigan

In mammals and birds, tanycytes are known to regulate thyroid hormone conversion, and this process is central to the control of seasonal reproduction. In mammals, this cell type is also implicated in retinoic acid signalling, neurogenesis, and nutritional gatekeeping, all of which have been linked to hypothalamic regulation of energy metabolism. Less is known about these potential wider roles of tanycytes in birds. To address this gap, we combined LASER capture microdissection and transcriptomics to profile the tanycytic region in male Svalbard ptarmigan, a High Arctic species with photoperiod-dependent seasonal rhythms in reproductive activation and body mass. Short photoperiod (SP) adapted birds were transferred to constant light (LL) to trigger breeding and body mass loss. After five months under LL, the development of photorefractoriness led to spontaneous re-emergence of the winter phenotype, marked by the termination of breeding and gain in body mass. The transfer from SP to LL initiated gene expression changes in both thyroid hormone and retinoic acid pathways, as described in seasonal mammals. Furthermore, transcriptomic signatures of cell differentiation and migration were observed. Comparison to data from Siberian hamsters demonstrated that a photoperiod-dependent re-organisation of the hypothalamic tanycytic region is likely a conserved feature. Conversely, the spontaneous development of photorefractoriness showed a surprisingly small number of genes that reverted in expression level, despite reversal of the reproductive and metabolic phenotype. Our data suggest general conservation of tanycyte biology between photoperiodic birds and mammals and raise questions about the mechanistic origins of the photorefractory state.

Melatonin and Seasonal Synchrony in Mammals

In mammals, seasonal opportunities and challenges are anticipated through programmed changes in physiology and behavior. Appropriate anticipatory timing depends on synchronization to the external solar year, achieved through the use of day length (photoperiod) as a synchronizing signal. In mammals, nocturnal production of melatonin by the pineal gland is the key hormonal mediator of photoperiodic change, exerting its effects via the hypothalamopituitary axis. In this review/perspective, we consider the key developments during the history of research into the seasonal synchronizer effect of melatonin, highlighting the role that the pars tuberalis-tanycyte module plays in this process. We go on to consider downstream pathways, which include discrete hypothalamic neuronal populations. Neurons that express the neuropeptides kisspeptin and (Arg)(Phe)-related peptide-3 (RFRP-3) govern seasonal reproductive function while neurons that express somatostatin may be involved in seasonal metabolic adaptations. Finally, we identify several outstanding questions, which need to be addressed to provide a much thorough understanding of the deep impact of melatonin upon seasonal synchronization.

Photoperiod, but not progesterone, has a strong impact upon the transcriptome of the medio-basal hypothalamus in female goats and ewes

Photoperiod is the main environmental driver of seasonal responses in organisms living at temperate and polar latitudes. Other external cues such as food and temperature, and internal cues including hormones, intervene to fine-tune phasing of physiological functions to the solar year. In mammals, the medio-basal hypothalamus (MBH) is the key integrator of these cues, which orchestrates a wide array of seasonal functions, including breeding. Here, using RNAseq and RT-qPCR, we demonstrate that molecular components of the photoperiodic response previously identified in ewes are broadly conserved in does (female goats, Capra hircus), with a common core of ∼50 genes. This core group can be defined as the "MBH seasonal trancriptome", which includes key players of the pars tuberalis-tanycytes neuroendocrine retrograde pathway that governs intra-MBH photoperiodic switches of triiodothyronine (T3) production (Tshb, Eya3, Dio2 and SlcO1c1), the two histone methyltransferases Suv39H2 and Ezh2 and the secreted protein Vmo1. Prior data in ewes revealed that T3 and estradiol (E2), both key hormones for the proper timing of seasonal breeding, differentially impact the MBH seasonal transcriptome, and identified cellular and molecular targets through which these hormones might act. In contrast, information regarding the potential impact of progesterone (P4) upon the MBH transcriptome was nonexistent. Here, we demonstrate that P4 has no discernible transcriptional impact in either does or ewes. Taken together, our data show that does and ewes possess a common core set of photoperiod-responsive genes in the MBH and conclusively demonstrate that P4 is not a key regulator of the MBH transcriptome.

Hypothalamic tanycytes as mediators of maternally programmed seasonal plasticity

In mammals, maternal photoperiodic programming (MPP) provides a means whereby juvenile development can be matched to forthcoming seasonal environmental conditions.1,2,3,4 This phenomenon is driven by in utero effects of maternal melatonin5,6,7 on the production of thyrotropin (TSH) in the fetal pars tuberalis (PT) and consequent TSH receptor-mediated effects on tanycytes lining the 3rd ventricle of the mediobasal hypothalamus (MBH).8,9,10 Here we use LASER capture microdissection and transcriptomic profiling to show that TSH-dependent MPP controls the attributes of the ependymal region of the MBH in juvenile animals. In Siberian hamster pups gestated and raised on a long photoperiod (LP) and thereby committed to a fast trajectory for growth and reproductive maturation, the ependymal region is enriched for tanycytes bearing sensory cilia and receptors implicated in metabolic sensing. Contrastingly, in pups gestated and raised on short photoperiod (SP) and therefore following an over-wintering developmental trajectory with delayed sexual maturation, the ependymal region has fewer sensory tanycytes. Post-weaning transfer of SP-gestated pups to an intermediate photoperiod (IP), which accelerates reproductive maturation, results in a pronounced shift toward a ciliated tanycytic profile and formation of tanycytic processes. We suggest that tanycytic plasticity constitutes a mechanism to tailor metabolic development for extended survival in variable overwintering environments.

Ambient Temperature Effects on the Spring and Autumn Somatic Growth Trajectory Show Plasticity in the Photoneuroendocrine Response Pathway in the Tundra Vole

Seasonal mammals register photoperiodic changes through the photoneuroendocrine system enabling them to time seasonal changes in growth, metabolism, and reproduction. To a varying extent, proximate environmental factors like ambient temperature (Ta) modulate timing of seasonal changes in physiology, conferring adaptive flexibility. While the molecular photoneuroendocrine pathway governing the seasonal responses is well defined, the mechanistic integration of nonphotoperiodic modulatory cues is poorly understood. Here, we explored the interaction between Ta and photoperiod in tundra voles, Microtus oeconomus, a boreal species in which the main impact of photoperiod is on postnatal somatic growth. We demonstrate that postweaning growth potential depends on both gestational and postweaning patterns of photoperiodic exposure, with the highest growth potential seen in voles experiencing short (8 h) gestational and long (16 h) postweaning photoperiods-corresponding to a spring growth program. Modulation by Ta was asymmetric: low Ta (10 °C) enhanced the growth potential of voles gestated on short photoperiods independent of postweaning photoperiod exposure, whereas in voles gestated on long photoperiods, showing a lower autumn-programmed growth potential, the effect of Ta was highly dependent on postweaning photoperiod. Analysis of the primary molecular elements involved in the expression of a neuroendocrine response to photoperiod, thyrotropin beta subunit (tshβ) in the pars tuberalis, somatostatin (srif) in the arcuate nucleus, and type 2/3 deiodinase (dio2/dio3) in the mediobasal hypothalamus identified dio2 as the most Ta-sensitive gene across the study, showing increased expression at higher Ta, while higher Ta reduced somatostatin expression. Contrastingly dio3 and tshβ were largely insensitive to Ta. Overall, these observations reveal a complex interplay between Ta and photoperiodic control of postnatal growth in M. oeconomus, and suggest that integration of Ta into the control of growth occurs downstream of the primary photoperiodic response cascade revealing potential adaptivity of small herbivores facing rising temperatures at high latitudes.

Characterization of the pars tuberalis and hypothalamus transcriptome in female sheep under different reproductive stages

For understanding the molecular events underlying the follicular (F) and luteal (L) phases of estrous cycle, and anestrous (A) phase, the pars tuberalis (PT), and hypothalamus (HT) transcriptomes of 21 ewes were studied. In HT, 72 and 3 differential expression genes (DEGs) were found when comparing F vs. A and L vs. A, respectively. In PT, 6 and 4 DEGs were found in F vs. A and L vs. A comparisons, respectively. Enrichment analysis for DEGs between the F and A phases in the HT revealed significant clusters, mainly associated with actin-binding, and cytoskeleton, that are related to neural plasticity modulated by gonadal steroid hormones, as well as with oxytocin signaling. We found that DEGs in PT had higher differences in expression levels than those found in HT. In this sense, the ITLN was highly upregulated in the F and L vs. A phases, being MRPL57 and IRX4 highly downregulated in L vs. A comparison. The DDC gene in PT, related to LH regulation, was upregulated in the F phase. The gene set enrichment analysis (GSEA) revealed multiple pathways related to neurotransmission and neuronal plasticity. Our study reveals new candidate genes involved in the reproductive stages' transitions in seasonal sheep.

Highly-Sensitive In Vitro Bioassays for FSH, TSH, PTH, Kp, and OT in Addition to LH in Mouse Leydig Tumor Cell

We demonstrate here that highly sensitive in vitro bioassays for FSH, TSH, and PTH can be set up in mouse Leydig Tumor Cells (mLTC), in addition to the normal LH/CG bioassay, after they were transfected with expression vectors encoding the corresponding Gs Protein-Coupled Receptors (GsPCR), such as FSHR, TSHR, or PTHR. Although the β2 adrenergic receptor is also a GsPCR, its expression in mLTC led to a significant but very low cAMP response compared to those observed with FSH, TSH, or PTH. Similarly, after transfection of the GiPCR MT1 melatonin receptor, we did not observe any inhibitory effect by melatonin of the LH or hCG stimulation. Interestingly, after transfection of mLTC with the human kisspeptin receptor (hKpR), which is a GqPCR, we observed a dose-dependent synergy of 10-12-10-7 M kisspeptin variants with a fixed concentration of 0.3 nM LH or hCG. Without any exogenous receptor transfection, a 2 h preincubation with OT or AVP led to a dose-dependent cAMP response to a fixed dose of LH or hCG. Therefore, highly sensitive in vitro bioassays for various hormones and other GPCR ligands can be set up in mLTC to measure circulating concentrations in only 3-10 µL of blood or other body fluids. Nevertheless, the development of an LHRKO mLTC cell line will be mandatory to obtain strict specificity for these bioassays to eliminate potential cross-reaction with LH or CG.

Day-night and seasonal variation of human gene expression across tissues

Circadian and circannual cycles trigger physiological changes whose reflection on human transcriptomes remains largely uncharted. We used the time and season of death of 932 individuals from GTEx to jointly investigate transcriptomic changes associated with those cycles across multiple tissues. Overall, most variation across tissues during day-night and among seasons was unique to each cycle. Although all tissues remodeled their transcriptomes, brain and gonadal tissues exhibited the highest seasonality, whereas those in the thoracic cavity showed stronger day-night regulation. Core clock genes displayed marked day-night differences across multiple tissues, which were largely conserved in baboon and mouse, but adapted to their nocturnal or diurnal habits. Seasonal variation of expression affected multiple pathways, and it was enriched among genes associated with the immune response, consistent with the seasonality of viral infections. Furthermore, they unveiled cytoarchitectural changes in brain regions. Altogether, our results provide the first combined atlas of how transcriptomes from human tissues adapt to major cycling environmental conditions. This atlas may have multiple applications; for example, drug targets with day-night or seasonal variation in gene expression may benefit from temporally adjusted doses.

Epigenetic Regulation of miR-25 and Lnc107153 on Expression of Seasonal Estrus Key Gene CHGA in Sheep

Pituitary pars tuberalis (PT) plays an important role as the transmission center in the seasonal reproduction of animals. It helps convert external photoperiod signals into intrinsic seasonal reproduction signals. In sheep PT, specific expression patterns of several genes (including short photoperiod-induced gene CHGA and long photoperiod genes EYA3 and TSHβ) under different photoperiods are crucial characteristics during this signal transduction. Recent studies have revealed the role of epigenetics in regulating the expression of seasonal reproductive key genes. Therefore, we explored whether microRNAs and LncRNAs regulated the expressions of the above key genes. Firstly, the expression of miR-25 and CHGA showed a significant negative correlation in sheep PT. Results of the dual luciferase reporter assay and miR-25 overexpression indicated that miR-25 could inhibit the expression of CHGA by specifically binding to its 3'UTR region in pituitary cells. Then, expression negative correlation and dual luciferase reporter analyses were used to screen and identify the candidate LncRNA (Lnc107153) targeted by miR-25. Finally, the results of fluorescence in situ hybridization and Lnc107153 overexpression suggested that Lnc107153 and miR-25 were involved in the epigenetic regulation of CHGA expression. However, the expressions of EYA3 and TSHβ were not regulated by miRNAs. These results will provide new insights into the epigenetic regulatory network of key genes in sheep seasonal reproduction.

Exploring differentially expressed genes in hypothalamic, pars tuberalis and pineal gland transcriptomes in different sexual behavior phenotypes in rams using RNA-Seq

Reproductive seasonality is a limiting factor in sheep production. Sexual behavior is a key element in reproductive efficiency, and this function is regulated by the hypothalamus-pituitary-gonadal (HPG) axis. To understand the mechanisms of sexual behavior, transcriptomic sequencing technology was used to identify differentially expressed genes (DEGs) in the hypothalamus (HT), pars tuberalis (PT) and pineal gland (PG) in Rasa Aragonesa rams with different sexual behavior. Bioinformatics analysis of the 16,401 identified genes by RNA-Seq revealed 103 and 12 DEGs in the HT and the PG, respectively, at a false discovery rate (FDR) of 5% with an absolute value of expression ≥ 1 (log2FC). However, no DEGs were found in the PT. Functional annotation and pathway enrichment analysis showed that DEGs of HT were enriched mainly in neuroactive ligand-receptor interactions and signaling pathways, including notable candidate genes such as MTNR1A, CHRNA2, FSHB, LHB, GNRHR, AVP, PRL, PDYN, CGA, GABRD, and TSHB, which play a crucial role in sexual behavior. The GnRH and cAMP signaling pathways were also highlighted. In addition, gene set enrichment analysis (GSEA) identified potential pathways, dominated mainly by biological process category, that could be responsible for the differences in sexual behavior observed in rams. The intracellular protein transport and pattern specification process were enriched within the PT and the transcription factor binding and protein ubiquitination pathways for the PG. Thus, these pathways together may play an important role in the regulation of the sexual behavior in Rasa Aragonesa rams through the hypothalamic-pituitary-gonadal axis. The validation of 5 DEGs using reverse transcription quantitative polymerase chain reaction (RT-qPCR) showed expression patterns like the found with RNA-Seq. Overall, these results contribute to understanding the genomic basis of sexual behavior in rams. Our study demonstrates that multiple networks and pathways orchestrate sexual behavior in sheep.

Impact of food restriction on the medio-basal hypothalamus of intact ewes as revealed by a large-scale transcriptomics study

In mammals, the medio-basal hypothalamus (MBH) integrates photoperiodic and food-related cues to ensure timely phasing of physiological functions, including seasonal reproduction. The current human epidemics of obesity and associated reproductive disorders exemplifies the tight link between metabolism and reproduction. Yet, how food-related cues impact breeding at the level of the MBH remains unclear. In this respect, the sheep, which is a large diurnal mammal with a marked dual photoperiodic/metabolic control of seasonal breeding, is a relevant model. Here, we present a large-scale study in ewes (n = 120), which investigated the impact of food restriction (FRes) on the MBH transcriptome using unbiased RNAseq, followed by RT-qPCR. Few genes (~100) were impacted by FRes and the transcriptional impact was very modest (<2-fold increase or < 50% decrease for most genes). As anticipated, FRes increased expression of Npy/AgRP/LepR and decreased expression of Pomc/Cartpt, while Kiss1 expression was not impacted. Of particular interest, Eya3, Nmu and Dio2, genes involved in photoperiodic decoding within the MBH, were also affected by FRes. Finally, we also identified a handful of genes not known to be regulated by food-related cues (e.g., RNase6, HspA6, Arrdc2). In conclusion, our transcriptomics study provides insights into the impact of metabolism on the MBH in sheep, which may be relevant to human, and identifies possible molecular links between metabolism and (seasonal) reproduction.

Antimicrobial Proteins and Peptides in Avian Eggshell: Structural Diversity and Potential Roles in Biomineralization

The calcitic avian eggshell provides physical protection for the embryo during its development, but also regulates water and gaseous exchange, and is a calcium source for bone mineralization. The calcified eggshell has been extensively investigated in the chicken. It is characterized by an inventory of more than 900 matrix proteins. In addition to proteins involved in shell mineralization and regulation of its microstructure, the shell also contains numerous antimicrobial proteins and peptides (AMPPs) including lectin-like proteins, Bacterial Permeability Increasing/Lipopolysaccharide Binding Protein/PLUNC family proteins, defensins, antiproteases, and chelators, which contribute to the innate immune protection of the egg. In parallel, some of these proteins are thought to be crucial determinants of the eggshell texture and its resulting mechanical properties. During the progressive solubilization of the inner mineralized eggshell during embryonic development (to provide calcium to the embryo), some antimicrobials may be released simultaneously to reinforce egg defense and protect the egg from contamination by external pathogens, through a weakened eggshell. This review provides a comprehensive overview of the diversity of avian eggshell AMPPs, their three-dimensional structures and their mechanism of antimicrobial activity. The published chicken eggshell proteome databases are integrated for a comprehensive inventory of its AMPPs. Their biochemical features, potential dual function as antimicrobials and as regulators of eggshell biomineralization, and their phylogenetic evolution will be described and discussed with regard to their three-dimensional structural characteristics. Finally, the repertoire of chicken eggshell AMPPs are compared to orthologs identified in other avian and non-avian eggshells. This approach sheds light on the similarities and differences exhibited by AMPPs, depending on bird species, and leads to a better understanding of their sequential or dual role in biomineralization and innate immunity.

Revisiting TSHβ's Role in Avian Seasonal Reproduction, Insights and Challenges from Mammalian Models

The core neuroendocrine pathways regulating seasonal reproduction in vertebrates were characterized over a decade ago. This has led to the development of a "consensus" model of seasonal reproduction that appears to be largely conserved across mammals, birds, amphibians, reptile and fish. This model centers around the photoinduced increase in TSHβ expression in the pars tuberalis (PT) of the anterior pituitary gland as the key transducer of photic information from sensory cells to the critical switch in hypothalamic deiodinase enzyme expression that drives changes in localized thyroid hormone signaling. These changes in localized thyroid hormone signaling in the medial basal hypothalamus ultimately activate the reproductive axis. This model has in turn been consistently supported by studies in a variety of taxa. As such it has become the definitional standard against which subsequent work is compared, particularly in the non-mammalian literature. However, as new studies move away from the handful of canonical model systems and begin to explore the effects of naturalistic rather than artificial photoperiod manipulations a more nuanced picture has begun to emerge. Yet, progress in elucidating the detailed events of reproductive photostimulation has been uneven across the research community. In this perspective I draw on emerging data from studies in free living animals that challenges some of the established assumptions of the avian consensus model of reproduction. Specifically, the role of TSHβ and its dissociation from deiodinase signaling. I then discuss how these apparently surprising findings can be contextualized within the context of the mammalian seasonal literature. In turn this ability to contextualize from the mammalian literature highlights the breadth of the current gap I our understanding of the molecular neuroendocrine mechanisms of seasonality in mammals versus birds and other non-mammalian seasonal breeders.

GnRH and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and RFRP-3

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

Transcriptomic Changes of Photoperiodic Response in the Hypothalamus Were Identified in Ovariectomized and Estradiol-Treated Sheep

Accurate timing of seasonal changes is an essential ability for an animal’s survival, and the change in the photoperiod is the key factor affecting reproductive seasonality in mammals. Emerging evidence has suggested that multiple hypothalamic genes participate in the photoperiod-induced regulation of reproductive activities in sheep, but the mechanism is still unclear. In this study, we initially examined the plasma level of two major reproductive hormones, namely, follicle-stimulating hormone (FSH) and prolactin (PRL), under different photoperiods in ovariectomized and estradiol-treated (OVX + E2) sheep using radioimmunoassay (RIA). Of the two hormones, the concentration of PRL significantly increased with the extension of the photoperiod, while FSH showed the opposite trend. Subsequently, an examination of the transcriptomic variation between the short photoperiod (SP) and long photoperiod (LP) was conducted. Differential expression analyses and functional annotation showed that several key genes in the insulin secretion (VAMP2, PRKACB, PRKCG, and PLCB1), GnRH (MAPK13, CGA, CDC42, ATF4, and LHB) pathways, and circadian entrainment (KCNJ5, PER1, GNB2, MTNR1A, and RASD1), as well as numerous lncRNAs, including XR_173257.3, XR_173415.3, XR_001435315.1, XR_001024596.2, and XR_001023464.2, were shown potentially vital for the hypothalamic photoperiodic response. Four of the differentially expressed mRNAs and lncRNAs were validated by qPCR. The constructed mRNA–mRNA interaction networks further revealed that transcripts potentially participated in hypothalamic thyroid hormone synthesis, endocrine resistance, and neuroactive ligand–receptor interactions. The interactome analysis of lncRNAs and their targets implied that XR_173257.3 and its target arylalkylamine N-acetyltransferase (AANAT) and XR_173415.3 and its target TH might participate in the regulation of seasonal reproduction. Together, the changes in reproductive hormones and transcriptome will help to determine the important photoperiod-induced lncRNAs and mRNAs and provide a valuable resource for further research on reproductive seasonality in sheep.

Day-night and seasonal variation of human gene expression across tissues

Circadian and circannual cycles trigger physiological changes whose reflection on human transcriptomes remains largely uncharted. We used the time and season of death of 932 individuals from GTEx to jointly investigate transcriptomic changes associated with those cycles across multiple tissues. Overall, most variation across tissues during day-night and among seasons was unique to each cycle. Although all tissues remodeled their transcriptomes, brain and gonadal tissues exhibited the highest seasonality, whereas those in the thoracic cavity showed stronger day-night regulation. Core clock genes displayed marked day-night differences across multiple tissues, which were largely conserved in baboon and mouse, but adapted to their nocturnal or diurnal habits. Seasonal variation of expression affected multiple pathways and it was enriched among genes associated with the immune response, consistent with the seasonality of viral infections. Furthermore, they unveiled cytoarchitectural changes in brain regions. Altogether, our results provide the first combined atlas of how transcriptomes from human tissues adapt to major cycling environmental conditions.

Photoperiods induced the circRNA differential expression in the thyroid gland of OVX+E2 ewes

Circular RNAs (circRNAs) are non-coding RNAs newly identified and play important roles in RNA regulation. However, little is known regarding photoperiods induced circRNAs in the thyroid gland. In this study, we performed a comprehensive analysis of circRNA profiles in the thyroid gland of OVX+E₂ ewes at different photoperiods by whole transcriptome sequencing. A total of 37,470 novel circRNAs were detected in different photoperiods (42 days of short photoperiod treatment, SP42; 42 days of long photoperiod treatment, LP42; SP42 transfer to LP42, SPLP42), with a total of 817 circRNAs for SP42-LP42 (down: 132; up: 114), LP42-SPLP42 (down: 136; up: 112) and SP42-SPLP42 (down: 182; up: 141) having differentially expressed. Functional enrichment annotation analysis of DE-circRNAs for GO and KEGG by R package, features that influence photoperiod response in Sunite ewes through the Inositol phosphate metabolism, cGMP-PKG signaling pathway, Calcium signaling pathway, MAPK signaling pathway, and Oocyte meiosis. In addition, competitive endogenous RNA (ceRNA) network analysis revealed target binding sites for identified miRNAs in DE-cirRNAs such as oar-miR-10b, oar-miR-200c, oar-miR-21, oar-miR-370-3p, oar-miR-377-3p, oar-miR-181a, oar-miR-432, and oar-miR-495-3p. These results of this study will provide some new information for understanding circRNA function as well as the changes in the sheep thyroid gland under different photoperiods.

Photoperiod is involved in the regulation of seasonal breeding in male water voles (Arvicola terrestris)

Mammals living at temperate latitudes typically display annual cyclicity in their reproductive activity: births are synchronized when environmental conditions are most favorable. In a majority of these species, day length is the main proximate factor used to anticipate seasonal changes and to adapt physiology. The brain integrates this photoperiodic signal through key hypothalamic structures, which regulate the reproductive axis. In this context, our study aimed to characterize regulations that occur along the hypothalamo-pituitary-gonadal (HPG) axis in male fossorial water voles (Arvicola terrestris, also known as Arvicola amphibius) throughout the year and to further probe the implication of photoperiod in these seasonal regulations. Our monthly field monitoring showed dramatic seasonal changes in the morphology and activity of reproductive organs, as well as in the androgen-dependent lateral scent glands. Moreover, our data uncovered seasonal variations at the hypothalamic level. During the breeding season, kisspeptin expression in the arcuate nucleus (ARC) decreases, while RFRP3 expression in the dorsomedial hypothalamic nucleus (DMH) increases. Our follow-up laboratory study revealed activation of the reproductive axis and confirmed a decrease in kisspeptin expression in males exposed to a long photoperiod (summer condition) compared with those maintained under a short photoperiod (winter condition) that retain all features reminiscent of sexual inhibition. Altogether, our study characterizes neuroendocrine and anatomical markers of seasonal reproductive rhythmicity in male water voles and further suggests that these seasonal changes are strongly impacted by photoperiod.

Identification of Photoperiod-Induced LncRNAs and mRNAs in Pituitary Pars Tuberalis of Sheep

The pituitary pars tuberalis (PT) is the regulating center of seasonal reproduction, which can sense the melatonin signal and eventually cause downstream changes of GnRH secretion through TSHβ. Recently, lncRNAs have been identified in animal reproductive-related tissues, and they play important roles in reproductive regulation. Therefore, in this study, we expect to identify photoperiod-induced lncRNAs and genes in pituitary PT of sheep by comparison of expression profiles between short photoperiod (SP) and long photoperiod (LP). Through RNA-Seq, a total of 55,472 lncRNAs were identified in pituitary PT of Sunite ewes. The number of differentially expressed (DE) genes and lncRNAs between SP and LP increased gradually with the extension of LP (from LP7 to LP42). The notable LP-induced candidate genes included EYA3, TSHB, SIX1, DCT, VMO1, AREG, SUV39H2, and EZH2, and SP-induced genes involved ENSOARG00000012585, CHGA, FOS, SOCS3, and TH. In enriched pathways for DE genes and lncRNA target genes between SP and LP, the reproduction- and circadian-related pathways were highlighted. In addition, the interactome analysis of lncRNAs and their targets implied that MSTRG.209166 and its trans-target TSHB, MSTRG.288068 and its cis-target SIX1, and ENSOARG00000026131 and its cis-target TH might participate in regulation of seasonal reproduction. Together, these results will help to determine important photoperiod-induced lncRNAs and genes and give us some new insights into the epigenetic regulation of seasonal reproduction in sheep.

Mechanisms of temperature modulation in mammalian seasonal timing

Global warming is predicted to have major effects on the annual time windows during which species may successfully reproduce. At the organismal level, climatic shifts engage with the control mechanism for reproductive seasonality. In mammals, laboratory studies on neuroendocrine mechanism emphasize photoperiod as a predictive cue, but this is based on a restricted group of species. In contrast, field-oriented comparative analyses demonstrate that proximate bioenergetic effects on the reproductive axis are a major determinant of seasonal reproductive timing. The interaction between proximate energetic and predictive photoperiodic cues is neglected. Here, we focused on photoperiodic modulation of postnatal reproductive development in common voles (Microtus arvalis), a herbivorous species in which a plastic timing of breeding is well documented. We demonstrate that temperature-dependent modulation of photoperiodic responses manifest in the thyrotrophin-sensitive tanycytes of the mediobasal hypothalamus. Here, the photoperiod-dependent expression of type 2 deiodinase expression, associated with the summer phenotype was enhanced by 21°C, whereas the photoperiod-dependent expression of type 3 deiodinase expression, associated with the winter phenotype, was enhanced by 10°C in spring voles. Increased levels of testosterone were found at 21°C, whereas somatic and gonadal growth were oppositely affected by temperature. The magnitude of these temperature effects was similar in voles photoperiodical programmed for accelerated maturation (ie, born early in the breeding season) and in voles photoperiodical programmed for delayed maturation (ie, born late in the breeding season). The melatonin-sensitive pars tuberalis was relatively insensitive to temperature. These data define a mechanistic hierarchy for the integration of predictive temporal cues and proximate thermo-energetic effects in mammalian reproduction.

Evidence for circadian-based photoperiodic timekeeping in Svalbard ptarmigan, the northernmost resident bird

The high Arctic archipelago of Svalbard (74°-81° north) experiences extended periods of uninterrupted daylight in summer and uninterrupted night in winter, apparently relaxing the major driver for the evolution of circadian rhythmicity. Svalbard ptarmigan (Lagopus muta hyperborea) is the only year-round resident terrestrial bird species endemic to the high Arctic and is remarkably adapted to the extreme annual variation in environmental conditions.¹ Here, we demonstrate that, although circadian control of behavior disappears rapidly upon transfer to constant light conditions, consistent with the loss of daily activity patterns observed during the polar summer and polar night, Svalbard ptarmigans nonetheless employ a circadian-based mechanism for photoperiodic timekeeping. First, we show the persistence of rhythmic clock gene expression under constant light within the mediobasal hypothalamus and pars tuberalis, the key tissues in the seasonal neuroendocrine cascade. We then employ a "sliding skeleton photoperiod" protocol, revealing that the driving force behind seasonal biology of the Svalbard ptarmigan is rhythmic sensitivity to light, a feature that depends on a functioning circadian rhythm. Hence, the unusual selective pressures of life in the high Arctic have favored decoupling of the circadian clock from organization of daily activity patterns, while preserving its importance for seasonal synchronization.

Expression characteristics of pineal miRNAs at ovine different reproductive stages and the identification of miRNAs targeting the AANAT gene

Background

Many recent studies have shown that miRNAs play important roles in the regulation of animal reproduction, including seasonal reproduction. The pineal gland is a crucial hub in the regulation of seasonal reproduction. However, little is known about the expression characteristics of pineal miRNAs in different reproductive seasons (anestrus and breeding season). Therefore, the expression profiles and regulatory roles of ovine pineal miRNAs were investigated during different reproductive stages using Solexa sequencing technology and dual luciferase reporter assays.

Results

A total of 427 miRNAs were identified in the sheep pineal gland. Significant differences in miRNA expression were demonstrated between anestrus and the breeding season in terms of the frequency distributions of miRNA lengths, number of expressed miRNAs, and specifically and highly expressed miRNAs in each reproductive stage. KEGG analysis of the differentially expressed (DE) miRNAs between anestrus and the breeding season indicated that they are significantly enriched in pathways related to protein synthesis, secretion and uptake. Furthermore, transcriptome analysis revealed that many target genes of DE miRNAs in the ribosome pathway showed relatively low expression in the breeding season. On the other hand, analyses combining miRNA-gene expression data with target relationship validation in vitro implied that miR-89 may participate in the negative regulation of aralkylamine N-acetyltransferase (AANAT) mRNA expression by targeting its 3’UTR at a unique binding site.

Conclusions

Our results provide new insights into the expression characteristics of sheep pineal miRNAs at different reproductive stages and into the negative regulatory effects of pineal miRNAs on AANAT mRNA expression.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07536-y.

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Thyroid hormone and hypothalamic stem cells in seasonal functions

Seasonal rhythms are a pervasive feature of most living organisms, which underlie yearly timeliness in breeding, migration, hibernation or weight gain and loss. To achieve this, organisms have developed inner timing devices (circannual clocks) that endow them with the ability to predict then anticipate changes to come, usually using daylength as the proximate cue. In Vertebrates, daylength interpretation involves photoperiodic control of TSH production by the pars tuberalis (PT) of the pituitary, which governs a seasonal switch in thyroid hormone (TH) availability in the neighboring hypothalamus. Tanycytes, specialized glial cells lining the third ventricle (3V), are responsible for this TH output through the opposite, PT-TSH-driven, seasonal control of deiodinases 2/3 (Dio 2/3). Tanycytes comprise a photoperiod-sensitive stem cell niche and TH is known to play major roles in cell proliferation and differentiation, which suggests that seasonal control of tanycyte proliferation may be involved in the photoperiodic synchronization of seasonal rhythms. Here we review our current knowledge of the molecular and neuroendocrine pathway linking photoperiodic information to seasonal changes in physiological functions and discuss the potential implication of tanycytes, TH and cell proliferation in seasonal timing.

Regional Differences in Height, Weight, and Body Composition may Result from Photoperiodic Responses: An Ecological Analysis of Japanese Children and Adolescents

This ecological study examined whether geographical differences in the physique of Japanese children and adolescents can be explained from the perspective of photoperiodicity induced by effective day length (light duration exceeding a certain threshold of illuminance) using prefecture-level anatomical data and Mesh Climatic Data. Multiple regression analysis for height prediction demonstrated that when controlled by weight, effective day lengths of the longest and shortest months were inversely correlated with height distribution. Conversely, for weight prediction, when controlled by height, the effective day lengths of the longest and shortest months were positively correlated with weight distribution. The regression coefficients were greater for the effective day length of the shortest month in both height and weight prediction. This phenomenon where the same two explanatory variables are negatively correlated with height and positively correlated with weight in a significant manner is rare, and there may be no physiological interpretation of this phenomenon other than one based on changes in thyroid hormone signaling. These distribution characteristics are common to the photoperiodicity by which seasonal breeding vertebrates reciprocally switch thyroid hormone signaling according to prior photoperiodic history through epigenetic functions. From these perspectives, thyroid hormone signaling in a certain region was assumed to be activated in summer according to the prior shorter winter day length and inactivated in winter according to the prior longer summer day length. Regarding the prevalence of obesity, the coexistence of longer summer and winter day lengths was thought to set body composition to be short and fat in early adolescence.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

Expression analysis of DIO2, EYA3, KISS1 and GPR54 genes in year-round estrous and seasonally estrous rams

The expression characteristics of the hypothalamic-pituitary-gonadal (HPG) axis-related candidate genes, DIO2, EYA3, KISS1 and GPR54, were analyzed in year-round estrous rams (small-tail Han sheep, STH) and seasonally estrous rams (Sunite sheep, SNT) using qPCR. The results were as follows: DIO2 was mainly expressed in pituitary, and KISS1 was specifically expressed in hypothalamus in the two groups. However, EYA3 and GPR54 were widely expressed in the cerebrum, cerebellum, hypothalamus, pituitary, testis, epididymis, vas deferens and adrenal gland tissues in both breeds, with significant differences in the cerebellum, hypothalamus, pituitary, testis and vas deferens tissues. We speculated that DIO2 and KISS1 may have positive roles in different regions in ram year-round estrus. Moreover, the expression patterns of EYA3 and GPR54 suggested that they may regulate the estrous mode of ram via testis and vas deferens. This is the first study to systematically analyze the expression patterns of HPG axis-related genes in rams.

Photoperiod-induced changes in hypothalamic de novo DNA methyltransferase expression are independent of triiodothyronine in female Siberian hamsters (Phodopus sungorus)

Many temperate zone animals engage in seasonal reproductive physiology and behavior as a strategy to maximise the propagation of the species. The hypothalamus integrates environmental cues and hormonal signalling to optimize the timing of reproduction. Recent work has revealed that epigenetic modifications, such as DNA methylation, vary across seasonal reproductive states. Multiple hormones act in the hypothalamus to permit or inhibit reproductive physiology, and the increase in thyroid hormone triiodothyronine (T₃) has been implicated in the initiation of breeding in many species. The objective of this study was to examine the effect of T₃ on the photoperiod-dependent regulation of reproductive physiology and hypothalamic DNA methyltransferase enzyme expression in female Siberian hamsters (Phodopus sungorus). We tested the hypothesis that T₃ in short days (SD) would stimulate hypothalamic Rfrp3 and de novo DNA methyltransferase (Dnmt) expression in female Siberian hamsters. 10 weeks of SD lengths induced a decrease in body and uterine mass. Hamsters maintained in SD were found to express lower levels of GnRH, Rfrp3, Dnmt3a and Dnmt3b. Two weeks of daily T₃ injections did not affect body mass, uterine mass, Gnrh, Rfrp3, Dnmt3a or Dnmt3b expression in neuroendocrine tissues. SD significantly lowered Tshβ mRNA expression and T3 reduced Tshβ in LD hamsters. Our data indicate sex-dependent effects of T₃ for the neuroendocrine regulation of seasonal reproduction in hamsters.

No evidence that Spexin impacts LH release and seasonal breeding in the ewe

Spexin (SPX) is a recently identified peptide hormone of 14 amino acids. Interestingly, Spx and Kiss1 genes share a common ancestor gene. Considering that KISS1 peptides are key controllers of breeding in mammals and circumstantial evidence that SPX regulates gonadotropins in some fish species, we hypothesized that SPX may play a KISS1-related role in sheep. Here, we cloned the ovine Spx cDNA, performed in vivo injection and infusion of SPX (i.c.v. route, with or without concomittant KISS1 presence) and assessed a potential regulation of Spx expression by season, thyroid hormone and estradiol in the medio-basal hypothalamus of the ewe. Our data do not provide support for a role of SPX in the control of the gonadotropic axis in the ewe.

Photoperiodic induction without light-mediated circadian entrainment in a High Arctic resident bird

Organisms use changes in photoperiod to anticipate and exploit favourable conditions in a seasonal environment. While species living at temperate latitudes receive day length information as a year-round input, species living in the Arctic may spend as much as two-thirds of the year without experiencing dawn or dusk. This suggests that specialised mechanisms may be required to maintain seasonal synchrony in polar regions. Svalbard ptarmigan (Lagopus muta hyperborea) are resident at 74-81°N latitude. They spend winter in constant darkness (DD) and summer in constant light (LL); extreme photoperiodic conditions under which they do not display overt circadian rhythms. Here, we explored how Arctic adaptation in circadian biology affects photoperiodic time measurement in captive Svalbard ptarmigan. For this purpose, DD-adapted birds, showing no circadian behaviour, either remained in prolonged DD, were transferred into a simulated natural photoperiod (SNP) or were transferred directly into LL. Birds transferred from DD to LL exhibited a strong photoperiodic response in terms of activation of the hypothalamic thyrotropin-mediated photoperiodic response pathway. This was assayed through expression of the Eya3, Tshβ and deiodinase genes, as well as gonadal development. While transfer to SNP established synchronous diurnal activity patterns, activity in birds transferred from DD to LL showed no evidence of circadian rhythmicity. These data show that the Svalbard ptarmigan does not require circadian entrainment to develop a photoperiodic response involving conserved molecular elements found in temperate species. Further studies are required to define how exactly Arctic adaptation modifies seasonal timer mechanisms.

Circuit-level analysis identifies target genes of sex steroids in ewe seasonal breeding

Thyroid hormone (TH) and estradiol (E2) direct seasonal switches in ovine reproductive physiology. In sheep, as in other mammals and birds, control of thyrotropin (TSH) production by the pars tuberalis (PT) links photoperiod responsiveness to seasonal breeding. PT-derived TSH governs opposite seasonal patterns of the TH deiodinases Dio2/Dio3 expression in tanycytes of the neighboring medio-basal hypothalamus (MBH), which explain the key role of TH. We recently used RNA-Seq to identify seasonal markers in the MBH and define the impact of TH. This impact was found to be quite limited, in terms of number of target genes, and very restricted with regards to neuroanatomical location, as TH specifically impacts genes expressed in tanycytes and hypothalamus, not in the PT. Here we address the impact of E2 on these seasonal markers, which are specifically expressed in either PT, tanycytes or hypothalamus. We also investigate if progesterone (P4) may be involved in timing the seasonal transition to anestrus. Our analysis provides circuit-level insights into the impact of sex steroids on the ewe seasonal breeding cycle. First, seasonal gene expression in the PT is independent of the sex steroid status. The fact that seasonal gene expression in the PT is also TH-independent strengthens the view that the PT is a circannual timer. Second, select tanycytic markers display some level of responsiveness to E2 and P4, which indicates another potential level of feedback control by sex steroids. Third, Kiss1 neurons of the arcuate nucleus are responsive to both TH and E2, which places them at the crossroads of photoperiodic transduction pathway and sex steroid feedback. This provides strong support to the concept that these Kiss1 neurons are pivotal to the long-recognized "seasonal switch in the ability of E2 to exert negative feedback", which drives seasonal breeding.

Debunking the Myth of the Endogenous Antiangiogenic Vegfaxxxb Transcripts

In this opinion article we critically assess evidence for the existence of a family of antiangiogenic vascular endothelial growth factor (Vegfaxxxb) transcripts, arising from the use of a phylogenetically conserved alternative distal splice site within exon 8 of the VEGFA gene. We explain that prior evidence for Vegfaxxxb transcripts in tissues rests heavily upon flawed RT-PCR methodologies, with the extensive use of 5'-tailing in primer design being the main issue. Furthermore, our analysis of large RNA-seq data sets (human and ovine) fails to identify a single Vegfaxxxb transcript. Therefore, we challenge the very existence of Vegfaxxxb transcripts, which further questions the physiological relevance of studies based on the use of 'anti-VEGFAxxxb' antibodies. Our analysis has implications for the proposed therapeutic use of isoform-specific anti-VEGFA strategies for treating cancer and retinopathies.

Photoperiodic regulation in a wild-derived mouse strain

Mus musculus molossinus (MSM) is a wild-derived mouse strain which maintains the ability to synthesize melatonin in patterns reflecting the ambient photoperiod. The objective of this study was to characterize the effects of photoperiodic variation on metabolic and reproductive traits, and the related changes in pituitary-hypothalamic gene expression in MSM mice. MSM mice were kept in long (LP) or short photoperiod (SP) for 6 weeks. Our results demonstrate that MSM mice kept in LP, as compared to mice kept in SP, display higher expression of genes encoding thyrotropin (TSH) in the pars tuberalis, thyroid hormone deiodinase 2 (dio2) in the tanycytes, RFamide-related peptide (RFRP3) in the hypothalamus and lower expression of dio3 in the tanycytes, along with larger body and reproductive organ mass. Additionally, to assess the effects of the gestational photoperiodic environment on the expression of these genes, we kept MSM mice in LP or SP from gestation and studied offspring. We show that the gestational photoperiod affects the TSH/dio pathway in newborn MSM mice in a similar way to adults. This result indicates a transgenerational effect of photoperiod from the mother to the fetus in utero. Overall, these results indicate that photoperiod can influence neuroendocrine regulation in a melatonin-proficient mouse strain, in a manner similar that documented in other seasonal rodent species. MSM mice may therefore become a useful model for research into the molecular basis of photoperiodic regulation of seasonal biology.

Discontinuity in the molecular neuroendocrine response to increasing daylengths in Ile-de-France ewes: Is transient Dio2 induction a key feature of circannual timing?

In mammals, melatonin is responsible for the synchronisation of seasonal cycles to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via the pituitary pars tuberalis (PT), which in turn modulates hypothalamic thyroid hormone status via seasonal changes in the production of locally-acting thyrotrophin. Recently, we demonstrated that, in the Soay sheep, photoperiodic induction of Tshb expression and consequent downstream hypothalamic changes occur over a narrow range of photoperiods between 12 and 14 hours in duration. In the present study, we aimed to extend our molecular characterisation of this pathway, based on transcriptomic analysis of photoperiodic changes in the pituitary and hypothalamus of ovariectomised, oestradiol-implanted Ile-de-France ewes. We demonstrate that photoperiodic treatments applied before the winter solstice elicit two distinctive modes of accelerated reproductive switch off compared to ewes held on a simulated natural photoperiod, with shut-down occurring markedly faster on photoperiods of 13 hours or more than on photoperiods of 12 hours and less. This pattern of response was reflected in gene expression profiles of photoperiodically sensitive markers, both in the PT (Tshb, Fam150b, Vmo1, Ezh2 and Suv39H2) and in tanycytes (Tmem252 and Dct). Unexpectedly, the expression of Dio2 in tanycytes did not show any noticeable increase in expression with lengthening photoperiods. Finally, the expression of Kiss1, the key activator of gonadotrophin-releasing hormone release, was proportionately decreased by lengthening photoperiods, in a pattern that correlated strongly with gonadotrophin suppression. These data show that stepwise increases in photoperiod lead to graded molecular responses at the level of the PT, a progressive suppression of Kiss1 in the hypothalamic arcuate nucleus and luteinising hormone/follicle-stimulating hormone release by the pituitary, despite apparently unchanged Dio2 expression in tanycytes. We hypothesise that this apparent discontinuity in the seasonal neuroendocrine response illustrates the transient nature of the thyroid hormone-mediated response to long days in the control of circannual timing.

An integrative view of mammalian seasonal neuroendocrinology

Seasonal neuroendocrine cycles that govern annual changes in reproductive activity, energy metabolism and hair growth are almost ubiquitous in mammals that have evolved at temperate and polar latitudes. Changes in nocturnal melatonin secretion regulating gene expression in the pars tuberalis (PT) of the pituitary stalk are a critical common feature in seasonal mammals. The PT sends signal(s) to the pars distalis of the pituitary to regulate prolactin secretion and thus the annual moult cycle. The PT also signals in a retrograde manner via thyroid-stimulating hormone to tanycytes, which line the ventral wall of the third ventricle in the hypothalamus. Tanycytes show seasonal plasticity in gene expression and play a pivotal role in regulating local thyroid hormone (TH) availability. Within the mediobasal hypothalamus, the cellular and molecular targets of TH remain elusive. However, two populations of hypothalamic neurones, which produce the RF-amide neuropeptides kisspeptin and RFRP3 (RF-amide related peptide 3), are plausible relays between TH and the gonadotrophin-releasing hormone-pituitary-gonadal axis. By contrast, the ways by which TH also impinges on hypothalamic systems regulating energy intake and expenditure remain unknown. Here, we review the neuroendocrine underpinnings of seasonality and identify several areas that warrant further research.

A unifying hypothesis for control of body weight and reproduction in seasonally breeding mammals

Animals have evolved diverse seasonal variations in physiology and reproduction to accommodate yearly changes in environmental and climatic conditions. These changes in physiology are initiated by changes in photoperiod (daylength) and are mediated through melatonin, which relays photoperiodic information to the pars tuberalis of the pituitary gland. Melatonin drives thyroid-stimulating hormone transcription and synthesis in the pars tuberalis, which, in turn, regulates thyroid hormone and retinoic acid synthesis in the tanycytes lining the third ventricle of the hypothalamus. Seasonal variation in central thyroid hormone signalling is conserved among photoperiodic animals. Despite this, different species adopt divergent phenotypes to cope with the same seasonal changes. A common response amongst different species is increased hypothalamic cell proliferation/neurogenesis in short photoperiod. That cell proliferation/neurogenesis may be important for seasonal timing is based on (i) the neurogenic potential of tanycytes; (ii) the fact that they are the locus of striking seasonal morphological changes; and (iii) the similarities to mechanisms involved in de novo neurogenesis of energy balance neurones. We propose that a decrease in hypothalamic thyroid hormone and retinoic acid signalling initiates localised neurodegeneration and apoptosis, which leads to a reduction in appetite and body weight. Neurodegeneration induces compensatory cell proliferation from the neurogenic niche in tanycytes and new cells are born under short photoperiod. Because these cells have the potential to differentiate into a number of different neuronal phenotypes, this could provide a mechanistic basis to explain the seasonal regulation of energy balance, as well as reproduction. This cycle can be achieved without changes in thyroid hormone/retinoic acid and explains recent data obtained from seasonal animals held in natural conditions. However, thyroid/retinoic acid signalling is required to synchronise the cycles of apoptosis, proliferation and differentiation. Thus, hypothalamic neurogenesis provides a framework to explain diverse photoperiodic responses.

Functional Implications of RFRP-3 in the Central Control of Daily and Seasonal Rhythms in Reproduction

Adaptation of reproductive activity to environmental changes is essential for breeding success and offspring survival. In mammals, the reproductive system displays regular cycles of activation and inactivation which are synchronized with seasonal and/or daily rhythms in environmental factors, notably light intensity and duration. Thus, most species adapt their breeding activity along the year to ensure that birth and weaning of the offspring occur at a time when resources are optimal. Additionally, female reproductive activity is highest at the beginning of the active phase during the period of full oocyte maturation, in order to improve breeding success. In reproductive physiology, it is therefore fundamental to delineate how geophysical signals are integrated in the hypothalamo-pituitary-gonadal axis, notably by the neurons expressing gonadotropin releasing hormone (GnRH). Several neurochemicals have been reported to regulate GnRH neuronal activity, but recently two hypothalamic neuropeptides belonging to the superfamily of (Arg)(Phe)-amide peptides, RFRP-3 and kisspeptin, have emerged as critical for the integration of environmental cues within the reproductive axis. The goal of this review is to survey the current understanding of the role played by RFRP-3 in the temporal regulation of reproduction, and consider how its effect might combine with that of kisspeptin to improve the synchronization of reproduction to environmental challenges.

Maternal photoperiodic programming enlightens the internal regulation of thyroid-hormone deiodinases in tanycytes

Seasonal rhythms in physiology are widespread among mammals living in temperate zones. These rhythms rely on the external photoperiodic signal being entrained to the seasons, although they persist under constant conditions, revealing their endogenous origin. Internal long-term timing (circannual cycles) can be revealed in the laboratory as photoperiodic history-dependent responses, comprising the ability to respond differently to similar photoperiodic cues based on prior photoperiodic experience. In juveniles, history-dependence relies on the photoperiod transmitted by the mother to the fetus in utero, a phenomenon known as "maternal photoperiodic programming" (MPP). The response to photoperiod in mammals involves the nocturnal pineal hormone melatonin, which regulates a neuroendocrine network including thyrotrophin in the pars tuberalis and deiodinases in tanycytes, resulting in changes in thyroid hormone in the mediobasal hypothalamus. This review addresses MPP and discusses the latest findings on its impact on the thyrotrophin/deiodinase network. Finally, commonalities between MPP and other instances of endogenous seasonal timing are considered, and a unifying scheme is suggested in which timing arises from a long-term communication between the pars tuberalis and the hypothalamus and resultant spontaneous changes in local thyroid hormone status, independently of the pineal melatonin signal.

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

How can a binary switch within the pars tuberalis control seasonal timing of reproduction?

Life in seasonally changing environments is challenging. Biological systems have to not only respond directly to the environment, but also schedule life history events in anticipation of seasonal changes. The cellular and molecular basis of how these events are scheduled is unknown. Cellular decision-making processes in response to signals above certain thresholds regularly occur i.e. cellular fate determination, apoptosis and firing of action potentials. Binary switches, the result of cellular decision-making processes, are defined as a change in phenotype between two stable states. A recent study presents evidence of a binary switch operating in the pars tuberalis (PT) of the pituitary, seemingly timing seasonal reproduction in sheep. Though, how a binary switch would allow for anticipation of seasonal environmental changes, not just direct responsiveness, is unclear. The purpose of this review is to assess the evidence for a binary switching mechanism timing seasonal reproduction and to hypothesize how a binary switch would allow biological processes to be timed over weeks to years. I draw parallels with mechanisms used in development, cell fate determination and seasonal timing in plants. I propose that the adult PT is a plastic tissue, showing a seasonal cycle of cellular differentiation, and that the underlying processes are likely to be epigenetic. Therefore, considering the mechanisms behind adult cellular plasticity offers a framework to hypothesize how a long-term timer functions within the PT.

Photoperiod and thyroid hormone regulate expression of l-dopachrome tautomerase (Dct), a melanocyte stem-cell marker, in tanycytes of the ovine hypothalamus

The pars tuberalis (PT) of the pituitary is central to the control of seasonal breeding. In mammals, the PT translates the photoperiodic message carried by melatonin into an endocrine thyroid-stimulating hormone output, which controls local thyroid hormone (TH) signalling in tanycytes of the neighbouring hypothalamus. In the present study, we identify l-dopachrome tautomerase (Dct) as a novel marker of ovine tanycytes and show that Dct displays marked seasonal variations in expression, with higher levels during spring and summer. This seasonal profile is photoperiod-dependent because an acute exposure to long days induces Dct expression. In addition, we find that TH also modulates Dct expression. DCT functions as an enzyme in the melanin synthesis pathway within skin melanocytes, whereas expression in other tissues is comparatively low. We demonstrate that both Tyr and Tyrp1, which are enzymes that intervene upstream and downstream of Dct in the melanin synthesis pathway, respectively, are expressed at very low levels in the ovine hypothalamus. This suggests that Dct in tanycytes may not be involved in melanin synthesis. We speculate that DCT function is linked to its protective role towards oxidative stress and/or its function in the control of neural progenitor cell proliferation.

Neuroendocrine correlates of the critical day length response in the Soay sheep

In mammals, melatonin is the hormone responsible for synchronisation of seasonal physiological cycles of physiology to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via melatonin-responsive cells in the pituitary pars tuberalis (PT), which in turn modulate hypothalamic thyroid hormone status. Recent models suggest that the actions of melatonin in the PT depend critically on day length-dependent changes in the expression of eyes absent 3 (Eya3), which is a coactivator for thyrotrophin β-subunit (Tshβ) gene transcription. According to this model, short photoperiods suppress Eya3 and hence Tshβ expression, whereas long photoperiods produce the inverse effect. Studies underpinning this model have relied on step changes in photoperiod (from 8 to 16 hours of light/24 hours) and have not compared the sensitive ranges of photoperiods for changes in Eya3 and Tshβ expression with those for relevant downstream molecular and endocrine responses. We therefore performed a "critical day length" experiment in Soay sheep, in which animals acclimated to 8 hours of light/24 hours (SP) were exposed to a range of increased photoperiods spanning the range 11.75 to 16 hours (LP) and then responses at the level of the PT, hypothalamus and hormonal output were assessed. Although Eya3 and Tshβ both showed the predicted SP vs LP differences, they responded quite differently to intermediate photoperiods within this range and, at the individual animal level, no clear Eya3-Tshβ relationship could be seen. This result is inconsistent with a simple coactivator model for EYA3 action in the PT. Further downstream layers of nonlinearity were also seen in terms of the Tshβ-dio2 and the dio2-testosterone relationships. We conclude that the transduction of progressive changes in photoperiod into transitions in endocrine output is an emergent property of a multistep signalling cascade within the mammalian neuroendocrine system.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

Anti-angiogenic VEGFAxxxb transcripts are not expressed in the medio-basal hypothalamus of the seasonal sheep

This study investigated Vegfa expression in the pars tuberalis (PT) of the pituitary and medio-basal hypothalamus (MBH) of sheep, across seasons and reproductive states. It has recently been proposed that season impacts alternative splicing of Vegfa mRNA in the PT, which shifts the balance between angiogenic VEGFAxxx and anti-angiogenic VEGFAxxxb isoforms (with xxx the number of amino acids of the mature VEGFA proteins) to modulate seasonal breeding. Here, we used various RT-PCR methodologies and analysis of RNAseq datasets to investigate seasonal variation in expression and splicing of the ovine Vegfa gene. Collectively, we identify 5 different transcripts for Vegfa within the ewe PT/MBH, which correspond to splicing events previously described in mouse and human. All identified transcripts encode angiogenic VEGFAxxx isoforms, with no evidence for alternative splicing within exon 8. These findings led us to investigate in detail how "Vegfaxxxb-like" PCR products could be generated by RT-PCR and misidentified as endogenous transcripts, in sheep and human HEK293 cells. In conclusion, our findings do not support the existence of anti-angiogenic VEGFAxxxb isoforms in the ovine PT/MBH and shed new light on the interpretation of prior studies, which claimed to identify Vegfaxxxb isoforms by RT-PCR.

Downregulation of Deiodinase 3 is the earliest event in photoperiodic and photorefractory activation of the gonadotropic axis in seasonal hamsters

In seasonal rodents, reproduction is activated by a long photoperiod. Furthermore, maintaining an inhibitory short photoperiod for over 20 weeks triggers a spontaneous reactivation of the gonadotropic axis called photorefractoriness. Photoactivation is proposed to involve melatonin, hypothalamic thyroid hormones (TH) and (Arg) (Phe)-amide peptides. The mechanisms involved in photorefractoriness are so far unknown. We analyzed the dynamic changes in long photoperiod- and photorefractory-induced activation of reproduction in both Syrian and Djungarian hamsters to validate the current model of photoactivation and to uncover the mechanisms involved in photorefractoriness. We detected a conserved early inhibition of expression of the TH catabolizing enzyme deiodinase 3 (Dio3) in tanycytes, associated with a late decrease of the TH transporter MCT8. This suggests that an early peak of hypothalamic TH may be involved in both photoinduced and photorefractory reactivation. In photoactivation, Dio3 downregulation is followed by an upregulation of Dio2, which is not observed in photorefraction. The upregulation of (Arg) (Phe)-amides occurs several weeks after the initial Dio3 inhibition. In conclusion, we uncovered a so far unreported early inhibition of Dio3. This early downregulation of Dio3 is reinforced by an upregulation of Dio2 in photoactivated hamsters. In photorefractoriness, the Dio3 downregulation might be sufficient to reactivate the gonadotropic axis.