A molecular switch for photoperiod responsiveness in mammals

CRISPR-Cas9 gene-editing to assess the role of RF-amide-related peptide 3 in ovine seasonal breeding

Seasonal breeding is an adaptive strategy that ensures the birth of the offspring during the best time of year and allows energy saving in times of food scarcity and adverse environmental conditions. At temperate and polar latitudes, photoperiod is the main synchroniser of seasonal functions, which exerts its action through melatonin. Within the pars tuberalis of the anterior pituitary, melatonin triggers a series of events that lead to enhanced local triiodothyronine (T3) production in the medio-basal hypothalamus specifically under long days and ultimately drive the appropriate GnRH output at the median eminence. How T3 governs GnRH output is mostly unknown but neuronal populations that respectively produce KISS1 and RFRP3 appear to be involved. However, while the role of KISS1 as a major GnRH secretagogue is undisputed, the function of RFRP3 in the control of (seasonal) breeding remains enigmatic, with conflicting results hinting at elusive mechanisms. Therefore, we launched an extensive series of experiments in sheep, aimed at invalidating the NPVF gene, which encodes RFRP3, using CRISPR-Cas9 technology. Here, we report on the generation of six sheep for which the NPVF gene has been successfully edited. Four of these animals bear at least one allele expected to behave as a null and constitute founders for distinct ovine lines. These founder sheep will now enter a standard breeding scheme in order to obtain male and female homozygotes for distinct mutations. These animals are expected to provide a clear delineation of the function of RFRP3 in seasonal breeding.

Transcriptome analysis of the hypothalamus and testes in Brandt's Vole: new insights into mechanisms of photoperiodic plasticity in postnatal testicular development

Postnatal gonadal development is regulated by photoperiod via the hypothalamus, especially in seasonal breeding small rodents. However, the precise molecular mechanisms remain unclear. In this study, we conducted a comparative analysis of the transcriptomes of the hypothalamus and testes in 10-week-old male Brandt's voles born under long (LP, 16L:8D) and short photoperiod (SP, 8L:16D) conditions. Results indicate that the SP group exhibited significantly smaller testes with spermatogenesis halted before meiosis, identifying 129 differentially expressed genes (DEGs) in the hypothalamus and 21,673 DEGs in the testes. In the hypothalamus, genes involved in the thyroid hormone and retinoic acid (RA) pathway were notably altered under SP conditions, including decreased Tshb and Cga expression, increased Dio3, and reduced Crabp1 and Lrat, highlighting their key roles in SP signaling. In the testes, downregulated genes were significantly enriched in male reproduction-related GO terms and metabolic KEGG pathways, such as steroid hormone biosynthesis and retinol metabolism. Key genes for testosterone synthesis (e.g. Star, Cyp11a1) and RA synthesis (e.g. Rdh10, Rdh11) were downregulated, while those linked to RA degradation (Cyp26b1) and undifferentiated spermatogonia maintenance (e.g. Gdnf, Gfra1) were upregulated. These findings outline a molecular microenvironment that favors the preservation of undifferentiated spermatogonia over their differentiation from the hypothalamus to the testes. This study firstly provides valuable insights into the transcriptomic basis of SP-inhibited testicular development in Brandt's voles.

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.

Stationarity of the Special Relationship Between the Geographical Distribution of Body Size and Day Length in Japanese Adolescents: Spatial and Temporal Analysis Using a GTWR Model

OBJECTIVES

Northern Japanese children tend to have larger physiques; however, the underlying cause remains unknown. Previous geographical correlation analyses revealed an unusual trend; effective day length was negatively correlated with height and positively correlated with weight (adjusted for height). This paradoxical relationship suggests a thyroid hormone-like effect and possible photoperiodic response. This study aimed to determine whether this phenomenon remains consistent over time and across different regions of Japan.

METHODS

We used geographically and temporally weighted regression (GTWR) to examine whether the relationship between height and effective day length, which differs from the relationship between weight and effective day length, varies by location and time. GTWR models each observation point separately, allowing for spatial and temporal variations. The analysis included the average height and weight data of children and adolescents by prefecture from 1989 to 2019, along with effective day length considering illuminance above 5000 lx derived from the agrometeorological grid square data.

RESULTS

Height was used as the dependent variable, whereas weight and effective day length were used as independent variables. For height estimation, the coefficients of weight and effective day length were consistently positive and negative, respectively, although the regression coefficients showed minor geographical and temporal variations.

CONCLUSION

The opposite correlation between height and effective day length and that between weight and effective day length were consistent. This suggests that the phenomenon is more likely driven by environmental factors than by economic or genetic influences.

Transcriptomic profiling of gill biopsies to define predictive markers for seawater survival in farmed Atlantic salmon

Wild Atlantic salmon migrate to sea following completion of a developmental process known as parr-smolt transformation (PST), which establishes a seawater (SW) tolerant phenotype. Effective imitation of this aspect of anadromous life history is a crucial aspect of commercial salmon production, with current industry practice being marred by significant losses during transition from the freshwater (FW) to SW phase of production. The natural photoperiodic control of PST can be mimicked by exposing farmed juvenile fish to a reduced duration photoperiod for at least 6 weeks before increasing the photoperiod in the last 1-2 months before SW transfer. While it is known that variations in this general protocol affect subsequent SW performance, there is no uniformly accepted industry standard; moreover, reliable prediction of SW performance from fish attributes in the FW phase remains a major challenge. Here we describe an experiment in which we took gill biopsies 1 week prior to SW transfer from 3000 individually tagged fish raised on three different photoperiod regimes during the FW phase. Biopsies were subjected to RNA profiling by Illumina sequencing, while individual fish growth and survival was monitored over 300 days in a SW cage environment, run as a common garden experiment. Using a random forest machine learning algorithm, we developed gene expression-based predictive models for initial survival and stunted growth in SW. Stunted growth phenotypes could not be predicted based on gill transcriptomes, but survival the first 40 days in SW could be predicted with moderate accuracy. While several previously identified marker genes contribute to this model, a surprisingly low weighting is ascribed to sodium potassium ATPase subunit genes, contradicting advocacy for their use as SW readiness markers. However, genes with photoperiod-history sensitive regulation were highly enriched among the genes with highest importance in the prediction model. This work opens new avenues for understanding and exploiting developmental changes in gill physiology during smolt development.

Seasonal changes in activity of hypothalamic thyroid hormone system in different winter phenotypes of Djungarian hamster (Phodopus sungorus)

Although the Djungarian hamster (Phodopus sungorus) is a seasonality model, it presents substantial variability in winter acclimation. In response to short photoperiod, some individuals express a suite of winter traits such as low body mass, regressed gonads, white fur, and daily torpor, while others develop only some adjustments or maintain a summer phenotype. Despite comprehensive research, the mechanisms underlying polymorphism of winter phenotype are still unknown. We compared key elements of the hypothalamic thyroid hormone system, as well as the tanycyte architecture in hamsters of both sexes. Individuals presented different responses to short photoperiod characterized either as phenotypes (non-responder, partial-responder and full-responder) or photoresponsive index. We measured the expression of genes coding iodothyronine deiodinase 2 and 3, monocarboxylate transporter 8, thyrotropin-releasing hormone, and somatostatin in 40 individuals and counted the number of immunolabeled tanycyte processes in standardized regions of interest around the third ventricle in 30 individuals. Animals acclimated to short photoperiod presented a downregulation of diodinase 2 and somatostatin and an upregulation of deiodinase 3, as well as a decreased number of tanycyte processes, compared to long photoperiod-exposed individuals. Although phenotypes did not differ in gene expression, the higher the photoresponsive index, the lower was the deiodinase 2 expression and the higher the deiodinase 3 expression. Partial-responders and full-responders had less tanycyte processes than non-responders, and the number of tanycyte processes correlated with the photoresponsive index. Sexes differed neither in their seasonal response, nor hypothalamic gene expression, but females had more tanycyte processes. Our results are in accordance with studies emphasizing the pivotal role of thyroid hormones in seasonal response. We suggest that the whole spectrum of winter phenotypes exists within the population of Djungarian hamsters and that it is reflected also at the level of neuroendocrine regulation. However, the neuroendocrine underpinnings of winter phenotype polymorphism require further investigation.

Photoperiod effects on corticosterone and seasonal clocks in cafeteria-induced obese fischer 344 rats are influenced by gut microbiota

Seasonal rhythms are gaining attention given their impact on metabolic disorders development such as obesity gut microbiota is emerging as a key factor in mediating this link. However, the underlying mechanisms are still poorly understood. In this regard, corticosterone may play a role as it has been shown to be affected by gut bacteria and seasonal rhythms, and has been linked to obesity. Thus, this study aimed to investigate if seasonal rhythms effects on corticosterone are influenced by gut microbiota in obese rats and whether this may be related to seasonal and clock genes expression in the pituitary gland and colon. Fischer 344 male rats fed with cafeteria diet (CAF) were housed under different photoperiods for 9 weeks and treated with an antibiotic cocktail (ABX) in drinking water during the last 4 weeks. Rats fed with standard chow and CAF-fed rats without ABX were included as controls. ABX altered gut microbiota, corticosterone levels and seasonal clock expression in the pituitary depending on photoperiod conditions. These results suggest a link between gut bacteria, seasonal rhythms and corticosterone and a novel nutrigenomic target for obesity.

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.

One seasonal clock fits all?

Adaptation of physiology and behavior to seasonal changes in the environment are for many organisms essential for survival. Most of our knowledge about the underlying mechanisms comes from research on photoperiodic regulation of reproduction in plants, insects and mammals. However, even humans, who mostly live in environments with minimal seasonal influences, show annual rhythms in physiology (e.g., immune activity, brain function), behavior (e.g., sleep-wake cycles) and disease prevalence (e.g., infectious diseases). As seasonal variations in environmental conditions may be drastically altered due to climate change, the understanding of the mechanisms underlying seasonal adaptation of physiology and behavior becomes even more relevant. While many species have developed specific solutions for dedicated tasks of photoperiodic regulation, we find a number of common principles and mechanisms when comparing insect and mammalian systems: (1) the circadian system contributes to photoperiodic regulation; (2) similar signaling molecules (VIP and PDF) are used for transferring information from the circadian system to the neuroendocrine system controlling the photoperiodic response; (3) the hormone melatonin participates in seasonal adaptation in insects as well as mammals; and (4) changes in photoperiod affect neurotransmitter function in both animal groups. The few examples of overlap elaborated in this perspective article, as well as the discussion on relevance for humans, should be seen as encouragement to unravel the machinery of seasonal adaptation in a multitude of organisms.

Variation in photoperiod response corresponds to differences in circadian light sensitivity in northern and southern Nasonia vitripennis lines

The circadian clock times physiological and behavioural processes and resets on a daily basis to synchronize with the environment. The involvement of the circadian clock in photoperiodic time measurement synchronising annual rhythms is still under debate and different models have been proposed explaining their integration. Insects overcome unfavourable conditions in diapause, a form of dormancy. A latitudinal cline in diapause induction in the parasitoid wasp Nasonia vitripennis as well as a difference in circadian light sensitivity between north and south provide us with additional evidence that the circadian system of Nasonia is involved in photoperiodic time measurement and that latitude-specific seasonality drives adaptive evolution in photoperiodism partly through adaptation responses in the circadian system. We tested diapause induction in a range of T-cycles and photoperiods and found diapause induction in short photoperiods in all T-cycles in the northern line but in the southern line, diapause only occurred in T-cycles close to 24 h. Due to a lower light sensitivity in the southern line, a wider distribution of phase angles of entrainment can be expected at a specific T-cycle duration, while the range of entrainment will decrease. Taking these oscillator properties into account, our data can be explained by an external coincidence model involving a single oscillator with a light-sensitive phase that drives annual timing of diapause in Nasonia vitripennis.

Uncoupling of behavioral and metabolic 24-h rhythms in reindeer

Reindeer in the Arctic seasonally suppress daily circadian patterns of behavior present in most animals.1 In humans and mice, even when all daily behavioral and environmental influences are artificially suppressed, robust endogenous rhythms of metabolism governed by the circadian clock persist and are essential to health.2,3 Disrupted rhythms foster metabolic disorders and weight gain.4 To understand circadian metabolic organization in reindeer, we performed behavioral measurements and untargeted metabolomics from blood plasma samples taken from Eurasian tundra reindeer (Rangifer tarandus tarandus) across 24 h at 2-h intervals in four seasons. Our study confirmed the absence of circadian rhythms of behavior under constant darkness in the Arctic winter and constant daylight in the Arctic summer, as reported by others.1 We detected and measured the intensity of 893 metabolic features in all plasma samples using untargeted ultra-high-performance liquid chromatography-mass spectrometry (UPLC-MS). A core group of metabolites (66/893 metabolic features) consistently displayed 24-h rhythmicity. Most metabolites displayed a robust 24-h rhythm in winter and spring but were arrhythmic in summer and fall. Half of all measured metabolites displayed ultradian sleep-wake dependence in summer. Irrespective of the arrhythmic behavior, metabolism is rhythmic (24 h) in seasons of low food availability, potentially favoring energy efficiency. In seasons of food abundance, 24-h rhythmicity in metabolism is drastically reduced, again irrespective of behavioral rhythms, potentially fostering weight gain.

Hormonal basis of seasonal metabolic changes in mammalian species

Seasonal changes in external conditions (photoperiod, meteorological conditions, diet) cause adaptive changes in both energy and substrate metabolism in the animals of mammalian species. In summer, long days and a rich diet contribute to relative elevation in the levels of thyroid hormones (TH), but warmer weather lowers their levels. In winter, short days and a poor diet inhibit TH synthesis, but low temperatures increase their secretion. In addition, the results of our meta-analyses revealed a significant role of atmospheric pressure in circannual fluctuations of metabolic parameters in humans. The changes in photoperiod are generally viewed as a major factor contributing to seasonal rhythm regulation However, numerous data show that season-dependent metabolic changes in mammals could be also accounted for by meteorological factors and diet.

The dynamic landscape of chromatin accessibility and active regulatory elements in the mediobasal hypothalamus influences the seasonal activation of the reproductive axis in the male quail under long light exposure

BACKGROUND

In cold and temperate zones, seasonal reproduction plays a crucial role in the survival and reproductive success of species. The photoperiod influences reproductive processes in seasonal breeders through the hypothalamic-pituitary-gonadal (HPG) axis, in which the mediobasal hypothalamus (MBH) serves as the central region responsible for transmitting light information to the endocrine system. However, the cis-regulatory elements and the transcriptional activation mechanisms related to seasonal activation of the reproductive axis in MBH remain largely unclear. In this study, an artificial photoperiod program was used to induce the HPG axis activation in male quails, and we compared changes in chromatin accessibility changes during the seasonal activation of the HPG axis.

RESULTS

Alterations in chromatin accessibility occurred in the mediobasal hypothalamus (MBH) and stabilized at LD7 during the activation of the HPG axis. Most open chromatin regions (OCRs) are enriched mainly in introns and distal intergenic regions. The differentially accessible regions (DARs) showed enrichment of binding motifs of the RFX, NKX, and MEF family of transcription factors that gained-loss accessibility under long-day conditions, while the binding motifs of the nuclear receptor (NR) superfamily and BZIP family gained-open accessibility. Retinoic acid signaling and GTPase-mediated signal transduction are involved in adaptation to long days and maintenance of the HPG axis activation. According to our footprint analysis, three clock-output genes (TEF, DBP, and HLF) and the THRA were the first responders to long days in LD3. THRB, NR3C2, AR, and NR3C1 are the key players associated with the initiation and maintenance of the activation of the HPG axis, which appeared at LD7 and tended to be stable under long-day conditions. By integrating chromatin and the transcriptome, three genes (DIO2, SLC16A2, and PDE6H) involved in thyroid hormone signaling showed differential chromatin accessibility and expression levels during the seasonal activation of the HPG axis. TRPA1, a target of THRB identified by DAP-seq, was sensitive to photoactivation and exhibited differential expression levels between short- and long-day conditions.

CONCLUSION

Our data suggest that trans effects were the main factors affecting gene expression during the seasonal activation of the HPG axis. This study could lead to further research on the seasonal reproductive behavior of birds, particularly the role of MBH in controlling seasonal reproductive behavior.

Genome-Wide Detection for Runs of Homozygosity in Baoshan Pigs Using Whole Genome Resequencing

Baoshan pigs (BS) are a local breed in Yunnan Province that may face inbreeding owing to its limited population size. To accurately evaluate the inbreeding level of the BS pig population, we used whole-genome resequencing to identify runs of homozygosity (ROH) regions in BS pigs, calculated the inbreeding coefficient based on pedigree and ROH, and screened candidate genes with important economic traits from ROH islands. A total of 22,633,391 SNPS were obtained from the whole genome of BS pigs, and 201 ROHs were detected from 532,450 SNPS after quality control. The number of medium-length ROH (1-5 Mb) was the highest (98.43%), the number of long ROH (>5 Mb) was the lowest (1.57%), and the inbreeding of BS pigs mainly occurred in distant generations. The inbreeding coefficient FROH, calculated based on ROH, was 0.018 ± 0.016, and the FPED, calculated based on the pedigree, was 0.027 ± 0.028, which were positively correlated. Forty ROH islands were identified, containing 507 genes and 891 QTLs. Several genes were associated with growth and development (IGFALS, PTN, DLX5, DKK1, WNT2), meat quality traits (MC3R, ACSM3, ECI1, CD36, ROCK1, CACNA2D1), and reproductive traits (NPW, TSHR, BMP7). This study provides a reference for the protection and utilization of BS pigs.

Regulation of circulating thyroid hormone levels by hypothalamic tanycytes and hypophysial pars tuberalis-specific cells and their morphological and gene- and protein-expression changes under different photoperiods

Thyroid hormone in the hypothalamus acts as a key determinant of seasonal transitions. Thyroid hormone-levels in the brain are mainly regulated by the hypothalamic tanycytes and pituitary pars tuberalis (PT)-specific cells. TSHβ produced by the PT-specific cells stimulates Dio2 expression and decreases Dio3 expression of the tanycytes. Both tanycytes and PT-specific cells in photosensitive animals exhibit remarkable changes of morphological appearance and expressions of genes and proteins under different photoperiods. Long photoperiods induce increased gene- and protein-expressions and active features. Short photoperiods cause the decreased gene- and protein-expressions and inactive features. In the PT, expressions of TSHβ, common α-subunit of glycoprotein hormones (α-GSU), and MT1 receptor of melatonin receptors and eyes absent 3 change under different photoperiods. Diurnal rhythms of α-GSU mRNA expression are observed in the PT of Djungarian hamsters. Hes1, Nkx2.1, and LIM homeodomain gene 2 (Lhx2) are involved in the differentiation of PT. In the hypothalamic tanycytes, expressions of Dio2, Dio3, vimentin, serine/threonine kinase 33, GPR50, Nestin, Retinoid signaling genes (retinaldehyde dehydrogenase 1, cellular retinol binding protein 1, and Stra6), monocarboxylate transporter 8, and neural cell adhesion molecule change under different photoperiods. Rax, Lhx2, Nfia/b/x, and fibroblast growth factor 10 are involved in the differentiation of tanycytes.

Summer fades, deer change: Photoperiodic control of cellular seasonal acclimatization of skeletal muscle

We found major seasonal changes of polyunsaturated fatty acids (PUFAs) in muscular phospholipids (PL) in a large non-hibernating mammal, the red deer (Cervus elaphus). Dietary supply of essential linoleic acid (LA) and α-linolenic acid (ALA) had no, or only weak influence, respectively. We further found correlations of PL PUFA concentrations with the activity of key metabolic enzymes, independent of higher winter expression. Activity of the sarcoplasmic reticulum (SR) Ca++-ATPase increased with SR PL concentrations of n-6 PUFA, and of cytochrome c oxidase and citrate synthase, indicators of ATP-production, with concentrations of eicosapentaenoic acid in mitochondrial PL. All detected cyclic molecular changes were controlled by photoperiod and are likely of general relevance for mammals living in seasonal environments, including humans. During winter, these changes at the molecular level presumably compensate for Arrhenius effects in the colder peripheral body parts and thus enable a thrifty life at lower body temperature.

Thyroid-stimulating hormone-thyroid hormone signaling contributes to circadian regulation through repressing clock2/npas2 in zebrafish

Thyroid-stimulating hormone (TSH) is important for the thyroid gland, development, growth, and metabolism. Defects in TSH production or the thyrotrope cells within the pituitary gland cause congenital hypothyroidism (CH), resulting in growth retardation and neurocognitive impairment. While human TSH is known to display rhythmicity, the molecular mechanisms underlying the circadian regulation of TSH and the effects of TSH-thyroid hormone (TH) signaling on the circadian clock remain elusive. Here we show that TSH, thyroxine (T4), triiodothyronine (T3), and tshba display rhythmicity in both larval and adult zebrafish and tshba is regulated directly by the circadian clock via both E'-box and D-box. Zebrafish tshba-/- mutants manifest congenital hypothyroidism, with the characteristics of low levels of T4 and T3 and growth retardation. Loss or overexpression of tshba alters the rhythmicity of locomotor activities and expression of core circadian clock genes and hypothalamic-pituitary-thyroid (HPT) axis-related genes. Furthermore, TSH-TH signaling regulates clock2/npas2 via the thyroid response element (TRE) in its promoter, and transcriptome analysis reveals extensive functions of Tshba in zebrafish. Together, our results demonstrate that zebrafish tshba is a direct target of the circadian clock and in turn plays critical roles in circadian regulation along with other functions.

Photoperiodic physiology of summer breeding birds and a search for the role of eye

Photoperiod regulation of gonadal cycles is well studied and documented in both birds and mammals. Change in photoperiod is considered as the most effective and important cue to time the initiation of the annual physiological cycles in birds. Approaching of long days (as observed in summer months), signal long-day breeding birds to initiation reproduction and other related functions. Birds and other non-mammalian vertebrates use the extraocular photoreceptors which may be present in the mediobasal hypothalamus (MBH) or associated regions to measure the photoperiodic time and so are different from mammals where only the eyes are lone photoreceptive organs. The downstream signaling involves thyroid responsive genes playing a crucial role in mediating photoperiodic signals in both birds and mammals. Role of eyes in the avian seasonal cycle has been a questionable issue with evidences both favoring and negating any role. We propose that morphological as well as physiological data argue that retinal photoreceptors can participate in gonadal cycle, at least in the quail and duck. The present review details the studies of photoneuroendocrine control of gonadal axis in birds and review evidences to decipher the role eyes in photoperiodic mediated physiologies in birds.

Multiparametric MR Evaluation of the Photoperiodic Regulation of Hypothalamic Structures in Sheep

Most organisms on earth, humans included, have developed strategies to cope with environmental day-night and seasonal cycles to survive. For most of them, their physiological and behavioral functions, including the reproductive function, are synchronized with the annual changes of day length, to ensure winter survival and subsequent reproductive success in the following spring. Sheep are sensitive to photoperiod, which also regulates natural adult neurogenesis in their hypothalamus. We postulate that the ovine model represents a good alternative to study the functional and metabolic changes occurring in response to photoperiodic changes in hypothalamic structures of the brain. Here, the impact of the photoperiod on the neurovascular coupling and the metabolism of the hypothalamic structures was investigated at 3T using BOLD fMRI, perfusion-MRI and proton magnetic resonance spectroscopy (1H-MRS). A longitudinal study involving 8 ewes was conducted during long days (LD) and short days (SD) revealing significant BOLD, rCBV and metabolic changes in hypothalamic structures of the ewe brain between LD and SD. More specifically, the transition between LD and SD revealed negative BOLD responses to hypercapnia at the beginning of SD period followed by significant increases in BOLD, rCBV, Glx and tNAA concentrations towards the end of the SD period. These observations suggest longitudinal mechanisms promoting the proliferation and differentiation of neural stem cells within the hypothalamic niche of breeding ewes. We conclude that multiparametric MRI studies including 1H-MRS could be promising non-invasive translational techniques to investigate the existence of natural adult neurogenesis in-vivo in gyrencephalic brains.

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.

Biological timekeeping in polar environments: lessons from terrestrial vertebrates

The polar regions receive less solar energy than anywhere else on Earth, with the greatest year-round variation in daily light exposure; this produces highly seasonal environments, with short summers and long, cold winters. Polar environments are also characterised by a reduced daily amplitude of solar illumination. This is obvious around the solstices, when the Sun remains continuously above (polar 'day') or below (polar 'night') the horizon. Even at the solstices, however, light levels and spectral composition vary on a diel basis. These features raise interesting questions about polar biological timekeeping from the perspectives of function and causal mechanism. Functionally, to what extent are evolutionary drivers for circadian timekeeping maintained in polar environments, and how does this depend on physiology and life history? Mechanistically, how does polar solar illumination affect core daily or seasonal timekeeping and light entrainment? In birds and mammals, answers to these questions diverge widely between species, depending on physiology and bioenergetic constraints. In the high Arctic, photic cues can maintain circadian synchrony in some species, even in the polar summer. Under these conditions, timer systems may be refined to exploit polar cues. In other instances, temporal organisation may cease to be dominated by the circadian clock. Although the drive for seasonal synchronisation is strong in polar species, reliance on innate long-term (circannual) timer mechanisms varies. This variation reflects differing year-round access to photic cues. Polar chronobiology is a productive area for exploring the adaptive evolution of daily and seasonal timekeeping, with many outstanding areas for further investigation.

Sleep-light interventions that shift melatonin rhythms earlier improve perimenopausal and postmenopausal depression: preliminary findings

OBJECTIVE

Testing the hypothesis that a sleep-light intervention, which phase-advances melatonin rhythms, will improve perimenopausal-postmenopausal (P-M; by follicle-stimulating hormone) depression.

METHODS

In at-home environments, we compared two contrasting interventions: (1) an active phase-advance intervention: one night of advanced/restricted sleep from 9 pm to 1 am , followed by 8 weeks of morning bright white light for 60 min/d within 30 minutes of awakening, and (2) a control phase-delay intervention: one night of delayed/restricted sleep (sleep from 3 to 7 am ) followed by 8 weeks of evening bright white light for 60 min/d within 90 minutes of bedtime. We tested 17 P-M participants, 9 normal controls and 8 depressed participants (DPs) (by Diagnostic and Statistical Manual of Mental Disorders [Fifth Edition] criteria). Clinicians assessed mood by structured interviews and subjective mood ratings. Participants wore actigraphs to measure sleep and activity and collected overnight urine samples for the melatonin metabolite, 6-sulfatoxymelatonin (6-SMT), before, during, and after interventions.

RESULTS

Baseline depressed mood correlated with delayed 6-SMT offset time (cessation of melatonin metabolite [6-SMT] secretion) ( r = +0.733, P = 0.038). After phase-advance intervention versus phase-delay intervention, 6-SMT offset (start of melatonin and 6-SMT decrease) was significantly advanced in DPs (mean ± SD, 2 h 15 min ± 12 min; P = 0.042); advance in 6-SMT acrophase (time of maximum melatonin and 6-SMT secretion) correlated positively with mood improvement ( r = +0.978, P = 0.001). Mood improved (+70%, P = 0.007) by both 2 and 8 weeks.

CONCLUSIONS

These preliminary findings reveal significantly phase-delayed melatonin rhythms in DP versus normal control P-M women. Phase-advancing melatonin rhythms improves mood in association with melatonin advance. Thus, sleep-light interventions may potentially offer safe, rapid, nonpharmaceutical, well-tolerated, affordable home treatments for P-M depression.

Unveiling Subtle Geographical Clines: Phenotypic Effects and Dynamics of Circadian Clock Gene Polymorphisms

Our understanding of the gene regulatory network that constitutes the circadian clock has greatly increased in recent decades, notably due to the use of Drosophila as a model system. In contrast, the analysis of natural genetic variation that enables the robust function of the clock under a broad range of environments has developed more slowly. In the current study, we analyzed comprehensive genome sequencing data from wild European populations of Drosophila, which were densely sampled through time and space. We identified hundreds of single nucleotide polymorphisms (SNPs) in nine genes associated with the clock, 276 of which exhibited a latitudinal cline in their allele frequencies. While the effect sizes of these clinal patterns were small, indicating subtle adaptations driven by natural selection, they provided important insights into the genetic dynamics of circadian rhythms in natural populations. We selected nine SNPs in different genes and assessed their impact on circadian and seasonal phenotypes by reconstructing outbred populations fixed for either of the SNP alleles, from inbred DGRP strains. The circadian free-running period of the locomotor activity rhythm was affected by an SNP in doubletime (dbt) and eyes absent (Eya). The SNPs in Clock (Clk), Shaggy (Sgg), period (per), and timeless (tim) affected the acrophase. The alleles of the SNP in Eya conferred different levels of diapause and the chill coma recovery response.

Novel Candidate Regulators and Developmental Trajectory of Pituitary Thyrotropes

The pituitary gland regulates growth, metabolism, reproduction, the stress response, uterine contractions, lactation, and water retention. It secretes hormones in response to hypothalamic input, end organ feedback, and diurnal cues. The mechanisms by which pituitary stem cells are recruited to proliferate, maintain quiescence, or differentiate into specific cell types, especially thyrotropes, are not well understood. We used single-cell RNA sequencing in juvenile P7 mouse pituitary cells to identify novel factors in pituitary cell populations, with a focus on thyrotropes and rare subtypes. We first observed cells coexpressing markers of both thyrotropes and gonadotropes, such as Pou1f1 and Nr5a1. This was validated in vivo by both immunohistochemistry and lineage tracing of thyrotropes derived from Nr5a1-Cre; mTmG mice and demonstrates that Nr5a1-progenitors give rise to a proportion of thyrotropes during development. Our data set also identifies novel factors expressed in pars distalis and pars tuberalis thyrotropes, including the Shox2b isoform in all thyrotropes and Sox14 specifically in Pou1f1-negative pars tuberalis thyrotropes. We have therefore used single-cell transcriptomics to determine a novel developmental trajectory for thyrotropes and potential novel regulators of thyrotrope populations.

The gut signals to AGRP-expressing cells of the pituitary to control glucose homeostasis

Glucose homeostasis can be improved after bariatric surgery, which alters bile flow and stimulates gut hormone secretion, particularly FGF15/19. FGFR1 expression in AGRP-expressing cells is required for bile acids' ability to improve glucose control. We show that the mouse Agrp gene has 3 promoter/enhancer regions that direct transcription of each of their own AGRP transcripts. One of these Agrp promoters/enhancers, Agrp-B, is regulated by bile acids. We generated an Agrp-B knockin FLP/knockout allele. AGRP-B-expressing cells are found in endocrine cells of the pars tuberalis and coexpress diacylglycerol lipase B - an endocannabinoid biosynthetic enzyme - distinct from pars tuberalis thyrotropes. AGRP-B expression is also found in the folliculostellate cells of the pituitary's anterior lobe. Mice without AGRP-B were protected from glucose intolerance induced by high-fat feeding but not from excess weight gain. Chemogenetic inhibition of AGRP-B cells improved glucose tolerance by enhancing glucose-stimulated insulin secretion. Inhibition of the AGRP-B cells also caused weight loss. The improved glucose tolerance and reduced body weight persisted up to 6 weeks after cessation of the DREADD-mediated inhibition, suggesting the presence of a biological switch for glucose homeostasis that is regulated by long-term stability of food availability.

Biological timing: Linking the circadian clock to the season

The circadian clock is thought to provide the internal time reference for measuring day length, allowing organisms to prepare in advance for the coming winter and summer. A new study sheds light on the neural link between the circadian clock and seasonal timing.

Seasonal cues act through the circadian clock and pigment-dispersing factor to control EYES ABSENT and downstream physiological changes

Organisms adapt to seasonal changes in photoperiod and temperature to survive; however, the mechanisms by which these signals are integrated in the brain to alter seasonal biology are poorly understood. We previously reported that EYES ABSENT (EYA) shows higher levels in cold temperature or short photoperiod and promotes winter physiology in Drosophila. Nevertheless, how EYA senses seasonal cues is unclear. Pigment-dispersing factor (PDF) is a neuropeptide important for regulating circadian output rhythms. Interestingly, PDF has also been shown to regulate seasonality, suggesting that it may mediate the function of the circadian clock in modulating seasonal physiology. In this study, we investigated the role of EYA in mediating the function of PDF on seasonal biology. We observed that PDF abundance is lower on cold and short days as compared with warm and long days, contrary to what was previously observed for EYA. We observed that manipulating PDF signaling in eya+ fly brain neurons, where EYA and PDF receptor are co-expressed, modulates seasonal adaptations in daily activity rhythm and ovary development via EYA-dependent and EYA-independent mechanisms. At the molecular level, altering PDF signaling impacted EYA protein abundance. Specifically, we showed that protein kinase A (PKA), an effector of PDF signaling, phosphorylates EYA promoting its degradation, thus explaining the opposite responses of PDF and EYA abundance to changes in seasonal cues. In summary, our results support a model in which PDF signaling negatively modulates EYA levels to regulate seasonal physiology, linking the circadian clock to the modulation of seasonal adaptations.

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.

Seasonal Adaptation: Geographic Photoperiod-Temperature Patterns Explain Genetic Variation in the Common Vole Tsh Receptor

The vertebrate photoperiodic neuroendocrine system uses the photoperiod as a proxy to time the annual rhythms in reproduction. The thyrotropin receptor (TSHR) is a key protein in the mammalian seasonal reproduction pathway. Its abundance and function can tune sensitivity to the photoperiod. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; twenty-two intronic and twenty-seven exonic) were found, with a weak or lack of correlation with pairwise geographical distance, latitude, longitude, and altitude. By applying a temperature threshold to the local photoperiod-temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for the spring onset of local primary food production (grass). The obtained pCPP explains the distribution of the genetic variation in Tshr in Western Europe through highly significant correlations with five intronic and seven exonic SNPs. The relationship between pCPP and SNPs was lacking in Eastern Europe. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection in Western European vole populations, resulting in the optimized timing of seasonal reproduction.

Photoperiodic regulation of avian physiology: From external coincidence to seasonal reproduction

Seasonal cycles of environmental cues generate variation in the timing of life-history transition events across taxa. It is through the entrainment of internal, endogenous rhythms of organisms to these external, exogenous rhythms in environment, such as cycling temperature and daylight, by which organisms can regulate and time life history transitions. Here, we review the current understanding of how photoperiod both stimulates and terminates seasonal reproduction in birds. The review describes the role of external coincidence timing, the process by which photoperiod is proposed to stimulate reproductive development. Then, the molecular basis of light detection and the photoperiodic regulation of neuroendocrine timing of seasonal reproduction in birds is presented. Current data indicates that vertebrate ancient opsin is the predominant photoreceptor for light detection by the hypothalamus, compared to neuropsin and rhodopsin. The review then connects light detection to well-characterized hypothalamic and pituitary gland molecules involved in the photoperiodic regulation of reproduction. In birds, Gonadotropin-releasing hormone synthesis and release are controlled by photoperiodic cues via thyrotropin-stimulating hormone-β (TSHβ) independent and dependent pathways, respectively. The review then highlights the role of D-box and E-box binding motifs in the promoter regions of photoperiodic genes, in particular Eyes-absent 3, as the key link between circadian clock function and photoperiodic time measurement. Based on the available evidence, the review proposes that at least two molecular programs form the basis for external coincidence timing in birds: photoperiodic responsiveness by TSHβ pathways and endogenous internal timing by gonadotropin synthesis.

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.

Medaka as a model for seasonal plasticity: Photoperiod-mediated changes in behaviour, cognition, and hormones

Teleosts display the highest level of brain plasticity of all vertebrates. Yet we still know little about how seasonality affects fish behaviour and the underlying cognitive mechanisms since the common neurobehavioral fish models are native to tropical environments where seasonal variation is absent or reduced. The medaka, Oryzias latipes, which inhabits temperate zone habitats, represents a promising model in this context given its large phenotypic changes associated with seasonality and the possibility to induce seasonal plasticity by only manipulating photoperiod. Here, we report the first extended investigation of seasonal plasticity in medaka behaviour and cognition, as well as the potential underlying molecular mechanisms. We compared medaka exposed to summer photoperiod (16 h light:8 h dark) with medaka exposed to winter photoperiod (8 h light:16 h dark), and detected substantial differences. Medaka were more active and less social in summer photoperiod conditions, two effects that emerged in the second half of an open-field and a sociability test, respectively, and might be at least in part related to habituation to the testing apparatus. Moreover, the cognitive phenotype was significantly affected: in the early response to a social stimulus, brain functional lateralisation shifted between the two hemispheres under the two photoperiod conditions, and inhibitory and discrimination learning performance were reduced in summer conditions. Finally, the expression of genes encoding key pituitary hormones, tshß and gh, and of the tshß regulatory transcription factor tef in the brain was increased in summer photoperiod conditions. This work reveals remarkable behavioural and cognitive phenotypic plasticity in response to photoperiod in medaka, and suggests a potential regulatory role for the same hormones involved in seasonal plasticity of other vertebrates.

A cross-taxonomic perspective on the integration of temperature cues in vertebrate seasonal neuroendocrine pathways

The regulation of seasonality has been an area of interest for decades, yet global climate change has created extra urgency in the quest to understand how sensory circuits and neuroendocrine control systems interact to generate flexibility in biological timekeeping. The capacity of temperature to alter endogenous or photoperiod-regulated neuroendocrine mechanisms driving seasonality, either as a direct cue or through temperature-dependent effects on energy and metabolism, is at the heart of this phenological flexibility. However, until relatively recently, little research had been done on the integration of temperature information in canonical seasonal neuroendocrine pathways, particularly in vertebrates. We review recent advances from research in vertebrates that deepens our understanding of how temperature cues are perceived and integrated into seasonal hypothalamic thyroid hormone (TH) signaling, which is a critical regulator of downstream seasonal phenotypic changes such as those regulated by the BPG (brain-pituitary-gonadal) axis. Temperature perception occurs through cutaneous transient receptor potential (TRP) neurons, though sensitivity of these neurons varies markedly across taxa. Although photoperiod is the dominant cue used to trigger seasonal physiology or entrain circannual clocks, across birds, mammals, fish, reptiles and amphibians, seasonality appears to be temperature sensitive and in at least some cases this appears to be related to phylogenetically conserved TH signaling in the hypothalamus. Nevertheless, the exact mechanisms through which temperature modulates seasonal neuroendocrine pathways remains poorly understood.

Vertebrate Phenological Plasticity: from Molecular Mechanisms to Ecological and Evolutionary Implications

Seasonal variation in the availability of essential resources is one of the most important drivers of natural selection on the phasing and duration of annually recurring life-cycle events. Shifts in seasonal timing are among the most commonly reported responses to climate change and the capacity of organisms to adjust their timing, either through phenotypic plasticity or evolution, is a critical component of resilience. Despite growing interest in documenting and forecasting the impacts of climate change on phenology, our ability to predict how individuals, populations, and species might alter their seasonal timing in response to their changing environments is constrained by limited knowledge regarding the cues animals use to adjust timing, the endogenous genetic and molecular mechanisms that transduce cues into neural and endocrine signals, and the inherent capacity of animals to alter their timing and phasing within annual cycles. Further, the fitness consequences of phenological responses are often due to biotic interactions within and across trophic levels, rather than being simple outcomes of responses to changes in the abiotic environment. Here, we review the current state of knowledge regarding the mechanisms that control seasonal timing in vertebrates, as well as the ecological and evolutionary consequences of individual, population, and species-level variation in phenological responsiveness. Understanding the causes and consequences of climate-driven phenological shifts requires combining ecological, evolutionary, and mechanistic approaches at individual, populational, and community scales. Thus, to make progress in forecasting phenological responses and demographic consequences, we need to further develop interdisciplinary networks focused on climate change science.

Effects of spring warming on seasonal neuroendocrinology and activation of the reproductive axis in hibernating arctic ground squirrels

Many animals adjust the timing of seasonal events, such as reproduction, molt, migration, and hibernation, in response to interannual variation and directional climate-driven changes in temperature. However, the mechanisms by which temperature influences seasonal timing are relatively under-explored. Seasonal timing involves retrograde signaling in which thyrotropin (TSH) in the pars tuberalis (PT) alters expression of thyroid hormone (TH) deiodinases (Dio2/Dio3) in tanycyte cells lining the third ventricle of the hypothalamus. This, in turn, affects the availability of triiodothyronine (T3) within the mediobasal hypothalamus - increased hypothalamic T3 restores a summer phenotype and activates the reproductive axis in long-day breeders. Recently, we showed that retrograde TH signaling is activated during late hibernation in arctic ground squirrels (Urocitellus parryii) held in constant darkness and constant ambient temperature. Sensitivity of seasonal pathways to non-photic cues, such as temperature, is likely particularly important to hibernating species that are sequestered in hibernacula during spring. To address this issue, we exposed captive arctic ground squirrels of both sexes to an ecologically relevant increase in ambient temperature (from -6°C to -1°C) late in hibernation and examined the effects of warming on the seasonal retrograde TSH/Dio/T3 signaling pathway, as well as downstream elements of the reproductive axis. We found that warmed males tended to have higher PT TSHβ expression and significantly heavier testis mass whereas the TSH/Dio/T3 signaling pathway was unaffected by warming in females, although warmed females exhibited a slight decrease in ovarian mass. Our findings suggest that temperature could have different effects on gonadal growth in male and female arctic ground squirrels, which could lead to mismatched timing in response to rapid climate change.

Differential temperature effects on photoperiodism in female voles: A possible explanation for declines in vole populations

Many mammalian species use photoperiod as a predictive cue to time seasonal reproduction. In addition, metabolic effects on the reproductive axis may also influence seasonal timing, especially in female small, short-lived mammals. To get a better understanding of how annual cycling environmental cues impact reproductive function and plasticity in small, short-lived herbivores with different geographic origins, we investigated the mechanisms underlying integration of temperature in the photoperiodic-axis regulating female reproduction in a Northern vole species (tundra vole, Microtus oeconomus) and in a Southern vole species (common vole, Microtus arvalis). We show that photoperiod and temperature interact to determine appropriate physiological responses; there is species-dependent annual variation in the sensitivity to temperature for reproductive organ development. In common voles, temperature can overrule photoperiodical spring-programmed responses, with reproductive organ mass being higher at 10°C than at 21°C, whereas in autumn they are less sensitive to temperature. These findings are in line with our census data, showing an earlier onset of spring reproduction in cold springs, while reproductive offset in autumn is synchronized to photoperiod. The reproductive organs of tundra voles were relatively insensitive to temperature, whereas hypothalamic gene expression was generally upregulated at 10°C. Thus, both vole species use photoperiod, whereas only common voles use temperature as a cue to control spring reproduction, which indicates species-specific reproductive strategies. Due to global warming, spring reproduction in common voles will be delayed, perhaps resulting in shorter breeding seasons and thus declining populations, as observed throughout Europe.

Hypothalamic remodeling of thyroid hormone signaling during hibernation in the arctic ground squirrel

Hibernation involves prolonged intervals of profound metabolic suppression periodically interrupted by brief arousals to euthermy, the function of which is unknown. Annual cycles in mammals are timed by a photoperiodically-regulated thyroid-hormone-dependent mechanism in hypothalamic tanycytes, driven by thyrotropin (TSH) in the pars tuberalis (PT), which regulates local TH-converting deiodinases and triggers remodeling of neuroendocrine pathways. We demonstrate that over the course of hibernation in continuous darkness, arctic ground squirrels (Urocitellus parryii) up-regulate the retrograde TSH/Deiodinase/TH pathway, remodel hypothalamic tanycytes, and activate the reproductive axis. Forcing the premature termination of hibernation by warming animals induced hypothalamic deiodinase expression and the accumulation of secretory granules in PT thyrotrophs and pituitary gonadotrophs, but did not further activate the reproductive axis. We suggest that periodic arousals may allow for the transient activation of hypothalamic thyroid hormone signaling, cellular remodeling, and re-programming of brain circuits in preparation for the short Arctic summer.

Arctic ground squirrels hibernating in darkness activate the pars tuberalis - hypothalamus thyroid hormone signaling pathway, remodel hypothalamic tanycytes, and activate the reproductive axis.

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.

Molecular and epigenetic regulation of seasonal reproduction in Terai tree frog (Polypedates teraiensis)

Seasonal breeders predominantly use photoperiod as the predictable environmental cue to time their reproduction. Terai tree frogs are long-day seasonal breeders, but the molecular mechanism is unknown. We tested the role of different photoperiodic conditions on expression levels of candidate genes involved in seasonal reproduction and epigenetic regulation. Four experiments were performed. In experiment 1, frogs were exposed to long (LD: 16L:8D) or short photoperiod (SD: 8L:16D). In experiment 2, animals were procured at four different phases of breeding, i.e., during April (emergence just after hibernation), June (breeding phase), August (post-breeding), and October (just before hibernation). In experiments 3 and 4, frogs were exposed to equinox photoperiod but different (10, 100, or 500 lx) light intensities (exp. 3) or wavelength (red: 640 nm, green: 540 nm, blue: 450 nm or white; exp. 4). After 2 weeks, animals were euthanized, and their brain was harvested. mRNA levels of transcripts involved in photoperiodic transduction (Eya3 and Opn5), reproduction (Tshß, GnRH, Dio2, and Dio3), and epigenetics regulation (Dnmt1, Dnmt3a, Hdac1, Hdac3, and Tet2) were measured. Results show that LD promotes the upregulation of Eya3, Opn5, Tshß, GnRH, and Dio2. Differential expression of Opn5 during LD and SD suggests its involvement in light perception. Dio3 levels were upregulated in SD (exp.1) and during the post-breeding phase (exp. 2). These results employ the limited role of light intensity and spectrum in reproduction. This is the first study showing molecular machinery involved in the amphibian system's seasonal reproduction and epigenetic regulation.

Comparative transcriptomics of the Djungarian hamster hypothalamus during short photoperiod acclimation and spontaneous torpor

The energy-saving strategy of Djungarian hamsters (Phodopus sungorus, Cricetidae) to overcome harsh environmental conditions comprises of behavioral, morphological, and physiological adjustments, including spontaneous daily torpor, a metabolic downstate. These acclimatizations are triggered by short photoperiod and orchestrated by the hypothalamus. Key mechanisms of long-term photoperiodic acclimatizations have partly been described, but specific mechanisms that acutely control torpor remain incomplete. Here, we performed comparative transcriptome analysis on hypothalamus of normometabolic hamsters in their summer- and winter-like state to enable us to identify changes in gene expression during photoperiodic acclimations. Comparing nontorpid and torpid hamsters may also be able to pin down mechanisms relevant for torpor control. A de novo assembled transcriptome of the hypothalamus was generated from hamsters acclimated to long photoperiod or to short photoperiod. The hamsters were sampled either during long photoperiod normothermia, short photoperiod normothermia, or short photoperiod-induced spontaneous torpor with a body temperature of 24.6 ± 1.0 °C, or. The mRNA-seq analysis revealed that 32 and 759 genes were differentially expressed during photoperiod or torpor, respectively. Biological processes were not enriched during photoperiodic acclimatization but were during torpor, where transcriptional and metabolic processes were reinforced. Most extremely regulated genes (those genes with |log2(FC)| > 2.0 and padj < 0.05 of a pairwise group comparison) underpinned the role of known key players in photoperiodic comparison, but these genes exhibit adaptive and protective adjustments during torpor. Targeted analyses of genes from potentially involved hypothalamic systems identified gene regulation of previously described torpor-relevant systems and a potential involvement of glucose transport.

Circadian clocks and their role in lactation competence

Circadian rhythms are 24 h cycles of behavior, physiology and gene expression that function to synchronize processes across the body and coordinate physiology with the external environment. Circadian clocks are central to maintaining homeostasis and regulating coordinated changes in physiology in response to internal and external cues. Orchestrated changes occur in maternal physiology during the periparturient period to support the growth of the fetus and the energetic and nutritional demands of lactation. Discoveries in our lab made over a decade ago led us to hypothesize that the circadian timing system functions to regulate metabolic and mammary specific changes that occur to support a successful lactation. Findings of studies that ensued are summarized, and point to the importance of circadian clocks in the regulation of lactation competence. Disruption of the circadian timing system can negatively affect mammary gland development and differentiation, alter maternal metabolism and impair milk production.

Neuroendocrine Regulation of Reproductive Dormancy in the Fruit Fly Drosophila melanogaster: A Review of Juvenile Hormone-Dependent Regulation

Animals can adjust their physiology, helping them survive and reproduce under a wide range of environmental conditions. One of the strategies to endure unfavorable environmental conditions such as low temperature and limited food supplies is dormancy. In some insect species, this may manifest as reproductive dormancy, which causes their reproductive organs to be severely depleted under conditions unsuitable for reproduction. Reproductive dormancy in insects is induced by a reduction in juvenile hormones synthesized in the corpus allatum (pl. corpora allata; CA) in response to winter-specific environmental cues, such as low temperatures and short-day length. In recent years, significant progress has been made in the study of dormancy-inducing conditions dependent on CA control mechanisms in Drosophila melanogaster. This review summarizes dormancy control mechanisms in D. melanogaster and discusses the implications for future studies of insect dormancy, particularly focusing on juvenile hormone-dependent regulation.

Photoperiod integration in C3H rd1 mice

In mammals, the suprachiasmatic nuclei (SCN) constitute the main circadian clock, receiving input from the retina which allows synchronization of endogenous biological rhythms with the daily light/dark cycle. Over the year, the SCN encodes photoperiodic variations through duration of melatonin secretion, with abundant nocturnal levels in winter and lower levels in summer. Thus, light information is critical to regulate seasonal reproduction in many species and is part of the central photoperiodic integration. Since intrinsically photosensitive retinal ganglion cells (ipRGCs) are vital for circadian photoentrainment and other nonvisual functions, we studied the contribution of ipRGCs in photoperiod integration in C3H retinal degeneration 1 (rd1) mice. We assessed locomotor activity and melatonin secretion in mice exposed to short or long photoperiods. Our results showed that rd1 mice are still responsive to photoperiod variations in term of locomotor activity, melatonin secretion, and regulation of the reproductive axis. In addition, retinas of animals exposed to short photoperiod exhibit higher melanopsin labeling intensity compared with the long photoperiod condition, suggesting seasonal-dependent changes within this photoreceptive system. These results show that ipRGCs in rd1 mice can still measure photoperiod and suggest a key role of melanopsin cells in photoperiod integration and the regulation of seasonal physiology.

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.

Seasonal differences in hypothalamic thyroid-stimulating hormone β, gonadotropin-releasing hormone-I and deiodinase expression between migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in the deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to type 2 (DIO2). This reciprocal switch is considered to be mediated by increasing thyroid-stimulating hormone β (TSHβ) release from the pars tuberalis, which occurs in response to a change in photoperiod. Although well characterised in a handful of model organisms in controlled laboratory settings, this pathway remains largely unexplored in free-living animals under natural environmental conditions. In this comparative gene expression study, we investigated hypothalamic thyroid hormone signalling in two seasonally breeding subspecies of white-crowned sparrow (Zonotrichia leucophrys), across the entirety of their annual cycles. The migratory Gambel's (Z. l. gambelii) and resident Nuttall's (Z. l. nuttalii) subspecies differ with respect to timing of reproduction, as well as life history stage and migratory strategies. Although DIO3 mRNA expression was elevated and DIO2 mRNA expression was reduced in the wintering period in both subspecies, DIO2 peaked in both subspecies prior to the onset of reproduction. However, there was differential timing between subspecies in peak DIO2 expression. Intriguingly, seasonal modulation of TSHβ mRNA was only observed in migrants, where expression was elevated at the start of breeding, consistent with observations from other highly photoperiodic species. There was no correlation between TSHβ, DIO2 and gonadotropin-releasing hormone-I mRNA or reproductive metrics in residents. Based on these observed differences, we discuss potential implications for our understanding of how changes in medial basal hypothalamic gene expression mediates initiation of seasonal reproduction.

Long photoperiod impairs learning in male but not female medaka

Day length in conjunction with seasonal cycles affects many aspects of animal biology. We have studied photoperiod-dependent alterations of complex behavior in the teleost, medaka (Oryzias latipes), a photoperiodic breeder, in a learning paradigm whereby fish have to activate a sensor to obtain a food reward. Medaka were tested under a long (14:10 LD) and short (10:14 LD) photoperiod in three different groups: mixed-sex, all-males, and all-females. Under long photoperiod, medaka mixed-sex groups learned rapidly with a stable response. Unexpectedly, males-only groups showed a strong learning deficit, whereas females-only groups performed efficiently. In mixed-sex groups, female individuals drove group learning, whereas males apparently prioritized mating over feeding behavior resulting in strongly reduced learning performance. Under short photoperiod, where medaka do not mate, male performance improved to a level similar to that of females. Thus, photoperiod has sex-specific effects on the learning performance of a seasonal vertebrate.

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.