Researchers at the University of California, Davis, led by Sara K. Pintwala and Patrick M. Fuller, used single-nucleus RNA sequencing to profile individual neurons from the lateral hypothalamus of adult female and male mice. The technique captures the complete transcriptome — the full inventory of genes being actively read — from thousands of individual cell nuclei at once. Tissue was collected precisely at the onset of the dark phase, zeitgeber time 12, a biologically charged moment when nocturnal animals transition from rest to activity and when competing drives like feeding, exploration, and wakefulness surge together.

The team sequenced more than 85,000 nuclei in total, ultimately isolating 33,235 high-quality neuronal nuclei — 16,432 from females and 16,803 from males. After correcting for ambient RNA contamination, removing doublets, and integrating the datasets to control for batch effects, the researchers asked a deceptively simple question: do male and female lateral hypothalamic neurons differ in the genes that build and receive circadian signals?

The first surprise was what they did not find. When the researchers clustered all the nuclei together using unsupervised computational methods, the analysis revealed 24 distinct neuronal clusters — and every single one contained cells from both sexes. There was no evidence of sex-specific neuron classes, and the relative proportions of major cell types were nearly identical between males and females. Glutamatergic, GABAergic, orexin-producing, and melanin-concentrating hormone-associated populations were all represented at matching ratios, with odds ratios hovering close to 1.0. Structurally, the male and female lateral hypothalamus looked like the same brain region.

But beneath that conserved architecture, the molecular machinery told a different story. The researchers focused on a curated panel of clock and circadian signaling genes: the core activators Clock, Arntl (which encodes BMAL1), and Npas2; the repressors Per1, Per2, Per3, Cry1, and Cry2; auxiliary regulators such as Nr1d1, Nr1d2, and the ROR family; and receptors for the suprachiasmatic nucleus’s signature neuropeptides, including vasoactive intestinal peptide (VIP), arginine vasopressin (AVP), gastrin-releasing peptide, neuromedin S, cholecystokinin, and melatonin. Differential expression testing, corrected for false discovery, uncovered 15 genes whose expression differed significantly between the sexes.

The pattern of those differences was strikingly coherent. Female-derived neurons were enriched for the vasopressin receptor Avpr1a, the melatonin receptor Mtnr1b, the clock repressors Per2 and Per3, and the auxiliary regulators Nr1d1, Nr1d2, and Rora. Male-derived neurons, by contrast, showed enrichment for the VIP receptor Vipr2, along with Nmur2 and Cckar — receptors for neuromedin and cholecystokinin signaling — and, notably, for the core positive-limb oscillator genes Clock, Npas2, and Arntl, plus Per1, Cry1, and Rorb. In other words, the two sexes appeared to weight different limbs of the same circadian feedback loop, and different neuromodulatory input channels, across shared neuronal populations.

These motifs repeated across cell types. In glutamatergic neurons, females preferentially expressed AVP and melatonin-related transcripts while males emphasized VIP and CCK receptors alongside Npas2 and Clock. In GABAergic neurons, female enrichment again centered on Nr1d2, Per2, Per3, Rora, and the melatonin receptor Mtnr1a, while males showed elevated Nmur2, Rorb, Vipr2, Per1, Cry1, Arntl, and Clock. Among the peptidergic populations, differences were subtler but still present: Npas2 was enriched in female orexin neurons, while male melanin-concentrating hormone neurons showed enrichment for Rora, Cry1, Arntl, Npas2, and Mtnr1a. The consistency across excitatory, inhibitory, and neuropeptide-specific populations suggests a coordinated, system-level shift in how temporal information is processed, rather than random noise.

The functional implications are considerable. AVP signaling from the suprachiasmatic nucleus is a principal conduit of circadian output, critical for synchrony and phase regulation, while VIP is essential for keeping pacemaker neurons firing in unison. Neuromedin S-producing neurons act as pacemakers that couple clock cells across the circadian network, and cholecystokinin pathways have been implicated in circadian adaptation under changing photoperiods. If female lateral hypothalamic neurons are transcriptionally primed to receive vasopressin-dominant signals while male neurons favor VIP, neuromedin, and CCK inputs, then the same upstream timing information from the master clock could be translated into different downstream behavioral and physiological outputs in each sex — different thresholds for arousal, feeding, stress responsiveness, and sleep-wake transitions at the same time of day.

The study also mapped sex hormone receptor expression across all lateral hypothalamic neuron types, finding that androgen receptor transcripts were most abundant, estrogen receptor 1 moderate, and estrogen receptor 2 lowest, in both sexes. Because estrogen receptor 1 functions as a ligand-activated transcription factor known to drive expression of Per2 and Clock in hypothalamic neurons, its widespread presence identifies a concrete molecular route through which circulating steroids could reshape circadian transcription in a sex-dependent manner. This aligns with earlier work showing that androgens alter the magnitude of light-induced phase shifts and that estrogen signaling stabilizes firing synchrony in the suprachiasmatic nucleus.

The authors are careful about what their data can and cannot show. Because tissue was sampled at a single circadian timepoint in gonad-intact animals without estrous-cycle staging, the findings cannot establish whether the observed differences reflect stable sex dimorphism, fluctuating endocrine state, circadian phase, or interactions among all three. Nor can a snapshot at dark-phase onset reveal oscillatory amplitude or phase relationships across the full 24-hour cycle. What the study does establish is a framework: sexual dimorphism in the lateral hypothalamus emerges not through fundamentally distinct neuronal populations but through subtle, reproducible, and coordinated shifts in molecular signaling architecture — a differential tuning of shared circuits. As circadian and sleep medicine increasingly confront sex differences in rhythm period, amplitude, and disease vulnerability, the message is clear: the same clock may tick in everyone, but males and females may hear it through different molecular ears.

Subject of Research: Sex differences in circadian clock gene and neuropeptide receptor expression in the lateral hypothalamus

Article Title: Sex-associated circadian signaling signatures in the lateral hypothalamus

Article References: Pintwala, S. K., & Fuller, P. M. (2026). Sex-associated circadian signaling signatures in the lateral hypothalamus. Frontiers in Sleep, 5, 1895831. https://doi.org/10.3389/frsle.2026.1895831

Image Credits: AI Generated

DOI: 10.3389/frsle.2026.1895831

Keywords: lateral hypothalamus, circadian rhythms, single-nucleus RNA sequencing, sex differences, suprachiasmatic nucleus, orexin neurons, melanin-concentrating hormone, vasoactive intestinal peptide, arginine vasopressin, clock genes, hypothalamus, sleep-wake regulation

The discovery matters because the evolutionary origin of endogenous, self-sustaining rhythms has long been murky. Complex transcriptional clock networks are well characterized in flowering plants, built from interlocked feedback loops involving genes such as CCA1, LHY, TOC1, GIGANTEA, and the evening complex components ELF3, ELF4, and LUX. But when and how such temporal coordination emerged during the transition from aquatic algae to terrestrial flora has remained an open question. Chara braunii, whose genome was fully sequenced in 2018, occupies a pivotal phylogenetic position near the base of the streptophyte lineage, making it an ideal organism in which to probe the deep history of plant circadian biology.

The research team cultivated Chara braunii from germinated oospores and grew young thalli under a regular 14-hour light, 10-hour dark cycle. Using a custom-built imaging chamber fitted with LED strips and a smartphone camera capturing frames every ten minutes, they tracked the angle between each branchlet and its basal node over three consecutive days. The results were striking: every morning, about an hour after lights came on, the branchlets began to unfurl from a nearly vertical position, opening to angles between 130 and 180 degrees by midday. As evening approached, they folded back toward vertical, settling at angles between 90 and 110 degrees. Statistical analysis with the JTK_CYCLE algorithm confirmed significant rhythmicity with a consistent midday peak, and all eight biological replicates remained significant after correction for multiple testing.

Crucially, the team then transferred the algae to continuous light to see whether the rhythm would persist without any external day-night signal. The answer depended dramatically on brightness. Under continuous regular light of 30 micromoles of photons per square meter per second, the branchlets initially closed, then partially reopened and settled into a stable, semiopen position. No statistically significant circadian oscillation could be detected. But under continuous dim light at half that intensity, the rhythmic opening and closing continued for the full 60-hour recording period, with highly significant periodicity. In other words, the alga’s internal rhythm did not vanish under constant conditions—it was masked or damped by brighter light, a phenomenon also documented in Arabidopsis, where continuous light suppresses rhythmic outputs despite an intact clock.

The researchers propose a plausible physical mechanism for this irradiance effect. Under dim light, a sufficient brightness gradient may persist between the upper and lower sides of each branchlet, maintaining the differential elongation that earlier studies in other Chara species suggested drives bending. Under brighter light, photons may penetrate more effectively through the chloroplast layer, erasing the asymmetry between the two sides of the branchlet. With more uniform illumination, growth becomes more even, and the visible oscillation disappears. The authors caution, however, that branchlet angle is a geometric readout that cannot distinguish growth-driven changes from reversible, turgor-driven movements, and both processes may contribute to the observed rhythm.

The team also examined how the alga acclimates physiologically to continuous illumination. Pigment analysis revealed that chlorophyll a, chlorophyll b, and carotenoid concentrations rose significantly under dim continuous light after 36 hours, consistent with a classic low-light acclimation response that boosts light-harvesting capacity. By 60 hours, absolute pigment levels had declined again, leaving only the chlorophyll a to chlorophyll b ratio elevated. Photosynthetic parameters measured by pulsed-amplitude modulation chlorophyll fluorometry—including maximum electron transport rate, the initial slope of the photosynthesis-irradiance curve, and the light saturation point—showed no detectable temporal variation across days or times of day, suggesting that rhythmic regulation of photosynthesis in this organism, if present, is process-specific rather than global.

Metabolite profiling added another layer of insight. Using liquid chromatography tandem mass spectrometry, the researchers quantified 35 primary metabolites across three sampling points after transfer to continuous light. Under dim light, metabolites related to carbon and nitrogen metabolism accumulated steadily, with malate, fumarate, nicotinate, and glutamate significantly elevated at 36 hours, and malate, fumarate, nicotinate, and alanine remaining high at 60 hours. Under brighter continuous light, changes were smaller and more transient: serine, phenylalanine, and alpha-aminobutyrate rose at 36 hours, while pyruvate declined by 60 hours. The serine increase likely reflects intensified photorespiration, a hallmark of metabolic stress, while the persistent accumulation of TCA-cycle intermediates under dim light suggests enhanced anabolic activity. The divergence between treatments points to an altered carbon-nitrogen balance at higher irradiance and hints that continuous bright light decouples metabolic outputs from temporal control.

Perhaps most intriguingly, a phylogenetic survey of the Chara braunii genome revealed homologs of several major land-plant clock-associated gene families, though with strikingly incomplete conservation. The REVEILLE family is represented by a single protein carrying the characteristic MYB DNA-binding domain but lacking the full architecture of CCA1 or LHY. A putative PRR homolog retains only the CCT domain and cannot be classified as a canonical PRR. GIGANTEA is well conserved in Chara and other late-diverging streptophytes, while the evening complex components ELF3, ELF4, and LUX are all present and broadly conserved. Notably, no canonical ZEITLUPE ortholog with both LOV and Kelch domains was found—only a related ADAGIO-family F-box protein—suggesting that light-input pathways in Chara differ fundamentally from those of flowering plants.

Together, these findings sketch a compelling picture: an endogenous regulatory component capable of driving circadian branchlet movements existed in the streptophyte ancestors of land plants, but its observable output is modulated by light intensity, and its molecular machinery was assembled from parts that only partially overlap with the canonical angiosperm clock. The co-occurrence of attenuated rhythmic movement and a distinct metabolic profile under brighter continuous light provides a framework for future mechanistic work, even though the present data cannot establish a causal link between the two. For evolutionary biologists, the message is tantalizing: the seeds of the plant circadian clock were already sown in aquatic algae, long before the conquest of land, waiting for brighter light—or better experiments—to reveal their full rhythm.

Subject of Research: Circadian regulation of branchlet movement and irradiance-dependent acclimation in the charophyte alga Chara braunii

Article Title: Circadian Oscillations in Branchlet Angle and Irradiance‐Dependent Acclimation in Chara braunii Under Continuous Light

Article References: Maidel, A.-M., Kurtović, K., Heise, C. M., Vosolsobě, S., Petrášek, J., Timm, S., & Schubert, H. (2026). Circadian Oscillations in Branchlet Angle and Irradiance‐Dependent Acclimation in Chara braunii Under Continuous Light. Plant Direct, 10(10), Article e70194. https://doi.org/10.1002/pld3.70194

Image Credits: AI Generated

DOI: 10.1002/pld3.70194

Keywords: Chara braunii, circadian clock, charophyte algae, branchlet movement, continuous light, irradiance acclimation, chlorophyll, metabolomics, streptophyte evolution, plant physiology, circadian rhythm, clock genes

Obesity has long been recognized as a state of chronic low-grade inflammation, a smoldering immune activation that contributes to insulin resistance, fatty liver disease, and cardiovascular risk. At the same time, obesity is marked by circadian disruption: the molecular clocks that orchestrate daily cycles of metabolism, hormone secretion, and immune activity fall out of sync. What has remained unclear is whether these two phenomena are merely parallel consequences of excess weight or whether they are mechanistically intertwined, and, crucially, whether a dietary intervention can pull both back into alignment at the same time.

The new study, led by Claudia Pivonello and senior author Annamaria Colao, set out to address exactly that question. Thirty-eight adults with obesity completed a twelve-week hypocaloric Mediterranean diet program. Before and after the intervention, the researchers collected detailed anthropometric and metabolic measurements, including body weight, body mass index, waist circumference, fat mass percentage, fat-free mass, muscle mass, and HDL cholesterol. They also assessed each participant’s chronotype using the Morningness–Eveningness Questionnaire, a validated instrument that captures whether a person’s natural tendencies lean toward morning or evening activity.

The most technically ambitious part of the study involved the participants’ peripheral blood mononuclear cells, or PBMCs, a mixed population of immune cells that includes lymphocytes and monocytes. Blood samples were drawn at two fixed time points, eight in the morning and four in the afternoon, allowing the researchers to track how gene expression shifted across the day. Using quantitative reverse-transcription PCR, they measured the expression of five core clock genes: BMAL1, PER1, PER2, CRY1, and CRY2. These genes form the interlocking feedback loops that constitute the molecular circadian oscillator, with BMAL1 acting as the positive driving element and the PER and CRY proteins as the negative limbs that restrain its activity on a roughly twenty-four-hour cycle. Alongside the clock genes, the team quantified expression of two innate immune receptors, TLR4 and TLR8, members of the Toll-like receptor family that detect microbial components and trigger inflammatory signaling.

The metabolic results were consistent with what clinicians have come to expect from a well-executed Mediterranean diet intervention. Participants showed significant reductions in body weight, body mass index, waist circumference, and fat mass percentage, together with increases in fat-free mass, muscle mass, and HDL cholesterol. Their MEQ scores also rose significantly, indicating a shift toward greater morningness, although the distribution of chronotype categories did not change in a statistically significant way. In other words, participants drifted somewhat toward morning preference without crossing into a new chronotype classification.

The molecular findings were more striking. At baseline, before any dietary change, expression of TLR4 and TLR8 in PBMCs was significantly higher in the afternoon than in the morning, a diurnal pattern in innate immune receptor expression that the researchers documented for the first time in this cohort. After twelve weeks of the Mediterranean diet, the picture had changed in a coordinated fashion. BMAL1 expression increased significantly in the morning samples, while PER1, TLR4, and TLR8 expression decreased significantly in the afternoon samples. Moreover, all five clock genes examined, BMAL1, PER1, PER2, CRY1, and CRY2, displayed a more pronounced morning-to-afternoon expression pattern following the intervention, which the authors interpret as evidence of improved diurnal organization of the peripheral clock.

Perhaps the most intriguing observation was the correlation structure linking the two systems. The researchers found significant positive correlations between BMAL1 expression and both TLR4 and TLR8 expression at both sampling time points. This statistical relationship supports the hypothesis of a bidirectional interaction between the molecular clock and innate immune pathways, a connection that has been suggested by animal and cellular studies but rarely examined in the context of a human dietary intervention. If BMAL1 activity helps govern the daily rhythm of Toll-like receptor expression, then dietary signals that strengthen the clock could, in principle, dampen the inappropriate immune activation that characterizes obesity.

The authors frame their results as evidence of partial restoration of diurnal–immune homeostasis. Increased morning BMAL1 together with reduced afternoon TLR4 and TLR8 suggests that the intervention did not simply suppress inflammation indiscriminately but instead rebalanced the temporal architecture of immune readiness. They propose that dietary modulation of what they call the BMAL1–TLR axis may represent a mechanism linking metabolic improvement to diurnal regulation, and they identify peripheral clock gene expression as a potential translational biomarker of response to lifestyle interventions. If validated, such a biomarker could allow clinicians to monitor, at the level of gene expression in a simple blood draw, whether a patient’s body clock and immune system are responding to dietary therapy.

Important caveats temper these conclusions. The study lacked a comparator arm, meaning there was no control group of patients with obesity who did not receive the intervention or who received an alternative diet. Without such a group, the findings must be interpreted as associations rather than proof of causation, and the authors are explicit on this point, calling for confirmation in randomized controlled studies. The sample size of thirty-eight participants, while adequate for an exploratory interventional design, is modest, and the two-time-point sampling strategy captures only a coarse snapshot of the full twenty-four-hour rhythm. The study was registered at ClinicalTrials.gov as NCT06236932 and was approved by the Ethical Committee of Federico II University of Naples, and it was funded by the Italian Ministry of University and Research through a Projects of Relevant National Interest grant.

Even with those limitations, the study adds a compelling piece to a rapidly growing body of work on the circadian–immune axis. It suggests that the Mediterranean diet, already celebrated for its cardiovascular and metabolic benefits, may exert part of its influence by entraining the peripheral clocks embedded in immune cells and by smoothing out the aberrant daily oscillations of innate immune receptors. For patients with obesity, the practical message is familiar but now carries deeper mechanistic weight: what you eat, and presumably when you eat it, may help decide not only how much you weigh but how well your body’s internal timekeeping and immune surveillance work together. For researchers, the message is that the blood-borne clock may be a readable, and potentially modifiable, target of nutritional therapy.

Subject of Research: Effects of a hypocaloric Mediterranean diet on circadian clock gene and innate immune receptor expression in patients with obesity

Article Title: A 12-week hypocaloric Mediterranean diet is associated with metabolic improvement and modulation of diurnal clock gene and innate immune receptor expression in peripheral blood mononuclear cells of patients with obesity

Article References: Pivonello, C., Magnacca, N., Graziadio, C., Negri, M., Vetrani, C., Amatrudo, F., Vozza, E., Vecchio, G. D., Tini, S., Celano, S., Prodam, F., Auriemma, R. S., Pivonello, R., & Colao, A. (2026). A 12-week hypocaloric Mediterranean diet is associated with metabolic improvement and modulation of diurnal clock gene and innate immune receptor expression in peripheral blood mononuclear cells of patients with obesity. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-09013-0

Image Credits: AI Generated

DOI: 10.1186/s12967-026-09013-0

Keywords: Mediterranean diet, obesity, circadian rhythm, clock genes, BMAL1, Toll-like receptors, TLR4, TLR8, peripheral blood mononuclear cells, inflammation, chronotype, nutritional intervention