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Depression is a leading cause of disability. Current pharmacological treatment of depression is insufficient, and development of improved treatments especially for treatment-resistant depression is desired. Understanding the neurobiology of antidepressant actions may lead to development of improved therapeutic approaches. Here, we demonstrate that dopamine D1 receptors in the dentate gyrus act as a pivotal mediator of antidepressant actions in mice. Chronic administration of a selective serotonin reuptake inhibitor (SSRI), fluoxetine, increases D1 receptor expression in mature granule cells in the dentate gyrus. The increased D1 receptor signaling, in turn, contributes to the actions of chronic fluoxetine treatment, such as suppression of acute stress-evoked serotonin release, stimulation of adult neurogenesis and behavioral improvement. Importantly, under severely stressed conditions, chronic administration of a D1 receptor agonist in conjunction with fluoxetine restores the efficacy of fluoxetine actions on D1 receptor expression and behavioral responses. Thus, our results suggest that stimulation of D1 receptors in the dentate gyrus is a potential adjunctive approach to improve therapeutic efficacy of SSRI antidepressants.

Major depressive disorder (MDD) lacks reliable laboratory tests for diagnosis and treatment monitoring, underscoring the need for robust molecular readouts in blood. Beyond symptom-based classification, MDD can also be viewed as a condition involving impaired homeostatic regulation across stress-responsive, immune, metabolic, and neural systems. Consistent with this perspective, altered intron retention (IR) patterns have been observed in peripheral blood in depression-related and treatment-response contexts, supporting the translational relevance of this RNA-processing layer to mood disorders. A key observation underpinning this review is that IR can function as a reversible, intervention-responsive readout of physiological state. In a pre-symptomatic stress-like state in klotho mutant mice (a premature-aging model), widespread IR increases revert toward a healthy pattern upon treatment, suggesting that IR is embedded in a controllable homeostatic layer. Against the backdrop of limited cross-cohort transferability of differential gene expression (DGE) signatures, we propose that IR provides a mechanistically grounded biomarker layer because it reports regulated RNA processing states rather than context-fragile abundance endpoints. We operationalize IR as a post-transcriptional “throttle” on effective gene output, with increased IR/detained intron (DI) states acting as a reversible brake and decreased IR acting as an accelerator that increases translation-competent mRNA supply. Mechanistic exemplars across immune, metabolic, and neuronal systems (e.g., IFNG, OGT, MAT2A, neuronal activity-triggered intron excision, and intron detention-mediated stemness/differentiation switching in adult neural stem cells) show that defined inputs can switch IR/DI states to tune output kinetics. Integrating these findings, we propose an “Intron Retention Homeostat” (IR-Homeostat) model in which cells sense deviations from physiological set points and implement feedback control of gene output through switchable IR/DI regulation. This framework positions IR not only as a robust state readout for stratification, treatment response prediction, and pharmacodynamic profiling, but also as a tractable entry point to identify the molecular sensors and mediators that couple homeostatic signals to RNA processing control.

Clinically robust molecular biomarkers for depression have remained elusive, despite extensive transcriptomic research. This gap is consequential: depression is prevalent and heterogeneous, yet objective measures to quantify burden, stratify patients, and track recovery remain limited. Here, we review evidence that intron retention (IR) can serve as a homeostatic state variable—and therefore a sensitive biomarker—reporting stress adaptation and recovery at an upstream regulatory layer, often preceding or outperforming differential gene expression (DEG) readouts. Mechanistically, IR enables bidirectional fine-tuning of effective gene output: increased IR (IncIR) can throttle output under overload, whereas decreased IR (DecIR) releases this brake to restore gene output. Because these shifts are reversible and treatment-responsive, IR signatures can function not only as disease markers but also as pharmacodynamic metrics for blood-based monitoring of drug response and recovery. To evaluate the clinical utility of IR, we use depression as a proof of concept and focus on two interventions: (i) the Kampo formula hangekobokuto (HKT), which is associated with IR normalization consistent with reduced peripheral inflammatory load; and (ii) ketamine, where IR patterns measured before ketamine treatment in non-responders are linked to stronger innate-immune/antiviral activity, suggesting a higher inflammatory load that may limit treatment benefit. Finally, we discuss transdiagnostic extensions beyond depression, using early cognitive decline (mild cognitive impairment, MCI) as a stringent, biologically distal test case for blood-based IR/DI readouts and motivating independent cohort replication and longitudinal validation.

The traditional Japanese medicine Hangeshashinto (HST) is a multicompound drug used for stomatitis. The identification of HST's active ingredients is essential for understanding its mechanism of action and establishing pharmacological quality control markers, but remains unclear due to its multicomponent drug. To systematically explore anti-inflammatory ingredients in HST using a combined approach involving a cell-based bioassay and multicomponent analysis, and to identify potential quality marker compounds. A cell-based bioassay modeling stomatitis was established using human oral keratinocytes (HOK), with interleukin (IL)-1β-induced prostaglandin E2 (PGE2) as an inflammatory indicator. The PGE2 inhibitory effects of quality-controlled 101 HST manufacturing lots were compared. Multicomponent analysis was performed on the 101 HST samples, correlating the intensity of 121 component peaks with the pharmacological activities. Ingredients with the highest correlation coefficients were validated for their PGE2 inhibitory effects. A total of 101 HST samples (30 µg/mL: approximately IC ) showed consistent inhibition of IL-1β-induced PGE2 production in HOK (41.33%-67.38% with 100% as IL-1β). Correlation analysis between PGE2 inhibitory activities and peak intensity of 121 components revealed the contribution of each ingredient to the pharmacological effect. Eight ingredients (glycycoumarin, neoglycyrol, [6]-shogaol, [8]-shogaol, [10]-shogaol, [6]-gingerol, [8]-gingerol, and [10]-gingerol) with the highest correlation coefficients (below -0.30) exhibited PGE2-inhibitory potential. The standardization of current quality control markers contributes to the stability of HST's anti-inflammatory effects. The eight components discovered by this integrated analysis method may become novel quality control marker ingredients for further improving the pharmacological quality of HST.

The greater part of the striatum is composed of striosomes and matrix compartments, but we recently demonstrated the presence of a region that has a distinct structural organization in the ventral half of the mouse caudal striatum (Miyamoto et al. in Brain Struct Funct 223:4275–4291, 2018). This region, termed the tri-laminar part based upon its differential immunoreactivities for substance P and enkephalin, consists of medial, intermediate, and lateral divisions. In this study, we quantitatively analyzed the distributions of both projection neurons and interneurons in each division using immunohistochemistry. Two types of projection neurons expressing either the dopamine D1 receptor (D1R) or D2 receptor (D2R) showed complementary distributions throughout the tri-laminar part, but the proportions significantly differed among the three divisions. The proportion of D1R-expressing neurons in the medial, intermediate, and lateral divisions was 88.6 ± 8.2% (651 cells from 3 mice), 14.7 ± 3.8% (1025 cells), and 49.3 ± 4.5% (873 cells), respectively. The intermediate division was further characterized by poor innervation of tyrosine hydroxylase immunoreactive axons. The numerical density of choline acetyltransferase immunoreactive neurons differed among the three divisions following the order from the medial to lateral divisions. In contrast, PV-positive somata were distributed throughout all three divisions at a constant density. Two types of GABAergic interneurons labeled for nitric oxide synthase and calretinin showed the highest cell density in the medial division. The present results characterize the three divisions of the mouse caudal striatum as distinct structures, which will facilitate studies of novel functional loops in the basal ganglia.

Hypomyelinating leukodystrophies (HLDs) are a group of hereditary CNS disorders characterized by hypomyelination and, sometimes, repeated cycles of demyelination and remyelination. In HLDs, various genetic mutations in the responsible genes disrupt the morphogenesis of oligodendrocytes (oligodendroglial cells), which wrap neuronal axons with their differentiated myelin sheaths. A stop-loss mutation (c.622T-C or c.622T-G) in the gene encoding claudin family tetraspan plasma membrane protein claudin-11 (CLDN11) is associated with HLD22, which is characterized by incomplete differentiation and hypomyelination or delayed myelination in the brain. Herein, we describe for the first time that a CLDN11 mutant protein with an additional amino acid sequence due to the stop-loss mutation, but not the wild-type protein, leads to decreased expression of oligodendroglial differentiation marker proteins in the FBD-102b oligodendroglial progenitor cell line, the model undergoing its differentiation, at both the molecular and morphological levels. Consistently, mutant CLDN11 exhibited decreased morphological differentiation with a reduced ability to extend processes. These cells contained punctate structures that were partially localized in the endoplasmic reticulum (ER) and stimulated phosphorylation of c-Jun N-terminal kinase (JNK) and eukaryotic translation initiation factor 2A (eIF2A) kinase, ER stress-responsible kinases. Hesperetin, a neuroprotective flavonoid that can downregulate ER stress, recovered the differentiation abilities of these cells. Notably, the effects were related to decreased phosphorylation of ER stress-responsible kinases. JNK was found to be present in a co-precipitate with the hesperetin core, whereby hesperetin inhibited signaling through c-Jun as a negative regulator of differentiation. These findings indicate that the HLD22-associated mutant protein can cause an ER stress response, decreasing cell morphological differentiation. In addition, this study offers possible therapeutic implications for the as-yet-unexplored mechanisms involved in HLD22, at least at the molecular and cellular levels.

Genetic truncation or mutation of the gene encoding band 4.1, ezrin, radixin, and moesin (FERM) domain protein containing 4A (FRMD4A) is associated with brain developmental diseases, including microcephaly with global developmental delay. It has also been identified as a risk factor for Alzheimer’s disease. By analogy with other FERM domain-containing proteins, FRMD4A is believed to regulate cell morphogenesis and/or cell polarization in central nervous system (CNS) cells; however, it remains unclear whether and how dysfunction of FRMD4A and/or its closely homologous protein FRMD4B causes abnormal morphogenesis in neuronal cells. Here, we describe for the first time the roles of FRMD4A and FRMD4B in process elongation in neuronal cells. Knockdown of Frmd4a or Frmd4b using specific RNA-targeting clustered regularly interspaced short palindromic repeat (CRISPR) and Cas13-fitted gRNAs led to decreased process elongation in primary cortical neurons. Similar decreases in neuronal marker expression were observed in the N1E-115 cell line, a model of neuronal differentiation. Furthermore, hesperetin, an aglycone of the citrus flavonoid hesperidin known to promote neuroprotective signaling, recovered the decreased process elongation induced by the knockdown of Frmd4a or Frm4b. Hesperetin also stimulated phosphorylation of mitogen-activated protein kinases/extracellular signal-regulated kinases (MAPKs/ERKs), which could help promote neuronal processes. These results suggest that FRMD4A and FRMD4B regulate process elongation through a possible signaling pathway linked to the sustained phosphorylation of MAPKs/ERKs. Crucially, this study reveals that, at the molecular and cellular levels, hesperetin can restore normal phenotypes when FRMD4A protein or FRMD4B protein is impaired.

Maoto, a traditional kampo medicine, has been clinically prescribed for influenza infection and is reported to relieve symptoms and tissue damage. In this study, we evaluated the effects of maoto as an herbal multi-compound medicine on host responses in a mouse model of influenza infection. On the fifth day of oral administration to mice intranasally infected with influenza virus [A/PR/8/34 (H1N1)], maoto significantly improved survival rate, decreased viral titer, and ameliorated the infection-induced phenotype as compared with control mice. Analysis of the lung and plasma transcriptome and lipid mediator metabolite profile showed that maoto altered the profile of lipid mediators derived from ω-6 and ω-3 fatty acids to restore a normal state, and significantly up-regulated the expression of macrophage- and T-cell-related genes. Collectively, these results suggest that maoto regulates the host’s inflammatory response by altering the lipid mediator profile and thereby ameliorating the symptoms of influenza.

In contemporary aging societies, preventing and ameliorating mental and physical frailty is essential. Kampo formulas, including ninjinyoeito (NYT) and juzentaihoto (JTT), have been used traditionally to treat frailty in the elderly. NYT has been reported to alleviate psychological frailty such as depression and anxiety. This study aimed to clarify the mechanisms underlying the effects of these two Kampo formulas in the early stages of neurodegeneration associated with psychiatric disorders. Genes affected by Kampo formulas were comprehensively investigated by administering JTT or NYT to senescence accelerated mouse prone 8 (SAMP8) mice, from 7 weeks, and by RNA sequencing of the hippocampus at 19 weeks when depression and anxiety behaviors typically emerge. Additionally, we examined the impact of these Kampo formulas on neuroinflammation induced by lipopolysaccharide (LPS). The two Kampo formulas alleviated the depressive-like behavior of SAMP8 mice, as demonstrated by the restoration of microglial cell activation, DNA repair, stress-responsive transcription factor expression, and nervous system development-related gene expression. However, the NYT-administrated group presented a greater number of recovered genes than did the JTT-administrated group, and NYT additionally suggests that the potential inhibition of age-related mitochondrial dysfunction and increased oxidative stress. The administration of LPS resulted in elevated expression levels of immune and inflammation-related genes and increased astrocyte activity in SAMP8 mice. JTT mitigated these effects by suppressing the expression of the LPS receptor TLR4 and its downstream target NF-κB. In contrast to JTT, NYT maintained and increased the expression of genes associated with neuroprotective functions in microglia. The two Kampo formulas exerted neuroprotective effects by enhancing neural and glial stress responses in the early stages of neurodegeneration. Under condition of acute inflammation, JTT and NYT alleviated neuronal damage via the suppression of microglial activity and the enhancement of microglial neuroprotection, respectively. These findings provide novel insights into the mechanism of action of NYT, which has been reported to ameliorate psychological frailty associated with aging, and further suggest that JTT may exert effects against inflammatory neurodegeneration.

Temperature perception is essential for humans to discern the environment and maintain homeostasis. However, some individuals experience cold hypersensitivity, characterized by a subjective feeling of coldness despite ambient environmental temperatures being normal, the underlying mechanisms of which are unknown. In this study, we aimed to investigate the relationship between subjective cold symptoms and somatic burden or single nucleotide polymorphisms to understand the causes of cold hypersensitivity. We conducted an online questionnaire survey [comprising 30 questions, including past medical history, subjective symptoms of cold hypersensitivity, and the Somatic Symptom Scale-8 (SSS-8)]. Respondents were 1200 Japanese adult female volunteers (age: 20–59 years), recruited between April 21 and May 25, 2022, who were customers of MYCODE, a personal genome service in Japan. Among the 1111 participants, 599 (54%) reported cold hypersensitivity. Higher cold hypersensitivity severity was positively associated with the SSS-8 scores. Additionally, a genome-wide association study for cold hypersensitivity was conducted using array-based genomic data obtained from genetic testing. We identified 11 lead variants showing suggestive associations ( P  < 1 × 10 –5 ) with cold hypersensitivity, some of which showed a reasonable change in expression in specific tissues in the Genotype-Tissue Expression database. The study findings shed light on the underlying causes of cold hypersensitivity.