This cross-sectional observational case-control study evaluated serum levels of leucine-rich repeat and fibronectin type III domain-containing protein 5 (LRFN5) and olfactomedin-4 (OLFM4) in subjects with bipolar disorder (BD) during acute manic episodes and in healthy controls (HC). The study also assessed systemic inflammation using the Aggregate Index of Systemic Inflammation (AISI) and examined associations between biomarkers and clinical symptom severity, including Young Mania Rating Scale (YMRS) and Beck Depression Inventory (BDI) scores. The study aimed to investigate the potential interaction between synaptic dysregulation and immunoinflammatory abnormalities in BD.

Bipolar disorder (BD) is a chronic psychiatric disorder characterized by recurrent episodes of mania, hypomania, and depression, interspersed with periods of euthymia. Increasing evidence suggests that BD is associated with both synaptic dysregulation and immunoinflammatory abnormalities. Alterations in synaptic plasticity, dendritic architecture, and glutamatergic signaling have been implicated in mood instability. In parallel, immune activation, altered cytokine profiles, and elevated peripheral inflammatory markers have been reported in individuals with BD. These findings support the hypothesis that synaptic and immune-inflammatory mechanisms may interact in the pathophysiology of BD.

Leucine-rich repeat and fibronectin type III domain-containing protein 5 (LRFN5), also known as synaptic adhesion-like molecule 5 (SALM5), is a synaptic adhesion molecule primarily expressed in the central nervous system. It is enriched at the postsynaptic density of excitatory synapses and contributes to synapse formation, maintenance, and glutamatergic organization. Olfactomedin-4 (OLFM4) is a secreted extracellular glycoprotein involved in innate immune regulation, apoptosis modulation, and inflammatory signaling pathways. OLFM4 is expressed in hematopoietic tissues and immune cells, particularly neutrophils, and participates in immune homeostasis and stress-response mechanisms.

Despite their biological relevance, circulating levels of LRFN5 and OLFM4 have not previously been systematically investigated in individuals with BD. In addition, the Aggregate Index of Systemic Inflammation (AISI), calculated as (neutrophils × monocytes × platelets) / lymphocytes, represents a composite marker reflecting systemic inflammatory burden. AISI integrates components of innate and adaptive immunity and may provide a practical peripheral indicator of inflammatory activation.

The present study was designed as a cross-sectional observational case-control investigation conducted at Elazığ Mental Health and Diseases Hospital between July 7, 2025, and January 2, 2026. The study enrolled 72 participants, including 37 inpatients diagnosed with BD during acute manic episodes and 35 healthy control (HC) subjects.

Diagnosis of BD was established according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), through structured clinical interviews conducted by an experienced psychiatrist. All BD participants were hospitalized during an acute manic episode and were medication-free for at least one month prior to admission. Healthy controls were evaluated by the hospital medical board and had no psychiatric or systemic medical disorders. None of the participants had hypertension, diabetes mellitus, chronic kidney disease, autoimmune disease, severe neurological disorder, or other systemic inflammatory conditions.

For BD participants, all procedures were conducted at hospital admission prior to initiation of psychotropic treatment. Venous blood samples were collected at baseline. Routine biochemical analyses, including complete blood count (CBC), were performed. Serum samples were centrifuged, aliquoted, and stored at -80°C until analysis. Serum LRFN5 and OLFM4 levels were measured using commercially available enzyme-linked immunosorbent assay (ELISA) kits in accordance with the manufacturer's instructions. AISI values were calculated from CBC parameters.

Sociodemographic data and clinical characteristics were recorded. Manic symptom severity was assessed using the Young Mania Rating Scale (YMRS), and depressive symptoms were assessed using the Beck Depression Inventory (BDI).

The primary objective of the study was to compare serum LRFN5 and OLFM4 levels between BD patients and healthy controls. Secondary objectives included:

Comparison of AISI levels between groups Evaluation of correlations between biomarkers and clinical severity measures Assessment of the predictive value of biomarkers for BD status using hierarchical binary logistic regression Evaluation of discriminative performance using receiver operating characteristic (ROC) curve analysis Statistical analyses were performed using IBM SPSS Statistics Version 26. Continuous variables were expressed as mean ± standard deviation or median values, and categorical variables as frequencies. Group comparisons were conducted using independent samples t-test or Mann-Whitney U test as appropriate. Categorical variables were analyzed using chi-square or Fisher's exact test. Partial correlation analyses were performed controlling for age, gender, and body mass index (BMI). Hierarchical binary logistic regression analysis was used to evaluate independent predictors of BD status. ROC curve analysis was conducted to determine area under the curve (AUC) values for each biomarker. Statistical significance was set at p < 0.05.

The BD group demonstrated significantly lower serum LRFN5 and OLFM4 levels and significantly higher AISI levels compared to healthy controls. Within the BD group, a significant positive correlation was observed between LRFN5 and OLFM4 levels. AISI showed a significant positive correlation with manic symptom severity after controlling for age, gender, and BMI. In hierarchical logistic regression analysis, AISI emerged as an independent predictor of BD status. ROC analysis indicated that AISI demonstrated the highest discriminative performance among the evaluated biomarkers.

This study provides data on circulating synaptic and immunoinflammatory biomarkers in individuals with BD during acute manic episodes and evaluates their potential association with systemic inflammation and symptom severity.

1. For Bipolar Disorder (BD) Group:

   *Inclusion Criteria:

   • Diagnosis of BD according to DSM-5-TR
   • Acute manic episode
   • Medication-free for at least one month prior to admission
   • Age ≥ 18 years and <65 years
   • Provided informed consent

   For Bipolar Disorder (BD) Group: *

   Exclusion Criteria:

   • Hypertension

   • Diabetes mellitus

   • Chronic kidney disease

   • Rheumatoid arthritis

   • Systemic lupus erythematosus

   • Cardiac illness

   • Severe neurological disorders
   • Immunological or systemic illness
   • Primary psychiatric disorders other than BD
   • Alcohol/drug/substance use

2. For Healthy Control Group:

   *Inclusion Criteria:

   • No psychiatric diagnosis

   • No systemic or immunological illness

   • Medication-free for at least one month
   • Age ≥ 18 years and < 65 years
   • Provided informed consent

   For Healthy Control Group:

   *Exclusion Criteria:

   • Hypertension
   • Diabetes mellitus
   • Chronic kidney disease
   • Rheumatoid arthritis
   • Systemic lupus erythematosus
   • Cardiac illness
   • Severe neurological disorders
   • Immunological or systemic illness
   • Having psychiatric disorders
   • Alcohol/drug/substance use

The study population consisted of adult participants aged 18-65 years. The bipolar disorder (BD) group included consecutive patients diagnosed with BD according to DSM-5-TR criteria who were admitted to the psychiatry clinic of Elazığ Mental Health and Diseases Hospital (Turkey). The healthy control (HC) group consisted of individuals from the general population who applied to the hospital medical board and had no current or past psychiatric or significant medical disorders. All participants provided informed consent prior to enrollment.

Deidentified individual participant data (IPD) underlying the results reported in this study (including demographic variables, serum leucine-rich repeat and fibronectin type III domain-containing protein 5 (LRFN5) and Olfactomedin-4 (OLFM4) levels, complete blood count parameters, and Young Mania Rating Scale (YMRS) for manic symptom severity and the Beck Depression Inventory (BDI) for depressive symptoms) will be made available to qualified researchers upon reasonable request for academic purposes. Data will be shared after removal of all direct identifiers and in accordance with applicable ethical approvals and data protection regulations. Access to the data will require a methodologically sound research proposal and a data use agreement. Requests should be directed to the corresponding author.

Data will be available beginning 6 months after publication and will remain available for 5 years.

Access will be granted to researchers who provide a methodologically sound proposal. Requests must be approved by the principal investigator and may require a data use agreement in accordance with institutional and ethical regulations.