Plasma Glial Fibrillary Acidic Protein and Neurofilament Light are Elevated in Bipolar Disorder: Evidence for Neuroprogression and Astrocytic Activation

ABSTRACT Importance: Recent methodological developments allow us to measure small amounts of brain-specific proteins in the blood, including neurofilament light chain (NfL), a marker of axonal pathology, and glial fibrillary acidic protein (GFAP), a marker of astrocytic activation. Given the evidence of potential astroglial pathology and neuronal dysfunction in bipolar disorder, these markers may provide further insight into its pathophysiology. Objective: We investigated plasma NfL and GFAP levels in people with bipolar depression and compared them with unaffected individuals. Design, Setting, and Participants: This cross-sectional study included 216 individuals, of which 120 participants had bipolar depression and 96 healthy controls. The blood samples were analysed between November 2023 and April 2024. Main outcomes and measures: We used bootstrapped general linear models (GLM) to compare plasma NfL and GFAP levels between people with bipolar depression and healthy controls, adjusted adjusting for age, sex, and weight. We examined associations between these biomarkers and clinical variables, including mood symptom severity, past psychiatric history, and functioning, adjusting for multiple comparisons. For additional sensitivity analyses, predictors were evaluated using Bayesian model averaging (BMA). Results: GFAP and NfL levels in plasma were elevated in people with bipolar depression (n = 120) compared to healthy controls (n = 96) after adjusting for age, sex and weight. The duration of illness was positively associated with NfL. The BMA analysis also identified duration of illness as a strong predictor of NfL (Posterior Inclusion Probability, PIP = 0.85). Age of onset was positively associated with GFAP. The BMA analysis similarly found age of onset to be a moderately strong predictor (PIP = 0.67). Conclusions and Relevance: This study found elevated levels of plasma NfL and GFAP in bipolar depression compared to unaffected individuals, with significant associations with the duration of illness and age at onset, suggesting a degree of neuronal injury and astrocytic dysfunction in bipolar depression. These biomarkers may reflect specific illness stages, including neuroprogression and the later onset of bipolar disorder. ### Competing Interest Statement This study was supported by: MACH MRFF RART 2.2, NHMRC (1185180), and Psychiatry and Rehabilitation Division, Region Skane, Sweden. The role of these funding sources was to support research study staff and biosample analyses. The healthy controls cohort was part of a major Australian Department of Industry Co-operative Research Centre (CRC) grant - https://researchdata.ands.org.au/treatment-resistant-schizophrenia-biobank/1325206 MK is supported by the Nick Christopher PhD scholarship and the Research Training Program Scholarship from the Department of Psychiatry, University of Melbourne with contributions from the Australian Commonwealth Government, and the Ramsay Hospital Research Foundation. AFS is supported by the Swedish federal government under the ALF agreement. MB is supported by a NHMRC Leadership 3 Investigator grant (GNT2017131). C Pantelis was supported by a National Health and Medical Research Council (NHMRC) Senior Principal Research Fellowship (1105825), an NHMRC L3 Investigator Grant (1196508). PBM is supported by an NHMRC Investigator Grant (1177991). In the last 3 years he has received remuneration from Janssen (Australia) for lectures and advisory board membership OHs work at Lund University was supported by Alzheimers Association (ZEN24-1069572, SG-23-1061717), GHR Foundation, Swedish Research Council (2022-00775), ERA PerMed (ERAPERMED2021-184), Knut and Alice Wallenberg foundation (2022-0231), Strategic Research Area MultiPark (Multidisciplinary Research in Parkinsons disease) at Lund University, Swedish Alzheimer Foundation (AF-980907), Swedish Brain Foundation (FO2021-0293), Parkinson foundation of Sweden (1412/22), Cure Alzheimers fund, Ronstrom Family Foundation, Konung Gustaf Vsoch Drottning Victorias Frimurarestiftelse, Skane University Hospital Foundation (2020-O000028), Regionalt Forskningsstod (2022-1259) and Swedish federal government under the ALF agreement (2022-Projekt0080). AJW was previously supported by a Trisno Family Research Fellowship. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication. OH has acquired research support (for the institution) from AVID Radiopharmaceuticals, Biogen, C2N Diagnostics, Eli Lilly, Eisai, Fujirebio, GE Healthcare, and Roche. In the past 2 years, he has received consultancy/speaker fees from Alzpath, BioArctic, Biogen, Bristol Meyer Squibb, Eisai, Eli Lilly, Fujirebio, Merck, Novartis, Novo Nordisk, Roche, Sanofi and Siemens. OMD has received grant support from the Brain and Behavior Foundation, Simons Autism Foundation, Stanley Medical Research Institute, Deakin University, Lilly, NHMRC and ASBDD/Servier. She has also received in kind support from BioMedica Nutracuticals, NutritionCare and Bioceuticals. ### Funding Statement This study was supported by: MACH MRFF RART 2.2, NHMRC (1185180), and Psychiatry and Rehabilitation Division, Region Skane, Sweden. The role of these funding sources was to support research study staff and biosample analyses. The healthy controls cohort was part of a major Australian Department of Industry Co-operative Research Centre (CRC) grant - https://researchdata.ands.org.au/treatment-resistant-schizophrenia-biobank/1325206 MK is supported by the Nick Christopher PhD scholarship and the Research Training Program Scholarship from the Department of Psychiatry, University of Melbourne with contributions from the Australian Commonwealth Government, and the Ramsay Hospital Research Foundation. AFS is supported by the Swedish federal government under the ALF agreement. MB is supported by a NHMRC Leadership 3 Investigator grant (GNT2017131). C Pantelis was supported by a National Health and Medical Research Council (NHMRC) Senior Principal Research Fellowship (1105825), an NHMRC L3 Investigator Grant (1196508). PBM is supported by an NHMRC Investigator Grant (1177991). In the last 3 years he has received remuneration from Janssen (Australia) for lectures and advisory board membership OHs work at Lund University was supported by Alzheimers Association (ZEN24-1069572, SG-23-1061717), GHR Foundation, Swedish Research Council (2022-00775), ERA PerMed (ERAPERMED2021-184), Knut and Alice Wallenberg foundation (2022-0231), Strategic Research Area MultiPark (Multidisciplinary Research in Parkinsons disease) at Lund University, Swedish Alzheimer Foundation (AF-980907), Swedish Brain Foundation (FO2021-0293), Parkinson foundation of Sweden (1412/22), Cure Alzheimers fund, Ronstrom Family Foundation, Konung Gustaf Vs och Drottning Victorias Frimurarestiftelse, Skane University Hospital Foundation (2020-O000028), Regionalt Forskningsstod (2022-1259) and Swedish federal government under the ALF agreement (2022-Projekt0080). AJW was previously supported by a Trisno Family Research Fellowship. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study, which is part of The Markers in Neuropsychiatric Disorders Study (The MiND Study, https://themindstudy.org), was approved by the Melbourne Health Human Research Ethics Committee (MH HREC 2020.142). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Deidentified and pooled data is available at request to the corresponding author.