Bipolar disorder and major depressive disorder are notoriously challenging to differentiate clinically, with overlapping symptoms but divergent treatment responses, underscoring the urgent need for molecular markers that delineate their neurobiological distinctions and commonalities. The investigators obtained postmortem brain specimens from multiple brain regions critically involved in mood regulation, including the prefrontal cortex and hippocampus, to delineate cell-type-specific transcriptomic changes that contribute to these psychiatric conditions.

A key innovation in this study was the utilization of single-nucleus RNA sequencing, a cutting-edge technique that isolates nuclei from archived frozen brain tissue, enabling fine-grained analysis of gene expression at single-cell resolution. This approach circumvents the issue of cellular heterogeneity within bulk tissue samples, revealing how discrete neuronal and glial populations are differentially affected in BD and MDD. By integrating single-nucleus and bulk RNA-seq data, the research team robustly characterized both cell-type-specific expression patterns and more global transcriptomic signatures associated with mood disorders.

The results illuminated both convergent and divergent pathological processes. For instance, inflammatory responses and synaptic signaling pathways were dysregulated across both BD and MDD, suggesting common neuroimmune mechanisms contributing to mood dysregulation. However, certain pathways, such as mitochondrial function and calcium signaling, exhibited distinct alterations exclusive to bipolar disorder, potentially explaining its episodic mood swings and treatment resistance. Conversely, dysregulation in neuroplasticity-related genes was more prominent in major depressive disorder, providing molecular evidence for the chronic and often treatment-resistant nature of depressive states.

In particular, inhibitory interneurons, which play a pivotal role in maintaining excitatory/inhibitory balance within cortical circuits, showed altered gene expression profiles predominantly in the bipolar disorder samples. These perturbations may underlie the episodic manic and depressive phases characteristic of BD, by disrupting cortical oscillations and neuronal synchrony. Contrastingly, astrocytes and microglial cells prioritized in the depressive brain tissue exhibited genes consistent with a pro-inflammatory phenotype and impaired support for neuronal survival and plasticity.

Intriguingly, the study identified a set of “hub” genes with altered expression patterns that are common to both disorders, interfacing with known psychiatric risk loci from genome-wide association studies. These genes appear to orchestrate complex networks involving neuroinflammation, neurotransmission, and cellular metabolism, positing them as potential therapeutic targets that could modulate trajectories of mood disorder progression. Such convergence strengthens the conceptualization of mood disorders as spectrum diseases sharing overlapping pathogenic pathways.

The authors further investigated how chronic mood symptoms might imprint on gene expression by comparing early versus late-stage individuals with BD and MDD. Progressive dysregulation in synaptic and mitochondrial genes suggested a cumulative neurobiological burden, providing molecular correlates for illness chronicity and cognitive decline observed clinically. This temporal dimension paves the way for identifying early biomarkers and intervention windows before irreversible neuropathology ensues.

Advanced computational modeling and network analyses underscored the power of integrating multi-omic datasets for unraveling neuropsychiatric complexity. By employing weighted gene co-expression network analysis, the team uncovered modules heavily enriched for cell-type-specific functions altered in mood disorders. This systems biology perspective highlights how individual gene changes ripple through interconnected pathways, culminating in the multifaceted symptomatology seen in BD and MDD.

Importantly, this research leveraged well-characterized human brain samples, overcoming the limitations of animal models which frequently fail to recapitulate the full spectrum of human mood disorder biology. Postmortem studies like this bridge the translational gap, anchoring preclinical findings to human pathology and enhancing the relevance and precision of future therapeutic development.

The implications for treatment are manifold. Identifying distinct molecular signatures opens avenues for personalized medicine, enabling clinicians to tailor interventions based on an individual’s unique gene expression profile. For example, therapeutics targeting mitochondrial dysfunction may benefit BD patients exhibiting specific transcriptomic disturbances, while anti-inflammatory strategies could be prioritized for MDD cases marked by neuroimmune dysregulation.

Looking forward, the integration of single-nucleus transcriptomics with other emerging modalities—such as epigenomic mapping, spatial transcriptomics, and proteomics—promises to construct even richer cellular atlases of the human brain in health and disease. These comprehensive data layers will be instrumental in deciphering the dynamic interplay of genes, environment, and neural circuitry that culminate in complex psychiatric illnesses.

This study by Gao and colleagues represents a landmark in neuropsychiatric research by elegantly demonstrating how advanced genomic technologies can dissect heterogeneity within and between mood disorders at an unparalleled resolution. Such mechanistic insights are crucial for the rational design of next-generation therapeutics with improved efficacy and fewer side effects, addressing the considerable unmet needs in mental health care.

Beyond its scientific merit, this research challenges existing diagnostic frameworks, advocating for a biology-driven reclassification of mood disorders that transcends symptomatic overlap. As psychiatric medicine moves toward precision psychiatry, findings like these underscore the urgent imperative to redefine mental illnesses based on molecular pathology rather than solely clinical presentation.

In conclusion, by unveiling both shared and distinct transcriptomic landscapes of bipolar disorder and major depressive disorder, this landmark study illuminates the molecular intricacies underlying these enigmatic illnesses. It sets a new gold standard for postmortem brain research and opens transformative pathways toward more effective diagnostics and interventions that can profoundly improve the lives of millions suffering from mood disorders globally.

Subject of Research: Molecular and cellular abnormalities in bipolar disorder and major depressive disorder as revealed by postmortem brain single-nucleus and bulk gene expression analyses.

Article Title: Postmortem brain single-nucleus and bulk gene expression analyses identify shared and distinct abnormalities in bipolar disorder and major depressive disorder.

Article References:
Gao, R., Otsuka, I., Shirai, T. et al. Postmortem brain single-nucleus and bulk gene expression analyses identify shared and distinct abnormalities in bipolar disorder and major depressive disorder. Transl Psychiatry (2026). https://doi.org/10.1038/s41398-026-04200-5

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41398-026-04200-5

Traditionally, neuroscience and psychiatric disorders have been studied through tissue sourced exclusively from postmortem donations. This has perpetuated the assumption that molecular profiles obtained from deceased brain tissue adequately reflect the molecular state of living brains. However, this assumption remained largely untested due to the rarity and technical complexity of collecting living brain samples. Mount Sinai’s Living Brain Project has now systematically addressed this gap by developing a safe, scalable biopsy technique that harvests small quantities of brain tissue during deep brain stimulation (DBS) surgeries.

Leveraging transcriptomics, which scrutinizes the comprehensive expression of RNA transcripts, alongside proteomics, the large-scale analysis of protein content, the researchers studied approximately 300 samples extracted from the prefrontal cortex of living patients undergoing neurosurgical procedures. The project’s methodological sophistication allowed for intricate comparisons between living brain tissue and standard postmortem samples, exposing profound discrepancies that have important implications for brain science and disease research.

The centerpiece publication in Molecular Psychiatry lays this foundation, providing compelling evidence that gene expression profiles acquired from postmortem brains do not always faithfully mirror the gene expression in living brain tissue. This gap in molecular congruence spans across neurological and psychiatric disease signatures as well as normative phenotypes such as aging. As such, this research cautions against overreliance on postmortem data to model living brain function and pathology without validation.

Alexander W. Charney, MD, PhD, co-leader of the Living Brain Project and Director of The Charles Bronfman Institute for Personalized Medicine, underscores the profound significance of these findings. He calls for an integrative approach to brain research that incorporates the molecular insights gleaned exclusively from living tissue to complement traditional postmortem studies. His remarks emphasize that these revelations enhance, rather than diminish, the value of postmortem research by highlighting the vital, previously inaccessible molecular dimension of living brain tissue.

Expanding upon these insights, the subsequent PLOS ONE publication explores the molecular underpinnings in even greater biochemical depth—specifically, RNA splicing, intron usage, and proteomic variation. This study unveils an extraordinary degree of difference, with over 60% of proteins and a staggering 95% of RNA transcripts processed or expressed differently when comparing living brain tissue with postmortem samples. These disparities emphasize the dynamic and context-dependent nature of brain molecular biology that is lost upon death.

Brian Kopell, MD, Director of Mount Sinai’s Center for Neuromodulation and lead author of the PLOS ONE study, articulates the sheer scale of these differentiated molecular phenomena. He notes that nearly all RNA transcripts examined exhibited altered primary or mature RNA levels or splicing rates in the living brain versus postmortem brain. Importantly, even key interactions between RNA and protein co-expression networks were disrupted in postmortem samples, suggesting that molecular dysregulation post-death goes beyond mere static degradation.

Building on these discoveries, this research advocates for a transformative shift in brain biobanking. With millions worldwide undergoing neurosurgical interventions annually, the feasibility of systematically collecting living brain tissue for diverse biomedical research objectives is within reach. This could catalyze revolutionary advances in deciphering real-time molecular changes related to mood regulation, cognitive processing, and therapeutic responsiveness in a wide array of neuropsychiatric disorders and normal brain functions.

The safety profile of the tissue collection technique developed by the team ensures that brain biopsies can be conducted without compromising patient outcomes, offering a scalable method to construct living brain tissue libraries. Such biobanks would support an unprecedented level of molecular neuroscience inquiry with longitudinal and personalized data sets, reshaping biomedical research trajectories over the decades ahead.

Mount Sinai Health System, the institutional powerhouse behind the Living Brain Project, exemplifies cutting-edge clinical and scientific integration. Housing expansive research infrastructure—including hundreds of clinical and research labs and a vast clinician-scientist workforce—it remains at the vanguard of medical innovation. Their multidisciplinary approach harnesses advances in AI, informatics, and personalized medicine to address complex neurological conditions through a molecular lens.

Beyond the hospital and lab, Mount Sinai’s commitment extends to education and community outreach, ensuring that these transformative discoveries translate into tangible therapeutic breakthroughs and enhanced care paradigms accessible to all patients. The system’s standing—repeatedly validated through top rankings in national and global hospital assessments—affirms its capacity to spearhead bold initiatives like the Living Brain Project.

As neuroscience embraces these revelations, the research community faces a paradigm recalibration. Understanding the living human brain at the molecular level will no longer be extrapolated solely from postmortem proxies. Instead, it will derive directly from the tissue of living subjects, powering forward novel insights into brain health, disease mechanisms, and precision interventions that can drastically improve lives worldwide.

Subject of Research: People

Article Title: A study of gene expression in the living human brain

News Publication Date: 23-Aug-2025

Web References:

• https://icahn.mssm.edu/research/friedman/living-brain
• https://www.nature.com/articles/s41380-025-03163-1
• https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0332651

References:

• Molecular Psychiatry (DOI: 10.1038/s41380-025-03163-1)
• PLOS ONE (2025 study on RNA splicing and protein expression differences)

Image Credits: Mount Sinai Health System

Keywords:
Molecular neuroscience, Human brain, Proteomics, Omics, Brain, Brain tissue

This extensive investigation leverages ex vivo microglia isolated from 189 postmortem human brains, encompassing 58 cognitively normal elderly individuals and 131 subjects exhibiting a range of neurological conditions, including 63 with documented Alzheimer’s pathology spanning from early to advanced clinical stages. Such a comprehensive sampling provides a rare opportunity to dissect how microglial gene expression adapts—or maladapts—in response to progressive neuropathology, cognitive decline, and dementia severity. By harnessing high-resolution RNA sequencing technologies, the researchers mapped microglial transcriptomes with exquisite detail, unveiling complex shifts that extend far beyond previously known molecular markers.

One of the most striking findings of the study is the delineation of a broad, disease-associated transcriptional signature reflecting multiple facets of AD pathology. This signature does not merely correspond to the presence or absence of classic amyloid-beta plaques or tau tangles; rather, it correlates with a composite measure of cognitive impairment, neuritic and diffuse plaques, neurofibrillary tangles, and other neuropathological hallmarks. Such an integrative transcriptional signature underscores the multifactorial nature of microglial involvement in AD and highlights the potential for gene expression patterns to serve as molecular barometers of disease progression.

Delving deeper into transcript-level nuances, the analysis reveals considerable heterogeneity in isoform usage among AD-associated genes. Alternative splicing events and isoform shifts suggest sophisticated layers of post-transcriptional regulation in microglia that may fine-tune their functional repertoire in response to evolving pathology. These isoform dynamics underscore a complexity hitherto underappreciated in neuro-immune cross talk and pave the way for isoform-specific biomarkers or therapeutic targets with enhanced precision.

An equally compelling dimension of this work pertains to the altered coordination of gene expression networks within microglia during AD. The authors identify significant dysregulation in gene-gene coexpression modules, indicative of disrupted molecular governance. Such perturbations may reflect or drive maladaptive microglial phenotypes, contributing to a vicious cycle of inflammation, synaptic dysfunction, and neurodegeneration. In particular, modules associated with immune activation, metabolic processes, and lipid handling show conspicuous rewiring, highlighting critical pathways potentially amenable to intervention.

Beyond these molecular disruptions, the study uncovers heterogeneity within the AD microglial response itself. Using unsupervised clustering of gene expression patterns, distinct disease subtypes emerge, each characterized by unique transcriptional profiles. These microglial subpopulations may embody diverse functional states, ranging from protective surveillance phenotypes to deleterious pro-inflammatory states, thereby offering new insights into disease resilience and vulnerability. The stratification of AD into molecularly defined subtypes based on microglial states could revolutionize personalized approaches to diagnosis and treatment.

Importantly, the findings extend knowledge beyond simple disease associations by nominating specific candidate genes for therapeutic targeting. Several genes with previously unappreciated roles in microglial biology or AD pathogenesis show marked dysregulation, positioning them as attractive prospects for drug development. The integration of isoform-specific data further enriches this candidate pool, allowing nuanced targeting strategies that account for transcript diversity and microglial heterogeneity.

Central to the study’s strength is the use of postmortem human brain tissue, which faithfully captures disease-relevant states in situ. This approach overcomes limitations of animal models and in vitro cultures that often fail to recapitulate human microglial complexity or AD pathology. By directly profiling microglia from well-characterized clinical cohorts, the research provides a valuable translational bridge linking molecular insights to patient phenotypes.

This transcriptional atlas also sets the stage for downstream functional studies aimed at elucidating how microglial gene expression changes translate into altered cellular behavior. For instance, shifts in immune-modulatory genes could alter microglial phagocytic activity or cytokine production, while metabolic gene rewiring might impact energy utilization and survival. Understanding these functional consequences is imperative for harnessing microglia’s dualistic roles as both protectors and potential offenders in the AD brain.

Furthermore, the data reinforce the concept of microglia as dynamic responders that evolve in response to changing microenvironments within the aging and diseased brain. Rather than being static custodians, microglia exhibit a spectrum of activation states that likely influence the trajectory of neurodegeneration. Mapping these trajectories with such depth provides an invaluable framework for temporal dissection of AD progression and identification of optimal therapeutic windows.

The implications for biomarker discovery are equally profound. Transcriptional signatures and isoform profiles from microglia could inform peripheral readouts or imaging surrogates, enabling earlier and more accurate diagnosis of AD subtypes. Non-invasive monitoring of microglial states might also facilitate real-time assessment of therapeutic efficacy, accelerating the pipeline from bench to bedside.

As the field moves toward precision medicine, this study’s revelations about molecular subtypes and network dysregulation within microglia highlight the necessity of tailored interventions. Drugs that selectively modulate harmful microglial phenotypes without impairing their essential homeostatic functions hold promise for more effective and safer AD treatments. The identification of novel candidate genes further enriches the drug discovery landscape, potentially yielding targets that circumvent the pitfalls of amyloid- or tau-centric strategies.

Moreover, the interplay of microglial transcriptional changes with other brain cell types and systemic factors remains a fertile area for investigation. Integration of single-cell multi-omics, spatial transcriptomics, and longitudinal clinical data promises to refine the mechanistic models of AD. This comprehensive approach will be critical to disentangling cause-effect relationships and identifying intervention points amenable to disease modification.

In sum, this landmark study presents a compelling narrative on how human microglia transcriptionally respond to Alzheimer’s disease, revealing intricacies of isoform dynamics, gene network dysregulation, and cellular heterogeneity previously uncharted. By marrying clinical phenotyping with deep molecular profiling, it forges new pathways toward understanding and ultimately combating one of the world’s most devastating neurodegenerative disorders.

The transformative potential of these findings lies not only in elucidating fundamental disease biology but also in shaping the next generation of diagnostics and therapeutics. As microglia move to center stage in AD research, their transcriptional landscape will undoubtedly be a critical roadmap guiding future scientific endeavors and clinical breakthroughs.

Subject of Research: Transcriptional profiling of primary human microglia revealing molecular changes associated with Alzheimer’s disease pathology and clinical phenotypes.

Article Title: Alzheimer’s disease transcriptional landscape in ex vivo human microglia.

Article References:
Kosoy, R., Fullard, J.F., Bendl, J. et al. Alzheimer’s disease transcriptional landscape in ex vivo human microglia. Nat Neurosci (2025). https://doi.org/10.1038/s41593-025-02020-2

Image Credits: AI Generated

Progressive supranuclear palsy is a relentlessly progressive neurodegenerative disease marked by motor dysfunctions, cognitive decline, and characteristic brainstem and basal ganglia pathology. Tau protein abnormalities—specifically the pathological aggregation of hyperphosphorylated tau—are known hallmarks of PSP. However, the precise molecular events that facilitate the spread of these tau species throughout neural circuits have remained elusive until now. The research led by McGeachan, Keavey, Simzer, and colleagues presents direct human evidence that oligomeric tau, a soluble prefibrillar tau species increasingly implicated in toxicity, propagates trans-synaptically between neurons in PSP.

The study utilized highly advanced imaging and biochemical methods to interrogate postmortem brain tissues from individuals diagnosed with PSP, focusing on cortical and subcortical regions known to undergo characteristic tau pathology. Sophisticated immunohistochemical staining coupled with super-resolution microscopy allowed the researchers to delineate the subcellular localization of tau oligomers at synaptic terminals. Remarkably, they observed tau oligomers colocalizing with synaptic markers, suggesting not only neuronal accumulation but active involvement in synaptic transmission and potentially in inter-neuronal transfer.

A particularly striking aspect of the findings is the identification of tau oligomers within pre- and post-synaptic compartments, providing unprecedented evidence that these pathogenic tau forms can traverse synaptic clefts, thereby facilitating a prion-like spread of tau pathology. This mechanism is reminiscent of the spread observed with other aggregation-prone proteins such as alpha-synuclein in Parkinson’s disease, highlighting a possible common pathological motif in neurodegeneration.

The authors meticulously characterized the biochemical properties of the tau oligomers extracted from affected brain regions. Utilizing size-exclusion chromatography combined with tau-specific antibodies, they confirmed the oligomeric state of tau species, distinct from monomeric or fully fibrillar tau. Moreover, biochemical assays demonstrated increased seeding activity of these oligomers, underscoring their pathological relevance in initiating tau aggregation cascades in recipient neurons.

Further reinforcing the trans-synaptic propagation hypothesis, the team identified spatial gradients of tau oligomers corresponding with known neuroanatomical connectivity patterns in PSP brains. This anatomical correlation strongly supports the notion that tau pathology does not randomly distribute but follows synaptically connected neural networks, progressively compromising brain function in a predictable manner as the disease advances.

Critically, the study also employed ultrastructural electron microscopy to visualize tau oligomers at nanometer resolution within synaptic vesicles and synaptic membranes. These observations provide compelling morphological evidence of tau oligomer involvement in synaptic vesicle trafficking and potentially synaptic dysfunction, a mechanism that may contribute directly to the clinical symptoms of PSP.

The research integrates these morphological and biochemical findings into a coherent model wherein extracellular release and subsequent uptake of tau oligomers occur via synaptic contacts, enabling a cell-to-cell propagation that amplifies tau aggregation neuropathology. This model explains the characteristic spread of tau lesions observed in PSP and suggests novel therapeutic windows targeting early tau oligomer transmission at the synapse.

Notably, this investigation builds on prior in vitro and animal model studies by delivering pivotal data derived from human brain specimens, thereby bridging experimental observations and clinical reality. This translational leap is vital, as it validates the relevance of trans-synaptic tau propagation mechanisms in human neurodegenerative diseases beyond theoretical constructs.

The implications of this research are vast, suggesting that interventions designed to inhibit tau oligomer formation, disrupt their synaptic release or uptake, or bolster synaptic resilience against tau-induced toxicity could arrest or slow the progression of PSP and other tauopathies. It also raises the intriguing possibility that synaptic transmission pathways can be manipulated pharmacologically to mitigate the insidious spread of tau pathology.

Furthermore, these insights enrich our comprehension of synaptic pathobiology in neurodegeneration. The synapse, traditionally viewed as a passive victim of neurodegenerative protein accumulation, emerges here as an active conduit and amplifier of pathological tau spread. This paradigm shift may redefine therapeutic targets prioritizing synaptic health and inter-neuronal communication pathways.

The study also underscores the importance of oligomeric tau species, distinct from fibrillar tangles, as key mediators of neurotoxicity and disease progression. Previous focus on fibrillar tau may have obscured the pathogenic roles played by soluble oligomers, which appear more mobile and capable of intercellular transfer. Recognizing oligomeric tau as the pathogenic species opens new research avenues exploring their formation, stabilization, and clearance.

Moreover, the findings raise compelling questions regarding the cell biology underlying tau release and uptake mechanisms at synapses. Whether tau oligomers exploit exosomal pathways, receptor-mediated endocytosis, or direct membrane penetration remains to be elucidated. Understanding these processes in detail may reveal novel molecular players amenable to therapeutic modulation.

This study also invites deeper examination into the role of neuronal activity in modulating tau propagation. Since synaptic transmission is activity-dependent, it is conceivable that hyperactive or aberrantly firing neural circuits could exacerbate tau spread, implicating neural network dynamics in disease trajectory. Future research integrating electrophysiological and imaging techniques might illuminate this interplay.

Importantly, the authors note that while tau propagation likely contributes to pathological and clinical progression, it operates within a multifactorial landscape including neuroinflammation, mitochondrial dysfunction, and genetic factors influencing tau metabolism. Integrated multimodal studies combining neuropathology, genetics, and clinical phenotyping will be essential to construct a comprehensive model of PSP pathogenesis.

In conclusion, the discovery of trans-synaptic propagation of oligomeric tau in human progressive supranuclear palsy marks a transformative advance in neurodegenerative disease research. It defines critical molecular events that bridge cellular pathology and clinical progression, creating opportunities for targeted therapeutic interventions. As the global burden of tauopathies escalates, such mechanistic insights provide crucial hope for developing disease-modifying treatments that can alter the devastating course of these disorders.

Subject of Research: Progressive supranuclear palsy and the mechanisms underlying tau protein propagation in human neurodegeneration.

Article Title: Evidence for trans-synaptic propagation of oligomeric tau in human progressive supranuclear palsy.

Article References:
McGeachan, R.I., Keavey, L., Simzer, E.M. et al. Evidence for trans-synaptic propagation of oligomeric tau in human progressive supranuclear palsy. Nat Neurosci (2025). https://doi.org/10.1038/s41593-025-01992-5

Image Credits: AI Generated