Scientists have uncovered two surprising forces that help guide how the human brain forms before birth. Before birth, the human brain is shaped in large part by an unusual class of stem cells known as radial glia. These cells help determine which brain cells are produced, when they appear, and how the cerebral cortex, the [...]A hidden molecular defect in inflammatory bowel disease may help explain why flare-ups can return even when patients appear well. Even when inflammatory bowel disease appears under control, intestinal cells may remain primed to die. In a WEHI-led study, researchers found this hidden molecular defect in patients with few or no symptoms, and higher levels [...]Cell-culture brain models called organoids can be used to study far more advanced stages of neurodevelopment than researchers once imagined, according to new research.
A team funded by the National Institutes of Health (NIH) reports that organoids were sustained for almost six years—far longer than previous efforts. The findings, detailed in a recent paper published in Nature, showed that a developmental clock similar to the one governing real brains also appears to govern organoid maturation.
The work could significantly expand researchers’ ability to study later stages of human brain development and neurological conditions that emerge over longer periods.
Organoids have been a major and growing tool in neurological research. They consist of small cell cultures arranged into 3D models of human brain tissue. Typically, these have been limited to only basic reconstructions, yet the new work significantly advances their modeling capabilities by expressing much later-stage brain development attributes.
Significantly, the NIH-funded team says that the work could pave the way for previously impossible neurodevelopmental studies.’
Organoids occupy a crucial space in research because they can be used to study processes that are difficult to investigate directly at the cellular level in living human brains. However, their typically limited lifespans have largely restricted neuroscientists to studying earlier stages of development. Another challenge has been the high variability between batches of cells, which the researchers addressed by producing cerebral cortex organoids with more consistent cellular compositions.
Consistency was not the team’s only advancement. In laboratory experiments, the organoids also survived and continued developing for much longer periods.
The team tracked biological age and gene expression by observing chemical changes to the genome and examining cellular characteristics down to the level of individual cells. Many key developmental milestones in the organoids, including the emergence and maturation of neurons, occurred on timelines resembling those seen during normal human brain development. The biological clocks in the organoids closely corresponded with the molecular and chronological ages observed in humans.
Although these organoids developed outside the human body, their maturation proceeded at a similarly slow pace.
Notably, neurons in the organoids continued establishing connections and displaying electrical activity for years. The researchers attributed their ability to maintain this activity in part to a specialized fluid designed to support long-term neuronal function.
“The brain doesn’t develop in a vacuum. It’s an organ of incredible complexity that interacts with so many other systems. It was not a given at all that our simplified model would match natural development in this many ways,” said co-first author Irene Faravelli, M.D., Ph.D.
During their experiments, the team transplanted neurons between organoids of different ages to determine how their new environments would affect their development. Remarkably, transplanted neurons largely retained developmental characteristics associated with the organoids from which they originated rather than resetting their developmental trajectories to match their younger surroundings.
“I like to think of this as a sort of ‘warping of developmental time’ indicating that the organoid cells record and recall the time they have already spent in culture. This suggests their development is driven by a cell-intrinsic clock, reflecting mechanisms of endogenous human brain development,” said co-author Paola Arlotta, Ph.D, of Harvard University.
Having significantly expanded researchers’ understanding of what organoids can model, the team now hopes to increase their complexity further, potentially allowing scientists to investigate additional developmental processes and test interventions.
“There is still much to learn about how the embryo naturally builds a progressively more complex and mature brain,” Arlotta concluded. “Applying these lessons to organoids will allow us to model unexplored events of human brain maturation that occur after birth.”
The paper, “Human Brain Organoids Record the Passage of Time over Multiple Years,” appeared in Nature on August 19, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org.
In a groundbreaking study poised to reshape our understanding of mitochondrial genetics and neurological disorders, researchers have uncovered profound cortical neuronal disturbances driven by mitochondrial DNA (mtDNA) heteroplasmy in human brain organoids. This revelation centers on the common m.3243A>G mutation, a genetic defect frequently linked to a spectrum of neurodegenerative diseases. The work, published in Nature Communications in 2026 by Hathazi, Lyons, Lagos, and colleagues, offers unprecedented insights into how subtle shifts in mitochondrial DNA populations might disrupt cortical neuron function at a cellular and network level—potentially illuminating new therapeutic avenues for conditions that have long eluded effective treatment.
Mitochondria, often described as the cell’s powerhouses, maintain cellular energy homeostasis by generating adenosine triphosphate (ATP). However, mitochondrial DNA holds a unique genetic code separate from that of the nuclear genome, and it exists in multiple copies within each mitochondrion. Heteroplasmy—an admixture of normal (wild-type) and mutant mtDNA molecules—can fluctuate extensively between cells and tissues. This heterogeneity significantly affects mitochondrial function, but the precise pathological consequences in human neurons have remained elusive. The study by Hathazi and collaborators bridges this gap by meticulously linking mtDNA heteroplasmy directly to measurable disturbances within cortical neurons derived from human organoids.
The researchers employed human pluripotent stem cell-derived cerebral organoids, three-dimensional cellular models that recapitulate many aspects of human cortical development and cellular diversity. By engineering these organoids to harbor varying levels of the m.3243A>G point mutation in the mitochondrial tRNA^Leu(UUR) gene, they generated a physiologically relevant system that models how mtDNA heteroplasmy inherently affects neuronal populations. This mutation impairs mitochondrial protein translation, leading to compromised oxidative phosphorylation—a known driver of metabolic dysfunction in affected cells.
Single-cell analyses revealed marked mitochondrial dysfunction in neurons exhibiting high mutation loads. The mutant mitochondria produced increased reactive oxygen species (ROS), exhibited diminished membrane potential, and showed perturbed calcium handling. These biochemical aberrations culminated in altered electrophysiological properties, including disrupted synaptic transmission and impaired action potential firing patterns. The affected cortical neurons displayed reduced synaptic density and connectivity, phenotypes consistent with impaired network synchronization and cognitive deficits observed in mitochondrial encephalopathies.
Remarkably, the study delineates a threshold effect whereby neuronal function remains relatively preserved below a critical heteroplasmy level but deteriorates precipitously once mutant load surpasses this point. This finding is pivotal in explaining the clinical heterogeneity seen among patients with the m.3243A>G mutation, where symptom severity correlates with mtDNA mutant load. It also underscores the fine balance within mitochondria between normal and mutant genomes that regulates cellular health in the brain.
To establish causality, the investigators utilized gene-editing techniques to reduce the mutant mtDNA proportion within organoids. This intervention partially rescued mitochondrial respiration and restored electrophysiological parameters toward normal, confirming that the mutation-induced mitochondrial defects directly drive neuronal impairment. Pharmacological modulation of mitochondrial stress pathways, such as enhancing mitophagy and antioxidant defenses, further ameliorated neuronal dysfunction, highlighting potential therapeutic strategies.
The implications of these findings extend beyond the m.3243A>G mutation alone. Mitochondrial heteroplasmy is a pervasive phenomenon implicated in aging and numerous neurodegenerative diseases, including Parkinson’s and Alzheimer’s disease. This study suggests that subtle shifts in mitochondrial genetic composition may critically influence disease progression in ways previously unappreciated. Moreover, the use of human cerebral organoids enables modeling of human-specific neuronal vulnerabilities to mitochondrial perturbations, overcoming the limitations inherent in animal models that often lack analogous heteroplasmy dynamics.
The technological advancements leveraged in this research, including high-resolution single-cell mitochondrial assays and precise heteroplasmy editing, represent a tour de force in disease modeling. By recapitulating human cortical development and neuronal network formation in vitro, organoids provide a window into how mitochondrial mutations disrupt developmentally timed neuronal processes leading to widespread cortical dysfunction. These insights could fuel the design of novel mitochondrial-targeted therapies aiming to maintain heteroplasmy below pathological thresholds or to enhance mitochondrial quality control mechanisms.
Furthermore, the findings provoke reconsideration of diagnostic and prognostic approaches in mitochondrial disease. Current clinical assessments largely measure systemic heteroplasmy levels, which may not reflect the mosaicism present within specific brain regions. Non-invasive biomarkers that detect mitochondrial dysfunction in cortical neurons or imaging modalities sensitive to mitochondrial bioenergetics could aid in more accurate disease staging and treatment monitoring.
The study also opens intriguing questions regarding mitochondrial-nuclear genome interactions in the context of heteroplasmy. Given that mitochondrial function is coordinated with nuclear-encoded metabolic pathways, disturbed crosstalk in the presence of mutant mtDNA could exacerbate neuronal pathology. Future research may uncover compensatory nuclear genetic factors that modulate the impact of mtDNA mutations, potentially identifying new therapeutic targets.
In summary, Hathazi and colleagues provide a compelling narrative linking mitochondrial genetic heterogeneity to neuronal network disturbances using a cutting-edge human brain organoid model. This work underscores the critical importance of mtDNA heteroplasmy in shaping neuronal health and disease, particularly within the cerebral cortex, a region central to cognition and behavior. The insights gained promise to accelerate translational efforts aimed at mitigating mitochondrial dysfunction in neurological disorders, paving the way for therapies that restore mitochondrial harmony and preserve brain function.
By marrying sophisticated genetic engineering with organoid technology and comprehensive functional assays, this research sets a new benchmark in mitochondrial neuroscience. The concept that distinct levels of a common mtDNA mutation can finely tune neuronal fate and function introduces a paradigm shift with far-reaching implications. As the field progresses, these findings may herald a future where mitochondrial genomic interventions become integral components of personalized neurotherapeutics, ultimately improving the lives of patients affected by devastating mitochondrial diseases.
Subject of Research:
Mitochondrial DNA heteroplasmy and its impact on cortical neuronal function in human brain organoids bearing the m.3243A>G mutation.
Article Title:
Mitochondrial DNA heteroplasmy drives cortical neuronal disturbances in human organoids harbouring the common m.3243A>G mutation.
Article References:
Hathazi, D., Lyons, C., Lagos, D. et al. Mitochondrial DNA heteroplasmy drives cortical neuronal disturbances in human organoids harbouring the common m.3243A>G mutation. Nat Commun (2026). https://doi.org/10.1038/s41467-026-74103-y
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