Summary: Researchers used induced pluripotent stem cells reprogrammed from the skin of people with Tourette syndrome to grow three-dimensional basal ganglia organoids. These patient-derived models reproduce developmental differences in the basal ganglia, reveal a deficit of inhibitory interneurons and disrupted patterning, and point to a potential developmental mechanism involving altered Sonic Hedgehog signaling.
Source: Yale
Tourette syndrome (TS) is a common neurodevelopmental disorder marked by involuntary motor and vocal tics that begin in childhood and can affect learning, social interactions, and daily functioning.
A team at Yale used patient-derived stem cells to build three-dimensional organoid models of the basal ganglia—an area previously implicated in TS—to study early developmental processes that may underlie the disorder.
Previous clinical and post-mortem studies have shown that the basal ganglia, a set of subcortical nuclei that regulate motor patterns and skilled actions, tend to be smaller in people with TS and contain fewer particular neuronal subtypes. Those missing cells are inhibitory interneurons, local circuit neurons that control inhibition and shape how the basal ganglia respond to signals from the cortex.
To determine whether these interneurons are lost later in life or fail to develop, the researchers converted skin cells from TS patients into induced pluripotent stem cells (iPSCs) and guided them to form basal ganglia-like organoids. These 3D cultures mimic early human brain development and allow researchers to observe cellular differentiation and patterning in a patient-specific context.
The study, published in Molecular Psychiatry on November 29, uses these organoids to trace the origin of the interneuron deficit and identify molecular pathways that may be disrupted in TS.
“To prevent or treat developmental disorders such as Tourette syndrome, we need to study the process from the beginning,” says Flora Vaccarino, MD, Harris Professor in the Yale Child Study Center and senior author of the study. “Organoids let us go back to those earliest stages and examine how disease mechanisms unfold.”
Tourette syndrome linked to basal ganglia differences
TS typically emerges between ages five and ten and often co-occurs with attention-deficit/hyperactivity disorder (ADHD) and obsessive-compulsive disorder (OCD). Tics range from simple movements like eye blinking and throat clearing to more complex vocal or motor behaviors. Although some people can transiently suppress tics, doing so is effortful, and severe cases can cause physical harm or substantial social impairment. Symptoms frequently lessen in adulthood for many individuals, but the disorder is highly heritable and its genetic contributors remain incompletely defined.
“The basal ganglia store motor programs that we perform automatically, like speaking or riding a bicycle,” explains Vaccarino. “When developmental processes in this circuit are altered, unwanted motor acts such as tics can emerge—actions that interrupt what you are doing and appear involuntarily.”
Imaging studies have consistently found subtle reductions in basal ganglia volume in children and young adults with TS. Prior post-mortem work from Vaccarino’s group identified a loss of inhibitory interneurons in the basal ganglia of people with TS. That finding raised a central question: are these interneurons produced normally and then lost, or do they fail to develop during early brain formation?
Tourette organoids reveal neurological anomalies
To address this question, the researchers recruited five individuals with TS and 11 control participants. They reprogrammed a small skin biopsy from each person into iPSC lines and differentiated these lines into three-dimensional basal ganglia organoids. Organoids allow researchers to follow neural progenitors as they differentiate into diverse neuronal types under conditions that recapitulate aspects of embryonic brain development.
Using immunocytochemistry to visualize cell-type specific proteins and RNA sequencing to profile gene expression, the team compared TS-derived organoids with control organoids. Both approaches revealed a marked reduction of inhibitory interneurons in organoids from TS patients.
“The organoid data indicate these interneurons are not being generated during development,” says Melanie Brady, Ph.D., a lead investigator on the project. “We observed from the earliest stages of differentiation that progenitor populations that normally yield interneurons were redirected toward other cell fates, producing abnormal neuronal patterning.”
In TS organoids, progenitors that should adopt a medial ganglionic eminence (MGE) identity—an embryonic region that gives rise to cholinergic and GABAergic interneurons—showed impaired specification. Instead, there was a shift toward dorsolateral identities at the expense of ventromedial fates, which helps explain the interneuron shortfall.
At the molecular level, transcriptome analysis implicated altered signaling downstream of the Sonic Hedgehog (SHH) pathway. TS neural stem cells displayed a reduced response to SHH, a morphogen added early in culture to promote basal ganglia identity. The researchers also observed changes in the expression of GLI transcription factors and evidence of ciliary disruption—both of which could impair SHH signaling and contribute to the mispatterning.
Abnormalities and tic severity appear related
The study further suggested that the degree of developmental abnormality in organoids correlates with clinical severity. One TS participant who had much milder symptoms produced organoids with a smaller interneuron deficit and less pronounced mispatterning than the other patients. This finding raises the possibility that organoid phenotypes could reflect, and potentially predict, differences in disease severity among individuals.
“Detecting neurobiological differences at the developmental level could one day enable earlier diagnosis and open new avenues for intervention,” Brady notes.
With these developmental differences and a candidate mechanism identified, the team plans to investigate the genetic and epigenetic factors that might underlie the blunted SHH response in TS. Understanding those contributors could suggest therapeutic strategies—for example, drugs that enhance the activity of remaining interneurons or modify developmental signaling to restore proper basal ganglia patterning.
“Once we understand why a developmental process goes awry, we can begin to design ways to correct it,” Vaccarino says.
About this Tourette syndrome research news
Author: Isabella Backman
Source: Yale
Contact: Isabella Backman – Yale
Image: The image is in the public domain
Original Research: Closed access.
“Mispatterning and interneuron deficit in Tourette Syndrome basal ganglia organoids” by Melanie V. Brady et al. Molecular Psychiatry
Abstract
Mispatterning and interneuron deficit in Tourette Syndrome basal ganglia organoids
Tourette Syndrome (TS) is a neuropsychiatric disorder thought to involve a reduction of basal ganglia (BG) interneurons and malfunctioning of BG circuitry. Whether interneurons fail to develop or are lost postnatally has been unclear.
To investigate early developmental pathophysiology in TS, induced pluripotent stem cell (iPSC)-derived basal ganglia organoids from TS patients and healthy controls were compared across multiple assays.
BG organoids from TS individuals demonstrated impaired medial ganglionic eminence fate and decreased differentiation of cholinergic and GABAergic interneurons. Transcriptome analyses revealed organoid mispatterning in TS, favoring dorsolateral over ventromedial identities.
The results point to altered expression of GLI transcription factors downstream of the Sonic Hedgehog signaling pathway, with evidence of ciliary disruption at the earliest stages of BG organoid differentiation, as a potential mechanism for BG mispatterning in TS.
This study uncovers early neurodevelopmental underpinnings of TS neuropathology using organoids as a model system.