FGF pathway overactivation underlies reduced neurogenesis in cerebellar organoid models of neurodevelopmental ciliopathy

This study demonstrates that FGF/MAPK pathway overactivation at the ciliary base drives reduced neurogenesis and impaired Purkinje cell formation in Joubert syndrome cerebellar organoids, a defect that can be rescued by pharmacological inhibition of FGF receptors.

The Gist

The Big Picture: A Broken Antenna and a Stuck Switch

Imagine the human brain is a massive, bustling construction site. To build a perfect building (a healthy brain), the workers (cells) need to know exactly when to stop building new rooms (proliferation) and when to start furnishing them (differentiation into specific neurons).

In this study, scientists investigated Joubert Syndrome, a rare genetic disorder that causes the "vermis" (a specific part of the brain called the cerebellum, which controls balance and coordination) to be underdeveloped. People with this condition often have trouble walking and learning.

The culprit is a broken gene called RPGRIP1L. This gene is like the blueprint for a tiny, antenna-like structure on the surface of cells called a primary cilium. Think of the cilium as a cell's "Wi-Fi antenna." It receives signals from the outside world to tell the cell what to do.

The Problem: The Antenna is Broken, But the Signal is Too Loud

The researchers grew tiny, 3D models of human brains (called organoids) in a lab using stem cells from healthy people and from patients with Joubert Syndrome.

What they found:

1. The Broken Antenna: In the patient's cells, the "antenna" (cilium) was defective. It was missing a crucial part of its base.
2. The Stuck Switch: Usually, when a cell receives a signal to grow, it listens, grows a bit, and then the signal turns off so the cell can mature. However, in the patient's cells, the "antenna" was broken in a way that made the FGF signal (a growth signal) get stuck in the "ON" position.
3. The Result: Because the "growth" switch was stuck on, the brain cells kept multiplying like crazy (hyperproliferation) and refused to stop and become the specific types of neurons needed for the cerebellum (specifically Purkinje cells, which are the "conductors" of the cerebellum).
   • Analogy: Imagine a construction crew that keeps hiring more workers instead of building the house. You end up with a huge pile of workers but no finished rooms.

The Discovery: It's Not Just About the Antenna

For a long time, scientists thought the problem was just that the antenna was broken, so the cell couldn't hear anything. But this study found something surprising: the cell could hear the signal, but it couldn't turn it off.

The broken antenna (RPGRIP1L) failed to act as a "dimmer switch." Instead of dampening the signal, the broken antenna let the FGF signal roar at full volume right at the base of the antenna. This caused the cells to stay in a "baby" state (progenitors) forever, preventing them from growing up into the specialized neurons needed for balance and coordination.

The Solution: Flipping the Switch Back

The most exciting part of the study is the potential cure. The researchers asked: If we can't fix the broken antenna, can we just turn down the volume of the signal?

They treated the sick brain organoids with a drug called BGJ-398, which acts like a "volume knob" for the FGF signal.

The Results:

• The Growth Stopped: The cells stopped multiplying uncontrollably.
• The Cells Matured: The cells finally started turning into the correct types of neurons (Purkinje cells).
• The Antenna Stayed Broken: Interestingly, the drug did not fix the antenna itself. The antenna was still broken, but because the "volume" of the signal was turned down, the brain tissue developed correctly anyway.

Why This Matters

This study changes how we understand Joubert Syndrome and similar brain disorders.

• Old View: The antenna is broken, so the cell is deaf.
• New View: The antenna is broken, so the cell is deaf to the "stop" signal and keeps listening to the "go" signal too loudly.

The Takeaway:
Even if we can't fix the genetic defect (the broken antenna) right now, we might be able to treat the disease by using drugs to balance the chemical signals. It's like fixing a noisy room not by repairing the broken speaker, but by turning down the volume on the amplifier. This opens the door for new drug therapies that could help people with Joubert Syndrome and other neurodevelopmental disorders.

Technical Summary

1. Problem Statement

Joubert syndrome (JBTS) is a rare neurodevelopmental disorder characterized by cerebellar vermis hypoplasia/dysplasia and the "molar tooth sign" on brain imaging. It is caused by mutations in genes encoding primary cilia proteins, specifically those involved in the ciliary transition zone (e.g., RPGRIP1L). While mouse models have shown cerebellar defects, the specific molecular mechanisms linking ciliary dysfunction to human cerebellar malformations remain poorly understood. Previous studies suggested that SHH-dependent granule cell progenitor (GCP) proliferation defects might be the cause, but human fetal samples showed vermis hypoplasia independent of GCP defects, leaving the primary origin of the pathology unresolved. There is a critical need for human-specific models to elucidate the early cellular and molecular events driving JBTS cerebellar defects and to identify potential therapeutic targets.

2. Methodology

The authors utilized human induced pluripotent stem cell (hiPSC)-derived cerebellar organoids to model early human cerebellar development and JBTS pathology.

• Cell Lines:
  • Controls: Two independent hiPSC lines (Ph2 and Wtc11).
  • RPGRIP1L Knockout (KO): CRISPR-engineered KO lines derived from the same control backgrounds.
  • JBTS Patient Line: hiPSCs from a patient (NG2266) with severe JBTS carrying biallelic RPGRIP1L mutations (a nonsense mutation p.Q684X and a splice variant c.2304+1G>T).
• Differentiation Protocol: Organoids were generated using a modified protocol involving sequential treatment with FGF2, SB431542 (TGFβ inhibitor), FGF19, and SDF1, with dynamic culture conditions (orbital agitation) to promote midbrain-hindbrain boundary formation and tissue self-organization.
• Experimental Approaches:
  • Transcriptomics: Bulk RNA-seq performed at multiple timepoints (days 7, 14, 21, 28, 35) to identify differentially expressed genes (DEGs).
  • Immunofluorescence & Microscopy: Confocal and Airyscan microscopy to assess cell lineage markers (Purkinje, Granule), progenitor proliferation (SOX2, EdU), cilia structure (INPP5E, $\gamma$-TUBULIN), and signaling pathway activation (pMEK1/2, SP8, FGF8).
  • Pharmacological Rescue: Treatment with BGJ-398 (Infigratinib), a pan-FGFR inhibitor, administered from day 11 to day 18 to test if blocking FGF signaling rescues the phenotype.
  • Proliferation Assays: EdU pulse-chase experiments to quantify cell cycle dynamics and label retention.

3. Key Contributions

• Human-Specific Mechanism: The study establishes that RPGRIP1L deficiency in human neural progenitors leads to FGF/MAPK pathway overactivation, a mechanism distinct from the SHH-centric defects often observed in mouse models.
• Ciliary Signaling Defect: It identifies that the primary ciliary base acts as a signaling hub where RPGRIP1L is required to attenuate FGF signaling. Loss of RPGRIP1L results in the accumulation of activated MEK1/2 at the ciliary base, even in the absence of a visible axoneme in some cells.
• Therapeutic Rescue: The study demonstrates that pharmacological inhibition of FGF receptors can rescue the neurogenic defects and restore Purkinje cell lineage formation, suggesting a potential therapeutic avenue for JBTS.
• Cell-Type Specificity: It reveals that RPGRIP1L is differentially required for ciliogenesis in polarized (apical) vs. non-polarized neural progenitors, with severe ciliary defects observed primarily in non-polarized cells.

4. Key Results

A. Phenotypic Characterization of RPGRIP1L-Deficient Organoids

• Morphology: RPGRIP1L-deficient organoids exhibited a significant increase in size (2.3 to 3.3-fold larger than controls) starting from day 21.
• Neurogenesis Defect: There was a severe reduction in Purkinje cell markers (OLIG2, SKOR2, CALB1, FOXP2) and a modest decrease in glutamatergic lineage markers (BARHL1, ATOH1).
• Progenitor Expansion: The organoids showed an increased proportion of neural progenitors (SOX2+) and an expanded apical surface area (ZO1+), indicating a failure to exit the progenitor state.

B. Molecular Mechanism: FGF/MAPK Overactivation

• Transcriptomics: RNA-seq at day 14 revealed massive upregulation of FGF pathway genes (ligands FGF8, FGF17, FGF19 and targets SPRY1-4, ETV4/5, SP8) and downregulation of neurogenic genes (NEUROG1, NEUROD1).
• Signaling Localization: Immunostaining showed that pMEK1/2 (activated MAPK effector) was enriched at the ciliary base in control cells. In RPGRIP1L-deficient cells, pMEK1/2 levels at the ciliary base were significantly increased, even in cells lacking axonemes.
• Feedback Loop: The study proposes a self-sustaining loop where FGF8 produced by apical progenitors activates the pathway in neighboring cells, upregulating SP8, which in turn further drives FGF8 expression.

C. Ciliogenesis Analysis

• Differential Requirement: In polarized apical progenitors (forming rosettes), cilia were present but occasionally displayed bulbous tips. In non-polarized regions, ciliogenesis was severely impaired (reduced density, longer but defective cilia).
• Independence of Rescue: Crucially, blocking FGF signaling did not rescue the ciliary structural defects (density, length, or INPP5E content), indicating that the ciliary defects are a direct consequence of RPGRIP1L loss, while the neurogenic defects are a downstream consequence of FGF overactivation.

D. Pharmacological Rescue

• Treatment: Administration of the FGFR inhibitor BGJ-398 from day 11 to 18.
• Outcomes:
  • Restored organoid size to control levels.
  • Reduced pMEK1/2 levels at the ciliary base.
  • Downregulated FGF pathway targets (SP8, ETV4/5).
  • Rescued Neurogenesis: Significantly restored the expression of proneural genes (NEUROD1) and Purkinje cell markers (OLIG2, CALB1).
  • Proliferation: Restored the balance between proliferation and differentiation, reducing the number of EdU-retaining cells (indicating reduced hyperproliferation).

5. Significance

• Pathogenic Insight: The paper redefines the etiology of cerebellar hypoplasia in JBTS, shifting the focus from a primary defect in SHH-driven granule cell proliferation to an early defect in FGF signal attenuation mediated by ciliary transition zone proteins.
• Developmental Origin: It suggests that the vermis hypoplasia seen in patients may result from early progenitor hyperproliferation that disrupts the precise spatiotemporal balance required for midline fusion and vermis formation, rather than a simple lack of cell production.
• Therapeutic Potential: The successful rescue of neurogenesis defects via FGFR inhibition in a human model provides a strong rationale for exploring repurposed FGFR inhibitors (like Infigratinib) as a potential treatment strategy for Joubert syndrome and related ciliopathies.
• Model Validation: It reinforces the utility of human cerebellar organoids for studying human-specific developmental mechanisms that differ from murine models, particularly regarding the timing and nature of ciliary signaling pathways.