⚗️ biochemistry

INPP5E interactome reveals novel connections to growth factor signaling

This study elucidates the molecular mechanisms of the Joubert syndrome protein INPP5E by identifying its tyrosine phosphorylation-dependent interactome with growth factor signaling regulators and demonstrating its differential modulation of PDGF and TGF{beta} pathways in primary cilia.

Original authors: Martin-Morales, R., Barbeito, P., Sierra-Rodero, B., Jimenez, L., Schroder Holm, M., Cilleros-Rodriguez, D., Ebert, L. K., Cortabarria, M., Pedersen, L. B., Badano, J. L., Irigoin, F., Schermer, B., L

Published 2026-02-15

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CC BY 4.0

Original authors: Martin-Morales, R., Barbeito, P., Sierra-Rodero, B., Jimenez, L., Schroder Holm, M., Cilleros-Rodriguez, D., Ebert, L. K., Cortabarria, M., Pedersen, L. B., Badano, J. L., Irigoin, F., Schermer, B., Link, W., Christensen, S. T., Garcia-Gonzalo, F. R.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your body is a bustling city, and every cell is a house. Sticking out of the roof of each house is a tiny, delicate antenna called a primary cilium. This antenna is crucial because it listens to the weather reports (chemical signals) from the outside world. If this antenna is broken or missing, the house gets confused, leading to a chaotic neighborhood. This is what happens in a group of diseases called ciliopathies, one of which is Joubert syndrome (JBTS), a condition that can cause brain and kidney problems.

The star of this story is a protein named INPP5E. Think of INPP5E as the specialized maintenance crew for that antenna. Its job is to manage the "fuel" (lipids) on the antenna's surface so it can hear signals correctly. But until now, scientists didn't fully understand how this crew gets its instructions or who it talks to.

Here is what this new research discovered, explained simply:

1. The "On" Switch

The researchers found that when the cell receives a "growth" signal (like a message from a protein called PDGF), it doesn't just turn on the antenna; it actually pokes the INPP5E maintenance crew with a specific chemical tag (tyrosine phosphorylation). It's like a foreman tapping the crew chief on the shoulder and saying, "Hey, we have a big job to do, get ready!"

Interestingly, this "poke" doesn't change how the crew works (their enzyme activity), but it does change who they hang out with.

2. The New Team-Up

Once INPP5E gets that "poke," it starts shaking hands with a new group of helpers.

SH3GL1 and SNX9: Think of these as the delivery trucks. They help the antenna shed old, worn-out parts (called ectovesicles) to make room for new signals. INPP5E and these trucks work together to keep the antenna clean and ready for new messages.
The Big Network: The study also found INPP5E hanging out with other major players in the cell's communication network, including proteins that control cell growth (mTORC2), cell shape (TGF-beta), and even other proteins known to be broken in Joubert syndrome. It's like discovering that the antenna maintenance crew is actually part of the city's entire emergency response team.

3. The "Velcro" Connection

There was a special moment where INPP5E needed a specific tag (a phosphorylated serine-85) to stick to a group of proteins called 14-3-3. Imagine 14-3-3 proteins as Velcro strips. Without the specific tag on INPP5E, the Velcro doesn't stick. But once the tag is there, they lock together tightly, ensuring the crew stays in the right place at the right time.

4. The Double-Agent Effect

The most surprising discovery is that INPP5E acts like a traffic cop that directs different types of traffic in opposite directions.

When the cell gets a PDGF signal (one type of growth message), INPP5E hits the brakes, slowing down a specific pathway (AKT).
However, when the cell gets a TGF-beta signal (a different type of message), INPP5E steps on the gas, speeding up a different pathway (SMAD2 and ERK).

Why Does This Matter?

Think of the cell as a car. If the antenna (cilium) is broken, the car can't see the road, leading to a crash (disease). This paper shows us exactly how the maintenance crew (INPP5E) communicates with the engine, the brakes, and the steering wheel.

By understanding these connections, scientists can better figure out why the "antenna" fails in diseases like Joubert syndrome. It's like finding the missing link in the instruction manual for the car's navigation system, which could one day help engineers (doctors) fix the broken parts and keep the city running smoothly.

Technical Summary: INPP5E Interactome and Growth Factor Signaling in Ciliopathies

1. Problem Statement

Primary cilia are essential sensory organelles, and their dysfunction leads to a class of disorders known as ciliopathies. Joubert syndrome (JBTS) is a specific recessive ciliopathy characterized by severe brain malformations and renal cysts. A critical molecular player in JBTS is INPP5E, a ciliary phosphoinositide lipid phosphatase. While it is established that INPP5E regulates growth factor signaling within the cilium, the precise molecular mechanisms governing its regulation, protein interactions, and downstream signaling effects remain poorly understood. Specifically, the link between INPP5E post-translational modifications (such as phosphorylation) and its functional interactome in the context of growth factor signaling was unclear.

2. Methodology

The study employed a multi-faceted approach combining cell biology, biochemistry, and interactomics:

Cellular Models: Fibroblasts were utilized to study cilium-dependent signaling.
Stimulation Models: Cells were stimulated with a constitutively active growth factor receptor (PDGFR-D842V) and the SRC tyrosine kinase to induce specific signaling states.
Biochemical Analysis:
- Assessment of INPP5E tyrosine phosphorylation status.
- Evaluation of INPP5E enzymatic activity under phosphorylated vs. non-phosphorylated states.
- Co-immunoprecipitation (Co-IP) and binding assays to determine protein-protein interactions.
Interactomics: Systematic identification of the INPP5E protein interaction network (interactome).
Site-Directed Mutagenesis: Investigation of specific residues, particularly Serine-85, to determine its role in binding 14-3-3 proteins.
Signaling Assays: Measurement of downstream phosphorylation events (AKT, SMAD2, ERK) in response to PDGF and TGF $\beta$ stimulation.

3. Key Contributions

This research provides a comprehensive map of the INPP5E interactome and elucidates the regulatory mechanisms controlling INPP5E function:

Elucidation of Regulatory Mechanisms: Demonstrated that INPP5E is a direct substrate for tyrosine kinases (PDGFR-D842V and SRC).
Functional Decoupling: Established that while tyrosine phosphorylation regulates INPP5E's protein-binding partners, it does not alter its intrinsic phosphatase enzymatic activity.
Identification of Novel Interactors: Mapped a network of growth factor signaling regulators and JBTS-associated proteins interacting with INPP5E.
Mechanism of 14-3-3 Binding: Defined the specific requirement for Serine-85 phosphorylation for the interaction between INPP5E and 14-3-3 proteins.
Pathway Specificity: Revealed that INPP5E exerts differential, opposing effects on distinct growth factor signaling pathways within the cilium.

4. Key Results

Tyrosine Phosphorylation & Binding: Constitutively active PDGFR and SRC induce tyrosine phosphorylation of INPP5E. This modification enhances the binding affinity of INPP5E to:
- SH3GL1: An endocytic regulator.
- SNX9: A phosphoinositide-binding protein involved in growth factor-induced ciliary ectovesicle release.
- Note: This phosphorylation did not change INPP5E's lipid phosphatase activity.
The INPP5E Interactome: The study identified several critical interaction partners, including:
- SIN1: The phosphoinositide-binding subunit of the mTORC2 complex.
- STRAP: A regulator of TGF $\beta$ and PI3K/AKT signaling.
- GRB2: A canonical growth factor receptor adaptor.
- JBTS-associated proteins: AHI1 and NPHP1, reinforcing the link between INPP5E and the ciliopathy complex.
14-3-3 Interaction: INPP5E strongly interacts with 14-3-3 proteins, but this interaction is strictly dependent on the phosphorylation of Serine-85.
Differential Signaling Regulation: In fibroblasts, INPP5E modulates two major pathways differently:
- PDGF Pathway: INPP5E downregulates PDGF-induced AKT phosphorylation.
- TGF $\beta$ Pathway: INPP5E upregulates TGF $\beta$ -induced SMAD2 and ERK phosphorylations.

5. Significance

This work significantly advances the understanding of ciliary biology and Joubert syndrome by:

Mechanistic Insight: Providing a molecular explanation for how INPP5E integrates growth factor signals via post-translational modifications (tyrosine and serine phosphorylation) rather than just enzymatic regulation.
Network Context: Placing INPP5E within a broader signaling network involving mTORC2, TGF $\beta$ , and PI3K/AKT pathways, suggesting that JBTS pathology may arise from the dysregulation of these specific signaling nodes.
Therapeutic Implications: By identifying specific interaction partners (like SH3GL1, SNX9, and 14-3-3) and regulatory residues (Ser-85), the study highlights potential targets for therapeutic intervention in ciliopathies.
Paradigm Shift: Demonstrating that a single ciliary protein (INPP5E) can act as a bifurcation point, suppressing one growth factor pathway (PDGF/AKT) while promoting another (TGF $\beta$ /SMAD/ERK), thereby fine-tuning cellular responses to external cues.