Developmental regulation of kinetochore phosphorylation determines mitotic fidelity

This study reveals that the low mitotic fidelity observed in human pluripotent stem cells is caused by developmentally regulated hypophosphorylation of the kinetochore protein HEC1, rather than reduced protein abundance, and can be corrected by enhancing phosphorylation through PP2A inhibition or cellular differentiation.

The Gist

The Big Picture: Why Stem Cells Make Mistakes

Imagine your body is a massive construction site. Every time a cell divides to make a new one, it has to copy its instruction manual (DNA) and split it perfectly in half so both new buildings get the exact same blueprint.

Usually, this happens perfectly. But Human Pluripotent Stem Cells (hPSCs)—the "master builder" cells that can turn into any type of tissue in the body—are notoriously bad at this. They drop the blueprints, lose pages, or send the wrong instructions to the new buildings. This is called aneuploidy (having the wrong number of chromosomes), and it can lead to birth defects or cancer.

Scientists have known for a while that these stem cells make more mistakes than regular adult cells, but they didn't know why. This paper solves that mystery.

The Two Suspects: The "Brick Count" vs. The "Traffic Light"

When a cell divides, it uses a complex machine called the kinetochore to grab the DNA and pull it apart. Think of the kinetochore as a claw that grabs the DNA rope.

The scientists investigated two possible reasons why stem cells drop the rope:

1. The "Brick Count" (Protein Levels): Maybe the claw is just too small or weak because it's missing building blocks (proteins).
2. The "Traffic Light" (Phosphorylation): Maybe the claw is the right size, but the signal telling it when to let go is broken.

Clue #1: The Claw is Small, But That's Not the Problem

The researchers found that stem cells do have fewer building blocks. Their kinetochore claws are smaller and have about 50% fewer proteins than regular cells.

• The Experiment: They tried to fix this by gluing extra building blocks onto the stem cells' claws, making them as big as regular cells.
• The Result: The cells still dropped the rope!
• The Lesson: It's not about the size of the machine; it's about how the machine is controlled.

Clue #2: The Traffic Light is Stuck on "Red"

This is where the real discovery happened. The key to a perfect division is a protein called HEC1. Think of HEC1 as the grip on the claw.

• How it works: To fix mistakes, the cell needs to be able to let go of the rope and try again. To do this, the cell uses chemical "tags" (phosphorylation) to loosen HEC1's grip.
  • Tags ON (Phosphorylated): The grip loosens. If the rope is attached wrong, it slips off so it can be re-attached correctly.
  • Tags OFF (Hypophosphorylated): The grip tightens. The rope is stuck. If it was attached wrong, it stays wrong, and the cell divides with a mistake.

The Discovery: In stem cells, the HEC1 grip is too tight. The chemical tags that tell it to loosen up are missing.

• Why? Stem cells have a "super-phosphatase" (a chemical eraser) called PP2A that is working overtime, wiping away the tags before they can do their job.
• The Analogy: Imagine a construction worker (the cell) trying to untangle a knot. In regular cells, the worker has a tool to loosen the knot. In stem cells, someone keeps pouring super-glue on the knot, making it impossible to untangle.

The Solution: Differentiation is the Fix

The paper shows that when these stem cells stop being "master builders" and turn into regular cells (a process called differentiation), the super-glue stops working.

• The chemical tags return.
• The grip loosens up just enough to allow for error correction.
• The division becomes perfect.

They also tested a drug (LB-100) that temporarily stops the "super-glue" (PP2A) in stem cells. Even without turning them into regular cells, this drug made the stem cells divide much more accurately.

The Takeaway

The reason stem cells are so prone to genetic errors isn't because they are broken or missing parts. It's because their "safety brakes" are turned off.

• In Stem Cells: The system is set to "Hold Tight" to maintain their special, flexible state. This makes them great at being stem cells, but terrible at dividing accurately.
• In Regular Cells: The system switches to "Check and Correct," allowing them to divide with high precision.

Why does this matter?
If we want to use stem cells for therapies (like growing new heart tissue for a patient), we need to make sure they don't accidentally become cancerous due to these division errors. This paper suggests that by tweaking the chemical signals (the "traffic lights") that control how tightly they hold onto DNA, we might be able to make stem cell treatments safer and more reliable.

Technical Summary

1. Problem Statement

Human pluripotent stem cells (hPSCs), including embryonic (hESCs) and induced (hiPSCs) stem cells, exhibit high rates of genomic instability and aneuploidy during in vitro culture. This instability is a major barrier to their clinical application, as aneuploidy impairs differentiation potential and drives tumorigenesis. Previous research established that hPSCs have a low fidelity of chromosome segregation, primarily caused by lagging chromosomes in anaphase due to the persistence of erroneous merotelic kinetochore-microtubule (k-MT) attachments.

While it was known that hPSCs have reduced levels of centromere and kinetochore proteins compared to somatic cells, it remained unclear whether this reduction in protein abundance or a dysregulation of post-translational modifications (specifically phosphorylation) was the primary driver of this low mitotic fidelity.

2. Methodology

The authors employed a comparative approach using isogenic cell lines to eliminate genetic variation as a confounding factor:

• Cell Models: They compared human induced pluripotent stem cells (WTC-11, GM) and human embryonic stem cells (H1) against their respective isogenic primary somatic fibroblasts.
• Protein Quantification: Quantitative immunofluorescence microscopy was used to measure the abundance of key centromere/kinetochore proteins (CENP-A, CENP-C, HEC1) and kinases (Aurora B, Cyclin B1/2) at individual kinetochores during mitosis.
• Genetic Manipulation: To test the sufficiency of protein levels, they generated stable hPSC lines overexpressing GFP-CENP-A to restore centromeric protein levels to somatic ranges.
• Pharmacological Inhibition: They used LB-100, a specific catalytic inhibitor of the phosphatase PP2A, to acutely inhibit phosphatase activity in both hPSCs and somatic cells.
• Differentiation: hPSCs were differentiated using all-trans retinoic acid (RA) to assess changes in phosphorylation and mitotic fidelity upon loss of pluripotency.
• Metrics: Key readouts included interkinetochore distance (IKD), spindle size, HEC1 phosphorylation status (at sites S44 and T31), and the frequency of lagging chromosomes in anaphase.

3. Key Contributions & Results

A. Protein Abundance is Reduced but Not the Primary Cause of Errors

• Finding: hPSCs exhibit a ~50% reduction in centromeric CENP-A and CENP-C, and a lesser reduction in kinetochore HEC1, compared to isogenic somatic cells.
• Intervention: Overexpression of CENP-A in hPSCs successfully restored CENP-A, CENP-C, and HEC1 levels to near-somatic quantities.
• Result: Despite restoring protein abundance to somatic levels, chromosome segregation error rates (lagging chromosomes) did not improve. This indicates that low protein abundance is not the causal mechanism for hPSC genomic instability; the reduced levels are sufficient for structural integrity but insufficient for high-fidelity segregation.

B. Hypophosphorylation of HEC1 Drives Instability

• Finding: The study identified that HEC1 (a critical subunit of the NDC80 complex) is hypophosphorylated at key regulatory sites in hPSCs compared to somatic cells.
  • Site S44: Phosphorylated by Aurora B/A kinases. hPSCs showed ~50% reduction in S44 phosphorylation. This was linked to reduced Aurora B abundance and activity (shorter spindles).
  • Site T31: Phosphorylated by Cyclin B/Cdk1. hPSCs showed ~50% reduction in T31 phosphorylation.
• Mechanism: Unlike S44, the reduction in T31 phosphorylation was not due to a lack of kinase (Cyclin B1/2 and Cdk1 were actually enriched at hPSC kinetochores). Instead, the data suggests elevated phosphatase activity (specifically PP2A) is counteracting kinase activity, leading to net hypophosphorylation.
• Consequence: Hypophosphorylation of HEC1 increases its binding affinity for microtubules. This leads to hyperstable k-MT attachments, which prevents the cell from correcting merotelic errors, resulting in lagging chromosomes.

C. Reversibility via Phosphatase Inhibition and Differentiation

• Pharmacological Rescue: Acute inhibition of PP2A using LB-100 in hPSCs increased HEC1 T31 phosphorylation levels. This increase correlated with a significant decrease in lagging chromosome rates, effectively improving mitotic fidelity.
• Developmental Regulation: Upon differentiation of hPSCs with retinoic acid (RA), HEC1 T31 phosphorylation levels increased naturally, and lagging chromosome rates decreased. This confirms that the low-fidelity state is an inherent feature of the pluripotent state, reversible upon differentiation.

4. Significance

• Paradigm Shift: The study challenges the assumption that low protein abundance is the sole driver of mitotic errors in stem cells. It establishes that developmental regulation of the kinase-phosphatase network is the critical determinant of mitotic fidelity.
• Mechanistic Insight: It reveals that pluripotency programs actively suppress HEC1 phosphorylation (via elevated PP2A activity and reduced Aurora B), creating a "tunable" state of hyperstable microtubule attachments that is detrimental to error correction.
• Therapeutic Implications: The findings suggest that transiently modulating phosphatase activity (e.g., via PP2A inhibition) could be a strategy to preserve genome integrity in hPSCs during culture, reducing the risk of aneuploidy in stem cell-based therapies.
• Broader Context: The work highlights that mitotic fidelity is not a static cellular property but is dynamically regulated by developmental signaling pathways, linking cell fate decisions directly to the mechanics of chromosome segregation.

Conclusion

The paper concludes that the low mitotic fidelity of human pluripotent stem cells is caused by the developmental regulation of HEC1 phosphorylation. A pluripotent state promotes hypophosphorylation of HEC1 (specifically at T31 and S44) through elevated PP2A phosphatase activity and reduced Aurora B kinase activity. This leads to hyperstable kinetochore-microtubule attachments that fail to correct merotelic errors. Restoring phosphorylation levels, either through differentiation or PP2A inhibition, rescues chromosome segregation fidelity, demonstrating that the kinase/phosphatase balance is the primary gatekeeper of genomic stability in stem cells.