Discovered: Hidden weaknesses in microtubules stabilized by cancer drug paclitaxel
Tiny structural defects can trigger the breakdown of microtubules, the microscopic structures that help cells maintain their shape, transport materials and divide, researchers at the Nano Life Science Institute (WPI-NanoLSI) at Kanazawa University have discovered
Kanazawa (Japan): Tiny structural defects can trigger the breakdown of microtubules, the microscopic structures that help cells maintain their shape, transport materials and divide, researchers at the Nano Life Science Institute (WPI-NanoLSI) at Kanazawa University have discovered.
Using high-speed atomic force microscopy, the team observed how these defects develop - and sometimes cause microtubules to collapse, even in the presence of paclitaxel, a widely used cancer drug that stabilizes them.
The findings, published in ACS Nano, challenge the conventional view that microtubule breakdown begins primarily at their ends and offer new insights into the structural processes that may influence responses to cancer treatment.
About Microscopic
Inside almost every cell is a network of microscopic structures known as the cytoskeleton. An important part of this network consists of microtubules, long, hollow tubes assembled from proteins called tubulin. These structures act as internal supports and tracks for transporting materials. They also play an essential role in cell division by helping separate chromosomes into two daughter cells.
Microtubules are constantly changing. They grow by adding tubulin molecules and shrink when these molecules are removed. Sometimes a growing microtubule suddenly begins to shorten rapidly, a process scientists call catastrophe.
For many years, researchers have explained this behavior mainly through changes at the ends of microtubules. In particular, the loss of a stabilizing molecular structure known as the GTP cap can cause the microtubule to begin breaking down.
However, exactly how structural damage develops within the walls of microtubules, and whether such damage can initiate their collapse, has remained difficult to observe directly.
Understanding these processes is particularly important in cancer research. Paclitaxel, also known by the brand name Taxol, is used to treat several types of cancer, including breast and ovarian cancer. It works by stabilizing microtubules and interfering with the changes needed for normal cell division. However, not all cancer cells respond equally to the drug.
Discovering defects inside stabilized microtubules
To investigate how microtubules behave when exposed to paclitaxel, Xin Zhang and colleagues, led by Richard W. Wong at Kanazawa University, used high-speed atomic force microscopy (HS-AFM).
This technique uses an extremely fine probe to scan biological structures in liquid and produce a rapid sequence of images with nanometer-scale detail. Unlike conventional methods that provide static pictures, HS-AFM allows researchers to follow structural changes as they occur.
The team prepared microtubules from purified tubulin proteins, stabilized them with paclitaxel and observed their behavior over time.
At first, many microtubules appeared stable. Their overall tubular shape remained intact, and the researchers could resolve the regularly arranged protein units that make up their walls.
Closer examination, however, revealed something unexpected.
Small defects appeared within otherwise intact regions of the microtubule wall. Some began as shallow depressions only a few nanometers across before gradually developing into larger structural disruptions.
These defects matter because they can weaken the microtubule wall and determine whether the structure remains intact or begins to collapse. Some defects underwent partial repair, while others grew and triggered the breakdown of the microtubule, sometimes spreading in both directions from the damaged region.
Importantly, these changes occurred within the microtubule itself rather than exclusively at its ends, even though the structures had been treated with paclitaxel.
The observations suggest that microtubule stability depends not only on what happens at its ends but also on the condition of the protein framework along its entire length. Tiny structural defects can therefore have consequences for the stability of the whole microtubule.
Four different ways microtubules can collapse
One of the study's most significant findings was the identification of four distinct patterns of microtubule breakdown.
By analyzing 73 collapse events, the researchers found that 42 percent began at one end of a microtubule, consistent with the conventional model.
However, almost as many events, 40 percent, began at defects within the microtubule wall. In these cases, the structure broke down in both directions from the damaged region.
Another 10 percent of events involved simultaneous breakdown from both ends, while the remaining 8 percent began at multiple sites within the microtubule.
The results show that internal structural defects are a substantial source of microtubule instability under the experimental conditions.
The team also observed temporary structural changes in which parts of the microtubule surface formed curved or arc-like features. Some of these regions underwent further remodeling or partial recovery before either stabilizing or progressing toward collapse.
These observations suggest that microtubule breakdown can develop through a series of structural changes rather than always occurring as a single abrupt event.
Based on these observations, the researchers propose a model called Spontaneous Lattice Decoupling (SLD). The model describes how tiny defects within the microtubule wall can follow two different paths: some undergo repair, allowing the structure to remain intact, while others develop into larger disruptions that eventually cause the microtubule to collapse.
"Microtubules may look stable from the outside, but our observations show that tiny defects can develop within their structure and sometimes lead to complete collapse," said Richard W. Wong of Kanazawa University's Nano Life Science Institute. "What is particularly interesting is that these changes can occur even when the microtubules are treated with a drug designed to stabilize them."
What the findings mean for cancer treatment
Paclitaxel (Taxol) is widely used to treat breast, ovarian, lung and other cancers. It works by stabilizing microtubules and disrupting the structural changes needed for cell division, thereby preventing cancer cells from multiplying.
However, resistance to paclitaxel remains a major clinical challenge. Although several mechanisms of resistance have been identified, the nanoscale structural behavior of microtubules exposed to the drug is not yet fully understood.
The new findings raise an important question: why do microtubules remain vulnerable to structural damage even when treated with a drug designed to stabilize them?
The new findings reveal that paclitaxel-treated microtubules are not necessarily rigid or structurally uniform. Although the drug preserves their overall shape, localized regions can remain vulnerable to structural changes.
This raises the possibility that differences in microtubule structure and the ability to repair or propagate small defects could influence how cells respond to drugs that target microtubules.
The researchers propose a framework in which small structural defects develop, undergo local remodeling, and sometimes progress to microtubule collapse. This sequence provides a way to understand how apparently stable microtubules can still undergo substantial structural changes.
The findings could eventually contribute to research aimed at understanding why some cancer cells respond differently to paclitaxel and related drugs.
However, the present study does not establish a mechanism of drug resistance in cancer cells or demonstrate a new treatment strategy.
The researchers now hope to investigate microtubules associated with cancer cells that are resistant to paclitaxel. By comparing their nanoscale structural behavior with that of microtubules from drug-sensitive cells, they aim to determine whether differences in defect formation, structural stability and repair contribute to resistance. Such studies could provide new insights into why some cancers respond less effectively to microtubule-targeting drugs.
A new view of microtubule stability
Beyond cancer treatment, the findings have implications for understanding the basic biology of microtubules.
These structures are involved in many essential cellular processes, including chromosome separation, intracellular transport and the organization of cell components. Their stability depends on interactions between the individual protein units that form their walls.
The study shows that small local disruptions can have very different outcomes. Some remain temporary and recover, while others spread and lead to the breakdown of an entire microtubule.
The researchers also found that the overall bending of a microtubule was not significantly associated with whether it collapsed. Instead, localized defects appeared to be more closely related to where breakdown began and how it progressed.
By observing these changes directly, the study provides a more detailed picture of how microtubules maintain their structure and how that structure can fail.
Study limitations
The experiments were conducted on microtubules assembled from purified proteins and observed under laboratory conditions rather than inside living cells.
The paclitaxel concentration was selected to provide strong stabilization during prolonged imaging and should not be interpreted as reproducing drug exposure in patients.
Although control experiments showed similar structural changes under different sample preparation conditions, the researchers cannot completely exclude the possibility that interactions between the HS-AFM probe and the microtubules influenced some of the observations
Further studies will be needed to determine how these structural processes occur inside cells and whether they contribute directly to differences in cancer drug sensitivity or resistance.
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