Photopharmacology: Light-gated control of the cytoskeleton
Paclitaxel (Taxol) is derived from the bark of the pacific yew tree. Image: imago images / Gottfried Czepluch
The cells of higher organisms rely on three dynamically reconfigurable systems of protein filaments (collectively referred to as the cytoskeleton), which play crucial roles in all fundamental cellular processes involving motion and directionality. One of these systems consists of massive hollow polymer tubes called microtubules, which are in turn made up of globular subunits called tubulins. Microtubules serve as highways for intracellular transport of mitochondria, neurotransmitters and other biochemical cargos – and coordinated microtubule assemblies form the spindle apparatus that is responsible for the ordered segregation of chromosomes to the daughter cells during cell division (mitosis). Hence, compounds that bind specifically to microtubules and stabilize or destabilize them, provide vital tools for research on cellular cargo trafficking, regulation of mitosis and patterning of embryonic development. Such compounds also find application as powerful anticancer drugs that inhibit tumor-cell proliferation – and drugs such as paclitaxel (Taxol), vinca alkaloids, epothilones, auristatins and dolastatins have been used to treat millions of cancer patients worldwide.
"The problem with using these drugs as research tools is that they are not precise enough to tell us what we need to know,” says Oliver Thorn-Seshold, who is in LMU's Department of Pharmacy. Biology is regulated at the subcellular level and with extreme temporal accuracy, but these drugs act on all the cells they reach, and it has not been possible to modulate their dynamics over time. Now, he and his colleagues have solved this problem. In cooperation with Dirk Trauner (New York University) and Anna Akhmanova (Utrecht University), he has developed light-responsive analogs of these drugs, which can be locally activated at specific times. This allows one to control their interactions with microtubules far more precisely. “These light-responsive reagents give us access to a range of powerful, high-precision biology studies,” explains Thorn-Seshold. The researchers have used their light-responsive compounds to optically control cell division, cell survival, cytoskeleton structure and remodelling rates down to the level of individual cells, and even to subcellular regions in neurons.
By developing light-responsive analogs of one of the most powerful and clinically important anticancer drugs – paclitaxel, a taxane class drug that stabilizes microtubules – the team hopes also to impact applied research. These high-precision photoresponsive taxanes may be useful for decoding how the clinical drugs exert both their desirable antitumour activity, and their undesirable side-effects, which are primarily caused by damage to neurons. “Since our photoresponsive analogues are also powerfully active in neurons, but can uniquely be applied to selected neurons within a given sample for tightly controlled studies, we believe that our compounds will give us a better understanding of how these side-effects arise,” Thorn-Seshold says.
The new compounds are the latest in a set of high-precision, light-responsive cytoskeleton research reagents developed by Dr. Thorn-Seshold and Prof. Trauner since 2013. Taxol is a difficult molecule to work on, because only a few modifications can be made easily. Most modifications could require months or years of effort to synthesize, before even a single compound can be tested. “Taxanes are also very water-insoluble compounds, which makes them tricky to apply reliably to cells or animals; we struggled to tune our compounds' polarity and solubility, and still cannot explain the patterns we observed,” Thorn-Seshold explains. However, the researchers are confident that this new reagent approach brings significant benefits. “There are important differences in both the biological and therapeutic effects of microtubule destabilisers, such as those we previously developed, compared to the stabilisers we have now created.” By bringing microtubule stabilisation under high-precision control, Thorn-Seshold and his colleagues are convinced that these new reagents will open up entirely new perspectives for cell biology research on topics where temporally or subcellularly specific roles of microtubules determine downstream biological effects, such as in cargo transport, cell migration, mitotic progression, and especially in neurobiology during neuron development or axonal regeneration.
Nature Communications 2020