Dr. Schlebach, you published a paper on SLC6A8 in the Journal of Biochemistry. What inspired you to start working on it?“
While finishing my postdoctoral studies at Vanderbilt University (circa 2015), I started to consider problems I was interested in pursuing as an independent investigator. I became aware of the creatine transporter and its link to CCDS through a discussion with Dr. Randy Blakely (Florida Atlantic University), a renowned expert on SLC6 transporters, who encouraged me to consider this topic. After reading more on the topic, it seemed like an excellent avenue for research that dovetailed nicely with my own interests and expertise. The creatine transporter immediately seemed like a good drug target due to the fact that it has a clearly defined pocket that binds a naturally abundant metabolite.”
What does your lab do?
“Our lab studies the molecular basis of various diseases of membrane protein misfolding including: cystic fibrosis, retinitis pigmentosa, and cerebral creatine deficiency syndromes.”
What are you doing for Creatine Deficiencies?
“We originally set out to survey how the mutations that cause creatine transporter deficiencies cause a loss of creatine uptake. Our findings to date have shown that these mutations cause a spectrum of molecular defects in the creatine transporter, and suggest the molecular basis of this disease is comparable to that of cystic fibrosis. These observations suggest a pathway for the development of new therapeutics.”
Dr. Schlebach, your lab cultivates a more broad set of experimental and computational approaches on structural modeling of proteins. How does this help us understand the function of the protein?
“Our lab actually tends to focus on the questions and problems first, and tries to pick the best tools for the job. In the case of creatine transporter deficiency, our experimental measurements suggested a subset of known mutations specifically perturb the transporter function. We therefore pursued molecular modeling through a collaboration with Dr. Julia Koehler Leman (Simons Foundation) in order to gain insights into the structural basis for this distinction. Dr. Koehler Leman spearheaded these modeling efforts, and deserves all of the credit for this. Her models revealed that the variants that compromise function do so by directly disrupting the creatine binding pocket. This will again help to inform drug development efforts.”
Can you elaborate on how molecular modeling can help us in therapeutic discovery?
“Molecular structure is closely associated with function. Modeling the structure of creatine transporter variants can therefore provide insights as to how they disrupt its function. With this information, we can distinguish mutations that perturb function and those that disrupt folding and expression, which can provide a decisive advantage in the development and targeting of new therapeutics.”
What is in-silico screening?
“Virtual or in-silico screening utilizes structural models to identify small molecules that are likely to bind to a target protein–in this case, the creatine transporter. While screening can be carried out experimentally (in real life), virtual screens are generally cheaper, faster, and can be used to search much larger libraries of compounds. Identifying compounds in this manner allows one to efficiently narrow down the pool of candidates to a more manageable set of compounds that can then be evaluated in greater detail.”
Are there examples of drugs that have been discovered with the help of structural modeling and subsequent in-silico screening?
“Virtual screening is widely used in drug discovery, and there are many cases where these approaches have successfully identified molecules with the desired properties. Nevertheless, it is typically used early on in the drug discovery process, and pharmaceutical companies do not necessarily publish these sorts of data. It is therefore difficult to say how many molecules that were identified in this manner have since been developed into approved therapeutics.”
What are some of the challenges unique to SLC6A8 modeling?
“One challenge is the lack of an experimentally derived structural model of the creatine transporter. Nevertheless, there are numerous structures of homologous transporters that helped Dr. Koehler Leman to develop our current structural models. Previous docking studies on models of related transporters lend confidence to the feasibility of virtual screening using these structural models.”
Do you consider the SLC6A8 channel to be druggable? What are the similarities of SLC6A8 with other druggable transporters?
“Yes, I think there is good reason to believe this protein is druggable. First, I would say that there are a variety of creatine analogs that are already known to bind the transporter with moderate affinity. It is quite possible that some of these compounds may help to refold mutant transporters. Second, the Sucic and Freissmuth groups have previously shown that the function of misfolded variants of the dopamine transporter, which is quite similar to the creatine transporter, can be partially restored by inhibitors that bind and stabilize its native conformation. Third, our structural models clearly reveal a discrete binding pocket that is an excellent target for structure-based drug design. Fourth, pharmacological chaperones that bind and stabilize mutant forms of CFTR, another type of misfolded transporter, have recently revolutionized the treatment of cystic fibrosis. Based on these considerations, this seems like a tractable problem. We just need to come together as a community and make this happen!”
What are some of the tools, which if available, would be beneficial to work on the SLC6A8 transporter?
“Current mouse models that lack the creatine transporter have been immensely successful at revealing the physiological basis of CCDS. However, it would be great to have mouse models that express misfolded variants of the creatine transporter. This would eventually provide a more powerful approach to evaluate the physiological effects of small molecules designed to rescue the expression of defective transporter variants.”
If you were to hazard a guess on the involvement of SLC6A8 channels in other disorders or health states, what would they be?
“There was a publication in Cell a few years back showing that creatine transporters are upregulated in certain types of pancreatic cancer, and that the increased expression of these transporters helps fuel the growth of these cancer cells. This makes sense to me–creatine acts as a battery for the cell and cancer cells need extra juice to fuel their growth. This might be worth watching–if the creatine transporter becomes a major cancer target, there may be others engaged in the discovery of compounds that bind the creatine transporter. In this case, we may be able to repurpose some of these compounds for CCDS.”
What are the next steps in the study of SLC6A8?
“Current knowledge on the molecular basis of creatine transporter deficiency suggests it may be a favorable target for the development of pharmacological chaperones (aka correctors) that bind to the mutant forms of the protein and correct their molecular defects. We believe virtual screening approaches represent the next logical step in the drug discovery process.”
Are you wondering how you can help more research in CCDS happen? Here are a few simple ways you can get involved:
- Participate in research by filling out surveys or donating biosamples to Coriell
- Tell your story to advocate for CCDS and participate in ACD events!
- Help the ACD fundraise to fund more research!