The Curious Case of Glioblastoma: Back to the Progenitors

Did you ever want to be a super hero? To have the ability to fly, read minds, or travel through time? With much frustration, I realized in my childhood that these powers were not in my metaphorical deck of cards. Unfortunately, the biological mayhem maker, Glioblastoma, has found a way to travel through time. No DeLorean. No Lightning. Just a steady diet of TGFα (Transforming Growth Factor).

Well... sort of through time. Astrocytes are a form of mature glial cells and are part of the brain’s native defense system. In the case of brain injury, they may also be produced from quiescent progenitors and stem cells to fill damaged voids. This is a good thing. This study highlights what is a bad thing and making efficient treatment of Glioblastoma Multiform extremely difficult. Researchers found that prolonged exposure to TGF promotes the conversion of “adult” astrocytes into neural progenitors and then stem cells (like the ones mentioned above). This is called anaplasia, and is a daunting characteristic of Glioblastoma Multiform (GBM, Stage IV glioma). These cells violate a critical feature of healthy brain function: absolute control over cellular replication.

Figure: Anaplastic transformation of astrocytes (a, day 0) to progenitor cells (c, day 7) in the presence of TGF. Cells are immuno-stained with a glial-specific actin required for cell structure and movement.

“Does prolonged TGF exposure affect the stability of wild type, mature astrocytes?”

      To address this question, researchers isolated healthy, adult, mouse and human astrocytes that were cultured in the presence of TGF. These cells were observed to take on the characteristic phenotype of bonafide progenitor cells (radial glial cells) after 2-3 days of exposure! The control-condition astrocytes, without TGF, displayed no changes of their healthy morphology. Within 7 days, 53% of +TGF cells had transformed. [Check out the progression above] After one month of exposure, the derived progenitor cells further transformed into pluripotent stem cells! If an astrocyte could indeed revert like this, as the Cancer Stem Cell theory may support, they are likely in a stem cell niche and may comfortably proliferate and dominate. For surgery, this means that if every last tumorigenic stem cell is not removed, then even one can repopulate the region. 

Figure: TGF induces morphological changes of healthy astrocytes into immunofluorescently labeled radial glial cells recognizable by their bipolar, long cell body. DAPI identifies cell nuclei by labeling chromosomes. GLAST is a glial-specific transporter. BLBP is a lipid binding protein specific to the brain. RC2 is a very specific spliced variant of Nestin only found in radial glial cells. All together, it means they are brain cells, they are glial cells, and most specifically are radial glial cells. **Note, they clearly chose the most representative samples of each stain, rather than showing the same cells each time. This is commonly done, but worth noting for comparison.

In a cited study, aberrant TGF activity was observed in 80% of early-phase, resected gliomas. Also, 20-40% of later-stage tumors also had increased expression of EGF-receptors.  I would expect that most neuronal cells do not have any, or many, growth factor receptors, as the brain is under such tight lock and key. So perhaps TGF induces early-stage morphological anaplasia while EGF-receptors follow with a re-found mechanism to replicate. This is kind of like finding 20 bucks in your pocket, but really like finding the fountain of youth. 

Also experimentally:

•   erbB1 (an EGF-receptor) knockout mice: TGF exposure significantly increased rates of apoptosis compared to wild type mice. (Wagner et al., 2006)

•   In vitro human and mouse cells with over expressed erbB1, without TGF, did not transform.

•   Derived radial glial cells later were shown to support the migration of young, healthy neuroblasts. The glial cells are neurotrophic.

Figure: Derived radial cells support the migrations of embryonic neurons. Data are confocal microscopic images of real-time migration. **The neurons were seeded onto the derived cells. This was confirmed elsewhere in the paper, but still very tricky!

•   Alas, there may be a shred of hope. Upon treating transformed cells with an inhibitor of EGF-receptors the progenitor cells reversed phenotypes! (after 7 days in vitro) I find this incredibly puzzling. And if any one has a guess on how this happens, I would love to hear it.

  So, what does this all add up to? Glioblastoma Multiform derives from astrocytes and neural stem cells. This team is essentially mapping out a recruiting process that may occur in the early stages of GBM. Also, I found the derived radial cells’ ability to guide healthy neuroblasts to be startling but a great way to show off and say, “Hey, they work.” The team suggests that TGF may be an early player in cellular deregulation and eventual transformation.  I found their data fairly convincing on some levels, did you? I would love to hear. 

  I buy into the idea that TGF can transform cells, but the team has not yet addressed one major issue in my book: which cells are potentially over expressing TGF, in vivo? TGF is a neurotrophin, and functionally can guide developing neurons. However, cells do have other options as far as guidance cues, so this is not proof that the derived cells are able to further recruit. This is where the experiment fell short without western and southern blot analyses. No genes, no probes, not much protein expression comparison. It would have been easy to do a Western blot at least! It makes a bit skeptical that maybe they found that the derived cells were not expressing "extra" TGF. I am curious to know if these transformed cells are indeed capable of recruiting healthy friends. Is this where the “multiform” comes in? Is one phenotypic form driving another? I assume the authors are cellular biologists by their amazing use of immunofluorescent labeling and sophisticatedly simple methodology. Molecular Biology, the ball is in your court. Approaching the glial, cancer stem cells’ induced differentiation and potential TGF over expression on a genetic level is on the horizon. Yet, there is still a vast leap to be made from vitro to vivo.




*****For more information on Cancer Stem Cells, check out this video by Dr. Robert Weinberg. (Sound familiar??) It is about an hour and 20 minute video, the beginning is a bit slow. I definitely recommend the whole video but maybe for time's sake, skipping to the middle or just watching random clips.