A recently published study (1), reinforces the importance of the Mitogen-Activated Protein Kinase (MAPK) cascade for bone growth. Geister and coworkers, researchers from Michigan and San Francisco, US, described the consequences of a newly identified mutation in the gene that encodes (carry the biochemical instruction to generate) the C-type natriuretic peptide (CNP) receptor (Npr2) which makes it inactive.
This syndrome is characterized by short bone dwarfism. In summary, they described the effect in bone growth of the lack of CNP signaling in the chondrocytes and, consequently, in the growth plate. Now, guess what they did to overcome the presence of a malfunctioning CNP signaling in their animal models. If you thought on targeting the fibroblast growth factor receptor 3 (FGFR3) cascade, you are right. They targeted the enzymes called MEK1 and MEK2 and, using specific inhibitors, they were able to rescue bone growth of their animal models. Obviously, they concluded by implying that this strategy could be used to treat other conditions where the MAPK pathway is important, such as achondroplasia.
You can also visit other sources to check this enzymatic cascade. For instance, the review by Dr. William Horton published here (free access) contains a nice figure showing this cascade.
In summary, when a FGF (green) bind to the extracellular part of the FGFR3 (red) it will produce the coupling (we call dimerization) of two FGFR3 molecules and this reaction causes a change in the position of some intracellular parts of the receptors, which in turn, will attract neighbor molecules and proteins like FRS2a, Sos and Grb, causing the transference of phosphorus ions, a phenomenon that represents the activation of the enzymes. The activation of these proteins triggers the cascade activation of enzymes of the MAPK pathway starting by Ras, Raf, MEK and ERK and p38. The activated ERK and p38 translocate (move) to the nucleus and trigger the activation of target genes to cause changes in cell behavior. This is not easy to copy. It needs time to digest the complexity of these molecular reactions. We will not go further on the chemistry details.
The authors found that the malfunctioning of the CNP receptor leaded to a growth disturbance which resembles a rare human syndrome called acromesomelic dysplasia, Maroteux type.
Achondroplasia and the FGFR3
It is always good to strengthen our knowledge about the biology of achondroplasia. Achondroplasia is caused by a gain-in-function mutation in the gene that encodes the FGFR3. In the growth plate, the natural function of FGFR3 is to reduce the growth pace, acting in concert with several other agents, such as CNP. Because of the mutation, FGFR3 is working (signaling) excessively and this causes marked reduction of the proliferation (multiplication) and maturation (hypertrophy) of the chondrocytes, the cells responsible for building the scaffold which will give base for the new forming bone. The result of the strong FGFR3 signaling is that the affected individual will have shorter bones and a narrow spinal canal, which in turn will lead to the common clinical consequences seen in achondroplasia. We have already reviewed this topic here, but we are visiting this again because of the insights studies like the one by Geister et al. and from other investigators are bringing to light.
FGFR3 is an antenna. An antenna which works using MAPK
FGFR3 is a kind of cell antenna. It is located across the chondrocyte cell membrane and works as a transmitter of environmental signals to the cell nucleus, to allow the cell to respond appropriately to those external stimuli. But how does FGFR3 work? As you know, proteins like FGFR3 are organic molecules which have active chemical properties (and are also called enzymes). FGFR3 reacts to the binding of an FGF coming from outside the cell and, in turn, activates a cascade of other enzymes located inside the cell, which will trigger a cell response by stimulating the nucleus to turn on or off genes encoding other proteins. These newly formed proteins will allow the cell to respond to the original stimulus produced by that binding of the FGF to FGFR3.
FGFR3 and MAPK
Let’s see in more detail the FGFR3 enzymatic cascade. The main signaling cascade triggered by FGFR3 is the MAPK pathway. Take a look in the following figure showing the main enzymes involved in the reaction started by FGFR3.
CNP acts inhibiting the MAPK pathway. Could we do the same with other compounds?
CNP works blocking the signaling produced by Raf. Visit Dr. Horton’s review and take a look in the figure presenting the FGFR3 pathway. It is also showing the interaction of CNP and the MAPK cascade. Reducing the activity of the MAPK cascade, CNP restores bone growth. Can we do the same using synthetic compounds? Can we block the MAPK pathway? The answer is yes. The main issue here is that the MAPK enzymes are key players in so many cell reactions in all body tissues that it seems excessively risky to block one of those molecules to obtain bone growth.
But, isn't this exactly what CNP does? Is it working in all tissues of the body when injected in the test animals described in the published studies?
The reason why a CNP analogue is being developed to become a therapy for achondroplasia is that this peptide has been showing to have its main biological relevance specifically and almost only in the growth plate. However, for a manufactured compound designed to block, for instance, the enzymes MEK, how could we make it to only block MEKs produced by chondrocytes in the growth plate?
Imagine this: a pill containing the MEK inhibitor is swallowed and then absorbed in the intestines. How would this anti-MEK compound “know” it shouldn’t block the MEKs produced in the intestinal cells? Man-made classical drugs usually don’t have GPS to help them find the way; they don’t know which the right target cell is.
There are several anti- Raf, anti-MEK and anti-ERK compounds already in the market or being developed to treat a number of conditions, notably cancer (2). Some, like vemurafenib, an anti-Raf (a mutated Raf), is already being used to treat melanoma.
When the disease to be treated is one such as this kind of devastating cancer, there is less concern about potential side effects, what is called off-target effects. But for non-malignant conditions, usually chronic, long term disorders, a more balanced approach must be taken. If the drug causes so many potential dangerous adverse events, it is not worth to pursue its use for that condition.
Drugs for the growth plate need GPS
That’s why although there are several drugs targeting the MAPK cascade we do not see research directed to examine the possibility of treating achondroplasia with them. It is difficult and “risky” to expend resources trying to find a good GPS for these drugs. You know, the growth plate is not exactly Times Square or the Eiffel Tower, it is more like a remote village in the Amazon forest, where you will need maps, equipment and GPS to find it. We have also reviewed this topic in this previous article.
Nevertheless, the good news studies like this one by Geister and coworkers bring is that more and more evidence about the concept of blocking the MAPK to treat achondroplasia is being published.
This article is also reinforcing that the MAPK pathway, and especially the ERK enzyme and its targets in the chondrocyte nucleus seem to be key for bone growth.
As I mentioned in a recent article, we are reaching a time when we will be asking no more about the existence of a therapy for achondroplasia; we will be asking which one we will use to treat our children.
1. Geister KA et al. A novel loss-of-function mutation in Npr2 clarifies primary role in female reproduction and reveals a potential therapy for acromesomelic dysplasia, Maroteaux type. Hum Mol Gen 2012; 1–13. doi:10.1093/hmg/dds432
2. Santarpia L et al. Targeting the MAPK--RAS—RAF signaling pathway in cancer therapy. Expert Opin Ther Targets 2012;16(1):103-119. Free access
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