Let’s start to see how
things work in achondroplasia by reviewing how the fibroblast growth
factor receptor 3 (FGFR3) is created, how it exerts its actions and how its
fate is. The text may seem not exactly like a cookies’ recipe. However, talking
a little bit about these topics will help the reader to understand why some
potential approaches to treat achondroplasia are attractive and worth to be explored and
others are not.
Before diving in the more
technical parts, it will be useful to explain some terms and expressions being
applied here:
Amino acids: these are molecules essential for life and
when combined in predetermined sequences – imagine a sequence of bricks, one
after another – they constitute the Proteins.
Cell matrix or interstitium: the medium inside the growth plate where
the chondrocytes are located.
Cell membrane: this is the external cover of the cell,
keeping the cell content safe and functioning as an in-and-out gate.
Cytoplasm: this is the medium inside the cell where
the cell organelles and molecules lie and exert their functions.
Domains: the different parts of a protein. This
expression may be used to denote a specific part of a protein bearing a
distinctive pattern.
Nucleotides: these are the molecules that constitute the
genetic code. There are four nucleotides and the combination of them produces
all information needed to create and maintain life.
Protein: proteins are large molecules made of
predetermined sequences of amino acids. When they cause chemical reactions,
they are also called enzymes.
The FGFR family
FGFR3 pertains to a family
of four similar proteins (named 1 to 4). These four proteins (or enzymes) are
called receptors because they are located across the cell membrane,
which makes one of their extremities being positioned outside the cell membrane
(extracellular domain), in contact with the local environment (often
called cell matrix or interstitium) and the other, inside the cell,
exposed in the cytoplasm (intracellular or tyrosine kinase domain).
Their function is to work
as communication channels between the matrix (or, in a broader vision, the
body) and the cell. How does this communication work? It is through chemical
contacts between locally circulating molecules or activators and these
receptors. Messages are passed on in a chemical language. There are many other
classes of receptors with distinct functions also placed across the cell
membrane. These communication channels allow cells to easily respond to local
changes in the external environment, the matrix. Although they have distinct
functions, in many of them the way used to exert their actions is similar. We
must save this information for later, because it has consequences.
The four FGFRs share a
common basic structure and respond to the same activators (or, in the
jargon, ligands) called fibroblast growth factors (FGFs). Compared
to many other receptor proteins, the FGFR structure is quite simple. The
extracellular part (or domain) is composed of three amino
acid loops that resemble the structure ofimmunoglobulins (we
know these structures by their popular name: antibodies). There is a transmembranedomain
(naturally, the part of the protein placed across the cell membrane). And,
finally, the intracellular domain, where lies the part of the protein which,
when the receptor is turned on, will trigger series of intracellular reactions,
commonly called signaling pathways or cascades. You
can see a schematic figure of the FGFRs at Dr. Mohammadi’s website.
Now, let’s focus on the
FGFR3 and the chondrocyte.
We will be talking about
the steps FGFR3 makes from its production to its fate, with emphasis in its
synthesis (production) and activation:
- Synthesis
- Transport
- Activation
- Degradation
- It reduces the speed of the cell multiplication and
- It slows the pace the cell get older (differentiate, becomes hypertrophic).
- It reduces the cell proliferation rate
- It reduces the cell maturation rhythm (differentiation, hypertrophy)
FGFR3 synthesis
Like any other protein,
FGFR3 is produced (expressed) when its specific gene is read and the
instructions it holds in the form of a chain of four molecules called nucleotides are
translated to a chain of amino acids. In a very simplified model, each sequence
of three nucleotides in the DNA sequence will give origin to the choice of one
amino acid. You may read more about this here.
Simplifying again, let’s
see the FGFR3 production sequence: the part of the DNA where lies the gene for
FGFR3 is opened like a zipper. Then, tailor proteins, reading the nucleotide
sequence, build a copy of the DNA guideline with free nucleotides available in
the vicinity. The tailored DNA copy is called messenger RNA, or mRNA, and the
process in which the gene is read is called transcription. The mRNA
will leave the nucleus towards the ribosome, a small cell organelle.
There, the code in the mRNA will be read again and translated into
amino acids, which will be assembled in a chain, creating the protein.
In summary, the FGFR3
gene is read and copied through a process called transcription and
the RNA copy is further read and translated into a chain of
amino acids, the protein. You can learn more about gene transcription and
protein expression watching this interesting animation.
The whole process,
basically described here, is tightly regulated. There are many proteins and
small pieces of other kinds of RNA molecules involved in the gene reading and
in protein assembly, working as quality controls or checkpoints. They identify
reading errors and correct them or send the defective product for degradation.
Some of these small RNAs can also either start the gene frame reading or stop
it, thus having a regulatory role (save this information for later).
The system is not devoid of errors and many diseases and conditions are a
result of failures in the quality control or when the defect is not identified
as so, allowing changed proteins to become active. This is the case of the
mutation of the FGFR3 in achondroplasia.
FGFR3 shaping and transport
After the amino acid
sequence is built into a new protein, another chemical reaction starts. A group
of proteins called chaperones make the ‘final arrangements’ in
the new protein, putting it in the right shape, ready to be positioned across
the cell membrane, which is a step mediated by an intracellular transport
mechanism. In the case of FGFR3, the commonly involved chaperone is the one called
HSP-90. If chaperones fail to finalize the protein, the new product usually is
directed to degradation, too. Although carrying a composition error, the
mutated FGFR3 keeps its functionality and is normally released for transport to
the cell membrane. The transport system is a common path for many other
proteins.
FGFR3 activation
FGFR3 is activated (turned
on, like an electric switch) when a FGF binds to the extracellular domain of
the receptor. In a reaction in which other matrix components also participate,
the binding of the FGF will attract another molecule of FGFR3, which will be
positioned with the first one in a structure called dimmer. The
dimmer suffers conformational changes, we could say turnings,
which will expose specific sites, like electric plugs, in the intracellular
domain. When these plugs, called literally adenosine triphosphate (ATP)
pockets, are exposed they attract phosphate carriers and the cascade begins:
the signaling downstream pathway is activated through this
process called phosphorylation. Several proteins are attracted to
the FGFR3 activated tail, one becoming activated by the previous one and
activating the next one: the cascade. Take a look in this article by
Dr. William Horton, where there is a figure showing the cascade.
There are two main
different cascades turned on with the activation of FGFR3: the so called
RAS-RAF-MAPK pathway and the STAT1 pathway. Let’s go here step-by-step, no
rush.
The RAS and RAF proteins
are extremely important to many biological processes. They are a kind of cell
signal distributors lying just in the first steps of the signaling cascades of
many receptor enzymes. In the case of FGFR3, the first important cascade they
will trigger is the one headed by the mitogen activated protein kinase (MAPK)
enzymes.
Once activated, RAF will
‘call’ the MAPK enzymes named MEKs (1 to 6). Which ones will be activated will
depend on the origin of the signal upstream. For FGFR3, the most important are
MEK1, MEK2, MEK3 and MEK6. MEK1 and MEK2 will then trigger a couple of enzymes
called extracellular-signal regulated kinases (ERK1 and ERK2),
while MEK3 and MEK6 will activate another enzyme called p38. The ERKs and p38
are the enzymes responsible for delivering the message from the matrix outside
to the cell nucleus, where the response to the external signal will be
generated, through the expression of some genes and/or the repression
(inhibition) of others.
STAT1 (signal transducer
and activator of transcription 1) is a cellular agent working directly in the
nucleus in response to extracellular stimuli. When activated, it will tell the
cell nucleus to reduce or stop the actions needed to let the cell multiply. In
other words, it will reduce the cell proliferation rate.
ERKs and p38, in response
to the signal initiated by FGFR3, will stimulate the nucleus to reduce the pace
that chondrocyte is maturing (progressing to the hypertrophic state).
So, here are the main
consequences of having the FGFR3 overactive in achondroplasia:
FGFR3 Degradation
The last phase the FGFR3
passes through is the deactivation or degradation. You can have an excellent
description of the process reading Dr. Horton’s Lab Blog in Growing Stronger:
his group is working exactly with the steps involved in the metabolism
(cleavage) of FGFR3. This is a field with still many questions to be answered
but may bring one of the potential therapeutic solutions for achondroplasia.
We have made this complex
trip through the chondrocyte to understand the machinery involved in the
production of FGFR3. We also made a brief review of the main reactions FGFR3
causes in the chondrocyte. Now, we are ready to explore the potential
approaches to counteract the defective FGFR3.
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