2016-10-14T11:15:06Z

Z5015544:

{{ANAT2341Project2016header}}
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=Fibroblast Growth Factor Receptor (FGFR) Pathway=
==Introduction==
The Fibroblast Growth Factor (FGF) signalling pathway is critical for regulating progenitor cell proliferation, differentiation, survival and patterning. It is involved in the regulation and development of the early embryo, and is considered to be critical for normal organ, vascular and skeletal development. Furthermore, this pathway is also involved in maintaining adult tissues through the regulation of metabolic functions and tissue repair (which is often through the reactivation of the same signalling pathways involved in early development.) <ref><pubmed>25772309</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref>

This page will outline the FGFR signaling pathway, the history of scientific discoveries relevant to this pathway, the receptor subtypes and a description of signal transduction. It also outlines its various roles in embryonic development including in the patterning of embryonic axis, as well as limb bud, bone, kidney, external genitalia and inner ear development. There is also a brief explanation discussing relevant animals models, such as those of the chick embryo, as well as abnormalities in this pathway relevant to embryonic development, including Achondroplasia, Pfeiffer syndrome and Apert syndrome are discussed. There is also a short informative quiz accompanied with feedback at the bottom of the page for readers to challenge their knowledge on the information provided. There is a glossary listed at the bottom explaining some terms mentioned throughout the page, as well as links to relevant information from UNSW embryology lectures.

==History==
Fibroblast growth factor (FGF) was initially discovered in pituitary extracts through experiments conducted in 1973. Researchers had noticed the growth stimulating effects that these isolated factors had, in that they induced fibroblast proliferation. Due to their ability to stimulate fibroblast proliferation they were termed "FGFs". Today, a variety of subtypes of FGFs have been discovered and categorised into a large family that exist in organisms including humans as well as nematodes. In addition, it was soon discovered that not all FGFs can stimulate fibroblasts.

{| class="pretty table"
| '''1973'''
| FGF first identified in pituitary extracts<ref><pubmed> PMC427087</pubmed>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC427087/]</ref>
|-
|'''1999'''
| FGFs were categorised into 2 groups using acidic and basic pH; they where referred to as "Acidic FGF" (FGF1) and "Basic FGF" (FGF2)<ref><pubmed>PMC25296 </pubmed>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC25296/]</ref>
|}

== Overview Of The FGFR Pathway==
22 protein families of have been identified from the FGF signalling pathway, 18 of which are secreted signalling proteins (FGF1-10, and FGF16-23) that interact with 4 tyrosine kinase FGF Receptors (FGFR1-4) and the other 4 are intracellular non-signalling proteins (iFGFs; FGF11-14).
<ref><pubmed>25772309</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref>

As illustrated in the image below, FGFRs are comprised of 3 immunoglobulin domains (IgI, IgII, IgIII), with IgIII being the closest to the transmembrane, and IgI being the furthest away. Some notable features of this receptor include an acidic box (AD) located in-between IgI and IgII, a heparin-binding domain (HBD) within IgII which is important in signal transduction, and the transmembrane (TM) structure of IgIII which has both with kinase and interkinase domains (KD and IKD) within the intracellular space. FGF ligands linked to heparin sulfate proteoglycan (HSPG) bind to both the IgII and IgIII domain of the receptor (with the heparin component specificially binding to IgII) resulting in dimerisation of the receptors and activation of signal transduction pathways through the phosphorylation of tyrosine residues, as discussed in more detail under the subheading signal transduction. <ref><pubmed>16216232</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/16216232]</ref>

[[File:FGFR receptor subtype.jpeg|thumb|none|300px|Simplistic illustration of the FGFR receptors adapted from review article [http://www.ncbi.nlm.nih.gov/pubmed/16216232 Functions and regulations of fibroblast growth factor signaling during embryonic development]]]

==Subtypes of FGFR==
{| class="pretty table"
|-
| '''FGFR Subtype''' || '''Function''' || '''Abnormalities'''
|-
| FGFR1 ||
*Involved in morphogenesis as well as orchestrating the patterning of the mesodermal germ layer at gastrulation<ref><pubmed>16207751</pubmed></ref> 
*Involved in formation of the organ of corti and auditory sensory epithelium <ref><pubmed>12194867</pubmed></ref> *<ref><pubmed>12194867</pubmed></ref>
||
*Pfeiffer Syndrome (Type 1)
*Kallmann syndrome
*Osteoglophonic dysplasia
*8p11 myeloproliferative syndrome
|-
| FGFR2 ||
*Activated prior to gastrulation with the purpose of repressing cellular movements in the presumptive anterior neural plate and preventing normal retinal progenitor cells from adopting retinal fates<ref><pubmed>14723847</pubmed></ref> 
||
*Pfeiffer Syndrome (Type 1-3)
*Apert Syndrome
*Crouzon Syndrome
|-
| FGFR3 ||
*Induces complete growth arrest of cells<ref><pubmed>11779141 </pubmed></ref> 
*Is required to promote differentiation of prechondrogenic mesenchymal cells to cartilage-producing chondrocytes
||
*Achondroplasia
*Thanatophoric Dysplasia
*Hypochondroplasia
|-
| FGFR4 || ADD INFO HERE || ADD INFO HERE
|-
|}

===Signal Transduction===
 [[File:FGF signalling pathway.jpg|thumb|500px|FGFR Signalling Pathway (Image based upon<ref><pubmed>27458533</pubmed></ref>)]]

The process of signal transduction commence with the binding of a cognate ligand to FGFRs ligand binding site which in turn triggers receptor dimerization. This dimerization of the receptor will cause activation of intrinsic kinase activity<ref><pubmed>1655404</pubmed></ref>. This will activate multiple signal transduction pathways intracellularly including RAS, Mitogen-activated protein kinase (MAPK), p38 MAPKs, Phospholipase-C-Gamma, Crk, Protein Kinase-C and Phospholipase-C-Gamma and Extracellular signal-regulated kinases. Activation of FGFRs induces tyrosine phosphorylation of FRS2 (FGFR stimulated2 Grb2 binding protein) which in turn stimulates the recruitment of GRB2 (Growth factor receptor bound protein-2) and SHP2 ( Src homology 2 phosphatase-2) <ref><pubmed>11021964</pubmed></ref>.
 
 
In turn, these sequence of events promote sustained activation of RAS, which leads to changes in gene transcription through interactions with DNA. In addition, FGF receptors will also induce the activation of PI3K (phosphatidylinositol-3-Kinase), STAT1 and Src tyrosine kinase, which will contribute to certain FGF-stimulated biological responses <ref><pubmed>1656221</pubmed></ref>.
 
 
With respect to embryonic development, both the PI3K and RAS pathways are essential in order for normal mesoderm to occur in the embryo. Additionally, receptor-mediated induction of the SHP2-RAS-ERK pathway is a key mechanism through which FGF can activate a variety of biological signalling pathways including cell growth, cellular differentiation as well as morphogenesis <ref><pubmed>9632781</pubmed></ref>.


==Role In Embryonic Development==

===Patterning Of The Embryonic Axis===
In the process of patterning of the embryonic axis, the caudal primordium that is part of the neural plate, contains cells that are rapidly dividing and is able to maintain itself as a growth region (this region is considered to be of "stem cell" status). The expanding populations of dividing cells us spread along the neural tube by cell movements of convergence and extension. In the process by which cells are driven out of the tube, they change their pattern of movement which eventually causes a gradual restriction in space<ref><pubmed>8575335</pubmed></ref>. Within this process, it is the misexpression of a dominant negative FGFR construct in the tissue which causes these cells prematurely to leave the stem cell region and to change their movement patters as if they had aged<ref><pubmed>11389440</pubmed></ref>.

Furthermore, Mathias et al. (2001) suggest that FGFR is required in order to maintain this stem cell status in the caudal neural plate during patterning of the nervous system. In addition, it is possible that FGF serves the purpose of acting as a caudalizing factor for the neural tube because it is capable of prolonging the window of time during which cells are exposed to a caudalizing factor.

In summary, FGF signalling is important in regulating the maturation of developing cells which are gradually being laid down in a caudal direction along the axis of the neural tube.

===Limb Bud Formation===
[[File:LIMB BUD.png|200px|thumb|400px|FGFR Signalling Pathway (Image based upon<ref><pubmed>25772309 </pubmed></ref>)]]

Limb buds are structures formed early in [[Lecture - Limb Development| limb development]] which are comprised of lateral plate mesoderm (LPM) cells and an overlying surface ectoderm. They are roughly formed around week 4 of embryonic development as a result of interactions between the mesoderm and ectoderm germ layers.

FGF proteins and its interactions with other signalling pathways, are critical for the initiation and proximal-distal growth of limbs from a limb bud structure.<ref><pubmed>9620845</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/9620845]</ref> Prior to limb bud formation, FGF10 is widely expressed in the LPM and is stabilized by the WNT signaling proteins. FGF10 is responsible for stimulating the expression WNT3 (and downstream transcription factors including SP6 and SP8<ref><pubmed>15358670</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/15358670]</ref>) in the overlying ectoderm, which results in the formation of the Apical Ectodermal Ridge (AER), a specialised thickening of epithelium located towards the proximal end of the bud that is required for growth,<ref><pubmed>25772309</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref> which subsequently stimulates FGF8. FGF8 is responsible for continued growth of the underlying mesoderm by keeping in mitotically active state, and stimulating a positive feedbacks loop on FGF10 (which in turn stimulates increased FGF8 expression). FGF8 is the known AER-specific FGF to be expressed throughout it, although other FGFs are expressed in the posterior of the AER (including Fgf4, Fgf9 and Fgf17) and are thought to have supporting roles.<ref><pubmed>11101846</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/11101846]</ref><ref><pubmed>12152071</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/12152071]</ref>
FGFs in the AER signal FGFR1 and FGR2 in distal mesenchyme, activating ETV1 and EWSR1 which function to help to maintain FGF10 expression.<ref><pubmed>25109552</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/25109552]</ref>

Furthermore, the Zone of Polarising Activity (ZPA) is a region located on the posterior side of the limb bud composed of mesenchyme which signals its anterior-posterior growth (for example this region signals the position of the thumb relative to the little finger.) The Fibroblast Growth Factors FGF2<ref><pubmed>7908145</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/7908145]</ref>, FGF4<ref><pubmed>8001146</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/8001146]</ref> and FGF8<ref><pubmed> 8598907</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/8598907]</ref> induce Sonic Hedgehog (SHH) within ZPA region and is critical for its growth along the anterior-posterior axis.

Therefore together these interactions of the FGFs from the AER help to maintain proliferating cells near the distal tip of the limb bud, and are known to be critical in limb bud development, both along the proximal-distal axis and the anterior-posterior axis. It is also important to note that growth along the dorsal-vental axis is dependent on the involvement of growth factors from the Wnt family on the ectoderm layer.

FGF signaling is also involved in lung bud initiation and development, with a similar underlying process.

===Bone Development===
[[File:FGF and FGFR expression patterns during endochondral and intramembranous bone development.jpeg|thumb|500px|FGF and FGFR expression patterns during endochondral and intramembranous bone development <ref><pubmed>PMC4526732</pubmed>
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526732/]</ref>]]

Much of what we now understand about the involvement of the FGF signalling pathway in bone development is a result of discovering missense mutations responsible for conditions characterised by abnormal bone structure, including but not limited to skeletal dysplasias and craniosysnostosis syndromes (some of which discussed in more detail later under the subheading abnormalities.) The first and questionably the most important mutation discovered affecting skeletal development was a point mutation of the FGFR3 protein, which was found to be responsible for achondroplasia.

FGF signalling is involved in both endochondral and intramembranous [[Lecture - Musculoskeletal Development| bone development]], which are critical in the early stages of embryonic bone formation, as shown in the diagram to the right the presence of FGFR1-3 and FGF2, FGF9, FGF18 are shown in various stages of bone development. Endochondral bone development is responsible for forming the long bones of the appendicular skeleton, face and spinal column. This involves an intermediate cartilage template (which helps control the growth and patterning of the development of the bony structure.) In comparison intramembranous bone development is responsible for forming bones of the skull and clavicles, and doesn’t require a cartilage template, it directly forms bone. <ref><pubmed>PMC4526732</pubmed>
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526732/]</ref>

TBC

===Kidney development===
 
The metanephric kidney is an organ which arises primarily form two tissues, the nephrogenic cord and the Wolffian duct, which will eventually give rise to the metanephric mesenchyme and the ureteric bud respectively <ref><pubmed>18835385</pubmed></ref>. Around week 5 of gestation in the developing human embryo, the metanephric mesenchyme will release signalling molecules that stimulate the ureteric bud to grow out from the Wolffian duct and invade the metanephric mesenchyme. The stromal mesenchyme that exists between the Wolffian duct and the metanephric mesenchyme restricts the ureteric bud to its proper position and prevents ectopic budding<ref><pubmed>10749566</pubmed></ref>. The metanephric mesenchyme will continue to release signals which will stimulate the ureteric bud to elongate and repeatedly branch, leading to formation of the ureter, collecting duct system and the renal pelvis. Following its contact with the ureteric bud, the metanephric mesenchyme will then divide into a nephrogenic lineage lying adjacent to the bud, and a surrounding renal cortical stromal lineage <ref><pubmed>19272374</pubmed></ref>. Each terminal tip of the ureteric bud induces local areas of nephrogenic mesenchyme in order to differentiate into nephron epithelia, progressing from renal vesicles ,to comma-shaped bodies, to S-shaped bodies, and then to immature nephrons<ref><pubmed>19272374</pubmed></ref>. The renal cortical stroma will provide a framework and likely a niche for the other renal lineages and vasculature, and ultimately differentiates into interstitial and other supportive cells within the kidney <ref><pubmed>10594778</pubmed></ref>.
 
 
In terms of the development of the metanephric kidney, all FGFRs have been detected in the process of development, however studies using animal models have revealed that it is FGFR1, FGFR2 and FGFR11 which play a key role in renal development <ref><pubmed>10691305</pubmed></ref>. FGFR1 is a receptor which is expressed mostly in the metanephric mesenchyme lineages, these including the early metanephric mesenchyme, the cap mesenchyme and the developing nephrons beginning with vesicles. However, FGFR1 is present at lower levels in the ureteric lineage and in the renal cortical stroma<ref><pubmed>10385628</pubmed></ref>. In contrast, FGFR2 is strongly expressed in the Wolffian duct and the ureteric bud tree as well as the differentiating nephrons. Despite this, FGFR2 is present at lower levels in the early metanephric mesenchyme and stomal mesenchyme adjacent to the Wolffian duct<ref><pubmed>1315677</pubmed></ref>. In addition, FGFR11 is present in renal vesicles <ref><pubmed>1315677</pubmed></ref>.
 
 

===External Genitalia development===
 
 
The genital tubercle (GT) is a structure from which characteristics in the external genitalia in the adult develop. The GT differentiates into a penis in males and a clitoris in females. The process of proximodistal elongation of this GT involves multiple interactions between growth factors and transcription factors <ref><pubmed>3723059</pubmed></ref>. Interactions between epithelium and mesenchyme has an essential role in the regulation of various development processes throughout the embryo. Such signalling controls many aspects of organogenesis, from the initiation of organ development to differentiation <ref><pubmed>8896986</pubmed></ref>.
 
 
The FGFR signalling pathway is involved in epithelial to mesenchymal interactions during organogenesis. Studies have revealed that the first morphological sign of GT outgrowth occurs at approximately 10.5 days post coitum, and will continue throughout the perinatal period <ref><pubmed>12004962</pubmed></ref>. Initially within the developing embryo, the external genitalia of the male and female foetuses are morphologically identical and consist of the GT. Several growth factors including FGF proteins have been shown to control external genitalia development in mice <ref><pubmed>10021340</pubmed></ref>. FGF8, FGF10 and FGFR2 expression has been found during GT developing, thus suggesting that a combination of these factors may constitute redundant developmental functions during GT morphogenesis<ref><pubmed>10804187</pubmed></ref>. As the GT elongates, a groove appears on its ventral aspect called the urethral groove. At the distal end, this groove is made up of a solid plate of epithelial cells, the distal urethral epithelium (DUE) that extends into the glans penis. The solid urethral plate canalizes and thus extends the urethral groove distally into the glans. It was found that FGFR2IIIb is expressed in the DUE and urethral plate epithelia of the GT. Deletion of this receptor and FGF10 was shown to cause urethral dysmorphogenesis.
[[File:External genitalia.jpg|thumb|500px|centre|<ref><pubmed>25772309 </pubmed></ref>]]
 
 

===Inner Ear Development===

==Animal Models==
Jocelyn
 
===Site of FGF10 expression in the chick embryo===
 
==Abnormalities ==
As discussed above, the FGF signalling pathway is critical for regulating many early embryonic developmental processes, and is critical for normal organ, vascular and skeletal development. Consequently, abnormalities in genes coding for the proteins within this signalling pathway (including signalling proteins, non-signalling proteins, and receptors) can result in many visible structural abnormalities such as short statue and face deformations. Not to mention that a large majority of these conditions, if not all, influence an individual’s quality of life, and in some cases increase risk of fatality. Some of these FGF abnormalities are outlined in more detail below, including Achondroplasia, Pfeiffer and Apert Syndrome which particularly emphasise the significance of FGF signalling in early skeletal development.

===Achondroplasia===
Achondroplasia is the most common form of skeletal dysplasia, and is often characterised by shortened proximal limbs, a curved spine, a large prominent forehead and a fattened nasal bridge. This condition is inherited genetically as an autosomal dominant trait, although a large proportion of cases are sporadic. <ref><pubmed>7913883</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/7913883]</ref> This condition results in reduced inhibition of endochondral ossification, which is one of the main way in which bone tissue is created during embryonic development (the other being intramembranous ossification.) Endochondral ossification is essential during development for both the formation and growth of long bones as well as healing fractures. For the majority of affected individuals, it is a result of a missense mutation in FGFR3, specifically due to a substitution of arginine for glycine (G380R).<ref><pubmed>12816345</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/12816345]</ref> As originally postulated by Bonaventure et al. (1996) this introduction of a hydrophilic residue in a hydrophobic receptor domain results in a disruption of alpha-helical structure of the transmembrane portion of the protein and consequently interferes with the signal transduction pathway of which it is involved in. <ref><pubmed>8723101</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/8723101]</ref>

There are other mutations in FGFR3 which are responsible for different skeletal developmental conditions, including a more severe (usually fatal) form of skeletal dysplasia, Thanatophoric Dysplasia, which is due to two different mutations, K650E and R248C in FGFR3 (type 1 and type 2 respectively) and a milder form, hypochondroplasia, which is due to the mutations, N540K or K650N in FGFR3.

===Pfeiffer Syndrome===

Pfeiffer syndrome is characterised by craniosynostosis, meaning that is it a condition where the cranial fibrous sutures prematurely fuse (ossify) resulting in a reduced space for the growing brain. Subsequently, abnormal growth of the skull, in an attempt to increase the space available for the brain and reduce cranial pressure, results in the development of abnormal facial features including, but not limited to, proptosis (abnormal placement of the eye), hypertelorism (abnormal increase in distance between the eyes), maxillary deficiency, and a beaked nose. Other notable features include those of the hands, broad thumbs and the feet, medially deviated broad great toes.
<ref><pubmed>9300656</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/9300656]</ref><ref><pubmed>25679016</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/25679016]</ref> This condition is inherited genetically as an autosomal dominant trait. There are 3 types of Pfeiffer syndrome. Type 1 is a result of either a gain of function P252R mutation of FGFR1 (5%), which increases the receptor’s ligand binding affinity resulting in over-activation of the receptor, or sequence variants of FGFR2 gene (95%.)<ref><pubmed>25679016</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/25679016]</ref> Type 2 and 3 are similar, both appear more severe and generally have a worse prognosis compared to Type 1, and are a result of mutations of the FGFR2 gene.
<ref><pubmed>8434615</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/8434615]</ref><ref><pubmed>10394936</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/10394936]</ref>

===Apert Syndrome===

Mutations in FGFR2: S252W

==New and Emerging Research Into FGF==

===Emerging Research Into The Role Of FGF In The Development Of The Growth Plate===
https://www.ncbi.nlm.nih.gov/pubmed/25114206
 
===Autoregulatory loop of induction between FGF10 and FGF8 ===
 
==Quiz: How much do you really know about FGF? Take the quiz and find out!==

<quiz display=simple>

{Which of the following statements regarding FGFR3 is true?
|type="()"}
- Mutation in the receptor causes Pfeiffer Syndrome
+ Induces complete growth arrest of cells
- Prevents chondrocytes from developing
- Associated with Kallmann syndrome
|| Option X is correct. EXPLAIN

{What...
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

{The ...:
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

{Which of the following is false:
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

</quiz>
==Glossary==

{| class="pretty table"
| '''Fibroblast Growth Factors (FGFs)'''
| Family of 22 proteins, 18 of which are secreted signalling proteins (FGF1-10, and FGF16-23) and the other 4 are intracellular non-signalling proteins (iFGFs; FGF11-14)
|-
| '''Fibroblast Growth Factor Receptors (FGFRs)'''
| 4 tyrosine kinase receptors (FGFR1-4) that interact with with the signalling FGF proteins
|-
|}

Some more words to include:
-Gastrulation
-Embryonic axis
-Germ layers (endoderm, ectoderm, mesoderm)
-Endochondral bone development
-Intramembranous bone development
-Missense mutations
-Metanephric kidney
-Craniosysnostosis syndromes
-Skeletal dysplasia

Manraaj

==References==

<references/>

2016 Group Project 3

2016-10-14T11:10:05Z

Z5015544:

{{ANAT2341Project2016header}}
{{Group Assessment Criteria table}}


=Fibroblast Growth Factor Receptor (FGFR) Pathway=
==Introduction==
The Fibroblast Growth Factor (FGF) signalling pathway is critical for regulating progenitor cell proliferation, differentiation, survival and patterning. It is involved in the regulation and development of the early embryo, and is considered to be critical for normal organ, vascular and skeletal development. Furthermore, this pathway is also involved in maintaining adult tissues through the regulation of metabolic functions and tissue repair (which is often through the reactivation of the same signalling pathways involved in early development.) <ref><pubmed>25772309</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref>

This page will outline the FGFR signaling pathway, the history of scientific discoveries relevant to this pathway, the receptor subtypes and a description of signal transduction. It also outlines its various roles in embryonic development including in the patterning of embryonic axis, as well as limb bud, bone, kidney, external genitalia and inner ear development. There is also a brief explanation discussing relevant animals models, such as those of the chick embryo, as well as abnormalities in this pathway relevant to embryonic development, including Achondroplasia, Pfeiffer syndrome and Apert syndrome are discussed. There is also a short informative quiz accompanied with feedback at the bottom of the page for readers to challenge their knowledge on the information provided. There is a glossary listed at the bottom explaining some terms mentioned throughout the page, as well as links to relevant information from UNSW embryology lectures.

==History==
Fibroblast growth factor (FGF) was initially discovered in pituitary extracts through experiments conducted in 1973. Researchers had noticed the growth stimulating effects that these isolated factors had, in that they induced fibroblast proliferation. Due to their ability to stimulate fibroblast proliferation they were termed "FGFs". Today, a variety of subtypes of FGFs have been discovered and categorised into a large family that exist in organisms including humans as well as nematodes. In addition, it was soon discovered that not all FGFs can stimulate fibroblasts.

{| class="pretty table"
| '''1973'''
| FGF first identified in pituitary extracts<ref><pubmed> PMC427087</pubmed>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC427087/]</ref>
|-
|'''1999'''
| FGFs were categorised into 2 groups using acidic and basic pH; they where referred to as "Acidic FGF" (FGF1) and "Basic FGF" (FGF2)<ref><pubmed>PMC25296 </pubmed>[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC25296/]</ref>
|}

== Overview Of The FGFR Pathway==
22 protein families of have been identified from the FGF signalling pathway, 18 of which are secreted signalling proteins (FGF1-10, and FGF16-23) that interact with 4 tyrosine kinase FGF Receptors (FGFR1-4) and the other 4 are intracellular non-signalling proteins (iFGFs; FGF11-14).
<ref><pubmed>25772309</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref>

As illustrated in the image below, FGFRs are comprised of 3 immunoglobulin domains (IgI, IgII, IgIII), with IgIII being the closest to the transmembrane, and IgI being the furthest away. Some notable features of this receptor include an acidic box (AD) located in-between IgI and IgII, a heparin-binding domain (HBD) within IgII which is important in signal transduction, and the transmembrane (TM) structure of IgIII which has both with kinase and interkinase domains (KD and IKD) within the intracellular space. FGF ligands linked to heparin sulfate proteoglycan (HSPG) bind to both the IgII and IgIII domain of the receptor (with the heparin component specificially binding to IgII) resulting in dimerisation of the receptors and activation of signal transduction pathways through the phosphorylation of tyrosine residues, as discussed in more detail under the subheading signal transduction. <ref><pubmed>16216232</pubmed>[http://www.ncbi.nlm.nih.gov/pubmed/16216232]</ref>

[[File:FGFR receptor subtype.jpeg|thumb|none|300px|Simplistic illustration of the FGFR receptors adapted from review article [http://www.ncbi.nlm.nih.gov/pubmed/16216232 Functions and regulations of fibroblast growth factor signaling during embryonic development]]]

==Subtypes of FGFR==
{| class="pretty table"
|-
| '''FGFR Subtype''' || '''Function''' || '''Abnormalities'''
|-
| FGFR1 ||
*Involved in morphogenesis as well as orchestrating the patterning of the mesodermal germ layer at gastrulation<ref><pubmed>16207751</pubmed></ref> 
*Involved in formation of the organ of corti and auditory sensory epithelium <ref><pubmed>12194867</pubmed></ref> *<ref><pubmed>12194867</pubmed></ref>
||
*Pfeiffer Syndrome (Type 1)
*Kallmann syndrome
*Osteoglophonic dysplasia
*8p11 myeloproliferative syndrome
|-
| FGFR2 ||
*Activated prior to gastrulation with the purpose of repressing cellular movements in the presumptive anterior neural plate and preventing normal retinal progenitor cells from adopting retinal fates<ref><pubmed>14723847</pubmed></ref> 
||
*Pfeiffer Syndrome (Type 1-3)
*Apert Syndrome
*Crouzon Syndrome
|-
| FGFR3 ||
*Induces complete growth arrest of cells<ref><pubmed>11779141 </pubmed></ref> 
*Is required to promote differentiation of prechondrogenic mesenchymal cells to cartilage-producing chondrocytes
||
*Achondroplasia
*Thanatophoric Dysplasia
*Hypochondroplasia
|-
| FGFR4 || ADD INFO HERE || ADD INFO HERE
|-
|}

===Signal Transduction===
 [[File:FGF signalling pathway.jpg|thumb|500px|FGFR Signalling Pathway (Image based upon<ref><pubmed>27458533</pubmed></ref>)]]

The process of signal transduction commence with the binding of a cognate ligand to FGFRs ligand binding site which in turn triggers receptor dimerization. This dimerization of the receptor will cause activation of intrinsic kinase activity<ref><pubmed>1655404</pubmed></ref>. This will activate multiple signal transduction pathways intracellularly including RAS, Mitogen-activated protein kinase (MAPK), p38 MAPKs, Phospholipase-C-Gamma, Crk, Protein Kinase-C and Phospholipase-C-Gamma and Extracellular signal-regulated kinases. Activation of FGFRs induces tyrosine phosphorylation of FRS2 (FGFR stimulated2 Grb2 binding protein) which in turn stimulates the recruitment of GRB2 (Growth factor receptor bound protein-2) and SHP2 ( Src homology 2 phosphatase-2) <ref><pubmed>11021964</pubmed></ref>.
 
 
In turn, these sequence of events promote sustained activation of RAS, which leads to changes in gene transcription through interactions with DNA. In addition, FGF receptors will also induce the activation of PI3K (phosphatidylinositol-3-Kinase), STAT1 and Src tyrosine kinase, which will contribute to certain FGF-stimulated biological responses <ref><pubmed>1656221</pubmed></ref>.
 
 
With respect to embryonic development, both the PI3K and RAS pathways are essential in order for normal mesoderm to occur in the embryo. Additionally, receptor-mediated induction of the SHP2-RAS-ERK pathway is a key mechanism through which FGF can activate a variety of biological signalling pathways including cell growth, cellular differentiation as well as morphogenesis <ref><pubmed>9632781</pubmed></ref>.


==Role In Embryonic Development==

===Patterning Of The Embryonic Axis===
In the process of patterning of the embryonic axis, the caudal primordium that is part of the neural plate, contains cells that are rapidly dividing and is able to maintain itself as a growth region (this region is considered to be of "stem cell" status). The expanding populations of dividing cells us spread along the neural tube by cell movements of convergence and extension. In the process by which cells are driven out of the tube, they change their pattern of movement which eventually causes a gradual restriction in space<ref><pubmed>8575335</pubmed></ref>. Within this process, it is the misexpression of a dominant negative FGFR construct in the tissue which causes these cells prematurely to leave the stem cell region and to change their movement patters as if they had aged<ref><pubmed>11389440</pubmed></ref>.

Furthermore, Mathias et al. (2001) suggest that FGFR is required in order to maintain this stem cell status in the caudal neural plate during patterning of the nervous system. In addition, it is possible that FGF serves the purpose of acting as a caudalizing factor for the neural tube because it is capable of prolonging the window of time during which cells are exposed to a caudalizing factor.

In summary, FGF signalling is important in regulating the maturation of developing cells which are gradually being laid down in a caudal direction along the axis of the neural tube.

===Limb Bud Formation===
[[File:LIMB BUD.png|200px|thumb|400px|FGFR Signalling Pathway (Image based upon<ref><pubmed>25772309 </pubmed></ref>)]]

Limb buds are structures formed early in [[Lecture - Limb Development| limb development]] which are comprised of lateral plate mesoderm (LPM) cells and an overlying surface ectoderm. They are roughly formed around week 4 of embryonic development as a result of interactions between the mesoderm and ectoderm germ layers.

FGF proteins and its interactions with other signalling pathways, are critical for the initiation and proximal-distal growth of limbs from a limb bud structure.<ref><pubmed>9620845</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/9620845]</ref> Prior to limb bud formation, FGF10 is widely expressed in the LPM and is stabilized by the WNT signaling proteins. FGF10 is responsible for stimulating the expression WNT3 (and downstream transcription factors including SP6 and SP8<ref><pubmed>15358670</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/15358670]</ref>) in the overlying ectoderm, which results in the formation of the Apical Ectodermal Ridge (AER), a specialised thickening of epithelium located towards the proximal end of the bud that is required for growth,<ref><pubmed>25772309</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/25772309]</ref> which subsequently stimulates FGF8. FGF8 is responsible for continued growth of the underlying mesoderm by keeping in mitotically active state, and stimulating a positive feedbacks loop on FGF10 (which in turn stimulates increased FGF8 expression). FGF8 is the known AER-specific FGF to be expressed throughout it, although other FGFs are expressed in the posterior of the AER (including Fgf4, Fgf9 and Fgf17) and are thought to have supporting roles.<ref><pubmed>11101846</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/11101846]</ref><ref><pubmed>12152071</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/12152071]</ref>
FGFs in the AER signal FGFR1 and FGR2 in distal mesenchyme, activating ETV1 and EWSR1 which function to help to maintain FGF10 expression.<ref><pubmed>25109552</pubmed> [http://www.ncbi.nlm.nih.gov/pubmed/25109552]</ref>

Furthermore, the Zone of Polarising Activity (ZPA) is a region located on the posterior side of the limb bud composed of mesenchyme which signals its anterior-posterior growth (for example this region signals the position of the thumb relative to the little finger.) The Fibroblast Growth Factors FGF2<ref><pubmed>7908145</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/7908145]</ref>, FGF4<ref><pubmed>8001146</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/8001146]</ref> and FGF8<ref><pubmed> 8598907</pubmed> [https://www.ncbi.nlm.nih.gov/pubmed/8598907]</ref> induce Sonic Hedgehog (SHH) within ZPA region and is critical for its growth along the anterior-posterior axis.

Therefore together these interactions of the FGFs from the AER help to maintain proliferating cells near the distal tip of the limb bud, and are known to be critical in limb bud development, both along the proximal-distal axis and the anterior-posterior axis. It is also important to note that growth along the dorsal-vental axis is dependent on the involvement of growth factors from the Wnt family on the ectoderm layer.

FGF signaling is also involved in lung bud initiation and development, with a similar underlying process.

===Bone Development===
[[File:FGF and FGFR expression patterns during endochondral and intramembranous bone development.jpeg|thumb|500px|FGF and FGFR expression patterns during endochondral and intramembranous bone development <ref><pubmed>PMC4526732</pubmed>
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526732/]</ref>]]

Much of what we now understand about the involvement of the FGF signalling pathway in bone development is a result of discovering missense mutations responsible for conditions characterised by abnormal bone structure, including but not limited to skeletal dysplasias and craniosysnostosis syndromes (some of which discussed in more detail later under the subheading abnormalities.) The first and questionably the most important mutation discovered affecting skeletal development was a point mutation of the FGFR3 protein, which was found to be responsible for achondroplasia.

FGF signalling is involved in both endochondral and intramembranous [[Lecture - Musculoskeletal Development| bone development]], which are critical in the early stages of embryonic bone formation, as shown in the diagram to the right the presence of FGFR1-3 and FGF2, FGF9, FGF18 are shown in various stages of bone development. Endochondral bone development is responsible for forming the long bones of the appendicular skeleton, face and spinal column. This involves an intermediate cartilage template (which helps control the growth and patterning of the development of the bony structure.) In comparison intramembranous bone development is responsible for forming bones of the skull and clavicles, and doesn’t require a cartilage template, it directly forms bone. <ref><pubmed>PMC4526732</pubmed>
[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526732/]</ref>

TBC

===Kidney development===
 
The metanephric kidney is an organ which arises primarily form two tissues, the nephrogenic cord and the Wolffian duct, which will eventually give rise to the metanephric mesenchyme and the ureteric bud respectively <ref><pubmed>18835385</pubmed></ref>. Around week 5 of gestation in the developing human embryo, the metanephric mesenchyme will release signalling molecules that stimulate the ureteric bud to grow out from the Wolffian duct and invade the metanephric mesenchyme. The stromal mesenchyme that exists between the Wolffian duct and the metanephric mesenchyme restricts the ureteric bud to its proper position and prevents ectopic budding<ref><pubmed>10749566</pubmed></ref>. The metanephric mesenchyme will continue to release signals which will stimulate the ureteric bud to elongate and repeatedly branch, leading to formation of the ureter, collecting duct system and the renal pelvis. Following its contact with the ureteric bud, the metanephric mesenchyme will then divide into a nephrogenic lineage lying adjacent to the bud, and a surrounding renal cortical stromal lineage <ref><pubmed>19272374</pubmed></ref>. Each terminal tip of the ureteric bud induces local areas of nephrogenic mesenchyme in order to differentiate into nephron epithelia, progressing from renal vesicles ,to comma-shaped bodies, to S-shaped bodies, and then to immature nephrons<ref><pubmed>19272374</pubmed></ref>. The renal cortical stroma will provide a framework and likely a niche for the other renal lineages and vasculature, and ultimately differentiates into interstitial and other supportive cells within the kidney <ref><pubmed>10594778</pubmed></ref>.
 
 
In terms of the development of the metanephric kidney, all FGFRs have been detected in the process of development, however studies using animal models have revealed that it is FGFR1, FGFR2 and FGFR11 which play a key role in renal development <ref><pubmed>10691305</pubmed></ref>. FGFR1 is a receptor which is expressed mostly in the metanephric mesenchyme lineages, these including the early metanephric mesenchyme, the cap mesenchyme and the developing nephrons beginning with vesicles. However, FGFR1 is present at lower levels in the ureteric lineage and in the renal cortical stroma<ref><pubmed>10385628</pubmed></ref>. In contrast, FGFR2 is strongly expressed in the Wolffian duct and the ureteric bud tree as well as the differentiating nephrons. Despite this, FGFR2 is present at lower levels in the early metanephric mesenchyme and stomal mesenchyme adjacent to the Wolffian duct<ref><pubmed>1315677</pubmed></ref>. In addition, FGFR11 is present in renal vesicles <ref><pubmed>1315677</pubmed></ref>.
 
 

===External Genitalia development===
 
 
The genital tubercle (GT) is a structure from which characteristics in the external genitalia in the adult develop. The GT differentiates into a penis in males and a clitoris in females. The process of proximodistal elongation of this GT involves multiple interactions between growth factors and transcription factors <ref><pubmed>3723059</pubmed></ref>. Interactions between epithelium and mesenchyme has an essential role in the regulation of various development processes throughout the embryo. Such signalling controls many aspects of organogenesis, from the initiation of organ development to differentiation <ref><pubmed>8896986</pubmed></ref>.
 
 
The FGFR signalling pathway is involved in epithelial to mesenchymal interactions during organogenesis. Studies have revealed that the first morphological sign of GT outgrowth occurs at approximately 10.5 days post coitum, and will continue throughout the perinatal period <ref><pubmed>12004962</pubmed></ref>. Initially within the developing embryo, the external genitalia of the male and female foetuses are morphologically identical and consist of the GT. Several growth factors including FGF proteins have been shown to control external genitalia development in mice <ref><pubmed>10021340</pubmed></ref>. FGF8, FGF10 and FGFR2 expression has been found during GT developing, thus suggesting that a combination of these factors may constitute redundant developmental functions during GT morphogenesis<ref><pubmed>10804187</pubmed></ref>. As the GT elongates, a groove appears on its ventral aspect called the urethral groove. At the distal end, this groove is made up of a solid plate of epithelial cells, the distal urethral epithelium (DUE) that extends into the glans penis. The solid urethral plate canalizes and thus extends the urethral groove distally into the glans. It was found that FGFR2IIIb is expressed in the DUE and urethral plate epithelia of the GT. Deletion of this receptor and FGF10 was shown to cause urethral dysmorphogenesis.
[[File:External genitalia.jpg]]
 
 

===Inner Ear Development===

==Animal Models==
Jocelyn
 
===Site of FGF10 expression in the chick embryo===
 
==Abnormalities ==
As discussed above, the FGF signalling pathway is critical for regulating many early embryonic developmental processes, and is critical for normal organ, vascular and skeletal development. Consequently, abnormalities in genes coding for the proteins within this signalling pathway (including signalling proteins, non-signalling proteins, and receptors) can result in many visible structural abnormalities such as short statue and face deformations. Not to mention that a large majority of these conditions, if not all, influence an individual’s quality of life, and in some cases increase risk of fatality. Some of these FGF abnormalities are outlined in more detail below, including Achondroplasia, Pfeiffer and Apert Syndrome which particularly emphasise the significance of FGF signalling in early skeletal development.

===Achondroplasia===
Achondroplasia is the most common form of skeletal dysplasia, and is often characterised by shortened proximal limbs, a curved spine, a large prominent forehead and a fattened nasal bridge. This condition is inherited genetically as an autosomal dominant trait, although a large proportion of cases are sporadic. <ref><pubmed>7913883</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/7913883]</ref> This condition results in reduced inhibition of endochondral ossification, which is one of the main way in which bone tissue is created during embryonic development (the other being intramembranous ossification.) Endochondral ossification is essential during development for both the formation and growth of long bones as well as healing fractures. For the majority of affected individuals, it is a result of a missense mutation in FGFR3, specifically due to a substitution of arginine for glycine (G380R).<ref><pubmed>12816345</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/12816345]</ref> As originally postulated by Bonaventure et al. (1996) this introduction of a hydrophilic residue in a hydrophobic receptor domain results in a disruption of alpha-helical structure of the transmembrane portion of the protein and consequently interferes with the signal transduction pathway of which it is involved in. <ref><pubmed>8723101</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/8723101]</ref>

There are other mutations in FGFR3 which are responsible for different skeletal developmental conditions, including a more severe (usually fatal) form of skeletal dysplasia, Thanatophoric Dysplasia, which is due to two different mutations, K650E and R248C in FGFR3 (type 1 and type 2 respectively) and a milder form, hypochondroplasia, which is due to the mutations, N540K or K650N in FGFR3.

===Pfeiffer Syndrome===

Pfeiffer syndrome is characterised by craniosynostosis, meaning that is it a condition where the cranial fibrous sutures prematurely fuse (ossify) resulting in a reduced space for the growing brain. Subsequently, abnormal growth of the skull, in an attempt to increase the space available for the brain and reduce cranial pressure, results in the development of abnormal facial features including, but not limited to, proptosis (abnormal placement of the eye), hypertelorism (abnormal increase in distance between the eyes), maxillary deficiency, and a beaked nose. Other notable features include those of the hands, broad thumbs and the feet, medially deviated broad great toes.
<ref><pubmed>9300656</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/9300656]</ref><ref><pubmed>25679016</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/25679016]</ref> This condition is inherited genetically as an autosomal dominant trait. There are 3 types of Pfeiffer syndrome. Type 1 is a result of either a gain of function P252R mutation of FGFR1 (5%), which increases the receptor’s ligand binding affinity resulting in over-activation of the receptor, or sequence variants of FGFR2 gene (95%.)<ref><pubmed>25679016</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/25679016]</ref> Type 2 and 3 are similar, both appear more severe and generally have a worse prognosis compared to Type 1, and are a result of mutations of the FGFR2 gene.
<ref><pubmed>8434615</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/8434615]</ref><ref><pubmed>10394936</pubmed>[https://www.ncbi.nlm.nih.gov/pubmed/10394936]</ref>

===Apert Syndrome===

Mutations in FGFR2: S252W

==New and Emerging Research Into FGF==

===Emerging Research Into The Role Of FGF In The Development Of The Growth Plate===
https://www.ncbi.nlm.nih.gov/pubmed/25114206
 
===Autoregulatory loop of induction between FGF10 and FGF8 ===
 
==Quiz: How much do you really know about FGF? Take the quiz and find out!==

<quiz display=simple>

{Which of the following statements regarding FGFR3 is true?
|type="()"}
- Mutation in the receptor causes Pfeiffer Syndrome
+ Induces complete growth arrest of cells
- Prevents chondrocytes from developing
- Associated with Kallmann syndrome
|| Option X is correct. EXPLAIN

{What...
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

{The ...:
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

{Which of the following is false:
|type="()"}
- OPTION
+ OPTION
- OPTION
- OPTION
|| Option X is correct. EXPLAIN

</quiz>
==Glossary==

{| class="pretty table"
| '''Fibroblast Growth Factors (FGFs)'''
| Family of 22 proteins, 18 of which are secreted signalling proteins (FGF1-10, and FGF16-23) and the other 4 are intracellular non-signalling proteins (iFGFs; FGF11-14)
|-
| '''Fibroblast Growth Factor Receptors (FGFRs)'''
| 4 tyrosine kinase receptors (FGFR1-4) that interact with with the signalling FGF proteins
|-
|}

Some more words to include:
-Gastrulation
-Embryonic axis
-Germ layers (endoderm, ectoderm, mesoderm)
-Endochondral bone development
-Intramembranous bone development
-Missense mutations
-Metanephric kidney
-Craniosysnostosis syndromes
-Skeletal dysplasia

Manraaj

==References==

<references/>