Glia-activating factor is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that in humans is encoded by the ''FGF9''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

Function

The protein encoded by this gene is a member of the

fibroblast growth factor Fibroblast growth factors (FGF) are a family of cell signalling proteins produced by the macrophages. They are involved in a wide variety of processes, most notably as crucial elements for normal development in animal cells. Any irregularities in ...

(FGF) family. FGF family members possess broad mitogenic and cell survival activities, and are involved in a variety of biological processes, including

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

, cell growth,

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

, tissue repair, tumor growth and invasion. This protein was isolated as a secreted factor that exhibits a growth-stimulating effect on cultured

glial cell Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up ...

s. In nervous system, this protein is produced mainly by neurons and may be important for glial cell development. Expression of the mouse homolog of this gene was found to be dependent on

Sonic hedgehog Sonic hedgehog protein (SHH) is a major signaling molecule of embryonic development in humans and animals, encoded by the ''SHH'' gene. This signaling molecule is key in regulating embryonic morphogenesis in all animals. SHH controls organoge ...

(Shh) signaling. Mice lacking the homolog gene displayed a male-to-female sex reversal phenotype, which suggested a role in testicular embryogenesis. This gene is involved in the patterning of sex determination, lung development, and skeletal development.

Sex determination

FGF9 has also been shown to play a vital role in male sex development. FGF9’s role in sex determination begins with its expression in the bi-potent gonads for both females and males. Once activated by

SOX9 Transcription factor SOX-9 is a protein that in humans is encoded by the ''SOX9'' gene. Function SOX-9 recognizes the sequence CCTTGAG along with other members of the HMG-box class DNA-binding domain, DNA-binding proteins. It is expressed by ...

, it is responsible for forming a feedforward loop with Sox9, increasing the levels of both genes. It forms a

positive feedback loop Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects ...

upregulating SOX9, while simultaneously inactivating the female

Wnt4 WNT4 is a secreted protein that, in humans, is encoded by the ''WNT4'' gene, found on chromosome 1. It promotes female sex development and represses male sex development. Loss of function may have consequences, such as female to male sex revers ...

signaling pathway.

Lung development

In lung development, FGF9 is expressed in the

mesothelium The mesothelium is a membrane composed of simple squamous epithelium, simple squamous epithelial cells of mesodermal origin, which forms the lining of several body cavities: the pleura (pleural cavity around the lungs), peritoneum (abdominopelvic ...

and pulmonary epithelium, where its purpose is to retain lung mesenchymal proliferation. Inactivation of FGF9 results in diminished epithelial branching. By the end of gestation, the lungs that are developed cannot sustain life and will result in a prenatal death.

Skeletal development

Another biological role presented by this gene is its involvement in skeletal development and repair. FGF9 and FGF18 both stimulate

chondrocyte Chondrocytes (, ) are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although the word '' chondroblast'' is commonly used to describe an immatu ...

proliferation. FGF9 heterozygous mutant mice had a compromised bone repair after an injury with less expression of VEGF and VEGFR2 and lower

osteoclast An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and bone remodeling, remodeling of bones of the vertebrate, vertebral skeleton. The osteoclast disassembles and digests th ...

recruitment. One disease associated with this gene is multiple synostoses syndrome (SYNS), a rare bone disease that has to do with the fusion of the fingers and toes. A missense mutation in the second exon of the FGF9 gene, the S99N mutation, seems to be the third cause of SYNS. A mutation in Noggin (NOG) and the Growth Differentiation Factor 5 ( GDF5) are the other two causes of SYNS. The S99N mutation results in cell signaling irregularities that interfere with chondrogenesis and

osteogenesis Osteoblasts (from the Greek language, Greek combining forms for "bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cell (biology), cells with a single Cell nucleus, nucleus that synthesize bone. However, in the p ...

causing the fusion of the joints during development.

Overexpression of FGF9

FGF9 is a gene within the larger family of fibroblast growth factors (FGF), a type of cell signaling protein. This gene signals embryonic stem cell development and sex determination. FGF9 gene expression is also essential for development of the prostate and maintaining prostate tissue homeostasis. The prostate is a male reproductive organ that is composed of epithelial and stromal cells. Overexpression of FGF9 in prostate epithelial cells can lead to high grade prostate intraepithelial neoplasia, which is a precursor for prostate cancer. Additionally, high expression of the gene in prostate epithelial cells disrupts prostate tissue homeostasis, and promotes a high frequency of metastasis. On the other hand, overexpression of FGF9 in the alternate, prostate stromal cells, promotes the communication with prostate cancer cells. It has been reported that abnormal expression of FGF9 has oncogenic effects in various human cancers including; ovarian, brain, lung, and colon cancers. In studies with mice, high expression of FGF9 resulted in fusion of the prostate and seminal vesicles, and penis protrusion. More importantly, it caused hyperplasia in both stromal and epithelial compartments. Due to the enlargement of tissue caused by an increase in the reproduction rate of its cells, hyperplasia is frequently the primary stage in the development of cancer. Although several studies have proven that high expression of FGF9 correlates to the progression of prostate cancer, the question of whether overexpression of FGF9 initiates prostate tumorigenesis is still being tested.

Interactions

FGF9 has been shown to interact with Fibroblast growth factor receptor 3.