FGF9

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.^[7] Once activated by SOX9, it is responsible for forming a feedforward loop with Sox9, increasing the levels of both genes. It forms a positive feedback loop upregulating SOX9, while simultaneously inactivating the female Wnt4 signaling pathway.^[7][8]

Lung development

In lung development, FGF9 is expressed in the mesothelium and pulmonary epithelium, where its purpose is to retain lung mesenchymal proliferation. Inactivation of FGF9 results in diminished epithelial branching.^[9] By the end of gestation, the lungs that are developed cannot sustain life and will result in a prenatal death.^[9]

FGF9, which encodes for a fibroblast growth factor signaling protein, is vital for lung development. It is expressed in both the mesothelium, which will later differentiate into the pleura, as well as the pulmonary epithelium, which differentiates into the airways. The gene controls mesenchymal proliferation and as development progresses, there is a lower expression of the gene present in the epithelium although it persists in the mesenchymal cells. Inactivation causes diminished epithelial branching, as well as underdeveloped lungs, which often results in neonatal death. In the FGF9 knockout mice, death is common before, or within a few weeks of, birth due to the absence of adequate lung development. Lower ratios of mesenchymal tissue are also observed in these mice. Research has been conducted on knockout mice to assess the development of lungs without the presence of Fgf9. It has been found that Fgf9 is also vital for Shh (Sonic Hedgehog) signaling and function, as both genes work in tandem to determine lung structure and branching.^[10]

Skeletal development

Another biological role presented by this gene is its involvement in skeletal development and repair. FGF9 and FGF18 both stimulate chondrocyte proliferation.^[11] FGF9 heterozygous mutant mice had a compromised bone repair after an injury with less expression of VEGF and VEGFR2 and lower osteoclast recruitment.^[11] 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.^[12] A missense mutation in the second exon of the FGF9 gene, the S99N mutation, seems to be the third cause of SYNS.^[13] A mutation in Noggin (NOG) and the Growth Differentiation Factor 5 (GDF5) are the other two causes of SYNS.^[13] The S99N mutation results in cell signaling irregularities that interfere with chondrogenesis and osteogenesis causing the fusion of the joints during development.^[13]

Metabolism and bone-adipose homeostasis

Furthermore, FGF9 serves as an important regulator of metabolic processes, particularly in bone and adipose tissue homeostasis. In the bone marrow microenvironment, FGF9 influences the fate of bone marrow mesenchymal stem cells (BMSCs), which can differentiate into either osteoblasts or adipocytes. Studies demonstrate that FGF9 promotes adipogenic differentiation while suppressing osteogenesis, thereby regulating the balance between bone formation and bone marrow adipose tissue accumulation.^[14] Mechanistically, this effect is mediated through signaling pathways such as PI3K/AKT, Hippo, and MAPK/ERK.

Knockout and functional evidence

Lastly, functional studies using animal models have demonstrated that FGF9 is essential for normal development and physiological function. Loss-of-function mutations and gene ablation studies reveal that disruption of FGF9 signaling leads to significant developmental abnormalities, including defects in lung formation and male-to-female sex reversal.

In addition to these developmental defects, experimental knockdown models demonstrate that FGF9 plays a critical role in metabolic homeostasis. Reduced expression of FGF9 leads to increased lipid accumulation in hepatocytes and exacerbates metabolic disease phenotypes, including insulin resistance and fatty liver disease.^[15] In contrast, overexpression models show protective effects against these conditions.

Additionally, FGF9 knockout and mutation models also provide insight into skeletal biology. In particular, Fgf9⁻/⁻ mice exhibit reduced blood vessel formation in long bones, leading to delayed mineralization center formation and ultimately resulting in shortened skeletal segments.^[14] Histomorphological and cytodifferentiation analyses also demonstrate that Fgf9 loss-of-function mutations alter bone marrow adipose tissue dynamics, including reduced BMAT formation and alleviation of ovariectomy-induced bone loss and BMAT accumulation.

Finally, beyond the aforementioned roles, studies have also demonstrated that FGF9 is critical during early pregnancy. FGF9 expression is tightly regulated and peaks at the time of embryo implantation, where it is localized to the uterine epithelium surrounding the blastocyst and contributes to the establishment of a microenvironment necessary for successful implantation and pregnancy maintenance.^[16]