The intracellular interplay between galectin-1 and FGF12 in the assembly of ribosome biogenesis complex.

Galectins constitute a class of lectins that specifically interact with ß-galactoside sugars in glycoconjugates and are implicated in diverse cellular processes, including transport, autophagy or signaling. Since most of the activity of galectins depends on their ability to bind sugar chains, galectins exert their functions mainly in the extracellular space or at the cell surface, which are microenvironments highly enriched in glycoconjugates. Galectins are also abundant inside cells, but their specific intracellular functions are largely unknown. Here we report that galectin-1, -3, -7 and -8 directly interact with the proteinaceous core of fibroblast growth factor 12 (FGF12) in the cytosol and in nucleus. We demonstrate that binding of galectin-1 to FGF12 in the cytosol blocks FGF12 secretion. Furthermore, we show that intracellular galectin-1 affects the assembly of FGF12-containing nuclear/nucleolar ribosome biogenesis complexes consisting of NOLC1 and TCOF1. Our data provide a new link between galectins and FGF proteins, revealing an unexpected glycosylation-independent intracellular interplay between these groups of proteins.

FGF12: biology and function.

Fibroblast growth factor 12 (FGF12) belongs to the fibroblast growth factor homologous factors (FHF) subfamily, which is also known as the FGF11 subfamily. The human FGF12 gene is located on chromosome 3 and consists of four introns and five coding exons. Their alternative splicing results in two FGF12 isoforms - the shorter 'b' isoform and the longer 'a' isoform. Structurally, the core domain of FGF12, is highly homologous to that of the other FGF proteins, providing the classical tertiary structure of ß-trefoil. FGF12 is expressed in various tissues, most abundantly in excitable cells such as neurons and cardiomyocytes. For many years, FGF12 was thought to be exclusively an intracellular protein, but recent studies have shown that it can be secreted despite the absence of a canonical signal for secretion. The best-studied function of FGF12 relates to its interaction with sodium channels. In addition, FGF12 forms complexes with signaling proteins, regulates the cytoskeletal system, binds to the FGF receptors activating signaling cascades to prevent apoptosis and interacts with the ribosome biogenesis complex. Importantly, FGF12 has been linked to nervous system disorders, cancers and cardiac diseases such as epileptic encephalopathy, pulmonary hypertension and cardiac arrhythmias, making it a potential target for gene therapy as well as a therapeutic agent.

FGF homologous factors are secreted from cells to induce FGFR-mediated anti-apoptotic response.

FGF homologous factors (FHFs) are the least described group of fibroblast growth factors (FGFs). The FHF subfamily consists of four proteins: FGF11, FGF12, FGF13, and FGF14. Until recently, FHFs were thought to be intracellular, non-signaling molecules, despite sharing structural and sequence similarities with other members of FGF family that can be secreted and activate cell signaling by interacting with surface receptors. Here, we show that despite lacking a canonical signal peptide for secretion, FHFs are exported to the extracellular space. Furthermore, we propose that their secretion mechanism is similar to the unconventional secretion of FGF2. The secreted FHFs are biologically active and trigger signaling in cells expressing FGF receptors (FGFRs). Using recombinant proteins, we demonstrated their direct binding to FGFR1, resulting in the activation of downstream signaling and the internalization of the FHF-FGFR1 complex. The effect of receptor activation by FHF proteins is an anti-apoptotic response of the cell.

FGF12 is a novel component of the nucleolar NOLC1/TCOF1 ribosome biogenesis complex.

Among the FGF proteins, the least characterized superfamily is the group of fibroblast growth factor homologous factors (FHFs). To date, the main role of FHFs has been primarily seen in the modulation of voltage-gated ion channels, but a full picture of the function of FHFs inside the cell is far from complete. In the present study, we focused on identifying novel FGF12 binding partners to indicate its intracellular functions. Among the identified proteins, a significant number were nuclear proteins, especially RNA-binding proteins involved in translational processes, such as ribosomal processing and modification. We have demonstrated that FGF12 is localized to the nucleolus, where it interacts with NOLC1 and TCOF1, proteins involved in the assembly of functional ribosomes. Interactions with both NOLC1 and TCOF1 are unique to FGF12, as other FHF proteins only bind to TCOF1. The formation of nucleolar FGF12 complexes with NOLC1 and TCOF1 is phosphorylation-dependent and requires the C-terminal region of FGF12. Surprisingly, NOLC1 and TCOF1 are unable to interact with each other in the absence of FGF12. Taken together, our data link FHF proteins to nucleoli for the first time and suggest a novel and unexpected role for FGF12 in ribosome biogenesis. Video Abstract.