FGFRL1: Structure, Primary Function, and Evolutionary Perspective

Fibroblast Growth Factor Receptor-Like 1 (FGFRL1) – sometimes called FGFR5 – is the fifth member of the FGFR family. It shares the general architecture of classical FGFRs, with three extracellular immunoglobulin-like (Ig) domains and a single-pass transmembrane helix, but crucially lacks the intracellular tyrosine kinase domain[1][2]. Instead, FGFRL1 ends in a short \~100-amino-acid cytoplasmic tail containing unique sequence motifs. Below, we detail FGFRL1’s structural features, its role in cell signaling and development, and an evolutionary view of this atypical receptor. We focus on FGFRL1’s primary molecular functions – how it interacts with ligands and cells – rather than downstream phenotypic effects (though key developmental phenotypes that illuminate function are included). Finally, we highlight open questions one might pose to experts and suggest experiments (in various model organisms) to further elucidate FGFRL1’s function.

Evolutionary Origin and Conservation of FGFRL1

FGFRL1 is an evolutionarily ancient gene present across a broad range of metazoans. Orthologs have been identified in species from cnidarians (e.g. Nematostella sea anemone) to vertebrates, suggesting FGFRL1 emerged early alongside the FGF signaling system[3][4]. In fact, some researchers speculate FGFRL1 may have been an ancestral FGFR that predated the evolution of the canonical FGFR1–4 receptors[5][6]. Most animals possess a single FGFRL1 gene; for instance, mammals and birds carry one FGFRL1 copy, as do basal chordates like amphioxus[7]. (Teleost fishes are an exception – due to genome duplication they have multiple fgfrl1 paralogs[7].) The FGFRL1 protein sequence is highly conserved across species, especially in its extracellular region. For example, chicken FGFRL1 shares \~74% identity with human FGFRL1 (and \~72% with rat)[8]. This level of conservation implies strong evolutionary pressure on the ectodomain, likely due to its critical interactions (ligand binding, etc.). By contrast, the short intracellular tail of FGFRL1 is poorly conserved between species – little sequence similarity is seen apart from three small motifs: a dileucine sequence, a tandem YXXΦYXXΦ motif, and a histidine-rich segment[9]. These motifs, rather than exact sequence, appear functionally important (as discussed below).

Notably, FGFRL1 orthologs can be difficult to recognize in some invertebrate genomes because of rapid sequence divergence[10][11]. Early on, FGFRL1 was not obvious in fruit flies (Drosophila) or nematodes (C. elegans), leading to speculation it might be a chordate-only gene[12]. However, subsequent analysis found FGFRL1 in echinoderms (e.g. sea urchin) and even cnidarians[13][4]. It’s possible that insects and some worms either lost FGFRL1 or that their versions are highly diverged and thus “escaped attention” in genome searches[10][11]. Overall, the weight of evidence indicates FGFRL1 (or a proto-FGFRL1) co-evolved with the FGF signaling pathway in early metazoans[5][6]. Indeed, one hypothesis is that FGFRL1 was an ancestral FGFR which later gave rise to the kinase-bearing FGFRs – or conversely, that it arose by duplication of an FGFR followed by loss of the kinase domain[6]. In either case, its conservation points to an important biological role maintained over hundreds of millions of years.

Structural Features of FGFRL1

Structure of the FGFRL1 protein, highlighting domains and their functions. FGFRL1 has an N-terminal signal peptide (cleaved upon secretion), three Ig-like extracellular domains (D1–D3) separated by a flexible linker (“acidic box”), a single transmembrane region, and a short intracellular C-terminus. Key functional elements include a basic region for heparin binding, an FGF-binding site in D2–D3, a hydrophobic patch in D3 implicated in cell–cell fusion, and cytosolic motifs for endocytic trafficking (tandem YXXΦ motifs) and a histidine-rich Zn-binding segment[14][15].

Domain architecture: FGFRL1 is a single-pass transmembrane glycoprotein in the immunoglobulin superfamily. Its extracellular portion consists of three Ig-like domains (D1, D2, D3), arranged similarly to FGFR1–4[1][2]. A short linker (sometimes termed an “acidic box” in FGFRs) connects D1 and D2, providing flexibility for D1 to fold back toward D2[16]. Each Ig domain is stabilized by conserved disulfide bonds[17]. Like classical FGFRs, FGFRL1’s ectodomain is N-glycosylated at multiple sites[18], and indeed 3–4 N-linked glycans are present on human FGFRL1[19][18]. At the extreme N-terminus is a signal peptide that directs the nascent protein into the secretory pathway; this signal sequence is cleaved off in the endoplasmic reticulum (the cleavage occurs between Gly-17 and Ala-18 in human FGFRL1)[20]. Following the Ig domains is a single hydrophobic transmembrane (TM) helix anchoring FGFRL1 in the plasma membrane.

Crucially, FGFRL1 lacks any kinase or other conventional signaling domain intracellularly[21][2]. Instead, its cytoplasmic tail (\~100 amino acids) contains unusual motifs: a tandem tyrosine-based motif (sequence YXXΦYXXΦ, where Φ is a hydrophobic residue) and a histidine-rich segment near the C-terminus[15][22]. These motifs are highly conserved in vertebrates despite overall divergence of the tail[9]. They serve as signals for intracellular trafficking and protein interactions. The tandem YXXΦ motifs resemble canonical endocytic sorting signals; indeed, mutating or deleting either the tyrosine motifs or the histidine-rich tail causes FGFRL1 to remain at the cell surface longer rather than being efficiently internalized[23][24]. In normal FGFRL1, these signals mediate rapid trafficking of the receptor from the plasma membrane into endosomes and lysosomes[15][25]. This means that wild-type FGFRL1 doesn’t linger long on the cell surface – it cycles inward, whereas a mutant lacking these motifs accumulates at the membrane[26][27]. The histidine-rich sequence (in human, ten histidines alternating with other residues) has been shown to bind divalent metal ions, notably Zn\2+\ and Ni\2+\[28][29]. In vitro, the FGFRL1 tail can coordinate about three Zn ions[29], though the in vivo significance of this metal-binding is not fully understood. It might confer pH-sensitive or metal-regulated properties to FGFRL1, or simply be a relic motif. Interestingly, rodents have a natural frameshift that replaces part of the histidine-rich stretch with an unrelated sequence (\~54 residues), yet their FGFRL1 still functions in trafficking[22]. This suggests that while the presence of a C-terminal “bulk” sequence is needed for proper localization, the exact amino acid composition can vary as long as key signals (e.g. the dileucine and tyrosines) are present or functionally compensated[22][30].

Ligand binding: Despite lacking a kinase domain, FGFRL1 does bind canonical FGF ligands and heparan sulfate – a key clue to its function. The FGFRL1 ectodomain is \~40% similar in sequence to FGFR1–4 and retains many residues known to contact FGFs in those receptors[31][16]. Structural modeling and mutational analyses indicate FGFRL1 likely binds FGFs in a manner analogous to FGFRs: the ligand contacts primarily the D2–D3 interface of FGFRL1, with heparan sulfate (or heparin) aiding the interaction[32][16]. Indeed, FGFRL1 has an unusually basic region at the beginning of domain D2 (ten positively charged residues in a row) that is thought to serve as a heparin-binding site[33][31] – heparin (or heparan sulfates on cell surfaces) strongly enhances FGF–FGFR interactions. FGFRL1 binds heparin with high affinity, similar to FGFRs[34][35]. Direct binding assays have confirmed that FGFRL1 can physically bind a subset of FGF proteins: notably FGF2, FGF3, FGF4, FGF8, FGF10, FGF18, and FGF22 (among others)[36]. In one study, FGFRL1 showed strongest binding to FGF3, 4, 8, 10, and 22; intermediate affinity for FGF2, 5, 17, 23; and little/no binding to FGF1 or certain others[37]. This pattern suggests FGFRL1 may preferentially interact with FGFs involved in developmental processes (e.g. FGF8 and FGF10 are key in organogenesis, FGF18 in osteogenesis, etc.). Importantly, FGFRL1’s three Ig domains appear to act together for optimal ligand binding and function – for example, deletions of the Ig domains abrogate its effects in vivo (discussed later). The first Ig domain (D1) may have a regulatory role, potentially folding back to autoinhibit ligand binding (a mechanism seen in FGFR1)[38], but FGFRL1’s D1 is sometimes alternatively spliced out (two minor FGFRL1 mRNA isoforms lacking D1 have been reported[39][40], though their significance is unclear).

Shedding and soluble FGFRL1: Like many membrane receptors, FGFRL1 can be proteolytically shed from the cell surface. A fraction of FGFRL1 is released as a soluble ectodomain, which can be detected in conditioned media[41]. This shedding was observed to increase when myoblasts differentiate into myotubes in culture (coinciding with cell fusion events)[42], and also occurs in FGFRL1-overexpressing HEK293 cells[43]. The cleavage site has been mapped to a region just outside the transmembrane domain (within a cluster of four serines), although the responsible protease remains unknown[44]. It does not appear to be a canonical sheddase like furin, BACE, or ADAM9[45]. The soluble FGFRL1 ectodomain can still bind FGF ligands[46], effectively acting as a decoy receptor in the extracellular space. Thus, shedding provides another mechanism for FGFRL1 to modulate FGF signaling (by scavenging ligands before they reach signaling receptors). In summary, the structural features of FGFRL1 – an FGF-binding ectodomain coupled to an inert (kinase-lacking) but traffickable membrane anchor – position it as an atypical receptor that can intercept growth factors and mediate cell interactions without directly activating canonical tyrosine-kinase signaling pathways.

Modulation of FGF Signaling and Cellular Effects

Given its ability to bind FGFs but inability to signal via phosphorylation, FGFRL1 was long hypothesized to function as a “decoy” receptor that dampens FGF signaling. Early experiments supported this notion: for example, ectopic expression of FGFRL1 in frog embryos antagonized FGF-mediated developmental signals[47]. By sequestering FGF ligands or forming non-signaling complexes, FGFRL1 can prevent excessive activation of the classical FGFRs. In cell culture, overexpression of FGFRL1 tends to reduce cell proliferation and DNA synthesis[48]. For instance, in osteoblast-like cells FGFRL1 overexpression inhibited thymidine incorporation (DNA replication), suggesting an anti-proliferative effect[48]. Conversely, some studies indicated FGFRL1 promotes differentiation: cells with high FGFRL1 showed enhanced morphological differentiation even as proliferation slowed[34][49]. This led to a paradigm where FGFRL1 acts as a negative regulator of growth signals (restricting cell proliferation) but a positive facilitator of differentiation[34][49].

However, the role of FGFRL1 in signaling is more complex than a simple off-switch. Notably, a 2016 study by Yang et al. found that manipulating FGFRL1 levels in vitro did not measurably change cell proliferation or ERK phosphorylation in their system[50][51]. Using inducible expression and siRNA knockdown, they observed “no effect on cell growth” and no activation of 250 signaling phosphoproteins tested[52][51]. This was surprising, and the authors proposed that FGFRL1’s primary role may not be to modulate mitogenic signaling after all, but rather to function in cell adhesion (discussed in the next section)[53]. They also noted that FGFRL1 knockout mice did not show the expected increase in FGF target gene expression that one might predict if a negative regulator were removed[54]. These findings suggest FGFRL1’s in vivo function may be less about globally suppressing FGF signaling intensity and more about contextual modulation or structural roles.

One way FGFRL1 might modulate signaling contextually is through its interaction with intracellular inhibitors of the pathway. Indeed, FGFRL1 has been shown to bind Spred1, a member of the Sprouty/Spred family which are natural inhibitors of the Ras/MAPK cascade[55]. Specifically, FGFRL1’s C-terminal region (notably the histidine-rich tail) can bind the SPR (Sprouty-related) domain of Spred1[56][55]. By tethering Spred1 (and possibly Sprouty2) to certain cellular locations, FGFRL1 could facilitate localized suppression of FGF-induced Ras–ERK signaling[57]. For example, during kidney development FGFRL1 might recruit Spred1 at sites of FGF8 signaling, ensuring that the signal does not overshoot. This is speculative but supported by co-immunoprecipitation evidence of FGFRL1–Spred binding[58]. In line with this, Fgfrl1 is essential for kidney organogenesis – knockout mice completely lack metanephric kidneys[59], a phenotype consistent with dysregulated FGF signaling (FGF8 and other FGFs are critical in kidney inductive interactions). Indeed, Fgfrl1\-/-\ mouse kidney rudiments fail to express normal levels of Wnt4, Fgf8, Pax8, Lim1, etc., indicating that the mesenchyme was not properly induced[60]. FGFRL1’s presence is thus required for that FGF-driven mesenchymal-to-epithelial transition – likely not by propagating the signal itself, but by finely tuning it (perhaps ensuring the right spatial distribution or timing of FGF activity).

Another mechanism by which FGFRL1 influences signaling is via its rapid internalization and ligand sequestration. FGFRL1 on the cell surface can bind FGFs, then get endocytosed (thanks to its YXXΦ motifs), carrying the ligand into endosomes. From there, the FGFRL1–FGF complex may be routed to lysosomes for degradation or to the Golgi and recycled[61][62]. This effectively removes FGF ligands from the extracellular milieu, limiting their availability to signaling receptors. The soluble FGFRL1 generated by ectodomain shedding likely has a similar ligand-trapping effect in the extracellular space[63][64]. Thus, FGFRL1 can act as a sink or buffer for FGFs, helping to shape morphogen gradients or prevent overactivation of FGFR signaling loops. It’s worth noting that FGFRL1 does not appear to form heterodimers with FGFR1–4 to signal; instead, it functions in parallel or as a competitor. For example, FGFRL1 transgenic overexpression can rescue certain FGF-driven phenotypes by soaking up ligands[47], rather than by transmitting signals.

While generally associated with dampening proliferation, FGFRL1 has also been linked to pro-differentiation and pro-maturation signals in cells. In chondrocytes and osteoblasts, FGFRL1 expression is upregulated as cells exit the cell cycle and begin to differentiate[34]. Appropriately, FGFRL1 contributes to cartilage and bone development: mice lacking FGFRL1 show defects in skeletal ossification and a human FGFRL1 mutation causes craniosynostosis (premature fusion of skull bones)[65]. These phenotypes reflect perturbations in cell differentiation in cartilage and bone lineages. FGFRL1 may normally restrain chondrocyte proliferation in the growth plate while promoting their hypertrophy and maturation into bone. In support of this, FGFRL1 is highly expressed in cartilaginous tissues (it was originally discovered in a screen for cartilage-specific genes[66]), and also enriched in developing bone and muscle[67][68]. Conversely, in contexts like cancer, loss of FGFRL1 might remove a brake on cell growth. Indeed, FGFRL1 overexpression has been shown to suppress tumor growth in xenograft models[69]. For example, introducing FGFRL1 into HEK293 cells slowed their proliferation and reduced tumor formation in nude mice[69], consistent with FGFRL1 acting as a tumor suppressor. Some cancers, however, show elevated FGFRL1 and there is emerging evidence that FGFRL1 might also influence pathways like PI3K/AKT or Hedgehog to promote survival or migration in specific contexts[70]. These effects appear cell-type specific and may involve cross-talk outside of classical FGF signaling. For instance, FGFRL1 has been reported to interact with phosphatase SHP-1 in pancreatic β-cells, potentially modulating ERK activity in an unusual (possibly ligand-independent) way[71]. Such findings hint that FGFRL1’s role is multifaceted – it may primarily act as a scaffold or modulator at the cell membrane, with outcomes that can either inhibit or fine-tune signaling depending on the cellular context.

In summary, FGFRL1 modulates cell signaling chiefly by ligand binding and sequestration, and by recruiting inhibitory proteins, rather than by transducing signals itself. The net effect in most developmental contexts is to temper FGF-driven proliferation and to facilitate proper differentiation of cells (ensuring signals occur in the right place and time). However, recent studies challenge the simplistic “decoy” model, suggesting FGFRL1 is not merely a passive sink but could also participate in organizing cell architecture and interacting with other pathways. This leads into FGFRL1’s intriguing functions in cell adhesion and fusion, which are integral to its developmental role.

Roles in Development: Cartilage, Bone, Kidney, and Diaphragm

FGFRL1 plays essential roles in mammalian development, as evidenced by knockout studies. Mice lacking Fgfrl1 die at birth (100% penetrance) due to a combination of organ defects[72][73]. The most striking phenotypes are: absence of kidneys, a malformed diaphragm, and a dome-shaped skull[73][74]. The renal agenesis (no metanephric kidneys) underscores FGFRL1’s importance in branching morphogenesis and mesenchymal induction in the kidney. Normally, FGF signals (e.g. FGF8 from the nephric duct tip) induce the metanephric mesenchyme to form nephrons. In Fgfrl1 knockouts, this induction fails – markers of mesenchyme differentiation (Wnt4, Pax8, etc.) are not upregulated[60]. Thus, FGFRL1 is required for the kidney-forming signal to take effect, likely by controlling the availability or distribution of FGF8 and related factors. Interestingly, FGFRL1 is also expressed in other branching organs (lung, salivary gland), but kidney is especially sensitive to its loss[75][60], possibly because of the precise FGF threshold needed in that context.

The diaphragm defect in Fgfrl1\-/-\ mice is lethal – pups cannot inflate their lungs and suffocate at birth[73][76]. Closer examination revealed that the diaphragm muscle in knockouts lacks slow-twitch muscle fibers[74][77]. Without these fatigue-resistant fibers, the diaphragm is too weak to function. This indicates FGFRL1 is important in muscle development and differentiation, particularly in specifying muscle fiber types or ensuring muscle integrity. FGFRL1 is highly expressed in skeletal muscle (especially during development)[78][68], and its upregulation correlates with myoblast differentiation. Why would a “decoy receptor” be needed for muscle fiber formation? One possibility is that FGFRL1 moderates FGF signaling in muscle progenitors, allowing them to exit the cell cycle and differentiate (excess FGF can keep myoblasts proliferating). Alternatively, FGFRL1’s adhesion/fusion role (see next section) could be critical for myoblasts to fuse into myotubes. Indeed, FGFRL1 is shed during myotube formation[42], and FGFRL1’s Ig3 domain can induce cultured cells to fuse (even non-muscle cells), hinting that it might act as a fusogen or fusion facilitator in muscle development. Consistent with this, the absence of FGFRL1 disrupts the normal formation of slow fibers in the diaphragm, perhaps because myoblast fusion or differentiation cues are impaired[74][77]. It would be interesting to see if other muscles are also affected in the knockout (the diaphragm defect is most prominent because it’s essential for breathing).

Skeletal development is another major domain of FGFRL1 function. Fgfrl1 knockout mice that die at birth have cranial abnormalities – described as a dome-shaped skull[73] – and a specific craniosynostosis (premature fusion of skull sutures) was observed in a human patient with an FGFRL1 mutation[79][80]. This is reminiscent of mutations in classical FGFRs (FGFR2, FGFR3) that cause craniosynostosis syndromes, albeit those are typically gain-of-function mutations. In FGFRL1’s case, a loss of function led to abnormal suture fusion[80]. One interpretation is that FGFRL1 normally helps keep suture mesenchyme in an undifferentiated state (or promotes balanced osteogenic signals); without it, bone-forming signals (possibly FGF or Hedgehog pathways) go unrestrained, causing early ossification of sutures[79]. FGFRL1 is expressed in developing cartilage and growth plates, and it was initially cloned from cartilage tissue[66]. In vitro, FGFRL1 can promote chondrocyte differentiation and matrix production[34]. It likely ensures proper transition of chondrocytes to osteoblasts during endochondral ossification. The antley-Bixler syndrome (ABS) has recently been linked to FGFRL1: ABS patients have skeletal malformations, and one reported case had a mutation in FGFRL1[81], reinforcing FGFRL1’s relevance in human bone development. Moreover, FGFRL1 may contribute to bone homeostasis in adults – mice with one Fgfrl1 allele deleted develop osteoporosis-like changes, suggesting haploinsufficiency can affect bone density (though this is a subject of ongoing research).

In summary, FGFRL1 is indispensable for proper formation of certain organs and tissues. Its absence in mice reveals critical requirements in the kidney (inductive signaling), diaphragm muscle (fiber formation), and craniofacial skeleton (suture patterning). In each case, FGFRL1’s role can be understood as modulating developmental signals to achieve the correct outcome: e.g. fine-tuning FGFs in kidney, allowing myoblast fusion in muscle, and restraining premature bone differentiation in skull sutures. Notably, the vital functions of FGFRL1 appear to reside in its extracellular domain – mice engineered to lack the intracellular tail of FGFRL1 are actually viable and healthy[82], implying that the cytosolic part is not required for its developmental roles. This striking result means FGFRL1 carries out its key functions via external interactions (with ligands or other cell-surface molecules), not through any intracellular signaling cascade[82][83]. That finding dovetails with FGFRL1 acting as a scaffold/adhesion or decoy receptor in development, rather than a classic signal-transducing receptor.

FGFRL1 in Cell Adhesion and Cell–Cell Fusion

Beyond ligand scavenging, FGFRL1 has emerged as an important player in cell adhesion. It localizes to sites of cell–cell contact, and can mediate cells sticking and fusing together. In fact, some researchers have concluded that “FGFRL1 is a cell adhesion protein similar to the nectins rather than a signaling receptor”[53]. Nectins are Ig-domain cell adhesion molecules that help form junctions between cells. FGFRL1, with its three Ig-like domains, appears capable of a comparable function.

Experiments have shown that the extracellular portion of FGFRL1 can promote cell–cell adhesion in culture. For example, when purified FGFRL1 ectodomain (Ig1–Ig3) is coated on plastic, various cell lines readily attach to it[84][85]. Conversely, cells engineered to express high levels of FGFRL1 form tighter intercellular contacts and clusters. In a Tet-inducible FGFRL1 system, adding doxycycline (to induce FGFRL1 expression) caused previously dispersed cells to aggregate into clusters, indicating FGFRL1 drove them to adhere to one another[86][87]. FGFRL1’s adhesion activity depends on heparan sulfate proteoglycans (HSPGs) on the cell surface – if HSPGs are removed or soluble heparin is added to compete, FGFRL1-mediated cell adhesion is blocked[88][89]. This suggests a model where FGFRL1 on one cell binds to HSPG molecules on an adjacent cell (or vice versa), creating a bridge. Heparan sulfate chains could cross-link FGFRL1 molecules or serve as co-receptors that stabilize FGFRL1–FGFRL1 interactions between cells. FGFRL1’s Ig domains (particularly D3) are likely involved in the homophilic or heterophilic binding that underlies adhesion[90][91]. Indeed, deletion analyses showed that both the Ig3 domain and the transmembrane domain of FGFRL1 are required for its full cell–cell adhesion and fusion activity[92][93]. Ig3 contains the binding interface for a partner on the neighboring cell, while the TM domain may be needed for proper membrane localization or clustering.

A remarkable property of FGFRL1 is its ability to induce cell–cell fusion under certain conditions. When FGFRL1 is overexpressed in CHO cells or similar lines, the cells can fuse into multinucleated syncytia[94][95]. Trueb and colleagues demonstrated this fusogenic effect and traced it to FGFRL1’s Ig3 domain. Specifically, a small hydrophobic patch on the Ig3 domain’s surface is critical: mutating a single amino acid in this patch abolishes FGFRL1’s fusion ability[96][97]. A set of four hydrophobic residues in Ig3 (e.g. L281, F303, L339, V304 in human FGFRL1) form a pocket that likely interacts with a “target protein” on adjacent cells[70][92]. Although the exact partner is not yet identified, it is presumably another membrane protein that triggers the fusion process upon binding FGFRL1. (It might even be FGFRL1 itself on the opposing cell, i.e. homotypic binding, but involvement of additional molecules is also possible.) When this interaction occurs, ultrastructural studies have observed “net-like” membrane structures with \~1 μm pores at contacting cell surfaces – thought to be intermediate structures in the fusion process[84][85]. Eventually, the membranes merge and a large syncytium forms containing many nuclei[94]. Intriguingly, FGFRL1’s fusion activity seems to have arisen or enhanced during vertebrate evolution: FGFRL1 from human, mouse, chicken, and fish can induce fusion, whereas FGFRL1 from more ancient lineages like lancelet (amphioxus) or sea urchin cannot[98][99]. This correlates with differences in the Ig3 domain; the vertebrate FGFRL1 Ig3 has the requisite hydrophobic motif, while invertebrate versions lack an effective fusion site[98][99]. The “fusogenic” function of FGFRL1 might therefore be a vertebrate innovation, potentially related to new requirements in vertebrate tissue development (such as muscle cell fusion or bone formation).

Biologically, where might FGFRL1-mediated cell fusion be important? One strong candidate is skeletal muscle development – the formation of multinucleated muscle fibers (myotubes) requires fusion of precursor cells (myoblasts). FGFRL1 is expressed in developing muscle, and as noted, Fgfrl1 knockout mice fail to form normal slow-twitch fibers in the diaphragm[74]. It’s tempting to speculate FGFRL1 aids the fusion of myoblasts, perhaps specifically promoting the fusion events that generate oxidative (slow) fibers. FGFRL1 is also expressed in the developing heart and could conceivably influence cardiomyocyte adhesion, though this is less clear. Another context is the placenta: placental trophoblast cells fuse into a syncytium (syncytiotrophoblast) – it’s unknown if FGFRL1 plays a role there, but given its fusogenic capacity, it would be an interesting question (placental defects haven’t been reported in the mouse knockouts, but that may warrant closer examination). FGFRL1’s adhesive role is also likely relevant in cartilage, where chondrocytes interact closely as they mature. In culture, FGFRL1 promotes chondrogenic cell aggregation (a step in cartilage nodule formation). Additionally, FGFRL1 could contribute to cell–matrix adhesion indirectly via HSPGs in the extracellular matrix.

It’s important to note that FGFRL1’s adhesive and fusogenic functions rely on its extracellular domain, independent of any signaling through its cytoplasmic tail. In fact, the finding that mice lacking the FGFRL1 intracellular tail are normal implies that all crucial functions – including adhesion/fusion – are carried out by the ectodomain tethered in the membrane[82][83]. FGFRL1 can thus be viewed as a multifunctional surface protein: part decoy receptor (binding and neutralizing FGFs), part adhesion molecule (connecting cells via Ig domain interactions), and part fusogen (in specific contexts causing membrane merging). These roles are not mutually exclusive – for instance, at a developing organ tip, FGFRL1 might both sequester excess FGF and simultaneously help cells stick together as they differentiate.

In summary, FGFRL1 acts almost like a molecular organizer at the cell surface, rather than a classical signaling receptor. It ensures proper cell–cell interactions (adhesion/fusion) and modulates growth factor availability, thereby coordinating morphogenesis. Its unique combination of structural domains allows it to bridge cells together (via Ig domains and heparan sulfate binding) and regulate signaling microenvironments (via ligand binding and internalization). This places FGFRL1 at a crossroads of cell communication and tissue architecture – an evolutionary repurposing of an FGFR-like scaffold for functions beyond signal transduction.

Key Open Questions and Future Directions

FGFRL1 has intrigued researchers as an outlier in the FGFR family, and many aspects of its function remain to be fully clarified. Here are important questions one could pose to experts in the field, along with experimental approaches that could address them:

How exactly does FGFRL1 regulate FGF signaling in vivo? Is it purely by sequestering FGFs (acting as a sink), or does it form inhibitory complexes with the signaling receptors? To answer this, one could perform FGFRL1–FGFR1 (and FGFR2/3/4) co-immunoprecipitation experiments to see if FGFRL1 ever associates with active FGFR complexes. Live-cell imaging of fluorescent FGFRL1 and FGFRs during ligand stimulation could reveal whether FGFRL1 colocalizes at signaling sites or remains segregated. Additionally, analyzing FGF gradient diffusion in tissues with and without FGFRL1 (e.g. using fluorescent FGF in organ culture) might show whether FGFRL1 alters the spread or persistence of FGF signals.
What are the binding partners of FGFRL1’s Ig3 domain on adjacent cells? The “target protein” involved in FGFRL1-mediated cell fusion is currently unknown. Proteomic approaches could be employed: for instance, use FGFRL1-coated beads or labeled FGFRL1 ectodomain as bait to pull down binding proteins from cell membranes, followed by mass spectrometry. CRISPR screens for cells that fail to fuse in the presence of FGFRL1 might also pinpoint genes required for the fusion partner. Understanding this partner (be it an HSPG core protein, another IgCAM, or something like integrins) would illuminate the mechanism of adhesion/fusion. Structural biology could help too – solving the crystal or cryo-EM structure of FGFRL1 Ig3 in complex with a candidate ligand (or with heparan sulfate) could identify interaction surfaces and suggest how cell-cell binding occurs.
In what developmental processes is FGFRL1’s fusogenic activity physiologically relevant? Muscle development is a prime candidate, but experiments are needed to confirm it. One could create a muscle-specific knockout of Fgfrl1 (using Cre-lox in myoblasts) to see if muscle fiber formation is impaired beyond the diaphragm. Conversely, a knock-in of an “fusion-defective” FGFRL1 mutant (e.g. a single amino-acid change in Ig3 that abolishes fusion but leaves FGF binding intact) would be incredibly informative – if such mice have muscle or bone defects, it would directly tie the fusion function to those tissues. In simpler organisms, zebrafish could be used: zebrafish have two fgfrl1 genes; CRISPR knockout of both, or expression of human vs. lancelet FGFRL1 in fish embryos, could test whether fusion activity correlates with certain developmental outcomes (e.g. muscle fiber formation, gill arch fusion, etc.). In vitro, primary myoblast fusion assays with FGFRL1-knockout vs. wild-type cells (or rescue with mutant FGFRL1 constructs) could clarify if FGFRL1 is required for efficient myotube formation.
Why do some tissues (kidney, diaphragm, skull) absolutely require FGFRL1, while others tolerate its absence? This touches on redundancy and compensation. Perhaps in kidney and diaphragm, FGFRL1 has a non-redundant role (no other decoy or adhesion molecule can substitute), whereas in other FGF-rich contexts there are compensators (like Sprouty proteins or other IgCAMs). To probe this, one could look at compound knockouts: e.g. Fgfrl1 and Sprouty2 double knockout – does that exacerbate phenotypes or unmask new ones? Or Fgfrl1 knockout combined with partial reduction in heparan sulfate (ext1^+/- mice) to see if that worsens signaling abnormalities. Another angle is single-cell RNA-seq of FGFRL1-deficient vs. normal tissues (kidney buds, etc.) to identify which signaling pathways and cell states are misregulated – this could reveal unexpected roles or compensations.
What is the physiological ligand (if any) that triggers FGFRL1’s own signaling capacity? While FGFRL1 isn’t a kinase, could it signal through binding partners? For example, does FGFRL1 binding to Spred1 or SHP-1 ever induce a signaling cascade (like the reported ERK activation in pancreatic β-cells[71])? An expert might investigate whether FGFRL1 gets phosphorylated on its tyrosines by another kinase (perhaps when dimerizing with a classical FGFR). Mass spectrometry on cells co-expressing FGFR1 and FGFRL1 could detect if FGFRL1 is tyrosine-phosphorylated in an FGF-dependent manner. If so, FGFRL1 might act as a docking platform for SH2-domain proteins (like SHP-1). Mutational analysis of the FGFRL1 YXXΦ motifs (Y→F mutations) in cells could test if any intracellular signaling or protein binding (e.g. SHP-1 association) depends on those tyrosines.
Could manipulating FGFRL1 be therapeutically useful in FGF-related diseases? For example, would increasing FGFRL1 expression mitigate conditions of excessive FGF signaling (such as achondroplasia caused by FGFR3 overactivation, or certain cancers with FGF/FGFR drivers)? Conversely, might inhibiting FGFRL1 help conditions where FGF signaling is deficient? Animal models could be used to test this: e.g. introduce an FGFRL1 transgene in an achondroplasia mouse model to see if it rescues bone growth by sequestering FGFs, or treat FGF-driven tumors with soluble FGFRL1 decoys. These experiments would help determine if FGFRL1 can serve as a “buffer” in vivo and if it has translational potential as a drug or therapeutic target.
Exploring FGFRL1 in non-mammalian systems: Since FGFRL1 is present in organisms like amphibians and fish (and perhaps functionally diverged), comparative studies could be enlightening. Creating FGFRL1-knockout frogs or fish and observing phenotypes in cartilage, kidney, etc., would tell us if the roles seen in mice are conserved or if new roles emerged in amniotes. Likewise, examining FGFRL1 function in an evolutionarily distant model (like Drosophila if a homolog exists, or planarians which have FGF-like pathways) might reveal minimal requirements of the FGF-modulation system. An expert may also pursue rescue experiments across species: e.g. can Drosophila or C. elegans FGFRL1 analog (if identified) substitute for mammalian FGFRL1 in knockout cells? Such cross-species complementation would map which functions are core and which are lineage-specific.

In conclusion, FGFRL1 stands at an intriguing intersection of growth factor signaling and cell adhesion/fusion biology. It has a clear structural role in assembling cells into functional tissues, and a subtler regulatory role in tuning signals that govern cell proliferation and differentiation. Unraveling the precise mechanisms of FGFRL1 – from the molecular partners it engages to the developmental events it choreographs – will likely require a combination of biochemical, genetic, and biophysical approaches. The questions above highlight that, despite two decades since its discovery, FGFRL1 remains a “mysterious” receptor. Its multifunctionality challenges our conventional categories of receptor vs. adhesion protein, but that is exactly what makes it a fascinating subject for further research. Each experiment designed to probe FGFRL1’s function not only illuminates this peculiar protein but also deepens our understanding of how cells integrate signaling with physical interactions to build organisms. The hope is that by studying FGFRL1 across different systems and scales, we will fully decipher its role in human biology – and perhaps harness its unique properties for therapeutic benefit.

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