FGF/FGFR system in the central nervous system demyelinating disease: Recent progress and implications for multiple sclerosis

Background With millions of victims worldwide, multiple sclerosis is the second most common cause of disability among young adults. Although formidable advancements have been made in understanding the disease, the neurodegeneration associated with multiple sclerosis is only partially counteracted by current treatments, and effective therapy for progressive multiple sclerosis remains an unmet need. Therefore, new approaches are required to delay demyelination and the resulting disability and to restore neural function by promoting remyelination and neuronal repair. Aims The article reviews the latest literature in this field. Materials and methods The fibroblast growth factor (FGF) signaling pathway is a promising target in progressive multiple sclerosis. Discussion FGF signal transduction contributes to establishing the oligodendrocyte lineage, neural stem cell proliferation and differentiation, and myelination of the central nervous system. Furthermore, FGF signaling is implicated in the control of neuroinflammation. In recent years, interventions targeting FGF, and its receptor (FGFR) have been shown to ameliorate autoimmune encephalomyelitis symptoms in multiple sclerosis animal models moderately. Conclusion Here, we summarize the recent findings and investigate the role of FGF/FGFR signaling in the onset and progression, discuss the potential therapeutic advances, and offer fresh insights into managing multiple sclerosis.

adulthood. 1 Approximately 20% of patients develop symptoms after age 40, and disability progression in the male population with late onset is faster than in young RRMS patients. 4 In 50% to 60% of RRMS cases, the condition develops into secondary progressive MS (SPMS), characterized by an infrequent or completely terminated relapse after 10-15 years with a slow progression of irreversible disabilities associated with neurodegeneration. 5,6 Unidirectional progressive disability from the outset is observed in 15% of MS cases and is referred to as primary progressive MS (PPMS). For this patient cohort, the rapid onset of early neurodegeneration is the best predictor of long-term progression rates. 6 In addition to experimental autoimmune encephalomyelitis (EAE), the most frequently employed model, 7 several demyelinating models have also been used in MS research. Both clinical and pathological characteristics of human MS are shared by the murine hepatitis virus (MHV) strain A59 that causes demyelination in animal models, 8 and lysophosphatidylcholine has been used to induce inflammatory demyelination, in which the myelin structures, as well as the blood-brain barrier (BBB), are disrupted, but neuronal loss is absent. 9 Moreover, the chronic cuprizone demyelination model leads to consistent demyelination followed by spontaneous remyelination within a short period. 10 Thus, these models are valuable supplements to EAE research and are appropriate for studying the mechanisms of demyelination and therapeutic interventions for MS.
The fibroblast growth factor (FGF) family contains 23 members, including 18 protein ligands (the murine FGF15 and the human FGF19 genes are orthologous) and four fibroblast homologous factors (FHFs).
FGF ligands exercise their functions by binding with their high-affinity receptor family (FGFR1-4) and are involved in fundamental physiological processes in adults, including wound repair, angiogenesis, and metabolism. 11,12 The roles of FGF/FGFR signaling on essential cellular function control indicates the relevance of this axis in the pathogenesis of MS. 13,14 Over the past few years, interventions targeting FGF/ FGFR have moderately ameliorated symptoms in animal models, and the conditional deletion of FGFR1 and FGFR2 has shown remarkable therapeutic promise. 15,16 Furthermore, several FGF family members are strongly associated with the pathogenesis and course of MS. [17][18][19] Here, we summarize the latest relevant findings, discuss the function of FGF/FGFR signaling in MS pathogenesis, and describe potential therapeutic advances, providing fresh perspectives on MS therapy.

| Inflammation
Multiple sclerosis is a chronic inflammatory demyelinating and degenerative condition of the CNS. Inflammation of the spinal cord and brain is invariably present in all phases of MS and declines with disease progression. 20 A dominant aspect of the early pathology in RRMS patients is active inflammatory demyelinating lesions, which arise through inflammatory infiltrates associated with disrupted BBB. 21 At this stage, lesions of relapsing MS have more plentiful macrophages than any type of progressive MS (PMS) to phagocytose the myelin degradation products. 22 In contrast, chronic lesions predominate in progressive disease. Lymphocyte infiltration is initially blocked in the leptomeninges and blood vessels behind an intact BBB. 23 Here, T cells attract immune cells into the CNS by interacting with major histocompatibility complex class II + microglia 24 and produce adhesion molecules, chemokines, and a variety of proinflammatory cytokines. 20 Another type of inflammation is densely populated by B cells of the brain's connective tissue spaces adjacent to an intact BBB, where they may form aggregates or tertiary lymph follicles. 21,25 Interestingly, the inflammatory state of the CNS might provide a favorable environment for lymphocyte proliferation and expansion. 20 Furthermore, mononuclear phagocytes (MPs), namely resident microglia and the macrophages differentiated from infiltrating monocytes, also act as a significant part in the pathologic mechanisms of PMS. According to experimental and clinical studies, MPs present in demyelinating lesions secrete chemokines that induce lymphocytes to infiltrate the CNS and thus provide an inflammatory environment. They also generate reactive oxygen and nitrogen species, which leads to oligodendrocyte and neuronal cell death. 1,26,27 In addition, CNS glial cells may initiate an immunological response in MS (particularly in PMS, as it is intimately linked to the chronic activation of the innate immune system). 28 Some studies have pointed out that, at least in certain situations, MS may originate from a primary injury within the CNS, possibly associated with oligodendrocytes, followed by glial activation and ultimately by immune-mediated inflammatory activation as a secondary response. 25

| Demyelination and neurodegeneration
Demyelination leads to a reduction in axonal integrity and, over time, to neuronal dysfunction. Neurodegeneration is a characteristic of MS and a major contributor to clinical impairment and decreased quality of life. Demyelinated axons become frangible and suffer damage from activated immunological and glial cells releasing cytokines, oxidative products, and free radicals. Even in the predominantly inflammatory demyelinating stage of the disease, transected axons are abundant, demonstrating that axonal loss occurs at disease onset and continues with time. In the initial phases of RRMS, the axonal loss has no immediate substantial clinical impact. As lesions accumulate with time, however, the clinical aspects of MS become driven by axonal loss. Thus, it is believed that the brain's ability to adjust for further axonal loss exhausts before RRMS and SPMS shift. At this stage, MS lesions include remyelination, inflammation resolution without repair, or a "smoldering" state of coexistence of inflammation and myelin degeneration. 29 In PMS, depletion of the myelination capacity by both oligodendrocyte precursor cells (OPCs) and residual oligodendrocytes is critical. Recent studies of MS using human single-nucleus RNAseq demonstrated that oligodendrocytes respond rapidly to oxidative stress, with the downregulation of homeostasis and myelin synthesis genes. 30 Moreover, when the dynamics of oligodendrocyte generation in MS brain tissue were assessed by 14 C methods, it was found that the demyelination is partly caused by the depletion of the myelination ability of the surviving oligodendrocytes rather than by an impairment in OPC differentiation. 31 These results may impact the establishment of disease models and the development of myelin regeneration strategies for PMS. RRMS has become a pharmacologically treatable condition. However, PMS continues to face treatment challenges because of the persistent accumulation of neurological impairments and disabilities.

| Remyelination failure
Demyelination can occur parallel to regeneration processes, which restore some of the destroyed myelin-generating cells and rebuild the myelin sheath around axons. This is accomplished by the activation, migration, and polarization of resident OPCs and neural stem cells (NSCs) into myelin cells, initiating an oligodendrocyte-driven repair process known as remyelination. The identity of cells that cause remyelination in the CNS of MS patients has been a subject of debate. OPCs and mature oligodendrocytes that have survived are two potential candidates. This discussion is crucial because therapeutic approaches to enhance remyelination may differ depending on the specific cellular pathways involved. Lineage tracing experiments revealed that newly generated oligodendrocytes derived from OPCs form new myelin sheaths in demyelinated regions. 32 However, it has also been found that surviving oligodendrocytes can expand and remyelinate axons in MS. 31 Moreover, myelin sheaths derived from OPCs are thinner and less functional than those generated by the surviving oligodendrocytes. Myelin cells in the adult CNS can also differentiate from NSCs in the subventricular region. The microenvironment of the demyelinating lesions substantially impacts OPC and NSC homeostasis, in addition to uncontrollable factors such as gender and age and may also be the target of future remyelination treatment strategies. Other glial cells, like microglia, are momentous to remyelination and aid in removing myelin debris and releasing neurotrophic factors that support OPC functions. 33,34 Among them, CX3CR1, a fractalkine receptor that is abundantly expressed on microglia, has been shown to affect the ability of these cells to phagocytose. Reduced microglial phagocytosis in cuprizone-treated CX3CR1-deficient animals causes a continuous accumulation of myelin debris, inhibiting remyelination due to insufficient OPC recruitment. 35 Additionally, the disequilibrium of pro-regeneration and inhibitory elements limits the remyelination capacity of OPCs and oligodendrocytes. OPC RNA sequencing revealed that the mTOR pathway plays a substantial role in remyelination failure. This pathway can be manipulated by caloric restriction or by administration of the AMPK-agonist metformin to reverse the decline in OPC differentiation and restore their ability to remyelinate axons. 5 Moreover, multiple OPC differentiation inhibitors, including PSA-NCAM, Lingo-1, Jagged, and Galectin-4, appear relatively overexpressed.
Additionally, IFNγ, Gli1, and Sirt1 inhibited the proliferation and differentiation of NSCs in demyelinating lesions. 34 Remyelination failure leads to axonal loss and neurodegenerative changes over time. Therefore, specific targeting of this pathological process is expected to deliver a breakthrough in MS treatment in the future.

| FGFs, FGFRs, and co-receptors
The 18 FGFs cluster into six subfamilies, with the FGF1, FGF4, FGF7, FGF8, and FGF9 subfamilies functioning in a paracrine manner and the FGF19 subfamily members operating as endocrine factors. 11 FGFRs contain a single transmembrane helix (termed TM), three extracellular immunoglobulin-like domains (termed D1-3), and two intracellular tyrosine kinase domains (termed TK1-2). An eightresidue acid box, a hallmark of FGFRs, is located between D1 and D2 and, together with the D1 loop, plays an autoinhibitory role in receptor activation. 36 The FGF-binding region is in D2 and interacts with D3 providing specificity. Two alternative splice sites (D3b and D3c) of the D3 protein show distinct FGF binding specificities ( Figure 1A). The FGFR D3b isoform is commonly seen in epithelial cells, whereas the FGFR D3c isoform is typically found in mesenchymal cells. Although FGFR1-3 exhibits frequent alternative splicing, there is no isoform due to the absence of alternative splicing exons in FGFR4. 36,37 Depending on the combination with co-receptors, which include heparin sulfate (HS)/proteoglycans (HSPGs) and Klotho proteins, can FGFs-FGFRs binding elicit a signal. Most of FGFs feature HS binding domains, and HSPGs are widely distributed in the extracellular matrix. Their different affinities determine whether they work in a paracrine, autocrine, or endocrine way. 11,38 Not only can HSPGs tether FGFs and enable them to function in an autocrine or paracrine way, but they also enhance FGFS signaling by forming FGF/FGFR/HSPG complexes. 39 In contrast, the endocrine FGF subclass ligands (FGF19, FGF21, and FGF23), with a weak affinity for HSPGs, utilize Klotho proteins as co-receptors for binding to their respective FGFRs 37,40 ( Figure 1B). However, they exhibit a strong affinity for FGFR/Klotho complexes but a limited affinity for individual FGFRs or Klotho proteins. 41 Klotho proteins are a class of transmembrane proteins consisting of the α-, β-, and γ-Klotho subunits. α-Klotho is necessary for the activity of FGF23, and the biological effects of FGF19 and FGF21 need β-Klotho (Table 1).

| ROLE OF FG F/FG FR S I G NALING IN CNS
FGF/FGFR signaling not only plays a crucial role in CNS formation during development but also has a broad function in the adult CNS.
A prototype member, FGF1, is expressed by neurons in adult neural tissue and acts as a mitogen in neurodevelopmental processes. [47][48][49] FGF1 stimulated NSC cell expansion and neurite outgrowth in neurons. 50,51 In vitro and in vivo, FGF2 controls NSC proliferation.
Under the induction of FGF2, it has been discovered that undifferentiated precursor cells in the adult mouse proliferated and differentiated into various CNS cells, such as neurons, oligodendrocytes, and astrocytes. 52 Several studies have shown that FGF2 is also crucial for NSC proliferation in vivo. Granule precursor neuron proliferation was four times higher after subcutaneous injection of FGF2 and had a 250% increase in the subventricular zone of the lateral ventricles. Furthermore, a 68% and 50% increase in DNA synthesis in hippocampal and whole cerebellar homogenates were observed F I G U R E 1 The FGF/FGFR signaling system. (A) FGFR monomer structures: FGFR is a form of the extracellular domains and intracellular catalytic domains linked by a single pass transmembrane domain. Except for FGFR4, the other three FGFR coding genes generate two major splice variants in D3, termed as D3b and D3c, which are essential determinants of ligand binding specificity. (B) The relative orientation of the FGF/FGFR/co-receptor complex. (C) The downstream pathways of FGF/FGFR signaling: binding of FGFs triggers the dimerization and activation of FGFRs. Activated FGFRs phosphorylate FRS2, which binds to SH2 domain-containing adaptor GRB2 and GRB2 will subsequently bind to SOS and GAB1 to activate RAS/RAF/MAPKs pathway, including ERK, p38 and JNK, as well as the PI3K/AKT pathway. Independent of the FRS2 binding, FGF signals also activate STATs and PLCγ. Activated PLCγ hydrolyzes PIP2 to DAG and PIP3, which stimulates calcium release from the endoplasmic reticulum and activation of calcium/calmodulin dependent protein kinases. FGFRL1 and SEF are transmembrane proteins and can interact directly with FGFRs to negatively regulate it. Phosphorylation of the MAPK/ERK cascade can be negatively regulated by SEF. SPRY acts at the level of Grb2 to attenuate FGF/FGFR signaling. MKP3 functions as a negative regulator by affecting the phosphorylation of the ERK. AKT, protein kinase B (AKT); DAG, diacylglycerol; ERK, extracellular signal regulated kinase; FRS2, FGFR substrate 2; GAB1, GRB2 associated binding protein 1; JNK, c-Jun N-terminal kinase; MAPK, mitogen activated protein kinase; PI3K, phosphatidylinositol 3-kinase; PKC, protein kinase C; PLCγ, phospholipase Cγ; SOS, son of seven.
following in vivo FGF2 treatment. 53 Moreover, the generation and dendritic development of new dentate granule cells was also enhanced after intracerebroventricular FGF2 infusion. 54 However, the positive regulatory effect of FGF2 on neuronal proliferation was reversed after treatment with FGF2-specific neutralizing antibodies. 55 These results indicate that FGF2 is an important neurogenic factor that directly acts on the mitosis of NSCs to promote their proliferation. Conversely, according to multiple studies, FGF2 is not necessary to proliferate neuronal precursors, as FGF2-deficient mice show normal neural progenitor proliferation during development.
However, these mice exhibited partial cerebral cortex loss, impaired neural stem cell differentiation, and increased CNS cell apoptosis, suggesting that FGF2 triggers neural progenitors to migrate and differentiate. 56,57 In addition, other FGFs, such as FGF4, FGF8, FGF9, FGF10, and FGF21, regulate neuronal fate. [58][59][60][61][62] The CNS myelin-producing cells, oligodendrocytes, play a central role in generating and preserving the pace and the power of axonal electrical impulses. Neurological deficits in MS result from myelin damage or insufficient remyelination. Understanding the signals involved in developing oligodendrocyte-driven myelination may shed light on demyelinating disease prevention and treatment. OPCs migrate to various brain regions during development, transforming into myelin-producing cells. 63,64 Several growth factors, including FGFs, control the development of oligodendrocytes. 65 However, the effect of FGF/FGFR signaling on oligodendrocyte development is regulated by the differential expression of FGFRs. Early and late OPCs express FGFR3, FGFR2 is expressed in mature oligodendrocytes, and both express FGFR1 but not FGFR4. 66 Immunity is continually regulated by FGF signaling, which is, thereby attenuating nerve damage. 77 Controlling inflammatory responses is thought to be a viable therapeutic approach for stroke.
Like other FGFs, after a stroke, recombinant human FGF21 has anti-inflammatory properties that attenuate inflammatory cell polarization and the infiltration of peripheral immune cells, showing its potential as an anti-inflammatory agent in stroke. 78 Taken together, FGF/FGFR signaling presumably exerts a pivotal role in neural tissue regeneration, remyelination, and neuroinflammation.

| FGF1 subfamily
In MS, FGF1 is predominantly expressed in remyelinated lesions, with its production being lower in the demyelinated lesion core than in the remyelinated rim. 79 In cerebellar slice cultures, FGF1 promoted remyelination and directly accelerated myelination. Furthermore, by inducing the upregulation of the leukemia inhibitory factor (LIF) and the chemokine CXCL8 in human astrocytes, which are involved in the recruitment of oligodendrocytes, FGF1 indirectly promoted the induction of remyelination. 79 LIF has been demonstrated to support oligodendrocyte development and survival, [80][81][82] in addition to promoting myelination. 83 In EAE, it has also been proven to prevent oligodendrocyte death 84 and promote remyelination. 85 Astrocytes at the edges of active MS lesions secrete high levels of CXCL8, thereby recruiting OPCs into the lesions and participating in their regeneration. 86,87 How FGF2 affects oligodendrocyte responses during demyelination and remyelination in MS is debatable. On one hand, FGF2 is regarded as a neuroprotective agent that promotes remyelination in MS. 17 FGF2 has mainly been detected in microglial and macrophages of active, chronic-active, and chronic-inactive lesions in MS. 88 An increase in serum FGF2 levels was also found in gadolinium-enhanced lesions in RRMS and disability progression of SPMS. 89 However, FGF2 peaked in the initial stage of remyelination. 90 100 However, dysfunction of OPC activation in MS affects remyelination processes. 101 A reduced capacity for myelination may be caused in part by the attenuation of OPC migration and differentiation, positioning FGF8 as a potential therapeutic target.

| FGF9 subfamily
The cascade of pathogenic events in MS eventually leads to the loss of neurons and axons, which can be measured by reduced brain volume on volumetric magnetic resonance imaging (MRI) in vivo.
Between 0.5% and 1.5% of MS patients develop brain atrophy each year, and during the progressive stages of the disease, the deep gray matter structures display a higher rate of degeneration. 2 It was shown that plasma FGF9 levels are strongly associated with brain volume loss in MS patients, and the annual percentage of brain volume change was inversely related to these levels. 102 FGF9 was high in early active lesions and was upregulated in ongoing lesions of MS patients with longstanding progressive disease. However, FGF9 levels were remarkably lower in healthy white matter and almost nonexistent in chronically demyelinated inactive lesions. 18 Furthermore, GFAP+ astrocytes and OLIG2+ and NOGO-A+ oligodendrocytes were shown to produce FGF9. This suggests that FGF9 was induced by a localized glial response toward ongoing tissue damage in MS.
Although FGF9 was discovered to prevent OPCs from developing into mature oligodendrocytes, the authors did not agree that this direct effect was responsible for the inhibition of remyelination.
Rather, FGF9 still serves as an OPC stimulant and contributes to the generation of proteolipid protein+ (PLP+) oligodendrocytes in the complex cellular environment. 18 However, this proliferation is of little importance since FGF9 inhibited the differentiation of precursor oligodendrocytes into mature myelination-competent oligodendrocytes through an astrocyte-dependent mechanism. Furthermore,

| FGF19 subfamily
The endocrine FGF subclass ligands (FGF19, FGF21, and FGF23) are also known as the FGF19 subfamily. FGF21, mainly released by the skeletal muscle, pancreas, liver, kidney, and adipose tissue, exerts pleiotropic effects in regulating glucose, lipid, and energy homeostasis. 103 Apart from metabolic regulation, it has been recently discovered that FGF21 is secreted by neurons and exhibits neuroprotective functions. 104 FGF21 was dramatically downregulated after cerebral ischemia, and the upregulation could restore brain function by reducing cerebral infarction and ameliorating neuronal cell death. 105 Moreover, FGF21 promoted remyelination after traumatic brain injury. 103 In the lysophosphatidylcholine-induced demyelination model, treatment with a neutralizing antibody against FGF21 or gene knockout abolished these positive effects on OPCs. 9 The effect of FGF21 on OPCs seems to be limited to promoting proliferation without affecting their differentiation or apoptosis.
Furthermore, FGF21 did not modulate the cell fate of astrocytes, and Kuroda et al. detected no significant proliferation of OPCs cultured in astrocyte supernatant with FGF21 pre-treatment, indicating that FGF21 acts directly as an OPC mitogen. Interestingly, FGF21mediated proliferation of human OPCs in autopsy samples from MS patients has been observed. 9 Therefore, we can infer that FGF21mediated OPC proliferation and consequent remyelination are conserved in CNS demyelinating models.
FGF23 was first identified in the ventrolateral thalamic nucleus of the mouse brain, and its physiological role has recently attracted significant attention. 106 The FGF23 protein is present in three distinct forms in the bloodstream: a full-length mature form and two inactive (C-and N-terminal) fragments. 107 It is generally accepted that intact FGF23 is a bioactive factor that controls the metabolism of phosphate and vitamin D. In contrast, high levels of the inactive FGF23 forms have been demonstrated to inhibit these effects. 108 Considered a bone-derived hormone, FGF23 is primarily released by osteocytes and osteoblasts in the skeleton. It is a component of the novel hormonal bone-parathyroid-kidney axis, which interacts to form hormonal homeostasis. 109,110 Several studies have shown that serum FGF23 concentrations were elevated in RRMS patients. However, calcitriol levels were reduced, indicating that elevated FGF23 levels in MS may disrupt the FGF23-PTH-vitamin D axis, resulting in pathological effects. 19,111 However, in a study that measured plasma levels of FGF23 in 91 MS patients and 92 healthy controls, no difference was observed. 112 Similar results were obtained in a study by Alagha et al. 113 In addition, they also found that  115 Compelling studies suggest that FGF23 targets macrophages, which express FGFR1 along with DCs, and exhibit increased α-Klotho expression upon inflammatory stimulation. 116

| Fibroblast growth factor receptors
To Additionally, single deletion of FGFR1 promoted remyelination and functional recovery in the chronic phase. 15 Therefore, these data  OPCs and the maturation of oligodendrocytes, which helps to enhance tissue repair and restore motor function ( In addition, more research is needed to detect the sensitivity, specificity, and stability of FGFs as biomarkers and they must be compared with existing oligoclonal bands and IgG to clarify their characteristics. 121 In contrast, FGFRs have shown promising therapeutic potential.
FGFRs are widely expressed in CNS and immune cells ( Jin critically revised the whole manuscript. All authors read and approved the final manuscript.

ACK N OWLED G M ENTS
We would like to thank Editage (www.edita ge.cn) for English language editing.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have no competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.