Weaving the neuronal net with target-derived fibroblast growth factors

Neuronal fibroblast growth factor 22 signaling during development, but not in adults, is involved in anhedonia

Growth factor signaling in the brain is implicated in many neuropsychiatric disorders, including depression, autism, and epilepsy. Fibroblast growth factor 22 is a growth factor that regulates excitatory synapse development and neurogenesis in the brain. We have previously shown that adult mice in which fibroblast growth factor 22 is constitutively inactivated in all cells throughout life (fibroblast growth factor 22-null mice) show anhedonia, a core feature of depression in humans, suggesting that fibroblast growth factor 22 signaling contributes to the regulation of affective behavior. Here we asked (1) whether inactivation of fibroblast growth factor 22 specifically in neurons is sufficient to induce anhedonia in mice and (2) whether fibroblast growth factor 22 signaling is important during development or in adults for the regulation of affective behavior. To address these questions, we performed the sucrose preference test, which is used as an indicator of anhedonia, with neuron-specific conditional fibroblast growth factor 22 knockout mice, in which fibroblast growth factor 22 is inactivated in neurons at birth (neonatal-fibroblast growth factor 22-knockout mice) or in adults (adult-fibroblast growth factor 22-knockout mice). We found that neonatal-fibroblast growth factor 22-knockout mice show anhedonia (decreased preference for sucrose), while adult-fibroblast growth factor 22-knockout mice do not. Therefore, neuronal fibroblast growth factor 22 signaling is critical during development, and not in adults, for the regulation of affective behavior. Our work also implies that defects in growth factor-dependent synapse development, neurogenesis, or both may underlie depression of a developmental origin.

FGF binding proteins (FGFBPs): Modulators of FGF signaling in the developing, adult, and stressed nervous system

Members of the fibroblast growth factor (FGF) family are involved in a variety of cellular processes. In the nervous system, they affect the differentiation and migration of neurons, the formation and maturation of synapses, and the repair of neuronal circuits following insults. Because of the varied yet critical functions of FGF ligands, their availability and activity must be tightly regulated for the nervous system, as well as other tissues, to properly develop and function in adulthood. In this regard, FGF binding proteins (FGFBPs) have emerged as strong candidates for modulating the actions of secreted FGFs in neural and non-neural tissues. Here, we will review the roles of FGFBPs in the peripheral and central nervous systems.

Selective Inactivation of Fibroblast Growth Factor 22 (FGF22) in CA3 Pyramidal Neurons Impairs Local Synaptogenesis and Affective Behavior Without Affecting Dentate Neurogenesis

Various growth factors regulate synapse development and neurogenesis, and are essential for brain function. Changes in growth factor signaling are implicated in many neuropsychiatric disorders such as depression, autism and epilepsy. We have previously identified that fibroblast growth factor 22 (FGF22) is critical for excitatory synapse formation in several brain regions including the hippocampus. Mice with a genetic deletion of FGF22 (FGF22 null mice) have fewer excitatory synapses in the hippocampus. We have further found that as a behavioral consequence, FGF22 null mice show a depression-like behavior phenotype such as increased passive stress-coping behavior and anhedonia, without any changes in motor, anxiety, or social cognitive tests, suggesting that FGF22 is specifically important for affective behavior. Thus, addressing the precise roles of FGF22 in the brain will help understand how synaptogenic growth factors regulate affective behavior. In the hippocampus, FGF22 is expressed mainly by CA3 pyramidal neurons, but also by a subset of dentate granule cells. We find that in addition to synapse formation, FGF22 also contributes to neurogenesis in the dentate gyrus: FGF22 null mice show decreased dentate neurogenesis. To understand the cell type-specific roles of FGF22, we generated and analyzed CA3-specific FGF22 knockout mice (FGF22-CA3KO). We show that FGF22-CA3KO mice have reduced excitatory synapses on CA3 pyramidal neurons, but do not show changes in dentate neurogenesis. Behaviorally, FGF22-CA3KO mice still show increased immobility and decreased latency to float in the forced swim test and decreased preference for sucrose in the sucrose preference test, which are suggestive of a depressive-like phenotype similar to FGF22 null mice. These results demonstrate that: (i) CA3-derived FGF22 serves as a target-derived excitatory synaptic organizer in CA3 in vivo; (ii) FGF22 plays important roles in dentate neurogenesis, but CA3-derived FGF22 is not involved in neurogenesis; and (iii) a depression-like phenotype can result from FGF22 inactivation selectively in CA3 pyramidal neurons. Our results link the role of CA3-derived FGF22 in synapse development, and not in neurogenesis, to affective behavior.

Fibroblast growth factor 22 is a novel modulator of depression through interleukin-1β

BACKGROUND AND AIMS

Emerging evidence shows that fibroblast growth factor 22 (FGF22) plays a critical role in the etiology of depression. However, the molecular mechanisms of FGF22 are not fully comprehended. Here, the effect of FGF22 in depression and its relationship with interleukin-1β (IL-1β) were investigated in clinical, animal, and cell experiments.

METHODS

Serum from depressive patients was collected, and the levels of FGF22 and IL-1β were analyzed by ELISA. The chronic unpredictable mild stress (CUMS) model was established, and primary hippocampal neuronal cells were cultured to examine changes in FGF22 and IL-1β levels in rat hippocampus.

RESULTS

The results revealed a negative correlation between serum FGF22 levels and serum IL-1β levels. The expression of IL-1β in the CUMS rat hippocampus decreased, and the apoptosis of hippocampal cells improved after the injection of lentiviral vector-mediated FGF22 (LV-FGF22). Further tests in primary hippocampal neuronal cells also showed a reduction in IL-1β and the cell apoptosis rate after treatment with FGF22.

CONCLUSION

In conclusion, the results revealed that FGF22 plays a role in alleviating depression, which may be mediated by reduced expression of IL-1β.

Muscle Fibers Secrete FGFBP1 to Slow Degeneration of Neuromuscular Synapses during Aging and Progression of ALS

The identity of muscle secreted factors critical for the development and maintenance of neuromuscular junctions (NMJs) remains largely unknown. Here, we show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. Although FGFBP1 expression increases during development, its expression decreases before NMJ degeneration during aging and in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis (ALS). Based on these findings, we examined the impact of deleting FGFBP1 on NMJs. In the absence of FGFBP1, NMJs exhibit structural abnormalities in developing and middle age mice. Deletion of FGFBP1 from SOD1^G93A mice also accelerates NMJ degeneration and death. Based on these findings, we sought to identify the mechanism responsible for decreased FGFBP1 in stressed skeletal muscles. We show that FGFBP1 expression is inhibited by increased accumulation of the transforming growth factor-β1 (TGF-β1) in skeletal muscles and at their NMJs. These findings suggest that targeting the FGFBP1 and TGF-β1 signaling axis holds promise for slowing age- and disease-related degeneration of NMJs.

SIGNIFICANCE STATEMENT

The neuromuscular junction (NMJ) is critical for all voluntary movement. Its malformation during development and degeneration in adulthood impairs motor function. Therefore, it is important to identify factors that function to maintain the structural integrity of NMJs. We show that muscle fibers secrete and concentrate the fibroblast growth factor binding protein 1 (FGFBP1) at NMJs. However, FGFBP1 expression decreases in skeletal muscles during aging and before NMJ degeneration in SOD1^G93A mice, a mouse model for amyotrophic lateral sclerosis. We show that transforming growth factor-β1 is responsible for the decreased levels of FGFBP1. Importantly, we demonstrate critical roles for FGFBP1 at NMJs in developing, aging and SOD1^G93A mice.

Deletion of fibroblast growth factor 22 (FGF22) causes a depression-like phenotype in adult mice

Specific growth factors induce formation and differentiation of excitatory and inhibitory synapses, and are essential for brain development and function. Fibroblast growth factor 22 (FGF22) is important for specifying excitatory synapses during development, including in the hippocampus. Mice with a genetic deletion of FGF22 (FGF22KO) during development subsequently have fewer hippocampal excitatory synapses in adulthood. As a result, FGF22KO mice are resistant to epileptic seizure induction. In addition to playing a key role in learning, the hippocampus is known to mediate mood and anxiety. Here, we explored whether loss of FGF22 alters affective, anxiety or social cognitive behaviors in mice. We found that relative to control mice, FGF22KO mice display longer duration of floating and decreased latency to float in the forced swim test, increased immobility in the tail suspension test, and decreased preference for sucrose in the sucrose preference test, which are all suggestive of a depressive-like phenotype. No differences were observed between control and FGF22KO mice in other behavioral assays, including motor, anxiety, or social cognitive tests. These results suggest a novel role for FGF22 specifically in affective behaviors.

Distinct sets of FGF receptors sculpt excitatory and inhibitory synaptogenesis

Neurons in the brain must establish a balanced network of excitatory and inhibitory synapses during development for the brain to function properly. An imbalance between these synapses underlies various neurological and psychiatric disorders. The formation of excitatory and inhibitory synapses requires precise molecular control. In the hippocampus, the structure crucial for learning and memory, fibroblast growth factor 22 (FGF22) and FGF7 specifically promote excitatory or inhibitory synapse formation, respectively. Knockout of either Fgf gene leads to excitatory-inhibitory imbalance in the mouse hippocampus and manifests in an altered susceptibility to epileptic seizures, underscoring the importance of FGF-dependent synapse formation. However, the receptors and signaling mechanisms by which FGF22 and FGF7 induce excitatory and inhibitory synapse differentiation are unknown. Here, we show that distinct sets of overlapping FGF receptors (FGFRs), FGFR2b and FGFR1b, mediate excitatory or inhibitory presynaptic differentiation in response to FGF22 and FGF7. Excitatory presynaptic differentiation is impaired in Fgfr2b and Fgfr1b mutant mice; however, inhibitory presynaptic defects are only found in Fgfr2b mutants. FGFR2b and FGFR1b are required for an excitatory presynaptic response to FGF22, whereas only FGFR2b is required for an inhibitory presynaptic response to FGF7. We further find that FGFRs are required in the presynaptic neuron to respond to FGF22, and that FRS2 and PI3K, but not PLCγ, mediate FGF22-dependent presynaptic differentiation. Our results reveal the specific receptors and signaling pathways that mediate FGF-dependent presynaptic differentiation, and thereby provide a mechanistic understanding of precise excitatory and inhibitory synapse formation in the mammalian brain.

The best-laid plans go oft awry: synaptogenic growth factor signaling in neuropsychiatric disease

Growth factors play important roles in synapse formation. Mouse models of neuropsychiatric diseases suggest that defects in synaptogenic growth factors, their receptors, and signaling pathways can lead to disordered neural development and various behavioral phenotypes, including anxiety, memory problems, and social deficits. Genetic association studies in humans have found evidence for similar relationships between growth factor signaling pathways and neuropsychiatric phenotypes. Accumulating data suggest that dysfunction in neuronal circuitry, caused by defects in growth factor-mediated synapse formation, contributes to the susceptibility to multiple neuropsychiatric diseases, including epilepsy, autism, and disorders of thought and mood (e.g., schizophrenia and bipolar disorder, respectively). In this review, we will focus on how specific synaptogenic growth factors and their downstream signaling pathways might be involved in the development of neuropsychiatric diseases.

Fibroblast growth factor 2 regulates adequate nigrostriatal pathway formation in mice

Fibroblast growth factor 2 (FGF-2) is an important neurotrophic factor that promotes survival of adult mesencephalic dopaminergic (mDA) neurons and regulates their adequate development. Since mDA neurons degenerate in Parkinson's disease, a comprehensive understanding of their development and maintenance might contribute to the development of causative therapeutic approaches. The current analysis addressed the role of FGF-2 in mDA axonal outgrowth, pathway formation, and innervation of respective forebrain targets using organotypic explant cocultures of ventral midbrain (VM) and forebrain (FB). An enhanced green fluorescent protein (EGFP) transgenic mouse strain was used for the VM explants, which allowed combining and distinguishing of individual VM and FB tissue from wildtype and FGF-2-deficient embryonic day (E)14.5 embryos, respectively. These cocultures provided a suitable model to study the role of target-derived FB and intrinsic VM-derived FGF-2. In fact, we show that loss of FGF-2 in both FB and VM results in significantly increased mDA fiber outgrowth compared to wildtype cocultures, proving a regulatory role of FGF-2 during nigrostriatal wiring. Further, we found in heterogeneous cocultures deficient for FGF-2 in FB and VM, respectively, similar phenotypes with wider fiber tracts compared to wildtype cocultures and shorter fiber outgrowth distance than cocultures completely deficient for FGF-2. Additionally, the loss of target-derived FGF-2 in FB explants resulted in decreased caudorostral glial migration. Together these findings imply an intricate interplay of target-derived and VM-derived FGF signaling, which assures an adequate nigrostriatal pathway formation and target innervation.

Specific sets of intrinsic and extrinsic factors drive excitatory and inhibitory circuit formation

How are excitatory (glutamatergic) and inhibitory (GABAergic) synapses established? Do distinct molecular mechanisms direct differentiation of glutamatergic and GABAergic synapses? In the brain, glutamatergic and GABAergic synaptic connections are formed with specific patterns. To establish such precise synaptic patterns, neurons pass through multiple checkpoints during development, such as cell fate determination, cell migration and localization, axonal guidance and target recognition, and synapse formation. Each stage offers key molecules for neurons/synapses to obtain glutamatergic or GABAergic specificity. Some mechanisms are based on intrinsic systems to induce gene expression, whereas others are based on extrinsic systems mediated by cell-cell or axon-target interactions. Recent studies indicate that specific formation of glutamatergic and GABAergic synapses is controlled by the expression or activation of different sets of molecules during development. In this review, the authors outline stages critical to the determination of glutamatergic or GABAergic specificity and describe molecules that act as determinants of specificities in each stage, with a particular focus on the synapse formation stage. They also discuss possible mechanisms underlying glutamatergic and GABAergic synapse formation via synapse-type specific synaptic organizers.

Analysis of the fibroblast growth factor system reveals alterations in a mouse model of spinal muscular atrophy

The monogenetic disease Spinal Muscular Atrophy (SMA) is characterized by a progressive loss of motoneurons leading to muscle weakness and atrophy due to severe reduction of the Survival of Motoneuron (SMN) protein. Several models of SMA show deficits in neurite outgrowth and maintenance of neuromuscular junction (NMJ) structure. Survival of motoneurons, axonal outgrowth and formation of NMJ is controlled by neurotrophic factors such as the Fibroblast Growth Factor (FGF) system. Besides their classical role as extracellular ligands, some FGFs exert also intracellular functions controlling neuronal differentiation. We have previously shown that intracellular FGF-2 binds to SMN and regulates the number of a subtype of nuclear bodies which are reduced in SMA patients. In the light of these findings, we systematically analyzed the FGF-system comprising five canonical receptors and 22 ligands in a severe mouse model of SMA. In this study, we demonstrate widespread alterations of the FGF-system in both muscle and spinal cord. Importantly, FGF-receptor 1 is upregulated in spinal cord at a pre-symptomatic stage as well as in a mouse motoneuron-like cell-line NSC34 based model of SMA. Consistent with that, phosphorylations of FGFR-downstream targets Akt and ERK are increased. Moreover, ERK hyper-phosphorylation is functionally linked to FGFR-1 as revealed by receptor inhibition experiments. Our study shows that the FGF system is dysregulated at an early stage in SMA and may contribute to the SMA pathogenesis.

Fibroblast growth factor 22 contributes to the development of retinal nerve terminals in the dorsal lateral geniculate nucleus

At least three forms of signaling between pre- and postsynaptic partners are necessary during synapse formation. First, “targeting” signals instruct presynaptic axons to recognize and adhere to the correct portion of a postsynaptic target cell. Second, trans-synaptic “organizing” signals induce differentiation in their synaptic partner so that each side of the synapse is specialized for synaptic transmission. Finally, in many regions of the nervous system an excess of synapses are initially formed, therefore “refinement” signals must either stabilize or destabilize the synapse to reinforce or eliminate connections, respectively. Because of both their importance in processing visual information and their accessibility, retinogeniculate synapses have served as a model for studying synaptic development. Molecular signals that drive retinogeniculate “targeting” and “refinement” have been identified, however, little is known about what “organizing” cues are necessary for the differentiation of retinal axons into presynaptic terminals. To identify such “organizing” cues, we used microarray analysis to assess whether any target-derived “synaptic organizers” were enriched in the mouse dorsal lateral geniculate nucleus (dLGN) during retinogeniculate synapse formation. One candidate “organizing” molecule enriched in perinatal dLGN was FGF22, a secreted cue that induces the formation of excitatory nerve terminals in muscle, hippocampus, and cerebellum. In FGF22 knockout mice, the development of retinal terminals in dLGN was impaired. Thus, FGF22 is an important “organizing” cue for the timely development of retinogeniculate synapses.

FGFs: Neurodevelopment's Jack-of-all-Trades - How Do They Do it?

From neurulation to postnatal processes, the requirements for FGF signaling in many aspects of neural precursor cell biology have been well documented. However, identifying a requirement for FGFs in a particular neurogenic process provides only an initial and superficial understanding of what FGF signaling is doing. How FGFs specify cell types in one instance, yet promote cell survival, proliferation, migration, or differentiation in other instances remains largely unknown and is key to understanding how they function. This review describes what we have learned primarily from in vivo vertebrate studies about the roles of FGF signaling in neurulation, anterior–posterior patterning of the neural plate, brain patterning from local signaling centers, and finally neocortex development as an example of continued roles for FGFs within the same brain area. The potential explanations for the diverse functions of FGFs through differential interactions with cell intrinsic and extrinsic factors is then discussed with an emphasis on how little we know about the modulation of FGF signaling in vivo. A clearer picture of the mechanisms involved is nevertheless essential to understand the behavior of neural precursor cells and to potentially guide their fates for therapeutic purposes.

FGF-2 deficiency does not influence FGF ligand and receptor expression during development of the nigrostriatal system

Secreted proteins of the fibroblast growth factor (FGF) family play important roles during development of various organ systems. A detailed knowledge of their temporal and spatial expression profiles, especially of closely related FGF family members, are essential to further identification of specific functions in distinct tissues. In the central nervous system dopaminergic neurons of the substantia nigra and their axonal projections into the striatum progressively degenerate in Parkinson's disease. In contrast, FGF-2 deficient mice display increased numbers of dopaminergic neurons. In this study, we determined the expression profiles of all 22 FGF-ligands and 10 FGF-receptor isoforms, in order to clarify, if FGF-2 deficiency leads to compensatory up-regulation of other FGFs in the nigrostriatal system. Three tissues, ventral mesencephalon (VM), striatum (STR) and as reference tissue spinal cord (SC) of wild-type and FGF-2 deficient mice at four developmental stages E14.5, P0, P28, and adult were comparatively analyzed by quantitative RT-PCR. As no differences between the genotypes were observed, a compensatory up-regulation can be excluded. Moreover, this analysis revealed that the majority of FGF-ligands (18/22) and FGF-receptors (9/10) are expressed during normal development of the nigrostriatal system and identified dynamic changes for some family members. By comparing relative expression level changes to SC reference tissue, general alterations in all 3 tissues, such as increased expression of FGF-1, -2, -22, FgfR-2c, -3c and decreased expression of FGF-13 during postnatal development were identified. Further, specific changes affecting only one tissue, such as increased FGF-16 (STR) or decreased FGF-17 (VM) expression, or two tissues, such as decreased expression of FGF-8 (VM, STR) and FGF-15 (SC, VM) were found. Moreover, 3 developmentally down-regulated FGFs (FGF-8b, FGF-15, FGF-17a) were functionally characterized by plasmid-based over-expression in dissociated E11.5 VM cell cultures, however, such a continuous exposure had no influence on the yield of dopaminergic neurons in vitro.

Orchestrating the synaptic network by tyrosine phosphorylation signalling

The establishment of a functional brain requires coordinated and stereotyped formation of synapses between neurons. For this, trans-synaptic molecular cues (synaptic organizers) are exchanged between a neuron and its target to organize appropriate synapses. The understanding of signalling mechanisms by which such synaptic organizers lead to synapse formation is just being elucidated. However, recent studies revealed that some of these cues act through receptor protein tyrosine kinases (RPTKs) or phosphatases (RPTPs). Synaptogenic RPTKs and RPTPs pattern synaptic network through affecting local protein-protein binding dynamics, changing the phosphorylation state of signalling cascades, or promoting gene expression. Each RPTK or RPTP has distinct roles in synapse formation, serving at different synapses or showing differential synaptogenic effects. Thus, tyrosine phosphorylation signalling plays critical roles in building the orchestrated synaptic circuitry in the brain.

Microarray identification of novel downstream targets of FoxD4L1/D5, a critical component of the neural ectodermal transcriptional network

FoxD4L1/D5 is a forkhead transcription factor that functions as both a transcriptional activator and repressor. FoxD4L1/D5 acts upstream of several other neural transcription factors to maintain neural fate, regulate neural plate patterning, and delay the expression of neural differentiation factors. To identify a more complete list of downstream genes that participate in these earliest steps of neural ectodermal development, we carried out a microarray analysis comparing gene expression in control animal cap ectodermal explants (ACs), which will form epidermis, to that in FoxD4L1/D5-expressing ACs. Forty-four genes were tested for validation by RT-PCR of ACs and/or in situ hybridization assays in embryos; 86% of those genes up-regulated and 100% of those genes down-regulated in the microarray were altered accordingly in one of these independent assays. Eleven of these 44 genes are of unknown function, and we provide herein their developmental expression patterns to begin to reveal their roles in ectodermal development.