Disruption of fibroblast growth factor receptor signaling in nonmyelinating Schwann cells causes sensory axonal neuropathy and impairment of thermal pain sensitivity

Fibroblast growth factor 3 contributes to neuropathic pain through Akt/mTOR signaling in mouse primary sensory neurons

Neuropathic pain (NP), a severe chronic pain condition, remains a substantial clinical challenge due to its complex pathophysiology and limited effective treatments. An association between the members of the Fibroblast Growth Factors (FGFs), particularly Fgf3, and the development of NP has become evident. In this study, utilizing a mouse model of NP, we observed a time-dependent increase in Fgf3 expression at both mRNA and protein levels within the dorsal root ganglia (DRG). Functional studies revealed that blocking Fgf3 expression mitigated nerve injury induced nociceptive hypersensitivity, suggesting its pivotal role in pain modulation. Moreover, our findings elucidate that Fgf3 contributes to pain hypersensitivity through the activation of the Akt/mTOR signaling in injured DRG neurons. These results not only shed light on the involvement of Fgf3 in nerve injury-induced NP but also highlight its potential as a promising therapeutic target for pain management. This study thereby advances our understanding of the molecular mechanisms underlying NP and opens new avenues for the development of effective treatment strategies.

Signaling Pathways Controlling Axonal Wrapping in Drosophila

The rapid transmission of action potentials is an important ability that enables efficient communication within the nervous system. Glial cells influence conduction velocity along axons by regulating the radial axonal diameter, providing electrical insulation as well as affecting the distribution of voltage-gated ion channels. Differentiation of these wrapping glial cells requires a complex set of neuron-glia interactions involving three basic mechanistic features. The glia must recognize the axon, grow around it, and eventually arrest its growth to form single or multiple axon wraps. This likely depends on the integration of numerous evolutionary conserved signaling and adhesion systems. Here, we summarize the mechanisms and underlying signaling pathways that control glial wrapping in Drosophila and compare those to the mechanisms that control glial differentiation in mammals. This analysis shows that Drosophila is a beneficial model to study the development of even complex structures like myelin.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

Discoidin domain receptor regulates ensheathment, survival and caliber of peripheral axons

Most invertebrate axons and small-caliber axons in mammalian peripheral nerves are unmyelinated but still ensheathed by glia. Here, we use Drosophila wrapping glia to study the development and function of non-myelinating axon ensheathment, which is poorly understood. Selective ablation of these glia from peripheral nerves severely impaired larval locomotor behavior. In an in vivo RNA interference screen to identify glial genes required for axon ensheathment, we identified the conserved receptor tyrosine kinase Discoidin domain receptor (Ddr). In larval peripheral nerves, loss of Ddr resulted in severely reduced ensheathment of axons and reduced axon caliber, and we found a strong dominant genetic interaction between Ddr and the type XV/XVIII collagen Multiplexin (Mp), suggesting that Ddr functions as a collagen receptor to drive axon wrapping. In adult nerves, loss of Ddr decreased long-term survival of sensory neurons and significantly reduced axon caliber without overtly affecting ensheathment. Our data establish essential roles for non-myelinating glia in nerve development, maintenance and function, and identify Ddr as a key regulator of axon-glia interactions during ensheathment and establishment of axon caliber.

Peripheral glia diversity

Recent years have seen an evolving appreciation for the role of glial cells in the nervous system. As we move away from the typical neurocentric view of neuroscience, the complexity and variability of central nervous system glia is emerging, far beyond the three main subtypes: astrocytes, oligodendrocytes, and microglia. Yet the diversity of the glia found in the peripheral nervous system remains rarely discussed. In this review, we discuss the developmental origin, morphology, and function of the different populations of glia found in the peripheral nervous system, including: myelinating Schwann cells, Remak Schwann cells, repair Schwann cells, satellite glia, boundary cap-derived glia, perineurial glia, terminal Schwann cells, glia found in the skin, olfactory ensheathing cells, and enteric glia. The morphological and functional heterogeneity of glia found in the periphery reflects the diverse roles the nervous system performs throughout the body. Further, it highlights a complexity that should be appreciated and considered when it comes to a complete understanding of the peripheral nervous system in health and disease.

Multiple Nf1 Schwann cell populations reprogram the plexiform neurofibroma tumor microenvironment

To define alterations early in tumor formation, we studied nerve tumors in neurofibromatosis 1 (NF1), a tumor predisposition syndrome. Affected individuals develop neurofibromas, benign tumors driven by NF1 loss in Schwann cells (SCs). By comparing normal nerve cells to plexiform neurofibroma (PN) cells using single-cell and bulk RNA sequencing, we identified changes in 5 SC populations, including a de novo SC progenitor-like (SCP-like) population. Long after Nf1 loss, SC populations developed PN-specific expression of Dcn, Postn, and Cd74, with sustained expression of the injury response gene Postn and showed dramatic expansion of immune and stromal cell populations; in corresponding human PNs, the immune and stromal cells comprised 90% of cells. Comparisons between injury-related and tumor monocytes/macrophages support early monocyte recruitment and aberrant macrophage differentiation. Cross-species analysis verified each SC population and unique conserved patterns of predicted cell-cell communication in each SC population. This analysis identified PROS1-AXL, FGF-FGFR, and MIF-CD74 and its effector pathway NF-κB as deregulated in NF1 SC populations, including SCP-like cells predicted to influence other types of SCs, stromal cells, and/or immune cells in mouse and human. These findings highlight remarkable changes in multiple types of SCs and identify therapeutic targets for PN.

Detection of repeat expansions in large next generation DNA and RNA sequencing data without alignment

Bioinformatic methods for detecting short tandem repeat expansions in short-read sequencing have identified new repeat expansions in humans, but require alignment information to identify repetitive motif enrichment at genomic locations. We present superSTR, an ultrafast method that does not require alignment. superSTR is used to process whole-genome and whole-exome sequencing data, and perform the first STR analysis of the UK Biobank, efficiently screening and identifying known and potential disease-associated STRs in the exomes of 49,953 biobank participants. We demonstrate the first bioinformatic screening of RNA sequencing data to detect repeat expansions in humans and mouse models of ataxia and dystrophy.

Inhibition of Oligodendrocyte Apoptosis in the Prelimbic Medial Prefrontal Cortex Prevents Fentanyl-induced Hyperalgesia in Rats

Opioid-induced hyperalgesia (OIH) is a problem associated with prolonged use of opioids in chronic pain management, and its effective treatment has been hampered by lack of mechanistic evidence. Oligodendrocytes have recently been linked with several pain-related diseases; however, little is known its role in OIH. The prelimbic medial prefrontal cortex (PL-mPFC) has emerged as a significant center of pain regulation, and is rich in oligodendrocytes. Herein we explored the effect of oligodendrocyte apoptosis of PL-mPFC on OIH. Using a fentanyl-induced rat model of OIH and proteomics analysis of the PL-mPFC, we observed a downregulation in 5 types of myelin-related proteins originating from oligodendrocytes; this was further verified by western blotting. Meanwhile, cleaved-caspase 3 (an apoptosis marker) was increased, whereas the oligodendrocyte precursor cell (OPC) marker NG2 remained unchanged. These results suggest that downregulated myelin-related proteins may be associated with oligodendrocyte apoptosis rather than a reduction in their generating source, and immunohistochemistry confirmed this hypothesis. Behaviorally, prophylactic blockade of oligodendrocyte apoptosis by microinjection of z-DEVD-fmk into the PL-mPFC prevented fentanyl-induced mechanical and thermal hyperalgesia, but downregulated myelin basic protein (mbp) gradually recovered in 12 h. We suggest that OIH may be primed in part via oligodendrocyte apoptosis in the PL-mPFC. PERSPECTIVE: In this study we showed that oligodendrocyte apoptosis in the PL-mPFC is a key trigger for fentanyl-induced hyperalgesia. Targeting oligodendrocyte apoptosis in the PL-mPFC may prevented hyperalgesia priming induced by fentanyl.

Fibroblast Growth Factor Signalling in the Diseased Nervous System

Fibroblast growth factors (FGFs) act as key signalling molecules in brain development, maintenance, and repair. They influence the intricate relationship between myelinating cells and axons as well as the association of astrocytic and microglial processes with neuronal perikarya and synapses. Advances in molecular genetics and imaging techniques have allowed novel insights into FGF signalling in recent years. Conditional mouse mutants have revealed the functional significance of neuronal and glial FGF receptors, not only in tissue protection, axon regeneration, and glial proliferation but also in instant behavioural changes. This review provides a summary of recent findings regarding the role of FGFs and their receptors in the nervous system and in the pathogenesis of major neurological and psychiatric disorders.

Transcriptomic signature of painful human neurofibromatosis type 2 schwannomas

Schwannomas are benign neoplasms that can cause gain‐ and loss‐of‐function neurological phenotypes, including severe, intractable pain. To investigate the molecular mechanisms underlying schwannoma‐associated pain we compared the RNA sequencing profile of painful and non‐painful schwannomas from NF2 patients. Distinct segregation of painful and non‐painful tumors by gene expression patterns was observed. Differential expression analysis showed the upregulation of fibroblast growth factor 7 (FGF7) in painful schwannomas. Behavioral support for this finding was observed using a xenograft human NF2‐schwannoma model in nude mice. In this model, over‐expression of FGF7 in intra‐sciatically implanted NF2 tumor cells generated pain behavior compared with controls.

Investigation Into the Role of ERK in Tyrosine Kinase Inhibitor-Induced Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a common and debilitating adverse event that can alter patient treatment options and halt candidate drug development. A case study is presented here describing the preclinical and clinical development of CC-90003, a small molecule extracellular signal-regulated kinase (ERK)1/2 inhibitor investigated as an oncology therapy. In a Phase Ia clinical trial, CC-90003 elicited adverse drug-related neuropathy and neurotoxicity that contributed to discontinued development of CC-90003 for oncology therapy. Preclinical evaluation of CC-90003 in dogs revealed clinical signs and electrophysiological changes consistent with peripheral neuropathy that was reversible. Mice did not exhibit signs of neuropathy upon daily dosing with CC-90003, supporting that rodents generally poorly predict CIPN. We sought to investigate the mechanism of CC-90003-induced peripheral neuropathy using a phenotypic in vitro assay. Translating preclinical neuropathy findings to humans proves challenging as no robust in vitro models of CIPN exist. An approach was taken to examine the influence of CIPN-associated drugs on human-induced pluripotent stem cell-derived peripheral neuron (hiPSC-PN) electrophysiology on multielectrode arrays (MEAs). The MEA assay with hiPSC-PNs was sensitive to CIPN-associated drugs cisplatin, sunitinib, colchicine, and importantly, to CC-90003 in concordance with clinical neuropathy incidence. Biochemical data together with in vitro MEA data for CC-90003 and 12 of its structural analogs, all having similar ERK inhibitory activity, revealed that CC-90003 disrupted in vitro neuronal electrophysiology likely via on-target ERK inhibition combined with off-target kinase inhibition and translocator protein inhibition. This approach could prove useful for assessing CIPN risk and interrogating mechanisms of drug-induced neuropathy.

A ligand-receptor interactome platform for discovery of pain mechanisms and therapeutic targets

In the peripheral nervous system, ligand-receptor interactions between cells and neurons shape sensory experience, including pain. We set out to identify the potential interactions between sensory neurons and peripheral cell types implicated in disease-associated pain. Using mouse and human RNA sequencing datasets and computational analysis, we created interactome maps between dorsal root ganglion (DRG) sensory neurons and an array of normal cell types, as well as colitis-associated glial cells, rheumatoid arthritis-associated synovial macrophages, and pancreatic tumor tissue. These maps revealed a common correlation between the abundance of heparin-binding EGF-like growth factor (HBEGF) in peripheral cells with that of its receptor EGFR (a member of the ErbB family of receptors) in DRG neurons. Subsequently, we confirmed that increased abundance of HBEGF enhanced nociception in mice, likely acting on DRG neurons through ErbB family receptors. Collectively, these interactomes highlight ligand-receptor interactions that may lead to treatments for disease-associated pain and, furthermore, reflect the complexity of cell-to-neuron signaling in chronic pain states.

Wrapping glia regulates neuronal signaling speed and precision in the peripheral nervous system of Drosophila

The functionality of the nervous system requires transmission of information along axons with high speed and precision. Conductance velocity depends on axonal diameter whereas signaling precision requires a block of electrical crosstalk between axons, known as ephaptic coupling. Here, we use the peripheral nervous system of Drosophila larvae to determine how glia regulates axonal properties. We show that wrapping glial differentiation depends on gap junctions and FGF-signaling. Abnormal glial differentiation affects axonal diameter and conductance velocity and causes mild behavioral phenotypes that can be rescued by a sphingosine-rich diet. Ablation of wrapping glia does not further impair axonal diameter and conductance velocity but causes a prominent locomotion phenotype that cannot be rescued by sphingosine. Moreover, optogenetically evoked locomotor patterns do not depend on conductance speed but require the presence of wrapping glial processes. In conclusion, our data indicate that wrapping glia modulates both speed and precision of neuronal signaling.

FGF5 Regulates Schwann Cell Migration and Adhesion

The fibroblast growth factor (FGF) family polypeptides play key roles in promoting tissue regeneration and repair. FGF5 is strongly up-regulated in Schwann cells of the peripheral nervous system following injury; however, a role for FGF5 in peripheral nerve regeneration has not been shown up to now. In this report, we examined the expression of FGF5 and its receptors FGFR1-4 in Schwann cells of the mouse sciatic nerve following injury, and then measured the effects of FGF5 treatment upon cultured primary rat Schwann cells. By microarray and mRNA sequencing data analysis, RT-PCR, qPCR, western blotting and immunostaining, we show that FGF5 is highly up-regulated in Schwann cells of the mouse distal sciatic nerve following injury, and FGFR1 and FGFR2 are highly expressed in Schwann cells of the peripheral nerve both before and following injury. Using cultured primary rat Schwann cells, we show that FGF5 inhibits ERK1/2 MAP kinase activity but promotes rapid Schwann cell migration and adhesion via the upregulation of N-cadherin. Thus, FGF5 is an autocrine regulator of Schwann cells to regulate Schwann cell migration and adhesion.

Sensory neuronopathy is a specific and disabling neurological manifestation of autoimmune hepatitis

BACKGROUND AND PURPOSE

Neurological manifestations have been identified in the context of autoimmune hepatitis (AIH). Previous case reports highlighted the association between AIH and sensory neuronopathy (SN). Despite that, little is known about the frequency of AIH-related SN and its clinical/neurophysiological profile. Moreover, it is not clear whether SN is an AIH-specific manifestation or related to chronic liver damage.

METHODS

Seventy consecutive AIH patients were enrolled and their characteristics were compared with 52 consecutive patients with chronic active hepatitis B. All subjects underwent clinical and neurophysiological evaluation. Further comparisons were performed between AIH SN and AIH non-SN patients.

RESULTS

Mean ages and male:female proportions in the AIH and chronic active hepatitis B groups were 42.2 ± 16.3/51.7 ± 13.6 years and 14:56/29:23, respectively. The frequencies of carpal tunnel syndrome, radiculopathy and polyneuropathy were similar between groups. In contrast, SN was identified only in AIH patients (5/70 vs. 0/52, P = 0.04); the overall prevalence of AIH-related SN was 7% with an average profile of a woman in her 40s with asymmetric onset of sensory deficits that chronically evolved to disabling proprioceptive ataxia associated with marked dysautonomia. Neurological disability and hepatocellular damage did not follow in parallel. Anti-fibroblast growth factor receptor type 3 antibodies were found in 3/5 (60%) of the patients with AIH-related SN. Clinical or demographic predictors of SN in the context of AIH could not be identified.

CONCLUSION

Sensory neuronopathy, but not other peripheral nervous system diseases, is a specific AIH neurological manifestation. It is often disabling and, in contrast to hepatocellular injury, does not respond to immunosuppression.

Peripheral FGFR1 Regulates Myofascial Pain in Rats via the PI3K/AKT Pathway

Myofascial pain syndrome (MPS) is a type of skeletal pain identified by myofascial trigger points (MTrPs). The formation of MTrPs is linked to muscle damage. The fibroblast growth factor receptor (FGFR1) has been found to cause pain sensitivity while repairing tissue damage. The aim of the current study was to explore the mechanism of FGFR1 in MTrPs. We used a RayBio human phosphorylation array kit to measure p-FGFR1 levels in human control subjects and patients with MTrPs. P-FGFR1 was upregulated in the patients with MTrPs. Then a rat model of MPS was established by a blunt strike on the left gastrocnemius muscles (GM) and eccentric-exercise for 8 weeks with 4 weeks of recovery. After establishing the MPS model, the morphology of the GM changed, and the differently augmented sizes of round fibers (contracture knots) in the transverse section and fusiform shapes in the longitudinal section were clearly seen in the rats with myofascial pain. The expression of p-FGFR1 was upregulated on the peripheral nerves and dorsal root ganglion neurons in the MTrPs group. The spinal Fos protein expression was increased in the MTrPs group. Additionally, the mechanical pain threshold was reduced, and the expression of FGF2, p-FGFR1, PI3K-p110γ, and p-AKT increased in the MTrPs group. PD173074 increased the mechanical pain threshold of the MTrPs group, and inhibited the expression of p-FGFR1, PI3K-p110γ, and p-AKT. Moreover, LY294002 increased the mechanical pain threshold of the MTrPs group. These findings suggest that FGFR1 may regulate myofascial pain in rats through the PI3K/AKT pathway.

Clinical characteristics of fibroblast growth factor receptor 3 antibody-related polyneuropathy: a retrospective study

BACKGROUND AND PURPOSE

Autoantibodies are increasingly being used as a diagnostic biomarker of chronic inflammatory neuropathies. However, their role and associated clinical syndrome are not well defined.

METHODS

This retrospective chart review evaluated the clinical presentation, diagnostic workup and therapeutic responses in fibroblast growth factor receptor 3 (FGFR3) antibody-associated neuropathy.

RESULTS

A total of 27 patients [14 men, aged 29-87 (65 ± 14) years] with positive FGFR3 antibody were included. Distal lower-extremity paresthesia (66%), unsteady gait (26%) and foot drop (11%) were common presenting symptoms. Symptom onset was acute in four (15%) cases. Distal lower-extremity weakness (mild in eight and severe in three patients) was the most frequent motor finding. Decreased distal sensation to pinprick (59%) and loss of vibration sensation (37%) were observed. Titer of FGFR3 ranged between 3100 and 30 000 (normal < 3000) with a mean of 10 688 ± 7284. Apart from the occasional association of other neuropathy-related autoantibodies, comprehensive neuropathy workup was otherwise unrevealing. Six patients had other autoimmune disease and seven patients had a history of cancer. Electromyography reflected sensorimotor neuropathy with mixed axonal and demyelinating features in 11 cases. Pure sensory neuropathy was noted in three patients. Demyelination was found in five of six nerve biopsies. Intravenous immunoglobulin response was observed in 8/10 treated patients.

CONCLUSIONS

The FGFR3 antibody appears not to be restricted to sensory neuropathy only. Its role in the pathogenicity of chronic inflammatory neuropathies is not yet well established and, although there may be a role for immunotherapy, larger studies are warranted.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

A Phase Ib Study of the FGFR/VEGFR Inhibitor Dovitinib With Gemcitabine and Capecitabine in Advanced Solid Tumor and Pancreatic Cancer Patients

OBJECTIVES

Preclinical studies demonstrated antitumor activity of dovitinib in pancreatic cancer models. This phase Ib study aimed to determine the maximum tolerated dose (MTD) of dovitinib in combination with gemcitabine and capecitabine and to characterize the safety and pharmacokinetic profile in patients with advanced pancreatic and biliary tract cancers and solid malignancies.

MATERIALS AND METHODS

Patients received gemcitabine 1000 mg/m² intravenously on days 1 and 8, capecitabine 1300 mg/m² oral daily from day 1 to 14, and dovitinib oral daily 5 days on and 2 days off, every 21-day cycle. The standard 3+3 dose escalation design was utilized and the study expanded to treat an additional 20 advanced pancreatic and biliary tract cancers patients at MTD.

RESULTS

A total of 29 patients were enrolled. One patient experienced dose-limiting grade 3 colitis. Two patients developed clinically significant neuropathy after the first cycle requiring dose reduction. The MTD was not reached and dovitinib 300 mg was declared the recommended dose for expansion. The most frequent grade 2 or worse adverse events were fatigue (45%), neutropenia (41%), thrombocytopenia (34%), anemia (24%), nausea (24%), and palmer-plantar erythrodysaesthesia syndrome (21%). Partial responses were observed in 5 patients. Pharmacokinetic studies showed no drug-drug interaction between dovitinib, capecitabine and gemcitabine. Fibroblast growth factor 23 plasma level increased in 4 of 5 patients during the first cycle of treatment.

CONCLUSIONS

Dovitinib 300 mg daily is the recommended dose when combined with gemcitabine and capecitabine, achieving clinically relevant plasma concentrations. The study combination demonstrated encouraging efficacy signals in advanced pancreatic cancer.

Treatment of spontaneous corneal perforation secondary to undiagnosed Sjögren's syndrome using regenerating agent and autologous serum eye drops

PURPOSE

The aim of this study was to report a case of sterile corneal ulcer leading to perforation, which was treated effectively with autologous serum eye drops, topical regenerative agent (poly-carboxymethylglucose sulfate), steroids, and systemic immunosuppression in a patient with undiagnosed primary Sjögren's syndrome.

METHODS

A 74-year-old female presented with a month's history of gradually worsening blurry vision in her left eye. Ophthalmic examination revealed a central descemetocele with excessive corneal stromal melting and absence of signs of infection. A bandage contact lens was applied for tectonic support along with topical corticosteroid and antibiotic drops. Autoimmune screen disclosed a diagnosis of Sjögren's syndrome, and the patient was commenced on systemic immunosuppression. Forty-eight hours after presentation, the patient developed a localized corneal perforation, presenting with a flat anterior chamber.

RESULTS

Urgent amniotic membrane transplantation was arranged while topical dexamethasone, moxifloxacin, and autologous serum eye drops were administered. After 24 h of intensive topical treatment, a significant reforming of the anterior chamber and subsequent gradual regeneration of the corneal stroma were noted, thus postponing amniotic grafting. The patient remained under close monitoring, showing progressive clinical improvement. Regenerating agent eye drops (Cacicol20^®) were also applied over the next month, with careful and slow tapering of topical dexamethasone. Further improvement of corneal thickness was observed, and visual acuity increased to 20/80.

CONCLUSION

This case report demonstrates the successful medical treatment of an autoimmune-related sterile corneal perforation without surgical intervention, highlighting the fact that early diagnosis and rigorous medical treatment with autologous serum and regenerating agent eye drops can effectively aid tissue regeneration and favorable visual rehabilitation.

Novel miRNA, miR-sc14, promotes Schwann cell proliferation and migration

MicroRNAs refer to a class of endogenous, short non-coding RNAs that mediate numerous biological functions. MicroRNAs regulate various physiological and pathological activities of peripheral nerves, including peripheral nerve repair and regeneration. Previously, using a rat sciatic nerve injury model, we identified many functionally annotated novel microRNAs, including miR-sc14. Here, we used real-time reverse transcription-polymerase chain reaction to examine miR-sc14 expression in rat sciatic nerve stumps. Our results show that miR-sc14 is noticeably altered following sciatic nerve injury, being up-regulated at 1 day and diminished at 7 days. EdU and transwell chamber assay results showed that miR-sc14 mimic promoted proliferation and migration of Schwann cells, while miR-sc14 inhibitor suppressed their proliferation and migration. Additionally, bioinformatic analysis examined potential target genes of miR-sc14, and found that fibroblast growth factor receptor 2 might be a potential target gene. Specifically, our results show changes of miR-sc14 expression in the sciatic nerve of rats at different time points after nerve injury. Appropriately, up-regulation of miR-sc14 promoted proliferation and migration of Schwann cells. Consequently, miR-sc14 may be an intervention target to promote repair of peripheral nerve injury. The study was approved by the Jiangsu Provincial Laboratory Animal Management Committee, China on March 4, 2015 (approval No. 20150304-004).

FGFR3 Antibodies in Neuropathy: What to Do With Them?

OBJECTIVES

To describe the variability of fibroblast growth factor receptor 3 (FGFR3) antibody titers in a small series of patients.

METHODS

We performed a retrospective review of patients with neuropathy and positive FGFR3 antibodies.

RESULTS

We report 7 patients (3 women) with an age range 44-81 years. Symptoms were acute onset in 3 and subacute onset in 4 patients. Five had neuropathic pain. Examination revealed normal large fiber function to mild/moderate predominantly sensory neuropathy and ataxia in one patient. Electrodiagnostic studies revealed normal large fiber function (3), demyelinating neuropathy (1), and mild/moderate axonal neuropathy (3). Four patients had high and 3 patients had low FGFR3 titers. Repeat testing revealed absence of antibodies in 2 patients and a significant reduction in one patient without any intervening immunotherapy.

CONCLUSIONS

Our case series highlights the variability and inconsistency in FGFR3 antibody titers through enzyme-linked immunosorbent assay testing. These antibody titers should always be interpreted with caution in clinical context.

The development of human digital Meissner's and Pacinian corpuscles

Meissner's and Pacinian corpuscles are cutaneous mechanoreceptors responsible for different modalities of touch. The development of these sensory formations in humans is poorly known, especially regarding the acquisition of the typical immunohistochemical profile related to their full functional maturity. Here we used a panel of antibodies (to specifically label the main corpuscular components: axon, Schwann-related cells and endoneurial-perineurial-related cells) to investigate the development of digital Meissner's and Pacinian corpuscles in a representative sample covering from 11 weeks of estimated gestational age (wega) to adulthood. Development of Pacinian corpuscles starts at 13 wega, and it is completed at 4 months of life, although their basic structure and immunohistochemical characteristics are reached at 36 wega. During development, around the axon, a complex network of S100 positive Schwann-related processes is progressively compacted to form the inner core, while the surrounding mesenchyme is organized and forms the outer core and the capsule. Meissner's corpuscles start to develop at 22 wega and complete their typical morphology and immunohistochemical profile at 8 months of life. In developing Meissner's corpuscles, the axons establish complex relationships with the epidermis and are progressively covered by Schwann-like cells until they complete the mature arrangement late in postnatal life. The present results demonstrate an asynchronous development of the Meissner's and Pacini's corpuscles and show that there is not a total correlation between morphological and immunohistochemical maturation. The correlation of the present results with touch-induced cortical activity in developing humans is discussed.

Preserved Expression of Skin Neurotrophic Factors in Advanced Diabetic Neuropathy Does Not Lead to Neural Regeneration despite Pancreas and Kidney Transplantation

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes with potential severe consequences. Its pathogenesis involves hyperglycemia-linked mechanisms, which may include changes in the expression of neurotrophic growth factors. We analyzed the expression of 29 factors potentially related to nerve degeneration and regeneration in skin biopsies from 13 type 1 diabetic pancreas and kidney recipients with severe DPN including severe depletion of intraepidermal nerve fibers (IENF) in lower limb skin biopsies (group Tx1 1st examination). The investigation was repeated after a median 28-month period of normoglycemia achieved by pancreas transplantation (group Tx1 2nd examination). The same tests were performed in 13 stable normoglycemic pancreas and kidney recipients 6-12 years posttransplantation (group Tx2), in 12 matched healthy controls (group HC), and in 12 type 1 diabetic subjects without severe DPN (group DM). Compared to DM and HC groups, we found a significantly higher (p < 0.05-0.001) expression of NGF (nerve growth factor), NGFR (NGF receptor), NTRK1 (neurotrophic receptor tyrosine kinase 1), GDNF (glial cell-derived neurotrophic factor), GFRA1 (GDNF family receptor alpha 1), and GFAP (glial fibrillary acidic protein) in both transplant groups (Tx1 and Tx2). Enhanced expression of these factors was not normalized following the median 28-month period of normoglycemia (Tx1 2nd examination) and negatively correlated with IENF density and with electrophysiological indices of DPN (vibration perception threshold, electromyography, and autonomic tests). In contrast to our expectation, the expression of most of 29 selected factors related to neural regeneration was comparable in subjects with severe peripheral nerve fiber depletion and healthy controls and the expression of six factors was significantly upregulated. These findings may be important for better understanding the pathophysiology of nerve regeneration and for the development of intervention strategies.

Gabapentin regulates expression of FGF2 and FGFR1 in dorsal root ganglia via microRNA-15a in the arthritis rat model

BACKGROUND

Arthritis is an inflammatory disease with a prevalence rate of approximately 10% in China, which commonly manifests as pain. The aim of the current study was to investigate the function of gabapentin in the dorsal root ganglion in an arthritis rat model, and assess the effect of gabapentin on the expression of fibroblast growth factor 2 (FGF2) and FGF receptor 1 (FGFR1).

METHODS

A total of 30 healthy male Sprague-Dawley rats were randomly divided into the following three groups: Untreated group, control group and gabapentin group. Rats in the control and the gabapentin groups were injected with Freund's complete adjuvant to induce arthritis. A total of 7 days subsequent to model establishment, the gabapentin group was administered intraperitoneally gabapentin for 8 days. The alterations in thickness of paw pad and paw withdrawal mechanical threshold (PWMT) were detected, which indicated that the rats in the control and gabapentin groups presented with the symptoms of arthritis.

RESULTS

In the control group, the PWMT value was significantly reduced (P < 0.05), whereas the PWMT value was significantly increased in the gabapentin group. Immunohistochemistry demonstrated that the expression levels of FGF2 and FGFR1 were increased in the control group compared with the untreated group, while the expression levels of FGF2 and FGFR1 were reduced in the gabapentin group. Moreover, the FGF2 antagonist PD173074 partially improved the plantar thickness and PWMT of the arthritic rats. Bioinformatics analysis predicted microRNA-15a binding sites in the 3'untranslated regions (UTR) of FGF2 and FGFR1. Furthermore, the expression of microRNA-15a was reduced in the control group compared with untreated rats, whereas microRNA-15a in the gabapentin group was upregulated compared with the control. Additionally, the luciferase reporter assay confirmed that microRNA-15a could inhibit the protein expression through pairing with the 3'UTR of FGF2 and FGFR1 mRNAs.

CONCLUSION

Gabapentin may relieve arthritis pain and reduce the expression of FGF2 and FGFR1 in dorsal root ganglia. Furthermore, microRNA-15a may be involved in the regulatory process.

New approach for the treatment of neuropathic pain: Fibroblast growth factor 1 gene-transfected adipose-derived mesenchymal stem cells

BACKGROUND

Neuropathic pain triggered by peripheral nerve lesion is extremely difficult to manage with current approaches, hence the importance of exploring therapeutic alternatives.

METHODS

We have analysed adipose-derived mesenchymal stem cells (AD-MSCs) and fibroblast growth factor 1 gene-transfected adipose-derived mesenchymal stem cells (AD-MSCs ^FGF1 ) on chronic constriction injury (CCI). The mechanical and thermal hypersensitivity were assessed using the von Frey filament, radiant heat and acetone drop tests. Histopathological and apoptotic changes and the level of FGF1, GFAP and TNFα proteins were assessed in the lumbar portion (L4-L6). Moreover, AD-MSCs ^FGF1 were labelled with ^99m Tc -HMPAO and isolated organ counting were performed upon AD-MSCs ^FGF1 administration.

RESULTS

Administration of AD-MSCs ^FGF1 attenuated the CCI-induced mechanical and thermal hypersensitivity. Spinal structural alterations and apoptosis were decreased in the AD-MSCs ^FGF1 group. The injection of either phosphate-buffered saline or normal NIH3T3 fibroblasts could not attenuate the behavioural symptoms of neuropathic pain. Increased genetically engineered cells were counted in the injured sciatic nerve and the elevated levels of FGF1 were detected in the spinal tissue. Stem cell therapy lead to decrement the level of the CCI-induced TNF-α and GFAP expression.

CONCLUSION

The intravenous administration of AD-MSCs ^FGF1 could be considered as a potential remedy for the management of neuropathic pain.

SIGNIFICANCE

AD-MSCs ^FGF1 attenuated the CCI-induced mechanical and thermal hypersensitivity. Spinal structural alterations and apoptosis were significantly decreased in the AD-MSCs ^FGF1 group. Elevated levels of FGF1 were detected in the spinal tissue.

The Src family kinase inhibitor dasatinib delays pain-related behaviour and conserves bone in a rat model of cancer-induced bone pain

Pain is a severe and debilitating complication of metastatic bone cancer. Current analgesics do not provide sufficient pain relief for all patients, creating a great need for new treatment options. The Src kinase, a non-receptor protein tyrosine kinase, is implicated in processes involved in cancer-induced bone pain, including cancer growth, osteoclastic bone degradation and nociceptive signalling. Here we investigate the role of dasatinib, an oral Src kinase family and Bcr-Abl tyrosine kinase inhibitor, in an animal model of cancer-induced bone pain. Daily administration of dasatinib (15 mg/kg, p.o.) from day 7 after inoculation of MRMT-1 mammary carcinoma cells significantly attenuated movement-evoked and non-evoked pain behaviour in cancer-bearing rats. Radiographic - and microcomputed tomographic analyses showed significantly higher relative bone density and considerably preserved bone micro-architecture in the dasatinib treated groups, suggesting a bone-preserving effect. This was supported by a significant reduction of serum TRACP 5b levels in cancer-bearing rats treated with 15 mg/kg dasatinib. Furthermore, immunoblotting of lumbar spinal segments showed an increased activation of Src but not the NMDA receptor subunit 2B. These findings support a role of dasatinib as a disease modifying drug in pain pathologies characterized by increased osteoclast activity, such as bone metastases.

Signaling by FGF Receptor 2, Not FGF Receptor 1, Regulates Myelin Thickness through Activation of ERK1/2-MAPK, Which Promotes mTORC1 Activity in an Akt-Independent Manner

FGF signaling has emerged as a significant "late-stage" regulator of myelin thickness in the CNS, independent of oligodendrocyte differentiation. Therefore, it is critically important to identify the specific FGF receptor type and its downstream signaling molecules in oligodendrocytes to obtain better insights into the regulatory mechanisms of myelin growth. Here, we show that FGF receptor type 2 (FGFR2) is highly enriched at the paranodal loops of myelin. Conditional ablation of this receptor-type, but not FGF receptor type 1 (FGFR1), resulted in attenuation of myelin growth, expression of major myelin genes, key transcription factor Myrf and extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) activity. This was rescued by upregulating ERK1/2 activity in these mice, strongly suggesting that ERK1/2 are key transducers of FGFR2 signals for myelin growth. However, given that the PI3K/Akt/mechanistic target of rapamycin (mTOR) pathway is also known to regulate myelin thickness, we examined FGFR2-deficient mice for the expression of key signaling molecules in this pathway. A significant downregulation of p-mTOR, p-Raptor, and p-S6RP was observed, which was restored to normal by elevating ERK1/2 activity in these mice. Similar downregulation of these molecules was observed in ERK1/2 knock-out mice. Interestingly, since p-Akt levels remained largely unchanged in these mice, it suggests a mechanism of mTORC1 activation by ERK1/2 in an Akt-independent manner in oligodendrocytes. Taken together, these data support a model in which FGFs, possibly from axons, activate FGFR2 in the oligodendrocyte/myelin compartment to increase ERK1/2 activation, which ultimately targets Myrf, as well as converges with the PI3K/Akt/mTOR pathway at the level of mTORC1, working together to drive the growth of the myelin sheath, thus increasing myelin thickness.SIGNIFICANCE STATEMENT It is well accepted that myelin is a biologically active membrane in active communication with the axons. However, the axonal signals, the receptors on myelin, and the integration of intracellular signaling pathways emanating downstream from these receptors that drive the growth of the myelin sheath remain poorly understood in the CNS. This study brings up the intriguing possibility that FGF receptor 2, in the oligodendrocyte/myelin compartment, may be one such signal. Importantly, it provides compelling evidence linking FGFR2 with the ERK1/2-MAPK pathway, which converges with the PI3K/Akt/mTOR (mechanistic target of rapamycin) pathway at the level of mTORC1 and also regulates the transcription factor Myrf, together providing a mechanistic framework for regulating both the transcriptional and translational machinery required for the proper growth of the myelin sheath.

Fibroblast growth factor signaling in oligodendrocyte-lineage cells facilitates recovery of chronically demyelinated lesions but is redundant in acute lesions

Remyelination is a potent regenerative process in demyelinating diseases, such as multiple sclerosis, the effective therapeutic promotion of which will fill an unmet clinical need. The development of proregenerative therapies requires the identification of key regulatory targets that are likely to be involved in the integration of multiple signaling mechanisms. Fibroblast growth factor (FGF) signaling system, which comprises multiple ligands and receptors, potentially provides one such target. Since the FGF/FGF receptor (FGFR) interactions are complex and regulate multiple diverse functions of oligodendrocyte lineage cells, it is difficult to predict their overall therapeutic potential in the regeneration of oligodendrocytes and myelin. Therefore, to assess the integrated effects of FGFR signaling on this process, we simultaneously inactivated both FGFR1 and FGFR2 in oligodendrocytes and their precursors using two Cre-driver mouse lines. Acute and chronic cuprizone-induced or lysolecithin-induced demyelination was established in Fgfr1/Fgfr2 double knockout mice (dKO). We found that in the acute cuprizone model, there was normal differentiation of oligodendrocytes and recovery of myelin in the corpus callosum of both control and dKO mice. Similarly, in the spinal cord, lysolecithin-induced demyelinated lesions regenerated similarly in the dKO and control mice. In contrast, in the chronic cuprizone model, fewer differentiated oligodendrocytes and less efficient myelin recovery were observed in the dKO compared to control mice. These data suggest that while cell-autonomous FGF signaling is redundant during recovery of acute demyelinated lesions, it facilitates regenerative processes in chronic demyelination. Thus, FGF-based therapies have potential value in stimulating oligodendrocyte and myelin regeneration in late-stage disease.

Effects of new biomimetic regenerating agents on corneal wound healing in an experimental model of post-surgical corneal ulcers

OBJECTIVE

The purpose of this study is to assess the effectiveness of the topical application of cacicol regenerating agent (RGTA) in an experimental model of corneal ulcer after photorefractive keratectomy (PRK) in mice.

METHODS

Mice were subjected to PRK surgery with a 2.0mm ablation zone on the central cornea and 45mm of depth on a VISX Star S2 excimer laser. Corneas were treated topically with cacicol drops 1hour and 48hours after injury. Control groups received balanced salt solution (BSS) in the same dosage. Clinical and histopathological events were evaluated at 1, 2, 3 and 7 days after surgery. Sections obtained through the central region of the corneas were used to analyze the histopathological events of injured and healed corneas. αSMA (myofibroblast transformation), E cadherin (assembly of epithelial cells) and neuronal class III β-tubulin (innervation) were performed.

RESULTS

Corneas treated topically with cacicol for 7 days showed a greater degree of transparency compared to controls. cacicol treated corneas showed improved epithelial cytoarchitecture. Analysis of αSMA profiles in the stroma showed that cacicol reduced or delayed the presence of myofibroblasts in the stroma compared to BSS (P<0.001). Finally, a putative neuroregenerative effect of cacicol was found in corneas subjected to an experimental PRK lesion. In some cases some interindividual variability could be observed due to the design of the experimental model. This is a limitation to consider, despite the statistical significance of the data.

CONCLUSIONS

In a model of laser induced surgical lesions in the cornea, topical application of an RGTA (i.e. cacicol) could be involved in avoiding myofibroblast scarring formation and promoting nerve regeneration.

Fibroblast growth factor 7 is a nociceptive modulator secreted via large dense-core vesicles

Fibroblast growth factor (FGF) 7, a member of FGF family, is initially found to be secreted from mesenchymal cells to repair epithelial tissues. However, its functions in the nervous system are largely unknown. The present study showed that FGF7 was a neuromodulator localized in the large dense-core vesicles (LDCVs) in nociceptive neurons. FGF7 was mainly expressed in small-diameter neurons of the dorsal root ganglion and could be transported to the dorsal spinal cord. Interestingly, FGF7 was mostly stored in LDCVs that did not contain neuropeptide substance P. Electrophysiological recordings in the spinal cord slice showed that buffer-applied FGF7 increased the amplitude of excitatory post-synaptic current evoked by stimulating the sensory afferent fibers. Behavior tests showed that intrathecally applied FGF7 potentiated the formalin-induced acute nociceptive response. Moreover, both acute and inflammatory nociceptive responses were significantly reduced in Fgf7-deficient mice. These results suggest that FGF7 exerts an excitatory modulation of nociceptive afferent transmission.

Role of ERK1/2 MAPK signaling in the maintenance of myelin and axonal integrity in the adult CNS

Oligodendrocytes form myelin during postnatal development and then maintain a functional myelin sheath throughout adult life. While many regulators of developmental myelination have been identified, the signal transduction mechanisms that regulate oligodendrocyte functions in adulthood are not well understood. The extracellular signal-regulated kinases-1 and -2 (ERK1/2), downstream mediators of mitogen-activated protein kinases (MAPKs), have emerged as prominent regulators of myelin formation. Here, we investigated whether these signaling molecules are also required for myelin maintenance in the adult CNS. Inducible conditional ablation of Erk1/2 in oligodendrocytes of the adult CNS resulted in a downregulation of myelin gene expression. Although myelin thickness was reduced and some axons were demyelinated, the majority of axons were wrapped by intact myelin sheaths that appeared structurally normal. However, late onset of progressive axonal degeneration, accompanied by astrogliosis, microglial activation, partial loss of oligodendrocytes, and functional impairment, occurred in the adult mice lacking ERK1/2 activity. Conditional ablation of Fibroblast Growth Factor receptors-1 and -2 (FGFR1/2) in oligodendrocytes also resulted in downregulation of myelin gene expression and development of axonal degeneration as the mice aged. Further, the level of the key transcription factor myelin gene regulatory factor (Myrf) was downregulated or upregulated in mice with genetic loss or gain of ERK1/2 function, respectively. Together, our studies demonstrate that ERK1/2-MAPK signaling is required for the long-term maintenance of myelin and axonal integrity in the adult CNS and suggest that FGFR1/2 and Myrf may, in part, contribute to signaling upstream and downstream of ERK1/2 in maintaining these oligodendrocyte functions during adulthood.

Increased expression of fibroblast growth factor receptor 1 in endometriosis and its correlation with endometriosis-related dysmenorrhea and recurrence

OBJECTIVE(S)

This study aims to identify a critical molecule that potentially participates in endometriosis pathogenesis and characterize its correlation with dysmenorrhea and recurrence.

STUDY DESIGN

We utilized a bioinformatics-based strategy to screen for candidate genes and fibroblast growth factor receptor 1(FGFR1) was chosen for further validation. FGFR1 expression was examined in specimens of ectopic and eutopic endometrium obtained from 48 patients with endometriosis and specimens of eutopic endometrium from 26 healthy control subjects using immunohistochemistry and Western blotting. In addition, FGFR shRNA treatment was applied in a nude mice endometriosis model to examine the functional role of FGFR1 in endometriosis formation in vivo.

RESULTS

FGFR1 was found commonly overexpressed in ectopic endometrium of endometriosis compared with either its eutopic counterpart or endometrium from normal patients (P < 0.05). FGFR shRNA treatment impaired endometriosis formation and alleviated endometriosis-related symptoms in vivo. FGFR1 expression in ectopic endometrium was correlated with dysmenorrhea severity (P < 0.05) and recurrence in endometriosis patients (P < 0.05).

CONCLUSION(S)

FGFR1 might be involved in endometriosis development, which could possibly serve as a novel therapeutic target and prognostic marker for this disease.

Clinical implications of Schwann cell biology

The neuroglia of the peripheral nervous system (PNS) are derived from the neural crest and are a diverse family of cells. They consist of myelinating Schwann cells, non-myelinating Schwann cells, satellite cells, and perisynaptic Schwann cells. Due to their prominent role in the formation of myelin, myelinating Schwann cells are the best recognised of these cells. However, Schwann cells and the other neuroglia of the PNS have many functions that are independent of myelination and contribute significantly to the functioning of the peripheral nerve in both health and disease. Here we discuss the contribution of PNS neuroglial cells to clinical deficit in neurodegenerative disease, peripheral neuropathy, and pain.

Concepts for regulation of axon integrity by enwrapping glia

Long axons and their enwrapping glia (EG; Schwann cells (SCs) and oligodendrocytes (OLGs)) form a unique compound structure that serves as conduit for transport of electric and chemical information in the nervous system. The peculiar cytoarchitecture over an enormous length as well as its substantial energetic requirements make this conduit particularly susceptible to detrimental alterations. Degeneration of long axons independent of neuronal cell bodies is observed comparatively early in a range of neurodegenerative conditions as a consequence of abnormalities in SCs and OLGs . This leads to the most relevant disease symptoms and highlights the critical role that these glia have for axon integrity, but the underlying mechanisms remain elusive. The quest to understand why and how axons degenerate is now a crucial frontier in disease-oriented research. This challenge is most likely to lead to significant progress if the inextricable link between axons and their flanking glia in pathological situations is recognized. In this review I compile recent advances in our understanding of the molecular programs governing axon degeneration, and mechanisms of EG’s non-cell autonomous impact on axon-integrity. A particular focus is placed on emerging evidence suggesting that EG nurture long axons by virtue of their intimate association, release of trophic substances, and neurometabolic coupling. The correction of defects in these functions has the potential to stabilize axons in a variety of neuronal diseases in the peripheral nervous system and central nervous system (PNS and CNS).

Fibroblast growth factor receptor signaling in oligodendrocytes regulates myelin sheath thickness

Formation of the CNS white matter is developmentally tightly regulated, but the molecules and mechanisms of myelination control in the postnatal CNS are poorly understood. Here, we show that myelin growth is controlled by fibroblast growth factor (FGF) signaling, originally identified as a proliferative signal for oligodendrocyte precursor cells (OPCs) in vitro. We created two lines of mice lacking both FGF receptor 1 (Fgfr1) and Fgfr2 in oligodendrocyte-lineage cells but found that in these mice OPC proliferation and differentiation were unaffected. In addition, axonal ensheathment and the initiation of myelination were on time. However, the rapid growth of CNS myelin, normally occurring in the second postnatal week, was strongly inhibited. Throughout adulthood, the myelin sheath remained disproportionately thin relative to the axon caliber. In adult mice, mutant oligodendrocytes were normal in number, whereas the transcription of major myelin genes was reduced. This FGF receptor-mediated stimulation of mature oligodendrocytes could also be modeled in vitro, demonstrating that enhanced expansion of oligodendroglial processes requires signaling by extracellular signal regulated kinase-1 and -2 (Erk1/2), downstream mediators of mitogen-activated protein kinase (MAPK). In vivo, Erk1/2-MAPK activity was reduced in the hypomyelinated CNS of Fgfr1/Fgfr2 mutant mice. These studies reveal a previously unrecognized function of FGF receptor signaling in oligodendrocytes that contributes to the regulation of myelin sheath thickness and that uncouples the initiation of ensheathment from the later phase of continued myelin growth.

Signals to promote myelin formation and repair

The myelin sheath wraps large axons in both the CNS and the PNS, and is a key determinant of efficient axonal function and health. Myelin is targeted in a series of diseases, notably multiple sclerosis (MS). In MS, demyelination is associated with progressive axonal damage, which determines the level of patient disability. The few treatments that are available for combating myelin damage in MS and related disorders, which largely comprise anti-inflammatory drugs, only show limited efficacy in subsets of patients. More-effective treatment of myelin disorders will probably be accomplished by early intervention with combinatorial therapies that target inflammation and other processes-for example, signaling pathways that promote remyelination. Indeed, evidence suggests that such pathways might be impaired in pathology and, hence, contribute to the failure of remyelination in such diseases. In this article, we review the molecular basis of signaling pathways that regulate myelination in the CNS and PNS, with a focus on signals that affect differentiation of myelinating glia. We also discuss factors such as extracellular molecules that act as modulators of these pathways. Finally, we consider the few preclinical and clinical trials of agents that augment this signaling.

Inactivation of fibroblast growth factor receptor signaling in myelinating glial cells results in significant loss of adult spiral ganglion neurons accompanied by age-related hearing impairment

Hearing loss has been attributed to many factors, including degeneration of sensory neurons in the auditory pathway and demyelination along the cochlear nerve. Fibroblast growth factors (FGFs), which signal through four receptors (Fgfrs), are produced by auditory neurons and play a key role in embryonic development of the cochlea and in neuroprotection against sound-induced injury. However, the role of FGF signaling in the maintenance of normal auditory function in adult and aging mice remains to be elucidated. Furthermore, the contribution of glial cells, which myelinate the cochlear nerves, is poorly understood. To address these questions, we generated transgenic mice in which Fgfr1 and Fgfr2 were specifically inactivated in Schwann cells and oligodendrocytes but not in neurons. Adult mutant mice exhibited late onset of hearing impairment, which progressed markedly with age. The hearing impairment was accompanied by significant loss of myelinated spiral ganglion neurons. The pathology extended into the cochlear nucleus, without apparent loss of myelin or of the deletion-bearing glial cells themselves. This suggests that perturbation of FGF receptor-mediated glial function leads to the attenuation of glial support of neurons, leading to their loss and impairment of auditory functions. Thus, FGF/FGF receptor signaling provides a potentially novel mechanism of maintaining reciprocal interactions between neurons and glia in adult and aging animals. Dysfunction of glial cells and FGF receptor signaling may therefore be implicated in neurodegenerative hearing loss associated with normal aging.