Neurturin-mediated ret activation is required for retinal function

Fovea-Involved Outer Retinal Abnormality Induced by a Novel RET Inhibitor

This case report describes an outer retinal abnormality associated with taking a RET inhibitor during a clinical trial in a 52-year-old patient with right lung adenocarcinoma and a RET gene fusion.

Post-Mortem Studies of Neurturin Gene Therapy for Parkinson's Disease: Two Subjects with 10 Years CERE120 Delivery

BACKGROUND

Neurturin is a member of the glial cell line-derived neurotrophic factor family of neurotrophic factors and has the potential to protectdegenerating dopaminergic neurons.

OBJECTIVE

Here, we performed post-mortem studies on two patients with advanced Parkinson's disease that survived 10 years following AAV-neurturin gene (Cere120) delivery to verify long-term effects of trophic factor neurturin.

METHODS

Cere120 was delivered to the putamen bilaterally in one case and to the putamen plus substantia nigra bilaterally in the second. Immunohistochemistry was used to examine neurturin, Rearranged during transfection(RET), phosphor-S6, and tyrosine hydroxylase expressions, inflammatory reactions, and α-synuclein accumulation.

RESULTS

In both patients there was persistent, albeit limited, neurturin expression in the putamen covering 1.31% to 5.92% of the putamen. Dense staining of tyrosine hydroxylase-positive fibers was observed in areas that contained detectable neurturin expression. In substantia nigra, neurturin expression was detected in 11% of remaining melanin-containing neurons in the patient with combined putamenal and nigral gene delivery, but not in the patient with putamenal gene delivery alone. Tyrosine hydroxylase positive neurons were 66% to 84% of remaining neuromelanin neurons in substantia nigra with Cere120 delivery and 23% to 24% in substantia nigra without gene delivery. More RET and phosphor-S6 positive neurons were observed in substantia nigra following nigral Cere120. Inflammatory and Lewy pathologies were similar in substantia nigra with or without Cere120 delivery.

CONCLUSIONS

This study provides evidence of long-term persistent transgene expression and bioactivity following gene delivery to the nigrostriatal system. Therefore, future efforts using gene therapy for neurodegenerative diseases should consider means to enhance remaining dopamine neuron function and stop pathological propagation. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Molecular and Regenerative Characterization of Repair and Non-repair Schwann Cells

Although evidence has accumulated to indicate that Schwann cells (SCs) differentiate into repair SCs (RSCs) upon injury and that the unique phenotype of these cells allow them to provide support for peripheral nerve regeneration, the details of the RSCs are not fully understood. The findings of the current study indicate that the RSCs have enhanced adherent properties and a greater capability to promote neurite outgrowth and axon regeneration after peripheral nerve injury, compared to the non-RSCs. Further, transcriptome analyses have demonstrated that the molecular signature of the RSCs is distinctly different from that of the non-RSCs. The RSCs upregulate a group of genes that are related to inflammation, repair, and regeneration, whereas non-RSCs upregulate genes related to myelin maintenance, Notch, and aging. These findings indicate that the RSCs have markedly different cellular, regenerative, and molecular characteristics compared to the non-RSCs, even though the RSCs were just derived from non-RSCs upon injury, thus providing the basis for understanding the mechanisms related to SC mediated repair after peripheral nerve injury.

Conditional Deletion of Activating Rearranged During Transfection Receptor Tyrosine Kinase Leads to Impairment of Photoreceptor Ribbon Synapses and Disrupted Visual Function in Mice

Purpose: The rearranged during transfection (RET) receptor tyrosine kinase plays a key role in transducing signals related to cell growth and differentiation. Ret mutant mice show abnormal retinal activity and abnormal levels and morphology of bipolar cells, yet die on the 21st day after birth as a result of renal underdevelopment. To extend the observation period, we generated the Ret conditional knockout Chx10-Cre;C-Ret lx/lx mouse model and analyzed the retinal function and morphological changes in mature and aging Chx10-Cre;C-Ret lx/lx mice. Methods: Retina-specific depletion of Ret was achieved using mice with floxed alleles of the Ret gene with CHX10-driven Cre recombinase; floxed mice without Cre expression were used as controls. Retinal function was examined using electroretinography (ERG), and 2-, 4-, 12-, and 24-month-old mice were analyzed by hematoxylin staining and immunohistochemistry to evaluate retinal morphological alterations. The ultrastructure of photoreceptor synapses was evaluated using electron microscopy. Results: The results of the ERG testing showed that b-wave amplitudes were reduced in Chx10-Cre;C-Ret lx/lx mice, whereas a-waves were not affected. A histopathological analysis revealed a thinner and disorganized outer plexiform layer at the ages of 12 and 24 months in Chx10-Cre;C-Ret lx/lx mice. Moreover, the data provided by immunohistochemistry showed defects in the synapses of photoreceptor cells. This result was confirmed at the ultrastructural level, thus supporting the participation of Ret in the morphological changes of the synaptic ribbon. Conclusion: Our results provide evidence of the role of Ret in maintaining the function of the retina, which was essential for preserving the structure of the synaptic ribbon and supporting the integrity of the outer plexiform layer.

Genetic interplay between transcription factor Pou4f1/Brn3a and neurotrophin receptor Ret in retinal ganglion cell type specification

Background

While the transcriptional code governing retinal ganglion cell (RGC) type specification begins to be understood, its interplay with neurotrophic signaling is largely unexplored. In mice, the transcription factor Brn3a/Pou4f1 is expressed in most RGCs, and is required for the specification of RGCs with small dendritic arbors. The Glial Derived Neurotrophic Factor (GDNF) receptor Ret is expressed in a subset of RGCs, including some expressing Brn3a, but its role in RGC development is not defined.

Methods

Here we use combinatorial genetic experiments using conditional knock-in reporter alleles at the Brn3a and Ret loci, in combination with retina- or Ret specific Cre drivers, to generate complete or mosaic genetic ablations of either Brn3a or Ret in RGCs. We then use sparse labelling to investigate Brn3a and Ret gene dosage effects on RGC dendritic arbor morphology. In addition, we use immunostaining and/or gene expression profiling by RNASeq to identify transcriptional targets relevant for the potential Brn3a-Ret interaction in RGC development.

Results

We find that mosaic gene dosage manipulation of the transcription factor Brn3a/Pou4f1 in neurotrophic receptor Ret heterozygote RGCs results in altered cell fate decisions and/or morphological dendritic defects. Specific RGC types are lost if Brn3a is ablated during embryogenesis and only mildly affected by postnatal Brn3a ablation. Sparse but not complete Brn3a heterozygosity combined with complete Ret heterozygosity has striking effects on RGC type distribution. Brn3a only mildly modulates Ret transcription, while Ret knockouts exhibit slightly skewed Brn3a and Brn3b expression during development that is corrected by adult age. Brn3a loss of function modestly but significantly affects distribution of Ret co-receptors GFRα1-3, and neurotrophin receptors TrkA and TrkC in RGCs.

Conclusions

Based on these observations, we propose that Brn3a and Ret converge onto developmental pathways that control RGC type specification, potentially through a competitive mechanism requiring signaling from the surrounding tissue.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13064-021-00155-z.

Long-term post-mortem studies following neurturin gene therapy in patients with advanced Parkinson's disease

We performed post-mortem studies on two patients with advanced Parkinson’s disease 8 and10 years following AAV2-neurturin (CERE120) gene therapy, the longest post-mortem trophic factor gene therapy cases reported to date. CERE120 was delivered to the putamen bilaterally in one case (10 years post-surgery), and to the putamen plus the substantia nigra bilaterally in the second (8 years post-surgery). In both patients there was persistent, albeit limited, neurturin expression in the putamen covering ∼3–12% of the putamen. In the putamen, dense staining of tyrosine hydroxylase-positive fibres was observed in areas that contained detectable neurturin expression. In the substantia nigra, neurturin expression was detected in 9.8–18.95% and 22.02–39% of remaining melanin-containing neurons in the patient with putamenal and combined putamenal and nigral gene delivery, respectively. Melanized neurons displayed intense tyrosine hydroxylase and RET proto-oncogene expression in nigral neurons in the patient where CERE120 was directly delivered to the nigra. There was no difference in the degree of Lewy pathology in comparison to untreated control patients with Parkinson’s disease, and α-synuclein aggregates were detected in neurons that also stained for neurturin, RET, and tyrosine hydroxylase. These changes were not associated with antiparkinsonian benefits likely due to the limited neurturin expression. This study provides the longest term evidence of persistent transgene expression following gene delivery to the CNS and the first human results when targeting both the terminal fields in the putamen as well as the originating nigral neurons.

Small-Molecule Ligands that Bind the RET Receptor Activate Neuroprotective Signals Independent of but Modulated by Coreceptor GFRα1

Glial cell line-derived neurotrophic factor (GDNF) binds the GFRα1 receptor, and the GDNF-GFRα1 complex binds to and activates the transmembrane RET tyrosine kinase to signal through intracellular Akt/Erk pathways. To dissect the GDNF-GFRα1-RET signaling complex, agents that bind and activate RET directly and independently of GFRα1 expression are valuable tools. In a focused naphthalenesulfonic acid library from the National Cancer Institute database, we identified small molecules that are genuine ligands binding to the RET extracellular domain. These ligands activate RET tyrosine kinase and afford trophic signals irrespective of GFRα1 coexpression. However, RET activation by these ligands is constrained by GFRα1, likely via an allosteric mechanism that can be overcome by increasing RET ligand concentration. In a mouse model of retinitis pigmentosa, monotherapy with a small-molecule RET agonist activates survival signals and reduces neuronal death significantly better than GDNF, suggesting therapeutic potential. SIGNIFICANCE STATEMENT: A genuine ligand of RET receptor ectodomain was identified, which acts as an agonist. Binding and agonism are independent of a coreceptor glial cell line-derived neurotrophic factor family receptor α, which is required by the natural growth factor glial cell line-derived neurotrophic factor, and are selective for cells expressing RET. The lead agent protects neurons from death in vivo. This work validates RET receptor as a druggable therapeutic target and provides for potential leads to evaluate in neurodegenerative states. We also report problems that arise when screening chemical libraries.

Ret and Substrate-Derived TGF-β Maverick Regulate Space-Filling Dendrite Growth in Drosophila Sensory Neurons

Dendrite morphogenesis is a highly regulated process that gives rise to stereotyped receptive fields, which are required for proper neuronal connectivity and function. Specific classes of neurons, including Drosophila class IV dendritic arborization (C4da) neurons, also feature complete space-filling growth of dendrites. In this system, we have identified the substrate-derived TGF-β ligand maverick (mav) as a developmental signal promoting space-filling growth through the neuronal Ret receptor. Both are necessary for radial spreading of C4da neuron dendrites, and Ret is required for neuronal uptake of Mav. Moreover, local changes in Mav levels result in directed dendritic growth toward regions with higher ligand availability. Our results suggest that Mav acts as a substrate-derived secreted signal promoting dendrite growth within not-yet-covered areas of the receptive field to ensure space-filling dendritic growth.

Effects of Neurotrophic Factors in Glial Cells in the Central Nervous System: Expression and Properties in Neurodegeneration and Injury

Astrocytes, oligodendrocytes, and microglia are abundant cell types found in the central nervous system and have been shown to play crucial roles in regulating both normal and disease states. An increasing amount of evidence points to the critical importance of glia in mediating neurodegeneration in Alzheimer’s and Parkinson’s diseases (AD, PD), and in ischemic stroke, where microglia are involved in initial tissue clearance, and astrocytes in the subsequent formation of a glial scar. The importance of these cells for neuronal survival has previously been studied in co-culture experiments and the search for neurotrophic factors (NTFs) initiated after finding that the addition of conditioned media from astrocyte cultures could support the survival of primary neurons in vitro. This led to the discovery of the potent dopamine neurotrophic factor, glial cell line-derived neurotrophic factor (GDNF). In this review, we focus on the relationship between glia and NTFs including neurotrophins, GDNF-family ligands, CNTF family, and CDNF/MANF-family proteins. We describe their expression in astrocytes, oligodendrocytes and their precursors (NG2-positive cells, OPCs), and microglia during development and in the adult brain. Furthermore, we review existing data on the glial phenotypes of NTF knockout mice and follow NTF expression patterns and their effects on glia in disease models such as AD, PD, stroke, and retinal degeneration.

Retinal de novo lipogenesis coordinates neurotrophic signaling to maintain vision

Membrane lipid composition is central to the highly specialized functions of neurological tissues. In the retina, abnormal lipid metabolism causes severe forms of blindness, often through poorly understood neuronal cell death. Here, we demonstrate that deleting the de novo lipogenic enzyme fatty acid synthase (FAS) from the neural retina, but not the vascular retina, results in progressive neurodegeneration and blindness with a temporal pattern resembling rodent models of retinitis pigmentosa. Blindness was not rescued by protection from light-evoked activity; by eating a diet enriched in palmitate, the product of the FAS reaction; or by treatment with the PPARα agonist fenofibrate. Vision loss was due to aberrant synaptic structure, blunted responsiveness to glial-derived neurotrophic factor and ciliary neurotrophic factor, and eventual apoptotic cell loss. This progressive neurodegeneration was associated with decreased membrane cholesterol content, as well as loss of discrete n-3 polyunsaturated fatty acid- and saturated fatty acid-containing phospholipid species within specialized membrane microdomains. Neurotrophic signaling was restored by exogenous cholesterol delivery. These findings implicate de novo lipogenesis in neurotrophin-dependent cell survival by maintaining retinal membrane configuration and lipid composition, and they suggest that ongoing lipogenesis may be required to prevent cell death in many forms of retinopathy.

Characterization of retinal ganglion cell, horizontal cell, and amacrine cell types expressing the neurotrophic receptor tyrosine kinase Ret

We report the retinal expression pattern of Ret, a receptor tyrosine kinase for the glial derived neurotrophic factor (GDNF) family ligands (GFLs), during development and in the adult mouse. Ret is initially expressed in retinal ganglion cells (RGCs), followed by horizontal cells (HCs) and amacrine cells (ACs), beginning with the early stages of postmitotic development. Ret expression persists in all three classes of neurons in the adult. Using RNA sequencing, immunostaining and random sparse recombination, we show that Ret is expressed in at least three distinct types of ACs, and ten types of RGCs. Using intersectional genetics, we describe the dendritic arbor morphologies of RGC types expressing Ret in combination with each of the three members of the POU4f/Brn3 family of transcription factors. Ret expression overlaps with Brn3a in 4 RGC types, with Brn3b in 5 RGC types, and with Brn3c in one RGC type, respectively. Ret⁺ RGCs project to the lateral geniculate nucleus (LGN), pretectal area (PTA) and superior colliculus (SC), and avoid the suprachiasmatic nucleus and accessory optic system. Brn3a⁺ Ret⁺ and Brn3c⁺ Ret⁺ RGCs project preferentially to contralateral retinorecipient areas, while Brn3b⁺ Ret⁺ RGCs shows minor ipsilateral projections to the olivary pretectal nucleus and the LGN. Our findings establish intersectional genetic approaches for the anatomic and developmental characterization of individual Ret⁺ RGC types. In addition, they provide necessary information for addressing the potential interplay between GDNF neurotrophic signaling and transcriptional regulation in RGC type specification.

Neuronal differentiation in the early human retinogenesis

AIM

Our study investigates the differentiation of retinal stem cells towards different neuronal subtypes during the critical period of human eye development.

METHODS

Expression of the neuronal marker neurofilament 200 (NF200), tyrosine hydroxilase (TH) and choline acetyltransferase (ChAT) was seen by immunofluorescence in the 5th-12th - week stage of development in the human eye. Data was analysed by Mann-Whitney, Kruskal-Wallis and Dunn's post hoc tests.

RESULTS

NF200, TH and ChAT cells appeared in the 5th/6th week and gradually increased during further development. The proportion of TH positive areas were distributed similarly to NF200, with a higher proportion in the outer neuroblastic layer. The proportion of a ChAT positive surface was highest in the 5th/6th - week whilst from the 7th week onwards, its proportion became higher in the optic nerve and inner neuroblastic layers than in the outer layer, where a decrease of ChAT positive areas were seen.

CONCLUSIONS

Our study indicates a high differentiation potential of early retinal cells, which decreased with the advancement of development. The observed great variety of retinal phenotypic expressions results from a large scale of influences, taking place at different developmental stages.

Neurotrophin receptor agonists and antagonists as therapeutic agents: An evolving paradigm

Neurodegenerative disorders are prevalent, complex and devastating conditions, with very limited treatment options currently available. While they manifest in many forms, there are commonalities that link them together. In this review, we will focus on neurotrophins - a family of related factors involved in neuronal development and maintenance. Neurodegenerative diseases often present with a neurotrophin imbalance, in which there may be decreases in trophic signaling through Trk receptors for example, and/or increases in pro-apoptotic activity through p75. Clinical trials with neurotrophins have continuously failed due to their poor pharmacological properties as well as the unavoidable activation of p75. Thus, there is a need for drugs without such setbacks. Small molecule neurotrophin mimetics are favorable options since they can selectively activate Trks or inactivate p75. In this review, we will initially present a brief outline of how these molecules are synthesized and their mechanisms of action; followed by an update in the current state of neurotrophins and small molecules in major neurodegenerative diseases. Although there has been significant progress in the development of potential therapeutics, more studies are needed to establish clear mechanisms of action and target specificity in order to transition from animal models to the assessment of safety and use in humans.

Multiple Endocrine Neoplasia: Genetics and Clinical Management

Early diagnosis of multiple endocrine neoplasia (MEN) syndromes is critical for optimal clinical outcomes; before the MEN syndromes can be diagnosed, they must be suspected. Genetic testing for germline alterations in both the MEN type 1 (MEN1) gene and RET proto-oncogene is crucial to identifying those at risk in affected kindreds and directing timely surveillance and surgical therapy to those at greatest risk of potentially life-threatening neoplasia. Pancreatic, thymic, and bronchial neuroendocrine tumors are the leading cause of death in patients with MEN1 and should be aggressively considered by at least biannual computed tomography imaging.

Artemin augments survival and axon regeneration in axotomized retinal ganglion cells

Artemin, a recently discovered member of the glial cell line-derived neurotrophic factor (GDNF) family, has neurotrophic effects on damaged neurons, including sympathetic neurons, dopamine neurons, and spiral ganglion neurons both in vivo and in vitro. However, its effects on retinal cells and its intracellular signaling remain relatively unexplored. During development, expression of GFRα3, a specific receptor for artemin, is strong in the immature retina and gradually decreases during maturation, suggesting a possible role in the formation of retinal connections. Optic nerve damage in mature rats causes levels of GFRα3 mRNA to increase tenfold in the retina within 3 days. GFRα3 mRNA levels continue to rise within the first week and then decline. Artemin, a specific ligand for GFRα3, has a neuroprotective effect on axotomized retinal ganglion cells (RGCs) in vivo and in vitro via activation of the extracellular signal-related kinase- and phosphoinositide 3-kinase-Akt signaling pathways. Artemin also has a substantial effect on axon regeneration in RGCs both in vivo and in vitro, whereas other GDNF family members do not. Therefore, artemin/GFRα3, but not other GDNF family members, may be of value for optic nerve regeneration in mature mammals.

Effect of neurturin deficiency on cholinergic and catecholaminergic innervation of the murine eye

Neurturin (NRTN) is a neurotrophic factor required for the development of many parasympathetic neurons and normal cholinergic innervation of the heart, lacrimal glands and numerous other tissues. Previous studies with transgenic mouse models showed that NRTN is also essential for normal development and function of the retina (J. Neurosci. 28:4123-4135, 2008). NRTN knockout (KO) mice exhibit a marked thinning of the outer plexiform layer (OPL) of the retina, with reduced abundance of horizontal cell dendrites and axons, and aberrant projections of horizontal cells and bipolar cells into the outer nuclear layer. The effects of NRTN deletion on specific neurotransmitter systems in the retina and on cholinergic innervation of the iris are unknown. To begin addressing this deficiency, we used immunohistochemical methods to study cholinergic and noradrenergic innervation of the iris and the presence and localization of cholinergic and dopaminergic neurons and nerve fibers in eyes from adult male wild-type (WT) and NRTN KO mice (age 4-6 months). Mice were euthanized, and eyes were removed and fixed in cold neutral buffered formalin or 4% paraformaldehyde. Formalin-fixed eyes were embedded in paraffin, and 5 μm cross-sections were collected. Representative sections were stained with hematoxylin and eosin or processed for fluorescence immunohistochemistry after treatment for antigen retrieval. Whole mount preparations were dissected from paraformaldehyde fixed eyes and used for immunohistochemistry. Cholinergic and catecholaminergic nerve fibers were labeled with primary antibodies to the vesicular acetylcholine transporter (VAChT) and tyrosine hydroxylase (TH), respectively. Cholinergic and dopaminergic cell bodies were labeled with antibodies to choline acetyltransferase (ChAT) and TH, respectively. Cholinergic innervation of the mouse iris was restricted to the sphincter region, and noradrenergic fibers occurred throughout the iris and in the ciliary processes. This pattern was unaffected by deletion of NRTN. Furthermore, functional experiments demonstrated that cholinergic regulation of the pupil diameter was retained in NRTN KO mice. Hematoxylin and eosin stains of the retina confirmed a marked thinning of the OPL in KO mice. VAChT and ChAT staining of the retina revealed two bands of cholinergic processes in the inner plexiform layer, and these were unaffected by NRTN deletion. Likewise, NRTN deletion did not affect the abundance of ChAT-positive ganglion and amacrine cells. In marked contrast, staining for TH showed an increased abundance of dopaminergic processes in the OPL of retina from KO mice. Staining of retinal whole mounts for TH showed no difference in the abundance of dopaminergic amacrine cells between WT and KO mice. These findings demonstrate that the neurotrophic factor NRTN is not required for the development or maintenance of cholinergic innervation of the iris, cholinergic control of pupil diameter, or for development of cholinergic and dopaminergic amacrine cells of the retina. However, NRTN deficiency causes a marked reduction in the size of the OPL and aberrant growth of dopaminergic processes into this region.

Gene expression changes in aging retinal microglia: relationship to microglial support functions and regulation of activation

Microglia, the resident immune cells of the central nervous system (CNS), are thought to contribute to the pathogenesis of age-related neurodegenerative disorders. It has been hypothesized that microglia undergo age-related changes in gene expression patterns that give rise to pathogenic phenotypes. We compared the gene expression profiles in microglia isolated ex vivo from the retinas of mice ranging from early adulthood to late senescence. We discovered that microglial gene expression demonstrated progressive change with increasing age, and involved genes that regulate microglial supportive functions and immune activation. Molecular pathways involving immune function and regulation, angiogenesis, and neurotrophin signaling demonstrated age-related change. In particular, expression levels of complement genes, C3 and CFB, previously associated with age-related macular degeneration (AMD), increased with aging, suggesting that senescent microglia may contribute to complement dysregulation during disease pathogenesis. Taken together, senescent microglia demonstrate age-related gene expression changes capable of altering their constitutive support functions and regulation of their activation status in ways relating to neuroinflammation and neurodegeneration in the CNS.

What can we learn about stroke from retinal ischemia models?

Retinal ischemia is a very useful model to study the impact of various cell death pathways, such as apoptosis and necrosis, in the ischemic retina. However, it is important to note that the retina is formed as an outpouching of the diencephalon and is part of the central nervous system. As such, the cell death pathways initiated in response to ischemic damage in the retina reflect those found in other areas of the central nervous system undergoing similar trauma. The retina is also more accessible than other areas of the central nervous system, thus making it a simpler model to work with and study. By utilizing the retinal model, we can greatly increase our knowledge of the cell death processes initiated by ischemia which lead to degeneration in the central nervous system. This paper examines work that has been done so far to characterize various aspects of cell death in the retinal ischemia model, such as various pathways which are activated, and the role neurotrophic factors, and discusses how these are relevant to the treatment of ischemic damage in both the retina and the greater central nervous system.

G-protein betagamma-complex is crucial for efficient signal amplification in vision

A fundamental question of cell signaling biology is how faint external signals produce robust physiological responses. One universal mechanism relies on signal amplification via intracellular cascades mediated by heterotrimeric G-proteins. This high amplification system allows retinal rod photoreceptors to detect single photons of light. Although much is now known about the role of the α-subunit of the rod-specific G-protein transducin in phototransduction, the physiological function of the auxiliary βγ-complex in this process remains a mystery. Here, we show that elimination of the transducin γ-subunit drastically reduces signal amplification in intact mouse rods. The consequence is a striking decline in rod visual sensitivity and severe impairment of nocturnal vision. Our findings demonstrate that transducin βγ-complex controls signal amplification of the rod phototransduction cascade and is critical for the ability of rod photoreceptors to function in low light conditions.

Optic nerve dysfunction in a mouse model of neurofibromatosis-1 optic glioma

Individuals with neurofibromatosis type 1 (NF1) are prone to develop optic pathway gliomas that can result in significant visual impairment. To explore the cellular basis for the reduced visual function resulting from optic glioma formation, we used a genetically engineered mouse model of Nf1 optic glioma (Nf1+/-(GFAP)CKO mice). We performed multimodal functional and structural analyses both before and after the appearance of macroscopic tumors. At 6 weeks of age, before obvious glioma formation, Nf1+/-(GFAP)CKO mice had decreased visual-evoked potential amplitudes and increased optic nerve axon calibers. By 3 months of age, Nf1+/-(GFAP)CKO mice exhibited pronounced optic nerve axonopathy and apoptosis of neurons in the retinal ganglion cell layer. Magnetic resonance diffusion tensor imaging showed a progressive increase in radial diffusivity between 6 weeks and 6 months of age in the optic nerve proximal to the tumor indicating ongoing deterioration of axons. These data suggest that optic glioma formation results in early axonal disorganization and damage, which culminates in retinal ganglion cell death. Collectively, this study shows that Nf1+/-(GFAP)CKO mice can provide a useful model for defining mechanisms of visual abnormalities in children with NF1 and lay the foundations for future interventional studies aimed at reducing visual loss.

Sominone enhances neurite outgrowth and spatial memory mediated by the neurotrophic factor receptor, RET

BACKGROUND AND PURPOSE

Orally administered withanoside IV (a compound isolated from the roots of Withania somnifera) improved memory deficits in mice with a model of Alzheimer's disease induced by the amyloid peptide Abeta(25-35). Sominone, an aglycone of withanoside IV, was identified as an active metabolite after oral administration of withanoside IV. We aimed to identify receptors or associated molecules of sominone, and to investigate the effects of sominone on memory in normal mice.

EXPERIMENTAL APPROACH

Phosphorylation levels of 71 molecules were compared between control and sominone-stimulated cortical cultured cells to search for target molecules of sominone. Object location memory and neurite density in the brain were evaluated in sominone-injected mice.

KEY RESULTS

Phosphorylation of RET (a receptor for the glial cell line-derived neurotrophic factor, GDNF) was increased in neurons by sominone, without affecting the synthesis and secretion of GDNF. Knockdown of RET prevented sominone-induced outgrowths of axons and dendrites. After a single i.p. injection of sominone into normal mice, they could better memorize scenery information than control mice. Sixty minutes after sominone injection, RET phosphorylation was increased, particularly in the hippocampus of mice. After the memory tests, the densities of axons and dendrites were increased in the hippocampus by sominone administration.

CONCLUSIONS AND IMPLICATIONS

Sominone could reinforce the morphological plasticity of neurons by activation of the RET pathway and thus enhance memory. Sominone, a compound with low molecular weight, may be a GDNF-independent stimulator of the RET pathway and/or a novel modulator of RET signalling.