Müller glia factors induce survival and neuritogenesis of peripheral and central neurons

The Mlc1 Promoter Directs Müller Cell-specific Gene Expression in the Retina

Purpose

Because the importance of glia in regulating brain functions has been demonstrated, genetic technologies that manipulate glial cell-specific gene expression in the brain have become essential and have made great progress. However, it is unknown whether the same strategy that is used in the brain can be applied to the retina because retinal glia differs from glia in the brain. Here, we aimed to find a method for selective gene expression in Müller cells (characteristic glial cells in the retina) and identified Mlc1 as a specific promoter of Müller cells.

Methods

Mlc1-tTA::Yellow-Cameleon-Nano^tetO/tetO (YC-Nano) mice were used as a reporter line. YC-Nano, a fluorescent protein, was ectopically expressed in the cell type controlled by the Mlc1 promotor. Immunofluorescence staining was used to identify the cell type expressing YC-Nano protein.

Results

YC-Nano-positive (⁺) signals were observed as vertical stalks in the sliced retina and spanned from the nerve fiber layer through the outer nuclear layer. The density of YC-Nano⁺ cells was higher around the optic nerve head and lower in the peripheral retina. The YC-Nano⁺ signals colocalized with vimentin, a marker of Müller cells, but not with the cell markers for blood vessels, microglia, neurons, or astrocytes.

Conclusions

The Mlc1 promoter allows us to manipulate gene expression in Müller cells without affecting astrocytes in the retina.

Translational Relevance

Gene manipulation under control of Mlc1 promoter offers novel technique to investigate the role of Müller cells.

The effect of extrinsic Wnt/β-catenin signaling in Muller glia on retinal ganglion cell neurite growth

Muller glia are the predominant glial cell type in the retina, and they structurally and metabolically support retinal neurons. Wnt/β-catenin signaling pathways play essential roles in the central nervous system, including glial and neuronal differentiation, axonal growth, and neuronal regeneration. We previously demonstrated that Wnt signaling activation in retinal ganglion cells (RGC) induces axonal regeneration after injury. However, whether Wnt signaling within the adjacent Muller glia plays an axongenic role is not known. In this study, we characterized the effect of Wnt signaling in Muller glia on RGC neurite growth. Primary Muller glia and RGC cells were grown in transwell co-cultures and adenoviral constructs driving Wnt regulatory genes were used to activate and inhibit Wnt signaling specifically in primary Muller glia. Our results demonstrated that activation of Wnt signaling in Muller glia significantly increased RGC average neurite length and branch site number. In addition, the secretome of Muller glia after induction or inhibition of Wnt signaling was characterized using protein profiling of conditioned media by Q Exactive mass spectrometry. The Muller glia secretome after activation of Wnt signaling had distinct and more numerous proteins involved in regulation of axon extension, axon projection and cell adhesion. Furthermore, we showed highly redundant expression of Wnt signaling ligands in Muller glia and Frizzled receptors in RGCs and Muller glia. Therefore, this study provides new information about potential neurite growth promoting molecules in the Muller glia secretome, and identified Wnt-dependent target proteins that may mediate the axonal growth.

Evidence of Müller Glia Conversion Into Retina Ganglion Cells Using Neurogenin2

Degenerative retinopathies are the leading causes of irreversible visual impairment in the elderly, affecting hundreds of millions of patients. Müller glia cells (MGC), the main type of glia found in the vertebrate retina, can resume proliferation in the rodent adult injured retina but contribute weakly to tissue repair when compared to zebrafish retina. However, postnatal and adult mouse MGC can be genetically reprogrammed through the expression of the transcription factor (TF) Achaete-scute homolog 1 (ASCL1) into induced neurons (iNs), displaying key hallmarks of photoreceptors, bipolar and amacrine cells, which may contribute to regenerate the damaged retina. Here, we show that the TF neurogenin 2 (NEUROG2) is also sufficient to lineage-reprogram postnatal mouse MGC into iNs. The efficiency of MGC lineage conversion by NEUROG2 is similar to that observed after expression of ASCL1 and both TFs induce the generation of functionally active iNs. Treatment of MGC cultures with EGF and FGF2 prior to Neurog2 or Ascl1 expression enhances reprogramming efficiencies, what can be at least partially explained by an increase in the frequency of MGCs expressing sex determining region Y (SRY)-box 2 (SOX2). Transduction of either Neurog2 or Ascl1 led to the upregulation of key retina neuronal genes in MGC-derived iNs, but only NEUROG2 induced a consistent increase in the expression of putative retinal ganglion cell (RGC) genes. Moreover, in vivo electroporation of Neurog2 in late progenitors from the neonatal rat retina, which are transcriptionally similar to MGCs, also induced a shift in the generation of retinal cell subtypes, favoring neuronal differentiation at the expense of MGCs and resuming the generation of RGCs. Altogether, our data indicate that NEUROG2 induces lineage conversion of postnatal rodent MGCs into RGC-like iNs in vitro and resumes the generation of this neuronal type from late progenitors of the retina in vivo.

Endogenous spontaneous ultraweak photon emission in the formation of eye-specific retinogeniculate projections before birth

In 1963, it was suggested [Sperry, R.W. (1963). Chemoaffinity in the orderly growth of nerve fiber patterns and connections. Proc. Natl. Acad. Sci. USA 50, 703-710.] that molecular cues can direct the development of orderly connections between the eye and the brain (the "chemoaffinity hypothesis"). In the same year, the amazing degree of functional accuracy of the visual pathway in the absence of any external light/photon perception prior to birth [Wiesel, T.N and Hubel, D.H. (1963). Single-cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol. 26, 1003-1017.] was discovered. These recognitions revealed that the wiring of the visual system relies on innate cues. However, how the eye-specific retinogeniculate pathway can be developed before birth without any visual experience is still an unresolved issue. In the present paper, we suggest that Müller cells (functioning as optical fibers), Müller cell cone (i.e. the inner half of the foveola that is created of an inverted cone-shaped zone of Müller cells), discrete retinal noise of rods, and intrinsically photosensitive retinal ganglion cells might have key functions by means of retinal spontaneous ultraweak photon emission in the development of eye-specific retinogeniculate pathways prior to birth.

A Ser75-to-Asp phospho-mimicking mutation in Src accelerates ageing-related loss of retinal ganglion cells in mice

Src knockout mice show no detectable abnormalities in central nervous system (CNS) post-mitotic neurons, likely reflecting functional compensation by other Src family kinases. Cdk1- or Cdk5-dependent Ser75 phosphorylation in the amino-terminal Unique domain of Src, which shares no homology with other Src family kinases, regulates the stability of active Src. To clarify the roles of Src Ser75 phosphorylation in CNS neurons, we established two types of mutant mice with mutations in Src: phospho-mimicking Ser75Asp (SD) and non-phosphorylatable Ser75Ala (SA). In ageing SD/SD mice, retinal ganglion cell (RGC) number in whole retinas was significantly lower than that in young SD/SD mice in the absence of inflammation and elevated intraocular pressure, resembling the pathogenesis of progressive optic neuropathy. By contrast, SA/SA mice and wild-type (WT) mice exhibited no age-related RGC loss. The age-related retinal RGC number reduction was greater in the peripheral rather than the mid-peripheral region of the retina in SD/SD mice. Furthermore, Rho-associated kinase activity in whole retinas of ageing SD/SD mice was significantly higher than that in young SD/SD mice. These results suggest that Src regulates RGC survival during ageing in a manner that depends on Ser75 phosphorylation.

Effect of Müller cells on the survival and neuritogenesis in retinal ganglion cells

OBJECTIVE

Retinal ganglion cells (RGCs) are the first affected cells in neuropathies like glaucoma, for that reason it is very important to explore new methods to neuroprotect these neurons. Müller cells are glial cells that provide the neurons with trophic factors and scaffold. The purpose of this study was to analyze the effect of Müller cells on survival and neurite formation in RGCs.

METHOD

Rat Müller cells were grown until a confluent culture on which rat RGCs were added, using pure culture of rat RGCs as controls. RGCs were labeled with βIII-tubulin, and Müller cells with glutamine synthetase antibodies. In addition, nuclei were stained with DAPI. The number of RGCs and number and neurite length were measured.

RESULTS

No differences were found in the number of RGCs between control and cells grown on the substrate of Müller cells. The proportion of RGCs with neurites increased when they grew on Müller (RGCs with 1-3 neurites increased from 19% to 43%. The length of neurites also increased in RGCs grown on Müller cells, with the number of RGCs with neurites from 50 to 200μm increasing from 21% to 41%, and with neurites of more than 200μm the increase was from 6% to 20%.

CONCLUSIONS

Müller cells support the survival of RGCs and induced an increase in the number and length of neurites of RGCs.

Transplantation of photoreceptors derived from human Muller glia restore rod function in the P23H rat

Müller glia possess stem cell characteristics that have been recognized to be responsible for the regeneration of injured retina in fish and amphibians. Although these cells are present in the adult human eye, they are not known to regenerate human retina in vivo. Human Müller glia with stem cell characteristics (hMSCs) can acquire phenotypic and genotypic characteristics of rod photoreceptors in vitro, suggesting that they may have potential for use in transplantation strategies to treat human photoreceptor degenerations. Much work has been undertaken in rodents using various sources of allogeneic stem cells to restore photoreceptor function, but the effect of human Müller glia-derived photoreceptors in the restoration of rod photoreceptor function has not been investigated. This study aimed to differentiate hMSCs into photoreceptor cells by stimulation with growth and differentiation factors in vitro to upregulate gene and protein expression of CRX, NR2E3, and rhodopsin and various phototransduction markers associated with rod photoreceptor development and function and to examine the effect of subretinal transplantation of these cells into the P23H rat, a model of primary photoreceptor degeneration. Following transplantation, hMSC-derived photoreceptor cells migrated and integrated into the outer nuclear layer of the degenerated retinas and led to significant improvement in rod photoreceptor function as shown by an increase in a-wave amplitude and slope using scotopic flash electroretinography. These observations suggest that hMSCs can be regarded as a cell source for development of cell-replacement therapies to treat human photoreceptor degenerations and may also offer potential for the development of autologous transplantation.

Characterization of NGF, trkA (NGFR) , and p75 (NTR) in Retina of Mice Lacking Reelin Glycoprotein

Both Reelin and Nerve Growth Factor (NGF) exert crucial roles in retinal development. Retinogenesis is severely impaired in E-reeler mice, a model of Reelin deficiency showing specific Green Fluorescent Protein expression in Rod Bipolar Cells (RBCs). Since no data are available on Reelin and NGF cross-talk, NGF and trkA^NGFR/ p75^NTR expression was investigated in retinas from E-reeler versus control mice, by confocal microscopy, Western blotting, and real time PCR analysis. A scattered increase of NGF protein was observed in the Ganglion Cell Layer and more pronounced in the Inner Nuclear Layer (INL). A selective increase of p75^NTR was detected in most of RBCs and in other cell subtypes of INL. On the contrary, a slight trend towards a decrease was detected for trkA^NGFR, albeit not significant. Confocal data were validated by Western blot and real time PCR. Finally, the decreased trkA^NGFR/ p75^NTR ratio, representative of p75^NTR increase, significantly correlated with E-reeler versus E-control. These data indicate that NGF-trkA^NGFR/ p75^NTR is affected in E-reeler retina and that p75^NTR might represent the main NGF receptor involved in the process. This first NGF-trkA^NGFR/ p75^NTR characterization suggests that E-reeler might be suitable for exploring Reelin-NGF cross-talk, representing an additional information source in those pathologies characterized by retinal degeneration.

Hydrocortisone stimulates neurite outgrowth from mouse retinal explants by modulating macroglial activity

PURPOSE

There is mounting evidence that retinal ganglion cells (RGCs) require a complex milieu of trophic factors to enhance cell survival and axon regeneration after optic nerve injury. The authors' goal was to examine the contribution of components of a combination of hormones, growth factors, steroids, and small molecules to creating a regenerative environment and to determine if any of these components modulated macroglial behavior to aid in regeneration.

METHODS

Postnatal day 7 mouse retinal explants embedded in collagen were used as an in vitro model of neurite regeneration. Explants were treated with the culture supplements fetal bovine serum, N2, and G5 and a mixture of G5 and N2 components, designated enhanced N2 (EN2). Explants were evaluated for neurite outgrowth over 7 days in culture. The effects of each treatment were also evaluated on cultured RGCs purified by Thy1 immunopanning. Immunohistochemistry and qPCR analysis were used to evaluate differences in gene expression in the explants due to different treatments.

RESULTS

EN2 stimulated significant neurite outgrowth from explants but not from purified RGCs. Elimination of hydrocortisone (HC) from EN2 reduced the mean neurites per explant by 37%. EN2-treated explants demonstrated increased expression of Gfap, Glul, Glt1, Cntf, Pedf, and VegfA compared with explants treated with EN2 without HC. Subsequent experiments showed that increased expression of Cntf and Glul was critical to the trophic effect of HC.

CONCLUSIONS

These data suggest that the HC in EN2 indirectly contributed to neurite outgrowth by activating macroglia to produce neurotrophic and neuroprotective molecules.

Roles of BDNF in spinal neuroplasticity in cats subjected to partial dorsal ganglionectomy

This study investigated the role of brain-derived neurotrophic factor (BDNF) in neuroplasticity in cats subjected to the removal of dorsal root ganglia (DRG). Following partial ganglionectomy, the number of BDNF-positive varicosities from spared L6 DRG decreased significantly. This reduction was observed at 3 days post operation (dpo) in spinal lamina II of L3 and L5. Whereas the percentages of positive neurons for BDNF and its mRNA in spared L6 DRG at 10 dpo were significantly increased, and accumulated BDNF was seen on the DRG side of the ligated axons. Importantly, BDNF antibody neutralization in vivo results in a significant reduction in the number of varicosities in spinal lamina II, evidenced by BDNF and calcitonin gene-related peptide immunohistochemical staining. These findings suggested that peripheral-derived BDNF could play a critical role in spinal neuroplasticity in cats subjected to partial ganglionectomy. This may underlie the basis of molecular therapy depending on gene drug-like BDNF release.

Brain-derived neurotrophic factor and its receptors in Bergmann glia cells

Brain-derived neurotrophic factor is an abundant and widely distributed neurotrophin expressed in the Central Nervous System. It is critically involved in neuronal differentiation and survival. The expression of brain-derived neurotrophic factor and that of its catalytic active cognate receptor (TrkB) has been extensively studied in neuronal cells but their expression and function in glial cells is still controversial. Despite of this fact, brain-derived neurotrophic factor is released from astrocytes upon glutamate stimulation. A suitable model to study glia/neuronal interactions, in the context of glutamatergic synapses, is the well-characterized culture of chick cerebellar Bergmann glia cells. Using, this system, we show here that BDNF and its functional receptor are present in Bergmann glia and that BDNF stimulation is linked to the activation of the phosphatidyl-inositol 3 kinase/protein kinase C/mitogen-activated protein kinase/Activator Protein-1 signaling pathway. Accordingly, reverse transcription-polymerase chain reaction (RT-PCR) experiments predicted the expression of full-length and truncated TrkB isoforms. Our results suggest that Bergmann glia cells are able to express and respond to BDNF stimulation favoring the notion of their pivotal role in neuroprotection.

Secreted human amyloid precursor protein binds semaphorin 3a and prevents semaphorin-induced growth cone collapse

The amyloid precursor protein (APP) is well known for giving rise to the amyloid-β peptide and for its role in Alzheimer's disease. Much less is known, however, on the physiological roles of APP in the development and plasticity of the central nervous system. We have used phage display of a peptide library to identify high-affinity ligands of purified recombinant human sAPPα₆₉₅ (the soluble, secreted ectodomain from the main neuronal APP isoform). Two peptides thus selected exhibited significant homologies with the conserved extracellular domain of several members of the semaphorin (Sema) family of axon guidance proteins. We show that sAPPα₆₉₅ binds both purified recombinant Sema3A and Sema3A secreted by transfected HEK293 cells. Interestingly, sAPPα₆₉₅ inhibited the collapse of embryonic chicken (Gallus gallus domesticus) dorsal root ganglia growth cones promoted by Sema3A (K_d≤8·10⁻⁹ M). Two Sema3A-derived peptides homologous to the peptides isolated by phage display blocked sAPPα binding and its inhibitory action on Sema3A function. These two peptides are comprised within a domain previously shown to be involved in binding of Sema3A to its cellular receptor, suggesting a competitive mechanism by which sAPPα modulates the biological action of semaphorins.

Otx2 promotes the survival of damaged adult retinal ganglion cells and protects against excitotoxic loss of visual acuity in vivo

Retinal ganglion cells (RGCs) are the projection neurons from the eye to the brain and their loss results in visual impairment in a number of diseases. Transcription factors with a homeodomain can translocate between cells and, in at least one reported case, can stimulate neuronal survival. Otx2 is a homeoprotein transcription factor expressed in the retina that is taken up by RGCs. We thus hypothesized that Otx2 capture could regulate the survival of adult RGCs. We report that Otx2 stimulates the survival of adult mouse and rat RGCs in vitro and protects RGCs against NMDA-induced toxicity in vivo in mice. In the latter model, Otx2 also preserves visual acuity.

Functional identification of cell phenotypes differentiating from mice retinal neurospheres using single cell calcium imaging

Degeneration of neural retina causes vision impairment and can lead to blindness. Neural stem and progenitor cells might be used as a tool directed to regenerative medicine of the retina. Here, we describe a novel platform for cell phenotype-specific drug discovery and screening of proneurogenic factors, able to boost differentiation of neural retinal progenitor cells. By using single cell calcium imaging (SCCI) and a rational-based stimulation protocol, a diversity of cells emerging from differentiated retinal neurosphere cultures were identified. Exposure of retinal progenitor cultures to KCl or to α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) stimulated Ca(2+) transients in microtubule-associated protein 2 (MAP-2) positive neurons. Doublecortin (DCX) and polysialated neural cell adhesion molecule (PSA-NCAM) positive neuroblasts were distinguished from differentiated neurons on the basis of their response to muscimol. Ca(2+) fluxes in glial fibrillary acidic protein (GFAP) or glutamine synthetase (GS) positive cells were induced by ATP. To validate the platform, neurospheres were treated with brain-derived neurotrophic factor (BDNF) (proneurogenic) or ciliary neurotrophic factor (CNTF) (gliogenic factor). BDNF increased the percentage of differentiated cells expressing Tuj-1 sensitive to KCl or AMPA and reduced the population of cells responding to muscimol. CNTF exposure resulted in a higher number of cells expressing GFAP responding to ATP. All together, our data may open new perspectives for cell type-specific discovery of drug targets and screening of novel proneurogenic factors to boost differentiation of neural retina cells to treat degenerative retinal diseases.

Trophic responsiveness of purified postnatal and adult rat retinal ganglion cells

Central neurons lose the ability for axonal re-growth during development and typically do not regenerate their axons following axotomy once they become mature unless given a growth-permissive environment i.e. peripheral nerve graft. In the present study, the growth responsiveness of purified retinal ganglion cells (RGCs) at different ages to neurotrophic factors and Schwann cell (SC)-secreted factors were examined directly. The purity of adult RGCs was 97% as assessed by retrograde labelling with 4,6-diamidino-2-phenylindole. The stability of cultures were demonstrated by long-term survival (30 days) in medium contained brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and forskolin (F) (BCF). RGCs from postnatal (P) (P0, P4, P8, P21) and adult (P90) rats showed decreasing levels of survival and neuritogenesis when grown in BCF. In contrast, the opposite was observed in SC-conditioned medium (CM)-treated P0-P8 RGCs which were increasingly responsive. SCCM induced maximal neurite outgrowth in P8 RGCs via the activation of extracellular regulated kinase 1/2 (Erk1/2). Inhibition of mitogen-activated protein kinase-Erk1/2 signaling using an Erk1/2-specific inhibitor (UO126) abolished SCCM-induced Erk1/2 phosphorylation and neuritogenesis completely. Although both SCCM and BCF failed to sustain the same levels of growth in P21 or P90 cultures as observed in P8 cultures, SCCM promoted higher survival and neuritogenesis than BCF-treated adult RGCs. This study is the first report of adult rat RGC purification and demonstrates that mature RGCs need multiple factors for survival and neurite outgrowth.

Activated retinal glia promote neurite outgrowth of retinal ganglion cells via apolipoprotein E

In the present study, we investigated the role and mechanism through which activated retinal glia stimulate retinal ganglion cell (RGC) neurite outgrowth. We have found that the level of retinal glial activation correlates directly with enhanced RGC neurite outgrowth after a preconditioning intravitreal Zymosan injection. Reduction in the number of activated glia in primary rat retinal cultures led to significantly reduced RGC neurite outgrowth. Glial-related neurite outgrowth appears to be, at least in part, mediated via apolipoprotein E (ApoE), which is expressed by activated retinal astrocytes and Müller glia. ApoE-deficient mice showed significantly reduced RGC neurite outgrowth potential after intravitreal Zymosan injection compared with age-matched wild-type animals. These observations suggest that ApoE, expressed by activated retinal glia, stimulates RGC neurite outgrowth after intravitreal Zymosan injection.

Optic nerve and vitreal inflammation are both RGC neuroprotective but only the latter is RGC axogenic

Intravitreal inflammation, induced by either lens injury, or intravitreal injection of zymosan (IVZ), protects RGC from apoptosis and stimulates axon regeneration after optic nerve transection. Here, we investigate the differential effects of intra-optic nerve zymosan (ONZ) and IVZ injections on RGC neuroprotection and axogenesis. After both IVZ and ONZ injection, zymosan-induced inflammation promoted a similar 4-/5-fold enhancement in RGC survival, compared to optic nerve transected controls, but only IVZ promoted RGC axon regeneration. IVZ was the most effective in activating retinal astrocyte/Müller cells while regulated intramembraneous proteolysis (RIP) of p75(NTR) and inactivation of Rho (key components of the axon growth inhibitory signalling cascade) occurred in both ONZ and IVZ, but only in the latter did RGC axons regenerate. We suggest that neuroprotective factors may be transported to RGC somata by retrograde transport after ONZ and diffuse into the retina after IVZ injection, but an axogenic agent is required to initiate and maintain disinhibited RGC axon regeneration that may be an exclusive property of a Müller cell-derived factor released after IVZ.

Off-target effects of epidermal growth factor receptor antagonists mediate retinal ganglion cell disinhibited axon growth

Inhibition of central nervous system axon growth is reportedly mediated in part by calcium-dependent phosphorylation of axonal epidermal growth factor receptor, with local administration of the epidermal growth factor receptor kinase inhibitors AG1478 and PD168393 to an optic nerve lesion site promoting adult retinal ganglion cell axon regeneration. Here, we show that epidermal growth factor receptor was neither constitutively expressed, nor activated in optic nerve axons in our non-regenerating and regenerating optic nerve injury models, a finding that is inconsistent with phosphorylated epidermal growth factor receptor-dependent intra-axonal signalling of central nervous system myelin-related axon growth inhibitory ligands. However, epidermal growth factor receptor was localized and activated within most glia in the retina and optic nerve post-injury, and thus an indirect glial-dependent mechanism for stimulated retinal ganglion cell axon growth by epidermal growth factor receptor inhibitors seemed plausible. Using primary retinal cultures with added central nervous system myelin extracts, we confirmed previous reports that AG1478/PD168393 blocks epidermal growth factor receptor activation and promotes disinhibited neurite outgrowth. Paradoxically, neurites did not grow in central nervous system myelin extract-containing cultures after short interfering ribonucleic acid-mediated knockdown of epidermal growth factor receptor. However, addition of AG1478 restored neurite outgrowth to short interfering ribonucleic acid-treated cultures, implying that epidermal growth factor receptor does not mediate AG1478-dependent effects. TrkA-/B-/C-Fc fusion proteins and the kinase blocker K252a abrogated the neuritogenic activity in these cultures, correlating with the presence of the neurotrophins brain derived neurotrophic factor, nerve growth factor and neurotrophin-3 in the supernatant and increased intracellular cyclic adenosine monophosphate activity. Neurotrophins released by AG1478 stimulated disinhibited retinal ganglion cell axon growth in central nervous system myelin-treated cultures by the induction of regulated intramembraneous proteolysis of p75(NTR) and Rho inactivation. Retinal astrocytes/Müller cells and retinal ganglion cells were the source of neurotrophins, with neurite outgrowth halved in the presence of glial inhibitors. We attribute AG1478-stimulated neuritogenesis to the induced release of neurotrophins together with raised cyclic adenosine monophosphate levels in treated cultures, leading to axon growth and disinhibition by neurotrophin-induced regulated intramembraneous proteolysis of p75(NTR). These off-target effects of epidermal growth factor receptor kinase inhibition suggest a novel therapeutic approach for designing treatments to promote central nervous system axon regeneration.

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.

Trophic activity derived from bone marrow mononuclear cells increases peripheral nerve regeneration by acting on both neuronal and glial cell populations

A rat model of complete sciatic nerve transection was used to evaluate the effect of bone marrow mononuclear cells (BMMC) transplanted to the injury site immediately after lesion. Rats treated with BMMC had both sensory and motor axons reaching the distal stump earlier compared to untreated animals. In addition, BMMC transplantation reduced cell death in dorsal root ganglia (DRG) compared to control animals. Transplanted BMMC remained in the lesion site for several days but there is no evidence of BMMC differentiation into Schwann cells. However, an increase in the number of Schwann cells, satellite cells and astrocytes was observed in the treated group. Moreover, neutralizing antibodies for nerve growth factor (NGF) (but not for brain-derived neurotrophic factor and ciliary-derived neurotrophic factor) added to the BMMC-conditioned medium reduced neurite growth of sensory and sympathetic neurons in vitro, suggesting that BMMC release NGF, improve regeneration of the sciatic nerve in the adult rat and stimulate Schwann and satellite cell proliferation or a combination of both.

Expression of functional dopaminergic phenotype in purified cultured Müller cells from vertebrate retina

Purified retina glial Müller cells can express the machinery for dopamine synthesis and release when maintained in culture. Dopamine is detected in cell extracts of cultures exposed to its precursor, L-DOPA. A large portion of synthesized dopamine is recovered in the superfusing medium showing the tendency of the accumulated dopamine to be released. Müller cells purified from developing chick and mouse retinas express L-DOPA decarboxylase (DDC; aromatic-L-amino-acid decarboxylase; EC 4.1.1.28) and the dopamine transporter DAT. The synthesis of dopamine from L-DOPA supplied to Müller cultures is inhibited by m-hydroxybenzylhydrazine, a DDC inhibitor. Dopamine release occurs via a transporter-mediated process and can activate dopaminergic D(1) receptors expressed by the glia population. The synthesis and release of dopamine were also observed in Müller cell cultures from mouse retina. Finally, cultured avian Müller cells display increased expression of tyrosine hydroxylase, under the influence of agents that increase cAMP levels, which results in higher levels of dopamine synthesized from tyrosine. A large proportion of glial cells in culture do express Nurr1 transcription factor, consistent with the dopaminergic characteristics displayed by these cells in culture. The results show that Müller cells, deprived of neuron influence, differentiate dopaminergic properties thought to be exclusive to neurons.