Subcellular second messenger networks drive distinct repellent-induced axon behaviors.

Second messengers, including cAMP, cGMP and Ca2+ are often placed in an integrating position to combine the extracellular cues that orient growing axons in the developing brain. This view suggests that axon repellents share the same set of cellular messenger signals and that axon attractants evoke opposite cAMP, cGMP and Ca2+ changes. Investigating the confinement of these second messengers in cellular nanodomains, we instead demonstrate that two repellent cues, ephrin-A5 and Slit1, induce spatially segregated signals. These guidance molecules activate subcellular-specific second messenger crosstalk, each signaling network controlling distinct axonal morphology changes in vitro and pathfinding decisions in vivo.

Modeling PRPF31 retinitis pigmentosa using retinal pigment epithelium and organoids combined with gene augmentation rescue.

Mutations in the ubiquitously expressed pre-mRNA processing factor (PRPF) 31 gene, one of the most common causes of dominant form of Retinitis Pigmentosa (RP), lead to a retina-specific phenotype. It is uncertain which retinal cell types are affected and animal models do not clearly present the RP phenotype observed in PRPF31 patients. Retinal organoids and retinal pigment epithelial (RPE) cells derived from human-induced pluripotent stem cells (iPSCs) provide potential opportunities for studying human PRPF31-related RP. We demonstrate here that RPE cells carrying PRPF31 mutations present important morphological and functional changes and that PRPF31-mutated retinal organoids recapitulate the human RP phenotype, with a rod photoreceptor cell death followed by a loss of cones. The low level of PRPF31 expression may explain the defective phenotypes of PRPF31-mutated RPE and photoreceptor cells, which were not observed in cells derived from asymptomatic patients or after correction of the pathogenic mutation by CRISPR/Cas9. Transcriptome profiles revealed differentially expressed and mis-spliced genes belonging to pathways in line with the observed defective phenotypes. The rescue of RPE and photoreceptor defective phenotypes by PRPF31 gene augmentation provide the proof of concept for future therapeutic strategies.

Pleiotropic Roles of Scavenger Receptors in Circadian Retinal Phagocytosis: A New Function for Lysosomal SR-B2/LIMP-2 at the RPE Cell Surface.

The retinal phagocytic machinery resembles the one used by macrophages to clear apoptotic cells. However, in the retina, the permanent contact between photoreceptor outer segments (POS) and retinal pigment epithelial (RPE) cells requires a tight control of this circadian machinery. In addition to the known receptors synchronizing POS internalization, several others are expressed by RPE cells. Notably, scavenger receptor CD36 has been shown to intervene in the internalization speed. We thus investigated members of the scavenger receptor family class A SR-AI and MARCO and class B CD36, SR-BI and SR-B2/LIMP-2 using immunoblotting, immunohisto- and immunocytochemistry, lipid raft flotation gradients, phagocytosis assays after siRNA/antibody inhibition, RT-qPCR and western blot analysis along the light:dark cycle. All receptors were expressed by RPE cell lines and tissues and colocalized with POS, except SR-BI. All receptors were associated with lipid rafts, and even more upon POS challenge. SR-B2/LIMP-2 inhibition suggested a role in the control of the internalization speed similar to CD36. In vivo, MARCO and CD36 displayed rhythmic gene and protein expression patterns concomitant with the phagocytic peak. Taken together, our results indicate that CD36 and SR-B2/LIMP-2 play a direct regulatory role in POS phagocytosis dynamics, while the others such as MARCO might participate in POS clearance by RPE cells either as co-receptors or via an indirect process.

Plexin-B2 controls the timing of differentiation and the motility of cerebellar granule neurons.

Plexin-B2 deletion leads to aberrant lamination of cerebellar granule neurons (CGNs) and Purkinje cells. Although in the cerebellum Plexin-B2 is only expressed by proliferating CGN precursors in the outer external granule layer (oEGL), its function in CGN development is still elusive. Here, we used 3D imaging, in vivo electroporation and live-imaging techniques to study CGN development in novel cerebellum-specific Plxnb2 conditional knockout mice. We show that proliferating CGNs in Plxnb2 mutants not only escape the oEGL and mix with newborn postmitotic CGNs. Furthermore, motility of mitotic precursors and early postmitotic CGNs is altered. Together, this leads to the formation of ectopic patches of CGNs at the cerebellar surface and an intermingling of normally time-stamped parallel fibers in the molecular layer (ML), and aberrant arborization of Purkinje cell dendrites. There results suggest that Plexin-B2 restricts CGN motility and might have a function in cytokinesis.

DCC regulates astroglial development essential for telencephalic morphogenesis and corpus callosum formation.

The forebrain hemispheres are predominantly separated during embryogenesis by the interhemispheric fissure (IHF). Radial astroglia remodel the IHF to form a continuous substrate between the hemispheres for midline crossing of the corpus callosum (CC) and hippocampal commissure (HC). Deleted in colorectal carcinoma (DCC) and netrin 1 (NTN1) are molecules that have an evolutionarily conserved function in commissural axon guidance. The CC and HC are absent in Dcc and Ntn1 knockout mice, while other commissures are only partially affected, suggesting an additional aetiology in forebrain commissure formation. Here, we find that these molecules play a critical role in regulating astroglial development and IHF remodelling during CC and HC formation. Human subjects with DCC mutations display disrupted IHF remodelling associated with CC and HC malformations. Thus, axon guidance molecules such as DCC and NTN1 first regulate the formation of a midline substrate for dorsal commissures prior to their role in regulating axonal growth and guidance across it.

cAMP-Dependent Co-stabilization of Axonal Arbors from Adjacent Developing Neurons.

Axonal arbors in many neuronal networks are exuberant early during development and become refined by activity-dependent competitive mechanisms. Theoretical work proposed non-competitive interactions between co-active axons to co-stabilize their connections, but the demonstration of such interactions is lacking. Here, we provide experimental evidence that reducing cyclic AMP (cAMP) signaling in a subset of retinal ganglion cells favors the elimination of thalamic projections from neighboring neurons, pointing to a cAMP-dependent interaction that promotes axon stabilization.

The 10q26 Risk Haplotype of Age-Related Macular Degeneration Aggravates Subretinal Inflammation by Impairing Monocyte Elimination.

A minor haplotype of the 10q26 locus conveys the strongest genetic risk for age-related macular degeneration (AMD). Here, we examined the mechanisms underlying this susceptibility. We found that monocytes from homozygous carriers of the 10q26 AMD-risk haplotype expressed high amounts of the serine peptidase HTRA1, and HTRA1 located to mononuclear phagocytes (MPs) in eyes of non-carriers with AMD. HTRA1 induced the persistence of monocytes in the subretinal space and exacerbated pathogenic inflammation by hydrolyzing thrombospondin 1 (TSP1), which separated the two CD47-binding sites within TSP1 that are necessary for efficient CD47 activation. This HTRA1-induced inhibition of CD47 signaling induced the expression of pro-inflammatory osteopontin (OPN). OPN expression increased in early monocyte-derived macrophages in 10q26 risk carriers. In models of subretinal inflammation and AMD, OPN deletion or pharmacological inhibition reversed HTRA1-induced pathogenic MP persistence. Our findings argue for the therapeutic potential of CD47 agonists and OPN inhibitors for the treatment of AMD.

SpiCee: A Genetic Tool for Subcellular and Cell-Specific Calcium Manipulation.

Calcium is a second messenger crucial to a myriad of cellular processes ranging from regulation of metabolism and cell survival to vesicle release and motility. Current strategies to directly manipulate endogenous calcium signals lack cellular and subcellular specificity. We introduce SpiCee, a versatile and genetically encoded chelator combining low- and high-affinity sites for calcium. This scavenger enables altering endogenous calcium signaling and functions in single cells in vitro and in vivo with biochemically controlled subcellular resolution. SpiCee paves the way to investigate local calcium signaling in vivo and directly manipulate this second messenger for therapeutic use.

SponGee: A Genetic Tool for Subcellular and Cell-Specific cGMP Manipulation.

cGMP is critical to a variety of cellular processes, but the available tools to interfere with endogenous cGMP lack cellular and subcellular specificity. We introduce SponGee, a genetically encoded chelator of this cyclic nucleotide that enables in vitro and in vivo manipulations in single cells and in biochemically defined subcellular compartments. SponGee buffers physiological changes in cGMP concentration in various model systems while not affecting cAMP signals. We provide proof-of-concept strategies by using this tool to highlight the role of cGMP signaling in vivo and in discrete subcellular domains. SponGee enables the investigation of local cGMP signals in vivo and paves the way for therapeutic strategies that prevent downstream signaling activation.

Mutations in the netrin-1 gene cause congenital mirror movements.

Netrin-1 is a secreted protein that was first identified 20 years ago as an axon guidance molecule that regulates midline crossing in the CNS. It plays critical roles in various tissues throughout development and is implicated in tumorigenesis and inflammation in adulthood. Despite extensive studies, no inherited human disease has been directly associated with mutations in NTN1, the gene coding for netrin-1. Here, we have identified 3 mutations in exon 7 of NTN1 in 2 unrelated families and 1 sporadic case with isolated congenital mirror movements (CMM), a disorder characterized by involuntary movements of one hand that mirror intentional movements of the opposite hand. Given the diverse roles of netrin-1, the absence of manifestations other than CMM in NTN1 mutation carriers was unexpected. Using multimodal approaches, we discovered that the anatomy of the corticospinal tract (CST) is abnormal in patients with NTN1-mutant CMM. When expressed in HEK293 or stable HeLa cells, the 3 mutated netrin-1 proteins were almost exclusively detected in the intracellular compartment, contrary to WT netrin-1, which is detected in both intracellular and extracellular compartments. Since netrin-1 is a diffusible extracellular cue, the pathophysiology likely involves its loss of function and subsequent disruption of axon guidance, resulting in abnormal decussation of the CST.

A plasma membrane microdomain compartmentalizes ephrin-generated cAMP signals to prune developing retinal axon arbors.

The development of neuronal circuits is controlled by guidance molecules that are hypothesized to interact with the cholesterol-enriched domains of the plasma membrane termed lipid rafts. Whether such domains enable local intracellular signalling at the submicrometre scale in developing neurons and are required for shaping the nervous system connectivity in vivo remains controversial. Here, we report a role for lipid rafts in generating domains of local cAMP signalling in axonal growth cones downstream of ephrin-A repulsive guidance cues. Ephrin-A-dependent retraction of retinal ganglion cell axons involves cAMP signalling restricted to the vicinity of lipid rafts and is independent of cAMP modulation outside of this microdomain. cAMP modulation near lipid rafts controls the pruning of ectopic axonal branches of retinal ganglion cells in vivo, a process requiring intact ephrin-A signalling. Together, our findings indicate that lipid rafts structure the subcellular organization of intracellular cAMP signalling shaping axonal arbors during the nervous system development.

Reverse Signaling by Semaphorin-6A Regulates Cellular Aggregation and Neuronal Morphology.

The transmembrane semaphorin, Sema6A, has important roles in axon guidance, cell migration and neuronal connectivity in multiple regions of the nervous system, mediated by context-dependent interactions with plexin receptors, PlxnA2 and PlxnA4. Here, we demonstrate that Sema6A can also signal cell-autonomously, in two modes, constitutively, or in response to higher-order clustering mediated by either PlxnA2-binding or chemically induced multimerisation. Sema6A activation stimulates recruitment of Abl to the cytoplasmic domain of Sema6A and phos¡phorylation of this cytoplasmic tyrosine kinase, as well as phosphorylation of additional cytoskeletal regulators. Sema6A reverse signaling affects the surface area and cellular complexity of non-neuronal cells and aggregation and neurite formation of primary neurons in vitro. Sema6A also interacts with PlxnA2 in cis, which reduces binding by PlxnA2 of Sema6A in trans but not vice versa. These experiments reveal the complex nature of Sema6A biochemical functions and the molecular logic of the context-dependent interactions between Sema6A and PlxnA2.

PlexinA1 is a new Slit receptor and mediates axon guidance function of Slit C-terminal fragments.

Robo-Slit and Plexin-Semaphorin signaling participate in various developmental and pathogenic processes. During commissural axon guidance in the spinal cord, chemorepulsion by Semaphorin3B and Slits controls midline crossing. Slit processing generates an N-terminal fragment (SlitN) that binds to Robo1 and Robo2 receptors and mediates Slit repulsive activity, as well as a C-terminal fragment (SlitC) with an unknown receptor and bioactivity. We identified PlexinA1 as a Slit receptor and found that it binds the C-terminal Slit fragment specifically and transduces a SlitC signal independently of the Robos and the Neuropilins. PlexinA1-SlitC complexes are detected in spinal cord extracts, and ex vivo, SlitC binding to PlexinA1 elicits a repulsive commissural response. Analysis of various ligand and receptor knockout mice shows that PlexinA1-Slit and Robo-Slit signaling have complementary roles during commissural axon guidance. Thus, PlexinA1 mediates both Semaphorin and Slit signaling, and Slit processing generates two active fragments, each exerting distinct effects through specific receptors.

Signaling switch of the axon guidance receptor Robo3 during vertebrate evolution.

Development of neuronal circuits is controlled by evolutionarily conserved axon guidance molecules, including Slits, the repulsive ligands for roundabout (Robo) receptors, and Netrin-1, which mediates attraction through the DCC receptor. We discovered that the Robo3 receptor fundamentally changed its mechanism of action during mammalian evolution. Unlike other Robo receptors, mammalian Robo3 is not a high-affinity receptor for Slits because of specific substitutions in the first immunoglobulin domain. Instead, Netrin-1 selectively triggers phosphorylation of mammalian Robo3 via Src kinases. Robo3 does not bind Netrin-1 directly but interacts with DCC. Netrin-1 fails to attract pontine neurons lacking Robo3, and attraction can be restored in Robo3(-/-) mice by expression of mammalian, but not nonmammalian, Robo3. We propose that Robo3 evolution was key to sculpting the mammalian brain by converting a receptor for Slit repulsion into one that both silences Slit repulsion and potentiates Netrin attraction.

Further insights into GPR179: expression, localization, and associated pathogenic mechanisms leading to complete congenital stationary night blindness.

PURPOSE: Mutations in GPR179, which encodes the G protein-coupled receptor 179, lead to autosomal recessive complete (c) congenital stationary night blindness (CSNB), which is characterized by an ON-bipolar retinal cell dysfunction. This study further defined the exact site of Gpr179 expression and its protein localization in human retina and elucidated the pathogenic mechanism of the reported missense and splice site mutations. METHODS: RNA in situ hybridization was performed with mouse retinal sections. A commercially available antibody was validated with GPR179-overexpressing COS-1 cells and applied to human retinal sections. Live-cell extracellular staining along with subsequent intracellular immunolocalization and ELISA studies were performed using mammalian cells overexpressing wild-type or missense mutated GPR179. Wild-type and splice site-mutated mini-gene constructs were transiently transfected, and RNA was extracted. RT-PCR-amplified products were cloned, and Sanger sequenced. RESULTS: Mouse Gpr179 transcript was expressed in the upper part of the inner nuclear layer, and the respective human protein localized at the dendritic tips of bipolar cells in human retina. The missense mutations p.Tyr220Cys, p.Gly455Asp, and p.His603Tyr led to severely reduced cell surface localization, whereas p.Asp126His did not. The mutated splice donor site altered GPR179 splicing. CONCLUSIONS: Our findings indicate that the site of expression and protein localization of human and mouse GPR179 is similar to that of other proteins implicated in cCSNB. For most of the mutations identified so far, loss of the GPR179 protein function seems to be the underlying pathogenic mechanism leading to this form of cCSNB.

Role of transmembrane semaphorin Sema6A in oligodendrocyte differentiation and myelination.

Myelination is regulated by extracellular proteins, which control interactions between oligodendrocytes and axons. Semaphorins are repulsive axon guidance molecules, which control the migration of oligodendrocyte precursors during normal development and possibly in demyelinating diseases. We show here that the transmembrane semaphorin 6A (Sema6A) is highly expressed by myelinating oligodendrocytes in the postnatal mouse brain. In adult mice, Sema6A expression is upregulated in demyelinating lesions in cuprizone-treated mice. The analysis of the optic nerve and anterior commissure of Sema6A-deficient mice revealed a marked delay of oligodendrocyte differentiation. Accordingly, the development of the nodes of Ranvier is also transiently delayed. We also observed an arrest in the in vitro differentiation of purified oligodendrocytes lacking Sema6A, with a reduction of the expression level of Myelin Basic Protein. Their morphology is also abnormal, with less complex and ramified processes than wild-type oligodendrocytes. In myelinating co-cultures of dorsal root ganglion neurons and purified oligodendrocytes we found that myelination is perturbed in absence of Sema6A. These results suggest that Sema6A might have a role in myelination by controlling oligodendrocyte differentiation.

Moving away from the midline: new developments for Slit and Robo.

In most tissues, the precise control of cell migration and cell-cell interaction is of paramount importance to the development of a functional structure. Several families of secreted molecules have been implicated in regulating these aspects of development, including the Slits and their Robo receptors. These proteins have well described roles in axon guidance but by influencing cell polarity and adhesion, they participate in many developmental processes in diverse cell types. We review recent progress in understanding both the molecular mechanisms that modulate Slit/Robo expression and their functions in neural and non-neural tissue.

Plexin-A2 and its ligand, Sema6A, control nucleus-centrosome coupling in migrating granule cells.

During their migration, cerebellar granule cells switch from a tangential to a radial mode of migration. We have previously demonstrated that this involves the transmembrane semaphorin Sema6A. We show here that plexin-A2 is the receptor that controls Sema6A function in migrating granule cells. In plexin-A2-deficient (Plxna2(-/-)) mice, which were generated by homologous recombination, many granule cells remained in the molecular layer, as we saw in Sema6a mutants. A similar phenotype was observed in mutant mice that were generated by mutagenesis with N-ethyl-N-nitrosourea and had a single amino-acid substitution in the semaphorin domain of plexin-A2. We found that this mutation abolished the ability of Sema6A to bind to plexin-A2. Mouse chimera studies further suggested that plexin-A2 acts in a cell-autonomous manner. We also provide genetic evidence for a ligand-receptor relationship between Sema6A and plexin-A2 in this system. Using time-lapse video microscopy, we found that centrosome-nucleus coupling and coordinated motility were strongly perturbed in Sema6a(-/-) and Plxna2(-/-) granule cells. This suggests that semaphorin-plexin signaling modulates cell migration by controlling centrosome positioning.

Detection of a raft-located estrogen receptor-like protein distinct from ER alpha.

17Beta-estradiol (17beta-E2) elicits at the cell membrane rapid actions that remain insensitive to the inhibitory effect of ICI 182,780, a pure estrogen antagonist, and therefore cannot be attributed to the classic nuclear receptors. We addressed the question of the identity of the protein involved in these rapid actions. We first examined the responses of several cell lines for intracellular calcium mobilization, an effect not inhibited by ICI 182,780, tamoxifen and raloxifen. We then demonstrated the presence of binding sites in the membranes, by incubating them with antibodies directed against different domains of ER alpha, and by flow cytometry analysis. The membrane proteins were eluted by affinity chromatography using E2 conjugated to bovine serum albumin as a ligand. Western blots of the elution fractions using an antibody directed against the ligand binding site of ER alpha showed the existence of a protein of approximately 50 kDa. The protein was concentrated in the lipid rafts, together with another heavier form of approximately 66 kDa. The 50 kDa protein was immunoprecipitable, and co-immunoprecipitation experiments showed that it was associated with the Gbeta(1-4) protein, but not with caveolin-1. The protein was expressed in ER alpha-null cells, like HO-23 and Cos-7 cells. Therefore, in the lipid rafts, there exists a protein, similar to, but molecularly distinct from ER alpha.

Signaling cross-talk from Gbeta4 subunit to Elk-1 in the rapid action of androgens.

Androgens act on transcription via intracellular androgen receptors (ARs), but they also have rapid AR-independent effects. We have identified the multistep processes involved in the rapid actions of androgens in male osteoblasts, which also possess the classical AR. Incubating cells with 5alpha-dihydroxytestosterone (100 pm, DHT) rapidly increased (1 min) the phosphorylation of the transcription factor Elk-1, and this was inhibited by pertussis toxin (PTX). DHT activated ERK1/2, a substrate of Elk-1, within 15 s but had no effect on p38 MAPK or JNK/SAPK. The inhibitors PD98059 (MEK1/2); Gö6976, Gö6983, and chelerythrine (protein kinase C); wortmannin and LY294002 (phosphatidylinositol 3-kinase); PP1 (Src); and PTX all blunted the DHT-stimulated phosphorylation of ERK1/2. DHT increased the phosphorylation of c-Raf-1 within 5 s; this was blocked by conventional protein kinase C and phosphatidylinositol 3-kinase inhibitors. The first activated membrane protein was the PTX-sensitive Gbeta(4) subunit coupled to phospholipase C-beta2, which triggered a rapid (5 s) increase in intracellular calcium and diacylglycerol formation. The androgen antagonist cyproterone acetate did not modify the responses to DHT. Lastly an anti-AR antibody directed against the ligand binding domain recognized a protein at the plasma membrane. The cascade of rapid effects triggered by androgens may involve the classical AR at the plasma membrane or an uncharacterized form of AR that is insensitive to nuclear antagonists.