Neural circuits are assembled through the coordinated innervation of pre- and postsynaptic partners. We show that connectivity between two interneurons, AIY and RIA, in Caenorhabditis elegans is orchestrated by a pair of glial cells that express UNC-6 (netrin). In the postsynaptic neuron RIA, the netrin receptor UNC-40 (DCC, deleted in colorectal cancer) plays a conventional guidance role, directing outgrowth of the RIA process ventrally toward the glia. In the presynaptic neuron AIY, UNC-40 (DCC) plays an unexpected and previously uncharacterized role: It cell-autonomously promotes assembly of presynaptic terminals in the immediate vicinity of the glial cell endfeet. These results indicate that netrin can be used both for guidance and local synaptogenesis and suggest that glial cells can function as guideposts during the assembly of neural circuits in vivo.

Neurons are highly polarized cells with morphologically and molecularly distinct axonal and dendritic compartments. It is not well understood how postsynaptic receptors are selectively enriched in dendrites in vivo. We investigated the molecular mechanisms of dendritically polarized localization of a glutamate receptor, an acetylcholine receptor, and a ROR-type receptor tyrosine kinase in the interneuron RIA in C. elegans. We found that the clathrin adaptor AP-1 complex 1 subunit UNC-101 functions cell autonomously to maintain the correct localization of these receptors in a dynamindependent manner. In unc-101 mutants, instead of being dendritically enriched, all 3 receptors are evenly distributed in the axonal and dendritic compartments. Surprisingly, UNC-101 predominantly localizes to the axonal compartment, suggesting a possible transcytosis model for the dendritic targeting of neurotransmitter receptors.polarity ͉ trafficking ͉ glutamate receptor ͉ dendrite N eurons are polarized cells that receive, process, and transmit information. These different functions are performed by morphologically and functionally distinct subcellular compartmentsdendrites and axons-that contain proteins specialized for signal input or output. In principle, many different cellular mechanisms could contribute to molecular polarization in neurons (1, 2). Proteins destined for the axon or the dendrite could be selectively sorted into different transport vesicles at the transGolgi network (TGN) and selectively delivered to their proper subcellular compartment. Alternatively, steady-state differences in axonal and dendritic protein distribution could be achieved by selective stabilization of proteins by scaffolding proteins at certain areas of the plasma membrane, as well as by selective sorting and redistribution following endocytosis.What is currently known about sorting and trafficking of axonal and dendritic proteins to their proper compartments? Three axonally localized proteins, NgCAM, Nav1.2, and VAMP, are delivered to both axons and dendrites in neurons and achieve polarization following selective endocytosis from the dendritic plasma membrane (3-6). In contrast, dendritically localized transferrin receptor appears to be directly transported to the dendrite (3). For a number of dendritically targeted proteins, structure-function analysis has revealed C-terminal tyrosine-based or dileucine-based cytoplasmic motifs that mediate dendritic targeting (7,8).Interesting parallels have been made between dendritic sorting in neurons and basolateral sorting in epithelia (9). A number of basolaterally sorted proteins localize to the somatodendritic region in cultured hippocampal neurons and rely on the same sorting sequences in both cell types (10). One of the key players involved in basolateral sorting in polarized epithelial cells is 1B, the medium subunit of clathrin adaptor AP-1 complex (11). AP complexes are cytosolic tetramers that mediate sorting in secretory and endocytic pathways by promoting budding of clathrin-coated vesicles (...

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Synapses are specialized junctions that mediate information flow between neurons and their targets. A striking feature of the nervous system is the specificity of its synaptic connections: an individual neuron will form synapses only with a small subset of available presynaptic and postsynaptic partners. Synaptic specificity has been classically thought to arise from homophilic or heterophilic interactions between adhesive molecules acting across the synaptic cleft. Over the past decade, many new mechanisms giving rise to synaptic specificity have been identified. Synapses can be specified by secreted molecules that promote or inhibit synaptogenesis, and their source can be a neighboring guidepost cell, not just presynaptic and postsynaptic neurons. Furthermore, lineage, fate, and timing of development can also play critical roles in shaping neural circuits. Future work utilizing large-scale screens will aim to elucidate the full scope of cellular mechanisms and molecular players that can give rise to synaptic specificity.

The increase in CD163+ cell number in eyes with mild and severe glaucoma is the first demonstration of macrophage infiltration in glaucomatous human optic nerves. This finding supports a role for macrophages in glaucoma pathogenesis and progression.

Prior studies have demonstrated that microglial activation is involved in the pathogenesis of primary open-angle glaucoma (POAG). Here we sought to identify genetic associations between POAG and variants in APOE and TREM2, genes associated with Alzheimer disease (AD) that critically regulate microglial neurodegenerationassociated molecular signature. METHODS. APOE genotypes were called using imputed data from the NEIGHBOR consortium (2120 POAG cases, 2262 controls) and a second cohort from the Massachusetts Eye and Ear Infirmary (MEEI; 486 cases, 344 controls). TREM2 coding variants were genotyped by means of the Illumina HumanExome BeadArray. The data set was analyzed for association with POAG overall, as well as the high-tension glaucoma (HTG) and normaltension glaucoma (NTG) subgroups, using logistic regression adjusting for age and sex. RESULTS. In the combined NEIGHBOR-MEEI data set, significant association was observed for APOE ε4 in POAG overall (odds ratio [OR], 0.83; 95% confidence interval [CI], 0.74-0.94; P = 0.0022) and in both the HTG subgroup (OR, 0.81; 95% CI, 0.70-0.94; P = 0.0052) and NTG subgroup (OR, 0.71; 95% CI, 0.58-0.87; P = 0.0014). A rare TREM2 variant (A105V) was found only in HTG cases (3 of 2863 cases) and in none of the controls (P = 0.03). Three TREM2 rare variants associated with AD were not significantly associated with POAG (P > 0.05). CONCLUSIONS. We have found that the APOE ε4 allele is associated with a reduced risk of POAG. Interestingly, the same allele is adversely associated with AD, suggesting a mechanistic difference between neurodegenerative diseases of the eye and the brain. TREM2 variants associated with AD did not significantly contribute to POAG risk.

Synapses are specialized sites of cell contact that mediate information flow between neurons and their targets. Genetic screens in the nematode C. elegans have led to the discovery of a number of molecules required for synapse patterning and assembly. Recent studies have demonstrated the importance of guidepost cells in the positioning of presynaptic sites at specific locations along the axon. Interestingly, these guideposts can promote or inhibit synapse formation, and do so by utilizing transmembrane adhesion molecules or secreted factors that act over relatively larger distances. Once the decision of where to build a presynaptic terminal has been made, key molecules are recruited to assemble synaptic vesicles and active zone proteins at that site. Multiple steps of this process are regulated by ubiquitin ligase complexes. Interestingly, some of the molecules involved in presynaptic assembly also play roles in regulating axon polarity and outgrowth, suggesting that different neurodevelopmental processes are molecularly integrated.

Single cell RNA sequencing studies identified novel neurodegeneration-associated microglial (MGnD/DAM) subtypes activated around cerebral amyloid plaques. Micro-RNA (miR)-155 of the TREM2-APOE pathway was shown to be a key transcriptional regulator of MGnD microglial phenotype. Despite growing interest in studying manifestations of Alzheimer’s disease (AD) in the retina, a CNS organ accessible to noninvasive high-resolution imaging, to date MGnD microglia have not been studied in the AD retina. Here, we discovered the presence and increased populations of Clec7a+ and Galectin-3+ MGnD microglia in retinas of transgenic APPSWE/PS1L166P AD-model mice. Conditionally targeting MGnD microglia by miR-155 ablation via the tamoxifen-inducible CreERT2 system in APPSWE/PS1L166P mice diminished retinal Clec7a+ and Galectin-3+ microglial populations while increasing homeostatic P2ry12+ microglia. Retinal MGnD microglia were often adhering to microvessels; their depletion protected the inner blood-retina barrier and reduced vascular amyloidosis. Microglial miR-155 depletion further limits retinal inflammation. Mass spectrometry analysis revealed enhanced retinal PI3K-Akt signaling and predicted IL-8 and Spp1 decreases in mice with microglia-specific miR-155 knockout. Overall, this study identified MGnD microglia in APPSWE/PS1L166P mouse retina. Transcriptional regulation of these dysfunctional microglia mitigated retinal inflammation and vasculopathy. The protective effects of microglial miR-155 ablation should shed light on potential treatments for retinal inflammation and vascular damage during AD and other ocular diseases.