Spatiotemporal Control of CNS Myelination by Oligodendrocyte Programmed Cell Death through the TFEB-PUMA Axis.

Nervous system function depends on proper myelination for insulation and critical trophic support for axons. Myelination is tightly regulated spatially and temporally, but how it is controlled molecularly remains largely unknown. Here, we identified key molecular mechanisms governing the regional and temporal specificity of CNS myelination. We show that transcription factor EB (TFEB) is highly expressed by differentiating oligodendrocytes and that its loss causes precocious and ectopic myelination in many parts of the murine brain. TFEB functions cell-autonomously through PUMA induction and Bax-Bak activation to promote programmed cell death of a subset of premyelinating oligodendrocytes, allowing selective elimination of oligodendrocytes in normally unmyelinated brain regions. This pathway is conserved across diverse brain areas and is critical for myelination timing. Our findings define an oligodendrocyte-intrinsic mechanism underlying the spatiotemporal specificity of CNS myelination, shedding light on how myelinating glia sculpt the nervous system during development.

Glia Get Neurons in Shape.

Glial cells are essential components of the nervous system. In this issue, Singhvi et al. uncover cellular and molecular mechanisms through which C. elegans glia shape sensory neuron terminals and thus control animal thermosensing behaviors.

Dlg5 regulates dendritic spine formation and synaptogenesis by controlling subcellular N-cadherin localization.

Most excitatory synapses in the mammalian brain are formed on dendritic spines, and spine density has a profound impact on synaptic transmission, integration, and plasticity. Membrane-associated guanylate kinase (MAGUK) proteins are intracellular scaffolding proteins with well established roles in synapse function. However, whether MAGUK proteins are required for the formation of dendritic spines in vivo is unclear. We isolated a novel disc large-5 (Dlg5) allele in mice, Dlg5(LP), which harbors a missense mutation in the DLG5 SH3 domain, greatly attenuating its ability to interact with the DLG5 GUK domain. We show here that DLG5 is a MAGUK protein that regulates spine formation, synaptogenesis, and synaptic transmission in cortical neurons. DLG5 regulates synaptogenesis by enhancing the cell surface localization of N-cadherin, revealing a key molecular mechanism for regulating the subcellular localization of this cell adhesion molecule during synaptogenesis.

Guidance-cue control of horizontal cell morphology, lamination, and synapse formation in the mammalian outer retina.

In the vertebrate retina, neuronal circuitry required for visual perception is organized within specific laminae. Photoreceptors convey external visual information to bipolar and horizontal cells at triad ribbon synapses established within the outer plexiform layer (OPL), initiating retinal visual processing. However, the molecular mechanisms that organize these three classes of neuronal processes within the OPL, thereby ensuring appropriate ribbon synapse formation, remain largely unknown. Here we show that mice with null mutations in Sema6A or PlexinA4 (PlexA4) exhibit a pronounced defect in OPL stratification of horizontal cell axons without any apparent deficits in bipolar cell dendrite or photoreceptor axon targeting. Furthermore, these mutant horizontal cells exhibit aberrant dendritic arborization and reduced dendritic self-avoidance within the OPL. Ultrastructural analysis shows that the horizontal cell contribution to rod ribbon synapse formation in PlexA4⁻/⁻ retinas is disrupted. These findings define molecular components required for outer retina lamination and ribbon synapse formation.

A luciferase fragment complementation assay to detect focal adhesion kinase (FAK) signaling events.

Integrin Adhesion Complexes (IACs) serve as links between the cytoskeleton and extracellular environment, acting as mechanosensing and signaling hubs. As such, IACs participate in many aspects of cellular motility, tissue morphogenesis, anchorage-dependent growth and cell survival. Focal Adhesion Kinase (FAK) has emerged as a critical organizer of IAC signaling events due to its early recruitment and diverse substrates, and thus has become a genetic and therapeutic target. Here we present the design and characterization of simple, reversible, and scalable Bimolecular Complementation sensors to monitor FAK phosphorylation in living cells. These probes provide novel means to quantify IAC signaling, expanding on the currently available toolkit for interrogating FAK phosphorylation during diverse cellular processes.

Autophagy collaborates with apoptosis pathways to control myelination specificity and function.

Oligodendrocytes are the sole myelin producing cells in the central nervous system. Oligodendrocyte numbers are tightly controlled across diverse brain regions to match local axon type and number, but the underlying mechanisms and functional significance remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under canonical settings, elicits premyelinating oligodendrocyte apoptosis during development and regulates critical aspects of nerve pulse propagation. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy acts in the TFEB-Bax/Bak pathway and elevates PUMA mRNA levels to trigger premyelinating oligodendrocyte apoptosis cell-autonomously. Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath numbers and fine-tune nerve pulse propagation. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrocyte number. HIGHLIGHTS: Autophagy flux increases in the premyelinating and myelinating oligodendrocytesAutophagy promotes premyelinating oligodendrocyte (pre-OL) apoptosis to control myelination location and timing Autophagy acts in the TFEB-PUMA-Bax/Bak pathway and elevates PUMA mRNA levels to determine pre-OL fate Autophagy continuously functions in the myelinating oligodendrocytes to limit myelin sheath thickness and finetune nerve pulse propagation.

Oligodendrocyte-lineage cell exocytosis and L-type prostaglandin D synthase promote oligodendrocyte development and myelination.

In the developing central nervous system, oligodendrocyte precursor cells (OPCs) differentiate into oligodendrocytes, which form myelin around axons. Oligodendrocytes and myelin are essential for the function of the central nervous system, as evidenced by the severe neurological symptoms that arise in demyelinating diseases such as multiple sclerosis and leukodystrophy. Although many cell-intrinsic mechanisms that regulate oligodendrocyte development and myelination have been reported, it remains unclear whether interactions among oligodendrocyte-lineage cells (OPCs and oligodendrocytes) affect oligodendrocyte development and myelination. Here, we show that blocking vesicle-associated membrane protein (VAMP) 1/2/3-dependent exocytosis from oligodendrocyte-lineage cells impairs oligodendrocyte development, myelination, and motor behavior in mice. Adding oligodendrocyte-lineage cell-secreted molecules to secretion-deficient OPC cultures partially restores the morphological maturation of oligodendrocytes. Moreover, we identified L-type prostaglandin D synthase as an oligodendrocyte-lineage cell-secreted protein that promotes oligodendrocyte development and myelination in vivo. These findings reveal a novel autocrine/paracrine loop model for the regulation of oligodendrocyte and myelin development.

Autophagy collaborates with apoptosis pathways to control oligodendrocyte number.

Oligodendrocytes are the sole myelin-producing cells in the central nervous system. Oligodendrocyte number is tightly controlled across diverse brain regions to match local axon type and number, yet the underlying mechanisms remain unclear. Here, we show that autophagy, an evolutionarily conserved cellular process that promotes cell survival under physiological conditions, elicits premyelinating oligodendrocyte apoptosis during development. Autophagy flux is increased in premyelinating oligodendrocytes, and its genetic blockage causes ectopic oligodendrocyte survival throughout the entire brain. Autophagy functions cell autonomously in the premyelinating oligodendrocyte to trigger cell apoptosis, and it genetically interacts with the TFEB pathway to limit oligodendrocyte number across diverse brain regions. Our results provide in vivo evidence showing that autophagy promotes apoptosis in mammalian cells under physiological conditions and reveal key intrinsic mechanisms governing oligodendrogenesis.

Distinct oligodendrocyte populations have spatial preference and different responses to spinal cord injury.

Mature oligodendrocytes (MOLs) show transcriptional heterogeneity, the functional consequences of which are unclear. MOL heterogeneity might correlate with the local environment or their interactions with different neuron types. Here, we show that distinct MOL populations have spatial preference in the mammalian central nervous system (CNS). We found that MOL type 2 (MOL2) is enriched in the spinal cord when compared to the brain, while MOL types 5 and 6 (MOL5/6) increase their contribution to the OL lineage with age in all analyzed regions. MOL2 and MOL5/6 also have distinct spatial preference in the spinal cord regions where motor and sensory tracts run. OL progenitor cells (OPCs) are not specified into distinct MOL populations during development, excluding a major contribution of OPC intrinsic mechanisms determining MOL heterogeneity. In disease, MOL2 and MOL5/6 present different susceptibility during the chronic phase following traumatic spinal cord injury. Our results demonstrate that the distinct MOL populations have different spatial preference and different responses to disease.

Developmental Heterogeneity of Microglia and Brain Myeloid Cells Revealed by Deep Single-Cell RNA Sequencing.

Microglia are increasingly recognized for their major contributions during brain development and neurodegenerative disease. It is currently unknown whether these functions are carried out by subsets of microglia during different stages of development and adulthood or within specific brain regions. Here, we performed deep single-cell RNA sequencing (scRNA-seq) of microglia and related myeloid cells sorted from various regions of embryonic, early postnatal, and adult mouse brains. We found that the majority of adult microglia expressing homeostatic genes are remarkably similar in transcriptomes, regardless of brain region. By contrast, early postnatal microglia are more heterogeneous. We discovered a proliferative-region-associated microglia (PAM) subset, mainly found in developing white matter, that shares a characteristic gene signature with degenerative disease-associated microglia (DAM). Such PAM have amoeboid morphology, are metabolically active, and phagocytose newly formed oligodendrocytes. This scRNA-seq atlas will be a valuable resource for dissecting innate immune functions in health and disease.

Functional assembly of accessory optic system circuitry critical for compensatory eye movements.

Accurate motion detection requires neural circuitry that compensates for global visual field motion. Select subtypes of retinal ganglion cells perceive image motion and connect to the accessory optic system (AOS) in the brain, which generates compensatory eye movements that stabilize images during slow visual field motion. Here, we show that the murine transmembrane semaphorin 6A (Sema6A) is expressed in a subset of On direction-selective ganglion cells (On DSGCs) and is required for retinorecipient axonal targeting to the medial terminal nucleus (MTN) of the AOS. Plexin A2 and A4, two Sema6A binding partners, are expressed in MTN cells, attract Sema6A(+) On DSGC axons, and mediate MTN targeting of Sema6A(+) RGC projections. Furthermore, Sema6A/Plexin-A2/A4 signaling is required for the functional output of the AOS. These data reveal molecular mechanisms underlying the assembly of AOS circuits critical for moving image perception.

Cas adaptor proteins organize the retinal ganglion cell layer downstream of integrin signaling.

Stratification of retinal neuronal cell bodies and lamination of their processes provide a scaffold upon which neural circuits can be built. However, the molecular mechanisms that direct retinal ganglion cells (RGCs) to resolve into a single-cell retinal ganglion cell layer (GCL) are not well understood. The extracellular matrix protein laminin conveys spatial information that instructs the migration, process outgrowth, and reorganization of GCL cells. Here, we show that the ß1-Integrin laminin receptor is required for RGC positioning and reorganization into a single-cell GCL layer. ß1-Integrin signaling within migrating GCL cells requires Cas signaling-adaptor proteins, and in the absence of ß1-Integrin or Cas function retinal neurons form ectopic cell clusters beyond the inner-limiting membrane (ILM), phenocopying laminin mutants. These data reveal an essential role for Cas adaptor proteins in ß1-Integrin-mediated signaling events critical for the formation of the single-cell GCL in the mammalian retina.

Sema6B, Sema6C, and Sema6D expression and function during mammalian retinal development.

In the vertebrate retina, the formation of neural circuits within discrete laminae is critical for the establishment of retinal visual function. Precise formation of retinal circuits requires the coordinated actions of adhesive and repulsive molecules, including repulsive transmembrane semaphorins (Sema6A, Sema5A, and Sema5B). These semaphorins signal through different Plexin A (PlexA) receptors, thereby regulating distinct aspects of retinal circuit assembly. Here, we investigate the physiological roles of three Class 6 transmembrane semaphorins (Sema6B, Sema6C, and Sema6D), previously identified as PlexA receptor ligands in non-retinal tissues, in mammalian retinal development. We performed expression analysis and also phenotypic analyses of mice that carry null mutations in each of genes encoding these proteins using a broad range of inner and outer retinal markers. We find that these Class 6 semaphorins are uniquely expressed throughout postnatal retinal development in specific domains and cell types of the developing retina. However, we do not observe defects in stereotypical lamina-specific neurite stratification of retinal neuron subtypes in Sema6B-/- or Sema6C-/-; Sema6D-/- retinas. These findings indicate these Class 6 transmembrane semaphorins are unlikely to serve as major PlexA receptor ligands for the assembly of murine retinal circuit laminar organization.

On and off retinal circuit assembly by divergent molecular mechanisms.

Direction-selective responses to motion can be to the onset (On) or cessation (Off) of illumination. Here, we show that the transmembrane protein semaphorin 6A and its receptor plexin A2 are critical for achieving radially symmetric arborization of On starburst amacrine cell (SAC) dendrites and normal SAC stratification in the mouse retina. Plexin A2 is expressed in both On and Off SACs; however, semaphorin 6A is expressed in On SACs. Specific On-Off bistratified direction-selective ganglion cells in semaphorin 6A(-/-) mutants exhibit decreased tuning of On directional motion responses. These results correlate the elaboration of symmetric SAC dendritic morphology and asymmetric responses to motion, shedding light on the development of visual pathways that use the same cell types for divergent outputs.