Neuronal apoptosis drives remodeling states of microglia and shifts in survival pathway dependence

Microglia serve critical remodeling roles that shape the developing nervous system, responding to the changing neural environment with phagocytosis or soluble factor secretion. Recent single-cell sequencing (scRNAseq) studies have revealed the context-dependent diversity in microglial properties and gene expression, but the cues promoting this diversity are not well defined. Here, we ask how interactions with apoptotic neurons shape microglial state, including lysosomal and lipid metabolism gene expression and independence from Colony-stimulating factor 1 receptor (CSF1R) for survival. Using early postnatal mouse retina, a CNS region undergoing significant developmental remodeling, we performed scRNAseq on microglia from mice that are wild-type, lack neuronal apoptosis (Bax KO), or are treated with CSF1R inhibitor (PLX3397). We find that interactions with apoptotic neurons drives multiple microglial remodeling states, subsets of which are resistant to CSF1R inhibition. We find that TAM receptor Mer and complement receptor 3 are required for clearance of apoptotic neurons, but that Mer does not drive expression of remodeling genes. We show TAM receptor Axl is negligible for phagocytosis or remodeling gene expression but is consequential for microglial survival in the absence of CSF1R signaling. Thus, interactions with apoptotic neurons shift microglia towards distinct remodeling states and through Axl, alters microglial dependence on survival pathway, CSF1R.

Silencing neuronal activity is required for developmental circuit remodeling

Postnatal refinement of neuronal connectivity shapes the mature nervous system. Pruning of exuberant connections involves both cell autonomous and non-cell autonomous mechanisms, such as neuronal activity. While the role of neuronal activity in the plasticity of excitatory synapses has been extensively studied, the involvement of inhibition is less clear. Furthermore, the role of activity during stereotypic developmental remodeling, where competition is not as apparent, is not well understood. Here we use the Drosophila mushroom body as a model to show that regulated silencing of neuronal activity is required for developmental axon pruning of the γ-Kenyon cells. We demonstrate that silencing neuronal activity is mechanistically achieved by cell autonomous expression of the inward rectifying potassium channel (irk1) combined with inhibition by the GABAergic APL neuron. These results support the Hebbian-like rule 'use it or lose it', where inhibition can destabilize connectivity and promote pruning while excitability stabilizes existing connections.

A changing signaling environment induces multiciliated cell trans-differentiation during developmental remodeling

Multiciliated cells (MCCs) are extremely highly-differentiated, presenting >100 cilia and basal bodies. We analyzed how MCCs are lost from the airway-like Xenopus embryonic epidermis during developmental tissue remodeling. We found that some MCCs undergo apoptosis, but that the majority trans-differentiate into secretory cells. Trans-differentiation involves loss of ciliary gene expression, cilia retraction and lysosomal degradation. Apoptosis and trans-differentiation are both induced by a changing signaling environment through Notch, Jak/STAT, Thyroid hormone and mTOR signaling, and trans-differentiation can be inhibited by Rapamycin. This demonstrates that even cells with extreme differentiation features can undergo direct fate conversion. Our data further suggest that the reactivation of this developmental mechanism in adults can drive tissue remodeling in human chronic airway disease, a paradigm resembling cancer formation and progression.

From a syncytium to mononucleate cells and back: Yki and JNK in symphony

Alary muscle syncytia in Drosophila larvae undergo a remarkable process of dedifferentiation into single cells that then fuse to become ventral longitudinal muscle in the adult. In this issue, Schaub et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201905048) identify the Hippo and JNK signaling pathways as key regulators of this process of developmental remodeling of cell fate.

N-terminal alternative splicing of GluN1 regulates the maturation of excitatory synapses and seizure susceptibility

The majority of NMDA receptors (NMDARs) in the brain are composed of 2 GluN1 and 2 GluN2 subunits. The inclusion or exclusion of 1 N-terminal and 2 C-terminal domains of GluN1 results in 8 splicing variants that exhibit distinct temporal and spatial patterns of expression and functional properties. However, previous functional analyses of Grin1 variants have been done using heterologous expression and the in vivo function of Grin1 splicing is unknown. Here we show that N-terminal splicing of GluN1 has important functions in the maturation of excitatory synapses. The inclusion of exon 5 of Grin1 is up-regulated in several brain regions such as the thalamus and neocortex. We find that deletion of Grin1 exon 5 disrupts the developmental remodeling of NMDARs in thalamic neurons and the effect is distinct from that of Grin2a (GluN2A) deletion. Deletion of Grin2a or exon 5 of Grin1 alone partially attenuates the shortening of NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSCs) during early life, whereas deletion of both Grin2a and exon 5 of Grin1 completely abolishes the developmental change in NMDAR-EPSC decay time. Deletion of exon 5 of Grin1 leads to an overproduction of excitatory synapses in layer 5 pyramidal neurons in the cortex and increases seizure susceptibility in adult mice. Our findings demonstrate that N-terminal splicing of GluN1 has important functions in synaptic maturation and neuronal network excitability.

Developmental synapse remodeling in the cerebellum and visual thalamus

Functional neural circuits of mature animals are shaped during postnatal development by eliminating early-formed redundant synapses and strengthening of necessary connections. In the nervous system of newborn animals, redundant synapses are only transient features of the circuit. During subsequent postnatal development, some synapses are strengthened whereas other redundant connections are weakened and eventually eliminated. In this review, we introduce recent studies on the mechanisms of developmental remodeling of climbing fiber–to–Purkinje cell synapses in the cerebellum and synapses from the retina to neurons in the dorsal lateral geniculate nucleus of the visual thalamus (retinogeniculate synapses). These are the two representative models of developmental synapse remodeling in the brain and they share basic principles, including dependency on neural activity. However, recent studies have disclosed that, in several respects, the two models use different molecules and strategies to establish mature synaptic connectivity. We describe similarities and differences between the two models and discuss remaining issues to be tackled in the future in order to understand the general schemes of developmental synapse remodeling.

Neuron-specific signatures in the chromosomal connectome associated with schizophrenia risk

To explore the developmental reorganization of the three-dimensional genome of the brain in the context of neuropsychiatric disease, we monitored chromosomal conformations in differentiating neural progenitor cells. Neuronal and glial differentiation was associated with widespread developmental remodeling of the chromosomal contact map and included interactions anchored in common variant sequences that confer heritable risk for schizophrenia. We describe cell type–specific chromosomal connectomes composed of schizophrenia risk variants and their distal targets, which altogether show enrichment for genes that regulate neuronal connectivity and chromatin remodeling, and evidence for coordinated transcriptional regulation and proteomic interaction of the participating genes. Developmentally regulated chromosomal conformation changes at schizophrenia-relevant sequences disproportionally occurred in neurons, highlighting the existence of cell type–specific disease risk vulnerabilities in spatial genome organization.