During corticogenesis, distinct subtypes of neurons are sequentially born from ventricular zone progenitors. How these cells are molecularly temporally patterned is poorly understood. We used single-cell RNA sequencing at high temporal resolution to trace the lineage of the molecular identities of successive generations of apical progenitors (APs) and their daughter neurons in mouse embryos. We identified a core set of evolutionarily conserved, temporally patterned genes that drive APs from internally driven to more exteroceptive states. We found that the Polycomb repressor complex 2 (PRC2) epigenetically regulates AP temporal progression. Embryonic age–dependent AP molecular states are transmitted to their progeny as successive ground states, onto which essentially conserved early postmitotic differentiation programs are applied, and are complemented by later-occurring environment-dependent signals. Thus, epigenetically regulated temporal molecular birthmarks present in progenitors act in their postmitotic progeny to seed adult neuronal diversity.

During corticogenesis, excitatory neurons are born from progenitors located in the ventricular zone (VZ), from where they migrate to assemble into circuits. How neuronal identity is dynamically specified upon progenitor division is unknown. Here, we study this process using a high-temporal-resolution technology allowing fluorescent tagging of isochronic cohorts of newborn VZ cells. By combining this in vivo approach with single-cell transcriptomics in mice, we identify and functionally characterize neuron-specific primordial transcriptional programs as they dynamically unfold. Our results reveal early transcriptional waves that instruct the sequence and pace of neuronal differentiation events, guiding newborn neurons toward their final fate, and contribute to a road map for the reverse engineering of specific classes of cortical neurons from undifferentiated cells.

During development, thalamocortical (TC) input has a critical role in the spatial delineation and patterning of cortical areas, yet the underlying cellular and molecular mechanisms that drive cortical neuron differentiation are poorly understood. In the primary (S1) and secondary (S2) somatosensory cortex, layer 4 (L4) neurons receive mutually exclusive input originating from two thalamic nuclei: the ventrobasalis (VB), which conveys tactile input, and the posterior nucleus (Po), which conveys modulatory and nociceptive input. Recently, we have shown that L4 neuron identity is not fully committed postnatally, implying a capacity for TC input to influence differentiation during cortical circuit assembly. Here we investigate whether the cell-type-specific molecular and functional identity of L4 neurons is instructed by the origin of their TC input. Genetic ablation of the VB at birth resulted in an anatomical and functional rewiring of Po projections onto L4 neurons in S1. This induced acquisition of Po input led to a respecification of postsynaptic L4 neurons, which developed functional molecular features of Po-target neurons while repressing VB-target traits. Respecified L4 neurons were able to respond both to touch and to noxious stimuli, in sharp contrast to the normal segregation of these sensory modalities in distinct cortical circuits. These findings reveal a behaviourally relevant TC-input-type-specific control over the molecular and functional differentiation of postsynaptic L4 neurons and cognate intracortical circuits, which instructs the development of modality-specific neuronal and circuit properties during corticogenesis.

Modality-specific sensory inputs from individual sense organs are processed in parallel in distinct areas of the neocortex. For each sensory modality, input follows a cortico-thalamo-cortical loop in which a 'first-order' exteroceptive thalamic nucleus sends peripheral input to the primary sensory cortex, which projects back to a 'higher order' thalamic nucleus that targets a secondary sensory cortex. This conserved circuit motif raises the possibility that shared genetic programs exist across sensory modalities. Here we report that, despite their association with distinct sensory modalities, first-order nuclei in mice are genetically homologous across somatosensory, visual, and auditory pathways, as are higher order nuclei. We further reveal peripheral input-dependent control over the transcriptional identity and connectivity of first-order nuclei by showing that input ablation leads to induction of higher-order-type transcriptional programs and rewiring of higher-order-directed descending cortical input to deprived first-order nuclei. These findings uncover an input-dependent genetic logic for the design and plasticity of sensory pathways, in which conserved developmental programs lead to conserved circuit motifs across sensory modalities.

Functional interplay between ionotropic and metabotropic receptors frequently involves complex intracellular signaling cascades. The group I metabotropic glutamate receptor mGlu5a co-clusters with the ionotropic N-methyl-D-aspartate (NMDA) receptor in hippocampal neurons. In this study, we report that a more direct cross-talk can exist between these types of receptors. Using bioluminescence resonance energy transfer in living HEK293 cells, we demonstrate that mGlu5a and NMDA receptor clustering reflects the existence of direct physical interactions. Consequently, the mGlu5a receptor decreased NMDA receptor current, and reciprocally, the NMDA receptor strongly reduced the ability of the mGlu5a receptor to release intracellular calcium. We show that deletion of the C terminus of the mGlu5a receptor abolished both its interaction with the NMDA receptor and reciprocal inhibition of the receptors. This direct functional interaction implies a higher degree of target-effector specificity, timing, and subcellular localization of signaling than could ever be predicted with complex signaling pathways.Presynaptic glutamate release activates multiple receptors that are assembled onto the postsynaptic membrane by scaffolding proteins (1) and determine the neuronal response. Activation of the glutamate ionotropic receptors of the ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype is responsible for the fast excitatory postsynaptic currents. Concomitant activation of glutamate ionotropic receptors of the N-methyl-D-aspartate (NMDA) 2 subtype mediates a slow excitatory postsynaptic current (2) that can trigger bidirectional modifications of synaptic strength: long-term potentiation (3) or long-term depression (4). The metabotropic glutamate (mGlu) receptors are subdivided into three groups based on sequence homology, preferred associated second messenger systems, and pharmacological properties (group I, mGlu1 and mGlu5; group II, mGlu2 and mGlu3; and group III, mGlu4 and mGlu6 -mGlu8). mGlu1a and mGlu5a/b receptor subtypes are mainly postsynaptic and positively coupled to G q proteins, stimulation of phospholipase C (5), protein kinase C, and mobilization of intracellular calcium via inositol 1,4,5-trisphosphate receptors. These receptors can also be synaptically activated by glutamate and participate in the modulation of postsynaptic ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor currents (4, 6 -8). Electrophysiological experiments have shown functional interplay between mGlu1a/5 and NMDA receptors in various structures of the brain. Indeed, activation of mGlu5 receptors can enhance NMDA-evoked responses in the hippocampus, striatum, cortex, and spinal cord (9 -18), suggesting that the functional interactions between the mGlu and NMDA receptors are of widespread significance.Because both NMDA and mGlu receptors are important players in synaptic transmission, there has been considerable interest in identifying the mechanisms underlying the functional interactions between these two classes of glutamate r...