Non-canonical role for Lpar1-EGFP subplate neurons in early postnatal mouse somatosensory cortex

Subplate neurons (SPNs) are thought to play a role in nascent sensory processing in neocortex. To better understand how heterogeneity within this population relates to emergent function, we investigated the synaptic connectivity of Lpar1-EGFP SPNs through the first postnatal week in whisker somatosensory cortex (S1BF). These SPNs comprise of two morphological subtypes: fusiform SPNs with local axons, and pyramidal SPNs with axons that extend through the marginal zone. The former receive translaminar synaptic input up until the emergence of the whisker barrels; a timepoint coincident with significant cell death. In contrast, pyramidal SPNs receive local input from the subplate at early ages but then – during the later time window, acquire input from overlying cortex. Combined electrical and optogenetic activation of thalamic afferents identified that Lpar1-EGFP SPNs receive sparse thalamic innervation. These data reveal components of the postnatal network that interpret sparse thalamic input to direct the emergent columnar structure of S1BF.

Chromatin remodeler Arid1a regulates subplate neuron identity and wiring of cortical connectivity

Loss-of-function mutations in chromatin remodeler gene ARID1A are a cause of Coffin-Siris syndrome, a developmental disorder characterized by dysgenesis of corpus callosum. Here, we characterize Arid1a function during cortical development and find unexpectedly selective roles for Arid1a in subplate neurons (SPNs). SPNs, strategically positioned at the interface of cortical gray and white matter, orchestrate multiple developmental processes indispensable for neural circuit wiring. We find that pancortical deletion of Arid1a leads to extensive mistargeting of intracortical axons and agenesis of corpus callosum. Sparse Arid1a deletion, however, does not autonomously misroute callosal axons, implicating noncell-autonomous Arid1a functions in axon guidance. Supporting this possibility, the ascending axons of thalamocortical neurons, which are not autonomously affected by cortical Arid1a deletion, are also disrupted in their pathfinding into cortex and innervation of whisker barrels. Coincident with these miswiring phenotypes, which are reminiscent of subplate ablation, we unbiasedly find a selective loss of SPN gene expression following Arid1a deletion. In addition, multiple characteristics of SPNs crucial to their wiring functions, including subplate organization, subplate axon-thalamocortical axon cofasciculation (“handshake”), and extracellular matrix, are severely disrupted. To empirically test Arid1a sufficiency in subplate, we generate a cortical plate deletion of Arid1a that spares SPNs. In this model, subplate Arid1a expression is sufficient for subplate organization, subplate axon-thalamocortical axon cofasciculation, and subplate extracellular matrix. Consistent with these wiring functions, subplate Arid1a sufficiently enables normal callosum formation, thalamocortical axon targeting, and whisker barrel development. Thus, Arid1a is a multifunctional regulator of subplate-dependent guidance mechanisms essential to cortical circuit wiring.

Early peripheral activity alters nascent subplate circuits in the auditory cortex

Cortical function can be shaped by sensory experience during a critical period. The onset of the critical period is thought to coincide with the onset of thalamocortical transmission to the thalamo-recipient layer 4 (L4). In early development, subplate neurons (SPNs), and not L4 neurons, are the first targets of thalamic afferents. SPNs are transiently involved in early development and are largely eliminated during development. Activation of L4 by thalamic afferents coincides with the opening of ear canal (~P11 in mice) and precedes the later critical period. Here, we show in mice that abolishing peripheral function or presenting sound stimuli even before P11 leads to bidirectionally altered functional connectivity of SPNs in auditory cortex. Thus, early sensory experience can sculpt subplate circuits before thalamocortical circuits to L4 are mature. Our results show that peripheral activity shapes cortical circuits in a sequential manner and from earlier ages than has been appreciated.

Altered White Matter and Layer VIb Neurons in Heterozygous Disc1 Mutant, a Mouse Model of Schizophrenia

Increased white matter neuron density has been associated with neuropsychiatric disorders including schizophrenia. However, the pathogenic features of these neurons are still largely unknown. Subplate neurons, the earliest generated neurons in the developing cortex have also been associated with schizophrenia and autism. The link between these neurons and mental disorders is also not well established. Since cortical layer VIb neurons are believed to be the remnant of subplate neurons in the adult rodent brain, in this study, we aimed to examine the cytoarchitecture of neurons in cortical layer VIb and the underlying white matter in heterozygous Disc1 mutant (Het) mice, a mouse model of schizophrenia. In the white matter, the number of NeuN-positive neurons was quite low in the external capsule; however, the density of these cells was found increased (54%) in Het mice compared with wildtype (WT) littermates. The density of PV-positive neurons was unchanged in the mutants. In the cortical layer VIb, the density of CTGF-positive neurons increased (21.5%) in Het mice, whereas the number of Cplx3-positive cells reduced (16.1%) in these mutants, compared with WT mice. Layer VIb neurons can be classified by their morphological characters. The morphology of Type I pyramidal neurons was comparable between genotypes while the dendritic length and complexity of Type II multipolar neurons were significantly reduced in Het mice. White matter neurons and layer VIb neurons receive synaptic inputs and modulate the process of sensory information and sleep/arousal pattern. Aberrances of these neurons in Disc1 mutants implies altered brain functions in these mice.

Chromatin remodeler Arid1a regulates subplate neuron identity and wiring of cortical connectivity

Subplate neurons indispensably orchestrate the developmental assembly of cortical neural circuits. Here, by cell type-specific dissection of Arid1a function, we uncover an unexpectedly selective role for this ubiquitous chromatin remodeler in subplate neuron molecular identity and circuit wiring function. We find that pan-cortical deletion of Arid1a, but not sparse deletion, leads to mistargeting of callosal and thalamocortical connectivities reminiscent of subplate ablation. These miswiring phenotypes are concomitant with disrupted subplate neuron organization, morphogenesis, axons, and extracellular matrix. Mechanistically, Arid1a is required to establish the transcriptional identity of subplate neurons. Remarkably, cortical plate deletion of Arid1a, which spares subplate neurons, restores subplate axons and extracellular matrix, and is sufficient to extensively correct callosal and thalamocortical axon misrouting, revealing an axon guidance function of Arid1a centered on the subplate. Thus, Arid1a regulates the molecular identity and function of subplate neurons, and thereby non-cell autonomously mediates the formation of cortical connectivity during development.

An early cortical progenitor-specific mechanism regulates thalamocortical innervation

The cortical subplate is critical in regulating the entry of thalamocortical sensory afferents into the cortex. These afferents reach the subplate at embryonic day (E)15.5 in the mouse, but “wait” for several days, entering the cortical plate postnatally. We report that when transcription factor Lhx2 is lost in E11.5 cortical progenitors, which give rise to subplate neurons, thalamocortical afferents display premature, exuberant innervation of the E15.5 cortex. Embryonic mutant subplate neurons are correctly positioned below the cortical plate, but they display an altered transcriptome and immature electrophysiological properties during the waiting period. The sensory thalamus in these cortex-specific Lhx2 mutants displays atrophy, eventually leading to severe deficits in thalamocortical innervation. Strikingly, these phenotypes do not manifest if Lhx2 is lost in postmitotic subplate neurons. These results demonstrate a mechanism operating in subplate progenitors that has profound consequences on the growth of thalamocortical axons into the cortex.

Non-canonical role for Lpar1-EGFP subplate neurons in early postnatal somatosensory cortex

ABSTRACTSubplate neurons (SPNs) are a transient neuronal population shown to play a key role in nascent sensory processing relaying thalamic information to the developing cerebral cortex. However there is little understanding of how heterogeneity within this population relates to emergent function. To address this question we employed optical and electrophysiological technologies to investigate the synaptic connectivity of SPNs defined by expression of the Lpar1-EGFP transgene through the first postnatal week in primary whisker somatosensory cortex (S1BF) in mouse. Our data identify that the Lpar1-EGFP SPNs represent two morphological subtypes: (1) transient, fusiform SPNs with axons largely restricted to the subplate zone; (2) pyramidal SPNs with axon collaterals that traverse the overlying cortex to extend through the marginal zone. Laser scanning photostimulation of caged glutamate was used to determine columnar glutamatergic and GABAergic input onto both of these SPN subtypes. These experiments revealed that the former receive translaminar input from more superficial cortical layers up until the emergence of the whisker barrels (~postnatal (P)5). In contrast, pyramidal SPNs only receive local input from the adjacent subplate network at early ages but then at later ages can acquire varied input from the overlying cortex. Combined electrical stimulation of the ventral posterior nucleus of the thalamus and optogenetic activation of thalamic afferents in thalamocortical slice preparations revealed that Lpar1-EGFP SPNs only receive sparse thalamic innervation during early postnatal development. Taken together, these data reveal two components of the postnatal network that interpret sparse thalamic input to direct the emergent columnar structure of neonatal somatosensory cortex.