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

ASPM and CITK regulate spindle orientation by affecting the dynamics of astral microtubules.

Correct orientation of cell division is considered an important factor for the achievement of normal brain size, as mutations in genes that affect this process are among the leading causes of microcephaly. Abnormal spindle orientation is associated with reduction of the neuronal progenitor symmetric divisions, premature cell cycle exit, and reduced neurogenesis. This mechanism has been involved in microcephaly resulting from mutation of ASPM, the most frequently affected gene in autosomal recessive human primary microcephaly (MCPH), but it is presently unknown how ASPM regulates spindle orientation. In this report, we show that ASPM may control spindle positioning by interacting with citron kinase (CITK), a protein whose loss is also responsible for severe microcephaly in mammals. We show that the absence of CITK leads to abnormal spindle orientation in mammals and insects. In mouse cortical development, this phenotype correlates with increased production of basal progenitors. ASPM is required to recruit CITK at the spindle, and CITK overexpression rescues ASPM phenotype. ASPM and CITK affect the organization of astral microtubules (MT), and low doses of MT-stabilizing drug revert the spindle orientation phenotype produced by their knockdown. Finally, CITK regulates both astral-MT nucleation and stability. Our results provide a functional link between two established microcephaly proteins

Characterization of VIP+ interneurons in the mouse whisker barrel cortex during development

GABAergic interneurons (INs) are thought to be important players in normal cortical circuit development, with a body of data highlighting the role of two major IN classes – parvalbumin and somatostatin-positive (PV+ and SST+) subtypes – in neonatal mouse sensory cortices (Marques-Smith et al., 2016; Takesian and Hensch, 2013; Tuncdemir et al., 2016). More recent evidence supports a developmental role for a third class of IN, defined by the expression of vasoactive intestinal peptide (VIP+ INs) (Batista-Brito et al., 2017; Goff and Goldberg, 2019; Mossner et al., 2017). This IN subtype has been shown to play a role in sensorimotor integration via preferential targeting of SST+ cells and consequent disinhibition of pyramidal cells in the mature cortex (e.g., Lee et al., 2013). However, it is unknown how and when VIP+ INs integrate into the local and long-range circuitry during early development. To address this, I first investigated the integration of VIP+ cells into the local circuit in postnatal whisker barrel cortex (S1BF). Data show that, while VIP+ INs acquire mature electrophysiological properties and integrate in the local glutamatergic network over the first two postnatal weeks, they engage with other INs and pyramidal cells already in the first postnatal week. Second, I employed a viral optogenetic strategy to test the emergence of long-range inputs from anterior-motor areas onto S1BF VIP+ cells. Data show that VIP+ INs start to be recruited already in the first postnatal week, but they are fully integrated in the long-range circuitry only at the end of the second postnatal week. Finally, I have used the VIP-Cre;Prox1c/c conditional knockout model (Miyoshi et al., 2015) to investigate whether genetic perturbation of VIP+ INs has an impact on synaptic integration and in vivo activity. Data suggest that conditional deletion of Prox1 leads to reorganisation of the local but not long-range glutamatergic input, and increased activity upon whisker stimulation. These findings define the emergence of the VIP+ circuitry and show their early influence in circuit maturation, further supporting the importance of IN signalling in cortical development

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Distinct roles for dopamine clearance mechanisms in regulating behavioral flexibility

Dopamine plays a crucial role in adaptive behavior, and dysfunctional dopamine is implicated in multiple psychiatric conditions characterized by inflexible or inconsistent choices. However, the precise relationship between dopamine and flexible decision making remains unclear. One reason is that, while many studies have focused on the activity of dopamine neurons, efficient dopamine signaling also relies on clearance mechanisms, notably the dopamine transporter (DAT), which predominates in striatum, and catechol-O-methyltransferase (COMT), which predominates in cortex. The exact locus, extent, and timescale of the effects of DAT and COMT are uncertain. Moreover, there is limited data on how acute disruption of either mechanism affects flexible decision making strategies mediated by cortico-striatal networks. To address these issues, we combined pharmacological modulation of DAT and COMT with electrochemistry and behavior in mice. DAT blockade, but not COMT inhibition, regulated sub-second dopamine release in the nucleus accumbens core, but surprisingly neither clearance mechanism affected evoked release in prelimbic cortex. This was not due to a lack of sensitivity, as both amphetamine and atomoxetine changed the kinetics of sub-second release. In a multi-step decision making task where mice had to respond to reversals in either reward probabilities or the choice sequence to reach the goal, DAT blockade selectively impaired, and COMT inhibition improved, performance after reward reversals, but neither manipulation affected the adaptation of choices after action-state transition reversals. Together, our data suggest that DAT and COMT shape specific aspects of behavioral flexibility by regulating different aspects of the kinetics of striatal and cortical dopamine, respectively