The expression patterns of the transcription factor FOXP2 in the developing mammalian forebrain have been described, and some studies have tested the role of this protein in the development and function of specific forebrain circuits by diverse methods and in multiple species. Clinically, mutations in FOXP2 are associated with severe developmental speech disturbances, and molecular studies indicate that impairment of Foxp2 may lead to dysregulation of genes involved in forebrain histogenesis. Here, anatomical and molecular phenotypes of the cortical neuron populations that express FOXP2 were characterized in mice. Additionally, Foxp2 was removed from the developing mouse cortex at different prenatal ages using two Cre-recombinase driver lines. Detailed molecular and circuit analyses were undertaken to identify potential disruptions of development. Surprisingly, the results demonstrate that Foxp2 function is not required for many functions that it has been proposed to regulate, and therefore plays a more limited role in cortical development than previously thought.

24The expression patterns of the transcription factor FOXP2 in the developing mammalian 25 forebrain have been described, and some studies have tested the role of this protein in the 26 development and function of specific forebrain circuits by diverse methods and in multiple 27 species. Clinically, mutations in FOXP2 are associated with severe developmental speech 28 disturbances, and molecular studies indicate that impairment of Foxp2 may lead to 29 dysregulation of genes involved in forebrain histogenesis. Here, anatomical and molecular 30 phenotypes of the cortical neuron populations that express FOXP2 were characterized in mice. 31Additionally, Foxp2 was removed from the developing mouse cortex at different prenatal ages 32 using two Cre-recombinase driver lines. Detailed molecular and circuit analyses were 33 undertaken to identify potential disruptions of development. Surprisingly, the results 34 demonstrate that Foxp2 function is not required for many functions that it has been proposed to 35 regulate, and therefore plays a more limited role in cortical development than previously 36 thought. 3738 prenatal ages to ascertain its putative roles in the normal histogenic processes that generate the 65 canonical 6 layers, specific cell types based on gene expression, and basic axon targeting to 66 subcortical structures. The results show that FOXP2 expression is limited to specific corticofugal 67 neuron populations and suggest that the gene plays a more limited role in mouse corticogenesis 68 than previously concluded based on results obtained by other experimental methodologies. 69 70 Results 71 Foxp2 expression is enriched in developing corticothalamic projection neurons 72 Initial Foxp2 expression mapping studies determined that Foxp2 transcript and protein 73 expression begin prenatally, with the onset of protein expression delayed relative to the mRNA, 74 and with protein present primarily in postmitotic neurons (Ferland et al., 2003). However, other 75 more recent studies have reported that FOXP2 protein is also expressed in mitotic progenitor 76 cells (Tsui et al., 2013), where it regulates cortical neurogenesis. FOXP2 immunohistochemistry 77 of coronal sections of the embryonic forebrain suggested that FOXP2 protein expression begins 78 between embryonic day (E) 14.5 and E16.5 within postmitotic neurons of the infragranular 79 layers (Figure 1A, B). Postnatally, it is well established that Foxp2 expression is limited to 80 glutamatergic neurons of the infragranular cortical layers, with robust expression predominantly 81 in layer 6 (Ferland et al., 2003; Hisaoka et al., 2010; Sundberg et al., 2018). Layer 6 contains 82 many FOXP2 + neurons, whereas layer 5 contains some FOXP2 + neurons that are more 83 abundant in medial cortical areas at early postnatal stages (Ferland et al., 2003; Campbell et al., 84 2009). Layer 6, where most of the FOXP2+ neurons are located, is comprised of two primary 85 glutamatergic cortical neuron populations, corticothalamic (CT) and corticocortical (CC) neurons 86 (Thomson...

Cognitive impairment is a common phenotype of neurodevelopmental disorders, but how these deficits arise remains elusive. Determining the onset of discrete cognitive capabilities facilitates studies in probing mechanisms underlying their emergence. The present study analyzed the emergence of contextual fear memory persistence (7-day memory retention) and remote memory (30-day memory retention). There was a rapid transition from postnatal day (P) 20 to P21, in which memory persistence emerged in C57Bl/6J male and female mice. Remote memory was present at P23, but expression was not robust compared to pubertal and adult mice. To address a potential molecular mechanism, the present study examined the MET receptor tyrosine kinase (MET), which when deleted results in fear memory deficits in adult mice and regulates timing of critical period in the visual cortex, positioning it as a regulator for onset of contextual fear memory. Sustaining Met past the normal window of peak cortical expression or deleting Met did not alter the timing of emergence of persistence or remote memory capabilities. However, failure to exhibit fear memory occurred by P90 in mice with reduction or deletion of Met. Remarkably, the number of FOS-expressing infragranular neurons in medial prefrontal cortex (mPFC) did not increase with contextual conditioning at P35 but exhibited enhanced activation at P90. Additionally, MET-expressing neurons were preferentially recruited at P90 compared to P35 during fear memory expression. The studies demonstrate a developmental profile of contextual fear memory capabilities. Further, developmental disruption of Met leads to a delayed functional deficit that arises in adulthood.