Fibroblast growth factors (FGFs) and FGF receptors (FGFRs) are known for their potent effects on cell proliferation/differentiation and cortical patterning in the developing brain. However, little is known regarding the roles of FGFs/FGFRs in cortical circuit formation. Here we show that //3 and /// mRNAs are expressed in the developing primary somatosensory (S1) barrel cortex. Barrel cortex layer IV spiny stellate cells (bSCs) are the primary recipients of ascending sensory information via thalamocortical axons (TCAs). Detail quantification revealed distinctive phases for bSC dendritogenesis: orienting dendrites toward TCAs, adding dendritic segments, and elongating dendritic length, while maintaining dendritic patterns. Deleting// in bSCs had minimal impact on dendritic polarity but transiently increased the number of dendritic segments. However, 6 d later, FGFR1/2/3 loss of function reduced dendritic branch numbers. These data suggest that FGFs/FGFRs have a role in stabilizing dendritic patterning. Depolarization of cultured mouse cortical neurons upregulated the levels of several / mRNAs within 2 h. , within 6 h of systemic kainic acid administration at postnatal day 6, mRNA levels of, ,, and in S1 cortices were enhanced, and this was accompanied by exuberant dendritogenesis of bSCs by 24 h. Deleting// abolished kainic acid-induced bSC dendritic overgrowth. Finally, FGF9/10 gain of function also resulted in extensive dendritogenesis. Together, our data suggest that FGFs/FGFRs can be regulated by glutamate transmission to modulate/stabilize bSC dendritic complexity. Both male and female mice were used for our study. Glutamatergic transmission plays critical roles in cortical circuit formation. Its dysregulation has been proposed as a core factor in the etiology of many neurological diseases. We found that excessive glutamate transmission upregulated mRNA expression of and their ligands Deleting // not only impaired bSC dendritogenesis but also abolished glutamate transmission-induced dendritic overgrowth. Overexpressing FGF9 or FGF10 in cortical glutamatergic neurons results in excessive dendritic outgrowth within 24 h, resembling the changes induced by excessive glutamate transmission. Our findings provide strong evidence for the physiological role of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) in establishing and maintaining cortical circuits. Perturbing the expression levels of FGFs/FGFRs by excessive glutamatergic neurotransmission could lead to abnormal neuronal circuits, which may contribute to neurological and psychiatric disease.

Abnormal levels of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) have been detected in various neurological disorders. The potent impact of FGF-FGFR in multiple embryonic developmental processes makes it challenging to elucidate their roles in postmitotic neurons. Taking an alternative approach to examine the impact of aberrant FGFR function on glutamatergic neurons, we generated a FGFR gain-of-function (GOF) transgenic mouse, which expresses constitutively activated FGFR3 (FGFR3K650E) in postmitotic glutamatergic neurons. We found that GOF disrupts mitosis of radial-glia neural progenitors (RGCs), inside-out radial migration of post-mitotic glutamatergic neurons, and axonal tract projections. In particular, late-born CUX1-positive neurons are widely dispersed throughout the GOF cortex. Such a cortical migration deficit is likely caused, at least in part, by a significant reduction of the radial processes projecting from RGCs. RNA-sequencing analysis of the GOF embryonic cortex reveals significant alterations in several pathways involved in cell cycle regulation and axonal pathfinding. Collectively, our data suggest that FGFR3 GOF in postmitotic neurons not only alters axonal growth of postmitotic neurons but also impairs RGC neurogenesis and radial glia processes.

Abnormal levels of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) have been detected in various neurological disorders. The potent impact of FGF-FGFR in multiple embryonic developmental processes makes it challenging to elucidate their roles in post-mitotic neurons. Taking an alternative approach, we directly examined the impact of aberrant FGFR function after neurogenesis by generating a FGFR gain-of-function (GOF) transgenic mouse which expresses constitutively activated FGFR3 (FGFR3 K650E ) in post-mitotic glutamatergic neurons. We found that enhanced FGFR activity in glutamatergic neurons results in abnormal radial migration and axonal miswiring. Regarding the lamination phenotype in GOF brains, we found later-born Cux1-positive neurons are dispersed throughout the GOF cortex. Such a cortical migration deficit is likely caused, at least in part, by a significant reduction of the radial processes normally projecting from the radial glia cells (RGCs). In addition, FGFR3 GOF also results in the misrouting of several long-range axonal projections, including the corpus callosum, anterior commissure, and postcommissural fornix. RNA-sequencing analysis of the GOF embryonic cortex reveals significant alterations in several pathways involved in cell cycle regulation and axonal pathfinding. Collectively, our results suggest that FGFR hyperfunction in post-mitotic neurons at the late embryonic stage result in cortical dysplasia and circuit miswiring.