Fgfr1 is required for cortical regeneration and repair after perinatal hypoxia.

Chronic postnatal hypoxia causes an apparent loss of cortical neurons that is reversed during recovery (Fagel et al., 2006). The cellular and molecular mechanisms underlying this plasticity are not understood. Here, we show that chronic hypoxia from postnatal days 3 (P3) to 10 causes a 30% decrease in cortical neurons and a 24% decrease in cortical volume. T-brain-1 (Tbr1)(+) and SMI-32(+) excitatory neuron numbers were completely recovered 1 month after the insult, but the mice showed a residual deficit in Parvalbumin(+) and Calretinin(+) GABAergic interneurons. In contrast, hypoxic mice carrying a disrupted fibroblast growth factor receptor-1 (Fgfr1) gene in GFAP+ cells [Fgfr1 conditional knock-out (cKO)], demonstrated a persistent loss of excitatory cortical neurons and a worsening of the interneuron defect. Labeling proliferating progenitors at P17 revealed increased generation of cortical NeuN(+) and Tbr1(+) excitatory neurons in wild-type mice subjected to hypoxic insult, whereas Fgfr1 cKO failed to mount a cortical neurogenetic response. Hypoxic wild-type mice also demonstrated a twofold increase in cell proliferation in the subventricular zone (SVZ) at P17 and a threefold increase in neurogenesis in the olfactory bulb (OB) at P48, compared with normoxic mice. In contrast, Fgfr1 cKO mice had decreased SVZ cell proliferation and curtailed reactive neurogenesis in the OB. Thus, the activation of FGFR-1 in GFAP+ cells is required for neuronal recovery after neonatal hypoxic injury, which is attributable in part to enhanced cortical and OB neurogenesis. In contrast, there is incomplete recovery of inhibitory neurons after injury, which may account for persistent behavioral deficits.

Midline radial glia translocation and corpus callosum formation require FGF signaling.

Midline astroglia in the cerebral cortex develop earlier than other astrocytes through mechanisms that are still unknown. We show that radial glia in dorsomedial cortex retract their apical endfeet at midneurogenesis and translocate to the overlaying pia, forming the indusium griseum. These cells require the fibroblast growth factor receptor 1 (Fgfr1) gene for their precocious somal translocation to the dorsal midline, as demonstrated by inactivating the Fgfr1 gene in radial glial cells and by RNAi knockdown of Fgfr1 in vivo. Dysfunctional astroglial migration underlies the callosal dysgenesis in conditional Fgfr1 knockout mice, suggesting that precise targeting of astroglia to the cortex has unexpected roles in axon guidance. FGF signaling is sufficient to induce somal translocation of radial glial cells throughout the cortex; furthermore, the targeting of astroglia to dorsolateral cortex requires FGFr2 signaling after neurogenesis. Hence, FGFs have an important role in the transition from radial glia to astrocytes by stimulating somal translocation of radial glial cells.

Decrease in excitatory neurons, astrocytes and proliferating progenitors in the cerebral cortex of mice lacking exon 3 from the Fgf2 gene.

BACKGROUND: The Fgf2 gene is expressed in the brain neuroepithelium during embryonic development and in astroglial cells throughout life. Previous knockout studies suggested that FGF2 plays a role in the proliferation of neural progenitors in the embryonic cerebral cortex. These studies exclusively used knockout alleles lacking the Fgf2 exon 1. However, the description of putative alternative exons located downstream from the canonical exon 1 raised the possibility that alternatively spliced transcripts may compensate for the lack of the canonical exon 1 in the Fgf2 -/- mice. RESULTS: We generated and characterized a new line of Fgf2 knockout mice lacking the expression of exon 3, which is conserved in all Fgf2 transcripts and contains essential heparin and receptor binding interfaces. The expression of Fgf2 exon 3 was prevented by inserting a transcriptional STOP cassette in the Fgf2 genomic locus. These mice demonstrate a phenotype in the adult neocortex characterized by decreased density and number of cortical excitatory neurons and astrocytes, which is virtually identical to that of the Fgf2 -/- mice lacking exon 1. In addition, we also show that the Fgf2 exon 3 knockout mice have decreased proliferation of precursors in the adult cerebral cortex, which had not been previously investigated in the other mutant lines. CONCLUSION: The results demonstrate that the phenotype of two completely different Fgf2 KO mouse lines, lacking exon 1 or exon 3, is remarkably similar. The combined results from these KO models clearly indicate that FGF2 plays a role in cortical cell genesis during embryonic development as well as in adulthood. Thus, FGF2 may be required for the maintenance of the pool of adult cortical progenitor cells.

Early postnatal astroglial cells produce multilineage precursors and neural stem cells in vivo.

To identify the fates that astroglial cells can attain in the postnatal brain, we generated mice carrying an inducible Cre recombinase (Cre-ER(T2)) controlled by the human GFAP promoter (hGFAP). In mice carrying the GCE (hGFAP-Cre-ER(T2)) transgene, OHT (4-hydroxy-tamoxifen) injections induced Cre recombination in astroglial cells at postnatal day 5 and allowed us to permanently tag these cells with reporter genes. Three days after recombination, reporter-tagged cells were quiescent astroglial cells that expressed the stem cell marker LeX in the subventricular zone (SVZ) and dentate gyrus (DG). After 2-4 weeks, the tagged GFAP lineage included proliferating progenitors expressing the neuronal marker Dcx (Doublecortin) in the SVZ and the DG. After 4 weeks, the GFAP lineage generated mature neurons in the olfactory bulb (OB), DG, and, strikingly, also in the cerebral cortex. A major portion of all neurons in the DG and OB born at the end of the first postnatal week were generated from GFAP+ cells. In addition to neurons, mature oligodendrocytes and astrocytes populating the cerebral cortex and white matter were also the progeny of GFAP+ astroglial ancestors. Thus, genetic fate mapping of postnatal GFAP+ cells reveals that they seed the postnatal brain with neural progenitors/stem cells that in turn give rise to neural precursors and their mature neuronal and oligodendrocytic progeny in many CNS regions, including the cerebral cortex.

Astroglial cells in development, regeneration, and repair.

Three main cellular components have been described in the CNS: neurons, astrocytes, and oligodendrocytes. In the past 10 years, lineage studies first based on retroviruses in the embryonic CNS and then by genetic fate mapping in both the prenatal and postnatal CNS have proposed that astroglial cells can be progenitors for neurons and oligodendrocytes. Hence, the population of astroglial cells is increasingly recognized as heterogeneous and diverse, encompassing cell types performing widely different roles in development and plasticity. Astroglial cells populating the neurogenic niches increase their proliferation after perinatal injury and in young mice can differentiate into neurons and oligodendrocytes that migrate to the cerebral cortex, replacing the cells that are lost. Although much remains to be learned about this process, it appears that the up-regulation of the Fibroblast growth factor receptor is critical for mediating the injury-induced increase in cell division and perhaps for the neuronal differentiation of astroglial cells.

Fibroblast growth factor receptor 1 is required for the proliferation of hippocampal progenitor cells and for hippocampal growth in mouse.

Fibroblast growth factor receptor 1 (Fgfr1) is expressed at high levels by progenitor cells of the ventricular zone (VZ) within the hippocampal primordium. To investigate the role of Fgfr1 in these cells, in vivo Cre recombination of "floxed" Fgfr1 alleles was directed to cells of the radial glial lineage by using the human glial fibrillary acidic protein promoter. Radial glial-like cells of the hippocampal VZ are the progenitors of pyramidal neurons and granule cells of hippocampal dentate gyrus (DG). Mice carrying null Fgfr1 alleles (Fgfr1(Deltaflox)) in cells of this lineage showed a dramatic loss of Fgfr1 gene expression throughout the embryonic dorsal telencephalon. These Fgfr1(Deltaflox) mice exhibited a approximately 30% decrease in dividing radial glial progenitor cells in the hippocampal VZ and DG in the late embryonic period, progressing to a approximately 50-60% loss at birth, without any changes in cell survival. In addition, no FGF2-sensitive neural stem cells could be isolated from the Fgfr1(Deltaflox) hippocampal neuroepithelium, whereas epidermal growth factor-sensitive neural stem cells were not affected. The number of hippocampal pyramidal neurons and DG granule cells was approximately 30-50% decreased from the perinatal period through adulthood, and the number of parvalbumin-containing interneurons was similarly decreased in both the DG and pyramidal cell fields. We conclude that Fgfr1 is necessary for hippocampal growth, because it promotes the proliferation of hippocampal progenitors and stem cells during development.

Deficiency in inhibitory cortical interneurons associates with hyperactivity in fibroblast growth factor receptor 1 mutant mice.

BACKGROUND: Motor hyperactivity due to hyper-dopaminergic neurotransmission in the basal ganglia is well characterized; much less is known about the role of the neocortex in controlling motor behavior. METHODS: Locomotor behavior and motor, associative, and spatial learning were examined in mice with conditional null mutations of fibroblast growth factor receptor 1 (Fgfr1) restricted to telencephalic neural precursors (Fgfr1(f/f;hGfapCre)). Locomotor responses to a dopamine agonist (Amphetamine 2 mg/kg and Methylphenidate 10 mg/kg) and antagonists (SCH233390 .025 mg/kg and Haloperidol .2 mg/kg) were assessed. Stereological and morphological characterization of various monoaminergic, excitatory, and inhibitory neuronal subtypes was performed. RESULTS: Fgfr1(f/f;hGfapCre) mice have spontaneous locomotor hyperactivity characterized by longer bouts of locomotion and fewer resting points that is significantly reduced by the D1 and D2 receptor antagonists. No differences in dopamine transporter, tyrosine hydroxylase, or serotonin immunostaining were observed in Fgfr1(f/f;hGfapCre) mice. There was no change in cortical pyramidal neurons, but parvalbumin+, somatostatin+, and calbindin+ inhibitory interneurons were reduced in number in the cerebral cortex. The decrease in parvalbumin+ interneurons in cortex correlated with the extent of hyperactivity. CONCLUSIONS: Dysfunction in specific inhibitory cortical circuits might account for deficits in behavioral control, providing insights into the neurobiology of psychiatric disorders.