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Final Report Summary - CORTICAL FOXP2 (Functional role of Foxp2 and Caspr2 in the cortex)

The identification of mutations of FOXP2 have provided the first example of a gene specifically implicated in a speech and language disorder initially in the multigenerational KE family. Here around half the members carry a heterozygous FOXP2 mutation which is inherited in an autosomal dominant manner. Affected individuals have difficulty mastering the orofacial motor sequences necessary for fluent speech (verbal dyspraxia) accompanied by other deficits in both oral and written language. It has long been suggested that critical roles of FOXP2 might lie further upstream than the motor system. Foxp2 is expressed in about 50% of cortical layer 6 pyramidal neurons in mice and humans, yet the role of Foxp2 in cortical development and function is largely unknown. Interestingly, subsequent studies revealed that the closely related FOXP1 gene also plays a role in neurodevelopmental processes. Novel rare disruptions in FOXP1 have been reported in multiple cases of cognitive dysfunction, including intellectual disability and autism spectrum disorder, together with language impairment.
The general objective of the project was to characterize the role of Foxp1 and Foxp2 in the development and function of cortical neurons.

In the first part of the project (first reporting period, 2013-2015), we took advantage of newly generated transgenic mice with a brain-specific Foxp1 deletion (Nestin-CreFoxp1−/−mice) to understand the role of Foxp1 in the context of neurodevelopment. The mutant mice allowed for the first time the analysis of neurodevelopmental phenotypes, which included anatomical alterations in the hippocampus. We performed a more detailed analysis in the CA1 region of the hippocampus. We showed that abnormal neuronal morphogenesis of CA1 pyramidal cells was associated with reduced excitability and an imbalance of excitatory to inhibitory input in CA1 hippocampal neurons in Nestin-CreFoxp1−/− mice. Interestingly, Foxp1 ablation was also associated with various cognitive and social deficits. These results were published in Molecular Psychiatry in 2015.
In the second part of the project (second reporting period, 2015-2017) we pursued the complementary characterization of the role of Foxp2 in the cortex using mice that carry a Foxp2 mutation (R552H) identical to the mutation identified in the KE family. Our studies revealed that cortical neurons expressing the mutant form of Foxp2 showed altered excitability and decreased potassium channel expression. Neuronal morphology was also affected in mutant mice. In addition, the frequency of miniature excitatory postsynaptic currents was decreased. The results were presented at the FENS meeting in Copenhagen in 2016. Together with T. Lemonnier, we also generated Foxp2-expressing human induced pluripotent stem cell (iPSC)-derived neurons that were characterized using patch-clamp electrophysiology. Cortical iPSC-derived neurons were successfully generated but lacked physiological features of maturity such as trains of action potentials.

Our studies have shown that Foxp1 and Foxp2 share similar roles in the control of neuronal morphology and neuronal excitability. While the amplitude of excitatory postsynaptic currents increased in Foxp1-deficient neurons, the frequency (but not the amplitude) of excitatory postsynaptic currents was decreased in Foxp2R552H neurons. Moreover, their pattern of expression differs in the mammalian brain. This may constitute the neurobiological basis for the distinct endophenotypes associated with mutations in either Foxp1 or Foxp2 in human patients.

Excitatory/inhibitory imbalance is a hallmark feature of several neurodevelopmental disorders including Autism Spectrum Disorder. Several studies have reported that ASD-related mutations selectively impact glutamatergic or GABAergic synapses without affecting the other, leading to an imbalance of excitatory and inhibitory inputs.
We have shown that the amplitude and/or frequency of excitatory postsynaptic currents but not inhibitory postsynaptic currents changed in Foxp1-/- neurons and Foxp2R552H neurons, suggesting that the Excitatory/inhibitory balance is changed in the brain of patients with decreased expression or function of either Foxp1 or Foxp2. In addition, the excitability of pyramidal cells was reduced in Foxp1 -/- and Foxp2R552H mice. Whether these physiological deficits occur independently of each other is not clear. It is possible that the excitability of Foxp1 KO neurons is reduced to compensate for the increased glutamatergic transmission we observed, thus maintaining neurons within their physiological range of firing rate. These results show that genes involved in language disorders impact the balance of excitatory and inhibitory inputs in the cortex and control neuronal excitability and dendritic length. This may constitute an important step towards molecular therapies aiming at treating these disorders.

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