During postnatal brain development, the maturation of inhibitory neuronal circuits and formation of perineuronal nets (PNNs) around GABAergic interneurons result in a transition from juvenile, highly malleable, forms of plasticity to adult restricted modalities. Emerging evidence from the visual cortex points to a key role for the orthodenticle homeobox 2 (OTX2) protein in such critical developmental transitions. OTX2 internalization in neurons ensheated by PNNs induces their maturation and is necessary to open, then close, critical periods of plasticity. PNNs, specialized extracellular matrix structures surrounding distinct neuronal populations, regulate synaptic functions and plasticity, and sustain intracellular OTX2 levels in mature neurons. Converging evidence suggests that OTX2/PNN interactions may affect brain regions beyond the visual cortex, including the amygdala and prefrontal cortex (PFC). Of note, OTX2 is not produced within the adult brain. Results from rodents, and preliminary data in human, point to the choroid plexus (ChP) as a global source of OTX2, implying that altered OTX2 synthesis outside the brain parenchyma may have a profound impact on key neuronal functions. Together, these findings suggest the intriguing possibility that availability of ChP-derived OTX2 may modulate inhibitory neuronal circuits and adult forms of plasticity, a concept with far-reaching physiological, pathological and therapeutic implications. Notably, each element of this mechanism is of particular interest to the pathophysiology of schizophrenia (SZ): I) Involvement of the ChP, and the cerebrospinal fluid (CSF)/blood barrier, have been long suspected, although somewhat neglected in recent years. II) GABAergic neuron and PNN abnormalities have been reported in several brain regions, including the amygdala and PFC, brain regions involved in cognitive and emotion processing and in the pathophysiology of SZ. III) Preliminary evidence shows OTX2 decreases in the CSF, amygdala and PFC of subjects with SZ. We postulate that deficits of ChP-derived OTX2 and abnormalities affecting GABAergic interneurons and PNNs in SZ may be mechanistically linked. The proposed investigations employ a complementary, truly translational approach, combining human studies on postmortem amygdala, PFC, visual cortex, and CSF and in vitro studies on human ChP epithelial cells, with animal model approaches including conditional gene-targeting in mice, and whole-cell electrophysiology. These investigations will test the hypothesis that OTX2 originating from the ChP is pivotal to neuronal maturation and circuit plasticity in the PFC and amygdala. In particular, we postulate that OTX2 affects maturation and maintenance of PNNs surrounding GABAergic neurons and their functional and behavioral correlates. In SZ, we hypothesize that OTX2 deficits occur in the ChP and CSF as well as in the amygdala and PFC in association with PNN loss and GABAergic neuron abnormalities. The potential for systemic modulation of these mechanisms through the CP, tested in these studies, may broaden our understanding of brain plasticity and open novel therapeutic approaches to SZ.
The proposed research links novel neurobiological concepts on the role of the choroid plexus, and its role as a global brain source of the homeoprotein OTX2, on postnatal brain maturation and plasticity to the pathophysiology of schizophrenia. These translational studies also bear relevance to a number of brain disorders with similarly strong developmental components, in which the postnatal maturation of intrinsic inhibitory neural circuits may be affected.
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