Functional interaction between BDNF and Kidins220: a study in primary mouse astrocytes and in an adult conditional knock-out mouse model

Neurotrophins are a family of growth factors known for their pleiotropic effects on neuronal survival, maturation and plasticity. Brain-derived neurotrophic factor (BDNF) is the most expressed in the brain. The activation of specific BDNF downstream pathways hinges on BDNF binding to its receptor TrkB. Kidins220 is a scaffold protein that interacts with neurotrophin receptors and is directly involved in the activation of neurotrophin signaling. It is also required for neuron differentiation, survival and plasticity. This protein has been linked to several diseases including psychiatric and neurodegenerative pathologies and for this reason, several KO animal models have been generated. So far none of them was viable after birth, making it impossible to investigate the role of this protein in postnatal/adult brain development. Astrocytes are fundamental in maintaining nervous system homeostasis. They are capable of perceiving a wide variety of extracellular cues and transducing them via the activation of specific intracellular signaling pathways into responses that may be protective or disruptive toward neighboring neurons. Moreover, astrocytes are key regulators of neuronal circuit formation and synaptic transmission. Several aspects of astrocyte physiology are controlled by neurotrophins. However, the role of Kidins220 in astrocytes, as well as in the adult brain remains largely unknown. Thus, in this thesis we aimed to deeply understand the role of Kidins220 using both in vivo and in vitro models. First we compared the signaling competence of embryonic and postnatal primary cortical astrocytes exposed to BDNF, and observed a shift from a kinase-based response in embryonic cells to a predominantly Ca2+-based response in postnatal cultures. We demonstrated that Kidins220 ablation is accompanied by a decreased expression of both BDNF receptor TrkB isoforms. We also described the role of Kidins220 in BDNF-induced signaling in astrocytes, showing that it contributes to both kinase and Ca2+-activated pathways. To evaluate the effect of Kidins220 ablation in the adult brain we used a floxed line that expresses only the full-length isoform, which we crossed with mice expressing Cre under the CamKII promoter, leading to a conditional knockout (cKO) line where Kidins220 is absent only in the excitatory neurons of the forebrain, starting at the second postnatal week. In this animal model, we have observed altered dendritic arborization and spine number in the cortico-hippocampal regions. The deletion of Kidins220 also leads to behavioral changes, such as reduced anxiety-like traits due to to alterations in TrkB-BDNF signaling. Our data increase the knowledge of the complex role played by Kidins220 both in astrocytes and in adult brains, reveal a previously unidentified role of this protein in astrocytes, controlling the response to BDNF and to Ca2+ dynamics during development. Finally, our data confirm the fundamental role of Kidins220 in adult mice, where its ablation leads to both behavioral and biochemical impairments.