Cooperative interaction between serotonergic signalling and extracellular matrix in regulation of synaptic plasticity under physiological and pathological conditions

突触传递效能的长期改变（突触可塑性）被认为是学习和记忆的核心机制。突触可塑性伴随着突触前和突触后的分子和形态变化，导致神经网络在更大范围内的重新布线，在广泛的神经和认知疾病中起着关键作用。我们之前已经证明5-羟色胺受体7 （5-HT7R）具有明显的形态发生特性。受体介导的异三聚体G12蛋白的激活导致小GTPase Cdc42的选择性激活，这反过来导致细胞形态的显著变化，包括神经突生长和突触发生。结构重塑和突触可塑性不仅依赖于神经元的形态发生信号，还依赖于周围细胞外基质（ECM）的重组。脑ECM的核心成分是透明质酸（HA），它构成了神经周围网络的骨干，并影响可塑性的结构和功能方面。血凝素的主要受体是跨膜蛋白CD44。我们还通过发现一个涉及5-HT7R、CD44、基质金属蛋白酶-9 （MMP-9）和小GTPase Cdc42的新的信号通路，确定了5-羟色胺能信号传导与ECM之间的功能相互作用。潜在的分子机制涉及5- ht7r介导的MMP-9的激活，这导致CD44裂解，随后Cdc42激活。我们还强调了5-HT7R和CD44之间的物理相互作用。然而，调控5-HT7R/CD44复合物形成和稳定性的机制以及这种相互作用在神经元中的功能影响尚不清楚。因此，本研究的一个重要目标是了解调节5-HT7R/CD44复合物形成的分子机制，并阐明其在神经元中的功能后果，包括细胞内信号通路的调节、结构重塑和突触可塑性。在我们的初步实验中，我们证明了CD44在脑中的翻译后被棕榈酰化修饰。此外，我们在CD44的c端区域内确定了假定的棕榈酰化位点。因此，我们的第二个目标是在分子水平上确定调节CD44棕榈酰化的机制，并分析CD44棕榈酰化在神经元中5-HT7R/CD44信号传导调节中的作用。最后，我们将在生理和病理条件下研究5-HT7R/CD44信号通路和CD44棕榈酰化在体内的影响。后者也意味着验证5-HT7R/CD44复合物作为治疗抑郁症的潜在治疗靶点。

Long-lasting alterations in the efficacy of synaptic transmission (synaptic plasticity) are regarded as a central mechanism for learning and memory. Synaptic plasticity goes along with molecular and morphological changes in the pre- and post-synapse and results in the rewiring of the neural network on a larger scale, which plays a key role in a wide range of neurological and cognitive disorders. We have previously demonstrated that the serotonin receptor 7 (5-HT7R) possesses pronounced morphogenic properties. Receptor-mediated activation of the heterotrimeric G12 protein results in a selective activation of small GTPase Cdc42, which in turn leads to prominent changes in cellular morphology, including neurite outgrowth and synaptogenesis. Structural remodeling and synaptic plasticity depend not only on morphogenic signaling in neurons, but also on the reorganization of the surrounding extracellular matrix (ECM). The central component of the brain’s ECM is hyaluronan (HA), which forms the backbone of perineuronal nets and influences both structural and functional aspects of plasticity. The main receptor for HA is the transmembrane protein CD44.We have also identified a functional interplay between serotonergic signaling and ECM by uncovering a novel signaling pathway involving the 5-HT7R, the CD44, the matrix metalloproteinase-9 (MMP-9), and the small GTPase Cdc42. The underlying molecular machinery involves 5-HT7R-mediated activation of MMP-9, which leads to CD44 cleavage followed by Cdc42 activation. We also highlighted physical interaction between 5-HT7R and CD44. However, mechanisms regulating formation and stability of the 5-HT7R/CD44 complex as well as functional impact of this interaction in neurons are still not known. One important goal of the present proposal is thus to understand the molecular mechanisms regulating the formation of 5-HT7R/CD44 complexes and to elucidate their functional consequences in neurons, including modulation of intracellular signaling pathways, structural remodeling, and synaptic plasticity.In our preliminary experiments we demonstrated that CD44 is post-translationally modified by palmitoylation in the brain. Furthermore, we identified putative palmitoylation site within the C-terminal domain of CD44. Therefore, our second goal is to define mechanisms regulating CD44 palmitoylation on a molecular level and to analyze the involvement of CD44 palmitoylation in the modulation of the 5-HT7R/CD44 signaling in neurons. Finally, we will investigate the impact of the 5-HT7R/CD44 signaling and CD44 palmitoylation in vivo under physiological and pathological conditions. The latter also implies verification of the 5-HT7R/CD44 complexes as a potential therapeutic target for the treatment of depressive disorders.