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

突触传递效率的长期变化(突触可塑性)被认为是学习和记忆的中心机制。突触的可塑性伴随着突触前后分子和形态的变化，导致神经网络在更大范围内重新布线，这在广泛的神经和认知障碍中起着关键作用。我们先前已经证明5-羟色胺受体7(5-HT7R)具有显著的形态发生特性。受体介导的异源三聚体G12蛋白的激活导致小GTP酶CDc42的选择性激活，这反过来又导致细胞形态的显著变化，包括轴突生长和突触形成。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。大脑细胞外基质的中心成分是透明质酸(HA)，它构成神经周围神经网络的主干，影响可塑性的结构和功能方面。HA的主要受体是跨膜蛋白CD44。我们还发现了一条新的信号通路，涉及5-HT7R、CD44、基质金属蛋白酶-9(MMP9)和小GTP酶CDc42，从而确定了5-羟色胺能信号和ECM之间的功能相互作用。其潜在的分子机制涉及5-HT7R介导的基质金属蛋白酶-9的激活，从而导致CD44的裂解，随后是CDC42的激活。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复合体的形成和稳定性的机制以及这种相互作用对神经元的功能影响仍不清楚。因此，本研究的一个重要目标是了解5-HT7R/CD44复合体形成的分子机制，并阐明它们在神经元中的功能后果，包括细胞内信号通路的调节、结构重构和突触可塑性。在我们的初步实验中，我们证明CD44是通过脑中的棕榈酰化作用进行翻译后修饰的。此外，我们还在CD44的C-末端结构域中确定了可能的棕榈酰化位点。因此，我们的第二个目标是在分子水平上确定CD44棕榈酰化的调控机制，并分析CD44棕榈酰化在神经元5-HT7R/CD44信号调节中的作用。最后，我们将研究生理和病理条件下5-HT7R/CD44信号转导和CD44棕榈酰化在体内的影响。后者还意味着5-HT7R/CD44复合体被证实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.