Neuritogenesis Induced by Thyroid Hormone-treated Astrocytes Is Mediated by Epidermal Growth Factor/Mitogen-activated Protein Kinase-Phosphatidylinositol 3-Kinase Pathways and Involves Modulation of Extracellular Matrix Proteins

Abstract

Thyroid hormone (T3) plays a crucial role in several steps of cerebellar ontogenesis. By using a neuron-astrocyte coculture model, we have investigated the effects of T3-treated astrocytes on cerebellar neuronal differentiation in vitro. Neurons plated onto T3-astrocytes presented a 40 -60% increase on the total neurite length and an increment in the number of neurites. Treatment of astrocytes with epidermal growth factor (EGF) yielded similar results, suggesting that this growth factor might mediate T3-induced neuritogenesis. EGF and T3 treatment increased fibronectin and laminin expression by astrocytes, suggesting that astrocyte neurite permissiveness induced by these treatments is mostly due to modulation of extracellular matrix (ECM) components. Such increase in ECM protein expression as well as astrocyte permissiveness to neurite outgrowth was reversed by the specific EGF receptor tyrosine kinase inhibitor, tyrphostin. Moreover, studies using selective inhibitors of several transductionsignaling cascades indicated that modulation of ECM proteins by EGF is mainly through a synergistic activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways. In this work, we provide evidence of a novel role of EGF as an intermediary factor of T3 action on cerebellar ontogenesis. By modulating the content of ECM proteins, EGF increases neurite outgrowth. Our data reveal an important role of astrocytes as mediators of T3-induced cerebellar development and partially elucidate the role of EGF and mitogen-activated protein kinase/phosphatidylinositol 3-kinase pathways on this process.

Modulation of Neuritogenesis by Astrocyte Muscarinic Receptors

“…Two of these proteins, fibronectin and laminin, play a pivotal role in neuronal differentiation mediated by astrocytes (15,21). We, thus, investigated the effect of carbachol on the release of fibronectin and laminin-1 from astrocytes.…”

“…Although the importance of neuron-glia communication in neuronal development is well established, the stimuli that induce astrocytes to produce neurite-promoting or -inhibiting factors have not been extensively investigated; few exceptions are thyroid hormone (T3), which has been shown to stimulate the release of epidermal growth factor from astrocytes, leading to neuritogenesis in cerebellar neurons (21), and the vasoactive intestinal polypeptide, which stimulates the release by astrocytes of activity-dependent neurotrophic factor, promoting morphological and functional differentiation of hippocampal neurons (22). Astrocytes express many neurotransmitter receptors (23) whose functions in these cells remain elusive; among others, astrocytes express M2 and M3 muscarinic receptors (24).…”

Voltage-Gated Na+Channel β1 Subunit-Mediated Neurite Outgrowth Requires Fyn Kinase and Contributes to Postnatal CNS DevelopmentIn Vivo

“…Given that EGF has also been shown to regulate neurite outgrowth (Martinez and Gomes, 2002;Povlsen et al, 2008) and that L1-CAM-mediated cell adhesion activates EGFR tyrosine kinase (Islam et al, 2004), we next tested whether EGFR signaling is involved in ␤1-dependent neurite outgrowth, using the EGFR inhibitor AG1478. In a control experiment, the activity of AG1478 was confirmed by assessing its dose-dependent effect on the proliferation of N/TERT human keratinocytes.…”

Kappa opioid receptor contributes to EGF-stimulated neurite extension in development

“…Although it has not been as widely emphasized, EGF is also known to exert neurotrophic and/or neuromodulatory effects, thereby regulating neurite extension and neuronal activity as shown in studies of primary neurons (17,18) and PC12 cells (19,20). Indirectly, EGF also modulates neurite extension stimulated by hormones/factors such as thyroid hormones (21), suppressor of cytokine signaling-2 (18), and Versican G3 domain (22). EGF signal is primarily mediated by its binding to the EGF receptor (EGFR), which activates two signal transduction pathways.…”