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MAGED2 is essential for Gαs and cAMP/PKA signaling under hypoxia, which promotes induction of HIF-1α and inhibits stress-induced autophagy
MAGED2 mutations cause a severe but transient form of antenatal Bartter syndrome, known as transient Bartter syndrome (tBS or Bartter type 5). This condition is characterized by significant renal salt wasting in affected fetuses and newborns, resulting in pronounced fetal urine production and excess...
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| Format: | Dissertation |
| Sprache: | Englisch |
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Philipps-Universität Marburg
2024
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| Zusammenfassung: | MAGED2 mutations cause a severe but transient form of antenatal Bartter syndrome, known as transient Bartter syndrome (tBS or Bartter type 5). This condition is characterized by significant renal salt wasting in affected fetuses and newborns, resulting in pronounced fetal urine production and excessive amounts of amniotic fluid leading to preterm birth, as well as increased perinatal mortality. Remarkably, all these symptoms resolve spontaneously starting at 30 weeks of gestational age. Given the physical interaction between Gαs and MAGED2, as well as the role of Gαs in activating the membrane-bound adenylyl cyclase, which subsequently generates cAMP and promotes renal salt reabsorption via protein kinase A (PKA), the impact of MAGED2 for the function of Gαs was investigated. These investigations were conducted under both normoxic and hypoxic conditions, considering that spontaneous recovery occurs alongside the developmental increase of renal oxygenation. In contrast to normoxia, physical and chemical hypoxia induced internalization of Gαs upon MAGED2 depletion in renal and cancer cell lines. This internalization was accompanied by a significant reduction in cAMP generation and PKA activity. Importantly, hypoxic internalization of Gαs was shown to require dynamin and was completely reversible upon re-oxygenation. The process was shown to be mediated by the ubiquitin E3 ligase MDM2, which ubiquitinates Gαs, resulting in its endocytosis. The latter was abrogated by mutating critical lysine-residues (ubiquitin receptor sites) of Gαs, by MDM2 inhibitors or knockdown of MDM2, respectively. Hence, MAGED2 is crucial under hypoxia to regulate Gαs endocytosis by blocking its MDM2-dependent ubiquitination, thereby maintaining proper induction of the cAMP/ PKA pathway. Reduced cAMP/ PKA activation upon MAGED2 depletion impaired HIF-1α induction. Notably, forskolin, a cAMP/ PKA activator acting downstream of Gαs rescued HIF-1α expression, highlighting the essential role of MAGED2 in Gαs functioning under hypoxic condition. Additionally, forskolin treatment increased the abundance of MAGED2 at both mRNA and protein levels. Interestingly, PKA type II specifically regulates the expression of HIF-1α and the latter reciprocally increases PKA activity under hypoxia and promotes MAGED2 expression. Finally, MAGED-2 was identified as inhibitor of autophagy under various stress conditions. Induction of autophagy by depleting Gαs and its reversal by forskolin demonstrate the inhibitory role of the cAMP/ PKA pathway governed by MAGED2. In contrast to other MAGE family members, such as MAGEA3/A6 whose absence initiates autophagy, MAGED-2 specifically blocks autophagy under stress conditions. Given that autophagy (at least under hypoxia) is induced upon Gαs depletion and inhibited by the cAMP/ PKA activator forskolin, the context-dependent regulation of Gαs by MAGED2 could be the underlying molecular switch of autophagy induction. |
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| DOI: | 10.17192/z2024.0134 |