Die Effekte der Depletion von MAGED2 auf den PKA-Signalweg und HIF-1⍺ in hypoxischer Umgebung und Aufhebung der Effekte durch Forskolin
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The Bartter syndrome (BS) is a rare genetic kidney disorder first described by Frederic Bartter in 1962. This syndrome is characterized by salt wasting, polyhydramnios, and metabolic alkalosis, with five distinct subtypes. The transient antenatal Bartter syndrome (taBS), Type 5, was first correlated with mutations in the MAGED2 gene in 2016, accounting for approximately 10% of aBS cases. Affected fetuses face the risk of a dangerous preterm delivery due to severe polyhydramnios. Among the 75% of patients who survive, symptoms resolve within a few weeks after birth.
The precise molecular mechanisms explaining the severe symptoms caused by mutations in the MAGED2 gene, as well as the spontaneous resolution of these symptoms after birth, are not fully understood. It has been shown that the expression of NKCC2- and NCC-cotransporters in the apical membrane is reduced in MAGED2 depleted cells. This could contribute to the impairment of renal salt and water reabsorption and thus explain the symptoms. One possible signaling pathway involved in this syndrome is the PKA signaling pathway.
To investigate this hypothesis, siMAGED2-transfections were performed in HEK293 and HeLa cell lines, resulting in MAGED2-depletion intended to mimic the truncating MAGED2 mutations observed in taBS-patients. To simulate the hypoxic milieu of the fetal circulation – which is assumed to underlie the transient nature of this syndrome – cells were exposed either to physical hypoxia or “chemical hypoxia” induced by cobalt chloride. PKA activity was measured using the PepTag® assay, while other protein analyses were examined out by Western blotting.
The results demonstrate a significant reduction of PKA activity in MAGED2 depleted cells under hypoxic conditions, with comparable effects on P‑CREB/CREB and MDM2. In contrast, Gαs did not show a consistent change in signaling, suggesting that further optimization of experimental conditions and analysis of its subcellular localization by immunocytochemistry are needed. Under hypoxia, MAGED2 knock-down also caused a significant reduction of HIF‑1α, which was reversed when forskolin, an adenylate cyclase stimulator, was added.
Taken together with additional experiments, these findings indicate that MAGED2 plays a key role in maintaining the PKA signaling pathway in a hypoxic environment by binding the E3-ligase MDM2. This interaction prevents internalization of Gαs and thereby supports the activation of adenylate cyclase and therefore maintenance of PKA signaling. In renal cells, an intact PKA pathway is essential for a cAMP-dependent expression of cotransporters and for the phosphorylation of HIF‑1α, which contributes to normal renal function. Postnatally, or after completion of nephrogenesis corresponding to the 32nd gestational week, HIF is no longer detectable, coinciding with the resolution of symptoms in the known taBS cases. MAGED2 mutations appear to have no further effect on renal function under normoxic conditions. This association could explain the transient nature of taBS symptoms. It can be hypothesized that surviving taBS patients may develop renal symptoms such as polyuria later in life under hypoxic stressors, e.g., acute kidney injury or high-altitude exposure with low oxygen partial pressure.
These data not only promote understanding of the pathophysiology of taBS but also open new perspectives for therapeutic intervention. On the one hand, MAGED2‑mediated stabilization of HIF‑1α under hypoxic conditions suggests a potential protective function in stress situations in which HIF‑1α exerts beneficial effects. On the other hand, the results highlight the therapeutic potential of forskolin, which has already been evaluated in previous clinical studies and may alleviate symptoms in taBS patients by sustaining the PKA signaling pathway. Finally, the role of MAGED2 in tumor biology, given its expression in some cancer types and its impact on HIF-1α and Gαs, raises intriguing possibilities for therapeutic interventions targeting MAGED2 in hypoxic tumors.
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