Magnesium (Mg2+) plays an essential role in many biological processes. Mg2+ deficiency is therefore associated with a wide range of clinical effects including muscle cramps, fatigue, seizures and arrhythmias. To maintain sufficient Mg2+ levels, (re)absorption of Mg2+ in the intestine and kidney is tightly regulated. Genetic defects that disturb Mg2+ uptake pathways, as well as drugs interfering with Mg2+ (re)absorption cause hypomagnesemia. The aim of this review is to provide an overview of the molecular mechanisms underlying genetic and drug-induced Mg2+ deficiencies. This leads to the identification of four main mechanisms that are affected by hypomagnesemia-causing mutations or drugs: luminal transient receptor potential melastatin type 6/7-mediated Mg2+ uptake, paracellular Mg2+ reabsorption in the thick ascending limb of Henle's loop, structural integrity of the distal convoluted tubule and Na+-dependent Mg2+ extrusion driven by the Na+/K+-ATPase. Our analysis demonstrates that genetic and drug-induced causes of hypomagnesemia share common molecular mechanisms. Targeting these shared pathways can lead to novel treatment options for patients with hypomagnesemia.
Actin plays a central role in many biological processes such as cell division, motility and contractility. In birds and mammals, actin has six, highly conserved isoforms, four of which are primarily present in muscles and two that are ubiquitously expressed across tissues. While each isoform has nonredundant biological functions, we currently lack the tools to investigate the molecular basis for isoform-specificity due to their high similarity and the limited possibilities to manipulate actin. To solve this technical challenge, we developed IntAct, an internally tagged actin system to study actin isoform organization in fixed and living cells. We performed a microscopy-based screen for 11 internal actin positions and identified one residue pair that allows for non-disruptive epitope tag integration. Using knockin cell lines with tags into the ubiquitously expressed β-actin, we demonstrate that IntAct actins are properly expressed and that their filament incorporation is indistinguishable from wildtype. We further show that IntAct actins can be visualized in living cells by exploiting the nanobody-targeted ALFA tag and that they keep their ability to interact with the actin-binding proteins profilin and cofilin. Lastly, we also introduced the tag in the ubiquitously expressed γ-actin and demonstrate that the differential localization observed for actin isoforms remains unaltered for IntAct actins. Together, our data demonstrate that IntAct is a promising tool to study actin isoform localization, dynamics and molecular interactions to finally enable the molecular characterization of actin isoforms in biological processes.
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