SMDT1 variants impair EMRE-mediated mitochondrial calcium uptake in patients with muscle involvement

Elianne P. Bulthuis; Merel J.W. Adjobo-Hermans; Bastiaan de Potter; Saskia Hoogstraten; Lisanne H.T. Wezendonk; Omar A.Z. Tutakhel; Liesbeth T. Wintjes; Bert van den Heuvel; Peter H.G.M. Willems; Erik-Jan Kamsteeg; M. Estela Rubio Gozalbo; Suzanne C.E.H. Sallevelt; Suzanne M. Koudijs; Joost Nicolai; Charlotte I. de Bie; Jessica E. Hoogendijk; Werner J.H. Koopman; Richard J. Rodenburg

doi:10.1101/2022.10.31.514480

Abstract

Ionic calcium (Ca²⁺) is a key messenger in signal transduction and its mitochondrial uptake plays an important role in cell physiology. This uptake is mediated by the mitochondrial Ca²⁺ uniporter (MCU), which is regulated by EMRE (essential MCU regulator) encoded by the SMDT1 (single-pass membrane protein with aspartate rich tail 1) gene. This work presents the genetic, clinical and cellular characterization of two patients harbouring SMDT1 variants and presenting with muscle problems. Analysis of patient fibroblasts and complementation experiments provide evidence that these variants lead to absence of EMRE protein, induce MCU subcomplex formation and impair mitochondrial Ca²⁺ uptake. However, the activity of the oxidative phosphorylation enzymes, mitochondrial morphology and membrane potential, as well as routine/ATP-linked respiration were not affected. We hypothesize that the muscle-related symptoms in the patients with SMDT1 variants result from aberrant mitochondrial Ca²⁺ uptake.

Competing Interest Statement

WJHK is a scientific advisor of Khondrion B.V. (Nijmegen, The Netherlands). This company was not involved in the data analysis and interpretation, writing of the manuscript, and in the decision to submit the manuscript for publication.

Footnotes

↵% Shared first authors.

Abbreviations

ΔΨ: trans-MIM mitochondrial membrane potential
[Ca²⁺]_c: cytosolic free Ca²⁺ concentration
[Ca²⁺]_m: mitochondrial free Ca²⁺ concentration
[Ca²⁺]_n: nuclear free Ca²⁺ concentration
BK: bradykinin
CK: creatine kinase
ECAR: extracellular acidification rate
EMRE: essential MCU Regulator
FOV: field of view
IF: immunofluorescence
LGMD: limb-girdle muscular dystrophy
MCU: mitochondrial calcium uniporter
MICU1/2: mitochondrial calcium uptake protein 1/2
MIM: mitochondrial inner membrane
MOM: mitochondrial outer membrane
MW: molecular weight
OCR: oxygen consumption rate
OXPHOS: oxidative phosphorylation
PHSFs: primary human skin fibroblasts
SMDT1: single-pass membrane protein with aspartate rich tail 1
TCA: tricarboxylic acid cycle
WB: Western blot
WES: whole exome sequencing.

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