Absence of the ER Cation Channel TMEM38B/TRIC-B Disrupts Intracellular Calcium Homeostasis and Dysregulates Collagen Synthesis in Recessive Osteogenesis Imperfecta

• Published in: PLoS genetics Vol. 12; no. 7; p. e1006156
• Main Authors: Cabral, Wayne A, Ishikawa, Masaki, Garten, Matthias, Makareeva, Elena N, Sargent, Brandi M, Weis, MaryAnn, Barnes, Aileen M, Webb, Emma A, Shaw, Nicholas J, Ala-Kokko, Leena, Lacbawan, Felicitas L, Högler, Wolfgang, Leikin, Sergey, Blank, Paul S, Zimmerberg, Joshua, Eyre, David R, Yamada, Yoshihiko, Marini, Joan C
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 21.07.2016; Public Library of Science (PLoS)
• Subjects: Adult; Biology and Life Sciences; Calcium; Calcium (Nutrient); Calcium - metabolism; Calcium Signaling; Collagen; Collagen Type I - biosynthesis; Collagen Type I - metabolism; Consanguinity; Defects; DNA Mutational Analysis; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum Stress; Enzymes; Experiments; Extracellular matrix; Female; Fibroblasts; Gene expression; Gene mutation; Genes, Recessive; Genetic aspects; Genetic Association Studies; Genetic Predisposition to Disease; Genotype & phenotype; Health aspects; Homeostasis; Humans; Infant; Ion channels; Ion Channels - genetics; Male; Medicine and Health Sciences; Mutation; Osteogenesis imperfecta; Osteogenesis Imperfecta - genetics; Pedigree; Protein Processing, Post-Translational; Proteins
• ISSN: 1553-7404; 1553-7390; 1553-7404
• DOI: 10.1371/journal.pgen.1006156

Recessive osteogenesis imperfecta (OI) is caused by defects in proteins involved in post-translational interactions with type I collagen. Recently, a novel form of moderately severe OI caused by null mutations in TMEM38B was identified. TMEM38B encodes the ER membrane monovalent cation channel, TRIC-B, proposed to counterbalance IP3R-mediated Ca2+ release from intracellular stores. The molecular mechanisms by which TMEM38B mutations cause OI are unknown. We identified 3 probands with recessive defects in TMEM38B. TRIC-B protein is undetectable in proband fibroblasts and osteoblasts, although reduced TMEM38B transcripts are present. TRIC-B deficiency causes impaired release of ER luminal Ca2+, associated with deficient store-operated calcium entry, although SERCA and IP3R have normal stability. Notably, steady state ER Ca2+ is unchanged in TRIC-B deficiency, supporting a role for TRIC-B in the kinetics of ER calcium depletion and recovery. The disturbed Ca2+ flux causes ER stress and increased BiP, and dysregulates synthesis of proband type I collagen at multiple steps. Collagen helical lysine hydroxylation is reduced, while telopeptide hydroxylation is increased, despite increased LH1 and decreased Ca2+-dependent FKBP65, respectively. Although PDI levels are maintained, procollagen chain assembly is delayed in proband cells. The resulting misfolded collagen is substantially retained in TRIC-B null cells, consistent with a 50-70% reduction in secreted collagen. Lower-stability forms of collagen that elude proteasomal degradation are not incorporated into extracellular matrix, which contains only normal stability collagen, resulting in matrix insufficiency. These data support a role for TRIC-B in intracellular Ca2+ homeostasis, and demonstrate that absence of TMEM38B causes OI by dysregulation of calcium flux kinetics in the ER, impacting multiple collagen-specific chaperones and modifying enzymes.