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Atypical cell death and insufficient matrix organization in long-bone growth plates from Tric-b-knockout mice
Atsuhiko Ichimura1, Yuu Miyazaki1, Hiroki Nagatomo1
1Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto, 606-8501, Japan.
Abstract:
TRIC-A and TRIC-B proteins form homotrimeric cation-permeable channels in the endoplasmic reticulum (ER) and nuclear membranes and are thought to contribute to counterionic flux coupled with store Ca2+ release in various cell types. Serious mutations in the TRIC-B (also referred to as TMEM38B) locus cause autosomal recessive osteogenesis imperfecta (OI), which is characterized by insufficient bone mineralization. We have reported that Tric-b-knockout mice can be used as an OI model; Tric-b deficiency deranges ER Ca2+ handling and thus reduces extracellular matrix (ECM) synthesis in osteoblasts, leading to poor mineralization. Here we report irregular cell death and insufficient ECM in long-bone growth plates from Tric-b-knockout embryos. In the knockout growth plate chondrocytes, excess pro-collagen fibers were occasionally accumulated in severely dilated ER elements. Of the major ER stress pathways, activated PERK/eIF2α (PKR-like ER kinase/ eukaryotic initiation factor 2α) signaling seemed to inordinately alter gene expression to induce apoptosis-related proteins including CHOP (CCAAT/enhancer binding protein homologous protein) and caspase 12 in the knockout chondrocytes. Ca2+ imaging detected aberrant Ca2+ handling in the knockout chondrocytes; ER Ca2+ release was impaired, while cytoplasmic Ca2+ level was elevated. Our observations suggest that Tric-b deficiency directs growth plate chondrocytes to pro-apoptotic states by compromising cellular Ca2+-handling and exacerbating ER stress response, leading to impaired ECM synthesis and accidental cell death.
Insights
TRIC-B protein deficiency disrupts calcium handling in growth plate chondrocytes, leading to endoplasmic reticulum stress, impaired extracellular matrix synthesis, and cell death, causing osteogenesis imperfecta.
Area of Science:
- Cell Biology
- Biochemistry
- Genetics
Background:
- TRIC-A and TRIC-B proteins form cation channels in the endoplasmic reticulum (ER) and nuclear membranes, influencing calcium (Ca2+) flux.
- Mutations in TRIC-B (TMEM38B) cause autosomal recessive osteogenesis imperfecta (OI), characterized by poor bone mineralization.
- Tric-b-knockout mice exhibit ER Ca2+ dysregulation, reduced extracellular matrix (ECM) synthesis in osteoblasts, and poor mineralization, serving as an OI model.
Purpose of the Study:
- Investigate the role of Tric-b deficiency in growth plate chondrocytes in the context of osteogenesis imperfecta.
- Elucidate the mechanisms underlying impaired ECM synthesis and cell death in Tric-b-deficient growth plates.
- Determine the impact of Tric-b deficiency on ER Ca2+ handling and ER stress pathways in chondrocytes.
Main Methods:
- Analysis of long-bone growth plates from Tric-b-knockout embryos.
- Assessment of ER morphology and pro-collagen fiber accumulation.
- Evaluation of ER stress pathways, including PERK/eIF2α signaling, CHOP, and caspase-12 expression.
- Ca2+ imaging to analyze ER and cytoplasmic Ca2+ levels in chondrocytes.
Main Results:
- Tric-b-knockout embryos displayed irregular cell death and insufficient ECM in long-bone growth plates.
- Knockout chondrocytes showed excess pro-collagen fibers and dilated ER elements.
- Activated PERK/eIF2α signaling led to increased apoptosis-related proteins (CHOP, caspase-12).
- Aberrant Ca2+ handling was observed, with impaired ER Ca2+ release and elevated cytoplasmic Ca2+.
Conclusions:
- Tric-b deficiency compromises cellular Ca2+ handling in growth plate chondrocytes.
- This leads to exacerbated ER stress responses, including apoptosis.
- Impaired ECM synthesis and premature cell death contribute to the pathogenesis of osteogenesis imperfecta.
