Plastin 3 in X-Linked Osteoporosis: Imbalance of Ca2+-Dependent Regulation Is Equivalent to Protein Loss

Osteogenesis imperfecta is a genetic disorder disrupting bone development and remodeling. The primary causes of osteogenesis imperfecta are pathogenic variants of collagen and collagen processing genes. However, recently variants of the actin bundling protein plastin 3 have been identified as another source of osteogenesis imperfecta. Plastin 3 is a highly conserved protein involved in several important cellular structures and processes and is controlled by intracellular Ca²⁺ which potently inhibits its actin-bundling activity. The precise mechanisms by which plastin 3 causes osteogenesis imperfecta remain unclear, but recent advances have contributed to our understanding of bone development and the actin cytoskeleton. Here, we review the link between plastin 3 and osteogenesis imperfecta highlighting in vitro studies and emphasizing the importance of Ca²⁺ regulation in the localization and functionality of plastin 3.

Osteogenesis imperfecta mutations in plastin 3 lead to impaired calcium regulation of actin bundling

Mutations in actin-bundling protein plastin 3 (PLS3) emerged as a cause of congenital osteoporosis, but neither the role of PLS3 in bone development nor the mechanisms underlying PLS3-dependent osteoporosis are understood. Of the over 20 identified osteoporosis-linked PLS3 mutations, we investigated all five that are expected to produce full-length protein. One of the mutations distorted an actin-binding loop in the second actin-binding domain of PLS3 and abolished F-actin bundling as revealed by cryo-EM reconstruction and protein interaction assays. Surprisingly, the remaining four mutants fully retained F-actin bundling ability. However, they displayed defects in Ca2+ sensitivity: two of the mutants lost the ability to be inhibited by Ca2+, while the other two became hypersensitive to Ca2+. Each group of the mutants with similar biochemical properties showed highly characteristic cellular behavior. Wild-type PLS3 was distributed between lamellipodia and focal adhesions. In striking contrast, the Ca2+-hyposensitive mutants were not found at the leading edge but localized exclusively at focal adhesions/stress fibers, which displayed reinforced morphology. Consistently, the Ca2+-hypersensitive PLS3 mutants were restricted to lamellipodia, while chelation of Ca2+ caused their redistribution to focal adhesions. Finally, the bundling-deficient mutant failed to co-localize with any F-actin structures in cells despite a preserved F-actin binding through a non-mutation-bearing actin-binding domain. Our findings revealed that severe osteoporosis can be caused by a mutational disruption of the Ca2+-controlled PLS3's cycling between adhesion complexes and the leading edge. Integration of the structural, biochemical, and cell biology insights enabled us to propose a molecular mechanism of plastin activity regulation by Ca2+.

The Roles of Actin-Binding Domains 1 and 2 in the Calcium-Dependent Regulation of Actin Filament Bundling by Human Plastins

The actin cytoskeleton is a complex network controlled by a vast array of intricately regulated actin-binding proteins. Human plastins (PLS1, PLS2, and PLS3) are evolutionary conserved proteins that non-covalently crosslink actin filaments into tight bundles. Through stabilization of such bundles, plastins contribute, in an isoform-specific manner, to the formation of kidney and intestinal microvilli, inner ear stereocilia, immune synapses, endocytic patches, adhesion contacts, and invadosomes of immune and cancer cells. All plastins comprise an N-terminal Ca²⁺-binding regulatory headpiece domain followed by two actin-binding domains (ABD1 and ABD2). Actin bundling occurs due to simultaneous binding of both ABDs to separate actin filaments. Bundling is negatively regulated by Ca²⁺, but the mechanism of this inhibition remains unknown. In this study, we found that the bundling abilities of PLS1 and PLS2 were similarly sensitive to Ca²⁺ (pCa₅₀ ~6.4), whereas PLS3 was less sensitive (pCa₅₀ ~5.9). At the same time, all three isoforms bound to F-actin in a Ca²⁺-independent manner, suggesting that binding of only one of the ABDs is inhibited by Ca²⁺. Using limited proteolysis and mass spectrometry, we found that in the presence of Ca²⁺ the EF-hands of human plastins bound to an immediately adjacent sequence homologous to canonical calmodulin-binding peptides. Furthermore, our data from differential centrifugation, Förster resonance energy transfer, native electrophoresis, and chemical crosslinking suggest that Ca²⁺ does not affect ABD1 but inhibits the ability of ABD2 to interact with actin. A structural mechanism of signal transmission from Ca²⁺ to ABD2 through EF-hands remains to be established.