nSMase2, which cleaves sphingomyelin to generate bioactive lipids, is required for chondrocyte apoptosis and, cell autonomously, for bone mineralization.

Matrix gla protein (MGP) is a potent inhibitor of extracellular matrix (ECM) mineralization. MGP-deficiency in humans leads to Keutel syndrome, a rare genetic disease hallmarked by abnormal soft tissue calcification. MGP-deficient (Mgp -/-) mice show progressive deposition of hydroxyapatite minerals in the arterial walls and die within 2 months of age. The mechanism of antimineralization function of MGP is not fully understood. We examined the progression of vascular calcification and expression of several chondrogenic/osteogenic markers in the thoracic aortas of Mgp -/-mice at various ages. Although cells with chondrocyte-like morphology have been reported in the calcified aorta, our gene expression data indicate that chondrogenic/osteogenic markers are not upregulated in the arteries prior to the initiation of calcification. Interestingly, arterial calcification in Mgp -/-mice appears first in the elastic laminae. Considering the known mineral scaffolding function of elastin (ELN), a major elastic lamina protein, we hypothesize that elastin content in the laminae is a critical determinant for arterial calcification in Mgp -/-mice. To investigate this, we performed micro-computed tomography (mCT) and histological analyses of the aortas of Mgp -/-;Eln þ/-mice and show that elastin haploinsufficiency significantly reduces arterial calcification in this strain. Our data suggest that MGP deficiency leads to alterations of vascular ECM that may in turn initiate arterial calcification.

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The regulators affecting skeletal tissue formation and its maintenance include a wide array of molecules with very diverse functions. More recently, sphingolipids have been added to this growing list of regulatory molecules in the skeletal tissues. Sphingolipids are integral parts of various lipid membranes present in the cells and organelles. For a long time, these macromolecules were considered as inert structural elements. This view, however, has radically changed in recent years as sphingolipids are now recognized as important second messengers for signal-transduction pathways that affect cell growth, differentiation, stress responses and programmed death. In the current review, we discuss the available data showing the roles of various sphingolipids in three different skeletal cell types-chondrocytes in cartilage and osteoblasts and osteoclasts in bone. We provide an overview of the biology of sphingomyelin phosphodiesterase 3 (SMPD3), an important regulator of sphingolipid metabolism in the skeleton. SMPD3 is localized in the plasma membrane and has been shown to cleave sphingomyelin to generate ceramide, a bioactive lipid second messenger, and phosphocholine, an essential nutrient. SMPD3 deficiency in mice impairs the mineralization in both cartilage and bone extracellular matrices leading to severe skeletal deformities. A detailed understanding of SMPD3 function may provide a novel insight on the role of sphingolipids in the skeletal tissues.

ObjectivesTo test the hypothesis that ROSAH (retinal dystrophy, optic nerve oedema, splenomegaly, anhidrosis and headache) syndrome, caused by dominant mutation in ALPK1, is an autoinflammatory disease.MethodsThis cohort study systematically evaluated 27 patients with ROSAH syndrome for inflammatory features and investigated the effect of ALPK1 mutations on immune signalling. Clinical, immunologic and radiographical examinations were performed, and 10 patients were empirically initiated on anticytokine therapy and monitored. Exome sequencing was used to identify a new pathogenic variant. Cytokine profiling, transcriptomics, immunoblotting and knock-in mice were used to assess the impact of ALPK1 mutations on protein function and immune signalling.ResultsThe majority of the cohort carried the p.Thr237Met mutation but we also identified a new ROSAH-associated mutation, p.Tyr254Cys.Nearly all patients exhibited at least one feature consistent with inflammation including recurrent fever, headaches with meningeal enhancement and premature basal ganglia/brainstem mineralisation on MRI, deforming arthritis and AA amyloidosis. However, there was significant phenotypic variation, even within families and some adults lacked functional visual deficits. While anti-TNF and anti-IL-1 therapies suppressed systemic inflammation and improved quality of life, anti-IL-6 (tocilizumab) was the only anticytokine therapy that improved intraocular inflammation (two of two patients).Patients’ primary samples and in vitro assays with mutated ALPK1 constructs showed immune activation with increased NF-κB signalling, STAT1 phosphorylation and interferon gene expression signature. Knock-in mice with the Alpk1 T237M mutation exhibited subclinical inflammation.Clinical features not conventionally attributed to inflammation were also common in the cohort and included short dental roots, enamel defects and decreased salivary flow.ConclusionROSAH syndrome is an autoinflammatory disease caused by gain-of-function mutations in ALPK1 and some features of disease are amenable to immunomodulatory therapy.