Bone marrow neutrophil aging in sickle cell disease mice is associated with impaired osteoblast functions

Bone loss is ameliorated by fecal microbiota transplantation through SCFA/GPR41/ IGF1 pathway in sickle cell disease mice

Bone loss is common in sickle cell disease (SCD), but the molecular mechanisms is unclear. Serum insulin-like growth factor 1 (IGF1) was low in SCD subjects and SCD mice. To determine if decreased IGF1 associated with low bone mass in SCD is due to reduced SCFA production by gut microbiota, we performed reciprocal fecal microbiota transplantation (FMT) between healthy control (Ctrl) and SCD mice. uCT and histomorphometry analysis of femur showed decreased bone volume/total volume (BV/TV), trabecular number (Tb.N), osteoblast surface/bone surface (Ob.S/BS), mineralizing surface/ bone surface (MS/BS), inter-label thickness (Ir.L.Th) in SCD mice were significantly improved after receiving Ctrl feces. Bone formation genes Alp, Col1, Runx2, and Dmp1 from SCD mice were significantly decreased and were rescued after FMT from Ctrl feces. Transplantation of Ctrl feces increased the butyrate, valerate, and propionate levels in cecal content of SCD mice. Decreased G-coupled protein receptors 41 and 43 (GPR41 and GPR43) mRNA in tibia and lower IGF1 in bone and serum of SCD mice were partially restored after FMT from Ctrl feces. These data indicate that the healthy gut microbiota of Ctrl mice is protective for SCD bone loss through regulating IGF1 in response to impaired bacterial metabolites SCFAs.

High Dickkopf-1 (DKK-1) levels are associated with chronic inflammation in children with Sickle Cell Disease

OBJECTIVES

Sickle bone disease (SBD) is a chronic complication of Sickle Cell Disease (SCD) whose pathogenesis is not completely understood. Chronic inflammation associated with SCD could alter bone remodeling. Our aim was to analyze the serum levels of bone remodeling markers in a group of SCD children to evaluate their involvement in the SBD.

METHODS

We enrolled 26 SCD subjects and 26 age-matched controls, who lived in the same geographic area. DKK-1, sclerostin, RANKL, and OPG serum levels were evaluated. Neutrophil-lymphocyte ratio (NLR) was also evaluated as a marker of inflammation.

RESULTS

The analysis of bone remodeling markers did not show any significant difference between the two groups except for DKK-1 levels that were significantly higher in the patients than controls (p<0.05). A significant direct correlation between NLR and DKK-1 (p=0.004) was found. An inverse correlation between NLR and osteocalcin (p=0.01) has also been observed.

CONCLUSIONS

The chronic inflammation, which represents a peculiar characteristic in SCD patients, would represent the primary causal agent of the activation of osteoblastogenesis inhibitors responsible of bone impairment in these subjects. Further studies will be needed to better explain the role of these inhibitors in SCD, to prevent or treat bone damage in this population.

Osteoblast Dysfunction in Non-Hereditary Sclerosing Bone Diseases

A review of the available literature was performed in order to summarize the existing evidence between osteoblast dysfunction and clinical features in non-hereditary sclerosing bone diseases. It has been known that proliferation and migration of osteoblasts are concerted by soluble factors such as fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF), bone morphogenetic protein (BMP) but also by signal transduction cascades such as Wnt signaling pathway. Protein kinases play also a leading role in triggering the activation of osteoblasts in this group of diseases. Post-zygotic changes in mitogen-activated protein kinase (MAPK) have been shown to be associated with sporadic cases of Melorheostosis. Serum levels of FGF and PDGF have been shown to be increased in myelofibrosis, although studies focusing on Sphingosine-1-phosphate receptor was shown to be strongly expressed in Paget disease of the bone, which may partially explain the osteoblastic hyperactivity during this condition. Pathophysiological mechanisms of osteoblasts in osteoblastic metastases have been studied much more thoroughly than in rare sclerosing syndromes: striking cellular mechanisms such as osteomimicry or complex intercellular signaling alterations have been described. Further research is needed to describe pathological mechanisms by which rare sclerosing non hereditary diseases lead to osteoblast dysfunction.

Mechanisms of Bone Impairment in Sickle Bone Disease

Sickle bone disease (SBD) is a chronic and invalidating complication of Sickle cell disease (SCD), a multisystem autosomal recessive genetic disorder affecting millions of people worldwide. Mechanisms involved in SBD are not completely known, especially in pediatric age. Among the hypothesized pathogenetic mechanisms underlying SBD are bone marrow compensatory hyperplasia and bone ischemic damage, both secondary to vaso-occlusive crisis (VOC), which leads to cell sickling, thus worsening local hypoxia with a negative impact on osteoblast recruitment. Furthermore, the hypoxia is a strong activator of erythropoietin, which in turn stimulates osteoclast precursors and induces bone loss. Hemolysis and iron overload due to a chronic transfusion regimen could also contribute to the onset of bone complications. Vitamin D deficiency, which is frequently seen in SCD subjects, may worsen SBD by increasing the resorptive state that is responsible for low bone mineral density, acute/chronic bone pain, and high fracture risk. An imbalance between osteoblasts and osteoclasts, with a relative decrease of osteoblast recruitment and activity, is a further possible mechanism responsible for the impairment of bone health in SCD. Moreover, delayed pubertal growth spurt and low peak bone mass may explain the high incidence of fracture in SCD adolescents. The aim of this review was to focus on the pathogenesis of SBD, updating the studies on biochemical, instrumental, and biological markers of bone metabolism. We also evaluated the growth development and endocrine complications in subjects affected with SCD.

Innate immune cells, major protagonists of sickle cell disease pathophysiology

Sickle cell disease (SCD), considered the most common monogenic disease worldwide, is a severe hemoglobin disorder. Although the genetic and molecular bases have long been characterized, the pathophysiology remains incompletely elucidated and therapeutic options are limited. It has been increasingly suggested that innate immune cells, including monocytes, neutrophils, invariant natural killer T cells, platelets and mast cells, have a role in promoting inflammation, adhesion and pain in SCD. Here we provide a thorough review of the involvement of these novel, major protagonists in SCD pathophysiology, highlighting recent evidence for innovative therapeutic perspectives.

Depletion of Intestinal Microbiome Partially Rescues Bone Loss in Sickle Cell Disease Male Mice

Osteoporosis or osteopenia are common clinical manifestations of sickle cell disease (SCD) with unclear mechanisms. Since senescence of circulating neutrophil can be modulated by signals derived from intestinal microbiome and neutrophils are abundant in bone marrow and can regulate osteoblasts and osteoclasts, we examined whether gut microbiome contributes to bone loss in SCD mice. SCD and their littermates control mice were treated with antibiotics to deplete gut microbiome. At the end of 7 weeks treatment, serum was collected for biochemistry marker measurements. Bone mass and remodeling were evaluated by dual beam X-ray absorptiometry, micro-computed tomography, and histomorphometry. Bone-related genes in tibia and barrier marker genes in the small intestine were analyzed by quantitative PCR. Antibiotic treatment rescued increased intestinal inflammatory cytokine marker genes (Tnfα, IL17, Ifnγ) expression, rescued decreased intestinal barrier marker genes (claudin 3 and claudin 15) expression, and rescued increased serum cytokines (IFNγ, IL27, IL10) in SCD mice. Antibiotic significantly improved decreased bone mass in SCD mice mainly through enhanced osteoblast function and increased osteoblast-related genes (Runx2 and Igf1) expression in SCD mice. Our findings support that increased bacteria load augments antigenic load traversing the impaired intestinal barrier through inflammation, leading to increased inflammatory cytokines, impaired osteoblast function, and bone loss in SCD mice.