Bone circuitry and interorgan skeletal crosstalk

The past decade has seen significant advances in our understanding of skeletal homeostasis and the mechanisms that mediate the loss of bone integrity in disease. Recent breakthroughs have arisen mainly from identifying disease-causing mutations and modeling human bone disease in rodents, in essence, highlighting the integrative nature of skeletal physiology. It has become increasingly clear that bone cells, osteoblasts, osteoclasts, and osteocytes, communicate and regulate the fate of each other through RANK/RANKL/OPG, liver X receptors (LXRs), EphirinB2-EphB4 signaling, sphingolipids, and other membrane-associated proteins, such as semaphorins. Mounting evidence also showed that critical developmental pathways, namely, bone morphogenetic protein (BMP), NOTCH, and WNT, interact each other and play an important role in postnatal bone remodeling. The skeleton communicates not only with closely situated organs, such as bone marrow, muscle, and fat, but also with remote vital organs, such as the kidney, liver, and brain. The metabolic effect of bone-derived osteocalcin highlights a possible role of skeleton in energy homeostasis. Furthermore, studies using genetically modified rodent models disrupting the reciprocal relationship with tropic pituitary hormone and effector hormone have unraveled an independent role of pituitary hormone in skeletal remodeling beyond the role of regulating target endocrine glands. The cytokine-mediated skeletal actions and the evidence of local production of certain pituitary hormones by bone marrow-derived cells displays a unique endocrine-immune-skeletal connection. Here, we discuss recently elucidated mechanisms controlling the remodeling of bone, communication of bone cells with cells of other lineages, crosstalk between bone and vital organs, as well as opportunities for treating diseases of the skeleton.

FSH-blocking therapeutic for osteoporosis

Pharmacological and genetic studies over the past decade have established the follicle-stimulating hormone (FSH) as an actionable target for diseases affecting millions, namely osteoporosis, obesity, and Alzheimer's disease. Blocking FSH action prevents bone loss, fat gain, and neurodegeneration in mice. We recently developed a first-in-class, humanized, epitope-specific FSH-blocking antibody, MS-Hu6, with a KD of 7.52 nM. Using a Good Laboratory Practice (GLP)-compliant platform, we now report the efficacy of MS-Hu6 in preventing and treating osteoporosis in mice and parameters of acute safety in monkeys. Biodistribution studies using 89Zr-labeled, biotinylated or unconjugated MS-Hu6 in mice and monkeys showed localization to bone and bone marrow. The MS-Hu6 displayed a β phase t½ of 7.5 days (180 hr) in humanized Tg32 mice. We tested 217 variations of excipients using the protein thermal shift assay to generate a final formulation that rendered MS-Hu6 stable in solution upon freeze-thaw and at different temperatures, with minimal aggregation, and without self-, cross-, or hydrophobic interactions or appreciable binding to relevant human antigens. The MS-Hu6 showed the same level of "humanness" as human IgG1 in silico and was non-immunogenic in ELISpot assays for IL-2 and IFN-γ in human peripheral blood mononuclear cell cultures. We conclude that MS-Hu6 is efficacious, durable, and manufacturable, and is therefore poised for future human testing.

[C-kit is a marker of human pancreatic endocrinocyte stem cells]

The aim of our study was to analyze the expression of one of the markers of progenitor cell of different cell types - CD 117 (C-kit) - in human pancreas during prenatal development. The pancreas of human embryos and fetuses at 4-28 weeks of gestation as well as of infants aged up to 2nd postnatal month, was studied. In histological sections, the immunocytochemical reactions were performed with the antibodies against C-kit, insulin and glucagon. In situ hybridization was used for detection of proinsulin mRNA. First cells expressing C-kit were found in human pancreas at 8.5 weeks of gestation among ductal epithelial cells. At 11.5 weeks of gestation these cells were found to segregate from the ductal epithelium and start to form islets. From 8.5 weeks of gestation C-kit positive cells started to express glucagon and proinsulin mRNA, and after 11.5 weeks they also expressed insulin. Islet C-kit positive cells coexpressing both glucagon and insulin, were also found after the birth. It may be concluded that C-kit positive endocrinocyte progenitor cells are common for pancreatic islet A- and B-cells and they are preserved in he islets after the birth.

Mesenchymal-epithelial transformation of ito cells in vitro

Cultured pure population of Ito cells isolated from adult rat liver expressed epithelial markers cytokeratin-8, alpha-fetoprotein, and gamma-glutamyl transpeptidase after forming a dense monolayer. Mesenchymal-epithelial transformation of these cells is possible, which suggests them as candidates of hepatic stem cells.

[Cell sources of liver development]

The work is devoted to consequent expression of different cell types' protein markers such as vimentin, desmin, cytokeratins 7, 18, 19, stem cell markers CD34 and Bcl-2 at early stages of human prenatal development. Desmin was revealed in sinusoidal liver cells on 3.5-12 weeks of gestation, in mesenchymal cells of ventral mesentery and hepatoblasts on the 4-7 accordingly. During hepatic period of blood formation such desmin positive sinusoidal cells were found to be located close to blood cells. So called "cholangio-" cytokeratins 7 and 19 showed different expression, the first one was found only in cholangiocytes, while cytokeratin 19 existed in hepatoblasts as well until week 15-16 of prenatal development. Mesenchymal cells of ventral mesentery are positive for cytokeratins 18 and 19 even brighter than hepatoblasts in the 4-7 weeks of gestation. Bcl-2 expression was seen in the same periods in most sinusoidal and mesenchymal cells of ventral mesentery. CD34 positive cells are strongly depicted in liver sinusoids from 4th until 9th weeks of gestation, but probably they are not a source of hepatocytes' development in embryonic ontogenesis. Ventral mesentery mesenchyme was negative for this very marker. These results let us suppose that hepatocytes and cholangiocytes may develop from quite different embryonic sources: cholangyocytes grow exceptionally from duodenum epithelial cells, while there is a strong possibility that hepatoblasts formation occurs with participation of mesenchymal cells.

Alteration of the liver in rats with experimental dysbiosis

We evaluated the relationship between pathological changes in the liver and the state of intestinal microflora in rats with experimental dysbiosis. Changes in the intestinal microflora were accompanied by alteration of the morphological structure in the liver. Enhanced proliferation of Ito cells served as an indirect evidence of damage to the liver. Ito cells did not undergo transformation into myofibroblasts that excluded the possibility of fibrosis.

[The Ito cells in ontogenesis and liver regeneration]

This review regards the Ito cells, which constitute one of the four types of liver sinusoidal cells. These cells synthesize many cytokines and extracellular matrix proteins. The Ito cells make an important link in retinoid metabolism. The role played by the Ito cells in liver hemopoiesis and differentiation of liver epithelium in prenatal ontogenesis is emphasized. The importance of the Ito cells in reparative liver regeneration is also discussed.

[The "direct mitogen" lead nitrate causes an intensification in lipid peroxidation and Ito cell activation in the rat liver]

We have used biochemical and immunohistochemical methods to study lipid peroxidation and activation of Ito cells in rat liver after a single administration of lead nitrate, a "direct mitogen". Lead nitrate was shown to injure hepatocytes through an increased lipid peroxidation. Response to the injury included increase in the proliferative activity of parenchymal and sinusoidal liver cells. In addition, activation of Ito cells has been noted, which manifested as increased expression of desmin and increased proliferation. However, no transformation of Ito cells into myofibroblasts has been observed. We discuss the possible role of Ito cell activation in creating conditions for the proliferation of liver parenchymal cells after the injury by lead nitrate.

[Cytokeratin expression in the pre- and postnatal ontogeny of the rat liver]

Expression of cytokeratins nos. 7 and 19 has been examined during pre- and postnatal ontogenesis in rat liver. Cells expressing cytokeratin no. 19 appeared around large incoming vessels in the liver at days 17-18 of gestation. From day 20 of gestation and during the first week of postnatal ontogenesis, cholangiocytes and periportal hepatocytes could be stained with antibodies against cytokeratin no. 19. The expression of cytokeratin no. 7 begins later than cytokeratin no. 19, and it is present only in cholangiocytes, throughout pre- and postnatal ontogenesis. Since pre- and postnatal hepatocytes are capable of expressing cytokeratin no. 19, we discuss possible use of this marker to study cytodifferentiation of epithelial cell lines in rat liver.