Inflammation, Oxidative Stress, and Bone in Chronic Kidney Disease in the Osteoimmunology Era

Bone is not only a mineralized and apparently non-vital structure that provides support for locomotion and protection to inner organs. An increasing number of studies are unveiling new biologic functions and connections to other systems, giving the rise to new fields of research, such as osteoimmunology. The bone marrow niche, a new entity in bone physiology, seems to represent the site where a complex crosstalk between bone and immune/inflammatory responses takes place. An impressive interplay with the immune system is realized in bone marrow, with reciprocal influences between bone cells and haematopoietic cells. In this way, systemic chronic inflammatory diseases realize a crosstalk with bone, resulting in bone disease. Thus, pathogenetic links between chronic kidney disease-mineral bone disorders and osteoporosis, cardiovascular disease, and ageing are common. The aim of this narrative review is to provide a general view of the progresses in the field of bone research and their potential clinical implications, with emphasis on the links with inflammation and the connections to osteoimmunology and chemokines.

Mineral and bone physiology in the foetus, preterm and full-term neonates

Mother is the major source of minerals in foetal life with placenta actively transporting against a concentration and electrochemical gradient. The foetal serum mineral concentration is thereby higher as compared to maternal values, which possibly help in its rapid accretion in developing bones and for counteracting postnatal fall in calcium levels at birth. Parathyroid hormone related peptide (PTHrP) and parathyroid hormone (PTH) play a major role in mineral physiology during foetal life with hormones like calcitriol, calcitonin, FGF-23 and sex steroids having minimal role. PTHrP and PTH also play a major role in endochondral bone formation and mineralization of skeleton. At the birth, as the cord is clamped, there is loss of active transport of minerals through placenta and the neonate has to rely on enteral intake of minerals to meet the demands of growing bones and metabolisms. The calcium levels fall after birth, reaching a nadir at 24-48 h and gradually rise to adult values over several days, probably resulting from a fall in PTHrP levels and hyporesponsiveness of parathyroid glands. As PTH and calcitriol levels increase postnatally, there is a rise in calcium levels with maturation in functioning of kidneys and intestines. However, there may be significant delay in intestinal maturation in preterm infants along with an increased demand for mineral accretion, which predispose them to osteopenia of prematurity.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Bone remodeling as a spatial evolutionary game

Bone remodeling is a complex process involving cell-cell interactions, biochemical signaling and mechanical stimuli. Early models of the biological aspects of remodeling were non-spatial and focused on the local dynamics at a fixed location in the bone. Several spatial extensions of these models have been proposed, but they generally suffer from two limitations: first, they are not amenable to analysis and are computationally expensive, and second, they neglect the role played by bone-embedded osteocytes. To address these issues, we developed a novel model of spatial remodeling based on the principles of evolutionary game theory. The analytically tractable framework describes the spatial interactions between zones of bone resorption, bone formation and quiescent bone, and explicitly accounts for regulation of remodeling by bone-embedded, mechanotransducing osteocytes. Using tools from the theory of interacting particle systems we systematically classified the different dynamic regimes of the spatial model and identified regions of parameter space that allow for global coexistence of resorption, formation and quiescence, as observed in physiological remodeling. In coexistence scenarios, three-dimensional simulations revealed the emergence of sponge-like bone clusters. Comparison between spatial and non-spatial dynamics revealed substantial differences and suggested a stabilizing role of space. Our findings emphasize the importance of accounting for spatial structure and bone-embedded osteocytes when modeling the process of bone remodeling. Thanks to the lattice-based framework, the proposed model can easily be coupled to a mechanical model of bone loading.