Biomolecules Orchestrating Cardiovascular Calcification

Vascular calcification, once considered a degenerative, end-stage, and inevitable condition, is now recognized as a complex process regulated in a manner similar to skeletal bone at the molecular and cellular levels. Since the initial discovery of bone morphogenetic protein in calcified human atherosclerotic lesions, decades of research have now led to the recognition that the regulatory mechanisms and the biomolecules that control cardiovascular calcification overlap with those controlling skeletal mineralization. In this review, we focus on key biomolecules driving the ectopic calcification in the circulation and their regulation by metabolic, hormonal, and inflammatory stimuli. Although calcium deposits in the vessel wall introduce rupture stress at their edges facing applied tensile stress, they simultaneously reduce rupture stress at the orthogonal edges, leaving the net risk of plaque rupture and consequent cardiac events depending on local material strength. A clinically important consequence of the shared mechanisms between the vascular and bone tissues is that therapeutic agents designed to inhibit vascular calcification may adversely affect skeletal mineralization and vice versa. Thus, it is essential to consider both systems when developing therapeutic strategies.

SLITRK5 is a negative regulator of hedgehog signaling in osteoblasts

Hedgehog signaling is essential for bone formation, including functioning as a means for the growth plate to drive skeletal mineralization. However, the mechanisms regulating hedgehog signaling specifically in bone-forming osteoblasts are largely unknown. Here, we identified SLIT and NTRK-like protein-5(Slitrk5), a transmembrane protein with few identified functions, as a negative regulator of hedgehog signaling in osteoblasts. Slitrk5 is selectively expressed in osteoblasts and loss of Slitrk5 enhanced osteoblast differentiation in vitro and in vivo. Loss of SLITRK5 in vitro leads to increased hedgehog signaling and overexpression of SLITRK5 in osteoblasts inhibits the induction of targets downstream of hedgehog signaling. Mechanistically, SLITRK5 binds to hedgehog ligands via its extracellular domain and interacts with PTCH1 via its intracellular domain. SLITRK5 is present in the primary cilium, and loss of SLITRK5 enhances SMO ciliary enrichment upon SHH stimulation. Thus, SLITRK5 is a negative regulator of hedgehog signaling in osteoblasts that may be attractive as a therapeutic target to enhance bone formation.

Fix That PHEX Loss

Phosphorous has a critical role in multiple biological functions in the body, such as skeletal mineralization, and an imbalance of this can lead to several musculoskeletal disorders. An important regulator of renal phosphate excretion is fibroblast growth factor 23 (FGF23) which is produced by osteocytes and osteoblasts themselves thus providing a mechanism for the skeletal system to influence its own mineralization needs. PHEX is a gene that regulates FGF23 secretion therefore a loss-of-function mutation in this gene would result in elevated circulating FGF23 and phosphate depletion. This mutation has been identified as a cause for X-linked hypophosphatemia (XLH). Treatment of XLH has been limited and mainly involved phosphorous replacement in combination with 1,25(OH) vitamin D. Antiresorptive osteoporosis treatment can exacerbate the skeletal mineralization process. Here we present a patient with multiple fractures who was on denosumab treatment for presumed osteoporosis before being found to have PHEX mutation. The patient is a 64 year-old female with past medical history of bilateral hip replacement and recurrent femur fractures who was seen in clinic in 2014 due to recurrent fractures and diagnosis of osteoporosis since early 2000s. She had only tried alendronate up until that point. Due to the recurrent fractures she was switched to denosumab therapy while workup was underway for secondary causes. She was found to have low phosphorous levels and elevated FGF23 therefore genetic counseling was pursued and was recommended to check for PHEX mutation. The testing came back positive for loss-of-function mutation in PHEX, and by that point she had received 3 doses of denosumab therapy. She suffered another femoral fracture which was determined to be an atypical fracture, and therefore denosumab treatment was stopped and she was continued on phosphorous replacement as well as 1,25(OH) vitamin D replacement. Most recently her phosphorous levels have been controlled with therapy, and there is current discussion underway to try burosumab, an antibody to FGF23. During evaluation for osteoporosis, it is important to consider phosphorous roles in skeletal mineralization. If recurrent fractures are seen in a patient with low phosphorous levels, especially while they are on conventional antiresorptive osteoporosis medications, genetic testing for PHEX mutations should be considered as well as safely stopping antiresorptive medications.

Ocean acidification and warming affect skeletal mineralization in a marine fish

Ocean acidification and warming are known to alter, and in many cases decrease, calcification rates of shell and reef building marine invertebrates. However, to date, there are no datasets on the combined effect of ocean pH and temperature on skeletal mineralization of marine vertebrates, such as fishes. Here, the embryos of an oviparous marine fish, the little skate ( Leucoraja erinacea ), were developmentally acclimatized to current and increased temperature and CO 2 conditions as expected by the year 2100 (15 and 20°C, approx. 400 and 1100 µatm, respectively), in a fully crossed experimental design. Using micro-computed tomography, hydroxyapatite density was estimated in the mineralized portion of the cartilage in jaws, crura, vertebrae, denticles and pectoral fins of juvenile skates. Mineralization increased as a consequence of high CO 2 in the cartilage of crura and jaws, while temperature decreased mineralization in the pectoral fins. Mineralization affects stiffness and strength of skeletal elements linearly, with implications for feeding and locomotion performance and efficiency. This study is, to my knowledge, the first to quantify a significant change in mineralization in the skeleton of a fish and shows that changes in temperature and pH of the oceans have complex effects on fish skeletal morphology.

Temperature affects growth allometry and development patterns in brown trout (Salmo trutta) fry: a multitrait approach

To study the influence of temperature (4, 6, and 12 °C) on the development of brown trout (Salmo trutta) from hatching to the end of metamorphosis, an analysis of allometric growth patterns was conducted to identify two different groups of individuals, namely developmental phases at total lengths (TL) ranging from 2.72 cm at 4 °C to 2.22 cm at 12 °C. Then, a multitrait approach considering different variables like the survival rate, development time, morphometric characteristics, energetic value, and skeletal mineralization was conducted on these two groups. Results indicated that the first growth phase was slower at 4 °C, whereas the second was also slower at this temperature, even though swimming behavior was already present. However, at 12 °C, fry showed a delay in their development (i.e., lower levels of skeletal mineralization and energetic content) during the first growth phase, but they compensated during the second growth phase, reaching the same size in TL when compared with the other temperatures (4 and 6 °C); fry at 12 C° showed low energy reserves. Our study demonstrated that the use of an allometric analysis to identify different developmental stages coupled with a multitrait approach was more efficient than a classical distinction between biological stages (hatching, emergence, first food intake, and exogenous feeding), and this procedure is of interest when evaluating the impact of rearing conditions on early development in fish.