A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Comparative morphology of the osteocyte lacunocanalicular system in various vertebrates

Osteocytes are embedded in the bone matrix, and they communicate with adjacent osteocytes, osteoblasts, and osteoclasts through the osteocyte lacunocanalicular system. Osteocytes are believed to be essential for the maintenance of bone homeostasis because they regulate mechanical sensing and mineral metabolism in mammalian bones; however, osteocyte morphology in other vertebrates has not been well documented. We conducted a comparative study on the morphology of osteocytes and the lacunocanalicular system of the following vertebrates: two teleost fishes [medaka (Oryzias latipes), and zebrafish (Danio rerio)], three amphibians [African clawed frog (Xenopus laevis), black-spotted pond frog (Rana nigromaculata), and Japanese fire-bellied newt (Cynops pyrrhogaster)], two reptiles [four-toed tortoise (Testudo horsfieldii) and green iguana (Iguana iguana)], and two mammals (laboratory mouse C57BL6 and human). The distribution of the osteocyte lacunocanalicular system in all these animals was investigated using the modified silver staining and the fluorescein-conjugated phalloidin staining methods. Bones of medaka had few osteocytes (acellular bone). Bones of zebrafish contained osteocytes (cellular bone) but had a poorly developed osteocyte lacunocanalicular system. Bones of Xenopus laevis, a freshwater species, and of other amphibians, reptiles, and mammals contained numerous osteocytes and a well-developed lacunocanalicular system. The present study indicates that development of the osteocyte lacunocanalicular system differs between teleost fishes and land vertebrates, but this pattern is not directly related to aquatic habitat.

Osteoclasts in bone modeling, as revealed by in vivo imaging, are essential for organogenesis in fish

Bone modeling is the central system controlling the formation of bone including bone growth and shape in early development, in which bone is continuously resorbed by osteoclasts and formed by osteoblasts. However, this system has not been well documented, because it is difficult to trace osteoclasts and osteoblasts in vivo during development. Here we showed the important role of osteoclasts in organogenesis by establishing osteoclast-specific transgenic medaka lines and by using a zebrafish osteoclast-deficient line. Using in vivo imaging of osteoclasts in the transgenic medaka carrying an enhanced GFP (EGFP) or DsRed reporter gene driven by the medaka TRAP (Tartrate-Resistant Acid Phosphatase) or Cathepsin K promoter, respectively, we examined the maturation and migration of osteoclasts. Our results showed that mononuclear or multinucleated osteoclasts in the vertebral body were specifically localized at the inside of the neural and hemal arches, but not at the vertebral centrum. Furthermore, transmission electron microscopic (TEM) analyses revealed that osteoclasts were flat-shaped multinucleated cells, suggesting that osteoclasts initially differentiate from TRAP-positive mononuclear cells residing around bone. The zebrafish panther mutant lacks a functional c-fms (receptor for macrophage colony-stimulating factor) gene crucial for osteoclast proliferation and differentiation and thus has a low number of osteoclasts. Analysis of this mutant revealed deformities in both its neural and hemal arches, which resulted in abnormal development of the neural tube and blood vessels located inside these arches. Our results provide the first demonstration that bone resorption during bone modeling is essential for proper development of neural and vascular systems associated with fish vertebrae.

Trpv5/6 is vital for epithelial calcium uptake and bone formation

Calcium is an essential ion serving a multitude of physiological roles. Aside from its role as a second messenger, it is an essential component of the vertebrate bone matrix. Efficient uptake and storage of calcium are therefore indispensable for all vertebrates. Transient receptor potential family, vanilloid type (TRPV)5 and TRPV6 channels are known players in transcellular calcium uptake, but the exact contribution of this pathway is unclear. We used forward genetic screening in zebrafish (Danio rerio) to identify genes essential in bone formation and identified a lethal zebrafish mutant (matt-und-schlapp) with severe defects in bone formation, including lack of ossification of the vertebral column and craniofacial structures. Mutant embryos show a 68% reduction in calcium content, and systemic calcium homeostasis is disturbed when compared with siblings. The phenotype can be partially rescued by increasing ambient calcium levels to 25 mM. We identified the mutation as a loss-of-function mutation in the single orthologue of TRPV5 and 6, trpv5/6. Expression in HEK293 cells showed that Trpv5/6 is a calcium-selective channel capable of inward calcium transport at physiological concentrations whereas the mutant channel is not. Taken together, this study provides both genetic and functional evidence that transcellular epithelial calcium uptake is vital to sustain life and enable bone formation.

Sustained swimming increases the mineral content and osteocyte density of salmon vertebral bone

This study addresses the effects of increased mechanical load on the vertebral bone of post-smolt Atlantic salmon by forcing them to swim at controlled speeds. The fish swam continuously in four circular tanks for 9 weeks, two groups at 0.47 body lengths (bl) × s(-1) (non-exercised group) and two groups at 2 bl × s(-1) (exercised group), which is just below the limit for maximum sustained swimming speed in this species. Qualitative data concerning the vertebral structure were obtained from histology and electron microscopy, and quantitative data were based on histomorphometry, high-resolution X-ray micro-computed tomography images and analysis of bone mineral content, while the mechanical properties were tested by compression. Our key findings are that the bone matrix secreted during sustained swimming had significantly higher mineral content and mechanical strength, while no effect was detected on bone in vivo architecture. mRNA levels for two mineralization-related genes bgp and alp were significantly upregulated in the exercised fish, indicating promotion of mineralization. The osteocyte density of the lamellar bone of the amphicoel was also significantly higher in the exercised than non-exercised fish, while the osteocyte density in the cancellous bone was similar in the two groups. The vertebral osteocytes did not form a functional syncytium, which shows that salmon vertebral bone responds to mechanical loading in the absence of an extensive connecting syncytial network of osteocytic cell processes as found in mammals, indicating the existence of a different mechanosensing mechanism. The adaptive response to increased load is thus probably mediated by osteoblasts or bone lining cells, a system in which signal detection and response may be co-located. This study offers new insight into the teleost bone biology, and may have implications for maintaining acceptable welfare for farmed salmon.

Matrix metalloproteinases in osteoclasts of ontogenetic and regenerating zebrafish scales

Matrix metalloproteinases (MMPs) are key enzymes in the turnover of extracellular matrix in health, disease, development and regeneration. We have studied zebrafish scale regeneration to ascertain the role of MMP-2 and MMP-9 in these processes. Scales were plucked from the surface of anaesthetised adult male zebrafish, and the scales that regenerated in the scale pocket were recovered at various time points after plucking. Analyses consisted of (i) mmp-9 in situ hybridisation; (ii) MMP-9+TRAcP double-staining; (iii) qRT-PCR for mmp-2 and mmp-9; (iv) zymography for gelatinolytic activity and (v) a hydroxyproline assay. We found that mmp-9 positive cells were confined to the episquamal side of the scales. Ontogenetic scales had irregular clusters of mono- and multinucleated mmp-9 expressing cells along their lateral margins and radii. During regeneration, mmp-9 positive cells were seen on the scale plate, but not along the lateral margins. Double staining for TRAcP and MMP-9 revealed the osteoclastic nature of these cells. During early scale regeneration, mmp-2 and mmp-9 transcripts increased in abundance in the scale, enzymatic MMP activity increased and collagen degradation was detected by means of hydroxyproline measurements. Near the end of regeneration, all of these parameters returned to the basal values seen in ontogenetic scales. These findings suggest that MMPs play an important role in remodelling of the scale plate during regeneration, and that this function resides in mononucleated and multinucleated osteoclasts which co-express TRAcP and mmp-9. Our findings suggest that the fish scale regeneration model may be a useful system in which to study the cells and mechanisms responsible for regeneration, development and skeletal remodelling.

Dietary fatty acids and inflammation in the vertebral column of Atlantic salmon, Salmo salar L., smolts: a possible link to spinal deformities

Vegetable oils (Vo) are an alternative to fish oil (Fo) in aquaculture feeds. This study aimed to evaluate the effect of dietary soybean oil (Vo diet), rich in linoleic acid, and of dietary fish oil (Fo diet) on the development of spinal deformities under bacterial lipopolysaccharide (LPS)-induced chronic inflammation conditions in Atlantic salmon, Salmo salar L. Fish [25 g body weight (BW)] were fed the experimental diets for 99 days. On day 47 of feeding (40 g BW), fish were subjected to four experimental regimes: (i) intramuscular injections with LPS, (ii) sham-injected phosphate-buffered saline (PBS), (iii) intraperitoneally injected commercial oil adjuvant vaccine, or (iv) no treatment. The fish continued under a common feeding regime in sea water for 165 more days. Body weight was temporarily higher in the Vo group than in the Fo group prior to immunization and was also affected by the type of immunization. At the end of the trial, no differences were seen between the dietary groups. The overall prevalence of spinal deformities was approximately 14% at the end of the experiment. The Vo diet affected vertebral shape but did not induce spinal deformities. In groups injected with LPS and PBS, spinal deformities ranged between 21% and 38%, diet independent. Deformed vertebrae were located at or in proximity to the injection point. Assessment of inflammatory markers revealed high levels of plasma prostaglandin E₂ (PGE₂) in the Vo-fed and LPS-injected groups, suggesting an inflammatory response to LPS. Cyclooxigenase 2 (COX-2) mRNA expression in bone was higher in fish fed Fo compared to Vo-fed fish. Gene expression of immunoglobulin M (IgM) was up-regulated in bone of all LPS-injected groups irrespective of dietary oil. In conclusion, the study suggests that Vo is not a risk factor for the development of inflammation-related spinal deformities. At the same time, we found evidence that localized injection-related processes could trigger the development of vertebral body malformations.

Hedgehog signalling is required for perichondral osteoblast differentiation in zebrafish

In tetrapod long bones, Hedgehog signalling is required for osteoblast differentiation in the perichondrium. In this work we analyse skeletogenesis in zebrafish larvae treated with the Hedgehog signalling inhibitor cyclopamine. We show that cyclopamine treatment leads to the loss of perichondral ossification of two bones in the head. We find that the Hedgehog co-receptors patched1 and patched2 are expressed in regions of the perichondrium that will form bone before the onset of ossification. We also show that cyclopamine treatment strongly reduces the expression of osteoblast markers in the perichondrium and that perichondral ossification is enhanced in patched1 mutant fish. This data suggests a conserved role for Hedgehog signalling in promoting perichondral osteoblast differentiation during vertebrate skeletal development. However, unlike what is seen during long bone development, we did not observe ectopic chondrocytes in the perichondrium when Hedgehog signalling is blocked. This result may point to subtle differences between the development of the skeleton in the skull and limb.

Molecular pathology of vertebral deformities in hyperthermic Atlantic salmon (Salmo salar)

Background

Hyperthermia has been shown in a number of organisms to induce developmental defects as a result of changes in cell proliferation, differentiation and gene expression. In spite of this, salmon aquaculture commonly uses high water temperature to speed up developmental rate in intensive production systems, resulting in an increased frequency of skeletal deformities. In order to study the molecular pathology of vertebral deformities, Atlantic salmon was subjected to hyperthermic conditions from fertilization until after the juvenile stage.

Results

Fish exposed to the high temperature regime showed a markedly higher growth rate and a significant higher percentage of deformities in the spinal column than fish reared at low temperatures. By analyzing phenotypically normal spinal columns from the two temperature regimes, we found that the increased risk of developing vertebral deformities was linked to an altered gene transcription. In particular, down-regulation of extracellular matrix (ECM) genes such as col1a1, osteocalcin, osteonectin and decorin, indicated that maturation and mineralization of osteoblasts were restrained. Moreover, histological staining and in situ hybridization visualized areas with distorted chondrocytes and an increased population of hypertrophic cells. These findings were further confirmed by an up-regulation of mef2c and col10a, genes involved in chondrocyte hypertrophy.

Conclusion

The presented data strongly indicates that temperature induced fast growth is severely affecting gene transcription in osteoblasts and chondrocytes; hence change in the vertebral tissue structure and composition. A disrupted bone and cartilage production was detected, which most likely is involved in the higher rate of deformities developed in the high intensive group. Our results are of basic interest for bone metabolism and contribute to the understanding of the mechanisms involved in development of temperature induced vertebral pathology. The findings may further conduce to future molecular tools for assessing fish welfare in practical farming.

Structural and micro-anatomical changes in vertebrae associated with idiopathic-type spinal curvature in the curveback guppy model

BACKGROUND

The curveback lineage of guppy is characterized by heritable idiopathic-type spinal curvature that develops during growth. Prior work has revealed several important developmental similarities to the human idiopathic scoliosis (IS) syndrome. In this study we investigate structural and histological aspects of the vertebrae that are associated with spinal curvature in the curveback guppy and test for sexual dimorphism that might explain a female bias for severe curve magnitudes in the population.

METHODS

Vertebrae were studied from whole-mount skeletal specimens of curved and non-curved adult males and females. A series of ratios were used to characterize structural aspects of each vertebra. A three-way analysis of variance tested for effects of sex, curvature, vertebral position along the spine, and all 2-way interactions (i.e., sex and curvature, sex and vertebra position, and vertebra position and curvature). Histological analyses were used to characterize micro-architectural changes in affected vertebrae and the intervertebral region.

RESULTS

In curveback, vertebrae that are associated with curvature demonstrate asymmetric shape distortion, migration of the intervertebral ligament, and vertebral thickening on the concave side of curvature. There is sexual dimorphism among curved individuals such that for several vertebrae, females have more slender vertebrae than do males. Also, in the region of the spine where lordosis typically occurs, curved and non-curved females have a reduced width at the middle of their vertebrae, relative to males.

CONCLUSIONS

Based on similarities to human spinal curvatures and to animals with induced curves, the concave-convex biases described in the guppy suggest that there is a mechanical component to curve pathogenesis in curveback. Because idiopathic-type curvature in curveback is primarily a sagittal deformity, it is structurally more similar to Scheuermann kyphosis than IS. Anatomical differences between teleosts and humans make direct biomechanical comparisons difficult. However, study of basic biological systems involved in idiopathic-type spinal curvature in curveback may provide insight into the relationship between a predisposing aetiology, growth, and biomechanics. Further work is needed to clarify whether observed sex differences in vertebral characteristics are related to the female bias for severe curves that is observed in the population.

No bones about it: an enigmatic Devonian fossil reveals a new skeletal framework--a potential role of loss of gene regulation

Palaeospondylus gunni (Devonian, Scotland) is an enigmatic vertebrate, assigned to various jawless and jawed groups since its original description. New sections through the whole body allow description of a novel skeletal tissue for Palaeospondylus, comprising the entire skeleton. This tissue is mineralized cartilage and is characterized by large cell spaces embedded in minimal matrix. Bone is completely absent. Calcium phosphate mineralization has a differential topography of deposition within the cartilage that reflects a biogenic origin, despite subsequent diagenetic modification. This combination of hypertrophied cell spaces surrounded by regionalized mineralized matrix differs from all other cartilage in fossil and extant vertebrates. However, it compares most closely to gnathostome endochondral bone in early developmental stages. For example, Palaeospondylus skeletal histology differs from the Devonian agnathan Euphanerops and extant lamprey cartilage. Comparison with mineralized cartilage of armored fossil agnathans and placoderms shows the histology is not comparable to globular calcified cartilage. It also differs from that in extant chondrichthyan mineralized tesserae, which is restricted to a subperichondral zone. Amongst this diversity of calcified cartilage types we discuss various interpretations, including one that implicates tissue either in developmental stasis, before osteoblasts can deposit bone, or at a phylogenetic stage when this step has not evolved. These very different hypotheses highlight difficulties in interpreting fossil ontogenies when phylogenetic relationships are uncertain. Nevertheless, we propose that the composition of the Palaeospondylus skeleton represents a fossilized ontogenetic stage of endochondral bone, a type of bone characteristic of osteichthyan vertebrates.