Effect of chronic undernutrition on body mass and mechanical bone quality under normoxic and altitude hypoxic conditions

Hypobaric hypoxia causes low fecundity in zebrafish parents and impairment of skeletal development in zebrafish embryos and rat offspring

Hypobaric Hypoxia (HH) negatively affects the cardiovascular and respiratory systems as well as gonadal development and the therefore next generation. This study investigated the effects of HH on zebrafish and SD rats, by exposing them to a low-pressure environment at 6000 m elevation for 30 days to simulate high-altitude conditions. It was indicated that parental zebrafish reared amh under HH had increased embryo mortality, reduced hatchability, and abnormal cartilage development in the offspring. Furthermore, the HH-exposed SD rats had fewer reproductive cells and smaller litters. Moreover, the transcriptome analysis revealed the down-regulation of steroid hormone biosynthesis pathways. The expression of the gonad-associated genes (amh, pde8a, man2a2 and lhcgr), as well as the gonad and cartilage-related gene bmpr1a, were also down-regulated. In addition, Western blot analysis validated reduced bmpr1a protein expression in the ovaries of HH-treated rats. In summary, these data indicate the negative impact of HH on reproductive organs and offspring development, emphasizing the need for further research and precautions to protect future generations' health.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Hypobaric hypoxia deteriorates bone mass and strength in mice

Mountaineers at high altitude are at increased risk of acute mountain sickness as well as high altitude pulmonary and cerebral edema. A densitometric study in mountaineers has suggested that expeditions at high altitude decrease bone mineral density. Surprisingly, the in vivo skeletal effects of hypobaric hypoxia are largely unknown, and have not been studied using advanced contemporary methods to assess bone microstructure. Eighty-four 22-week-old female mice were divided into seven groups with 12 mice in each group: 1. Baseline; 2. Normobaric, 4 weeks; 3. Hypobaric hypoxia, 4 weeks; 4. Normobaric, 8 weeks; 5. Hypobaric hypoxia, 8 weeks; 6. Normobaric, 12 weeks; and 7. Hypobaric hypoxia, 12 weeks. Hypobaric hypoxia mice were housed in hypobaric chambers at an ambient pressure of 500 mbar (5500 m altitude), while normobaric mice were housed at sea level atmospheric pressure for 4, 8, or 12 weeks, respectively. Hypobaric hypoxia had a profound impact on femoral cortical bone and L4 trabecular bone, while the effect on femoral trabecular bone was less pronounced. Hypobaric hypoxia reduced the bone strength of the femoral mid-diaphysis and L4 at all time-points. At femoral cortical bone, hypobaric hypoxia reduced bone formation through fewer mineralizing surfaces and lower bone formation rate after 2 weeks. In addition, bone strength decreased, and C-terminal telopeptide of type I collagen (CTX-I) increased independently of the duration of exposure to simulated high altitude. At L4, hypobaric hypoxia resulted in a substantial reduction in bone volume fraction, trabecular thickness, and trabecular number after 4 weeks of exposure. Hypobaric hypoxia reduced bone strength and femoral bone mass, while femoral trabecular bone was much less affected, indicating the skeletal response to hypobaric hypoxia differ between cortical and trabecular bone. These findings provide initial preclinical support for future clinical studies in mountaineers to assess bone status and bone strength after exposure to prolonged high altitude exposure.

A review of the skeletal effects of exposure to high altitude and potential mechanisms for hypobaric hypoxia-induced bone loss

Mountaineering and exposure to high altitude result in physiological adaptations to the reduced inspiratory oxygen availability. Acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and high altitude cerebral edema (HACE) are well-described harmful effects of exposure to high altitude. Common to AMS, HAPE, and HACE are distinct clinical signs and symptoms of impaired function. However, several studies have suggested that high altitude might result in a substantial bone loss, which usually does not produce any apparent symptoms. This review aims to provide a comprehensive overview of, and map current knowledge of the skeletal effects of hypobaric hypoxia and high altitude. PubMed and Embase were searched from inception to September 6, 2021, to identify studies investigating the skeletal effects of exposure to hypobaric hypoxia and high altitude. Three hundred sixty titles and abstracts were screened, and 20 full-text articles were included (16 in vivo studies and four real-world human studies). In rodents, simulated high altitude up to 2900 m did not result in any adverse skeletal effects. In contrast, studies exposing animals to very high altitude (3500-5500 m) reported substantial reductions in BMD, cortical morphology, and bone strength, as well as deteriorated trabecular microstructure. Detrimental microstructural effects were also reported in rats exposed to simulated extreme altitude (6000 m). Finally, real-world human studies in mountaineers suggested high altitude exposure reduced bone mineral density (BMD) and that the harmful skeletal effects of hypobaric hypoxia were not entirely recovered after 12 months. In conclusion, in vivo and real-world studies demonstrated high altitude exposure results in adverse skeletal effects. The underlying mechanism for hypobaric hypoxia-induced bone loss is not elucidated.

Impact of High-Altitude Hypoxia on Early Osseointegration With Bioactive Titanium

Nowadays, the bone osseointegration in different environments is comparable, but the mechanism is unclear. This study aimed to investigate the osseointegration of different bioactive titanium surfaces under normoxic or high-altitude hypoxic environments. Titanium implants were subjected to one of two surface treatments: (1) sanding, blasting, and acid etching to obtain a rough surface, or (2) extensive polishing to obtain a smooth surface. Changes in the morphology, proliferation, and protein expression of osteoblasts on the rough and smooth surfaces were examined, and bone formation was studied through western blotting and animal-based experiments. Our findings found that a hypoxic environment and rough titanium implant surface promoted the osteogenic differentiation of osteoblasts and activated the JAK1/STAT1/HIF-1α pathway in vitro. The animal study revealed that following implant insertion in tibia of rabbit, bone repair at high altitudes was slower than that at low altitudes (i.e., in plains) after 2weeks; however, bone formation did not differ significantly after 4weeks. The results of our study showed that: (1) The altitude hypoxia environment would affect the early osseointegration of titanium implants while titanium implants with rough surfaces can mitigate the effects of this hypoxic environment on osseointegration, (2) the mechanism may be related to the activation of JAK1/STAT1/HIF-1α pathway, and (3) our results suggest the osteogenesis of titanium implants, such as oral implants, is closely related to the oxygen environment. Clinical doctors, especially dentists, should pay attention to the influence of hypoxia on early osseointegration in patients with high altitude. For example, it is better to choose an implant system with rough implant surface in the oral cavity of patients with tooth loss at high altitude.

Mechanical mandible competence in rats with nutritional growth retardation

OBJECTIVE

In order to provide a better understanding of the sympathetic nervous system as a negative regulator of bone status, the aim of the study was to establish the biomechanical mandible response to different doses of a β-adrenergic antagonist such as propranolol (P) in a stress-induced food restriction model of growth retardation.

METHODS

Rats were assigned to eight groups: Control (C), C+P3.5 (CP3.5), C+P7 (CP7), C+P14 (CP14), NGR, NGR+P3.5 (NGRP3.5), NGR+P7 (NGRP7) and NGR+P14 (NGRP14). C, CP3.5, CP7 and CP14 rats were freely fed with the standard diet. NGR, NGRP3.5, NGRP7 and NGRP14 rats received, for 4 weeks (W4), 80% of the amount of controls food consumed. Propranolol 3.5, 7 and 14mg/kg/day was injected ip 5days per week in CP3.5 and NGRP3.5, CP7 and NGRP7, CP14 and NGRP14, respectively. At W4, zoometry, mandible morphometry, static histomorphometric and biomechanical competence were performed.

RESULTS

A dose of Propranolol 7mg/kg/day induced interradicular bone volume accretion reaching a mandible stiffness according to chronological age.

CONCLUSION

These findings evidenced that sympathetic nervous system activity is a negative regulator of mandible mechanical competence in the nutritional growth retardation model. Propranolol 7mg/kg/day, under the regimen usage, seems to be appropriate to blockade SNS activity on mandible mechanical performance in NGR rats, probably associated to an effect on bone mechanostat system ability to detect disuse mode as an error.