Selective β2-Adrenoceptor Blockade Rescues Mandibular Growth Retardation in Adolescent Rats Exposed to Chronic Intermittent Hypoxia

Emerging roles of nerve-bone axis in modulating skeletal system

Over the past decades, emerging evidence in the literature has demonstrated that the innervation of bone is a crucial modulator for skeletal physiology and pathophysiology. The nerve-bone axis sparked extensive preclinical and clinical investigations aimed at elucidating the contribution of nerve-bone crosstalks to skeleton metabolism, homeostasis, and injury repair through the perspective of skeletal neurobiology. To date, peripheral nerves have been widely reported to mediate bone growth and development and fracture healing via the secretion of neurotransmitters, neuropeptides, axon guidance factors, and neurotrophins. Relevant studies have further identified several critical neural pathways that stimulate profound alterations in bone cell biology, revealing a complex interplay between the skeleton and nerve systems. In addition, inspired by nerve-bone crosstalk, novel drug delivery systems and bioactive materials have been developed to emulate and facilitate the process of natural bone repair through neuromodulation, eventually boosting osteogenesis for ideal skeletal tissue regeneration. Overall, this work aims to review the novel research findings that contribute to deepening the current understanding of the nerve-bone axis, bringing forth some schemas that can be translated into the clinical scenario to highlight the critical roles of neuromodulation in the skeletal system.

Gestational intermittent hypoxia reduces mandibular growth with decreased Sox9 expression and increased Hif1a expression in male offspring rats

Introduction

Maternal obstructive sleep apnea (OSA) during pregnancy is the risk factor for impaired fetal growth with low birth weight in the offspring. However, it is unclear whether gestational intermittent hypoxia (IH, a hallmark of maternal OSA) has long-term detrimental consequences on the skeletal development of offspring. This study aimed to investigate postnatal maxillofacial bone growth and cartilage metabolism in male and female offspring that were exposed to gestational IH.

Methods

Mother rats underwent IH at 20 cycles/h (nadir, 4% O2; peak, 21% O2; 0% CO2) for 8 h per day during gestational days (GD) 7-20, and their male and female offspring were analyzed postnatally at 5 and 10 weeks of age. All male and female offspring were born and raised under normoxic conditions.

Results

There was no significant difference in whole-body weight and tibial length between the IH male/female offspring and their control counterparts. In contrast, the mandibular condylar length was significantly shorter in the IH male offspring than in the control male offspring at 5 and 10 weeks of age, while there was no significant difference in the female offspring. Real-time polymerase chain reaction (PCR) showed that gestational IH significantly downregulated the mRNA level of SOX9 (a chondrogenesis marker) and upregulated the mRNA level of HIF-1α (a hypoxia-inducible factor marker) in the mandibular condylar cartilage of male offspring, but not in female offspring.

Conclusion

Gestational IH induced underdeveloped mandibular ramus/condyles and reduced mRNA expression of SOX9, while enhancing mRNA expression of HIF-1α in a sex-dependent manner.

Effect of breathing patterns on mandibular cortical bone quality in children and establishment of a preliminary screening model

OBJECTIVE

This retrospective study analyzed breathing patterns and age subgroups effect on cortical bone quality of the mandible in growing subjects, aiming to explore the application value of facial skeletal pattern combined with cortical bone density detection in early screening diagnosis of mouth breathing.

METHODS

One hundred twenty-six participants were divided into four groups: mouth breathing group (7-9, 10-12 years old) and nasal breathing group (7-9, 10-12 years old). The mandibular anterior, middle, and posterior cortical bone mineral density (CBMD), cortical bone width (MCW), ANB, and FMA values were measured. Independent T-test and Mann-Whitney U test were used to compare the measured values. Binary logistic regression was employed to analyze the correlation between measured variables and the children's breathing patterns. ROC analysis was used to determine the ability of the cortical bone density measurements in early screening diagnosis of MB.

RESULTS

Mouth breathing had a negative impact on CBMD and MCW of the pre-mandibular (Pog) in subjects aged 7-9 years and also impacted the development of (Pog) and submandibular (Me) sites in subjects aged 10-12 years. Older children in the nasal breathing group have higher CBMD, MCW, and SNB values and lower FMA values. Single-factor and multiple-factor logistic binary regression analysis showed that FMA, MSPogCBMD, MSPogMCW, and ANB are correlated factors for children at risk of mouth breathing.

CONCLUSION

Mouth breathing pattern is closely associated with decreased mandibular CBMD and MCW values in children aged 7-12, where the anterior (Pog) and inferior (Me) sites of anterior mandible are more significantly affected. Furthermore, in combination with facial skeletal pattern, it provides a basis for the early warning diagnosis of mouth breathing.

Metabolic dysregulation and decreased capillarization in skeletal muscles of male adolescent offspring rats exposed to gestational intermittent hypoxia

Gestational intermittent hypoxia (IH) is a hallmark of obstructive sleep apnea that occurs frequently during pregnancy, and effects caused by this environmental change during pregnancy may be transmitted to the offspring. In this study, we aimed to clarify the effects of IH in pregnant rats on the skeletal muscle of adolescent offspring rats. Mother rats underwent IH from gestation day 7-21, and their 5-weeks-old male offspring were analyzed. All male offspring rats were born and raised under normoxia conditions. Although no general growth retardation was observed, we found that exposure to gestational IH reduces endurance running capacity of adolescent offspring rats. Both a respiratory muscle (diaphragm; DIA) and a limb muscle (tibialis anterior; TA) showed no histological abnormalities, including fiber size and fiber type distribution. To identify the possible mechanism underlying the reduced running capacity, regulatory factors associated with energy metabolism were analyzed in different parts of skeletal muscles. Compared with rats born under conditions of gestational normoxia, gestational IH offspring rats showed significantly lower expression of genes associated with glucose and lipid metabolism, and lower protein levels of phosphorylated AMPK and AKT. Furthermore, gene expression of adiponectin receptors one and two was significantly decreased in the DIA and TA muscles. In addition, the DIA muscle from adolescent rats had significantly decreased capillary density as a result of gestational IH. However, these changes were not observed in a sucking muscle (geniohyoid) and a masticating muscle (masseter) of these rats. These results suggest that respiratory and limb muscles are vulnerable to gestational IH, which induces altered energy metabolism with decreased aerobic motor function. These changes were partially owing to the decreased expression of adiponectin receptors and decreased capillary density in adolescent offspring rats.

Gestational Intermittent Hypoxia Induces Mitochondrial Impairment in the Geniohyoid Muscle of Offspring Rats

Introduction Gestational intermittent hypoxia (IH), a hallmark of obstructive sleep apnea during gestation, alters respiratory neural control and diaphragm muscle contractile function in the offspring. The geniohyoid (GH) muscle is innervated by the respiratory-related hypoglossal nerve and plays a role in tongue traction and suckling, motor behaviors that then give way to chewing. Here, we aimed to investigate the effects of gestational exposure to IH on the muscle development and metabolism of GH and masseter muscles in male offspring rats. Materials and methods Pregnant Sprague-Dawley rats were exposed to IH (3-min periods of 4-21% O2) for eight hours/day during gestational days 7-20. The GH and masseter muscles from 35-day-old male offspring (n = 6 in each group) were analyzed.  Results Gestational IH induction reduced type IIA fiber size in the GH muscle of the offspring but not in the masseter muscle. Western blot analysis showed that gestational IH-induced significant downregulation of peroxisome proliferator-activated receptor (PPAR)-gamma coactivator 1-alpha (PGC1α) protein in the GH muscle but not in the masseter muscle. Moreover, optic atrophy 1 and mitofusin-2 proteins were decreased and mitochondrial fission 1 protein levels were increased in the GH muscle of the offspring exposed to gestational IH. Mitochondrial adenosine triphosphate (ATP) synthase subunit alpha and transcriptional factor A (TFAM) were decreased in the GH muscle post-gestational IH. Conclusion These findings suggest that gestational IH-induced impaired mitochondrial metabolism and alteration of oxidative myofibers of the GH muscle in the pre-adolescent offspring, but not the masseter muscle, owing to the susceptibility of GH muscular mitochondria to gestational IH.