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Bone Morphogenetic Protein 7 Regulates Nasal Cartilage Properties Under Physiological And Pathological Conditions

  • Author / Creator
    Baddam, Pranidhi
  • Background: The nasal septum is a cartilaginous structure that provides critical structural and growth support for the midface. Nasal septum deviation (NSD) is a relatively frequent abnormality, which can be associated with nasal airway obstruction and disordered breathing in children. Although many children present with NSD, not everyone presents with secondary co-morbidities such as difficulty breathing. Additionally, it has been established that the incidence of NSD increases during midfacial growth spurt in children. However, the cellular and molecular mechanisms contributing to midfacial growth and NSD are poorly understood due to limited access to pediatric clinical samples. As a result, there is a need to understand the biological underpinning of midfacial growth and nasal septum development. Since the nasal septum cartilage is a hyaline cartilage similar to the knee cartilage and the nasal cartilage contributes to the growth of the midface, we hypothesize that nasal septum deviation is a consequence of underlying cartilage differentiation and growth defects. Bone Morphogenetic Proteins (BMP) are signalling molecules important for proliferation and differentiation of chondrocytes to make cartilage. BMPs are thought to regulate extracellular matrix. However, how they do this is yet to be identified. BMP7, in particular, has been extensively investigated in articular cartilage repair models. Articular cartilage is a form of hyaline cartilage. Literature suggests that BMP7, an anabolic growth factor, can repair and maintain articular cartilage both in vivo and in vitro. However, no studies have demonstrated the role BMP7 plays in craniofacial cartilages such as the nasal septum. Here we assess changes to chondrocyte properties underlying NSD in a mouse model for midfacial hypoplasia to better understand the etiology of NSD as a consequence of growth defects in the face.
    Methods: To understand the relationship between severity of NSD and onset of comorbidities, a retrospective study assessing nasal cavity abnormalities was quantified subjectively and objectively and then correlated with their risk for developing sleep-disordered breathing. To understand molecular and cellular changes contributing to normal development of nasal septum and midfacial growth, histological and molecular properties of wild-type nasal septum were characterized in mice. To identify changes to chondrocyte properties in a pathological setting, mice carrying a conditional deletion of Bmp7 in neural crest (Bmp7ncko) present with midfacial hypoplasia and NSD were used. Firstly, a timeline of when NSD and its associated co-morbidity (difficulty breathing) were established. This was followed by morphometric, histological, immunohistochemical, genomic and proteomic analyses of the nasal septum to correlate molecular differences to physiological structural changes.
    Results: The retrospective study revealed that assessing severity of NSD and onset of associated co-morbidities is not possible using cone-beam computed tomography scans. Nasal cartilage assessment of wild-type mice revealed that nasal septum grows asynchronous across its length with phases of rapid growth interrupted by more stationary growth. Growth appears to be driven predominantly by acquisition of chondrocyte hypertrophy. Morphometric, physiological and metabolomic analysis of Bmp7ncko mice revealed that NSD develops during midfacial growth and precedes the development of an upper-airway obstruction as observed by reduced breathing frequency, apneas and overall reduced oxygen consumption. The nasal cartilage was histologically inconspicuous at 2 weeks; however, structural changes were evident at 4 weeks. The normally hyaline cartilage showed expression of proteins characteristic of hypertrophic chondrocytes, acquisition of elastic cartilage properties, a switch to glucose metabolism, and acquisition of molecular characteristics commonly associated with early-stage osteoarthritis (OA). We also found that genetic reduction of Bmp2 in these Bmp7ncko mice prevented the deviation. 3D in vitro pellet cultures using Bmp7ctrl and Bmp7ncko nasal septum chondrocytes stimulated with/without recombinant BMP7 (rhBMP7) and induced to differentiate using Insulin-Transferrin-Selenium (ITS) demonstrated that pellets treated with both rhBMP7 and ITS completely suppressed induction of Elastin.
    Conclusion: Loss of BMP7 resulted in cellular changes in chondrocytes, which altered nasal cartilage properties and compromised its growth, placing BMP7 as a critical factor regulating chondrocyte differentiation. Additionally, there is strong evidence that absence of BMP7 might lead to formation of elastic cartilage in place of hyaline cartilage. Our findings suggest that NSD can occur because of underlying nasal septum cartilage growth and differentiation defects. Overall, NSD, in connection with midfacial growth reduction, might indicate altered chondrocyte biology in affected children.

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  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.