Skeletal Characterization of the Fgfr3 Mouse Model of Achondroplasia Using Micro-CT and MRI Volumetric Imaging

Impact loading intensifies cortical bone (re)modeling and alters longitudinal bone growth of pubertal rats

Physical exercise is important for musculoskeletal development during puberty, which builds bone mass foundation for later in life. However, strenuous levels of training might bring adverse effects to bone health, reducing longitudinal bone growth. Animal models with various levels of physical exercise were largely used to provide knowledge to clinical settings. Experiments from our previous studies applied different levels of mechanical loading on rat tibia during puberty accompanied by weekly in vivo micro-CT scans. In the present article, we apply 3D image registration-based methods to retrospectively analyze part of the previously acquired micro-CT data. Longitudinal bone growth, growth plate thickness, and cortical bone (re)modeling were evaluated from rats' age of 28-77 days. Our results show that impact loading inhibited proximal bone growth throughout puberty. We hypothesize that impact loading might bring different growth alterations to the distal and proximal growth plates. High impact loading might lead to pathological consequence of osteochondrosis and catch-up growth due to growth inhibition. Impact loading also increased cortical bone (re)modeling before and after the peak proximal bone growth period of young rats, of which the latter case might be caused by the shift from modeling to remodeling as the dominant activity toward the end of rat puberty. We confirm that the tibial endosteum is more mechano-sensitive than the periosteum in response to mechanical loading. To our knowledge, this is the first study to follow up bone growth and bone (re)modeling of young rats throughout the entire puberty with a weekly time interval.

Meeting Report from 2nd ICCBH-ERN BOND Spinal Complications in Children and Adults with Achondroplasia Workshop, Dublin, Ireland, 2nd July 2022

A pre-meeting workshop on spinal complications in children and adults with achondroplasia was held in Dublin, Ireland at the 10th International Conference on Children's Bone Health (ICCBH) 2-5 July 2022. The pathophysiology, natural history and medical/surgical management of thoraco-lumbar kyphosis and spinal stenosis remains poorly described in the literature. The structure of the workshop consisted of lectures, a debate and an interactive round table discussion. In total over 100 delegates affiliated to over 70 institutions from 20 countries were in attendance.

An orally available small molecule that targets soluble TNF to deliver anti-TNF biologic-like efficacy in rheumatoid arthritis

Tumor necrosis factor (TNF) is a pleiotropic cytokine belonging to a family of trimeric proteins with both proinflammatory and immunoregulatory functions. TNF is a key mediator in autoimmune diseases and during the last couple of decades several biologic drugs have delivered new therapeutic options for patients suffering from chronic autoimmune diseases such as rheumatoid arthritis and chronic inflammatory bowel disease. Attempts to design small molecule therapies directed to this cytokine have not led to approved products yet. Here we report the discovery and development of a potent small molecule inhibitor of TNF that was recently moved into phase 1 clinical trials. The molecule, SAR441566, stabilizes an asymmetrical form of the soluble TNF trimer, compromises downstream signaling and inhibits the functions of TNF in vitro and in vivo. With SAR441566 being studied in healthy volunteers we hope to deliver a more convenient orally bioavailable and effective treatment option for patients suffering with chronic autoimmune diseases compared to established biologic drugs targeting TNF.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Phenotype assessment for neurodegenerative murine models with ataxia and application to Niemann-Pick disease, type C1

Identifying meaningful predictors of therapeutic efficacy from preclinical studies is challenging. However, clinical manifestations occurring in both patients and mammalian models offer significant translational value. Many neurological disorders, including inherited, metabolic Niemann-Pick disease, type C (NPC), exhibit ataxia. Both individuals with NPC and murine models manifest ataxia, and investigational therapies impacting this phenotype in mice have been reported to slow disease progression in patients (e.g. miglustat, intrathecal 2-hydroxypropyl-beta-cyclodextrin, and acetyl-L-leucine). Reproducible phenotypic scoring of animal models can facilitate comparisons between genotypes, sexes, disease course, and therapies. Previously, other groups have developed a composite phenotypic scoring system (CPSS), which was subsequently used to distinguish strain-dependent phenotypes and, with modifications, to evaluate potential therapies. However, high inter-rater reliability is paramount to widespread use. We have created a comprehensive, easy-to-follow phenotypic assessment based on the CPSS and have verified its reproducibility using murine models of NPC disease. Application of this scoring system is not limited to NPC disease and may be applicable to other models of neurodegeneration exhibiting motor incoordination, thereby increasing its utility in translational studies.

Impairment of motor skills in children with achondroplasia-usefulness of brain and cranio-cervical junction evaluation by quantitative magnetic resonance imaging: a case-control study

BACKGROUND

Most infants and children with achondroplasia show delayed motor skill development; however, some patients may have clinical consequences related to cranio-cervical junction stenosis and compression.

PURPOSE

To assess, using brain magnetic resonance imaging (MRI), quantitative variables linked to neuromotor impairment in achondroplasic children.

MATERIAL AND METHODS

In total, 24 achondroplasic children underwent pediatric neurological assessment and were grouped in two cohorts according to relevant motor skill impairment. Achondroplasic children with (n=12) and without (n=12) motor symptoms were identified, and brain MRI scans were quantitatively evaluated. 3D fast spoiled gradient echo T1-weighted images were used to assess: supratentorial intracranial volumes (SICV); supratentorial intracranial brain volume (SICBV); SICV/SICBV ratio; posterior cranial fossa volume (PCFV); posterior cranial fossa brain volume (PCBFV); PCFV/PCFBV ratio; ventricular and extra-ventricular cerebrospinal fluid (CSF) volumes; foramen magnum (FM) area; and jugular foramina (JF) areas.

RESULTS

In both groups, SICV/SICBV ratio, supratentorial ventricular and extra-ventricular space volumes were increased while SICBV was increased only in the asymptomatic group (P < 0.05). PCFV/PCFBV ratio, IV ventricle, infratentorial extra-ventricular spaces volumes were reduced (P < 0.05) in the symptomatic group while PCFBV was increased only in the asymptomatic group (P < 0.05). Foramen magnum (FM) area was more reduced in the symptomatic group than the asymptomatic group (P < 0.05) but no correlation between FM area and ventriculomegaly was found (P > 0.05).

CONCLUSION

Evaluation of the FM area together with infratentorial ventricular and extra-ventricular space volume reduction may be helpful in differentiating patients at risk of developing motor skill impairment. Further investigation is needed to better understand the temporal profile between imaging and motor function in order to propose possible personalized surgical treatment.

Loss of Sprouty Produces a Ciliopathic Skeletal Phenotype in Mice Through Upregulation of Hedgehog Signaling

The Sprouty family is a highly conserved group of intracellular modulators of receptor tyrosine kinase (RTK)-signaling pathways, which have been recently linked to primary cilia. Disruptions in the structure and function of primary cilia cause inherited disorders called ciliopathies. We aimed to evaluate Sprouty2 and Sprouty4 gene-dependent alterations of ciliary structure and to focus on the determination of its association with Hedgehog signaling defects in chondrocytes. Analysis of the transgenic mice phenotype with Sprouty2 and Sprouty4 deficiency revealed several defects, including improper endochondral bone formation and digit patterning, or craniofacial and dental abnormalities. Moreover, reduced bone thickness and trabecular bone mass, skull deformities, or chondroma-like lesions were revealed. All these pathologies might be attributed to ciliopathies. Elongation of the ciliary axonemes in embryonic and postnatal growth plate chondrocytes was observed in Sprouty2^-/- and Sprouty2^+/- /Sprouty4^-/- mutants compared with corresponding littermate controls. Also, cilia-dependent Hedgehog signaling was upregulated in Sprouty2/4 mutant animals. Ptch1 and Ihh expression were upregulated in the autopodium and the proximal tibia of Sprouty2^-/- /Sprouty4^-/- mutants. Increased levels of the GLI3 repressor (GLI3R) form were detected in Sprouty2/4 mutant primary fibroblast embryonic cell cultures and tissues. These findings demonstrate that mouse lines deficient in Sprouty proteins manifest phenotypic features resembling ciliopathic phenotypes in multiple aspects and may serve as valuable models to study the association between overactivation of RTK and dysfunction of primary cilia during skeletogenesis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Pleckstrin homology (PH) domain and Leucine Rich Repeat Phosphatase 1 (Phlpp1) Suppresses Parathyroid Hormone Receptor 1 (Pth1r) Expression and Signaling During Bone Growth

Endochondral ossification is tightly controlled by a coordinated network of signaling cascades including parathyroid hormone (PTH). Pleckstrin homology (PH) domain and leucine rich repeat phosphatase 1 (Phlpp1) affects endochondral ossification by suppressing chondrocyte proliferation in the growth plate, longitudinal bone growth, and bone mineralization. As such, Phlpp1-/- mice have shorter long bones, thicker growth plates, and proportionally larger growth plate proliferative zones. The goal of this study was to determine how Phlpp1 deficiency affects PTH signaling during bone growth. Transcriptomic analysis revealed greater PTH receptor 1 (Pth1r) expression and enrichment of histone 3 lysine 27 acetylation (H3K27ac) at the Pth1r promoter in Phlpp1-deficient chondrocytes. PTH (1-34) enhanced and PTH (7-34) attenuated cell proliferation, cAMP signaling, cAMP response element-binding protein (CREB) phosphorylation, and cell metabolic activity in Phlpp1-inhibited chondrocytes. To understand the role of Pth1r action in the endochondral phenotypes of Phlpp1-deficient mice, Phlpp1-/- mice were injected with Pth1r ligand PTH (7-34) daily for the first 4 weeks of life. PTH (7-34) reversed the abnormal growth plate and long-bone growth phenotypes of Phlpp1-/- mice but did not rescue deficits in bone mineral density or trabecular number. These results show that elevated Pth1r expression and signaling contributes to increased proliferation in Phlpp1-/- chondrocytes and shorter bones in Phlpp1-deficient mice. Our data reveal a novel molecular relationship between Phlpp1 and Pth1r in chondrocytes during growth plate development and longitudinal bone growth. © 2021 American Society for Bone and Mineral Research (ASBMR).

Macrocerebellum in Achondroplasia: A Further CNS Manifestation of FGFR3 Mutations?

Achondroplasia is the result of a mutation in the fibroblast growth factor receptor 3 gene (FGFR3). Appearances suggestive of macrocerebellum have not been described in this patient group. We retrospectively reviewed MR imaging studies of the brain in 23 children with achondroplasia. A constellation of imaging findings that are recognized in macrocerebellum was observed, including cerebellar hemisphere enlargement (inferior and superior extension, wrapping around the brainstem); an effaced retro- and infravermian cerebellar subarachnoid CSF space; a shortened midbrain; distortion of the tectal plate; and mass effect on the brainstem. All MR imaging studies exhibited some of these findings. Quantitative analysis confirmed an increased cerebellar volume compared with age- and sex-matched controls. We hypothesized that this may be due to direct effects of the FGFR3 mutation on cerebellar morphogenesis.

Fibroblast growth factors in skeletal development

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are expressed throughout all stages of skeletal development. In the limb bud and in cranial mesenchyme, FGF signaling is important for formation of mesenchymal condensations that give rise to bone. Once skeletal elements are initiated and patterned, FGFs regulate both endochondral and intramembranous ossification programs. In this chapter, we review functions of the FGF signaling pathway during these critical stages of skeletogenesis, and explore skeletal malformations in humans that are caused by mutations in FGF signaling molecules.