Proteoglycan Errors in Embryonic Bone Development: A Closer Look

The critical role of proteoglycans in embryonic bone development is highlighted when genetic mutations are linked to skeletal disorders. Understanding the genetic basis of these disorders enhances our insight into critical design features of biomaterials for skeletal repair. One promising avenue is the use of carbohydrate-based scaffolds for tissue engineering. These materials, including Osteo-PTM BGS, structurally mimic GAGs and have shown potential for bone regeneration. For example, a novel hyper-crosslinked carbohydrate polymer demonstrated efficacy in repairing bone defects in vivo. To learn more, refer to “Hyper-Crosslinked Carbohydrate Polymer for Repair of Critical-Sized Bone Defects” (Koleva et al., 2019). 

Proteoglycans are vital macromolecules that play a crucial role in embryonic bone development. These complex molecules consist of a core protein with covalently attached glycosaminoglycan (GAG) side chains. Proper synthesis and modification of proteoglycans are essential for normal skeletal formation. However, mutations in the genes responsible for these processes can severely disrupt bone development, leading to skeletal abnormalities such as chondrodysplasias and connective tissue diseases (Sao & Risbud, 2024; Taylan & Mäkitie, 2016). 

The Role of GAG Sulfation in Skeletal Development 

GAG sulfation, a critical modification process catalyzed by sulfotransferases within the Golgi apparatus, is essential for the proper function of all GAGs except hyaluronic acid. Disruptions in this sulfation process can significantly alter the physiological roles of proteoglycans, particularly in developing tissues. Such disruptions have been linked to various skeletal disorders, including chondrodystrophies, characterized by abnormal cartilage and bone development.  Mutations in genes related to sulfation or ion transport can lead to a range of skeletal disorders, as summarized in Table 1 (Sao & Risbud, 2024; Taylan & Mäkitie, 2016). 

Table 1: Gene Mutations and Associated Skeletal Disorders 

Mutated Genes	Disease/ Disorders
PAPSS2, CHST3	Skeletal dysplasia
IMPAD1, SLC35B2, CHST11	Chondrodysplasia
CHST11	Brachydactyly
CHSY1	Temtamy preaxial brachydactyly syndrome
B4GALT7, CHST14, DSE	Ehlers-Danlos syndrome
SDC4	Intervertebral Disc degeneration
XYLT1	Desbuquois dysplasia type 2
XYLT2	Spondylo-ocular syndrome
FAM20B	Neonatal short limb dysplasia
EXT1 and EXT2	Hereditary multiple exostosis
B3GAT3	Larsen-like syndrome, Gerodermia osteodysplastica-like phenotype
EXTL3	Skeletal dysplasia, Hereditary multiple exostosis
SLC35D1	Schneckenbecken dysplasia
SLC26A2	Chondrodysplasias, Achondrogenesis, Diastrophic dysplasia, Recessive multiple epiphyseal dysplasia
GORAB	Gerodermia osteodysplastica

Proteoglycans and Intervertebral Disc Disease 

Proteoglycans are also key players in maintaining the structural integrity of the intervertebral discs, particularly through their role in collagen fibril organization and tissue hydration.  In individuals with intervertebral disc disease, elevated levels of inflammatory cytokines such as tumor necrosis factor-α (TNF- α), interleukin (IL)-6, IL-1β can exacerbate disc degeneration. This process is mediated by the upregulation of the SDC4 gene which encodes for syndecan-4, a proteoglycan involved in matrix regulation. Activation of SDC4 triggers proteolytic cascades that lead to the disintegration of the disc extracellular matrix, primarily through enzymes like ADAMTS-5 and matrix metalloproteinases (Sao & Risbud, 2024). 

Consequences of Faulty Proteoglycan Synthesis 

Genetic mutations affecting each step of proteoglycan biosynthesis – from the initial synthesis of the protein core to elongation and sulfation of GAG chains – can lead to a wide array of disorders. Table 2 outlines some of the key genetic disorders associated with defects in proteoglycan synthesis (Chan et al., 2018; Paganini et al., 2019; Sao & Risbud, 2024; Wen et al., 2014). 

Table 2: Genetic Disorders Linked to Proteoglycan Biosynthesis Errors 

Gene	Normal Function	Dysfunction Due to Mutation	Disorder/Disease and Symptoms
XYLT1	Xylosyltransferase , catalyzes the first step in GAG biosynthesis	Early chondrocyte maturation, ossification, leading to disproportionate dwarfism.	Desbuquois dysplasia type 2,   Baratela–Scott syndrome
FAM20B	Xylose kinase for GAG chain extension	Immature, nonfunctional proteoglycans	Lethal neonatal short limb dysplasia,   Skeletal phenotypes with reduced cartilage proteoglycans
B4GALT7	Initiates GAG chain on the protein core	Proteoglycans lacking GAG chains, disorganized matrix	Progeroid Ehlers-Danlos syndrome
B3GAT3	Adds glucuronic acid to the linker region	Reduced GAG chains, disorganized collagen	Larsen-like syndrome,  Severe skeletal and cardiovascular anomalies
CHSY1	Chondroitin sulfate synthase 1	Disrupted bone morphogenesis, joint formation	Temtamy preaxial brachydactyly syndrome,  Limb patterning defects
EXTL3	N-acetylglucosaminyltransferase for  GAG binding	Heparan sulfate synthesis	Skeletal dysplasia,   Hereditary multiple exostosis
EXT1 EXT2	Heparan sulfate polymerization in Golgi	Loss of function, skeletal develpment	Hereditary multiple exostosis
SLC35D1	Golgi transporter for UDP sugars	Reduced GAG chain synthesis	Schneckenbecken dysplasia
SLC26A2	Sulfate transporter	Reduced cartilage proteoglycan sulfation	Chondrodysplasias,  Achondrogenesis,  Diastrophic dysplasia
BGN	Biglycan, essential for bone and muscle	Severe osteopenia, collagen fibril abnormalities	Early onset of osteoporosis, disc degeneration
DCN	Decorin, regulates collagen fibril formation	Fragile skin and tendons	Irregular collagen fibrils, skin fragility
GORAB	Golgi protein recycling	Disrupted proteoglycan synthesis, collagen organization	Gerodermia osteodysplastica,  Extreme bone fragility

Conclusions 

Current therapeutic interventions for skeletal and connective tissue disorders largely focus on symptomatic management, such as orthopedic surgery and physiotherapy (Paganini et al., 2019). Understanding the genetic basis of these disorders enhances our insight into critical design features of biomaterials for skeletal repair. One promising avenue is the use of carbohydrate-based scaffolds for tissue engineering. These materials structurally mimic GAGs and have shown potential for bone regeneration. For example, a novel hyper-crosslinked carbohydrate polymer demonstrated efficacy in repairing bone defects in vivo. To learn more, refer to “Hyper-Crosslinked Carbohydrate Polymer for Repair of Critical-Sized Bone Defects” (Koleva et al., 2019). 

By elucidating the intricate connections between genetic mutations and skeletal disorders, this post highlights the critical role of proteoglycans in bone development. With this knowledge, potential avenues for advanced therapeutic interventions offer hope for more effective bone and cartilage repair and treatment in the future. 

References: 

Chan, W. L., Steiner, M., Witkos, T., Egerer, J., Busse, B., Mizumoto, S., Pestka, J. M., Zhang, H., Hausser, I., Khayal, L. A., Ott, C.-E., Kolanczyk, M., Willie, B., Schinke, T., Paganini, C., Rossi, A., Sugahara, K., Amling, M., Knaus, P., … Kornak, U. (2018). Impaired proteoglycan glycosylation, elevated TGF-β signaling, and abnormal osteoblast differentiation as the basis for bone fragility in a mouse model for gerodermia osteodysplastica. PLOS Genetics, 14(3), e1007242. https://doi.org/10.1371/journal.pgen.1007242 

Koleva, P. M., Keefer, J. H., Ayala, A. M., Lorenzo, I., Han, C. E., Pham, K., Ralston, S. E., Kim, K. D., & Lee, C. C. (2019). Hyper-Crosslinked Carbohydrate Polymer for Repair of Critical-Sized Bone Defects. BioResearch Open Access, 8(1), 111–120. https://doi.org/10.1089/biores.2019.0021 

Paganini, C., Costantini, R., Superti‐Furga, A., & Rossi, A. (2019). Bone and connective tissue disorders caused by defects in glycosaminoglycan biosynthesis: A panoramic view. The FEBS Journal, 286(15), 3008–3032. https://doi.org/10.1111/febs.14984 

Sao, K., & Risbud, M. V. (2024). Proteoglycan Dysfunction: A Common Link Between Intervertebral Disc Degeneration and Skeletal Dysplasia. Neurospine, 21(1), 162–178. https://doi.org/10.14245/ns.2347342.671 

Taylan, F., & Mäkitie, O. (2016). Abnormal Proteoglycan Synthesis Due to Gene Defects Causes Skeletal Diseases with Overlapping Phenotypes. Hormone and Metabolic Research, 48(11), 745–754. https://doi.org/10.1055/s-0042-118706 

Wen, J., Xiao, J., Rahdar, M., Choudhury, B. P., Cui, J., Taylor, G. S., Esko, J. D., & Dixon, J. E. (2014). Xylose phosphorylation functions as a molecular switch to regulate proteoglycan biosynthesis. Proceedings of the National Academy of Sciences, 111(44), 15723–15728. https://doi.org/10.1073/pnas.1417993111