TSP1 and TSP2 deficiencies affect LOX protein distribution in the femoral diaphysis and pro-peptide removal in marrow-derived mesenchymal stem cells in vitro

Thrombospondins modulate cell function and tissue structure in the skeleton

Thrombospondins (TSPs) belong to a functional class of ECM proteins called matricellular proteins that are not primarily structural, but instead influence cellular interactions within the local extracellular environment. The 3D arrangement of TSPs allow interactions with other ECM proteins, sequestered growth factors, and cell surface receptors. They are expressed in mesenchymal condensations and limb buds during skeletal development, but they are not required for patterning. Instead, when absent, there are alterations in musculoskeletal connective tissue ECM structure, organization, and function, as well as altered skeletal cell phenotypes. Both functional redundancies and unique contributions to musculoskeletal tissue structure and physiology are revealed in mouse models with compound TSP deletions. Crucial roles of individual TSPs are revealed during musculoskeletal injury and regeneration. The interaction of TSPs with mesenchymal stem cells (MSC), and their influence on cell fate, function, and ultimately, musculoskeletal phenotype, suggest that TSPs play integral, but as yet poorly understood roles in musculoskeletal health. Here, unique and overlapping contributions of trimeric TSP1/2 and pentameric TSP3/4/5 to musculoskeletal cell and matrix physiology are reviewed. Opportunities for new research are also noted.

TSP-1 increases autophagy level in cartilage by upregulating HSP27 which delays progression of osteoarthritis

This study aimed to determine whether Thrombospondin-1 (TSP-1) can be used as a biomarker to diagnose early osteoarthritis (OA) and whether it has a chondroprotective effect against OA. We examined TSP-1 expression in cartilage, synovial fluid, and serum at different time points after anterior cruciate ligament transection (ACLT) surgery in rats. Subsequently, TSP-1 was overexpressed or silenced to detect its effects on extracellular matrix (ECM) homeostasis, autophagy level, proliferation and apoptosis in chondrocytes. Adenovirus encoding TSP-1 was injected into the knee joints of ACLT rats to test its effect against OA. Combined with proteomic analysis, the molecular mechanism of TSP-1 in cartilage degeneration was explored. Intra-articular injection of an adenovirus carrying the TSP-1 sequence showed chondroprotective effects against OA. Moreover, TSP-1 expression decreases with OA progression and can effectively promote cartilage proliferation, inhibit apoptosis, and helps to sustain the balance between ECM anabolism and catabolism. Overexpression of TSP-1 also can increase autophagy by upregulating Heat Shock Protein 27 (HSP27, hspb1), thereby enhancing its effect as a stimulator of autophagy. TSP-1 is a hopeful strategy for OA treatment.

EMILIN1 deficiency causes arterial tortuosity with osteopenia and connects impaired elastogenesis with defective collagen fibrillogenesis

EMILIN1 (elastin-microfibril-interface-located-protein-1) is a structural component of the elastic fiber network and localizes to the interface between the fibrillin microfibril scaffold and the elastin core. How EMILIN1 contributes to connective tissue integrity is not fully understood. Here, we report bi-allelic EMILIN1 loss-of-function variants causative for an entity combining cutis laxa, arterial tortuosity, aneurysm formation, and bone fragility, resembling autosomal-recessive cutis laxa type 1B, due to EFEMP2 (FBLN4) deficiency. In both humans and mice, absence of EMILIN1 impairs EFEMP2 extracellular matrix deposition and LOX activity resulting in impaired elastogenesis, reduced collagen crosslinking, and aberrant growth factor signaling. Collagen fiber ultrastructure and histopathology in EMILIN1- or EFEMP2-deficient skin and aorta corroborate these findings and murine Emilin1^-/- femora show abnormal trabecular bone formation and strength. Altogether, EMILIN1 connects elastic fiber network with collagen fibril formation, relevant for both bone and vascular tissue homeostasis.

Compound deletion of thrombospondin-1 and -2 results in a skeletal phenotype not predicted by the single gene knockouts

The trimeric thrombospondin homologs, TSP1 and TSP2, are both components of bone tissue and contribute in redundant and distinct ways to skeletal physiology. TSP1-null mice display increased femoral cross-sectional area and thickness due to periosteal expansion, as well as diminished matrix quality and impaired osteoclast function. TSP2-null mice display increased femoral cross-sectional thickness and reduced marrow area due to increased endosteal osteoblast activity, with very little periosteal expansion. Osteoblast lineage cells are reduced in TSP2-null mice, but not in TSP1-null. The functional effects of combined TSP1 and TSP2 deficiency remain to be elucidated. Here, we examined the spectrum of detergent soluble proteins in diaphyseal cortical bone of growing (6-week old) male and female mice deficient in both thrombospondins (double knockout (DKO)). Of 3429 detected proteins, 195 were differentially abundant in both male and female DKO bones. Physiologically relevant annotation terms identified by Ingenuity Pathway Analysis included "ECM degradation" and "Quantity of Monocytes." Manual inspection revealed that a number of proteins with shared expression among osteoclasts and osteocytes were reduced in DKO bones. To associate changes in protein content with phenotype, we examined 12-week old male and female DKO and WT mice. DKO mice were smaller than WT and in male DKO, femoral cross section area was reduced. Some of the male DKO femora also had a flattened, less circular cross-section. Male DKO bones were less stiff in bending and they displayed reduced ultimate load. Displacements at yield load and at max load were both elevated in male DKO. However, the ratios of post-yield to pre-yield displacements significantly diminished in DKO suggesting proportionally reduced post-yield behavior. Male DKO mice also exhibited reductions in trabecular bone mass, which were surprisingly associated with equivalent osteoblast numbers and accordingly increased osteoblast surface. Marrow-derived colony forming unit-fibroblastic was reduced in male and female DKO mice. Together our data suggest that when both TSP1 and TSP2 are absent, a unique, sex-specific bone phenotype not predicted by the single knockouts, is manifested.

Thrombospondins in bone remodeling and metastatic bone disease

Thrombospondins (TSPs) are a family of five multimeric matricellular proteins. Through a wide range of interactions, TSPs play pleiotropic roles in embryogenesis and in tissue remodeling in adult physiology as well as in pathological conditions, including cancer development and metastasis. TSPs are active in bone remodeling, the process of bone resorption (osteolysis) and deposition (osteogenesis) that maintains bone homeostasis. TSPs are particularly involved in aberrant bone remodeling, including osteolytic and osteoblastic skeletal cancer metastasis, frequent in advanced cancers such as breast and prostate carcinoma. TSPs are major players in the bone metastasis microenvironment, where they finely tune the cross talk between tumor cells and bone resident cells in the metastatic niche. Each TSP family member has different effects on the differentiation and activity of bone cells-including the bone-degrading osteoclasts and the bone-forming osteoblasts-with different outcomes on the development and growth of osteolytic and osteoblastic metastases. Here, we overview the involvement of TSP family members in the bone tissue microenvironment, focusing on their activity on osteoclasts and osteoblasts in bone remodeling, and present the evidence to date of their roles in bone metastasis establishment and growth.

Thrombospondin 1 and Its Diverse Roles as a Regulator of Extracellular Matrix in Fibrotic Disease

Thrombospondin 1 (TSP1) is a matricellular extracellular matrix protein that has diverse roles in regulating cellular processes important for the pathogenesis of fibrotic diseases. We will present evidence for the importance of TSP1 control of latent transforming growth factor beta activation in renal fibrosis with an emphasis on diabetic nephropathy. Other functions of TSP1 that affect renal fibrosis, including regulation of inflammation and capillary density, will be addressed. Emerging roles for TSP1 N-terminal domain regulation of collagen matrix assembly, direct effects of TSP1-collagen binding, and intracellular functions of TSP1 in mediating endoplasmic reticulum stress responses in extracellular matrix remodeling and fibrosis, which could potentially affect renal fibrogenesis, will also be discussed. Finally, we will address possible strategies for targeting TSP1 functions to treat fibrotic renal disease.