Diverse effects of Perlecan-HS on cell and matrix regulation: Tendon destabilization in Hspg2 Exon 3 null HS deficient mice reveals essential homeostatic roles for HS in cellular regulation and tendon functionality

The aim of this study was to determine the regulatory role of the perlecan HS side chains on cell and matrix homeostasis in tail and Achilles tendons in 3 and 12 week old Hspg2 exon 3 null HS deficient ( Hspg2 ∆3-/∆3- ) and C57 BL/6 Wild Type (WT) mice. Tendons were biomechanically tested (ultimate tensile stress [UTS], tensile modulus [TM]) and glycosaminoglycan (GAG) and collagen (hydroxyproline) compositional analyses undertaken. Monolayer cultures of Hspg2 ∆3-/∆3- tenocytes stimulated with FGF-2 showed elevated Adamts4 , Mmp2, 3, 13 gene expression compared to WT mice . Col1A1, Vcan, Bgn, Dcn, Lum, Hspg2, Ltbp1, Ltbp2, Eln and Fbn1 showed no major differences between genotypes. Type VI collagen and perlecan were immunolocalised in tail tendon and collagen fibrils imaged using transmission electron microscopy (TEM). The amplified catabolic phenotype of Hspg2 ∆3-/∆3- mice may account for the age-dependent decline in GAG observed in tail tendon and changes in UTS/TM biomechanics. Collagen fibril diameter increased in WT but decreased in Hspg2 ∆3-/∆3- tail tendons over 3 to 12 weeks. Achilles tenotomy resulted in changes in tendon material properties in both genotypes but, Hspg2 ∆3-/∆3- mice had a slower recovery of UTS after tenotomy. HS deficiency in Hspg2 ∆3-/∆3- tendon impaired tenocyte repair responses to FGF-2 and led to deleterious changes in tendon organization which was reflected in changes in their material properties. .