A Novel Role of Hyaluronic Acid and Proteoglycan Link Protein 1 (HAPLN1) in Delaying Vascular Endothelial Cell Senescence

The effects of hyaluronan and proteoglycan link protein 1 (HAPLN1) in ameliorating spinal cord injury mediated by Nrf2

Excessive inflammatory response and oxidative stress (OS) play an important role in the pathogenesis of spinal cord injury (SCI). Balance of inflammation and prevention of OS have been considered an effective strategy for the treatment of SCI. Hyaluronan and proteoglycan link protein 1 (HAPLN1), also known as cartilage link protein, has displayed a wide range of biological and physiological functions in different types of tissues and cells. However, whether HAPLN1 regulates inflammation and OS during SCI is unknown. Therefore, we aimed to examine whether HAPLN1 can have a protective effect on SCI. In this study, both in vitro and in vivo SCI models were established. Nissl staining and terminal deoxynucleotidyl transferase dUTP nick end labeling staining assays were used. Western blotting and enzyme-linked immunosorbent assay were employed to assess the expression of proteins. Our results demonstrate that the administration of HAPLN1 promoted the recovery of motor neurons after SCI by increasing the Basso mouse scale score, increasing the numbers of motor neurons, and preventing apoptosis of spinal cord cells. Additionally, HAPLN1 mitigated OS in spinal cord tissue after SCI by increasing the content of superoxide dismutase SOD and the activity of glutathione peroxidase but reducing the levels of malondialdehyde. Importantly, we found that HAPLN1 stimulated the activation of the nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) pathway and stimulated the expression of heme oxygenase-1 and nicotinamide adenine dinucleotide phosphate quinone oxidoreductase-1, which mediated the attenuation of HAPLN1 in activation of the NOD-like receptor protein 3 (NLRP3) inflammasome by reducing the levels of NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), caspase-1, and interleukin-1β. Correspondingly, in vitro experiments show that the presence of HAPLN1 suppressed the NLRP3 inflammasome and prevented cell injury against H2O2 in PC12 cells. These effects were mediated by the Nrf2/ARE pathway, and inhibition of Nrf2 with ML385 abolished the beneficial effects of HAPLN1. Based on these findings, we conclude that HAPLN1 inhibits the NLRP3 inflammasome through the stimulation of the Nrf2/ARE pathway, thereby suppressing neuroinflammation, enhancing motor neuronal survival, and improving the recovery of nerve function after SCI.

Comparison of sialylated and fucosylated N-glycans attached to Asn 6 and Asn 41 with different roles in hyaluronan and proteoglycan link protein 1 (HAPLN1)

Hyaluronan and proteoglycan link protein 1 (HAPLN1) is an extracellular matrix protein stabilizing interactions between hyaluronan and proteoglycan. Although HAPLN1 is being investigated for various biological roles, its N-glycosylation is poorly understood. In this study, the structure of N-glycopeptides of trypsin-treated recombinant human HAPLN1 (rhHAPLN1) expressed from CHO cells were identified by nano-liquid chromatography-tandem mass spectrometry. A total of 66 N-glycopeptides were obtained, including 16 and 12 N-glycans at sites Asn 6 (located in the N-terminal region) and Asn 41 (located in the Ig-like domain, which interacts with proteoglycan), respectively. The quantities (%) of each N-glycan relative to the totals (100 %) at each site were calculated. Tri- and tetra-sialylation (to resist proteolysis and extend half-life) were more abundant at Asn 6, and di- (core- and terminal-) fucosylation (to increase binding affinity and stability) and sialyl-Lewis X/a epitope (a major ligand for E-selectin) were more abundant at Asn 41. These results indicate that N-glycans attached to Asn 6 (protecting HAPLN1) and Asn 41 (supporting molecular interactions) play different roles in HAPLN1. This is the first study of site-specific N-glycosylation in rhHAPLN1, which will be useful for understanding its molecular interactions in the extracellular matrix.