TSG-6 mediated extracellular matrix modifications regulate hypoxic-ischemic brain injury

Proteoglycans containing link-domains modify the extracellular matrix (ECM) to regulate cellular homeostasis and can also sensitize tissues/organs to injury and stress. Hypoxic-Ischemic (H-I) injury disrupts cellular homeostasis by activating inflammation and attenuating regeneration and repair pathways. In the brain, the main component of the ECM is the glycosaminoglycan (GAG), Hyaluronic Acid (HA), but whether HA modifications of the ECM regulate cellular homeostasis and response to H-I injury is not known. In this report, employing both male and female mice, we demonstrate that link-domain containing proteoglycan, TNF-stimulated gene-6 (TSG-6), is active in the brain from birth onwards and differentially modifies ECM HA during discrete neurodevelopmental windows. ECM HA modification by TSG-6 enables it to serve as a developmental switch to regulate activity of the Hippo pathway effector protein, Yes Associated Protein 1 (YAP1) in the maturing brain and in response to H-I injury. Mice that lack TSG-6 expression display dysregulated expression of YAP1 targets, excitatory amino acid transporter 1 (EAAT1; GLAST) and 2 (EAAT2; GLT-1). Dysregulation of YAP1 activation in TSG-6-/- mice coincides with age- and sex-dependent sensitization of the brain to H-I injury such that 1-week-old neonates display an anti-inflammatory response in contrast to an enhanced pro-inflammatory injury reaction in 3-month-old adult males but not females. Our findings thus support that a key regulator of age- and sex-dependent H-I injury response in the mouse brain is modulation of the Hippo-YAP1 pathway by TSG-6 dependent ECM modifications.