Mechanisms underlying hypoxic ischemic injury to the developing brain: The significance of matrix metalloproteinase 2 and 9

Abstract

Perinatal hypoxic ischemic (HI) injury is a leading cause of long-term neurological complications in newborn babies. Matrix metalloproteinases (MMPs) are a family of endopeptidases that are capable of degrading the extracellular matrix (ECM) components. They are considered to be integral in many physiological processes. However, recently it has been demonstrated that the inappropriate activity of these proteases, particularly MMP-2 and 9, contribute to the pathogenesis of cerebral ischemia in the adult brain. Given that ECM disruption is frequently observed following injury to the developing brain, it is possible that MMPs play an important role in HI injury processes in the developing brain. Therefore, this thesis evaluated the hypothesis that MMP-2 and 9 participate in the pathophysiology of HI injury to the developing brain. Since ECM remodelling is a fundamental process during brain development it was important to first characterise the MMP-2 and 9 profiles in the normal developing forebrain. We demonstrated that MMP-2, which mainly was observed in cortical plate neurons, declined with age, thus indicating a potential role in the development and differentiation of the cortical plate. Conversely, MMP-9 was increased with age, particularly during active myelination, indicating that it may contribute in myelination. Secondly, we showed an upregulation of MMP-9 within the ischemic core during the early hours following HI injury, suggesting that MMP-9 may be involved in the development of delayed injury processes following hypoxic ischemia. On the contrary, MMP-2 was strongly upregulated during a later stage following injury surrounding the ischemic core possibly suggesting that it plays a role in wound repair processes. Thirdly, the profiles of tissue (tPA) and urokinase (uPA) plasminogen activators were characterised following HI injury since they are known to be major upstream activators of MMPs. uPA upregulation paralleled that of MMP-2 suggesting a function for uPA in wound repair processes following HI injury to the developing brain through activation of MMP-2. In contrast with uPA, tPA activity remained unaffected following injury at both ages. Finally, MMP-9 activity was inhibited using a very specific MMP-2/9 inhibitor, SB-3CT, to determine if the MMP-9 deficiency protects the developing brain from HI injury. The elevated MMP-9 activity following HI injury was attenuated by the SB-3CT treatment. Although SB-3CT failed to confer any significant neuroprotection, we recommend that further investigations are needed before discounting the role of MMP-9 during HI injury to the developing brain. In conclusion, we suggest that MMP-9 is induced following an insult to the developing brain potentially contributing to the delayed neuronal death whilst MMP-2 is involved in essential developmental, differentiation and wound repair processes.