Developmental regulation of matrix metalloproteinases in response to multi-factorial, severe TBI injuries during immaturity.

Introduction: A striking pattern in young children after severe TBI is when the entire cortical ribbon displays tissue damage: hemispheric hypodensity (HH). HH is often a result of abusive head trauma (AHT). We previously reported a model of HH in a gyrencephalic species where a combination of injuries consisting of 1) cortical impact, 2) midline shift, 3) subdural hematoma/subarachnoid hemorrhage, 4) traumatic seizures, and 5) brief apnea and hypoventilation, resulted in extensive, hypoxic-ischemic type injury. Importantly, this mechanism closely resembles that seen in children, with relative sparing of the contralateral cortex, thus, ruling out a pure asphyxia mechanism. In this model, piglets of similar developmental stage to human toddlers (postnatal day 30, PND30) have extensive hypoxic-ischemic damage to the cortical ribbon with sparing of the contralateral hemisphere and deep gray matter areas. However, piglets of similar developmental stage to human infants (postnatal day 7, PND7) have less hypoxic-ischemic damage that is notably bilateral and patchy. We therefore sought to discover whether the extensive tissue damage observed in PND30 was due to a greater upregulation of matrix metalloproteinases (MMPs).

Materials and methods: In PND 7 or PND 30 piglets receiving AHT injuries (cortical impact, midline shift, subdural hematoma/subarachnoid hemorrhage, traumatic seizures, and brief apnea and hypoventilation) or a sham injury, the pattern of albumin extravasation and MMP-9 upregulation throughout the brain was determined via immunohistochemistry, brain tissue adjacent to the cortical impact where the tissue damage spreads was collected for Western blots, and the gelatinase activity was determined over time in peripheral plasma. EEG was recorded and piglets survived up to 24 hours after injury administration.

Results: The pattern of albumin extravasation, indicating vasogenic edema, as well as increase in MMP-9, were both present at the same areas of hypoxic-ischemic tissue damage. Evidence from immunohistochemistry, western blot, and zymogens demonstrate that MMP- 2,- 3 or -9 are constitutively expressed during immaturity and are not different between developmental stages; however, active forms are upregulated in PND30 but not PND7 after in response to AHT model injuries. Furthermore, peripheral active MMP-9 was downregulated after model injuries in PND7.

Conclusions: This differential response to AHT model injuries might confer protection to the PND7 brain. Additionally, we find that immature gyrencephalic species have a greater baseline and array of MMP's than previously demonstrated in rodent species. Treatment with an oral or intravenous broad-spectrum matrix metalloproteinase inhibitor might reduce the extensive spread of injury in PND30, but the exposure to metalloproteinase inhibitors must be acute as to not interfere with the homeostatic role of matrix metalloproteinases in normal postnatal brain development and plasticity as well as post-injury synaptogenesis and tissue repair.