Astrocytic demise in the developing rat and human brain after hypoxic-ischemic damage.

In order to approach the physiopathological mechanism underlying the selective susceptibility of the immature brain to hypoxia-ischemia (HI), we have compared the lesions experimentally induced in postnatal day 7 rats using a model of neonatal stroke with those occurring in human fetal and neonatal brains. We first observed that gray and white matter lesions demonstrated a similar organization (core with cell loss and/or cavity and penumbra) and evolutionary pattern between experimental and human HI lesions. We then observed that, in the intermediate white matter, GFAP- and vimentin-positive astrocytes exhibited clasmatodendrosis and represent a major cell population involved in cell death in human brains (56.3 and 67.9%, respectively). In rat brains, GFAP- and TUNEL-positive astrocytes were also highly vulnerable, increasing between 6 (31%) and 72 (58%) hours after ischemia. Together, these results indicate that astroglial dysfunction may play a critical role in determining the progress and outcome of acute hypoxic-ischemic injury particularly in the developing brain.

Survivin and heat shock protein 25/27 colocalize with cleaved caspase-3 in surviving reactive astrocytes following excitotoxicity to the immature brain.

Following immature excitotoxic brain damage, distinct patterns of caspase activation have been described in neurons and glial cells. Neuronal cells show activation of the mitochondrial apoptosis pathway, caspase-3 cleavage and apoptotic cell death, while reactive astrocytes show caspase-3 cleavage that is not always correlated with enzymatic protease activity and does not generally terminate in cell death. Accordingly, the aim of the present study was to evaluate the astrocytic colocalization of cleaved caspase-3 and several anti-apoptotic proteins of the inhibitor of apoptosis proteins family (IAPs), such as survivin and cellular inhibitor of apoptosis-2 (cIAP-2), and the heat shock proteins (HSPs) family, Hsp25/27 and Hsc70/Hsp70, which can all prevent caspases from cleaving their substrates. At several survival times ranging from 4 h to 14 days after cortical excitotoxic damage induced by N-methyl-d-aspartate (NMDA) injection at postnatal day 9 in rat pups, single and double immunohistochemical techniques were performed in free floating cryostat sections and sections were analyzed by confocal microscopy. Our results show that survivin and Hsp25/27 are primarily expressed in reactive astrocytes of the damaged cortex and the adjacent white matter. In addition, both molecules strongly colocalize with cleaved caspase-3. Survivin is primarily located in the nucleus, like cleaved caspase-3; while Hsp25/27 is cytoplasmic but very frequently found in cells showing nuclear caspase-3. cIAP-2 was mostly found in damaged neurons but also in some glial scar reactive astrocytes and showed fewer correlation with caspase-3. Hsc70/Hsp70 was only expressed in injured neurons and did not correlate with caspase-3. Thus, we conclude that primarily survivin and Hsp25/27 may participate in the inhibition of cleaved caspase-3 in reactive astrocytes and may be involved in protecting astrocytes after injury.