Hypoxia‐Ischemia in Perinatal Rat Brain Induces the Formation of a Low Molecular Weight Isoform of Striatal Enriched Tyrosine Phosphatase (STEP)

Abstract

Abstract:Protein tyrosine phosphatases play a critical role in controlling tyrosine phosphorylation levels of proteins. Ischemia induces changes in tyrosine phosphorylation. As part of our investigations of the mechanisms responsible for these changes, we studied the effects of cerebral hypoxia-ischemia in 7-day-old (P7) and P21 rat brains on expression of the STEP (striatal enriched phosphatase) family of protein tyrosine phosphatases. P7 and P21 rats were subjected to unilateral hypoxia-ischemia, and brains were analyzed at various intervals of recovery for the presence of STEP. Hypoxia-ischemia induced the formation of a low M r isoform of STEP, STEP 33 , in the ipsilateral (damaged) hemisphere but not in the contralateral (undamaged) side. STEP 33 produced as a result of ischemia was located exclusively in the cell soluble fraction. In P21 rats, the ischemia-induced elevation in STEP 33 was delayed relative to P7 rats. STEP 33 was produced by digestion of postsynaptic densities with calpain I and by exposure of NT2/D1 cells expressing STEP to the calcium ionophore A23187. The results suggest that ischemia-induced calcium influx results in the calcium-dependent proteolysis of membrane-associated STEP 61 and the concomitant release of STEP 33 into the cytoplasm. Key Words: Perinatal hypoxia-ischemia-Tyrosine phosphorylation-Protein tyrosine phosphataseStriatal enriched phosphatase. J. Neurochem. 73, 1990Neurochem. 73, -1994Neurochem. 73, (1999.Perinatal hypoxic-ischemic brain damage is a major cause of cerebral palsy, mental retardation, and epilepsy. The design of effective therapeutic interventions requires an understanding of the specific molecular events responsible for ischemia-induced cell damage. One of the early changes induced by cerebral ischemia in mature brain is a marked increase in the tyrosine phosphorylation of proteins (Hu and Weiloch, 1994;Takagi et al., 1997). This change may reflect modulation of the activities of protein tyrosine kinases (Ohtsuki et al., 1996;Braunton et al., 1998), protein tyrosine phosphatases (Takano et al., 1995), or both.The STEP (striatal enriched phosphatase) family of protein tyrosine phosphatases was originally identified as a family of intracellular protein tyrosine phosphatases enriched within neurons of the CNS (Lombroso et al., 1991(Lombroso et al., , 1993. Subsequent studies have identified two subfamilies of STEP proteins: a group of higher molecular weight (M r ) isoforms that are enriched in membrane compartments, and a group of lower M r isoforms, some of which are membrane-associated and others of which are cytosolic proteins (Boulanger et al., 1995;Sharma et al., 1995;Bult et al., 1996Bult et al., , 1997.The exact relationship between the higher and lower M r isoforms remains to be determined. However, one hypothesis that has been suggested is that the larger, membrane-associated proteins are proteolytically cleaved to release some of the cytosolic variants. The prototypical membrane-associated isoform, STEP 61 , has two PEST sequences within its am...