A caspase-independent proapoptotic role of OMI/HTRA2 has been described in B-lymphoid cells, following interleukin-3 withdrawal (18). frontal cortex and hippocampus, two brain areas particularly affected by AD, indicated similar levels KU 59403 in patients with AD (n=10) and matched control subjects (n=10). In addition, we analyzed the occurrence of theOMI/HTRA2variants A141S and G399S in Swedish case-control materials for AD and PD and found a poor association of A141S with AD, but not with PD. In conclusion, our genetic, histological, and biochemical findings give further support to an involvement of OMI/HTRA2 in the pathology of AD; however, further studies KU 59403 are needed to clarify the role of this gene in neurodegeneration.Westerlund, M., Behbahani, H., Gellhaar, S.,Forsell, C., Carmine Belin, A., Anvret, A., Zettergren, A., Nissbrandt, H., Lind, C., Sydow, O., Graff, C., Olson, L., Ankarcrona, M., Galter, D. Altered enzymatic activity and allele frequency of OMI/HTRA2 in Alzheimer’s disease. Keywords:postmortem tissue, PRSS25, Western blot,in situhybridization, protease assay The serine-protease OMI, originally identified as a homologue of the bacterial chaperone high-temperature requirement A (HTRA), has been implicated in the pathogenesis of neurodegenerative disorders, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Involvement of OMI/HTRA2 in degenerative processes typical of AD pathology were first suggested when the enzyme was identified as presenilin-1 (PS1) interacting factor in a 2-hybrid system (1). Binding of PS1 increased the protease activity of OMI/HTRA2 (2), whereas the conversation between PS1 as part of the active -secretase complex and mouse Omi/HtrA2 in mitochondria resulted in reduction of the -secretase activity (3). The C-terminal region of OMI/HTRA2 was also identified as a binding site for amyloid -peptide (A) (4), and the protease has been implicated in the intracellular amyloid-precursor protein (APP) metabolism. In a cell-freein vitrosystem, OMI/HTRA2 delayed aggregation of A, indicating a chaperone function of the enzyme (5). These findings are in line with an earlier study suggesting that a portion of OMI/HTRA2 localizes to the endoplasmic reticulum, where it binds to immature APP and regulates APP degradation (6). A chaperone activity of OMI/HTRA2 KU 59403 has also been highlighted recently through the finding that its PDZ domain name selectively binds and detoxifies neurotoxic oligomeric forms of A (7). Other lines of evidence linked OMI/HTRA2 to PD. The mouse strain mnd2 (motor neuron degeneration-2) displays striatal degeneration, microglia activation and Parkinson-like features, which are caused by a spontaneously occurring point mutation in Omi/HtrA2 (8,9). More recently, OMI/HTRA2 was implicated in PD through a possible link to the PTEN-induced putative kinase-1 (PINK1) at thePARK6locus (10). Phosphorylation of OMI/HTRA2 by PINK1 has been suggested to alter the protease activity, thereby increasing the resistance to mitochondria-induced stress. In agreement with these findings, decreased OMI/HTRA2 phosphorylation has been observed in brains of patients with PD transporting mutations in PINK1 (11). Finally, genetic studies have also implicatedOMI/HTRA2in PD (11,12). Using a candidate gene approach, Strausset al.(12)discovered twoOMI/HTRA2genetic variants in German PD patients. The two variants A141S (Ala141Ser; c.421G>T, rs72470544) and G399S (Gly399Ser; c.1195G>A, rs72470545), are located in exon 1 and 7, respectively, and KU 59403 result in defective OMI/HTRA2 protease activity, similar to the mouse mutation in the mnd-2 strain. On the basis of these data, it has been suggested that mutations abolishing OMI/HTRA2 protease activity may cause increased susceptibility to mitochondrial stress and neuronal death, thereby increasing the risk of developing PD. TheOMI/HTRA2gene, at chromosome 2p1213, codes for an inactive precursor protein (50 kDa), as is the case for most trypsin-like serine proteases. The precursor protein is mostly targeted to the mitochondrial intermembrane space due to an N-terminal signal and is processed to the proteolytically active form (35 kDa), most likely through autocleavage at amino acid residue 133, forming active noncovalent homotrimers (13). The first function explained for OMI/HTRA2 was its role during apoptosis, when the active enzyme is usually released into the cytosol (14), binds to inhibitor of apoptosis proteins (IAPs), and relieves the inhibition of caspases (1517). A caspase-independent proapoptotic role of OMI/HTRA2 has been explained in B-lymphoid cells, following interleukin-3 withdrawal (18). Additional functions of OMI/HTRA2 are emerging, implicating the enzyme in modulation of mitochondrial activity and morphology through conversation with the fusion factor OPA1 (19), as well as a role in mitochondrial homeostasis and quality control of the mitochondrial network (20). A recent study demonstrated a further neuroprotective mechanism of OMI/HTRA2 by showing that this proteolytic activity of OMI/HTRA2 enhances autophagy in cell lines and increases degradation of mutant proteins involved in neurodegeneration such as -synuclein and huntingtin (21). Despite the biochemical studies implicating OMI/HTRA2 in AD pathology, few data are available on possible changes in expression levels or enzyme activity in AD brain tissue. A recent statement showed that in AD brain tissue, OMI/HTRA2 immunoreactivity localizes to senile plaques and neurofibrillary tangles IgG2b/IgG2a Isotype control antibody (FITC/PE) (22). We investigated proteolytic activity and expression of OMI/HTRA2 in human postmortem brain.