Mitochondria are central in the regulation of cell death. Apart from providing the cell with ATP, mitochondria also harbor several death factors that are released upon apoptotic stimuli. Alterations in mitochondrial functions, increased oxidative stress, and neurons dying by apoptosis have been detected in Alzheimer's disease patients. These findings suggest that mitochondria may trigger the abnormal onset of neuronal cell death in Alzheimer's disease. We previously reported that presenilin 1 (PS1), which is often mutated in familial forms of Alzheimer's disease, is located in mitochondria and hypothesized that presenilin mutations may sensitize cells to apoptotic stimuli at the mitochondrial level. Presenilin forms an active ␥-secretase complex together with Nicastrin (NCT), APH-1, and PEN-2, which among other substrates cleaves the ␤-amyloid precursor protein (␤-APP) generating the amyloid ␤-peptide and the ␤-APP intracellular domain. Here we have identified dual targeting sequences (for endoplasmic reticulum and mitochondria) in NCT and showed expression of NCT in mitochondria by immunoelectron microscopy. We also showed that NCT together with APH-1, PEN-2, and PS1 form a high molecular weight complex located in mitochondria. ␥-Secretase activity in isolated mitochondria was demonstrated using C83 (␣-secretasecleaved C-terminal 83-residue ␤-APP fragment from BD8 cells lacking presenilin and thus ␥-secretase activity) or recombinant C100-Flag (C-terminal 100-residue ␤-APP fragment) as substrates. Both systems generated an APP intracellular domain, and the activity was inhibited by the ␥-secretase inhibitors L-685,458 or Compound E. This novel localization of NCT, PS1, APH-1, and PEN-2 expands the role and importance of ␥-secretase activity to mitochondria.

c-Secretase is a key enzyme involved in the processing of the b-amyloid precursor protein into amyloid b-peptides (Ab). Ab accumulates and forms plaques in Alzheimer's disease (AD) brains. A progressive neurodegeneration and cognitive decline occurs during the course of the disease, and Ab is believed to be central for the molecular pathogenesis of AD. Apoptosis has been implicated as one of the mechanisms behind the neuronal cell loss seen in AD. We have studied preservation and activity of the c-secretase complex during apoptosis in neuroblastoma cells (SH-SY5Y) exposed to staurosporine (STS). We report that the known components (presenilin, Nicastrin, Aph-1 and Pen-2) interact and form active c-secretase complexes in apoptotic cells. In addition, the fragments corresponding to the PS1 N-terminal fragment and the caspase-cleaved PS1 C-terminal fragment (PS1-caspCTF) were found to form active c-secretase complexes when co-expressed in presenilin (PS) knockout cells. Interestingly, PS1-caspCTF replaced the normal PS1 C-terminal fragment and was co-immunoprecipitated with the c-secretase complex in SH-SY5Y cells exposed to STS. In addition, Ab was detected in medium from apoptotic HEK APP swe cells. Together, the data show that c-secretase complexes containing PS1-caspCTF are active, and suggest that this proteolytic activity is also important in dying cells and may affect the progression of AD.

Our findings suggest how recurrent back and neck pain can develop, interact with medical, individual, work-related and structural factors and result in incapacity to work. To avoid this incapacity pain has to be followed and the other factors considered in the design and timing of rehabilitation.

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