Alzheimer disease (AD) associated (gamma)-secretase components presenilin-1 and -2 accumulate in MAM, an LR-like ER subcompartment connected to mitochondria. MAM function increases in patients with familial or sporadic AD and may be linked to AD pathogenesis.

Presenilin-1 (PS1) and -2 (PS2), which when mutated cause familial Alzheimer disease, have been localized to numerous compartments of the cell, including the endoplasmic reticulum, Golgi, nuclear envelope, endosomes, lysosomes, the plasma membrane, and mitochondria. Using three complementary approaches, subcellular fractionation, ␥-secretase activity assays, and immunocytochemistry, we show that presenilins are highly enriched in a subcompartment of the endoplasmic reticulum that is associated with mitochondria and that forms a physical bridge between the two organelles, called endoplasmic reticulum-mitochondria-associated membranes. A localization of PS1 and PS2 in mitochondria-associated membranes may help reconcile the disparate hypotheses regarding the pathogenesis of Alzheimer disease and may explain many seemingly unrelated features of this devastating neurodegenerative disorder. Alzheimer disease (AD) is a late onset neurodegenerative disorder characterized by progressive neuronal loss, especially in the cortex and the hippocampus. 1 The two main histopathological hallmarks of AD are the accumulation of extracellular neuritic plaques, consisting predominantly of ␤-amyloid (A␤), and of neurofibrillary tangles, consisting mainly of hyperphosphorylated forms of the microtubule-associated protein tau. 1The vast majority of AD is sporadic, but mutations in amyloid precursor protein (APP), presenilin-1 (PS1), and presenilin-2 (PS2) have been identified in the rarer familial form, which is similar to sporadic AD but has an earlier age of onset.PS1 and PS2 are aspartyl proteases that cleave their substrates within transmembrane regions. The active forms of PS1 and PS2 are N-and C-terminal fragments, which are produced by cleavage of full-length presenilin in its "loop" domain.2 PS1 and PS2 are components of the ␥-secretase complex that processes a number of plasma-membrane proteins, including Notch, Jagged, E-cadherin, and, most relevant to AD, APP. The ␥-secretase complex also contains three other structural subunits: APH1, nicastrin (also called APH2), and presenilin enhancer protein 2.2 Following cleavage of APP by ␤-secretase, ␥-secretase cleaves the ϳ100-aa C-terminal "␤-stub" to release small amyloidogenic fragments, 40-and 42-aa in length (A␤40 and A␤42), that have been implicated in the pathogenesis of AD, as well as a ϳ60-aa APP intracellular domain.

Skeletal muscles respond with high plasticity to pathobiological conditions or changes in physiological demand by remodeling cytoarchitectural and metabolic characteristics of individual myocytes. We have previously shown that muscles of mice without mitochondrial and/or cytosolic creatine kinases (ScCKmit(-/-) and/or M-CK(-/-)) partly compensate for the defect(s) by redirecting metabolic pathways and ultrastructural characteristics. Here, we show by semiquantitative Western blot analysis that the compensatory changes involve mutation- and fiber-type-specific coordinated regulation of divergent but functionally coupled groups of proteins. Fast-twitch gastrocnemius muscle of CK(--/--) mice display a two- to fourfold upregulation of mitochondrial cytochrome c oxidase, inorganic phosphate carrier, adenine nucleotide translocator, and voltage-dependent anion channel proteins. In parallel, cytosolic myoglobin is upregulated. Slow-twitch soleus muscle responds with changes in the glycolytic enzyme pattern, including a shift in lactate dehydrogenase isoenzyme composition. Adaptations in the network for oxidative adenosine triphosphate (ATP) production are already apparent at 17 days of age.

Human mitochondrial complex I (CI) deficiency is associated with progressive neurological disorders. To better understand the CI pathomechanism, we here studied how deletion of the CI gene NDUFS4 affects cell metabolism. To this end we compared immortalized mouse embryonic fibroblasts (MEFs) derived from wildtype (wt) and whole-body NDUFS4 knockout (KO) mice. Mitochondria from KO cells lacked the NDUFS4 protein and mitoplasts displayed virtually no CI activity, moderately reduced CII, CIII and CIV activities and normal citrate synthase and CV (F(o)F(1)-ATPase) activity. Native electrophoresis of KO cell mitochondrial fractions revealed two distinct CI subcomplexes of ~830kDa (enzymatically inactive) and ~200kDa (active). The level of fully-assembled CII-CV was not affected by NDUFS4 gene deletion. KO cells exhibited a moderately reduced maximal and routine O(2) consumption, which was fully inhibited by acute application of the CI inhibitor rotenone. The aberrant CI assembly and reduced O(2) consumption in KO cells were fully normalized by NDUFS4 gene complementation. Cellular [NAD(+)]/[NADH] ratio, lactate production and mitochondrial tetramethyl rhodamine methyl ester (TMRM) accumulation were slightly increased in KO cells. In contrast, NDUFS4 gene deletion did not detectably alter [NADP(+)]/[NADPH] ratio, cellular glucose consumption, the protein levels of hexokinases (I and II) and phosphorylated pyruvate dehydrogenase (P-PDH), total cellular adenosine triphosphate (ATP) level, free cytosolic [ATP], cell growth rate, and reactive oxygen species (ROS) levels. We conclude that the NDUFS4 subunit is of key importance in CI stabilization and that, due to the metabolic properties of the immortalized MEFs, NDUFS4 gene deletion has only modest effects at the live cell level. This article is part of a special issue entitled: 17th European Bioenergetics Conference (EBEC 2012).