A γ-Secretase Independent Role for Presenilin in Calcium Homeostasis Impacts Mitochondrial Function and Morphology in Caenorhabditis elegans

Abstract

Mutations in the presenilin (PSEN) encoding genes (PSEN1 and PSEN2) occur in most early onset familial Alzheimer's Disease. Despite the identification of the involvement of PSEN in Alzheimer's Disease (AD) 20 years ago, the underlying role of PSEN in AD is not fully understood. To gain insight into the biological function of PSEN, we investigated the role of the PSEN homolog SEL-12 in Caenorhabditis elegans. Using genetic, cell biological, and pharmacological approaches, we demonstrate that mutations in sel-12 result in defects in calcium homeostasis, leading to mitochondrial dysfunction. Moreover, consistent with mammalian PSEN, we provide evidence that SEL-12 has a critical role in mediating endoplasmic reticulum (ER) calcium release. Furthermore, we found that in SEL-12-deficient animals, calcium transfer from the ER to the mitochondria leads to fragmentation of the mitochondria and mitochondrial dysfunction. Additionally, we show that the impact that SEL-12 has on mitochondrial function is independent of its role in Notch signaling, g-secretase proteolytic activity, and amyloid plaques. Our results reveal a critical role for PSEN in mediating mitochondrial function by regulating calcium transfer from the ER to the mitochondria.KEYWORDS Caenorhabditis elegans; calcium; endoplasmic reticulum; mitochondria; presenilin M ITOCHONDRIA are highly dynamic organelles that are responsible for a myriad of functions including ATP production, generation and metabolism of reactive oxygen species (ROS), and apoptosis (Detmer and Chan 2007). Thus cells, in particular muscle cells and neurons due to their highenergy demands, are extremely reliant on mitochondrial function. Mitochondria also play a critical role in calcium homeostasis by acting as a calcium buffer. Calcium, in turn, regulates mitochondrial morphology, activity, and movement. The relevance of mitochondria is highlighted by the fact that many neurodegenerative diseases, such as Parkinson's and Alzheimer's Disease (AD) are characterized by abnormal mitochondrial activity and morphology (Cali et al. 2012).Mutations in the genes encoding presenilins (PSENs), PSEN1 and PSEN2, are the most common cause of early onset familial Alzheimer's Disease (FAD). PSENs are 50-kDa multipass transmembrane proteins that reside predominantly on the endoplasmic reticulum (ER) (Bezprozvanny and Mattson 2008) and have been shown to be enriched in an ER subcompartment that is in contact with mitochondria (Area-Gomez et al. 2009). Although altered PSEN function has been known to have a role in AD for 20 years (Sherrington et al. 1995), the functional consequences of mutations in PSENs are controversial and not understood. The prevailing model for the pathogenesis of AD is the "amyloid hypothesis," which states that an increase in b amyloid (Ab) 42 peptide is the major cause of neuronal degradation in AD (Hardy and Selkoe 2002); support for this hypothesis stems from the accumulation of amyloid plaques in the brains of AD patients (Hardy 2006). In addition to the "amyloid hypot...

Role of Presenilin in Mitochondrial Oxidative Stress and Neurodegeneration in <em>Caenorhabditis elegans</em>

“…These Drp1 spirals will then constrict mitochondrial tubules through conformational changes, driven by GTP hydrolysis resulting in mitochondrial fission (Cereghetti et al, 2008;Cribbs and Strack, 2007;Xu et al, 2013). Mitochondrial morphological changes manifesting as fragmented mitochondria with damaged inner membrane structures have been observed in neurons in AD patients (Hirai et al, 2001) and consistent with this, mitochondrial structural disorganization has been observed in the body wall muscle and neurons of sel-12 mutants (Sarasija and Norman, 2015), suggestive of elevated Drp1 activity. Interestingly, ER-mediated calcium release and subsequent mitochondrial calcium uptake can impact the activity of Drp1 (Cereghetti et al, 2008;Cribbs and Strack, 2007;Xu et al, 2013).…”

“…Using optogenetic, behavioral and pharmacological assays, it was demonstrated that there is increased ER-calcium signaling in sel-12 mutants (Sarasija and Norman, 2015) similar to what is observed in vertebrate systems with presenilin mutations (Chakroborty et al, 2009;Chan et al, 2000;Leissring et al, 2000a;Smith et al, 2005;Stutzmann et al, 2004;Tu et al, 2006). Calcium homeostasis is crucial for organismal health and therefore, mechanisms exist to ensure its maintenance.…”

“…Given the role of oxidative stress in neurodegeneration, ROS levels were measured in sel-12 animals. Markedly, ROS levels were significantly higher in sel-12 mutants compared to wild type control animals (Sarasija and Norman, 2015). Moreover, reduction of ER-mitochondrial calcium signaling reduces the levels of ROS observed in sel-12 mutants (Sarasija et al, 2018), indicating that the high ROS production in sel-12 mutants is caused by increased ER to mitochondria calcium transfer.…”

“…To determine if Notch or γ-secretase activity has a role in mitochondrial or neurodegenerative phenotype observed in sel-12 mutants, several approaches were taken. First, analyses of C. elegans Notch mutants did not reveal any mitochondrial or touch behavioral defects as observed in sel-12 mutants (Sarasija et al, 2018;Sarasija and Norman, 2015), suggesting Notch does not have a role in the mitochondrial dysfunction or neurodegeneration observed in sel-12 mutants. Moreover, γ-secretase activity was investigated by both pharmacological inhibition and the generation of a protease dead SEL-12 protein.…”

“…Interestingly, ER-mediated calcium release and subsequent mitochondrial calcium uptake can impact the activity of Drp1 (Cereghetti et al, 2008;Cribbs and Strack, 2007;Xu et al, 2013). The higher incidence of structurally disorganized mitochondria in sel-12 mutants is rescued by reducing ER-calcium release, mitochondrial calcium uptake or knocking down Drp1 (Sarasija and Norman, 2015), suggesting that the loss of sel-12 results in enhanced ER-mitochondria calcium transfer, which activates Drp1 causing elevated mitochondrial fission.…”