Mitochondrial dysfunction and subsequent metabolic deregulation is observed in neurodegenerative diseases and aging. Mutations in the presenilin (PSEN) encoding genes (PSEN1 and PSEN2) cause most cases of familial Alzheimer’s disease (AD); however, the underlying mechanism of pathogenesis remains unclear. Here, we show that mutations in the C. elegans gene encoding a PSEN homolog, sel-12 result in mitochondrial metabolic defects that promote neurodegeneration as a result of oxidative stress. In sel-12 mutants, elevated endoplasmic reticulum (ER)-mitochondrial Ca2+ signaling leads to an increase in mitochondrial Ca2+ content which stimulates mitochondrial respiration resulting in an increase in mitochondrial superoxide production. By reducing ER Ca2+ release, mitochondrial Ca2+ uptake or mitochondrial superoxides in sel-12 mutants, we demonstrate rescue of the mitochondrial metabolic defects and prevent neurodegeneration. These data suggest that mutations in PSEN alter mitochondrial metabolic function via ER to mitochondrial Ca2+ signaling and provide insight for alternative targets for treating neurodegenerative diseases.

Aging and age-related diseases are associated with a decline of protein homeostasis (proteostasis), but the mechanisms underlying this decline are not clear. In particular, decreased proteostasis is a widespread molecular feature of neurodegenerative diseases, such as Alzheimer's disease (AD). Familial AD is largely caused by mutations in the presenilin encoding genes; however, their role in AD is not understood. In this study, we investigate the role of presenilins in proteostasis using the model system Caenorhabditis elegans. Previously, we found that mutations in C. elegans presenilin cause elevated ER to mitochondria calcium signaling, which leads to an increase in mitochondrial generated oxidative stress. This, in turn, promotes neurodegeneration.To understand the cellular mechanisms driving neurodegeneration, using several molecular readouts of protein stability in C. elegans, we find that presenilin mutants have widespread defects in proteostasis. Markedly, we demonstrate that these defects are independent of the protease activity of presenilin and that reduction in ER to mitochondrial calcium signaling can significantly prevent the proteostasis defects observed in presenilin mutants. Furthermore, we show that supplementing presenilin mutants with antioxidants suppresses the proteostasis defects. Our findings indicate that defective ER to mitochondria calcium signaling promotes proteostatic collapse in presenilin mutants by increasing oxidative stress.

The complex molecular and cellular mechanisms underlying neuronal control of animal movement are not well understood. Locomotion of Caenorhabditis elegans is mediated by a neuronal circuit that produces coordinated sinusoidal movement. Here we utilize this simple, yet elegant, behavior to show that VAV-1, a conserved guanine nucleotide exchange factor for Rho-family GTPases, negatively regulates motor circuit activity and the rate of locomotion. While vav-1 is expressed in a small subset of neurons, we find that VAV-1 function is required in a single interneuron ALA to regulate motor neuron circuit activity. Furthermore, we show by genetic and optogenetic manipulation of ALA that VAV-1 is required for the excitation and activation of this neuron. We find that ALA signaling inhibits command interneuron activity by abrogating excitatory signaling in the command interneurons that is responsible for promoting motor neuron circuit activity. Togther, our data describe a novel neuromodulatory role for VAV-1-dependent signaling in the regulation of motor circuit activity and locomotion.

Sleep is evolutionarily conserved and required for organism homeostasis and survival. Despite this importance, the molecular and cellular mechanisms underlying sleep are not well understood. Caenorhabditis elegans exhibits sleep-like behavioral quiescence and thus provides a valuable, simple model system for the study of cellular and molecular regulators of this process. In C. elegans, epidermal growth factor receptor (EGFR) signaling is required in the neurosecretory neuron ALA to promote sleep-like behavioral quiescence after cellular stress. We describe a novel role for VAV-1, a conserved guanine nucleotide exchange factor (GEF) for Rho-family GTPases, in regulation of sleep-like behavioral quiescence. VAV-1, in a GEF-dependent manner, acts in ALA to suppress locomotion and feeding during sleep-like behavioral quiescence in response to cellular stress. Additionally, VAV-1 activity is required for EGF-induced sleep-like quiescence and normal levels of EGFR and secretory dense core vesicles in ALA. Importantly, the role of VAV-1 in promoting cellular stress-induced behavioral quiescence is vital for organism health because VAV-1 is required for normal survival after cellular stress.KEYWORDS behavioral quiescence; Caenorhabditis elegans; sleep; Vav D ESPITE being a subject of formal study for over 150 years, sleep is not clearly understood. Moreover, various explanations for the functional role of sleep have been proposed, such as allowing "recharging" of cells following high metabolic activity (Benington and Heller 1995;Tu and McKnight 2006;Scharf et al. 2008) and remodeling of synapses built during wakefulness (Tononi and Cirelli 2006). Yet sleep problems have a significant impact on human health and are a leading reason for seeking medical attention (Mahowald and Schenck 2005). Therefore, there is a great need for understanding the cellular and molecular mechanisms that regulate sleep-wake cycles.Simple model organisms such as Caenorhabditis elegans have the potential to provide valuable information regarding sleep regulation. C. elegans is known for its easily manipulated genetics, small nervous system with mapped neuronal connectivity, stereotypical behaviors, and the ability to be studied efficiently in large numbers. Numerous studies have shown that C. elegans lethargus, a restful period that occurs before each molt of the cuticle during larval development, is neuronally regulated and likely orthologous to sleep in mammals (Van Buskirk and Sternberg 2007;Raizen et al. 2008;Van Buskirk and Sternberg 2010;Choi et al. 2013;Iwanir et al. 2013;Turek et al. 2013;Cho and Sternberg 2014;Singh et al. 2014). Lethargus quiescence in C. elegans shares several characteristics with mammalian sleep: inactivity (decreased locomotion and cessation of pharyngeal pumping, or feeding), a specific posture, reduced response to aversive stimuli, and rapid reversibility (Cassada and Russell 1975;Raizen et al. 2008;Schwarz et al. 2012;Iwanir et al. 2013;Cho and Sternberg 2014). In addition, lethargus quiescence is under ...

BackgroundMetabolic dysfunction and protein aggregation are common characteristics that occur in age-related neurodegenerative disease, such as Alzheimer’s disease (AD). However, the mechanisms underlying these abnormalities remain poorly understood. Mutations in the presenilin genes are the primary cause of early onset familial AD, but despite their identification over 20 years ago, their role in the disease remains unclear. MethodsThe model system Caenorhabditis elegans was utilized to study the in vivo function of the highly conserved presenilin ortholog SEL-12 in the nervous system. Cell biological and biochemical assays were employed to monitor changes to proteostasis and autophagic flux in sel-12 mutants. Immunoblotting was used to assess alterations to the activity of the mTORC1 pathway, a central inhibitor of autophagy. Genetic and pharmaceutical strategies to reduce mTORC1 activity, and fluorescent reporters and biosensors were expressed in the mechanosensory neurons to measure mTORC1’s influence on proteotoxicity, neuronal health and mitochondrial morphology. Additionally, behavioral response to touch was employed to determine the role mTORC1 activity has in neuronal function in sel-12 mutants. RNA interference by standard feeding methods was used to assess the contribution of autophagy to mTORC1-mediated sel-12 defects. ResultsLoss of SEL-12 results in the hyperactivation of the mTORC1 pathway and mTORC1-dependent reduction in autophagy. This hyperactivation is caused by elevated mitochondrial calcium signaling and concomitant mitochondrial hyperactivity. Reducing mTORC1 activity improves proteostasis defects and neurodegenerative phenotypes associated with loss of SEL-12 function. Consistent with high mTORC1 activity, we find that SEL-12 loss reduces autophagy, and this reduction is prevented by limiting mitochondrial calcium uptake or mitochondrial respiration. Moreover, the improvements in proteostasis and neuronal defects in sel-12 mutants due to mTORC1 inhibition require the induction of autophagy.ConclusionSEL-12 has a critical role in mediating mitochondrial calcium homeostasis and activity. In the absence of presenilin function mitochondrial calcium uptake and mitochondrial activity is increased. This mitochondrial hyperactivity stimulates mTORC1 signaling, which inhibits autophagy and promotes proteostasis decline and neuronal dysfunction in sel-12 mutants. These data suggest that the mTORC1 pathway is a potential therapeutic target for treating AD.

Metabolic dysfunction and protein aggregation are common characteristics that occur in age‐related neurodegenerative disease. However, the mechanisms underlying these abnormalities remain poorly understood. We have found that mutations in the gene encoding presenilin in Caenorhabditis elegans, sel‐12, results in elevated mitochondrial activity that drives oxidative stress and neuronal dysfunction. Mutations in the human presenilin genes are the primary cause of familial Alzheimer's disease. Here, we demonstrate that loss of SEL‐12/presenilin results in the hyperactivation of the mTORC1 pathway. This hyperactivation is caused by elevated mitochondrial calcium influx and, likely, the associated increase in mitochondrial activity. Reducing mTORC1 activity improves proteostasis defects and neurodegenerative phenotypes associated with loss of SEL‐12 function. Consistent with high mTORC1 activity, we find that SEL‐12 loss reduces autophagosome formation, and this reduction is prevented by limiting mitochondrial calcium uptake. Moreover, the improvements of proteostasis and neuronal defects in sel‐12 mutants due to mTORC1 inhibition require the induction of autophagy. These results indicate that mTORC1 hyperactivation exacerbates the defects in proteostasis and neuronal function in sel‐12 mutants and demonstrate a critical role of presenilin in promoting neuronal health.

Mitochondrial dysfunction and oxidative stress are major contributors to the pathophysiology of neurodegenerative diseases, including Alzheimer’s disease (AD). However, the mechanisms driving mitochondrial dysfunction and oxidative stress are unclear. Familial AD (fAD) is an early onset form of AD caused primarily by mutations in the presenilin-encoding genes. Previously, using Caenorhabditis elegans as a model system to study presenilin function, we found that loss of C. elegans presenilin orthologue SEL-12 results in elevated mitochondrial and cytosolic calcium levels. Here, we provide evidence that elevated neuronal mitochondrial generated reactive oxygen species (ROS) and subsequent neurodegeneration in sel-12 mutants are a consequence of the increase of mitochondrial calcium levels and not cytosolic calcium levels. We also identify mTORC1 signaling as a critical factor in sustaining high ROS in sel-12 mutants in part through its repression of the ROS scavenging system SKN-1/Nrf. Our study reveals that SEL-12/presenilin loss disrupts neuronal ROS homeostasis by increasing mitochondrial ROS generation and elevating mTORC1 signaling, which exacerbates this imbalance by suppressing SKN-1/Nrf antioxidant activity.

In Caenorhabditis elegans, rhythmic posterior body wall muscle contractions mediate the highly regular defecation cycle. These contractions are regulated by inositol-1,4,5-trisphosphate (InsP3) receptor-dependent Ca2+ oscillations in intestinal epithelial cells. Here, we find that mutations in dec-7, which encodes the nematode ortholog of the human Sushi domain containing 2 protein (SUSD2), lead to an increase in InsP3 receptor-dependent rhythmic posterior body wall muscle contractions. DEC-7 is highly expressed in the intestinal epithelia and localizes to the cell-cell junction. The increase in rhythmic activity caused by loss of dec-7 is dependent on the innexin gap junction protein INX-16. Moreover, DEC-7 is required for the clustering of INX-16 to the cell-cell junction of the intestinal epithelia. We hypothesize that DEC-7/SUSD2 regulates INX-16 activity to mediate the rhythmic frequency of the defecation motor program. Thus, our data indicate a critical role of a phylogenetically conserved cell-cell junction protein in mediating an ultradian rhythm in the intestinal epithelia of C. elegans.