Spontaneous age-related neurite branching in Caenorhabditis elegans

Strategies for studying sex differences in brain aging

Studying sex effects and their underlying mechanisms is of major relevance to understanding brain health. Despite growing interests, experimentally studying sex differences, particularly in the context of aging, remains challenging. Since sex chromosomal content influences gonadal development, separating the effects of gonadal hormones and chromosomal factors requires specific model systems. Here, we highlight rodent and tractable models for examining sex dimorphism in brain and cognitive aging. In addition, we discuss multi-omic and bioinformatic approaches that yield biological insights from animal and human studies. This review provides a comprehensive overview of the diverse toolkit now available to advance our understanding of sex differences in brain aging.

Context-specific interaction of the lipid regulator DIP-2 with phospholipid synthesis in axon regeneration and maintenance

Neurons maintain their morphology over prolonged periods of adult life with limited regeneration after injury. C. elegans DIP-2 is a conserved regulator of lipid metabolism that affects axon maintenance and regeneration after injury. Here, we investigated genetic interactions of dip-2 with mutants in genes involved in lipid biosynthesis and identified roles of phospholipids in axon regrowth and maintenance. CEPT-2 and EPT-1 are enzymes catalyzing the final steps in the de novo phospholipid synthesis (Kennedy) pathway. Loss of function mutants of cept-2 or ept-1 show reduced axon regrowth and failure to maintain axon morphology. We demonstrate that CEPT-2 is cell-autonomously required to prevent age-related axonal defects. Interestingly, loss of function in dip-2 led to suppression of the axon regrowth phenotype observed in either cept-2 or ept-2 mutants, suggesting that DIP-2 acts to counterbalance phospholipid synthesis. Our findings reveal the genetic regulation of lipid metabolism to be critical for axon maintenance under injury and during aging.

Article Summary

Little is known about how adult neurons live long with limited regenerative capacity. This study investigates the role of lipid metabolism in sustaining neuronal health in C. elegans. Mutating phospholipid synthetic genes impairs axon regrowth after injury. Lack of DIP-2, a lipid regulator, restores regrowth, suggesting DIP-2 counterbalances phospholipid synthesis. Moreover, neuronal phospholipid synthesis is essential for preventing age-dependent axonal defects. These findings reveal phospholipid biosynthesis is key to axon integrity during aging and injury. As lipid metabolism is implicated in neurological disorders, this study serves as an entry point into investigating neuronal lipid biology under various conditions.

Structural changes shaping the Drosophila ellipsoid body ER-neurons during development and aging

The ellipsoid body (EB) of the insect brain performs pivotal functions in controlling navigation. Input and output of the EB is provided by multiple classes of R-neurons (now referred to as ER-neurons) and columnar neurons which interact with each other in a stereotypical and spatially highly ordered manner. The developmental mechanisms that control the connectivity and topography of EB neurons are largely unknown. One indispensable prerequisite to unravel these mechanisms is to document in detail the sequence of events that shape EB neurons during their development. In this study, we analyzed the development of the Drosophila EB. In addition to globally following the ER-neuron and columnar neuron (sub)classes in the spatial context of their changing environment we performed a single cell analysis using the multi-color flip out (MCFO) system to analyze the developmental trajectory of ER-neurons at different pupal stages, young adults (4d) and aged adults (∼60d). We show that the EB develops as a merger of two distinct elements, a posterior and anterior EB primordium (prEBp and prEBa, respectively. ER-neurons belonging to different subclasses form growth cones and filopodia that associate with the prEBp and prEBa in a pattern that, from early pupal stages onward, foreshadows their mature structure. Filopodia of all ER-subclasses are initially much longer than the dendritic and terminal axonal branches they give rise to, and are pruned back during late pupal stages. Interestingly, extraneous branches, particularly significant in the dendritic domain, are a hallmark of ER-neuron structure in aged brains. Aging is also associated with a decline in synaptic connectivity from columnar neurons, as well as upregulation of presynaptic protein (Brp) in ER-neurons. Our findings advance the EB (and ER-neurons) as a favorable system to visualize and quantify the development and age-related decline of a complex neuronal circuitry.

Ortholog of autism candidate gene RBM27 regulates mitoribosomal assembly factor MALS-1 to protect against mitochondrial dysfunction and axon degeneration during neurodevelopment

Mitochondrial dysfunction is thought to be a key component of neurodevelopmental disorders such as autism, intellectual disability, and attention-deficit hyperactivity disorder (ADHD). However, little is known about the molecular mechanisms that protect against mitochondrial dysfunction during neurodevelopment. Here, we address this question through the investigation of rbm-26, the Caenorhabditis elegans ortholog of the RBM27 autism candidate gene, which encodes an RNA-binding protein whose role in neurons is unknown. We report that RBM-26 (RBM26/27) protects against axonal defects by negatively regulating expression of the MALS-1 (MALSU1) mitoribosomal assembly factor. Autism-associated missense variants in RBM-26 cause a sharp decrease in RBM-26 protein expression along with defects in axon overlap and axon degeneration that occurs during larval development. Using a biochemical screen, we identified the mRNA for the MALS-1 mitoribosomal assembly factor as a binding partner for RBM-26. Loss of RBM-26 function causes a dramatic overexpression of mals-1 mRNA and MALS-1 protein. Moreover, genetic analysis indicates that this overexpression of MALS-1 is responsible for the mitochondrial and axon degeneration defects in rbm-26 mutants. These observations reveal a mechanism that regulates expression of a mitoribosomal assembly factor to protect against axon degeneration during neurodevelopment.

Autism candidate gene rbm-26 (RBM26/27) regulates MALS-1 to protect against mitochondrial dysfunction and axon degeneration during neurodevelopment

Mitochondrial dysfunction is thought to be a key component of neurodevelopmental disorders such as autism, intellectual disability, and ADHD. However, little is known about the molecular mechanisms that protect against mitochondrial dysfunction during neurodevelopment. Here, we address this question through the investigation of rbm-26, the C. elegans ortholog of the RBM27 autism candidate gene, which encodes an RNA-binding protein whose role in neurons is unknown. We report that RBM-26 (RBM26/27) protects against axonal defects by negatively regulating expression of the MALS-1 (MALSU1) mitoribosomal assembly factor. Autism-associated missense variants in RBM-26 cause a sharp decrease in RBM-26 protein expression along with defects in in axon overlap and axon degeneration that occurs during larval development. Using a biochemical screen, we identified the mRNA for the MALS-1 mitoribosomal assembly factor as a binding partner for RBM-26. Loss of RBM-26 function causes a dramatic overexpression of mals-1 mRNA and MALS-1 protein. Moreover, genetic analysis indicates that this overexpression of MALS-1 is responsible for the mitochondrial and axon degeneration defects in rbm-26 mutants. These observations reveal a mechanism that regulates expression of a mitoribosomal assembly factor to protect against axon degeneration during neurodevelopment.

Consolidating multiple evolutionary theories of ageing suggests a need for new approaches to study genetic contributions to ageing decline

Understanding mechanisms of ageing remains a complex challenge for biogerontologists, but recent adaptations of evolutionary ageing theories offer a compelling lens in which to view both age-related molecular and physiological deterioration. Ageing is commonly associated with progressive declines in biochemical and molecular processes resulting from damage accumulation, yet the role of continued developmental gene activation is less appreciated. Natural selection pressures are at their highest in youthful periods to modify gene expression towards maximising reproductive capacity. After sexual maturation, selective pressure diminishes, subjecting individuals to maladaptive pleiotropic gene functions that were once beneficial for developmental growth but become pathogenic later in life. Due to this selective 'shadowing' in ageing, mechanisms to counter such hyper/hypofunctional genes are unlikely to evolve. Interventions aimed at targeting gene hyper/hypofunction during ageing might, therefore, represent an attractive therapeutic strategy. The nematode Caenorhabditis elegans offers a strong model for post-reproductive mechanistic and therapeutic investigations, yet studies examining the mechanisms of, and countermeasures against, ageing decline largely intervene from larval stages onwards. Importantly, however, lifespan extending conditions frequently impair early-life fitness and fail to correspondingly increase healthspan. Here, we consolidate multiple evolutionary theories of ageing and discuss data supporting hyper/hypofunctional changes at a global molecular and functional level in C. elegans, and how classical lifespan-extension mutations alter these dynamics. The relevance of such mutant models for exploring mechanisms of ageing are discussed, highlighting that post-reproductive gene optimisation represents a more translatable approach for C. elegans research that is not constrained by evolutionary trade-offs. Where some genetic mutations in C. elegans that promote late-life health map accordingly with healthy ageing in humans, other widely used genetic mutations that extend worm lifespan are associated with life-limiting pathologies in people. Lifespan has also become the gold standard for quantifying 'ageing', but we argue that gerospan compression (i.e., 'healthier' ageing) is an appropriate goal for anti-ageing research, the mechanisms of which appear distinct from those regulating lifespan alone. There is, therefore, an evident need to re-evaluate experimental approaches to study the role of hyper/hypofunctional genes in ageing in C. elegans.

The neuron-specific IIS/FOXO transcriptome in aged animals reveals regulatory mechanisms of cognitive aging

Cognitive decline is a significant health concern in our aging society. Here, we used the model organism C. elegans to investigate the impact of the IIS/FOXO pathway on age-related cognitive decline. The daf-2 Insulin/IGF-1 receptor mutant exhibits a significant extension of learning and memory span with age compared to wild-type worms, an effect that is dependent on the DAF-16 transcription factor. To identify possible mechanisms by which aging daf-2 mutants maintain learning and memory with age while wild-type worms lose neuronal function, we carried out neuron-specific transcriptomic analysis in aged animals. We observed downregulation of neuronal genes and upregulation of transcriptional regulation genes in aging wild-type neurons. By contrast, IIS/FOXO pathway mutants exhibit distinct neuronal transcriptomic alterations in response to cognitive aging, including upregulation of stress response genes and downregulation of specific insulin signaling genes. We tested the roles of significantly transcriptionally-changed genes in regulating cognitive functions, identifying novel regulators of learning and memory. In addition to other mechanistic insights, a comparison of the aged vs young daf-2 neuronal transcriptome revealed that a new set of potentially neuroprotective genes is upregulated; instead of simply mimicking a young state, daf-2 may enhance neuronal resilience to accumulation of harm and take a more active approach to combat aging. These findings suggest a potential mechanism for regulating cognitive function with age and offer insights into novel therapeutic targets for age-related cognitive decline.

Male-specific behavioral and transcriptomic changes in aging C. elegans neurons

Aging is a complex biological process with sexually dimorphic aspects. Although cognitive aging of Caenorhabditis elegans hermaphrodites has been studied, less is known about cognitive decline in males. We found that cognitive aging has both sex-shared and sex-dimorphic characteristics, and we identified neuron-specific age-associated sex-differential targets. In addition to sex-shared neuronal aging genes, males differentially downregulate mitochondrial metabolic genes and upregulate GPCR genes with age, while the X chromosome exhibits increased gene expression in hermaphrodites and altered dosage compensation complex expression with age, indicating possible X chromosome dysregulation that contributes to sexual dimorphism in cognitive aging. Finally, the sex-differentially expressed gene hrg-7, an aspartic-type endopeptidase, regulates male cognitive aging but does not affect hermaphrodites' behaviors. These results suggest that males and hermaphrodites exhibit different age-related neuronal changes. This study will strengthen our understanding of sex-specific vulnerability and resilience and identify pathways to target with treatments that could benefit both sexes.

Considering Caenorhabditis elegans Aging on a Temporal and Tissue Scale: The Case of Insulin/IGF-1 Signaling

The aging process is inherently complex, involving multiple mechanisms that interact at different biological scales. The nematode Caenorhabditis elegans is a simple model organism that has played a pivotal role in aging research following the discovery of mutations extending lifespan. Longevity pathways identified in C. elegans were subsequently found to be conserved and regulate lifespan in multiple species. These pathways intersect with fundamental hallmarks of aging that include nutrient sensing, epigenetic alterations, proteostasis loss, and mitochondrial dysfunction. Here we summarize recent data obtained in C. elegans highlighting the importance of studying aging at both the tissue and temporal scale. We then focus on the neuromuscular system to illustrate the kinetics of changes that take place with age. We describe recently developed tools that enabled the dissection of the contribution of the insulin/IGF-1 receptor ortholog DAF-2 to the regulation of worm mobility in specific tissues and at different ages. We also discuss guidelines and potential pitfalls in the use of these new tools. We further highlight the opportunities that they present, especially when combined with recent transcriptomic data, to address and resolve the inherent complexity of aging. Understanding how different aging processes interact within and between tissues at different life stages could ultimately suggest potential intervention points for age-related diseases.

Autophagy protein ATG-16.2 and its WD40 domain mediate the beneficial effects of inhibiting early-acting autophagy genes in C. elegans neurons

While autophagy genes are required for lifespan of long-lived animals, their tissue-specific roles in aging remain unclear. Here, we inhibited autophagy genes in Caenorhabditis elegans neurons, and found that knockdown of early-acting autophagy genes, except atg-16.2, increased lifespan, and decreased neuronal PolyQ aggregates, independently of autophagosomal degradation. Neurons can secrete protein aggregates via vesicles called exophers. Inhibiting neuronal early-acting autophagy genes, except atg-16.2, increased exopher formation and exopher events extended lifespan, suggesting exophers promote organismal fitness. Lifespan extension, reduction in PolyQ aggregates and increase in exophers were absent in atg-16.2 null mutants, and restored by full-length ATG-16.2 expression in neurons, but not by ATG-16.2 lacking its WD40 domain, which mediates noncanonical functions in mammalian systems. We discovered a neuronal role for C. elegans ATG-16.2 and its WD40 domain in lifespan, proteostasis and exopher biogenesis. Our findings suggest noncanonical functions for select autophagy genes in both exopher formation and in aging.

Silica Nanoparticles Disclose a Detailed Neurodegeneration Profile throughout the Life Span of a Model Organism

The incidence of age-related neurodegenerative diseases is rising globally. However, the temporal sequence of neurodegeneration throughout adult life is poorly understood. To identify the starting points and schedule of neurodegenerative events, serotonergic and dopaminergic neurons were monitored in the model organism C. elegans, which has a life span of 2-3 weeks. Neural morphology was examined from young to old nematodes that were exposed to silica nanoparticles. Young nematodes showed phenotypes such as dendritic beading of serotonergic and dopaminergic neurons that are normally not seen until late life. During aging, neurodegeneration spreads from specifically susceptible ADF and PDE neurons in young C. elegans to other more resilient neurons, such as dopaminergic CEP in middle-aged worms. Investigation of neurodegenerative hallmarks and animal behavior revealed a temporal correlation with the acceleration of neuromuscular defects, such as internal hatch in 2-day-old C. elegans. Transcriptomics and proteomics of young worms exposed to nano silica showed a change in gene expression concerning the gene ontology groups serotonergic and dopaminergic signaling as well as neuropeptide signaling. Consistent with this, reporter strains for nlp-3, nlp-14 and nlp-21 confirmed premature degeneration of the serotonergic neuron HSN and other neurons in young C. elegans. The results identify young nematodes as a vulnerable age group for nano silica-induced neural defects with a significantly reduced health span. Neurodegeneration of specific neurons impairs signaling by classical neurotransmitters as well as neuropeptides and compromises related neuromuscular behaviors in critical phases of life, such as the reproductive phase.

Caenorhabditis elegans as an in vivo model for the identification of natural antioxidants with anti-aging actions

Natural antioxidants have recently emerged as a highly exciting and significant topic in anti-aging research. Diverse organism models present a viable protocol for future research. Notably, many breakthroughs on natural antioxidants have been achieved in the nematode Caenorhabditis elegans, an animal model frequently utilized for the study of aging research and anti-aging drugs in vivo. Due to the conservation of signaling pathways on oxidative stress resistance, lifespan regulation, and aging disease between C. elegans and multiple high-level organisms (humans), as well as the low and controllable cost of time and labor, it gradually develops into a trustworthy in vivo model for high-throughput screening and validation of natural antioxidants with anti-aging actions. First, information and models on free radicals and aging are presented in this review. We also describe indexes, detection methods, and molecular mechanisms for studying the in vivo antioxidant and anti-aging effects of natural antioxidants using C. elegans. It includes lifespan, physiological aging processes, oxidative stress levels, antioxidant enzyme activation, and anti-aging pathways. Furthermore, oxidative stress and healthspan improvement induced by natural antioxidants in humans and C. elegans are compared, to understand the potential and limitations of the screening model in preclinical studies. Finally, we emphasize that C. elegans is a useful model for exploring more natural antioxidant resources and uncovering the mechanisms underlying aging-related risk factors and diseases.

Individual cell types in C. elegans age differently and activate distinct cell-protective responses

Aging is characterized by a global decline in physiological function. However, by constructing a complete single-cell gene expression atlas, we find that Caenorhabditis elegans aging is not random in nature but instead is characterized by coordinated changes in functionally related metabolic, proteostasis, and stress-response genes in a cell-type-specific fashion, with downregulation of energy metabolism being the only nearly universal change. Similarly, the rates at which cells age differ significantly between cell types. In some cell types, aging is characterized by an increase in cell-to-cell variance, whereas in others, variance actually decreases. Remarkably, multiple resilience-enhancing transcription factors known to extend lifespan are activated across many cell types with age; we discovered new longevity candidates, such as GEI-3, among these. Together, our findings suggest that cells do not age passively but instead react strongly, and individualistically, to events that occur during aging. This atlas can be queried through a public interface.

Age-associated anatomical and physiological alterations in Caenorhabditis elegans

Since its introduction by Sydney Brenner, Caenorhabditis elegans has become a widely studied organism. Given its highly significant properties, including transparency, short lifespan, self-fertilization, high reproductive yield and ease in manipulation and genetic modifications, the nematode has contributed to the elucidation of several fundamental aspects of biology, such as development and ageing. Moreover, it has been extensively used as a platform for the modelling of ageing-associated human disorders, especially those related to neurodegeneration. The use of C. elegans for such purposes requires, and at the same time promotes the investigation of its normal ageing process. In this review we aim to summarize the major organismal alterations during normal worm ageing, in terms of morphology and functionality.

Targeting the "hallmarks of aging" to slow aging and treat age-related disease: fact or fiction?

Aging is a major risk factor for a number of chronic diseases, including neurodegenerative and cerebrovascular disorders. Aging processes have therefore been discussed as potential targets for the development of novel and broadly effective preventatives or therapeutics for age-related diseases, including those affecting the brain. Mechanisms thought to contribute to aging have been summarized under the term the "hallmarks of aging" and include a loss of proteostasis, mitochondrial dysfunction, altered nutrient sensing, telomere attrition, genomic instability, cellular senescence, stem cell exhaustion, epigenetic alterations and altered intercellular communication. We here examine key claims about the "hallmarks of aging". Our analysis reveals important weaknesses that preclude strong and definitive conclusions concerning a possible role of these processes in shaping organismal aging rate. Significant ambiguity arises from the overreliance on lifespan as a proxy marker for aging, the use of models with unclear relevance for organismal aging, and the use of study designs that do not allow to properly estimate intervention effects on aging rate. We also discuss future research directions that should be taken to clarify if and to what extent putative aging regulators do in fact interact with aging. These include multidimensional analytical frameworks as well as designs that facilitate the proper assessment of intervention effects on aging rate.

BAM15 Relieves Neurodegeneration in Aged Caenorhabditis elegans and Extends Lifespan

BAM15 was recently screened as a protonophore uncoupler specifically for the mitochondrial membrane but not the plasma membrane. It is equally as potent as FCCP, but less toxic. Previously, mitochondrial uncoupling via DNP alleviates neurodegeneration in the nematode Caenorhabditis elegans during aging. Therefore, we investigated whether BAM15 uncouplers could phenotypically and functionally reduce neuronal defects in aged nematodes. We observed green fluorescence protein-tagged mechanosensory neurons and performed touch and chemotaxis assays during aging. Wild-type animals treated with both 50 µM BAM15 and 10 µM DNP showed reduced mechanosensory neuronal defects during aging, which correlates with the maintenance of touch responses and short-term memory during aging. Uncoupler mutant ucp-4 also responded the same way as the wild-type, reducing neurodegeneration in 50 µM BAM15 and 10 µM DNP-treated animals compared to the DMSO control. These results suggest that 50 µM BAM15 alleviates neurodegeneration phenotypically and functionally in C. elegans during aging, potentially through mitochondrial uncoupling. In accordance with the preserved neuronal shape and function in aged C. elegans, 50 µM BAM15 extended the mean lifespan of both wild-type and ucp-4 mutants.

Deregulation of Mitochondrial Calcium Handling Due to Presenilin Loss Disrupts Redox Homeostasis and Promotes Neuronal Dysfunction

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.

Age-associated changes to neuronal dynamics involve a disruption of excitatory/inhibitory balance in C. elegans

In the aging brain, many of the alterations underlying cognitive and behavioral decline remain opaque. C. elegans offers a powerful model for aging research, with a simple, well-studied nervous system to further our understanding of the cellular modifications and functional alterations accompanying senescence. We perform multi-neuronal functional imaging across the aged C. elegans nervous system, measuring an age-associated breakdown in system-wide functional organization. At single-cell resolution, we detect shifts in activity dynamics toward higher frequencies. In addition, we measure a specific loss of inhibitory signaling that occurs early in the aging process and alters the systems critical excitatory/inhibitory balance. These effects are recapitulated with mutation of the calcium channel subunit UNC-2/CaV2a. We find that manipulation of inhibitory GABA signaling can partially ameliorate or accelerate the effects of aging. The effects of aging are also partially mitigated by disruption of the insulin signaling pathway, known to increase longevity, or by a reduction of caspase activation. Data from mammals are consistent with our findings, suggesting a conserved shift in the balance of excitatory/inhibitory signaling with age that leads to breakdown in global neuronal dynamics and functional decline.

Invited review: Unearthing the mechanisms of age-related neurodegenerative disease using Caenorhabditis elegans

As human life expectancy increases, neurodegenerative diseases present a growing public health threat, for which there are currently few effective treatments. There is an urgent need to understand the molecular and genetic underpinnings of these disorders so new therapeutic targets can be identified. Here we present the argument that the simple nematode worm Caenorhabditis elegans is a powerful tool to rapidly study neurodegenerative disorders due to their short lifespan and vast array of genetic tools, which can be combined with characterization of conserved neuronal processes and behavior orthologous to those disrupted in human disease. We review how pre-existing C. elegans models provide insight into human neurological disease as well as an overview of current tools available to study neurodegenerative diseases in the worm, with an emphasis on genetics and behavior. We also discuss open questions that C. elegans may be particularly well suited for in future studies and how worms will be a valuable preclinical model to better understand these devastating neurological disorders.

Macroautophagy and normal aging of the nervous system: Lessons from animal models

Aging represents a cumulative form of cellular stress, which is thought to challenge many aspects of proteostasis. The non-dividing, long-lived neurons are particularly vulnerable to stress, and, not surprisingly, even normal aging is highly associated with a decline in brain function in humans, as well as in other animals. Macroautophagy is a fundamental arm of the proteostasis network, safeguarding proper protein turnover during different cellular states and against diverse cellular stressors. An intricate interplay between macroautophagy and aging is beginning to unravel, with the emergence of new tools, including those for monitoring autophagy in cultured neurons and in the nervous system of different organisms in vivo. Here, we review recent findings on the impact of aging on neuronal integrity and on neuronal macroautophagy, as they emerge from studies in invertebrate and mammalian models.

Increased mitochondrial calcium uptake and concomitant mitochondrial activity by presenilin loss promotes mTORC1 signaling to drive neurodegeneration

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.

Comparative toxicities of BPA, BPS, BPF, and TMBPF in the nematode Caenorhabditis elegans and mammalian fibroblast cells

Bisphenol A (BPA) is a chemical compound commonly used in the production of plastics for daily lives and industry. As BPA is well known for its adverse health effects, several alternative materials have been developed. This study comprehensively analyzed the toxicity of BPA and its three substitutes including bisphenol S (BPS), bisphenol F (BPF), and tetramethyl bisphenol F (TMBPF) on aging, healthspan, and mitochondria using an in vivo Caenorhabditis elegans (C. elegans) model animal and cultured mammalian fibroblast cells. C. elegans treated with 1 mM BPA exhibited abnormalities in the four tested parameters related to development and growth, including delayed development, decreased body growth, reduced reproduction, and abnormal tissue morphology. Exposure to the same concentration of each alternative including TMBPF, which has been proposed as a relatively safe BPA alternative, detrimentally affected at least three of these events. Moreover, all bisphenols (except BPS) remarkably shortened the organismal lifespan and increased age-related changes in neurons. Exposure to BPA and BPF resulted in mitochondrial abnormalities, such as reduced oxygen consumption and mitochondrial membrane potential. In contrast, the ATP levels were noticeably higher after treatment with all bisphenols. In mammalian fibroblast cells, exposure to increasing concentrations of all bisphenols (ranging from 50 μM to 500 μM) caused a severe decrease in cell viability in a dose-dependent manner. BPA increased ATP levels and decreased ROS but did not affect mitochondrial permeability transition pores (mPTP). Notably, TMBPF was the only bisphenol that caused a significant increase in mitochondrial ROS and mPTP opening. These results suggest that the potentially harmful physiological effects of BPA alternatives should be considered.

An economical and highly adaptable optogenetics system for individual and population-level manipulation of Caenorhabditis elegans

Background

Optogenetics allows the experimental manipulation of excitable cells by a light stimulus without the need for technically challenging and invasive procedures. The high degree of spatial, temporal, and intensity control that can be achieved with a light stimulus, combined with cell type-specific expression of light-sensitive ion channels, enables highly specific and precise stimulation of excitable cells. Optogenetic tools have therefore revolutionized the study of neuronal circuits in a number of models, including Caenorhabditis elegans. Despite the existence of several optogenetic systems that allow spatial and temporal photoactivation of light-sensitive actuators in C. elegans, their high costs and low flexibility have limited wide access to optogenetics. Here, we developed an inexpensive, easy-to-build, modular, and adjustable optogenetics device for use on different microscopes and worm trackers, which we called the OptoArm.

Results

The OptoArm allows for single- and multiple-worm illumination and is adaptable in terms of light intensity, lighting profiles, and light color. We demonstrate OptoArm’s power in a population-based multi-parameter study on the contributions of motor circuit cells to age-related motility decline. We found that individual components of the neuromuscular system display different rates of age-dependent deterioration. The functional decline of cholinergic neurons mirrors motor decline, while GABAergic neurons and muscle cells are relatively age-resilient, suggesting that rate-limiting cells exist and determine neuronal circuit ageing.

Conclusion

We have assembled an economical, reliable, and highly adaptable optogenetics system which can be deployed to address diverse biological questions. We provide a detailed description of the construction as well as technical and biological validation of our set-up. Importantly, use of the OptoArm is not limited to C. elegans and may benefit studies in multiple model organisms, making optogenetics more accessible to the broader research community.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01085-2.

Phosphorylation of GAP-43 T172 is a molecular marker of growing axons in a wide range of mammals including primates

GAP-43 is a vertebrate neuron-specific protein and that is strongly related to axon growth and regeneration; thus, this protein has been utilized as a classical molecular marker of these events and growth cones. Although GAP-43 was biochemically characterized more than a quarter century ago, how this protein is related to these events is still not clear. Recently, we identified many phosphorylation sites in the growth cone membrane proteins of rodent brains. Two phosphorylation sites of GAP-43, S96 and T172, were found within the top 10 hit sites among all proteins. S96 has already been characterized (Kawasaki et al., 2018), and here, phosphorylation of T172 was characterized. In vitro (cultured neurons) and in vivo, an antibody specific to phosphorylated T172 (pT172 antibody) specifically recognized cultured growth cones and growing axons in developing mouse neurons, respectively. Immunoblotting showed that pT172 antigens were more rapidly downregulated throughout development than those of pS96 antibody. From the primary structure, this phosphorylation site was predicted to be conserved in a wide range of animals including primates. In the developing marmoset brainstem and in differentiated neurons derived from human induced pluripotent stem cells, immunoreactivity with pT172 antibody revealed patterns similar to those in mice. pT172 antibody also labeled regenerating axons following sciatic nerve injury. Taken together, the T172 residue is widely conserved in a wide range of mammals including primates, and pT172 is a new candidate molecular marker for growing axons.

Multiple genetic pathways regulating lifespan extension are neuroprotective in a G2019S LRRK2 nematode model of Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most frequent cause of late-onset, familial Parkinson's disease (PD), and LRRK2 variants are associated with increased risk for sporadic PD. While advanced age represents the strongest risk factor for disease development, it remains unclear how different age-related pathways interact to regulate LRRK2-driven late-onset PD. In this study, we employ a C. elegans model expressing PD-linked G2019S LRRK2 to examine the interplay between age-related pathways and LRRK2-induced dopaminergic neurodegeneration. We find that multiple genetic pathways that regulate lifespan extension can provide robust neuroprotection against mutant LRRK2. However, the level of neuroprotection does not strictly correlate with the magnitude of lifespan extension, suggesting that lifespan can be experimentally dissociated from neuroprotection. Using tissue-specific RNAi, we demonstrate that lifespan-regulating pathways, including insulin/insulin-like growth factor-1 (IGF-1) signaling, target of rapamycin (TOR), and mitochondrial respiration, can be directly manipulated in neurons to mediate neuroprotection. We extend this finding for AGE-1/PI3K, where pan-neuronal versus dopaminergic neuronal restoration of AGE-1 reveals both cell-autonomous and non-cell-autonomous neuroprotective mechanisms downstream of insulin signaling. Our data demonstrate the importance of distinct lifespan-regulating pathways in the pathogenesis of LRRK2-linked PD, and suggest that extended longevity is broadly neuroprotective via the actions of these pathways at least in part within neurons. This study further highlights the complex interplay that occurs between cells and tissues during organismal aging and disease manifestation.

Structural aspects of the aging invertebrate brain

Aging is characterized by a decline in neuronal function in all animal species investigated so far. Functional changes are accompanied by and may be in part caused by, structurally visible degenerative changes in neurons. In the mammalian brain, normal aging shows abnormalities in dendrites and axons, as well as ultrastructural changes in synapses, rather than global neuron loss. The analysis of the structural features of aging neurons, as well as their causal link to molecular mechanisms on the one hand, and the functional decline on the other hand is crucial in order to understand the aging process in the brain. Invertebrate model organisms like Drosophila and C. elegans offer the opportunity to apply a forward genetic approach to the analysis of aging. In the present review, we aim to summarize findings concerning abnormalities in morphology and ultrastructure in invertebrate brains during normal aging and compare them to what is known for the mammalian brain. It becomes clear that despite of their considerably shorter life span, invertebrates display several age-related changes very similar to the mammalian condition, including the retraction of dendritic and axonal branches at specific locations, changes in synaptic density and increased accumulation of presynaptic protein complexes. We anticipate that continued research efforts in invertebrate systems will significantly contribute to reveal (and possibly manipulate) the molecular/cellular pathways leading to neuronal aging in the mammalian brain.

Programmed cell fusion in development and homeostasis

During multicellular organism development, complex structures are sculpted to form organs and tissues, which are maintained throughout adulthood. Many of these processes require cells to fuse with one another, or with themselves. These plasma membrane fusions merge endoplasmic cellular content across external, exoplasmic, space. In the nematode Caenorhabditis elegans, such cell fusions serve as a unique sculpting force, involved in the embryonic morphogenesis of the skin-like multinuclear hypodermal cells, but also in refining delicate structures, such as valve openings and the tip of the tail. During post-embryonic development, plasma membrane fusions continue to shape complex neuron structures and organs such as the vulva, while during adulthood fusion participates in cell and tissue repair. These processes rely on two fusion proteins (fusogens): EFF-1 and AFF-1, which are part of a broader family of structurally related membrane fusion proteins, encompassing sexual reproduction, viral infection, and tissue remodeling. The established capabilities of these exoplasmic fusogens are further expanded by new findings involving EFF-1 and AFF-1 in endocytic vesicle fission and phagosome sealing. Tight regulation by cell-autonomous and non-cell autonomous mechanisms orchestrates these diverse cell fusions at the correct place and time-these processes and their significance are discussed in this review.

FOXO Regulates Neuromuscular Junction Homeostasis During Drosophila Aging

The transcription factor foxo is a known regulator of lifespan extension and tissue homeostasis. It has been linked to the maintenance of neuronal processes across many species and has been shown to promote youthful characteristics by regulating cytoskeletal flexibility and synaptic plasticity at the neuromuscular junction (NMJ). However, the role of foxo in aging neuromuscular junction function has yet to be determined. We profiled adult Drosophila foxo- null mutant abdominal ventral longitudinal muscles and found that young mutants exhibited morphological profiles similar to those of aged wild-type flies, such as larger bouton areas and shorter terminal branches. We also observed changes to the axonal cytoskeleton and an accumulation of late endosomes in foxo null mutants and motor neuron-specific foxo knockdown flies, similar to those of aged wild-types. Motor neuron-specific overexpression of foxo can delay age-dependent changes to NMJ morphology, suggesting foxo is responsible for maintaining NMJ integrity during aging. Through genetic screening, we identify several downstream factors mediated through foxo-regulated NMJ homeostasis, including genes involved in the MAPK pathway. Interestingly, the phosphorylation of p38 was increased in the motor neuron-specific foxo knockdown flies, suggesting foxo acts as a suppressor of p38/MAPK activation. Our work reveals that foxo is a key regulator for NMJ homeostasis, and it may maintain NMJ integrity by repressing MAPK signaling.

Flavonoids mitigate neurodegeneration in aged Caenorhabditis elegans by mitochondrial uncoupling

Dietary supplementation of flavonoids has been shown to reduce the severity of neurodegenerative disorders such as dementia, Parkinson's disease, and Alzheimer's disease by their antioxidant effects. However, their low bioavailabilityin vivo raises the question of how much their antioxidant capacity actually contributes to the mitigating effects. The physicochemical properties of flavonoids suggest they could function as mitochondrial uncouplers. Moreover, mitochondrial uncoupling alleviated neurodegeneration in Caenorhabditis elegans during aging in previous research. Therefore, we investigated whether various flavonoids (fisetin, quercetin, apigenin, chrysin, catechin, and naringenin) could reduce neuronal defects by mitochondrial uncoupling in C. elegans. Both neuronal defects and mitochondrial membrane potential were reduced in aged worms in nearly all of the flavonoid treatments suggesting that flavonoids may reduce neurodegeneration in C. elegans. However, there was no significant reduction of neuronal defects in mitophagy-deficient pink-1/pdr-1 double mutants under flavonoid treatments. These results suggest that flavonoids could function as mitochondrial uncouplers to mitigate neurodegeneration in aged C. elegans, possibly via a PINK1/Parkin mitophagy process.

A C. elegans model for neurodegeneration in Cockayne syndrome

Cockayne syndrome (CS) is a congenital syndrome characterized by growth and mental retardation, and premature ageing. The complexity of CS and mammalian models warrants simpler metazoan models that display CS-like phenotypes that could be studied in the context of a live organism. Here, we provide a characterization of neuronal and mitochondrial aberrations caused by a mutation in the csb-1 gene in Caenorhabditis elegans. We report a progressive neurodegeneration in adult animals that is enhanced upon UV-induced DNA damage. The csb-1 mutants show dysfunctional hyperfused mitochondria that degrade upon DNA damage, resulting in diminished respiratory activity. Our data support the role of endogenous DNA damage as a driving factor of CS-related neuropathology and underline the role of mitochondrial dysfunction in the disease.

Deep learning-enabled analysis reveals distinct neuronal phenotypes induced by aging and cold-shock

BACKGROUND

Access to quantitative information is crucial to obtain a deeper understanding of biological systems. In addition to being low-throughput, traditional image-based analysis is mostly limited to error-prone qualitative or semi-quantitative assessment of phenotypes, particularly for complex subcellular morphologies. The PVD neuron in Caenorhabditis elegans, which is responsible for harsh touch and thermosensation, undergoes structural degeneration as nematodes age characterized by the appearance of dendritic protrusions. Analysis of these neurodegenerative patterns is labor-intensive and limited to qualitative assessment.

RESULTS

In this work, we apply deep learning to perform quantitative image-based analysis of complex neurodegeneration patterns exhibited by the PVD neuron in C. elegans. We apply a convolutional neural network algorithm (Mask R-CNN) to identify neurodegenerative subcellular protrusions that appear after cold-shock or as a result of aging. A multiparametric phenotypic profile captures the unique morphological changes induced by each perturbation. We identify that acute cold-shock-induced neurodegeneration is reversible and depends on rearing temperature and, importantly, that aging and cold-shock induce distinct neuronal beading patterns.

CONCLUSION

The results of this work indicate that implementing deep learning for challenging image segmentation of PVD neurodegeneration enables quantitatively tracking subtle morphological changes in an unbiased manner. This analysis revealed that distinct patterns of morphological alteration are induced by aging and cold-shock, suggesting different mechanisms at play. This approach can be used to identify the molecular components involved in orchestrating neurodegeneration and to characterize the effect of other stressors on PVD degeneration.

Short-term enhancement of motor neuron synaptic exocytosis during early aging extends lifespan in Caenorhabditis elegans

IMPACT STATEMENT

The functional decline of motor activity is a common feature in almost all aging animals that leads to frailty, loss of independence, injury, and even death in the elderly population. Thus, understanding the molecular mechanism that drives the initial stage of this functional decline and developing strategies to increase human healthspan and even lifespan by targeting this process would be of great interests to the field. In this study, we found that by precisely targeting the motor neurons to potentiate its synaptic releases either genetically or pharmacologically, we can not only delay the functional aging at NMJs but also slow the rate of aging at the organismal level. Most importantly, we have demonstrated that a critical window of time, that is the early stage of NMJs functional decline, is required for the beneficial effects. A short-term treatment within this time period is sufficient to extend the animals' lifespan.

Neuronal Signaling Involved in Neuronal Polarization and Growth: Lipid Rafts and Phosphorylation

Neuronal polarization and growth are developmental processes that occur during neuronal cell differentiation. The molecular signaling mechanisms involved in these events in in vivo mammalian brain remain unclear. Also, cellular events of the neuronal polarization process within a given neuron are thought to be constituted of many independent intracellular signal transduction pathways (the "tug-of-war" model). However, in vivo results suggest that such pathways should be cooperative with one another among a given group of neurons in a region of the brain. Lipid rafts, specific membrane domains with low fluidity, are candidates for the hotspots of such intracellular signaling. Among the signals reported to be involved in polarization, a number are thought to be present or translocated to the lipid rafts in response to extracellular signals. As part of our analysis, we discuss how such novel molecular mechanisms are combined for effective regulation of neuronal polarization and growth, focusing on the significance of the lipid rafts, including results based on recently introduced methods.

Age-dependent changes in response property and morphology of a thermosensory neuron and thermotaxis behavior in Caenorhabditis elegans

Age‐dependent cognitive and behavioral deterioration may arise from defects in different components of the nervous system, including those of neurons, synapses, glial cells, or a combination of them. We find that AFD, the primary thermosensory neuron of Caenorhabditis elegans, in aged animals is characterized by loss of sensory ending integrity, including reduced actin‐based microvilli abundance and aggregation of thermosensory guanylyl cyclases. At the functional level, AFD neurons in aged animals are hypersensitive to high temperatures and show sustained sensory‐evoked calcium dynamics, resulting in a prolonged operating range. At the behavioral level, senescent animals display cryophilic behaviors that remain plastic to acute temperature changes. Excessive cyclase activity of the AFD‐specific guanylyl cyclase, GCY‐8, is associated with developmental defects in AFD sensory ending and cryophilic behavior. Surprisingly, loss of the GCY‐8 cyclase domain reduces these age‐dependent morphological and behavioral changes, while a prolonged AFD operating range still exists in gcy‐8 animals. The lack of apparent correlation between age‐dependent changes in the morphology or stimuli‐evoked response properties of primary sensory neurons and those in related behaviors highlights the importance of quantitative analyses of aging features when interpreting age‐related changes at structural and functional levels. Our work identifies aging hallmarks in AFD receptive ending, temperature‐evoked AFD responses, and experience‐based thermotaxis behavior, which serve as a foundation to further elucidate the neural basis of cognitive aging.

A hormone receptor pathway cell-autonomously delays neuron morphological aging by suppressing endocytosis

Neurons have a lifespan that parallels that of the organism and are largely irreplaceable. Their unusually long lifespan predisposes neurons to neurodegenerative disease. We sought to identify physiological mechanisms that delay neuron aging in Caenorhabditis elegans by asking how neuron morphological aging is arrested in the long-lived, alternate organismal state, the dauer diapause. We find that a hormone signaling pathway, the abnormal DAuer Formation (DAF) 12 nuclear hormone receptor (NHR) pathway, functions cell-intrinsically in the dauer diapause to arrest neuron morphological aging, and that same pathway can be cell-autonomously manipulated during normal organismal aging to delay neuron morphological aging. This delayed aging is mediated by suppressing constitutive endocytosis, which alters the subcellular localization of the actin regulator T cell lymphoma Invasion And Metastasis 1 (TIAM-1), thereby decreasing age-dependent neurite growth. Intriguingly, we show that suppressed endocytosis appears to be a general feature of cells in diapause, suggestive that this may be a mechanism to halt the growth and other age-related programs supported by most endosome recycling.

Can Proprioceptive Training Reduce Muscle Fatigue in Patients With Motor Neuron Diseases? A New Direction of Treatment

Muscle fatigue is a serious problem in patients with motor neuron diseases (MNDs). It commonly disturbs both daily life activity and rehabilitation tolerance. A particular concern should be taken when MNDs occur in older ages. Older patients with MNDs usually have a worse clinical presentation and a lower survival rate. This could increase the occurrence of muscle fatigue. Muscle fatigue occurs due to a dysfunction in either motor or sensory systems. Current exercise interventions performed to decrease the occurrence of muscle fatigue focused only on treating motor causes of muscle fatigue. It has been demonstrated that these interventions have a high debate in their effectiveness on decreasing the occurrence of muscle fatigue. Also, these exercise interventions ignored training the affected sensory part of muscle fatigue, however, the important role of the sensory system in driving the motor system. Thus, this review aimed to develop a novel exercise intervention by using proprioceptive training as an intervention to decrease the occurrence of muscle fatigue in patients with MNDs particularly, older ones. The physiological effects of proprioceptive training to decrease the occurrence of muscle fatigue could include two effects. The first effect includes the ability of the proprioceptive training to increase the sensitivity of muscle spindles as an attempt to normalize the firing rate of α-motoneurons, which their abnormalities have major roles in the occurrence of muscle fatigue. The second effect includes its ability to correct the abnormal movement-compensations, which develop due to the biomechanical constraints imposed on patients with MNDs.

In-Vivo Quantitative Image Analysis of Age-Related Morphological Changes of C. elegans Neurons Reveals a Correlation between Neurite Bending and Novel Neurite Outgrowths

The aging of the human brain in the absence of diseases is accompanied by subtle changes of neuronal morphology, such as dendrite restructuring, neuronal sprouting, and synaptic deteriorations, rather than neurodegeneration or gross deterioration. Similarly, the nervous system of Caenorhabditis elegans does not show neurodegeneration or gross deterioration during normal aging, but displays subtle alterations in neuronal morphology. The occurrence of these age-dependent abnormalities is stochastic and dynamic, which poses a major challenge to fully capture them for quantitative comparison. Here, we developed a semi-automated pipeline for quantitative image analysis of these features during aging. We employed and evaluated this pipeline herein to reproduce findings from previous studies using visual inspection of neuronal morphology. Importantly, our approach can also quantify additional features, such as soma volume, the length of neurite outgrowths, and their location along the aged neuron. We found that, during aging, the soma of neurons decreases in volume, whereas the number and length of neurite outgrowths from the soma both increase. Long-lived animals showed less decrease in soma volume, fewer and shorter neurite outgrowths, and protection against abnormal sharp bends preferentially localized at the distal part of the dendrites during aging. We found a correlation of sharp bends with neurite outgrowth, suggesting the hypothesis that sharp bends might proceed neurite outgrowths. Thus, our semi-automated pipeline can help researchers to obtain and analyze quantitative datasets of this stochastic process for comparison across genotypes and to identify correlations to facilitate the generation of novel hypothesis.

Lessons from Worm Dendritic Patterning

The structural and functional properties of neurons have intrigued scientists since the pioneering work of Santiago Ramón y Cajal. Since then, emerging cutting-edge technologies, including light and electron microscopy, electrophysiology, biochemistry, optogenetics, and molecular biology, have dramatically increased our understanding of dendritic properties. This advancement was also facilitated by the establishment of different animal model organisms, from flies to mammals. Here we describe the emerging model system of a Caenorhabditis elegans polymodal neuron named PVD, whose dendritic tree follows a stereotypical structure characterized by repeating candelabra-like structural units. In the past decade, progress has been made in understanding PVD's functions, morphogenesis, regeneration, and aging, yet many questions still remain.

Age-dependent changes and biomarkers of aging in Caenorhabditis elegans

Caenorhabditis elegans is an exceptionally valuable model for aging research because of many advantages, including its genetic tractability, short lifespan, and clear age‐dependent physiological changes. Aged C. elegans display a decline in their anatomical and functional features, including tissue integrity, motility, learning and memory, and immunity. Caenorhabditis elegans also exhibit many age‐associated changes in the expression of microRNAs and stress‐responsive genes and in RNA and protein quality control systems. Many of these age‐associated changes provide information on the health of the animals and serve as valuable biomarkers for aging research. Here, we review the age‐dependent changes in C. elegans and their utility as aging biomarkers indicative of the physiological status of aging.

Microtubule regulators act in the nervous system to modulate fat metabolism and longevity through DAF-16 in C. elegans

Microtubule (MT) regulation is involved in both neuronal function and the maintenance of neuronal structure, and MT dysregulation appears to be a general downstream indicator and effector of age‐related neurodegeneration. But the role of MTs in natural aging is largely unknown. Here, we demonstrate a role of MT regulators in regulating longevity. We find that loss of EFA‐6, a modulator of MT dynamics, can delay both neuronal aging and extend the lifespan of C. elegans. Through the use of genetic mutants affecting other MT‐regulating genes in C. elegans, we find that loss of MT stabilizing genes (including ptrn‐1 and ptl‐1) shortens lifespan, while loss of MT destabilizing gene hdac‐6 extends lifespan. Via the use of tissue‐specific transgenes, we further show that these MT regulators can act in the nervous system to modulate lifespan. Through RNA‐seq analyses, we found that genes involved in lipid metabolism were differentially expressed in MT regulator mutants, and via the use of Nile Red and Oil Red O staining, we show that the MT regulator mutants have altered fat storage. We further find that the increased fat storage and extended lifespan of the long‐lived MT regulator mutants are dependent on the DAF‐16/FOXO transcription factor. Our results suggest that neuronal MT status might affect organismal aging through DAF‐16‐regulated changes in fat metabolism, and therefore, MT‐based therapies might represent a novel intervention to promote healthy aging.

Convergence of longevity and immunity: lessons from animal models

An increasing amount of data implicate immunity-mostly innate immunity-in the ageing process; both during healthy ageing as well as in neurodegenerative diseases. Despite the aetiology however, the underlying mechanisms are poorly understood. Here we review what we know from model organisms (worms, flies and mice) on the possible mechanistic details that connect immunity and ageing. These links provide evidence that inter-tissue communication (especially the interaction between gut and brain), hormonal control mechanisms and intestinal microbiota determine immune system activity and thus influence lifespan.

DIP-2 suppresses ectopic neurite sprouting and axonal regeneration in mature neurons

Neuronal morphology and circuitry established during early development must often be maintained over the entirety of animal lifespans. Compared with neuronal development, the mechanisms that maintain mature neuronal structures and architecture are little understood. The conserved disco-interacting protein 2 (DIP2) consists of a DMAP1-binding domain and two adenylate-forming domains (AFDs). We show that the Caenorhabditis elegans DIP-2 maintains morphology of mature neurons. dip-2 loss-of-function mutants display a progressive increase in ectopic neurite sprouting and branching during late larval and adult life. In adults, dip-2 also inhibits initial stages of axon regeneration cell autonomously and acts in parallel to DLK-1 MAP kinase and EFA-6 pathways. The function of DIP-2 in maintenance of neuron morphology and in axon regrowth requires its AFD domains and is independent of its DMAP1-binding domain. Our findings reveal a new conserved regulator of neuronal morphology maintenance and axon regrowth after injury.

Genetic and pharmacological interventions in the aging motor nervous system slow motor aging and extend life span in C. elegans

As animals and humans age, the motor system undergoes a progressive functional decline, leading to frailty. Age-dependent functional deteriorations at neuromuscular junctions (NMJs) contribute to this motor aging. However, it is unclear whether one can intervene in this process to slow motor aging. The Caenorhabditis elegans BK channel SLO-1 dampens synaptic transmission at NMJs by repressing synaptic release from motor neurons. Here, we show that genetic ablation of SLO-1 not only reduces the rate of age-dependent motor activity decline to slow motor aging but also surprisingly extends life span. SLO-1 acts in motor neurons to mediate both functions. Genetic knockdown or pharmacological inhibition of SLO-1 in aged, but not young, worms can slow motor aging and prolong longevity. Our results demonstrate that genetic and pharmacological interventions in the aging motor nervous system can promote both health span and life span.

Keeping Neurons Young and Foxy: FoxOs Promote Neuronal Plasticity

Any adult who has tried to take up the piano or learn a new language is faced with the sobering realization that acquiring such skills is more challenging as an adult than as a child. Neuronal plasticity, or the malleability of brain circuits, declines with age. Young neurons tend to be more adaptable and can alter the size and strength of their connections more readily than can old neurons. Myriad circuit- and synapse-level mechanisms that shape plasticity have been identified. Yet, molecular mechanisms setting the overall competence of young neurons for distinct forms of plasticity remain largely obscure. Recent studies indicate evolutionarily conserved roles for FoxO proteins in establishing the capacity for cell-fate, morphological, and synaptic plasticity in neurons.

A microfluidic platform for lifelong high-resolution and high throughput imaging of subtle aging phenotypes in C. elegans

Aging produces a number of changes in the neuronal structure and function throughout a variety of organisms. These aging-induced changes encompass a wide range of phenotypes, from loss of locomotion ability to defective production of synaptic vesicles. C. elegans is one of the primary systems used to elucidate phenotypes associated with aging processes. Conventional aging studies in C. elegans are typically labor-intensive, low-throughput, and incorporate fluorodeoxyuridine (FUdR) as a sterilizing agent to keep the population age-synchronized throughout the assay. However, FUdR exposure induces lifespan extension, and can potentially mask the phenotypes associated with the natural aging process. In addition, studying cellular or subcellular structures requires anesthetics or adhesives to immobilize nematodes while acquiring high-resolution images. In this platform, we are able to maintain a population (∼1000 worms) age-synchronized throughout its lifespan and perform a series of high-resolution microscopy studies in a drug-free environment. The device is composed of two main interconnected sections, one with the purpose of filtering progeny while keeping the parent population intact, and one for trapping nematodes in individual compartments for microscopy. Immobilization is carried out by decreasing the temperature of the device where nematodes are trapped by placing a heat sink on top of the chip. We were able to perform periodic high-resolution microscopy of fluorescently tagged synapses located at the dorsal side of the nematode's tail throughout the worms' lifespan. To characterize the subtle phenotypes that emerge as nematodes age, computer vision was implemented to perform automated unbiased detection of synapses and quantitative analysis of aging-induced synaptic changes.

Role of Presenilin in Mitochondrial Oxidative Stress and Neurodegeneration in Caenorhabditis elegans

Neurodegenerative diseases like Alzheimer’s disease (AD) are poised to become a global health crisis, and therefore understanding the mechanisms underlying the pathogenesis is critical for the development of therapeutic strategies. Mutations in genes encoding presenilin (PSEN) occur in most familial Alzheimer’s disease but the role of PSEN in AD is not fully understood. In this review, the potential modes of pathogenesis of AD are discussed, focusing on calcium homeostasis and mitochondrial function. Moreover, research using Caenorhabditis elegans to explore the effects of calcium dysregulation due to presenilin mutations on mitochondrial function, oxidative stress and neurodegeneration is explored.

Cellular clearance of circulating transthyretin decreases cell-nonautonomous proteotoxicity in Caenorhabditis elegans

Cell-autonomous and cell-nonautonomous mechanisms of neurodegeneration appear to occur in the proteinopathies, including Alzheimer's and Parkinson's diseases. However, how neuronal toxicity is generated from misfolding-prone proteins secreted by nonneuronal tissues and whether modulating protein aggregate levels at distal locales affects the degeneration of postmitotic neurons remains unknown. We generated and characterized animal models of the transthyretin (TTR) amyloidoses that faithfully recapitulate cell-nonautonomous neuronal proteotoxicity by expressing human TTR in the Caenorhabditis elegans muscle. We identified sensory neurons with affected morphological and behavioral nociception-sensing impairments. Nonnative TTR oligomer load and neurotoxicity increased following inhibition of TTR degradation in distal macrophage-like nonaffected cells. Moreover, reducing TTR levels by RNAi or by kinetically stabilizing natively folded TTR pharmacologically decreased TTR aggregate load and attenuated neuronal dysfunction. These findings reveal a critical role for in trans modulation of aggregation-prone degradation that directly affects postmitotic tissue degeneration observed in the proteinopathies.

Presenilin mutations deregulate mitochondrial Ca2+ homeostasis and metabolic activity causing neurodegeneration in Caenorhabditis elegans

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.

Alzheimer's disease is the most common type of dementia. A hallmark of this condition is progressive loss of memory, accompanied by a buildup of hard clumps of protein between the brain cells. These protein clumps, known as amyloid plaques, are a key focus of research into Alzheimer's disease. They are likely to be toxic to brain cells, but their role in the development and progression of the disease is not yet known.

Though the cause of Alzheimer's disease remains unclear, an inherited form of the disease may hold some clues. Mutations in genes for proteins called presenilins cause an earlier onset form of Alzheimer's disease, in which symptoms can develop in people who are in their 40s or 50s. The presenilin proteins appear in a cell structure called the endoplasmic reticulum, which plays many roles in the normal activities of a cell. Among other things, this structure stores and releases calcium ions, and cells use these ions to send and process many signals.

The cell's energy-producing powerhouses, the mitochondria, use calcium to boost their metabolic activity. This allows them to make more energy for the cell, but in the process they also make damaging byproducts. These byproducts include oxygen-containing chemicals, known as reactive oxygen species (ROS), which react strongly with other molecules. While low levels of ROS are a normal part of cell activity, if the levels get too high, these chemicals can attack and damage structures within the cell.

Untangling the effects of amyloid plaques and presenilins on brain cells in humans is challenging. But, a nematode worm called Caenorhabditis elegans does not form plaques, making it possible to look at presenilins on their own. Previous work in these worms has shown that presenilin mutations affect the endoplasmic reticulum and change the appearance of mitochondria. Here, Sarasija et al. extend this work to find out more about the effects presenilin mutations have on living cells.

Presenilin mutations in young adult worms increased the amount of calcium released by the endoplasmic reticulum. This increased the activity of the mitochondria and caused ROS levels to rise to damaging levels. This caused stress inside the cells, and the worms started to show early signs damage to their nervous systems. Mutations that decreased the movement of calcium from the endoplasmic reticulum to the mitochondria helped to prevent the damage. Treating the mitochondria with antioxidants to mop up the extra ROS also protected the cells.

This kind of damage to brain cells did not depend on amyloid plaques. Whilst the plaques are likely to be toxic, these new findings highlights the role that other chemical and biological processes might play in Alzheimer's disease. Further work to reveal the underlying cause of Alzheimer's disease may lead to new therapies to treat this condition in the future.