Emergence of Caenorhabditis elegans as a Model Organism for Dissecting the Gut-Brain Axis

Ageing, proteostasis, and the gut: Insights into neurological health and disease

Recent research has illuminated the profound bidirectional communication between the gastrointestinal tract and the brain, furthering our understanding of neurological ailments facilitating possible therapeutic strategies. Technological advancements in high-throughput sequencing and multi-omics have unveiled significant alterations in gut microbiota and their metabolites in various neurological disorders. This review provides a thorough analysis of the role of microbiome-gut-brain axis in neurodegenerative disease pathology, linking it to reduced age-associated proteostasis. We discuss evidences that substantiate the existence of a gut-brain cross talk ranging from early clinical accounts of James Parkinson to Braak's hypothesis. In addition to understanding of microbes, the review particularly entails specific metabolites which are altered in neurodegenerative diseases. The regulatory effects of microbial metabolites on protein clearance mechanisms, proposing their potential therapeutic implications, are also discussed. By integrating this information, we advocate for a combinatory therapeutic strategy that targets early intervention, aiming to restore proteostasis and ameliorate disease progression. This approach not only provides a new perspective on the pathogenesis of neurodegenerative diseases but also highlights innovative strategies to combat the increasing burden of these age-related disorders.

Loss of cilia in chemosensory neurons inhibits pathogen avoidance in Caenorhabditis elegans

Pathogen avoidance is a crucial and evolutionarily conserved behavior that enhances survival by preventing infection in diverse species, including Caenorhabditis elegans (C. elegans). This behavior relies on multiple chemosensory neurons equipped with cilia that are exposed to the external environment. However, the specific role of neuronal cilia in pathogen avoidance has not been completely elucidated. Herein, we discovered that osm-3(p802) mutants, which lack chemosensory neuronal cilia, exhibit slower avoidance of the pathogen Pseudomonas aeruginosa PA14, but not Escherichia coli OP50. This observation was consistent when osm-3(p802) mutants were exposed to P. aeruginosa PAO1. Following an encounter with PA14, the pumping, thrashing, and defecation behaviors of osm-3 mutants were comparable to those of the wild-type. However, the osm-3 mutants demonstrated reduced intestinal colonization of PA14, suggesting that they have stronger intestinal clearance ability. We conducted RNA-seq to identify genes responding to external stimuli that were differentially expressed owing to the loss of osm-3 and PA14 infection. Using RNAi, we demonstrated that three of these genes were essential for normal pathogen avoidance. In conclusion, our findings demonstrate that the loss of chemosensory neuronal cilia reduces pathogen avoidance in C. elegans while delaying intestinal colonization.

Utility of zebrafish-based models in understanding molecular mechanisms of neurotoxicity mediated by the gut-brain axis

The gut microbes perform several beneficial functions which impact the periphery and central nervous systems of the host. Gut microbiota dysbiosis is acknowledged as a major contributor to the development of several neuropsychiatric and neurological disorders including bipolar disorder, depression, anxiety, Parkinson's disease, Alzheimer's disease, attention deficit hyperactivity disorder, and autism spectrum disorder. Thus, elucidation of how the gut microbiota-brain axis plays a role in health and disease conditions is a potential novel approach to prevent and treat brain disorders. The zebrafish (Danio rerio) is an invaluable vertebrate model that possesses conserved brain and intestinal features with those of humans, thus making zebrafish a valued model to investigate the interplay between the gut microbiota and host health. This chapter describes current findings on the utility of zebrafish in understanding molecular mechanisms of neurotoxicity mediated via the gut microbiota-brain axis. Specifically, it highlights the utility of zebrafish as a model organism for understanding how anthropogenic chemicals, pharmaceuticals and bacteria exposure affect animals and human health via the gut-brain axis.

How neurobehavior and brain development in alternative whole-organism models can contribute to prediction of developmental neurotoxicity

Developmental neurotoxicity (DNT) is not routinely evaluated in chemical risk assessment because current test paradigms for DNT require the use of mammalian models which are ethically controversial, expensive, and resource demanding. Consequently, efforts have focused on revolutionizing DNT testing through affordable novel alternative methods for risk assessment. The goal is to develop a DNT in vitro test battery amenable to high-throughput screening (HTS). Currently, the DNT in vitro test battery consists primarily of human cell-based assays because of their immediate relevance to human health. However, such cell-based assays alone are unable to capture the complexity of a developing nervous system. Whole organismal systems that qualify as 3 R (Replace, Reduce and Refine) models are urgently needed to complement cell-based DNT testing. These models can provide the necessary organismal context and be used to explore the impact of chemicals on brain function by linking molecular and/or cellular changes to behavioural readouts. The nematode Caenorhabditis elegans, the planarian Dugesia japonica, and embryos of the zebrafish Danio rerio are all suited to low-cost HTS and each has unique strengths for DNT testing. Here, we review the strengths and the complementarity of these organisms in a novel, integrative context and highlight how they can augment current cell-based assays for more comprehensive and robust DNT screening of chemicals. Considering the limitations of all in vitro test systems, we discuss how a smart combinatory use of these systems will contribute to a better human relevant risk assessment of chemicals that considers the complexity of the developing brain.

New methods to unveil host-microbe interaction mechanisms along the microbiota-gut-brain-axis

Links between the gut microbiota and human health have been supported throughout numerous studies, such as the development of neurological disease disorders. This link is referred to as the "microbiota-gut-brain axis" and is the focus of an emerging field of research. Microbial-derived metabolites and gut and neuro-immunological metabolites regulate this axis in health and many diseases. Indeed, assessing these signals, whether induced by microbial metabolites or neuro-immune mediators, could significantly increase our knowledge of the microbiota-gut-brain axis. However, this will require the development of appropriate techniques and potential models. Methods for studying the induced signals originating from the microbiota remain crucial in this field. This review discusses the methods and techniques available for studies of microbiota-gut-brain interactions. We highlight several much-debated elements of these methodologies, including the widely used in vivo and in vitro models, their implications, and perspectives in the field based on a systematic review of PubMed. Applications of various animal models (zebrafish, mouse, canine, rat, rabbit) to microbiota-gut-brain axis research with practical examples of in vitro methods and innovative approaches to studying gut-brain communications are highlighted. In particular, we extensively discuss the potential of "organ-on-a-chip" devices and their applications in this field. Overall, this review sheds light on the most widely used models and methods, guiding researchers in the rational choice of strategies for studies of microbiota-gut-brain interactions.

Human gut homeostasis and regeneration: the role of the gut microbiota and its metabolites

The healthy human gut is a balanced ecosystem where host cells and representatives of the gut microbiota interact and communicate in a bidirectional manner at the gut epithelium. As a result of these interactions, many local and systemic processes necessary for host functionality, and ultimately health, take place. Impairment of the integrity of the gut epithelium diminishes its ability to act as an effective gut barrier, can contribute to conditions associated to inflammation processes and can have other negative consequences. Pathogens and pathobionts have been linked with damage of the integrity of the gut epithelium, but other components of the gut microbiota and some of their metabolites can contribute to its repair and regeneration. Here, we review what is known about the effect of bacterial metabolites on the gut epithelium and, more specifically, on the regulation of repair by intestinal stem cells and the regulation of the immune system in the gut. Additionally, we explore the potential therapeutic use of targeted modulation of the gut microbiota to maintain and improve gut homeostasis as a mean to improve health outcomes.

3'sialyllactose and 6'sialyllactose enhance performance in endurance-type exercise through metabolic adaptation

Human milk oligosaccharides (HMOs) belong to a group of multifunctional glycans that are abundantly present in human breast milk. While health effects of neutral oligosaccharides have been investigated extensively, a lot remains unknown regarding health effects of acidic oligosaccharides, such as the two sialyllactoses (SLs), 3'sialyllactose (3'SL), and 6'sialyllactose (6'SL). We utilized Caenorhabditis elegans (C. elegans) to investigate the effects of SLs on exercise performance. Using swimming as an endurance-type exercise, we found that SLs decrease exhaustion, signifying an increase in endurance that is strongest for 6'SL. Through an unbiased metabolomics approach, we identified changes in energy metabolism that correlated with endurance performance. Further investigation suggested that these metabolic changes were related to adaptations of muscle mitochondria that facilitated a shift from beta oxidation to glycogenolysis during exercise. We found that the effect of SLs on endurance performance required AMPK- (aak-1/aak-2) and adenosine receptor (ador-1) signaling. We propose a model where SLs alter the metabolic status in the gut, causing a signal from the intestine to the nervous system toward muscle cells, where metabolic adaptation increases exercise performance. Together, our results underline the potential of SLs in exercise-associated health and contribute to our understanding of the molecular processes involved in nutritionally-induced health benefits.

Caenorhabditis elegans as an emerging model in food and nutrition research: importance of standardizing base diet

As a model organism that has helped revolutionize life sciences, Caenorhabditis elegans has been increasingly used in nutrition research. Here we explore the tradeoffs between pros and cons of its use as a dietary model based primarily on literature review from the past decade. We first provide an overview of its experimental strengths as an animal model, focusing on lifespan and healthspan, behavioral and physiological phenotypes, and conservation of key nutritional pathways. We then summarize recent advances of its use in nutritional studies, e.g. food preference and feeding behavior, sugar status and metabolic reprogramming, lifetime and transgenerational nutrition tracking, and diet-microbiota-host interactions, highlighting cutting-edge technologies originated from or developed in C. elegans. We further review current challenges of using C. elegans as a nutritional model, followed by in-depth discussions on potential solutions. In particular, growth scales and throughputs, food uptake mode, and axenic culture of C. elegans are appraised in the context of food research. We also provide perspectives for future development of chemically defined nematode food ("NemaFood") for C. elegans, which is now widely accepted as a versatile and affordable in vivo model and has begun to show transformative potential to pioneer nutrition science.

Prolonged Lifespan, Improved Perception, and Enhanced Host Defense of Caenorhabditis elegans by Lactococcus cremoris subsp. cremoris

Lactic acid bacteria are beneficial to Caenorhabditis elegans; however, bacteria acting as probiotics in nematodes may not necessarily have probiotic functions in humans. Lactococcus cremoris subsp. cremoris reportedly has probiotic functions in humans. Therefore, we determined whether the strain FC could exert probiotic effects in C. elegans in terms of improving host defenses and extending life span. Live FC successfully extended the life span and enhanced host defense compared to Escherichia coli OP50 (OP50), a standard food source for C. elegans. The FC-fed worms were tolerant to Salmonella enterica subsp. enterica serovar Enteritidis or Staphylococcus aureus infection and had better survival than the OP50-fed control worms. Further, the chemotaxis index, an indicator of perception ability, was more stable and significantly higher in FC-fed worms than in the control worms. The increase in autofluorescence from advanced glycation end products (AGEs) with aging was also ameliorated in FC-fed worms. FC showed beneficial effects in daf-16 and pmk-1 mutants, but not in skn-1 mutants. Since SKN-1 is the C. elegans ortholog of Nrf2, we measured the transcription of heme oxygenase-1 (HO-1), which is regulated by Nrf2, in murine macrophages and found that HO-1 mRNA expression was increased >5 times by inoculation with FC cells. Thus, FC could exert antisenescence effects via the SKN-1/Nrf2 pathway. This study showed for the first time that FC supported perceptive function and suppressed AGEs in nematodes as probiotic bacteria. Therefore, C. elegans can be an alternative model to screen for probiotic bacteria that can be used for antisenescence effects in humans.

IMPORTANCE Aging is one of our greatest challenges. The World Health Organization proposed that “active aging” might encourage people to continue to work according to their capacities and preferences as they grow old and would prevent or delay disabilities and chronic diseases that are costly to both individuals and the society, considering that disease prevention is more economical than treatment. Probiotic bacteria, such as lactobacilli, are live microorganisms that exert beneficial effects on human health when ingested in sufficient amounts and can promote longevity. The significance of this study is that it revealed the antisenescence and various beneficial effects of the representative probiotic bacterium Lactococcus cremoris subsp. cremoris strain FC exerted via the SKN-1/Nrf2 pathway in the nematode Caenorhabditis elegans.