Modelling Toxoplasma gondii infection in human cerebral organoids

Human Brain In Vitro Model for Pathogen Infection-Related Neurodegeneration Study

Recent advancements in stem cell biology and tissue engineering have revolutionized the field of neurodegeneration research by enabling the development of sophisticated in vitro human brain models. These models, including 2D monolayer cultures, 3D organoids, organ-on-chips, and bioengineered 3D tissue models, aim to recapitulate the cellular diversity, structural organization, and functional properties of the native human brain. This review highlights how these in vitro brain models have been used to investigate the effects of various pathogens, including viruses, bacteria, fungi, and parasites infection, particularly in the human brain cand their subsequent impacts on neurodegenerative diseases. Traditional studies have demonstrated the susceptibility of different 2D brain cell types to infection, elucidated the mechanisms underlying pathogen-induced neuroinflammation, and identified potential therapeutic targets. Therefore, current methodological improvement brought the technology of 3D models to overcome the challenges of 2D cells, such as the limited cellular diversity, incomplete microenvironment, and lack of morphological structures by highlighting the need for further technological advancements. This review underscored the significance of in vitro human brain cell from 2D monolayer to bioengineered 3D tissue model for elucidating the intricate dynamics for pathogen infection modeling. These in vitro human brain cell enabled researchers to unravel human specific mechanisms underlying various pathogen infections such as SARS-CoV-2 to alter blood-brain-barrier function and Toxoplasma gondii impacting neural cell morphology and its function. Ultimately, these in vitro human brain models hold promise as personalized platforms for development of drug compound, gene therapy, and vaccine. Overall, we discussed the recent progress in in vitro human brain models, their applications in studying pathogen infection-related neurodegeneration, and future directions.

The Promise and Potential of Brain Organoids

Brain organoids are 3D in vitro culture systems derived from human pluripotent stem cells that self-organize to model features of the (developing) human brain. This review examines the techniques behind organoid generation, their current and potential applications, and future directions for the field. Brain organoids possess complex architecture containing various neural cell types, synapses, and myelination. They have been utilized for toxicology testing, disease modeling, infection studies, personalized medicine, and gene-environment interaction studies. An emerging concept termed Organoid Intelligence (OI) combines organoids with artificial intelligence systems to generate learning and memory, with the goals of modeling cognition and enabling biological computing applications. Brain organoids allow neuroscience studies not previously achievable with traditional techniques, and have the potential to transform disease modeling, drug development, and the understanding of human brain development and disorders. The aspirational vision of OI parallels the origins of artificial intelligence, and efforts are underway to map a roadmap toward its realization. In summary, brain organoids constitute a disruptive technology that is rapidly advancing and gaining traction across multiple disciplines.

Protein Lactylation and Metabolic Regulation of the Zoonotic Parasite Toxoplasma gondii

The biology of Toxoplasma gondii, the causative pathogen of one of the most widespread parasitic diseases (toxoplasmosis), remains poorly understood. Lactate, which is derived from glucose metabolism, is not only an energy source in a variety of organisms, including T. gondii, but also a regulatory molecule that participates in gene activation and protein function. Lysine lactylation (Kla) is a type of post-translational modifications (PTMs) that has been recently associated with chromatin remodeling; however, Kla of histone and non-histone proteins has not yet been studied in T. gondii. To examine the prevalence and function of lactylation in T. gondii parasites, we mapped the lactylome of proliferating tachyzoite cells and identified 1964 Kla sites on 955 proteins in the T. gondii RH strain. Lactylated proteins were distributed in multiple subcellular compartments and were closely related to a wide variety of biological processes, including mRNA splicing, glycolysis, aminoacyl-tRNA biosynthesis, RNA transport, and many signaling pathways. We also performed a chromatin immunoprecipitation sequencing (ChIP-seq) analysis using a lactylation-specific antibody and found that the histones H4K12la and H3K14la were enriched in the promoter and exon regions of T. gondii associated with microtubule-based movement and cell invasion. We further confirmed the delactylase activity of histone deacetylases TgHDAC2-4, and found that treatment with anti-histone acetyltransferase (TgMYST-A) antibodies profoundly reduced protein lactylation in T. gondii. This study offers the first dataset of the global lactylation proteome and provides a basis for further dissecting the functional biology of T. gondii.

Leveraging iPSC technology to assess neuro-immune interactions in neurological and psychiatric disorders

Communication between the immune and the nervous system is essential for human brain development and homeostasis. Disruption of this intricately regulated crosstalk can lead to neurodevelopmental, psychiatric, or neurodegenerative disorders. While animal models have been essential in characterizing the role of neuroimmunity in development and disease, they come with inherent limitations due to species specific differences, particularly with regard to microglia, the major subset of brain resident immune cells. The advent of induced pluripotent stem cell (iPSC) technology now allows the development of clinically relevant models of the central nervous system that adequately reflect human genetic architecture. This article will review recent publications that have leveraged iPSC technology to assess neuro-immune interactions. First, we will discuss the role of environmental stressors such as neurotropic viruses or pro-inflammatory cytokines on neuronal and glial function. Next, we will review how iPSC models can be used to study genetic risk factors in neurological and psychiatric disorders. Lastly, we will evaluate current challenges and future potential for iPSC models in the field of neuroimmunity.

Organoids as a novel tool in modelling infectious diseases

Infectious diseases worldwide affect human health and have important societal impacts. A better understanding of infectious diseases is urgently needed. In vitro and in vivo infection models have brought notable contributions to the current knowledge of these diseases. Organoids are multicellular culture systems resembling tissue architecture and function, recapitulating many characteristics of human disease and elucidating mechanisms of host-infectious agent interactions in the respiratory and gastrointestinal systems, the central nervous system and the skin. Here, we discuss the applicability of the organoid technology for modeling pathogenesis, host response and features, which can be explored for the development of preventive and therapeutic treatments.

Modelling host- Trypanosoma brucei gambiense interactions in vitro using human induced pluripotent stem cell-derived cortical brain organoids

Background: Sleeping sickness is caused by the extracellular parasite Trypanosoma brucei and is associated with neuroinflammation and neuropsychiatric disorders, including disruption of sleep/wake patterns, and is now recognised as a circadian disorder. Sleeping sickness is traditionally studied using murine models of infection due to the lack of alternative in vitro systems that fully recapitulate the cellular diversity and functionality of the human brain. The aim of this study is to develop a much-needed in vitro system that reduces and replaces live animals for the study of infections in the central nervous system, using sleeping sickness as a model infection. Methods: We developed a co-culture system using induced pluripotent stem cell (iPSC)-derived cortical human brain organoids and the human pathogen T. b. gambiense to model host-pathogen interactions in vitro. Upon co-culture, we analysed the transcriptional responses of the brain organoids to T. b. gambiense over two time points. Results: We detected broad transcriptional changes in brain organoids exposed to T. b. gambiense, mainly associated with innate immune responses, chemotaxis, and blood vessel differentiation compared to untreated organoids. Conclusions: Our co-culture system provides novel, more ethical avenues to study host-pathogen interactions in the brain as alternative models to experimental infections in mice. Although our data support the use of brain organoids to model host-pathogen interactions during T. brucei infection as an alternative to in vivo models, future work is required to increase the complexity of the organoids ( e.g., addition of microglia and vasculature). We envision that the adoption of organoid systems is beneficial to researchers studying mechanisms of brain infection by protozoan parasites. Furthermore, organoid systems have the potential to be used to study other parasites that affect the brain significantly reducing the number of animals undergoing moderate and/or severe protocols associated with the study of neuroinflammation and brain infections.

Human pluripotent stem cell (hPSC) and organoid models of autism: opportunities and limitations

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder caused by genetic or environmental perturbations during early development. Diagnoses are dependent on the identification of behavioral abnormalities that likely emerge well after the disorder is established, leaving critical developmental windows uncharacterized. This is further complicated by the incredible clinical and genetic heterogeneity of the disorder that is not captured in most mammalian models. In recent years, advancements in stem cell technology have created the opportunity to model ASD in a human context through the use of pluripotent stem cells (hPSCs), which can be used to generate 2D cellular models as well as 3D unguided- and region-specific neural organoids. These models produce profoundly intricate systems, capable of modeling the developing brain spatiotemporally to reproduce key developmental milestones throughout early development. When complemented with multi-omics, genome editing, and electrophysiology analysis, they can be used as a powerful tool to profile the neurobiological mechanisms underlying this complex disorder. In this review, we will explore the recent advancements in hPSC-based modeling, discuss present and future applications of the model to ASD research, and finally consider the limitations and future directions within the field to make this system more robust and broadly applicable.

Advances towards the complete in vitro life cycle of Toxoplasma gondii

The full life cycle of Toxoplasma gondii cannot be recapitulated in vitro, and access to certain stages, such as mature tissue cysts (bradyzoites) and oocysts (sporozoites), traditionally requires animal experiments. This has greatly hindered the study of the biology of these morphologically and metabolically distinct stages, which are essential for the infection of humans and animals. However, several breakthrough advances have been made in recent years towards obtaining these life stages in vitro, such as the discovery of several molecular factors that induce differentiation and commitment to the sexual cycle, and different culture methods that use, for example, myotubes and intestinal organoids to obtain mature bradyzoites and different sexual stages of the parasite. We review these novel tools and approaches, highlight their limitations and challenges, and discuss what research questions can already be answered with these models. We finally identify future routes for recapitulating the entire sexual cycle in vitro.

Cerebral toxoplasmosis in HIV-infected patients: a review

Toxoplasma gondii infection in the central nervous system commonly occurs among immunodeficient patients. Its prevalence is high in countries with a high burden of HIV and low coverage of antiretroviral drugs. The brain is one of the predilections for T. gondii infection due to its low inflammatory reaction, and cerebral toxoplasmosis occurs solely due to the reactivation of a latent infection rather than a new infection. Several immune elements have recently been recognized to have an essential role in the immunopathogenesis of cerebral toxoplasmosis. Although real-time isothermal amplification, next-generation sequencing, and enzyme-linked aptamer assays from blood samples have been the recommended diagnostic tools in some in-vivo studies, a combination of clinical symptoms, serology examination, and neuroimaging are still the daily standard for the presumptive diagnosis of cerebral toxoplasmosis and early anti-toxoplasma administration. Clinical trials are needed to find a new therapy that is less likely to affect folate synthesis, have neuroprotective properties, or cure the latent phase of infection. The development of a vaccine is being extensively tested in animals, but its efficacy and safety for humans are still not proven.

Use of in vitro derived human neuronal models to study host-parasite interactions of Toxoplasma gondii in neurons and neuropathogenesis of chronic toxoplasmosis

Toxoplasma gondii infects approximately one-third of the world's population resulting in a chronic infection with the parasite located in cysts in neurons in the brain. In most immunocompetent hosts the chronic infection is asymptomatic, but several studies have found correlations between Toxoplasma seropositivity and neuropsychiatric disorders, including Schizophrenia, and some other neurological disorders. Host-parasite interactions of bradyzoites in cysts in neurons is not well understood due in part to the lack of suitable in vitro human neuronal models. The advent of stem cell technologies in which human neurons can be derived in vitro from human induced pluripotent stem cells (hiPSCs) or direct conversion of somatic cells generating induced neurons (iNs), affords the opportunity to develop in vitro human neuronal culture systems to advance the understanding of T. gondii in human neurons. Human neurons derived from hiPSCs or iNs, generate pure human neuron monolayers that express differentiated neuronal characteristics. hiPSCs also generate 3D neuronal models that better recapitulate the cytoarchitecture of the human brain. In this review, an overview of iPSC-derived neurons and iN protocols leading to 2D human neuron cultures and hiPSC-derived 3D cerebral organoids will be given. The potential applications of these 2D and 3D human neuronal models to address questions about host-parasite interactions of T. gondii in neurons and the parasite in the CNS, will be discussed. These human neuronal in vitro models hold the promise to advance the understanding of T. gondii in human neurons and to improve the understanding of neuropathogenesis of chronic toxoplasmosis.

The imbalance in the relationship between inflammatory and regulatory cytokines during gestational toxoplasmosis can be harmful to fetuses: A systematic review

Objective

To evaluate the available information on inflammatory and regulatory plasma mediators in pregnant women (PW) diagnosed with toxoplasmosis. Source: The PubMed, Embase, Scopus, and Lilacs databases were evaluated until October 2022. Study eligibility criteria: This review was carried out following the PRISMA and registered on the PROSPERO platform (CRD42020203951). Studies that reported inflammatory mediators in PW with toxoplasmosis were considered.

Evaluation methods

After excluding duplicate articles, two authors independently carried out the process of title and abstract exclusion, and a third resolved disagreements when necessary. The full text was evaluated to detect related articles. The extraction table was built from the following data: Author, year of publication, journal name and impact factors, country, study design, number of gestations and maternal age (years), gestational period, diagnosis of toxoplasmosis, levels of inflammatory markers, laboratory tests, and clinical significance. Methodological quality was assessed using Joanna Briggs Institute tools.

Results

Of the 1,024 studies reported, only eight were included. Of the 868 PW included in this review, 20.2% were IgM+/IgG- and 50.8% were IgM-/IgG+ to T. gondii, and 29.0% uninfected. Infected PW presented higher plasma levels ofIL-5, IL-6, IL-8, IL-17, CCL5, and IL-10. Regarding the methodological quality, four studies obtained high quality. Data from this review pointed out the maintenance of the inflammatory pattern during pregnancy with a closely related to the parasite.

Conclusion

Immune status in PW defined the course of the T. gondii infection, where the equilibrium between inflammatory and regulatory cytokines mitigated the harmful placenta and fetus effects.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD420203951.

Modeling infectious diseases of the central nervous system with human brain organoids

Bacteria, fungi, viruses, and protozoa are known to infect and induce diseases in the human central nervous system (CNS). Modeling the mechanisms of interaction between pathogens and the CNS microenvironment is essential to understand their pathophysiology and develop new treatments. Recent advancements in stem cell technologies have allowed for the creation of human brain organoids, which more closely resembles the human CNS microenvironment when compared to classical 2-dimensional (2D) cultures. Now researchers can utilize these systems to investigate and reinvestigate questions related to CNS infection in a human-derived brain organoid system. Here in this review, we highlight several infectious diseases which have been tested in human brain organoids and compare similarities in response to these pathogens across different investigations. We also provide a brief overview of some recent advancements which can further enrich this model to develop new and better therapies to treat brain infections.

The determinants regulating Toxoplasma gondii bradyzoite development

Toxoplasma gondii is an obligate intracellular zoonotic pathogen capable of infecting almost all cells of warm-blooded vertebrates. In intermediate hosts, this parasite reproduces asexually in two forms, the tachyzoite form during acute infection that proliferates rapidly and the bradyzoite form during chronic infection that grows slowly. Depending on the growth condition, the two forms can interconvert. The conversion of tachyzoites to bradyzoites is critical for T. gondii transmission, and the reactivation of persistent bradyzoites in intermediate hosts may lead to symptomatic toxoplasmosis. However, the mechanisms that control bradyzoite differentiation have not been well studied. Here, we review recent advances in the study of bradyzoite biology and stage conversion, aiming to highlight the determinants associated with bradyzoite development and provide insights to design better strategies for controlling toxoplasmosis.

Hastening Progress in Cyclospora Requires Studying Eimeria Surrogates

Cyclospora cayetanensis is an enigmatic human parasite that sickens thousands of people worldwide. The scarcity of research material and lack of any animal model or cell culture system slows research, denying the produce industry, epidemiologists, and regulatory agencies of tools that might aid diagnosis, risk assessment, and risk abatement. Fortunately, related species offer a strong foundation when used as surrogates to study parasites of this type. Species of Eimeria lend themselves especially well as surrogates for C. cayetanensis. Those Eimeria that infect poultry can be produced in abundance, share many biological features with Cyclospora, pose no risk to the health of researchers, and can be studied in their natural hosts. Here, we overview the actual and potential uses of such surrogates to advance understanding of C. cayetanensis biology, diagnostics, control, and genomics, focusing on opportunities to improve prevention, surveillance, risk assessment, and risk reduction. Studying Eimeria surrogates accelerates progress, closing important research gaps and refining promising tools for producers and food safety regulators to monitor and ameliorate the food safety risks imposed by this emerging, enigmatic parasite.

Microfluidic model for in vitro acute Toxoplasma gondii infection and transendothelial migration

The protozoan parasite Toxoplasma gondii (T. gondii) causes one of the most common human zoonotic diseases and infects approximately one-third of the global population. T. gondii infects nearly every cell type and causes severe symptoms in susceptible populations. In previous laboratory animal studies, T. gondii movement and transmission were not analyzed in real time. In a three-dimensional (3D) microfluidic assay, we successfully supported the complex lytic cycle of T. gondii in situ by generating a stable microvasculature. The physiology of the T. gondii-infected microvasculature was monitored in order to investigate the growth, paracellular and transcellular migration, and transmission of T. gondii, as well as the efficacy of T. gondii drugs.

Bioengineered Co-culture of organoids to recapitulate host-microbe interactions

The recent spike in the instances of complex physiological host-microbe interactions has raised the demand for developing in vitro models that recapitulate the microbial microenvironment in the human body. Organoids are steadily emerging as an in vitro culture system that closely mimics the structural, functional, and genetic features of complex human organs, particularly for better understanding host-microbe interactions. Recent advances in organoid culture technology have become new avenues for assessing the pathogenesis of symbiotic interactions, pathogen-induced infectious diseases, and various other diseases. The co-cultures of organoids with microbes have shown great promise in simulating host-microbe interactions with a high level of complexity for further advancement in related fields. In this review, we provide an overview of bioengineering approaches for microbe-co-cultured organoids. Latest developments in the applications of microbe-co-cultured organoids to study human physiology and pathophysiology are also highlighted. Further, an outlook on future research on bioengineered organoid co-cultures for various applications is presented.

Cerebral malaria - modelling interactions at the blood-brain barrier in vitro

The blood–brain barrier (BBB) is a continuous endothelial barrier that is supported by pericytes and astrocytes and regulates the passage of solutes between the bloodstream and the brain. This structure is called the neurovascular unit and serves to protect the brain from blood-borne disease-causing agents and other risk factors. In the past decade, great strides have been made to investigate the neurovascular unit for delivery of chemotherapeutics and for understanding how pathogens can circumvent the barrier, leading to severe and, at times, fatal complications. One such complication is cerebral malaria, in which Plasmodium falciparum-infected red blood cells disrupt the barrier function of the BBB, causing severe brain swelling. Multiple in vitro models of the BBB are available to investigate the mechanisms underlying the pathogenesis of cerebral malaria and other diseases. These range from single-cell monolayer cultures to multicellular BBB organoids and highly complex cerebral organoids. Here, we review the technologies available in malaria research to investigate the interaction between P. falciparum-infected red blood cells and the BBB, and discuss the advantages and disadvantages of each model.

Summary: This Review discusses the available in vitro models to investigate the impact of adhesion of Plasmodium falciparum-infected red blood cells on the blood–brain barrier, a process associated with cerebral malaria.

Innate immune cell response to host-parasite interaction in a human intestinal tissue microphysiological system

Protozoan parasites that infect humans are widespread and lead to varied clinical manifestations, including life-threatening illnesses in immunocompromised individuals. Animal models have provided insight into innate immunity against parasitic infections; however, species-specific differences and complexity of innate immune responses make translation to humans challenging. Thus, there is a need for in vitro systems that can elucidate mechanisms of immune control and parasite dissemination. We have developed a human microphysiological system of intestinal tissue to evaluate parasite-immune-specific interactions during infection, which integrates primary intestinal epithelial cells and immune cells to investigate the role of innate immune cells during epithelial infection by the protozoan parasite, Toxoplasma gondii, which affects billions of people worldwide. Our data indicate that epithelial infection by parasites stimulates a broad range of effector functions in neutrophils and natural killer cell-mediated cytokine production that play immunomodulatory roles, demonstrating the potential of our system for advancing the study of human-parasite interactions.

Transcending Dimensions in Apicomplexan Research: from Two-Dimensional to Three-Dimensional In Vitro Cultures

Parasites belonging to the Apicomplexa phylum are among the most successful pathogens known in nature. They can infect a wide range of hosts, often remain undetected by the immune system, and cause acute and chronic illness. In this phylum, we can find parasites of human and veterinary health relevance, such as Toxoplasma, Plasmodium, Cryptosporidium, and Eimeria. There are still many unknowns about the biology of these pathogens due to the ethical and practical issues of performing research in their natural hosts. Animal models are often difficult or nonexistent, and as a result, there are apicomplexan life cycle stages that have not been studied. One recent alternative has been the use of three-dimensional (3D) systems such as organoids, 3D scaffolds with different matrices, microfluidic devices, organs-on-a-chip, and other tissue culture models. These 3D systems have facilitated and expanded the research of apicomplexans, allowing us to explore life stages that were previously out of reach and experimental procedures that were practically impossible to perform in animal models. Human- and animal-derived 3D systems can be obtained from different organs, allowing us to model host-pathogen interactions for diagnostic methods and vaccine development, drug testing, exploratory biology, and other applications. In this review, we summarize the most recent advances in the use of 3D systems applied to apicomplexans. We show the wide array of strategies that have been successfully used so far and apply them to explore other organisms that have been less studied.

Modeling Developmental Brain Diseases Using Human Pluripotent Stem Cells-Derived Brain Organoids - Progress and Perspective

Developmental brain diseases encompass a group of conditions resulting from genetic or environmental perturbations during early development. Despite the increased research attention in recent years following recognition of the prevalence of these diseases, there is still a significant lack of knowledge of their etiology and treatment options. The genetic and clinical heterogeneity of these diseases, in addition to the limitations of experimental animal models, contribute to this difficulty. In this regard, the advent of brain organoid technology has provided a new means to study the cause and progression of developmental brain diseases in vitro. Derived from human pluripotent stem cells, brain organoids have been shown to recapitulate key developmental milestones of the early human brain. Combined with technological advancements in genome editing, tissue engineering, electrophysiology, and multi-omics analysis, brain organoids have expanded the frontiers of human neurobiology, providing valuable insight into the cellular and molecular mechanisms of normal and pathological brain development. This review will summarize the current progress of applying brain organoids to model human developmental brain diseases and discuss the challenges that need to be overcome to further advance their utility.

Understanding parasite-brain microvascular interactions with engineered 3D blood-brain barrier models

Microbial interactions with the blood-brain barrier (BBB) can be highly pathogenic and are still not well understood. Among these, parasites present complex interactions with the brain microvasculature that are difficult to decipher using experimental animal models or reductionist 2D in vitro cultures. Novel 3D engineered blood-brain barrier models hold great promise to overcome limitations in traditional research approaches. These models better mimic the intricate 3D architecture of the brain microvasculature and recapitulate several aspects of BBB properties, physiology, and function. Moreover, they provide improved control over biophysical and biochemical experimental parameters and are compatible with advanced imaging and molecular biology techniques. Here, we review design considerations and methodologies utilized to successfully engineer BBB microvessels. Finally, we highlight the advantages and limitations of existing engineered models and propose applications to study parasite interactions with the BBB, including mechanisms of barrier disruption.

Organoids in modelling infectious diseases

Approaches based on animal and two-dimensional (2D) cell culture models cannot ensure reliable results in modeling novel pathogens or in drug testing in the short term; therefore, there is rising interest in platforms such as organoids. To develop a toolbox that can be used successfully to overcome current issues in modeling various infections, it is essential to provide a framework of recent achievements in applying organoids. Organoids have been used to study viruses, bacteria, and protists that cause, for example, respiratory, gastrointestinal, and liver diseases. Their future as models of infection will be associated with improvements in system complexity, including abilities to model tissue structure, a dynamic microenvironment, and coinfection. Teaser. Organoids are a flexible tool for modelling viral, bacterial and protist infections. They can provide fast and reliable information on the biology of pathogens and in drug screening, and thus have become essential in combatting emerging infectious diseases.

Progress and Challenges in the Use of a Liver-on-a-Chip for Hepatotropic Infectious Diseases

The liver is a target organ of life-threatening pathogens and prominently contributes to the variation in drug responses and drug-induced liver injury among patients. Currently available drugs significantly decrease the morbidity and mortality of liver-dwelling pathogens worldwide; however, emerging clinical evidence reveals the importance of host factors in the design of safe and effective therapies for individuals, known as personalized medicine. Given the primary adherence of cells in conventional two-dimensional culture, the use of these one-size-fit-to-all models in preclinical drug development can lead to substantial failures in assessing therapeutic safety and efficacy. Advances in stem cell biology, bioengineering and material sciences allow us to develop a more physiologically relevant model that is capable of recapitulating the human liver. This report reviews the current use of liver-on-a-chip models of hepatotropic infectious diseases in the context of precision medicine including hepatitis virus and malaria parasites, assesses patient-specific responses to antiviral drugs, and designs personalized therapeutic treatments to address the need for a personalized liver-like model. Second, most organs-on-chips lack a monitoring system for cell functions in real time; thus, the review discusses recent advances and challenges in combining liver-on-a-chip technology with biosensors for assessing hepatocyte viability and functions. Prospectively, the biosensor-integrated liver-on-a-chip device would provide novel biological insights that could accelerate the development of novel therapeutic compounds.

Advances in modelling the human microbiome-gut-brain axis in vitro

The human gut microbiome has emerged as a key player in the bidirectional communication of the gut–brain axis, affecting various aspects of homeostasis and pathophysiology. Until recently, the majority of studies that seek to explore the mechanisms underlying the microbiome–gut–brain axis cross-talk, relied almost exclusively on animal models, and particularly gnotobiotic mice. Despite the great progress made with these models, various limitations, including ethical considerations and interspecies differences that limit the translatability of data to human systems, pushed researchers to seek for alternatives. Over the past decades, the field of in vitro modelling of tissues has experienced tremendous growth, thanks to advances in 3D cell biology, materials, science and bioengineering, pushing further the borders of our ability to more faithfully emulate the in vivo situation. The discovery of stem cells has offered a new source of cells, while their use in generating gastrointestinal and brain organoids, among other tissues, has enabled the development of novel 3D tissues that better mimic the native tissue structure and function, compared with traditional assays. In parallel, organs-on-chips technology and bioengineered tissues have emerged as highly promising alternatives to animal models for a wide range of applications. Here, we discuss how recent advances and trends in this area can be applied in host–microbe and host–pathogen interaction studies. In addition, we highlight paradigm shifts in engineering more robust human microbiome-gut-brain axis models and their potential to expand our understanding of this complex system and hence explore novel, microbiome-based therapeutic approaches.

The Stem Cell Revolution Revealing Protozoan Parasites' Secrets and Paving the Way towards Vaccine Development

Protozoan infections are leading causes of morbidity and mortality in humans and some of the most important neglected diseases in the world. Despite relentless efforts devoted to vaccine and drug development, adequate tools to treat and prevent most of these diseases are still lacking. One of the greatest hurdles is the lack of understanding of host-parasite interactions. This gap in our knowledge comes from the fact that these parasites have complex life cycles, during which they infect a variety of specific cell types that are difficult to access or model in vitro. Even in those cases when host cells are readily available, these are generally terminally differentiated and difficult or impossible to manipulate genetically, which prevents assessing the role of human factors in these diseases. The advent of stem cell technology has opened exciting new possibilities to advance our knowledge in this field. The capacity to culture Embryonic Stem Cells, derive Induced Pluripotent Stem Cells from people and the development of protocols for differentiation into an ever-increasing variety of cell types and organoids, together with advances in genome editing, represent a huge resource to finally crack the mysteries protozoan parasites hold and unveil novel targets for prevention and treatment.

Modeling SARS-CoV-2 infection in individuals with opioid use disorder with brain organoids

The COVID-19 pandemic has aggravated a preexisting epidemic: the opioid crisis. Much literature has shown that the circumstances imposed by COVID-19, such as social distancing regulations, medical and financial instability, and increased mental health issues, have been detrimental to those with opioid use disorder (OUD). In addition, unexpected neurological sequelae in COVID-19 patients suggest that COVID-19 compromises neuroimmunity, induces hypoxia, and causes respiratory depression, provoking similar effects as those caused by opioid exposure. Combined conditions of COVID-19 and OUD could lead to exacerbated complications. With limited human in vivo options to study these complications, we suggest that iPSC-derived brain organoid models may serve as a useful platform to investigate the physiological connection between COVID-19 and OUD. This mini-review highlights the advances of brain organoids in other neuropsychiatric and infectious diseases and suggests their potential utility for investigating OUD and COVID-19, respectively.