In Vitro Culture of Cryptosporidium parvum Using Hollow Fiber Bioreactor: Applications for Simultaneous Pharmacokinetic and Pharmacodynamic Evaluation of Test Compounds

Protists: Eukaryotic single-celled organisms and the functioning of their organelles

Organelles are membrane bound structures that compartmentalize biochemical and molecular functions. With improved molecular, biochemical and microscopy tools the diversity and function of protistan organelles has increased in recent years, providing a complex panoply of structure/function relationships. This is particularly noticeable with the description of hydrogenosomes, and the diverse array of structures that followed, having hybrid hydrogenosome/mitochondria attributes. These diverse organelles have lost the major, at one time, definitive components of the mitochondrion (tricarboxylic cycle enzymes and cytochromes), however they all contain the machinery for the assembly of Fe-S clusters, which is the single unifying feature they share. The plasticity of organelles, like the mitochondrion, is therefore evident from its ability to lose its identity as an aerobic energy generating powerhouse while retaining key ancestral functions common to both aerobes and anaerobes. It is interesting to note that the apicoplast, a non-photosynthetic plastid that is present in all apicomplexan protozoa, apart from Cryptosporidium and possibly the gregarines, is also the site of Fe-S cluster assembly proteins. It turns out that in Cryptosporidium proteins involved in Fe-S cluster biosynthesis are localized in the mitochondrial remnant organelle termed the mitosome. Hence, different organisms have solved the same problem of packaging a life-requiring set of reactions in different ways, using different ancestral organelles, discarding what is not needed and keeping what is essential. Don't judge an organelle by its cover, more by the things it does, and always be prepared for surprises.

Genomic and virulence analysis of in vitro cultured Cryptosporidium parvum

Recent advances in the in vitro cultivation of Cryptosporidium parvum using hollow fiber bioreactor technology (HFB) have permitted continuous growth of parasites that complete all life cycle stages. The method provides access to all stages of the parasite and provides a method for non-animal production of oocysts for use in clinical trials. Here we examined the effect of long-term (>20 months) in vitro culture on virulence-factors, genome conservation, and in vivo pathogenicity of the host by in vitro cultured parasites. We find low-level sequence variation that is consistent with that observed in calf-passaged parasites. Further using a calf model infection, oocysts obtained from the HFB caused diarrhea of the same volume, duration and oocyst shedding intensity as in vivo passaged parasites.

Organoid-based in vitro systems to model Cryptosporidium parvum infection in 2D and 3D

Typical cancer cell-based culture systems cannot support the full life cycle of Cryptosporidium parvum, despite its monoxenous life cycle which is completed in the small intestine of a single host. There is a block to fertilization and zygote formation in vitro. In this paper, we adapted a 2D organoid derived monolayer system and a 3D inverted enteroid system for use in C. parvum culture. 3D inverted enteroids were successfully infected by C. parvum without the need for microinjection and supported subculture of C. parvum. Using the 2D organoid derived monolayer (ODM) system, the infection can be maintained for at least 3 weeks with new oocyst production throughout. Fertilization was confirmed based on successful mating of two strains of C. parvum. We demonstrated that the apparent block to fertilization in typical cell culture is overcome using ODMs.

An update on Cryptosporidium biology and therapeutic avenues

Cryptosporidium species has been identified as an important pediatric diarrheal pathogen in resource-limited countries, particularly in very young children (0–24 months). However, the only available drug (nitazoxanide) has limited efficacy and can only be prescribed in a medical setting to children older than one year. Many drug development projects have started to investigate new therapeutic avenues. Cryptosporidium’s unique biology is challenging for the traditional drug discovery pipeline and requires novel drug screening approaches. Notably, in recent years, new methods of oocyst generation, in vitro processing, and continuous three-dimensional cultivation capacities have been developed. This has enabled more physiologically pertinent research assays for inhibitor discovery. In a short time, many great strides have been made in the development of anti-Cryptosporidium drugs. These are expected to eventually turn into clinical candidates for cryptosporidiosis treatment in the future. This review describes the latest development in Cryptosporidium biology, genomics, transcriptomics of the parasite, assay development, and new drug discovery.

Oxygen levels are key to understanding "Anaerobic" protozoan pathogens with micro-aerophilic lifestyles

Publications abound on the physiology, biochemistry and molecular biology of "anaerobic" protozoal parasites as usually grown under "anaerobic" culture conditions. The media routinely used are poised at low redox potentials using techniques that remove O₂ to "undetectable" levels in sealed containers. However there is growing understanding that these culture conditions do not faithfully resemble the O₂ environments these organisms inhabit. Here we review for protists lacking oxidative energy metabolism, the oxygen cascade from atmospheric to intracellular concentrations and relevant methods of measurements of O₂, some well-studied parasitic or symbiotic protozoan lifestyles, their homeodynamic metabolic and redox balances, organism-drug-oxygen interactions, and the present and future prospects for improved drugs and treatment regimes.

Absorption and Distribution of Toltrazuril and Toltrazuril Sulfone in Plasma, Intestinal Tissues and Content of Piglets after Oral or Intramuscular Administration

Piglet coccidiosis due to Cystoisospora suis is a major cause of diarrhea and poor growth worldwide. It can effectively be controlled by application of toltrazuril (TZ), and oral formulations have been licensed for many years. Recently, the first parenteral formulation containing TZ in combination with iron (gleptoferron) was registered in the EU for the prevention of coccidiosis and iron deficiency anemia, conditions in suckling piglets requiring routine preventive measures. This study evaluated the absorption and distribution of TZ and its main metabolite, toltrazuril sulfone (TZ-SO₂), in blood and intestinal tissues after single oral (20 mg/kg) or single intramuscular (45 mg/piglet) application of TZ. Fifty-six piglets were randomly allocated to the two treatment groups. Animals were sacrificed 1-, 5-, 13-, and 24-days post-treatment and TZ and TZ-SO₂ levels were determined in blood, jejunal tissue, ileal tissue, and mixed jejunal and ileal content (IC) by high performance liquid chromatography (HPLC). Intramuscular application resulted in significantly higher and more sustained concentrations of both compounds in plasma, intestinal tissue, and IC. Higher concentrations after oral dosing were only observed one day after application of TZ in jejunum and IC. Toltrazuril was quickly metabolized to TZ-SO₂ with maximum concentrations on day 13 for both applications. Remarkably, TZ and TZ-SO₂ accumulated in the jejunum, the primary predilection site of C. suis, independently of the administration route, which is key to their antiparasitic effect.

The Truman Show for protozoan parasites: A review of in vitro cultivation platforms

Protozoan parasites are responsible for severe disease and suffering in humans worldwide. Apart from disease transmission via insect vectors and contaminated soil, food, or water, transmission may occur congenitally or by way of blood transfusion and organ transplantation. Several recent outbreaks associated with fresh produce and potable water emphasize the need for vigilance and monitoring of protozoan parasites that cause severe disease in humans globally. Apart from the tropical parasite Plasmodium spp., other protozoa causing debilitating and fatal diseases such as Trypanosoma spp. and Naegleria fowleri need to be studied in more detail. Climate change and socioeconomic issues such as migration continue to be major drivers for the spread of these neglected tropical diseases beyond endemic zones. Due to the complex life cycles of protozoa involving multiple hosts, vectors, and stringent growth conditions, studying these parasites has been challenging. While in vivo models may provide insights into host–parasite interaction, the ethical aspects of laboratory animal use and the challenge of ready availability of parasite life stages underline the need for in vitro models as valid alternatives for culturing and maintaining protozoan parasites. To our knowledge, this review is the first of its kind to highlight available in vitro models for protozoa causing highly infectious diseases. In recent years, several research efforts using new technologies such as 3D organoid and spheroid systems for protozoan parasites have been introduced that provide valuable tools to advance complex culturing models and offer new opportunities toward the advancement of parasite in vitro studies. In vitro models aid scientists and healthcare providers in gaining insights into parasite infection biology, ultimately enabling the use of novel strategies for preventing and treating these diseases.

In light of the far-reaching social and economic repercussions of communicable, zoonotic parasitic diseases on human health, it is imperative to continue to strive toward developing in vitro models for in-depth scrutiny and understanding of pathogenicity, as well as for innovations toward combating these infections. This review, to our knowledge, is the first to offer a qualitative summary of the existing models for culturing protozoan parasites with major relevance to human health in vitro. The present work aims to provide a reference guide on the current state of in vitro culture of these protozoan parasites and offers a foundation to facilitate exchange of expertise among researchers, clinicians, and healthcare workers. This comprehensive review would aid in enabling discussions on new intervention approaches to fill in the knowledge gaps in the field of parasitic diseases affecting the global population.

Challenges for Cryptosporidium Population Studies

Cryptosporidiosis is ranked sixth in the list of the most important food-borne parasites globally, and it is an important contributor to mortality in infants and the immunosuppressed. Recently, the number of genome sequences available for this parasite has increased drastically. The majority of the sequences are derived from population studies of Cryptosporidium parvum and Cryptosporidium hominis, the most important species causing disease in humans. Work with this parasite is challenging since it lacks an optimal, prolonged, in vitro culture system, which accurately reproduces the in vivo life cycle. This obstacle makes the cloning of isolates nearly impossible. Thus, patient isolates that are sequenced represent a population or, at times, mixed infections. Oocysts, the lifecycle stage currently used for sequencing, must be considered a population even if the sequence is derived from single-cell sequencing of a single oocyst because each oocyst contains four haploid meiotic progeny (sporozoites). Additionally, the community does not yet have a set of universal markers for strain typing that are distributed across all chromosomes. These variables pose challenges for population studies and require careful analyses to avoid biased interpretation. This review presents an overview of existing population studies, challenges, and potential solutions to facilitate future population analyses.

Target Product Profile and Development Path for Shigellosis Treatment with Antibacterials

Enteric infection with Shigella spp. can lead to symptoms ranging from acute watery diarrhea to sudden, severe dysentery. Approximately 212 000 diarrheal deaths annually are attributed to Shigella with a disproportionate impact in low-resource countries. The impact in under-resourced countries was illustrated by a reanalysis of the Global Enteric Multicenter Study which found that Shigella was the leading pathogen associated with moderate-to severe diarrhea in children under 5 years old. While recent studies have highlighted the burden of the disease, there has been a concurrent reduction in therapeutic options for the treatment of shigellosis as drug resistant strains increase in prevalence. In addition, increasing reports of drug resistant shigellosis cases in the men who have sex with men community confirm that the impact is not limited to low-resource countries. Despite the urgent need for new treatments, a target product profile (TPP) has not been established, and there is no clear development path for antibacterial treatments. To address this troubling concern, this manuscript describes a TPP for antishigellosis small molecule therapeutics and a development path that integrates currently available preclinical and clinical models of Shigella infection.

Current status and challenges in drug discovery against the globally important zoonotic cryptosporidiosis

The zoonotic cryptosporidiosis is globally distributed, one of the major diarrheal diseases in humans and animals. Cryptosporidium oocysts are also one of the major environmental concerns, making it a pathogen that fits well into the One Health concept. Despite its importance, fully effective drugs are not yet available. Anti-cryptosporidial drug discovery has historically faced many unusual challenges attributed to unique parasite biology and technical burdens. While significant progresses have been made recently, anti-cryptosporidial drug discovery still faces a major obstacle: identification of systemic drugs that can be absorbed by patients experiencing watery diarrhea and effectively pass through electron-dense (ED) band at the parasite-host cell interface to act on the epicellular parasite. There may be a need to develop an in vitro assay to effectively screen hits/leads for their capability to cross ED band. In the meantime, non-systemic drugs with strong mucoadhesive properties for extended gastrointestinal exposure may represent another direction in developing anti-cryptosporidial therapeutics. For developing both systemic and non-systemic drugs, a non-ruminant animal model exhibiting diarrheal symptoms suitable for routine evaluation of drug absorption and anti-cryptosporidial efficacy may be very helpful.

Analysis of Long Non-Coding RNA in Cryptosporidium parvum Reveals Significant Stage-Specific Antisense Transcription

Cryptosporidium is a protist parasite that has been identified as the second leading cause of moderate to severe diarrhea in children younger than two and a significant cause of mortality worldwide. Cryptosporidium has a complex, obligate, intracellular but extra cytoplasmic lifecycle in a single host. How genes are regulated in this parasite remains largely unknown. Long non-coding RNAs (lncRNAs) play critical regulatory roles, including gene expression across a broad range of organisms. Cryptosporidium lncRNAs have been reported to enter the host cell nucleus and affect the host response. However, no systematic study of lncRNAs in Cryptosporidium has been conducted to identify additional lncRNAs. In this study, we analyzed a C. parvum in vitro strand-specific RNA-seq developmental time series covering both asexual and sexual stages to identify lncRNAs associated with parasite development. In total, we identified 396 novel lncRNAs, mostly antisense, with 86% being differentially expressed. Surprisingly, nearly 10% of annotated mRNAs have an antisense transcript. lncRNAs occur most often at the 3' end of their corresponding sense mRNA. Putative lncRNA regulatory regions were identified and many appear to encode bidirectional promoters. A positive correlation between lncRNA and upstream mRNA expression was observed. Evolutionary conservation and expression of lncRNA candidates was observed between C. parvum, C. hominis and C. baileyi. Ten C. parvum protein-encoding genes with antisense transcripts have P. falciparum orthologs that also have antisense transcripts. Three C. parvum lncRNAs with exceptional properties (e.g., intron splicing) were experimentally validated using RT-PCR and RT-qPCR. This initial characterization of the C. parvum non-coding transcriptome facilitates further investigations into the roles of lncRNAs in parasite development and host-pathogen interactions.

Phenotypic screening techniques for Cryptosporidium drug discovery

Introduction: Two landmark epidemiological studies identified Cryptosporidium spp. as a significant cause of diarrheal disease in pediatric populations in resource-limited countries. Notably, nitazoxanide is the only approved drug for treatment of cryptosporidiosis but shows limited efficacy. As a result, many drug discovery efforts have commenced to find improved treatments. The unique biology of Cryptosporidium presents challenges for traditional drug discovery methods, which has inspired new assay platforms to study parasite biology and drug screening. Areas covered: The authors review historical advancements in phenotypic-based assays and techniques for Cryptosporidium drug discovery, as well as recent advances that will define future drug discovery. The reliance on phenotypic-based screens and repositioning of phenotypic hits from other pathogens has quickly created a robust pipeline of potential cryptosporidiosis therapeutics. The latest advances involve new in vitro culture methods for oocyst generation, continuous culturing capabilities, and more physiologically relevant assays for testing compounds. Expert opinion: Previous phenotypic screening techniques have laid the groundwork for recent cryptosporidiosis drug discovery efforts. The resulting improved methodologies characterize compound activity, identify, and validate drug targets, and prioritize new compounds for drug development. The most recent improvements in phenotypic assays are poised to help advance compounds into clinical development.

P-Glycoprotein-Mediated Efflux Reduces the In Vivo Efficacy of a Therapeutic Targeting the Gastrointestinal Parasite Cryptosporidium

Recent studies have illustrated the burden Cryptosporidium infection places on the lives of malnourished children and immunocompromised individuals. Treatment options remain limited, and efforts to develop a new therapeutic are currently underway. However, there are unresolved questions about the ideal pharmacokinetic characteristics of new anti-Cryptosporidium therapeutics. Specifically, should drug developers optimize therapeutics and formulations to increase drug exposure in the gastrointestinal lumen, enterocytes, or systemic circulation? Furthermore, how should researchers interpret data suggesting their therapeutic is a drug efflux transporter substrate? In vivo drug transporter–mediated alterations in efficacy are well recognized in multiple disease areas, but the impact of intestinal transporters on therapeutic efficacy against enteric diseases has not been established. Using multiple in vitro models and a mouse model of Cryptosporidium infection, we characterized the effect of P-glycoprotein efflux on bumped kinase inhibitor pharmacokinetics and efficacy. Our results demonstrated P-glycoprotein decreases bumped kinase inhibitor enterocyte exposure, resulting in reduced in vivo efficacy against Cryptosporidium. Furthermore, a hollow fiber model of Cryptosporidium infection replicated the in vivo impact of P-glycoprotein on anti-Cryptosporidium efficacy. In conclusion, when optimizing drug candidates targeting the gastrointestinal epithelium or gastrointestinal epithelial infections, drug developers should consider the adverse impact of active efflux transporters on efficacy.