Cryptosporidium parvum is cytopathic for cultured human biliary epithelia via an apoptotic mechanism

miR-181d targets BCL2 to regulate HCT-8 cell apoptosis and parasite burden in response to Cryptosporidium parvum infection via the intrinsic apoptosis pathway

Cryptosporidium parvum is an important zoonotic pathogen that is studied worldwide. MicroRNAs (miRNAs) act as post-transcriptional regulators and may play a key role in modulating host epithelial responses following Cryptosporidium infection. Our previous study has shown that C. parvum downregulates the expression of miR-181d through the p50-dependent TLRs/NF-κB pathway. However, the mechanism by which miR-181d regulates host cells in response to C. parvum infection remains unclear. The present study found that miR-181d downregulation inhibited cell apoptosis and increased parasite burden in HCT-8 cells after C. parvum infection. Bioinformatics analysis and luciferase reporter assays have shown that BCL2 was a target gene for miR-181d. Moreover, BCL2 overexpression and miR-181d downregulation had similar results. To further investigate the mechanism by which miR-181d regulated HCT-8 cell apoptosis during C. parvum infection, the expression of molecules involved in the intrinsic apoptosis pathway was detected. Bax, caspase-9, and caspase-3 expression was decreased at 4, 8, 12, and 24 hpi and upregulated at 36 and 48 hpi. Interfering with the expression of miR-181d or BCL2 significantly affected the expression of molecules in the intrinsic apoptosis pathway. These data indicated that miR-181d targeted BCL2 to regulate HCT-8 cell apoptosis and parasite burden in response to C. parvum infection via the intrinsic apoptosis pathway. These results allowed us to further understand the regulatory mechanisms of host miRNAs during Cryptosporidium infection, and provided a theoretical foundation for the design and development of anti-cryptosporidiosis drugs.

MiR-4521 affects the propagation of Cryptosporidium parvum in HCT-8 cells through targeting foxm1 by regulating cell apoptosis

Cryptosporidium parvum could regulate the expression of microRNAs of epithelial cells to facilitate its intracellular propagation. MiR-4521 has been reported to play an important role during the development and progression of tumors and infectious diseases by regulating cell proliferation, apoptosis, and autophagy. However, the implication of miR-4521 during C. parvum infection was still unknown. In this study, the expression of miR-4521 was found to be upregulated in HCT-8 cells infected with C. parvum from 8 h post-infection (pi) to 48 hpi, and its upregulation would be related with the TLR/NF-κB signal pathway during C. parvum infection. One potential target of miR-4521, foxm1, was down-regulated in HCT-8 cells from 24 hpi to 48 hpi, and the expression of foxm1 was negatively regulated by miR-4521. The target relationship between miR-4521 and foxm1 was further validated by using dual luciferase reporter assay. Further studies showed that miR-4521 promoted the propagation of C. parvum in HCT-8 cells through targeting foxm1 by regulating BCL2-mediating cell apoptosis. These results contribute to further understanding of the regulatory mechanisms of host miRNAs during Cryptosporidium infection.

Gene expression profile of HCT-8 cells following single or co-infections with Cryptosporidium parvum and bovine coronavirus

Among the causative agents of neonatal diarrhoea in calves, two of the most prevalent are bovine coronavirus (BCoV) and the intracellular parasite Cryptosporidium parvum. Although several studies indicate that co-infections are associated with greater symptom severity, the host-pathogen interplay remains unresolved. Here, our main objective was to investigate the modulation of the transcriptome of HCT-8 cells during single and co-infections with BCoV and C. parvum. For this, HCT-8 cells were inoculated with (1) BCoV alone, (2) C. parvum alone, (3) BCoV and C. parvum simultaneously. After 24 and 72 h, cells were harvested and analyzed using high-throughput RNA sequencing. Following differential expression analysis, over 6000 differentially expressed genes (DEGs) were identified in virus-infected and co-exposed cells at 72 hpi, whereas only 52 DEGs were found in C. parvum-infected cells at the same time point. Pathway (KEGG) and gene ontology (GO) analysis showed that DEGs in the virus-infected and co-exposed cells were mostly associated with immune pathways (such as NF-κB, TNF-α or, IL-17), apoptosis and regulation of transcription, with a more limited effect exerted by C. parvum. Although the modulation observed in the co-infection was apparently dominated by the virus, over 800 DEGs were uniquely expressed in co-exposed cells at 72 hpi. Our findings provide insights on possible biomarkers associated with co-infection, which could be further explored using in vivo models.

Hide-and-Seek: A Game Played between Parasitic Protists and Their Hosts

After invading the host organism, a battle occurs between the parasitic protists and the host's immune system, the result of which determines not only whether and how well the host survives and recovers, but also the fate of the parasite itself. The exact weaponry of this battle depends, among others, on the parasite localisation. While some parasitic protists do not invade the host cell at all (extracellular parasites), others have developed successful intracellular lifestyles (intracellular parasites) or attack only the surface of the host cell (epicellular parasites). Epicellular and intracellular protist parasites have developed various mechanisms to hijack host cell functions to escape cellular defences and immune responses, and, finally, to gain access to host nutrients. They use various evasion tactics to secure the tight contact with the host cell and the direct nutrient supply. This review focuses on the adaptations and evasion strategies of parasitic protists on the example of two very successful parasites of medical significance, Cryptosporidium and Leishmania, while discussing different localisation (epicellular vs. intracellular) with respect to the host cell.

Prospect of microbial food borne diseases in Pakistan: a review

Nowadays food borne illness is most common in people due to their epidemic nature. These diseases affect the human digestive system through bacteria, viruses and parasites. The agents of illness are transmitted in our body through various types of food items, water and uncooked. Pathogens show drastic changes in immunosuppressant people. This review gives general insights to harmful microbial life. Pakistan is a developed country and because of its improper food management, a lot of gastrointestinal problems are noted in many patients. Bacteria are most common agents to spread diarrhoea, villi infection, constipation and dysenteric disease in human and induce the rejection of organ transplant. Enhancement of their lifestyle, properly cooked food should be used and to overcome the outbreak of the diseases.

MicroRNA expression profile of HCT-8 cells in the early phase of Cryptosporidium parvum infection

Background

Cryptosporidium parvum is an important zoonotic parasitic disease worldwide, but the molecular mechanisms of the host–parasite interaction are not fully understood. Noncoding microRNAs (miRNAs) are considered key regulators of parasitic diseases. Therefore, we used microarray, qPCR, and bioinformatic analyses to investigate the intestinal epithelial miRNA expression profile after Cryptosporidium parvum infection.

Results

Twenty miRNAs were differentially expressed after infection (four upregulated and 16 downregulated). Further analysis of the differentially expressed miRNAs revealed that many important cellular responses were triggered by Cryptosporidium parvum infection, including cell apoptosis and the inflammatory and immune responses.

Conclusions

This study demonstrates for the first time that the miRNA expression profile of human intestinal epithelium cells is altered by C. parvum infection. This dysregulation of miRNA expression may contribute to the regulation of host biological processes in response to C. parvum infection, including cell apoptosis and the immune responses. These results provide new insight into the regulatory mechanisms of host miRNAs during cryptosporidiosis, which may offer potential targets for future C. parvum control strategies.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5410-6) contains supplementary material, which is available to authorized users.

Past and future trends of Cryptosporidium in vitro research

Cryptosporidium is a genus of single celled parasites capable of infecting a wide range of animals including humans. Cryptosporidium species are members of the phylum apicomplexa, which includes well-known genera such as Plasmodium and Toxoplasma. Cryptosporidium parasites cause a severe gastro-intestinal disease known as cryptosporidiosis. They are one of the most common causes of childhood diarrhoea worldwide, and infection can have prolonged detrimental effects on the development of children, but also can be life threatening to HIV/AIDS patients and transplant recipients. A variety of hosts can act as reservoirs, and Cryptosporidium can persist in the environment for prolonged times as oocysts. While there has been substantial interest in these parasites, there is very little progress in terms of treatment development and understanding the majority of the life cycle of this unusual organism. In this review, we will provide an overview on the existing knowledge of the biology of the parasite and the current progress in developing in vitro cultivation systems. We will then describe a synopsis of current and next generation approaches that could spearhead further research in combating the parasite.

Cryptosporidium parvum infection attenuates the ex vivo propagation of murine intestinal enteroids

Cryptosporidium, a ubiquitous coccidian protozoan parasite that infects the gastrointestinal epithelium and other mucosal surfaces, is an important opportunistic pathogen for immunocompromised individuals and a common cause of diarrhea in young children in the developing countries. One of the pathological hallmarks of intestinal cryptosporidiosis is villous atrophy, which results in a shorter height of intestinal villi. Here, we investigated the effects of Cryptosporidium infection on intestinal epithelial growth, using an ex vivo model of intestinal cryptosporidiosis employing enteroids from mice. We detected infection of enteroids isolated from immunocompetent adult and neonatal mice after ex vivo exposure to Cryptosporidium sporozoites. We observed a significant inhibition of enteroid propagation following infection. Intriguingly, we identified a decreased expression level of intestinal stem cell markers in enteroids following C. parvum infection. We further measured the expression levels of several Wnt antagonists or agonists in infected enteroids, as induction of the Wnt/β‐catenin activation is a key factor for intestinal stem cell function. We detected a markedly increased level of the Dickkopf‐related protein 1 and decreased level of the Wnt family member 5a in enteroids after infection. The low density lipoprotein receptor‐related protein 5, one of the Wnt co‐receptors, is downregulated in the infected enteroids. In addition, increased apoptotic cell death and cell senescence were observed in the infected enteroids. Our results demonstrate a significant inhibitory effect of Cryptosporidium infection on the ex vivo propagation of enteroids from mice, providing additional insights into the impact of Cryptosporidium infection on intestinal epithelial growth.

Concomitant AIDS cholangiopathy and Fanconi syndrome as complications of HIV in a single patient

We describe the case of a 50-year-old woman presenting to our acute medicine department with generalised non-specific symptoms on a background of HIV managed on triple therapy (tenofovir, lamivudine and zidovudine). On admission, she was noted to be acidotic with proteinuria, glycosuria, hypophosphataemia and generalised body pain, and was diagnosed with Fanconi's renotubular syndrome secondary to tenofovir. It was also noted that she had elevated liver dysfunction markers, and an MRI of the liver revealed a focal stricture near the ampulla of Vater, resulting in a diagnosis of AIDS cholangiopathy. These two diagnoses are rare complications of HIV, and the presence of both these pathologies in a single patient has never been reported in the literature before, and we therefore believe that this case is the first of its kind.

Biology of Cholangiocytes: From Bench to Bedside

Cholangiocytes, the lining epithelial cells in bile ducts, are an important subset of liver cells. They are activated by endogenous and exogenous stimuli and are involved in the modification of bile volume and composition. They are also involved in damaging and repairing the liver. Cholangiocytes have many functions including bile production. They are also involved in transport processes that regulate the volume and composition of bile. Cholangiocytes undergo proliferation and cell death under a variety of conditions. Cholangiocytes have functional and morphological heterogenecity. The immunobiology of cholangiocytes is important, particularly for understanding biliary disease. Secretion of different proinflammatory mediators, cytokines, and chemokines suggests the major role that cholangiocytes play in inflammatory reactions. Furthermore, paracrine secretion of growth factors and peptides mediates extensive cross-talk with other liver cells, including hepatocytes, stellate cells, stem cells, subepithelial myofibroblasts, endothelial cells, and inflammatory cells. Cholangiopathy refers to a category of chronic liver diseases whose primary disease target is the cholangiocyte. Cholangiopathy usually results in end-stage liver disease requiring liver transplant. We summarize the biology of cholangiocytes and redefine the concept of cholangiopathy. We also discuss the recent progress that has been made in understanding the pathogenesis of cholangiopathy and how such progress has influenced therapy.

Delivery of Parasite RNA Transcripts Into Infected Epithelial Cells During Cryptosporidium Infection and Its Potential Impact on Host Gene Transcription

Cryptosporidium parvum is an important opportunistic parasite pathogen for immunocompromised individuals and a common cause of diarrhea in young children. Previous studies have identified a panel of RNA transcripts of very low protein-coding potential in C. parvum. Using an in vitro model of human intestinal cryptosporidiosis, we report here that some of these C. parvum RNA transcripts were selectively delivered into the nuclei of host epithelial cells during C. parvum infection. Nuclear delivery of several such parasitic RNAs, including Cdg7_FLc_0990, involved heat-shock protein 70-mediated nuclear importing mechanism. Overexpression of Cdg7_FLc_0990 in intestinal epithelial cells resulted in significant changes in expression levels of specific genes, with significant overlapping with alterations in gene expression profile detected in host cells after C. parvum infection. Our data demonstrate that C. parvum transcripts of low protein-coding potential are selectively delivered into epithelial cells during infection and may modulate gene transcription in infected host cells.

Infection Strategies of Intestinal Parasite Pathogens and Host Cell Responses

Giardia lamblia, Cryptosporidium sp., and Entamoeba histolytica are important pathogenic intestinal parasites and are amongst the leading causes worldwide of diarrheal illness in humans. Diseases caused by these organisms, giardiasis, cryptosporidiosis, and amoebiasis, respectively, are characterized by self-limited diarrhea but can evolve to long-term complications. The cellular and molecular mechanisms underlying the pathogenesis of diarrhea associated with these three pathogens are being unraveled, with knowledge of both the strategies explored by the parasites to establish infection and the methods evolved by hosts to avoid it. Special attention is being given to molecules participating in parasite–host interaction and in the mechanisms implicated in the diseases’ pathophysiologic processes. This review focuses on cell mechanisms that are modulated during infection, including gene transcription, cytoskeleton rearrangements, signal transduction pathways, and cell death.

Cryptosporidiuminfections: molecular advances

Cryptosporidiumhost cell interaction remains fairly obscure compared with other apicomplexans such asPlasmodiumorToxoplasma. The reason for this is probably the inability of this parasite to complete its life cyclein vitroand the lack of a system to genetically modifyCryptosporidium. However, there is a substantial set of data about the molecules involved in attachment and invasion and about the host cell pathways involved in actin arrangement that are altered by the parasite. Here we summarize the recent advances in research on host cell infection regarding the excystation process, attachment and invasion, survival in the cell, egress and the available data on omics.

Innate immune responses against Cryptosporidium parvum infection

Cryptosporidium parvum infects intestinal epithelial cells and is commonly the parasite species involved in mammalian cryptosporidiosis, a major health problem for humans and neonatal livestock. In mice, immunologically mediated elimination of C. parvum requires CD4+ T cells and IFN-γ. However, innate immune responses also have a significant protective role in both adult and neonatal mice. NK cells and IFN-γ have been shown to be important components in immunity in T and B cell-deficient mice, but IFN-γ-dependent resistance has also been demonstrated in alymphocytic mice. Epithelial cells may play a vital role in immunity as once infected these cells have increased expression of inflammatory chemokines and cytokines and demonstrate antimicrobial killing mechanisms, including production of NO and antimicrobial peptides. Toll-like receptors facilitate the establishment of immunity in mice and are involved in the development of inflammatory responses of infected epithelial cells and also dendritic cells.

Cryptic parasite revealed improved prospects for treatment and control of human cryptosporidiosis through advanced technologies

Cryptosporidium is an important genus of parasitic protozoa of humans and other vertebrates and is a major cause of intestinal disease globally. Unlike many common causes of infectious enteritis, there are no widely available, effective vaccine or drug-based intervention strategies for Cryptosporidium, and control is focused mainly on prevention. This approach is particularly deficient for infections of severely immunocompromised and/or suppressed, the elderly or malnourished people. However, cryptosporidiosis also presents a significant burden on immunocompetent individuals, and can, for example have lasting effects on the physical and mental development of children infected at an early age. In the last few decades, our understanding of Cryptosporidium has expanded significantly in numerous areas, including the parasite life-cycle, the processes of excystation, cellular invasion and reproduction, and the interplay between parasite and host. Nonetheless, despite extensive research, many aspects of the biology of Cryptosporidium remain unknown, and treatment and control are challenging. Here, we review the current state of knowledge of Cryptosporidium, with a focus on major advances arising from the recently completed genome sequences of the two species of greatest relevance in humans, namely Cryptosporidium hominis and Cryptosporidium parvum. In addition, we discuss the potential of next-generation sequencing technologies, new advances in in silico analyses and progress in in vitro culturing systems to bridge these gaps and to lead toward effective treatment and control of cryptosporidiosis.

Evolution of Cryptosporidium in vitro culture

This overview discusses findings from culturing Cryptosporidium spp. in cell and axenic cultures as well as factors limiting the development of this parasite in cultivation systems during recent years. A systematic review is undertaken of findings regarding the life cycle of the parasite, taking into account physiological, biochemical and genetic aspects, in the hope that this attempt will facilitate future approaches to research and developments in the understanding of Cryptosporidium biology.

The cell biology of cryptosporidium infection

Cryptosporidiosis remains a significant cause of enteric disease worldwide. Basic investigations of host: pathogen interactions have revealed the intricate processes mediating infection. The following summarizes the interactions that mediate infection and the host responses that both permit and ultimately clear the infection.

NF-kappaB p65-dependent transactivation of miRNA genes following Cryptosporidium parvum infection stimulates epithelial cell immune responses

Cryptosporidium parvum is a protozoan parasite that infects the gastrointestinal epithelium and causes diarrheal disease worldwide. Innate epithelial immune responses are key mediators of the host's defense to C. parvum. MicroRNAs (miRNAs) regulate gene expression at the posttranscriptional level and are involved in regulation of both innate and adaptive immune responses. Using an in vitro model of human cryptosporidiosis, we analyzed C. parvum-induced miRNA expression in biliary epithelial cells (i.e., cholangiocytes). Our results demonstrated differential alterations in the mature miRNA expression profile in cholangiocytes following C. parvum infection or lipopolysaccharide stimulation. Database analysis of C. parvum-upregulated miRNAs revealed potential NF-κB binding sites in the promoter elements of a subset of miRNA genes. We demonstrated that mir-125b-1, mir-21, mir-30b, and mir-23b-27b-24-1 cluster genes were transactivated through promoter binding of the NF-κB p65 subunit following C. parvum infection. In contrast, C. parvum transactivated mir-30c and mir-16 genes in cholangiocytes in a p65-independent manner. Importantly, functional inhibition of selected p65-dependent miRNAs in cholangiocytes increased C. parvum burden. Thus, we have identified a panel of miRNAs regulated through promoter binding of the NF-κB p65 subunit in human cholangiocytes in response to C. parvum infection, a process that may be relevant to the regulation of epithelial anti-microbial defense in general.

MicroRNAs (miRNAs) are newly identified small non-coding RNAs that regulate gene expression at the posttranscriptional level. While much of our understanding of the cellular processes modulated by miRNAs has come from studies on development and tumorigenesis, the role of miRNAs in immune responses is now being gradually uncovered. Nevertheless, whether miRNA-mediated posttranscriptional gene regulation is involved in the fine-tuning of epithelial cell immune responses against pathogen infection remains undefined. Cryptosporidium parvum is a protozoan parasite that infects gastrointestinal epithelium. TLR/NF-κB-mediated innate immune responses by epithelial cells are critical to the host's defense to infection. Using an in vitro model of human cryptosporidiosis, we show here differential alterations in the miRNA expression profile in biliary epithelial cells following C. parvum infection. Promoter binding of NF-κB p65 subunit accounts for the upregulation of a panel of miRNA genes in cells infected by C. parvum. Importantly, functional inhibition of several NF-κB p65-dependent miRNAs in epithelial cells increases C. parvum infection burden. Our findings suggest that host epithelial cells activate NF-κB signaling to regulate miRNA expression in response to C. parvum infection. Moreover, NF-κB-mediated miRNA expression is involved in epithelial anti-microbial defense. Our study provides new insights into epithelial cell immunoregulation.

Over-expression and localization of a host protein on the membrane of Cryptosporidium parvum infected epithelial cells

The genus Cryptosporidium includes several species of intestinal protozoan parasites which multiply in intestinal epithelial cells. The impact of this infection on the transcriptome of cultured host cells was investigated using DNA microarray hybridizations. The expression of 14 genes found to be consistently up- or down-regulated in infected cell monolayers was validated with RT PCR. Using immunofluorescence we examined the expression of Protease Activated Receptor-2, which is encoded by one of the up-regulated genes. In infected cells this receptor localized to the host cell membrane which covers the intracellular trophozoites and meronts. This observation indicates that the composition of the host cell membrane is affected by the developing trophozoite, a phenomenon which has not been described previously.

MicroRNA-98 and let-7 confer cholangiocyte expression of cytokine-inducible Src homology 2-containing protein in response to microbial challenge

Posttranscriptional gene regulation by microRNAs (miRNAs) has been implicated in the fine-tuning of TLR-mediated inflammatory response. The cytokine-inducible Src homology 2-containing protein (CIS), one member of the suppressors of cytokine signaling family of proteins, is an important negative regulator for inflammatory cytokine signaling. Using in vitro models using normal human biliary epithelial cells (cholangiocytes), we demonstrated that LPS stimulation or infection with the parasitic protozoan Cryptosporidium parvum induced expression of CIS protein without a change in CIS mRNA levels by activating the TLR signaling pathway. Of those miRNAs expressed in cholangiocytes, we found that targeting of the 3'-untranslated region of CIS by microRNA-98 (miR-98) or let-7 resulted in translational repression, but not CIS mRNA degradation. LPS stimulation or C. parvum infection decreased cholangiocyte expression of miR-98 and let-7. Down-regulation of miR-98 and let-7 relieved miRNA-mediated translational suppression of CIS and contributed to LPS- and C. parvum-stimulated CIS protein expression. Moreover, gain-of-function (by overexpression of CIS) and loss-of-function (by siRNA interference) studies revealed that CIS could enhance IkappaBalpha degradation and regulate NF-kappaB activation in cholangiocytes in response to LPS stimulation or C. parvum infection. Our data suggest that miR-98 and let-7 confer cholangiocyte expression of CIS in response to microbial challenge, a process that may be relevant to the regulation of TLR-mediated epithelial innate immune response.

Cryptosporidium parvum DNA replication in cell-free culture

The lack of robust methods for culturing Cryptosporidium parasites remains a major challenge and is hampering efforts to screen for anti-cryptosporidial drugs. In existing culture methods, monolayers of mammalian epithelial cells are inoculated with oocysts. The system supports an initial phase of asexual proliferation of the parasite. For reasons that are not clear, development rapidly declines within 2-3 days. The unexpected report of Cryptosporidium parvum culture in the absence of host cells, and the failure of others to reproduce the method, prompted us to apply quantitative PCR to measure changes in C. parvum DNA levels in cell-free cultures, and parasite-specific antibodies to identify different life cycle stages. Based on this approach, which has not been applied previously to analyze C. parvum growth in cell-free culture, we found that the concentration of C. parvum DNA increased by about 5-fold over 5 days of culture. Immuno-labeling of cultured organisms revealed morphologically distinct stages, only some of which reacted with Cryptosporidium-specific monoclonal antibodies. These observations are indicative of a modest proliferation of C. parvum in cell-free culture.

Pathophysiology of diarrhea in calves

Infectious diarrhea in calves is most commonly associated with enterotoxigenic Escherichia coli, Cryptosporidium parvum, rotavirus, coronavirus, or some combination of these pathogens. Each of these agents leads to diarrhea through either secretion or malabsorption/maldigestion, though the specific mechanisms and pathways may differ. Specific pharmacologic control and treatment are dependent on gaining a greater understanding of the pathophysiology of these organisms.

Minireview: clinical cryptosporidiosis

Cryptosporidium has emerged as an important cause of diarrhoeal illness worldwide, especially amongst young children and patients with immune deficiencies. Usually presenting as a gastro-enteritis-like syndrome, disease ranges in seriousness from mild to severe and signs and symptoms depend on the site of infection, nutritional and immune status of the host, and parasite-related factors. Sources and routes of transmission are multiple, involving both zoonotic and anthroponotic spread, and facilitated by the resistance of the parasite to many commonly used disinfectants. Prevention and control measures are important for the protection of vulnerable groups since treatment options are limited. This review covers the life cycle, pathogenesis, clinical presentations, diagnosis, prevention and management of cryptosporidiosis in humans.

[Physiologic and pathologic experimental models for studying cholangiocytes]

Cholangiocytes (epithelial cells lining the intra- and extrahepatic bile ducts) and hepatocytes are two major components of liver epithelia. Although cholangiocytes are less numerous than hepatocytes, they are involved in both bile secretion and diverse cellular processes such as cell-cycle phenomena, cell signaling, and interactions with other cells, matrix components, foreign organisms, and xenobiotics. Cholangiocytes are also targets in several human diseases including cholangiocarcinoma, primary sclerosing cholangitis, autoimmune cholangitis, and vanishing bile-duct syndrome. The rapid advances in experimental biology technologies are greatly expanding interest in and knowledge of the physiology and pathophysiology of cholangiocytes. This review focuses on the progress of in vivo and in vitro experimental models in elucidating the physiologic functions of cholangiocytes and the pathophysiology of various cholangiopathies. The following aspects are reviewed: isolation of cholangiocytes from the liver and their heterogeneity, various culture systems, establishment of cholangiocyte cell lines, isolation and usage of intrahepatic bile-duct units, three-dimensional modeling of the bile duct, experimental models for inducing cholangiocyte proliferation, and various cholangiopathies such as cholangiocarcinoma, primary sclerosing cholangitis, and autoimmune cholangitis.

Cryptosporidium infection of human intestinal epithelial cells increases expression of osteoprotegerin: a novel mechanism for evasion of host defenses

Cryptosporidium parasites are pathogens of human intestinal epithelial cells. To determine which genes are regulated during early infection, human ileal mucosa cultured as explants was infected with C. parvum or C. hominis, and gene expression was analyzed by microarray. The gene for osteoprotegerin (OPG) was up-regulated by both parasites. OPG mRNA was also significantly increased in biopsy specimens obtained from a volunteer experimentally infected with C. meleagridis, compared with levels in a prechallenge biopsy specimen. After in vitro infection of HCT-8 cells, there was an early peak in production of OPG mRNA protein. Treatment of infected cells with the OPG ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induced epithelial cell apoptosis and reduced parasite numbers, and recombinant OPG blocked these effects. These results suggest a novel TRAIL-mediated pathway for elimination of Cryptosporidium infection and a role for OPG in modulating this host response.

Inhibition of apoptosis in Cryptosporidium parvum-infected intestinal epithelial cells is dependent on survivin

Cryptosporidium parvum is an obligate intracellular protozoan capable of causing severe diarrheal disease in a wide variety of mammals, including humans. C. parvum infection has been associated with induction of apoptosis in exposed epithelial cells, and we now demonstrate that apoptosis is restricted to a subset of cells actively infected with C. parvum. Approximately 20% of the infected cells underwent apoptosis within 48 h of infection, suggesting that the majority of the infected cells are rescued from apoptosis. C. parvum infection resulted in low-level activation of multiple members of the caspase family, including caspase-2, -3, -4, -6, -8, and -9. The kinetics of caspase activation correlated with apoptosis over a 48-h time course. Pan caspase inhibitors reduced apoptosis of epithelial cells infected by C. parvum. Furthermore, C. parvum infection inhibited staurosporine-induced apoptosis and caspase-3/7 activation at 24 h and 48 h. Infection with C. parvum led to upregulation of genes encoding inhibitors of apoptosis proteins (IAPs), including c-IAP1, c-IAP2, XIAP, and survivin. Knockdown of survivin gene expression, but not that of c-IAP1, c-IAP2, or XIAP expression, increased caspase-3/7 activity as well as apoptosis of infected cells and decreased C. parvum 18S rRNA levels. These data suggest that the apoptotic response of infected intestinal epithelial cells is actively suppressed by C. parvum via upregulation of survivin, favoring parasite infection.

The immunobiology of cholangiocytes

Cholangiocytes, the epithelial cells lining bile ducts, provide the first line of defense against lumenal microbes in the biliary system. Recent advances in biliary immunity indicate that cholangiocytes express a variety of pathogen-recognition receptors and can activate a set of intracellular signaling cascades to initiate a profound antimicrobial defense, including release of proinflammatory cytokines and chemokines, production of antimicrobial peptides and maintenance of biliary epithelial integrity. Cholangiocytes also interact with other cell types in the liver (for example, lymphocytes and Kupffer cells) through expression and release of adhesion molecules and immune mediators. Subsequently, through an intricate feedback mechanism involving both epithelial and other liver cells, a set of intracellular signaling pathways are activated to regulate the functional state of cholangiocyte responses during microbial infection. Thus, cholangiocytes are actively involved in mucosal immunity of the biliary system and represent a fine-tuned, integral component of liver immunity.

Preferential infection of dividing cells by Cryptosporidium parvum

In spite of its limitations, the culture of Cryptosporidium parvum in monolayers of epithelial cells is a suitable model to study the interaction of this protozoan parasite with the host cell, to assay oocyst infectivity, and to screen drugs for anti-cryptosporidial activity. For unknown reasons, growth of Cryptosporidium in culture is limited in time and generally does not lead to the production of significant numbers of oocysts. In monolayers infected with high doses of oocysts, we observed that many cells remain uninfected, suggesting that some cells are less susceptible to the infection. Since C. parvum and the related species C. hominis lack many essential biosynthetic pathways, we tested whether the dependence of the parasite on host cell metabolites may favour the infection of cells in mitosis. The proportion of monolayer cells in stationary (G0/G1) phase and in mitosis (S/G2/M) was determined and the prevalence of infected cells in each subpopulation was quantified. Although C. parvum infects and develops in dividing and stationary cells, a significant preference for cells in S/G2/M phase was observed. Consistent with previous observations showing that C. parvum induces apoptosis in cell monolayers, infection was accompanied by a significant increase in the proportion of mitotic cells.

DISTRIBUTION OF CRYPTOSPORIDIUM PARVUM SPOROZOITE APICAL ORGANELLES DURING ATTACHMENT TO AND INTERNALIZATION BY CULTURED BILIARY EPITHELIAL CELLS

Although accumulating evidence supports an active role for host cells during Cryptosporidium parvum invasion of epithelia, our knowledge of the underlying parasite-specific processes triggering such events is limited. In an effort to better understand the invasion strategy of C. parvum, we characterized the presence and distribution of the apical organelles (micronemes, dense granules, and rhoptry) through the stages of attachment to, and internalization by, human biliary epithelia, using serial-section electron microscopy. Novel findings include an apparent organized rearrangement of micronemes upon host cell attachment. The apically segregated micronemes were apposed to a central microtubule-like filamentous structure, and the more distal micronemes localized to the periphery and apical region of the parasite during internalization, coinciding with the formation of the anterior vacuole. The morphological observations presented here extend our understanding of parasite-specific processes that occur during attachment to, and internalization by, host epithelial cells.

ELECTRON MICROSCOPIC OBSERVATION OF THE INVASION PROCESS OF CRYPTOSPORIDIUM PARVUM IN SEVERE COMBINED IMMUNODEFICIENCY MICE

Cryptosporidium parvum mainly invades the intestinal epithelium and causes watery diarrhea in humans and calves. However, the invasion process has not yet been clarified. In the present study, the invasion process of C. parvum in severe combined immunodeficiency (SCID) mice was examined. Infected mice were necropsied; the ilea were double-fixed routinely and observed by scanning and transmission electron microscopy. In addition, the microvillus membrane was observed by ruthenium red staining. Scanning electron micrographs showed elongation of the microvilli at the periphery of the parasite. The microvilli were shown to be along the surface of the parasite in higher magnification. Transmission electron microscopy confirmed that the invading parasites were located among microvilli. Parasites existed in the parasitophorous vacuole formed by the microvillus membrane. The parasite pellicle attached to the host cell membrane at the bottom of the parasite, and then the pellicle and host cell membrane became unclear. Subsequently, the pellicle became complicated and formed a feeder organelle. In addition, invasion of the parasite was not observed in either a microvillus or the cytoplasm of the host cell. Therefore, C. parvum invades among microvilli, is covered with membranes derived from numerous microvilli, and develops within the host cell.

Cryptosporidium parvum at different developmental stages modulates host cell apoptosis in vitro

We studied apoptosis in a human ileocecal adenocarcinoma tumor cell line (HCT-8) infected with Cryptosporidium parvum, from 2 to 72 h postinfection (h.p.i.). At 2 h.p.i., the percentage of annexin V-positive cells in the cell culture had increased to 10% compared to 2.5% in noninfected control culture; sorted infected cells expressed mRNA of FasL, the active form of caspase 3, and high caspase 3 activity, whereas the noninfected neighboring cells sorted from the same culture showed no signs of apoptosis. At 24 h.p.i., the percentages of early (annexin V positive) and late (DNA fragment) apoptotic cells were 13 and 2%, respectively, in the entire cell culture, and these percentages were not statistically significant in comparison with those from noninfected control cultures. At this time, sorted infected cells expressed the inactive form of caspase 3, a low caspase 3 activity, and the antiapoptotic protein Bcl-2. Noninfected cells sorted from the same culture showed expression of the active form of caspase 3, a moderate caspase 3 activity, and no Bcl-2 expression. At 48 h.p.i., the percentages of early and late apoptotic cells and caspase 3 activity had increased in the total cell culture, and both sorted infected and noninfected cells showed the active form of caspase 3. These results show that C. parvum, depending on its developmental stage, can inhibit (at the trophozoite stage) or promote (at the sporozoite and merozoite stages) host cell apoptosis, suggesting that it is able to interact with and regulate the host-cell gene expression.

Host cell tropism underlies species restriction of human and bovine Cryptosporidium parvum genotypes

It has been recognized recently that human cryptosporidiosis is usually caused by Cryptosporidium parvum genotype I ("human" C. parvum), which is not found in animals. Compared to C. parvum genotype II, little is known of the biology of invasion of the human-restricted C. parvum genotype I. The aims of the present study were (i) to explore and compare with genotype II the pathogenesis of C. parvum genotype I infection by using an established in vitro model of infection and (ii) to examine the possibility that host-specific cell tropism determines species restriction among C. parvum genotypes by using a novel ex vivo small intestinal primary cell model of infection. Oocysts of C. parvum genotypes I and II were used to infect HCT-8 cells and primary intestinal epithelial cells in vitro. Primary cells were harvested from human endoscopic small-bowel biopsies and from bovine duodenum postmortem. C. parvum genotype I infected HCT-8 cells with lower efficiency than C. parvum genotype II. Actin colocalization at the host parasite interface and reduction in levels of invasion after treatment with microfilament inhibitors (cytochalasin B and cytochalasin D) were observed for both genotypes. C. parvum genotype II invaded primary intestinal epithelial cells, regardless of the species of origin. In contrast, C. parvum genotype I invaded only human small-bowel cells. The pathogenesis of C. parvum genotype I differs from C. parvum genotype II. C parvum genotype I does not enter primary bovine intestinal cells, suggesting that the species restriction of this genotype is due to host tissue tropism of the infecting isolate.

Structural analysis of Cryptosporidium parvum

Cryptosporidium parvum (Apicomplexa, formerly Sporozoa) is the causative agent of cryptosporidiosis, an enteric disease of substantial medical and veterinary importance. C. parvum shows a number of unique features that differ from the rest of the class of coccidea in which it is currently grouped taxonomically. Differences occur in the overall structure of the transmission form and the invasive stages of the parasite, its intracellular location, the presence of recently described additional extracellular stages, the host range and target cell tropism, the ability to autoinfection, the nonresponsiveness to anticoccidial drugs, the immune response of the host, and immunochemical and genetic characteristics. These differences have an important impact on the infectivity, the epidemiology, the therapy, and the taxonomy of the parasite. The present article describes the structural analysis of the parasite using light and electron microscopy with an emphasis on structural details unique to C. parvum.

Cdc42 and the actin-related protein/neural Wiskott-Aldrich syndrome protein network mediate cellular invasion by Cryptosporidium parvum

Cryptosporidium parvum invasion of epithelial cells involves host cell membrane alterations which require a remodeling of the host cell actin cytoskeleton. In addition, an actin plaque, possibly associated with the dense-band region, forms within the host cytoplasm at the host-parasite interface. Here we show that Cdc42 and RhoA, but not Rac1, members of the Rho family of GTPases, are recruited to the host-parasite interface in an in vitro model of human biliary cryptosporidiosis. Interestingly, activation of Cdc42, but not RhoA, was detected in the infected cells. Neural Wiskott-Aldrich syndrome protein (N-WASP) and p34-Arc, actin-regulating downstream effectors of Cdc42, were also recruited to the host-parasite interface. Whereas cellular expression of a constitutively active mutant of Cdc42 promoted C. parvum invasion, overexpression of a dominant negative mutant of Cdc42, or depletion of Cdc42 mRNA by short interfering RNA-mediated gene silencing, inhibited C. parvum invasion. Expression of the WA fragment of N-WASP to block associated actin polymerization also inhibited C. parvum invasion. Moreover, inhibition of host cell Cdc42 activation by dominant negative mutation inhibited C. parvum-associated actin remodeling, membrane protrusion, and dense-band formation. In contrast, treatment of cells with a Rho inhibitor, exoenzyme C3, or cellular overexpression of dominant negative mutants of RhoA and Rac1 had no effect on C. parvum invasion. These data suggest that C. parvum invasion of target epithelia results from the organism's ability to activate a host cell Cdc42 GTPase signaling pathway to induce host cell actin remodeling at the attachment site.

CRYPTOSPORIDIUM PARVUM ATTACHMENT TO AND INTERNALIZATION BY HUMAN BILIARY EPITHELIA IN VITRO: A MORPHOLOGIC STUDY

To explore the mechanisms by which Cryptosporidium parvum infects epithelial cells, we performed a detailed morphological study by serial electron microscopy to assess attachment to and internalization of biliary epithelial cells by C. parvum in an in vitro model of human biliary cryptosporidiosis. When C. parvum sporozoites initially attach to the host cell membrane, the rhoptry of the sporozoite extends to the attachment site; both micronemes and dense granules are recruited to the apical complex region of the attached parasite. During internalization, numerous vacuoles covered by the parasite's plasma membrane are formed and cluster together to establish a preparasitophorous vacuole. This preparasitophorous vacuole comes in contact with host cell membrane to form a host cell-parasite membrane interface, beneath which an electron-dense band begins to appear within the host cell cytoplasm. Simultaneously, host cells display membrane protrusion along the edge of the host cell-parasite membrane interface, resulting in the formation of a mature parasitophorous vacuole that completely covers the parasite. During internalization, vacuole-like structures appear in the apical complex region of the attached sporozoite, which bud out into host cells. A tunnel directly connecting the parasite to the host cell cytoplasm forms during internalization and remains when the parasite is totally internalized. Immunoelectron microscopy showed that sporozoite-associated proteins were localized along the dense band and at the parasitophorous vacuole membrane. These morphological observations provide evidence that secretion of parasite apical organelles and protrusion of host cell membrane play an important role in the attachment and internalization of host epithelial cells by C. parvum.

Stem cell factor restores hepatocyte proliferation in IL-6 knockout mice following 70% hepatectomy

Stem cell factor (SCF) is a molecule with known proliferative effects on hematopoietic cells. More recent studies suggest that this molecule may also have effects on cellular differentiation and proliferation in other types of cells. The current investigations demonstrate that there is a large reservoir of SCF in the liver, that hepatic SCF levels change dramatically following partial hepatectomy in mice, and that SCF blockade, either by administration of anti-SCF antibodies or by using genetically altered, SCF-deficient mice, inhibits hepatocyte proliferation after partial hepatectomy; if SCF is replaced in the genetically SCF-deficient mice after partial hepatectomy, hepatocyte proliferation is restored to that seen in WT animals. Furthermore, SCF administration to IL-6 knockout mice also restores hepatocyte proliferation to normal. In vitro studies using primary mouse hepatocytes demonstrate that SCF causes hepatocyte proliferation and is induced by IL-6 and that treatment with anti-SCF antibodies inhibits IL-6-induced hepatocyte proliferation. Further in vivo studies in IL-6 knockout mice demonstrate that SCF administration to these animals increases p-stat3 levels, suggesting that the SCF-induced increase in hepatocyte proliferation in this system is stat3-mediated.

Host cell fate on Cryptosporidium parvum egress from MDCK cells

Cryptosporidium parvum is an intracellular protozoan parasite that causes a severe diarrheal illness of unclear etiology. Also unclear is the fate of the host cell upon parasite egress. We show in an MDCK cell model that the host cell is killed upon parasite egress; this death is necrotic, rather than apoptotic, in nature.

Cryptosporidium parvum invasion of biliary epithelia requires host cell tyrosine phosphorylation of cortactin via c-Src

BACKGROUND & AIMS

Cryptosporidium parvum invasion of epithelia requires polymerization of host cell actin at the attachment site. We analyzed the role of host cell c-Src, a cytoskeleton-associated protein tyrosine kinase, in C. parvum invasion of biliary epithelia.

METHODS

In vitro models of biliary cryptosporidiosis using a human biliary epithelial cell line were used to assay the role of c-Src signaling pathway in C. parvum invasion.

RESULTS

c-Src and cortactin, an actin-binding protein and a substrate for c-Src, were recruited to the parasite-host cell interface during C. parvum invasion. Tyrosine phosphorylation of cortactin in infected cells was also detected. Inhibition of host cell c-Src significantly blocked C. parvum -induced accumulation and tyrosine phosphorylation of cortactin and actin polymerization at the attachment sites, thereby inhibiting C. parvum invasion of biliary epithelial cells. A triple mutation of tyrosine of cortactin in the epithelia also diminished C. parvum invasion. In addition, proteins originating from the parasite were detected within infected cells at the parasite-host cell interface. Antiserum against C. parvum membrane proteins blocked accumulation of c-Src and cortactin and significantly decreased C. parvum invasion. No accumulation of the endocytosis-related proteins, dynamin 2 and clathrin, was found at the parasite-host cell interface; also, inhibition of dynamin 2 did not block C. parvum invasion.

CONCLUSIONS

C. parvum invasion of biliary epithelial cells requires host cell tyrosine phosphorylation of cortactin by a c-Src-mediated signaling pathway to induce actin polymerization at the attachment site, a process associated with microbial secretion but independent of host cell endocytosis.

Cryptosporidium dose-response studies: variation between hosts

The issue of variation is highly important in dose-response analysis: variation among genetically related pathogens infecting the same host, but also variation among hosts, in susceptibility to infection by the same pathogen. This latter issue is addressed here for the protozoan parasite Cryptosporidium parvum, the causative agent for many outbreaks of water-borne gastrointestinal illness. In human feeding studies, infectivity has been shown to be low in subjects with high preexisting anti-Cryptosporidium IgG-levels. Here we adapt the hit theory model of microbial infection to incorporate covariables, characterizing the immune status of the susceptible host. The probability of any single oocyst in the inoculum to cause infection appears to depend on preexisting IgG-levels. This does not necessarily imply direct protection by the humoral immune system; high IgG-levels may reflect a recent episode of infection/illness, and be an epi-phenomenon associated with other protective responses. The IgG-dependence of the dose-response relation can be easily applied in quantitative risk analysis. The distribution of anti-Cryptosporidium IgG levels in the general population is accessible by analyzing serum banks, which are maintained in many Western countries. Using such an approach provides first insights into the variation of susceptibility to infection in the general population.

The role of cytokines in the pathogenesis of Cryptosporidium infection

First described in 1912, the importance of the coccidian parasite Cryptosporidium parvum as an enteropathogen in humans was not recognized until the early 1980s, when it was found to be a common opportunistic infection in AIDS. Infection with this organism triggers a complex array of innate and cell-mediated immune responses within the intestinal mucosa. How cytokines and chemokines interact to regulate these responses in order to achieve clearance of the parasite yet preserve the integrity of the intestinal mucosa is still being unravelled. T helper type 1 cytokines, and particularly interferon-gamma, have long been considered to be the main orchestrators of the immune response to this infection, but recent studies suggest that T helper type 2 cytokines may also be involved. In addition, transforming growth factor-beta 1, although having little effect on parasite development, is an important modulator of the immune response and plays a role in protecting the epithelial integrity from the effects of the inflammatory process.

The pathobiology of biliary epithelia

The morbidity and mortality from chronic biliary diseases (i.e., the cholangiopathies) remains substantial. End-stage liver disease from biliary causes of cirrhosis (e.g., primary biliary cirrhosis [PBC], and primary sclerosing cholangitis) account for approximately one third of patients referred for liver transplantation. A single-topic conference sponsored by the American Association for the Studies of Liver Diseases entitled "The Pathobiology of Biliary Epithelia" brought together investigators to review the status of the field of cholangiocyte pathobiology, identify new areas of interest, and propose future directions. This information was presented in 6 sessions: "Structural and Functional Characteristics of the Bile Duct System," "Biological Topics from Nonbiliary Epithelia," "Malignant Transformation of Cholangiocytes," "Cholangiocyte Proliferation and Death," "Transport Mechanisms in Bile Duct Epithelia," and "Pathobiology of Biliary Epithelia." In the 7 years since the first symposium on this topic, major advances have been made in our understanding of ductal bile formation, including, greater insight into the hormones, intracellular signaling mechanisms, and effector proteins responsible for bile secretion and absorption. More sophisticated imaging technologies have increased our understanding of the polarity of cholangiocytes, their embryology and ultrastructural anatomy, and in vivo human secretory responses to current medical therapy. Information on mediators of inflammation permeated many sessions, having potentially important roles in malignant transformation of cholangiocytes, cholangiocyte apoptosis, fluid and electrolyte transport, and have begun to be specifically characterized for certain biliary diseases, e.g., acquired immunodeficiency syndrome (AIDS) cholangiopathy and graft-versus-host disease (GVHD).

Experimental models to study cholangiocyte biology

Cholangiocytes-the epithelial cells which line the bile ducts-are increasingly recognized as important transporting epithelia actively involved in the absorption and secretion of water, ions, and solutes. This recognition is due in part to the recent development of new experimental models. New biologic concepts have emerged including the identification and topography of receptors and flux proteins on the apical and/or basolateral membrane which are involved in the molecular mechanisms of ductal bile secretion. Individually isolated and/or perfused bile duct units from livers of rats and mice serve as new,physiologically relevant in vitro models to study cholangiocyte transport. Biliary tree dimensions and novel insights into anatomic remodeling of proliferating bile ducts have emerged from three-dimensional reconstruction using CT scanning and sophisticated software. Moreover, new pathologic concepts have arisen regarding the interaction of cholangiocytes with pathogens such as Cryptosporidium parvum. These concepts and associated methodologies may provide the framework to develop new therapies for the cholangiopathies, a group of important hepatobiliary diseases in which cholangiocytes are the target cell.

Intracellular protozoan parasites and apoptosis: diverse strategies to modulate parasite-host interactions

Programmed cell death (apoptosis) is an important regulator of the host's response during infection with a variety of intracellular protozoan parasites. Parasitic pathogens have evolved diverse strategies to induce or inhibit host-cell apoptosis, thereby modulating the host's immune response, aiding dissemination within the host or facilitating intracellular survival. Here, we review the molecular and cell-biological mechanisms of the pathogen-induced modulation of host-cell apoptosis and its effects on the parasite-host interaction and the pathogenesis of parasitic diseases. We also discuss the previously unrecognized phenomenon of apoptotic cell death in (unicellular) protozoan parasites and its potential implications.