Variants in the SPINT2 gene, encoding HAI-2, cause the syndromic form of autosomal recessive Congenital Sodium Diarrhea (CSD), a life-threatening disease with early-onset persistent diarrhea and stools containing elevated levels of sodium. A newly discovered variant, HAI-2 Y68C, causes a milder non-syndromic CSD-like condition in an isolated case. The present investigation compares biochemical features of the HAI-2 Y68C with two newly discovered syndromic CSD-causing variants, HAI-2 Y129C and R148H, and two previously characterized syndromic CSD-causing variants, HAI-2 F161V and G168S. HAI-2 contains two Kunitz domains that inhibit a range of serine proteases, including matriptase and prostasin. HAI-2 is co-located with target proteases mainly in the ER and the early secretory pathway but also on the apical plasma membrane of polarized cells. All investigated variants of HAI-2 were expressed with a similar steady-state protein level and weak pericellular staining as wild-type HAI-2. However, none of the CSD-causing variants were capable of inhibiting prostasin as efficiently as wild-type HAI-2. In addition, the HAI-2 Y68C variant was unable to inhibit matriptase, constituting a clear biochemical difference between the single variant causing non-syndromic CSD and the variants causing syndromic CSD. The HAI-2 Y68C variant may affect the phenotype by altering matriptase catalyzed activation of prostasin in the early secretory pathway. We hypothesize that CSD, whether syndromic or non-syndromic, is caused by a lack of inhibitory activity of HAI-2 in the early secretory pathway and/or on the apical plasma membrane, resulting in uncontrolled proteolytic activity leading to proteolytic modification of sodium transporters/channels like NHE3 and ENaC. NHE3 and ENaC are normally located on the apical plasma membrane and are mainly responsible for the uptake of sodium from the gut lumen. Oral administration of a protease inhibitor mimicking HAI-2 wild-type could potentially help to reduce the degradation of proteins important for sodium uptake across the apical plasma membrane.

Diarrheal disease caused by Bacillus cereus is driven by pore-forming enterotoxins, with hemolysin BL (Hbl) and non-hemolytic enterotoxin (Nhe) as major virulence factors. Toxin structure and mode of action are well described, while especially human intestinal epithelial responses remain incompletely characterized. With the co-culture system of Caco-2 and mucus-producing HT29-MTX cells (9:1), this study describes a suitable model for investigating enterotoxin effects. Cell line-, dose- and enterotoxin-dependent changes in cell viability were detected. Undifferentiated Caco-2 generally showed higher susceptibility than HT29-MTX cells. Twenty-four-hour co-cultures were more susceptible than 28-day co-cultures but displayed partial recovery after toxin removal. Enterotoxin exposure caused a time-dependent decline in transepithelial electrical resistance, consistent with barrier dysfunction. In Ussing chamber experiments, enterotoxin-containing supernatants, especially Nhe+Hbl, increased short-circuit currents, indicating altered electrogenic ion transport consistent with pro-secretory phenotypes relevant to diarrhea. Transcriptome analyses demonstrated a pronounced time- and dose-dependent response toward Nhe+Hbl. Two-hour sublethal enterotoxin exposure elicited few transcriptional changes, whereas lethal enterotoxin exposure rapidly suppressed proliferation- and cell-cycle-associated programs. Pathway analyses consistently highlighted TNFα/NF-κB-driven inflammation, MAPK-associated stress signaling, and p53/apoptosis pathways; metabolic remodeling and senescence-/autophagy-related processes became more apparent at longer exposure (8 h) and/or higher toxin levels. Dose-dependent caspase-3/7 activation, LDH release, and minimal caspase-1 activation were also consistent with early inflammatory response, apoptotic cell death and membrane damage. Collectively, this study integrates barrier function, ion transport, cell viability, and transcriptomics to define host intestine epithelial response programs triggered by B. cereus enterotoxins and to connect toxin exposure to barrier dysfunction and diarrheal pathophysiology.

Vasoactive intestinal peptide (VIP)-secreting neuroblastic tumors are rare pediatric neoplasms that may present with chronic secretory diarrhea prior to tumor detection, posing significant diagnostic and therapeutic challenges. VIP-induced diarrhea is typically profuse, watery, and refractory to standard management, often resulting in dehydration, electrolyte imbalance, weight loss, and failure to thrive. Herein, we report two pediatric patients with neuroblastoma who presented with chronic secretory diarrhea as the initial and predominant symptom. In both cases, the persistent diarrhea remained unexplained by common etiologies and was unresponsive to conventional therapies. After approximately three months of ongoing symptoms accompanied by progressive abdominal distension, radiologic evaluation revealed intra-abdominal masses in both cases. The diagnosis of neuroblastoma was confirmed by tumor biopsy in one patient and bone marrow biopsy in the other. Serum VIP levels were not available in either case; however, the clinical presentation was consistent with VIP-mediated secretory diarrhea. Although surgical resection of VIP-secreting neuroblastic tumors in localized disease is typically associated with resolution of diarrhea and favorable outcomes, delayed diagnosis could result in advanced, metastatic, and inoperable disease. Both of our cases had metastatic, inoperable disease at the time of diagnosis, and proved fatal in one. These cases underscore the importance of considering VIP-secreting neuroblastic tumors in the differential diagnosis of children presenting with unexplained, treatment-resistant secretory diarrhea. Early recognition may facilitate timely diagnosis and improve clinical outcomes.

BEST4[+] cells are a recently identified, functionally specialized intestinal epithelial cell population characterized through single-cell and spatial transcriptomic analyses in humans, pigs, rats, and other vertebrates. These cells characteristically express BEST4, OTOP2, CA7, and GUCY2C, and exhibit particularly high CFTR expression in the small intestine, supporting their roles in luminal pH sensing, fluid-electrolyte homeostasis, and mucus hydration. Emerging evidence from organoid and cross-species studies supports a predominant role for the NOTCH-SPIB signaling axis in driving BEST4[+] cell differentiation, but their lineage identity and regulatory mechanisms remain incompletely resolved. This review focuses on intestinal epithelial BEST4[+] cells as a critical cellular hub that connects ion transport physiology with the pathophysiology of secretory diarrhea. We summarize the molecular identity, regional distribution, and cross-species conservation of BEST4[+] cells. We then evaluate their roles in luminal pH regulation, electrogenic fluid secretion, and mucus barrier integrity. We detail how bacterial enterotoxins activate the GC-C/cGMP/CFTR and cAMP/PKA/CFTR pathways in BEST4[+] cells to drive pathological fluid hypersecretion, and discuss how viral infections may indirectly engage or amplify BEST4[+] cell-associated ion-transport pathways. We also outline their relevance to viral diarrhea, inflammatory bowel disease, and cystic fibrosis-associated intestinal dysfunction. Finally, we assess established antidiarrheal approaches and investigational strategies that modulate BEST4[+] cell-associated pathways, and emphasize the need for cell-type-specific validation in physiologically relevant in vivo models.

Cryptosporidium parvum is a prevalent protozoan pathogen responsible for moderate to severe diarrhea in humans and animals, posing a significant threat to global public health. Host-parasite interactions play a crucial role in the infection process of C. parvum. Cyclophilin 23 (CpCyP23), a secreted protein of C. parvum, is implicated in parasite development and pathogenicity. As a secretory protein, its function often relies on interactions with host cell proteins. However, it remains unclear whether CpCyP23 has host interaction partners and what specific roles such interactions play during infection. In this study, using CpCyP23 as bait, we screened for potential interacting proteins in HCT-8 cells through pull-down assays combined with mass spectrometry (MS) analysis. The interaction was subsequently confirmed by co-immunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assays. Immunofluorescence (IF) was employed to determine subcellular localization. Furthermore, after confirming the altered expression of the interacting protein during parasite infection, we analyzed its impact on parasite infection using small interfering RNA (siRNA) knockdown and overexpression techniques, coupled with parasite infection assays. The results identified the host cell protein Prohibitin-2 (PHB2) as an interacting partner of CpCyP23. Co-IP and BiFC assays validated their interaction, and IF revealed their colocalization in the cytoplasm of HCT-8 cells. The mRNA level of PHB2 was significantly upregulated at 3 and 12 h postinfection (hpi) with C. parvum. Functional experiments further demonstrated that the expression level of PHB2 positively correlates with the infection burden of C. parvum. In summary, this study reveals an interaction between the host cell protein PHB2 and CpCyP23 and identifies PHB2 expression level as a key host factor significantly influencing C. parvum infection. These findings provide experimental evidence and a potential target for elucidating the pathogenic mechanisms of C. parvum and developing novel anti-infection strategies.

Zinc (Zn) deficiency affects approximately 1 billion people worldwide with severe consequences for their health, including increased intestinal infections, inflammation, and diarrhea. Accordingly, the intestinal defense barrier is compromised, leading to epithelial destruction and alteration of mucus. However, the processes and the extent to which Zn deficiency affects mucin synthesis in intestinal goblet cells (GCs) remain poorly understood. To this end, we investigated the impact of Zn deficiency on mucin expression and glycosylation in the human GC model HT-29-MTX. Zn deprivation altered the GC transcriptome, affecting genes involved in Zn transport, mucin synthesis and glycosylation. Accordingly, mucus composition was changed in Zn-deficient GCs, significantly increasing MUC2 and MUC17 on the mRNA and protein level. Several Zn transporters, mostly those associated with the early secretory pathway (ESP), were dysregulated, indicating an adaptive response of cellular Zn homeostasis. Additionally, free Zn was markedly reduced in the ESP, a critical location for glycosylation. Zn deficit substantially changed mucin glycosylation, characterized by an increase in sialylation and a strong decrease in complex N-glycans. All these changes involved widespread dysregulation of glycosyltransferase expression, including an increase in COSMC, a Zn-binding chaperone essential for the core 1 O-glycan formation. Collectively, our in vitro findings demonstrate that Zn is a critical regulator of mucin production and glycosylation in GCs. Zn deficiency might weaken the protective and functional qualities of intestinal mucus, increasing the risk of infections and potentially disrupting host-microbiome interactions.

Inflammation of the small intestine can be caused by infections, immune-mediated disorders (such as celiac disease) and medications, leading to malabsorption, chronic diarrhea, and subclinical alterations including increased gut permeability and systemic inflammation. It has been suggested that probiotics can restore barrier integrity and reduce inflammation. The objective of the present study was to evaluate the effects of the probiotic yeast Saccharomyces boulardii CNCM I-745 (Sb) in a mouse model of sterile, inflammation-induced enteropathy based on intragastric administration of the gliadin peptide p31-43. Challenge with this peptide induces histological changes in the small intestinal mucosa, causes an elevation in inflammatory markers, and promotes cell death.

Porcine epidemic diarrhea virus (PEDV) remains one of the most critical enteric coronaviruses affecting the global swine industry. PEDV causes severe diarrhea, dehydration, and high mortality in neonatal piglets. Despite the widespread use of commercial vaccines, persistent PEDV outbreaks worldwide indicate that current vaccination strategies provide suboptimal protection. Increasing evidence suggests that the limited effectiveness of current control strategies is primarily attributable to two interrelated factors: continuous antigenic variation among circulating PEDV strains and inadequate induction of maternal mucosal immunity. Unlike systemic viral infections, effective protection against PEDV in neonatal piglets predominantly depends on lactational immunity mediated by the gut-mammary gland-secretory IgA (sIgA) axis. However, most currently available vaccines predominantly induce systemic IgG responses and fail to effectively stimulate intestinal immune imprinting or sustain sIgA secretion in colostrum and milk. In this review, we summarize the mechanisms underlying PEDV evolution and maternal mucosal immunity, with particular emphasis on the gut-mammary gland-sIgA axis. We further discuss the recent advances in and limitations of current vaccine platforms and propose an integrated framework for broadly protective PEDV vaccines based on structural antigen optimization, mucosal-targeted immunization, and sIgA-oriented evaluation systems. This framework may provide new insights into the rational design of more effective maternal vaccines against PEDV and other enteric coronaviruses affecting pigs.

The weaning period represents a critical developmental window during which the intestinal host-microbiota interaction network in piglets undergoes profound remodeling and becomes highly vulnerable to disruption. Abrupt dietary transition, reduced feed intake, environmental stressors, and pathogen exposure can collectively interfere with gut microbial succession, epithelial barrier maturation, mucosal immune homeostasis, and neuroendocrine regulation, thereby increasing the susceptibility of piglets to post-weaning diarrhea, intestinal inflammation, and growth retardation. Gut microorganisms contribute to intestinal homeostasis by regulating epithelial renewal, barrier function, immune tolerance, inflammatory thresholds, and host metabolic adaptation through diverse microbial-derived signals, including short-chain fatty acids, tryptophan metabolites, bile acid derivatives, and amino acid-derived metabolites. Conversely, the host actively shapes microbial composition, spatial organization, and functional activity through mucins, antimicrobial peptides, secretory immunoglobulin A, intestinal alkaline phosphatase, epithelial oxygen gradients, and genetic factors. Dysregulation of these bidirectional interactions under weaning stress may shift the intestinal ecosystem from a homeostatic state toward dysbiosis and inflammation. In this review, we summarize current knowledge regarding the establishment and succession of the piglet gut microbiota, the molecular mechanisms underlying host-microbiota crosstalk, the disruption of these interaction networks during weaning stress, and the application of multi-omics approaches to decipher causal mechanisms. We further discuss microbiota-targeted nutritional strategies, including probiotics, prebiotics, synbiotics, postbiotics, functional amino acids, trace elements, and bioactive compounds, for improving intestinal health in weaned piglets. This review aims to provide a mechanistic framework for precision nutritional regulation of piglet intestinal health and for the development of antibiotic-alternative strategies in swine production.

Chronic diarrhea is a common clinical sign in cats, caused by various underlying conditions. Chronic diarrhea can be classified as either small or large bowel diarrhea based on clinical signs. The mechanisms involved are complex, including osmotic and secretory diarrhea, impaired intestinal barrier function, and dysregulated intestinal motility. Given the complexity of the causes of chronic diarrhea, accurate diagnosis of the underlying mechanisms is crucial for effective treatment. Additionally, nutritional management is indispensable for the long-term treatment of chronic diarrhea. This article will review the diseases that may cause chronic diarrhea, introduce their pathophysiology and diagnostic methods, and provide related nutritional management strategies.

Brucellosis is a zoonotic infection with diverse clinical manifestations that frequently delay diagnosis because of its nonspecific presentation. Hematologic abnormalities have been described in affected patients; however, severe thrombocytopenia remains an uncommon finding. We present the case of a 50-year-old woman who was admitted with a four-day history of fever up to 39°C, diffuse abdominal pain, vomiting, and secretory diarrhea. Initial laboratory evaluation revealed leukopenia (2.7 × 10³/μL), severe thrombocytopenia (29 × 10³/μL), hyponatremia, and elevated liver enzymes. The patient remained hemodynamically stable and showed progressive clinical improvement with supportive management. As part of the etiologic workup, two independent blood cultures yielded Brucella melitensis, establishing the diagnosis of systemic brucellosis. Further evaluation identified a history of unpasteurized milk consumption, and the 2-mercaptoethanol test was positive. Treatment with doxycycline and rifampin was initiated, and the patient subsequently demonstrated progressive clinical recovery, recovery of leukocyte and platelet counts, and improvement in biochemical abnormalities. This case highlights the importance of considering brucellosis in the differential diagnosis of patients presenting with acute febrile illness, severe thrombocytopenia, and leukopenia, even in the absence of classic focal manifestations of the disease.