The small intestine contains crypt-villus units that repeat. The pioneering experiments of Barker et al. (2007) demonstrated that Lgr5 ⁺ CBC stem cells are the progenitors of a variety of epithelial cells, which inhabit the base of the crypt and are intercalated between Paneth cells. Lgr5⁺ CBCs rapidly create transit-amplifying (TA) progenitor cells that move upwards and completely develop before entering the crypt (Barker et al., 2007). Based on morphology and expression features, differentiated epithelial cells may be generally separated into two types: secretory cells and absorptive cells (Flier and Clevers, 2009). Although as many as seven lineages of cells have been described in the intestinal epithelium, including cup cells and “membranous” (M) cells (Madara, 1982; Neutra, 1998), only five of which are usually considered. (Figure 2 depicts an IEC differentiation diagram).

Absorptive epithelial cells comprise the majority of differentiated epithelial cells, while secretory cells account for only 1%. Absorptive IECs play various roles in digestion, nutrition absorption, and mucosal defense. Secretory IECs are in charge of secreting antimicrobial peptides and growth factors, as well as the controlling the gut flora and surrounding stem cells (Gerbe et al., 2011).

Previous research has emphasized the transcriptional start sequence, the participation of particular transcription factors, and epigenetic modification. It is assumed that a multitude of mechanisms is involved in IEC differentiation, however, it is disputed whether transcriptional modulation is involved.

The lack of identifiable biomarkers has hampered the study of TCs since their discovery in the 1950s. Identifying more viable unambiguous, and specific markers has enhanced the research situation, allowing for a more detailed examination of TCs. With updated biomarker information, far more complete research is expected.

According to double immunostaining evidence, DCLK1/5HT-IR cells contain serotonin (5HT) and are a novel subtype of DCLK1-immunoreactive (IR) TCs. These cells shrank distally from the small to the large intestine. 5-HT has a wide range of biological roles, including cognition, learning, memory, emotional control and vasoconstriction (Young, 2007). Approximately 90% of the serotonin in the human body is located in the enterochromaffin cells of the GI tract, where it also involves in the accommodation of gut homeostasis (Berger et al., 2009). In a word, DCLK1/5HT-IR cells, as a non-negligible neo-subtype of TCs, may contribute to the intestinal physiological function (Cheng et al., 2019).

Transcriptome analysis revealed two additional TC subgroups: neuronal TCs (tuft 1) and immunological TCs (tuft 2). Despite the fact that DCLK1 and IL-25 are expressed by both TC subtypes, their roles are distinct (Haber et al., 2017). Tuft 1 has higher levels of neuronal gene expression profile, including Ninj1, Nrep, and Nradd. Immunological genes, such as those encoding CD45 and thymic stromal lymphopoietin (TSLP), were expressed at higher levels in tuft 2 (Haber et al., 2017). When parasite infections occur, tuft 2 outnumbers tuft 1 to form the majority of mouse gastrointestinal TCs (Haber et al., 2017).

According to a 2020 research, there may be another subtype of TC that mimics intestinal endocrine cells following the treatment of scopolamine (Middelhoff et al., 2020). The properties and functions of this novel subtype TCs need to be investigated urgently.

Banerjee et al. (2020) identified heterogeneous TC populations that respectively undergo ATOH1-dependent and ATOH1-independent pathways. Both ATOH1-independent and dependent TCs can be observed in the small intestine, but only ATOH1-dependent TCs can be observed in the colon. Banerjee et al. also found that ATOH1-independent TCs are a flexible cell population that can expand in the presence of luminal perturbations, whereas the ATOH1-dependent cell population is constant. Specifically, succinate drives ATOH1-dependent TC gene expression and growth in symbiotic bacteria (Stumhofer et al., 2006; Langille et al., 2013).

Previous research has revealed that TCs release various chemicals, including NO, leukotrienes, IL-25, opioids, fatty acid metabolism-related proteins, and components of the eicosanoid pathway. These molecular secretions demonstrate that TCs may perform a variety of roles in the digestive tract, as summarized in Table 3. These secretion-related activities might provide deeper insight into inflammation and tumor-related pathways. Box 4 depicts the function of TCs in various organs or tissues.

Changes in nutrition, pH, and microbiota can be detected by TCs, which are found in the airway and digestive tract. Because of their physical similarities to lingual taste bud cells, TCs were assumed to have a role in chemoreception. Members of the pancreatic and intestinal taste transduction pathways support this theory (Hofer et al., 1996; Hofer and Drenckhahn, 1998). TCs express several signaling molecules, including α-gustducin (also known as the guanine nucleotide binding protein alpha transducing 3, or GNAT3) (Hofer and Drenckhahn, 1998), TRPM5 (Kaske et al., 2007), G protein-coupled taste receptor type 1 member 3 (TAS1R3), the calcium signal transducer phospholipase Cβ2 (PLCβ2) (Ogura et al., 2010), β-endorphin, uroguanylin, and Met-Enkephalin, (Perez et al., 2002; Bezencon et al., 2007; Kaske et al., 2007). According to some study, TCs are a component of the diffuse chemosensory system (Sbarbati and Osculati, 2005). Furthermore, succinate receptor 1 (SUCNR1) was found to be expressed in TCs, and Lei et al. (2018) identified SUCNR1 as a TC-specific marker in mice, suggesting that SUCNR1 might aid in detecting infectious pathogens, triggering the proliferation of TCs and GCs involving in type 2 immune response.

Helminth infection is still regarded as a major worldwide health issue by scientists and practitioners, owing to its widespread occurrence and severe societal effect, particularly in less developed countries and regions. However, the early sensing and signaling mechanisms that initiate type 2 immunity against helminths remain unclear. The identification of these pathways might pave the way for the development of vaccines and medicines that target type 2 immunity. A recent study found that helminth infection can cause the synthesis of immunoregulatory substances that attract immune cells, resulting in infestations and inflammatory responses (Lightowlers and Rickard, 1988). Nontheless, the fundamental process, as well as the chemicals and cells involved, remain unclear. TCs were previously unseen to have great importance in this immunoreaction. TCs have been discovered as a significant activator of type 2 immunity in the small intestine by three distinct groups during the last decade (Gerbe et al., 2016; Howitt et al., 2016; von Moltke et al., 2016). Through a chemosensory mechanism, TCs in the small intestine detect helminths such as Heligmosomoides polygyrus, Trichinella spiralis, Nippostrongylus brasiliensis, and various species of Tritrichomonad protists.

In response to helminth infection (such as H. polygyrus), impaired epithelial cells release mediators such as leukotrienes, IL-22, and IL-33 (Artis and Grencis, 2008). Upon detecting the ligand, TCs transmit signals to the underlying lamina propria’s group 2 innate lymphoid cells (ILC2s), evoking an inflammation response. TCs are the only cell lineage that expresses IL-25 continuously (von Moltke et al., 2016). IL-25 stimulates ILC2s via the IL-17RB receptor. However, studies have observed that parasite-secreted H. polygyrus alarmin released inhibitor (HpARI) could hamper the “weep and sweep” immune response by limiting the IL-33 synthesis from injured epithelial cells (Osbourn et al., 2017). When subjected to helminth chemosensing, TCs produce cysteinyl leukotrienes (cysLTs), which rapidly activate type 2 immunity, accordng to McGinty et al.. CysLTs in collaboration with IL-25 stimulate ILC2s, and TC-specific leukotriene synthesis suppresses type 2 immunity and delays helminth clearance (McGinty et al., 2020). ILC2 activation may acquire additional signals to regulate the circuit in addition to IL-25. TCs in the colon, unlike those in the small intestine, respond to bacteria rather than parasites. Bacterial microflora can control colonic TC populations and stimulate TC growth, whereas colonic TCs have been shown to inhibit bacterial penetration and promote epithelial repair (McKinley et al., 2017; Wilen et al., 2018; Yi et al., 2019; Banerjee et al., 2020).

As a member of the chemokine family, IL-13 could stimulate secretory epithelial cells proliferation to boost mucus production and promote smooth muscle contraction to expel parasites in the intestine (Kamal et al., 2002; Gerbe et al., 2016; Howitt et al., 2016; Sharpe et al., 2018). IL-13 signals act directly on ISCs and bias their development towards the TCs and GCs, resulting in proliferation and a feed-forward loop in the tuft-ILC2 circuit. Upon the process, the quantity of TCs might rise tenfold within a few days of parasite infection (Gerbe et al., 2016; von Moltke et al., 2016). Given that IL-4 and IL-13 share a component, IL-4R, they may promote the proliferation of TCs (Gerbe et al., 2016), which also initiate smooth muscle contraction by releasing acetylcholine (ACh) and facilitate TCs to expel worms (Jonsson et al., 2007).

In mouse models, intestinal TCs appear around two weeks after birth, coinciding with epithelial changes in metabolic and nutritional behavior (Gerbe et al., 2011), as well as ILC2 and ILC3 growth (Hoyler et al., 2012), and the formation of solitary lymphoid clusters in the gut (Kiss et al., 2011). This research reveals a relationship between the ILC-epithelial cell axis and metabolic adaptation, showing that the innate immune system is important in homeostasis. A better knowledge of the innate immune system might pave the way for potential immunological advancements. The process of type 2 immune response orchestrated by TCs is shown in Figure 3.

Researchers revealed in 2022 that, in addition to their role in the immunological response to helminth infection, TCs (tuft 2) also contribute to bacterial clearance via a Vmn2r26-mediated mechanism. Animals lacking CD45⁺ tuft 2 were more vulnerable to pathogenic bacteria, indicating that tuft 2 might develop and respond to harmful bacteria. Tuft 2 was also shown to recognize the microbial chemical N-undecanoylglycine via its vomeronasal receptor Vmn2r26, which can activate the GPCR-PLC2-Ca²⁺ signaling axis and produce prostaglandin D2 (PGD2), causing GCs to generate mucus and increases gut immunity (Xiong et al., 2022).

Inflammatory bowel disease (IBD), also known as ulcerative colitis (UC) and Crohn’s disease (CD), is a chronic inflammatory illness characterized by inflammation and mucosal destruction that threatens the intestine’s integrity. The primary objective of IBD treatment is to repair the inflammatory mucosa, which improves clinical symptoms, decreases disease recurrence, and increases survival without resection (Pineton de Chambrun et al., 2010; Colombel et al., 2011; Neurath and Travis, 2012).

TCs serve as important sentinels in the intestine, directing host responses to particular injuries, including helminth infection (Gerbe et al., 2016; Grencis and Worthington, 2016; Howitt et al., 2016; von Moltke et al., 2016; Gerbe and Jay, 2016), as well as facilitating epithelial repair after tumorigenesis and acute injury (Westphalen et al., 2014).

Although helminth infection itself is a global health issue, it may have an impact on the treatment of CD (Summers et al., 2005; Broadhurst et al., 2010). It is widely accepted that anti-parasitic immune responses can neutralize CD’s pro-inflammatory signals (Summers et al., 2005). Banerjee et al. (2020) observed a reduction in the number of TCs in ileal tissues of mouse models and CD patients, therefore they postulated that TCs could act as a hub between parisites and the host, thus can be used to counteract pro-inflammatory signals in the gut. In pathological situation, the absence of TCs and DCLK1 causes a regeneration deficiency, resulting in impaired recovery of the epithelium (Yi et al., 2019). In addition, the helminth-induced tuft-ILC2 loop promotes mucus secretion by GCs and TCs and protects the intestinal mucosa, which may contribute to alleviating the symptoms of IBD. To conclude, TC is a clinically feasible strategy for reducing IBD symptoms and prognosis.

Radiotherapy has become a popular treatment in many cancers, although it has certain unavoidable adverse effects. Chronic radiation enteritis has been documented in up to one in every five patients treated with pelvic irradiation, with the real number being greater (Daly et al., 1989; Yeoh et al., 1993; Miller et al., 1999; Ooi et al., 1999). Colonic inflammation should not be ignored as it is one of the key factors for colon cancer (Kim and Chang, 2014). Experiments showed that DCLK1 ablation in the intestinal epithelium worsens the outcome during acute intestinal injury induced by radaition and dextran-sodium sulfate (DSS), since inadequate DCLK1 promotes protective intestinal epithelial regeneration (May et al., 2014; Qu et al., 2015). These findings prove that DCLK1 maintains integrity of the intestinal epithelial barrier and modulates the inflammatory response (Qu et al., 2015).

TCs are assumed to be engaged in the gut-brain axis and metabolic control due to their closeness to metabolic-regulating enteroendocrine and enteric neurons in the gut (Cheng et al., 2018). Although the underlying mechanisms of TC participation are unknown, intestinal TCs boost secretory ability while suppressing absorptive capacity during type 2 immune response, indicating that TCs are engaged in satiety mice and energy metabolism. Furthermore, the population of TCs rises in starved mice and persists even after refeeding (McKinley et al., 2017). Evidence above suggests that TCs may aid in adapting to various dietary situations (Arora et al., 2021).

MNV is the primary cause of acute viral gastroenteritis worldwide, with similar incidence in high and low-income nations (Mans, 2019). Evidence shows that TCs is the principal target of chronic MNV in both the small and large intestines and may enhance immune evasion (Baldridge et al., 2015; Tomov et al., 2017; Wilen et al., 2018). In mice, TCs express high levels of MNV receptor CD300lf, which acts as a target for viral infection. Viral shedding occurs several weeks after the acute phase of infection (Teunis et al., 2015; Wilen et al., 2018).

IBD is a collection of chronic idiopathic inflammatory illnesses that are widespread in Europe and North America. However, with industrialization and urbanization in the last 20 years, the prevalence of IBD in China has increased, attracting the attention of clinical practitioners and strengthening research into the condition.

IBD is distinguished by hidden asymptomatic intervals and repeated bouts of various degrees of gastrointestinal inflammation (Chang, 2020; Kobayashi et al., 2020; Roda et al., 2020). Blocking the p40 subunit shared by IL-12 and IL-23 was shown to induce colitis, leading to the conclusion that both the IL-12/Th1 and IL-23/Th17 axis may be implicated in the pathophysiology of CD and UC. (Gulati and Dubinsky, 2009; Strober and Fuss, 2011; Chang, 2020).

The considerable decrease in IL-25 in both inflamed intestinal mucosa and serum in patients with dilated IBD and healthy controls, as well as non-inflamed tissues and serum in patients with quiescent UC and CD, is cause for concern. When IBD was treated with infliximab, a TNF-α inhibitor, serum IL-25 levels returned to normal (Su et al., 2013). IL-25 may have a role in the etiology of IBD. Because TC acts as the sole generator of IL-25 in the mucosa, increasing IL-25 expression by TCs is a possible treatment strategy for IBD. The particular role of TC and the location of IL-25 expression, however, remain unknown. Yet, this simply suggests a correlation between IBD and the aberrant TC decrease, not a causal link. The findings presented here, that the quantity of TC in IBD may be altered, will provide vital insights into the underlying mechanism of IBD and clinical practice in the future.

Obesity is a globally increasing disease that is a risk factor for the development of a variety of ailments, including numerous cardiovascular issues and digestive system changes. In both rats and humans, diet-induced obesity is characterized by chronic low-grade systemic inflammation as well as alterations in gut flora (Lee et al., 2018). Especially, the proportion of TCs to total epithelial cells was not altered, and TC-specific expression of IL-25 and TLSP was reduced (Arora et al., 2021) along with activation of the GABA_A/B receptor pathways, which is positively correlated with alterations in the expression of the TC signature genes IL-25 and TSLP (Arora et al., 2021). This may provide solution for obesity by modulating TC secretion of IL-25 and TLSP.

DCLK1 is recognized as a possible marker since it is over-expressed in a variety of solid malignant tumors and has been associated to malignant biological activity and poor tumor prognosis (Chandrakesan et al., 2014; Ji et al., 2018).

Under normal circumstances, the only source of DCLK1 is TCs. DCLK1 has been detected in cancer stem cells (CSCs) from esophageal, pancreatic, and colon cancers (May et al., 2009; Vega et al., 2012; Weygant et al., 2015; Cao et al., 2020), suggesting that CSCs are derived from malignant TCs. CSCs interact with the immunosuppressive tumor microenvironment (TME) and aid in the activity of stem cells. An increasing body of data suggests that DCLK1⁺ TCs influence the formation and progression of inflammation-related malignancies (May et al., 2009; May et al., 2010; Vega et al., 2012; Weygant et al., 2015). Konishi et al. (2019) discovered in 2019 that TCs can induce Lgr5⁺ stem cells in the gastrointestinal tract, hence hastening cancer growth. Recent studies have further revealed that gastrointestinal TCs can promote hepatocellular carcinoma (HCC) development by secreting IL-25 to activate macrophages in TME (Friedrich et al., 2019). This “long-distance communication channel of the gut-liver axis” adds a new dimension to the study of TC function. Although TC markers can be found in mouse adenomas, they are uncommon in human cancer cell biopsies (Gerbe et al., 2011; Saqui-Salces et al., 2011), implying that animal studies are not yet useful for speculating on the association between human cancer and TC.

DCLK1 is expressed by certain pancreatic acinar and epithelial cells. Acinar-ductal metaplasia in pancreatic acinar cells may lead to cancer; DCLK1⁺ pancreatic epithelial cells are involved in regeneration following injury or inflammation (according to the lineage-tracing experiment); KRAS mutation in DCLK1⁺ pancreatic epithelial cells in pancreatitis may lead to pancreatic cancer (Nakanishi et al., 2013). Notably, utilizing a DCLK1 kinase inhibitor can reduce these DCLK1⁺ cells in the pancreas (Ferguson et al., 2020). These results suggest that DCLK1 may be a potential target for pancreatic cancer in clinical practice (Cao et al., 2020).

TC has been considered as a source of mature cell-derived carcinogenesis, alongside Paneth cells. In one word, DCLK1⁺ TCs (Nakanishi et al., 2013) or IL17RB⁺ TC-like cells (Goto et al., 2019) have been shown to act as stem cells in an intestinal tumor model. Similarly, in the context of further DSS-induced inflammation, Apc deletion in DCLK1⁺ TCs resulted in the development of colon tumors, whereas no DCLK1-expressing cells developed tumors in the steady state. Furthermore, following an acute assault, intestinal TC can act as colon cancer beginning cells (Westphalen et al., 2014). During validation, however, multiple essential pathways may be implicated in limiting TC activity and TC-derived tumor growth. For example, NF-κB signaling activation may be necessary for non-stem cell dedifferentiation and tumor development. At present, there are still many mysteries in this field. Future research will need to address this issue (Schwitalla et al., 2013).