The role of TTC7A in directing Apical Lumen Formation and Polarized Trafficking in the Intestinal Epithelium

  • Background: Loss-of-function mutations in TTC7A result in severe infantile-onset congenital diarrhea and intestinal inflammation. A hallmark of TTC7A disease is profound intestinal epithelial architectural defects with altered apico-basolateral polarity and multi-lumen formation in crypt glands, though the mechanisms by which this occurs remains unclear. Several events are required for appropriate apico-basolateral membrane organization, including generation of specific phosphoinositides (PIs) at plasma membranes (apical PIP2 and basolateral PIP3) and endosomal trafficking of proteins to the correct polarized membrane. TTC7A is thought to function as a chaperone protein for the PI kinase, PI4KIIIα at cellular membranes and therefore involved in shaping membrane PI generation. The maintenance of specific PIs at cell membranes has been hypothesized to be responsible for directing vesicular cargo to their correct destination and therefore critical for initiation of cell polarity and guiding lumen formation. Here we investigate the role of TTC7A mutations and subsequent loss of function on the processes of lumen formation and polarity during epithelial morphogenesis.

    Methods: We investigated role of TTC7A in epithelial lumen formation and endosomal trafficking using primary colonoids derived from patients with TTC7A mutations. Colonoids were cultured in 3D and imaged to assess lumen formation, and as monolayers to study endosomal trafficking. LS174T:W4 human colon cells (TTC7A-/-, TTC7A+/+), were used to quantitatively assess polarity formation, and membrane phosphoinositide, protein and brush-border localization over time.

    Results: Patient-derived TTC7A colonoids with loss-of-function mutations developed multiple lumens. Volumetric lightsheet imaging confirmed that secondary lumens are extracellular facing. Secondary lumens in TTC7A patient colonoids contained correctly oriented apical membrane and junctional proteins (i.e. villin, ZO-1) and basolateral markers (e-cadherin) suggesting that TTC7A function is important in the initial steps of polarity generation. To investigate this quantitatively we used a tetracycline-inducible polarity model - LS174T:W4 cells. We found that at early stages of polarity formation LS174T:W4-TTC7A-/- cells retain apical membrane proteins intracellularly vs WT cells, and exhibit a significant loss of plasma membrane PI34P2, but not PI45P2 or PI345P3. As polarity develops this results in the formation of multiple mature brush-borders in cells. TTC7A patient primary monolayers exhibited a significant impairment in polarized endosomal trafficking for both apical and basolateral transcytosis and recycling.

    Conclusion: Loss of TTC7A function leads to early disruption of polarity generation and endosomal trafficking leading to mis-orientation of lumen formation in intestinal epithelial cells.

  • https://eposters.ddw.org/ddw/2022/ddw-2022/361392/katlynn.bugda.gwilt.the.role.of.ttc7a.in.directing.apical.lumen.formation.and.html?f=menu%3D6%2Abrowseby%3D8%2Asortby%3D2%2Amedia%3D2%2Ace_id%3D2236%2Aot_id%3D27149%2Amarker%3D1773

  • https://answers.childrenshospital.org/khori-congenital-enteropathy/

Actions of Trace Amines in the Brain-Gut-Microbiome Axis via Trace Amine-Associated Receptor-1 (TAAR1)

  • Trace amines and their primary receptor, Trace Amine-Associated Receptor-1 (TAAR1) are widely studied for their involvement in the pathogenesis of neuropsychiatric disorders despite being found in the gastrointestinal tract at physiological levels. With the emergence of the "brain-gut-microbiome axis," we take the opportunity to review what is known about trace amines in the brain, the defined sources of trace amines in the gut, and emerging understandings on the levels of trace amines in various gastrointestinal disorders. Similarly, we discuss localization of TAAR1 expression in the gut, novel findings that TAAR1 may be implicated in inflammatory bowel diseases, and the reported comorbidities of neuropsychiatric disorders and gastrointestinal disorders. With the emergence of TAAR1 specific compounds as next-generation therapeutics for schizophrenia (Roche) and Parkinson's related psychoses (Sunovion), we hypothesize a therapeutic benefit of these compounds in clinical trials in the brain-gut-microbiome axis, as well as a potential for thoughtful manipulation of the brain-gut-microbiome axis to modulate symptoms of neuropsychiatric disease.

  • https://pubmed.ncbi.nlm.nih.gov/31836967/

Membrane Lipids in Epithelial Polarity: Sorting out the PIPs

  • The development of cell polarity in epithelia, is critical for tissue morphogenesis and vectorial transport between the environment and the underlying tissue. Epithelial polarity is defined by the development of distinct plasma membrane domains: the apical membrane interfacing with the exterior lumen compartment, and the basolateral membrane directly contacting the underlying tissue. The de novo generation of polarity is a tightly regulated process, both spatially and temporally, involving changes in the distribution of plasma membrane lipids, localization of apical and basolateral membrane proteins, and vesicular trafficking. Historically, the process of epithelial polarity has been primarily described in relation to the localization and function of protein ‘polarity complexes.’ However, a critical and foundational role is emerging for plasma membrane lipids, and in particular phosphoinositide species. Here, we broadly review the evidence for a primary role for membrane lipids in the generation of epithelial polarity and highlight key areas requiring further research. We discuss the complex interchange that exists between lipid species and briefly examine how major membrane lipid constituents are generated and intersect with vesicular trafficking to be preferentially localized to different membrane domains with a focus on some of the key protein-enzyme complexes involved in these processes.

  • https://www.frontiersin.org/articles/10.3389/fcell.2022.893960/full

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