Role of CAR and JAM-A in Regulation of Intestinal Epithelial Cell Function
Creators: name: Smith, Michelle
Publisher: name: My University
Year: 2023
Subjects: subject: CTX Proteins
subject: Inflammatory Bowel Disease
subject: Wound Healing
subject: Cell Migration
subject: Coxsackie Adenovirus Receptor
subject: Junctional Adhesion Molecule A
subject: Pathology
subject: Health Sciences

description: The Cortical thymocyte marker in Xenopus (CTX) family are transmembrane adhesive proteins characterized by a shared structure and broad tissue expression patterns, including epithelia, endothelia, and leukocytes. The adhesive properties of CTX proteins confer two primary functions: formation of tissue barriers and facilitation of leukocyte trafficking. Recently, a growing number of studies have described additional functions of CTX family members that extend beyond adhesive properties, including regulation of cell migration and proliferation. In addition to this, alterations in CTX protein expression, post-translational modifications, and localization is perturbed in ulcerative colitis and Crohn’s disease (Inflammatory Bowel Disease, termed IBD), as well as numerous types of cancer. These findings highlight a need for a more detailed understanding of the multiple roles CTX proteins play in coordination of cell function under homeostatic and inflammatory conditions. Junctional Adhesion Molecule A (JAM-A) and the Coxsackie Adenovirus Receptor (CAR) are two CTX proteins expressed in intestinal epithelial cells (IECs) that line the gastrointestinal tract. To date, JAM-A has been identified as a negative regulator of IEC proliferation via an Akt/β-catenin signaling axis, although current evidence suggests that JAM-A participates in other pathways that modulate IEC proliferation. It is unknown what the endogenous role of IEC-expressed CAR is in regulation of intestinal homeostasis in health and disease relative to studies of JAM-A. The focus of this dissertation is two-fold: first to determine other mechanisms by which JAM-A suppresses IEC proliferation and second to define the role of CAR in mucosal wound repair. To study JAM-A suppression of IEC proliferation, we employed an IEC-specific JAM-A knockout mouse, cultures of primary murine colonoids, and transformed human colon cancer cell lines. We observed that loss of JAM-A dimerization results in a delay in the initiation of Hippo signaling, a key pathway responsible for regulation of cellular proliferation, resulting in a pro-proliferative phenotype. Further mechanistic studies revealed that JAM-A homodimerization initiates Hippo pathway signaling to suppress cellular proliferation upstream of Hippo pathway constituents LATS1 and NF2. Moreover, we identify this pro-proliferative phenotype to be the result of YAP/TEAD targeting of a well-known oncogenic transcription factor, Evi1. These conclusions raise important implications in understanding tumorigenesis in cancers that exhibit loss of JAM-A protein levels, as well as the higher risk of colorectal cancer in IBD patients, where JAM-A expression is decreased in the mucosa. To study the role of CAR in IEC wound repair, I used a novel IEC-specific CAR knockout mouse to model wound recovery following acute injury in vivo and in vitro. Additionally, a transformed human colon cancer cell line was leveraged to corroborate my findings, and publicly available RNA sequencing data from ulcerative colitis (UC) patients was analyzed to determine the involvement of CAR in IBD. From these studies, it was determined that CAR plays an important role in regulating wound repair by mediating cell-extracellular matrix adhesion and migration efficiency in IECs. Further studies revealed that CAR stabilizes the protein expression levels of focal adhesion proteins β1 integrin and Talin and activates the GTPase Rap2. Additionally, CAR expression was decreased in UC patients, indicating its potential contributions to IBD pathophysiology. Collectively, this work defines new roles for CAR and JAM-A in regulation of IEC function, revealing pertinent future avenues for investigation in disease.