PROJECT SUMMARY
The goal of this grant application is to understand how mutations in tetratricopeptide repeat domain 7a (TTC7A)
affect formation of the polarized apical membrane; and how these mutations cause human disease. TTC7A loss
of function mutations result in severe infantile-onset gastrointestinal disease, with phenotypes related to both gut
epithelial and immune cell dysfunction. Previous studies suggest that loss of function in TTC7A results in altered
apico-basolateral polarity and lumen formation in intestinal epithelial cells, although how this occurs remains
unclear. Prevailing models suggest that initial organization of the apical membrane occurs at an initiation site
(AMIS), which is enriched with distinct phosphoinositides compared to the basolateral membrane. The putative
function of TTC7A is to serve as a chaperone and scaffolding protein for the phosphoinositide (PI) kinase –
PI4KIIIα - at the plasma membrane. This kinase is principally responsible for the generation phosphoinositide
PI4-phosphate (PI4P), which is one of the major precursors to the apically enriched PI(4,5P)2 and basolaterally
enriched PI(3,4,5)P3. The role of PIs in specialized compartments is thought to partly direct vesicular cargo to
the correct subcellular compartment – also responsible for proper polarity and lumenogenesis. Given the known
function of TTC7A in coordinating PI4KIIIα localization, we hypothesize that TTC7A mutations perturb PI4KIIIαs
normal functionality in cells, resulting in disordered spatiotemporal production of PI(4,5)P2 and PI(3,4,5)P3, thus
driving improper cellular polarity, lumenogenesis and endosomal trafficking. To test this idea, we investigate the
role of TTC7A in cell polarization, epithelial lumen formation, and endosomal trafficking. We propose studies
using patient-derived enteroids, intestinal epithelial cell (Caco2) monolayers and Caco2 cyst cultures to
determine the contribution of TTC7A to cell polarity and lumenogenesis. In Aim 1 of this proposal, we will further
develop novel technology to monitor the formation of the early apical membrane, protein and lipid movement
during apical membrane formation (and lumenogenesis), and how the proper subcellular localization of TTC7A
contributes to these processes. We will carry out high resolution confocal and lightsheet imaging of
phosphoinositide and apical and basolateral proteins localization in the polarization and lumen formation of
Caco2- cysts and enteroids. In Aim 2 of this proposal we further translate our recently established high-
throughput, quantitative endosomal trafficking assays for studies on primary patient derived enteroids. Further,
we develop novel imaging to spatially probe endosomal trafficking in live enteroids. Our studies seek to identify
if the membrane scaffolding function of TTC7A is important for correct spatial orientation of epithelial cells, and
how the loss of TTC7A induces abnormal intestinal lumen formation. The results of these experiments may
plausibly reveal general rules underlying epithelial development and suggest therapeutic approaches for TTC7A
deficiency as well as other more common GI pathologies. Further, successful completion of these aims will
provide extraordinary training towards my goal in becoming an epithelial biologist.
