Tissue-specific amino acid transporter partners ACE2 and collectrin differentially interact with hartnup mutations.
OBJECTIVE
Hartnup amino acid transporter B(0)AT1 (SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of small intestine and kidney proximal tubule. The expression of B(0)AT1 in kidney was recently shown to depend on its association with collectrin (Tmem27), a protein homologous to the membrane-anchoring domain of angiotensin-converting enzyme (ACE) 2.


METHODS
Because collectrin is almost absent from small intestine, we tested the hypothesis that it is ACE2 that interacts with B(0)AT1 in enterocytes. Furthermore, because B(0)AT1 expression depends on an associated protein, we tested the hypothesis that Hartnup-causing B(0)AT1 mutations differentially impact on B(0)AT1 interaction with intestinal and kidney accessory proteins.


RESULTS
Immunofluorescence, coimmunoprecipitation, and functional experiments using wild-type and ace2-null mice showed that expression of B(0)AT1 in small intestine critically depends on ACE2. Coexpressing new and previously identified Hartnup disorder-causing missense mutations of B(0)AT1 with either collectrin or ACE2 in Xenopus laevis oocytes showed that the high-frequency D173N and the newly identified P265L mutant B(0)AT1 transporters can still be activated by ACE2 but not collectrin coexpression. In contrast, the human A69T and R240Q B(0)AT1 mutants cannot be activated by either of the associated proteins, although they function as wild-type B(0)AT1 when expressed alone.


CONCLUSIONS
We thus show that ACE2 is necessary for the expression of the Hartnup transporter in intestine and suggest that the differential functional association of mutant B(0)AT1 transporters with ACE2 and collectrin in intestine and kidney, respectively, participates in the phenotypic heterogeneity of human Hartnup disorder.
Camargo
Simone M R
SM
Institute of Physiology and Zürich Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland.
Singer
Dustin
D
Makrides
Victoria
V
Huggel
Katja
K
Pos
Klaas M
KM
Wagner
Carsten A
CA
Kuba
Keiji
K
Danilczyk
Ursula
U
Skovby
Flemming
F
Kleta
Robert
R
Penninger
Josef M
JM
Verrey
François
F
D026921:Amino Acid Transport Systems, Neutral / Q000235:genetics / Q000378:metabolism*; D000085962:Angiotensin-Converting Enzyme 2; D000818:Animals; D005786:Gene Expression Regulation; D006250:Hartnup Disease / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007421:Intestine, Small / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000502:physiology; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation; D009865:Oocytes / Q000502:physiology; D018408:Patch-Clamp Techniques; D007703:Peptidyl-Dipeptidase A / Q000235:genetics / Q000378:metabolism*; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide; D014982:Xenopus laevis
D026921:Amino Acid Transport Systems, Neutral; D008562:Membrane Glycoproteins; C505021:SLC6A19 protein, mouse; C504407:Tmem27 protein, mouse; D007703:Peptidyl-Dipeptidase A; C000705307:ACE2 protein, human; C000705328:Ace2 protein, mouse; D000085962:Angiotensin-Converting Enzyme 2