Interplay between primary familial brain calcification-associated SLC20A2 and XPR1 phosphate transporters requires inositol polyphosphates for control of cellular phosphate homeostasis

Abstract

<jats:p>Solute carrier family 20 member 2 (SLC20A2) and xenotropic and polytropic retrovirus receptor 1 (XPR1) are transporters with phosphate uptake and efflux functions, respectively. Both are associated with primary familial brain calcification (PFBC), a genetic disease characterized by cerebral calcium-phosphate deposition and associated with neuropsychiatric symptoms. The association of the two transporters with the same disease suggests that they jointly regulate phosphate fluxes and cellular homeostasis, but direct evidence is missing. Here, we found that cross-talk between SLC20A2 and XPR1 regulates phosphate homeostasis, and we identified XPR1 as a key inositol polyphosphate (IP)-dependent regulator of this process. We found that overexpression of WT SLC20A2 increased phosphate uptake, as expected, but also unexpectedly increased phosphate efflux, whereas PFBC-associated SLC20A2 variants did not. Conversely, SLC20A2 depletion decreased phosphate uptake only slightly, most likely compensated for by the related SLC20A1 transporter, but strongly decreased XPR1-mediated phosphate efflux. The SLC20A2-XPR1 axis maintained constant intracellular phosphate and ATP levels, which both increased in XPR1 KO cells. Elevated ATP levels are a hallmark of altered inositol pyrophosphate (PP-IP) synthesis, and basal ATP levels were restored after phosphate efflux rescue with WT XPR1 but not with XPR1 harboring a mutated PP-IP–binding pocket. Accordingly, inositol hexakisphosphate kinase 1-2 (<jats:italic>IP6K1-2</jats:italic>) gene inactivation or IP6K inhibitor treatment abolished XPR1-mediated phosphate efflux regulation and homeostasis. Our findings unveil an SLC20A2-XPR1 interplay that depends on IPs such as PP-IPs and controls cellular phosphate homeostasis via the efflux route, and alteration of this interplay likely contributes to PFBC.</jats:p>

Phosphate dysregulation via the XPR1:KIDINS220 protein complex is a therapeutic vulnerability in ovarian cancer

“…We also observed downregulation of phosphate importers at both the mRNA (Figure 3f) and protein level (Supplemental Figure 1b) after XPR1 inactivation. Consistent with this, XPR1 inactivation led to a 60% decrease in phosphate uptake (Figure 3g), suggesting that phosphate uptake is regulated by intracellular phosphate concentrations 34,35 . We note, however, that this partial compensatory response is not sufficient to protect cells from XPR1-mediated cell death.…”

“…We mapped the XPR1:KIDINS220 interaction to the C-terminus of XPR1 containing the EXS domain, an evolutionarily conserved domain known to be required for XPR1 trafficking between the golgi apparatus and the plasma membrane in order to achieve phosphate efflux 42–44 . In contrast, the N-terminal SPX domain of XPR1, which has been implicated in phosphate efflux and regulation 35,45 , was neither necessary nor sufficient to bind KIDINS220 (Supplemental Figure 9b).…”

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

“…Cells overexpressing PIT2 showed a concomitant efflux in response to the resulting increase of P i uptake, possibly to maintain IC ATP and P i levels [ 211 ]. The P i efflux depends on IP7/IP8 signaling, which promotes efflux through XPR1 [ 211 ] and is absent in XPR1 KO cells or when IP6Ks are blocked pharmacologically [ 211 ]. Thus, IP7/IP8 may be important for intracellular P i homeostasis controlled by PIT2 and XPR1 [ 211 ].…”

“…The P i efflux depends on IP7/IP8 signaling, which promotes efflux through XPR1 [ 211 ] and is absent in XPR1 KO cells or when IP6Ks are blocked pharmacologically [ 211 ]. Thus, IP7/IP8 may be important for intracellular P i homeostasis controlled by PIT2 and XPR1 [ 211 ].…”

Role of Inositols and Inositol Phosphates in Energy Metabolism

“…Since XPR1 is the only protein known mammalian protein involved in phosphate export; thus, its interaction was investigated. Specifically, XPR1 contains SPX domain that has been known to interact with inositol polyphosphates, which was confirmed in various knock out models that the interaction between XPR1 and PPIPs can regulate mammalian cell phosphate homeostasis [44,49,50].…”