Biology subjects

Lundkvist, S.

Publications and source records attributed to Lundkvist, S..

3 recordsLinked to original sources

bioRxiv2025

ABCC6 and ANK regulate extracellular homeostasis of pyrophosphate and citrate and affect mineral deposition in bones and soft connective tissues

ABCC6 and ANK are integral membrane proteins involved in the efflux of specific organic anions. ABCC6, primarily expressed in the liver, regulates circulating levels of the mineralization inhibitor pyrophosphate (PPi). In contrast, ANK is widely expressed and plays a dual role, maintaining extracellular PPi homeostasis and, notably, mediating cellular citrate efflux. We studied how both proteins affected extracellular metabolite levels, ectopic calcification, and bone homeostasis. We observed that plasma PPi was reduced by [~]36% in Ankank/ankmice and [~]60% in Abcc6-/- mice. However, plasma citrate levels depended primarily on ANK, dropping [~]75% in Ankank/ank and double mutants, but remaining unchanged in Abcc6-/- mice. MicroCT revealed extreme ectopic calcification in double mutants, far exceeding either single knockout, affecting muzzle skin and ear cartilage. Oral citrate was bioavailable and, at high doses, prevented soft tissue calcification in Abcc6-/- mice, suggesting a systemic protective role. In bone, ANK was essential for incorporating both PPi and citrate, while ABCC6 mainly affected PPi. ANK deficiency led to reduced trabecular volume, cortical thickness, cortical area fraction, and mineral density, with more pronounced effects in males. Biomechanical testing showed decreased ultimate moment, bending rigidity, and energy in ANK-deficient femora, alongside increased post-yield displacement, indicating compensatory matrix changes. Collectively, our findings identify ANK as a dual regulator of PPi and citrate, with a previously unrecognized role in preventing soft tissue calcification. This study positions ANK as a potential therapeutic target for mineralization disorders like pseudoxanthoma elasticum (caused by ABCC6 deficiency) and conditions of low bone mineral density like osteoporosis.

developmental biology↗

bioRxiv2021

Ankylosis homologue mediates cellular efflux of ATP, not pyrophosphate

The plasma membrane protein Ankylosis Homologue (ANKH, mouse ortholog: Ank) prevents pathological mineralization of joints by controlling extracellular levels of the mineralization inhibitor pyrophosphate (PPi). It was long thought that ANKH acts by transporting PPi into the joints, but we recently showed that ANKH releases large amounts of nucleoside triphosphates (NTPs), predominantly ATP, into the culture medium. This ATP is converted extracellularly into PPi and AMP by the ectoenzyme Ectonucleotide Pyrophosphatase Phosphodiesterase 1 (ENPP1). We could not rule out, however, that cells also release PPi directly via ANK. We now addressed this question by determining the effect of a complete absence of ENPP1 on ANKH-dependent extracellular PPi concentrations. Introduction of ANKH in ENPP1-deficient HEK293 cells resulted in robust cellular ATP release without the concomitant increase in extracellular PPi seen in ENPP1-proficient cells. Ank-activity was previously shown to be responsible for about 75% of the PPi found in mouse bones. However, bones of Enpp1-/- mice contained < 2.5% of the PPi found in bones of wild type mice, showing that Enpp1-activity is also a prerequisite for Ank-dependent PPi incorporation into the mineralized bone matrix in vivo. Hence, ATP release precedes ENPP1-mediated PPi formation. We find that ANKH also provides about 25% of plasma PPi, whereas we have previously shown that 60-70 % of plasma PPi is derived from the NTPs extruded by the ABC transporter, ABCC6. Both transporters that keep plasma PPi at sufficient levels to prevent pathological calcification, therefore do so by extruding NTPs rather than PPi itself.

biochemistry↗

bioRxiv2019

Ankylosis homologue (ANKH) controls extracellular citrate and pyrophosphate homeostasis and affects bone mechanical performance

The membrane protein Ankylosis homologue (ANKH, mouse orthologue: ANK) prevents mineralization of joint-space and articular cartilage. The accepted view is that ANKH mediates cellular release of inorganic pyrophosphate (PPi), a strong physiological inhibitor of mineralization. Using global metabolite profiling, we identified citrate as the most prominent metabolite leaving HEK293 cells in an ANKH-dependent manner. Although PPi levels were increased in culture medium of HEK293-ANKH cells, PPi was formed extracellularly after release of ATP and other nucleoside triphosphates. Ankank/ank mice, which lack functional ANK, had substantially reduced concentrations of citrate in plasma and urine, while citrate was undetectable in urine of a human patient lacking functional ANKH. Bone hydroxyapatite of Ankank/ank mice also contained markedly reduced levels of citrate and PPi and displayed diminished strength. Together, our data show that ANKH is a crucial factor in extracellular citrate and PPi homeostasis that is essential for normal bone development.

developmental biology↗