Common traffic routes for imported spermine and endosomal glypican-1-derived heparan sulfate in fibroblasts

Ephrin-B1 regulates cell surface residency of heparan sulfate proteoglycans (HSPGs) and complexes with the HSPG CD44V3-10 and fibroblast growth factor receptors

The ephrin family of membrane proteins mediate intracellular signalling as ligands of transmembrane Eph tyrosine kinase receptors during cell-cell interactions. Ephrin/Eph signalling regulates processes like cell migration and angiogenesis and is of particular importance during embryonic development. Ephrins-A3 and -B3 can also bind to cell surface-associated and soluble heparan sulfate proteoglycans (HSPGs) that also play important roles during early development. Here we show that ephrins-B1, -B2, and -B3 all can bind in cis to cell surface HSPGs, while only ephrin-B1 interacts with cell surface HSPGs in a way that retards HSPG endocytosis. Expressing ephrin-B1 in HEK293T cells, using polyethyleneimine (PEI) as transfection agent, increased cell surface levels of HSPGs which were detected by an anti-heparan sulfate (HS) antibody or by ephrin-B3-Fc binding. Ephrin-B1 in the plasma membrane seemed to retard PEI-induced HSPG internalisation and degradation. Binding of HSPGs by ephrin-B1 was observed for the human, mouse, xenopus, and zebrafish homologs, and did not require the cytoplasmic tail of ephrin-B1 that contains tyrosines shown to be involved in intracellular signalling. Furthermore, ephrin-B1 could bind the HSPG variant of CD44 (CD44V3-10), a complex that could further associate with fibroblast growth factor receptors (1 and 4) after co-expression with one of these receptors. In summary, our data indicate that ephrin-B1 can regulate cellular HSPG turnover and is able to form complexes of potential biological importance with CD44V3-10 and fibroblast growth factor receptors.

Proteomic Investigation of Differential Interactomes of Glypican 1 and a Putative Disease-Modifying Variant of Ataxia

In a currently 13-year-old girl of consanguineous Turkish parents, who developed unsteady gait and polyneuropathy at the ages of 3 and 6 years, respectively, we performed whole genome sequencing and identified a biallelic missense variant c.424C>T, p.R142W in glypican 1 (GPC1) as a putative disease-associated variant. Up to date, GPC1 has not been associated with a neuromuscular disorder, and we hypothesized that this variant, predicted as deleterious, may be causative for the disease. Using mass spectrometry-based proteomics, we investigated the interactome of GPC1 WT and the missense variant. We identified 198 proteins interacting with GPC1, of which 16 were altered for the missense variant. This included CANX as well as vacuolar ATPase (V-ATPase) and the mammalian target of rapamycin complex 1 (mTORC1) complex members, whose dysregulation could have a potential impact on disease severity in the patient. Importantly, these proteins are novel interaction partners of GPC1. At 10.5 years, the patient developed dilated cardiomyopathy and kyphoscoliosis, and Friedreich's ataxia (FRDA) was suspected. Given the unusually severe phenotype in a patient with FRDA carrying only 104 biallelic GAA repeat expansions in FXN, we currently speculate that disturbed GPC1 function may have exacerbated the disease phenotype. LC-MS/MS data are accessible in the ProteomeXchange Consortium (PXD040023).

Novel Green Fluorescent Polyamines to Analyze ATP13A2 and ATP13A3 Activity in the Mammalian Polyamine Transport System

Cells acquire polyamines putrescine (PUT), spermidine (SPD) and spermine (SPM) via the complementary actions of polyamine uptake and synthesis pathways. The endosomal P_5B-type ATPases ATP13A2 and ATP13A3 emerge as major determinants of mammalian polyamine uptake. Our biochemical evidence shows that fluorescently labeled polyamines are genuine substrates of ATP13A2. They can be used to measure polyamine uptake in ATP13A2- and ATP13A3-dependent cell models resembling radiolabeled polyamine uptake. We further report that ATP13A3 enables faster and stronger cellular polyamine uptake than does ATP13A2. We also compared the uptake of new green fluorescent PUT, SPD and SPM analogs using different coupling strategies (amide, triazole or isothiocyanate) and fluorophores (symmetrical BODIPY, BODIPY-FL and FITC). ATP13A2 promotes the uptake of various SPD and SPM analogs, whereas ATP13A3 mainly stimulates the uptake of PUT and SPD conjugates. However, the polyamine linker and coupling position on the fluorophore impacts the transport capacity, whereas replacing the fluorophore affects polyamine selectivity. The highest uptake in ATP13A2 or ATP13A3 cells is observed with BODIPY-FL-amide conjugated to SPD, whereas BODIPY-PUT analogs are specifically taken up via ATP13A3. We found that P_5B-type ATPase isoforms transport fluorescently labeled polyamine analogs with a distinct structure-activity relationship (SAR), suggesting that isoform-specific polyamine probes can be designed.

Beneficial effects of spermidine on cardiovascular health and longevity suggest a cell type-specific import of polyamines by cardiomyocytes

Recent and exciting in vivo studies show that supplementation with the polyamine spermidine (Spd) is cardioprotective and prolongs lifespan in both mice and humans. The mechanisms behind Spd-induced cardioprotection are supposed to involve Spd-evoked stimulation of autophagy, mitophagy and mitochondrial respiration and improved the mechano-elastical function of cardiomyocytes. Although cellular uptake of Spd was not characterized, these results suggest that Spd is imported by the cardiomyocytes and acts intracellularly. In the light of these new and thrilling data, we discuss in the present review cellular polyamine import with a special focus on mechanisms that may be relevant for Spd uptake by electrically excitable cells such as cardiomyocytes.