Polyamines in Parkinson's Disease: Balancing Between Neurotoxicity and Neuroprotection.

The polyamines putrescine, spermidine, and spermine are abundant polycations of vital importance in mammalian cells. Their cellular levels are tightly regulated by degradation and synthesis, as well as by uptake and export. Here, we discuss the delicate balance between the neuroprotective and neurotoxic effects of polyamines in the context of Parkinson's disease (PD). Polyamine levels decline with aging and are altered in patients with PD, whereas recent mechanistic studies on ATP13A2 (PARK9) demonstrated a driving role of a disturbed polyamine homeostasis in PD. Polyamines affect pathways in PD pathogenesis, such as α-synuclein aggregation, and influence PD-related processes like autophagy, heavy metal toxicity, oxidative stress, neuroinflammation, and lysosomal/mitochondrial dysfunction. We formulate outstanding research questions regarding the role of polyamines in PD, their potential as PD biomarkers, and possible therapeutic strategies for PD targeting polyamine homeostasis.

ATP13A2 deficiency disrupts lysosomal polyamine export.

ATP13A2 (PARK9) is a late endolysosomal transporter that is genetically implicated in a spectrum of neurodegenerative disorders, including Kufor-Rakeb syndrome-a parkinsonism with dementia1-and early-onset Parkinson's disease2. ATP13A2 offers protection against genetic and environmental risk factors of Parkinson's disease, whereas loss of ATP13A2 compromises lysosomes3. However, the transport function of ATP13A2 in lysosomes remains unclear. Here we establish ATP13A2 as a lysosomal polyamine exporter that shows the highest affinity for spermine among the polyamines examined. Polyamines stimulate the activity of purified ATP13A2, whereas ATP13A2 mutants that are implicated in disease are functionally impaired to a degree that correlates with the disease phenotype. ATP13A2 promotes the cellular uptake of polyamines by endocytosis and transports them into the cytosol, highlighting a role for endolysosomes in the uptake of polyamines into cells. At high concentrations polyamines induce cell toxicity, which is exacerbated by ATP13A2 loss due to lysosomal dysfunction, lysosomal rupture and cathepsin B activation. This phenotype is recapitulated in neurons and nematodes with impaired expression of ATP13A2 or its orthologues. We present defective lysosomal polyamine export as a mechanism for lysosome-dependent cell death that may be implicated in neurodegeneration, and shed light on the molecular identity of the mammalian polyamine transport system.

A needs assessment for enhancing workplace-based assessment: a grounded theory study.

OBJECTIVES: Workplace-based assessment (WBA) has been vigorously criticized for not fulfilling its educational purpose by medical educators. A comprehensive exploration of stakeholders' needs regarding WBA is essential to optimize its implementation in clinical practice. METHOD: Three homogeneous focus groups were conducted with three groups of stakeholders: General Practitioner (GP) trainees, GP trainers, and GP tutors. Due to COVID-19 measures, we opted for an online asynchronous form to enable participation. An constructivist grounded theory approach was used to employ this study and allow the identification of stakeholders' needs for using WBA. RESULTS: Three core needs for WBA were identified in the analysis. Within GP Training, stakeholders found WBA essential, primarily, for establishing learning goals, secondarily, for assessment purposes, and, lastly, for providing or receiving feedback. CONCLUSION: All stakeholders perceive WBA as valuable when it fosters learning. The identified needs were notably influenced by agency, trust, availability, and mutual understanding. These were facilitating factors influencing needs for WBA. Embracing these insights can significantly illuminate the landscape of workplace learning culture for clinical educators and guide a successful implementation of WBA.

Co-designing Entrustable Professional Activities in General Practitioner's training: a participatory research study.

BACKGROUND: In medical education, Entrustable Professional Activities (EPAs) have been gaining momentum for the last decade. Such novel educational interventions necessitate accommodating competing needs, those of curriculum designers, and those of users in practice, in order to be successfully implemented. METHODS: We employed a participatory research design, engaging diverse stakeholders in designing an EPA framework. This iterative approach allowed for continuous refinement, shaping a comprehensive blueprint comprising 60 EPAs. Our approach involved two iterative cycles. In the first cycle, we utilized a modified-Delphi methodology with clinical competence committee (CCC) members, asking them whether each EPA should be included. In the second cycle, we used semi-structured interviews with General Practitioner (GP) trainers and trainees to explore their perceptions about the framework and refine it accordingly. RESULTS: During the first cycle, 14 CCC members agreed that all the 60 EPAs should be included in the framework. Regarding the formulation of each EPAs, 20 comments were given and 16 adaptations were made to enhance clarity. In the second cycle, the semi-structured interviews with trainers and trainees echoed the same findings, emphasizing the need of the EPA framework for improving workplace-based assessment, and its relevance to real-world clinical scenarios. However, trainees and trainers expressed concerns regarding implementation challenges, such as the large number of EPAs to be assessed, and perception of EPAs as potentially high-stakes. CONCLUSION: Accommodating competing stakeholders' needs during the design process can significantly enhance the EPA implementation. Recognizing users as experts in their own experiences empowers them, enabling a priori identification of implementation barriers and potential pitfalls. By embracing a collaborative approach, wherein diverse stakeholders contribute their unique viewpoints, we can only create effective educational interventions to complex assessment challenges.

ATP13A2-mediated endo-lysosomal polyamine export counters mitochondrial oxidative stress.

Recessive loss-of-function mutations in ATP13A2 (PARK9) are associated with a spectrum of neurodegenerative disorders, including Parkinson's disease (PD). We recently revealed that the late endo-lysosomal transporter ATP13A2 pumps polyamines like spermine into the cytosol, whereas ATP13A2 dysfunction causes lysosomal polyamine accumulation and rupture. Here, we investigate how ATP13A2 provides protection against mitochondrial toxins such as rotenone, an environmental PD risk factor. Rotenone promoted mitochondrial-generated superoxide (MitoROS), which was exacerbated by ATP13A2 deficiency in SH-SY5Y cells and patient-derived fibroblasts, disturbing mitochondrial functionality and inducing toxicity and cell death. Moreover, ATP13A2 knockdown induced an ATF4-CHOP-dependent stress response following rotenone exposure. MitoROS and ATF4-CHOP were blocked by MitoTEMPO, a mitochondrial antioxidant, suggesting that the impact of ATP13A2 on MitoROS may relate to the antioxidant properties of spermine. Pharmacological inhibition of intracellular polyamine synthesis with α-difluoromethylornithine (DFMO) also increased MitoROS and ATF4 when ATP13A2 was deficient. The polyamine transport activity of ATP13A2 was required for lowering rotenone/DFMO-induced MitoROS, whereas exogenous spermine quenched rotenone-induced MitoROS via ATP13A2. Interestingly, fluorescently labeled spermine uptake in the mitochondria dropped as a consequence of ATP13A2 transport deficiency. Our cellular observations were recapitulated in vivo, in a Caenorhabditis elegans strain deficient in the ATP13A2 ortholog catp-6 These animals exhibited a basal elevated MitoROS level, mitochondrial dysfunction, and enhanced stress response regulated by atfs-1, the C. elegans ortholog of ATF4, causing hypersensitivity to rotenone, which was reversible with MitoTEMPO. Together, our study reveals a conserved cell protective pathway that counters mitochondrial oxidative stress via ATP13A2-mediated lysosomal spermine export.

Fitness-for-purpose of the CanMEDS competencies for workplace-based assessment in General Practitioner's Training: a Delphi study.

BACKGROUND: In view of the exponential use of the CanMEDS framework along with the lack of rigorous evidence about its applicability in workplace-based medical trainings, further exploring is necessary before accepting the framework as accurate and reliable competency outcomes for postgraduate medical trainings. Therefore, this study investigated whether the CanMEDS key competencies could be used, first, as outcome measures for assessing trainees' competence in the workplace, and second, as consistent outcome measures across different training settings and phases in a postgraduate General Practitioner's (GP) Training. METHODS: In a three-round web-based Delphi study, a panel of experts (n = 25-43) was asked to rate on a 5-point Likert scale whether the CanMEDS key competencies were feasible for workplace-based assessment, and whether they could be consistently assessed across different training settings and phases. Comments on each CanMEDS key competency were encouraged. Descriptive statistics of the ratings were calculated, while content analysis was used to analyse panellists' comments. RESULTS: Out of twenty-seven CanMEDS key competencies, consensus was not reached on six competencies for feasibility of assessment in the workplace, and on eleven for consistency of assessment across training settings and phases. Regarding feasibility, three out of four key competencies under the role "Leader", one out of two competencies under the role "Health Advocate", one out of four competencies under the role "Scholar", and one out of four competencies under the role "Professional" were deemed as not feasible for assessment in a workplace setting. Regarding consistency, consensus was not achieved for one out of five competencies under "Medical Expert", two out of five competencies under "Communicator",one out of three competencies under "Collaborator", one out of two under "Health Advocate", one out of four competencies under "Scholar", one out of four competencies under "Professional". No competency under the role "Leader" was deemed to be consistently assessed across training settings and phases. CONCLUSIONS: The findings indicate a mismatch between the initial intent of the CanMEDS framework and its applicability in the context of workplace-based assessment. Although the CanMEDS framework could offer starting points, further contextualization of the framework is required before implementing in workplace-based postgraduate medical trainings.

ATP13A3 is a major component of the enigmatic mammalian polyamine transport system.

Polyamines, such as putrescine, spermidine, and spermine, are physiologically important polycations, but the transporters responsible for their uptake in mammalian cells remain poorly characterized. Here, we reveal a new component of the mammalian polyamine transport system using CHO-MG cells, a widely used model to study alternative polyamine uptake routes and characterize polyamine transport inhibitors for therapy. CHO-MG cells present polyamine uptake deficiency and resistance to a toxic polyamine biosynthesis inhibitor methylglyoxal bis-(guanylhydrazone) (MGBG), but the molecular defects responsible for these cellular characteristics remain unknown. By genome sequencing of CHO-MG cells, we identified mutations in an unexplored gene, ATP13A3, and found disturbed mRNA and protein expression. ATP13A3 encodes for an orphan P5B-ATPase (ATP13A3), a P-type transport ATPase that represents a candidate polyamine transporter. Interestingly, ATP13A3 complemented the putrescine transport deficiency and MGBG resistance of CHO-MG cells, whereas its knockdown in WT cells induced a CHO-MG phenotype demonstrated as a decrease in putrescine uptake and MGBG sensitivity. Taken together, our findings identify ATP13A3, which has been previously genetically linked with pulmonary arterial hypertension, as a major component of the mammalian polyamine transport system that confers sensitivity to MGBG.

Remote versus on-site proctored exam: comparing student results in a cross-sectional study.

BACKGROUND: The COVID-19 pandemic has profoundly affected assessment practices in medical education necessitating distancing from the traditional classroom. However, safeguarding academic integrity is of particular importance for high-stakes medical exams. We utilised remote proctoring to administer safely and reliably a proficiency-test for admission to the Advanced Master of General Practice (AMGP). We compared exam results of the remote proctored exam group to those of the on-site proctored exam group. METHODS: A cross-sectional design was adopted with candidates applying for admission to the AMGP. We developed and applied a proctoring software operating on three levels to register suspicious events: recording actions, analysing behaviour, and live supervision. We performed a Mann-Whitney U test to compare exam results from the remote proctored to the on-site proctored group. To get more insight into candidates' perceptions about proctoring, a post-test questionnaire was administered. An exploratory factor analysis was performed to explore quantitative data, while qualitative data were thematically analysed. RESULTS: In total, 472 (79%) candidates took the proficiency-test using the proctoring software, while 121 (20%) were on-site with live supervision. The results indicated that the proctoring type does not influence exam results. Out of 472 candidates, 304 filled in the post-test questionnaire. Two factors were extracted from the analysis and identified as candidates' appreciation of proctoring and as emotional distress because of proctoring. Four themes were identified in the thematic analysis providing more insight on candidates' emotional well-being. CONCLUSIONS: A comparison of exam results revealed that remote proctoring could be a viable solution for administering high-stakes medical exams. With regards to candidates' educational experience, remote proctoring was met with mixed feelings. Potential privacy issues and increased test anxiety should be taken into consideration when choosing a proctoring protocol. Future research should explore generalizability of these results utilising other proctoring systems in medical education and in other educational settings.

An N-terminal Ca2+-binding motif regulates the secretory pathway Ca2+/Mn2+-transport ATPase SPCA1.

The Ca2+/Mn2+ transport ATPases 1a and 2 (SPCA1a/2) are closely related to the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and are implicated in breast cancer and Hailey-Hailey skin disease. Here, we purified the human SPCA1a/2 isoforms from a yeast recombinant expression system and compared their biochemical properties after reconstitution. We observed that the purified SPCA1a displays a lower Ca2+ affinity and slightly lower Mn2+ affinity than SPCA2. Remarkably, the turnover rates of SPCA1a in the presence of Mn2+ and SPCA2 incubated with Ca2+ and Mn2+ were comparable, whereas the turnover rate of SPCA1a in Ca2+ was 2-fold higher. Moreover, we noted an unusual biphasic activation curve for the SPCA1a ATPase and autophosphorylation activity, not observed with SPCA2. We also found that the biphasic pattern and low apparent ion affinity of SPCA1a critically depends on ATP concentration. We further show that the specific properties of SPCA1a at least partially depend on an N-terminal EF-hand-like motif, which is present only in the SPCA1a isoform and absent in SPCA2. This motif binds Ca2+, and its mutation lowered the Ca2+ turnover rate relative to that of Mn2+, increased substrate affinity, and reduced the level of biphasic activation of SPCA1a. A biochemical analysis indicated that Ca2+ binding to the N-terminal EF-hand-like motif promotes the activity of SPCA1a by facilitating autophosphorylation. We propose that this regulation may be physiologically relevant in cells with a high Ca2+ load, such as mammary gland cells during lactation, or in cells with a low ATP content, such as keratinocytes.

Mutated ATP10B increases Parkinson's disease risk by compromising lysosomal glucosylceramide export.

Parkinson's disease (PD) is a progressive neurodegenerative brain disease presenting with a variety of motor and non-motor symptoms, loss of midbrain dopaminergic neurons in the substantia nigra pars compacta and the occurrence of α-synuclein-positive Lewy bodies in surviving neurons. Here, we performed whole exome sequencing in 52 early-onset PD patients and identified 3 carriers of compound heterozygous mutations in the ATP10B P4-type ATPase gene. Genetic screening of a Belgian PD and dementia with Lewy bodies (DLB) cohort identified 4 additional compound heterozygous mutation carriers (6/617 PD patients, 0.97%; 1/226 DLB patients, 0.44%). We established that ATP10B encodes a late endo-lysosomal lipid flippase that translocates the lipids glucosylceramide (GluCer) and phosphatidylcholine (PC) towards the cytosolic membrane leaflet. The PD associated ATP10B mutants are catalytically inactive and fail to provide cellular protection against the environmental PD risk factors rotenone and manganese. In isolated cortical neurons, loss of ATP10B leads to general lysosomal dysfunction and cell death. Impaired lysosomal functionality and integrity is well known to be implicated in PD pathology and linked to multiple causal PD genes and genetic risk factors. Our results indicate that recessive loss of function mutations in ATP10B increase risk for PD by disturbed lysosomal export of GluCer and PC. Both ATP10B and glucocerebrosidase 1, encoded by the PD risk gene GBA1, reduce lysosomal GluCer levels, emerging lysosomal GluCer accumulation as a potential PD driver.

Proficiency testing for identifying underperforming students before postgraduate education: a longitudinal study.

BACKGROUND: Efficient selection of medical students in GP training plays an important role in improving healthcare quality. The aim of this study was to collect quantitative and qualitative validity evidence of a multicomponent proficiency-test for identifying underperforming students in cognitive and non-cognitive competencies, prior to entering postgraduate GP Training. From 2016 to 2018, 894 medical GP students in four Flemish universities in Belgium registered to take a multicomponent proficiency-test before admission to postgraduate GP Training. Data on students were obtained from the proficiency-test as a test-score and from traineeship mentors' narrative reports. RESULTS: In total, 849 students took the multicomponent proficiency-test during 2016-2018. Test scores were normally distributed. Five different descriptive labels were extracted from mentors' narrative reports based on thematic analysis, considering both cognitive and non-cognitive competences. Chi-square tests and odds ratio showed a significant association between students scoring low on the proficiency-test and having gaps in cognitive and non-cognitive competencies during GP traineeship. CONCLUSION: A multicomponent proficiency-test could detect underperforming students prior to postgraduate GP Training. Students that ranked in the lowest score quartile had a higher likelihood of being labelled as underperforming than students in the highest score quartile. Therefore, a low score in the multicomponent proficiency-test could indicate the need for closer guidance and early remediating actions focusing on both cognitive and non-cognitive competencies.

Store-independent coupling between the Secretory Pathway Ca2+ transport ATPase SPCA1 and Orai1 in Golgi stress and Hailey-Hailey disease.

The Secretory Pathway Ca2+ ATPases SPCA1 and SPCA2 transport Ca2+ and Mn2+ into the Golgi and Secretory Pathway. SPCA2 mediates store-independent Ca2+ entry (SICE) via STIM1-independent activation of Orai1, inducing constitutive Ca2+ influx in mammary epithelial cells during lactation. Here, we show that like SPCA2, also the overexpression of the ubiquitous SPCA1 induces cytosolic Ca2+ influx, which is abolished by Orai1 knockdown and occurs independently of STIM1. This process elevates the Ca2+ concentration in the cytosol and in the non-endoplasmic reticulum (ER) stores, pointing to a functional coupling between Orai1 and SPCA1. In agreement with this, we demonstrate via Total Internal Reflection Fluorescence microscopy that Orai1 and SPCA1a co-localize near the plasma membrane. Interestingly, SPCA1 overexpression also induces Golgi swelling, which coincides with translocation of the transcription factor TFE3 to the nucleus, a marker of Golgi stress. The induction of Golgi stress depends on a combination of SPCA1 activity and SICE, suggesting a role for the increased Ca2+ level in the non-ER stores. Finally, we tested whether impaired SPCA1a/Orai1 coupling may be implicated in the skin disorder Hailey-Hailey disease (HHD), which is caused by SPCA1 loss-of-function. We identified HHD-associated SPCA1a mutations that impair either the Ca2+ transport function, Orai1 activation, or both, while all mutations affect the Ca2+ content of the non-ER stores. Thus, the functional coupling between SPCA1 and Orai1 increases cytosolic and intraluminal Ca2+ levels, representing a novel mechanism of SICE that may be affected in HHD.

Structure/activity relationship of thapsigargin inhibition on the purified Golgi/secretory pathway Ca2+/Mn2+-transport ATPase (SPCA1a).

The Golgi/secretory pathway Ca2+/Mn2+-transport ATPase (SPCA1a) is implicated in breast cancer and Hailey-Hailey disease. Here, we purified recombinant human SPCA1a from Saccharomyces cerevisiae and measured Ca2+-dependent ATPase activity following reconstitution in proteoliposomes. The purified SPCA1a displays a higher apparent Ca2+ affinity and a lower maximal turnover rate than the purified sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA1a). The lipids cholesteryl hemisuccinate, linoleamide/oleamide, and phosphatidylethanolamine inhibit and phosphatidic acid and sphingomyelin enhance SPCA1a activity. Moreover, SPCA1a is blocked by micromolar concentrations of the commonly used SERCA1a inhibitors thapsigargin (Tg), cyclopiazonic acid, and 2,5-di-tert-butylhydroquinone. Because tissue-specific targeting of SERCA2b by Tg analogues is considered for prostate cancer therapy, the inhibition of SPCA1a by Tg might represent an off-target risk. We assessed the structure-activity relationship (SAR) of Tg for SPCA1a by in silico modeling, site-directed mutagenesis, and measuring the potency of a series of Tg analogues. These indicate that Tg and the analogues are bound via the Tg scaffold but with lower affinity to the same homologous cavity as on the membrane surface of SERCA1a. The lower Tg affinity may depend on a more flexible binding cavity in SPCA1a, with low contributions of the Tg O-3, O-8, and O-10 chains to the binding energy. Conversely, the protein interaction of the Tg O-2 side chain with SPCA1a appears comparable with that of SERCA1a. These differences define a SAR of Tg for SPCA1a distinct from that of SERCA1a, indicating that Tg analogues with a higher specificity for SPCA1a can probably be developed.

A lipid switch unlocks Parkinson's disease-associated ATP13A2.

ATP13A2 is a lysosomal P-type transport ATPase that has been implicated in Kufor-Rakeb syndrome and Parkinson's disease (PD), providing protection against α-synuclein, Mn(2+), and Zn(2+) toxicity in various model systems. So far, the molecular function and regulation of ATP13A2 remains undetermined. Here, we demonstrate that ATP13A2 contains a unique N-terminal hydrophobic extension that lies on the cytosolic membrane surface of the lysosome, where it interacts with the lysosomal signaling lipids phosphatidic acid (PA) and phosphatidylinositol(3,5)bisphosphate [PI(3,5)P2]. We further demonstrate that ATP13A2 accumulates in an inactive autophosphorylated state and that PA and PI(3,5)P2 stimulate the autophosphorylation of ATP13A2. In a cellular model of PD, only catalytically active ATP13A2 offers cellular protection against rotenone-induced mitochondrial stress, which relies on the availability of PA and PI(3,5)P2. Thus, the N-terminal binding of PA and PI(3,5)P2 emerges as a key to unlock the activity of ATP13A2, which may offer a therapeutic strategy to activate ATP13A2 and thereby reduce α-synuclein toxicity or mitochondrial stress in PD or related disorders.

Evaluating Feedback Comments in Entrustable Professional Activities: A Cross-Sectional Study.

INTRODUCTION: Competency-based medical education (CBME) has transformed postgraduate medical training, prioritizing competency acquisition over traditional time-based curricula. Integral to CBME are Entrustable Professional Activities (EPAs), that aim to provide high-quality feedback for trainee development. Despite its importance, the quality of feedback within EPAs remains underexplored. METHODS: We employed a cross-sectional study to explore feedback quality within EPAs, and to examine factors influencing length of written comments and their relationship to quality. We collected and analyzed 1163 written feedback comments using the Quality of Assessment for Learning (QuAL) score. The QuAL aims to evaluate written feedback from low-stakes workplace assessments, based on 3 quality criteria (evidence, suggestion, connection). Afterwards, we performed correlation and regression analyses to examine factors influencing feedback length and quality. RESULTS: EPAs facilitated high-quality written feedback, with a significant proportion of comments meeting quality criteria. Task-oriented and actionable feedback was prevalent, enhancing value of low-stakes workplace assessments. From the statistical analyses, the type of assessment tool significantly influenced feedback length and quality, implicating that direct and video observations can yield superior feedback in comparison to case-based discussions. However, no correlation between entrustment scores and feedback quality was found, suggesting potential discrepancies between the feedback and the score on the entrustability scale. CONCLUSION: This study indicates the role of the EPAs to foster high-quality feedback within CBME. It also highlights the multifaceted feedback dynamics, suggesting the influence of factors such as feedback length and assessment tool on feedback quality. Future research should further explore contextual factors for enhancing medical education practices.

The lipid flippase ATP10B enables cellular lipid uptake under stress conditions.

Pathogenic ATP10B variants have been described in patients with Parkinson's disease and dementia with Lewy body disease, and we previously established ATP10B as a late endo-/lysosomal lipid flippase transporting both phosphatidylcholine (PC) and glucosylceramide (GluCer) from the lysosomal exoplasmic to cytoplasmic membrane leaflet. Since several other lipid flippases regulate cellular lipid uptake, we here examined whether also ATP10B impacts cellular lipid uptake. Transient co-expression of ATP10B with its obligatory subunit CDC50A stimulated the uptake of fluorescently (NBD-) labeled PC in HeLa cells. This uptake is dependent on the transport function of ATP10B, is impaired by disease-associated variants and appears specific for NBD-PC. Uptake of non-ATP10B substrates, such as NBD-sphingomyelin or NBD-phosphatidylethanolamine is not increased. Remarkably, in stable cell lines co-expressing ATP10B/CDC50A we only observed increased NBD-PC uptake following treatment with rotenone, a mitochondrial complex I inhibitor that induces transport-dependent ATP10B phenotypes. Conversely, Im95m and WM-115 cells with endogenous ATP10B expression, present a decreased NBD-PC uptake following ATP10B knockdown, an effect that is exacerbated under rotenone stress. Our data show that the endo-/lysosomal lipid flippase ATP10B contributes to cellular PC uptake under specific cell stress conditions.

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 P5B-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 P5B-type ATPase isoforms transport fluorescently labeled polyamine analogs with a distinct structure-activity relationship (SAR), suggesting that isoform-specific polyamine probes can be designed.

ATP13A4 Upregulation Drives the Elevated Polyamine Transport System in the Breast Cancer Cell Line MCF7.

Polyamine homeostasis is disturbed in several human diseases, including cancer, which is hallmarked by increased intracellular polyamine levels and an upregulated polyamine transport system (PTS). Thus far, the polyamine transporters contributing to the elevated levels of polyamines in cancer cells have not yet been described, despite the fact that polyamine transport inhibitors are considered for cancer therapy. Here, we tested whether the upregulation of candidate polyamine transporters of the P5B transport ATPase family is responsible for the increased PTS in the well-studied breast cancer cell line MCF7 compared to the non-tumorigenic epithelial breast cell line MCF10A. We found that MCF7 cells presented elevated expression of a previously uncharacterized P5B-ATPase, ATP13A4, which was responsible for the elevated polyamine uptake activity. Furthermore, MCF7 cells were more sensitive to polyamine cytotoxicity, as demonstrated by cell viability, cell death and clonogenic assays. Importantly, the overexpression of ATP13A4 WT in MCF10A cells induced a MCF7 polyamine phenotype, with significantly higher uptake of BODIPY-labeled polyamines and increased sensitivity to polyamine toxicity. In conclusion, we established ATP13A4 as a new polyamine transporter in the human PTS and showed that ATP13A4 may play a major role in the increased polyamine uptake of breast cancer cells. ATP13A4 therefore emerges as a candidate therapeutic target for anticancer drugs that block the PTS.

Thapsigargin affinity purification of intracellular P(2A)-type Ca(2+) ATPases.

The ubiquitous sarco(endo)plasmic reticulum (SR/ER) Ca(2+) ATPase (SERCA2b) and secretory-pathway Ca(2+) ATPase (SPCA1a) belong both to the P(2A)-type ATPase subgroup of Ca(2+) transporters and play a crucial role in the Ca(2+) homeostasis of respectively the ER and Golgi apparatus. They are ubiquitously expressed, but their low abundance precludes purification for crystallization. We have developed a new strategy for purification of recombinant hSERCA2b and hSPCA1a that is based on overexpression in yeast followed by a two-step affinity chromatography method biasing towards properly folded protein. In a first step, these proteins were purified with the aid of an analogue of the SERCA inhibitor thapsigargin (Tg) coupled to a matrix. Wild-type (WT) hSERCA2b bound efficiently to the gel, but its elution was hampered by the high affinity of SERCA2b for Tg. Therefore, a mutant was generated carrying minor modifications in the Tg-binding site showing a lower affinity for Tg. In a second step, reactive dye chromatography was performed to further purify and concentrate the properly folded pumps and to exchange the detergent to one more suitable for crystallization. A similar strategy was successfully applied to purify WT SPCA1a. This study shows that it is possible to purify functionally active intracellular Ca(2+) ATPases using successive thapsigargin and reactive dye affinity chromatography for future structural studies. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

P5B-ATPases in the mammalian polyamine transport system and their role in disease.

Polyamines (PAs) are physiologically relevant molecules that are ubiquitous in all organisms. The vitality of PAs to the healthy functioning of a cell is due to their polycationic nature causing them to interact with a vast plethora of cellular players and partake in numerous cellular pathways. Naturally, the homeostasis of such essential molecules is tightly regulated in a strictly controlled interplay between intracellular synthesis and degradation, uptake from and secretion to the extracellular compartment, as well as intracellular trafficking. Not surprisingly, dysregulated PA homeostasis and signaling are implicated in multiple disorders, ranging from cancer to neurodegeneration; leading many to propose rectifying the PA balance as a potential therapeutic strategy. Despite being well characterized in bacteria, fungi and plants, the molecular identity and properties of the PA transporters in animals are poorly understood. This review brings together the current knowledge of the cellular function of the mammalian PA transport system (PTS). We will focus on the role of P5B-ATPases ATP13A2-5 which are PA transporters in the endosomal system that have emerged as key players in cellular PA uptake and organelle homeostasis. We will discuss recent breakthroughs on their biochemical and structural properties as well as their implications for disease and therapy.