Transcatheter Aortic-Valve Replacement with a Self-Expanding Valve in Low-Risk Patients.

BACKGROUND: Transcatheter aortic-valve replacement (TAVR) is an alternative to surgery in patients with severe aortic stenosis who are at increased risk for death from surgery; less is known about TAVR in low-risk patients. METHODS: We performed a randomized noninferiority trial in which TAVR with a self-expanding supraannular bioprosthesis was compared with surgical aortic-valve replacement in patients who had severe aortic stenosis and were at low surgical risk. When 850 patients had reached 12-month follow-up, we analyzed data regarding the primary end point, a composite of death or disabling stroke at 24 months, using Bayesian methods. RESULTS: Of the 1468 patients who underwent randomization, an attempted TAVR or surgical procedure was performed in 1403. The patients' mean age was 74 years. The 24-month estimated incidence of the primary end point was 5.3% in the TAVR group and 6.7% in the surgery group (difference, -1.4 percentage points; 95% Bayesian credible interval for difference, -4.9 to 2.1; posterior probability of noninferiority >0.999). At 30 days, patients who had undergone TAVR, as compared with surgery, had a lower incidence of disabling stroke (0.5% vs. 1.7%), bleeding complications (2.4% vs. 7.5%), acute kidney injury (0.9% vs. 2.8%), and atrial fibrillation (7.7% vs. 35.4%) and a higher incidence of moderate or severe aortic regurgitation (3.5% vs. 0.5%) and pacemaker implantation (17.4% vs. 6.1%). At 12 months, patients in the TAVR group had lower aortic-valve gradients than those in the surgery group (8.6 mm Hg vs. 11.2 mm Hg) and larger effective orifice areas (2.3 cm2 vs. 2.0 cm2). CONCLUSIONS: In patients with severe aortic stenosis who were at low surgical risk, TAVR with a self-expanding supraannular bioprosthesis was noninferior to surgery with respect to the composite end point of death or disabling stroke at 24 months. (Funded by Medtronic; ClinicalTrials.gov number, NCT02701283.).

The Two-pore channel (TPC) interactome unmasks isoform-specific roles for TPCs in endolysosomal morphology and cell pigmentation.

The two-pore channels (TPC1 and TPC2) belong to an ancient family of intracellular ion channels expressed in the endolysosomal system. Little is known about how regulatory inputs converge to modulate TPC activity, and proposed activation mechanisms are controversial. Here, we compiled a proteomic characterization of the human TPC interactome, which revealed that TPCs complex with many proteins involved in Ca(2+) homeostasis, trafficking, and membrane organization. Among these interactors, TPCs were resolved to scaffold Rab GTPases and regulate endomembrane dynamics in an isoform-specific manner. TPC2, but not TPC1, caused a proliferation of endolysosomal structures, dysregulating intracellular trafficking, and cellular pigmentation. These outcomes required both TPC2 and Rab activity, as well as their interactivity, because TPC2 mutants that were inactive, or rerouted away from their endogenous expression locale, or deficient in Rab binding, failed to replicate these outcomes. Nicotinic acid adenine dinucleotide phosphate (NAADP)-evoked Ca(2+) release was also impaired using either a Rab binding-defective TPC2 mutant or a Rab inhibitor. These data suggest a fundamental role for the ancient TPC complex in trafficking that holds relevance for lysosomal proliferative scenarios observed in disease.

Timing in cellular Ca2+ signaling.

Calcium (Ca2+) signals are generated across a broad time range. Kinetic considerations impact how information is processed to encode and decode Ca2+ signals, the choreography of responses that ensure specific and efficient signaling and the overall temporal amplification such that ephemeral Ca2+ signals have lasting physiological value. The reciprocal importance of timing for Ca2+ signaling, and Ca2+ signaling for timing is exemplified by the altered kinetic profiles of Ca2+ signals in certain diseases and the likely role of basal Ca2+ fluctuations in the perception of time itself.

Computed Tomography Annular Dimensions: A Novel Method to Compare Prosthetic Valve Hemodynamics.

BACKGROUND: The Cardiac Surgical Societies Valve Labeling Task Force consensus document acknowledged inconsistent sizing and labeling of prosthetic heart valves. This study compared the labeled size, internal diameter, and hemodynamics of different surgical and transcatheter valve types implanted into the same size annulus, measured by preprocedural computed tomography (CT). METHODS: Patients were retrospectively sorted into 3 CT annular diameter size groups: small (less than 23 mm), medium (23 to less than 26 mm), and large (26 mm or greater). Surgical valves were sorted into 4 categories based on tissue and design: (stentless porcine, standard stented bovine, wraparound stented bovine, and stented porcine). Comparisons were made within the surgical types and with a transcatheter valve. Echocardiograms were independently assessed and CTs were centrally measured. RESULTS: We analyzed 726 surgical and 923 transcatheter valve paired data sets. Among the various valve types implanted into the same size CT annulus, there were significant differences regarding size, internal diameter, and hemodynamics within all 3 size groups. Root enlargement procedures occurred in 1.2% with no differences across valve types or size groups. Transcatheter valve hemodynamics were similar to stentless valves and were significantly better than all stented valves. There was no difference in hemodynamics between the 2 bovine stented valve types, and stented porcine valves were inferior to all valve types. CONCLUSIONS: This study documents that prosthetic heart valve sizing and labeling inconsistencies exist. Use of preoperative CT annular dimensions is the most accurate method to compare size, internal diameter, and hemodynamics of bioprosthetic aortic valves because it compares values among various valve types implanted into the same size annulus.

Computed Tomography-Based Indexed Aortic Annulus Size to Predict Prosthesis-Patient Mismatch.

BACKGROUND: Hemodynamic performance of prostheses after transcatheter aortic valve replacement (TAVR) is generally better than after surgical aortic valve replacement (SAVR), especially in patients with a small native annulus size. However, it remains unclear whether differences are consistent for patients with a different propensity for developing prosthesis-patient mismatch (PPM), considering annulus size and body size of patients. METHODS AND RESULTS: The SURTAVI trial (Surgical Replacement and Transcatheter Aortic Implantation) compared TAVR using a self-expandable valve with SAVR in intermediate-risk patients. Multidetector computed tomography-based aortic annulus size consisted of the perimeter-derived diameter, which was divided by body surface area to produce an indexed annulus size. Patients were categorized into a small (9-12 mm/m2), medium (>12-14 mm/m2), and large (>14-18 mm/m2) group according to indexed annulus size. We compared TAVR and SAVR for PPM, hemodynamics, and clinical, and functional outcomes through 1-year follow-up within the size groups. Patients who underwent TAVR received a larger prosthesis with increasing indexed annulus size ( P<0.001), while there was no difference in prosthesis size in patients who underwent SAVR ( P=0.74). Patients in all size groups had significantly larger indexed effective orifice area and lower mean gradients at discharge after TAVR versus SAVR. Rates of PPM were significantly lower with TAVR versus SAVR in all groups ( P<0.001) and declined with larger indexed annulus sizes with both TAVR ( P=0.04) and SAVR ( P=0.03). Indexed annulus size was an independent predictor of PPM after TAVR and SAVR. Clinical outcomes were comparable between TAVR and SAVR across all groups, apart from a significantly higher rate of reintervention after TAVR versus SAVR in the large indexed annulus size group (2.5% versus 0%; P=0.01) but without significant interaction ( Pint=0.81). CONCLUSIONS: Rates of PPM were significantly lower after TAVR than after SAVR across all groups of indexed annulus size, reflecting better hemodynamic performance of transcatheter versus surgical valves, irrespective of the propensity to develop PPM. More attention should be directed to prevention of PPM after SAVR. This information should be considered by the Heart Team to recommend a specific procedure or valve. CLINICAL TRIAL REGISTRATION: URL: https://www.clinicaltrials.gov . Unique identifier: NCT01586910.

Self-Expanding Transcatheter Aortic Valve Replacement in Patients With Low-Gradient Aortic Stenosis.

OBJECTIVES: The authors sought to compare clinical and hemodynamic outcomes in patients receiving transcatheter aortic valve replacement (TAVR) for low-gradient (LG) aortic stenosis in the CoreValve EUS (Expanded Use Study) versus those with high-gradient (HG) aortic stenosis from the CoreValve U.S. Pivotal Extreme Risk Trial and CAS (Continued Access Study). BACKGROUND: The EUS examined the impact of TAVR in patients unsuitable for surgical aortic valve replacement who were excluded from the U.S. Pivotal Extreme Risk Trial due to LG aortic stenosis. METHODS: EUS patients were stratified by left ventricular ejection fraction: normal (≥50%, LG-normal ejection fraction), and low (<50%, did not respond to dobutamine by generating a mean gradient >40 mm Hg and/or velocity >4.0 m/s, "nonresponders"), and compared with extreme-risk patients from U.S. Pivotal and CAS that had either low resting gradient and responded to dobutamine ("responders"), or a high resting gradient (HG) or velocity. The primary endpoint was all-cause mortality or major stroke at 1 year. Hemodynamics and quality of life are reported at 30 days and 1 year. RESULTS: At 30 days, patients with LG/low left ventricular ejection fraction (nonresponders and responders) had significantly higher rates of all-cause mortality or major stroke, all-cause mortality, and cardiovascular mortality than both HG and LG-normal ejection fraction patients. At 1 year, only the responders had higher rates of these outcomes in comparison to the other 3 groups. Mean gradient and effective orifice area improved significantly in all patients and were maintained through 1 year. New York Heart Association functional classification and Kansas City Cardiomyopathy Questionnaire overall summary scores improved (p < 0.05) in all cohorts through 1 year. When all 4 subgroups were pooled, both decreasing mean gradient and stroke volume index were associated with increased mortality. Pre-procedural mean gradient was the only hemodynamic independent predictor of 1-year mortality by multivariate analysis. CONCLUSIONS: In this study, TAVR provided EUS patients significant hemodynamic relief with both 1-year survival and quality of life outcomes comparable to Pivotal and CAS patients (Safety & Efficacy Study of the Medtronic CoreValve System-Treatment of Symptomatic Severe Aortic Stenosis With Significant Comorbidities in Extreme Risk Subjects Who Need Aortic Valve Replacement, NCT01675440; Safety and Efficacy Study of the Medtronic CoreValve System in the Treatment of Symptomatic Severe Aortic Stenosis in High Risk and Very High Risk Subjects Who Need Aortic Valve Replacement, NCT01240902; Safety and Efficacy Continued Access Study of the Medtronic CoreValve System in the Treatment of Symptomatic Severe Aortic Stenosis in Very High Risk Subjects and High Risk Subjects Who Need Aortic Valve Replacement, NCT01531374).

Nuclear pore disassembly from endoplasmic reticulum membranes promotes Ca2+ signalling competency.

The functionality of the endoplasmic reticulum (ER) as a Ca(2+) storage organelle is supported by families of Ca(2+) pumps, buffers and channels that regulate Ca(2+) fluxes between the ER lumen and cytosol. Although many studies have identified heterogeneities in Ca(2+) fluxes throughout the ER, the question of how differential functionality of Ca(2+) channels is regulated within proximal regions of the same organelle is unresolved. Here, we studied the in vivo dynamics of an ER subdomain known as annulate lamellae (AL), a cytoplasmic nucleoporin-containing organelle widely used in vitro to study the mechanics of nuclear envelope breakdown. We show that nuclear pore complexes (NPCs) within AL suppress local Ca(2+) signalling activity, an inhibitory influence relieved by heterogeneous dissociation of nucleoporins to yield NPC-denuded ER domains competent at Ca(2+) signalling. Consequently, we propose a novel generalized role for AL - reversible attenuation of resident protein activity - such that regulated AL (dis)assembly via a kinase/phosphatase cycle allows cells to support rapid gain/loss-of-function transitions in cellular physiology.

IP3 receptor activity is differentially regulated in endoplasmic reticulum subdomains during oocyte maturation.

Fertilization competency results from hormone-induced remodeling of oocytes into eggs. The signaling pathways that effect this change exemplify bistability, where brief hormone exposure irrevocably switches cell fate. In Xenopus, changes in Ca(2+) signaling epitomize such remodeling: The reversible Ca(2+) signaling phenotype of oocytes rapidly adapts to support irreversible propagation of the fertilization Ca(2+) wave. Here, we simultaneously resolved IP(3) receptor (IP(3)R) activity with endoplasmic reticulum (ER) structure to optically dissect the functional architecture of the Ca(2+) release apparatus underpinning this reorganization. We show that changes in Ca(2+) signaling correlate with IP(3)R redistribution from specialized ER substructures called annulate lamellae (AL), where Ca(2+) release activity is attenuated, into IP(3)R-replete patches in the cortical ER of eggs that support the fertilization Ca(2+) wave. These data show: first, that IP(3)R sensitivity is regulated with high spatial acuity even between contiguous ER regions; and second, that drastic reorganization of Ca(2+) signaling dynamics can be driven by subcellular redistribution in the absence of changes in channel number or molecular or familial Ca(2+) channel diversity. Finally, these results define a novel role for AL in Ca(2+) signaling. Because AL are prevalent in other scenarios of rapid cell division, further studies of their impact on Ca(2+) signaling are warranted.

The inositol 1,4,5-trisphosphate receptor (Itpr) gene family in Xenopus: identification of type 2 and type 3 inositol 1,4,5-trisphosphate receptor subtypes.

Studies in the Xenopus model system have provided considerable insight into the developmental role of intracellular Ca2+ signals produced by activation of IP3Rs (inositol 1,4,5-trisphosphate receptors). However, unlike mammalian systems where three IP3R subtypes have been well characterized, our molecular understanding of the IP3Rs that underpin Ca2+ signalling during Xenopus embryogenesis relate solely to the original characterization of the 'Xenopus IP3R' cloned and purified from Xenopus laevis oocytes several years ago. In the present study, we have identified Xenopus type 2 and type 3 IP3Rs and report the full-length sequence, genomic architecture and developmental expression profile of these additional IP3R subtypes. In the light of the emerging genomic resources and opportunities for genetic manipulation in the diploid frog Xenopus tropicalis, these data will facilitate manipulations to resolve the contribution of IP3R diversity in Ca2+ signalling events observed during vertebrate development.

Evolution of postmenopausal hormone therapy between 2002 and 2009.

OBJECTIVE: The results of the Women's Health Initiative led to a sharp decline in postmenopausal hormone therapy use. Subsequently, treatment guidelines were revised to recommend hormone therapy at the lowest effective dose for the shortest possible duration. The objective of this analysis was to assess trends in nationwide hormone therapy prescription claims from 2002 to 2009. METHODS: This study was a retrospective database analyses of pharmacy claims from MedImpact Healthcare Systems Inc. Data from women with claims for oral or transdermal hormone therapy were analyzed to assess trends in hormone therapy claims, including route of administration, dose, and physician specialty. RESULTS: By the end of 2002, the total number of hormone therapy claims dropped approximately 30% from 2002 second quarter claims. This trend continued during the next 7 years, and by 2009, hormone therapy claims were reduced by more than 70%. The proportion of low-dose oral claims rose fourfold, whereas the proportion of standard/high-dose claims decreased 30%. The proportion of claims for transdermal formulations more than doubled, and the proportion of claims for low-dose transdermal hormone therapy increased 10-fold. Although reductions in overall claims, routes of administration, and dose categories were similar between physician specialties, obstetrician/ gynecologists prescribed transdermal hormone therapy nearly twice as often as all other types of providers. CONCLUSIONS: Since the publication of the Women's Health Initiative results, there has been a sustained decrease in hormone therapy claims. The proportional use of low-dose oral and transdermal formulations has increased, but as of 2009, claims for these formulations accounted for approximately one in four total hormone therapy claims.

Safety and Efficacy of Self-Expanding TAVR in Patients With Aortoventricular Angulation.

OBJECTIVES: The aim of this study was to determine the relationship between aortoventricular (AoV) angulation on clinical outcomes after self-expanding transcatheter aortic valve replacement (TAVR) in patients with severe aortic stenosis who were deemed suboptimal for surgery. BACKGROUND: Multidetector computed tomographic (MDCT) imaging of the aortovalvular complex has become a prerequisite for case planning with self-expanding TAVR. The effect of aortic angulation, an index of an unfolded or "horizontal" aorta, on procedural outcome after self-expanding TAVR is not known. METHODS: The clinical course of 3,578 patients who received implants in the CoreValve US Clinical Trials and who had prospective MDCT estimation of the AoV angle before the procedure was reviewed. Clinical site echocardiogram assessments were used to determine the degree of residual aortic regurgitation 24 to 48 h after the procedure and at 30 days. On the basis of the measurement of the AoV angle on MDCT, patients were categorized into septiles, ranging from the lowest septile of an AoV angle <37.0° to the highest AoV angle septile of >55.0°. RESULTS: Patients were elderly (age 83.3 ± 7.8 years) and were at high risk for surgical valve replacement (Society of Thoracic Surgeons Predicted Risk of Mortality 8.8 ± 4.7). Greater degrees of AoV angulation were correlated with older age (p < 0.0001). Although procedure time was 6.9 min longer in the highest septile (59.4 ± 35.9 min vs. 52.5 ± 35.3 min in the lowest septile; p = 0.004), there were no linear trends (p > 0.05) in the frequencies of device success, procedural success, frequencies of moderate or greater aortic regurgitation at 30 days, number of valves implanted, or need for balloon post-dilation or new pacemakers among the AoV angle septiles. CONCLUSIONS: The degree of AoV angulation does not affect early clinical outcomes self-expanding transcatheter aortic valve replacement. (Safety and Efficacy Study of the Medtronic CoreValve® System in the Treatment of Symptomatic Severe Aortic Stenosis in High Risk and Very High Risk Subjects Who Need Aortic Valve Replacement [Medtronic CoreValve® U.S. Pivotal Trial]; NCT01240902).

Polarized expression of members of the solute carrier SLC19A gene family of water-soluble multivitamin transporters: implications for physiological function.

Humans lack biochemical pathways for the synthesis of the micro-nutrients thiamine and folate. Cellular requirements are met through membrane transport activity, which is mediated by proteins of the SLC19A gene family. By using live-cell confocal imaging methods to resolve the localization of all SLC19A family members, we show that the two human thiamine transporters are differentially targeted in polarized cells, establishing a vectorial transport system. Such polarization decreases functional redundancy between transporter isoforms and allows for independent regulation of thiamine import and export pathways in cells.

Evolution of postmenopausal hormone therapy between 2002 and 2009.

OBJECTIVE: The results of the Women's Health Initiative led to a sharp decline in postmenopausal hormone therapy use. Subsequently, treatment guidelines were revised to recommend hormone therapy at the lowest effective dose for the shortest possible duration. The objective of this analysis was to assess trends in nationwide hormone therapy prescription claims from 2002 to 2009. METHODS: This study was a retrospective database analyses of pharmacy claims from MedImpact Healthcare Systems Inc. Data from women with claims for oral or transdermal hormone therapy were analyzed to assess trends in hormone therapy claims, including route of administration, dose, and physician specialty. RESULTS: By the end of 2002, the total number of hormone therapy claims dropped approximately 30% from 2002 second quarter claims. This trend continued during the next 7 years, and by 2009, hormone therapy claims were reduced by more than 70%. The proportion of low--dose oral claims rose fourfold, whereas the proportion of standard/high-dose claims decreased 30%. The proportion of claims for transdermal formulations more than doubled, and the proportion of claims for low-dose transdermal hormone therapy increased 10-fold. Although reductions in overall claims, routes of administration, and dose categories were similar between physician specialties, obstetrician/gynecologists prescribed transdermal hormone therapy nearly twice as often as all other types of providers. CONCLUSIONS: Since the publication of the Women's Health Initiative results, there has been a sustained decrease in hormone therapy claims. The proportional use of low-dose oral and transdermal formulations has increased, but as of 2009, claims for these formulations accounted for approximately one in four total hormone therapy claims.

Membrane targeting and intracellular trafficking of the human sodium-dependent multivitamin transporter in polarized epithelial cells.

The human sodium-dependent multivitamin transporter (hSMVT) mediates sodium-dependent uptake of biotin in renal and intestinal epithelia. To date, however, there is nothing known about the structure-function relationship or targeting sequences in the hSMVT polypeptide that control its polarized expression within epithelia. Here, we focused on the role of the COOH-terminal tail of hSMVT in the targeting and functionality of this transporter. A full-length hSMVT-green fluorescent protein (GFP) fusion protein was functional and expressed at the apical membrane in renal and intestinal cell lines. Microtubule disrupting agents disrupted the mobility of trafficking vesicles and impaired cell surface delivery of hSMVT, which was also prevented in cells treated with dynamitin (p50), brefeldin, or monensin. Progressive truncation of the COOH-terminal tail impaired the functionality and targeting of the transporter. First, biotin transport decreased by approximately 20-30% on deletion of up to 15 COOH-terminal amino acids of hSMVT, a decrease mimicked solely by deletion of the terminal PDZ motif (TSL). Second, deletions into the COOH-terminal tail (between residues 584-612, containing a region of predicted high surface accessibility) resulted in a further drop in hSMVT transport (to approximately 40% of wild-type). Third, apical targeting was lost on deletion of a helical-prone region between amino acids 570-584. We conclude that the COOH tail of hSMVT contains several determinants important for polarized targeting and biotin transport.

Essential requirement for two-pore channel 1 in NAADP-mediated calcium signaling.

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a widespread and potent calcium-mobilizing messenger that is highly unusual in activating calcium channels located on acidic stores. However, the molecular identity of the target protein is unclear. In this study, we show that the previously uncharacterized human two-pore channels (TPC1 and TPC2) are endolysosomal proteins, that NAADP-mediated calcium signals are enhanced by overexpression of TPC1 and attenuated after knockdown of TPC1, and that mutation of a single highly conserved residue within a putative pore region abrogated calcium release by NAADP. Thus, TPC1 is critical for NAADP action and is likely the long sought after target channel for NAADP.

Molecular characterization of a novel cell surface ADP-ribosyl cyclase from the sea urchin.

The sea urchin is an extensively used model system for the study of calcium signalling by the messenger molecules NAADP and cyclic ADP-ribose. Both are synthesized by ADP-ribosyl cyclases but our molecular understanding of these enzymes in the sea urchin is limited. We have recently reported the cloning of an extended family of sea urchin ADP-ribosyl cyclases and shown that one of these enzymes (SpARC1) is active within the endoplasmic reticulum lumen. These studies suggest that production of messengers is compartmentalized. Here we characterize the properties of SpARC2. SpARC2 catalyzed both NAADP and cyclic ADP-ribose production. Unusually, the NAD surrogate, NGD was a poor substrate. In contrast to SpARC1, heterologously expressed SpARC2 localized to the plasma membrane via a glycosylphosphatidylinositol (GPI)-anchor. Transcripts for SpARC2 were readily detectable in sea urchin eggs and a majority of the endogenous membrane bound activity was found to be GPI-anchored. Our data reveal striking differences in the properties of sea urchin ADP-ribosyl cyclases and provide further evidence that messenger production may occur outside of the cytosol.

Molecular characterization of a novel intracellular ADP-ribosyl cyclase.

BACKGROUND: ADP-ribosyl cyclases are remarkable enzymes capable of catalyzing multiple reactions including the synthesis of the novel and potent intracellular calcium mobilizing messengers, cyclic ADP-ribose and NAADP. Not all ADP-ribosyl cyclases however have been characterized at the molecular level. Moreover, those that have are located predominately at the outer cell surface and thus away from their cytosolic substrates. METHODOLOGY/PRINCIPAL FINDINGS: Here we report the molecular cloning of a novel expanded family of ADP-ribosyl cyclases from the sea urchin, an extensively used model organism for the study of inositol trisphosphate-independent calcium mobilization. We provide evidence that one of the isoforms (SpARC1) is a soluble protein that is targeted exclusively to the endoplasmic reticulum lumen when heterologously expressed. Catalytic activity of the recombinant protein was readily demonstrable in crude cell homogenates, even under conditions where luminal continuity was maintained. CONCLUSIONS/SIGNIFICANCE: Our data reveal a new intracellular location for ADP-ribosyl cyclases and suggest that production of calcium mobilizing messengers may be compartmentalized.

A C-terminal region dictates the apical plasma membrane targeting of the human sodium-dependent vitamin C transporter-1 in polarized epithelia.

The human sodium-dependent vitamin C transporter (hSVCT1) mediates sodium-dependent cellular uptake of the essential micronutrient l-ascorbic acid (vitamin C). However, the molecular determinants that control the cell surface expression, subcellular distribution, and dynamics of hSVCT1 remain undefined. To identify molecular determinants involved in hSVCT1 targeting in polarized epithelia, we used live cell imaging approaches to resolve the targeting and trafficking dynamics of hSVCT1 truncation mutants in renal and intestinal cells. Confocal imaging demonstrated that hSVCT1 was expressed at the apical cell surface and video rate measurements revealed hSVCT1 also resided in a heterogeneous population of intracellular organelles with discrete dynamic properties. By progressive truncation of the cytoplasmic C-terminal tail of hSVCT1, we delimited an essential role for an embedded ten amino acid sequence PICPVFKGFS (amino acids 563-572) in defining the physiological targeting of hSVCT1. Intriguingly, this sequence bears significant homology to recently identified apical targeting motifs in two other sodium-dependent transporters, and we suggest this conservation is reflected topologically through the adoption of a beta-turn confirmation in the cytoplasmic C-tail of each transporter. Our results provide the first direct resolution of functional hSVCT1 expression at the apical cell surface of polarized epithelia and define an apical targeting signal of relevance to transporters of diverse substrate specificity.