Structure and function of the SIT1 proline transporter in complex with the COVID-19 receptor ACE2.

Proline is widely known as the only proteogenic amino acid with a secondary amine. In addition to its crucial role in protein structure, the secondary amino acid modulates neurotransmission and regulates the kinetics of signaling proteins. To understand the structural basis of proline import, we solved the structure of the proline transporter SIT1 in complex with the COVID-19 viral receptor ACE2 by cryo-electron microscopy. The structure of pipecolate-bound SIT1 reveals the specific sequence requirements for proline transport in the SLC6 family and how this protein excludes amino acids with extended side chains. By comparing apo and substrate-bound SIT1 states, we also identify the structural changes that link substrate release and opening of the cytoplasmic gate and provide an explanation for how a missense mutation in the transporter causes iminoglycinuria.

Pho pictures provide powerful perspectives of phosphate importing proteins.

In this issue of Structure, Schneider et al.1 report multiple structures of the low-affinity inorganic-phosphate transporter Pho90 from Saccharomyces cerevisiae. With remarkable resolution of the Divalent Anion Sodium Symporter family member, their cryo-EM studies of this fungal protein reveal new modes of sodium, substrate, and lipid binding.

Protein Binder Toolbox for Studies of Solute Carrier Transporters.

Transporters of the solute carrier superfamily (SLCs) are responsible for the transmembrane traffic of the majority of chemical substances in cells and tissues and are therefore of fundamental biological importance. As is often the case with membrane proteins that can be heavily glycosylated, a lack of reliable high-affinity binders hinders their functional analysis. Purifying and reconstituting transmembrane proteins in their lipidic environments remains challenging and standard approaches to generate binders for multi-transmembrane proteins, such as SLCs, channels or G protein-coupled receptors (GPCRs) are lacking. While generating protein binders to 27 SLCs, we produced full length protein or cell lines as input material for binder generation by selected binder generation platforms. As a result, we obtained 525 binders for 22 SLCs. We validated the binders with a cell-based validation workflow using immunofluorescent and immunoprecipitation methods to process all obtained binders. Finally, we demonstrated the potential applications of the binders that passed our validation pipeline in structural, biochemical, and biological applications using the exemplary protein SLC12A6, an ion transporter relevant in human disease. With this work, we were able to generate easily renewable and highly specific binders against SLCs, which will greatly facilitate the study of this neglected protein family. We hope that the process will serve as blueprint for the generation of binders against the entire superfamily of SLC transporters.

Still in Search for an EAAT Activator: GT949 Does Not Activate EAAT2, nor EAAT3 in Impedance and Radioligand Uptake Assays.

Excitatory amino acid transporters (EAATs) are important regulators of amino acid transport and in particular glutamate. Recently, more interest has arisen in these transporters in the context of neurodegenerative diseases. This calls for ways to modulate these targets to drive glutamate transport, EAAT2 and EAAT3 in particular. Several inhibitors (competitive and noncompetitive) exist to block glutamate transport; however, activators remain scarce. Recently, GT949 was proposed as a selective activator of EAAT2, as tested in a radioligand uptake assay. In the presented research, we aimed to validate the use of GT949 to activate EAAT2-driven glutamate transport by applying an innovative, impedance-based, whole-cell assay (xCELLigence). A broad range of GT949 concentrations in a variety of cellular environments were tested in this assay. As expected, no activation of EAAT3 could be detected. Yet, surprisingly, no biological activation of GT949 on EAAT2 could be observed in this assay either. To validate whether the impedance-based assay was not suited to pick up increased glutamate uptake or if the compound might not induce activation in this setup, we performed radioligand uptake assays. Two setups were utilized; a novel method compared to previously published research, and in a reproducible fashion copying the methods used in the existing literature. Nonetheless, activation of neither EAAT2 nor EAAT3 could be observed in these assays. Furthermore, no evidence of GT949 binding or stabilization of purified EAAT2 could be observed in a thermal shift assay. To conclude, based on experimental evidence in the present study GT949 requires specific assay conditions, which are difficult to reproduce, and the compound cannot simply be classified as an activator of EAAT2 based on the presented evidence. Hence, further research is required to develop the tools needed to identify new EAAT modulators and use their potential as a therapeutic target.

The binding and mechanism of a positive allosteric modulator of Kv3 channels.

Small-molecule modulators of diverse voltage-gated K+ (Kv) channels may help treat a wide range of neurological disorders. However, developing effective modulators requires understanding of their mechanism of action. We apply an orthogonal approach to elucidate the mechanism of action of an imidazolidinedione derivative (AUT5), a highly selective positive allosteric modulator of Kv3.1 and Kv3.2 channels. AUT5 modulation involves positive cooperativity and preferential stabilization of the open state. The cryo-EM structure of the Kv3.1/AUT5 complex at a resolution of 2.5 Å reveals four equivalent AUT5 binding sites at the extracellular inter-subunit interface between the voltage-sensing and pore domains of the channel's tetrameric assembly. Furthermore, we show that the unique extracellular turret regions of Kv3.1 and Kv3.2 essentially govern the selective positive modulation by AUT5. High-resolution apo and bound structures of Kv3.1 demonstrate how AUT5 binding promotes turret rearrangements and interactions with the voltage-sensing domain to favor the open conformation.

High-Throughput Expression and Purification of Human Solute Carriers for Structural and Biochemical Studies.

Solute carriers (SLCs) are membrane transporters that import and export a range of endogenous and exogenous substrates, including ions, nutrients, metabolites, neurotransmitters, and pharmaceuticals. Despite having emerged as attractive therapeutic targets and markers of disease, this group of proteins is still relatively underdrugged by current pharmaceuticals. Drug discovery projects for these transporters are impeded by limited structural, functional, and physiological knowledge, ultimately due to the difficulties in the expression and purification of this class of membrane-embedded proteins. Here, we demonstrate methods to obtain high-purity, milligram quantities of human SLC transporter proteins using codon-optimized gene sequences. In conjunction with a systematic exploration of construct design and high-throughput expression, these protocols ensure the preservation of the structural integrity and biochemical activity of the target proteins. We also highlight critical steps in the eukaryotic cell expression, affinity purification, and size-exclusion chromatography of these proteins. Ultimately, this workflow yields pure, functionally active, and stable protein preparations suitable for high-resolution structure determination, transport studies, small-molecule engagement assays, and high-throughput in vitro screening.

The solute carrier SPNS2 recruits PI(4,5)P2 to synergistically regulate transport of sphingosine-1-phosphate.

Solute carrier spinster homolog 2 (SPNS2), one of only four known major facilitator superfamily (MFS) lysolipid transporters in humans, exports sphingosine-1-phosphate (S1P) across cell membranes. Here, we explore the synergistic effects of lipid binding and conformational dynamics on SPNS2's transport mechanism. Using mass spectrometry, we discovered that SPNS2 interacts preferentially with PI(4,5)P2. Together with functional studies and molecular dynamics (MD) simulations, we identified potential PI(4,5)P2 binding sites. Mutagenesis of proposed lipid binding sites and inhibition of PI(4,5)P2 synthesis reduce S1P transport, whereas the absence of the N terminus renders the transporter essentially inactive. Probing the conformational dynamics of SPNS2, we show how synergistic binding of PI(4,5)P2 and S1P facilitates transport, increases dynamics of the extracellular gate, and stabilizes the intracellular gate. Given that SPNS2 transports a key signaling lipid, our results have implications for therapeutic targeting and also illustrate a regulatory mechanism for MFS transporters.

Structural basis of ion - substrate coupling in the Na+-dependent dicarboxylate transporter VcINDY.

The Na+-dependent dicarboxylate transporter from Vibrio cholerae (VcINDY) is a prototype for the divalent anion sodium symporter (DASS) family. While the utilization of an electrochemical Na+ gradient to power substrate transport is well established for VcINDY, the structural basis of this coupling between sodium and substrate binding is not currently understood. Here, using a combination of cryo-EM structure determination, succinate binding and site-directed cysteine alkylation assays, we demonstrate that the VcINDY protein couples sodium- and substrate-binding via a previously unseen cooperative mechanism by conformational selection. In the absence of sodium, substrate binding is abolished, with the succinate binding regions exhibiting increased flexibility, including HPinb, TM10b and the substrate clamshell motifs. Upon sodium binding, these regions become structurally ordered and create a proper binding site for the substrate. Taken together, these results provide strong evidence that VcINDY's conformational selection mechanism is a result of the sodium-dependent formation of the substrate binding site.

Structural basis for inhibition of the drug efflux pump NorA from Staphylococcus aureus.

Membrane protein efflux pumps confer antibiotic resistance by extruding structurally distinct compounds and lowering their intracellular concentration. Yet, there are no clinically approved drugs to inhibit efflux pumps, which would potentiate the efficacy of existing antibiotics rendered ineffective by drug efflux. Here we identified synthetic antigen-binding fragments (Fabs) that inhibit the quinolone transporter NorA from methicillin-resistant Staphylococcus aureus (MRSA). Structures of two NorA-Fab complexes determined using cryo-electron microscopy reveal a Fab loop deeply inserted in the substrate-binding pocket of NorA. An arginine residue on this loop interacts with two neighboring aspartate and glutamate residues essential for NorA-mediated antibiotic resistance in MRSA. Peptide mimics of the Fab loop inhibit NorA with submicromolar potency and ablate MRSA growth in combination with the antibiotic norfloxacin. These findings establish a class of peptide inhibitors that block antibiotic efflux in MRSA by targeting indispensable residues in NorA without the need for membrane permeability.

An eight centre, retrospective, clinical practice data analysis of algorithm-based treatment with follitropin delta.

RESEARCH QUESTION: Can efficacy and success rates of the first recombinant FSH expressed in a human cell line with an individualized dosing algorithm based on body weight and anti-Müllerian hormone (AMH) as shown in the ESTHER-1 trial be confirmed in routine clinical practice? DESIGN: In eight reproductive medicine centres in Germany, observational data of 360 women who underwent ovarian stimulation with follitropin delta were evaluated as part of the quality control from January 2018 to June 2019. The data were analysed retrospectively. RESULTS: Mean age of patients was 33.5 (±3.8) years. Pretreatment AMH concentrations ranged from <0.5 ng/ml or 3.6 pmol/l (2.5%) to >5.6 ng/ml or 40 pmol/l (19.7%), with 79.7% of all AMH measurements above 2.0 ng/ml or 14.5 pmol/l. The mean number of oocytes obtained in n = 359 first follitropin delta cycles was 11.2 (±6.7) oocytes with 42.1% of patients having between eight and 14 oocytes retrieved at oocyte retrieval. The average clinical pregnancy rate in the first cycle with a fresh embryo transfer was 38.2% with a mean of 1.4 embryos per transfer. The cumulative pregnancy rate was 49.4% for the first stimulation cycle (including cryopreservation cycles generated from the first stimulation cycle). CONCLUSION: The goal of obtaining an adequate number of oocytes (8-14 oocytes) using the follitropin delta dosing algorithm was reached in 42.1% of patients despite a wide range of pretreatment AMH values, while achieving very good clinical pregnancy rates. Hence, algorithm-based ovarian stimulation with follitropin delta remains highly effective in clinical practice.

Mucoepidermoid Carcinomas of the Larynx.

OBJECTIVES: Discussions regarding the specific management and outcomes for laryngeal MEC are limited to very small, single-institution case series. To look further into the diagnosis and management of these uncommon non-squamous cell carcinomas of the larynx, we present 3 recent cases of laryngeal MEC treated at our institution. METHODS: Patients at a tertiary hospital treated for MEC between October 2019 and December 2020 were retrospectively identified. Chart review, imaging analysis, and histologic slide creation were completed for all patients. RESULTS: We identified and treated 2 patients with high-grade supraglottic and 1 patient with intermediate-grade glottic MEC. These patients presented to our clinic with a primary complaint of either gradual, worsening dysphonia, dysphagia, or both. All patients underwent laryngovideostroboscopy as well as panendoscopy with directed submucosal biopsy, which was consistent with MEC. MRI was performed in 2 of the cases further elucidating the extent of submucosal spread. PET-CT was performed in all 3 cases, and none demonstrated evidence of regional or distal metastases. Surgically, high-grade MEC lesions were treated with a total laryngectomy. The intermediate MEC lesion was managed with a supracricoid partial laryngectomy (SCPL). Surgical margins were free of tumor in all cases with no nodal metastases by modified radical neck dissection. Radiation therapy was offered to both high-grade MEC patients and declined by one. Radiation was not recommended to the patient with intermediate-grade MEC as we believed that the risk of additional treatment outweighed the benefit. CONCLUSION: We believe that MEC of the larynx should be considered in patients with atypical submucosal laryngeal masses. Laryngovideostroboscopy, MRI, and PET imaging may be valuable in determining the extent of the lesions and planning appropriate surgery. Postoperative radiation therapy should be considered a per tumor grade in other more studied sites, as there is no data on efficacy in laryngeal MEC.

The ups and downs of elevator-type di-/tricarboxylate membrane transporters.

The divalent anion sodium symporter (DASS) family contains both sodium-driven anion cotransporters and anion/anion exchangers. The family belongs to a broader ion transporter superfamily (ITS), which comprises 24 families of transporters, including those of AbgT antibiotic efflux transporters. The human proteins in the DASS family play major physiological roles and are drug targets. We recently determined multiple structures of the human sodium-dependent citrate transporter (NaCT) and the succinate/dicarboxylate transporter from Lactobacillus acidophilus (LaINDY). Structures of both proteins show high degrees of structural similarity to the previously determined VcINDY fold. Conservation between these DASS protein structures and those from the AbgT family indicates that the VcINDY fold represents the overall protein structure for the entire ITS. The new structures of NaCT and LaINDY are captured in the inward- or outward-facing conformations, respectively. The domain arrangements in these structures agree with a rigid body elevator-type transport mechanism for substrate translocation across the membrane. Two separate NaCT structures in complex with a substrate or an inhibitor allowed us to explain the inhibition mechanism and propose a detailed classification scheme for grouping disease-causing mutations in the human protein. Structural understanding of multiple kinetic states of DASS proteins is a first step toward the detailed characterization of their entire transport cycle.

Mediastinal Mass: An Unusual Location for a Paraganglioma.

A 50-year-old man presented to the hospital for workup of a symptomatic inguinal hernia. At presurgical workup, findings from a contrast material-enhanced CT of the chest, abdomen, and pelvis revealed a large, well-defined and enhancing middle mediastinal mass arising from the right ventricular outflow tract. The mass was ultimately deemed resectable due to preserved fat planes between the mass and other mediastinal structures and the preservation of the right and left coronary arteries. The tumor was diagnosed as a mediastinal paraganglioma at histologic assessment. Keywords: Adults, Angiography, CT-Angiography, MR-Angiography, Cardiac, Neoplasms-Primary ©RSNA, 2021.

Circulating hybrid cells predict presence of occult nodal metastases in oral cavity carcinoma.

BACKGROUND: Levels of circulating hybrid cells (CHCs), a newly identified circulating tumor cell (CTC), correlate with disease stage and progression in cancer. We investigated their utility to risk-stratify patients with clinically N0 (cN0) oral cavity squamous cell carcinoma (OCSCC), and to identify patients with occult cervical lymph node metastases (pN+). METHODS: We analyzed peripheral blood samples for CHCs with co-expression of cytokeratin (tumor) and CD45 (leukocyte) from 22 patients with cN0 OCSCC using immunofluorescence microscopy, then correlated levels with pathologic lymph node status. RESULTS: CHC levels exceeded CTCs and correlated with the presence of both clinically overt (p = 0.002) and occult nodal metastases (p = 0.006). CONCLUSIONS: For evaluated cN0 OCSCC patients, those with cN0 → pN+ status harbored elevated CHC levels compared to patients without occult disease. Our findings highlight a promising blood-based biologic assay with potential utility to determine the necessity of surgical neck dissection for staging and treatment.

Thoracic Endemic Fungi in the United States: Importance of Patient Location.

The digitization of radiographic studies along with high-speed transmission of images has formed the basis of teleradiology, which has become an integral component in the workflow of a contemporary radiology practice. It is with this advent and growing utilization of teleradiology that the significance of the source location of images has gained importance. Specifically, the importance of where the patient resides and what endemic fungi occur in that location cannot be underestimated. In the United States, histoplasmosis, coccidioidomycosis, blastomycosis, and cryptococcosis are caused by endemic fungi occurring in the Ohio and Mississippi river valleys, the Southwest, the Upper Midwest, and the Pacific Northwest, respectively. All of these organisms enter the body through the respiratory system and have the potential to cause significant morbidity and mortality. Patients infected with these fungi are often asymptomatic but may present with acute flulike symptoms such as fever, cough, or dyspnea. Patients may also present with vague chronic symptoms including cough, fever, malaise, and weight loss. Thoracic manifestations at radiography and CT include consolidation, nodules, cavities, lymphadenopathy, and pleural disease. PET may show fluorine 18-fluorodeoxyglucose uptake with active acute or chronic infections, and it is difficult to distinguish infections from malignancy. Imaging findings may be nonspecific and can be confused with other disease processes, including malignancy. The patient demographics, clinical history, and location are clues that may lead to a proper diagnosis of endemic fungal disease. The radiologist should be cognizant of the patient location to provide a correct and timely radiologic diagnosis that helps guide the clinician to initiate appropriate therapy. ©RSNA, 2021.

Structure and inhibition mechanism of the human citrate transporter NaCT.

Citrate is best known as an intermediate in the tricarboxylic acid cycle of the cell. In addition to this essential role in energy metabolism, the tricarboxylate anion also acts as both a precursor and a regulator of fatty acid synthesis1-3. Thus, the rate of fatty acid synthesis correlates directly with the cytosolic concentration of citrate4,5. Liver cells import citrate through the sodium-dependent citrate transporter NaCT (encoded by SLC13A5) and, as a consequence, this protein is a potential target for anti-obesity drugs. Here, to understand the structural basis of its inhibition mechanism, we determined cryo-electron microscopy structures of human NaCT in complexes with citrate or a small-molecule inhibitor. These structures reveal how the inhibitor-which binds to the same site as citrate-arrests the transport cycle of NaCT. The NaCT-inhibitor structure also explains why the compound selectively inhibits NaCT over two homologous human dicarboxylate transporters, and suggests ways to further improve the affinity and selectivity. Finally, the NaCT structures provide a framework for understanding how various mutations abolish the transport activity of NaCT in the brain and thereby cause epilepsy associated with mutations in SLC13A5 in newborns (which is known as SLC13A5-epilepsy)6-8.

Mucosal Eosinophilia and Neutrophilia Are Not Associated With QOL or Olfactory Function in Chronic Rhinosinusitis.

BACKGROUND: Chronic rhinosinusitis (CRS) is often differentiated by histopathologic phenotypes (eosinophilic versus neutrophilic), which may impact disease severity measures and outcomes. As such, it has been suggested that counts of cellular elements be included as part of a standard pathological report following endoscopic sinus surgery (ESS). OBJECTIVES: This cross-sectional study evaluated associations of mucosal eosinophilia and neutrophilia with measures of quality-of-life (QoL) and olfactory function. METHODS: Patients with medically refractory CRS completed the SNOT-22 survey and Brief Smell Identification Test (BSIT) at enrollment. In addition, baseline Lund-Mackay computed tomography (CT) and Lund-Kennedy endoscopy scores were collected. Ethmoid mucosa was biopsied during ESS and reviewed using microscopy to quantify densest infiltrate of eosinophils or neutrophils per high-powered-field (HPF). Eosinophilic CRS (eCRS) and neutrophilic CRS (nCRS), both with and without nasal polyposis (NP), were compared across SNOT-22 and BSIT scores. RESULTS: 77/168 patients demonstrated mucosal eosinophilia (eCRS) while a total of 42/168 patients demonstrated mucosal neutrophilia (nCRS). After adjusting for polyp status, 35/168 had eCRSsNP, 42/168 eCRSwNP, 75/168 non-eCRSsNP, 16/168 non-eCRSwNP. Additionally, 22/161 were noted to have nCRSsNP, 20/161 nCRSwNP, 84/161 non-nCRSwNP, and 35/161 non-nCRSsNP. A small subset of patients demonstrated both eosinophilia and neutrophilia: 14 CRSwNP and 7 CRSsNP. When evaluating average Lund-Mackay Scores (LMS), significant differences existed between non-eCRSsNP and eCRSsNP (p = 0.006). However, after controlling for nasal polyps, eosinophilia did not significantly associate with differences in the Lund-Kennedy Score. Neutrophilia did not significantly associate with any changes in LMS or LKS after controlling for NP. Eosinophilic and neutrophilic histopathologic subtypes did not significantly associate with differences in baseline SNOT-22 or BSIT measures after controlling for NP. CONCLUSION: Neither the presence of mucosal eosinophilia nor mucosal neutrophilia demonstrated significant associations with SNOT-22 quality-of-life or BSIT olfactory function scores when controlling for comorbid nasal polyposis.

Allergic Fungal Sinusitis Imitating an Aggressive Skull Base Lesion in the Setting of Pembrolizumab Immunotherapy.

OBJECTIVES: We report a case of acutely worsening allergic fungal sinusitis in a patient receiving immunotherapy with pembrolizumab, a programmed cell death protein 1 (PD-1) inhibitor. METHODS: A 53-year-old man with a history of metastatic melanoma and recent initiation of pembrolizumab therapy presented with acutely worsening headaches, left abducens nerve palsy, and neuroimaging demonstrating an erosive skull base lesion with bilateral cavernous sinus involvement. RESULTS: Intraoperative findings were consistent with non-invasive allergic fungal sinus disease. Microbiology and histopathologic data ruled out malignancy and demonstrated Aspergillus fumigatus without concern for angioinvasion. After treatment with antifungal therapy, the patient's symptoms and abducens nerve palsy resolved. Symptoms were well-controlled 7 months after his initial presentation. CONCLUSIONS: Inflammatory sinusitis in patients receiving anti-PD-1 therapy may be secondary to T-cell infiltration, a similar pathophysiology as immune-related adverse events, and warrants appreciation by otolaryngologists given our increasing exposure to immunotherapy and its head and neck manifestations.

Patient-reported sleep outcomes lack association with mucosal eosinophilia or neutrophilia in patients with chronic rhinosinusitis undergoing functional endoscopic sinus surgery.

BACKGROUND: Chronic rhinosinusitis (CRS) is associated with sleep dysfunction, but the underlying pathophysiology is poorly understood. The purpose of this study was to determine if mucosal eosinophilia or neutrophilia were associated with sleep dysfunction severity or altered the improvement in sleep dysfunction following functional endoscopic sinus surgery (FESS). METHODS: A total of 104 patients with medically refractory CRS with nasal polyposis (CRSwNP) and CRS without nasal polyposis (CRSsNP), completed the Pittsburgh Sleep Quality Index (PSQI) before and after FESS. Anterior ethmoid mucosa was collected during FESS and densest infiltrates of eosinophilia and neutrophilia per high-power field (HPF) were determined by microscopy. Eosinophilic (>10 eosinophils/HPF) and neutrophilic (>4 neutrophils/HPF) CRS were then compared to preoperative and postoperative PSQI measures. RESULTS: Of 104 study participants, 88 (85%) reported preoperative PSQI scores consistent with "poor sleep," (PSQI total > 5). The cohort overall demonstrated significant improvement in poor sleep (65%; χ2 = 12.03; p < 0.001) 16.8 ± 5.0 months after FESS. Regardless of nasal polyposis, neither eosinophilic nor neutrophilic CRS was associated with differences in mean postoperative PSQI improvement. However, in patients with neutrophilic CRSsNP, there was a significant relationship between severity of neutrophilia and improvement in sleep latency (R = -0.798, p = 0.003) and sleep efficacy (R = -0.777, p = 0.005). CONCLUSION: Chronic inflammation has been hypothesized to play a pathophysiologic role in sleep dysfunction associated with CRS. This study suggests that in patients with medically refractory CRS, evidence of mucosal eosinophilia and neutrophilia lack strong associations with patient-reported sleep dysfunction or improvements in sleep quality after FESS, overall. However, neutrophilia may impact sleep latency and efficacy in patients with CRSsNP.