Structural basis for ion selectivity in potassium-selective channelrhodopsins.

KCR channelrhodopsins (K+-selective light-gated ion channels) have received attention as potential inhibitory optogenetic tools but more broadly pose a fundamental mystery regarding how their K+ selectivity is achieved. Here, we present 2.5-2.7 Å cryo-electron microscopy structures of HcKCR1 and HcKCR2 and of a structure-guided mutant with enhanced K+ selectivity. Structural, electrophysiological, computational, spectroscopic, and biochemical analyses reveal a distinctive mechanism for K+ selectivity; rather than forming the symmetrical filter of canonical K+ channels achieving both selectivity and dehydration, instead, three extracellular-vestibule residues within each monomer form a flexible asymmetric selectivity gate, while a distinct dehydration pathway extends intracellularly. Structural comparisons reveal a retinal-binding pocket that induces retinal rotation (accounting for HcKCR1/HcKCR2 spectral differences), and design of corresponding KCR variants with increased K+ selectivity (KALI-1/KALI-2) provides key advantages for optogenetic inhibition in vitro and in vivo. Thus, discovery of a mechanism for ion-channel K+ selectivity also provides a framework for next-generation optogenetics.

Nasopharyngeal rhinosporidiosis with intracranial extension masquerading as juvenile angiofibroma: an unusual entity.

Rhinosporidiosis is a chronic fungal inflammatory disease prevalent in India and Sri Lanka. Its manifestations are mostly nasal and extranasal lesions are relatively rare. Occasional atypical presentations of this disease lead to diagnostic dilemma. Herein we report on a case of nasopharyngeal rhinosporidiosis having extensive involvement of paranasal sinuses along with intracranial extension which mimicked radiologically as juvenile nasopharyngeal angiofibroma. To our knowledge, this is the first reported case of rhinosporidiosis having intracranial extension. We discuss the pathology, treatment and briefly review the literature of this rare disease.

Equine rhinosporidiosis in Buenos Aires, Argentina.

Rhinosporidiosis is caused by Rhinosporidium seeberi, a parasitic organism of the family Rhinosporideacea family, class Micomycetozoa. The disease is endemic in India; however, some cases were reported in Europe, Africa, North America, and South America. The aim of the present study is to report three cases of rhinosporidiosis in wild horses in different cities of Buenos Aires province, Argentina. We confirm the presence of R. seeberi in the analyzed samples using histopathological and PCR sequencing techniques.

Rhinosporidiosis-Factors predicting disease recurrence.

BACKGROUND: Rhinosporidiosis is a chronic granulomatous disease of the nose caused by Rhinosporidium seeberi. The disease is largely non-amenable to medical therapy and shows high recurrence rates requiring patients to undergo multiple surgeries often resulting in increased morbidity. OBJECTIVE: To analyse the epidemiological, clinical, histopathological characteristics, treatment and outcome in rhinosporidiosis and to identify factors which predispose to recurrence of the disease. PATIENTS/METHODS: Retrospective analysis of data of all patients with a diagnosis of rhinosporidiosis confirmed by histopathology at a tertiary care hospital from 2015 to 2019. RESULTS: There were 42 patients, 40 males and two females, with a mean age of 37.37 years. Disease showed bilateral involvement in 17 (40.48%) patients. Nineteen (45.24%) patients had more than two sites involved at initial presentation. Most patients had nasal cavity involvement followed by nasopharynx. Among the 28 patients who had a follow-up, 12 showed recurrent disease. However, 21 patients were disease free following a revision excision. Involvement of more than two sites was an independent significant factor for recurrence. On univariate analysis, other factors which showed statistically significant odds of developing recurrence were previous surgery (p = .054), involvement of nasal septum (p = .022), middle turbinate (p = .024), nasopharynx (p = .049) and posterior pharyngeal wall (p = .05). Factors which showed significantly less likelihood of developing a recurrence included patients who had less than 12 months duration from first symptom to intervention (p = .016), involvement of less than two sites (p = .0003) and unilateral disease (p = .019). CONCLUSION: Early intervention in rhinosporidiosis especially when the disease is unilateral and involves less than two sites improves the outcome.

Cluster of Nasal Rhinosporidiosis, Eastern Province, Rwanda.

We report 4 recent cases of nasal rhinosporidiosis in Rwanda. All patients were boys or young men living in the same district (Gatsibo District, Eastern Province), suggesting a reservoir in the area. The recent reemergence of rhinosporidiosis in Rwanda might reflect increased availability of diagnostic services rather than emerging disease.

Partial regression of large anterior scleral staphyloma secondary to rhinosporidiosis after corneoscleral graft - a case report.

BACKGROUND: Rhinosporidiosis is a rare chronic infection of the mucous membranes caused by the Rhinosporidium seeberi. Approximately 15% of cases of rhinosporidiosis are ocular, occurring mainly in the tarsal conjunctiva. There are only 11 cases of scleral melt with staphyloma formation associated with bulbar conjuctival oculosporidiosis and none of them was associated with partial regression of the scleral ectasia after a corneoscleral tectonic graft. CASE PRESENTATION: a 13-year-old girl with a progressively increasing black mass in the upper nasal part above the cornea of the left eye. The biomicroscopy revealed an oval, bluish mass measuring 10x10x5 mm with congestion of the overlying conjunctiva. Conjunctival biopsy showed sporoblasts of Rinosporidium seeberi. Treatment was conducted by conjunctival resection and tectonic corneoscleral graft (13x13mm) over the staphyloma. Within 1 year of follow-up the patient presented a partial staphyloma reduction, 9x9x2.5 mm, and the patch detached from the lesion. A novel surgical approach was done reducing the corneal patch and no recurrence was seen after 9 months. CONCLUSIONS: This case is one of the largest anterior scleral staphylomas secondary to rhinosporidiosis described in the literature. Scleral anterior staphyloma partial regression is an unusual outcome after a tectonic corneoscleral graft. Infection resolution and graft covering of thinned area contributed to scleral reepithelization.

Educational report: A case of lacrimal sac rhinosporidiosis.

This article reports a presentation of lacrimal sac rhinosporidiosis and informs the reader of this uncommon but important diagnosis. A 36-year-old man from Pakistan presented with a 3-month history of swelling at the nasal aspect of the left lower lid. This was associated with occasional crepitus and slight localised discomfort, but no epiphora. There was a palpable fullness near the left medial canthus associated with telecanthus but a normal sac washout and normal eye examination otherwise. Previous medical history included an ipsilateral nasal polypectomy and inferior meatal antrostomy around 10 years previously, whilst living in Pakistan. Various imaging modalities were useful in identifying a soft tissue mass within the left nasolacrimal duct. Following excision biopsy, histological examination confirmed the presence of rhinosporidiosis, likely caused by the organism Rhinosporidium seeberi. Rhinosporidiosis should be considered as a potential cause in any case of lacrimal sac pathology. Imaging studies may be helpful in measuring the extent of disease, although histological examination is required to confirm the diagnosis. Although rare, the complications of rhinosporidiosis can be potentially blinding or fatal. As discussed in this case, the presence of telecanthus may represent a lacrimal system tumour, either malignant or benign, and should always prompt further investigation.

Rhinosporidiosis in African reed frogs Hyperolius spp. caused by a new species of Rhinosporidium.

We report the identification of a new Rhinosporidium species (Dermocystida, Mesomycetozoea) infecting amphibian hosts, while showing a species specificity for African reed frogs of the genus Hyperolius. Large dermal cysts (sporangia) of R. rwandae sp. nov. were observed in 18% of H. lateralis and similar cysts in 0.7% of H. viridiflavus surveyed. Fully developed R. rwandae cysts are about 500 to 600 µm in diameter and sealed from the frog tissue by a thick chitinous wall. Some cysts were filled with numerous round-oval basophilic microspores of 8 to 12 µm diameter. With the exception of legs, nodules were visible over the complete torso surface including the vocal sac of males, but the most affected skin region was the area around the cloaca. Behavior, condition, and lifespan of infected frogs do not seem to be distinct from that of healthy individuals. The mode of infection remains unknown, but we hypothesize that the infectious life stage reaches the dermis via the intraepidermal ducts of the skin glands. Molecular evidence places the new frog pathogen as a sister species of the human pathogen R. seeberi.

Ocular rhinosporidiosis mimicking conjunctival squamous papilloma in Kenya - a case report.

BACKGROUND: Ocular rhinosporidiosis is a chronic granulomatous infection caused by a newly classified organism that is neither a fungus nor bacterium. It often presents as a benign conjunctival tumour but may mimic other ocular conditions. It is most often described in India. In Africa cases have been reported from South Africa, Kenya, Tanzania, Malawi, Uganda, Congo and Ivory Coast. CASE PRESENTATION: A 54 year old man was seen in Kenya with a lesion that resembled a conjunctival papilloma. We report resemblance to conjunctival papilloma and the result of vital staining with 0.05% Toluidine Blue. CONCLUSION: Ocular rhinosporidiosis occurs in East Africa. It may resemble conjunctival squamous papilloma. Vital staining with 0.05% Toluidine blue dye did not distinguish the two lesions well.

Lacrimal sac rhinosporidiosis: clinical profile and surgical management by modified dacryocystorhinostomy.

PURPOSE: To describe the clinical profile and surgical management of patients with lacrimal sac rhinosporidiosis. METHODS: This is a retrospective interventional case series. Eighteen patients, who were clinically diagnosed as isolated lacrimal sac rhinosporidiosis between October 2009 to January 2012, were included in the study. Detailed history, including whether there was exposure to stagnant bathing water, was noted. All patients underwent modified dacryocystorhinostomy (DCR) under general anaesthesia. RESULT: Out of 18 patients, 14 were male and 4 were female. Rhinosporidiosis was more prevalent in the 25 to 34 years age group. The most common clinical presentation was a doughy swelling over the lacrimal sac area, present in all cases. Twelve patients (66.67%) had a history of bathing in stagnant water. After modified DCR operation, all patients were followed for one year and only two patients had recurrence of their disease. CONCLUSION: Patients with lacrimal sac rhinosporidiosis usually present with a soft, doughy swelling in the lacrimal sac area. Bathing in stagnant water is a common risk factor. A modified DCR yields excellent outcomes.

Channel Gating in Kalium Channelrhodopsin Slow Mutants.

Kalium channelrhodopsin 1 from Hyphochytrium catenoides (HcKCR1) is the first discovered natural light-gated ion channel that shows higher selectivity to K+ than to Na+ and therefore is used to silence neurons with light (optogenetics). Replacement of the conserved cysteine residue in the transmembrane helix 3 (Cys110) with alanine or threonine results in a >1,000-fold decrease in the channel closing rate. The phenotype of the corresponding mutants in channelrhodopsin 2 is attributed to breaking of a specific interhelical hydrogen bond (the "DC gate"). Unlike CrChR2 and other ChRs with long distance "DC gates", the HcKCR1 structure does not reveal any hydrogen bonding partners to Cys110, indicating that the mutant phenotype is likely caused by disruption of direct interaction between this residue and the chromophore. In HcKCR1_C110A, fast photochemical conversions corresponding to channel gating were followed by dramatically slower absorption changes. Full recovery of the unphotolyzed state in HcKCR1_C110A was extremely slow with two time constants 5.2 and 70 min. Analysis of the light-minus-dark difference spectra during these slow processes revealed accumulation of at least four spectrally distinct blue light-absorbing photocycle intermediates, L, M1 and M2, and a UV light-absorbing form, typical of bacteriorhodopsin-like channelrhodopsins from cryptophytes. Our results contribute to better understanding of the mechanistic links between the chromophore photochemistry and channel conductance, and provide the basis for using HcKCR1_C110A as an optogenetic tool.

Presumptive synchronized nuclear divisions without cytokinesis in the Rhinosporidium seeberi parasitic life cycle.

Despite numerous studies of the Rhinosporidum seeberi parasitic phase, the stages of its nuclear cycle leading to the formation of endoconidia have yet to be properly described. R. seeberi resists culture and can only be investigated on histological preparations. We have evaluated tissue sections collected from 35 host species with rhinosporidosis searching for the presence of mitotic figures during sporangia development. This study found that soon after endoconidia release, the prominent reddish vesicles typical of this stage vanished leading to the development of juvenile sporangia (JS) 12-70 µm in diameter. This stage possesses granular cytoplasm, a thick cell wall, and a central reddish nucleus with a conspicuous nucleolus. The first nuclear division takes place in the JS. It is a rarely encountered event characterized by the development of a distorted nucleus leading to the formation of two nuclei without cytokinesis. The finding of multiple nuclear divisions at prophase-, metaphase- and telophase-like stages without cytokinesis was detected in intermediate sporangia (IS). IS with multiple dividing nuclei seem to be at the same stage of nuclear partitioning, suggesting synchronized nuclear division. In these sporangia, the nuclei continue divisions without cytokinesis until the sporangia reach ≥300 µm in diameter. The last nuclear division prior to cytokinesis appears to take place in very large sporangia with thousands of nuclei. The build-up of cytoplasm around each nucleus and the formation of a thin cell wall lead to the formation of endoconidia. This study revealed the presence of several mechanisms of pathogenesis in R. seeberi that deserved further investigation.

Tumoral cutaneous rhinosporidiosis: Case report and review of literature.

Rhinosporidiosis is a chronic granulomatous disease caused by Rhinosporidium seeberi commonly affecting nasal mucosa, conjunctiva, and urethra. Subcutaneous tumor nodule presentation is rare and often mimics as sarcoma. Such tumoral rhinosporidiosis has been reported rarely. This report describes a 60-year male who presented with a solitary, firm, nontender swelling in posterior aspect of right leg with an ulcer and mimicking clinically as soft tissue sarcoma. Histopathology was diagnostic. Surgical excision was found to be useful.

Light-gated channelrhodopsin sparks proton-induced calcium release in guard cells.

Although there has been long-standing recognition that stimuli-induced cytosolic pH alterations coincide with changes in calcium ion (Ca2+) levels, the interdependence between protons (H+) and Ca2+ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin HcKCR2 from the pseudofungus Hyphochytrium catenoides, which operates as a H+ conductive, Ca2+ impermeable ion channel on the plasma membrane of plant cells. Light activation of HcKCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca2+ release from the endoplasmic reticulum. A H+-induced cytosolic Ca2+ signal results in membrane depolarization through the activation of Ca2+-dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H+-induced Ca2+ release mechanism in plant cells and establishes HcKCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca2+ signaling.

Naso-Bronchial Rhinosporidiosis.

Rhinosporidiosis is a chronic mucocutaneous granulomatous disease caused by Rhinosporidium seeberi, commonly affecting the nose and nasopharynx. Endobronchial involvement is of rare occurrence but can pose challenging problems for diagnosis, surgical excision and anaesthetic management. We report a 40-year-old man with a history of recurrent nasal rhinosporidiosis who presented with unilateral nasal obstruction, cough, shortness of breath and a radiological feature of left lung collapse. Eight years since the last surgery, he presented with a recurrent lesion in the nose with concurrent endobronchial involvement. The patient underwent excision of the nasal and the endobronchial lesion successfully under general anaesthesia without any complication and good symptomatic improvement. The clinical presentation and the management of endobronchial rhinosporidiosis are discussed here. The surgical difficulties faced during the procedure are highlighted.

Nasal rhinosporidiosis in humans: new interpretations and a review of the literature of this enigmatic disease.

Rhinosporidiosis is a disease caused by Rhinosporidium seeberi which primarily affects the mucosa of the nose, conjunctiva and urethra. While it is endemic in some Asian regions, isolated cases are reported in other parts of the world as a result of the socio-cultural phenomenon of the migration. Its manifestation is a polypoid mass growing inside the affected cavity and the only treatment is surgical excision. Rhinosporidiosis is a condition which both clinicians and microbiologists should keep in mind when managing patients with nasal masses even those from non endemic areas. It is critical in such cases to follow the clinical course to ensure against recurrence of the disease. This study describes the clinical features, diagnosis, and treatment of rhinosporidiosis of the nose and nasopharynx in a series of three cases in East Delhi, India.