Size is a primary feature of biological systems that varies at many levels, from the organism to its constituent cells and subcellular structures. Amphibians populate some of the extremes in biological size and have provided insight into scaling mechanisms, upper and lower size limits, and their physiological significance. Body size variation is a widespread evolutionary tactic among amphibians, with miniaturization frequently correlating with direct development that occurs without a tadpole stage. The large genomes of salamanders lead to large cell sizes that necessitate developmental modification and morphological simplification. Amphibian extremes at the cellular level have provided insight into mechanisms that accommodate cell-size differences. Finally, how organelles scale to cell size between species and during development has been investigated at the molecular level, because subcellular scaling can be recapitulated using Xenopus in vitro systems.

MinK-related peptides (MiRPs) are single-span membrane proteins that assemble with specific voltage-gated K+ (Kv) channel alpha-subunits to establish gating kinetics, unitary conductance, expression level, and pharmacology of the mixed complex. MiRP3 (encoded by the KCNE4 gene) has been shown to alter the behavior of some Kv alpha-subunits in vitro but its natural partners and physiologic functions are unknown. Seeking in vivo partners for MiRP3, immunohistochemistry was used to localize its expression to a unique subcellular site, the apical membrane of renal intercalated cells, where one potassium channel type has been recorded, the calcium- and voltage-gated channel BK. Overlapping staining of these two proteins was found in rabbit intercalated cells, and MiRP3 and BK subunits expressed in tissue culture cells were found to form detergent-stable complexes. Electrophysiologic and biochemical evaluation showed MiRP3 to act on BK to reduce current density in two fashions: shifting the current-voltage relationship to more depolarized voltages in a calcium-dependent fashion ( approximately 10 mV at normal intracellular calcium levels) and accelerating degradation of MiRP3-BK complexes. The findings suggest a role for MiRP3 modulation of BK-dependent urinary potassium excretion.

Background

The 2P domain potassium (K2P) channels are a recently discovered ion channel superfamily. Structurally, K2P channels are distinguished by the presence of two pore forming loops within one channel subunit. Functionally, they are characterized by their ability to pass potassium across the physiologic voltage range. Thus, K2P channels are also called open rectifier, background, or leak potassium channels. Patch clamp studies of renal tubules have described several open rectifier potassium channels that have as yet eluded molecular identification. We sought to determine the segment-specific expression of transcripts for the 14 known K2P channel genes in human nephron to identify potential correlates of native leak channels.

Methods

Human kidney samples were obtained from surgical cases and specific nephron segments were dissected. RNA was extracted and used as template for the generation of cDNA libraries. Real-time polymerase chain reaction (PCR) (TaqMan) was used to analyze gene expression.

Results

We found significant (P < 0.05) expression of K2P10 in glomerulus, K2P5 in proximal tubule and K2P1 in cortical thick ascending limb of Henle's loop (cTAL) and in distal nephron segments. In addition, we repeatedly detected message for several other K2P channels with less abundance, including K2P3 and K2P6 in glomerulus, K2P10 in proximal tubule, K2P5 in thick ascending limb of Henle's loop, and K2P3, K2P5, and K2P13 in distal nephron segments.

Conclusion

K2P channels are expressed in specific segments of human kidney. These results provide a step toward assigning K2P channels to previously described native renal leaks.

Exosomes are cell-derived extracellular vesicles that have great potential in the field of nano-medicine. However, a fundamental challenge in the engineering of exosomes is the design of biocompatible molecular scaffolds on their surface to enable cell targeting and therapeutic functions. CD63 is a hallmark protein of natural exosomes that is highly enriched on the external surface of the membrane. We have previously described engineering of CD63 for use as a molecular scaffold in order to introduce cell-targeting features to the exosome surface. Despite this initial success, the restrictive M-shaped topology of full-length CD63 may hinder specific applications that require N- or C-terminal display of cell-targeting moieties on the outer surface of the exosome. In this study, we describe new and topologically distinct CD63 scaffolds that enable robust and flexible surface engineering of exosomes. In particular, we conducted sequential deletions of the transmembrane helix of CD63 to generate a series of CD63 truncates, each genetically-fused to a fluorescent protein. Molecular and cellular characterization studies showed truncates of CD63 harboring the transmembrane helix 3 (TM3) correctly targeted and anchored to the exosome membrane and exhibited distinct n-, N-, Ω-, or I-shaped membrane topologies in the exosomal membrane. We further established that these truncates retained robust membrane-anchoring and exosome-targeting activities when stably expressed in the HEK293 cells. Moreover, HEK293 cells produced engineered exosomes in similar quantities to cells expressing full-length CD63. Based on the results of our systematic sequential deletion studies, we propose a model to understand molecular mechanisms that underlie membrane-anchoring and exosome targeting features of CD63. In summary, we have established new and topologically distinct scaffolds based on engineering of CD63 that enables flexible engineering of the exosome surface for applications in disease-targeted drug delivery and therapy.

MiRP3, the single-span membrane protein encoded by KCNE4, is localized by immunofluorescence microscopy to the transverse tubules of murine cardiac myocytes. MiRP3 is found to co-localize with Kv4.2 subunits that contribute to cardiac transient outward potassium currents (I(to)). Whole-cell, voltage-clamp recordings of human MiRP3 and Kv4.2 expressed in a clonal cell line (tsA201) reveal MiRP3 to modulate Kv4.2 current activation, inactivation and recovery from inactivation. MiRP3 shifts the half-maximal voltage for activation (V(1/2)) approximately 20 mV and slows time to peak approximately 100%. In addition, MiRP3 slows inactivation approximately 100%, speeds recovery from inactivation approximately 30%, and enhances restored currents so they 'overshoot' baseline levels. The cytoplasmic accessory subunit KChIP2 also assembles with Kv4.2 in tsA201 cells to increase peak current, shift V(1/2) approximately 5 mV, slow time to peak approximately 10%, slow inactivation approximately 100%, and speed recovery from inactivation approximately 250% without overshoot. Simultaneous expression of all three subunits yields a biophysical profile unlike either accessory subunit alone, abolishes MiRP3-induced overshoot, and allows biochemical isolation of the ternary complex. Thus, regional heterogeneity in cardiac expression of MiRP3, Kv4.2 and KChIP2 in health and disease may establish the local attributes and magnitude of cardiac I(to).

Targeted delivery of lysosomal enzymes to the endocytic compartment of human cells represents a transformative technology for treating a large family of lysosomal storage diseases (LSDs). Gaucher disease is one of the most common types of LSDs caused by mutations to the lysosomal β-glucocerebrosidase (GBA). Here, we describe a genetic strategy to produce engineered exosomes loaded with GBA in two different spatial configurations for targeted delivery to the endocytic compartment of recipient cells. By fusing human GBA to an exosome-anchoring protein: vesicular stomatitis virus glycoprotein (VSVG), we demonstrate that the chimeric proteins were successfully integrated into exosomes which were secreted as extracellular vesicles (EVs) by producer cells. Isolation and molecular characterization of EVs confirmed that the fusion proteins were loaded onto exosomes without altering their surface markers, particle size or distribution. Further, enzyme-loaded exosomes/EVs added to cultured medium were taken up by recipient cells. Further, the endocytosed exosomes/EVs targeted to endocytic compartments exhibited a significant increase in GBA activity. Together, we have developed a novel method for targeting and delivery of lysosomal enzymes to their natural location: the endocytic compartment of recipient cells. Since exosomes/EVs have an intrinsic ability to cross the blood-brain-barrier, our technology may provide a new approach to treat severe types of LSDs, including Gaucher disease with neurological complications.

Myocardial repolarization capacity varies with sex, age, and pathology; the molecular basis for this variation is incompletely understood. Here, we show that the transcript for KCNE4, a voltage-gated potassium (Kv) channel β subunit associated with human atrial fibrillation, was 8-fold more highly expressed in the male left ventricle compared with females in young adult C57BL/6 mice (P < 0.05). Similarly, Kv current density was 25% greater in ventricular myocytes from young adult males (P < 0.05). Germ-line Kcne4 deletion eliminated the sex-specific Kv current disparity by diminishing ventricular fast transient outward current (Ito,f) and slowly activating K(+) current (IK,slow1). Kcne4 deletion also reduced Kv currents in male mouse atrial myocytes, by >45% (P < 0.001). As we previously found for Kv4.2 (which generates mouse Ito,f), heterologously expressed KCNE4 functionally regulated Kv1.5 (the Kv α subunit that generates IKslow1 in mice). Of note, in postmenopausal female mice, ventricular repolarization was impaired by Kcne4 deletion, and ventricular Kcne4 expression increased to match that of males. Moreover, castration diminished male ventricular Kcne4 expression 2.8-fold, whereas 5α-dihydrotestosterone (DHT) implants in castrated mice increased Kcne4 expression >3-fold (P = 0.01) to match noncastrated levels. KCNE4 is thereby shown to be a DHT-regulated determinant of cardiac excitability and a molecular substrate for sex- and age-dependent cardiac arrhythmogenesis.

Abstract Background and Aims The burden of disease associated with FSGS has not been well characterized, especially with regard to health care resource utilization (HCRU) and related costs. The aim of this study was to evaluate all-cause HCRU and estimate associated costs in patients with FSGS compared with a matched non-FSGS cohort; a secondary aim was to evaluate the impact of nephrotic range proteinuria on these outcomes. Method Data were from the Optum Clinformatics® Data Mart Database. Patients with ≥ 1 claim (1st claim = index event) for FSGS between April 2016 and December 2018 were identified based on ICD-10-CM codes and matched 1:2 (FSGS:controls) on index date, age, sex, and race to non-FSGS controls; continuous enrollment 6 months pre- and 12 months post-index was required. FSGS nephrotic range (either UPCR >3000 mg/g or ACR >2000 mg/g) and non-nephrotic subpopulations were also identified. Quan-Charlson Comorbidity Index (CCI) and individual comorbidities at baseline, and 12-month post-index all-cause HCRU and associated costs (per patient per year [PPPY]) as well as medication prescriptions related to FSGS treatment were compared between the matched cohorts and between the FSGS subpopulations; t-tests were used for continuous variables and chi-square tests for categorical variables. Results 844 patients with FSGS were matched with 1688 non-FSGS controls; 57.4% male, 56.9% white, mean (SD) age 54.7 (18.4) years. Mean (SD) CCI was higher in the FSGS cohort relative to matched controls (2.72 [2.12] vs 0.55 [1.29]; P < .0001), with prevalence of most individual comorbidities higher in the FSGS cohort. Only 308 FSGS patients (36.5%) had UPCR or ACR tests with available results during the review period; 112 (36.4%) were in the nephrotic range and 196 were non- nephrotic (63.6%). The FSGS cohort was characterized by higher rates of all-cause HCRU across resource categories (all P < .0001) (Table 1); outpatient visits was the most frequently used category (99.1% vs 69.0%), followed by prescription medications. Among patients who used these resources, units of use were significantly higher in FSGS vs matched controls except for length of stay (Table 1). Readmission rates following 1st post-index hospitalization were higher in the FSGS cohort vs matched controls at 30 days (16.1% vs 6.0%; P < .05) and 365 days (39.1% vs 22.9%; P < .05). Glucocorticoids were the most frequently prescribed FSGS-related medication in both cohorts, with a higher rate in FSGS vs matched controls (50.6% vs 23.3%; P < .0001); other FSGS-related medications were infrequently prescribed (< 14%). Inpatient, outpatient, and prescription costs were higher in the FSGS cohort vs matched controls (all P < .0001) resulting in mean total annual medical costs of $59,753 vs $8,431 PPPY (P < .0001) that were driven by outpatient costs (Fig. 1A). Nephrotic range proteinuria was associated with higher all-cause inpatient, outpatient, and prescription costs vs non-nephrotic patients (all P < .0001; Fig. 1B), resulting in higher total costs ($70,481 vs $36,099 PPPY; P < .0001). A higher proportion of nephrotic range patients were prescribed FSGS-modifying medications (73.2% vs 54.1%; P = 0.001), with glucocorticoids the most frequent medication. However, 26.8% of nephrotic range patients were not prescribed any FSGS-related medications. Conclusion FSGS is associated with significant clinical and economic burdens with total annual medical costs > 7-fold higher than matched controls that were driven by outpatient costs. The presence of nephrotic range proteinuria substantially and significantly increased the economic burden. New treatment modalities leading to lower rates of proteinuria may help improve patient outcomes while reducing HCRU and their associated costs.