Role of Mitochondria in Physiology of Chondrocytes and Diseases of Osteoarthritis and Rheumatoid Arthritis

Purpose of Review Mitochondria are recognized to be one of the most important organelles in chondrocytes for their role in triphosphate (ATP) generation through aerobic phosphorylation. Mitochondria also participate in many intracellular processes involving modulating reactive oxygen species (ROS), responding to instantaneous hypoxia stress, regulating cytoplasmic transport of calcium ion, and directing mitophagy to maintain the homeostasis of individual chondrocytes. Designs To summarize the specific role of mitochondria in chondrocytes, we screened related papers in PubMed database and the search strategy is ((mitochondria) AND (chondrocyte)) AND (English [Language]). The articles published in the past 5 years were included and 130 papers were studied. Results In recent years, the integrity of mitochondrial structure has been regarded as a prerequisite for normal chondrocyte survival and defect in mitochondrial function has been found in cartilage-related diseases, such as osteoarthritis (OA) and rheumatoid arthritis (RA). However, the understanding of mitochondria in cartilage is still largely limited. The mechanism on how the changes in mitochondrial structure and function directly lead to the occurrence and development of cartilage-related diseases remains to be elusive. Conclusion This review aims to summarize the role of mitochondria in chondrocytes under the physiological and pathological changes from ATP generation, calcium homeostasis, redox regulation, mitophagy modulation, mitochondria biogenesis to immune response activation. The enhanced understanding of molecular mechanisms in mitochondria might offer some new cues for cartilage remodeling and pathological intervention.

Therapeutic Targeting of Exportin-1 in Childhood Cancer

Overexpression of Exportin-1 (XPO1), a key regulator of nuclear-to-cytoplasmic transport, is associated with inferior patient outcomes across a range of adult malignancies. Targeting XPO1 with selinexor has demonstrated promising results in clinical trials, leading to FDA approval of its use for multiple relapsed/refractory cancers. However, XPO1 biology and selinexor sensitivity in childhood cancer is only recently being explored. In this review, we will focus on the differential biology of childhood and adult cancers as it relates to XPO1 and key cargo proteins. We will further explore the current state of pre-clinical and clinical development of XPO1 inhibitors in childhood cancers. Finally, we will outline potentially promising future therapeutic strategies for, as well as potential challenges to, integrating XPO1 inhibition to improve outcomes for children with cancer.

CD47 interactions with exportin-1 limit the targeting of m7G-modified RNAs to extracellular vesicles

CD47 is a marker of self and a signaling receptor for thrombospondin-1 that is also a component of extracellular vesicles (EVs) released by various cell types. Previous studies identified CD47-dependent functional effects of T cell EVs on target cells, mediated by delivery of their RNA contents, and enrichment of specific subsets of coding and noncoding RNAs in CD47+ EVs. Mass spectrometry was employed here to identify potential mechanisms by which CD47 regulates the trafficking of specific RNAs to EVs. Specific interactions of CD47 and its cytoplasmic adapter ubiquilin-1 with components of the exportin-1/Ran nuclear export complex were identified and confirmed by coimmunoprecipitation. Exportin-1 is known to regulate nuclear to cytoplasmic trafficking of 5’-7-methylguanosine (m7G)-modified microRNAs and mRNAs that interact with its cargo protein EIF4E. Interaction with CD47 was inhibited following alkylation of exportin-1 at Cys528 by its covalent inhibitor leptomycin B. Leptomycin B increased levels of m7G-modified RNAs, and their association with exportin-1 in EVs released from wild type but not CD47-deficient cells. In addition to perturbing nuclear to cytoplasmic transport, transcriptomic analyses of EVs released by wild type and CD47-deficient Jurkat T cells revealed a global CD47-dependent enrichment of m7G-modified microRNAs and mRNAs in EVs released by CD47-deficient cells. Correspondingly, decreasing CD47 expression in wild type cells or treatment with thrombospondin-1 enhanced levels of specific m7G-modified RNAs released in EVs, and re-expressing CD47 in CD47-deficient T cells decreased their levels. Therefore, CD47 signaling limits the trafficking of m7G-modified RNAs to EVs through physical interactions with the exportin-1/Ran transport complex.

A novel mechanism of bulk cytoplasmic transport by cortical dynein in Drosophila ovary

Cytoplasmic dynein, a major minus-end directed microtubule motor, plays essential roles in eukaryotic cells. Drosophila oocyte growth is mainly dependent on the contribution of cytoplasmic contents from the interconnected sister cells, nurse cells. We have previously shown that cytoplasmic dynein is required for Drosophila oocyte growth, and assumed that it transports cargoes along microtubule tracks from nurse cells to the oocyte. Here we report that instead transporting cargoes along microtubules into the oocyte, cortical dynein actively moves microtubules in nurse cells and from nurse cells to the oocyte via the cytoplasmic bridges, the ring canals. We demonstrate this microtubule movement is sufficient to drag even inert cytoplasmic particles through the ring canals to the oocyte. Furthermore, replacing dynein with a minus-end directed plant kinesin linked to the actin cortex is sufficient for transporting organelles and cytoplasm to the oocyte and driving its growth. These experiments show that cortical dynein can perform bulk cytoplasmic transport by gliding microtubules along the cell cortex and through the ring canals to the oocyte. We propose that the dynein-driven microtubule flow could serve as a novel mode of cargo transport for fast cytoplasmic transfer to support rapid oocyte growth.

Bacterial chemotaxis to saccharides is governed by a trade-off between sensing and uptake

To swim up gradients of nutrients, E. coli senses nutrient concentrations within its periplasm. For small nutrient molecules, periplasmic concentrations typically match extracellular concentrations. However, this is not necessarily the case for saccharides, such as maltose, which is transported into the periplasm via a specific porin. Previous observations have shown that under various conditions E. coli limits maltoporin abundance so that, for extracellular micromolar concentrations of maltose, there are predicted to be only nanomolar concentrations of free maltose in the periplasm. Thus, in the micromolar regime, the total uptake of maltose from the external environment into the cytoplasm is limited not by the abundance of cytoplasmic transport proteins but by the abundance of maltoporins. Here we present results from experiments and modeling showing that this porin-limited transport enables E. coli to sense micromolar gradients of maltose despite having a high-affinity ABC transport system that is saturated at these micromolar levels. We used microfluidic assays to study chemotaxis of E. coli in various gradients of maltose and methyl-aspartate and leveraged our experimental observations to develop a mechanistic transport-and-sensing chemotaxis model. Incorporating this model into agent-based simulations, we discover a trade-off between uptake and sensing: although high-affinity transport enables higher uptake rates at low nutrient concentrations, it severely limits dynamic sensing range. We thus propose that E. coli may limit periplasmic uptake to increase its chemotactic sensitivity, enabling it to use maltose as an environmental cue.

Value of SERCA2a as a Biomarker for the Identification of Patients with Heart Failure Requiring Circulatory Support

Background: Heart failure (HF) alters the nucleo-cytoplasmic transport of cardiomyocytes and reduces SERCA2a levels, essential for intracellular calcium homeostasis. We consider in this study whether the molecules involved in these processes can differentiate those patients with advanced HF and the need for mechanical circulatory support (MCS) as a bridge to recovery or urgent heart transplantation from those who are clinically stable and who are transplanted in an elective code. Material and method: Blood samples from 29 patients with advanced HF were analysed by ELISA, and the plasma levels of Importin5, Nucleoporin153 kDa, RanGTPase-Activating Protein 1 and sarcoplasmic reticulum Ca2+ ATPase were compared between patients requiring MCS and those patients without a MCS need prior to heart transplantation. Results: SERCA2a showed significantly lower levels in patients who had MCS compared to those who did not require it (0.501 ± 0.530 ng/mL vs. 1.123 ± 0.661 ng/mL; p = 0.01). A SERCA2a cut-off point of 0.84 ng/mL (AUC 0.812 ± 0.085, 95% CI: 0.646–0.979; p = 0.004) provided a 92% sensitivity, 62% specificity, 91% negative predictive value and 67% positive predictive value. Conclusions: In this cohort, patients with advanced HF and a need for MCS have shown significantly lower levels of SERCA2a as compared to stable patients without a need for MCS prior to heart transplantation. This is a small study with preliminary findings, and larger-powered dedicated studies are required to confirm and validate these results.

Cryo-EM structure of human Nup155 reveals the biochemical basis for atrial fibrillation linked genetic mutation R391H

SummaryHuman nuclear pore complexes are composed of ∼32 distinct nucleoporins to facilitate bidirectional nucleo-cytoplasmic transport. Many of them have been associated with various human diseases such as an inherited mutation (R391H) in Nup155 is shown as the clinical cause of atrial fibrillation and sudden cardiac arrest. Due to the lack of structural knowledge and mechanistic insights, the roles of Nups in NPC assembly and relevance in human diseases are very restricted. Here, we show the cryo-EM structure of human Nup155 at 5.2-5.7. Å resolution deciphered from 3 distinct particle classes: N-terminus (19-863), C-terminus (864-1337), and longer N-terminus (19-1069). It revealed intrinsic plasticity at the middle domain of Nup155 and the role of species-specific loop regions in an atypical 7-bladed β-propeller domain to provide a distinct interface for Nup93 and Nup35. Due to the proximity of these Nups interacting sites near the Arginine-391 position, atrial fibrillation linked genetic mutation (R391H) causes dissociation from NPC in absence of N-terminal 112 residues.HighlightsCryo-EM structure of human Nup155 at 5.2 Å resolutionSeven bladed β-propeller domain at N-terminus of Nup155 exhibited distinct features for interaction with Nup35 and Nup93The middle domain of Nup155 is highly dynamic in natureStructural mapping allows mechanistic interpretation of AF linked R391H mutation

Population-level survey of loss-of-function mutations revealed that background dependent fitness genes are rare and functionally related in yeast

In natural populations, the same mutation can lead to different phenotypic outcomes due to the genetic variation that exists among individuals. Such genetic background effects are commonly observed, including in the context of many human diseases. However, systematic characterization of these effects at the species level is still lacking to date. Here, we sought to comprehensively survey background-dependent traits associated with gene loss-of-function (LoF) mutations in 39 natural isolates of Saccharomyces cerevisiae using a transposon saturation strategy. By analyzing the modeled fitness variability of a total of 4,469 genes, we found that 15% of them, when impacted by a LoF mutation, exhibited a significant gain- or loss-of-fitness phenotype in certain natural isolates compared to the reference strain S288C. Out of these 632 genetic background-dependent fitness genes identified, a total of 2/3 show a continuous variation across the population while 1/3 are specific to a single genetic background. Genes related to mitochondrial function are significantly overrepresented in the set of genes showing a continuous variation and display a potential functional rewiring with other genes involved in transcription and chromatin remodeling as well as in nuclear-cytoplasmic transport. Such rewiring effects are likely modulated by both the genetic background and the environment. While background-specific cases are rare and span diverse cellular processes, they can be functionally related at the individual level. All background-dependent fitness genes tend to have an intermediate connectivity in the global genetic interaction network and have shown relaxed selection pressure at the population level, highlighting their potential evolutionary characteristics.