Potassium homeostasis: sensors, mediators, and targets.

Transmembrane potassium (K) gradients are key determinants of membrane potential that can modulate action potentials, control muscle contractility, and influence ion channel and transporter activity. Daily K intake is normally equal to the amount of K in the entire extracellular fluid (ECF) creating a critical challenge - how to maintain ECF [K] and membrane potential in a narrow range during feast and famine. Adaptations to maintain ECF [K] include sensing the K intake, sensing ECF [K] vs. desired set-point and activating mediators that regulate K distribution between ECF and ICF, and regulate renal K excretion. In this focused review, we discuss the basis of these adaptions, including (1) potential mechanisms for rapid feedforward signaling to kidney and muscle after a meal (before a rise in ECF [K]), (2) how skeletal muscles sense and respond to changes in ECF [K], (3) effects of K on aldosterone biosynthesis, and (4) how the kidney responds to changes in ECF [K] to modify K excretion. The concepts of sexual dimorphisms in renal K handling adaptation are introduced, and the molecular mechanisms that can account for the benefits of a K-rich diet to maintain cardiovascular health are discussed. Although the big picture of K homeostasis is becoming more clear, we also highlight significant pieces of the puzzle that remain to be solved, including knowledge gaps in our understanding of initiating signals, sensors and their connection to homeostatic adjustments of ECF [K].

Hydroxysafflor Yellow A Confers Neuroprotection from Focal Cerebral Ischemia by Modulating the Crosstalk Between JAK2/STAT3 and SOCS3 Signaling Pathways.

Natural bioactive compounds have increasingly proved to be promising in evidence- or target-directed treatment or modification of a spectrum of diseases including cerebral ischemic stroke. Hydroxysafflor yellow A (HSYA), a major active component of the safflower plant, has drawn more interests in recent year for its multiple pharmacological actions in the treatment of cerebrovascular and cardiovascular diseases. Although the Janus kinase signaling, such as JAK2/STAT3 pathway, has been implicated in the modulation of the disease, the inhibition or activation of the pathway that contributed to the neuronal prevention from ischemic damages remains controversial. In this study, a series of experiments were performed to examine the dose- and therapeutic time window-related pharmacological efficacies of HSYA with emphasis on the HSYA-modulated interaction of JAK2/STAT3 and SOCS3 signaling in the MCAO rats. We found that HSYA treatment significantly rescued the neurological and functional deficits in a dose-dependent manner in the MCAO rats within 3 h after ischemia. HSYA treatment with a dosage of 8 mg/kg or higher markedly downregulated the expression of the JAK2-mediated signaling that was activated in response to ischemic insult, while it also promoted the expression of SOCS3 coordinately. In the subsequent experiments with the use of the JAK2 inhibitor WP1066, we found that the treatment of WP1066 alone or combination of WP1066/HSYA all exhibited inhibitory effects on JAK2-mediated signaling, while there was no influence on the SOCS3 activity of corresponding efficacious data in the MCAO rats, suggesting that excessive activation of JAK2/STAT3 might be necessary for HSYA to provoke SOCS3-negative feedback signaling. Taking together, our study demonstrates that HSYA might modulate the crosstalk between JAK2/STAT3 and SOCS3 signaling pathways that eventually contributed to its therapeutic roles against cerebral ischemic stroke.

Pten Regulates Retinal Amacrine Cell Number by Modulating Akt, Tgfß, and Erk Signaling.

UNLABELLED: All tissues are genetically programmed to acquire an optimal size that is defined by total cell number and individual cellular dimensions. The retina contains stereotyped proportions of one glial and six neuronal cell types that are generated in overlapping waves. How multipotent retinal progenitors know when to switch from making one cell type to the next so that appropriate numbers of each cell type are generated is poorly understood. Pten is a phosphatase that controls progenitor cell proliferation and differentiation in several lineages. Here, using a conditional loss-of-function strategy, we found that Pten regulates retinal cell division and is required to produce the full complement of rod photoreceptors and amacrine cells in mouse. We focused on amacrine cell number control, identifying three downstream Pten effector pathways. First, phosphoinositide 3-kinase/Akt signaling is hyperactivated in Pten conditional knock-out (cKO) retinas, and misexpression of constitutively active Akt (Akt-CA) in retinal explants phenocopies the reduction in amacrine cell production observed in Pten cKOs. Second, Akt-CA activates Tgfß signaling in retinal explants, which is a negative feedback pathway for amacrine cell production. Accordingly, Tgfß signaling is elevated in Pten cKO retinas, and epistatic analyses placed Pten downstream of TgfßRII in amacrine cell number control. Finally, Pten regulates Raf/Mek/Erk signaling levels to promote the differentiation of all amacrine cell subtypes, which are each reduced in number in Pten cKOs. Pten is thus a positive regulator of amacrine cell production, acting via multiple downstream pathways, highlighting its diverse actions as a mediator of cell number control. SIGNIFICANCE STATEMENT: Despite the importance of size for optimal organ function, how individual cell types are generated in correct proportions is poorly understood. There are several ways to control cell number, including readouts of organ function (e.g., secreted hormones reach functional levels when enough cells are made) or counting of cell divisions or cell number. The latter applies to the retina, where cell number is regulated by negative feedback signals, which arrest differentiation of particular cell types at threshold levels. Herein, we show that Pten is a critical regulator of amacrine cell number in the retina, acting via multiple downstream pathways. Our studies provide molecular insights into how PTEN loss in humans may lead to uncontrolled cell division in several pathological conditions.

Ntf3 acts downstream of Sip1 in cortical postmitotic neurons to control progenitor cell fate through feedback signaling.

Cortical progenitors undergo progressive fate restriction, thereby sequentially producing the different layers of the neocortex. However, how these progenitors precisely change their fate remains highly debatable. We have previously shown the existence of cortical feedback mechanisms wherein postmitotic neurons signal back to the progenitors and promote a switch from neurogenesis to gliogenesis. We showed that Sip1 (Zeb2), a transcriptional repressor, controls this feedback signaling. A similar mechanism was also suggested to control neuronal cell type specification; however, the underlying mechanism was not identified. Here, we provide direct evidence that in the developing mouse neocortex, Ntf3, a Sip1 target neurotrophin, acts as a feedback signal between postmitotic neurons and progenitors, promoting both apical progenitor (AP) to basal progenitor (BP) and deep layer (DL) to upper layer (UL) cell fate switches. We show that specific overexpression of Ntf3 in neocortical neurons promotes an overproduction of BP at the expense of AP. This shift is followed by a decrease in DL and an increase in UL neuronal production. Loss of Ntf3, by contrast, causes an increase in layer VI neurons but does not rescue the Sip1 mutant phenotype, implying that other parallel pathways also control the timing of progenitor cell fate switch.

The yin and yang of cell wall integrity control: brassinosteroid and FERONIA signaling.

Understanding how developmental and environmental signals control plant cell expansion requires an intimate knowledge of the architecture of the primary cell wall and the chemo-rheological processes that underlie cell wall relaxation. In this review I discuss recent findings that reveal a more prominent role than previously suspected for covalent bonds and pectin cross-links in primary cell wall architecture. In addition, genetic studies have uncovered a role for receptor kinases in the control of cell wall homeostasis in growing cells. The emerging view is that, upon cell wall disruption, compensatory changes are induced in the cell wall through the interplay between the brassinosteroid signaling module, which positively regulates wall extensibility and receptor kinases of the CrRLKL1 family, which may act as negative regulators of cell wall stiffness. These findings lift the tip of the veil of a complex signaling network allowing normal homeostasis in walls of growing cells but also crisis management under stress conditions.

Small Extracellular Vesicle-Derived vWF Induces a Positive Feedback Loop between Tumor and Endothelial Cells to Promote Angiogenesis and Metastasis in Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is a hypervascular malignancy by which its growth and dissemination are largely driven by the modulation of tumor-derived small extracellular vesicles (sEVs). Proteomic profiling of circulating sEVs of control individuals and HCC patients identifies von Willibrand factor (vWF) to be upregulated progressively along HCC stages. Elevated sEV-vWF levels are found in a larger cohort of HCC-sEV samples and metastatic HCC cell lines compared to their respective normal counterparts. Circulating sEVs of late-stage HCC patients markedly augment angiogenesis, tumor-endothelial adhesion, pulmonary vascular leakiness, and metastasis, which are significantly compromised by anti-vWF antibody. The role of vWF is further corroborated by the enhanced promoting effect of sEVs collected from vWF-overexpressing cells. sEV-vWF modulates endothelial cells through an elevated level of vascular endothelial growth factor A (VEGF-A) and fibroblast growth factor 2 (FGF2). Mechanistically, secreted FGF2 elicits a positive feedback response in HCC via the FGFR4/ERK1 signaling pathway. The co-administration of anti-vWF antibody or FGFR inhibitor significantly improves the treatment outcome of sorafenib in a patient-derived xenograft mouse model. This study reveals mutual stimulation between HCC and endothelial cells by tumor-derived sEVs and endothelial angiogenic factors, facilitating angiogenesis and metastasis. It also provides insights into a new therapeutic strategy involving blocking tumor-endothelial intercellular communication.

The Molecular Chaperone Heat Shock Protein 70 Controls Liver Cancer Initiation and Progression by Regulating Adaptive DNA Damage and Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase Signaling Pathways.

Delineating the mechanisms that drive hepatic injury and hepatocellular carcinoma (HCC) progression is critical for development of novel treatments for recurrent and advanced HCC but also for the development of diagnostic and preventive strategies. Heat shock protein 70 (HSP70) acts in concert with several cochaperones and nucleotide exchange factors and plays an essential role in protein quality control that increases survival by protecting cells against environmental stressors. Specifically, the HSP70-mediated response has been implicated in the pathogenesis of cancer, but the specific mechanisms by which HSP70 may support malignant cell transformation remains to be fully elucidated. Here, we show that genetic ablation of HSP70 markedly impairs HCC initiation and progression by distinct but overlapping pathways. This includes the potentiation of the carcinogen-induced DNA damage response, at the tumor initiation stage, to increase the p53-dependent surveillance response leading to the cell cycle exit or death of genomically damaged differentiated pericentral hepatocytes, and this may also prevent their conversion into more proliferating HCC progenitor cells. Subsequently, activation of a mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) negative feedback pathway diminishes oncogenic signals, thereby attenuating premalignant cell transformation and tumor progression. Modulation of HSP70 function may be a strategy for interfering with oncogenic signals driving liver cell transformation and tumor progression, thus providing an opportunity for human cancer control.

Inter- and intra-cellular mechanism of NF-kB-dependent survival advantage and clonal expansion of radio-resistant cancer cells.

Understanding the underlying mechanism by which cancer cells acquire resistance to radiation and favorably selected for its clonal expansion will provide molecular insight into tumor recurrence at the treatment site. In the present study, we investigated the molecular mechanisms prompted in MCF-7 breast cancer cells in response to clinical radiation and the associated coordination of intra- and inter-cellular signaling that orchestrate radio-resistance and tumor relapse/recurrence. Our findings showed that 2 or 10Gy of 137Cs γ-rays at a dose rate of 1.03Gy/min trigger the activation of nuclear factor kappa B (NF-κB), its DNA-binding activity and recycles its own transcription. NF-κB DNA-binding kinetic analysis demonstrated both sustained and dual phase NF-κB activation with radiation. Gene manipulation approach revealed that radiation triggered NF-κB-mediated TNF-α transcriptional activity. TNF-α blocking approach confirmed that the de novo synthesis and secretion of TNF-α serves as a pre-requisite for the second phase of NF-κB activation and sustained maintenance. Radiation-associated NF-κB-dependent secretion of TNF-α from irradiated cells, in parallel, activates NF-κB in the non-targeted un-irradiated bystander cells. Together, these findings demonstrated that radiation-triggered NF-κB-dependent TNFα secretion is critical for self-sustenance of NF-κB (through autocrine positive feedback signaling) and for coordinating bystander response (through inter-cellular paracrine mechanism) after radiation exposure. Further, the data suggest that this self-sustained NF-κB in the irradiated cells determines radio-resistance, survival advantage and clonal expansion of the tumor cells at the treatment site. Parallel maintenance of NF-ΚB-TNF-α-NF-κB feedback-cycle in the un-irradiated non-targeted bystander cells initiates supportive mechanism for the promotion and progression of surviving tumor cells. Intervening this molecular pathway would help us to achieve disease-free cancer survivors.

Heterochronic Pellet Assay to Test Cell-cell Communication in the Mouse Retina.

All seven retinal cell types that make up the mature retina are generated from a common, multipotent pool of retinal progenitor cells (RPCs) (Wallace, 2011). One way that RPCs know when sufficient numbers of particular cell-types have been generated is through negative feedback signals, which are emitted by differentiated cells and must reach threshold levels to block additional differentiation of that cell type. A key assay to assess whether negative feedback signals are emitted by differentiated cells is a heterochronic pellet assay in which early stage RPCs are dissociated and labeled with BrdU, then mixed with a 20-fold excess of dissociated differentiated cells. The combined cells are then re-aggregated and cultured as a pellet on a membrane for 7-10 days in vitro. During this time frame, RPCs will differentiate, and the fate of the BrdU+ RPCs can be assessed using cell type-specific markers. Investigators who developed this pellet assay initially demonstrated that neonatal RPCs give rise to rods on an accelerated schedule compared to embryonic RPCs when the two cell types are mixed together (Watanabe and Raff, 1990; Watanabe et al., 1997). We have used this assay to demonstrate that sonic hedgehog (Shh), which we found acts as a negative regulator of retinal ganglion cell (RGC) differentiation, promotes RPC proliferation (Jensen and Wallace, 1997; Ringuette et al., 2014). More recently we modified the heterochronic pellet assay to assess the role of feedback signals for retinal amacrine cells, identifying transforming growth factor ß2 (Tgfß2) as a negative feedback signal, and Pten as a modulator of the Tgfß2 response (Ma et al., 2007; Tachibana et al., 2016). This assay can be adapted to other lineages and tissues to assess cell-cell interactions between two different cell-types (heterotypic) in either an isochronic or heterochronic manner.

Retrograde fibroblast growth factor 22 (FGF22) signaling regulates insulin-like growth factor 2 (IGF2) expression for activity-dependent synapse stabilization in the mammalian brain.

Communication between pre- and postsynaptic cells promotes the initial organization of synaptic specializations, but subsequent synaptic stabilization requires transcriptional regulation. Here we show that fibroblast growth factor 22 (FGF22), a target-derived presynaptic organizer in the mouse hippocampus, induces the expression of insulin-like growth factor 2 (IGF2) for the stabilization of presynaptic terminals. FGF22 is released from CA3 pyramidal neurons and organizes the differentiation of excitatory nerve terminals formed onto them. Local application of FGF22 on the axons of dentate granule cells (DGCs), which are presynaptic to CA3 pyramidal neurons, induces IGF2 in the DGCs. IGF2, in turn, localizes to DGC presynaptic terminals and stabilizes them in an activity-dependent manner. IGF2 application rescues presynaptic defects of Fgf22(-/-) cultures. IGF2 is dispensable for the initial presynaptic differentiation, but is required for the following presynaptic stabilization both in vitro and in vivo. These results reveal a novel feedback signal that is critical for the activity-dependent stabilization of presynaptic terminals in the mammalian hippocampus.

TGF-ß signal rewiring sustains epithelial-mesenchymal transition of circulating tumor cells in prostate cancer xenograft hosts.

Activation of TGF-ß signaling is known to promote epithelial-mesenchymal transition (EMT) for the development of metastatic castration-resistant prostate cancer (mCRPC). To determine whether targeting TGF-ß signaling alone is sufficient to mitigate mCRPC, we used the CRISPR/Cas9 genome-editing approach to generate a dominant-negative mutation of the cognate receptor TGFBRII that attenuated TGF-ß signaling in mCRPC cells. As a result, the delicate balance of oncogenic homeostasis is perturbed, profoundly uncoupling proliferative and metastatic potential of TGFBRII-edited tumor xenografts. This signaling disturbance triggered feedback rewiring by enhancing ERK signaling known to promote EMT-driven metastasis. Circulating tumor cells displaying upregulated EMT genes had elevated biophysical deformity and an increase in interactions with chaperone macrophages for facilitating metastatic extravasation. Treatment with an ERK inhibitor resulted in decreased aggressive features of CRPC cells in vitro. Therefore, combined targeting of TGF-ß and its backup partner ERK represents an attractive strategy for treating mCRPC patients.