Chloride oscillation in pacemaker neurons regulates circadian rhythms through a chloride-sensing WNK kinase signaling cascade

Function and regulation of the insect NaCCC2 sodium transport proteins

NaCCC2 transport proteins, including those from Drosophila melanogaster (Ncc83) and Aedes aegypti (aeCCC2), are an insect-specific clade with sequence similarity to Na+-K+-2Cl- cotransporters. Whereas the Na+-K+-2Cl- cotransporters and other cation-chloride cotransporters are electroneutral, recent work indicates that Ncc83 and aeCCC2 carry charge across membranes. Here, we further characterize the regulation and transport properties of Ncc83 and aeCCC2 expressed in Xenopus oocytes. In cation uptake experiments, Li+ was used as a tracer for Na+ and Rb+ was used as a tracer for K+. Li+ uptake of oocytes expressing either aeCCC2 or Ncc83 was greater than uptake in water-injected controls, activated by hypotonic swelling, and not inhibited by ouabain or ethyl cinnamate. Rb+ uptake of oocytes expressing either aeCCC2 or Ncc83 was not different than water injected controls. In oocytes expressing either aeCCC2 or Ncc83, Li+ uptake plateaued with increasing Li+ concentrations, with apparent Km values in the range of 10 to 20 mM. Following exposure to ouabain, intracellular [Na+] was greater in oocytes expressing aeCCC2 than in controls. Elevating intracellular cAMP (via 8-bromo-cAMP) in Ncc83 oocytes significantly stimulated both Li+ uptake and membrane conductances. Elevating intracellular cAMP in aeCCC2 oocytes did not affect Li+ uptake, but stimulated membrane conductances. Overall, these results confirm that the NaCCC2s resemble other cation-chloride cotransporters in their regulation and some transport properties. However, unlike other cation-chloride cotransporters, they carry charge across membranes.

Circadian Rhythm Regulation by Pacemaker Neuron Chloride Oscillation in Flies

Circadian rhythms in physiology and behavior sync organisms to external environmental cycles. Here, circadian oscillation in intracellular chloride in central pacemaker neurons of the fly, Drosophila melanogaster, is reviewed. Intracellular chloride links SLC12 cation-coupled chloride transporter function with kinase signaling and the regulation of inwardly rectifying potassium channels.

Engineering the ChlorON Series: Turn-On Fluorescent Protein Sensors for Imaging Labile Chloride in Living Cells

Beyond its role as the "queen of electrolytes", chloride can also serve as an allosteric regulator or even a signaling ion. To illuminate this essential anion across such a spectrum of biological processes, researchers have relied on fluorescence imaging with genetically encoded sensors. In large part, these have been derived from the green fluorescent protein found in the jellyfish Aequorea victoria. However, a standalone sensor with a turn-on intensiometric response at physiological pH has yet to be reported. Here, we address this technology gap by building on our discovery of the anion-sensitive fluorescent protein mNeonGreen (mNG). The targeted engineering of two non-coordinating residues, namely K143 and R195, in the chloride binding pocket of mNG coupled with an anion walking screening and selection strategy resulted in the ChlorON sensors: ChlorON-1 (K143W/R195L), ChlorON-2 (K143R/R195I), and ChlorON-3 (K143R/R195L). In vitro spectroscopy revealed that all three sensors display a robust turn-on fluorescence response to chloride (20- to 45-fold) across a wide range of affinities (Kd ≈ 30-285 mM). We further showcase how this unique sensing mechanism can be exploited to directly image labile chloride transport with spatial and temporal resolution in a cell model overexpressing the cystic fibrosis transmembrane conductance regulator. Building from this initial demonstration, we anticipate that the ChlorON technology will have broad utility, accelerating the path forward for fundamental and translational aspects of chloride biology.

Contribution of membrane-associated oscillators to biological timing at different timescales

Environmental rhythms such as the daily light-dark cycle selected for endogenous clocks. These clocks predict regular environmental changes and provide the basis for well-timed adaptive homeostasis in physiology and behavior of organisms. Endogenous clocks are oscillators that are based on positive feedforward and negative feedback loops. They generate stable rhythms even under constant conditions. Since even weak interactions between oscillators allow for autonomous synchronization, coupling/synchronization of oscillators provides the basis of self-organized physiological timing. Amongst the most thoroughly researched clocks are the endogenous circadian clock neurons in mammals and insects. They comprise nuclear clockworks of transcriptional/translational feedback loops (TTFL) that generate ∼24 h rhythms in clock gene expression entrained to the environmental day-night cycle. It is generally assumed that this TTFL clockwork drives all circadian oscillations within and between clock cells, being the basis of any circadian rhythm in physiology and behavior of organisms. Instead of the current gene-based hierarchical clock model we provide here a systems view of timing. We suggest that a coupled system of autonomous TTFL and posttranslational feedback loop (PTFL) oscillators/clocks that run at multiple timescales governs adaptive, dynamic homeostasis of physiology and behavior. We focus on mammalian and insect neurons as endogenous oscillators at multiple timescales. We suggest that neuronal plasma membrane-associated signalosomes constitute specific autonomous PTFL clocks that generate localized but interlinked oscillations of membrane potential and intracellular messengers with specific endogenous frequencies. In each clock neuron multiscale interactions of TTFL and PTFL oscillators/clocks form a temporally structured oscillatory network with a common complex frequency-band comprising superimposed multiscale oscillations. Coupling between oscillator/clock neurons provides the next level of complexity of an oscillatory network. This systemic dynamic network of molecular and cellular oscillators/clocks is suggested to form the basis of any physiological homeostasis that cycles through dynamic homeostatic setpoints with a characteristic frequency-band as hallmark. We propose that mechanisms of homeostatic plasticity maintain the stability of these dynamic setpoints, whereas Hebbian plasticity enables switching between setpoints via coupling factors, like biogenic amines and/or neuropeptides. They reprogram the network to a new common frequency, a new dynamic setpoint. Our novel hypothesis is up for experimental challenge.

Unanticipated domain requirements for Drosophila Wnk kinase in vivo

WNK (With no Lysine [K]) kinases have critical roles in the maintenance of ion homeostasis and the regulation of cell volume. Their overactivation leads to pseudohypoaldosteronism type II (Gordon syndrome) characterized by hyperkalemia and high blood pressure. More recently, WNK family members have been shown to be required for the development of the nervous system in mice, zebrafish, and flies, and the cardiovascular system of mice and fish. Furthermore, human WNK2 and Drosophila Wnk modulate canonical Wnt signaling. In addition to a well-conserved kinase domain, animal WNKs have a large, poorly conserved C-terminal domain whose function has been largely mysterious. In most but not all cases, WNKs bind and activate downstream kinases OSR1/SPAK, which in turn regulate the activity of various ion transporters and channels. Here, we show that Drosophila Wnk regulates Wnt signaling and cell size during the development of the wing in a manner dependent on Fray, the fly homolog of OSR1/SPAK. We show that the only canonical RF(X)V/I motif of Wnk, thought to be essential for WNK interactions with OSR1/SPAK, is required to interact with Fray in vitro. However, this motif is unexpectedly dispensable for Fray-dependent Wnk functions in vivo during fly development and fluid secretion in the Malpighian (renal) tubules. In contrast, a structure function analysis of Wnk revealed that the less-conserved C-terminus of Wnk, that recently has been shown to promote phase transitions in cell culture, is required for viability in vivo. Our data thus provide novel insights into unexpected in vivo roles of specific WNK domains.

Intracellular Ion Control of WNK Signaling

The with no lysine (K) (WNK) kinases are an evolutionarily ancient group of kinases with atypical placement of the catalytic lysine and diverse physiological roles. Recent studies have shown that WNKs are directly regulated by chloride, potassium, and osmotic pressure. Here, we review the discovery of WNKs as chloride-sensitive kinases and discuss physiological contexts in which chloride regulation of WNKs has been demonstrated. These include the kidney, pancreatic duct, neurons, and inflammatory cells. We discuss the interdependent relationship of osmotic pressure and intracellular chloride in cell volume regulation. We review the recent demonstration of potassium regulation of WNKs and speculate on possible physiological roles. Finally, structural and mechanistic aspects of intracellular ion and osmotic pressure regulation of WNKs are discussed.

Discovery of a monomeric green fluorescent protein sensor for chloride by structure-guided bioinformatics

Chloride is an essential anion for all forms of life. Beyond electrolyte balance, an increasing body of evidence points to new roles for chloride in normal physiology and disease. Over the last two decades, this understanding has been advanced by chloride-sensitive fluorescent proteins for imaging applications in living cells. To our surprise, these sensors have primarily been engineered from the green fluorescent protein (GFP) found in the jellyfish Aequorea victoria. However, the GFP family has a rich sequence space that could already encode for new sensors with desired properties, thereby minimizing protein engineering efforts and accelerating biological applications. To efficiently sample this space, we present and validate a stepwise bioinformatics strategy focused first on the chloride binding pocket and second on a monomeric oligomerization state. Using this, we identified GFPxm163 from GFPxm found in the jellyfish Aequorea macrodactyla. In vitro characterization shows that the binding of chloride as well as bromide, iodide, and nitrate rapidly tunes the ground state chromophore equilibrium from the phenolate to the phenol state generating a pH-dependent, turn-off fluorescence response. Furthermore, live-cell fluorescence microscopy reveals that GFPxm163 provides a reversible, yet indirect readout of chloride transport via iodide exchange. With this demonstration, we anticipate that the pairing of bioinformatics with protein engineering methods will provide an efficient methodology to discover and design new chloride-sensitive fluorescent proteins for cellular applications.

Role of daytime variation in pharmaceutical effects of sufentanil, dezocine, and tramadol: A matched observational study

The role of daytime variation in the comprehensive pharmaceutical effects of commonly used opioid analgesics in clinical setting remains unclear. This study aimed to explore the differences in daytime variation among elective surgery patients who were scheduled to receive preemptive analgesia with equivalent doses of sufentanil, dezocine, and tramadol in the morning and afternoon. The analgesic effect was assessed by changes in the pressure pain threshold before and after intravenous administration of sufentanil, dezocine, and tramadol. Respiratory effects were evaluated using pulse oximetry, electrical impedance tomography, and arterial blood gas analysis. Other side effects, including nausea, sedation, and dizziness, were also recorded, and blood concentration was measured. The results showed that the analgesic effects of sufentanil, dezocine, and tramadol were significantly better in the morning than in afternoon. In the afternoon, sufentanil had a stronger sedative effect, whereas dezocine had a stronger inhibitory respiratory effect. The incidence of nausea was higher in the morning with tramadol. Additionally, significant differences in different side effects were observed among three opioids. Our results suggest that the clinical use of these three opioids necessitates the formulation of individualized treatment plans, accounting for different administration times, to achieve maximum analgesic effect with minimal side effects.

The opposing chloride cotransporters KCC and NKCC control locomotor activity in constant light and during long days

Cation chloride cotransporters (CCCs) regulate intracellular chloride ion concentration ([Cl-]i) within neurons, which can reverse the direction of the neuronal response to the neurotransmitter GABA.1 Na+ K+ Cl- (NKCC) and K+ Cl- (KCC) cotransporters transport Cl- into or out of the cell, respectively. When NKCC activity dominates, the resulting high [Cl-]i can lead to an excitatory and depolarizing response of the neuron upon GABAA receptor opening, while KCC dominance has the opposite effect.1 This inhibitory-to-excitatory GABA switch has been linked to seasonal adaption of circadian clock function to changing day length,2-4 and its dysregulation is associated with neurodevelopmental disorders such as epilepsy.5-8 In Drosophila melanogaster, constant light normally disrupts circadian clock function and leads to arrhythmic behavior.9 Here, we demonstrate a function for CCCs in regulating Drosophila locomotor activity and GABA responses in circadian clock neurons because alteration of CCC expression in circadian clock neurons elicits rhythmic behavior in constant light. We observed the same effects after downregulation of the Wnk and Fray kinases, which modulate CCC activity in a [Cl-]i-dependent manner. Patch-clamp recordings from the large LNv clock neurons show that downregulation of KCC results in a more positive GABA reversal potential, while KCC overexpression has the opposite effect. Finally, KCC and NKCC downregulation reduces or increases morning behavioral activity during long photoperiods, respectively. In summary, our results support a model in which the regulation of [Cl-]i by a KCC/NKCC/Wnk/Fray feedback loop determines the response of clock neurons to GABA, which is important for adjusting behavioral activity to constant light and long-day conditions.