Src regulates membrane trafficking of the Kv3.1b channel

Oxidative stress-related cellular aging causes dysfunction of the Kv3.1/KCNC1 channel reverted by melatonin

The voltage-gated Kv3.1/KCNC1 channel is abundantly expressed in fast-spiking principal neurons and GABAergic inhibitory interneurons throughout the ascending auditory pathway and in various brain regions. Inactivating mutations in the KCNC1 gene lead to forms of epilepsy and a decline in the expression of the Kv3.1 channel is involved in age-related hearing loss. As oxidative stress plays a fundamental role in the pathogenesis of epilepsy and age-related hearing loss, we hypothesized that an oxidative insult might affect the function of this channel. To verify this hypothesis, the activity and expression of endogenous and ectopic Kv3.1 were measured in models of oxidative stress-related aging represented by cell lines exposed to 100 mM d-galactose. In these models, intracellular reactive oxygen species, thiobarbituric acid reactive substances, sulfhydryl groups of cellular proteins, and the activity of catalase and superoxide dismutase were dysregulated, while the current density of Kv3.1 was significantly reduced. Importantly, the antioxidant melatonin reverted all these effects. The reduction of function of Kv3.1 was not determined by direct oxidation of amino acid side chains of the protein channel or reduction of transcript or total protein levels but was linked to reduced trafficking to the cell surface associated with Src phosphorylation as well as metabolic and endoplasmic reticulum stress. The data presented here specify Kv3.1 as a novel target of oxidative stress and suggest that Kv3.1 dysfunction might contribute to age-related hearing loss and increased prevalence of epilepsy during aging. The pharmacological use of the antioxidant melatonin can be protective in this setting.

The dual role of c-src in cell-to-cell transmission of α-synuclein

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons located in the substantia nigra pars compacta and the presence of proteinaceous inclusions called Lewy bodies and Lewy neurites in numerous brain regions. Increasing evidence indicates that Lewy pathology progressively involves additional regions of the nervous system as the disease advances, and the prion-like propagation of α-synuclein (α-syn) pathology promotes PD progression. Accordingly, the modulation of α-syn transmission may be important for the development of disease-modifying therapies in patients with PD. Here, we demonstrate that α-syn fibrils induce c-src activation in neurons, which depends on the FcγRIIb-SHP-1/-2-c-src pathway and enhances signals for the uptake of α-syn into neurons. Blockade of c-src activation inhibits the uptake of α-syn and the formation of Lewy body-like inclusions. Furthermore, the blockade of c-src activation also inhibits the release of α-syn via activation of autophagy. The brain-permeable c-src inhibitor, saracatinib, efficiently reduces α-syn propagation into neighboring regions in an in vivo model system. These results suggest a new therapeutic target against progressive PD.

Hyperactivated PTP1B phosphatase in parvalbumin neurons alters anterior cingulate inhibitory circuits and induces autism-like behaviors

Individuals with autism spectrum disorder (ASD) have social interaction deficits and difficulty filtering information. Inhibitory interneurons filter information at pyramidal neurons of the anterior cingulate cortex (ACC), an integration hub for higher-order thalamic inputs important for social interaction. Humans with deletions including LMO4, an endogenous inhibitor of PTP1B, display intellectual disabilities and occasionally autism. PV-Lmo4KO mice ablate Lmo4 in PV interneurons and display ASD-like repetitive behaviors and social interaction deficits. Surprisingly, increased PV neuron-mediated peri-somatic feedforward inhibition to the pyramidal neurons causes a compensatory reduction in (somatostatin neuron-mediated) dendritic inhibition. These homeostatic changes increase filtering of mediodorsal-thalamocortical inputs but reduce filtering of cortico-cortical inputs and narrow the range of stimuli ACC pyramidal neurons can distinguish. Simultaneous ablation of PTP1B in PV-Lmo4KO neurons prevents these deficits, indicating that PTP1B activation in PV interneurons contributes to ASD-like characteristics and homeostatic maladaptation of inhibitory circuits may contribute to deficient information filtering in ASD.

Inhibition of non-receptor tyrosine kinase Src induces phosphoserine 256-independent aquaporin-2 membrane accumulation

KEY POINTS

Aquaporin-2 (AQP2) is crucial for water homeostasis, and vasopressin (VP) induces AQP2 membrane trafficking by increasing intracellular cAMP, activating PKA and causing phosphorylation of AQP2 at serine 256, 264 and 269 residues and dephosphorylation of serine 261 residue on the AQP2 C-terminus. It is thought that serine 256 is the master regulator of AQP2 trafficking, and its phosphorylation has to precede the change of phosphorylation state of other serine residues. We found that Src inhibition causes serine 256-independent AQP2 membrane trafficking and induces phosphorylation of serine 269 independently of serine 256. This targeted phosphorylation of serine 269 is important for Src inhibition-induced AQP2 membrane accumulation; without serine 269, Src inhibition exerts no effect on AQP2 trafficking. This result helps us better understand the independent pathways that can target different AQP2 residues, and design new strategies to induce or sustain AQP2 membrane expression when VP signalling is defective.

ABSTRACT

Aquaporin-2 (AQP2) is essential for water homeostasis. Upon stimulation by vasopressin, AQP2 is phosphorylated at serine 256 (S256), S264 and S269, and dephosphorylated at S261. It is thought that S256 is the master regulator of AQP2 trafficking and membrane accumulation, and that its phosphorylation has to precede phosphorylation of other serine residues. In this study, we found that VP reduces Src kinase phosphorylation: by suppressing Src using the inhibitor dasatinib and siRNA, we could increase AQP2 membrane accumulation in cultured AQP2-expressing cells and in kidney collecting duct principal cells. Src inhibition increased exocytosis and inhibited clathrin-mediated endocytosis of AQP2, but exerted its effect in a cAMP, PKA and S256 phosphorylation (pS256)-independent manner. Despite the lack of S256 phosphorylation, dasatinib increased phosphorylation of S269, even in S256A mutant cells in which S256 phosphorylation cannot occur. To confirm the importance of pS269 in AQP2 re-distribution, we expressed an AQP2 S269A mutant in LLC-PK1 cells, and found that dasatinib no longer induced AQP2 membrane accumulation. In conclusion, Src inhibition causes phosphorylation of S269 independently of pS256, and induces AQP2 membrane accumulation by inhibiting clathrin-mediated endocytosis and increasing exocytosis. We conclude that S269 can be phosphorylated without pS256, and pS269 alone is important for AQP2 apical membrane accumulation under some conditions. These data increase our understanding of the independent pathways that can phosphorylate different residues in the AQP2 C-terminus, and suggest new strategies to target distinct AQP2 serine residues to induce membrane expression of this water channel when VP signalling is defective.

K+ Channel Kv3.4 Is Essential for Axon Growth by Limiting the Influx of Ca2+ into Growth Cones

Membrane excitability in the axonal growth cones of embryonic neurons influences axon growth. Voltage-gated K⁺ (Kv) channels are key factors in controlling membrane excitability, but whether they regulate axon growth remains unclear. Here, we report that Kv3.4 is expressed in the axonal growth cones of embryonic spinal commissural neurons, motoneurons, dorsal root ganglion neurons, retinal ganglion cells, and callosal projection neurons during axon growth. Our in vitro (cultured dorsal spinal neurons of chick embryos) and in vivo (developing chick spinal commissural axons and rat callosal axons) findings demonstrate that knockdown of Kv3.4 by a specific shRNA impedes axon initiation, elongation, pathfinding, and fasciculation. In cultured dorsal spinal neurons, blockade of Kv3.4 by blood depressing substance II suppresses axon growth via an increase in the amplitude and frequency of Ca²⁺ influx through T-type and L-type Ca²⁺ channels. Electrophysiological results show that Kv3.4, the major Kv channel in the axonal growth cones of embryonic dorsal spinal neurons, is activated at more hyperpolarized potentials and inactivated more slowly than it is in postnatal and adult neurons. The opening of Kv3.4 channels effectively reduces growth cone membrane excitability, thereby limiting excessive Ca²⁺ influx at subthreshold potentials or during Ca²⁺-dependent action potentials. Furthermore, excessive Ca²⁺ influx induced by an optogenetic approach also inhibits axon growth. Our findings suggest that Kv3.4 reduces growth cone membrane excitability and maintains [Ca²⁺]_i at an optimal concentration for normal axon growth.SIGNIFICANCE STATEMENT Accumulating evidence supports the idea that impairments in axon growth contribute to many clinical disorders, such as autism spectrum disorders, corpus callosum agenesis, Joubert syndrome, Kallmann syndrome, and horizontal gaze palsy with progressive scoliosis. Membrane excitability in the growth cone, which is mainly controlled by voltage-gated Ca²⁺ (Cav) and K⁺ (Kv) channels, modulates axon growth. The role of Cav channels during axon growth is well understood, but it is unclear whether Kv channels control axon outgrowth by regulating Ca²⁺ influx. This report shows that Kv3.4, which is transiently expressed in the axonal growth cones of many types of embryonic neurons, acts to reduce excessive Ca²⁺ influx through Cav channels and thus permits normal axon outgrowth.

Role of Sodium Bicarbonate Cotransporters in Intracellular pH Regulation and Their Regulatory Mechanisms in Human Submandibular Glands

Sodium bicarbonate cotransporters (NBCs) are involved in the pH regulation of salivary glands. However, the roles and regulatory mechanisms among different NBC isotypes have not been rigorously evaluated. We investigated the roles of two different types of NBCs, electroneutral (NBCn1) and electrogenic NBC (NBCe1), with respect to pH regulation and regulatory mechanisms using human submandibular glands (hSMGs) and HSG cells. Intracellular pH (pH_i) was measured and the pH_i recovery rate from cell acidification induced by an NH₄Cl pulse was recorded. Subcellular localization and protein phosphorylation were determined using immunohistochemistry and co-immunoprecipitation techniques. We determined that NBCn1 is expressed on the basolateral side of acinar cells and the apical side of duct cells, while NBCe1 is exclusively expressed on the apical membrane of duct cells. The pH_i recovery rate in hSMG acinar cells, which only express NBCn1, was not affected by pre-incubation with 5 μM PP2, an Src tyrosine kinase inhibitor. However, in HSG cells, which express both NBCe1 and NBCn1, the pH_i recovery rate was inhibited by PP2. The apparent difference in regulatory mechanisms for NBCn1 and NBCe1 was evaluated by artificial overexpression of NBCn1 or NBCe1 in HSG cells, which revealed that the pH_i recovery rate was only inhibited by PP2 in cells overexpressing NBCe1. Furthermore, only NBCe1 was significantly phosphorylated and translocated by NH₄Cl, which was inhibited by PP2. Our results suggest that both NBCn1 and NBCe1 play a role in pH_i regulation in hSMG acinar cells, and also that Src kinase does not regulate the activity of NBCn1.