TCF4 Mutations Disrupt Synaptic Function Through Dysregulation of RIMBP2 in Patient-Derived Cortical Neurons.

BACKGROUND: Genetic variation in the TCF4 (transcription factor 4) gene is associated with risk for a variety of developmental and psychiatric conditions, which includes a syndromic form of autism spectrum disorder called Pitt-Hopkins syndrome (PTHS). TCF4 encodes an activity-dependent transcription factor that is highly expressed during cortical development and in animal models has been shown to regulate various aspects of neuronal development and function. However, our understanding of how disease-causing mutations in TCF4 confer pathophysiology in a human context is lacking. METHODS: To model PTHS, we differentiated human cortical neurons from human induced pluripotent stem cells that were derived from patients with PTHS and neurotypical individuals. To identify pathophysiology and disease mechanisms, we assayed cortical neurons with whole-cell electrophysiology, Ca2+ imaging, multielectrode arrays, immunocytochemistry, and RNA sequencing. RESULTS: Cortical neurons derived from patients with TCF4 mutations showed deficits in spontaneous synaptic transmission, network excitability, and homeostatic plasticity. Transcriptomic analysis indicated that these phenotypes resulted in part from altered expression of genes involved in presynaptic neurotransmission and identified the presynaptic binding protein RIMBP2 as the most differentially expressed gene in PTHS neurons. Remarkably, TCF4-dependent deficits in spontaneous synaptic transmission and network excitability were rescued by increasing RIMBP2 expression in presynaptic neurons. CONCLUSIONS: Taken together, these results identify TCF4 as a critical transcriptional regulator of human synaptic development and plasticity and specifically identifies dysregulation of presynaptic function as an early pathophysiology in PTHS.

Transient receptor potential canonical type 6 (TRPC6) O-GlcNAcylation at Threonine-221 plays potent role in channel regulation.

Transient receptor potential canonical type 6 (TRPC6) is a non-voltage-gated channel that principally conducts calcium. Elevated channel activation contributes to fibrosis, hypertrophy, and proteinuria, often coupled to stimulation of nuclear factor of activated T-cells (NFAT). TRPC6 is post-translationally regulated, but a role for O-linked ß-N-acetyl glucosamine (O-GlcNAcylation) as elevated by diabetes, is unknown. Here we show TRPC6 is constitutively O-GlcNAcylated at Ser14, Thr70, and Thr221 in the N-terminus ankryn-4 (AR4) and linker (LH1) domains. Mutagenesis to alanine reveals T221 as a critical controller of resting TRPC6 conductance, and associated NFAT activity and pro-hypertrophic signaling. T→A mutations at sites homologous in closely related TRPC3 and TRPC7 also increases their activity. Molecular modeling predicts interactions between Thr221-O-GlcNAc and Ser199, Glu200, and Glu246, and combined alanine substitutions of the latter similarly elevates resting NFAT activity. Thus, O-GlcNAcylated T221 and interactions with coordinating residues is required for normal TRPC6 channel conductance and NFAT activation.

TCF4 mutations disrupt synaptic function through dysregulation of RIMBP2 in patient-derived cortical neurons.

Genetic variation in the transcription factor 4 ( TCF4) gene is associated with risk for a variety of developmental and psychiatric conditions, which includes a syndromic form of ASD called Pitt Hopkins Syndrome (PTHS). TCF4 encodes an activity-dependent transcription factor that is highly expressed during cortical development and in animal models is shown to regulate various aspects of neuronal development and function. However, our understanding of how disease-causing mutations in TCF4 confer pathophysiology in a human context is lacking. Here we show that cortical neurons derived from patients with TCF4 mutations have deficits in spontaneous synaptic transmission, network excitability and homeostatic plasticity. Transcriptomic analysis indicates these phenotypes result from altered expression of genes involved in presynaptic neurotransmission and identifies the presynaptic binding protein, RIMBP2 as the most differentially expressed gene in PTHS neurons. Remarkably, TCF4-dependent deficits in spontaneous synaptic transmission and network excitability were rescued by increasing RIMBP2 expression in presynaptic neurons. Together, these results identify TCF4 as a critical transcriptional regulator of human synaptic development and plasticity and specifically identifies dysregulation of presynaptic function as an early pathophysiology in PTHS.

Electrophysiological measures from human iPSC-derived neurons are associated with schizophrenia clinical status and predict individual cognitive performance.

Neurons derived from human induced pluripotent stem cells (hiPSCs) have been used to model basic cellular aspects of neuropsychiatric disorders, but the relationship between the emergent phenotypes and the clinical characteristics of donor individuals has been unclear. We analyzed RNA expression and indices of cellular function in hiPSC-derived neural progenitors and cortical neurons generated from 13 individuals with high polygenic risk scores (PRSs) for schizophrenia (SCZ) and a clinical diagnosis of SCZ, along with 15 neurotypical individuals with low PRS. We identified electrophysiological measures in the patient-derived neurons that implicated altered Na+ channel function, action potential interspike interval, and gamma-aminobutyric acid-ergic neurotransmission. Importantly, electrophysiological measures predicted cardinal clinical and cognitive features found in these SCZ patients. The identification of basic neuronal physiological properties related to core clinical characteristics of illness is a potentially critical step in generating leads for novel therapeutics.

Ca2+-dependent regulation of sodium channels NaV1.4 and NaV1.5 is controlled by the post-IQ motif.

Skeletal muscle voltage-gated Na+ channel (NaV1.4) activity is subject to calmodulin (CaM) mediated Ca2+-dependent inactivation; no such inactivation is observed in the cardiac Na+ channel (NaV1.5). Taken together, the crystal structures of the NaV1.4 C-terminal domain relevant complexes and thermodynamic binding data presented here provide a rationale for this isoform difference. A Ca2+-dependent CaM N-lobe binding site previously identified in NaV1.5 is not present in NaV1.4 allowing the N-lobe to signal other regions of the NaV1.4 channel. Consistent with this mechanism, removing this binding site in NaV1.5 unveils robust Ca2+-dependent inactivation in the previously insensitive isoform. These findings suggest that Ca2+-dependent inactivation is effected by CaM's N-lobe binding outside the NaV C-terminal while CaM's C-lobe remains bound to the NaV C-terminal. As the N-lobe binding motif of NaV1.5 is a mutational hotspot for inherited arrhythmias, the contributions of mutation-induced changes in CDI to arrhythmia generation is an intriguing possibility.

NaV Channels: Assaying Biosynthesis, Trafficking, Function.

Integral to the cell surface is channels, pumps, and exchanger proteins that facilitate the movement of ions across the membrane. Ion channels facilitate the passive movement of ions down an electrochemical gradient. Ion pumps actively use energy to actively translocate ions, often against concentration or voltage gradients, while ion exchangers utilize energy to couple the transport of different ion species such that one ion moves down its gradient and the released free energy is used to drive the movement of a different ion against its electrochemical gradient. Some ion pumps and exchangers may be electrogenic, i.e., the ion transport they support is not electrically neutral and generates a current. Functions of these pore-forming membrane proteins include the establishment of membrane potentials, gating of ions flows across the cell membrane to elicit action potentials and other electrical signals, as well as the regulation of cell volumes. The major forms of ion channels include voltage-, ligand-, and signal-gated channels. In this review, we describe mammalian voltage dependent Na (NaV) channels.

Regulation of the NaV1.5 cytoplasmic domain by calmodulin.

Voltage-gated sodium channels (Na(v)) underlie the rapid upstroke of action potentials in excitable tissues. Binding of channel-interactive proteins is essential for controlling fast and long-term inactivation. In the structure of the complex of the carboxy-terminal portion of Na(v)1.5 (CTNa(v)1.5) with calmodulin (CaM)-Mg(2+) reported here, both CaM lobes interact with the CTNa(v)1.5. On the basis of the differences between this structure and that of an inactivated complex, we propose that the structure reported here represents a non-inactivated state of the CTNa(v), that is, the state that is poised for activation. Electrophysiological characterization of mutants further supports the importance of the interactions identified in the structure. Isothermal titration calorimetry experiments show that CaM binds to CTNa(v)1.5 with high affinity. The results of this study provide unique insights into the physiological activation and the pathophysiology of Na(v) channels.

A mutation causing Brugada syndrome identifies a mechanism for altered autonomic and oxidant regulation of cardiac sodium currents.

BACKGROUND: The mechanisms of the electrocardiographic changes and arrhythmias in Brugada syndrome (BrS) remain controversial. Mutations in the sodium channel gene, SCN5A, and regulatory proteins that reduce or eliminate sodium current (INa) have been linked to BrS. We studied the properties of a BrS-associated SCN5A mutation in a protein kinase A (PKA) consensus phosphorylation site, R526H. METHODS AND RESULTS: In vitro PKA phosphorylation was detected in the I-II linker peptide of wild-type (WT) channels but not R526H or S528A (phosphorylation site) mutants. Cell surface expression of R526H and S528A channels was reduced compared with WT. Whole-cell INa through all channel variants revealed no significant differences in the steady-state activation, inactivation, and recovery from inactivation. Peak current densities of the mutants were significantly reduced compared with WT. Infection of 2D cultures of neonatal rat ventricular myocytes with WT and mutant channels increased conduction velocity compared with noninfected cells. PKA stimulation significantly increased peak INa and conduction velocity of WT but not mutant channels. Oxidant stress inhibits cardiac INa; WT and mutant INa decreases with the intracellular application of reduced nicotinamide adenine dinucleotide (NADH), an effect that is reversed by PKA stimulation in WT but not in R526H or S528A channels. CONCLUSIONS: We identified a family with BrS and an SCN5A mutation in a PKA consensus phosphorylation site. The BrS mutation R526H is associated with a reduction in the basal level of INa and a failure of PKA stimulation to augment the current that may contribute to the predisposition to arrhythmias in patients with BrS, independent of the precipitants.

Phosphodiesterase 5 inhibition ameliorates angiontensin II-induced podocyte dysmotility via the protein kinase G-mediated downregulation of TRPC6 activity.

The emerging role of the transient receptor potential cation channel isotype 6 (TRPC6) as a central contributor to various pathological processes affecting podocytes has generated interest in the development of therapeutics to modulate its function. Recent insights into the regulation of TRPC6 have revealed PKG as a potent negative modulator of TRPC6 conductance and associated signaling via its phosphorylation at two highly conserved amino acid residues: Thr(69)/Thr(70) (Thr(69) in mice and Thr(70) in humans) and Ser(321)/Ser(322) (Ser(321) in mice and Ser(322) in humans). Here, we tested the role of PKG in modulating TRPC6-dependent responses in primary and conditionally immortalized mouse podocytes. TRPC6 was phosphorylated at Thr(69) in nonstimulated podocytes, but this declined upon ANG II stimulation or overexpression of constitutively active calcineurin phosphatase. ANG II induced podocyte motility in an in vitro wound assay, and this was reduced 30-60% in cells overexpressing a phosphomimetic mutant TRPC6 (TRPC6T70E/S322E) or activated PKG (P < 0.05). Pretreatment of podocytes with the PKG agonists S-nitroso-N-acetyl-dl-penicillamine (nitric oxide donor), 8-bromo-cGMP, Bay 41-2772 (soluble guanylate cyclase activator), or phosphodiesterase 5 (PDE5) inhibitor 4-{[3',4'-(methylenedioxy)benzyl]amino}[7]-6-methoxyquinazoline attenuated ANG II-induced Thr(69) dephosphorylation and also inhibited TRPC6-dependent podocyte motility by 30-60%. These data reveal that PKG activation strategies, including PDE5 inhibition, ameliorate ANG II-induced podocyte dysmotility by targeting TRPC6 in podocytes, highlighting the potential therapeutic utility of these approaches to treat hyperactive TRPC6-dependent glomerular disease.

Tonotopic relationships reveal the charge density varies along the lateral wall of outer hair cells.

Outer hair cells amplify and improve the frequency selectivity of sound within the mammalian cochlea through a sound-evoked receptor potential that induces an electromechanical response in their lateral wall membrane. We experimentally show that the membrane area and linear membrane capacitance of outer hair cells increases exponentially with the electrically evoked voltage-dependent charge movement (Q(T)) and peak membrane capacitance (C(peak)). We determine the size of the different functional regions (e.g., lateral wall, synaptic basal pole) of the polarized cells from the tonotopic relationships. We then establish that Q(T) and C(peak) increase with the logarithm of the lateral wall area (A(LW)) and determine from the functions that the charge (σ(LW,) pC/µm(2)) and peak (ρ(LW,) pF/µm(2)) densities vary inversely with A(LW) (σ(LW) = 1.3/A(LW) and ρ(LW) = 9/A(LW)). This shows contrary to conventional wisdom that σ(LW) and ρ(LW) are not constant along the length of an individual outer hair cell.

Ca2+ current of frog vestibular hair cells is modulated by intracellular ATP but not by long-lasting depolarisation.

Some aspects of Ca(2+) channel modulation in hair cells isolated from semicircular canals of the frog (Rana esculenta) have been investigated using the whole-cell technique and intra and extracellular solutions designed to modify the basic properties of the Ca(2+) macrocurrent. With 1 mM ATP in the pipette solution, about 60% of the recorded cells displayed a Ca(2+) current constituted by a mix of an L and a drug-resistant (R2) component; the remaining 40% exhibited an additional drug-resistant fraction (R1), which inactivated in a Ca-dependent manner. If the pipette ATP was raised to 10 mM, cells exhibiting the R1 current fraction displayed an increase of both the R1 and L components by approximately 280 and approximately 70%, respectively, while cells initially lacking R1 showed a similar increase in the L component with R1 becoming apparent and raising up to a mean amplitude of approximately 44 pA. In both cell types the R2 current fraction was negligibly affect by ATP. The current run-up was unaffected by cyclic nucleotides, and was not triggered by 10 mM ATPgammaS, ADP, AMP or GTP. Long-lasting depolarisations (>5 s) produced a progressive, reversible decay in the inward current despite the presence of intracellular ATP. Ca(2+) channel blockade by Cd(2+) unmasked a slowly activating outward Cs(+) current flowing through a non-Ca(2+) channel type, which became progressively unblocked by prolonged depolarisation even though Cs(+) and TEA(+) were present on both sides of the channel. The outward current waveform could be erroneously ascribed to a Ca- and/or voltage dependence of the Ca(2+) macrocurrent.

IP3 receptor in the hair cells of frog semicircular canal and its possible functional role.

The presence and functional role of inositol trisphosphate receptors (IP3R) was investigated by electrophysiology and immunohistochemistry in hair cells from the frog semicircular canal. Intracellular recordings were performed from single fibres of the posterior canal in the isolated, intact frog labyrinth, at rest and during rotation, in the presence of IP3 receptor inhibitors and drugs known to produce Ca2+ release from the internal stores or to increase IP3 production. Hair cell immunolabelling for IP3 receptor was performed by standard procedures. The drug 2-aminoethoxydiphenyl borate (2APB), an IP3 receptor inhibitor, produced a marked decrease of mEPSP and spike frequency at low concentration (0.1 mm), without affecting mEPSP size or time course. At high concentration (1 mm), 2APB is reported to block the sarcoplasmic-endoplasmic reticulum Ca2+-ATPase (SERCA pump) and increase [Ca2+]i; at the labyrinthine cytoneural junction, it greatly enhanced the resting and mechanically evoked sensory discharge frequency. The selective agonist of group I metabotropic glutamate receptors (RS)-3,5-dihydroxyphenylglycine (DHPG, 0.6 mm), produced a transient increase in resting mEPSP and spike frequency at the cytoneural junction, with no effects on mEPSP shape or amplitude. Pretreatment with cyclopiazonic acid (CPA, 0.1 mm), a SERCA pump inhibitor, prevented the facilitatory effect of both 2APB and DHPG, suggesting a link between Ca2+ release from intracellular stores and quantal emission. Consistently, diffuse immunoreactivity for IP3 receptors was observed in posterior canal hair cells. Our results indicate the presence and a possibly relevant functional role of IP3-sensitive stores in controlling [Ca2+]i and modulating the vestibular discharge.

Intracellular Ca2+ buffers can dramatically affect Ca2+ conductances in hair cells.

The effects of endogenous and exogenous Ca(2+) buffers on Ca(2+) current kinetics have been investigated by patch clamp in hair cells mechanically isolated from frog semicircular canals. This preparation displays at least three different Ca(2+) channel types: transient currents flow through a drug-resistant channel ("R1"), while non-inactivating channels sustain a steady, plateau current comprised of a large L component and a small drug-resistant fraction ("R2"). In the perforated-patch condition a large and stable Ca(2+) current was recorded, with all three components. In whole-cell, a buffer-free pipette solution did not prevent a complete Ca(2+) response. The size of the transient and plateau current fractions were greatly reduced, but the ratio between the two fractions, as well as the activation, inactivation and deactivation kinetics, were substantially unmodified. Current amplitude partially recovered with 5 mM EGTA in the pipette solution. With 50 mM EGTA all the kinetic parameters were slowed down and the transient component, but not the plateau component, markedly increased in size. Response kinetics slowed down even more with 30 mM Cs-BAPTA and the Ca(2+) waveform was substantially modified. The transient component was very large and inactivated slowly; the remaining very small plateau fraction deactivated along a slow, single exponential time. Under this condition nifedipine (10 microM) produced a great reduction of the transient current, leaving plateau and deactivation phase unaltered. This suggests that only R2 channels were still active at the end of the test and that the minor remaining transient component flowed through slowly but completely inactivating R1 channels. These results confirm the presence of several channel types in semicircular canal receptors, at difference with cochlear hair cells, and highlight a dramatic alteration of L-type channel behavior when intracellular Ca(2+) buffers are sufficiently concentrated and fast to interfere with rapid and local changes in Ca(2+) levels.