Andrew W. Varga

The A‐Type Potassium Channel Kv4.2 Is a Substrate for the Mitogen‐Activated Protein Kinase ERK

Abstract: The mitogen‐activated protein kinase ERK has recentlybecome a focus of studies of synaptic plasticity and learning and memory. Dueto the prominent role of potassium channels in regulating the electricalproperties of membranes, modulation of these channels by ERK could play animportant role in mediating learning‐related synaptic plasticity in the CNS.Kv4.2 is a Shal‐type potassium channel that passes an A‐type current and islocalized to dendrites and cell bodies in the hippocampus. The sequence ofKv4.2 contains several consensus sites for ERK phosphorylation. In the presentstudies, we tested the hypothesis that Kv4.2 is an ERK substrate. Wedetermined that the Kv4.2 C‐terminal cytoplasmic domain is an effective ERK2substrate, and that it is phosphorylated at three sites: Thr602,Thr607, and Ser616. We used this information to developantibodies that recognize Kv4.2 phosphorylated by ERK2. One of ourphospho‐site‐selective antibodies was generated using a triply phosphorylatedpeptide as the antigen. We determined that this antibody recognizesERK‐phosphorylated Kv4.2 in COS‐7 cells transfected with Kv4.2 and nativeERK‐phosphorylated Kv4.2 in the rat hippocampus. These observations indicatethat Kv4.2 is a substrate for ERK in vitro and in vivo, and suggest that ERKmay regulate potassium‐channel function by direct phosphorylation of thepore‐forming α subunit.

Calcium–Calmodulin-Dependent Kinase II Modulates Kv4.2 Channel Expression and Upregulates Neuronal A-Type Potassium Currents

Calcium–calmodulin-dependent kinase II (CaMKII) has a long history of involvement in synaptic plasticity, yet little focus has been given to potassium channels as CaMKII targets despite their importance in repolarizing EPSPs and action potentials and regulating neuronal membrane excitability. We now show that Kv4.2 acts as a substrate for CaMKII in vitro and have identified CaMKII phosphorylation sites as Ser438 and Ser459. To test whether CaMKII phosphorylation of Kv4.2 affects channel biophysics, we expressed wild-type or mutant Kv4.2 and the K+ channel interacting protein, KChIP3, with or without a constitutively active form of CaMKII in Xenopus oocytes and measured the voltage dependence of activation and inactivation in each of these conditions. CaMKII phosphorylation had no effect on channel biophysical properties. However, we found that levels of Kv4.2 protein are increased with CaMKII phosphorylation in transfected COS cells, an effect attributable to direct channel phosphorylation based on site-directed mutagenesis studies. We also obtained corroborating physiological data showing increased surface A-type channel expression as revealed by increases in peak K+ current amplitudes with CaMKII phosphorylation. Furthermore, endogenous A-currents in hippocampal pyramidal neurons were increased in amplitude after introduction of constitutively active CaMKII, which results in a decrease in neuronal excitability in response to current injections. Thus CaMKII can directly modulate neuronal excitability by increasing cell-surface expression of A-type K+ channels.

Sleep-disordered breathing advances cognitive decline in the elderly

Objective: To examine whether the presence of sleep-disordered breathing (SDB) is associated with an earlier age at mild cognitive impairment (MCI) or Alzheimer disease (AD)-dementia onset in participants from the Alzheimers Disease Neuroimaging Initiative (ADNI) cohort. We also examined whether continuous positive airway pressure (CPAP) use is associated with delayed onset of cognitive decline. Methods: From the ADNI cohort, 3 subsets with progressively stringent criteria were created in a step-wise manner. Age at MCI or AD-dementia onset was the main outcome variable. Analyses were performed separately for each subset in untreated SDB+ vs SDB− and untreated SDB+ vs CPAP+ groups. Chi-square and t tests were performed to examine between-group differences. Survival analyses were performed using the Kaplan–Meier method, compared by the log-rank test, and assessed by multivariate Cox regression adjusting for potential confounders. Results: SDB+ patients had a younger age at MCI onset in all subsets (MC1: 72.63 vs 83.67; MC2: 72.15 vs 83.45; MC3: 77.40 vs 89.89; p < 0.01). SDB+ patients had a younger age at AD-dementia onset only in our most conservative subset (AC3: 83.46 vs 88.13; p < 0.05). In a combined outcome analysis, SDB+ patients had a younger age at onset to MCI or AD-dementia in all subsets. In subsets 1 and 2, CPAP use delayed the age at MCI onset (CMC1: 72.63 vs 82.10; CMC2: 72.11 vs 82.10; p < 0.01). Conclusions: Consistent with our hypothesis, the presence of SDB was associated with an earlier age at cognitive decline. Our findings in CPAP+ participants suggest that CPAP treatment of SDB may delay progression of cognitive impairment.

The unipolar Shigella surface protein IcsA is targeted directly to the bacterial old pole: IcsP cleavage of IcsA occurs over the entire bacterial surface

Shigella flexneri is an intracellular pathogen that is able to move within the cytoplasm of infected cells by the continual assembly of actin onto one pole of the bacterium. IcsA, an outer membrane protein, is localized to the old pole of the bacterium and is both necessary and sufficient for actin assembly. IcsA is slowly cleaved from the bacterial surface by the protease IcsP (SopA). Absence of IcsP leads to an alteration in the distribution of surface IcsA, such that the polar cap is maintained and some IcsA is distributed along the lateral walls of the bacillus. The mechanism of unipolar localization of IcsA and the role of IcsP in its unipolar localization are incompletely understood. Here, we demonstrate that cleavage of IcsA occurs exclusively in the outer membrane and that IcsP is localized to the outer membrane. In addition, we show that IcsA at the old pole is susceptible to cleavage by IcsP and that native IcsP is active at the pole. Taken together, these data indicate that IcsP cleaves IcsA over the entire bacterial surface. Finally, we show that, immediately after induction from a tightly regulated promoter, IcsA is expressed exclusively at the old pole in both the icsP−icsA− and the icsA− background. These data demonstrate that unipolar localization of IcsA results from its direct targeting to the pole, followed by its diffusion laterally in the outer membrane.

White Matter Hemodynamic Abnormalities precede Sub-cortical Gray Matter Changes in Multiple Sclerosis

BACKGROUND Hypoperfusion has been reported in lesions, normal-appearing white (NAWM) and gray matter (NAGM) of patients with clinically definite multiple sclerosis (MS) by using perfusion MRI. However, it is still unknown how early such changes in perfusion occur. The aim of our study was to assess the presence of hemodynamic changes in the NAWM and subcortical NAGM of patients with clinically isolated syndrome (CIS) in comparison to healthy controls and to patients with early relapsing-remitting (RR) MS. METHODS Absolute cerebral blood flow (CBF), blood volume (CBV) and mean transit time (MTT) were measured in the periventricular and frontal NAWM, thalamus and putamen nuclei of 12 patients with CIS, 12 with early RR-MS and 12 healthy controls using dynamic susceptibility contrast enhanced (DSC) T2*-weighted MRI. RESULTS Compared to controls, CBF was significantly decreased in the periventricular NAWM of CIS patients and in the periventricular NAWM and putamen of RR-MS patients. Compared to CIS, RR-MS patients showed a significant CBF decrease in the putamen. CONCLUSIONS CBF was decreased in the NAWM of both CIS and RR-MS patients and in the subcortical NAGM of RR-MS patients suggesting a continuum of tissue perfusion decreases beginning in white matter and spreading to gray matter, as the disease progresses.

Book Review: Protein Kinase Signal Transduction Cascades in Mammalian Associative Conditioning

One of the most intriguing and intensely studied questions facing contemporary neuroscientists involves the elucidation of the physiological mechanisms that underlie learning and memory. Recent advances have given us a much more detailed understanding of the signal transduction mechanisms subserving learning in the intact animal. One fact that has become clear is that activation of protein kinases and phosphorylation of their downstream effectors play a critical role. Four protein kinase cascades have garnered considerable attention in the study of information storage at both the synaptic and behavioral levels: Ca++/phospholipid-dependent protein kinase (PKC), Ca++/calmodulin-dependent protein kinase II (CaMKII), cAMP-dependent protein kinase (PKA), and extracellular signal-regulated kinase (ERK). This review will concentrate on studies of two behavioral tasks, conditioned fear and conditioned taste aversion, that provide evidence for the involvement of these kinase systems in associative learning. The authors will also examine a number of potential kinase substrates and how each could participate in the formation of long-term memories.

The interaction between sleep-disordered breathing and apolipoprotein E genotype on cerebrospinal fluid biomarkers for Alzheimer's disease in cognitively normal elderly individuals

Previous studies have suggested a link between sleep disordered breathing (SDB) and dementia risk. In the present study, we analyzed the relationship between SDB severity, cerebrospinal fluid (CSF) Alzheimers disease-biomarkers, and the ApoE alleles. A total of 95 cognitively normal elderly participants were analyzed for SDB severity, CSF measures of phosphorylated-tau (p-tau), total-tau (t-tau), and amyloid beta 42 (Aβ-42), as well as ApoE allele status. In ApoE3+ subjects, significant differences were found between sleep groups for p-tau (F[df2] = 4.3, p = 0.017), and t-tau (F[df2] = 3.3, p = 0.043). Additionally, among ApoE3+ subjects, the apnea and/or hypopnea with 4% O2-desaturation index was positively correlated with p-tau (r = 0.30, p = 0.023), t-tau (r = 0.31, p = 0.021), and Aβ-42 (r = 0.31, p = 0.021). In ApoE2+ subjects, the apnea and/or hypopnea with 4% O2-desaturation index was correlated with lower levels of CSF Aβ-42 (r = -0.71, p = 0.004), similarly to ApoE4+ subjects where there was also a trend toward lower CSF Aβ-42 levels. Our observations suggest that there is an association between SDB and CSF Alzheimers disease-biomarkers in cognitively normal elderly individuals. Existing therapies for SDB such as continuous positive airway pressure could delay the onset to mild cognitive impairment or dementia in normal elderly individuals.

Candidate mechanisms underlying the association between sleep-wake disruptions and Alzheimer's disease

SUMMARY During wakefulness, extracellular levels of metabolites in the brain increase. These include amyloid beta (Aβ), which contributes to the pathogenesis of Alzheimer’s disease (AD). Counterbalancing their accumulation in the brain, sleep facilitates the removal of these metabolites from the extracellular space by convective flow of the interstitial fluid from the para-arterial to the para-venous space. However, when the sleep-wake cycle is disrupted (characterized by increased brain levels of the wake-promoting neuropeptide orexin and increased neural activity), the central nervous system (CNS) clearance of extracellular metabolites is diminished. Disruptions to the sleep-wake cycle have furthermore been linked to increased neuronal oxidative stress and impaired blood–brain barrier function – conditions that have also been proposed to play a role in the development and progression of AD. Notably, recent human and transgenic animal studies have demonstrated that AD-related pathophysiological processes that occur long before the clinical onset of AD, such as Aβ deposition in the brain, disrupt sleep and circadian rhythms. Collectively, as proposed in this review, these findings suggest the existence of a mechanistic interplay between AD pathogenesis and disrupted sleep-wake cycles, which is able to accelerate the development and progression of this disease.

The other half of Hebb: K+ channels and the regulation of neuronal excitability in the hippocampus.

Historically, much attention has focused on the mechanisms of activity-dependent plasticity since the description of long-term potentiation by Bliss and Lomo in the early 1970s, while extrasynaptic changes have received much less interest. However, recent work has concentrated on the role of back-propagating action potentials in hippocampal dendrites in synaptic plasticity. In this review, we focus on the modulation of back-propagating action potentials by K+ currents in the dendrites of hippocampal cells. We described the primary K+-channel subunits and their interacting subunits that most likely contribute to these currents, and how these sites can be regulated by phosphorylation and other mechanisms. In conclusion, we provide a model for an alternative form of coincidence detection through K+ channels in the hippocampus.

Apnea-Induced Rapid Eye Movement Sleep Disruption Impairs Human Spatial Navigational Memory

Hippocampal electrophysiology and behavioral evidence support a role for sleep in spatial navigational memory, but the role of particular sleep stages is less clear. Although rodent models suggest the importance of rapid eye movement (REM) sleep in spatial navigational memory, a similar role for REM sleep has never been examined in humans. We recruited subjects with severe obstructive sleep apnea (OSA) who were well treated and adherent with continuous positive airway pressure (CPAP). Restricting CPAP withdrawal to REM through real-time monitoring of the polysomnogram provides a novel way of addressing the role of REM sleep in spatial navigational memory with a physiologically relevant stimulus. Individuals spent two different nights in the laboratory, during which subjects performed timed trials before and after sleep on one of two unique 3D spatial mazes. One night of sleep was normally consolidated with use of therapeutic CPAP throughout, whereas on the other night, CPAP was reduced only in REM sleep, allowing REM OSA to recur. REM disruption via this method caused REM sleep reduction and significantly fragmented any remaining REM sleep without affecting total sleep time, sleep efficiency, or slow-wave sleep. We observed improvements in maze performance after a night of normal sleep that were significantly attenuated after a night of REM disruption without changes in psychomotor vigilance. Furthermore, the improvement in maze completion time significantly positively correlated with the mean REM run duration across both sleep conditions. In conclusion, we demonstrate a novel role for REM sleep in human memory formation and highlight a significant cognitive consequence of OSA.