A Molecular Framework for Delirium.

Over 2.6 million adults over the age of 65 develop delirium each year in the United States (US). Delirium is associated with a significant increase in mortality and the US health care costs associated with delirium are estimated at over $164 billion annually. Despite the prevalence of the condition, the molecular pathophysiology of delirium remains unexplained, limiting the development of pharmacotherapies. Delirious patients can be identified by prominent impairments in attention and working memory (WM), two cognitive domains that localize to the dorsolateral prefrontal cortex (dlPFC). The dlPFC is also a key site for Alzheimer's disease (AD) pathology, and given the high risk of delirium in AD patients, suggests that efforts at understanding delirium might focus on the dlPFC as a final common endpoint for cognitive changes. Preclinical studies of the dlPFC reproduce many of the pharmacological observations made of delirious patients, including sensitivity to anticholinergics and an 'inverted U' pattern of dependence on monoaminergic input, with diminished performance outside a narrow range of signaling. Medications like guanfacine, which influence the dlPFC in the context of attention-deficit/hyperactivity disorder (ADHD), have emerged as therapies for delirium and motivate a detailed understanding of the influence of α-2 agonists on WM. In this review, I will discuss the neural circuitry and molecular mechanisms underlying WM and the function of the dlPFC. Localizing the cognitive deficits that are commonly seen in delirious patients may help identify new molecular targets for this highly prevalent disease.

Characterization of hyperpolarization-activated cyclic nucleotide-gated channels in oligodendrocytes.

Mature oligodendrocytes (OLG) are the myelin-forming cells of the central nervous system. Recent work has shown a dynamic role for these cells in the plasticity of neural circuits, leading to a renewed interest in voltage-sensitive currents in OLG. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and their respective current (Ih) were recently identified in mature OLG and shown to play a role in regulating myelin length. Here we provide a biochemical and electrophysiological characterization of HCN channels in cells of the oligodendrocyte lineage. We observed that mice with a nonsense mutation in the Hcn2 gene (Hcn2ap/ap) have less white matter than their wild type counterparts with fewer OLG and fewer oligodendrocyte progenitor cells (OPCs). Hcn2ap/ap mice have severe motor impairments, although these deficits were not observed in mice with HCN2 conditionally eliminated only in oligodendrocytes (Cnpcre/+; Hcn2F/F). However, Cnpcre/+; Hcn2F/F mice develop motor impairments more rapidly in response to experimental autoimmune encephalomyelitis (EAE). We conclude that HCN2 channels in OLG may play a role in regulating metabolism.

An Electronic Health Record Intervention to Limit Viral Testing of Cerebrospinal Fluid.

Meningitis and encephalitis are neurologic emergencies that require immediate management and current guidelines recommend empiric treatment with broad-spectrum antimicrobials. Cerebrospinal fluid (CSF) testing algorithms are heterogeneous and largely institution-specific, reflecting a lack of consensus on how to effectively identify CSF pathogens while conserving resources and avoiding false positives. Moreover, many lumbar punctures (LPs) performed in the inpatient setting are done for noninfectious workups, such as evaluation for leptomeningeal metastasis. As such, tailoring CSF testing to clinical context has been a focus of multiple prior reports and several healthcare systems have focused on efforts to limit low-yield diagnostic testing when a positive result is unlikely. To curb ordering viral PCRs when pre-test probability is low, some peer institutions have implemented pleocytosis criteria for virus-specific polymerase chain reaction (PCR) tests from CSF. In this report, we retrospectively analyzed the diagnostic testing of CSF from patients who had an LP while admitted to a single, large academic medical center and found that many cases of Herpes Simplex Virus (HSV) meningoencephalitis were diagnosed by non-neurologists. The rate of positive virus-specific PCR tests was very low, and tests were frequently ordered in duplicate with a multiplexed meningitis/encephalitis PCR panel (M/E panel, BioFire, Salt Lake City, UT). We designed and implemented a systems-level intervention to promote a revised stepwise testing algorithm that minimizes unnecessary tests. This intervention led to a significant reduction in the number of low-yield virus-specific PCR tests ordered without implementing a policy of cancelling virus-specific PCRs.

Discovery of a small-molecule inhibitor of the TRIP8b-HCN interaction with efficacy in neurons.

Major depressive disorder is a critical public health problem with a lifetime prevalence of nearly 17% in the United States. One potential therapeutic target is the interaction between hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and an auxiliary subunit of the channel named tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b). HCN channels regulate neuronal excitability in the mammalian hippocampus, and recent work has established that antagonizing HCN function rescues cognitive impairment caused by chronic stress. Here, we utilize a high-throughput virtual screen to find small molecules capable of disrupting the TRIP8b-HCN interaction. We found that the hit compound NUCC-0200590 disrupts the TRIP8b-HCN interaction in vitro and in vivo. These results provide a compelling strategy for developing new small molecules capable of disrupting the TRIP8b-HCN interaction.

Hippocampal cAMP regulates HCN channel function on two time scales with differential effects on animal behavior.

Hyperpolarization-activated cyclic nucleotide­gated (HCN) channels regulate neuronal excitability and represent a possible therapeutic target for major depressive disorder (MDD). These channels are regulated by intracellular cyclic adenosine monophosphate (cAMP). However, the relationship between cAMP signaling and the influence of HCN channels on behavior remains opaque. In this study, we investigated the role of hippocampal cAMP signaling on behavior using chemogenetic technology in mice. Acutely increasing cAMP limited spatial memory and motivated behavior by increasing HCN function. However, chronically elevated cAMP limited surface trafficking of HCN channels by disrupting the interaction between HCN and tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b), an auxiliary subunit. Chronically increased cAMP in the dorsal hippocampus was also sufficient to rescue cognitive deficits induced by chronic stress in mice. These results reveal a behaviorally relevant form of regulation of HCN channel surface expression that has potential as a therapeutic target for cognitive deficits related to chronic stress.

The structure and function of TRIP8b, an auxiliary subunit of hyperpolarization-activated cyclic-nucleotide gated channels.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed throughout the mammalian central nervous system (CNS). These channels have been implicated in a wide range of diseases, including Major Depressive Disorder and multiple subtypes of epilepsy. The diversity of functions that HCN channels perform is in part attributable to differences in their subcellular localization. To facilitate a broad range of subcellular distributions, HCN channels are bound by auxiliary subunits that regulate surface trafficking and channel function. One of the best studied auxiliary subunits is tetratricopeptide-repeat containing, Rab8b-interacting protein (TRIP8b). TRIP8b is an extensively alternatively spliced protein whose only known function is to regulate HCN channels. TRIP8b binds to HCN pore-forming subunits at multiple interaction sites that differentially regulate HCN channel function and subcellular distribution. In this review, we summarize what is currently known about the structure and function of TRIP8b isoforms with an emphasis on the role of this auxiliary subunit in health and disease.

Moving Metabolism to Make Inroads in a Model of Mitochondrial Epilepsy.

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Phosphorylation of the HCN channel auxiliary subunit TRIP8b is altered in an animal model of temporal lobe epilepsy and modulates channel function.

Temporal lobe epilepsy (TLE) is a prevalent neurological disorder with many patients experiencing poor seizure control with existing anti-epileptic drugs. Thus, novel insights into the mechanisms of epileptogenesis and identification of new drug targets can be transformative. Changes in ion channel function have been shown to play a role in generating the aberrant neuronal activity observed in TLE. Previous work demonstrates that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate neuronal excitability and are mislocalized within CA1 pyramidal cells in a rodent model of TLE. The subcellular distribution of HCN channels is regulated by an auxiliary subunit, tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b), and disruption of this interaction correlates with channel mislocalization. However, the molecular mechanisms responsible for HCN channel dysregulation in TLE are unclear. Here we investigated whether changes in TRIP8b phosphorylation are sufficient to alter HCN channel function. We identified a phosphorylation site at residue Ser237 of TRIP8b that enhances binding to HCN channels and influences channel gating by altering the affinity of TRIP8b for the HCN cytoplasmic domain. Using a phosphospecific antibody, we demonstrate that TRIP8b phosphorylated at Ser237 is enriched in CA1 distal dendrites and that phosphorylation is reduced in the kainic acid model of TLE. Overall, our findings indicate that the TRIP8b-HCN interaction can be modulated by changes in phosphorylation and suggest that loss of TRIP8b phosphorylation may affect HCN channel properties during epileptogenesis. These results highlight the potential of drugs targeting posttranslational modifications to restore TRIP8b phosphorylation to reduce excitability in TLE.

Oligodendrocyte Kir4.1 Channels Clear Out Congested K.

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Current Opinions and Consensus for Studying Tremor in Animal Models.

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

CRAFTing a New Approach to Antiepileptic Drug Discovery.

Srivastava PK, van Eyll J, Godard P, Mazzuferi M, Delahaye-Duriez A, Steenwinckel JV, et al. A systems-level framework for drug discovery identifies Csf1R as an anti-epileptic drug target. Nat Commun. 2018;9(1):3561. doi:10.1038/s41467-018-06008-4. The identification of drug targets is highly challenging, particularly for diseases of the brain. To address this problem, we developed and experimentally validated a general computational framework for drug target discovery that combines gene regulatory information with causal reasoning ("Causal Reasoning Analytical Framework for Target discovery"-CRAFT). Using a systems genetics approach and starting from gene expression data from the target tissue, CRAFT provides a predictive framework for identifying cell membrane receptors with a direction-specified influence over disease-related gene expression profiles. As proof of concept, we applied CRAFT to epilepsy and predicted the tyrosine kinase receptor Csf1R as a potential therapeutic target. The predicted effect of Csf1R blockade in attenuating epilepsy seizures was validated in 3 preclinical models of epilepsy. These results highlight CRAFT as a systems-level framework for target discovery and suggest Csf1R blockade as a novel therapeutic strategy in epilepsy. The CRAFT is applicable to disease settings other than epilepsy.

PUMILIO1 Links Epilepsy to Spinocerebellar Ataxia.

A Mild PUM1 Mutation Is Associated With Adult-Onset Ataxia, Whereas Haploinsufficiency Causes Developmental Delay and Seizures Gennarino VA, Palmer EE, McDonell LM, et al. Cell. 2018;172(5):924-936.e11. doi:10.1016/j.cell.2018.02.006. Certain mutations can cause proteins to accumulate in neurons, leading to neurodegeneration. We recently showed, however, that upregulation of a wild-type protein, Ataxin1, caused by haploinsufficiency of its repressor, the RNA-binding protein Pumilio1 (PUM1), also causes neurodegeneration in mice. We therefore searched for human patients with PUM1 mutations. We identified 11 individuals with either PUM1 deletions or de novo missense variants who suffer a developmental syndrome (PUM1-associated developmental disability, ataxia, and seizure). We also identified a milder missense mutation in a family with adult-onset ataxia with incomplete penetrance (PUM1-related cerebellar ataxia). Studies in patient-derived cells revealed that the missense mutations reduced PUM1 protein levels by ∼25% in the adult-onset cases and by ∼50% in the infantile-onset cases; levels of known PUM1 targets increased accordingly. Changes in protein levels thus track with phenotypic severity, and identifying posttranscriptional modulators of protein expression should identify new candidate disease genes.

HCN channels in the hippocampus regulate active coping behavior.

Active coping is an adaptive stress response that improves outcomes in medical and neuropsychiatric diseases. To date, most research into coping style has focused on neurotransmitter activity and little is known about the intrinsic excitability of neurons in the associated brain regions that facilitate coping. Previous studies have shown that HCN channels regulate neuronal excitability in pyramidal cells and that HCN channel current (Ih ) in the CA1 area increases with chronic mild stress. Reduction of Ih in the CA1 area leads to antidepressant-like behavior, and this region has been implicated in the regulation of coping style. We hypothesized that the antidepressant-like behavior achieved with CA1 knockdown of Ih is accompanied by increases in active coping. In this report, we found that global loss of TRIP8b, a necessary subunit for proper HCN channel localization in pyramidal cells, led to active coping behavior in numerous assays specific to coping style. We next employed a viral strategy using a dominant negative TRIP8b isoform to alter coping behavior by reducing HCN channel expression. This approach led to a robust reduction in Ih in CA1 pyramidal neurons and an increase in active coping. Together, these results establish that changes in HCN channel function in CA1 influences coping style.

Allostery between two binding sites in the ion channel subunit TRIP8b confers binding specificity to HCN channels.

Tetratricopeptide repeat (TPR) domains are ubiquitous structural motifs that mediate protein-protein interactions. For example, the TPR domains in the peroxisomal import receptor PEX5 enable binding to a range of type 1 peroxisomal targeting signal motifs. A homolog of PEX5, tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b), binds to and functions as an auxiliary subunit of hyperpolarization-activated cyclic nucleotide (HCN)-gated channels. Given the similarity between TRIP8b and PEX5, this difference in function raises the question of what mechanism accounts for their binding specificity. In this report, we found that the cyclic nucleotide-binding domain and the C terminus of the HCN channel are critical for conferring specificity to TRIP8b binding. We show that TRIP8b binds the HCN cyclic nucleotide-binding domain through a 37-residue domain and the HCN C terminus through the TPR domains. Using a combination of fluorescence polarization- and co-immunoprecipitation-based assays, we establish that binding at either site increases affinity at the other. Thus, allosteric coupling of the TRIP8b TPR domains both promotes binding to HCN channels and limits binding to type 1 peroxisomal targeting signal substrates. These results raise the possibility that other TPR domains may be similarly influenced by allosteric mechanisms as a general feature of protein-protein interactions.