The Ste20 Family Kinases MAP4K4, MINK1, and TNIK Converge to Regulate Stress-Induced JNK Signaling in Neurons.

The c-Jun-N-terminal kinase (JNK) signaling pathway regulates nervous system development, axon regeneration, and neuronal degeneration after acute injury or in chronic neurodegenerative disease. Dual leucine zipper kinase (DLK) is required for stress-induced JNK signaling in neurons, yet the factors that initiate DLK/JNK pathway activity remain poorly defined. In the present study, we identify the Ste20 kinases MAP4K4, misshapen-like kinase 1 (MINK1 or MAP4K6) and TNIK Traf2- and Nck-interacting kinase (TNIK or MAP4K7), as upstream regulators of DLK/JNK signaling in neurons. Using a trophic factor withdrawal-based model of neurodegeneration in both male and female embryonic mouse dorsal root ganglion neurons, we show that MAP4K4, MINK1, and TNIK act redundantly to regulate DLK activation and downstream JNK-dependent phosphorylation of c-Jun in response to stress. Targeting MAP4K4, MINK1, and TNIK, but not any of these kinases individually, is sufficient to protect neurons potently from degeneration. Pharmacological inhibition of MAP4Ks blocks stabilization and phosphorylation of DLK within axons and subsequent retrograde translocation of the JNK signaling complex to the nucleus. These results position MAP4Ks as important regulators of the DLK/JNK signaling pathway.SIGNIFICANCE STATEMENT Neuronal degeneration occurs in disparate circumstances: during development to refine neuronal connections, after injury to clear damaged neurons, or pathologically during disease. The dual leucine zipper kinase (DLK)/c-Jun-N-terminal kinase (JNK) pathway represents a conserved regulator of neuronal injury signaling that drives both neurodegeneration and axon regeneration, yet little is known about the factors that initiate DLK activity. Here, we uncover a novel role for a subfamily of MAP4 kinases consisting of MAP4K4, Traf2- and Nck-interacting kinase (TNIK or MAP4K7), and misshapen-like kinase 1 (MINK1 or MAP4K6) in regulating DLK/JNK signaling in neurons. Inhibition of these MAP4Ks blocks stress-induced retrograde JNK signaling and protects from neurodegeneration, suggesting that these kinases may represent attractive therapeutic targets.

Loss of dual leucine zipper kinase signaling is protective in animal models of neurodegenerative disease.

Hallmarks of chronic neurodegenerative disease include progressive synaptic loss and neuronal cell death, yet the cellular pathways that underlie these processes remain largely undefined. We provide evidence that dual leucine zipper kinase (DLK) is an essential regulator of the progressive neurodegeneration that occurs in amyotrophic lateral sclerosis and Alzheimer's disease. We demonstrate that DLK/c-Jun N-terminal kinase signaling was increased in mouse models and human patients with these disorders and that genetic deletion of DLK protected against axon degeneration, neuronal loss, and functional decline in vivo. Furthermore, pharmacological inhibition of DLK activity was sufficient to attenuate the neuronal stress response and to provide functional benefit even in the presence of ongoing disease. These findings demonstrate that pathological activation of DLK is a conserved mechanism that regulates neurodegeneration and suggest that DLK inhibition may be a potential approach to treat multiple neurodegenerative diseases.

Identification of 12/15-lipoxygenase as a regulator of axon degeneration through high-content screening.

Axon degeneration is a programed process that takes place during development, in response to neuronal injury, and as a component of neurodegenerative disease pathology, yet the molecular mechanisms that drive this process remain poorly defined. In this study, we have developed a semi-automated, 384-well format axon degeneration assay in rat dorsal root ganglion (DRG) neurons using a trophic factor withdrawal paradigm. Using this setup, we have screened a library of known drugs and bioactives to identify several previously unappreciated regulators of axon degeneration, including lipoxygenases. Multiple structurally distinct lipoxygenase inhibitors as well as mouse DRG neurons lacking expression of 12/15-lipoxygenase display protection of axons in this context. Retinal ganglion cell axons from 12/15-lipoxygenase-null mice were similarly protected from degeneration following nerve crush injury. Through additional mechanistic studies, we demonstrate that lipoxygenases act cell autonomously within neurons to regulate degeneration, and are required for mitochondrial permeabilization and caspase activation in the axon. These findings suggest that these enzymes may represent an attractive target for treatment of neuropathies and provide a potential mechanism for the neuroprotection observed in various settings following lipoxygenase inhibitor treatment.

Discovery of inhibitors of microglial neurotoxicity acting through multiple mechanisms using a stem-cell-based phenotypic assay.

Stem cells, through their ability to both self-renew and differentiate, can produce a virtually limitless supply of specialized cells that behave comparably to primary cells. We took advantage of this property to develop an assay for small-molecule-based neuroprotection using stem-cell-derived motor neurons and astrocytes, together with activated microglia as a stress paradigm. Here, we report on the discovery of hit compounds from a screen of more than 10,000 small molecules. These compounds act through diverse pathways, including the inhibition of nitric oxide production by microglia, activation of the Nrf2 pathway in microglia and astrocytes, and direct protection of neurons from nitric-oxide-induced degeneration. We confirm the activity of these compounds using human neurons. Because microglial cells are activated in many neurological disorders, our hit compounds could be ideal starting points for the development of new drugs to treat various neurodegenerative and neurological diseases.

Raising cytosolic Cl- in cerebellar granule cells affects their excitability and vestibulo-ocular learning.

Cerebellar cortical throughput involved in motor control comprises granule cells (GCs) and Purkinje cells (PCs), both of which receive inhibitory GABAergic input from interneurons. The GABAergic input to PCs is essential for learning and consolidation of the vestibulo-ocular reflex, but the role of GC excitability remains unclear. We now disrupted the Kcc2 K-Cl cotransporter specifically in either cell type to manipulate their excitability and inhibition by GABA(A)-receptor Cl(-) channels. Although Kcc2 may have a morphogenic role in synapse development, Kcc2 disruption neither changed synapse density nor spine morphology. In both GCs and PCs, disruption of Kcc2, but not Kcc3, increased [Cl(-)](i) roughly two-fold. The reduced Cl(-) gradient nearly abolished GABA-induced hyperpolarization in PCs, but in GCs it merely affected excitability by membrane depolarization. Ablation of Kcc2 from GCs impaired consolidation of long-term phase learning of the vestibulo-ocular reflex, whereas baseline performance, short-term gain-decrease learning and gain consolidation remained intact. These functions, however, were affected by disruption of Kcc2 in PCs. GC excitability plays a previously unknown, but specific role in consolidation of phase learning.

Behavioral deficits and subregion-specific suppression of LTP in mice expressing a population of mutant NMDA receptors throughout the hippocampus.

The NMDA receptor (NMDAR) subunit GluN1 is an obligatory component of NMDARs without a known functional homolog and is expressed in almost every neuronal cell type. The NMDAR system is a coincidence detector with critical roles in spatial learning and synaptic plasticity. Its coincidence detection property is crucial for the induction of hippocampal long-term potentiation (LTP). We have generated a mutant mouse model expressing a hypomorph of the Grin1(N598R) allele, which leads to a minority (about 10%) of coincidence detection-impaired NMDARs. Surprisingly, these animals revealed specific functional changes in the dentate gyrus (DG) of the hippocampal formation. Early LTP was expressed normally in area CA1 in vivo, but was completely suppressed at perforant path-granule cell synapses in the DG. In addition, there was a pronounced reduction in the amplitude of the evoked population spike in the DG. These specific changes were accompanied by behavioral impairments in spatial recognition, spatial learning, reversal learning, and retention. Our data show that minor changes in GluN1-dependent NMDAR physiology can cause dramatic consequences in synaptic signaling in a subregion-specific fashion despite the nonredundant nature of the GluN1 gene and its global expression.

Neuron-glia communication via EphA4/ephrin-A3 modulates LTP through glial glutamate transport.

Astrocytes are critical participants in synapse development and function, but their role in synaptic plasticity is unclear. Eph receptors and their ephrin ligands have been suggested to regulate neuron-glia interactions, and EphA4-mediated ephrin reverse signaling is required for synaptic plasticity in the hippocampus. Here we show that long-term potentiation (LTP) at the CA3-CA1 synapse is modulated by EphA4 in the postsynaptic CA1 cell and by ephrin-A3, a ligand of EphA4 that is found in astrocytes. Lack of EphA4 increased the abundance of glial glutamate transporters, and ephrin-A3 modulated transporter currents in astrocytes. Pharmacological inhibition of glial glutamate transporters rescued the LTP defects in EphA4 (Epha4) and ephrin-A3 (Efna3) mutant mice. Transgenic overexpression of ephrin-A3 in astrocytes reduces glutamate transporter levels and produces focal dendritic swellings possibly caused by glutamate excitotoxicity. These results suggest that EphA4/ephrin-A3 signaling is a critical mechanism for astrocytes to regulate synaptic function and plasticity.

NKCC1-dependent GABAergic excitation drives synaptic network maturation during early hippocampal development.

A high intracellular chloride concentration in immature neurons leads to a depolarizing action of GABA that is thought to shape the developing neuronal network. We show that GABA-triggered depolarization and Ca2+ transients were attenuated in mice deficient for the Na-K-2Cl cotransporter NKCC1. Correlated Ca2+ transients and giant depolarizing potentials (GDPs) were drastically reduced and the maturation of the glutamatergic and GABAergic transmission in CA1 delayed. Brain morphology, synaptic density, and expression levels of certain developmental marker genes were unchanged. The expression of lynx1, a protein known to dampen network activity, was decreased. In mice deficient for the neuronal Cl(-)/HCO(3)(-) exchanger AE3, GDPs were also diminished. These data show that NKCC1-mediated Cl(-) accumulation contributes to GABAergic excitation and network activity during early postnatal development and thus facilitates the maturation of excitatory and inhibitory synapses.

Mice with targeted Slc4a10 gene disruption have small brain ventricles and show reduced neuronal excitability.

Members of the SLC4 bicarbonate transporter family are involved in solute transport and pH homeostasis. Here we report that disrupting the Slc4a10 gene, which encodes the Na(+)-coupled Cl(-)-HCO(3)(-) exchanger Slc4a10 (NCBE), drastically reduces brain ventricle volume and protects against fatal epileptic seizures in mice. In choroid plexus epithelial cells, Slc4a10 localizes to the basolateral membrane. These cells displayed a diminished recovery from an acid load in KO mice. Slc4a10 also was expressed in neurons. Within the hippocampus, the Slc4a10 protein was abundant in CA3 pyramidal cells. In the CA3 area, propionate-induced intracellular acidification and attenuation of 4-aminopyridine-induced network activity were prolonged in KO mice. Our data indicate that Slc4a10 is involved in the control of neuronal pH and excitability and may contribute to the secretion of cerebrospinal fluid. Hence, Slc4a10 is a promising pharmacological target for the therapy of epilepsy or elevated intracranial pressure.

Disruption of erythroid K-Cl cotransporters alters erythrocyte volume and partially rescues erythrocyte dehydration in SAD mice.

K-Cl cotransport activity in rbc is a major determinant of rbc volume and density. Pathologic activation of erythroid K-Cl cotransport activity in sickle cell disease contributes to rbc dehydration and cell sickling. To address the roles of individual K-Cl cotransporter isoforms in rbc volume homeostasis, we disrupted the Kcc1 and Kcc3 genes in mice. As rbc K-Cl cotransport activity was undiminished in Kcc1(-/-) mice, decreased in Kcc3(-/-) mice, and almost completely abolished in mice lacking both isoforms, we conclude that K-Cl cotransport activity of mouse rbc is mediated largely by KCC3. Whereas rbc of either Kcc1(-/-) or Kcc3(-/-) mice were of normal density, rbc of Kcc1(-/-)Kcc3(-/-) mice exhibited defective volume regulation, including increased mean corpuscular volume, decreased density, and increased susceptibility to osmotic lysis. K-Cl cotransport activity was increased in rbc of SAD mice, which are transgenic for a hypersickling human hemoglobin S variant. Kcc1(-/-)Kcc3(-/-) SAD rbc lacked nearly all K-Cl cotransport activity and exhibited normalized values of mean corpuscular volume, corpuscular hemoglobin concentration mean, and K(+) content. Although disruption of K-Cl cotransport rescued the dehydration phenotype of most SAD rbc, the proportion of the densest red blood cell population remained unaffected.

CaMKII translocation requires local NMDA receptor-mediated Ca2+ signaling.

Excitatory synaptic transmission and plasticity are critically modulated by N-methyl-D-aspartate receptors (NMDARs). Activation of NMDARs elevates intracellular Ca(2+) affecting several downstream signaling pathways that involve Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Importantly, NMDAR activation triggers CaMKII translocation to synaptic sites. NMDAR activation failed to induce Ca(2+) responses in hippocampal neurons lacking the mandatory NMDAR subunit NR1, and no EGFP-CaMKIIalpha translocation was observed. In cells solely expressing Ca(2+)-impermeable NMDARs containing NR1(N598R)-mutant subunits, prolonged NMDA application elevated internal Ca(2+) to the same degree as in wild-type controls, yet failed to translocate CaMKIIalpha. Brief local NMDA application evoked smaller Ca(2+) transients in dendritic spines of mutant compared to wild-type cells. CaMKIIalpha mutants that increase binding to synaptic sites, namely CaMKII-T286D and CaMKII-TT305/306VA, rescued the translocation in NR1(N598R) cells in a glutamate receptor-subtype-specific manner. We conclude that CaMKII translocation requires Ca(2+) entry directly through NMDARs, rather than other Ca(2+) sources activated by NMDARs. Together with the requirement for activated, possibly ligand-bound, NMDARs as CaMKII binding partners, this suggests that synaptic CaMKII accumulation is an input-specific signaling event.

Subcellular localisation of recombinant alpha- and gamma-synuclein.

alpha-Synuclein, a protein implicated in neurodegenerative diseases and of elusive physiological function owes its name to an observed presence in presynaptic and nuclear compartments. However, its nuclear localisation has remained controversial. We expressed synuclein-eGFP fusion proteins in organotypic rat hippocampal slice cultures and murine hippocampal primary neurons using a Sindbis virus expression system. Recombinant full-length alpha-synuclein accumulated in presynaptic locations, mimicking its native distribution. Expression of deletion mutant alpha-synuclein revealed that presynaptic targeting depended on the presence of its N-terminal and core region. This domain also causes nuclear exclusion of the alpha-synuclein fusion protein. In contrast, the C-terminal domain of alpha-synuclein directs fusion proteins into the nuclear compartment. The related protein gamma-synuclein contains a similar N-terminal and core domain as alpha-synuclein. However, gamma-synuclein lacks a C-terminal domain that causes nuclear localisation of the fusion protein, suggesting that the two synucleins might have different roles relating to the cell nucleus.

Absence of Whisker-related pattern formation in mice with NMDA receptors lacking coincidence detection properties and calcium signaling.

Precise refinement of synaptic connectivity is the result of activity-dependent mechanisms in which coincidence-dependent calcium signaling by NMDA receptors (NMDARs) under control of the voltage-dependent Mg2+ block might play a special role. In the developing rodent trigeminal system, the pattern of synaptic connections between whisker-specific inputs and their target cells in the brainstem is refined to form functionally and morphologically distinct units (barrelettes). To test the role of NMDA receptor signaling in this process, we introduced the N598R mutation into the native NR1 gene. This leads to the expression of functional NMDARs that are Mg2+ insensitive and Ca2+ impermeable. Newborn mice expressing exclusively NR1 N598R-containing NMDARs do not show any whisker-related patterning in the brainstem, whereas the topographic projection of trigeminal afferents and gross brain morphology appear normal. Furthermore, the NR1 N598R mutation does not affect expression levels of NMDAR subunits and other important neurotransmitter receptors. Our results show that coincidence detection by, and/or Ca2+ permeability of, NMDARs is necessary for the development of somatotopic maps in the brainstem and suggest that highly specific signaling underlies synaptic refinement.