Optogenetic Control of Spine-Head JNK Reveals a Role in Dendritic Spine Regression

miR-193b-365 microcluster downstream of Fezf2 coordinates neuron-subtype identity and dendritic morphology in cortical projection neurons

Different neuron types develop characteristic axonal and dendritic arborizations that determine their inputs, outputs, and functions. Expression of fate-determinant transcription factors is essential for specification of their distinct identities. However, the mechanisms downstream of fate-determinant factors coordinating different aspects of neuron identity are not understood. Specifically, how distinct projection neurons develop appropriate dendritic arbors that determine their inputs is unknown. Here, we investigate this question in corticospinal and callosal projection neurons. We identified a mechanism linking the corticospinal/corticofugal identity gene Fezf2 with the regulation of dendritic development. We show that miR-193b∼365 microRNA cluster is regulated by Fezf2 and enriched in corticospinal neurons. miR-193b∼365 represses mitogen-activated protein kinase 8 (MAPK8) to regulate corticospinal dendritic development. miR-193b∼365 overexpression in callosal neurons abnormally reduces MAPK8 signal and dendritic complexity. Our findings show that regulation of MAPK8 via miR-193b∼365 cluster regulates dendritic development, providing a mechanism that coordinates projection neuron identity, specified by Fezf2, and neuron-specific dendritic morphology.

Jnk1 and downstream signalling hubs regulate anxiety-like behaviours in a zebrafish larvae phenotypic screen

Current treatments for anxiety and depression show limited efficacy in many patients, indicating the need for further research into the underlying mechanisms. JNK1 has been shown to regulate anxiety- and depressive-like behaviours in mice, however the effectors downstream of JNK1 are not known. Here we compare the phosphoproteomes from wild-type and Jnk1-/- mouse brains and identify JNK1-regulated signalling hubs. We next employ a zebrafish (Danio rerio) larvae behavioural assay to identify an antidepressant- and anxiolytic-like (AA) phenotype based on 2759 measured stereotypic responses to clinically proven antidepressant and anxiolytic (AA) drugs. Employing machine learning, we classify an AA phenotype from extracted features measured during and after a startle battery in fish exposed to AA drugs. Using this classifier, we demonstrate that structurally independent JNK inhibitors replicate the AA phenotype with high accuracy, consistent with findings in mice. Furthermore, pharmacological targeting of JNK1-regulated signalling hubs identifies AKT, GSK-3, 14-3-3 ζ/ε and PKCε as downstream hubs that phenocopy clinically proven AA drugs. This study identifies AKT and related signalling molecules as mediators of JNK1-regulated antidepressant- and anxiolytic-like behaviours. Moreover, the assay shows promise for early phase screening of compounds with anti-stress-axis properties and for mode of action analysis.

LOV2-based photoactivatable CaMKII and its application to single synapses: Local Optogenetics

Optogenetic techniques offer a high spatiotemporal resolution to manipulate cellular activity. For instance, Channelrhodopsin-2 with global light illumination is the most widely used to control neuronal activity at the cellular level. However, the cellular scale is much larger than the diffraction limit of light (<1 μm) and does not fully exploit the features of the "high spatial resolution" of optogenetics. For instance, until recently, there were no optogenetic methods to induce synaptic plasticity at the level of single synapses. To address this, we developed an optogenetic tool named photoactivatable CaMKII (paCaMKII) by fusing a light-sensitive domain (LOV2) to CaMKIIα, which is a protein abundantly expressed in neurons of the cerebrum and hippocampus and essential for synaptic plasticity. Combining photoactivatable CaMKII with two-photon excitation, we successfully activated it in single spines, inducing synaptic plasticity (long-term potentiation) in hippocampal neurons. We refer to this method as "Local Optogenetics", which involves the local activation of molecules and measurement of cellular responses. In this review, we will discuss the characteristics of LOV2, the recent development of its derivatives, and the development and application of paCaMKII.

JNK Activation Correlates with Cognitive Impairment and Alteration of the Post-Synaptic Element in the 5xFAD AD Mouse Model

The c-Jun N-terminal kinases (JNKs) are a family of proteins that, once activated by stress stimuli, can alter neuronal functions and survival. The JNK cascade plays a crucial role in the post-synaptic neuronal compartment by altering its structural organization and leading, at worst, to an overall impairment of neuronal communication. Increasing evidence suggests that synaptic impairment is the first neurodegenerative event in Alzheimer’s disease (AD). To better elucidate this mechanism, we longitudinally studied 5xFAD mice at three selected time points representative of human AD symptom progression. We tested the mice cognitive performance by using the radial arm water maze (RAWM) in parallel with biochemical evaluations of post-synaptic enriched protein fraction and total cortical parenchyma. We found that 5xFAD mice presented a strong JNK activation at 3.5 months of age in the post-synaptic enriched protein fraction. This JNK activation correlates with a structural alteration of the post-synaptic density area and with memory impairment at this early stage of the disease that progressively declines to cause cell death. These findings pave the way for future studies on JNK as a key player in early neurodegeneration and as an important therapeutic target for the development of new compounds able to tackle synaptic impairment in the early phase of AD pathology.

c-Jun N-Terminal Kinases in Alzheimer's Disease: A Possible Target for the Modulation of the Earliest Alterations

Given the highly multifactorial origin of Alzheimer's disease (AD) neuropathology, disentangling and orderly knowing mechanisms involved in sporadic onset are arduous. Nevertheless, when the elements involved are dissected into smaller pieces, the task becomes more accessible. This review aimed to describe the link between c-Jun N-terminal Kinases (JNKs), master regulators of many cellular functions, and the early alterations of AD: synaptic loss and dysregulation of neuronal transport. Both processes have a role in the posterior cognitive decline observed in AD. The manuscript focuses on the molecular mechanisms of glutamatergic, GABA, and cholinergic synapses altered by the presence of amyloid-β aggregates and hyperphosphorylated tau, as well as on several consequences of the disruption of cellular processes linked to neuronal transport that is controlled by the JNK-JIP (c-jun NH2-terminal kinase (JNK)-interacting proteins (JIPs) complex, including the transport of AβPP or autophagosomes.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

Photoactivatable CaMKII induces synaptic plasticity in single synapses

Optogenetic approaches for studying neuronal functions have proven their utility in the neurosciences. However, optogenetic tools capable of inducing synaptic plasticity at the level of single synapses have been lacking. Here, we engineered a photoactivatable (pa)CaMKII by fusing a light-sensitive domain, LOV2, to CaMKIIα. Blue light or two-photon excitation reversibly activated paCaMKII. Activation in single spines was sufficient to induce structural long-term potentiation (sLTP) in vitro and in vivo. paCaMKII activation was also sufficient for the recruitment of AMPA receptors and functional LTP in single spines. By combining paCaMKII with protein activity imaging by 2-photon FLIM-FRET, we demonstrate that paCaMKII activation in clustered spines induces robust sLTP via a mechanism that involves the actin-regulatory small GTPase, Cdc42. This optogenetic tool for dissecting the function of CaMKII activation (i.e., the sufficiency of CaMKII rather than necessity) and for manipulating synaptic plasticity will find many applications in neuroscience and other fields.

Optogenetic control of molecules is important in cell biology and neuroscience. Here, the authors describe an optogenetic tool to control the Ca^²+/calmodulin-dependent protein kinase II and use it to control plasticity at the single synapse level.

Photobiomodulation suppresses JNK3 by activation of ERK/MKP7 to attenuate AMPA receptor endocytosis in Alzheimer's disease

Alzheimer's disease (AD), a severe age‐related neurodegenerative disorder, lacks effective therapeutic methods at present. Physical approaches such as gamma frequency light flicker that can effectively reduce amyloid load have been reported recently. Our previous research showed that a physical method named photobiomodulation (PBM) therapy rescues Aβ‐induced dendritic atrophy in vitro. However, it remains to be further investigated the mechanism by which PBM affects AD‐related multiple pathological features to improve learning and memory deficits. Here, we found that PBM attenuated Aβ‐induced synaptic dysfunction and neuronal death through MKP7‐dependent suppression of JNK3, a brain‐specific JNK isoform related to neurodegeneration. The results showed PBM‐attenuated amyloid load, AMPA receptor endocytosis, dendrite injury, and inflammatory responses, thereby rescuing memory deficits in APP/PS1 mice. We noted JNK3 phosphorylation was dramatically decreased after PBM treatment in vivo and in vitro. Mechanistically, PBM activated ERK, which subsequently phosphorylated and stabilized MKP7, resulting in JNK3 inactivation. Furthermore, activation of ERK/MKP7 signaling by PBM increased the level of AMPA receptor subunit GluR 1 phosphorylation and attenuated AMPA receptor endocytosis in an AD pathological model. Collectively, these data demonstrated that PBM has potential therapeutic value in reducing multiple pathological features associated with AD, which is achieved by regulating JNK3, thus providing a noninvasive, and drug‐free therapeutic strategy to impede AD progression.

Resonance energy transfer sensitises and monitors in situ switching of LOV2-based optogenetic actuators

Engineered light-dependent switches provide uniquely powerful opportunities to investigate and control cell regulatory mechanisms. Existing tools offer high spatiotemporal resolution, reversibility and repeatability. Cellular optogenetics applications remain limited with diffusible targets as the response of the actuator is difficult to independently validate. Blue light levels commonly needed for actuation can be cytotoxic, precluding long-term experiments. We describe a simple approach overcoming these obstacles. Resonance energy transfer can be used to constitutively or dynamically modulate actuation sensitivity. This simultaneously offers on-line monitoring of light-dependent switching and precise quantification of activation-relaxation properties in intact living cells. Applying this approach to different LOV2-based switches reveals that flanking sequences can lead to relaxation times up to 11-fold faster than anticipated. In situ–measured parameter values guide the design of target-inhibiting actuation trains with minimal blue-light exposure, and context-based optimisation can increase sensitivity and experimental throughput a further 10-fold without loss of temporal precision.

Cellular optogenetics applications are limited by difficulties in quantification and blue light toxicity. Here the authors design LOV2-based switches that use resonance energy transfer to overcome these concerns.

Protein synthesis is suppressed in sporadic and familial Parkinson's disease by LRRK2

Gain of function LRRK2-G2019S is the most frequent mutation found in familial and sporadic Parkinson's disease. It is expected therefore that understanding the cellular function of LRRK2 will provide insight on the pathological mechanism not only of inherited Parkinson's, but also of sporadic Parkinson's, the more common form. Here, we show that constitutive LRRK2 activity controls nascent protein synthesis in rodent neurons. Specifically, pharmacological inhibition of LRRK2, Lrrk2 knockdown or Lrrk2 knockout, all lead to increased translation. In the rotenone model for sporadic Parkinson's, LRRK2 activity increases, dopaminergic neuron translation decreases, and the neurites atrophy. All are prevented by LRRK2 inhibitors. Moreover, in striatum and substantia nigra of rotenone treated rats, phosphorylation changes are observed on eIF2α-S52(↑), eIF2s2-S2(↓), and eEF2-T57(↑) in directions that signify protein synthesis arrest. Significantly, translation is reduced by 40% in fibroblasts from Parkinson's patients (G2019S and sporadic cases alike) and this is reversed upon LRRK2 inhibitor treatment. In cells from multiple system atrophy patients, translation is unchanged suggesting that repression of translation is specific to Parkinson's disease. These findings indicate that repression of translation is a proximal function of LRRK2 in Parkinson's pathology.

Involvement of JNK1 in Neuronal Polarization During Brain Development

The c-Jun N-terminal Kinases (JNKs) are a group of regulatory elements responsible for the control of a wide array of functions within the cell. In the central nervous system (CNS), JNKs are involved in neuronal polarization, starting from the cell division of neural stem cells and ending with their final positioning when migrating and maturing. This review will focus mostly on isoform JNK1, the foremost contributor of total JNK activity in the CNS. Throughout the text, research from multiple groups will be summarized and discussed in order to describe the involvement of the JNKs in the different steps of neuronal polarization. The data presented support the idea that isoform JNK1 is highly relevant to the regulation of many of the processes that occur in neuronal development in the CNS.

Impact of JNK and Its Substrates on Dendritic Spine Morphology

The protein kinase JNK1 exhibits high activity in the developing brain, where it regulates dendrite morphology through the phosphorylation of cytoskeletal regulatory proteins. JNK1 also phosphorylates dendritic spine proteins, and Jnk1-/- mice display a long-term depression deficit. Whether JNK1 or other JNKs regulate spine morphology is thus of interest. Here, we characterize dendritic spine morphology in hippocampus of mice lacking Jnk1-/- using Lucifer yellow labelling. We find that mushroom spines decrease and thin spines increase in apical dendrites of CA3 pyramidal neurons with no spine changes in basal dendrites or in CA1. Consistent with this spine deficit, Jnk1-/- mice display impaired acquisition learning in the Morris water maze. In hippocampal cultures, we show that cytosolic but not nuclear JNK, regulates spine morphology and expression of phosphomimicry variants of JNK substrates doublecortin (DCX) or myristoylated alanine-rich C kinase substrate-like protein-1 (MARCKSL1), rescue mushroom, thin, and stubby spines differentially. These data suggest that physiologically active JNK controls the equilibrium between mushroom, thin, and stubby spines via phosphorylation of distinct substrates.