Activity-Dependent Acetylation of Alpha Tubulin in the Hippocampus

Microtubule function and dysfunction in the nervous system

Microtubules are core components of the neuronal cytoskeleton, providing structural support for the complex cytoarchitecture of neurons and serving as tracks for long-distance transport. The properties and functions of neuronal microtubules are controlled by tubulin isoforms and a variety of post-translational modifications, collectively known as the "tubulin code." The tubulin code exerts direct control over the intrinsic properties of neuronal microtubules and regulates the repertoire of proteins that read the code, which in turn, has a significant impact on microtubule stability and dynamics. Here, we review progress in the understanding of the tubulin code in the nervous system, with a particular focus on tubulin post-translational modifications that have been proposed as potential contributors to the development and maintenance of the mammalian nervous system. Furthermore, we also discuss the potential links between disruptions in the tubulin code and neurological disorders, including neurodevelopmental abnormalities and neurodegenerative diseases.

Crebinostat facilitates memory formation

Protein modifications importantly contribute to memory formation. Protein acetylation is a post-translational modification of proteins that regulates memory formation. Acetylation level is determined by the relative activities of acetylases and deacetylases. Crebinostat is a histone deacetylase inhibitor. Here we show that in an object recognition task, crebinostat facilitates memory formation by a weak training. Further, this compound enhances acetylation of α-tubulin, and reduces the level of histone deacetylase 6, an α-tubulin deacetylase. The results suggest that enhanced acetylation of α-tubulin by crebinostat contributes to its facilitatory effect on memory formation.

Sulfotransferase activity contributes to long-term potentiation and long-term memory

A critical role of protein modifications such as phosphorylation and acetylation in synaptic plasticity and memory is well documented. Tyrosine sulfation plays important roles in several biological processes. However, its role in synaptic plasticity and memory is not well understood. Here, we show that sulfation contributes to long-term potentiation (LTP) in the hippocampal slices. In addition, inhibition of sulfation impairs long-term memory in a spatial memory task without affecting acquisition or short-term memory. Furthermore, LTP-inducing stimulus enhances protein tyrosine sulfation. These results suggest an important role for tyrosine sulfation in LTP and memory.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Potassium Titanate Assembled Titanium Dioxide Nanotube Arrays Endow Titanium Implants Excellent Osseointegration Performance and Nerve Formation Potential

Titanium based materials have been widely applied in bone-tissue engineering. However, inefficient bone repair remains to be solved due to the lack of neural network reconstruction at the bone-implant interface. Herein, we propose a functional surface modification approach to promote neurogenesis. Using an electrochemical technique and a hydrothermal approach, a potassium titanate nanorod-decorated titanium oxide (K₂Ti₆O₁₃-TiO₂) nanotube array is constructed on the surface of titanium implants. The K₂Ti₆O₁₃-TiO₂ hybrid nanotube array on titanium implants can enhance the osteogenic differentiation of mesenchymal stem cells due to the special nanostructures of titanium oxide nanorods. Meanwhile, the release of potassium ions is able to accelerate the neural differentiation of neural stem cells. This study provides a new approach to promote neuralization on the surface of implants, which is promising for future applications in constructing a fully functional interface in bone repair.

Activity- and memory training-induced acetylation of α-tubulin in the hippocampus

Posttranslational modifications play crucial roles in synaptic plasticity and memory formation. The important role of histone acetylation is well established in these processes. However, activity-dependent regulation of acetylation of non-histone proteins is not well understood. We previously showed that α-tubulin is acetylated in an activity-dependent manner. Here, we show that cyclin-dependent kinase 5 (CDK5) plays an important role in α-tubulin acetylation induced by KCl depolarization or N-methyl-D-aspartate stimulation of the hippocampal slices. In addition, KCl depolarization inhibits the activity of SIRT2, an α-tubulin deacetylase. The inhibitory effect of KCl on SIRT2 activity requires CDK5 activity. Furthermore, α-tubulin acetylation is enhanced by memory training in object recognition task. These results suggest that memory formation may involve α-tubulin acetylation.

Specific ion channels contribute to key elements of pathology during secondary degeneration following neurotrauma

BACKGROUND

Following partial injury to the central nervous system, cells beyond the initial injury site undergo secondary degeneration, exacerbating loss of neurons, compact myelin and function. Changes in Ca²⁺ flux are associated with metabolic and structural changes, but it is not yet clear how flux through specific ion channels contributes to the various pathologies. Here, partial optic nerve transection in adult female rats was used to model secondary degeneration. Treatment with combinations of three ion channel inhibitors was used as a tool to investigate which elements of oxidative and structural damage related to long term functional outcomes. The inhibitors employed were the voltage gated Ca²⁺ channel inhibitor Lomerizine (Lom), the Ca²⁺ permeable AMPA receptor inhibitor YM872 and the P2X₇ receptor inhibitor oxATP.

RESULTS

Following partial optic nerve transection, hyper-phosphorylation of Tau and acetylated tubulin immunoreactivity were increased, and Nogo-A immunoreactivity was decreased, indicating that axonal changes occurred acutely. All combinations of ion channel inhibitors reduced hyper-phosphorylation of Tau and increased Nogo-A immunoreactivity at day 3 after injury. However, only Lom/oxATP or all three inhibitors in combination significantly reduced acetylated tubulin immunoreactivity. Most combinations of ion channel inhibitors were effective in restoring the lengths of the paranode and the paranodal gap, indicative of the length of the node of Ranvier, following injury. However, only all three inhibitors in combination restored to normal Ankyrin G length at the node of Ranvier. Similarly, HNE immunoreactivity and loss of oligodendrocyte precursor cells were only limited by treatment with all three ion channel inhibitors in combination.

CONCLUSIONS

Data indicate that inhibiting any of a range of ion channels preserves certain elements of axon and node structure and limits some oxidative damage following injury, whereas ionic flux through all three channels must be inhibited to prevent lipid peroxidation and preserve Ankyrin G distribution and OPCs.

Axonal transport deficits in multiple sclerosis: spiraling into the abyss

The transport of mitochondria and other cellular components along the axonal microtubule cytoskeleton plays an essential role in neuronal survival. Defects in this system have been linked to a large number of neurological disorders. In multiple sclerosis (MS) and associated models such as experimental autoimmune encephalomyelitis (EAE), alterations in axonal transport have been shown to exist before neurodegeneration occurs. Genome-wide association (GWA) studies have linked several motor proteins to MS susceptibility, while neuropathological studies have shown accumulations of proteins and organelles suggestive for transport deficits. A reduced effectiveness of axonal transport can lead to neurodegeneration through inhibition of mitochondrial motility, disruption of axoglial interaction or prevention of remyelination. In MS, demyelination leads to dysregulation of axonal transport, aggravated by the effects of TNF-alpha, nitric oxide and glutamate on the cytoskeleton. The combined effect of all these pathways is a vicious cycle in which a defective axonal transport system leads to an increase in ATP consumption through loss of membrane organization and a reduction in available ATP through inhibition of mitochondrial transport, resulting in even further inhibition of transport. The persistent activity of this positive feedback loop contributes to neurodegeneration in MS.

The growing landscape of tubulin acetylation: lysine 40 and many more

Tubulin heterodimers are the building block of microtubules, which are major elements of the cytoskeleton. Several types of post-translational modifications are found on tubulin subunits as well as on the microtubule polymer to regulate the multiple roles of microtubules. Acetylation of lysine 40 (K40) of the α-tubulin subunit is one of these post-translational modifications which has been extensively studied. We summarize the current knowledge about the structural aspects of K40 acetylation, the functional consequences, the enzymes involved and their regulation. Most importantly, we discuss the potential importance of the recently discovered additional acetylation acceptor lysines in tubulin subunits and highlight the urgent need to study tubulin acetylation in a more integrated perspective.

Tubulin acetylation: responsible enzymes, biological functions and human diseases

Microtubules have important functions ranging from maintenance of cell morphology to subcellular transport, cellular signaling, cell migration, and formation of cell polarity. At the organismal level, microtubules are crucial for various biological processes, such as viral entry, inflammation, immunity, learning and memory in mammals. Microtubules are subject to various covalent modifications. One such modification is tubulin acetylation, which is associated with stable microtubules and conserved from protists to humans. In the past three decades, this reversible modification has been studied extensively. In mammals, its level is mainly governed by opposing actions of α-tubulin acetyltransferase 1 (ATAT1) and histone deacetylase 6 (HDAC6). Knockout studies of the mouse enzymes have yielded new insights into biological functions of tubulin acetylation. Abnormal levels of this modification are linked to neurological disorders, cancer, heart diseases and other pathological conditions, thereby yielding important therapeutic implications. This review summarizes related studies and concludes that tubulin acetylation is important for regulating microtubule architecture and maintaining microtubule integrity. Together with detyrosination, glutamylation and other modifications, tubulin acetylation may form a unique 'language' to regulate microtubule structure and function.

Deceivingly dynamic: Learning-dependent changes in stathmin and microtubules

Microtubules, one of the major cytoskeletal structures, were previously considered stable and only indirectly involved in synaptic structure and function in mature neurons. However, recent evidence demonstrates that microtubules are dynamic and have an important role in synaptic structure, synaptic plasticity, and memory. In particular, learning induces changes in microtubule turnover and stability, and pharmacological manipulation of microtubule dynamics alters synaptic plasticity and long-term memory. These learning-induced changes in microtubules are controlled by the phosphoprotein stathmin, whose only known cellular activity is to negatively regulate microtubule formation. During the first eight hours following learning, changes in the phosphorylation of stathmin go through two phases causing biphasic shifts in microtubules stability/instability. These shifts, in turn, regulate memory formation by controlling in the second phase synaptic transport of the GluA2 subunit of AMPA receptors. Improper regulation of stathmin and microtubule dynamics has been observed in aged animals and in patients with Alzheimer's disease and depression. Thus, recent work on stathmin and microtubules has identified new molecular players in the early stages of memory encoding.

Neuronal protein trafficking: emerging consequences of endoplasmic reticulum dynamics

The highly polarized morphology and complex geometry of neurons is determined to a great extent by the structural and functional organization of the secretory pathway. It is intuitive to propose that the spatial arrangement of secretory organelles and their dynamic behavior impinge on protein trafficking and neuronal function, but these phenomena and their consequences are not well delineated. Here we analyze the architecture and motility of the archetypal endoplasmic reticulum (ER), and their relationship to the microtubule cytoskeleton and post-translational modifications of tubulin. We also review the dynamics of the ER in axons, dendrites and spines, and discuss the role of ER dynamics on protein mobility and trafficking in neurons.