Direct and indirect effects of tubulin post-translational modifications on microtubule stability: Insights and regulations

Eml1 promotes axonal growth by enhancing αTAT1-mediated microtubule acetylation

Microtubule stabilization is critical for axonal growth and regeneration, and many microtubule-associated proteins are involved in this process. In this study, we found that the knockdown of echinoderm microtubule-associated protein-like 1 (EML1) hindered axonal growth in cultured cortical and dorsal root ganglion neurons. We further revealed that EML1 facilitated the acetylation of microtubules and that the impairment of axonal growth due to EML1 inhibition could be restored by treatment with deacetylase inhibitors, suggesting that EML1 affected tubulin acetylation. Moreover, we verified an interaction between EML1 and the alpha-tubulin acetyltransferase 1, which is responsible for the acetylation of alpha-tubulin. We thus proposed that EML1 might regulate microtubule acetylation and stabilization via alpha-tubulin acetyltransferase 1 and then promote axon growth. Finally, we verified that the knockdown of EML1 in vivo also inhibited sciatic nerve regeneration. Our findings revealed a novel effect of EML1 on microtubule acetylation during axonal regeneration.

The α-tubulin acetyltransferase ATAT1: structure, cellular functions, and its emerging role in human diseases

The acetylation of α-tubulin on lysine 40 is a well-studied post-translational modification which has been associated with the presence of long-lived stable microtubules that are more resistant to mechanical breakdown. The discovery of α-tubulin acetyltransferase 1 (ATAT1), the enzyme responsible for lysine 40 acetylation on α-tubulin in a wide range of species, including protists, nematodes, and mammals, dates to about a decade ago. However, the role of ATAT1 in different cellular activities and molecular pathways has been only recently disclosed. This review comprehensively summarizes the most recent knowledge on ATAT1 structure and substrate binding and analyses the involvement of ATAT1 in a variety of cellular processes such as cell motility, mitosis, cytoskeletal organization, and intracellular trafficking. Finally, the review highlights ATAT1 emerging roles in human diseases and discusses ATAT1 potential enzymatic and non-enzymatic roles and the current efforts in developing ATAT1 inhibitors.

Effectiveness of fixation methods for wholemount immunohistochemistry across cellular compartments in chick embryos

The choice of fixation method significantly impacts tissue morphology and protein visualization after immunohistochemistry (IHC). In this study, we compared the effects of paraformaldehyde (PFA) and trichloroacetic acid (TCA) fixation prior to IHC on chicken embryos. Our findings underscore the importance of validating fixation methods for accurate interpretation of IHC results, with implications for antibody validation and tissue-specific protein localization studies. We found that TCA fixation resulted in larger and more circular nuclei compared to PFA fixation. Additionally, TCA fixation altered the appearance of subcellular localization and fluorescence intensity of various proteins, including transcription factors and cytoskeletal proteins. Notably, TCA fixation revealed protein localization domains that may be inaccessible with PFA fixation. These results highlight the need for optimization of fixation protocols depending on the target epitope and model system, emphasizing the importance of methodological considerations in biological analyses.

A cytoskeleton symphony: Actin and microtubules in microglia dynamics and aging

Microglia dynamically reorganize their cytoskeleton to perform essential functions such as phagocytosis of toxic protein aggregates, surveillance of the brain parenchyma, and regulation of synaptic plasticity during neuronal activity bursts. Recent studies have shed light on the critical role of the microtubule cytoskeleton in microglial reactivity and function, revealing key regulators like cyclin-dependent kinase 1 and centrosomal nucleation in the remodeling of microtubules in activated microglia. Concurrently, the role of the actin cytoskeleton is also pivotal, particularly in the context of small GTPases like RhoA, Rac1, and Cdc42 and actin-binding molecules such as profilin-1 and cofilin. This article delves into the intricate molecular landscape of actin and microtubules, exploring their synergistic roles in driving microglial cytoskeletal dynamics. We propose a more integrated view of actin and microtubule cooperation, which is fundamental to understanding the functional coherence of the microglial cytoskeleton and its pivotal role in propelling brain homeostasis. Furthermore, we discuss how alterations in microglial cytoskeleton dynamics during aging and in disease states could have far-reaching implications for brain function. By unraveling the complexities of microglia cytoskeletal dynamics, we can deepen our understanding of microglial functional states and their implications in health and disease, offering insights into potential therapeutic interventions for neurologic disorders.

Microtubules as a signal hub for axon growth in response to mechanical force

Microtubules are highly polar structures and are characterized by high anisotropy and stiffness. In neurons, they play a key role in the directional transport of vesicles and organelles. In the neuronal projections called axons, they form parallel bundles, mostly oriented with the plus-end towards the axonal termination. Their physico-chemical properties have recently attracted attention as a potential candidate in sensing, processing and transducing physical signals generated by mechanical forces. Here, we discuss the main evidence supporting the role of microtubules as a signal hub for axon growth in response to a traction force. Applying a tension to the axon appears to stabilize the microtubules, which, in turn, coordinate a modulation of axonal transport, local translation and their cross-talk. We speculate on the possible mechanisms modulating microtubule dynamics under tension, based on evidence collected in neuronal and non-neuronal cell types. However, the fundamental question of the causal relationship between these mechanisms is still elusive because the mechano-sensitive element in this chain has not yet been identified.

Polarized microtubule remodeling transforms the morphology of reactive microglia and drives cytokine release

Microglial reactivity is a pathological hallmark in many neurodegenerative diseases. During stimulation, microglia undergo complex morphological changes, including loss of their characteristic ramified morphology, which is routinely used to detect and quantify inflammation in the brain. However, the underlying molecular mechanisms and the relation between microglial morphology and their pathophysiological function are unknown. Here, proteomic profiling of lipopolysaccharide (LPS)-reactive microglia identifies microtubule remodeling pathways as an early factor that drives the morphological change and subsequently controls cytokine responses. We find that LPS-reactive microglia reorganize their microtubules to form a stable and centrosomally-anchored array to facilitate efficient cytokine trafficking and release. We identify cyclin-dependent kinase 1 (Cdk-1) as a critical upstream regulator of microtubule remodeling and morphological change in-vitro and in-situ. Cdk-1 inhibition also rescues tau and amyloid fibril-induced morphology changes. These results demonstrate a critical role for microtubule dynamics and reorganization in microglial reactivity and modulating cytokine-mediated inflammatory responses.

NAP1L1 is a novel microtubule-associated protein

Microtubule-associated proteins (MAPs) regulate assembly and stability of microtubules (MTs) during cell cytokinesis, cell migration, neuronal growth, axon guidance, and synapse formation. Using data mining of the Human Protein Atlas database and experimental screening, we identified nucleosome assembly protein 1 like 1 (NAP1L1) as a new MAP. The Human Protein Atlas and PubMed database screening identified 99 potential new MAPs. Twenty candidate proteins that highly co-localized with MTs were exogenously expressed with green fluorescent protein (GFP) or hemagglutinin (HA) tags in tissue culture cells and MTs were co-stained for immunofluorescent microscopy. We found that NAP1L1 is mainly localized in the cytosol with MTs during interphase. Using bacterially expressed recombinant NAP1L1 fragments and purified MTs, we biochemically mapped the MT-binding site on the N-terminal region (1-72aa) and the central region (164-269aa) of NAP1L1. NAP1L1 dimerizes through the long helix region (73-163aa), and full-length NAP1L1 induces the formation of thick MTs, indicating that NAP1L1 has the ability to bundle MTs in cells. Analysis of publicly available RNA-seq data of NAP1L1 depleted cells suggested that NAP1L1 is involved in cell adhesion and migration in agreement with the function of NAP1L1 as a MAP.

In vivo impact of tubulin polymerization promoting protein (Tppp) knockout to the airway inflammatory response

Microtubule dysfunction has been implicated as a mediator of inflammation in multiple diseases such as disorders of the cardiovascular and neurologic systems. Tubulin polymerization promoting protein (Tppp) facilitates microtubule elongation and regulates tubulin acetylation through inhibition of cytosolic deacetylase enzymes. Pathologic alterations in microtubule structure and dynamics have been described in cystic fibrosis (CF) and associated with inflammation, however the causality and mechanism remain unclear. Likewise, Tppp has been identified as a potential modifier of CF airway disease severity. Here we directly assess the impact of microtubule dysfunction on infection and inflammation by interrogating wild type and a Tppp knockout mouse model (Tppp - / -). Mice are challenged with a clinical isolate of Pseudomonas aeruginosa-laden agarose beads and assessed for bacterial clearance and inflammatory markers. Tppp - / - mouse model demonstrate impaired bacterial clearance and an elevated inflammatory response compared to control mice. These data are consistent with the hypothesis microtubule dysregulation is sufficient to lead to CF-like airway responses in mice.

Deciphering the Tubulin Language: Molecular Determinants and Readout Mechanisms of the Tubulin Code in Neurons

Microtubules (MTs) are dynamic components of the cell cytoskeleton involved in several cellular functions, such as structural support, migration and intracellular trafficking. Despite their high similarity, MTs have functional heterogeneity that is generated by the incorporation into the MT lattice of different tubulin gene products and by their post-translational modifications (PTMs). Such regulations, besides modulating the tubulin composition of MTs, create on their surface a "biochemical code" that is translated, through the action of protein effectors, into specific MT-based functions. This code, known as "tubulin code", plays an important role in neuronal cells, whose highly specialized morphologies and activities depend on the correct functioning of the MT cytoskeleton and on its interplay with a myriad of MT-interacting proteins. In recent years, a growing number of mutations in genes encoding for tubulins, MT-interacting proteins and enzymes that post-translationally modify MTs, which are the main players of the tubulin code, have been linked to neurodegenerative processes or abnormalities in neural migration, differentiation and connectivity. Nevertheless, the exact molecular mechanisms through which the cell writes and, downstream, MT-interacting proteins decipher the tubulin code are still largely uncharted. The purpose of this review is to describe the molecular determinants and the readout mechanisms of the tubulin code, and briefly elucidate how they coordinate MT behavior during critical neuronal events, such as neuron migration, maturation and axonal transport.

βIII-Tubulin Gene Regulation in Health and Disease

Microtubule proteins form a dynamic component of the cytoskeleton, and play key roles in cellular processes, such as vesicular transport, cell motility and mitosis. Expression of microtubule proteins are often dysregulated in cancer. In particular, the microtubule protein βIII-tubulin, encoded by the TUBB3 gene, is aberrantly expressed in a range of epithelial tumours and is associated with drug resistance and aggressive disease. In normal cells, TUBB3 expression is tightly restricted, and is found almost exclusively in neuronal and testicular tissues. Understanding the mechanisms that control TUBB3 expression, both in cancer, mature and developing tissues will help to unravel the basic biology of the protein, its role in cancer, and may ultimately lead to the development of new therapeutic approaches to target this protein. This review is devoted to the transcriptional and posttranscriptional regulation of TUBB3 in normal and cancerous tissue.

Ubiquitin Proteasome System and Microtubules Are Master Regulators of Central and Peripheral Nervous System Axon Degeneration

Axonal degeneration is an active process that differs from neuronal death, and it is the hallmark of many disorders affecting the central and peripheral nervous system. Starting from the analyses of Wallerian degeneration, the simplest experimental model, here we describe how the long projecting neuronal populations affected in Parkinson’s disease and chemotherapy-induced peripheral neuropathies share commonalities in the mechanisms and molecular players driving the earliest phase of axon degeneration. Indeed, both dopaminergic and sensory neurons are particularly susceptible to alterations of microtubules and axonal transport as well as to dysfunctions of the ubiquitin proteasome system and protein quality control. Finally, we report an updated review on current knowledge of key molecules able to modulate these targets, blocking the on-going axonal degeneration and inducing neuronal regeneration. These molecules might represent good candidates for disease-modifying treatment, which might expand the window of intervention improving patients’ quality of life.