Dynamic Interaction Between Microtubules and the Nucleus Regulates Nuclear Movement During Neuronal Migration

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.

Microcephaly gene Cenpj regulates axonal growth in cortical neurons through microtubule destabilization

Neocortex development comprises of a complex series of time- and space-specific processes to generate the typical interconnected six-layered architecture of adult mammals. Axon growth is required for the proper establishment of cortical circuits. Malformations in axonal growth and pathfinding might lead to severe neuropathologies, such as the Corpus Callosum dysgenesis. Cenpj, a microcephaly gene, encodes a scaffold protein that regulates centrosome biogenesis and microtubule stabilization. During cortical development, Cenpj regulates progenitor division and neuronal migration during corticogenesis. Since microtubule stabilization is crucial for axon extension, we investigated the role of Cenpj in axon extension during cortical development in a mouse model. Using performed loss- and gain-of-function assays ex vivo and in utero, we quantified callosal axonal length, branching and growth cone size compared to controls. We observed that silencing Cenpj results in an increased axonal length. Ex vivo, we assessed the number of branches, the area of growth cones and the stability of microtubules. In silenced Cenpj axons, there were more branches, larger growth cone area and more stable microtubules. Rescue experiments confirmed that neurons present axonal length comparable to controls. Here we propose that Cenpj regulates axon growth by destabilizing microtubules during cortical development. Finally, our findings suggest that Cenpj might be a novel target for axonal regeneration.

Mechanical Regulation of Nuclear Translocation in Migratory Neurons

Neuronal migration is a critical step during the formation of functional neural circuits in the brain. Newborn neurons need to move across long distances from the germinal zone to their individual sites of function; during their migration, they must often squeeze their large, stiff nuclei, against strong mechanical stresses, through narrow spaces in developing brain tissue. Recent studies have clarified how actomyosin and microtubule motors generate mechanical forces in specific subcellular compartments and synergistically drive nuclear translocation in neurons. On the other hand, the mechanical properties of the surrounding tissues also contribute to their function as an adhesive support for cytoskeletal force transmission, while they also serve as a physical barrier to nuclear translocation. In this review, we discuss recent studies on nuclear migration in developing neurons, from both cell and mechanobiological viewpoints.

Cytoskeletal control of nuclear migration in neurons and non-neuronal cells

Cell migration is a complex molecular event that requires translocation of a large, stiff nucleus, oftentimes through interstitial pores of submicron size in tissues. Remarkable progress in the past decade has uncovered an ever-increasing array of diverse nuclear dynamics and underlying cytoskeletal control in various cell models. In many cases, the microtubule motors dynein and kinesin directly interact with the nucleus via the LINC complex and steer directional nuclear movement, while actomyosin contractility and its global flow exert forces to deform and move the nucleus. In this review, I focus on the synergistic interplay of the cytoskeletal motors and spatiotemporal sites of force transmission in various nuclear migration models, with a special focus on neuronal migration in the vertebrate brain.