Post-Translational Incorporation of L-Phenylalanine into the C-Terminus of α-Tubulin as a Possible Cause of Neuronal Dysfunction

Post-translational Modifications of the Protein Termini

Post-translational modifications (PTM) involve enzyme-mediated covalent addition of functional groups to proteins during or after synthesis. These modifications greatly increase biological complexity and are responsible for orders of magnitude change between the variety of proteins encoded in the genome and the variety of their biological functions. Many of these modifications occur at the protein termini, which contain reactive amino- and carboxy-groups of the polypeptide chain and often are pre-primed through the actions of cellular machinery to expose highly reactive residues. Such modifications have been known for decades, but only a few of them have been functionally characterized. The vast majority of eukaryotic proteins are N- and C-terminally modified by acetylation, arginylation, tyrosination, lipidation, and many others. Post-translational modifications of the protein termini have been linked to different normal and disease-related processes and constitute a rapidly emerging area of biological regulation. Here we highlight recent progress in our understanding of post-translational modifications of the protein termini and outline the role that these modifications play in vivo.

Irreversible incorporation of L-dopa into the C-terminus of α-tubulin inhibits binding of molecular motor KIF5B to microtubules and alters mitochondrial traffic along the axon

L-dopa is the most effective drug used to date for management of Parkinson's disease symptoms. Unfortunately, long-term administration of L-dopa often results in development of motor disorders, including dyskinesias. Despite extensive research on L-dopa-induced dyskinesia, its pathogenesis remains poorly understood. We demonstrated previously that L-dopa can be post-translationally incorporated into the C-terminus of α-tubulin in living cells. In the present study, we investigated the effect of the presence of L-dopa-tubulin-enriched microtubules on mitochondrial traffic mediated by molecular motor KIF5B. Using biochemical approaches in combination with experiments on neuronal cell lines and mouse hippocampal primary cultures, we demonstrated that L-dopa incorporation into tubulin is irreversible. Transport of mitochondria along the axon was altered after L-dopa treatment of cells. In L-dopa-treated cells, mitochondria had reduced ability to reach the distal segment of the axon, spent more time in pause, and showed reduced velocity of anterograde movement. KIF5B motor, a member of the kinesin family involved in mitochondrial transport in neurons, showed reduced affinity for Dopa-tubulin-containing microtubules. Our findings, taken together, suggest that tyrosination state of tubulin (and microtubules) is altered by L-dopa incorporation into tubulin; the gradual increase in amount of altered microtubules affects microtubule functioning, impairs mitochondrial traffic and distribution, and this could be relevant in Parkinson's disease patients chronically treated with L-dopa.

Tubulin post-translational modifications control neuronal development and functions

Microtubules (MTs) are an essential component of the neuronal cytoskeleton; they are involved in various aspects of neuron development, maintenance, and functions including polarization, synaptic plasticity, and transport. Neuronal MTs are highly heterogeneous due to the presence of multiple tubulin isotypes and extensive post‐translational modifications (PTMs). These PTMs—most notably detyrosination, acetylation, and polyglutamylation—have emerged as important regulators of the neuronal microtubule cytoskeleton. With this review, we summarize what is currently known about the impact of tubulin PTMs on microtubule dynamics, neuronal differentiation, plasticity, and transport as well as on brain function in normal and pathological conditions, in particular during neuro‐degeneration. The main therapeutic approaches to neuro‐diseases based on the modulation of tubulin PTMs are also summarized. Overall, the review indicates how tubulin PTMs can generate a large number of functionally specialized microtubule sub‐networks, each of which is crucial to specific neuronal features.

Tuba8 Drives Differentiation of Cortical Radial Glia into Apical Intermediate Progenitors by Tuning Modifications of Tubulin C Termini

Most adult neurons and glia originate from radial glial progenitors (RGs), a type of stem cell typically extending from the apical to the basal side of the developing cortex. Precise regulation of the choice between RG self-renewal and differentiation is critical for normal development, but the mechanisms underlying this transition remain elusive. We show that the non-canonical tubulin Tuba8, transiently expressed in cortical progenitors, drives differentiation of RGs into apical intermediate progenitors, a more restricted progenitor type lacking attachment to the basal lamina. This effect depends on the unique C-terminal sequence of Tuba8 that antagonizes tubulin tyrosination and Δ2 cleavage, two post-translational modifications (PTMs) essential for RG fiber maintenance and the switch between direct and indirect neurogenesis and ultimately distinct neuronal lineage outcomes. Our work uncovers an instructive role of a developmentally regulated tubulin isotype in progenitor differentiation and provides new insights into biological functions of the cellular tubulin PTM "code."

Vasohibins/SVBP are tubulin carboxypeptidases (TCPs) that regulate neuron differentiation

Reversible detyrosination of α-tubulin is crucial to microtubule dynamics and functions, and defects have been implicated in cancer, brain disorganization, and cardiomyopathies. The identity of the tubulin tyrosine carboxypeptidase (TCP) responsible for detyrosination has remained unclear. We used chemical proteomics with a potent irreversible inhibitor to show that the major brain TCP is a complex of vasohibin-1 (VASH1) with the small vasohibin binding protein (SVBP). VASH1 and its homolog VASH2, when complexed with SVBP, exhibited robust and specific Tyr/Phe carboxypeptidase activity on microtubules. Knockdown of vasohibins or SVBP and/or inhibitor addition in cultured neurons reduced detyrosinated α-tubulin levels and caused severe differentiation defects. Furthermore, knockdown of vasohibins disrupted neuronal migration in developing mouse neocortex. Thus, vasohibin/SVBP complexes represent long-sought TCP enzymes.