OCT 19 | 9:00 - 11:00

Cytoskeleton, its alterations, and neuronal defects

Gaston Bisig

CIQUIBIC-DQBRC. Fac. Cs Químicas. UNCba

New discoveries in cytoskeletons components and interactions, implication of tubulin c-terminus in neuronal alterations relevant in neuropathologies.

Tau, a key molecule in neurodegeneration from cytoskeleton to microtubule-non related functions.

Alejandra Alonso

City University of New York, CSI

Tau, a neuronal microtubule associated protein, plays a key role in cognitive processes. Deposits of abnormal forms of tau are associated with several neurodegenerative diseases, including Alzheimer disease (AD), the most prevalent, and Chronic Traumatic Encephalopathy (CTE), the most recently associated to abnormal tau. Tau post-translational modifications (PTMs) are responsible for its gain of toxic function. We were the first to show that the pathological tau isolated from AD brains has prion-like properties and can transfer its toxic function to the normal molecule. Furthermore, we reported that the pathological changes are associated with tau phosphorylation at Ser199 and 262 and Thr212 and 231. We have generated a transgenic mouse model that expresses pathological human tau (PH-Tau) in neurons at two different concentrations (4% and 14% of the total endogenous tau). Expression of PH-Tau causes cognitive decline by at least two different mechanisms: one that involves the cytoskeleton with axonal disruption (at high concentration), and another in which the apparent neuronal morphology is not grossly affected, but the synaptic terminals are altered (at lower concentration). We have evidence that tau interacts with proteins involved in the in the processing of mRNA, suggesting that the changes in tau might be involved in changes of proteostasis. Understanding tau’s biological activity on and off the microtubules will help shed light to the mechanism of neurodegeneration.

Implication of α-tubulin tyrosination/detyrosination cycle in synaptic activity and degeneration

Leticia Peris

U. Grenoble

Microtubules (MTs) are essential for neuronal morphogenesis and synaptic activity. While dynamic MTs are mainly composed of tyrosinated tubulin, long-lived MTs contain detyrosinated tubulin, suggesting that the tubulin tyrosination/detyrosination (Tyr/deTyr) cycle is a key player in the maintenance of MT dynamics and neuronal homeostasis, conditions which go awry in neurodegeneration. In the Tyr/deTyr cycle, the C-terminal tyrosine of α-tubulin is removed by tubulin carboxypeptidase complex composed of Vasohibin (VASH 1 and 2) and Small Vasohibin Binding Protein (SVBP) and re-added by tubulin-tyrosine-ligase (TTL). Reduced TTL expression induced a decreased tyrosinated dynamic MTs, reduced dendritic spine density, and defective synaptic plasticity and memory. TTL reduction and modified tubulin accumulation is also a feature of Alzheimer’s disease (AD), a neurodegenerative disorder characterized by progressive memory loss, amyloid accumulation and cognitive impairment. At neuronal level, the synapses visited by dynamic MTs are more resistant to amyloid toxicity and that expression of TTL, by restoring MT entry into spines, suppresses synapse loss induced by amyloid exposure. Our results demonstrate that a balanced Tyr/deTyr cycle is necessary for the maintenance of synaptic plasticity, is protective against amyloid-induced synaptic damage, and that this balance is lost in AD, providing evidence that defective tubulin re-tyrosination may contribute to circuit dysfunction in AD

In situ organization of the actin/spectrin membrane-associated periodic skeleton of axons with nanometer resolution

Nicolás Unsain

Laboratorio de Neurobiología, Instituto de Investigación Médica Mercedes y Martín Ferreyra, INIMEC-CONICET, Universidad Nacional de Córdoba (UNC)

Recently, actin, spectrin and associated proteins have been discovered to form a membrane-associated periodic skeleton (MPS) that is ubiquitously present in mature axons of all neuronal types evaluated up to the moment. MPS is a periodic protein structure consisting of actin "rings" located transversely to the axon and separated every 190 nm by α / β-spectrin "spacers" extended along the axon. Since its discovery, the characterization of MPS has been performed almost exclusively in cultured neurons; namely artificial environments in two dimensions. Hence, we proposed to study the spatial organization and biology of these structures in their “natural” environment, that is, in situ. Moreover, it is still not clear how spectrin tetramers are organized in each segment of this periodic scaffold. Taking all this into account, we have begun to analyze the transversal organization of spectrin tetramers in the MPS of axons within mouse nerves, namely optic and sciatic nerves. The MPS cannot be evidenced by conventional fluorescence microscopy, since its periodicity (~190 nm) is below the resolution limit (~250nm). To reach the needed resolution in tissue, we are combining 3D-STOchastic Reconstruction Microscopy (STORM) and tissue Expansion Microscopy (ExM) to gain both resolution and transparency. I am going to present data that allow us to identify common and distinct organization rules of the spectrin tetramers within the MPS in nerve tissue.

α-tubulin c-terminal modification: the importance of being tyrosinated.

Gaston Bisig

CIQUIBIC-DQBRC. Fac. Cs Químicas. UNCba

The C-terminal tyrosine (Tyr) of the α-tubulin chain is subjected to post-translational removal and readdition in a process termed the “detyrosination/tyrosination cycle”. L-Dopa and L-Phenylalanine (Phe) can be incorporated into tubulin in place of tyrosine. We described that the presence of L-Dopa in tubulin affects microtubules dynamics, mitochondrial traffic, and KIF5B affinity for Dopa-tubulin-containing microtubules. These findings could be relevant for the neuronal defects observed in Parkinson’s disease patients treated with L-dopa for long periods of time. When Phe was the analogue incorporated into tubulin, we observed that not only microtubules dynamics and mitochondrial traffic were altered, but in this case neurites retraction and cell proliferation were also affected. Since it is known that Phe is increased in phenylketonuria, it is conceivable the possibility that Phe incorporation into tubulin is the first event (or among the initial events) in the molecular pathways leading to brain dysfunctions that characterize this neurological disorder. We found that, even though the analogues of Tyr incorporated into tubulin are different, they similarly affect neuronal microtubules functions, and this raises the question whether these alterations are caused by the analogue presence by itself or by disbalancing the detyrosinated/tyrosinated tubulin content.