Microtubule-associated protein 1B (MAP1B)-deficient neurons show structural presynaptic deficiencies in vitro and altered presynaptic physiology.

Felipe J Bodaleo,Felipe A Court,Daniel R Henríquez,Christian Gonzalez-Billault,Carolina Montenegro-Venegas

doi:10.1038/srep30069

Abstract

Microtubule-associated protein 1B (MAP1B) is expressed predominantly during the early stages of development of the nervous system, where it regulates processes such as axonal guidance and elongation. Nevertheless, MAP1B expression in the brain persists in adult stages, where it participates in the regulation of the structure and physiology of dendritic spines in glutamatergic synapses. Moreover, MAP1B expression is also found in presynaptic synaptosomal preparations. In this work, we describe a presynaptic phenotype in mature neurons derived from MAP1B knockout (MAP1B KO) mice. Mature neurons express MAP1B, and its deficiency does not alter the expression levels of a subgroup of other synaptic proteins. MAP1B KO neurons display a decrease in the density of presynaptic and postsynaptic terminals, which involves a reduction in the density of synaptic contacts, and an increased proportion of orphan presynaptic terminals. Accordingly, MAP1B KO neurons present altered synaptic vesicle fusion events, as shown by FM4-64 release assay, and a decrease in the density of both synaptic vesicles and dense core vesicles at presynaptic terminals. Finally, an increased proportion of excitatory immature symmetrical synaptic contacts in MAP1B KO neurons was detected. Altogether these results suggest a novel role for MAP1B in presynaptic structure and physiology regulation in vitro.

Highlights

The proper function of the central nervous system (CNS) relies on the establishment of a neuronal network that conveys information through synaptic contacts
We determined that Microtubule-associated protein 1B (MAP1B) is expressed in mature WT hippocampal neurons in thin prolongations that correspond to axons and in the thick processes corresponding to dendrites, which are innervated by presynaptic terminals positive for Synaptophysin I (Synph I) (Fig. 1A)
While originally described as a protein involved in nervous system development, it has become apparent that MAP1B expression in adult brain is relevant for neuronal functions[21,22,56]

Summary

Introduction

The proper function of the central nervous system (CNS) relies on the establishment of a neuronal network that conveys information through synaptic contacts. Ultrastructure analyses show that microtubules (MT) are detected at presynaptic terminals[4,5,6,7] These findings raise the possibility that cytoskeleton associated proteins with the ability to interact with the actin and MT cytoskeleton could regulate synaptic structure and physiology. MAP1B is found in postsynaptic densities of the cerebellar cortex[23] and co-distributes with a subset of F-actin positive dendritic spines present in mature neurons[24]. Futsch regulates the density of presynaptic active zones and glutamate release, independent of its microtubule stabilizing properties[33] With these studies in mind, the aim of this work was to analyze the presynaptic phenotype of mature hippocampal neurons derived from wild-type (WT) and MAP1B knockout (MAP1B KO) mice. Our results support the idea that MAP1B-dependent synaptic regulation is not exclusively postsynaptic, and affects the presynaptic terminals

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