Size, Shape, and Distribution of Multivesicular Bodies in the Juvenile Rat Somatosensory Cortex: A 3D Electron Microscopy Study.

M Turegano-Lopez,A Merchan-Perez,A Santuy,J Defelipe

doi:10.1093/cercor/bhz211

Abstract

Multivesicular bodies (MVBs) are membrane-bound organelles that belong to the endosomal pathway. They participate in the transport, sorting, storage, recycling, degradation, and release of multiple substances. They interchange cargo with other organelles and participate in their renovation and degradation. We have used focused ion beam milling and scanning electron microscopy (FIB-SEM) to obtain stacks of serial sections from the neuropil of the somatosensory cortex of the juvenile rat. Using dedicated software, we have 3D-reconstructed 1618 MVBs. The mean density of MVBs was 0.21 per cubic micron. They were unequally distributed between dendrites (39.14%), axons (18.16%), and nonsynaptic cell processes (42.70%). About one out of five MVBs (18.16%) were docked on mitochondria, representing the process by which the endosomal pathway participates in mitochondrial maintenance. Other features of MVBs, such as the presence of tubular protrusions (6.66%) or clathrin coats (19.74%) can also be interpreted in functional terms, since both are typical of early endosomes. The sizes of MVBs follow a lognormal distribution, with differences across cortical layers and cellular compartments. The mean volume of dendritic MVBs is more than twice as large as the volume of axonic MVBs. In layer I, they are smaller, on average, than in the other layers.

Highlights

Multivesicular bodies (MVBs) are membrane-bound organelles that contain intraluminal vesicles
MVBs participate in the endosomal pathway, which begins with the formation of early endosomes by the fusion of endocytic vesicles coming from the plasma membrane
MVBs are ubiquitous in all components of the neuropil of all cortical layers, indicating that house-keeping functions of the endosomal pathway are distributed along all kinds of neuronal and glial processes

Summary

Introduction

Multivesicular bodies (MVBs) are membrane-bound organelles that contain intraluminal vesicles. They are involved in the transport, storage, sorting, recycling, and release of many substances (Von Bartheld and Altick 2011). Das et al 2016) They participate in autophagy and in the degradation and repair of organelles, proteins, and RNA (Fader and Colombo 2009). Molecules to be transported by MVBs (endocytosed macromolecules and cell membrane proteins) are mostly marked by monoubiquitination or by tetraspanins (Piper and Katzmann 2007; MacDonald et al 2015). The incorporation of molecules into intraluminal vesicles is mediated by the ESCRT complex (Hurley 2008; Wegner et al 2011), and they are sorted toward three possible routes: recycling, exocytosis, or degradation (Elkin et al 2016) (Fig. 1)

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Discussion

Conclusion