Research

Overview

The function of the nervous system is determined by the complex and highly polarized morphology of neurons. The generation and maintenance of the neuron’s functional morphology is directed by genetic programs, signaling pathways and environmental cues that impinge on the organization of the cytoskeleton and the secretory pathway. The interaction between these subcellular systems is essential for the establishment of functional domains such as axons, dendrites and synapses.

The ER in dendrites and axons

In all eukaryotic cells the availability of membrane components is regulated by coordinated mechanisms that deliver newly synthesized proteins to the plasma membrane and remove them for storage, recycling or degradation. Decades of studies support the clearly organized sequence of organelles along the biosynthetic secretory route. The structure-function relationships of these organelles are most likely conserved in all eukaryote cells. However, the size, complex geometry of neurons and their requirement for exquisitely controlled delivery of lipids and proteins strongly suggest that the spatial arrangement of secretory organelles – or topological organization – most likely provides an added level of regulation in intracellular trafficking that constitutes a critical element in neural morphogenesis and function.
The ER is responsible for the synthesis and modification of most membrane and secreted proteins and for regulating intracellular Ca2+ levels. The ER is a continuous membrane-bound network encompassing the nuclear membrane, the rough-ER (RER) predominantly composed of sheets and rich in ribosomes, and the smooth ER (SER), composed primarily by tubules joined by three way junctions that form irregular polygons with few ribosomes. The ER is not static, but a highly dynamic organelle. Sheets and tubules interconvert in response to intra/extracellular signals and the network suffers profound modifications in its architecture during the cell cycle. In addition, new tubules appear by attaching the ER membrane to molecular motors, or by attaching directly to the growing tip of microtubules, and may retract associated to the actin cytoskeleton.
In neurons, a continuous SER penetrates far into axons, dendrites and even dendritic spines. This intriguing spatial distribution was already implied by Camilo Golgi in his classical drawings of Purkinje cells more than a century ago. Today, compelling electron microscopy evidence indicates that in neuronal projections the SER is constituted by long tubules and three-way junctions that resemble the archetypal eukaryotic ER. The integrity of the ER network may be critical for the propagation of Ca2+ signals over long distances. Additionally, a non-canonical secretory modality including distal axo-dendritic sites for ER export and distal satellite Golgi organelles in dendrites strongly suggest that the integrity of the network may be crucial for the synthesis, mobility and trafficking of membrane constituents to distal sites (Figure 1). Mutations in genes involved in ER morphogenesis and dynamics are frequent causes of hereditary spastic paraplegias, but despite recent progress in identifying ER structure/function components as pathological threats, the link between axonopathies, trafficking and organelle morphogenesis are only beginning to emerge.
Thus, much remains to be learned about the relationship between ER function and the structure/dynamics of the organelle. A central scientific aim in the lab is to determine how protein transport and protein trafficking, and local Ca2+ release, are regulated by ER morphology and dynamics in dendrites and axons.

Neurotransmitter receptors and dendritic trafficking

Intracellular trafficking of neurotransmitter receptors plays a key role in the regulation of synaptic efficacy. For example it has been firmly established that rapid insertion or removal of AMPA, NMDA and GABAA receptors determine the strengthening and weakening of synapses and are dynamically regulated by activity, contributing to synaptic plasticity and revealing the importance of receptor trafficking in health and disease.
Metabotropic GABAB receptors are central players in the modulation of excitatory and inhibitory synaptic activity, but the molecular mechanisms controlling their availability remain poorly understood.
For many years, the central scientific goal of the lab was to discover the molecular mechanisms that control the abundance of GABABRs in neurons. By focusing on forward trafficking we explored the dynamics and the spatial features of the ER-retention-release mechanism in dendrites of hippocampal neurons. We reported our results in a series of GABAB-related articles.
We are currently investigation the dynamics of the ER to Golgi intermediate compartment in dendrites of hippocampal neurons.

Local trafficking of membrane proteins in axons

The regulation of the axonal proteome is key to generate and maintain neural function. The transport of proteins by fast and slow axoplasmic waves has been known for decades, but alternative mechanisms to control the abundance of axonal proteins based on local synthesis have also been identified. The presence of the endoplasmic reticulum has been documented in peripheral axons, but it is still unknown whether this localized organelle participates in the delivery of axonal membrane proteins. We are currently exploring whether axons contain components of the endoplasmic reticulum and other biosynthetic organelles. We are interested in understanding how they control the synthesis, processing and delivery of membrane proteins in axons (Figure 2).
Using mouse and rat models we evaluate how organelles of the axonal secretory pathway, especially the endoplasmic reticulum, contribute to the protein composition in axons of the central nervous or peripheral systems, and what is their role during injury and repair. More recently, using Drosophila we also seek to understand the role of the axonal endoplasmic reticulum in synaptic function.

Other fundamental properties of the endoplasmic reticulum

Using cell lines and cutting-edge optical microscopy we also seek to understand some fundamental properties of the structure and dynamics of the endoplasmic reticulum (Figure 3). And in dendrites of hippocampal neurons we are exploring the contribution of ER-shaping proteins in intracellular Ca2+ release.

From aging to brains… and machines

In collaboration with Felipe Salech, a young associated researcher in the laboratory, we explore aspects relating neurogenesis and aging.
In collaboration with Pedro Maldonado we participate in ventures that combine neuroscience and engineering.

Funding

The lab is currently funded by national grants: Fondecyt 1140617; ICM-Millenium Scientific Initiative – Biomedical Neuroscience Institute BNI P09-015-F; and CORFO grants; and by international collaboration grants: CONICYT USA2013-020.