Synapses enable neurons to communicate with each other and are therefore a prerequisite for normal brain function. Presynaptically, this communication requires energy and generates large fluctuations in calcium concentrations. Mitochondria are optimized for supplying energy and buffering calcium, and they are actively recruited to presynapses. However, not all presynapses contain mitochondria; thus, how might synapses with and without mitochondria differ? Mitochondria are also increasingly recognized to serve additional functions at the presynapse. Here, we discuss the importance of presynaptic mitochondria in maintaining neuronal homeostasis and how dysfunctional presynaptic mitochondria might contribute to the development of disease.

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Mitochondria at the neuronal presynapse in health and disease.

Mitostasis in Neurons: Maintaining Mitochondria in an Extended Cellular Architecture

Autophagy is an essential degradative pathway that maintains neuronal homeostasis and prevents axon degeneration. Initial observations suggest that autophagy is spatially regulated in neurons, but how autophagy is regulated in distinct neuronal compartments is unclear. Using live-cell imaging in mouse hippocampal neurons, we establish the compartment-specific mechanisms of constitutive autophagy under basal conditions, as well as in response to stress induced by nutrient deprivation. We find that at steady state, the cell soma contains populations of autophagosomes derived from distinct neuronal compartments and defined by differences in maturation state and dynamics. Axonal autophagosomes enter the soma and remain confined within the somatodendritic domain. This compartmentalization likely facilitates cargo degradation by enabling fusion with proteolytically active lysosomes enriched in the soma. In contrast, autophagosomes generated within the soma are less mobile and tend to cluster. Surprisingly, starvation did not induce autophagy in either the axonal or somatodendritic compartment. While starvation robustly decreased mTORC1 signaling in neurons, this decrease was not sufficient to activate autophagy. Furthermore, pharmacological inhibition of mammalian target of rapamycin with Torin1 also was not sufficient to markedly upregulate neuronal autophagy. These observations suggest that the primary physiological function of autophagy in neurons may not be to mobilize amino acids and other biosynthetic building blocks in response to starvation, in contrast to findings in other cell types. Rather, constitutive autophagy in neurons may function to maintain cellular homeostasis by balancing synthesis and degradation, especially within distal axonal processes far removed from the soma. SIGNIFICANCE STATEMENT Autophagy is an essential homeostatic process in neurons, but neuron-specific mechanisms are poorly understood. Here, we compare autophagosome dynamics within neuronal compartments. Axonal autophagy is a vectorial process that delivers cargo from the distal axon to the soma. The soma, however, contains autophagosomes at different maturation states, including input received from the axon combined with locally generated autophagosomes. Once in the soma, autophagosomes are confined to the somatodendritic domain, facilitating cargo degradation and recycling of biosynthetic building blocks to primary sites of protein synthesis. Neuronal autophagy is not robustly upregulated in response to starvation or mammalian target of rapamycin inhibition, suggesting that constitutive autophagy in neurons maintains homeostasis by playing an integral role in regulating the quality of the neuronal proteome.

https://www.jneurosci.org/content/jneuro/36/22/5933.full.pdf

Compartment-Specific Regulation of Autophagy in Primary Neurons.

That certain cell types in the central nervous system are more likely to undergo neurodegeneration in Parkinson’s disease is a widely appreciated but poorly understood phenomenon. Many vulnerable subpopulations, including dopamine neurons in the substantia nigra pars compacta, have a shared phenotype of large, widely distributed axonal networks, dense synaptic connections, and high basal levels of neural activity. These features come at substantial bioenergetic cost, suggesting that these neurons experience a high degree of mitochondrial stress. In such a context, mechanisms of mitochondrial quality control play an especially important role in maintaining neuronal survival. In this review, we focus on understanding the unique challenges faced by the mitochondria in neurons vulnerable to neurodegeneration in Parkinson’s and summarize evidence that mitochondrial dysfunction contributes to disease pathogenesis and to cell death in these subpopulations. We then review mechanisms of mitochondrial quality control mediated by activation of PINK1 and Parkin, two genes that carry mutations associated with autosomal recessive Parkinson’s disease. We conclude by pinpointing critical gaps in our knowledge of PINK1 and Parkin function, and propose that understanding the connection between the mechanisms of sporadic Parkinson’s and defects in mitochondrial quality control will lead us to greater insights into the question of selective vulnerability.

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PINK1 and Parkin mitochondrial quality control: a source of regional vulnerability in Parkinson’s disease

Axonal transport is the process whereby motor proteins actively navigate microtubules to deliver diverse cargoes, such as organelles, from one end of the axon to the other, and is widely regarded as essential for nerve development, function and survival. Mutations in genes encoding key components of the transport machinery, including motor proteins, motor adaptors and microtubules, have been discovered to cause neurological disease. Moreover, disruptions in axonal cargo trafficking have been extensively reported across a wide range of nervous system disorders. However, whether these impairments have a major causative role in, are contributing to or are simply a consequence of neuronal degeneration remains unclear. Therefore, the fundamental relevance of defective trafficking along axons to nerve dysfunction and pathology is often debated. In this article, we review the latest evidence emerging from human and in vivo studies on whether perturbations in axonal transport are indeed integral to the pathogenesis of neurological disease.

/pdf/axonal-transport-and-neurological-disease-20ubuj9f3o.pdf

Axonal transport and neurological disease.

Maintenance of healthy mitochondria is crucial in cells, such as neurons, with high metabolic demands, and dysfunctional mitochondria are thought to be selectively degraded. Studies of chemically uncoupled cells have implicated PINK1 mitochondrial kinase, and Parkin E3 ubiquitin ligase in targeting depolarized mitochondria for degradation. However, the role of the PINK1/Parkin pathway in mitochondrial turnover is unclear in the nervous system under normal physiological conditions, and we understand little about the changes that occur in the mitochondrial life cycle when turnover is disrupted. Here, we evaluated the nature, location, and regulation of quality control in vivo using quantitative measurements of mitochondria in Drosophila nervous system, with deletion and overexpression of genes in the PINK1/Parkin pathway. We tested the hypotheses that impairment of mitochondrial quality control via suppression of PINK1 function should produce failures of turnover, accumulation of senescent mitochondria in the axon, defects in mitochondrial traffic, and a significant shift in the mitochondrial fission–fusion steady state. Although mitochondrial membrane potential was diminished by PINK1 deletion, we did not observe the predicted increases in mitochondrial density or length in axons. Loss of PINK1 also produced specific, directionally balanced defects in mitochondrial transport, without altering the balance between stationary and moving mitochondria. Somatic mitochondrial morphology was also compromised. These results strongly circumscribe the possible mechanisms of PINK1 action in the mitochondrial life cycle and also raise the possibility that mitochondrial turnover events that occur in cultured embryonic axons might be restricted to the cell body in vivo, in the intact nervous system.

https://www.jneurosci.org/content/jneuro/35/25/9391.full.pdf

The Organization of Mitochondrial Quality Control and Life Cycle in the Nervous System In Vivo in the Absence of PINK1.

The extreme geometry of neurons spreads the need for mitochondrial functions out irregularly across vast cellular distances. This makes the long-distance transport of mitochondria a critical feature of their function in neurons. Axonal transport of mitochondria has been studied profitably in a variety of in vitro systems, particularly embryonic neurons grown in culture. This has allowed not only detailed motility analysis via light microscopy but also the ability to challenge the system with pharmacological agents and transfection. It does, however, carry caveats about its relevance to events in cells of the intact nervous system. In recent years, it has become possible to observe, quantify, and analyze the behavior of mitochondria within axons of the nervous system of live organisms. Here, we describe how to prepare the Drosophila larva for direct observation of mitochondrial axonal transport and how to gather and analyze motility data from this preparation, using confocal microscopy. This system takes advantage of our ability in Drosophila to express mitochondrially targeted fluorescent proteins in specific neuronal cell types, which allows us to visualize their traffic with ease, and to distinguish anterograde from retrograde traffic. Drosophila genetics also allows the analysis of mutations, gene overexpression, and knockdowns that affect mitochondrial function, including models of neurodegenerative disease. In addition, this preparation allows the visualization of the distribution and morphology of mitochondria in cell bodies within the central nervous system and in synapses. It is also possible to analyze mitochondrial functions other than transport, such as inner membrane potential, using this preparation.

Analysis of mitochondrial traffic in Drosophila.

The trans‐Golgi Network (TGN) sorts molecular “addresses” and sends newly synthesized proteins to their destination via vesicular transport carriers. Despite the functional significance of packaging processes at the TGN, the sorting of soluble proteins remains poorly understood. Recent research has shown that the Golgi resident protein Cab45 is a significant regulator of secretory cargo sorting at the TGN. Cab45 oligomerizes upon transient Ca2+ influx, recruits soluble cargo molecules (clients), and packs them in sphingomyelin‐rich transport carriers. However, the identity of client molecules packed into Cab45 vesicles is scarce. Therefore, we used a precise and highly efficient secretome analysis technology called hiSPECs. Intriguingly, we observed that Cab45 deficient cells manifest hypersecretion of lysosomal hydrolases. Specifically, Cab45 deficient cells secrete the unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN). In addition, lysosomes in these cells show an aberrant perinuclear accumulation suggesting a new role of Cab45 in lysosomal positioning. This work uncovers a yet unknown function of Cab45 in regulating lysosomal function.

Swathi Devireddy

Papers

The Organization of Mitochondrial Quality Control and Life Cycle in the Nervous System In Vivo in the Absence of PINK1.

Analysis of mitochondrial traffic in Drosophila.

Cab45 deficiency leads to the mistargeting of progranulin and prosaposin and aberrant lysosomal positioning