Small GTPases Of The Rab And Arf Families: Key Regulators Of Intracellular Trafficking in NeurodegenerationⅠ

Abstract: 

Small guanosine triphosphatases (GTPases) of the Rab and Arf families are key regulators of vesicle formation and membrane trafficking. Membrane transport plays an important role in the central nervous system. In this regard, neurons require a constant flow of membranes for the correct distribution of receptors, for the precise composition of proteins and organelles in dendrites and axons, for the continuous exocytosis/endocytosis of synaptic vesicles and the elimination of dysfunctional proteins. Thus, it is not surprising that Rab and Arf GTPases have been associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s. 

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Both pathologies share characteristics such as the presence of protein aggregates and/or the fragmentation of the Golgi apparatus, hallmarks that have been related to both Rab and Arf GTPases functions. Despite their relationship with neurodegenerative disorders, very few studies have focused on the role of these GTPases in the pathogenesis of neurodegeneration. In this review, we summarize their importance in the onset and progression of Alzheimer’s and Parkinson’s diseases, as well as their emergence as potential therapeutical targets for neurodegeneration. 

Keywords: Rab GTPase; Arf GTPase; small GTPase; Alzheimer; Parkinson; neurodegeneration; membrane trafficking; vesicle; transport

1 Introduction 

Eukaryotic cells constantly receive information from the extracellular medium by the binding of growth factors, hormones, peptides, and ions to specific receptors. This binding triggers the transmission of a message through signaling cascades in the cytoplasm to induce a precise biological response [1]. One of the central elements responsible for the diffusion of this message is the small guanosine triphosphatases (GTPases) of the Ras superfamily. These small GTPases participate in signaling cascades that control a wide range of cell responses, such as proliferation, differentiation, and apoptosis [2,3]. The small GTPases are molecular switches that can be found in two states: an inactive state in which the small GTPase is bound to GDP, and an active state in which it is bound to GTP. The process by which the GTPase changes from the inactive to the active state is known as the GTPase activation cycle. Three main molecules control the activation/deactivation cycle. 

The guanine exchange factors (GEFs) are in charge of activating the GTPase by favoring the release of GDP and the binding of GTP. The GTPase activating proteins (GAPs), on the contrary, are responsible for the inactivation of the GTPase by inducing the intrinsic GTPase activity that results in the hydrolysis of the GTP. Finally, guanine nucleotide dissociation inhibitors (GDIs) prevent the dissociation of the GDP from the GTPase, therefore maintaining the GTPase in an inactive state [4,5]. Moreover, small GTPases can also be regulated by post-translational modifications that permit their binding to either specific proteins or membranes. Thus, they can be farnesylated, geranylgeranylated, or palmitoylated in their C-terminal region and myristoylated in their N-terminal region [5,6]. The Ras superfamily of small GTPases is divided into five families: Ras, Rho, Rab, Arf, and Ran [2,3]. The Ras family is specialized in the control of cell growth and metabolism. Additionally, Ras family GTPases cooperate with the Rho family to regulate the cell cycle, gene expression, and cell transformation. Apart from those functions, the Rho family of GTPases is responsible for the actin cytoskeleton organization, whereas the Rab and the Arf families control the intracellular traffic of vesicles and membranes and the formation and intracellular transport of vesicles, respectively. Last, the GTPases of the Ran family are in charge of the nucleocytoplasmic transport [2,3,5,7]. 

Most of the intracellular compartments, such as the nucleus, mitochondria, or the Golgi apparatus (GA), are separated by membranes. Thus, eukaryotic cells require specific mechanisms for the traffic between these organelles. Furthermore, coordinated membrane trafficking between different cell types is needed in multicellular organisms [8]. The Rab GTPases, the largest family of the Ras superfamily, are key regulators of vesicle sorting and membrane trafficking. They can control this traffic by interacting with effector molecules such as the coat proteins (COPI, COPII, and clathrin), motor proteins (kinesins and dyneins), tethering complexes (early endosome antigen 1 (EEA1), Golgins, exocyst, and the homotypic fusion and protein sorting (HOPS) complex), and SNAREs [8]. Conversely, Arf GTPases participate in vesicle formation, especially in the GA [9], but they are also present in the plasma membrane, endosomes, and lipid droplets [9]. To regulate vesicle formation, like Rab, the Arf GTPases interact with effector molecules such as the coat proteins and their adaptors (COPI, Golgi-localized γ-ear containing Arf-binding proteins (GGA), and Munc18-interacting proteins (MINT)). Therefore, the Rab and Arf families of GTPases regulate the endomembrane system (Figure 1).

Membrane trafficking plays an important role in neurons. Neurons have a specific morphology that requires constant membrane trafficking between axons and dendrites to maintain synaptic function [8]. This enables synaptic transmission, the correct distribution of membrane receptors, and precise organelle and protein composition in dendrites and axons [8]. Synaptic function demands a continuous flux of membranes, as synaptic vesicles are constantly subjected to exocytosis and endocytosis. Additionally, proteins must be transported between the axon, dendrites, and cell body to transmit the signaling message or to be degraded. Besides, the retrograde transport of late endosomes and autophagosomes allows the removal of dysfunctional proteins, which is important for the correct neuronal function and survival. Hence, membrane trafficking is involved in all aspects of neuronal function, and its dysfunction has been linked to neurodegeneration [8].

Neurodegeneration consists of the progressive loss of specific subsets of neurons [10]. The main neurodegenerative diseases are Alzheimer’s disease (AD) and Parkinson’s disease (PD). AD is the most common form of dementia [11]. It is characterized by the progressive loss of neurons that results in the loss of memory and cognitive functions. The principal hallmarks of the disease are the extracellular amyloid-β (Aβ) plaques and the intracellular accumulation of neurofibrillary tangles (NFTs), formed by pTau aggregation. Despite being those the classical features, the molecular pathology of AD is not completely understood. On the one hand, the amyloidogenic processing of the amyloid precursor protein (APP) that leads to the generation of Aβ peptides occurs in the intracellular compartments that require endocytic trafficking. Under physiological conditions, the APP is processed by the β-secretase (BACE1) in the Rab5-positive early endosomes, giving rise to β-cleavage C-terminal fragments (β-CTFs). Such fragments are then processed in late endosomes or the trans-Golgi network (TGN) to produce Aβ peptides [12]. This highlights the importance of these GTPases and membrane trafficking in AD pathology. Besides, various genes related to endocytic trafficking have been associated with the risk of developing AD [12]. 

For instance, a low expression of phosphatidylinositol binding clathrin assembly protein (PICALM) has been described in AD, which plays an important role in the internalization, trafficking, and clearance of Aβ peptides [12,13]. Regarding PD, it is the second most common neurodegenerative disease. It is characterized by the accumulation of Lewy bodies, formed by α-synuclein (α-syn) aggregation, and by the selective degeneration of dopaminergic neurons of the substantia nigra pars compacta [14]. This results in disabilities in movements, including resting tremors and muscular rigidity. Mutations in α-syn, in PTEN-induced putative kinase 1 (PINK1), and in leucine-rich repeat kinase 2 (LRRK2) have been associated with the risk of developing PD [14]. Apart from these mutations, mutations in the Rab39B GTPase have been related to the development of this disease [15]. Rab39B controls the trafficking of the GluA2 subunit of the AMPA receptor and it is exclusively expressed in neurons [15]. Furthermore, various GTPases have been associated with defects in membrane trafficking that appear owing to α-syn accumulations [15]. Thus, in the same way as in AD, these GTPases and membrane trafficking are related to PD pathology. In summary, small GTPase-dependent membrane trafficking plays an important role in the nervous system, and dysregulations of such processes have been correlated with neurodegenerative diseases such as AD and PD (Table 1). As a result, in a similar fashion to the Ras and Rho families [5], the Rab and Arf family of GTPases have emerged as therapeutical targets for these pathologies.

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To be continued...

Alazne Arrazola Sastre 1,2, Miriam Luque Montoro 1 , Hadriano M. Lacerda 3 , Francisco Llavero 1,4,* and José L. Zugaza 1,2,5,*

1 Achucarro Basque Center for Neuroscience, Science Park of the UPV/EHU, 48940 Leioa, Spain; alazne.arrazola@ehu.eus (A.A.S.); miriamluquem@gmail.com (M.L.M.) 

2 Department of Genetics, Physical Anthropology and Animal Physiology, University of Basque Country UPV/EHU, 48940 Leioa, Spain 

3 Three R Labs, Science Park of the UPV/EHU, 48940 Leioa, Spain; hadrilac@gmail.com 

4 Hospital 12 de Octubre Research Institute (i+12), 28041 Madrid, Spain 

5 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain * Correspondence: fcollavero.imas12@h12o.es (F.L.); joseluis.zugaza@ehu.es (J.L.Z.)