Chapter 5 Nuclear Trafficking of Regulator of G Protein Signaling Proteins and Their Roles in the Nucleus

Short stature and combined immunodeficiency associated with mutations in RGS10

We report the clinical and molecular phenotype of three siblings from one family, who presented with short stature and immunodeficiency and carried uncharacterized variants in RGS10 (c.489_491del:p.E163del and c.G511T:p.A171S). This gene encodes regulator of G protein signaling 10 (RGS10), a member of a large family of GTPase-activating proteins (GAPs) that targets heterotrimeric G proteins to constrain the activity of G protein-coupled receptors, including receptors for chemoattractants. The affected individuals exhibited systemic abnormalities directly related to the RGS10 mutations, including recurrent infections, hypergammaglobulinemia, profoundly reduced lymphocyte chemotaxis, abnormal lymph node architecture, and short stature due to growth hormone deficiency. Although the GAP activity of each RGS10 variant was intact, each protein exhibited aberrant patterns of PKA-mediated phosphorylation and increased cytosolic and cell membrane localization and activity compared to the wild-type protein. We propose that the RGS10 p.E163del and p.A171S mutations lead to mislocalization of the RGS10 protein in the cytosol, thereby resulting in attenuated chemokine signaling. This study suggests that RGS10 is critical for both immune competence and normal hormonal metabolism in humans and that rare RGS10 variants may contribute to distinct systemic genetic disorders.

Human genetic variants disrupt RGS14 nuclear shuttling and regulation of LTP in hippocampal neurons

The human genome contains vast genetic diversity as naturally occurring coding variants, yet the impact of these variants on protein function and physiology is poorly understood. RGS14 is a multifunctional signaling protein that suppresses synaptic plasticity in dendritic spines of hippocampal neurons. RGS14 also is a nucleocytoplasmic shuttling protein, suggesting that balanced nuclear import/export and dendritic spine localization are essential for RGS14 functions. We identified genetic variants L505R (LR) and R507Q (RQ) located within the nuclear export sequence (NES) of human RGS14. Here we report that RGS14 encoding LR or RQ profoundly impacts protein functions in hippocampal neurons. RGS14 membrane localization is regulated by binding Gαi-GDP, whereas RGS14 nuclear export is regulated by Exportin 1 (XPO1). Remarkably, LR and RQ variants disrupt RGS14 binding to Gαi1-GDP and XPO1, nucleocytoplasmic equilibrium, and capacity to inhibit long-term potentiation (LTP). Variant LR accumulates irreversibly in the nucleus, preventing RGS14 binding to Gαi1, localization to dendritic spines, and inhibitory actions on LTP induction, while variant RQ exhibits a mixed phenotype. When introduced into mice by CRISPR/Cas9, RGS14-LR protein expression was detected predominantly in the nuclei of neurons within hippocampus, central amygdala, piriform cortex, and striatum, brain regions associated with learning and synaptic plasticity. Whereas mice completely lacking RGS14 exhibit enhanced spatial learning, mice carrying variant LR exhibit normal spatial learning, suggesting that RGS14 may have distinct functions in the nucleus independent from those in dendrites and spines. These findings show that naturally occurring genetic variants can profoundly alter normal protein function, impacting physiology in unexpected ways.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

Rab family proteins regulate the endosomal trafficking and function of RGS4

RGS4, a heterotrimeric G-protein inhibitor, localizes to plasma membrane (PM) and endosomal compartments. Here, we examined Rab-mediated control of RGS4 internalization and recycling. Wild type and constitutively active Rab5 decreased RGS4 PM levels while increasing its endosomal targeting. Rab5, however, did not appreciably affect the PM localization or function of the M1 muscarinic receptor (M1R)/Gq signaling cascade. RGS4-containing endosomes co-localized with subsets of Rab5-, transferrin receptor-, and Lamp1/Lysotracker-marked compartments suggesting RGS4 traffics through PM recycling or acidified endosome pathways. Rab7 activity promoted TGN association, whereas Rab7(dominant negative) trapped RGS4 in late endosomes. Furthermore, RGS4 was found to co-localize with an endosomal pool marked by Rab11, the protein that mediates recycling/sorting of proteins to the PM. The Cys-12 residue in RGS4 appeared important for its Rab11-mediated trafficking to the PM. Rab11(dominant negative) decreased RGS4 PM levels and increased the number of RGS4-containing endosomes. Inhibition of Rab11 activity decreased RGS4 function as an inhibitor of M1R activity without affecting localization and function of the M1R/Gq signaling complex. Thus, both Rab5 activation and Rab11 inhibition decreased RGS4 function in a manner that is independent from their effects on the localization and function of the M1R/Gq signaling complex. This is the first study to implicate Rab GTPases in the intracellular trafficking of an RGS protein. Thus, Rab GTPases may be novel molecular targets for the selective regulation of M1R-mediated signaling via their specific effects on RGS4 trafficking and function.

RGS10 exerts a neuroprotective role through the PKA/c-AMP response-element (CREB) pathway in dopaminergic neuron-like cells

Regulator of G-protein signaling-10 (RGS10) is a GTPase activating protein for Gαi/q/z subunits that is highly expressed in the immune system and in a broad range of brain regions including the hippocampus, striatum, dorsal raphe, and ventral midbrain. Previously, we reported that RGS10-null mice display increased vulnerability to chronic systemic inflammation-induced degeneration of nigral dopaminergic (DA) neurons. Given that RGS10 is expressed in DA neurons, we investigated the extent to which RGS10 regulates cell survival under conditions of inflammatory stress. Because of the inherent limitations associated with use of primary DA neurons for biochemical analyses, we employed a well-characterized ventral mesencephalon DA neuroblastoma cell line (MN9D) for our studies. We found that stable over-expression of RGS10 rendered them resistant to TNF-induced cytotoxicity; whereas MN9D cells expressing mutant RGS10-S168A (which is resistant to phosphorylation by protein kinase A at a serine residue that promotes its nuclear translocation) showed similar sensitivity to TNF as the parental MN9D cells. Using biochemical and pharmacologic approaches, we identified protein kinase A and the downstream phospho-cAMP response element-binding signaling pathway (and ruled out ERK 1/2, JNK, and NFkB) as key mediators of the neuroprotective effect of RGS10 against inflammatory stress.

Regulator of G-protein signaling-10 negatively regulates NF-κB in microglia and neuroprotects dopaminergic neurons in hemiparkinsonian rats

Microglia are the brain-resident macrophages responsible for immune surveillance that become activated in response to injury, infection, environmental toxins, and other stimuli that threaten neuronal survival. Previous work from our group demonstrated that mice deficient in Regulator of G-protein Signaling 10 (RGS10), a microglia-enriched GTPase activating protein (GAP) for G-protein α subunits, displayed increased microglial burden in the CNS at birth and developed a parkinsonian phenotype after exposure to chronic systemic inflammation, implicating a neuroprotective role for RGS10 in the nigrostriatal pathway. While it is known that RGS10 is expressed in both microglia and certain subsets of neurons, it is not known whether RGS10 functions similarly in both cells types. In this study we sought to delineate the specific role of RGS10 in microglia and identify the molecular pathway(s) required for RGS10 to exert its actions in microglia. Here, we identify RGS10 as a negative regulator of the nuclear factor κB(NF-κB) pathway in microglia and demonstrate that the proinflammatory and cytotoxic phenotype of Rgs10-null microglia can be reversed by lentiviral-mediated restoration of RGS10 expression. In vivo gene transfer of RGS10 into the substantia nigra pars compacta (SNpc) of rats reduced microgliosis and protected against 6-OHDA neurotoxin-induced death of dopaminergic (DA) neurons. Together, our findings suggest that modulation of RGS10 activity in microglia may afford therapeutic benefit in the treatment of chronic neuroinflammatory conditions as well as neuroprotection against inflammation-related degeneration in Parkinson's disease (PD), the second most common neurodegenerative disorder affecting individuals over age 65.