Rac1 and Rac3 GTPases and TPC2 are required for axonal outgrowth and migration of cortical interneurons

Rho GTPases, among them Rac1 and Rac3, are major transducers of extracellular signals and are involved in multiple cellular processes. In cortical interneurons, the neurons that control the balance between excitation and inhibition of cortical circuits, Rac1 and Rac3 are essential for their development. Ablation of both leads to a severe reduction in the numbers of mature interneurons found in the murine cortex, which is partially due to abnormal cell cycle progression of interneuron precursors and defective formation of growth cones in young neurons. Here, we present new evidence that upon Rac1 and Rac3 ablation, centrosome, Golgi complex and lysosome positioning is significantly perturbed, thus affecting both interneuron migration and axon growth. Moreover, for the first time, we provide evidence of altered expression and localization of the two-pore channel 2 (TPC2) voltage-gated ion channel that mediates Ca2+ release. Pharmacological inhibition of TPC2 negatively affected axonal growth and migration of interneurons. Our data, taken together, suggest that TPC2 contributes to the severe phenotype in axon growth initiation, extension and interneuron migration in the absence of Rac1 and Rac3.

Mice With Decreased Number of Interneurons Exhibit Aberrant Spontaneous and Oscillatory Activity in the Cortex

GABAergic (γ-aminobutyric acid) neurons are inhibitory neurons and protect neural tissue from excessive excitation. Cortical GABAergic neurons play a pivotal role for the generation of synchronized cortical network oscillations. Imbalance between excitatory and inhibitory mechanisms underlies many neuropsychiatric disorders and is correlated with abnormalities in oscillatory activity, especially in the gamma frequency range (30–80 Hz). We investigated the functional changes in cortical network activity in response to developmentally reduced inhibition in the adult mouse barrel cortex (BC). We used a mouse model that displays ∼50% fewer cortical interneurons due to the loss of Rac1 protein from Nkx2.1/Cre-expressing cells [Rac1 conditional knockout (cKO) mice], to examine how this developmental loss of cortical interneurons may affect basal synaptic transmission, synaptic plasticity, spontaneous activity, and neuronal oscillations in the adult BC. The decrease in the number of interneurons increased basal synaptic transmission, as examined by recording field excitatory postsynaptic potentials (fEPSPs) from layer II networks in the Rac1 cKO mouse cortex, decreased long-term potentiation (LTP) in response to tetanic stimulation but did not alter the pair-pulse ratio (PPR). Furthermore, under spontaneous recording conditions, Rac1 cKO brain slices exhibit enhanced sensitivity and susceptibility to emergent spontaneous activity. We also find that this developmental decrease in the number of cortical interneurons results in local neuronal networks with alterations in neuronal oscillations, exhibiting decreased power in low frequencies (delta, theta, alpha) and gamma frequency range (30–80 Hz) with an extra aberrant peak in high gamma frequency range (80–150 Hz). Therefore, our data show that disruption in GABAergic inhibition alters synaptic properties and plasticity, while it additionally disrupts the cortical neuronal synchronization in the adult BC.

Golgi sorting regulates organization and activity of GPI proteins at apical membranes

Here we combined classical biochemistry with new biophysical approaches to study the organization of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) with high spatial and temporal resolution at the plasma membrane of polarized epithelial cells. We show that in polarized MDCK cells, after sorting in the Golgi, each GPI-AP reaches the apical surface in homoclusters. Golgi-derived homoclusters are required for their subsequent plasma membrane organization into cholesterol-dependent heteroclusters. By contrast, in nonpolarized MDCK cells, GPI-APs are delivered to the surface as monomers in an unpolarized manner and are not able to form heteroclusters. We further demonstrate that this GPI-AP organization is regulated by the content of cholesterol in the Golgi apparatus and is required to maintain the functional state of the protein at the apical membrane. Thus, in contrast to fibroblasts, in polarized epithelial cells, a selective cholesterol-dependent sorting mechanism in the Golgi regulates both the organization and function of GPI-APs at the apical surface.

Rac-GTPases Regulate Microtubule Stability and Axon Growth of Cortical GABAergic Interneurons

Cortical interneurons are characterized by extraordinary functional and morphological diversity. Although tremendous progress has been made in uncovering molecular and cellular mechanisms implicated in interneuron generation and function, several questions still remain open. Rho-GTPases have been implicated as intracellular mediators of numerous developmental processes such as cytoskeleton organization, vesicle trafficking, transcription, cell cycle progression, and apoptosis. Specifically in cortical interneurons, we have recently shown a cell-autonomous and stage-specific requirement for Rac1 activity within proliferating interneuron precursors. Conditional ablation of Rac1 in the medial ganglionic eminence leads to a 50% reduction of GABAergic interneurons in the postnatal cortex. Here we examine the additional role of Rac3 by analyzing Rac1/Rac3 double-mutant mice. We show that in the absence of both Rac proteins, the embryonic migration of medial ganglionic eminence-derived interneurons is further impaired. Postnatally, double-mutant mice display a dramatic loss of cortical interneurons. In addition, Rac1/Rac3-deficient interneurons show gross cytoskeletal defects in vitro, with the length of their leading processes significantly reduced and a clear multipolar morphology. We propose that in the absence of Rac1/Rac3, cortical interneurons fail to migrate tangentially towards the pallium due to defects in actin and microtubule cytoskeletal dynamics.

Rac1-dependent cell cycle exit of MGE precursors and GABAergic interneuron migration to the cortex

Cortical γ-aminobutyric acid (GABA)ergic interneurons are characterized by extraordinary neurochemical and functional diversity. Although recent studies have uncovered some of the molecular components underlying interneuron development, including the cellular and molecular mechanisms guiding their migration to the cortex, the intracellular components involved are still unknown. Rac1, a member of the Rac subfamily of Rho-GTPases, has been implicated in various cellular processes such as cell cycle dynamics, axonogenesis, and migration. In this study, we have addressed the specific role of Rac1 in interneuron progenitors originating in the medial ganglionic eminence, via Cre/loxP technology. We show that ablation of Rac1 from Nkx2.1-positive progenitors, results in a migratory impairment. As a consequence, only half of GABAergic interneurons are found in the postnatal cortex. The rest remain aggregated in the ventral telencephalon and show morphological defects in their growing processes in vitro. Ablation of Rac1 from postmitotic progenitors does not result in similar defects, thus underlying a novel cell autonomous and stage-specific requirement for Rac1 activity, within proliferating progenitors of cortical interneurons. Rac1 is necessary for their transition from G1 to S phase, at least in part by regulating cyclin D levels and retinoblastoma protein phosphorylation.

Different GPI-attachment signals affect the oligomerisation of GPI-anchored proteins and their apical sorting

To understand the mechanism involved in the apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) we fused to the C-terminus of GFP the GPI-anchor-attachment signal of the folate receptor (FR) or of the prion protein (PrP), two native GPI-anchored proteins that are sorted apically or basolaterally, respectively, in MDCK cells. We investigated the behaviour of the resulting fusion proteins GFP-FR and GFP-PrP by analysing three parameters: their association with DRMs, their oligomerisation and their apical sorting. Strikingly, we found that different GPI-attachment signals differently modulate the ability of the resulting GFP-fusion protein to oligomerise and to be apically sorted. This is probably owing to differences in the GPI anchor and/or in the surrounding lipid microenvironment. Accordingly, we show that addition of cholesterol to the cells is necessary and sufficient to drive the oligomerisation and consequent apical sorting of GFP-PrP, which under control conditions does not oligomerise and is basolaterally sorted.

alpha-Adducin mutations increase Na/K pump activity in renal cells by affecting constitutive endocytosis: implications for tubular Na reabsorption

Genetic variation in alpha-adducin cytoskeletal protein is implicated in the polymerization and bundling of actin and alteration of the Na/K pump, resulting in abnormal renal sodium transport and hypertension in Milan hypertensive rats and humans. To investigate the molecular involvement of alpha-adducin in controlling Na/K pump activity, wild-type or mutated rat and human alpha-adducin forms were, respectively, transfected into several renal cell lines. Through multiple experimental approaches (microscopy, enzymatic assays, coimmunoprecipitation), we showed that rat and human mutated forms increased Na/K pump activity and the number of pump units; moreover, both variants coimmunoprecipitate with Na/K pump. The increased Na/K pump activity was not due to changes in its basolateral localization, but to an alteration of Na/K pump residential time on the plasma membrane. Indeed, both rat and human mutated variants reduced constitutive Na/K pump endocytosis and similarly affected transferrin receptor trafficking and fluid-phase endocytosis. In fact, alpha-adducin was detected in clathrin-coated vesicles and coimmunoprecipitated with clathrin. These results indicate that adducin, besides its modulatory effects on actin cytoskeleton dynamics, might play a direct role in clathrin-dependent endocytosis. The constitutive reduction of the Na/K pump endocytic rate induced by mutated adducin variants may be relevant in Na-dependent hypertension.

Coexpression of wild-type and mutant prion proteins alters their cellular localization and partitioning into detergent-resistant membranes

Transmissible spongiform encephalopathies (TSEs) are a group of diseases of infectious, sporadic and genetic origin, found in higher organisms and caused by the pathological form of the prion protein. The inheritable subgroup of TSEs is linked to insertional or point mutations in the prion gene prnp, which favour its misfolding and are passed on to offspring in an autosomal-dominant fashion. The large majority of patients with these diseases are heterozygous for the prnp gene, leading to the coexpression of the wild-type (wt) (PrP(C)) and the mutant forms (PrPmut) in the carriers of these mutations. To mimic this situation in vitro, we produced Fischer rat thyroid cells coexpressing PrPwt alongside mutant versions of mouse PrP including A117V, E200K and T182A relevant to the human TSE diseases Gestmann-Sträussler-Scheinker (GSS) disease and familial Creutzfeldt-Jakob disease (fCJD). We found that coexpression of mutant PrP with wt proteins does not affect the glycosylation pattern or the biochemical characteristics of either protein. However, FRET and co-immunoprecipitation experiments suggest an interaction occurring between the wt and mutant proteins. Furthermore, by comparing the intracellular localization and detergent-resistant membrane (DRM) association in single- and double-expressing clones, we found changes in the intracellular/surface ratio and an increased sequestration of both proteins in DRMs, a site believed to be involved in the pathological conversion (or protection thereof) of the prion protein. We, therefore, propose that the mutant forms alter the subcellular localization and the membrane environment of the wt protein in co-transfected cells. These effects may play a role in the development of these diseases.

Characterization of the Properties and Trafficking of an Anchorless Form of the Prion Protein

Conversion of PrP(C) into PrP(Sc) is the central event in the pathogenesis of transmissible prion diseases. Although the molecular basis of this event and the intracellular compartment where it occurs are not yet understood, the association of PrP with cellular membranes and in particular its presence in detergent-resistant microdomains appears to be of critical importance. In addition it appears that scrapie conversion requires membrane-bound glycosylphosphatidylinositol (GPI)-linked PrP. The GPI anchor may affect either the conformation, the intracellular localization, or the association of the prion protein with specific membrane domains. However, how this occurs is not known. To understand the relevance of the GPI anchor for the cellular behavior of PrP, we have studied the biosynthesis and localization of a PrP version which lacks the GPI anchor attachment signal (PrP Delta GPI). We found that PrP Delta GPI is tethered to cell membranes and associates to membrane detergent-resistant microdomains but does not assume a transmembrane topology. Differently to PrP(C), this protein does not localize at the cell surface but is mainly released in the culture media in a fully glycosylated soluble form. The cellular behavior of anchorless PrP explains why PrP Delta GPI Tg mice can be infected but do not show the classical signs of the disorder, thus indicating that the plasma membrane localization of PrP(C) and/or of the converted scrapie form might be necessary for the development of a symptomatic disease.

Oligomerization Is a Specific Requirement for Apical Sorting of Glycosyl-Phosphatidylinositol-Anchored Proteins but Not for Non-Raft-Associated Apical Proteins

Protein apical sorting in polarized epithelial cells is mediated by two different mechanisms, raft dependent and raft independent. In Madin-Darby canine kidney (MDCK) cells, an essential step for apical sorting of glycosyl-phosphatidylinositol (GPI)-anchored proteins (GPI-APs) is their coalescence into high-molecular-weight (HMW) oligomers. Here we show that this mechanism is also functional in Fischer rat thyroid cells, which possess a different sorting phenotype compared with MDCK cells. We demonstrate that, as in MDCK cells, both apical and basolateral GPI-APs associate with detergent-resistant microdomains, but that only the apical proteins are able to oligomerize into HMW complexes during their passage through the medial Golgi. We also show that oligomerization is a specific requirement for apical sorting of GPI-APs and is not used by transmembrane, non-raft-associated apical proteins.

Analysis of detergent-resistant membranes associated with apical and basolateral GPI-anchored proteins in polarized epithelial cells

Detergent-resistant membranes (DRMs) represent specialized membrane domains resistant to detergent extraction, which may serve to segregate proteins in a specific environment in order to improve their function. Segregation of glycosylphosphatidylinositol-anchored proteins (GPI-APs) in DRMs has been shown to be involved in their sorting to the apical membrane in polarized epithelial cells. Nonetheless, we have shown that both apical and basolateral GPI-APs associate with DRMs. In this report we investigated the lipid composition of DRMs associated with an apical and a basolateral GPI-AP. We found that apical and basolateral DRMs contain the same lipid species although in different ratios. This specific lipid ratio is maintained after mixing the cells before lysis indicating that DRMs maintain their identity after Triton extraction.

Protein oligomerization modulates raft partitioning and apical sorting of GPI-anchored proteins

An essential but insufficient step for apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) in epithelial cells is their association with detergent-resistant microdomains (DRMs) or rafts. In this paper, we show that in MDCK cells both apical and basolateral GPI-APs associate with DRMs during their biosynthesis. However, only apical and not basolateral GPI-APs are able to oligomerize into high molecular weight complexes. Protein oligomerization begins in the medial Golgi, concomitantly with DRM association, and is dependent on protein-protein interactions. Impairment of oligomerization leads to protein missorting. We propose that oligomerization stabilizes GPI-APs into rafts and that this additional step is required for apical sorting of GPI-APs. Two alternative apical sorting models are presented.