Vav3-deficient mice exhibit a transient delay in cerebellar development

Activation of TrkB signaling mitigates cerebellar anomalies caused by Rbm4-Bdnf deficiency

A molecular and functional link between neurotrophin signaling and cerebellar foliation is lacking. Here we show that constitutive knockout of two homologous genes encoding the RNA binding protein RBM4 results in foliation defects at cerebellar lobules VI-VII and delayed motor learning in mice. Moreover, the features of Rbm4 double knockout (dKO), including impaired differentiation of cerebellar granule cells and dendritic arborization of Purkinje cells, are reminiscent of neurotrophin deficiency. Loss of RBM4 indeed reduced brain-derived neurotrophic factor (BDNF). RBM4 promoted the expression of BDNF and full-length TrkB, implicating RBM4 in efficient BDNF-TrkB signaling. Finally, prenatal supplementation with 7,8-dihydroxyflavone, a TrkB agonist, restored granule cell differentiation, Purkinje cell dendritic complexity and foliation-the intercrural fissure in particular-in the neonatal cerebellum of Rbm4dKO mice, which also showed improved motor learning in adulthood. This study provides evidence that prenatal activation of TrkB signaling ameliorates cerebellar malformation caused by BDNF deficiency.

The guanine nucleotide exchange factor Vav3 intervenes in the migration pathway of oligodendrocyte precursor cells on tenascin-C

Oligodendrocyte precursor cells (OPCs) are the exclusive source of myelination in the central nervous system (CNS). Prior to myelination, OPCs migrate to target areas and mature into myelinating oligodendrocytes. This process is underpinned by drastic changes of the cytoskeleton and partially driven by pathways involving small GTPases of the Rho subfamily. In general, the myelination process requires migration, proliferation and differentiation of OPCs. Presently, these processes are only partially understood. In this study, we analyzed the impact of the guanine nucleotide exchange factor (GEF) Vav3 on the migration behavior of OPCs. Vav3 is known to regulate RhoA, Rac1 and RhoG activity and is therefore a promising candidate with regard to a regulatory role concerning the rearrangement of the cytoskeleton. Our study focused on the Vav3 knockout mouse and revealed an enhanced migration capacity of Vav3 -/- OPCs on the extracellular matrix (ECM) glycoprotein tenascin-C (TnC). The migration behavior of individual OPCs on further ECM molecules such as laminin-1 (Ln1), laminin-2 (Ln2) and tenascin-R (TnR) was not affected by the elimination of Vav3. The migration process was further investigated with regard to intracellular signal transmission by pharmacological blockade of downstream pathways of specific Rho GTPases. Our data suggest that activation of RhoA GTPase signaling compromises migration, as inhibition of RhoA-signaling promoted migration behavior. This study provides novel insights into the control of OPC migration, which could be useful for further understanding of the complex differentiation and myelination process.

A rigorous in silico genomic interrogation at 1p13.3 reveals 16 autosomal dominant candidate genes in syndromic neurodevelopmental disorders

Genome-wide chromosomal microarray is extensively used to detect copy number variations (CNVs), which can diagnose microdeletion and microduplication syndromes. These small unbalanced chromosomal structural rearrangements ranging from 1 kb to 10 Mb comprise up to 15% of human mutations leading to monogenic or contiguous genomic disorders. Albeit rare, CNVs at 1p13.3 cause a variety of neurodevelopmental disorders (NDDs) including development delay (DD), intellectual disability (ID), autism, epilepsy, and craniofacial anomalies (CFA). Most of the 1p13.3 CNV cases reported in the pre-microarray era encompassed a large number of genes and lacked the demarcating genomic coordinates, hampering the discovery of positional candidate genes within the boundaries. In this study, we present four subjects with 1p13.3 microdeletions displaying DD, ID, autism, epilepsy, and CFA. In silico comparative genomic mapping with three previously reported subjects with CNVs and 22 unreported DECIPHER CNV cases has resulted in the identification of four different sub-genomic loci harboring five positional candidate genes for DD, ID, and CFA at 1p13.3. Most of these genes have pathogenic variants reported, and their interacting genes are involved in NDDs. RT-qPCR in various human tissues revealed a high expression pattern in the brain and fetal brain, supporting their functional roles in NDDs. Interrogation of variant databases and interacting protein partners led to the identification of another set of 11 potential candidate genes, which might have been dysregulated by the position effect of these CNVs at 1p13.3. Our studies define 1p13.3 as a genomic region harboring 16 NDD candidate genes and underscore the critical roles of small CNVs in in silico comparative genomic mapping for disease gene discovery. Our candidate genes will help accelerate the isolation of pathogenic heterozygous variants from exome/genome sequencing (ES/GS) databases.

Vav3-Deficient Astrocytes Enhance the Dendritic Development of Hippocampal Neurons in an Indirect Co-culture System

Vav proteins belong to the class of guanine nucleotide exchange factors (GEFs) that catalyze the exchange of guanosine diphosphate (GDP) by guanosine triphosphate (GTP) on their target proteins. Here, especially the members of the small GTPase family, Ras homolog family member A (RhoA), Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division control protein 42 homolog (Cdc42) can be brought into an activated state by the catalytic activity of Vav-GEFs. In the central nervous system (CNS) of rodents Vav3 shows the strongest expression pattern in comparison to Vav2 and Vav1, which is restricted to the hematopoietic system. Several studies revealed an important role of Vav3 for the elongation and branching of neurites. However, little is known about the function of Vav3 for other cell types of the CNS, like astrocytes. Therefore, the following study analyzed the effects of a Vav3 knockout on several astrocytic parameters as well as the influence of Vav3-deficient astrocytes on the dendritic development of cultured neurons. For this purpose, an indirect co-culture system of native hippocampal neurons and Vav3-deficient cortical astrocytes was used. Interestingly, neurons cultured in an indirect contact with Vav3-deficient astrocytes showed a significant increase in the dendritic complexity and length after 12 and 17 days in vitro (DIV). Furthermore, Vav3-deficient astrocytes showed an enhanced regeneration in the scratch wound heal assay as well as an altered profile of released cytokines with a complete lack of CXCL11, reduced levels of IL-6 and an increased release of CCL5. Based on these observations, we suppose that Vav3 plays an important role for the development of dendrites by regulating the expression and the release of neurotrophic factors and cytokines in astrocytes.

New Functions of Vav Family Proteins in Cardiovascular Biology, Skeletal Muscle, and the Nervous System

Simple Summary

In this review, we provide information on the role of Vav proteins, a group of signaling molecules that act as both Rho GTPase activators and adaptor molecules, in the cardiovascular system, skeletal muscle, and the nervous system. We also describe how these functions impact in other physiological and pathological processes such as sympathoregulation, blood pressure regulation, systemic metabolism, and metabolic syndrome.

Abstract

Vav proteins act as tyrosine phosphorylation-regulated guanosine nucleotide exchange factors for Rho GTPases and as molecular scaffolds. In mammals, this family of signaling proteins is composed of three members (Vav1, Vav2, Vav3) that work downstream of protein tyrosine kinases in a wide variety of cellular processes. Recent work with genetically modified mouse models has revealed that these proteins play key signaling roles in vascular smooth and skeletal muscle cells, specific neuronal subtypes, and glia cells. These functions, in turn, ensure the proper regulation of blood pressure levels, skeletal muscle mass, axonal wiring, and fiber myelination events as well as systemic metabolic balance. The study of these mice has also led to the discovery of new physiological interconnection among tissues that contribute to the ontogeny and progression of different pathologies such as, for example, hypertension, cardiovascular disease, and metabolic syndrome. Here, we provide an integrated view of all these new Vav family-dependent signaling and physiological functions.

Rho Family GTPases and Rho GEFs in Glucose Homeostasis

Dysregulation of glucose homeostasis leading to metabolic syndrome and type 2 diabetes is the cause of an increasing world health crisis. New intriguing roles have emerged for Rho family GTPases and their Rho guanine nucleotide exchange factor (GEF) activators in the regulation of glucose homeostasis. This review summates the current knowledge, focusing in particular on the roles of Rho GEFs in the processes of glucose-stimulated insulin secretion by pancreatic β cells and insulin-stimulated glucose uptake into skeletal muscle and adipose tissues. We discuss the ten Rho GEFs that are known so far to regulate glucose homeostasis, nine of which are in mammals, and one is in yeast. Among the mammalian Rho GEFs, P-Rex1, Vav2, Vav3, Tiam1, Kalirin and Plekhg4 were shown to mediate the insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane and/or insulin-stimulated glucose uptake in skeletal muscle or adipose tissue. The Rho GEFs P-Rex1, Vav2, Tiam1 and β-PIX were found to control the glucose-stimulated release of insulin by pancreatic β cells. In vivo studies demonstrated the involvement of the Rho GEFs P-Rex2, Vav2, Vav3 and PDZ-RhoGEF in glucose tolerance and/or insulin sensitivity, with deletion of these GEFs either contributing to the development of metabolic syndrome or protecting from it. This research is in its infancy. Considering that over 80 Rho GEFs exist, it is likely that future research will identify more roles for Rho GEFs in glucose homeostasis.

Lysine Acetylation Reshapes the Downstream Signaling Landscape of Vav1 in Lymphocytes

Vav1 works both as a catalytic Rho GTPase activator and an adaptor molecule. These functions, which are critical for T cell development and antigenic responses, are tyrosine phosphorylation-dependent. However, it is not known whether other posttranslational modifications can contribute to the regulation of the biological activity of this protein. Here, we show that Vav1 becomes acetylated on lysine residues in a stimulation- and SH2 domain-dependent manner. Using a collection of both acetylation- and deacetylation-mimicking mutants, we show that the acetylation of four lysine residues (Lys²²², Lys²⁵², Lys⁵⁸⁷, and Lys⁷¹⁶) leads to the downmodulation of the adaptor function of Vav1 that triggers the stimulation of the nuclear factor of activated T cells (NFAT). These sites belong to two functional subclasses according to mechanistic criteria. We have also unveiled additional acetylation sites potentially involved in either the stimulation (Lys⁷⁸²) or the downmodulation (Lys³³⁵, Lys³⁷⁴) of specific Vav1-dependent downstream responses. Collectively, these results indicate that Nε-lysine acetylation can play variegated roles in the regulation of Vav1 signaling. Unlike the case of the tyrosine phosphorylation step, this new regulatory layer is not conserved in other Vav family paralogs.

Deletion of the Nucleotide Exchange Factor Vav3 Enhances Axonal Complexity and Synapse Formation but Tampers Activity of Hippocampal Neuronal Networks In Vitro

Vav proteins activate GTPases of the RhoA subfamily that regulate the cytoskeleton and are involved in adhesion, migration, differentiation, polarity and the cell cycle. While the importance of RhoA GTPases for neuronal morphology is undisputed, their regulation is less well understood. In this perspective, we studied the consequences of the deletion of Vav2, Vav3 and Vav2 and 3 (Vav2^-/-, Vav3^-/-, Vav2^-/-/3^-/-) for the development of embryonic hippocampal neurons in vitro. Using an indirect co-culture system of hippocampal neurons with primary wild-type (WT) cortical astrocytes, we analysed axonal and dendritic parameters, structural synapse numbers and the spontaneous network activity via immunocytochemistry and multielectrode array analysis (MEA). Here, we observed a higher process complexity in Vav3^-/-, but not in Vav2^-/- neurons after three and five days in vitro (DIV). Furthermore, an enhanced synapse formation was observed in Vav3^-/- after 14 days in culture. Remarkably, Vav2^-/-/3^-/- double knockout neurons did not display synergistic effects. Interestingly, these differences were transient and compensated after a cultivation period of 21 days. Network analysis revealed a diminished number of spontaneously occurring action potentials in Vav3^-/- neurons after 21 DIV. Based on these results, it appears that Vav3 participates in key events of neuronal differentiation.

Genomic signatures of high-altitude adaptation in Ethiopian sheep populations

BACKGROUND

Ethiopian sheep populations such as Arsi-Bale, Horro and Adilo (long fat-tailed, LFT) inhabit mid to high-altitude areas; and Menz sheep (MZ, short fat-tailed) are adapted to cool sub-alpine environments. In contrast, Blackhead Somali sheep (BHS, fat-rumped) thrive well in arid and semi-arid areas characterized by high temperature and low precipitation. The genomic investigation of Ethiopian sheep populations may help to identify genes and biological pathways enable to adapt to the different ecological conditions.

OBJECTIVE

To uncover genomic regions and genes showing evidence of positive selection for altitude adaptation in Ethiopian sheep populations.

METHODS

A total of 72 animals inhabiting high-versus low-altitude environments were genotyped on an Ovine Infinium HD array (~ 600 K). Pairwise genetic differentiation (Fst) was calculated in sliding windows of 20 SNPs and the upper 1% smoothed Fst values were considered to represent positive selection signatures. Genes within < 25 kb of the most differentiated SNPs were considered as selection candidates.

RESULTS

Signatures of selection were detected in genes known to be associated high with altitude adaptation in MZ-BHS pair comparison (PPP1R12A, RELN, PARP2, and DNAH9) and in LFT-BHS pair comparison (VAV3, MSRB3,EIF2AK4, MET, and TACR1). The candidate genes (MITF, FGF5, MTOR, TRHDE, and TUBB3) associated with altitude adaptation and shared between the MZ-BHS and LTF-BHS pair comparisons were also detected as under selection. Further functional analyses reveal that the candidate genes were involved in biological processes and pathways relevant to adaptation under extreme altitudes, including respiratory system development and smoothened signaling pathway.

CONCLUSION

The results of the present study could aid in-depth understanding and exploitation of the underlying genetic mechanisms for sheep and other livestock species adaptation to high-altitude environments.

The Vav GEF Family: An Evolutionary and Functional Perspective

Vav proteins play roles as guanosine nucleotide exchange factors for Rho GTPases and signaling adaptors downstream of protein tyrosine kinases. The recent sequencing of the genomes of many species has revealed that this protein family originated in choanozoans, a group of unicellular organisms from which animal metazoans are believed to have originated from. Since then, the Vav family underwent expansions and reductions in its members during the evolutionary transitions that originated the agnates, chondrichthyes, some teleost fish, and some neoaves. Exotic members of the family harboring atypical structural domains can be also found in some invertebrate species. In this review, we will provide a phylogenetic perspective of the evolution of the Vav family. We will also pay attention to the structure, signaling properties, regulatory layers, and functions of Vav proteins in both invertebrate and vertebrate species.

Novel role of Rac-Mid1 signaling in medial cerebellar development

Rac signaling impacts a relatively large number of downstream targets; however, few studies have established an association between Rac pathways and pathological conditions. In the present study, we generated mice with double knockout of Rac1 and Rac3 (Atoh1-Cre;Rac1^flox/flox;Rac3^-/- ) in cerebellar granule neurons (CGNs). We observed impaired tangential migration at E16.5, as well as numerous apoptotic CGNs at the deepest layer of the external granule layer (EGL) in the medial cerebellum of Atoh1-Cre;Rac1^flox/flox;Rac3^-/- mice at P8. Atoh1-Cre;Rac1^flox/flox;Rac3^-/- CGNs differentiated normally until expression of p27^kip1 and NeuN in the deep EGL at P5. Primary CGNs and cerebellar microexplants from Atoh1-Cre;Rac1^flox/flox;Rac3^-/- mice exhibited impaired neuritogenesis, which was more apparent in Map2-positive dendrites. Such findings suggest that impaired tangential migration and final differentiation of CGNs have resulted in decreased cerebellum size and agenesis of the medial internal granule layer, respectively. Furthermore, Rac depleted/deleted cells exhibited decreased levels of Mid1 and impaired mTORC1 signaling. Mid1 depletion in CGNs produced mild impairments in neuritogenesis and reductions in mTORC1 signaling. Thus, a novel Rac-signaling pathway (Rac1-Mid1-mTORC1) may be involved in medial cerebellar development.

Involvement of the guanine nucleotide exchange factor Vav3 in central nervous system development and plasticity

Small GTP-hydrolyzing enzymes (GTPases) of the RhoA family play manifold roles in cell biology and are regulated by upstream guanine nucleotide exchange factors (GEFs). Herein, we focus on the GEFs of the Vav subfamily. Vav1 was originally described as a proto-oncogene of the hematopoietic lineage. The GEFs Vav2 and Vav3 are more broadly expressed in various tissues. In particular, the GEF Vav3 may play important roles in the developing nervous system during the differentiation of neural stem cells into the major lineages, namely neurons, oligodendrocytes and astrocytes. We discuss its putative regulatory roles for progenitor differentiation in the developing retina, polarization of neurons and formation of synapses, migration of oligodendrocyte progenitors and establishment of myelin sheaths. We propose that Vav3 mediates the response of various neural cell types to environmental cues.

Regulation of oligodendrocyte precursor maintenance by chondroitin sulphate glycosaminoglycans

Chondroitin sulfate proteoglycans (CSPGs) have been proven to inhibit morphological maturation of oligodendrocytes as well as their myelination capabilities. Yet, it remained unclear, whether CSPGs and/or their respective chondroitin sulfate glycosaminoglycan (CS-GAG) side chains also regulate the oligodendrocyte lineage progression. Here, we initially show that CS-GAGs detected by the monoclonal antibody 473HD are expressed by primary rat NG2-positive oligodendrocyte precursor cells (OPCs) and O4-positive immature oligodendrocytes. CS-GAGs become down-regulated with ongoing oligodendrocyte differentiation. Enzymatic removal of the CS-GAG chains by the bacterial enzyme Chondroitinase ABC (ChABC) promoted spontaneous differentiation of proliferating rat OPCs toward O4-positive immature oligodendrocytes. Upon forced differentiation, the enzymatic removal of the CS-GAGs accelerated oligodendrocyte differentiation toward both MBP-positive and membrane forming oligodendrocytes. These processes were attenuated on enriched CSPG fractions, mainly consisting of Phosphacan/RPTPβ/ζ and to less extent of Brevican and NG2. To qualify CS-GAGs as universal regulators of oligodendrocyte biology, we finally tested the effect of CS-GAG removal on OPCs from different sources such as mouse cortical oligospheres, mouse spinal cord neurospheres, and most importantly human-induced pluripotent stem cell-derived radial glia-like neural precursor cells. For all culture systems used, we observed a similar inhibitory effect of CS-GAGs on oligodendrocyte differentiation. In conclusion, this study clearly suggests an important fundamental principle for complex CS-GAGs to regulate the oligodendrocyte lineage progression. Moreover, the use of ChABC in order to promote oligodendrocyte differentiation toward myelin gene expressing cells might be an applicable therapeutic option to enhance white matter repair.

Midbrain dopamine neurons in Parkinson's disease exhibit a dysregulated miRNA and target-gene network

The degeneration of substantia nigra (SN) dopamine (DA) neurons in sporadic Parkinson׳s disease (PD) is characterized by disturbed gene expression networks. Micro(mi)RNAs are post-transcriptional regulators of gene expression and we recently provided evidence that these molecules may play a functional role in the pathogenesis of PD. Here, we document a comprehensive analysis of miRNAs in SN DA neurons and PD, including sex differences. Our data show that miRNAs are dysregulated in disease-affected neurons and differentially expressed between male and female samples with a trend of more up-regulated miRNAs in males and more down-regulated miRNAs in females. Unbiased Ingenuity Pathway Analysis (IPA) revealed a network of miRNA/target-gene associations that is consistent with dysfunctional gene and signaling pathways in PD pathology. Our study provides evidence for a general association of miRNAs with the cellular function and identity of SN DA neurons, and with deregulated gene expression networks and signaling pathways related to PD pathogenesis that may be sex-specific.

Vav family exchange factors: an integrated regulatory and functional view

The Vav family is a group of tyrosine phosphorylation-regulated signal transduction molecules hierarchically located downstream of protein tyrosine kinases. The main function of these proteins is to work as guanosine nucleotide exchange factors (GEFs) for members of the Rho GTPase family. In addition, they can exhibit a variety of catalysis-independent roles in specific signaling contexts. Vav proteins play essential signaling roles for both the development and/or effector functions of a large variety of cell lineages, including those belonging to the immune, nervous, and cardiovascular systems. They also contribute to pathological states such as cancer, immune-related dysfunctions, and atherosclerosis. Here, I will provide an integrated view about the evolution, regulation, and effector properties of these signaling molecules. In addition, I will discuss the pros and cons for their potential consideration as therapeutic targets.

Identification of a Vav2-dependent mechanism for GDNF/Ret control of mesolimbic DAT trafficking

Dopamine (DA) homeostasis is essential for a variety of brain activities. Dopamine transporter (DAT)-mediated DA reuptake is one of the most critical mechanisms for normal DA homeostasis. However, the molecular mechanisms underlying the regulation of DAT activity in the brain remain poorly understood. Here we show that the Rho-family guanine nucleotide exchange factor protein Vav2 is required for DAT cell surface expression and transporter activity modulated by glial cell line-derived neurotrophic factor (GDNF) and its cognate receptor Ret. Mice deficient in either Vav2 or Ret displayed elevated DAT activity, which was accompanied by an increase in intracellular DA selectively in the nucleus accumbens. Vav2(-/-) mice exposed to cocaine showed reduced DAT activity and diminished behavioral cocaine response. Our data demonstrate that Vav2 is a determinant of DAT trafficking in vivo and contributes to the maintenance of DA homeostasis in limbic DA neuron terminals.

Regulation and function of P-Rex family Rac-GEFs

The P-Rex family are Dbl-type guanine-nucleotide exchange factors for Rac family small G proteins. They are distinguished from other Rac-GEFs through their synergistic mode of activation by the lipid second messenger phosphatidyl inositol (3,4,5) trisphosphate and the Gβγ subunits of heterotrimeric G proteins, thus acting as coincidence detectors for phosphoinositide 3-kinase and G protein coupled receptor signaling. Work in genetically-modified mice has shown that P-Rex1 has physiological importance in the inflammatory response and the migration of melanoblasts during development, whereas P-Rex2 controls the dendrite morphology of cerebellar Purkinje neurons as well as glucose homeostasis in liver and adipose tissue. Deregulation of P-Rex1 and P-Rex2 expression occurs in many types of cancer, and P-Rex2 is frequently mutated in melanoma. Both GEFs promote tumor growth or metastasis. This review critically evaluates the P-Rex literature and tools available and highlights exciting recent developments and open questions.

Regulating Rac in the nervous system: molecular function and disease implication of Rac GEFs and GAPs

Rho family GTPases, including RhoA, Rac1, and Cdc42 as the most studied members, are master regulators of actin cytoskeletal organization. Rho GTPases control various aspects of the nervous system and are associated with a number of neuropsychiatric and neurodegenerative diseases. The activity of Rho GTPases is controlled by two families of regulators, guanine nucleotide exchange factors (GEFs) as the activators and GTPase-activating proteins (GAPs) as the inhibitors. Through coordinated regulation by GEFs and GAPs, Rho GTPases act as converging signaling molecules that convey different upstream signals in the nervous system. So far, more than 70 members of either GEFs or GAPs of Rho GTPases have been identified in mammals, but only a small subset of them have well-known functions. Thus, characterization of important GEFs and GAPs in the nervous system is crucial for the understanding of spatiotemporal dynamics of Rho GTPase activity in different neuronal functions. In this review, we summarize the current understanding of GEFs and GAPs for Rac1, with emphasis on the molecular function and disease implication of these regulators in the nervous system.

Cross-species epigenetics identifies a critical role for VAV1 in SHH subgroup medulloblastoma maintenance

The identification of key tumorigenic events in Sonic Hedgehog subgroup medulloblastomas (MB_SHH) will be essential for the development of individualized therapies and improved outcomes. However, beyond confirmation of characteristic SHH-pathway mutations, recent genome-wide sequencing studies have not revealed commonly-mutated genes with widespread relevance as potential therapeutic targets. We therefore examined any role for epigenetic DNA methylation events in MB_SHH using a cross-species approach to candidate identification, prioritization and validation. MB_SHH–associated DNA methylation events were first identified in 216 subgrouped human medulloblastomas (50 MB_SHH, 28 WNT_, 44 Group 3, 94 Group 4) and their conservation then assessed in tumors arising from four independent murine models of Shh medulloblastoma, alongside any role in tumorigenesis using functional assessments in mouse and human models. This strategy identified widespread regional CpG hypo-methylation of VAV1, leading to its elevated expression, as a conserved aberrant epigenetic event which characterizes the majority of MB_SHH tumors in both species, and is associated with a poor outcome in MB_SHH patients. Moreover, direct modulation of VAV1 in mouse and human models revealed a critical role in tumor maintenance, and its abrogation markedly reduced medulloblastoma growth. Further, Vav1 activity regulated granule neuron precursor (GNP) germinal zone exit and migration initiation in an ex vivo model of early post-natal cerebellar development. These findings establish VAV1 as a critical epigenetically-regulated oncogene with a key role in MB_SHH maintenance, and highlight its potential as a validated therapeutic target and prognostic biomarker for the improved therapy of medulloblastoma.

The guanine nucleotide exchange factor Vav3 regulates differentiation of progenitor cells in the developing mouse retina

The seven main cell types in the mammalian retina arise from multipotent retinal progenitor cells, a process that is tightly regulated by intrinsic and extrinsic signals. However, the molecular mechanisms that control proliferation, differentiation and cell-fate decisions of retinal progenitor cells are not fully understood yet. Here, we report that the guanine nucleotide exchange factor Vav3, a regulator of Rho-GTPases, is involved in retinal development. We demonstrate that Vav3 is expressed in the mouse retina during the embryonic period. In order to study the role of Vav3 in the developing retina, we generate Vav3-deficient mice. The loss of Vav3 results in an accelerated differentiation of retinal ganglion cells and cone photoreceptors during early and late embryonic development. We provide evidence that more retinal progenitor cells express the late progenitor marker Sox9 in Vav3-deficient mice than in wild-types. This premature differentiation is compensated during the postnatal period and late-born cell types such as bipolar cells and Müller glia display normal numbers. Taken together, our data imply that Vav3 is a regulator of retinal progenitor cell differentiation, thus highlighting a novel role for guanine nucleotide exchange factors in retinogenesis.

RhoGEFs in cell motility: novel links between Rgnef and focal adhesion kinase

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

The small GTPases RhoA and Rac1 regulate cerebellar development by controlling cell morphogenesis, migration and foliation

The small GTPases RhoA and Rac1 are key cytoskeletal regulators that function in a mutually antagonistic manner to control the migration and morphogenesis of a broad range of cell types. However, their role in shaping the cerebellum, a unique brain structure composed of an elaborate set of folia separated by fissures of different lengths, remains largely unexplored. Here we show that dysregulation of both RhoA and Rac1 signaling results in abnormal cerebellar ontogenesis. Ablation of RhoA from neuroprogenitor cells drastically alters the timing and placement of fissure formation, the migration and positioning of granule and Purkinje cells, the alignment of Bergmann glia, and the integrity of the basement membrane, primarily in the anterior lobules. Furthermore, in the absence of RhoA, granule cell precursors located at the base of fissures fail to undergo cell shape changes required for fissure initiation. Many of these abnormalities can be recapitulated by deleting RhoA specifically from granule cell precursors but not postnatal glia, indicating that RhoA functions in granule cell precursors to control cerebellar morphogenesis. Notably, mice with elevated Rac1 activity due to loss of the Rac1 inhibitors Bcr and Abr show similar anterior cerebellar deficits, including ectopic neurons and defects in fissure formation, Bergmann glia organization and basement membrane integrity. Together, our results suggest that RhoA and Rac1 play indispensable roles in patterning cerebellar morphology.

The C-terminal SH3 domain contributes to the intramolecular inhibition of Vav family proteins

Vav proteins are phosphorylation-dependent guanine nucleotide exchange factors (GEFs) that catalyze the activation of members of the Rho family of guanosine triphosphatases (GTPases). The current regulatory model holds that the nonphosphorylated, catalytically inactive state of these GEFs is maintained by intramolecular interactions among the amino-terminal domains and the central catalytic core, which block the binding of Vav proteins to GTPases. We showed that this autoinhibition is mechanistically more complex, also involving the bivalent association of the carboxyl-terminal Src homology 3 (SH3) region of Vav with its catalytic and pleckstrin homology (PH) domains. Such interactions occurred through proline-rich region-independent mechanisms. Full release from this double-locked state required synergistic weakening effects from multiple phosphorylated tyrosine residues, thus providing an optimized system to generate gradients of Vav GEF activity depending on upstream signaling inputs. This mechanism is shared by mammalian and Drosophila melanogaster Vav proteins, suggesting that it may be a common regulatory feature for this protein family.

The Rho exchange factors Vav2 and Vav3 favor skin tumor initiation and promotion by engaging extracellular signaling loops

The catalytic activity of GDP/GTP exchange factors (GEFs) is considered critical to maintain the typically high activity of Rho GTPases found in cancer cells. However, the large number of them has made it difficult to pinpoint those playing proactive, nonredundant roles in tumors. In this work, we have investigated whether GEFs of the Vav subfamily exert such specific roles in skin cancer. Using genetically engineered mice, we show here that Vav2 and Vav3 favor cooperatively the initiation and promotion phases of skin tumors. Transcriptomal profiling and signaling experiments indicate such function is linked to the engagement of, and subsequent participation in, keratinocyte-based autocrine/paracrine programs that promote epidermal proliferation and recruitment of pro-inflammatory cells. This is a pathology-restricted mechanism because the loss of Vav proteins does not cause alterations in epidermal homeostasis. These results reveal a previously unknown Rho GEF-dependent pro-tumorigenic mechanism that influences the biology of cancer cells and their microenvironment. They also suggest that anti-Vav therapies may be of potential interest in skin tumor prevention and/or treatment.

The rho exchange factors vav2 and vav3 control a lung metastasis-specific transcriptional program in breast cancer cells

The guanosine triphosphatases of the Rho and Rac subfamilies regulate protumorigenic pathways and are activated by guanine nucleotide exchange factors (Rho GEFs), which could be potential targets for anticancer therapies. We report that two Rho GEFs, Vav2 and Vav3, play synergistic roles in breast cancer by sustaining tumor growth, neoangiogenesis, and many of the steps involved in lung-specific metastasis. The involvement of Vav proteins in these processes did not correlate with Rac1 and RhoA activity or cell migration, implying the presence of additional biological programs. Microarray analyses revealed that Vav2 and Vav3 controlled a vast transcriptional program in breast cancer cells through mechanisms that were shared between the two proteins, isoform-specific or synergistic. Furthermore, the abundance of Vav-regulated transcripts was modulated by Rac1-dependent and Rac1-independent pathways. This transcriptome encoded therapeutically targetable proteins that played nonredundant roles in primary tumorigenesis and lung-specific metastasis, such as integrin-linked kinase (Ilk), the transforming growth factor-β family ligand inhibin βA, cyclooxygenase-2, and the epithelial cell adhesion molecule Tacstd2. It also contained gene signatures that predicted disease outcome in breast cancer patients. These results identify possible targets for treating breast cancer and lung metastases and provide a potential diagnostic tool for clinical use.

Purkinje cell loss and motor coordination defects in profilin1 mutant mice

Profilin1 is an actin monomer-binding protein, essential for cytoskeletal dynamics. Based on its broad expression in the brain and the localization at excitatory synapses (hippocampal CA3-CA1 synapse, cerebellar parallel fiber (PF)-Purkinje cell (PC) synapse), an important role for profilin1 in brain development and synapse physiology has been postulated. We recently showed normal physiology of hippocampal CA3-CA1 synapses in the absence of profilin1, but impaired glial cell binding and radial migration of cerebellar granule neurons (CGNs). Consequently, brain-specific inactivation of profilin1 by exploiting conditional mutants and Nestin-mediated cre expression resulted in a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers, and ectopic CGNs. Apart from these findings we noted a loss of PCs and an irregularly shaped PC layer in adult mutants. In this study, we show that PC migration and development are not affected in profilin1 mutants, suggesting cell type-specific functions for profilin1 in PCs and CGNs. PC loss begins during the second postnatal week and progresses until adulthood with no further impairment in aged mutants. In Nestin-cre profilin1 mutants, defects in cerebellar cortex cytoarchitecture are associated with impaired motor coordination. However, in L7-cre mutants, lacking profilin1 specifically in PCs, the cerebellar cortex cytoarchitecture is unchanged. Thereby, our results demonstrate that the loss of PCs is not caused by cell-autonomous defects, but presumably by impaired CGN migration. Finally, we show normal functionality of PF-PC synapses in the absence of profilin1. In summary, we conclude that profilin1 is crucially important for brain development, but dispensable for the physiology of excitatory synapses.

Transcriptional architecture of the primate neocortex

Genome-wide transcriptional profiling was used to characterize the molecular underpinnings of neocortical organization in rhesus macaque, including cortical areal specialization and laminar cell-type diversity. Microarray analysis of individual cortical layers across sensorimotor and association cortices identified robust and specific molecular signatures for individual cortical layers and areas, prominently involving genes associated with specialized neuronal function. Overall, transcriptome-based relationships were related to spatial proximity, being strongest between neighboring cortical areas and between proximal layers. Primary visual cortex (V1) displayed the most distinctive gene expression compared to other cortical regions in rhesus and human, both in the specialized layer 4 as well as other layers. Laminar patterns were more similar between macaque and human compared to mouse, as was the unique V1 profile that was not observed in mouse. These data provide a unique resource detailing neocortical transcription patterns in a nonhuman primate with great similarity in gene expression to human.

Genetic adaptation to high altitude in the Ethiopian highlands

BACKGROUND

Genomic analysis of high-altitude populations residing in the Andes and Tibet has revealed several candidate loci for involvement in high-altitude adaptation, a subset of which have also been shown to be associated with hemoglobin levels, including EPAS1, EGLN1, and PPARA, which play a role in the HIF-1 pathway. Here, we have extended this work to high- and low-altitude populations living in Ethiopia, for which we have measured hemoglobin levels. We genotyped the Illumina 1M SNP array and employed several genome-wide scans for selection and targeted association with hemoglobin levels to identify genes that play a role in adaptation to high altitude.

RESULTS

We have identified a set of candidate genes for positive selection in our high-altitude population sample, demonstrated significantly different hemoglobin levels between high- and low-altitude Ethiopians and have identified a subset of candidate genes for selection, several of which also show suggestive associations with hemoglobin levels.

CONCLUSIONS

We highlight several candidate genes for involvement in high-altitude adaptation in Ethiopia, including CBARA1, VAV3, ARNT2 and THRB. Although most of these genes have not been identified in previous studies of high-altitude Tibetan or Andean population samples, two of these genes (THRB and ARNT2) play a role in the HIF-1 pathway, a pathway implicated in previous work reported in Tibetan and Andean studies. These combined results suggest that adaptation to high altitude arose independently due to convergent evolution in high-altitude Amhara populations in Ethiopia.

The role of Rho GTPase proteins in CNS neuronal migration

The architectonics of the mammalian brain arise from a remarkable range of directed cell migrations, which orchestrate the emergence of cortical neuronal layers and pattern brain circuitry. At different stages of cortical histogenesis, specific modes of cell motility are essential to the stepwise formation of cortical architecture. These movements range from interkinetic nuclear movements in the ventricular zone, to migrations of early-born, postmitotic polymorphic cells into the preplate, to the radial migration of precursors of cortical output neurons across the thickening cortical wall, and the vast, tangential migrations of interneurons from the basal forebrain into the emerging cortical layers. In all cases, actomyosin motors act in concert with cell adhesion receptor systems to provide the force and traction needed for forward movement. As key regulators of actin and microtubule cytoskeletons, cell polarity, and adhesion, the Rho GTPases play critical roles in CNS neuronal migration. This review will focus on the different types of migration in the developing neocortex and cerebellar cortex, and the role of the Rho GTPases, their regulators and effectors in these CNS migrations, with particular emphasis on their involvement in radial migration.

Transcriptional factor aryl hydrocarbon receptor (Ahr) controls cardiovascular and respiratory functions by regulating the expression of the Vav3 proto-oncogene

Aryl hydrocarbon receptor (Ahr) is a transcriptional factor involved in detoxification responses to pollutants and in intrinsic biological processes of multicellular organisms. We recently described that Vav3, an activator of Rho/Rac GTPases, is an Ahr transcriptional target in embryonic fibroblasts. These results prompted us to compare the Ahr(-/-) and Vav3(-/-) mouse phenotypes to investigate the implications of this functional interaction in vivo. Here, we show that Ahr is important for Vav3 expression in kidney, lung, heart, liver, and brainstem regions. This process is not affected by the administration of potent Ahr ligands such as benzo[a]pyrene. We also report that Ahr- and Vav3-deficient mice display hypertension, tachypnea, and sympathoexcitation. The Ahr gene deficiency also induces the GABAergic transmission defects present in the Vav3(-/-) ventrolateral medulla, a main cardiorespiratory brainstem center. However, Ahr(-/-) mice, unlike Vav3-deficient animals, display additional defects in fertility, perinatal growth, liver size and function, closure, spleen size, and peripheral lymphocytes. These results demonstrate that Vav3 is a bona fide Ahr target that is in charge of a limited subset of the developmental and physiological functions controlled by this transcriptional factor. Our data also reveal the presence of sympathoexcitation and new cardiorespiratory defects in Ahr(-/-) mice.

Vav3 is involved in GABAergic axon guidance events important for the proper function of brainstem neurons controlling cardiovascular, respiratory, and renal parameters

Vav3 is a phosphorylation-dependent activator of Rho/Rac GTPases that has been implicated in hematopoietic, bone, cerebellar, and cardiovascular roles. Consistent with the latter function, Vav3-deficient mice develop hypertension, tachycardia, and renocardiovascular dysfunctions. The cause of those defects remains unknown as yet. Here, we show that Vav3 is expressed in GABAegic neurons of the ventrolateral medulla (VLM), a brainstem area that modulates respiratory rates and, via sympathetic efferents, a large number of physiological circuits controlling blood pressure. On Vav3 loss, GABAergic cells of the caudal VLM cannot innervate properly their postsynaptic targets in the rostral VLM, leading to reduced GABAergic transmission between these two areas. This results in an abnormal regulation of catecholamine blood levels and in improper control of blood pressure and respiration rates to GABAergic signals. By contrast, the reaction of the rostral VLM to excitatory signals is not impaired. Consistent with those observations, we also demonstrate that Vav3 plays important roles in axon branching and growth cone morphology in primary GABAergic cells. Our study discloses an essential and nonredundant role for this Vav family member in axon guidance events in brainstem neurons that control blood pressure and respiratory rates.