Noninflammatory cerebral vasculopathy associated with recurrent ischemic strokes.

Recurrent ischemic strokes often have uncommon causes in young adults. Vascular abnormalities may be considered as a possible etiology. We report a 36-year-old man who experienced recurrent cryptogenic ischemic strokes despite medical therapy. Conventional cerebral angiography was unrevealing. Subsequent brain biopsy revealed a distinctive histopathological pattern of abnormal perivascular collagen deposition without inflammation. Recurrent cryptogenic strokes may have novel etiologies, and brain biopsy should be considered when standard diagnostic tests fail.

PyK2 and FAK connections to p190Rho guanine nucleotide exchange factor regulate RhoA activity, focal adhesion formation, and cell motility.

Integrin binding to matrix proteins such as fibronectin (FN) leads to formation of focal adhesion (FA) cellular contact sites that regulate migration. RhoA GTPases facilitate FA formation, yet FA-associated RhoA-specific guanine nucleotide exchange factors (GEFs) remain unknown. Here, we show that proline-rich kinase-2 (Pyk2) levels increase upon loss of focal adhesion kinase (FAK) in mouse embryonic fibroblasts (MEFs). Additionally, we demonstrate that Pyk2 facilitates deregulated RhoA activation, elevated FA formation, and enhanced cell proliferation by promoting p190RhoGEF expression. In normal MEFs, p190RhoGEF knockdown inhibits FN-associated RhoA activation, FA formation, and cell migration. Knockdown of p190RhoGEF-related GEFH1 does not affect FA formation in FAK(-/-) or normal MEFs. p190RhoGEF overexpression enhances RhoA activation and FA formation in MEFs dependent on FAK binding and associated with p190RhoGEF FA recruitment and tyrosine phosphorylation. These studies elucidate a compensatory function for Pyk2 upon FAK loss and identify the FAK-p190RhoGEF complex as an important integrin-proximal regulator of FA formation during FN-stimulated cell motility.

Disruption of neurofilament network with aggregation of light neurofilament protein: a common pathway leading to motor neuron degeneration due to Charcot-Marie-Tooth disease-linked mutations in NFL and HSPB1.

Mutations in neurofilament light (NFL) subunit and small heat-shock protein B1 (HSPB1) cause autosomal-dominant axonal Charcot-Marie-Tooth disease type 2E (CMT2E) and type 2F (CMT2F). Previous studies have shown that CMT mutations in NFL and HSPB1 disrupt NF assembly and cause aggregation of NFL protein. In this study, we investigate the role of aggregation of NFL protein in the neurotoxicity of CMT mutant NFL and CMT mutant HSPB1 in motor neurons. We find that expression of CMT mutant NFL leads to progressive degeneration and loss of neuronal viability of cultured motor neurons. Degenerating motor neurons show fragmentation and loss of neuritic processes associated with disruption of NF network and aggregation of NFL protein. Co-expression of wild-type HSPB1 diminishes aggregation of CMT mutant NFL, induces reversal of CMT mutant NFL aggregates and reduces CMT mutant NFL-induced loss of motor neuron viability. Like CMT mutant NFL, expression of S135F CMT mutant HSPB1 also leads to progressive degeneration of motor neurons with disruption of NF network and aggregation of NFL protein. Further studies show that wild-type and S135F mutant HSPB1 associate with wild-type and CMT mutant NFL and that S135F mutant HSPB1 has dominant effect on disruption of NF assembly and aggregation of NFL protein. Finally, we show that deletion of NFL markedly reduces degeneration and loss of motor neuron viability induced by S135F mutant HSPB1. Together, our data support the view that disruption of NF network with aggregation of NFL is a common triggering event of motor neuron degeneration in CMT2E and CMT2F disease.

The complex relation between genotype and phenotype in motor neuron disease.

The success in mapping genetic loci and identifying mutant genes in familial neurodegenerative disease has outpaced our ability to understand the linkage between genotype and phenotype of disease. The results have led to a backlog of genetic information with limited clarification of underlying disease mechanisms. A major dilemma is how mutations in widely expressed proteins lead to degeneration or dysfunction of small subsets of neurons. The problem raises fundamental questions as to the nature and interrelation of pathways that maintain the homeostasis of differentiated neurons. The issue also bears on the pathogenesis of sporadic forms of disease and prospective efficacy of therapeutic applications. This review examines the problem as it relates to motor neuron disease.

Role of neurofilament aggregation in motor neuron disease.

A major question in the pathogenesis of motor neuron disease is why motor neurons are selectively susceptible to mutations in widely expressed gene products. Reexamination of motor neuron degeneration due to alterations of neurofilament (NF) expression suggests that disruption of assembly with aggregation of the light neurofilament (NFL) protein may be an upstream event and contributing factor leading to the preferential degeneration of motor neurons. The implications of these findings are that aggregation of NFL is not only a triggering mechanism to account for the hallmark aggregates of NF protein in sporadic and familial forms of amyotrophic lateral sclerosis, but that aggregates of NFL may also promote aggregation of wildly expressed proteins that are destabilized by missense mutations, such as by mutations in superoxide dismutase-1 protein. This review examines the potential role of NFs in determining and promoting the preferential degeneration of motor neurons in motor neuron disease. The underlying premise is that motor neurons are selectively susceptible to alterations in NF expression, that alterations in NF expression lead to NF aggregates in motor neurons, and that elevated levels of NF aggregates provide a favorable microenvironment for the formation of neurotoxic aggregation and degeneration of motor neurons.

RNA-binding protein is involved in aggregation of light neurofilament protein and is implicated in the pathogenesis of motor neuron degeneration.

Abnormal protein aggregation is emerging as a common theme in the pathogenesis of neurodegenerative disease. Our previous studies have shown that overexpression of untranslated light neurofilament (NF-L) RNA causes motor neuron degeneration in transgenic mice, leads to accumulation of ubiquitinated aggregates in degenerating cultured motor neurons and triggers aggregation of NF-L protein and co-aggregation of mutant SOD1 protein in neuronal cells. Here, we report that p190RhoGEF, an RNA-binding protein that binds to a destabilizing element in NF-L mRNA, is involved in aggregation of NF-L protein and is implicated in the pathogenesis of motor neuron degeneration. We show that p190RhoGEF co-aggregates with unassembled NF-L protein and that co-aggregation is associated with down-regulation of parent NF-L mRNA in neuronal cells. Co-expression of NF-M increases NF assembly and reduces RNA-triggered aggregation as well as loss of solubility of NF-L protein. siRNA-induced down-regulation of p190RhoGEF not only reduces aggregation and promotes assembly of NF-L and NF-M, but also causes reversal of aggregation and recovery of NF assembly in transfected cells. Examination of transgenic models of motor neuron disease shows that prominent aggregates of p190RhoGEF and NF-L and down-regulation of NF-L expression occur in degenerating motor neurons of mice expressing untranslated NF-L RNA or a G93A mutant SOD1 transgene. Moreover, aggregates of p190RhoGEF and NF-L appear as early pathological changes in presymptomatic G93A mutant SOD1 transgenic mice. Together, the findings indicate that p190RhoGEF is involved in aggregation of NF-L protein and support a working hypothesis that aggregation of p190RhoGEF and NF-L is an upstream event triggering neurotoxicity in motor neuron disease.

HoxB2 binds mutant SOD1 and is altered in transgenic model of ALS.

Mutations in Cu/Zn superoxide dismutase (SOD1) cause approximately 20% of familial amyotrophic lateral sclerosis by a toxic gain of function; however, the precise mechanisms remain unclear. Here, we report the identification of HoxB2, a homeodomain-containing transcription factor, as a G93A mutant SOD1 interactive protein in a yeast two-hybrid screen. We show that HoxB2 co-precipitates and co-localizes with mutant SOD1 in neuronal cell lines, as well as in brain and spinal cord of G93A mutant SOD1 transgenic mice. Mutagenesis further shows that this interaction is mediated by the central homeodomain of HoxB2. In motor neuron-like NSC-34 cells, overexpression of HoxB2 or its homeodomain decreases the insolubility of mutant SOD1 and inhibits G93A or G86R mutant SOD1-induced neuronal cell death. In human and mouse tissues, we show that expression of HoxB2 persists in adult spinal cord and is primarily localized in nuclei of motor neurons. In G93A transgenic mice, HoxB2 co-localizes with mutant SOD1 and is redistributed to perikarya and proximal neurites of motor neurons. In addition, there is progressive accumulation of HoxB2 and mutant SOD1 as punctate inclusions in the neuropil surrounding motor neurons. Taken together, our findings demonstrate that interaction of HoxB2 with mutant SOD1 occurs in motor neurons of G93A mutant SOD1 transgenic mice and suggest that this interaction may modulate the neurotoxicity of mutant SOD1.

3' untranslated region in a light neurofilament (NF-L) mRNA triggers aggregation of NF-L and mutant superoxide dismutase 1 proteins in neuronal cells.

The pathogenesis of neurodegenerative diseases is believed to involve abnormal aggregation of proteins, but the mechanisms initiating protein aggregation are unclear. Here we report a novel phenomenon that could be instrumental in triggering protein aggregation in neurodegenerative diseases. We show that the 3' untranslated region (3'UTR) of a light neurofilament (NF-L) transcript enhances the reactivity of its own translated product and leads to loss of solubility and aggregation of NF-L protein and to coaggregation of mutant superoxide dismutase 1 (SOD1) protein. Full-length mouse NF-L cDNAs, with and without NF-L 3'UTR, were fused to the C terminus of a green fluorescent protein (GFP) reporter gene, and the GFP-tagged NF-L proteins were examined in transfected Neuro2a cells. The GFP-tagged NF-L protein expressed from the transgene containing NF-L 3'UTR, but not from the transgene lacking NF-L 3'UTR, colocalizes with endogenous heavy neurofilament protein and, at high-level expression, leads to loss of solubility and aggregation of GFP-tagged NF-L protein. Aggregation of GFP-tagged NF-L protein triggers coaggregation and loss of solubility of coexpressed DsRed-tagged mutant (G93A) SOD1 protein but not wild-type SOD1 protein. Deletional mutagenesis maps the RNA sequence causing aggregation of GFP-tagged NF-L protein to the proximal 45 nucleotides of NF-L 3'UTR. This is the site of a major destabilizing element in NF-L RNA and binding site for RNA-binding proteins. Our findings support a working model whereby NF-L RNA, or cognate RNA-binding factors, enhances the reactivity of NF-L protein and provides a triggering mechanism leading to aggregation of NF-L and other proteins in neurodegenerative diseases.

Neurofilament RNA causes neurodegeneration with accumulation of ubiquitinated aggregates in cultured motor neurons.

The mechanisms whereby mutant gene expression triggers neurodegeneration are poorly understood but have generally been attributed to translated gene products. We now demonstrate direct neuropathic effects of untranslated RNA on cultured motor neurons. We show that expression of untranslated light neurofilament (NF-L) RNA sequence in the 3'UTR of an EGFP transgene (pEGFP/NF-L RNA) or in a separate expression vector (pRc/NF-L RNA) causes dose-dependent, neuron-specific motor neuron degeneration. Neither unfused EGFP protein (pEGFP/wt) nor EGFP-tagged NF-L protein (pEGFP/NF-L protein) has similar neuropathic effects. The findings are the first demonstration of a direct RNA-mediated neurotoxic effect. Moreover, the resulting neuropathological changes show that untranslated RNA can lead to early degeneration of neuritic processes and accumulations of ubiquitinated aggregates in the perikarya and nuclei of degenerating motor neurons. The latter findings are hallmark neuropathological features of neurodegenerative diseases and their occurrence as a result of altered RNA expression raises the prospects of an RNA-mediated component in the pathogenesis of neurodegenerative states.

Cytoplasmic retention sites in p190RhoGEF confer anti-apoptotic activity to an EGFP-tagged protein.

p190RhoGEF is a large multi-functional protein with guanine nucleotide exchange (GEF) activity. The C-terminal region of p190RhoGEF is a highly interactive domain that binds multiple factors, including proteins with anti-apoptotic activities. We now report that transfection of EGFP-tagged p190RhoGEF protects Neuro 2a cells from stress-induced apoptosis and that anti-apoptotic activity is localized to cytoplasmic retention sequences (CRS-1 and CRS-2) in the C-terminal region of p190RhoGEF. Cytoplasmic retention is conferred to an EGFP fluorescent marker when fused to either CRS-1 or CRS-2. Both cytoplasmic retention and anti-apoptotic activity are lost by deleting CRS-1 and CRS-2 in the p190RhoGEF sequence and can be recovered by restoring either CRS-1 or CRS-2 to the EGFP-tagged sequence. Since the CRS-1 and CRS-2 contain the JIP-1 and 14-3-3 binding sites, we propose that anti-apoptotic activity may be conferred by the binding of p190RhoGEF to JIP-1 or 14-3-3, possibly by altering their interactive properties or nucleocytoplasmic movements. Taken together, our findings support a model whereby multiple interactions of p190RhoGEF confer homeostatic properties to differentiated neurons and may link neuronal homeostasis to the regulation of NF-L expression.

Direct interaction of focal adhesion kinase with p190RhoGEF.

Focal adhesion kinase (FAK) is a protein-tyrosine kinase that associates with multiple cell surface receptors and signaling proteins through which it can modulate the activity of several intracellular signaling pathways. FAK activity can influence the formation of distinct actin cytoskeletal structures such as lamellipodia and stress fibers in part through effects on small Rho GTPases, although the molecular interconnections of these events are not well defined. Here, we report that FAK interacts with p190RhoGEF, a RhoA-specific GDP/GTP exchange factor, in neuronal cells and in brain tissue extracts by co-immunoprecipitation and co-localization analyses. Using a two-hybrid assay and deletion mutagenesis, the binding site of the FAK C-terminal focal adhesion targeting (FAT) domain was identified within the C-terminal coiled-coil domain of p190RhoGEF. Binding was independent of a LD-like binding motif within p190RhoGEF, yet FAK association was disrupted by a mutation (Leu-1034 to Ser) that weakens the helical bundle structure of the FAK FAT domain. Neuro-2a cell binding to laminin increased endogenous FAK and p190RhoGEF tyrosine phosphorylation, and co-transfection of a dominant-negative inhibitor of FAK activity, termed FRNK, inhibited lamininstimulated p190RhoGEF tyrosine phosphorylation and p21 RhoA GTP binding. Overexpression of FAK in Neuro-2a cells increased both endogenous p190RhoGEF tyrosine phosphorylation and RhoA activity, whereas these events were inhibited by FRNK co-expression. Because insulin-like growth factor 1 treatment of Neuro-2a cells increased FAK tyrosine phosphorylation and enhanced p190RhoGEF-mediated activation of RhoA, our results support the conclusion that FAK association with p190RhoGEF functions as a signaling pathway downstream of integrins and growth factor receptors to stimulate Rho activity.

Binding of p190RhoGEF to a destabilizing element on the light neurofilament mRNA is competed by BC1 RNA.

The enhancement of RNA-mediated motor neuron degeneration in transgenic mice by mutating a major mRNA instability determinant in a light neurofilament (NF-L) transgene implicates cognate RNA binding factors in the pathogenesis of motor neuron degeneration. p190RhoGEF is a neuron-enriched guanine exchange factor (GEF) that binds to the NF-L-destabilizing element, to c-Jun N-terminal kinase-interactive protein-1 (JIP-1), and to 14-3-3 and may link neurofilament expression to pathways affecting neuronal homeostasis. This study was undertaken to identify additional RNA species that bind p190RhoGEF and could affect interactions of the exchange factor with NF-L transcripts. The C-terminal domain of p190RhoGEF, containing the RNA-binding site, was expressed as a glutathione S-transferase fusion protein and was used as an affinity probe to isolate interactive RNAs in rat brain extracts. As expected, NF-L mRNA was identified as an RNA specie eluted from the affinity column. In addition, BC1 RNA was also found enriched in the bound RNA fraction. BC1 is a 152-nucleotide RNA that is highly expressed but untranslated in differentiated neurons. We show that BC1 and NF-L mRNA bind to a similar site in the C-terminal domain of p190RhoGEF, and their bindings to p190RhoGEF are readily cross-competed. Moreover, we identify a novel binding site in BC1 to account for its interaction with p190RhoGEF. The findings suggest a novel role of BC1 in differentiated neurons involving RNA-protein interactions of p190RhoGEF.

Untranslated element in neurofilament mRNA has neuropathic effect on motor neurons of transgenic mice.

Studies of experimental motor neuron degeneration attributable to expression of neurofilament light chain (NF-L) transgenes have raised the possibility that the neuropathic effects result from overexpression of NF-L mRNA, independent of NF-L protein effects (Cañete-Soler et al., 1999). The present study was undertaken to test for an RNA-mediated pathogenesis. Transgenic mice were derived using either an enhanced green fluorescent protein reporter construct or modified chimeric constructs that differ only in their 3' untranslated regions (UTRs). Motor function and spinal cord histology were normal in mice expressing the unmodified reporter transgene. In mice expressing a chimeric transgene in which sequence of NF-L 3' UTR was inserted into the 3' UTR of the reporter transgene, we observed growth retardation and reduced kinetic activity during postnatal development. Older mice developed impairment of motor function and atrophy of nerve fibers in the ventral roots. A similar but more severe phenotype was observed when the chimeric transgene contained a 36 bp c-myc insert in an mRNA destabilizing element of the NF-L sequence. Our results suggest that neuropathic effects of overexpressing NF-L can occur at the level of transgene RNA and are mediated by sequences in the NF-L 3' UTR.