Solubilization of aggregates formed by expanded polyglutamine tract expression in cultured cells.

The expression of expanded polyglutamine in mammalian cells causes the formation of aggregates. For elucidation of the biochemical properties of the aggregates, isolation and solubilization of the aggregates are required. This chapter provides useful protocols for the solubilization of polyglutamine aggregates and further applications of protein analysis.

Effects of fibrillin-1 degradation on microfibril ultrastructure.

Current models of the elastic properties and structural organization of fibrillin-containing microfibrils are based primarily on microscopic analyses of microfibrils liberated from connective tissues after digestion with crude collagenase. Results presented here demonstrate that this digestion resulted in the cleavage of fibrillin-1 and loss of specific immunoreactive epitopes. The proline-rich region and regions near the second 8-cysteine domain in fibrillin-1 were easily cleaved by crude collagenase. Other sites that may also be cleaved during microfibril digestion and extraction were identified. In contrast to collagenase-digested microfibrils, guanidine-extracted microfibrils contained all fibrillin-1 epitopes recognized by available antibodies. The ultrastructure of guanidine-extracted microfibrils differed markedly from that of collagenase-digested microfibrils. Fibrillin-1 filaments splayed out, extending beyond the width of the periodic globular beads. Both guanidine-extracted and collagenase-digested microfibrils were subjected to extensive digestion by crude collagenase. Collagenase digestion of guanidine-extracted microfibrils removed the outer filaments, revealing a core structure. In contrast to microfibrils extracted from tissues, cell culture microfibrils could be digested into short units containing just a few beads. These data suggest that additional cross-links stabilize the long beaded microfibrils in tissues. Based on the microfibril morphologies observed after these experiments, on the crude collagenase cleavage sites identified in fibrillin-1, and on known antibody binding sites in fibrillin-1, a model is proposed in which fibrillin-1 molecules are staggered in microfibrils. This model further suggests that the N-terminal half of fibrillin-1 is asymmetrically exposed in the outer filaments, whereas the C-terminal half of fibrillin-1 is present in the interior of the microfibril.

Fibrillins 1 and 2 perform partially overlapping functions during aortic development.

Fibrillin-rich microfibrils are extracellular assemblies that impart structural properties to the connective tissue. To elucidate the contribution of fibrillin-rich microfibrils to organogenesis, we have examined the vascular phenotype of a newly created strain of mice that completely lacks fibrillin-1 and the consequences of combined deficiency of fibrillins 1 and 2 on tissue formation. The results demonstrated that fibrillins 1 and 2 perform partially overlapping functions during aortic development. Fbn1-/- mice died soon after birth from ruptured aortic aneurysm, impaired pulmonary function, and/or diaphragmatic collapse. Analysis of the neonatal Fbn1-/- aorta documented a disorganized and poorly developed medial layer but normal levels of elastin cross-links. Transcriptional profiling revealed that aneurysm progression in Fbn1 null mice is accompanied by unproductive up-regulation of gene products normally involved in tissue repair and vascular integrity, such as plasminogen activator inhibitor-1, activin A, and cysteine-rich angiogenic protein 61. In contrast to Fbn1-/- mice, Fbn2 null mice had a well developed and morphologically normal aortic wall. However, virtually all Fbn1-/-;Fbn2-/- embryos and about half of the Fbn1+/-;Fbn2-/- embryos died in utero and displayed a significantly more severe vascular phenotype than Fbn1-/- mice. Consistent with a specialized function of fibrillin-2, electron microscopy visualized ultrastructurally different microfibrils in Fbn1 null compared with control cell cultures. Collectively, these data demonstrate that involvement of fibrillin-2 in the initial assembly of the aortic matrix overlaps in part with fibrillin-1 and that continued fibrillin-1 deposition is absolutely required for the maturation and function of the vessel during neonatal life.

Histone H3 is aberrantly phosphorylated in glutamine-repeat diseases.

Double-labeling immunohistochemical studies staining with anti-ubiquitin and anti-phosphoserine antibodies and application of an enzymatic dephosphorylation technique reveal neuronal inclusions and affected nuclei to be aberrantly phosphorylated in brain tissues with patients with glutamine-repeat diseases. Regional distribution of the phosphorylated nuclei in neurons correlates with the pathology. To identify the target nuclear protein, transient expression of Huntington's disease exon 1 gene containing an expanded glutamine repeat was generated in a cell culture and nuclear inclusions were isolated with a fluorescence-activated cell sorting system. Immunoblotting studies of the aggregated nuclear proteins using anti-phosphoserine antibody demonstrate the protein of the aberrant phosphorylation as histone H3. The immunoblots of control and diseased brain tissues demonstrate that the phosphorylation of histone H3 is commonly increased in the diseased brains. Aberrant phosphorylation of histone H3 is surmised to be a shared pathological process in glutamine-repeat diseases.

Ultrastructure of nuclear aggregates formed by expressing an expanded polyglutamine.

Intranuclear inclusions have been observed in the brains of patients affected with Huntington's disease (HD). Neuro 2A cells that transiently expressed HD exon 1 bearing 74 glutamine repeats linked to the green fluorescent protein (GFP) and the nuclear localization sequence (NLS) contained aggregates in nuclei. The aggregates were purified by fractionation with centrifugation followed by fluorescence-activated cell sorting (FACS). Heat treatment of the aggregate in an SDS sample buffer caused the dense aggregate cores to disappear and generated a basket-like structure composed of fibrils. Biochemical analysis of the aggregates revealed that the HD exon 1-GFP fusion protein was the major component. The heterogeneous nuclear ribonucleoproteins F and H, histones and ubiquitin were found to be associated with the aggregates. Our observations suggest that the N-terminal fragment of huntingtin may organize the skeletal structure of the aggregates and may disturb normal cellular functions by trapping other proteins within the aggregates.