Production of monoclonal antibodies against neurofilament-associated proteins: demonstration of association with neurofilaments by a coimmunoprecipitation method

Overexpression of UBQLN1 reduces neuropathology in the P497S UBQLN2 mouse model of ALS/FTD

Missense mutations in UBQLN2 cause X-linked dominant inheritance of amyotrophic lateral sclerosis with frontotemporal dementia (ALS/FTD). UBQLN2 belongs to a family of four highly homologous proteins expressed in humans that play diverse roles in maintaining proteostasis, but whether one isoform can substitute for another is not known. Here, we tested whether overexpression of UBQLN1 can alleviate disease in the P497S UBQLN2 mouse model of ALS/FTD by crossing transgenic (Tg) mouse lines expressing the two proteins and characterizing the resulting genotypes using a battery of pathologic and behavioral tests. The pathologic findings revealed UBQLN1 overexpression dramatically reduced the burden of UBQLN2 inclusions, neuronal loss and disturbances in proteostasis in double Tg mice compared to single P497S Tg mice. The beneficial effects of UBQLN1 overexpression were primarily confirmed by behavioral improvements seen in rotarod performance and grip strength in male, but not female mice. Paradoxically, although UBQLN1 overexpression reduced pathologic signatures of disease in P497S Tg mice, female mice had larger percentage of body weight loss than males, and this correlated with a corresponding lack of behavioral improvements in the females. These findings lead us to speculate that methods to upregulate UBQLN1 expression may reduce pathogenicity caused by UBQLN2 mutations, but may also lead to gender-specific outcomes that will have to be carefully weighed with the therapeutic benefits of UBQLN1 upregulation.

Ubiquilin-1 overexpression increases the lifespan and delays accumulation of Huntingtin aggregates in the R6/2 mouse model of Huntington's disease

Huntington's Disease (HD) is a neurodegenerative disorder that is caused by abnormal expansion of a polyglutamine tract in huntingtin (htt) protein. The expansion leads to increased htt aggregation and toxicity. Factors that aid in the clearance of mutant huntingtin proteins should relieve the toxicity. We previously demonstrated that overexpression of ubiqulin-1, which facilitates protein clearance through the proteasome and autophagy pathways, reduces huntingtin aggregates and toxicity in mammalian cell and invertebrate models of HD. Here we tested whether overexpression of ubiquilin-1 delays or prevents neurodegeneration in R6/2 mice, a well-established model of HD. We generated transgenic mice overexpressing human ubiquilin-1 driven by the neuron-specific Thy1.2 promoter. Immunoblotting and immunohistochemistry revealed robust and widespread overexpression of ubiquilin-1 in the brains of the transgenic mice. Similar analysis of R6/2 animals revealed that ubiquilin is localized in huntingtin aggregates and that ubiquilin levels decrease progressively to 30% during the end-stage of disease. We crossed our ubiquilin-1 transgenic line with R6/2 mice to assess whether restoration of ubiquilin levels would delay HD symptoms and pathology. In the double transgenic progeny, ubiquilin levels were fully restored, and this correlated with a 20% increase in lifespan and a reduction in htt inclusions in the hippocampus and cortex. Furthermore, immunoblots indicated that endoplasmic reticulum stress response that is elevated in the hippocampus of R6/2 animals was attenuated by ubiquilin-1 overexpression. However, ubiquilin-1 overexpression neither altered the load of htt aggregates in the striatum nor improved motor impairments in the mice.

Excitable membranes and synaptic transmission: postsynaptic mechanisms. Localization of alpha-internexin in the postsynaptic density of the rat brain

The synaptic localization of alpha-internexin, a brain-specific intermediate filament protein, was investigated immunohistochemically in the rat brain. The specificity of the antibody used in this study was confirmed by Western blotting and the antibody specifically reacted with alpha-internexin in the neurofilament preparation and in the postsynaptic density (PSD) fraction. The alpha-internexin immunoreactivity was distributed in neurons, especially in the somata and dendrites, throughout the cerebral cortex. Immunoelectron microscopic examination showed the immunoreactivity in the PSD, while neurofilament M was not in the PSD. Thus alpha-internexin and neurofilament M are differentially localized in neuronal cells. Alpha-internexin content in the PSD fraction was relatively high even before the period of synaptogenesis and the content in the fraction was unchanged between young and adult rats (2-6 weeks old). These results suggest a role of alpha-internexin for early development and organization of the PSD.

Effects of insulin on the regulation of branched-chain alpha-keto acid dehydrogenase E1 alpha subunit gene expression

Alterations in dietary intake, especially of protein, may produce changes in the hepatic levels of the branched-chain alpha-keto acid dehydrogenase (BCKAD) complex. The possible role of insulin in the regulation of these observed changes in hepatic capacity for BCKAD expression was therefore examined. Steady-state RNA levels encoding three of the subunits, E1 alpha, E1 beta and E2, increased by 2-4-fold in the livers of mice starved for 3 days, a known hypoinsulinaemic state. In contrast, the levels of E1 beta and E2, but not E1 alpha, RNA were decreased when mice were fed 0% protein diets compared with the levels observed in mice fed standard (23%) or higher protein isocaloric diets. BCKAD subunit protein levels under these conditions changed co-ordinately even though the changes in RNA were not co-ordinate. The effects of hormonal changes that might be associated with these dietary changes were examined, using the rodent hepatoma cell line H4IIEC3. In these cells, the levels of E1 alpha protein and mRNA were significantly depressed in the presence of insulin. In contrast, the levels of E1 beta and E2 RNAs were not decreased by insulin. The half-lives of the E1 alpha and E2 RNAs were determined to be quite long, from 13 to 18 h, with insulin having no dramatic overall effect on the half-lives determined over 24 h. Therefore, it is likely that insulin directly affects the transcription of the E1 alpha gene rather than RNA stability in exerting its negative regulatory effect. This effect is specific to the E1 alpha subunit. The differences in BCKAD subunit RNA levels observed under various nutritional and developmental conditions may therefore be the result of the differential effects of insulin and other hormones on the multiple regulatory mechanisms modulating BCKAD subunit expression.

Mechanical effects of neurofilament cross-bridges. Modulation by phosphorylation, lipids, and interactions with F-actin

The structure of gels formed by bovine spinal cord neurofilaments was determined by fluorescence and electron microscopy and compared to mechanical properties measured by their elastic and viscous response to shear forces. Neurofilaments formed gels of high elastic modulus (>100 Pa) after addition of millimolar Mg2+. Gelation caused a slow increase in shear moduli to levels similar to those of vimentin intermediate filament networks, followed by a rapid rise due to formation of links between neurofilaments, mediated by cross-bridging structures that vimentin filaments lack. Neurofilament gels are more resistant to large deformations than are vimentin networks, suggesting the importance of cross-bridges for neurofilament mechanical properties. Fluorescence imaging of single neurofilaments showed flexible filaments that became straighter when they adhered to glass or were incorporated into filament bundles. Electron microscopy of neurofilament gels showed a system of bundles intertwined within a more isotropic network of individual filaments. Neurofilament gel formation was stimulated in vitro by acid phosphatase treatment or by inositol phospholipids. In contrast, addition of actin filaments reduced the resistance of neurofilament gels to large stresses. These results suggest that dynamic and regulated interactions occur between neurofilaments to form viscoelastic networks with properties distinct from other cytoskeletal structures.

A cdc2-like kinase distinct from cdk5 is associated with neurofilaments

An immunoprecipitation assay was used to identify protein kinases which are physically associated with neurofilaments (NF) in mouse brain extracts. Using this approach, we show that a cdc2-related kinase is associated with NF. The cdc2-related kinase was found to be distinct from cdk5 and the authentic cdc2 by a number of criteria. Firstly, it has a molecular mass on SDS-PAGE gels of 34 kDa, similar to that of cdc2, but differing from cdk5 (31 kDa). Secondly, it is not recognized by an antibody specific for cdk5. Thirdly, it is recognized by an antibody raised against the C-terminal region of authentic cdc2, but not by an antibody specific for the PSTAIRE motif. Using immunoblotting, we further show that the cdc2-related kinase copurifies with NF isolated from rat tissues. In vitro kinase assays further demonstrated that immunoprecipitated cdc2-related kinase phosphorylates recombinant NF-H protein. Phosphorylation of NF-H by the cdc2-like activity was not affected by 3 microM olomoucine but was inhibited by 10 microM of this kinase inhibitor. Phosphoamino acid analysis of in vitro phosphorylated NF-H indicates that the immunoprecipitated cdc2-related kinase phosphorylates serine residues.

Neurofilament phosphorylation is modulated by myelination

Axons undergo substantial changes in radial growth during the course of development. Recent evidence suggests that axonal diameter may be controlled by the state of neurofilament (NF) phosphorylation. Using dorsal root ganglion (DRG)-Schwann cell co-cultures, we provide direct evidence that phosphorylation of NF is regulated by myelination. NF phosphorylation increased upon myelination of DRG neurons by Schwann cells. The increase in NF phosphorylation was reflected both as an increase in immunoreactivity with the antibody SMI31, specific for phosphorylation-dependent NF epitopes, and a concomitant decrease in immunoreactivity with SMI32, specific for nonphosphorylated NF epitopes. The increase in NF phosphorylation induced by myelination in the neuron-glia co-cultures was similar to NF phosphorylation seen in sciatic nerve extracts of mice with normal myelination compared to Trembler J mouse littermates in which myelination of peripheral nerves is compromised. Using an in situ gel kinase assay, we have detected changes in individual NF kinase activities during myelination. In particular, a 35-kDa kinase activity was induced by myelination, whereas a 42-kDa kinase decreased in activity. We discuss the possibility that these and other kinases may be involved in signaling processes between neurons and glia during myelination.