Annette C. Smith

Polyglutamine-expanded ataxin-7 antagonizes CRX function and induces cone-rod dystrophy in a mouse model of SCA7.

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant disorder caused by a CAG repeat expansion. To determine the mechanism of neurotoxicity, we produced transgenic mice and observed a cone-rod dystrophy. Nuclear inclusions were present, suggesting that the disease pathway involves the nucleus. When yeast two-hybrid assays indicated that cone-rod homeobox protein (CRX) interacts with ataxin-7, we performed further studies to assess this interaction. We found that ataxin-7 and CRX colocalize and coimmunoprecipitate. We observed that polyglutamine-expanded ataxin-7 can dramatically suppress CRX transactivation. In SCA7 transgenic mice, electrophoretic mobility shift assays indicated reduced CRX binding activity, while RT-PCR analysis detected reductions in CRX-regulated genes. Our results suggest that CRX transcription interference accounts for the retinal degeneration in SCA7 and thus may provide an explanation for how cell-type specificity is achieved in this polyglutamine repeat disease.

Overexpression of amyloid precursor protein alters its normal processing and is associated with neurotoxicity

The recent discovery that point mutations in the beta/A4 amyloid precursor protein may be the cause of certain forms of familial Alzheimers disease provides strong support for the view that a thorough understanding of the metabolism of this protein may elucidate the pathogenesis of most forms of the disease and thus serve as a basis for rational prevention and therapy. Here we show that overexpression of a portion of the amyloid precursor protein molecule produces at least four distinct fragments of the COOH-terminus of amyloid precursor protein, suggesting altered proteolysis of amyloid precursor protein, and that such overexpression is associated with cytotoxicity. The degree of toxicity in the P19 cell culture model (differentiating mouse embryonal carcinoma cells) is shown to be related to the two larger novel COOH-terminal protein fragments (16 and 14 kilodalton), as well as to levels of expression of these two fragments. The toxicity is manifested in several differentiated cell lineages, including neuronal cells.

Polyglutamine-Expanded Androgen Receptor Truncation Fragments Activate a Bax-Dependent Apoptotic Cascade Mediated by DP5/Hrk

Spinal and bulbar muscular atrophy (SBMA) is an inherited neuromuscular disorder caused by a polyglutamine (polyQ) repeat expansion in the androgen receptor (AR). PolyQ-AR neurotoxicity may involve generation of an N-terminal truncation fragment, as such peptides occur in SBMA patients and mouse models. To elucidate the basis of SBMA, we expressed N-terminal truncated AR in motor neuron-derived cells and primary cortical neurons. Accumulation of polyQ-AR truncation fragments in the cytosol resulted in neurodegeneration and apoptotic, caspase-dependent cell death. Using primary neurons from mice transgenic or deficient for apoptosis-related genes, we determined that polyQ-AR apoptotic activation is fully dependent on Bax. Jun N-terminal kinase (JNK) was required for apoptotic pathway activation through phosphorylation of c-Jun. Expression of polyQ-AR in DP5/Hrk null neurons yielded significant protection against apoptotic activation, but absence of Bim did not provide protection, apparently due to compensatory upregulation of DP5/Hrk or other BH3-only proteins. Misfolded AR protein in the cytosol thus initiates a cascade of events beginning with JNK and culminating in Bax-dependent, intrinsic pathway activation, mediated in part by DP5/Hrk. As apoptotic mediators are candidates for toxic fragment generation and other cellular processes linked to neuron dysfunction, delineation of the apoptotic activation pathway induced by polyQ-expanded AR may shed light on the pathogenic cascade in SBMA and other motor neuron diseases.

Selective neurotoxicity of COOH-terminal fragments of the β-amyloid precursor protein

The primary component of amyloid deposits found in the brains of patients with Alzheimers disease is the beta-amyloid protein, a derivative of a much larger precursor protein (beta PP). We have previously reported that overexpression of carboxyl (COOH)-terminal fragments of beta PP from an integrated DNA construct leads to degeneration of neuronally differentiating mouse embryonic stem cells and that the neuronal degeneration is related to approximately 14- and 15-kDa COOH-terminal fragments of the precursor protein. We here demonstrate that these putative cytotoxic fragments contain intact beta-amyloid protein. When such transformed cell lines are treated with dimethyl sulfoxide to induce differentiation into muscle cells, however, the resulting muscle cells remain viable (as do control non-transformed cells), despite the production of comparable amounts of the 14- and 15-kDa fragments. These results are consistent with the hypothesis that particular COOH-terminal fragments of beta PP are amyloidogenic and neurotoxic.

Levels of DNA damage are unaltered in mice overexpressing human catalase in nuclei.

Two types of transgenic mice were generated to evaluate the role of hydrogen peroxide in the formation of nuclear DNA damage. One set of lines overexpresses wild-type human catalase cDNA, which is localized to peroxisomes. The other set overexpresses a human catalase construct that is targeted to the nucleus. Expression of the wild-type human catalase transgene was found in liver, kidney, skeletal muscle, heart, spleen, and brain with muscle and heart exhibiting the highest levels. Animals containing the nuclear-targeted construct had a similar pattern of expression with the highest levels in muscle and heart, but with lower levels in liver and spleen. In these animals, immunofluorescence detected catalase present in the nuclei of kidney, muscle, heart, and brain. Both types of transgenic animals had significant increases of catalase activities compared to littermate controls in most tissues examined. Despite enhanced activities of catalase, and its presence in the nucleus, there were no changes in levels of 8OHdG, a marker of oxidative damage to DNA. Nor were there differences in mutant frequencies at a Lac Z reporter transgene. This result suggests that in vivo levels of H(2)O(2) may not generate 8OHdG or other types of DNA damage. Alternatively, antioxidant defenses may be optimized such that additional catalase is unable to further protect nuclear DNA against oxidative damage.

High Frequency "Switching" at the Adenine Phosphoribosyltransferase Locus in Multipotent Mouse Teratocarcinoma Stem Cells

Clones of multipotent mouse tetratocarcinoma stem cells, presumptively heterozygous at the adenine phosphoribosyltransferase (APRT) locus (EC 2.4.2.7), were selected for partial resistance to the purine analog 2′,6′-diaminopurine (DAP). All had approximately 50% APRT activity as compared to the parental line and were found to segregate homozygous deficient cells at a high frequency (∼10−2). Homozygous deficient cells were isolated from one of the heterozygotes and were found to fall into a single class characterized by residual activity and the segregation of revertants at an equally high frequency. The revertants in turn gave rise to full mutants at comparably high frequencies. Chromosomal changes detectable with the light microscope were not associated with these transitions. Physical characterization of the APRT enzymes derived from mutant, revertant, and wild-type cells did not reveal any differences. We conclude that the reversible “switching” between heterozygosity and homozygosity is attributable to some form of gene inactivation and reactivation rather than to classical mutational events.

The Purkinje cell degeneration 5J mutation is a single amino acid insertion that destabilizes Nna1 protein.

In the mouse, Purkinje cell degeneration (pcd) is a recessive mutation characterized by degeneration of cerebellar Purkinje cells, retinal photoreceptors, olfactory bulb mitral neurons, and certain thalamic neurons, and is accompanied by defective spermatogenesis. Previous studies of pcd have led to the identification of Nna1 as the causal gene; however, how loss of Nna1 function results in neurodegeneration remains unresolved. One useful approach for establishing which functional domains of a protein underlie a recessive phenotype has been to determine the genetic basis of the various alleles at the locus of interest. Because none of the pcd alleles analyzed at the time of the identification of Nna1 provided insight into the molecular basis of Nna1 loss-of-function, we obtained a recent pcd remutation—pcd5J, and after determining that its phenotype is comparable to existing pcd severe alleles, we sought its genetic basis by sequencing Nna1. In this article we report that pcd5J results from the insertion of a single GAC triplet encoding an aspartic acid residue at position 775 of Nna1. Although this insertion does not affect Nna1 expression at the RNA level, Nna1pcd-5J protein expression is markedly decreased. Pulse-chase experiments reveal that the aspartic acid insertion dramatically destabilizes Nna1pcd-5J protein, accounting for the observation that pcd5J is a severe allele. The presence of a readily detectable genetic mutation in pcd5J confirms that Nna1 loss-of-function alone underlies the broad pcd phenotype and will facilitate further studies of how Nna1 loss-of-function produces neurodegeneration and defective spermatogenesis in pcd mice.

Activity assays of nine heterogeneous promoters in neural and other cultured cells

SummaryTo express high levels of proteins encoded by transfected DNA constructs in a variety of cultured cells, including neuronal cells, the activities of nine different promoters were evaluated usingEscherichia coli β-galactosidase (β-gal) (LacZ) as a reporter gene. These nine promoters were categorized into three distinct groups (high, intermediate, and low expresser), in terms of the levels ofβ-gal expression. An expression vector containing the cytomegalovirus enhancer and the chickβ-actin promoter (high expresser) showed the highest levels of expression, followed by vectors containing the cytomegalovirus promoter/enhancer and the SV40 promoter/enhancer (intermediate expresser). The rest of the promoters (thymidine kinase, adenovirus, murine proliferative sarcoma virus, nerve growth factor receptor, Rous sarcoma and mouse mammary tumor virus, andβ-amyloid precursor protein) expressed low levels ofβ-gal. These results were consistent for eight different cell types. A particularly attractive model is the stem cell, P19; cultures differentiating into progeny consisting predominantly of cholinergic neurons could be readily transfected with expression vectors using liposomes and expressedβ-gal without significant morphologic changes of the differentiated neurons. The systems should be useful for the study of promoters and various expressed proteins, including those involved in axonal transport.

Allelic variation linked to adenine phosphoribosyltransferase locus in mouse teratocarcinoma cell line and feral-derived mouse strains

Southern blot analysis reveals two distinct adenine phosphoribosyltransferase (APRT)alleles in the P-19 mouse teratocarcinoma cell line. One allele is identical to that observed in common laboratory mouse strains (Mus musculus domesticus).The restriction enzyme site variations between the two alleles occur in sequences located both upstream and downstream of the APRTgene, but not within it. Although the P-19 cell line was established from a C3H strain embryo (Mus musculus domesticus),a sixth generation ancestor of this embryo was a feral mouse (Mus musculus musculus).The restriction pattern of the variant APRTallele in P-19 is identical to that of a feral-derived Mus musculus musculusanimal, establishing the origin of this allele in the P-19 cell line. A third, distinct APRTallele was found in a Mus spretusferal-derived mouse. Exploiting the differences between the two APRTalleles in the P-19 cell line, we have demonstrated their sequential loss in APRT-deficient clones.

Overexpression of wild-type and nuclear-targeted catalase modulates resistance to oxidative stress but does not alter spontaneous mutant frequencies at APRT.

Animal cells generate hydrogen peroxide as a byproduct of energy metabolism. In the presence of reduced metals H(2)O(2) can decompose to a highly reactive hydroxyl radical that attacks essentially all organic molecules, including DNA. We wished to determine if overexpression of catalase and/or the targeting of the enzyme to the nucleus could protect cells from oxidative stress and reduce the frequency of mutation. Wild-type human catalase, which localizes to peroxisomes, and a modified construct, which targets catalase to the nucleus, were overexpressed in a murine line of embryonic carcinoma cells (P19). Both constructs enhanced the resistance of the cells to hydrogen peroxide, but sensitized them to bleomycin. Overexpression of wild-type catalase protected cells against paraquat, while nuclear targeting sensitized them to this agent. Expression of neither construct significantly altered spontaneous mutant frequencies at the endogenous murine adenosine phosphoribosyl transferase (APRT) locus; however, nuclear-targeted catalase prevented an increase in mutant frequency after H(2)O(2) treatment. These results suggest that endogenous levels of hydrogen peroxide may not generate DNA damage in vivo, or that such damage may be efficiently repaired in murine embryonic carcinoma cells.