The mouse gene retinal degeneration (rd) may reduce the number of neurons present in the adult hippocampal dentate gyrus

A study of long-term potentiation in transgenic mice over-expressing mutant forms of both amyloid precursor protein and presenilin-1

Synaptic transmission and long-term potentiation (LTP) in the CA1 region of hippocampal slices have been studied during ageing of a double transgenic mouse strain relevant to early-onset familial Alzheimer's disease (AD). This strain, which over-expresses both the 695 amino acid isoform of human amyloid precursor protein (APP) with K670N and M671L mutations and presenilin 1 with the A246E mutation, has accelerated amyloidosis and plaque formation. There was a decrease in synaptic transmission in both wildtype and transgenic mice between 2 and 9 months of age. However, preparing slices from 14 month old animals in kynurenic acid (1 mM) counteracted this age-related deficit. Basal transmission and paired-pulse facilitation was similar between the two groups at all ages (2, 6, 9 and 14 months) tested. Similarly, at all ages LTP, induced either by theta burst stimulation or by multiple tetani, was normal. These data show that a prolonged, substantially elevated level of Abeta are not sufficient to cause deficits in the induction or expression of LTP in the CA1 hippocampal region.

Impaired fear memories are correlated with subregion-specific deficits in hippocampal and amygdalar LTP

Inbred mouse strains have different genetic backgrounds that likely influence memory and long-term potentiation (LTP). LTP, a form of synaptic plasticity, is a candidate cellular mechanism for some forms of learning and memory. Strains with impaired fear memory may have selective LTP deficits in different hippocampal subregions or in the amygdala. The authors assessed fear memory in 4 inbred strains: C57BL/6NCrlBR (B6), 129S1/SvImJ (129), C3H/HeJ (C3H), and DBA/2J (D2). The authors also measured LTP in the hippocampal Schaeffer collateral (SC) and medial perforant pathways (MPP) and in the basolateral amygdala. Contextual and cued fear memory, and SC and amygdalar LTP, were intact in B6 and 129, but all were impaired in C3H and D2. MPP LTP was similar in all 4 strains. Thus, SC, but not MPP, LTP correlates with hippocampus-dependent contextual memory expression, and amygdalar LTP correlates with amygdala-dependent cued memory expression, in these inbred strains.

Hippocampal synaptic plasticity in mice carrying the rd mutation in the gene encoding cGMP phosphodiesterase type 6 (PDE6)

Cyclic GMP (cGMP) has been implicated in the modulation of long-term potentiation (LTP) and depression (LTD) in the hippocampus. Transcripts for subunits of several types of cGMP specific phosphodiesterase are found in the mammalian brain but their relative role in hippocampal function is unclear. The retinal degeneration (rd) mutation in the gene encoding the PDE6B subunit causes a loss of function in PDE6 enzyme and in adult mice homozygous to the mutation it causes blindness. We have used this natural mutation, and the cGMP phosphodiesterase inhibitor zaprinast, in wild-type and rd/rd mouse littermates to investigate whether PDE5 and/or PDE6 regulates excitatory synaptic transmission in the hippocampus. Mice were genotyped using two independent PCR methods. Glutamate-mediated synaptic transmission in the CA1 region or dentate gyrus was unaffected in hippocampal brain slices from mice carrying the rd mutation. Similarly the facilitation of synaptic events by paired-pulse stimuli, and LTP induced by a theta-burst (10 bursts of four events at 100 Hz with a 200-ms inter-burst interval) were normal in rd/rd mice. Inhibition of cGMP-specific PDE activity by zaprinast (10 microM, an inhibitor of PDE5 and PDE6) induced a slowly developing and sustained depression of field synaptic potentials that was quantitatively similar in both wild-type and rd/rd mice. Thus in the CA1 region synaptic plasticity is likely to be regulated by the PDE5 rather than the PDE6 isoform.

Age-related impairment of synaptic transmission but normal long-term potentiation in transgenic mice that overexpress the human APP695SWE mutant form of amyloid precursor protein

We have studied synaptic function in a transgenic mouse strain relevant to Alzheimer's disease (AD), overexpressing the 695 amino acid isoform of human amyloid precursor protein with K670N and M671L mutations (APP(695)SWE mice), which is associated with early-onset familial AD. Aged-transgenic mice had substantially elevated levels of Abeta (up to 22 micromol/gm) and displayed characteristic Abeta plaques. Hippocampal slices from 12-month-old APP(695)SWE transgenic animals displayed reduced levels of synaptic transmission in the CA1 region when compared with wild-type littermate controls. Inclusion of the ionotropic glutamate receptor antagonist kynurenate during preparation of brain slices abolished this deficit. At 18 months of age, a selective deficit in basal synaptic transmission was observed in the CA1 region despite treatment with kynurenate. Paired-pulse facilitation and long-term potentiation (LTP) were normal in APP(695)SWE transgenic mice at both 12 and 18 months of age. Thus, although aged APP(695)SWE transgenic mice have greatly elevated levels of Abeta protein, increased numbers of plaques, and reduced basal synaptic transmission, LTP can still be induced and expressed normally. We conclude that increased susceptibility to excitotoxicity rather than a specific effect on LTP is the primary cause of cognitive deficits in APP(695)SWE mice.

A PDE6A promoter fragment directs transcription predominantly in the photoreceptor

Rod photoreceptor cGMP phosphodiesterase (PDE6) is a key enzyme in the phototransduction cascade. Lines of transgenic mice were established to determine the spatial expression pattern directed by an upstream fragment of the PDE6A gene. RT-PCR analysis showed that three of four lines analyzed transcribed the transgene predominantly in the retina and weakly in brain. The line showing no transgene transcription did not contain an intact transgene. Transcription of the transgene in the three lines was found in retina and weakly in brain, but not in heart, kidney, liver, or lung. Transcripts were most predominant in the photoreceptors of the retina. These results demonstrate that a short segment of the upstream region of the PDE6A gene comprises a functional promoter that is most active in photoreceptors.

Responses to light after retinal degeneration

Transgenic rodless mice were given 1-h pulses of light of varying brightness at times of the night when they were normally active. The rodless mice showed decreases in locomotor activity during light pulses brighter than 2 lux; these decreases were significantly greater than those in wildtypes (ANOVA, P < 0.01). However, with very dim light, rodless mice showed no changes in activity, whereas wildtype mice actually increased their activity. It is suggested that irradiance detection could be enhanced by absence of image-forming vision. Enhanced inhibition of activity around twilight may be adaptive for mice in some circumstances and so help maintain genes for retinal degeneration in natural populations.

Search for a gene that may result in fewer neurons in the adult mouse hippocampus

Experimentally produced C57BL/6J-Pdebrd1 congenic mice carrying this gene for retinal degeneration were previously found to have fewer hippocampal neurons than partner strain C57BL/6J+Pdeb-rd1 mice possessing normal retinas. A linked passenger gene on the inserted chromosome segment containing the Pdebrd1 gene might have been responsible. An inbred strain segregating at the Pdebrd1 locus and with a genetic background on which the gene is normally present has now been examined. No neuron loss was observed. These new results indicate that, while Pdebrd1 was necessary for the occurrence of fewer hippocampal neurons in C57BL/6J-Pdebrd1 mice, it alone was not sufficient to produce this effect. The Pdebrd1 gene was acting in combination with at least one other gene not on the introduced chromosome segment. The results also provide evidence that genetic background effects can minimize or eliminate hippocampal neuron loss in some strains that normally carry the Pdebrd1 gene.

PCR analysis of DNA from 70-year-old sections of rodless retina demonstrates identity with the mouse rd defect

Rodless retina (gene symbol, r) was discovered in mice by Keeler 70 years ago and was first described in this journal as an autosomal recessive mutation leading to "the absence of the visual cells (rods), the external nuclear layer, and the external molecular layer" [Keeler, C. E. (1924) Proc. Natl. Acad. Sci. USA 10, 329-333]. The mutation was studied by Keeler and others in the United States and Europe over the next decade, but Keeler's stock was destroyed in 1939, and mice definitively related to his by pedigree and progeny tests also appeared to have been lost by the end of World War II. In the early 1950s Brückner in Basel recognized mice with a similar retinal phenotype. Investigators in London and Strasbourg analyzed descendants of Brückner's mice and concluded, on the basis of different pathogenesis from r, that they carried a new mutation, which came later to be called retinal degeneration, rd. The relationship of r and rd has been unsettled ever since. Now that the rd phenotype is known to be due to a nonsense mutation in the rod photoreceptor cGMP phosphodiesterase beta-subunit gene, we hoped to settle the question by direct analysis of r DNA. DNA was liberated from 70-year-old histological sections of +/r and r/r eyes, the only extant r DNA, and the regions encompassing the nonsense mutation amplified by the polymerase chain reaction (PCR). Sequence analysis of the PCR products revealed the presence of the same nonsense mutation and two intron polymorphisms in r DNA. PCR and direct sequence analysis of 11 strains of mice known to carry rd (or a similar allele) also revealed the presence of the nonsense mutation and the same intron polymorphisms. The fact that all r and rd mice contain an identical defect and intron polymorphisms in the phosphodiesterase beta-subunit gene settles beyond reasonable doubt that a single mutation arising > 70 years ago is now widely distributed through inbred mouse strains. Because of the extensive use of the name in publications of the past 40 years, we propose that the gene continue to be designated retinal degeneration, rd.

The human beta-subunit of rod photoreceptor cGMP phosphodiesterase: complete retinal cDNA sequence and evidence for expression in brain

We have identified and sequenced cDNA clones that encode for the human beta-subunit of rod cGMP phosphodiesterase (PDEB). A single 2565-bp open reading frame that codes for an 854-amino-acid protein was identified. The human beta-subunit protein is 90% identical to the bovine beta-subunit and 91% identical to the mouse protein. Northern blot analysis indicates that the gene is expressed as an abundant 3.5-kb transcript in retina and as a rare 2.9-kb transcript in brain. The isolation of cDNAs from human brain cDNA libraries confirms the brain as a site of expression for this gene. The molecular defect underlying retinal degeneration in the rd mouse has been found to be a nonsense mutation in the beta-subunit of the mouse cGMP PDE, resulting in a truncated protein (Pittler et al., 1991b, Proc. Natl. Acad. Sci. USA. 88: 8322-8326). The molecular cloning of the cDNA encoding for the PDEB represents the first step in establishing whether this gene plays a causative role in any one of the several human hereditary retinopathies or, based on its localization to chromosome 4p 16.3, in the pathogenesis of Huntington disease.

Exclusion of DNA changes in the beta-subunit of the c-GMP phosphodiesterase gene as the cause for Huntington's disease

To identify expressed sequences within candidate regions for the Huntington's disease (HD) gene in 4p16.3, we isolated the gene encoding the beta subunit of the human cGMP phosphodiesterase (PDEB). We formally assessed this as a candidate gene for HD based on it's expression in brain, the demonstration of linkage disequilibrium between intragenic DNA markers and HD, and the demonstration that mice with a mutation in this gene have a reduction of neurons in particular brain regions. We investigated all 22 exons of PDEB and 5'-flanking region for point mutations in 16 HD patients of different ethnic origins using single strand conformational polymorphism analysis. The underlying DNA changes found initially exclusively in HD patients were excluded as the cause for HD.

Chromosome mapping of the rod photoreceptor cGMP phosphodiesterase beta-subunit gene in mouse and human: tight linkage to the Huntington disease region (4p16.3)

The retinal degeneration mouse (gene symbol, rd) is an animal model for certain forms of human hereditary retinopathies. Recent findings of a nonsense mutation in the rd mouse PDE beta-subunit gene (Pdeb) prompted us to investigate the chromosome locations of the mouse and human genes. We have utilized backcross analysis in mice to verify and define more precisely the location of the Pdeb locus 6.1 +/- 2.3 cM distal of Mgsa on mouse chromosome 5. We have determined that the human gene (PDEB) maps to 4p16.3, very close to the Huntington disease (HD) region. Analysis of the comparative map for mice and humans shows that the mouse homologue of the HD gene will reside on chromosome 5. Linkage of the mouse Pdeb locus with other homologues in the human 4p16.3 region is maintained but gene order is not, suggesting at least three possible sites for the corresponding mouse HD gene.