Progressive behavioral deficits in DJ-1-deficient mice are associated with normal nigrostriatal function

Loss-of-function mutations in the DJ-1 gene account for an autosomal recessive form of Parkinson's disease (PD). To investigate the physiological functions of DJ-1 in vivo, we generated DJ-1 knockout (DJ-1(-/-)) mice. Younger (<1 year) DJ-1(-/-) mice were hypoactive and had mild gait abnormalities. Older DJ-1(-/-), however, showed decreased body weight and grip strength and more severe gait irregularities compared to wild-type littermates. The basal level of extracellular dopamine, evoked dopamine release and dopamine receptor D2 sensitivity appeared normal in the striatum of DJ-1(-/-) mice, which was consistent with similar results between DJ-1(-/-) and controls in behavioral paradigms specific for the dopaminergic system. An examination of spinal cord, nerve and muscle tissues failed to identify any pathological changes that were consistent with the noted motor deficits. Taken together, our findings suggest that loss of DJ-1 leads to progressive behavioral changes without significant alterations in nigrostriatal dopaminergic and spinal motor systems.

Moderate reduction of gamma-secretase attenuates amyloid burden and limits mechanism-based liabilities

Although gamma-secretase is recognized as a therapeutic target for Alzheimer's disease, side effects associated with strong inhibition of this aspartyl protease raised serious concerns regarding this therapeutic strategy. However, it is not known whether moderate inhibition of this enzyme will allow dissociation of beneficial effects in the CNS from mechanism-based toxicities in the periphery. We tested this possibility by using a series of mice with genetic reduction of gamma-secretase (levels ranging from 25 to 64% of control mice). Here, we document that even 30% reduction of gamma-secretase can effectively ameliorate amyloid burden in the CNS. However, global reduction of this enzyme below a threshold level increased the risk of developing squamous cell carcinoma as well as abnormal proliferation of granulocytes in a gamma-secretase dosage-dependent manner. Importantly, we demonstrate that there exists a critical gamma-secretase level that reduces the risk of amyloidosis in the CNS and limits tumorigenesis in epithelia. Our findings suggest that moderate inhibition of gamma-secretase represents an attractive anti-amyloid therapy for Alzheimer's disease.

BACE1, a major determinant of selective vulnerability of the brain to amyloid-beta amyloidogenesis, is essential for cognitive, emotional, and synaptic functions

A transmembrane aspartyl protease termed beta-site APP cleavage enzyme 1 (BACE1) that cleaves the amyloid-beta precursor protein (APP), which is abundant in neurons, is required for the generation of amyloid-beta (Abeta) peptides implicated in the pathogenesis of Alzheimer's disease (AD). We now demonstrate that BACE1, enriched in neurons of the CNS, is a major determinant that predisposes the brain to Abeta amyloidogenesis. The physiologically high levels of BACE1 activity coupled with low levels of BACE2 and alpha-secretase anti-amyloidogenic activities in neurons is a major contributor to the accumulation of Abeta in the CNS, whereas other organs are spared. Significantly, deletion of BACE1 in APPswe;PS1DeltaE9 mice prevents both Abeta deposition and age-associated cognitive abnormalities that occur in this model of Abeta amyloidosis. Moreover, Abeta deposits are sensitive to BACE1 dosage and can be efficiently cleared from the CNS when BACE1 is silenced. However, BACE1 null mice manifest alterations in hippocampal synaptic plasticity as well as in performance on tests of cognition and emotion. Importantly, memory deficits but not emotional alterations in BACE1(-/-) mice are prevented by coexpressing APPswe;PS1DeltaE9 transgenes, indicating that other potential substrates of BACE1 may affect neural circuits related to emotion. Our results establish BACE1 and APP processing pathways as critical for cognitive, emotional, and synaptic functions, and future studies should be alert to potential mechanism-based side effects that may occur with BACE1 inhibitors designed to ameliorate Abeta amyloidosis in AD.

Loss of ALS2 function is insufficient to trigger motor neuron degeneration in knock-out mice but predisposes neurons to oxidative stress

Amyotrophic lateral sclerosis (ALS), the most common motor neuron disease, is caused by a selective loss of motor neurons in the CNS. Mutations in the ALS2 gene have been linked to one form of autosomal recessive juvenile onset ALS (ALS2). To investigate the pathogenic mechanisms of ALS2, we generated ALS2 knock-out (ALS2(-/-)) mice. Although ALS2(-/-) mice lacked obvious developmental abnormalities, they exhibited age-dependent deficits in motor coordination and motor learning. Moreover, ALS2(-/-) mice showed a higher anxiety response in the open-field and elevated plus-maze tasks. Although they failed to recapitulate clinical or neuropathological phenotypes consistent with motor neuron disease by 20 months of age, ALS2(-/-) mice or primary cultured neurons derived from these mice were more susceptible to oxidative stress compared with wild-type controls. These observations suggest that loss of ALS2 function is insufficient to cause major motor deficits or motor neuron degeneration in a mouse model but predisposes neurons to oxidative stress.

Visuospatial attentional functioning in mice: interactions between cholinergic manipulations and genotype

Attentional functioning in mice was assessed in an analogue of the five-choice serial reaction time task in which the requirement was to detect brief visual stimuli presented across five spatial locations. Two hybrid strains of mice were assessed; F1 C57Bl/6xDBA/2 and C57Bl/6x129sv. Both strains acquired the task to high levels of performance with, in particular, no problems due to premature responding. At performance, systematic manipulation of the task parameters indicated a pattern of effects consistent with the task, taxing aspects of visuospatial attention. There were some differential effects of task manipulations at baseline across strain. However, the pattern of effects suggested these were likely to be the result of effects on factors other than attentional functioning per se, such as behavioural reactivity and inhibition. There was evidence in both strains of specific, centrally mediated effects of scopolamine on attentional functioning, with the C57Bl/6xDBA/2 hybrid showing greater sensitivity to the drug manipulation. Specific effects on discriminative accuracy were observed at doses of 0.02 and 0.2 mg/kg scopolamine. At the 2 mg/kg dose, large reductions in accuracy were associated with large effects on other measures, including omissions and response latencies, suggestive of nonspecific effects on task performance. These data indicate, for the first time, the utility of operant methods in assessing visuospatial attentional functioning in mice. They confirm the importance of cholinergic mechanisms in attentional processes across species, and suggest interactions between cholinergic mechanisms and genotype in the expression of attentional phenotypes.