Synaptic vesicle depletion correlates with attenuated synaptic responses to prolonged repetitive stimulation in mice lacking alpha-synuclein.

Although the mutation of alpha-synuclein, a protein associated with presynaptic vesicles, is implicated in the etiology and pathogenesis of Parkinson's disease, the biological function of the normal protein is unknown. Mice that lack alpha-synuclein have been generated by homologous recombination in embryonic stem cells. Electron microscopic examination of hippocampal synapses revealed a striking selective deficiency of undocked vesicles without affecting docked vesicles. Field recording of CA1 synapses in hippocampal slices from the mutant mice demonstrated normal basal synaptic transmission, paired-pulse facilitation, and response to a brief train of high-frequency stimulation (100 Hz, 40 pulses) that exhausts only docked vesicles. In contrast, the alpha-synuclein knock-out mice exhibited significant impairments in synaptic response to a prolonged train of repetitive stimulation (12.5 Hz, 300 pulses) capable of depleting docked as well as reserve pool vesicles. Moreover, the replenishment of the docked vesicles by reserve pool vesicles after depletion was slower in the mutant synapses. Thus, alpha-synuclein may be required for the genesis and/or maintenance of a subset of presynaptic vesicles, those in the "reserve" or "resting" pools. These results reveal, for the first time, the normal function of endogenous alpha-synuclein in regulating synaptic vesicle mobilization at nerve terminals.

Impaired conditioned fear and enhanced long-term potentiation in Fmr2 knock-out mice.

FRAXE mental retardation results from expansion and methylation of a CCG trinucleotide repeat located in exon 1 of the X-linked FMR2 gene, which results in transcriptional silencing. The product of FMR2 is a member of a family of proteins rich in serine and proline, members of which have been associated with transcriptional activation. We have developed a murine Fmr2 gene knock-out model by replacing a fragment containing parts of exon 1 and intron 1 with the Escherichia coli lacZ gene, placing lacZ under control of the Fmr2 promoter. Expression of lacZ in the knock-out animals indicates that Fmr2 is expressed in several tissues, including brain, bone, cartilage, hair follicles, lung, tongue, tendons, salivary glands, and major blood vessels. In the CNS, Fmr2 expression begins at the time that cells in the neuroepithelium differentiate into neuroblasts. Mice lacking Fmr2 showed a delay-dependent conditioned fear impairment. Long-term potentiation (LTP) was found to be enhanced in hippocampal slices of Fmr2 knock-out compared with wild-type littermates. To our knowledge, this mouse knock-out is the first example of an animal model of human mental retardation with impaired learning and memory performance and increased LTP. Thus, although a number of studies have suggested that diminished LTP is associated with memory impairment, our data suggest that increased LTP may be a mechanism that leads to impaired cognitive processing as well.

Knockout mouse model for Fxr2: a model for mental retardation.

Fragile X syndrome is a common form of mental retardation caused by the absence of the FMR1 protein, FMRP. Fmr1 knockout mice exhibit a phenotype with some similarities to humans, such as macro-orchidism and behavioral abnormalities. Two homologs of FMRP have been identified, FXR1P and FXR2P. These proteins show high sequence similarity, including all functional domains identified in FMRP, such as RNA binding domains. They have an overlap in tissue distribution to that of FMRP. Interactions between the three FXR proteins have also been described. FXR2P shows high expression in brain and testis, like FMRP. To study the function of FXR2P, we generated an Fxr2 knockout mouse model. No pathological differences between knockout and wild-type mice were found in brain or testis. Given the behavioral phenotype in fragile X patients and the phenotype previously reported for the Fmr1 knockout mouse, we performed a thorough evaluation of the Fxr2 knockout phenotype using a behavioral test battery. Fxr2 knockout mice were hyperactive (i.e. traveled a greater distance, spent more time moving and moved faster) in the open-field test, impaired on the rotarod test, had reduced levels of prepulse inhibition, displayed less contextual conditioned fear, impaired at locating the hidden platform in the Morris water task and were less sensitive to a heat stimulus. Interestingly, there are some behavioral phenotypes in Fxr2 knockout mice which are similar to those observed in Fmr1 knockout mice, but there are also some different behavioral abnormalities that are only observed in the Fxr2 mutant mice. The findings implicate a role for Fxr2 in central nervous system function.