in order to obtain mouse genomic dna clones containing the 5 ′ end of the dopamine transporter gene ( dat ; solute carrier family 6 , member 3 ) we screened a 129 / svj λ fix ii phage library ( stratagene ) using an 814 bp probe spanning from the promoter region over exon 1 and into intron 1 . the probe was generated by pcr using mouse genomic dna with the forward primer 5 ′- cag ggt cgg aga gtc ata caa c - 3 ′ ( seq id no 10 ) and the reverse primer 5 ′- atg aac cag gtc ttg agt ctg g - 3 ′ ( seq id no 11 ). five overlapping clones containing part of the dat gene were identified and isolated . restriction enzyme digestion followed by southern blotting and hybridization with oligo probes was used to make an approximate contig of the five clones . clone 1 , spanning from ˜ 3 . 7 kb upstream of exon 1 and downstream into intron 5 , was released with noti and subcloned into pbluescript ii sk , generating pme1 . an nls - cre / frt - neo - frt cassette was constructed by subcloning the needed elements in several steps . first the mcs of a pbluescript ii sk was released with sali - kpni and replaced with a synthetic oligo containing the necessary cloning sites , inactivating the 3 ′ kpni site ( saci / paci / sali / ecori / kpni / frt - sequence / xhoi / ecori / sacii / asci / kpni *), generating pme2 . an nls - cre fragment was released from pml78 ( mark b . lewandoski , national cancer institute at frederick ) using sali - kpni and ligated into pme2 , generating pme3 . next , a synthetic oligo containing an frt - sequence was introduced after the polya in pmc1neo - polya ( provided by thomas perlmann , karolinska institute ), using bamhi and sacii . this neo - frt construct was then released with xhoi - sacii and ligated into pme3 , generating pme4 which was sequenced from both directions . for final assembly of the targeting construct a new plasmid was generated by replacing the mcs of pbluescript ii sk with an oligo containing restriction sites for the insertion of 5 ′ and 3 ′ homology arms as well as for the nls - cre / frt - neo - frt cassette ( saci / noti / bamhi / smai / paci / bamhi / asci / ecorv / bamhi / ecori / kpni ), generating pme5 . the 5 ′ and 3 ′ homology arms were released from pme1 with noti - fspi and fspi - ecori , respectively , and inserted into pme5 using noti - smai and ecorv - ecori . the cassette from pme4 was released and inserted between the homology arms using paci - asci . the endogenous dat translational start site is located in exon 2 and since this design actually puts the cassette in intron 1 just before exon 2 we finally introduced a short synthetic oligo recreating the intron 1 / exon 2 border at the paci site just upstream of the cre gene , generating pme6 . the insertion site was sequenced to verify the correct integration and direction of the oligo . the targeting vector was linearized with noti and electroporated into ri embryonic stem ( es ) cells . candidate recombinant clones were selected for by growth in the presence of g418 . southern hybridization showed a ˜ 30 % targeting efficiency of the dat - locus with the knockin construct . three correctly targeted clones were injected into c57bl / 6j blastocysts which were then implanted into pseudopregnant females . two of the clones generated medium - to high - grade male chimeras that gave rise to agouti pups when bred to c57bl / 6j females , indicating germline transmission of the es - cell genome containing the targeted dat - locus . agouti pups were genotyped using southern blot as well as pcr . to remove the frt - flanked neo gene downstream of the cre gene , mice heterozygous for the targeted dat - locus (+/ dat - cre neo ) were mated to flpe deleter mice , expressing flpe from a human β - actin promoter . offspring lacking the neo gene (+/ dat - cre ) were identified with southern blot analysis and kept on a c57bl / 6j background . dat - cre mice were mated to mice homozygous for a loxp - flanked mitochondrial transcription factor a ( tfam ) allele ( tfam loxp / tfam loxp ) and offspring carrying the dat - cre locus (+/ dat - cre ; +/ tfam loxp ) were identified with pcr . in a second step such mice were again mated to tfam loxp / tfam loxp mice to generate homozygous knockout animals (+/ dat - cre ; tfam loxp / tfam loxp ) and controls (+/ tfam loxp or tfam loxp / tfam loxp ). for southern blot analysis , ˜ 10 μg of genomic tail dna was digested with bamhi , electrophoresed through a 0 . 8 % agarose gel and transferred to a hybond c + membrane ( amersham ). membranes were hybridized with a 3 ′ probe labeled with [ α - 32 p ] dctp ( 3000 ci / mmol ; amersham ), and visualized by autoradiography using standard procedures . routine pcr genotyping for the presence of cre was performed with the forward primer 5 ′- cac gac caa gtg aca gca at - 3 ′ ( seq id no 1 ) and the reverse primer 5 ′- aga gac gga aat cca tcg ct - 3 ′ ( seq id no 2 ), giving a product of 371 bp in the presence of cre . pcr genotyping to verify that the neo gene is excised by flpe recombination was done with the forward primer 5 ′- ccc aac ttg aga tgt atg aa - 3 ′ ( seq id no 3 ) and the reverse primer 5 ′- caa gaa gcg ctt tac tga c - 3 ′ ( seq id no 4 ), giving a product of 954 bp after excision . the tfam locus was genotyped with pcr using a three primer strategy ( mt31 : 5 ′- ctg cct tcc tct agc ccg gg - 3 ′ ( seq id no 5 ); mt12 : 5 ′- gta aca gca gac aac ttg tg - 3 ′ ( seq id no 6 ); mt36 : 5 ′- ctc tga agc aca tgg tca at - 3 ′) ( seq id no 12 ), generating 437 , 404 and 329 bp products for tfam loxp , tfam wt and tfam ko , respectively ( larsson et al ., nat . genet . 18 , 231 - 6 ( 1998 )). probes against the transcripts for tyrosine hydroxylase ( 5 ′- ggt gtg cag ctc atc ctg gac ccc ctc caa gga gcg ct - 3 ′) ( seq id no 7 ) and cre recombinase ( 5 ′- gcc cgg acc gac gat gaa gca tgt tta gct ggc cca aat gtt gct gga - 3 ′( seq id no 8 ) or 5 ′- cac cag aga cgg aaa tcc atc gct cga cca gtt tag tta ccc cca ggc - 3 ′) ( seq id no 9 ) were 3 ′- end labeled with 33 p and used to detect mtrna in 14 μm cryostat sections from fresh frozen brains by in situ hybridization ( dagerlind et al ., histochemistry 98 , 39 - 49 . ( 1992 )). a random oligonucleotide probe was used as a negative control . hybridizations were performed at 42 ° c . for 16 - 18 hr , and sections were rinsed five times in 1 × ssc , dehydrated , and exposed to photographic emulsion ( kodak ntb2 ; eastman kodak ). mice were perfused with ca 2 + - free tyrode &# 39 ; s solution , followed by 4 % paraformaldehyde with 0 . 4 % picric acid in 0 . 16 m phosphate buffer . the brains were dissected out , post - fixed , and equilibrated to 10 % sucrose containing 0 . 1 % sodium azide . primary antibodies used for indirect immunohistochemistry ( hökfelt et al ., histochemie 33 , 231 - 54 ( 1973 ); zetterström et al ., neuroscience 62 , 899 - 918 . ( 1994 )) included polyclonal antibodies against tyrosine hydroxylase ( th ) ( 1 : 400 ; pel - freez ), β - galactosidase ( 1 : 500 ; chemicon ), glial fibrillary acidic protein 19 ( gfap - 19 ) ( 1 : 500 ; sigma ) and α - synuclein ( 1 : 50 ; biogenesis ). cryostat sections ( 14 μm ) were incubated with primary antibodies overnight at 4 ° c ., rinsed , and incubated with appropriate fitc - or cy2 - labeled secondary antibodies . controls included omitting the primary antibody . sections were analyzed by fluorescence microscopy . for terminal deoxynucleotidyl transferase - mediated biotinylated utp nick end ( tunel ) labeling we used the in situ cell death detection ( pod ) kit ( roche ) according to the instructions of the manufacturer . open field activity was examined in automated activity cages . this consisted of plexiglas boxes ( 35 × 35 × 18 cm ) with a lower and a higher row of infrared sensitive photocells and a microcomputer registering interruption of photocell beams . animals were tested during the light phase of the light - dark cycle , between 9 : 00 and 13 : 00 h . locomotor activity was registered when animals interrupted the lower row of photocells and rearing was registered when the higher row was interrupted . animals were individually placed gently in the open field arena and remained there during 60 min . locomotion and rearing counts were registered every 10 min . for the l - dopa treatment , spontaneous locomotion and rearing were measured in activity cages as above . animals were then given i . p . injections of l - dopa combined with the peripheral dopa decarboxylase inhibitor benzerazide ( madopark 20 mg / kg in pbs ; roche ) or pbs and were then immediately put back into the activity cages for another 60 minutes of recording . the scoring of counts was given as mean +/− sem . statistical significance was tested using 2 - way anova analysis . the open field box was cleaned with 70 % alcohol after each test . locomotor skill and balance were tested using a rotating rod ( rota - rod ). the mice were given a short practice run in the morning and then three trials were done in the afternoon with one hour between every trial . the speed of the rod increased from 4 - 40 rpm and each trial was terminated after 300 seconds . mice with a cell - type specific expression of cre recombinase in dopaminergic ( da ) neurons were generated by knocking in cre at the dopamine transporter locus ( dat ; solute carrier family 6 , member 3 ). a targeting vector was constructed spanning 5 . 5 kb both upstream and downstream of the endogenous translational start site located in exon 2 . the vector contained a cassette with an nls - cre gene followed by an frt - flanked neomycin resistance ( neo ) gene inserted just before the start site in exon 2 to drive cre expression from the endogenous dat - promoter ( fig1 a ). the knockin locus was introduced into embryonic stem ( es ) cells by homologous recombination . after passage through the germline we verified the presence of both the wild - type ( wt ) and the knockin ( dat - cre neo ) allele in the heterozygous mice ( fig1 b ). the presence of the neo gene in this targeted locus could potentially lead to several problems . neo contains a cryptic splice site which can cause abnormal splicing and thereby affect the normal expression of the dat - locus . furthermore , the expression of neo is driven by the thymidine kinase ( tk ) promoter which may recruit excess amounts of transcriptional elements to the locus , thereby causing ectopic expression of cre in non - da cells . in our experimental strategy we circumvented this problem by flanking the neo gene with frt - sites , enabling us to later remove neo by crossing the dat - cre neo mice to flpe deleter mice thus generating dat - cre mice ( fig1 a ). we demonstrated the excision of neo both by southern blot ( fig1 b ) and by pcr ( fig1 c ). when dat - cre neo and dat - cre mice were crossed with mice homozygous for a loxp - flanked tfam allele ( tfam loxp / tfam loxp ) we only observed unspecific recombination of the tfam locus in tail dna of dat - cre neo mice but not in tail dna of dat - cre mice ( fig1 c ). when we repeated the tfam genotyping using dna from heart , kidney , liver , spleen , testis and cerebral cortex we again saw unspecific recombination of the tfam allele in all tissues of dat - cre neo , mice but not in any tissue from dat - cre mice ( data not shown ). to study the cre expression in the brain we performed in situ hybridization on brain sections from dat - cre mice using probes for both cre and tyrosine hydroxylase ( th ), a commonly used marker for catecholamine neurons . the cre probe gave a clear hybridization signal in the ventral tegmental area ( vta ) and in the substantia nigra pars compacta ( snpc ), overlapping perfectly with the signal from the th probe ( fig5 a ), thus demonstrating that the neurons expressing th mrna in the ventral mesencephalon ( dopamine neurons ) now co - expressed cre recombinase mrna . to assess cre activity we also crossed dat - cre mice with reporter mice that upon cre mediated excision of an upstream stop site express the lacz gene from the rosa26 locus ( rosa26 - r ). fluorescence immunohistochemistry with β - gal antibodies showed a highly specific labeling confined to the da neurons in the vta and snpc area very similar to the labeling achieved using th antibodies ( fig5 b ). thus , also at the protein level , there was a complete match between neurons expressing th immunoreactivity and those expressing β - gal immunoreactivity . we wanted to generate mice with a parkinsonian phenotype by knocking out the gene for mitochondrial transcription factor a ( tfam ) in dopaminergic neurons . tfam has previously been knocked out in different tissues always resulting in slow and delayed cell death due to respiratory chain failure . to generate the parkinson mice we used a two step breeding protocol . first dat - cre mice were crossed to mice homozygous for a loxp - flanked tfam allele ( tfam loxp / tfam loxp ). da t - cre ; +/ tfam loxp mice from this mating were identified and crossed again to tfam loxp / tfam loxp mice . this cross generated the four possible genotypes in approximately mendelian proportions ( 21 %+/ tfam loxp , 24 % tfam loxp / tfam loxp , 27 % dat - cre ; +/ tfam loxp and 29 % da t - cre ; tfam loxp / tfam loxp ). dat - cre ; tfam loxp / tfam loxp pups appeared normal , developed normally and gained weight as their littermates did ( data not shown ). at around 15 weeks of age they started to display pd - like symptoms such as abnormal gait and bradykinesia . decreased locomotion and reduced exploratory behaviour became apparent at that time . they still appeared to be relatively healthy though , with normal weight and a well groomed fur . this phenotype remained constant for several weeks with no further change in behavior or activity and no weight loss or other signs of decreased health status . we observed progression of symptoms with tremor , increased twitching and apparent limb rigidity at about 20 weeks of age . we provided the mice with moist food on the cage floor from about 20 weeks of age because motor impairment prevented them from reaching food and water normally provided on the grid covering the cage . however , even with this procedure , there was a slow gradual decline of weight as the neurodegeneration progressed ( fig2 i ), and the homozygous knockout mice had to be euthanized at about 45 weeks of age because of poor general condition . next , we used histological analyses to characterize the neuropathology underlying the progressive deterioration of motor function in homozygous knockout mice ( fig3 a - n ). we performed th immunofluorescence in newborn as well as 2 and 6 week old mice and found that the da innervation of striatum appeared normal in homozygous knockout mice at these ages ( fig3 a - c and data not shown ). we first observed loss of da nerve terminals in dorsolateral striatum at age 12 weeks and this loss progressed to involve most of the dorsal striatum as the homozygous knockout mice became older ( fig3 c - f and data not shown ). we also observed a subsequent reduction of da nerve terminals in ventral striatum , from 20 weeks of age , which progressed to involve most areas of the ventral striatum ( fig3 d - f ). the striking end result was an almost complete loss of th - immunoreactive nerve terminals ( fig3 m , n ) in dorsal as well as ventral striatum , except for parts of nucleus accumbens , in 43 weeks old homozygous knockout mice ( fig3 f ). cresyl violet staining ( not shown ) and th immunohistochemistry ( fig3 i - l ) revealed progressive loss of da neurons in the homozygous knockout mouse midbrain . we observed da nerve cell loss in snpc , which predominantly innervates dorsal striatum , from age 12 weeks , whereas da nerve cell loss in vta , which predominantly innervates ventral striatum and cortical areas , was not obvious until 20 weeks of age in homozygous knockout mice ( fig3 i - l ). there was thus a very good correlation between the temporal and spatial progression of striatal da denervation ( fig3 c - f ) and midbrain da nerve cell loss ( fig3 i - l ). neurochemical studies with hplc demonstrated a profound reduction of da and its metabolites in striatum ( caudate putamen and nucleus accumbens ), snpc , vta and cortex in 20 weeks old homozygous knockout mice ( fig3 - q ). in dorsal striatum da levels were reduced to about 1 . 5 % of normal levels . we also found markedly increased ratios of homovallinic acid ( hva ) and 3 , 4 - dihydroxyphenylacetic acid ( dopac ) to da , indicating an increased da turnover in striatum of homozygous knockout mice at this age ( fig3 q ). the loss of da nerve cells and nerve terminals was thus accompanied by a corresponding loss of da content in affected brain regions . many th positive neurons of homozygous knockout mice contained rounded th negative regions in their cytoplasm and we therefore performed confocal microscopy studies revealing that these areas were occupied by intracytoplasmic aggregates of α - synuclein - immunoreactivity ( fig4 a - b ). we detected small α - synuclein - immunoreactive aggregates in many da neurons in snpc and in a few da neurons in vta from 6 weeks of age in homozygous knockout mice . these α - synuclein aggregates became larger and more abundant as the homozygous knockout mice aged and were present in most remaining da neurons of snpc and vta from 16 weeks of age onwards . no α - synuclein - immunoreactive inclusions were observed in a12 da neurons of homozygous knockout mice ( fig4 d ) or in snpc , vta or a12 da neurons of control mice at different ages ( not shown ). da neurons in patients with parkinson &# 39 ; s disease often have intracytoplasmic lewy bodies , which contain ubiquitin - conjugated α - synuclein aggregates ( dauer et al ., neuron 39 , 889 - 909 ( 2003 )). we therefore performed additional confocal microscopy and found that the α - synuclein - immunoreactive aggregates were not immunoreactive for ubiquitin in younger homozygous knockout mice ( not shown ). however , a subset of the α - synuclein aggregates was also immunoreactive for ubiquitin in homozygous knockout mice older than 20 weeks ( fig4 c ). tunel labeling did not reveal any da neurons undergoing apoptotic death in mesencephalon of homozygous knockout mice at different ages ( not shown ). we also performed immunohistochemistry to detect glial fibrillary acidic protein ( gfap ) and found no significantly increased expression in striatum and a very slight increase in snpc ( not shown ) of homozygous knockout mice . these observations suggest that the cell death in snpc and vta of homozygous knockout mice is a continuous and slowly progressive process that is not associated with any marked astroglial response . we performed behavioral tests of homozygous knockout and control mice of different ages to obtain quantitative measurements of the clinically obvious motor impairment . we found a trend towards lower counts for both locomotion ( fig2 a ) and rearing ( fig2 d ) in 10 week old homozygous knockout mice , but these differences were not statistically significant . however , we found a clear reduction of both locomotion ( fig2 b ) and rearing ( fig2 e ) in 14 week old homozygous knockout mice and both of these activities declined progressively as homozygous knockout mice grew older ( fig2 c , f ). we also induced motor activity with the rota - rod and found a trend towards decreased performance in 10 week old homozygous knockout mice ( fig2 g ) and a significant reduction in 14 week old homozygous knockout mice ( fig2 h ). control mice typically improved their performance on the rota - rod as the test was repeated ( fig2 g , h ). most of the 14 week old control mice remained on the rod for the full 300 seconds of the experiment during the third trial , whereas homozygous knockout mice of the same age were unable to improve their locomotor performance ( fig2 h ). strikingly , the homozygous knockout mice were unable to move for an extended period of time after that they had fallen off the rota - rod , whereas control mice immediately started to explore the new surroundings . we used a standard parkinson &# 39 ; s disease drug containing l - dopa ( madopark , roche ) to treat 20 and 30 week old homozygous knockout mice and observed improved motor performance ( fig4 e - h ). interestingly , an identical l - dopa dose elicited a more pronounced and longer lasting locomotion response in the younger homozygous knockout mice ( fig4 e , f ), whereas the increase in rearing was similar in homozygous knockout mice at both ages ( fig4 g , h ). the locomotor response abruptly terminated in the older homozygous knockout mice resembling the “ wearing off ” effect seen clinically . the foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting . since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art , the invention should be construed to include everything within the scope of the appended claims and equivalents thereof . patent applications , patents and literature references cited herein indicate the knowledge in this field and are hereby incorporated by reference in their entirety . where inconsistent interpretations are possible , the disclosure herein controls .
the invention relates to a transgenic mouse model with deficient respiratory chain function in dopamine neurons . by suppressing or deleting the mitochondrial transcription factor a in da neurons , a mouse model is obtained , which reproduces key pathophysiological features of parkinson &# 39 ; s disease , i . e ., slow progressive loss of da terminals in striatum and loss of da neurons in substantia nigra pars compacta ; alpha - synuclein immunoreactivity including intracellular inclusions similar to lewy bodies in affected areas prior to and during cell loss ; progressive movement disorder associated with abnormal gait , tremor and rigid limbs . the mouse model can be used to develop pharmacological , gene therapy or cell therapy treatments for pd .