Do cognitive problems start before motor dysfunction in animal models for Huntington Disease

Huntington s disease (HD) is a devastating neurodegenerative disorder caused by a genetic mutation that produces an expansion of C-A-G repeats in the huntingtin (HTT) gene. This expansion results in a selective neuronal degeneration in striatal and cortical brain regions. Although HD is best known for chorea, the disease is actually a triad of progressive cognitive, psychiatric and motor symptoms and patients usually die within 15 to 20 years of onset. Cognitive impairments seem to occur decades prior to the onset of motor symptoms. This potentially offers a long time window for treatments to delay or halt disease progression. Development of such treatments requires the generation of transgenic animal models for HD. Such models are now available and have already greatly enhanced our understanding of the pathogenesis of HD. But unsolved questions such as: Do cognitive manifestations precede motor deficits in transgenic mouse and rat models for HD? µremain. We address this question in 3 studies that examine the development of motor, cognitive and psychiatric symptoms in BACHD mice and rats, two recently generated models for HD. Unlike other genetic models, BACHD mice and rats express the full length of the human mutant HTT (97 polyglutamine repeats) and present neuropathology similarities to that of patients. We found progressive motor, cognitive and psychiatric deficits in transgenics BACHD mice and rats compared to their wild type littermate controls, on different tasks such as catwalk, rotarod, open field, zero-maze, cross-maze, prepulse inhibition and fear conditioning. The findings indicate that progressive cognitive deficits can be reliably detected with methodologically rigorous protocols. However some phenotypic variability (body weight and emotional-like behavior, for example) was found and might involve species differences and ectopic expression of the human mutant HTT gene. Finally, the time course for the emergence of the various symptoms indicates that motor abnormalities might be the first to occur in transgenic BACHD mice and rats. In conclusion: we have identified robust cognitive and motor phenotypes for BACHD mice and rats that can be used to test novel compounds from HD drug discovery programs. Certain tests like fear conditioning, lend themselves for translational approaches and characterization of HD patients in such tests would be a logical next step. Confirmation of a fear conditioning phenotype in HD patients would offer an additional functional read-out for drug testing and decrease program risk by bridging the gap between rodent and human testing

Reversal learning and associative memory impairments in a BACHD rat model for Huntington disease

Chorea and psychiatric symptoms are hallmarks of Huntington disease (HD), a neurodegenerative disorder, genetically characterized by the presence of expanded CAG repeats (&gt;35) in the HUNTINGTIN (HTT) gene. HD patients present psychiatric symptoms prior to the onset of motor symptoms and we recently found a similar emergence of non motor and motor deficits in BACHD rats carrying the human full length mutated HTT (97 CAG-CAA repeats). We evaluated cognitive performance in reversal learning and associative memory tests in different age cohorts of BACHD rats. Male wild type (WT) and transgenic (TG) rats between 2 and 12 months of age were tested. Learning and strategy shifting were assessed in a cross-maze test. Associative memory was evaluated in different fear conditioning paradigms (context, delay and trace). The possible confound of a fear conditioning phenotype by altered sensitivity to a 'painful' stimulus was assessed in a flinch-jump test. In the cross maze, 6 months old TG rats showed a mild impairment in reversal learning. In the fear conditioning tasks, 4, 6 and 12 months old TG rats showed a marked reduction in contextual fear conditioning. In addition, TG rats showed impaired delay conditioning (9 months) and trace fear conditioning (3 months). This phenotype was unlikely to be affected by a change in 'pain' sensitivity as WT and TG rats showed no difference in their threshold response in the flinch-jump test. Our results suggest that BACHD rats have a profound associative memory deficit and, possibly, a deficit in reversal learning as assessed in a cross maze task. The time course for the emergence of these symptoms (i.e., before the occurrence of motor symptoms) in this rat model for HD appears similar to the time course in patients. These data suggest that BACHD rats may be a useful model for preclinical drug discovery.</p

Gehen kognitive Defizite motorischen Dysfunktionen in Tiermodellen für Chorea Huntington voraus.

Huntington s disease (HD) is a devastating neurodegenerative disorder caused by a genetic mutation that produces an expansion of C-A-G repeats in the huntingtin (HTT) gene. This expansion results in a selective neuronal degeneration in striatal and cortical brain regions. Although HD is best known for chorea, the disease is actually a triad of progressive cognitive, psychiatric and motor symptoms and patients usually die within 15 to 20 years of onset. Cognitive impairments seem to occur decades prior to the onset of motor symptoms. This potentially offers a long time window for treatments to delay or halt disease progression. Development of such treatments requires the generation of transgenic animal models for HD. Such models are now available and have already greatly enhanced our understanding of the pathogenesis of HD. But unsolved questions such as: "Do cognitive manifestations precede motor deficits in transgenic mouse and rat models for HD?" µremain. We address this question in 3 studies that examine the development of motor, cognitive and psychiatric symptoms in BACHD mice and rats, two recently generated models for HD. Unlike other genetic models, BACHD mice and rats express the full length of the human mutant HTT (97 polyglutamine repeats) and present neuropathology similarities to that of patients. We found progressive motor, cognitive and psychiatric deficits in transgenics BACHD mice and rats compared to their wild type littermate controls, on different tasks such as catwalk, rotarod, open field, zero-maze, cross-maze, prepulse inhibition and fear conditioning. The findings indicate that progressive cognitive deficits can be reliably detected with methodologically rigorous protocols. However some phenotypic variability (body weight and emotional-like behavior, for example) was found and might involve species differences and ectopic expression of the human mutant HTT gene. Finally, the time course for the emergence of the various symptoms indicates that motor abnormalities might be the first to occur in transgenic BACHD mice and rats. In conclusion: we have identified robust cognitive and motor phenotypes for BACHD mice and rats that can be used to test novel compounds from HD drug discovery programs. Certain tests like fear conditioning, lend themselves for translational approaches and characterization of HD patients in such tests would be a logical next step. Confirmation of a fear conditioning phenotype in HD patients would offer an additional functional read-out for drug testing and decrease program risk by bridging the gap between rodent and human testing

Motor, emotional and cognitive deficits in adult BACHD mice:A model for Huntington's disease

<p>Rationale: Huntington's disease (HD) is characterized by progressive motor dysfunction, emotional disturbances and cognitive deficits. It is a genetic disease caused by an elongation of the polyglutamine repeats in the huntingtin gene. Whereas HD is a complex disorder, previous studies in mice models have largely been confined to assessing motor deficits.</p><p>Objectives: The aim of the present studies is a comprehensive phenotypical assessment of not only motor and gait deficits, but also of emotional and cognitive deficits in adult BACHD mice.</p><p>Material and methods: 46 male BACHD mice between 9 and 10 months of age were used. Wild type (+/+) and transgenic (+/T) mice were tested for motor deficits on a Rotarod and Catwalk system. Emotional deficits were assessed with the zero-maze and fear conditioning tests. Cognitive deficits in a strategy shifting task were evaluated in a cross-maze test.</p><p>Results: Comparing +/T and +/+ mice, we replicated the motor deficits in the transgenic mice that were previously described in the Rotarod test. For the first time, motor coordination imbalances in +/T animals are described in the Catwalk gait analysis system. +/T mice showed more anxiety-like behavior in the zero-maze test and a higher freezing response in the fear conditioning test. Reversal and strategy shifting impairments were demonstrated in the cross-maze, indicative of a disturbed prefrontal striatal pathway.</p><p>Conclusion: The results suggest that BACHD mice represent an animal model with a high degree of face validity for HD and may be very useful for testing novel therapeutic strategies. (c) 2012 Elsevier B.V. All rights reserved.</p>

Assessment of Motor Function, Sensory Motor Gating and Recognition Memory in a Novel BACHD Transgenic Rat Model for Huntington Disease

<p>Rationale: Huntington disease (HD) is frequently first diagnosed by the appearance of motor symptoms; the diagnosis is subsequently confirmed by the presence of expanded CAG repeats (> 35) in the HUNTINGTIN (HTT) gene. A BACHD rat model for HD carrying the human full length mutated HTT with 97 CAG-CAA repeats has been established recently. Behavioral phenotyping of BACHD rats will help to determine the validity of this model and its potential use in preclinical drug discovery studies.</p><p>Objectives: The present study seeks to characterize the progressive emergence of motor, sensorimotor and cognitive deficits in BACHD rats.</p><p>Materials and Methods: Wild type and transgenic rats were tested from 1 till 12 months of age. Motor tests were selected to measure spontaneous locomotor activity (open field) and gait coordination. Sensorimotor gating was assessed in acoustic startle response paradigms and recognition memory was evaluated in an object recognition test.</p><p>Results: Transgenic rats showed hyperactivity at 1 month and hypoactivity starting at 4 months of age. Motor coordination imbalance in a Rotarod test was present at 2 months and gait abnormalities were seen in a Catwalk test at 12 months. Subtle sensorimotor changes were observed, whereas object recognition was unimpaired in BACHD rats up to 12 months of age.</p><p>Conclusion: The current BACHD rat model recapitulates certain symptoms from HD patients, especially the marked motor deficits. A subtle neuropsychological phenotype was found and further studies are needed to fully address the sensorimotor phenotype and the potential use of BACHD rats for drug discovery purposes.</p>

Strategy shifting.

<p>Number of rats that exhibited Place (P) or Response (R) learning strategy during each Probe trial P1, P2 and P3 (days 8, 16 and 23 respectively) are represented for WT and TG cohorts of 2 and 6 months of age. The size corresponds to animals that made (1) correct arm choices with a learning index greater than 0.5 during each training session, and (2) were successful for the two last trials prior to the probe trials in the cross maze.</p

Flinch-jump test.

<p>Sensitivity of 6-shocks for [<b>a</b>] flinch and [<b>b</b>] jump (WT n = 15, TG n = 15). Individual intensity response is plotted and bars indicate median values for each genotype. There was no difference between WT and TG rats in current intensities that elicited a flinch or a jump response.</p

Delay and Trace fear conditioning.

<p>[<b>a</b>] Schematic illustration of the delay fear conditioning paradigm. [<b>b</b>] Results in 9 months old BACHD rats are expressed as Mean ± SEM (WT n = 13, TG n = 15). TG rats presented a significant lower freezing response to the context and to the tone. [<b>c</b>] Schematic illustration of trace fear conditioning paradigm. [<b>d</b>] Results in 3 months old BACHD rats are expressed as Mean ± SEM (WT n = 13, TG n = 15). TG rats, compared to WT rats, showed a significantly lower freezing response to the context, to the tone and trace period during retention tests. Asterisks indicate significant differences between WT and TG rats (**p<0.01 and ***p<0.001).</p

Cross-maze task.

<p>2(WT n = 17, TG n = 17) and 6 months (WT n = 15, TG n = 15) BACHD rats were used. [<b>a</b>] Schematic representation of the cross-maze task. The north (N) arm is closed. The rat starts training in the south arm (S) and reaches the home cage through the hole located in the west arm (w, (1) acquisition) or east arm (E, (2) reversal). During (3) probe trial days 8 (P1), 16 (P2) and 23 (P3), the (S) arm is closed and the rat starts in the (N) arm. Rats reaching the home cage arm are Place learners, while those reaching the other arm are Response learners. [<b>b</b>] Learning index. Mean number of correct choices over acquisition (A), reversal (R) and extended reversal (ER) trials in BACHD rats. Both WT and TG rats of each age showed difficulties during (R); however, with (ER) training, 2 months old rats have improved learning whereas 6 months old TG rats have a learning index barely above chance level. Asterisks indicate significant difference from the hypothetical value (One sample test, *p<0.05, **p<0.01 and ***p<0.001). [<b>c</b> and <b>d</b>] Training trials. The percentage of correct choices made during acquisition (A), reversal (R) and extended reversal (ER) training are depicted. For the first 2 days, results where the goal arm was baited with sucrose food pellets are presented by dashed lines. There was no difference in acquisition training for both age cohorts. 6 months old TG rats (d) differed significantly from WT rats during reversal trial 7 and overall extended reversal trials (ER1 to ER6). Asterisks indicate significant difference (Chi square test, *p<0.05 and **p<0.01).</p

Contextual fear conditioning.

<p>[<b>a</b>] Schematic illustration of the contextual fear conditioning protocol. [<b>b–c</b>] Results are expressed as Mean ± SEM of percentage freezing. 4 months (WT n = 13, TG n = 13), 6 months (WT n = 7, TG n = 9) and 12 months (WT n = 16, TG, n = 6) BACHD rats were used. No difference in baseline responding to training was observed. TG rats showed less fear memory to the context as they freeze less in comparison with WT rats at 4, 6, and 12 months of age [<b>b</b>]. Long term memory was assessed 1 month and 2 months after retention testing were conducted in the 4 months old rats cohort (i.e. they were tested at 5 and 6 months of age respectively) [<b>c</b>]. TG compared to WT still had lower freezing to the context. A progressive freezing ‘extinction’ was observed. Asterisks indicate significant differences between WT and TG rats (***p<0.001).</p