Rescue from excitotoxicity and axonal degeneration accompanied by age-dependent behavioral and neuroanatomical alterations in caspase-6-deficient mice

Apoptosis, or programmed cell death, is a cellular pathway involved in normal cell turnover, developmental tissue remodeling, embryonic development, cellular homeostasis maintenance and chemical-induced cell death. Caspases are a family of intracellular proteases that play a key role in apoptosis. Aberrant activation of caspases has been implicated in human diseases. In particular, numerous findings implicate Caspase-6 (Casp6) in neurodegenerative diseases, including Alzheimer disease (AD) and Huntington disease (HD), highlighting the need for a deeper understanding of Casp6 biology and its role in brain development. The use of targeted caspase-deficient mice has been instrumental for studying the involvement of caspases in apoptosis. The goal of this study was to perform an in-depth neuroanatomical and behavioral characterization of constitutive Casp6-deficient (Casp6−/−) mice in order to understand the physiological function of Casp6 in brain development, structure and function. We demonstrate that Casp6−/− neurons are protected against excitotoxicity, nerve growth factor deprivation and myelin-induced axonal degeneration. Furthermore, Casp6-deficient mice show an age-dependent increase in cortical and striatal volume. In addition, these mice show a hypoactive phenotype and display learning deficits. The age-dependent behavioral and region-specific neuroanatomical changes observed in the Casp6−/− mice suggest that Casp6 deficiency has a more pronounced effect in brain regions that are involved in neurodegenerative diseases, such as the striatum in HD and the cortex in AD

Low Levels of Human HIP14 Are Sufficient to Rescue Neuropathological, Behavioural, and Enzymatic Defects Due to Loss of Murine HIP14 in Hip14−/− Mice

Huntingtin Interacting Protein 14 (HIP14) is a palmitoyl acyl transferase (PAT) that was first identified due to altered interaction with mutant huntingtin, the protein responsible for Huntington Disease (HD). HIP14 palmitoylates a specific set of neuronal substrates critical at the synapse, and downregulation of HIP14 by siRNA in vitro results in increased cell death in neurons. We previously reported that mice lacking murine Hip14 (Hip14−/−) share features of HD. In the current study, we have generated human HIP14 BAC transgenic mice and crossed them to the Hip14−/− model in order to confirm that the defects seen in Hip14−/− mice are in fact due to loss of Hip14. In addition, we sought to determine whether human HIP14 can provide functional compensation for loss of murine Hip14. We demonstrate that despite a relative low level of expression, as assessed via Western blot, BAC-derived human HIP14 compensates for deficits in neuropathology, behavior, and PAT enzyme function seen in the Hip14−/− model. Our findings yield important insights into HIP14 function in vivo

BAC-derived Human HIP14 is not sufficient to rescue body weight in the <i>Hip14</i>−/− mice.

<p><i>Hip14−/−</i> mice demonstrate reduced body weight as early as 2 weeks of age (data not shown). <b>a.</b> Body weight is decreased in <i>Hip14−/−</i> females (89.2% of WT) at 3 months of age, and this is partially rescued (93.8% of WT) in BAC mice (WT 23.47±0.76, <i>Hip14</i>−/− 20.94±0.23 g, BAC 22.02±0.44 g). At 6 months, a similar partial rescue is observed (WT 30.00±1.74, <i>Hip14</i>−/− 24.75±0.49 g and 82.5% of WT, BAC 26.29±0.69 g and 87.6% of WT). Repeated measures ANOVA reveals a significant effect of genotype in females alone (F(2,27) = 7.53, p = 0.0025). Bonferroni post-tests reveal that WT differs significantly from both <i>Hip14</i>−/− (p<0.0001) and BAC (p<0.01). n = 10, 10, and 9 for WT, <i>Hip14</i>−/−, and BAC respectively. <b>b.</b> In males, <i>Hip14−/−</i> body weight is similarly decreased at 3 months (86.7% of WT) with partial rescue (91.7% of WT) in BAC (WT 28.79±0.56, <i>Hip14</i>−/− 24.95±0.57 g, BAC 26.39±0.47 g). By 6 months, partial rescue is further apparent (WT 33.91±0.71, <i>Hip14</i>−/− 28.30±0.70 g and 83.45% of WT, BAC 30.78±0.44 g and 90.75% of WT). A highly significant effect of genotype is present in males (F(2,37) = 26.81, p<0.0001). Bonferroni post-tests reveal a highly significant difference between WT and <i>Hip14</i>−/− at 3 and 6 months of age (p<0.0001), and a significant, but less robust, difference between WT and BAC mice at both ages (3 months p<0.05, 6 months p<0.01). However, BAC mice are also significantly different from <i>Hip14−/−</i> at both 3 (p<0.05) and 6 months (p<0.0001). n = 15, 10, and 8 for WT, <i>Hip14</i>−/−, and BAC respectively.</p

Sequences of primers used in this study.

<p>Sequences of primers used in this study.</p

A table summarizing the genotyping results for the 11 founders.

<p>A “+" sign indicates that a PCR product was detected. A “−" sign indicates that no PCR product was detected, suggesting that a truncated BAC construct was integrated in these lines.</p

HIP14 rescues the neuropathological deficits seen in the <i>Hip14</i>−/− mouse.

<p>Mice aged 1 month were perfused with 4% PFA and sectioned using a cryostat, stained with anti-NeuN, mounted and volumes and neuronal counts were measured using Stereo investigator. <b>a.</b> Total brain weight shows a trend to rescue in mice aged 1 month, although one-way ANOVA analysis is not significant (WT: 321.5±3.78, <i>Hip14−/−:</i> 309.8±3.74, BAC: 319.2±5.32 g, ANOVA p = 0.14). Pairwise t-tests reveal that <i>Hip14−/−</i> whole brain weight is significantly decreased compared to WT (p = 0.04) and that BAC do not differ from WT (p = 0.7). <b>b.</b> Cerebellum weight is significantly decreased in <i>Hip14</i>−/− mice and is rescued to WT levels in BAC mice at 1 month (WT: 51.41±0.82, <i>Hip14−/−:</i> 46.45±0.83, BAC: 50.34±1.38 g, ANOVA p = 0.0038). Pairwise t-test analysis similarly reveals that <i>Hip14−/−</i> cerebellum is significantly decreased compared to WT (p = 0.0003) and that BAC do not differ from WT (p = 0.5). <b>c.</b> Forebrain weight shows a similar trend to rescue at 1 month of age (WT: 270.1±3.15, <i>Hip14−/−:</i> 263.4±3.06, BAC: 268.9±4.27 g, ANOVA p = 0.36). <b>d.</b> Striatal volume is significantly rescued in BAC mice at 1 month of age (WT:10.4±0.3, <i>Hip14</i>−/−: 8.8±0.4 mm³, BAC: 10.3±0.2 mm³; ANOVA p = 0.002). <b>e.</b> Striatal neuron count is likewise rescued by the human <i>HIP14</i> BAC at 1 month of age (WT:2.0±0.1, <i>Hip14</i>−/−: 1.7±0.08, BAC: 2.1±0.03 million cells; ANOVA p = 0.004). <b>f.</b> Finally, cortical volume loss observed in <i>Hip14−/−</i> mice is also rescued by human HIP14 at 1 month (WT:26.86±0.52, <i>Hip14</i>−/−: 23.66±0.75, BAC: 26.22±0.60 mm³; ANOVA p = 0.003). Similar observations are made at 3 months of age (data not shown). 1 month n = 12, 12, and 11 for WT, <i>Hip14</i>−/−, and BAC respectively. * p<0.05, **p<0.01, ***p<0.0001.</p

Human HIP14 rescues behavioural alterations in the <i>Hip14−/−</i> mouse.

<p>Deficits in motor coordination and balance are rescued by BAC-derived human HIP14. <b>a.</b> Repeated measures ANOVA in combined genders reveals a significant effect of genotype F(2,300) = 600, p = 0.0035. Bonferroni post-hoc analysis reveals that BAC mice perform consistently better on the accelerating rotarod task than <i>Hip14</i>−/− mice (p<0.05 in trials 1,4; p<0.01 in trials 5,6) and superior to WT mice in later trials (p<0.05 trial 5). All other post-hoc comparisons are non-significant (p>0.05). n = 25, 20, and 17 for WT, <i>Hip14</i>−/−, and BAC respectively. (<b>b–j</b>) Human HIP14 normalizes the hyperactivity observed in <i>Hip14</i>−/− mice to WT levels. Spontaneous activity was assessed in Med Associates boxes at 3 and 6 months of age. Repeated measures ANOVA reveals a significant effect of genotype in ambulatory time (<b>b;</b> F(2,53) = 5.05, p = 0.0099) and counts (<b>e;</b> F(2,53) = 5.36, p = 0.0076), stereotypic time (<b>c</b>; F(2,53) = 19.70, p<0.0001) and counts (<b>f</b>; F(2,53) = 15.13, p<0.0001), distance traveled <b>(d;</b> F(2,53) = 3.60, p = 0.034), time resting (<b>g;</b>(2,53) = 4.99, p = 0.010), and ambulatory episodes (<b>j</b>; F(2,53) = 8.68, p = 0.0005). A significant effect was not seen in vertical counts <b>(h;</b> F(2,51) = 1.89, p = 0.1611<b>)</b>, and average velocity <b>(i;</b> F(2,54) = 0.68, p = 0.51<b>)</b>. The most robust effects were observed for stereotypic time (<b>c</b>) and counts (<b>f</b>). Single-housed mice were excluded from all analyses.*p<0.05, **p<0.01, ***p<0.0001.</p

HIP14 mRNA and protein expression in <i>Hip14−/−</i> and <i>HIP14</i> BAC mice.

<p>Human <i>HIP14</i> mRNA (<b>a</b>) is expressed in BAC mice (p<0.0001) and no signal is detected in WT or <i>Hip14</i>−/− littermates (ANOVA p<0.0001, n = 6). Murine <i>Hip14</i> mRNA transcript (<b>b</b>) is significantly reduced in <i>Hip14−/−</i> mice (p<0.0001) and is not altered in the presence of human <i>HIP14</i> (ANOVA p<0.0001, n = 6). Primers for mouse actin were used as an endogenous control. HIP14 protein expression in striatum (<b>c</b>) and cortex (<b>d</b>) was assessed by western blot using an in-house HIP14 polyclonal antibody. Beta-tubulin was used as a loading control. Striatum and cortex both n = 5. Post-hoc Tukey tests: ***p<0.0001.</p

Rescue from excitotoxicity and axonal degeneration accompanied by age-dependent behavioral and neuroanatomical alterations in caspase-6-deficient mice

Apoptosis, or programmed cell death, is a cellular pathway involved in normal cell turnover, developmental tissue remodeling, embryonic development, cellular homeostasis maintenance and chemical-induced cell death. Caspases are a family of intracellular proteases that play a key role in apoptosis. Aberrant activation of caspases has been implicated in human diseases. In particular, numerous findings implicate Caspase-6 (Casp6) in neurodegenerative diseases, including Alzheimer disease (AD) and Huntington disease (HD), highlighting the need for a deeper understanding of Casp6 biology and its role in brain development. The use of targeted caspase-deficient mice has been instrumental for studying the involvement of caspases in apoptosis. The goal of this study was to perform an in-depth neuroanatomical and behavioral characterization of constitutive Casp6-deficient (Casp6−/−) mice in order to understand the physiological function of Casp6 in brain development, structure and function. We demonstrate that Casp6−/− neurons are protected against excitotoxicity, nerve growth factor deprivation and myelin-induced axonal degeneration. Furthermore, Casp6-deficient mice show an age-dependent increase in cortical and striatal volume. In addition, these mice show a hypoactive phenotype and display learning deficits. The age-dependent behavioral and region-specific neuroanatomical changes observed in the Casp6−/− mice suggest that Casp6 deficiency has a more pronounced effect in brain regions that are involved in neurodegenerative diseases, such as the striatum in HD and the cortex in AD