Research Articles, Neurobiology of Disease

Suppressing N-Acetyl-l-Aspartate Synthesis Prevents Loss of Neurons in a Murine Model of Canavan Leukodystrophy

Jiho Sohn, Peter Bannerman, Fuzheng Guo, Travis Burns, Laird Miers, Christopher Croteau, Naveen K. Singhal, Jennifer A. McDonough and David Pleasure
Journal of Neuroscience 11 January 2017, 37 (2) 413-421; DOI: https://doi.org/10.1523/JNEUROSCI.2013-16.2016

Abstract

Canavan disease is a leukodystrophy caused by aspartoacylase (ASPA) deficiency. The lack of functional ASPA, an enzyme enriched in oligodendroglia that cleaves N-acetyl-l-aspartate (NAA) to acetate and l-aspartic acid, elevates brain NAA and causes “spongiform” vacuolation of superficial brain white matter and neighboring gray matter. In children with Canavan disease, neuroimaging shows early-onset dysmyelination and progressive brain atrophy. Neuron loss has been documented at autopsy in some cases. Prior studies have shown that mice homozygous for the Aspa nonsense mutation Nur7 also develop brain vacuolation. We now report that numbers of cerebral cortical and cerebellar neurons are decreased and that cerebral cortex progressively thins in AspaNur7/Nur7 mice. This neuronal pathology is prevented by constitutive disruption of Nat8l, which encodes the neuronal NAA-synthetic enzyme N-acetyltransferase-8-like.

SIGNIFICANCE STATEMENT This is the first demonstration of cortical and cerebellar neuron depletion and progressive cerebral cortical thinning in an animal model of Canavan disease. Genetic suppression of N-acetyl-l-aspartate (NAA) synthesis, previously shown to block brain vacuolation in aspartoacylase-deficient mice, also prevents neuron loss and cerebral cortical atrophy in these mice. These results suggest that lowering the concentration of NAA in the brains of children with Canavan disease would prevent or slow progression of neurological deficits.

Introduction

Genetic screening for pathogenic aspartoacylase (ASPA) mutations has decreased the incidence of Canavan disease in Ashkenazi Jews, in whom ∼1 in 60 is an asymptomatic heterozygous mutation carrier (Feigenbaum et al., 2004). Pathogenic ASPA mutations also occur, although less frequently, in other human populations (Kaul et al., 1994; Shaag et al., 1995). No therapies are yet available to prevent or reverse cognitive and motor deficits in children with this fatal leukodystrophy.

ASPA is expressed by mature oligodendroglia and serves to cleave N-acetyl-l-aspartate (NAA), releasing acetate that can be used for myelin lipid synthesis (Burri et al., 1991; Chakraborty et al., 2001; Francis et al., 2011, 2016; Bakken et al., 2016; Jolly et al., 2016). The lack of functional ASPA blocks NAA catabolism and elevates the brain NAA concentration ([NAAB]). This pathognomonic increase in [NAAB] can be detected by in vivo brain proton nuclear magnetic resonance spectroscopy (Janson et al., 2006b; Francis et al., 2012). Macrocephaly often develops during the first year after birth in Canavan disease (Traeger and Rapin, 1998), likely as a consequence of the onset of brain spongiform vacuolation during infancy (Adachi et al., 1973; Mirimanoff, 1976; Janson et al., 2006b). Progressive lags in motor and cognitive development are often already evident in the first 6 months after birth but occasionally first appear only later in childhood (Jellinger and Seitelberger, 1969; Janson et al., 2006a).

Astroglia express Nadc3 (encoded by Slc13a3), an Na+-coupled dicarboxylate transporter with high affinity for NAA, and deploy NAA as an osmolyte to assist in maintaining CNS water homeostasis (Yodoya et al., 2006; Baslow and Guilfoyle, 2013; Clarner et al., 2014). The substantial increase in [NAAB] in Canavan disease may perturb this homeostatic mechanism. This perturbation has been hypothesized to cause the astroglial swelling and myelin lamellar splitting in brain superficial white matter and neighboring gray matter in this disorder (Gambetti et al., 1969; Mirimanoff, 1976; Fujita et al., 2005; Traka et al., 2008; Baslow and Guilfoyle, 2013; Clarner et al., 2014; Guo et al., 2015; Maier et al., 2015). A prediction of this hypothesis was that blocking NAA synthesis in the ASPA-deficient brain would prevent astroglial swelling and dysmyelination. To test this prediction, we bred mice homozygous for both an Aspa nonsense mutation (AspaNur7/Nur7) (Traka et al., 2008) and for deletion of Nat8l (Nat8l−/−), the gene that encodes N-acetyltransferase-8-like (also known as aspartate N-acetyltransferase), a neuron-enriched enzyme essential for brain NAA synthesis (Ariyannur et al., 2010; Wiame et al., 2009; Wang et al., 2016). Whereas AspaNur7/Nur7/Nat8l+/+ mice, in which [NAAB] was markedly elevated, developed ataxia soon after weaning and, at autopsy, demonstrated brain vacuolation, AspaNur7/Nur7/Nat8l−/− mice, in which [NAAB] was undetectably low, preserved normal motor function and did not develop brain vacuolation (Guo et al., 2015). These results were confirmed by another research group (Maier et al., 2015), which also reported that heterozygous constitutive Nat8l deletion in AspaNur7/Nur7 mice partially prevented brain vacuolation.

Serial neuroimaging reveals progressive brain atrophy in children with Canavan disease (Janson et al., 2006b; Leone et al., 2012), and diminished numbers of forebrain and cerebellar neurons have been reported in some autopsied cases (Jellinger and Seitelberger, 1969; Adachi et al., 1973; Mirimanoff, 1976). We now demonstrate that constitutive disruption of Nat8l prevents progressive cerebral cortical thinning and the loss of cerebral cortical, Purkinje, and internal granule neurons in AspaNur7/Nur7 mice.

Materials and Methods

Mice.

Heterozygous AspaNur7 mice (AspaNur7/+, C57BL/6J background; RRID:MGI:3027206) were from The Jackson Laboratory (stock number 008607), and heterozygous Nat8l knock-out mice (Nat8l+/−, C57BL/6J background; RRID:IMSR_KOMP:VG11213–1-Vlcg; Guo et al., 2015) were from the University of California Davis (UC Davis) KOMP Repository (project ID VG11213). We bred these two mutant strains to produce Aspa+/Nur7/Nat8l+/− mice, which were then crossed to generate mice of the desired Aspa+/+/Nat8l+/+, AspaNur7/Nur7/Nat8l+/+, AspaNur7/Nur7/Nat8l+/−, and AspaNur7/Nur7/Nat8l−/− genotypes, which were born in the predicted Mendelian ratios. Mixed groups of male and female mice were used in all studies reported here. All mouse protocols were approved by the UC Davis Institutional Animal Care and Use Committee (protocol number 18029).

Aspa Western blot and qRT-PCR for Nat8l, LacZ, and Cnp mRNAs.

The anti-Aspa antibody used for the Western blot was from Proteintech (catalog #13244-1-AP; RRID:AB_2274358). Primers for Nat8l mRNA were forward 5′-ATCTTCTACGACGGCATCTTGG-3′ and reverse 5′-GCGGGTCACAGCAAAACAG-3′. Primers for lacZ mRNA, which was inserted into the Nat8l locus during VelociGene Nat8l disruption (Valenzuela et al., 2003), were forward 5′-CGCTGACGGAAGCAAAACA-3′ and reverse 5′-GCCCGGATAAACGGAACTG-3′. Primers for 2′,3′-cyclic nucleotide 3-phosphodiesterase (Cnp) mRNA were forward 5′-TTTACCCGCAAAAGCCACACA-3′ and reverse 5′-CACCGTGTCCTCATCTTGAAG-3′.

[NAAB] assay.

Six-month-old mice were deeply anesthetized with ketamine/xylazine. Their brains (including forebrain, cerebellum, and upper brainstem) were flash-frozen and stored at −70°C. [NAAB] was measured by HPLC (Li et al., 2013). For this purpose, the brains were homogenized in ice-cold 90% methanol and centrifuged twice at 14,000 rpm for 10 min at 4°C. The supernatants were dried by speed-vac, dissolved in 0.5 ml of deionized water, and applied to AG50W x-8 poly-pre columns (Bio-Rad). The columns were then washed with 1 ml of deionized water, and all eluates were collected, lyophilized, and stored at 4°C. For HPLC analysis, the lyophilized eluates were resuspended in 300 μl of deionized H2O. Samples and NAA standards (Sigma-Aldrich) were applied to a Whatman Partisil 10 SAX anion-exchange column (4.6 × 250 mm) using an Agilent 1100 Series HPLC value system (Agilent Technologies). The mobile phase consisted of 0.1 m KH2PO4 and 0.025 m KCl, pH 4.5. After the column was washed with 50% acetonitrile and 50% deionized water, it was conditioned with 20–30 column volumes of new mobile phase. Retention data were collected at a flow rate of 1.5 ml/min and were monitored at 214 nm using an Agilent 1100 Series UV detector (Li et al., 2013). Under these conditions, the NAA retention time was 5.10 min. Peak areas were ascertained with Agilent Chemstation software. [NAAB] was determined by reference to an NAA standard curve, measured in duplicate for each mouse, and normalized to whole-brain weight.

Rotarod testing.

Motor function was assessed by measuring accelerating rotarod retention times at ages 2 and 12 months. The 12-month-old Aspa+/+/Nat8l+/+, AspaNur7/Nur7/Nat8l+/+, and AspaNur7/Nur7/Nat8l−/− mice used in these accelerating rotarod tests were identical to those previously tested by this means at age 2 months (Guo et al., 2015). An additional group of AspaNur7/Nur7/Nat8l+/− mice was also evaluated at 2 and 12 months of age. The rotarod starting speed was four rotations per minute (rpm), followed by a speed step of 1.3 rpm every 10 s. The mice were subjected to 5 min training sessions every day for 10 d, at which point they had achieved plateau performances. These plateau retention times were used for statistical analysis.

Histology.

Before laser-scanning confocal microscopy, 6-month-old mice were deeply anesthetized with ketamine/xylazine and perfused with 4% (w/v) paraformaldehyde in PBS, pH 7.4. Their brains were harvested, postfixed in 4% paraformaldehyde in PBS overnight at 4°C, washed in PBS, and embedded in OCT. Cryostat sections were air-dried at room temperature. The sections were then incubated in 8% normal donkey serum (v/v) and 0.1% Triton X-100 (v/v) in PBS, pH 7.4, for 1 h at room temperature, followed by incubation with one of the following primary antibodies overnight at 4°C in PBS containing 5% normal donkey serum (v/v) and 0.1% Triton X-100 (v/v): mouse anti-calbindin IgG (1:1000 dilution, Swant 300; RRID:AB_10000347), rabbit anti-glial fibrillary acidic protein (GFAP) IgG (1:500 dilution, Dako Z033429-02; RRID:AB_10013482), mouse anti-NeuN IgG1 (1:500 dilution, Millipore MAB377; RRID:AB_2298772), rat anti-myelin basic protein (MBP) IgG2b (1:5 dilution, Millipore MAB395; RRID:AB_240845), or rabbit anti-cleaved caspase-3 IgG (1:200 dilution, Cell Signaling 9661; RRID:AB_2341188). The sections were then incubated with Alexa Fluor-conjugated donkey anti-mouse, anti-rabbit, or anti-rat secondary antibodies (IgG, 1:600 dilution, Invitrogen) for 1 h at room temperature and mounted in Prolong Gold containing DAPI (Invitrogen).

Coronal sections of forebrain at bregma −1.1 mm were used for cerebral cortical thickness measurements and NeuN+ neuron counts. To determine cerebral cortical thickness, NeuN-immunostained sections were imaged with a Nikon laser-scanning confocal microscope, and the images were processed using the NIS-Elements Annotations and Measurements tool. Ten vertical lines were drawn from the pial surface to the most dorsal NeuN+ neuron in layer VI, and their lengths were averaged to calculate mean cortical thickness for each animal (Rattray et al., 2013). NeuN+ neuron counts were performed on the same sections using an Olympus BX61-DSU microscope with motorized stage and StereoInvestigator software (Giannaris and Rosene, 2012). The optical dissector and guard zone were set at 8 and 2 μm, respectively. Twenty random 50 × 50 μm counting grids were assigned, and the NeuN+ neuron counts were performed at 40× magnification.

Cerebella were sectioned from the midsagittal line through the vermis and imaged with a Nikon laser-scanning confocal microscope. The NIS-Elements Annotate and Measure auto-detect function was used to evaluate vacuole area in cerebellar white matter by an observer blinded to genotype. Linear densities of calbindin+ Purkinje neurons (Pinol et al., 1990) and the area occupied by NeuN+ internal granule cells (Sarnat et al., 1998) were determined in cerebellar posterior lobes by an observer blinded to genotype, using NIH ImageJ software.

For transmission electron microscopy, 1-year-old mice were deeply anesthetized with ketamine/xylazine and perfused with 4% (w/v) paraformaldehyde, followed by 3% (w/v) glutaraldehyde, both in 0.1 m phosphate buffer, pH 7.4. Lateral sagittal cerebellar specimens (1–2 mm thick) were washed with 0.2 m sodium cacodylate buffer, pH 7.2, postfixed in 2% (w/v) aqueous osmium tetroxide for 2 h, washed with cacodylate buffer, dehydrated through ascending alcohols, washed with propylene oxide, and embedded in EMBed-812 resin (Electron Microscopy Sciences). Ultrathin sections (70–80 nm) were cut on a Leica EM UC7 microtome and collected on 1 × 2 mm Formvar-coated copper slot grids, double stained with uranyl acetate and lead citrate, and examined on a CM120 electron microscope.

Statistical methods.

The numbers of mice used for each study are shown in the figures as individual dots or are specified in the figure legends. Vertical bars denote SEs of the mean. Statistical analyses were by Student's t test or by ANOVA followed by post hoc Tukey's multiple comparison test or Bonferroni's correction.

Results

AspaNur7/Nur7 and Nat8l−/− mice

AspaNur7/Nur7 mice did not express immunoreactive aspartoacylase (Fig. 1A). Nat8l−/− mice, in which a lacZ reporter gene had been introduced during gene deletion (Valenzuela et al., 2003), did not express enough Nat8l mRNA for detection (Fig. 1B), but did express lacZ mRNA (Fig. 1C). Levels of mRNA encoding the myelin protein Cnp were not significantly different in Aspa+/+/Nat8l−/− and Aspa+/+/Nat8l+/− mice than in wild-type (Aspa+/+/Nat8l+/+) mice (Fig. 1D).

Figure 1.
Figure 1.

Biochemical characterization of AspaNur7/Nur7 and Nat8l−/− mice. A, Western blot shows lack of detectable Aspa in an AspaNur7/Nur7 mouse brain. B, qRT-PCR shows lack of Nat8l mRNA in Nat8l−/− mouse brains. C, qRT-PCR shows presence of lacZ mRNA (inserted into the gene locus during Nat8l disruption) (Valenzuela et al., 2003) in Nat8l−/− mouse brains. D, qRT-PCR shows lack of effect of Nat8l knock-out on mouse brain expression of Cnp mRNA, which encodes myelin CNPase. Results in B–D are means of three mice per group. SEMs are indicated by vertical bars. *p < 0.05; ***p < 0.001.

Disrupting Nat8l lowers [NAAB] in AspaNur7/Nur7 mice

At age 6 months, mean [NAAB] was 5.8 mm in wild-type (Aspa+/+/Nat8l+/+) mice, 7.6 mm in AspaNur7/Nur7/Nat8l+/− mice, and 10.8 mm in AspaNur7/Nur7/Nat8l+/+ mice (Fig. 2). NAA was not detectable in brains of AspaNur7/Nur7/Nat8l−/− mice (data not shown).

Figure 2.
Figure 2.

Nat8l deletion lowers [NAAB] in AspaNur7/Nur7 mice. [NAAB] in 6-month-old AspaNur7/Nur7 mice with one functional Nat8l allele was intermediate between that in age-matched normal littermate control mice and in age-matched AspaNur7/Nur7 mice with two intact Nat8l alleles. Assays were done by HPLC. Each symbol represents a single mouse. Long horizontal bars indicate means, and vertical bars indicate SEMs. NAA was not detectable by HPLC in the brains of 6-month-old Aspa+/+/Nat8l−/− or AspaNur7/Nur7/Nat8l−/−mice (data not shown). Analysis was by ANOVA with post hoc Tukey's multiple comparison test. *p < 0.05; **p < 0.01.

Disrupting Nat8l reduces the severity of motor dysfunction in AspaNur7/Nur7 mice

We had previously reported that accelerating rotarod retention times of 2-month-old AspaNur7/Nur7/Nat8l+/+ mice were shorter than those in 2-month-old wild-type (Aspa+/+/Nat8l+/+) mice, whereas retention times of 2-month-old AspaNur7/Nur7/Nat8l−/− mice were not significantly different from those in wild-type mice (Guo et al. 2015). We retested these three groups of mice at age 1 year and obtained very similar results (Fig. 3A). We also tested AspaNur7/Nur7/Nat8l+/− mice at 2 and 12 months of age and found their rotarod retention times to be intermediate between those of AspaNur7/Nur7/Nat8l+/+ and AspaNur7/Nur7/Nat8l−/− mice at both ages (Fig. 3B). We concluded that the protective effects of constitutive disruption of Nat8l on the rotarod performance of AspaNur7/Nur7 mice extend to at least 1 year of age.

Figure 3.
Figure 3.

Nat8l deletion preserves motor function in AspaNur7/Nur7 mice. A, Mice were subjected to accelerating rotarod testing at 12 months of age. Each symbol represents a single mouse. Long horizontal bars indicate means, and vertical bars indicate SEMs. Analysis was by ANOVA with post hoc Tukey's multiple comparison test. *p < 0.05; **p < 0.01; ***p < 0.001. B, Accelerating rotarod retention time results in individual mice at 2 and 12 months of age are compared, with analysis within each of the four groups by Student's t test. The 2 month data for Aspa+/+/Nat8l+/+, AspaNur7/Nur7/Nat8l+/+, and AspaNur7/Nur7/Nat8l−/− mice that are shown in B were previously published (Guo et al., 2015).

Disrupting Nat8l diminishes brain vacuolation in AspaNur7/Nur7 mice

As previously reported in brains of 2-month-old mice (Traka et al., 2008; Clarner et al., 2014; Guo et al., 2015; Maier et al., 2015), cerebellar white matter and neighboring gray matter were vacuolated, immunoreactive MBP expression was diminished, and immunoreactive GFAP expression was increased in 6-month-old AspaNur7/Nur7/Nat8l+/+ mice (Fig. 4). Deletion of both Nat8l alleles prevented this vacuolation and also prevented loss of immunoreactive MBP and induction of immunoreactive GFAP. Deletion of a single Nat8l allele also diminished vacuolation but did not prevent loss of immunoreactive MBP or induction of immunoreactive GFAP (Fig. 4). Forebrain vacuolation in the 6-month-old AspaNur7/Nur7 mice was also prevented by homozygous deletion of Nat8l (data not shown).

Figure 4.
Figure 4.

Nat8l deletion diminishes cerebellar vacuolation in AspaNur7/Nur7 mice. A, Representative low-power microscopic views (scale bar, 100 μm) immunostained for MBP (green, top) or GFAP (red, bottom) of 6-month-old mice of the genotypes indicated above the photos. Neuronal nuclei were counterstained with DAPI. B, C, Ratio of vacuole area to total area measured (B) and mean cerebellar vacuole size (C) in 6-month-old mice of the indicated genotypes. Each symbol represents an individual mouse. Long horizontal bars indicate means, and vertical bars indicate SEMs. *p < 0.05; ***p < 0.001.

Transmission electron microscopy showed vacuoles and astroglial process swelling in the Purkinje cell layer of 1-year-old AspaNur7/Nur7/Nat8l+/+ mice. Vacuolation and astroglial swelling were less prominent in 1-year-old AspaNur7/Nur7/Nat8l+/− mice. Purkinje cell perikaryal morphology appeared normal in 1-year-old AspaNur7/Nur7/Nat8l+/+ and AspaNur7/Nur7/Nat8l+/− mice (Fig. 5).

Figure 5.
Figure 5.

Nat8l deletion diminishes vacuolation and astroglial swelling in the Purkinje cell layer of AspaNur7/Nur7 mice. Vacuoles (V) and nuclei of astroglia (A) with swollen processes are evident in an AspaNur7/Nur7/Nat8l+/+ mouse (right), less prominent in an age-matched AspaNur7/Nur7/Nat8l+/− mouse (middle), and are not seen in an age-matched wild-type control (Aspa+/+/Nat8l+/+) mouse (left). Results are representative of those in three mice of each of these three genotypes. P, Purkinje cell nucleus; BV, blood vessel. Arrows in normal control (left) point to normal-sized astroglial processes. Scale bar, 10 μm.

Nat8l deletion prevents loss of cerebral cortical NeuN+ neurons and progressive cerebral cortical thinning in AspaNur7/Nur7 mice

The number of NeuN+ neurons per unit area in somatosensory cortex of Aspa+/+/Nat8l+/+ mice was significantly greater than in AspaNur7/Nur7/Nat8l+/+ mice at both 2 and 12 months of age. Somatosensory cortical thickness in AspaNur7/Nur7/Nat8l+/+ mice was slightly diminished at age 2 months and more substantially diminished at age 12 months compared with Aspa+/+/Nat8l+/+ mice (Fig. 6). Somatosensory cortical thickness and NeuN+ neuron density in 12-month-old AspaNur7/Nur7/Nat8l−/− mice were not significantly different than in Aspa+/+/Nat8l+/+ mice. Constitutive deletion of a single Nat8l allele resulted in lesser protective effects against cerebral cortical atrophy and neuron loss in 12-month-old AspaNur7/Nur7 mice (Fig. 6).

Figure 6.
Figure 6.

Nat8l deletion diminishes cerebral cortical thinning and neuron loss in AspaNur7/Nur7 mice. In A and B, neurons were immunostained for NeuN (scale bar, 100 μm). A, Somatosensory cortex in a representative 2-month-old Aspa+/+/Nat8l+/+ mouse (left) and a representative 2-month-old AspaNur7/Nur7/Nat8l+/+ mouse (right). B, Somatosensory cortex in representative 12-month-old (from left to right) Aspa+/+/Nat8l+/+, AspaNur7/Nur7/Nat8l−/−, AspaNur7/Nur7/Nat8l+/−, and AspaNur7/Nur7/Nat8l+/+ mice. C, Somatosensory cortical NeuN+ neuron counts in 2- and 12-month-old mice. D, Somatosensory cortical thickness in 2- and 12-month-old mice. Each dot in C and D represents a single mouse. Vertical bars denote SEMs. Statistical analyses were by ANOVA with Tukey's multiple comparison test. ns, Not significantly different. *p < 0.05; **p < 0.01; ***p < 0.001.

Nat8l deletion prevents loss of Purkinje and internal granule cell neurons in AspaNur7/Nur7 mice

Compared with 6-month-old wild-type (Aspa+/+/Nat8l+/+) mice, the numbers of calbindin+ Purkinje neurons were significantly diminished in age-matched AspaNur7/Nur7/Nat8l+/+ mice. The loss of Purkinje neurons was less marked in age-matched AspaNur7/Nur7/Nat8l+/− mice and was not detected in age-matched AspaNur7/Nur7/Nat8l−/− mice (Fig. 7). The axons of some Purkinje neurons in AspaNur7/Nur7/Nat8l+/+ mice showed segmental swellings (“axonal torpedoes”; Babij et al., 2013), which resembled those reported in children with Canavan disease (Kamoshita et al., 1967; Jellinger and Seitelberger, 1969). Purkinje cell axon swellings were less frequent in 6-month-old AspaNur7/Nur7/Nat8l+/− mice and were not seen in 6-month-old AspaNur7/Nur7/Nat8l−/− or Aspa+/+/Nat8l+/+ mice (Fig. 7).

Figure 7.
Figure 7.

Nat8l deletion preserves cerebellar Purkinje neurons in AspaNur7/Nur7 mice. A–D, The cerebellar molecular layer (ML), granule cell layer (GL), and white matter (WM) of representative 6-month-old wild-type control (Aspa+/+, Nat8l+/+; A), AspaNur7/Nur7/Nat8l−/− (B), AspaNur7/Nur7/Nat8l+/− (C), and AspaNur7/Nur7/Nat8l+/+ (D) mice. Purkinje neurons and their axons were immunostained with a calbindin antibody. The sections were also immunostained for GFAP. Note the prominent GFAP induction and expansion of cerebellar white matter in the AspaNur7/Nur7/Nat8l+/+ mouse. Nuclei were labeled with DAPI. Segmental Purkinje axonal dilatations (“torpedoes”) are indicated by white arrows in C and D. Scale bar, 50 μm. E, Comparison of Purkinje neuron linear densities in wild-type control mice versus in age-matched AspaNur7/Nur7 mice in which zero, one, or two Nat8l alleles have been deleted. F, Comparison of the frequency of Purkinje axonal dilatations in the various genotypes. n = 3 mice in each group. Vertical bars denote SEMs. *p < 0.05; **p < 0.01. ND, Not detected.

The area occupied by NeuN+ cerebellar internal granule cells was smaller in 6-month-old AspaNur7/Nur7/Nat8l+/+ mice than in 6-month-old wild-type (Aspa+/+/Nat8l+/+) mice; this diminution was less marked in 6-month-old AspaNur7/Nur7/Nat8l+/− mice, and no diminution was detected in 6-month-old AspaNur7/Nur7/Nat8l−/− mice (Fig. 8). Some internal granule cells in the 6-month-old AspaNur7/Nur7/Nat8l+/+ mice expressed cleaved caspase-3, suggesting ongoing apoptosis (Hurtado de Mendoza et al., 2011). The frequency of these NeuN+/cleaved caspase-3+ internal granule neurons was diminished in 6-month-old AspaNur7/Nur7/Nat8l+/− mice and was further reduced in AspaNur7/Nur7/Nat8l−/− mice (Fig. 8).

Figure 8.
Figure 8.

Nat8l deletion preserves internal granule cell layer area and diminishes incidence of cleaved caspase-3+ granule cell neurons in AspaNur7/Nur7 mice. A, NeuN immunostaining of cerebellar internal granule neurons of representative 6-month-old mice of the genotypes indicated. The inset shows a NeuN+/cleaved caspase-3+ (double-stained) internal granule neuron. Scale bar, 100 μm. B, Fold change in areas occupied by NeuN+ internal granule neurons determined by NIH ImageJ and shown as means (n = 3 mice per group), with SEMs indicated by vertical bars. C, Numbers of NeuN+/cleaved caspase-3+ neurons/mm2 in the internal granule cell layer. *p < 0.05; **p < 0.01; ***p < 0.001. NS, Not significantly different.

Discussion

Magnetic resonance imaging in Canavan disease demonstrates brain dysmyelination and progressive brain atrophy, and magnetic resonance spectroscopy shows markedly elevated [NAAB] (Janson et al., 2006b; Leone et al., 2012). The most prominent neuropathological findings are vacuolation, dysmyelination, and astrogliosis in brain superficial white matter and adjacent gray matter, but brain neuron loss has also been noted (Jellinger and Seitelberger, 1969; Adachi et al., 1973; Mirimanoff, 1976) and may contribute to progression of neurological deficits.

AspaNur7/Nur7 mice express no immunochemically detectable Aspa, demonstrate markedly elevated [NAAB], and develop brain vacuolar degeneration beginning ∼2 weeks after birth. These mice, which are already ataxic when weaned, become progressively more tremulous thereafter (Traka et al., 2008) and often die prematurely (Maier et al., 2015), have already proven useful in demonstrating that genetic ablation of NAA synthesis prevents brain vacuolation (Guo et al., 2015; Maier et al., 2015). However, the neuron depletion and progressive brain atrophy that occur in Canavan disease have not thus far been reported to occur in these mice, nor in any other Aspa-deficient rodent Canavan model (Matalon et al., 2000; Klugmann et al., 2005; Traka et al., 2008; Mersmann et al., 2011; Clarner et al., 2014; Guo et al., 2015; Maier et al., 2015). Indeed, numbers of cerebellar internal granule cells were noted to be slightly increased in 14-d-old AspaNur7/Nur7 mice (Traka et al., 2008), and another study reported numbers of NeuN+ neurons in forebrains of 8 week postnatal AspaNur7/Nur7 mice to be normal, although the cerebral cortical region sampled in that study was not specified (Francis et al., 2012).

We have now documented decreased NeuN+ neuron density and mild thinning of somatosensory cortex at age 2 months and substantially more severe cortical thinning by age 12 months in AspaNur7/Nur7 mice. Also, in cerebellum, numbers of cerebellar Purkinje and internal granule neurons are diminished by age 6 months. These losses of cerebral cortical and cerebellar neurons are prevented by homozygous constitutive Nat8l deletion and partially prevented by heterozygous constitutive Nat8l deletion.

NAA has three known functions in brain: as an obligatory precursor for the neuromodulator N-acetyl-l-aspartyl-l-glutamate (NAAG; Pouwels and Frahm, 1997; Neale et al., 2011); as an intermediate in the transfer of acetate from neurons to oligodendroglia, where acetyl carbons can be converted to acetyl-CoA and incorporated into myelin lipids (Burri et al., 1991; Madhavarao et al., 2005; Moffett et al., 2013; Prokesch et al., 2016); and as an osmolyte that astroglia use to assist in maintaining CNS osmolar/water homeostasis (Fujita et al., 2005; Yodoya et al., 2006; Baslow and Guilfoyle, 2013). It has been hypothesized that the elevated [NAAB] in Canavan disease perturbs the astroglial osmolar regulatory mechanism, thus leading to astroglial swelling and myelin intraperiod line splitting (Baslow and Guilfoyle, 2013; Clarner et al., 2014). This hypothesis was supported by the demonstration that genetic ablation of NAA synthesis prevents vacuolation and dysmyelination in AspaNur7/Nur7 mice (Guo et al., 2015; Maier et al., 2015).

How might elevated [NAAB] cause neuron loss? Neurons might be injured by a direct toxic effect of NAA and/or NAAG. NAAG, although elevated in Canavan brain (Mochel et al., 2010), was not toxic for internal granule cells in cerebellar slices (Kolodziejczyk et al., 2009), but intracerebroventricular NAA caused cerebral cortical oxidative injury (Pederzolli et al., 2009) and killed hippocampal neurons (Pliss et al., 2003). Alternatively, high [NAAB] may have a secondary effect on neuronal viability by causing axonal dysmyelination (Traka et al., 2008) and/or by impairing other neuronotrophic effects of oligodendroglia and astroglia (Furuya et al., 2000; Wilkins et al., 2003; Morrison et al., 2013; Machler et al., 2016; Verkhratsky and Nedergaard, 2016).

Would interventions to diminish [NAAB] be safe for infants and children with Canavan disease? There is as yet no proof that the modest decreases in [NAAB] seen in neuroinflammatory/neurodegenerative brain disorders as a consequence of brain mitochondrial dysfunction are themselves deleterious, although they may cause changes in CNS myelin lipid composition (Ciccarelli et al., 2010; Li et al., 2013; Singhal et al., 2016). However, obliteration of NAA synthesis by inactivation of both Nat8l alleles, although not causing dysmyelination or obvious motor deficits in Nat8l−/− mice (Guo et al., 2015), has been associated with subtle behavioral abnormalities (Furukawa-Hibi et al., 2012; Toriumi et al., 2015), and the one known human with documented homozygous NAT8L deletion was developmentally delayed, ataxic, and microcephalic (Martin et al., 2001; Wiame et al., 2009). Homozygous inactivation of Slc25a12, which encodes the brain mitochondrial aspartate–glutamate carrier, also profoundly diminishes [NAAB] and causes developmental delay and hypomyelination (Wibom et al., 2009; Sakurai et al., 2010), but lack of NAA might not be responsible for this phenotype since Slc25a12 knock-out also disrupts lactate utilization by neurons and perhaps also by oligodendroglia (Rinholm and Bergersen, 2014; Llorente-Folch et al., 2016).

In conclusion, we have demonstrated that preventing [NAAB] from becoming markedly elevated can mitigate loss of cerebral cortical and cerebellar neurons and progressive cerebral cortical thinning in Aspa-deficient mice. Measures to suppress brain NAA overload, either by inhibiting NAA synthesis or by accelerating brain NAA clearance, may also enhance neuronal survival in infants and children with Canavan disease.

Footnotes

  • ↵*J.S., P.B., and F.G. are co-first authors.

  • This work was supported by NIH Grant R21NS096004, National Multiple Sclerosis Society Grant RG 5252-A-6, and the Shriners Hospitals for Children.

  • The authors declare no competing financial interests.

  • Correspondence should be addressed to Dr. David Pleasure, Department of Neurology, University of California Davis, c/o Shriners Hospital, 2425 Stockton Boulevard, Sacramento, CA 95817. depleasure{at}ucdavis.edu

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Suppressing N-Acetyl-l-Aspartate Synthesis Prevents Loss of Neurons in a Murine Model of Canavan Leukodystrophy
Jiho Sohn, Peter Bannerman, Fuzheng Guo, Travis Burns, Laird Miers, Christopher Croteau, Naveen K. Singhal, Jennifer A. McDonough, David Pleasure
Journal of Neuroscience 11 January 2017, 37 (2) 413-421; DOI: 10.1523/JNEUROSCI.2013-16.2016
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