Canavan disease: clinical features and recent advances in research

The full spectrum of MRI findings in 18 patients with Canavan disease: new insights into the areas of selective susceptibility

INTRODUCTION

Canavan disease (CD) is a rare autosomal recessive neurodegenerative disorder caused by a deficiency of aspartoacylase A, an enzyme that degrades N-acetylaspartate (NAA). The disease is characterized by progressive white matter degeneration, leading to intellectual disability, seizures, and death. This retrospective study aims to describe the full spectrum of magnetic resonance imaging (MRI) findings in a large case series of CD patients.

MATERIALS AND METHODS

MRI findings in 18 patients with confirmed CD were investigated, and the full spectrum of brain abnormalities was compared with the existing literature to provide new insights regarding the brain MRI findings in these patients. All the cases were proven based on genetic study or NAA evaluation in urine or brain.

RESULTS

Imaging analysis showed involvement of the deep and subcortical white matter as well as the globus pallidus in all cases, with sparing of the putamen, caudate, and claustrum. The study provides updates on the imaging characteristics of CD and validates some underreported findings such as the involvement of the lateral thalamus with sparing of the pulvinar, involvement of the internal capsules and corpus callosum, and cystic formation during disease progression.

CONCLUSION

To our knowledge, this is one of the largest case series of patients with CD which includes a detailed description of the brain MRI findings. The study confirmed many of the previously reported MRI findings but also identified abnormalities that were previously rarely or not described. We speculate that areas of ongoing myelination are particularly vulnerable to changes in CD.

Myelin lesion in the aspartoacylase (Aspa) knockout rat, an animal model for Canavan disease

Canavan disease (CD) is a fatal hereditary neurological disorder caused by a mutation in the aspartoacylase (ASPA) gene and characterized by neurological signs and vacuolation in the central nervous system (CNS). The mutation inhibits the hydrolysis of N-acetyl-aspartate (NAA) resulting in accumulation of NAA in the CNS. A new Aspa-knockout rat was generated by transcription activator-like effector nuclease (TALEN) technology. Herein we describe the pathological and morphometrical findings in the brain and spinal cords of Aspa-knockout rats. Although Aspa-knockout rats did not show any neurological signs, vacuolation with swollen axons, hypomyelination, and activated swollen astrocytes were observed mainly in the brainstem reticular formation, ascending and descending motor neuron pathway, and in the olfactory tract. Morphometrical analysis revealed no obvious change in the number of neurons. These changes in the CNS are similar to human CD, suggesting that this animal model would be useful for further study of treatment and understanding the pathophysiology of human CD.

Deep mutational scanning reveals a correlation between degradation and toxicity of thousands of aspartoacylase variants

Unstable proteins are prone to form non-native interactions with other proteins and thereby may become toxic. To mitigate this, destabilized proteins are targeted by the protein quality control network. Here we present systematic studies of the cytosolic aspartoacylase, ASPA, where variants are linked to Canavan disease, a lethal neurological disorder. We determine the abundance of 6152 of the 6260 ( ~ 98%) possible single amino acid substitutions and nonsense ASPA variants in human cells. Most low abundance variants are degraded through the ubiquitin-proteasome pathway and become toxic upon prolonged expression. The data correlates with predicted changes in thermodynamic stability, evolutionary conservation, and separate disease-linked variants from benign variants. Mapping of degradation signals (degrons) shows that these are often buried and the C-terminal region functions as a degron. The data can be used to interpret Canavan disease variants and provide insight into the relationship between protein stability, degradation and cell fitness.

Cellular and molecular mechanisms of aspartoacylase and its role in Canavan disease

Canavan disease is an autosomal recessive and lethal neurological disorder, characterized by the spongy degeneration of the white matter in the brain. The disease is caused by a deficiency of the cytosolic aspartoacylase (ASPA) enzyme, which catalyzes the hydrolysis of N-acetyl-aspartate (NAA), an abundant brain metabolite, into aspartate and acetate. On the physiological level, the mechanism of pathogenicity remains somewhat obscure, with multiple, not mutually exclusive, suggested hypotheses. At the molecular level, recent studies have shown that most disease linked ASPA gene variants lead to a structural destabilization and subsequent proteasomal degradation of the ASPA protein variants, and accordingly Canavan disease should in general be considered a protein misfolding disorder. Here, we comprehensively summarize the molecular and cell biology of ASPA, with a particular focus on disease-linked gene variants and the pathophysiology of Canavan disease. We highlight the importance of high-throughput technologies and computational prediction tools for making genotype-phenotype predictions as we await the results of ongoing trials with gene therapy for Canavan disease.

Leukodystrophy Imaging: Insights for Diagnostic Dilemmas

Leukodystrophies, a group of rare demyelinating disorders, mainly affect the CNS. Clinical presentation of different types of leukodystrophies can be nonspecific, and thus, imaging techniques like MRI can be used for a more definitive diagnosis. These diseases are characterized as cerebral lesions with characteristic demyelinating patterns which can be used as differentiating tools. In this review, we talk about these MRI study findings for each leukodystrophy, associated genetics, blood work that can help in differentiation, emerging diagnostics, and a follow-up imaging strategy. The leukodystrophies discussed in this paper include X-linked adrenoleukodystrophy, metachromatic leukodystrophy, Krabbe's disease, Pelizaeus-Merzbacher disease, Alexander's disease, Canavan disease, and Aicardi-Goutières Syndrome.

MRI pattern recognition in white matter disease

Magnetic resonance imaging (MRI) pattern recognition is a powerful tool for quick diagnosis of genetic and acquired white matter disorders. In many cases, distribution and character of white matter abnormalities directly point to a specific diagnosis and guide confirmatory testing. Knowledge of normal brain development is essential to interpret white matter changes in young children. MRI is also used for disease staging and treatment decisions in leukodystrophies and acquired disorders as multiple sclerosis, and as a biomarker to follow treatment effects.

Pathological Bergmann glia alterations and disrupted calcium dynamics in ataxic Canavan disease mice

Canavan disease (CD) is a recessively inherited pediatric leukodystrophy resulting from inactivating mutations to the oligodendroglial enzyme aspartoacylase (ASPA). ASPA is responsible for hydrolyzing the amino acid derivative N-acetyl-L-aspartate (NAA), and without it, brain NAA concentrations increase by 50% or more. Infants and children with CD present with progressive cognitive and motor delays, cytotoxic edema, astroglial vacuolation, and prominent spongiform brain degeneration. ASPA-deficient CD mice (Aspanur7/nur7 ) present similarly with elevated NAA, widespread astroglial dysfunction, ataxia, and Purkinje cell (PC) dendritic atrophy. Bergmann glia (BG), radial astrocytes essential for cerebellar development, are intimately intertwined with PCs, where they regulate synapse stability, functionality, and plasticity. BG damage is common to many neurodegenerative conditions and frequently associated with PC dysfunction and ataxia. Here, we report that, in CD mice, BG exhibit significant morphological alterations, decreased structural associations with PCs, loss of synaptic support proteins, and altered calcium dynamics. We also find that BG dysfunction predates cerebellar vacuolation and PC damage in CD mice. Previously, we developed an antisense oligonucleotide (ASO) therapy targeting Nat8l (N-acetyltransferase-8-like, "Nat8l ASO") that inhibits the production of NAA and reverses ataxia and PC atrophy in CD mice. Here, we show that Nat8l ASO administration in adult CD mice also leads to BG repair. Furthermore, blocking astroglial uptake of NAA is neuroprotective in astroglia-neuron cocultures exposed to elevated NAA. Our findings suggest that restoration of BG structural and functional integrity could be a mechanism for PC regeneration and improved motor function.

Developing a human iPSC-derived three-dimensional myelin spheroid platform for modeling myelin diseases

Myelin defects cause a collection of myelin disorders in the brain. The lack of human models has limited us from better understanding pathological mechanisms of myelin diseases. While human induced pluripotent stem cell (hiPSC)-derived spheroids or organoids have been used to study brain development and disorders, it has been difficult to recapitulate mature myelination in these structures. Here, we have developed a method to generate three-dimensional (3D) myelin spheroids from hiPSCs in a robust and reproducible manner. Using this method, we generated myelin spheroids from patient iPSCs to model Canavan disease (CD), a demyelinating disorder. By using CD patient iPSC-derived myelin spheroids treated with N-acetyl-aspartate (NAA), we were able to recapitulate key pathological features of the disease and show that high-level NAA is sufficient to induce toxicity on myelin sheaths. Our study has established a 3D human cellular platform to model human myelin diseases for mechanistic studies and drug discovery.

Adeno-associated virus-mediated gene therapy in a patient with Canavan disease using dual routes of administration and immune modulation

Gene replacement therapy is a rational therapeutic strategy and clinical intervention for neurodegenerative disorders like Canavan disease, a leukodystrophy caused by biallelic mutations in the aspartoacylase (ASPA) gene. We aimed to investigate whether simultaneous intravenous (i.v.) and intracerebroventricular (i.c.v.) administration of rAAV9-CB6-ASPA provides a safe and effective therapeutic strategy in an open-label, individual-patient, expanded-access trial for Canavan disease. Immunomodulation was given prophylactically prior to adeno-associated virus (AAV) treatment to prevent an immune response to ASPA or the vector capsid. The patient served as his own control, and change from baseline was assessed by clinical pathology tests, vector genomes in the blood, antibodies against ASPA and AAV capsids, levels of cerebrospinal fluid (CSF) N-acetylaspartate (NAA), brain water content and morphology, clinical status, and motor function tests. Two years post treatment, the patient's white matter myelination had increased, motor function was improved, and he remained free of typical severe epilepsy. NAA level was reduced at 3 months and remained stable up to 4 years post treatment. Immunomodulation prior to AAV exposure enables repeat dosing and has prevented an anti-transgene immune response. Dual-route administration of gene therapy may improve treatment outcomes.

Molecular Pathogenic Mechanisms of Hypomyelinating Leukodystrophies (HLDs)

Hypomyelinating leukodystrophies (HLDs) represent a group of congenital rare diseases for which the responsible genes have been identified in recent studies. In this review, we briefly describe the genetic/molecular mechanisms underlying the pathogenesis of HLD and the normal cellular functions of the related genes and proteins. An increasing number of studies have reported genetic mutations that cause protein misfolding, protein dysfunction, and/or mislocalization associated with HLD. Insight into the mechanisms of these pathways can provide new findings for the clinical treatments of HLD.

Metabolic partitioning in the brain and its hijacking by glioblastoma

The different cell types in the brain have highly specialized roles with unique metabolic requirements. Normal brain function requires the coordinated partitioning of metabolic pathways between these cells, such as in the neuron-astrocyte glutamate-glutamine cycle. An emerging theme in glioblastoma (GBM) biology is that malignant cells integrate into or "hijack" brain metabolism, co-opting neurons and glia for the supply of nutrients and recycling of waste products. Moreover, GBM cells communicate via signaling metabolites in the tumor microenvironment to promote tumor growth and induce immune suppression. Recent findings in this field point toward new therapeutic strategies to target the metabolic exchange processes that fuel tumorigenesis and suppress the anticancer immune response in GBM. Here, we provide an overview of the intercellular division of metabolic labor that occurs in both the normal brain and the GBM tumor microenvironment and then discuss the implications of these interactions for GBM therapy.

Emerging cellular themes in leukodystrophies

Leukodystrophies are a broad spectrum of neurological disorders that are characterized primarily by deficiencies in myelin formation. Clinical manifestations of leukodystrophies usually appear during childhood and common symptoms include lack of motor coordination, difficulty with or loss of ambulation, issues with vision and/or hearing, cognitive decline, regression in speech skills, and even seizures. Many cases of leukodystrophy can be attributed to genetic mutations, but they have diverse inheritance patterns (e.g., autosomal recessive, autosomal dominant, or X-linked) and some arise from de novo mutations. In this review, we provide an updated overview of 35 types of leukodystrophies and focus on cellular mechanisms that may underlie these disorders. We find common themes in specialized functions in oligodendrocytes, which are specialized producers of membranes and myelin lipids. These mechanisms include myelin protein defects, lipid processing and peroxisome dysfunction, transcriptional and translational dysregulation, disruptions in cytoskeletal organization, and cell junction defects. In addition, non-cell-autonomous factors in astrocytes and microglia, such as autoimmune reactivity, and intercellular communication, may also play a role in leukodystrophy onset. We hope that highlighting these themes in cellular dysfunction in leukodystrophies may yield conceptual insights on future therapeutic approaches.

Therapeutic development for Canavan disease using patient iPSCs introduced with the wild-type ASPA gene

Canavan disease (CD) is a devastating neurological disease that lacks effective therapy. Because CD is caused by mutations of the aspartoacylase (ASPA) gene, we introduced the wild-type (WT) ASPA gene into patient iPSCs through lentiviral transduction or CRISPR/Cas9-mediated gene editing. We then differentiated the WT ASPA-expressing patient iPSCs (ASPA-CD iPSCs) into NPCs and showed that the resultant ASPA-CD NPCs exhibited potent ASPA enzymatic activity. The ASPA-CD NPCs were able to survive in brains of transplanted CD mice. The engrafted ASPA-CD NPCs reconstituted ASPA activity in CD mouse brains, reduced the abnormally elevated level of NAA in both brain tissues and cerebrospinal fluid (CSF), and rescued hallmark pathological phenotypes of the disease, including spongy degeneration, myelination defects, and motor function impairment in transplanted CD mice. These genetically modified patient iPSC-derived NPCs represent a promising cell therapy candidate for CD, a disease that has neither a cure nor a standard treatment.

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The wild-type ASPA gene was introduced into CD patient iPSCs to make ASPA-CD iPSCs

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ASPA-CD iPSCs were differentiated into ASPA-CD NPCs with potent ASPA activity

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Engrafted ASPA-CD NPCs could rescue major disease phenotypes in CD mice

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CSF NAA level can be used as a biomarker to monitor the treatment outcome for CD

Practical Genetics for the Neuroradiologist: Adding Value in Neurogenetic Disease

Genetic discoveries have transformed our understanding of many neurologic diseases. Identification of specific causal pathogenic variants has improved understanding of pathophysiology and enabled replacement of many confusing eponyms and acronyms with more meaningful and clinically relevant genetics-based terminology. In this era of rapid scientific advancement, multidisciplinary collaboration among pediatricians, neurologists, geneticists, radiologists, and other members of the health care team is increasingly important in the care of patients with genetic neurologic diseases. Radiologists familiar with neurogenetic disease add value by (1) recognizing constellations of characteristic imaging findings that are associated with a genetic disease before one is clinically suspected; (2) predicting the most likely genotypes for a given imaging phenotype in clinically suspected genetic disease; and (3) providing detailed and accurate descriptions of the imaging phenotype in challenging cases with unknown or uncertain genotypes. This review aims to increase awareness and understanding of pathogenic variants relating to neurologic disease by (1) briefly reviewing foundational knowledge of chromosomes, inheritance patterns, and mutagenesis; (2) providing concrete examples of and detailed information about specific neurologic diseases resulting from pathogenic variants; and (3) highlighting clinical and imaging features that are of greatest relevance for the radiologist.

High Throughput Screening Cascade To Identify Human Aspartate N-Acetyltransferase (ANAT) Inhibitors for Canavan Disease

Canavan disease (CD) is a progressive, fatal neurological disorder that begins in infancy resulting from a mutation in aspartoacyclase (ASPA), an enzyme that catalyzes the deacetylation of N-acetyl aspartate (NAA) into acetate and aspartate. Increased NAA levels in the brains of affected children are one of the hallmarks of CD. Interestingly, genetic deletion of N-acetyltransferase-8-like (NAT8L), which encodes aspartate N-aceyltransferase (ANAT), an enzyme responsible for the synthesis of NAA from l-aspartate and acetyl-CoA, leads to normalization of NAA levels and improvement of symptoms in several genetically engineered mouse models of CD. Therefore, pharmacological inhibition of ANAT presents a promising therapeutic strategy for treating CD. Currently, however, there are no clinically viable ANAT inhibitors. Herein we describe the development of fluorescence-based high throughput screening (HTS) and radioactive-based orthogonal assays using recombinant human ANAT expressed in E. coli. In the fluorescence-based assay, ANAT activity was linear with respect to time of incubation up to 30 min and protein concentration up to 97.5 ng/μL with Km values for l-aspartate and acetyl-CoA of 237 μM and 11 μM, respectively. Using this optimized assay, we conducted a pilot screening of a 10 000-compound library. Hits from the fluorescence-based assay were subjected to an orthogonal radioactive-based assay using L-[U-14C] aspartate as a substrate. Two compounds were confirmed to have dose-dependent inhibition in both assays. Inhibitory kinetics studies of the most potent compound revealed an uncompetitive inhibitory mechanism with respect to l-aspartate and a noncompetitive inhibitory mechanism against acetyl-CoA. The screening cascade developed herein will enable large-scale compound library screening to identify novel ANAT inhibitors as leads for further medicinal chemistry optimization.

Canavan Disease: Clinical and Laboratory Profile from Southern Part of India

Background:

Canavan disease (CD) is an autosomal recessively inherited leukodystrophy. It affects one in 6,400 to 13,500 people in the Jewish population. However, prevalence and presentation of the disease in India is largely unknown; hence, we are reporting this series.

Methods:

This is a retrospective chart review in a tertiary care hospital from January 2015 to March 2020. CD was confirmed by elevated N- acetyl aspartate (NAA) levels in urinary gas chromatography and mass spectrometry (GCMS)/increased NAA peak in magnetic resonance spectroscopy (MRS) and/or detection of mutations. The data was extracted in a predesigned proforma and analyzed.

Results:

We had 12 children with mean age at presentation being 6.8 months (range 3 months to 10 months.). Males were more commonly affected (83.3%, n = 10). Ten children (83.3%) were born out of consanguineous parentage. All of them had visual impairment and pyramidal signs. Seizures were noted in five (42%) children. Normal head size in three (25%) and microcephaly in two (16.66%) cases were noted. Magnetic resonance imaging (MRI) revealed signal changes with bilateral symmetric T2W white matter (WM) hyperintensities in subcortical U fibers in all cases. MRS was done in ten children, all of which showed increased NAA peak. Increased level of NAA in urinary GCMS was noted in six out of eight children. Six cases had homozygous pathogenic variants in ASPA gene. Antenatal diagnosis helped in prevention of recurrence in three families.

Conclusion:

Urinary NAA and MRS showing NAA peak are useful in diagnosis of CD. Macrocephaly is not a necessary finding to diagnose CD. Early diagnosis helps in genetic counseling and prevention of subsequent conceptions.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Canavan Disease as a Model for Gene Therapy-Mediated Myelin Repair

In recent years, the scientific and therapeutic fields for rare, genetic central nervous system (CNS) diseases such as leukodystrophies, or white matter disorders, have expanded significantly in part due to technological advancements in cellular and clinical screenings as well as remedial therapies using novel techniques such as gene therapy. However, treatments aimed at normalizing the pathological changes associated with leukodystrophies have especially been complicated due to the innate and variable effects of glial abnormalities, which can cause large-scale functional deficits in developmental myelination and thus lead to downstream neuronal impairment. Emerging research in the past two decades have depicted glial cells, particularly oligodendrocytes and astrocytes, as key, regulatory modulators in constructing and maintaining myelin function and neuronal viability. Given the significance of myelin formation in the developing brain, myelin repair in a time-dependent fashion is critical in restoring homeostatic functionality to the CNS of patients diagnosed with white matter disorders. Using Canavan Disease (CD) as a leukodystrophy model, here we review the hypothetical roles of N-acetylaspartate (NAA), one of the brain's most abundant amino acid derivatives, in Canavan disease's CNS myelinating pathology, as well as discuss the possible functions astrocytes serve in both CD and other leukodystrophies' time-sensitive disease correction. Through this analysis, we also highlight the potential remyelinating benefits of gene therapy for other leukodystrophies in which alternative CNS cell targeting for white matter disorders may be an applicable path for reparative treatment.

Mapping the degradation pathway of a disease-linked aspartoacylase variant

Canavan disease is a severe progressive neurodegenerative disorder that is characterized by swelling and spongy degeneration of brain white matter. The disease is genetically linked to polymorphisms in the aspartoacylase (ASPA) gene, including the substitution C152W. ASPA C152W is associated with greatly reduced protein levels in cells, yet biophysical experiments suggest a wild-type like thermal stability. Here, we use ASPA C152W as a model to investigate the degradation pathway of a disease-causing protein variant. When we expressed ASPA C152W in Saccharomyces cerevisiae, we found a decreased steady state compared to wild-type ASPA as a result of increased proteasomal degradation. However, molecular dynamics simulations of ASPA C152W did not substantially deviate from wild-type ASPA, indicating that the native state is structurally preserved. Instead, we suggest that the C152W substitution interferes with the de novo folding pathway resulting in increased proteasomal degradation before reaching its stable conformation. Systematic mapping of the protein quality control components acting on misfolded and aggregation-prone species of C152W, revealed that the degradation is highly dependent on the molecular chaperone Hsp70, its co-chaperone Hsp110 as well as several quality control E3 ubiquitin-protein ligases, including Ubr1. In addition, the disaggregase Hsp104 facilitated refolding of aggregated ASPA C152W, while Cdc48 mediated degradation of insoluble ASPA protein. In human cells, ASPA C152W displayed increased proteasomal turnover that was similarly dependent on Hsp70 and Hsp110. Our findings underscore the use of yeast to determine the protein quality control components involved in the degradation of human pathogenic variants in order to identify potential therapeutic targets.

Feline Spongy Encephalopathy With a Mutation in the ASPA Gene

Canavan disease is an autosomal recessive leukodystrophy caused by mutations in the gene encoding aspartoacylase (ASPA), which hydrolyses N-acetylaspartate (NAA) to acetate and aspartate. A similar feline neurodegenerative disease associated with a mutation in the ASPA gene is reported herein. Comprehensive clinical, genetic, and pathological analyses were performed on 4 affected cats. Gait disturbance and head tremors initially appeared at 1 to 19 months of age. These cats eventually exhibited dysstasia and seizures and died at 7 to 53 months of age. Magnetic resonance imaging of the brain revealed diffuse symmetrical intensity change of the cerebral cortex, brainstem, and cerebellum. Gas chromatography-mass spectrometry analysis of urine showed significant excretion of NAA. Genetic analysis of the 4 affected cats identified a missense mutation (c.859G>C) in exon 6 of the ASPA gene, which was not detected in 4 neurologically intact cats examined as controls. Postmortem analysis revealed vacuolar changes predominantly distributed in the gray matter of the cerebrum and brain stem as well as in the cerebellar Purkinje cell layer. Immunohistochemically, these vacuoles were surrounded by neurofilaments and sometimes contained MBP- and Olig2-positive cells. Ultrastructurally, a large number of intracytoplasmic vacuoles containing mitochondria and electron-dense granules were detected in the cerebral cortex. All 4 cats were diagnosed as spongy encephalopathy with a mutation in the ASPA gene, a syndrome analogous to human Canavan disease. The histopathological findings suggest that feline ASPA deficiency induces intracytoplasmic edema in neurons and oligodendrocytes, resulting in spongy degeneration of the central nervous system.

Cell-Based Therapy for Canavan Disease Using Human iPSC-Derived NPCs and OPCs

Canavan disease (CD) is a fatal leukodystrophy caused by mutation of the aspartoacylase (ASPA) gene, which leads to deficiency in ASPA activity, accumulation of the substrate N-acetyl-L-aspartate (NAA), demyelination, and spongy degeneration of the brain. There is neither a cure nor a standard treatment for this disease. In this study, human induced pluripotent stem cell (iPSC)-based cell therapy is developed for CD. A functional ASPA gene is introduced into patient iPSC-derived neural progenitor cells (iNPCs) or oligodendrocyte progenitor cells (iOPCs) via lentiviral transduction or TALEN-mediated genetic engineering to generate ASPA iNPC or ASPA iOPC. After stereotactic transplantation into a CD (Nur7) mouse model, the engrafted cells are able to rescue major pathological features of CD, including deficient ASPA activity, elevated NAA levels, extensive vacuolation, defective myelination, and motor function deficits, in a robust and sustainable manner. Moreover, the transplanted mice exhibit much prolonged survival. These genetically engineered patient iPSC-derived cellular products are promising cell therapies for CD. This study has the potential to bring effective cell therapies, for the first time, to Canavan disease children who have no treatment options. The approach established in this study can also benefit many other children who have deadly genetic diseases that have no cure.

Maternal separation with early weaning impairs neuron-glia integrity: non-invasive evaluation and substructure demonstration

Astrocytes and oligodendrocytes play essential roles in regulating neural signal transduction along neural circuits in CNS. The perfect coordination of neuron/astrocyte and neuron/oligodendrocyte entities was termed as neuron-glia integrity recently. Here we monitored the status of neuron-glia integrity via non-invasive neuroimaging methods and demonstrated the substructures of it using other approaches in an animal model of maternal separation with early weaning (MSEW), which mimics early life neglect and abuse in humans. Compared to controls, MSEW rats showed higher glutamate level, but lower GABA in prefrontal cortex (PFC) detected by chemical exchange saturation transfer and ¹H-MRS methods, lower levels of glial glutamate transporter-1 and ATP-α, but increased levels of glutamate decarboxylase-65 and glutamine synthetase in PFC; reduced fractional anisotropy in various brain regions revealed by diffusion tensor imaging, along with increased levels of N-acetyl-aspartate measured by ¹H-MRS; and hypomyelination in PFC as evidenced by relevant cellular and molecular changes.

Severe retinal degeneration in a patient with Canavan disease

Background: Canavan disease is an autosomal recessive, neurodegenerative disorder caused by mutations in ASPA, a gene encoding the enzyme aspartoacylase. Patients present with macrocephaly, developmental delay, hypotonia, vision impairment and accumulation of N-acetylaspartic acid. Progressive white matter changes occur in the central nervous system. The disorder is often fatal in early childhood, but milder forms exist. Materials and methods: Case report. Results: We present the case of a 31-year-old male with mild/juvenile Canavan disease who had severe vision loss due to a retinal degeneration resembling retinitis pigmentosa. Prior to this case, vision loss in Canavan disease had been attributed to optic atrophy based on fundoscopic evidence of optic nerve pallor. Investigations for an alternative cause for our patient's retinal degeneration were non-revealing. Conclusion: We wonder if retinal degeneration may not have been previously recognized as a feature of Canavan disease. We highlight findings from animal models of Canavan disease to further support the association between Canavan disease and retinal degeneration.

Kinetic Modeling of the Blood Oxygenation Level Dependent (BOLD) Signals and Biocatalytic Reactions Observed in the Human Brain Using MRI: An Analysis of Normal and Pathological Conditions

A kinetic model describing the pulse of increased oxygen concentrations and the subsequent changes in the concentration of N-acetylaspartate in the excited nervous tissue of the human brain in response to an external signal is presented. The model is based on biochemical data, a multistage and nonlinear dynamic process the BOLD signal and N-acetylaspartate. The existence of multiple steady states explains the triggering effect of the system. The inhibitory effect of the substrate is a necessary factor for the autostabilization of N-acetylaspartate. The kinetic model allows the dynamic behavior of previously unmeasurable metabolites, namely, products of the hydrolysis of N-acetylaspartate, such as acetic and aspartic acid, and glutamic acid to be predicted. Kinetic modeling of the BOLD signal and the subsequent hydrolysis of N-acetylaspartate provides information about the biochemical and dynamic characteristics of some pathological conditions (schizophrenia, Canavan disease, and the superexcitation of the neural network).

Eye involvement in inherited metabolic disorders

Inherited metabolic disorders are a large group of rare disorders affecting normal biochemical pathways. Many metabolic disorders can present with symptoms affecting the eye, and eye disorders can evolve later in the natural history of an already diagnosed metabolic disorder. The ophthalmic involvement can be very varied affecting any part of the eye, including abnormalities of cornea, lens dislocation and cataracts, retina and the distal optic pathway, and extraocular muscles. Awareness of inherited metabolic disorders is important to facilitate early diagnosis and in some cases instigate early treatment if a patient presents with eye involvement suggestive of a metabolic disorder. Ophthalmological interventions are also an important component of the multisystem holistic approach to treating patients with metabolic disorders.

Glial mitochondrial function and dysfunction in health and neurodegeneration

Mitochondria play essential metabolic roles in neural cells. Mitochondrial dysfunction has profound effects on the brain. In primary mitochondrial diseases, mutations that impair specific oxidative phosphorylation (OXPHOS) proteins or OXPHOS assembly factors lead to isolated biochemical defects and a heterogeneous group of clinical phenotypes, including mitochondrial encephalopathies. A broader defect of OXPHOS function, due to mutations in proteins involved in mitochondrial DNA maintenance, mitochondrial biogenesis, or mitochondrial tRNAs can also underlie severe mitochondrial encephalopathies. While primary mitochondrial dysfunction causes rare genetic forms of neurological disorders, secondary mitochondrial dysfunction is involved in the pathophysiology of some of the most common neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Many studies have investigated mitochondrial function and dysfunction in bulk central nervous system (CNS) tissue. However, the interpretation of these studies has been often complicated by the extreme cellular heterogeneity of the CNS, which includes many different types of neurons and glial cells. Because neurons are especially dependent on OXPHOS for ATP generation, mitochondrial dysfunction is thought to be directly involved in cell autonomous neuronal demise. Despite being metabolically more flexible than neurons, glial mitochondria also play an essential role in the function of the CNS, and have adapted specific metabolic and mitochondrial features to support their diversity of functions. This review analyzes our current understanding and the gaps in knowledge of mitochondrial properties of glia and how they affect neuronal functions, in health and disease.

Clinical and Genetic Characteristics of Leukodystrophies in Africa

Recent understanding of the genetic basis of neurological disorders in Africa has grown rapidly in the last two decades. Africa harbors the largest genetic repertoire in the world which gives unique opportunity to discover novel variant, genes, and molecular pathways associated with various neurological diseases. Despite that, large-scale genomic and exome studies are severely lacking especially for neglected diseases such as leukodystrophies. This review aims to shed light on the currently developed research in leukodystrophies in Africa. We reviewed all research articles related to “Leukodystrophy in Africa” published in Medline/PubMed and Google Scholar databases up to date. We found very few studies in leukodystrophy from Africa, especially from the Sub-Saharan regions. Metachromatic leukodystrophy was the most studied type of leukodystrophy. Published studies from North Africa (Tunisia, Morocco, and Egypt) were very limited in either sample size (case studies or single/few family studies) or molecular methods (targeted sequencing or polymerase chain reaction-restriction fragment length polymorphisms). More studies (GWAS or large family studies) with advanced techniques such as exome or whole genome sequencing are needed to unveil the genetic basis of leukodystrophy in Africa. Unmasking novel genes and molecular pathways of leukodystrophies invariably lead to better detection and treatment for both Africans and worldwide populations.

Characteristics and advantages of adeno-associated virus vector-mediated gene therapy for neurodegenerative diseases

Common neurodegenerative diseases of the central nervous system are characterized by progressive damage to the function of neurons, even leading to the permanent loss of function. Gene therapy via gene replacement or gene correction provides the potential for transformative therapies to delay or possibly stop further progression of the neurodegenerative disease in affected patients. Adeno-associated virus has been the vector of choice in recent clinical trials of therapies for neurodegenerative diseases due to its safety and efficiency in mediating gene transfer to the central nervous system. This review aims to discuss and summarize the progress and clinical applications of adeno-associated virus in neurodegenerative disease in central nervous system. Results from some clinical trials and successful cases of central neurodegenerative diseases deserve further study and exploration.

Co-Ultramicronized Palmitoylethanolamide/Luteolin Facilitates the Development of Differentiating and Undifferentiated Rat Oligodendrocyte Progenitor Cells

Oligodendrocytes, the myelin-producing cells of the central nervous system (CNS), have limited capability to bring about repair in chronic CNS neuroinflammatory demyelinating disorders such as multiple sclerosis (MS). MS lesions are characterized by a compromised pool of undifferentiated oligodendrocyte progenitor cells (OPCs) unable to mature into myelin-producing oligodendrocytes. An attractive strategy may be to replace lost OLs and/or promote their maturation. N-palmitoylethanolamine (PEA) is an endogenous fatty acid amide signaling molecule with anti-inflammatory and neuroprotective actions. Recent studies show a co-ultramicronized composite of PEA and the flavonoid luteolin (co-ultraPEALut) to be more efficacious than PEA in improving outcome in CNS injury models. Here, we examined the effects of co-ultraPEALut on development of OPCs from newborn rat cortex cultured under conditions favoring either differentiation (Sato medium) or proliferation (fibroblast growth factor-2 and platelet-derived growth factor (PDGF)-AA-supplemented serum-free medium ("SFM")). OPCs in SFM displayed high expression of PDGF receptor alpha gene and the proliferation marker Ki-67. In Sato medium, in contrast, OPCs showed rapid decreases in PDGF receptor alpha and Ki-67 expression with a concomitant rise in myelin basic protein (MBP) expression. In these conditions, co-ultraPEALut (10 μM) enhanced OPC morphological complexity and expression of MBP and the transcription factor TCF7l2. Surprisingly, co-ultraPEALut also up-regulated MBP mRNA expression in OPCs in SFM. MBP expression in all cases was sensitive to inhibition of mammalian target of rapamycin. Within the context of strategies to promote endogenous remyelination in MS which focus on enhancing long-term survival of OPCs and stimulating their differentiation into remyelinating oligodendrocytes, co-ultraPEALut may represent a novel pharmacological approach.

Therapeutic AAV Gene Transfer to the Nervous System: A Clinical Reality

Gene transfer has long been a compelling yet elusive therapeutic modality. First mainly considered for rare inherited disorders, gene therapy may open treatment opportunities for more challenging and complex diseases such as Alzheimer's or Parkinson's disease. Today, examples of striking clinical proof of concept, the first gene therapy drugs coming onto the market, and the emergence of powerful new molecular tools have broadened the number of avenues to target neurological disorders but have also highlighted safety concerns and technology gaps. The vector of choice for many nervous system targets currently is the adeno-associated viral (AAV) vector due to its desirable safety profile and strong neuronal tropism. In aggregate, the clinical success, the preclinical potential, and the technological innovation have made therapeutic AAV drug development a reality, particularly for nervous system disorders. Here, we discuss the rationale, opportunities, limitations, and progress in clinical AAV gene therapy.

First Record Mutations in the Genes ASPA and ARSA Causing Leukodystrophy in Jordan

Leukodystrophies (LDs) are heterogeneous genetic disorders characterized by abnormal white matter in the central nervous system. Some of the LDs are progressive and often fatal. In general, LD is primarily diagnosed based on the neuroimaging; however, definitive diagnosis of the LD type is done using genetic testing such as next-generation sequencing. The aim of this study is to identify the genetic causes of LD in two independent Jordanian cases that exhibit MRI findings confirming LD with no definitive diagnosis using whole exome sequencing (WES). The most likely causative variants were identified. In one case, the homozygous pathogenic variant NM_000049.2:c.914C>A;p.Ala305Glu, which is previously reported in ClinVar, in the gene ASPA was identified causing Canavan disease. In the second case, the homozygous novel variant NM_000487.5:c.256C>G;p.Arg86Gly in the gene ARSA was identified causing metachromatic leukodystrophy. The two variants segregate in their families. The phenotypes of the two studied cases overlap with assigned diseases. The present study raises the importance of using WES to identify the precise neurodevelopmental diseases in Jordan.

Oligodendrocyte Bioenergetics in Health and Disease

The human brain weighs approximately 2% of the body; however, it consumes about 20% of a person’s total energy intake. Cellular bioenergetics in the central nervous system involves a delicate balance between biochemical processes engaged in energy conversion and those responsible for respiration. Neurons have high energy demands, which rely on metabolic coupling with glia, such as with oligodendrocytes and astrocytes. It has been well established that astrocytes recycle and transport glutamine to neurons to make the essential neurotransmitters, glutamate and GABA, as well as shuttle lactate to support energy synthesis in neurons. However, the metabolic role of oligodendrocytes in the central nervous system is less clear. In this review, we discuss the energetic demands of oligodendrocytes in their survival and maturation, the impact of altered oligodendrocyte energetics on disease pathology, and the role of energetic metabolites, taurine, creatine, N-acetylaspartate, and biotin, in regulating oligodendrocyte function.

Patent highlights October-November 2017

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Brain Nat8l Knockdown Suppresses Spongiform Leukodystrophy in an Aspartoacylase-Deficient Canavan Disease Mouse Model

Canavan disease, a leukodystrophy caused by loss-of-function ASPA mutations, is characterized by brain dysmyelination, vacuolation, and astrogliosis ("spongiform leukodystrophy"). ASPA encodes aspartoacylase, an oligodendroglial enzyme that cleaves the abundant brain amino acid N-acetyl-L-aspartate (NAA) to L-aspartate and acetate. Aspartoacylase deficiency results in a 50% or greater elevation in brain NAA concentration ([NAA_B]). Prior studies showed that homozygous constitutive knockout of Nat8l, the gene encoding the neuronal NAA synthesizing enzyme N-acetyltransferase 8-like, prevents aspartoacylase-deficient mice from developing spongiform leukodystrophy. We now report that brain Nat8l knockdown elicited by intracerebroventricular/intracisternal administration of an adeno-associated viral vector carrying a short hairpin Nat8l inhibitory RNA to neonatal aspartoacylase-deficient Aspa^Nur7/Nur7 mice lowers [NAA_B] and suppresses development of spongiform leukodystrophy.

Leukodystrophies: a proposed classification system based on pathological changes and pathogenetic mechanisms

Leukodystrophies are genetically determined disorders characterized by the selective involvement of the central nervous system white matter. Onset may be at any age, from prenatal life to senescence. Many leukodystrophies are degenerative in nature, but some only impair white matter function. The clinical course is mostly progressive, but may also be static or even improving with time. Progressive leukodystrophies are often fatal, and no curative treatment is known. The last decade has witnessed a tremendous increase in the number of defined leukodystrophies also owing to a diagnostic approach combining magnetic resonance imaging pattern recognition and next generation sequencing. Knowledge on white matter physiology and pathology has also dramatically built up. This led to the recognition that only few leukodystrophies are due to mutations in myelin- or oligodendrocyte-specific genes, and many are rather caused by defects in other white matter structural components, including astrocytes, microglia, axons and blood vessels. We here propose a novel classification of leukodystrophies that takes into account the primary involvement of any white matter component. Categories in this classification are the myelin disorders due to a primary defect in oligodendrocytes or myelin (hypomyelinating and demyelinating leukodystrophies, leukodystrophies with myelin vacuolization); astrocytopathies; leuko-axonopathies; microgliopathies; and leuko-vasculopathies. Following this classification, we illustrate the neuropathology and disease mechanisms of some leukodystrophies taken as example for each category. Some leukodystrophies fall into more than one category. Given the complex molecular and cellular interplay underlying white matter pathology, recognition of the cellular pathology behind a disease becomes crucial in addressing possible treatment strategies.

Identification of altered brain metabolites associated with TNAP activity in a mouse model of hypophosphatasia using untargeted NMR-based metabolomics analysis

Tissue non-specific alkaline phosphatase (TNAP) is a key player of bone mineralization and TNAP gene (ALPL) mutations in human are responsible for hypophosphatasia (HPP), a rare heritable disease affecting the mineralization of bones and teeth. Moreover, TNAP is also expressed by brain cells and the severe forms of HPP are associated with neurological disorders, including epilepsy and brain morphological anomalies. However, TNAP's role in the nervous system remains poorly understood. To investigate its neuronal functions, we aimed to identify without any a priori the metabolites regulated by TNAP in the nervous tissue. For this purpose we used ¹ H- and ³¹ P NMR to analyze the brain metabolome of Alpl (Akp2) mice null for TNAP function, a well-described model of infantile HPP. Among 39 metabolites identified in brain extracts of 1-week-old animals, eight displayed significantly different concentration in Akp2^-/- compared to Akp2^+/+ and Akp2^+/- mice: cystathionine, adenosine, GABA, methionine, histidine, 3-methylhistidine, N-acetylaspartate (NAA), and N-acetyl-aspartyl-glutamate, with cystathionine and adenosine levels displaying the strongest alteration. These metabolites identify several biochemical processes that directly or indirectly involve TNAP function, in particular through the regulation of ecto-nucleotide levels and of pyridoxal phosphate-dependent enzymes. Some of these metabolites are involved in neurotransmission (GABA, adenosine), in myelin synthesis (NAA, NAAG), and in the methionine cycle and transsulfuration pathway (cystathionine, methionine). Their disturbances may contribute to the neurodevelopmental and neurological phenotype of HPP.

Investigation of the motor system in two siblings with Canavan's disease: a combined transcranial magnetic stimulation (TMS) - diffusion tensor imaging (DTI) study

Canavan's disease (CD) is a hereditary leukodystrophy caused by mutations in the aspartoacylase gene (ASPA), leading to spongiform degeneration of the white matter and severe impairment of psychomotor development. We present the cases of two non-Jewish sisters with CD that have a milder and protracted clinical course compared to typical CD. MRI imaging revealed bilateral high-signal-intensity areas in the thalami and the internal capsule and MR spectroscopy showed typical findings for CD (a marked increase in N-acetylaspartate (NAA) levels). FA values of the right and left corticospinal tracts at the level of the posterior limb of the internal capsule, and the centrum semiovale were found to be significantly reduced compared to healthy controls. From a neurophysiological point of view, the peripheral motor system was normal. In contrast, cortical stimulation at maximal intensity failed to elicit facilitated or resting MEPs and silent periods (SPs) in upper and lower limbs, providing evidence for significant upper motor pathway dysfunction.

Two patients with Canavan disease and structural modeling of a novel mutation

Canavan disease (CD) is a rare fatal childhood neurological autosomal recessive genetic disease caused by mutations in the ASPA gene, which lead to catalytic deficiency of the ASPA enzyme, which catalyzes the hydrolysis of N-acetyl-L-aspartate (NAA) into aspartate and acetate. CD occurs frequently among Ashkenazi Jewish population, however it has been reported in many other ethnic groups with significantly lower frequency. Here, we report on two Egyptian patients diagnosed with CD, the first patient harbors five missense mutations (c.427 A > G; p. I143V, c.502C > T; p. R168C, c.530 T > C; p. I177T, c.557 T > C; p. V186D c.548C > T; p. P183L) and a silent mutation (c.693 C > T; p. Y231Y). The second patient was found to be homozygous for two missense mutations (c.427 A > G; p. I143V and c.557 T > A; p. V186D). Furthermore, molecular modeling of the novel mutation p. P183L provides an instructive explanation of the mutational impact on the protein structure that can affect the function of the ASPA. Here, the clinical, radiological, and biochemical profile of the two patients are reviewed in details.

Knockdown of Unconventional Myosin ID Expression Induced Morphological Change in Oligodendrocytes

Myelin is a special multilamellar structure involved in various functions in the nervous system. In the central nervous system, the oligodendrocyte (OL) produces myelin and has a unique morphology. OLs have a dynamic membrane sorting system associated with cytoskeletal organization, which aids in the production of myelin. Recently, it was reported that the assembly and disassembly of actin filaments is crucial for myelination. However, the partner myosin molecule which associates with actin filaments during the myelination process has not yet been identified. One candidate myosin is unconventional myosin ID (Myo1d) which is distributed throughout central nervous system myelin; however, its function is still unclear. We report here that Myo1d is expressed during later stages of OL differentiation, together with myelin proteolipid protein (PLP). In addition, Myo1d is distributed at the leading edge of the myelin-like membrane in cultured OL, colocalizing mainly with actin filaments, 2',3'-cyclic nucleotide phosphodiesterase and partially with PLP. Myo1d-knockdown with specific siRNA induces significant morphological changes such as the retraction of processes and degeneration of myelin-like membrane, and finally apoptosis. Furthermore, loss of Myo1d by siRNA results in the impairment of intracellular PLP transport. Together, these results suggest that Myo1d may contribute to membrane dynamics either in wrapping or transporting of myelin membrane proteins during formation and maintenance of myelin.

A case of Canavan disease with microcephaly

BACKGROUND

Canavan disease is an autosomal recessive disorder with spongy degeneration of white matter of the brain. It presents with developmental delay, visual problems and macrocephaly.

PATIENT DESCRIPTION

We report a ten-month old boy with Canavan disease who presented with global developmental delay, seizures, abnormal eye movements and microcephaly.

RESULTS

MRI brain revealed diffuse involvement of the supra tentorial white matter, globus pallidi, thalami, dentate nuclei and brainstem with sparing of the corpus callosum. The genetic testing revealed homozygous mutation of aspartoacylase gene [c.859 G>A (p.Ala287Thr)] in Exon 6.

CONCLUSION

Possibility of Canavan disease should be considered even in the presence of microcephaly.

The recovery trajectory of adolescent social defeat stress-induced behavioral, (1)H-MRS metabolites and myelin changes in Balb/c mice

Adolescent exposure to social stress precipitates emotion-related disorders and affects the development and function of medial prefrontal cortex (mPFC). However, this adversity-induced behavioral and neurological changes remain not fully explored. Adolescent Balb/c mice were subjected to intermittent social defeat stress during postnatal days 28 to 42. Proton magnetic resonance spectroscopy (¹H-MRS) measurements, behavioral tests and immunohistochemistry were performed one day or 3 weeks after the last stress episode. Defeated mice exhibited hypoactivity and social avoidance with the latter lasting into the early adulthood, while the anxiety level was unchanged. Social defeat experience lead to temporary decreases in the levels of total creatines (Cr + pCr) and Glx (Glu + Gln), but a delayed increase of N- acetylaspartate (NAA) levels. These alternations were accompanied with a persistent reduction of myelin basic protein expression although the number of mature oligodendrocyte did not change. These findings provide evidence that adolescent adverse social experience permanently impairs the emotion-related behavioral performance and induces biochemical and molecular changes in the brain which at least lasts into early adulthood, thus enhancing our understanding of the neurobiology of social defeat stress. Our finding also implicates that NAA signals on MRS may reflect myelin status.

Novel mutation in an Egyptian patient with infantile Canavan disease

Canavan disease (CD) is a rare fatal childhood neurological autosomal recessive genetic disease caused by mutations in the ASPA gene, which lead to catalytic deficiency of the ASPA enzyme that catalyzes the deacetylation of NAA. It is a severe progressive leukodystrophy characterized by spongiform degeneration of the white matter of the brain. CD occurs frequently among Ashkenazi Jewish population, however it has been reported in many other ethnic groups with significantly lower frequency. Here, we report on a 2 year-old Egyptian child with severe CD who harbors a novel homozygous missense variant (c.91G > T, p.V31F) in the ASPA gene. The clinical, radiological, and molecular genetic profiles are reviewed in details.

Inherited and acquired disorders of myelin: The underlying myelin pathology

Remyelination is a major therapeutic goal in human myelin disorders, serving to restore function to demyelinated axons and providing neuroprotection. The target disorders that might be amenable to the promotion of this repair process are diverse and increasing in number. They range primarily from those of genetic, inflammatory to toxic origin. In order to apply remyelinating strategies to these disorders, it is essential to know whether the myelin damage results from a primary attack on myelin or the oligodendrocyte or both, and whether indeed these lead to myelin breakdown and demyelination. In some disorders, myelin sheath abnormalities are prominent but demyelination does not occur. This review explores the range of human and animal disorders where myelin pathology exists and focusses on defining the myelin changes in each and their cause, to help define whether they are targets for myelin repair therapy.

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We reviewed myelin disorders of the CNS in humans and animals.

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Myelin damage results from primary attack on the oligodendrocyte or myelin sheath.

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All major categories of disease can affect CNS myelin.

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Myelin vacuolation is common, yet does not always result in demyelination.

Atypical clinical and radiological course of a patient with Canavan disease

Canavan disease (CD) is a rare metabolic disorder caused by aspartoacylase (ASPA) deficiency. It leads to severe neurological degeneration with spongiform brain degeneration. Accumulation of N-acetylaspartate (NAA) in brain and urine is specific to the disease and guides diagnosis. Magnetic resonance imaging (MRI) usually shows diffuse white matter abnormalities with involvement of the basal ganglia. Mild forms of the disease with a more favorable clinical course and radiological involvement of the basal ganglia without white matter abnormalities have also been reported. Here we report an atypical case of a girl aged nine years with CD. The disease started at the classical age of five months. Classical elevation of NAA in brain and urine was present and genetic analysis identified mutations in the ASPA gene. However, clinical evolution was milder than typical CD, with partial motor impairment and relatively well-preserved cognitive skills. MRI was also atypical with low white matter involvement and unusual topography and evolution of abnormalities in the basal ganglia.