Suppression of galactosylceramidase (GALC) expression in the twitcher mouse model of globoid cell leukodystrophy (GLD) is caused by nonsense-mediated mRNA decay (NMD)

Expression study of Krabbe Disease GALC missense variants - Insights from quantification profiles of residual enzyme activity, secretion and psychosine levels

Krabbe disease (KD) is an autosomal recessive lysosomal storage disorder caused by loss-of-function mutations in the GALC gene, which encodes for the enzyme galactosylceramidase (GALC). GALC is crucial for myelin metabolism. Functional deficiency of GALC leads to toxic accumulation of psychosine, dysfunction and death of oligodendrocytes, and eventual brain demyelination. To date, 46 clinically-relevant, pathogenic GALC missense mutations (MMs) have been identified in KD patients. These MMs are present in ∼70% of KD cases reported over 8 published studies between 1996 - 2019. However, the mechanisms by which these MMs lead to GALC functional deficiency and their correlations with clinical phenotype remain poorly understood. To address this, we generated a GALC -knockout human oligodendrocytic cell line (MO3.13/ GALC -KO) using CRISPR-Cas9 method to assess GALC function and GALC secretion. We evaluated 5 polymorphic and 31 clinically-relevant MM variants (MMVs) using transient expression assays. Our results showed that 26 MMVs, including 10 co-variants with p.I562T, reduced GALC activity by 92% - 100% compared to wild-type GALC (WT-GALC). MMVs from infantile-onset KD patients produced < 2% of WT activity, whereas those associated with juvenile- and adult-onset cases retained up to 7% of WT activity. Residual GALC activity was correlated with mature, lysosomal GALC protein levels (Pearson r = 0.93, P<0.0001). Many low-activity MMVs did not correspondingly impair GALC secretion. Twenty-one of the 26 low-activity MMVs showed a 21% - 100% reduction in sec-GALC levels, indicating varying degrees of GALC mis-trafficking among these variants. Importantly, GALC activity among MMVs strongly correlates with clinical disease severity, based on the age of symptom onset in patients with either homozygous MM (Pearson r = 0.98, P<0.0001, n = 7) or compound heterozygous (Pearson r = 0.94, P<0.0001, n = 12) MM-null mutation genotypes. Thus, our data suggests that GALC activity could serve as a prognostic disease indicator under specific experimental conditions. We further investigated the impact of pathogenic MMVs on psychosine accumulation, a key biomarker for KD. Psychosine levels were 21-fold higher in mock control cells compared to WT-GALC transfected cells (mock = 0.349 pmol/mg, WT-GALC = 0.016 pmol/mg), but negatively correlated with GALC activity among pathogenic MMVs (Pearson r = -0.63, P < 0.01, n = 15). Although psychosine levels were higher in most MMVs associated with infantile-onset KD, no significant correlations with clinical onset were detected. Overall, our study provides a comprehensive quantitative analysis of the functional deficits and mis-trafficking associated with clinically-relevant GALC MMVs, enhancing our understanding of the molecular genetics and genotype-phenotype correlations of the GALC gene in Krabbe disease.

Lysosomal TMEM106B interacts with galactosylceramidase to regulate myelin lipid metabolism

TMEM106B is an endolysosomal transmembrane protein not only associated with multiple neurological disorders including frontotemporal dementia, Alzheimer's disease, and hypomyelinating leukodystrophy but also potentially involved in COVID-19. Additionally, recent studies have identified amyloid fibrils of C-terminal TMEM106B in both aged healthy and neurodegenerative brains. However, so far little is known about physiological functions of TMEM106B in the endolysosome and how TMEM106B is involved in a wide range of human conditions at molecular levels. Here, we performed lipidomic analysis of the brain of TMEM106B-deficient mice. We found that TMEM106B deficiency significantly decreases levels of two major classes of myelin lipids, galactosylceramide and its sulfated derivative sulfatide. Subsequent co-immunoprecipitation assay showed that TMEM106B physically interacts with galactosylceramidase. We also found that galactosylceramidase activity was significantly increased in TMEM106B-deficient brains. Thus, our results suggest that TMEM106B interacts with galactosylceramidase to regulate myelin lipid metabolism and have implications for TMEM106B-associated diseases.

HDAC-6 inhibition ameliorates the early neuropathology in a mouse model of Krabbe disease

Introduction

In Krabbe disease (KD), mutations in β-galactosylceramidase (GALC), a lysosomal enzyme responsible for the catabolism of galactolipids, leads to the accumulation of its substrates galactocerebroside and psychosine. This neurologic condition is characterized by a severe and progressive demyelination together with neuron-autonomous defects and degeneration. Twitcher mice mimic the infantile form of KD, which is the most common form of the human disease. The Twitcher CNS and PNS present demyelination, axonal loss and neuronal defects including decreased levels of acetylated tubulin, decreased microtubule stability and impaired axonal transport.

Methods

We tested whether inhibiting the α-tubulin deacetylase HDAC6 with a specific inhibitor, ACY-738, was able to counteract the early neuropathology and neuronal defects of Twitcher mice.

Results

Our data show that delivery of ACY-738 corrects the low levels of acetylated tubulin in the Twitcher nervous system. Furthermore, it reverts the loss myelinated axons in the sciatic nerve and in the optic nerve when administered from birth to postnatal day 9, suggesting that the drug holds neuroprotective properties. The extended delivery of ACY-738 to Twitcher mice delayed axonal degeneration in the CNS and ameliorated the general presentation of the disease. ACY-738 was effective in rescuing neuronal defects of Twitcher neurons, stabilizing microtubule dynamics and increasing the axonal transport of mitochondria.

Discussion

Overall, our results support that ACY-738 has a neuroprotective effect in KD and should be considered as an add-on therapy combined with strategies targeting metabolic correction.

Brain Targeted AAV1-GALC Gene Therapy Reduces Psychosine and Extends Lifespan in a Mouse Model of Krabbe Disease

Krabbe disease (KD) is a progressive and devasting neurological disorder that leads to the toxic accumulation of psychosine in the white matter of the central nervous system (CNS). The condition is inherited via biallelic, loss-of-function mutations in the galactosylceramidase (GALC) gene. To rescue GALC gene function in the CNS of the twitcher mouse model of KD, an adeno-associated virus serotype 1 vector expressing murine GALC under control of a chicken β-actin promoter (AAV1-GALC) was administered to newborn mice by unilateral intracerebroventricular injection. AAV1-GALC treatment significantly improved body weight gain and survival of the twitcher mice (n = 8) when compared with untreated controls (n = 5). The maximum weight gain after postnatal day 10 was significantly increased from 81% to 217%. The median lifespan was extended from 43 days to 78 days (range: 74-88 days) in the AAV1-GALC-treated group. Widespread expression of GALC protein and alleviation of KD neuropathology were detected in the CNS of the treated mice when examined at the moribund stage. Functionally, elevated levels of psychosine were completely normalized in the forebrain region of the treated mice. In the posterior region, which includes the mid- and the hindbrain, psychosine was reduced by an average of 77% (range: 53-93%) compared to the controls. Notably, psychosine levels in this region were inversely correlated with body weight and lifespan of AAV1-GALC-treated mice, suggesting that the degree of viral transduction of posterior brain regions following ventricular injection determined treatment efficacy on growth and survivability, respectively. Overall, our results suggest that viral vector delivery via the cerebroventricular system can partially correct psychosine accumulation in brain that leads to slower disease progression in KD.

Preclinical studies in Krabbe disease: A model for the investigation of novel combination therapies for lysosomal storage diseases

Krabbe disease (KD) is a lysosomal storage disease (LSD) caused by mutations in the galc gene. There are over 50 monogenetic LSDs, which largely impede the normal development of children and often lead to premature death. At present, there are no cures for LSDs and the available treatments are generally insufficient, short acting, and not without co-morbidities or long-term side effects. The last 30 years have seen significant advances in our understanding of LSD pathology as well as treatment options. Two gene therapy-based clinical trials, NCT04693598 and NCT04771416, for KD were recently started based on those advances. This review will discuss how our knowledge of KD got to where it is today, focusing on preclinical investigations, and how what was discovered may prove beneficial for the treatment of other LSDs.

SPIONs mediated magnetic actuation promotes nerve regeneration by inducing and maintaining repair-supportive phenotypes in Schwann cells

Background

Schwann cells, the glial cells in the peripheral nervous system, are highly plastic. In response to nerve injury, Schwann cells are reprogrammed to a series of specialized repair-promoting phenotypes, known as repair Schwann cells, which play a pivotal role in nerve regeneration. However, repair Schwann cells represent a transient and unstable cell state, and these cells progressively lose their repair phenotypes and repair‐supportive capacity; the transience of this state is one of the key reasons for regeneration failure in humans. Therefore, the ability to control the phenotypic stability of repair Schwann cells is of great practical importance as well as biological interest.

Results

We designed and prepared a type of fluorescent–magnetic bifunctional superparamagnetic iron oxide nanoparticles (SPIONs). In the present study, we established rat sciatic nerve injury models, then applied SPIONs to Schwann cells and established an effective SPION-mediated magnetic actuation system targeting the sciatic nerves. Our results demonstrate that magnetic actuation mediated by SPIONs can induce and maintain repair-supportive phenotypes of Schwann cells, thereby promoting regeneration and functional recovery of the sciatic nerve after crush injury.

Conclusions

Our research indicate that Schwann cells can sense these external, magnetically driven mechanical forces and transduce them to intracellular biochemical signals that promote nerve regeneration by inducing and maintaining the repair phenotypes of Schwann cells. We hope that this study will provide a new therapeutic strategy to promote the regeneration and repair of injured peripheral nerves.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01337-5.

Chronic lithium administration in a mouse model for Krabbe disease

Krabbe disease (KD; or globoid cell leukodystrophy) is an autosomal recessive lysosomal storage disorder caused by deficiency of the galactosylceramidase (GALC) enzyme. No cure is currently available for KD. Clinical applied treatments are supportive only. Recently, we demonstrated that two differently acting autophagy inducers (lithium and rapamycin) can improve some KD hallmarks in-vitro, laying the foundation for their in-vivo pre-clinical testing. Here, we test lithium carbonate in-vivo, in the spontaneous mouse model for KD, the Twitcher (TWI) mouse. The drug is administered ad libitum via drinking water (600 mg/L) starting from post natal day 20. We longitudinally monitor the mouse motor performance through the grip strength, the hanging wire and the rotarod tests, and a set of biochemical parameters related to the KD pathogenesis [i.e., GALC enzymatic activity, psychosine (PSY) accumulation and astrogliosis]. Additionally, we investigate the expression of some crucial markers related to the two pathways that could be altered by lithium: the autophagy and the β-catenin-dependent pathways. Results demonstrate that lithium has not a significant rescue effect on the TWI phenotype, although it can slightly and transiently improves muscle strength. We also show that lithium, with this administration protocol, is unable to stimulate autophagy in the TWI mice central nervous system, whereas results suggest that it can restore the β-catenin activation status in the TWI sciatic nerve. Overall, these data provide intriguing inputs for further evaluations of lithium treatment in TWI mice.

CRISPR-Cas9 Knock-In of T513M and G41S Mutations in the Murine β-Galactosyl-Ceramidase Gene Re-capitulates Early-Onset and Adult-Onset Forms of Krabbe Disease

Krabbe Disease (KD) is a lysosomal storage disorder characterized by the genetic deficiency of the lysosomal enzyme β-galactosyl-ceramidase (GALC). Deficit or a reduction in the activity of the GALC enzyme has been correlated with the progressive accumulation of the sphingolipid metabolite psychosine, which leads to local disruption in lipid raft architecture, diffuse demyelination, astrogliosis, and globoid cell formation. The twitcher mouse, the most used animal model, has a nonsense mutation, which limits the study of how different mutations impact the processing and activity of GALC enzyme. To partially address this, we generated two new transgenic mouse models carrying point mutations frequently found in infantile and adult forms of KD. Using CRISPR-Cas9 gene editing, point mutations T513M (infantile) and G41S (adult) were introduced in the murine GALC gene and stable founders were generated. We show that GALC ^T513M/T513M mice are short lived, have the greatest decrease in GALC activity, have sharp increases of psychosine, and rapidly progress into a severe and lethal neurological phenotype. In contrast, GALC ^G41S/G41S mice have normal lifespan, modest decreases of GALC, and minimal psychosine accumulation, but develop adult mild inflammatory demyelination and slight declines in coordination, motor skills, and memory. These two novel transgenic lines offer the possibility to study the mechanisms by which two distinct GALC mutations affect the trafficking of mutated GALC and modify phenotypic manifestations in early- vs adult-onset KD.

Cell-autonomous expression of the acid hydrolase galactocerebrosidase

Lysosomal storage diseases (LSDs) are typically caused by a deficiency in a soluble acid hydrolase and are characterized by the accumulation of undegraded substrates in the lysosome. Determining the role of specific cell types in the pathogenesis of LSDs is a major challenge due to the secretion and subsequent uptake of lysosomal hydrolases by adjacent cells, often referred to as "cross-correction." Here we create and validate a conditional mouse model for cell-autonomous expression of galactocerebrosidase (GALC), the lysosomal enzyme deficient in Krabbe disease. We show that lysosomal membrane-tethered GALC (GALCLAMP1) retains enzyme activity, is able to cleave galactosylsphingosine, and is unable to cross-correct. Ubiquitous expression of GALCLAMP1 fully rescues the phenotype of the GALC-deficient mouse (Twitcher), and widespread deletion of GALCLAMP1 recapitulates the Twitcher phenotype. We demonstrate the utility of this model by deleting GALCLAMP1 specifically in myelinating Schwann cells in order to characterize the peripheral neuropathy seen in Krabbe disease.

Proteostasis network alteration in lysosomal storage disorders: Insights from the mouse model of Krabbe disease

In Krabbe disease, a mutation in GALC gene causes widespread demyelination determining cell death by apoptosis, mainly in oligodendrocytes and Schwann cells. Less is known on the molecular mechanisms induced by this deficiency. Here, we report an impairment in protein synthesis and degradation and in proteasomal clearance with a potential accumulation of the misfolded proteins and induction of the endoplasmic reticulum stress in the brain of 6-day-old twitcher mice (TM) (model of Krabbe disease). In particular, an imbalance of the immunoproteasome function was highlighted, useful for shaping adaptive immune response by neurological cells. Moreover, our data show an involvement of cytoskeleton remodeling in Krabbe pathogenesis, with a lamin meshwork disaggregation in twitcher oligodendrocytes in 6-day-old TM. This study provides interesting protein targets and mechanistic insight on the early onset of Krabbe disease that may be promising options to be tested in combination with currently available therapies to rescue Krabbe phenotype.

Unfolded protein response is activated in Krabbe disease in a manner dependent on the mutation type

Krabbe disease, one of the autosomal-recessive lysosomal storage disorders (LSDs), is caused by a deficiency of galactocerebrosidase (GALC) activity, resulting in the intracellular accumulation of psychosine, which is cytotoxic for neuronal cells. Genetically pathogenic mutations result in conformational changes in GALC and disrupt the lysosmal trafficking of cargos, which subsequently accumulate in the endoplasmic reticulum (ER). Recently, ER stress together with the activation of the unfolded protein response (UPR) has been suggested to play a key role in the pathogenesis of LSDs. In this study, we hence investigated whether the UPR is activated in Krabbe disease using COS-7 cells expressing pathogenic GALC mutants and skin fibroblasts (SFs) from Krabbe disease patients with various phenotypes, using a combination of semiquantitative and quantitative real-time polymerase chain reactions. We found that UPR activation in Krabbe disease depends on the mutations and cell types, and there is the possibility that multiple pathways, involving ER chaperones, inositol-requiring kinase 1, and protein kinase regulated by RNA-like ER kinase are activated by mutations associated with the infantile form. These results indicate that in Krabbe disease, each misfolded/unfolded protein evokes different UPR activation depending on the mutation, and that the activated pathways affect the phenotypes.

Suppression of galactocerebrosidase premature termination codon and rescue of galactocerebrosidase activity in twitcher cells

Krabbe's disease (KD) is a degenerative lysosomal storage disease resulting from deficiency of β-galactocerebrosidase activity. Over 100 mutations are known to cause the disease, and these usually occur in compound heterozygote patterns. In affected patients, nonsense mutations leading to a nonfunctional enzyme are often found associated with other mutations. The twitcher mouse is a naturally occurring model of KD, containing in β-galactocerebrosidase a premature stop codon, W339X. Recent studies have shown that selected compounds may induce the ribosomal bypass of premature stop codons without affecting the normal termination codons. The rescue of β-galactocerebrosidase activity induced by treatment with premature termination codon (PTC) 124, a well-characterized compound known to induce ribosomal read-through, was investigated on oligodendrocytes prepared from twitcher mice and on human fibroblasts from patients bearing nonsense mutations. The effectiveness of the nonsense-mediated mRNA decay (NMD) inhibitor 1 (NMDI1), a newly identified inhibitor of NMD, was also tested. Incubation of these cell lines with PTC124 and NMDI1 increased the levels of mRNA and rescued galactocerebrosidase enzymatic activity in a dose-dependent manner. The low but sustained expression of β-galactocerebrosidase in oligodendrocytes was sufficient to improve the morphology of the differentiated cells. Our in vitro approach provides the basis for further investigation of ribosomal read-through as an alternative therapeutic strategy to ameliorate the quality of life in selected KD patients. © 2016 Wiley Periodicals, Inc.

New therapeutic approaches for Krabbe disease: The potential of pharmacological chaperones

Missense mutations in the lysosomal hydrolase β‐galactocerebrosidase (GALC) account for at least 40% of known cases of Krabbe disease (KD). Most of these missense mutations are predicted to disrupt the fold of the enzyme, preventing GALC in sufficient amounts from reaching its site of action in the lysosome. The predominant central nervous system (CNS) pathology and the absence of accumulated primary substrate within the lysosome mean that strategies used to treat other lysosomal storage disorders (LSDs) are insufficient in KD, highlighting the still unmet clinical requirement for successful KD therapeutics. Pharmacological chaperone therapy (PCT) is one strategy being explored to overcome defects in GALC caused by missense mutations. In recent studies, several small‐molecule inhibitors have been identified as promising chaperone candidates for GALC. This Review discusses new insights gained from these studies and highlights the importance of characterizing both the chaperone interaction and the underlying mutation to define properly a responsive population and to improve the translation of existing lead molecules into successful KD therapeutics. We also highlight the importance of using multiple complementary methods to monitor PCT effectiveness. Finally, we explore the exciting potential of using combination therapy to ameliorate disease through the use of PCT with existing therapies or with more generalized therapeutics, such as proteasomal inhibition, that have been shown to have synergistic effects in other LSDs. This, alongside advances in CNS delivery of recombinant enzyme and targeted rational drug design, provides a promising outlook for the development of KD therapeutics. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Treatment for Krabbe's disease: Finding the combination

Globoid cell leukodystrophy (GLD) is an autosomal recessive neurodegenerative disorder caused by a deficiency of the lysosomal enzyme galactocerebrosidase (GALC). GALC is responsible for catabolism of certain glycolipids, including the toxic compound galactosylsphingosine (psychosine). Histological signs of disease include the widespread loss of myelin in the central and peripheral nervous systems, profound neruroinflammation, and axonal degeneration. Patients suffering from GLD also display neurological deterioration. Many different individual therapies have been investigated in the murine model of the GLD, the Twitcher mouse, with minimal success. The current standard of care for GLD patients, hematopoietic stem cell transplantation, serves only to delay disease progression and is not an effective cure. However, combination therapies that target different pathogenic mechanisms/pathways have been more effective at reducing histological signs of disease, delaying disease onset, prolonging life span, and improving behavioral/cognitive functions in rodent models of Krabbe's disease. In some cases, dramatic synergy between the various therapies has been observed. © 2016 Wiley Periodicals, Inc.

Altered Trafficking and Processing of GALC Mutants Correlates with Globoid Cell Leukodystrophy Severity

Globoid cell leukodystrophy (GLD, Krabbe disease) is due to autosomal recessive mutations in the lysosomal enzyme galactosylceramidase (GALC). Many GLD patients develop infantile-onset of progressive neurologic deterioration and death by 2 years of age, whereas others have a later-onset, milder disease. Cord blood transplant slows disease progression much more effectively when performed presymptomatically, highlighting the importance of early diagnosis. Current diagnosis is based on reduced GALC activity, DNA sequence, and clinical examination. However, presymptomatic diagnosis is hampered by imperfect genotype-GALC activity-phenotype correlations. In addition, three polymorphisms in the GALC gene are variably associated with disease mutations and have unknown effects on GALC activity and disease outcome. Here, we study mutations that cause infantile or later-onset GLD, and show that GALC activity is significantly lower in infantile versus later-onset mutants when measured in the lysosomal fraction, but not in whole-cell lysates. In parallel, infantile-onset mutant GALCs showed reduced trafficking to lysosomes and processing than later-onset mutant GALCs. Finally, the cis-polymorphisms also affected trafficking to the lysosome and processing of GALC. These differences potentially explain why the activity of different mutations appears similar in whole-cell extracts from lymphocytes, and suggest that measure of GALC activity in lysosomes may better predict the onset and severity of disease for a given GLD genotype.

SIGNIFICANCE STATEMENT

Globoid cell leukodystrophy (GLD, Krabbe disease) is diagnosed by measuring galactosylceramidase (GALC) activity and DNA analysis. However, genotype and phenotype often do not correlate due to considerable clinical variability, even for the same mutation, for unknown reasons. We find that altered trafficking to the lysosome and processing of GALC correlates with GLD severity and is modulated by cis-polymorphisms. Current diagnosis of GLD is based on GALC activity of total cell lysates from blood, which does not discriminate whether the activity comes from the lysosome or other subcellular organelles. Measurement of GALC activity in lysosomes may predict which infants are at high risk for the infantile phenotype while distinguishing other children who will develop later-onset phenotypes without onset of symptoms for years.

Globoid cell leukodystrophy (Krabbe’s disease) in a Japanese domestic cat

A male Japanese domestic cat developed progressive limb paralysis from 4 months of age. The cat showed visual disorder, trismus and cognitive impairment and died at 9 months of age. At necropsy, significant discoloration of the white matter was observed throughout the brain and spinal cord. Histologically, severe myelin loss and gliosis were observed, especially in the internal capsule and cerebellum. In the lesions, severe infiltration of macrophages with broad cytoplasm filled with PAS‐positive and non‐metachromatic granules (globoid cells) was evident. On the basis of these findings, the case was diagnosed as feline globoid cell leukodystrophy (Krabbe's disease). Immunohistochemical observation indicated the involvement of oxidative stress and small HSP in the disease.

Experimental therapies in the murine model of globoid cell leukodystrophy

BACKGROUND

Globoid cell leukodystrophy or Krabbe disease, is a rapidly progressive childhood lysosomal storage disorder caused by a deficiency in galactocerebrosidase. Galactocerebrosidase deficiency leads to the accumulation of galactosylsphingosine (psychosine), a cytotoxic lipid especially damaging to oligodendrocytes and Schwann cells. The progressive loss of cells involved in myelination results in a dysmyelinating phenotype affecting both the central and peripheral nervous systems. Current treatment for globoid cell leukodystrophy is limited to bone marrow or umbilical cord blood transplantation. However, these therapies are not curative and simply slow the progression of the disease. The Twitcher mouse is a naturally occurring biochemically faithful model of human globoid cell leukodystrophy that has been used extensively to study globoid cell leukodystrophy pathophysiology and experimental treatments. In this review, we present the major single and combination experimental therapies targeting specific aspects of murine globoid cell leukodystrophy.

METHODS

Literature review and analysis.

RESULTS

The evidence suggests that even with the best available therapies, targeting a single pathogenic mechanism provides minimal clinical benefit. More recently, combination therapies have demonstrated the potential to further advance globoid cell leukodystrophy treatment by synergistically increasing life span. However, such therapies must be designed and evaluated carefully because not all combination therapies yield such positive results.

CONCLUSIONS

A more complete understanding of the underlying pathophysiology and the interplay between various therapies holds the key to the discovery of more effective treatments for globoid cell leukodystrophy.

Early axonal loss accompanied by impaired endocytosis, abnormal axonal transport, and decreased microtubule stability occur in the model of Krabbe's disease

In Krabbe's disease (KD), a leukodystrophy caused by β-galactosylceramidase deficiency, demyelination and a myelin-independent axonopathy contributes to the severe neuropathology. Beyond axonopathy, we show that in Twitcher mice, a model of KD, a decreased number of axons both in the PNS and in the CNS, and of neurons in dorsal root ganglia (DRG), occurred before the onset of demyelination. Despite the early axonal loss, and although in vitro Twitcher neurites degenerated over time, Twitcher DRG neurons displayed an initial neurite overgrowth and, following sciatic nerve injury, Twitcher axons were regeneration-competent, at a time point where axonopathy was already ongoing. Psychosine, the toxic substrate that accumulates in KD, induced lipid raft clustering. At the mechanistic level, TrkA recruitment to lipid rafts was dysregulated in Twitcher neurons, and defective activation of the ERK1/2 and AKT pathways was identified. Besides defective recruitment of signaling molecules to lipid rafts, the early steps of endocytosis and the transport of endocytic and synaptic vesicles were impaired in Twitcher DRG neurons. Defects in axonal transport, specifically in the retrograde component, correlated with decreased levels of dynein, abnormal levels of post-translational tubulin modifications and decreased microtubule stability. The identification of the axonal defects that precede demyelination in KD, together with the finding that Twitcher axons are regeneration-competent when axonopathy is already installed, opens new windows of action to effectively correct the neuropathology that characterizes this disorder.

Missense mutation in mouse GALC mimics human gene defect and offers new insights into Krabbe disease

Krabbe disease is a devastating pediatric leukodystrophy caused by mutations in the galactocerebrosidase (GALC) gene. A significant subset of the infantile form of the disease is due to missense mutations that result in aberrant protein production. The currently used mouse model, twitcher, has a nonsense mutation not found in Krabbe patients, although it is similar to the human 30 kb deletion in abrogating GALC expression. Here, we identify a spontaneous mutation in GALC, GALCtwi-5J, that precisely matches the E130K missense mutation in patients with infantile Krabbe disease. GALCtwi-5J homozygotes show loss of enzymatic activity despite normal levels of precursor protein, and manifest a more severe phenotype than twitcher, with half the life span. Although neuropathological hallmarks such as gliosis, globoid cells and psychosine accumulation are present throughout the nervous system, the CNS does not manifest significant demyelination. In contrast, the PNS is severely hypomyelinated and lacks large diameter axons, suggesting primary dysmyelination, rather than a demyelinating process. Our data indicate that early demise is due to mechanisms other than myelin loss and support an important role for neuroinflammation in Krabbe disease progression. Furthermore, our results argue against a causative relationship between psychosine accumulation, white matter loss and gliosis.

Insights into Krabbe disease from structures of galactocerebrosidase

Krabbe disease is a devastating neurodegenerative disease characterized by widespread demyelination that is caused by defects in the enzyme galactocerebrosidase (GALC). Disease-causing mutations have been identified throughout the GALC gene. However, a molecular understanding of the effect of these mutations has been hampered by the lack of structural data for this enzyme. Here we present the crystal structures of GALC and the GALC-product complex, revealing a novel domain architecture with a previously uncharacterized lectin domain not observed in other hydrolases. All three domains of GALC contribute residues to the substrate-binding pocket, and disease-causing mutations are widely distributed throughout the protein. Our structures provide an essential insight into the diverse effects of pathogenic mutations on GALC function in human Krabbe variants and a compelling explanation for the severity of many mutations associated with fatal infantile disease. The localization of disease-associated mutations in the structure of GALC will facilitate identification of those patients that would be responsive to pharmacological chaperone therapies. Furthermore, our structure provides the atomic framework for the design of such drugs.

Widespread enzymatic correction of CNS tissues by a single intracerebral injection of therapeutic lentiviral vector in leukodystrophy mouse models

Leukodystrophies are rare diseases caused by defects in the genes coding for lysosomal enzymes that degrade several glycosphingolipids. Gene therapy for leukodystrophies requires efficient distribution of the missing enzymes in CNS tissues to prevent demyelination and neurodegeneration. In this work, we targeted the external capsule (EC), a white matter region enriched in neuronal projections, with the aim of obtaining maximal protein distribution from a single injection site. We used bidirectional (bd) lentiviral vectors (LV) (bdLV) to ensure coordinate expression of a therapeutic gene (beta-galactocerebrosidase, GALC; arylsulfatase A, ARSA) and of a reporter gene, thus monitoring simultaneously transgene distribution and enzyme reconstitution. A single EC injection of bdLV.GALC in early symptomatic twitcher mice (a murine model of globoid cell leukodystrophy) resulted in rapid and robust expression of a functional GALC protein in the telencephalon, cerebellum, brainstem and spinal cord. This led to global rescue of enzymatic activity, significant reduction of tissue storage and decrease of activated astroglia and microglia. Widespread protein distribution and complete metabolic correction were also observed after EC injection of bdLV.ARSA in a mouse model of metachromatic leukodystrophy. Our data indicated axonal transport, distribution through cerebrospinal fluid flow and cross-correction as the mechanisms contributing to widespread bioavailability of GALC and ARSA proteins in CNS tissues. LV-mediated gene delivery of lysosomal enzymes by targeting highly interconnected CNS regions is a potentially effective strategy that, combined with a treatment able to target the PNS and peripheral organs, may provide significant therapeutic benefit to patients affected by leukodystrophies.

Premature termination codons in PRPF31 cause retinitis pigmentosa via haploinsufficiency due to nonsense-mediated mRNA decay

Dominant mutations in the gene encoding the mRNA splicing factor PRPF31 cause retinitis pigmentosa, a hereditary form of retinal degeneration. Most of these mutations are characterized by DNA changes that lead to premature termination codons. We investigated 6 different PRPF31 mutations, represented by single-base substitutions or microdeletions, in cell lines derived from 9 patients with dominant retinitis pigmentosa. Five of these mutations lead to premature termination codons, and 1 leads to the skipping of exon 2. Allele-specific measurement of PRPF31 transcripts revealed a strong reduction in the expression of mutant alleles. As a consequence, total PRPF31 protein abundance was decreased, and no truncated proteins were detected. Subnuclear localization of the full-length PRPF31 that was present remained unaffected. Blocking nonsense-mediated mRNA decay significantly restored the amount of mutant PRPF31 mRNA but did not restore the synthesis of mutant proteins, even in conjunction with inhibitors of protein degradation pathways. Our results indicate that most PRPF31 mutations ultimately result in null alleles through the activation of surveillance mechanisms that inactivate mutant mRNA and, possibly, proteins. Furthermore, these data provide compelling evidence that the pathogenic effect of PRPF31 mutations is likely due to haploinsufficiency rather than to gain of function.

Does the nonsense-mediated mRNA decay mechanism prevent the synthesis of truncated BRCA1, CHK2, and p53 proteins?

The nonsense-mediated mRNA decay (NMD) mechanism is an evolutionarily conserved process ensuring the degradation of transcripts carrying premature termination codon(s). NMD is believed to prevent the synthesis of truncated proteins that could be detrimental to the cell. However, although numerous studies have assessed the efficiency of this mechanism at the mRNA level, data are lacking in regard to whether NMD fulfills its expected goal at the protein level. In this study, we have investigated whether endogenous alleles of breast cancer predisposing genes carrying nonsense codons were able to produce detectable amounts of truncated proteins in lymphoblastoid cell lines. A total of 20 truncating BRCA1 mutations were analyzed, along with the 1100delC CHEK2 and the 770delT TP53 mutations. All the studied alleles triggered NMD, the amount of mutant transcript ranging from 16 to 63% of that of the wild-type species. We found that BRCA1 and CHK2 truncated proteins could not be detected, even when NMD was inhibited. This suggests that BRCA1 and CHK2 truncated proteins are highly unstable. Conversely, the p53 protein encoded by the 770delT allele is as abundant as the wild-type protein, as removal of the C-terminal p53 domain leads to a stabilized mutant protein, whose abundance is markedly increased when NMD is inhibited. Therefore, our results show that it is not possible to infer the presence of truncated proteins in cells from carriers of a truncated mutation without experimental verification, as each case is expected to be different.

A mouse model for nonsense mutation bypass therapy shows a dramatic multiday response to geneticin

Aminoglycosides can bypass nonsense mutations and are the prototypic agents for translational bypass therapy (TBT). Initial results demonstrate the need for more potent drugs and an in vivo model system for quantitative assessment of TBT. Herein, we present an in vivo system for evaluating the efficacy of premature stop codon management therapies: in vivo quantitative stop codon management repli-sampling TBT efficacy assay (IQSCMaRTEA). Application of IQSCMaRTEA reveals that geneticin is much more efficacious in vivo than gentamicin. Treatment with geneticin elicits a multiday response, and residual F9 antigen can be detected after 3 weeks. These data demonstrate the utility of IQSCMaRTEA for evaluating drugs that bypass nonsense mutations. In addition, IQSCMaRTEA may be helpful for testing inhibitors of nonsense-mediated decay, as stop codon management therapy will sometimes require inhibition of nonsense-mediated decay and translational bypass of the nonsense mutation. Furthermore, geneticin, its metabolites, or better tolerated analogues should be evaluated as a general treatment with multiday response for severe genetic disease caused by nonsense mutation.

Single‐dose intracerebroventricular administration of galactocerebrosidase improves survival in a mouse model of globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD), also known as Krabbe disease, is a devastating, degenerative neurological disorder. It is inherited as an autosomal recessive trait caused by loss-of-function mutations in the galactocerebrosidase (GALC) gene. Previously, we have shown that peripheral injection of recombinant GALC, administered every other day, results in a substantial improvement in early clinical phenotype in the twitcher mouse model of GLD. While we did detect active enzyme in the brain following peripheral administration, most of the administered enzyme was localized to the periphery. Given the substantial central nervous system (CNS) involvement in this disease, we were interested in determining whether or not a single-dose administration of the recombinant enzyme directly to the CNS, which could potentially be achieved clinically, would result in any substantial improvement. Following intracerebroventricular (i.c.v.) administration of GALC we noted a significant, 16.5%, reduction in the GALC substrate psychosine, the abnormal accumulation of which is believed to play a pivotal role in the CNS pathology observed in this disease. Moreover, recombinant GALC was found not only in periventricular regions but also at sites distant to the injection such as the cerebral cortex and cerebellum. Most importantly, animals receiving a single i.c.v. dose of the enzyme at postnatal day 20 survived up to 51 days, which compares favorably to the control twitcher animals, which normally only live to postnatal day 40/42. These results indicate that even a single i.c.v. administration of the recombinant enzyme can have significant clinical impact and suggests that other lysosomal storage disorders with significant CNS involvement may similarly benefit.