Mucopolysaccharidosis type II zebrafish model exhibits early impaired proteasomal-mediated degradation of the axon guidance receptor Dcc.

Most of the patients affected by neuronopathic forms of Mucopolysaccharidosis type II (MPS II), a rare lysosomal storage disorder caused by defects in iduronate-2-sulfatase (IDS) activity, exhibit early neurological defects associated with white matter lesions and progressive behavioural abnormalities. While neuronal degeneration has been largely described in experimental models and human patients, more subtle neuronal pathogenic defects remain still underexplored. In this work, we discovered that the axon guidance receptor Deleted in Colorectal Cancer (Dcc) is significantly dysregulated in the brain of ids mutant zebrafish since embryonic stages. In addition, thanks to the establishment of neuronal-enriched primary cell cultures, we identified defective proteasomal degradation as one of the main pathways underlying Dcc upregulation in ids mutant conditions. Furthermore, ids mutant fish-derived primary neurons displayed higher levels of polyubiquitinated proteins and P62, suggesting a wider defect in protein degradation. Finally, we show that ids mutant larvae display an atypical response to anxiety-inducing stimuli, hence mimicking one of the characteristic features of MPS II patients. Our study provides an additional relevant frame to MPS II pathogenesis, supporting the concept that multiple developmental defects concur with early childhood behavioural abnormalities.

GBA1 inactivation in oligodendrocytes affects myelination and induces neurodegenerative hallmarks and lipid dyshomeostasis in mice.

BACKGROUND: Mutations in the ß-glucocerebrosidase (GBA1) gene do cause the lysosomal storage Gaucher disease (GD) and are among the most frequent genetic risk factors for Parkinson's disease (PD). So far, studies on both neuronopathic GD and PD primarily focused on neuronal manifestations, besides the evaluation of microglial and astrocyte implication. White matter alterations were described in the central nervous system of paediatric type 1 GD patients and were suggested to sustain or even play a role in the PD process, although the contribution of oligodendrocytes has been so far scarcely investigated. METHODS: We exploited a system to study the induction of central myelination in vitro, consisting of Oli-neu cells treated with dibutyryl-cAMP, in order to evaluate the expression levels and function of ß-glucocerebrosidase during oligodendrocyte differentiation. Conduritol-B-epoxide, a ß-glucocerebrosidase irreversible inhibitor was used to dissect the impact of ß-glucocerebrosidase inactivation in the process of myelination, lysosomal degradation and α-synuclein accumulation in vitro. Moreover, to study the role of ß-glucocerebrosidase in the white matter in vivo, we developed a novel mouse transgenic line in which ß-glucocerebrosidase function is abolished in myelinating glia, by crossing the Cnp1-cre mouse line with a line bearing loxP sequences flanking Gba1 exons 9-11, encoding for ß-glucocerebrosidase catalytic domain. Immunofluorescence, western blot and lipidomic analyses were performed in brain samples from wild-type and knockout animals in order to assess the impact of genetic inactivation of ß-glucocerebrosidase on myelination and on the onset of early neurodegenerative hallmarks, together with differentiation analysis in primary oligodendrocyte cultures. RESULTS: Here we show that ß-glucocerebrosidase inactivation in oligodendrocytes induces lysosomal dysfunction and inhibits myelination in vitro. Moreover, oligodendrocyte-specific ß-glucocerebrosidase loss-of-function was sufficient to induce in vivo demyelination and early neurodegenerative hallmarks, including axonal degeneration, α-synuclein accumulation and astrogliosis, together with brain lipid dyshomeostasis and functional impairment. CONCLUSIONS: Our study sheds light on the contribution of oligodendrocytes in GBA1-related diseases and supports the need for better characterizing oligodendrocytes as actors playing a role in neurodegenerative diseases, also pointing at them as potential novel targets to set a brake to disease progression.

Genome-wide screening in pluripotent cells identifies Mtf1 as a suppressor of mutant huntingtin toxicity.

Huntington's disease (HD) is a neurodegenerative disorder caused by CAG-repeat expansions in the huntingtin (HTT) gene. The resulting mutant HTT (mHTT) protein induces toxicity and cell death via multiple mechanisms and no effective therapy is available. Here, we employ a genome-wide screening in pluripotent mouse embryonic stem cells (ESCs) to identify suppressors of mHTT toxicity. Among the identified suppressors, linked to HD-associated processes, we focus on Metal response element binding transcription factor 1 (Mtf1). Forced expression of Mtf1 counteracts cell death and oxidative stress caused by mHTT in mouse ESCs and in human neuronal precursor cells. In zebrafish, Mtf1 reduces malformations and apoptosis induced by mHTT. In R6/2 mice, Mtf1 ablates motor defects and reduces mHTT aggregates and oxidative stress. Our screening strategy enables a quick in vitro identification of promising suppressor genes and their validation in vivo, and it can be applied to other monogenic diseases.

A novel CRISPR/Cas9-based iduronate-2-sulfatase (IDS) knockout human neuronal cell line reveals earliest pathological changes.

Multiple complex intracellular cascades contributing to Hunter syndrome (mucopolysaccharidosis type II) pathogenesis have been recognized and documented in the past years. However, the hierarchy of early cellular abnormalities leading to irreversible neuronal damage is far from being completely understood. To tackle this issue, we have generated two novel iduronate-2-sulfatase (IDS) loss of function human neuronal cell lines by means of genome editing. We show that both neuronal cell lines exhibit no enzymatic activity and increased GAG storage despite a completely different genotype. At a cellular level, they display reduced differentiation, significantly decreased LAMP1 and RAB7 protein levels, impaired lysosomal acidification and increased lipid storage. Moreover, one of the two clones is characterized by a marked decrease of the autophagic marker p62, while none of the two mutants exhibit marked oxidative stress and mitochondrial morphological changes. Based on our preliminary findings, we hypothesize that neuronal differentiation might be significantly affected by IDS functional impairment.

Monitoring Nrf2/ARE Pathway Activity with a New Zebrafish Reporter System.

Among multiple cytoprotective mechanisms, eukaryotic cells exhibit a complex transcriptional program relying on the Nrf2 transcription factor, which is generally recruited upon biological stressors including oxidative-stress-based cellular insults. The relevance of this master regulator has remarkably emerged in recent years in several research fields such as cancer, inflammatory disorders and age-related neurological diseases. Here, we document the generation and characterization of a novel Nrf2/ARE pathway biosensor fish which exhibits a dynamic spatiotemporal expression profile during the early developmental stages. The transgenic line is responsive to known Nrf2 pathway modulators but also to Edaravone, which direct activity on the Nrf2 pathway has never been documented in a live transgenic fish model. We also show that the reporter is faithfully activated during fin regeneration, and its degree of expression is slightly affected in a glucocerebrosidase (Gba1) morphant zebrafish model. Therefore, this novel transgenic fish may represent a valuable tool to be exploited for the characterization of zebrafish models of human diseases, as well as for primary high-throughput drug screening.

Lysosomal Storage Disorders: Molecular Basis and Therapeutic Approaches.

Lysosomal storage disorders (LSDs) are a group of 60 rare inherited diseases characterized by a heterogeneous spectrum of clinical symptoms, ranging from severe intellectual disabilities, cardiac abnormalities, visceromegaly, and bone deformities to slowly progressive muscle weakness, respiratory insufficiency, eye defects (corneal clouding and retinal degeneration), and skin alterations [...].

Lysosomal Function and Axon Guidance: Is There a Meaningful Liaison?

Axonal trajectories and neural circuit activities strongly rely on a complex system of molecular cues that finely orchestrate the patterning of neural commissures. Several of these axon guidance molecules undergo continuous recycling during brain development, according to incompletely understood intracellular mechanisms, that in part rely on endocytic and autophagic cascades. Based on their pivotal role in both pathways, lysosomes are emerging as a key hub in the sophisticated regulation of axonal guidance cue delivery, localization, and function. In this review, we will attempt to collect some of the most relevant research on the tight connection between lysosomal function and axon guidance regulation, providing some proof of concepts that may be helpful to understanding the relation between lysosomal storage disorders and neurodegenerative diseases.

"Janus" efficacy of CX-5011: CK2 inhibition and methuosis induction by independent mechanisms.

Methuosis has been described as a distinctive form of cell death characterized by the displacement of large fluid-filled vacuoles derived from uncontrolled macropinocytosis. Its induction has been proposed as a new strategy against cancer cells. Small molecules, such as indole-based calchones, have been identified as methuosis inducers and, recently, the CK2 inhibitor CX-4945 has been shown to have a similar effect on different cell types. However, the contribution of protein kinase CK2 to methuosis signalling is still controversial. Here we show that methuosis is not related to CK2 activity since it is not affected by structurally unrelated CK2 inhibitors and genetic reduction/ablation of CK2 subunits. Interestingly, CX-5011, a CK2 inhibitor related to CX-4945, behaves as a CK2-independent methuosis inducer, four times more powerful than its parental compound and capable to promote the formation on enlarged cytosolic vacuoles at low micromolar concentrations. We show that pharmacological inhibition of the small GTPase Rac-1, its downregulation by siRNA treatment, or the over-expression of the dominant-negative mutated form of Rac-1 (Rac-1 T17N), impairs CX-5011 ability to induce methuosis. Furthermore, cell treatment with CX-5011 induces a durable activation of Rac-1 that persists for at least 24 h. Worthy of note, CX-5011 is able to promote macropinocytosis not only in mammalian cells, but also in an in-vivo zebrafish model. Based on these evidences, CX-5011 is, therefore, proposed as a potential promising compound for cancer therapies for its dual efficacy as an inhibitor of the pro-survival kinase CK2 and inducer of methuosis.

Temporal control of Wnt signaling is required for habenular neuron diversity and brain asymmetry.

Precise temporal coordination of signaling processes is pivotal for cellular differentiation during embryonic development. A vast number of secreted molecules are produced and released by cells and tissues, and travel in the extracellular space. Whether they induce a signaling pathway and instruct cell fate, however, depends on a complex network of regulatory mechanisms, which are often not well understood. The conserved bilateral left-right asymmetrically formed habenulae of the zebrafish are an excellent model for investigating how signaling control facilitates the generation of defined neuronal populations. Wnt signaling is required for habenular neuron type specification, asymmetry and axonal connectivity. The temporal regulation of this pathway and the players involved have, however, have remained unclear. We find that tightly regulated temporal restriction of Wnt signaling activity in habenular precursor cells is crucial for the diversity and asymmetry of habenular neuron populations. We suggest a feedback mechanism whereby the tumor suppressor Wnt inhibitory factor Wif1 controls the Wnt dynamics in the environment of habenular precursor cells. This mechanism might be common to other cell types, including tumor cells.

A transcriptional and post-transcriptional dysregulation of Dishevelled 1 and 2 underlies the Wnt signaling impairment in type I Gaucher disease experimental models.

Bone differentiation defects have been recently tied to Wnt signaling alterations occurring in vitro and in vivo Gaucher disease (GD) models. In this work, we provide evidence that the Wnt signaling multi-domain intracellular transducers Dishevelled 1 and 2 (DVL1 and DVL2) may be potential upstream targets of impaired beta glucosidase (GBA1) activity by showing their misexpression in different type 1 GD in vitro models. We also show that in Gba mutant fish a miR-221 upregulation is associated with reduced dvl2 expression levels and that in type I Gaucher patients single-nucleotide variants in the DVL2 3' untranslated region are related to variable canonical Wnt pathway activity. Thus, we strengthen the recently outlined relation between bone differentiation defects and Wnt/ß-catenin dysregulation in type I GD and further propose novel mechanistic insights of the Wnt pathway impairment caused by glucocerebrosidase loss of function.

Impaired Mitochondrial ATP Production Downregulates Wnt Signaling via ER Stress Induction.

Wnt signaling affects fundamental development pathways and, if aberrantly activated, promotes the development of cancers. Wnt signaling is modulated by different factors, but whether the mitochondrial energetic state affects Wnt signaling is unknown. Here, we show that sublethal concentrations of different compounds that decrease mitochondrial ATP production specifically downregulate Wnt/ß-catenin signaling in vitro in colon cancer cells and in vivo in zebrafish reporter lines. Accordingly, fibroblasts from a GRACILE syndrome patient and a generated zebrafish model lead to reduced Wnt signaling. We identify a mitochondria-Wnt signaling axis whereby a decrease in mitochondrial ATP reduces calcium uptake into the endoplasmic reticulum (ER), leading to endoplasmic reticulum stress and to impaired Wnt signaling. In turn, the recovery of the ATP level or the inhibition of endoplasmic reticulum stress restores Wnt activity. These findings reveal a mechanism that links mitochondrial energetic metabolism to the control of the Wnt pathway that may be beneficial against several pathologies.

The Golgi 'casein kinase' Fam20C is a genuine 'phosvitin kinase' and phosphorylates polyserine stretches devoid of the canonical consensus.

Egg yolk phosvitins, generated through the fragmentation of vitellogenins (VTGs), are among the most heavily phosphorylated proteins ever described. Despite the early discovery in 1900 that chicken phosvitin is a phosphoprotein and its subsequent employment as an artificial substrate for a number of protein kinases, the identity of the enzyme(s) responsible for its phosphorylation remained a matter of conjecture until present. Here, we provide evidence that phosvitin phosphorylation is catalyzed by a family with sequence similarity 20, member C (Fam20C), an atypical protein kinase recently identified as the genuine casein kinase and responsible for the phosphorylation of many other secreted proteins at residues specified by the S-x-E/pS consensus. Such a conclusion is grounded on the following observations: (a) the levels of Fam20C and phosphorylated VTG rise in parallel upon treatment of zebrafish with oestrogens; (b) zebrafish phosvitin is readily phosphorylated upon coexpression in U2OS cells with Fam20C, but not with its catalytically inactive mutant; (c) a peptide reproducing a stretch of 12 serines, which are phosphorylated in chicken phosvitin despite lacking the C-terminal priming motif S-x-E, is efficiently phosphorylated by both recombinant and native Fam20C. The last finding expands the repertoire of potential targets of Fam20C to include several proteins known to harbor (p-Ser)n clusters not specified by any known kinase consensus.

The pathogenesis of lysosomal storage disorders: beyond the engorgement of lysosomes to abnormal development and neuroinflammation.

There is growing evidence that the complex clinical manifestations of lysosomal storage diseases (LSDs) are not fully explained by the engorgement of the endosomal-autophagic-lysosomal system. In this review, we explore current knowledge of common pathogenetic mechanisms responsible for the early onset of tissue abnormalities of two LSDs, Mucopolysaccharidosis type II (MPSII) and Niemann-Pick type C (NPC) diseases. In particular, perturbations of the homeostasis of glycosaminoglycans (GAGs) and cholesterol (Chol) in MPSII and NPC diseases, respectively, affect key biological processes, including morphogen signaling. Both GAGs and Chol finely regulate the release, reception and tissue distribution of Shh. Hence, not surprisingly, developmental processes depending on correct Shh signaling have been found altered in both diseases. Besides abnormal signaling, exaggerated activation of microglia and impairment of autophagy and mitophagy occur in both diseases, largely before the appearance of typical pathological signs.

FGF signaling deregulation is associated with early developmental skeletal defects in animal models for mucopolysaccharidosis type II (MPSII).

Skeletal abnormalities represent a major clinical burden in patients affected by the lysosomal storage disorder mucopolysaccharidosis type II (MPSII, OMIM #309900). While extensive research has emphasized the detrimental role of stored glycosaminoglycans (GAGs) in the bone marrow (BM), a limited understanding of primary cellular mechanisms underlying bone defects in MPSII has hampered the development of bone-targeted therapeutic strategies beyond enzyme replacement therapy (ERT). We here investigated the involvement of key signaling pathways related to the loss of iduronate-2-sulfatase activity in two different MPSII animal models, D. rerio and M. musculus. We found that FGF pathway activity is impaired during early stages of bone development in IDS knockout mice and in a newly generated Ids mutant fish. In both models the FGF signaling deregulation anticipated a slow but progressive defect in bone differentiation, regardless of any extensive GAGs storage. We also show that MPSII patient fibroblasts harboring different mutations spanning the IDS gene exhibit perturbed FGF signaling-related markers expression. Our work opens a new venue to discover possible druggable novel key targets in MPSII.

Perturbations in cell signaling elicit early cardiac defects in mucopolysaccharidosis type II.

Morphogens release and activity can be negatively affected by an impaired glycosaminoglycans (GAGs) turnover and proteoglycans assembly in the extracellular matrix, leading to altered tissue morphogenesis. In this work, we show that loss of Iduronate-2-sulfatase (IDS) activity, affecting GAGs catabolism and responsible for a life-threatening valvulopathy in mucopolysaccharidosis type II (MPSII), triggers early Sonic Hedgehog (Shh) and Wnt/ß-catenin signaling defects, leading to aberrant heart development and atrioventricular valve formation in a zebrafish model. In addition, we consistently found impaired Shh signaling activity and cardiac electrophysiological abnormalities in IDS knockout mice at postnatal stages before any evident massive GAGs accumulation. These results suggest that IDS activity substantially affect cardiac morphogenesis through impaired Shh signaling and document an unexplored role of the enzyme in the fine-tuning of cell signaling pathways.

Glucocerebrosidase deficiency in zebrafish affects primary bone ossification through increased oxidative stress and reduced Wnt/ß-catenin signaling.

Loss of lysosomal glucocerebrosidase (GBA1) function is responsible for several organ defects, including skeletal abnormalities in type 1 Gaucher disease (GD). Enhanced bone resorption by infiltrating macrophages has been proposed to lead to major bone defects. However, while more recent evidences support the hypothesis that osteoblastic bone formation is impaired, a clear pathogenetic mechanism has not been depicted yet. Here, by combining different molecular approaches, we show that Gba1 loss of function in zebrafish is associated with defective canonical Wnt signaling, impaired osteoblast differentiation and reduced bone mineralization. We also provide evidence that increased reactive oxygen species production precedes the Wnt signaling impairment, which can be reversed upon human GBA1 overexpression. Type 1 GD patient fibroblasts similarly exhibit reduced Wnt signaling activity, as a consequence of increased ß-catenin degradation. Our results support a novel model in which a primary defect in canonical Wnt signaling antecedes bone defects in type 1 GD.

A Smad3 transgenic reporter reveals TGF-beta control of zebrafish spinal cord development.

TGF-beta (TGFß) family mediated Smad signaling is involved in mesoderm and endoderm specifications, left-right asymmetry formation and neural tube development. The TGFß1/2/3 and Activin/Nodal signal transduction cascades culminate with activation of SMAD2 and/or SMAD3 transcription factors and their overactivation are involved in different pathologies with an inflammatory and/or uncontrolled cell proliferation basis, such as cancer and fibrosis. We have developed a transgenic zebrafish reporter line responsive to Smad3 activity. Through chemical, genetic and molecular approaches we have seen that this transgenic line consistently reproduces in vivo Smad3-mediated TGFß signaling. Reporter fluorescence is activated in phospho-Smad3 positive cells and is responsive to both Smad3 isoforms, Smad3a and 3b. Moreover, Alk4 and Alk5 inhibitors strongly repress the reporter activity. In the CNS, Smad3 reporter activity is particularly high in the subpallium, tegumentum, cerebellar plate, medulla oblongata and the retina proliferative zone. In the spinal cord, the reporter is activated at the ventricular zone, where neuronal progenitor cells are located. Colocalization methods show in vivo that TGFß signaling is particularly active in neuroD+ precursors. Using neuronal transgenic lines, we observed that TGFß chemical inhibition leads to a decrease of differentiating cells and an increase of proliferation. Similarly, smad3a and 3b knock-down alter neural differentiation showing that both paralogues play a positive role in neural differentiation. EdU proliferation assay and pH3 staining confirmed that Smad3 is mainly active in post-mitotic, non-proliferating cells. In summary, we demonstrate that the Smad3 reporter line allows us to follow in vivo Smad3 transcriptional activity and that Smad3, by controlling neural differentiation, promotes the progenitor to precursor switch allowing neural progenitors to exit cell cycle and differentiate.

Simplet/Fam53b is required for Wnt signal transduction by regulating ß-catenin nuclear localization.

Canonical ß-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of ß-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating ß-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating ß-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two ß-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of ß-catenin. Conversely, overexpression of smp increases ß-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the ß-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for ß-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating ß-catenin nuclear localization: the protein colocalizes with ß-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces ß-catenin nuclear localization and transcriptional activity; human SMP mediates ß-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human ß-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of ß-catenin-dependent Wnt signal transduction.

Intracardiac flow dynamics regulate atrioventricular valve morphogenesis.

AIMS: Valvular heart disease is responsible for considerable morbidity and mortality. Cardiac valves develop as the heart contracts, and they function throughout the lifetime of the organism to prevent retrograde blood flow. Their precise morphogenesis is crucial for cardiac function. Zebrafish is an ideal model to investigate cardiac valve development as it allows these studies to be carried out in vivo through non-invasive imaging. Accumulating evidence suggests a role for contractility and intracardiac flow dynamics in cardiac valve development. However, these two factors have proved difficult to uncouple, especially since altering myocardial function affects the intracardiac flow pattern. METHODS AND RESULTS: Here, we describe novel zebrafish models of developmental valve defects. We identified two mutant alleles of myosin heavy chain 6 that can be raised to adulthood despite having only one functional chamber-the ventricle. The adult mutant ventricle undergoes remodelling, and the atrioventricular (AV) valves fail to form four cuspids. In parallel, we characterized a novel mutant allele of southpaw, a nodal-related gene involved in the establishment of left-right asymmetry, which exhibits randomized heart and endoderm positioning. We first observed that in southpaw mutants the relative position of the two cardiac chambers is altered, affecting the geometry of the heart, while myocardial function appears unaffected. Mutant hearts that loop properly or exhibit situs inversus develop normally, whereas midline, unlooped hearts exhibit defects in their transvalvular flow pattern during AV valve development as well as defects in valve morphogenesis. CONCLUSION: Our data indicate that intracardiac flow dynamics regulate valve morphogenesis independently of myocardial contractility.