The mood stabilizer valproic acid improves defective neurite formation caused by Charcot-Marie-Tooth disease-associated mutant Rab7 through the JNK signaling pathway

RABGGTB plays a critical role in ALS pathogenesis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease with unknown causes, which mainly affects motor neurons in the anterior horn of the spinal cord, brain stem, and cerebral cortex, also known as motor neuron disease. An important pathological feature of ALS is the formation of aggregates of mutant SOD1 protein, CTF25 of TDP-43, or other abnormal proteins in motor neurons, which require autophagy for degradation. Protein prenylation is known to participate in membrane association and proper localization of proteins. RABGGTB is the β subunit of GGTase II (one of the prenyltransferases) that can regulate autophagy via Rab7 geranylgeranylation. In this study, we overexpressed RABGGTB via lentiviral transfection in NSC34-hSOD1G93A and TDP-43 cells. Overexpression of RABGGTB improved ALS cell proliferation by facilitating autophagosome-lysosome fusion. Furthermore, the abnormal aggregation of SOD1 protein was reduced. This indicates that protein prenylation is important for the proliferation and autophagy of cells autophagy. Enhanced autophagy has been observed in two of the most widely used ALS cell models. These findings indicate the widespread applicability of prenylation in ALS. In summary, overexpression of RABGGTB improved the geranylgeranylation of the Rab7 protein and had a positive effect on cells. These findings provide insights into the development of a novel therapeutic strategy for ALS.

Regulation of Endosomal Trafficking by Rab7 and Its Effectors in Neurons: Clues from Charcot-Marie-Tooth 2B Disease

Intracellular endosomal trafficking controls the balance between protein degradation and synthesis, i.e., proteostasis, but also many of the cellular signaling pathways that emanate from activated growth factor receptors after endocytosis. Endosomal trafficking, sorting, and motility are coordinated by the activity of small GTPases, including Rab proteins, whose function as molecular switches direct activity at endosomal membranes through effector proteins. Rab7 is particularly important in the coordination of the degradative functions of the pathway. Rab7 effectors control endosomal maturation and the properties of late endosomal and lysosomal compartments, such as coordination of recycling, motility, and fusion with downstream compartments. The spatiotemporal regulation of endosomal receptor trafficking is particularly challenging in neurons because of their enormous size, their distinct intracellular domains with unique requirements (dendrites vs. axons), and their long lifespans as postmitotic, differentiated cells. In Charcot-Marie-Tooth 2B disease (CMT2B), familial missense mutations in Rab7 cause alterations in GTPase cycling and trafficking, leading to an ulcero-mutilating peripheral neuropathy. The prevailing hypothesis to account for CMT2B pathologies is that CMT2B-associated Rab7 alleles alter endocytic trafficking of the neurotrophin NGF and its receptor TrkA and, thereby, disrupt normal trophic signaling in the peripheral nervous system, but other Rab7-dependent pathways are also impacted. Here, using TrkA as a prototypical endocytic cargo, we review physiologic Rab7 effector interactions and control in neurons. Since neurons are among the largest cells in the body, we place particular emphasis on the temporal and spatial regulation of endosomal sorting and trafficking in neuronal processes. We further discuss the current findings in CMT2B mutant Rab7 models, the impact of mutations on effector interactions or balance, and how this dysregulation may confer disease.

Knockdown of Rab7B, But Not of Rab7A, Which Antagonistically Regulates Oligodendroglial Cell Morphological Differentiation, Recovers Tunicamycin-Induced Defective Differentiation in FBD-102b Cells

In the central nervous system (CNS), insulative myelin sheaths are generated from the differentiated plasma membranes of oligodendrocytes (oligodendroglial cells) and surround neuronal axons to achieve saltatory conduction. Despite the functional involvement of myelin sheaths in the CNS, the molecular mechanism by which oligodendroglial cells themselves undergo differentiation of plasma membranes remains unclear. It also remains to be explored whether their signaling mechanisms can be applied to treating diseases of the oligodendroglial cells. Here, we describe that Rab7B of Rab7 subfamily small GTPases negatively regulates oligodendroglial cell morphological differentiation using FBD-102b cells, which are model cells undergoing differentiation of oligodendroglial precursors. Knockdown of Rab7B or Rab7A by the respective specific siRNAs in cells positively or negatively regulated morphological differentiation, respectively. Consistently, these changes were supported by changes on differentiation- and myelination-related structural protein and protein kinase markers. We also found that knockdown of Rab7B has the ability to recover inhibition of morphological differentiation following tunicamycin-induced endoplasmic reticulum (ER) stress, which mimics one of the major molecular pathological causes of hereditary hypomyelinating disorders in oligodendroglial cells, such as Pelizaeus-Merzbacher disease (PMD). These results suggest that the respective molecules among very close Rab7 homologues exhibit differential roles in morphological differentiation and that knocking down Rab7B can recover defective differentiating phenotypes under ER stress, thereby adding Rab7B to the list of molecular therapeutic cues taking advantage of signaling mechanisms for oligodendroglial diseases like PMD.

Hesperetin Ameliorates Inhibition of Neuronal and Oligodendroglial Cell Differentiation Phenotypes Induced by Knockdown of Rab2b, an Autism Spectrum Disorder-Associated Gene Product

Autism spectrum disorder (ASD) is a central nervous system (CNS) neurodevelopmental disorder that includes autism, pervasive developmental disorder, and Asperger’s syndrome. ASD is characterized by repetitive behaviors and social communication deficits. ASD is thought to be a multifactorial disorder with a range of genetic and environmental factors/candidates. Among such factors is the rab2b gene, although it remains unclear how Rab2b itself is related to the CNS neuronal and glial developmental disorganization observed in ASD patients. Rab2 subfamily members regulate intracellular vesicle transport between the endoplasmic reticulum and the Golgi body. To the best of our knowledge, we are the first to report that Rab2b positively regulates neuronal and glial cell morphological differentiation. Knockdown of Rab2b inhibited morphological changes in N1E-115 cells, which are often used as the neuronal cell differentiation model. These changes were accomplished with decreased expression levels of marker proteins in neuronal cells. Similar results were obtained for FBD-102b cells, which are used as the model of oligodendroglial cell morphological differentiation. In contrast, knockdown of Rab2a, which is another Rab2 family member not known to be associated with ASD, affected only oligodendroglial and not neuronal morphological changes. In contrast, treatment with hesperetin, a citrus flavonoid with various cellular protective effects, in cells recovered the defective morphological changes induced by Rab2b knockdown. These results suggest that knockdown of Rab2b inhibits differentiation in neuronal and glial cells and may be associated with pathological cellular phenotypes in ASD and that hesperetin can recover their phenotypes at the in vitro level at least.

Changes in uridine 5'-diphospho-glucuronosyltransferase 1A6 expression by histone deacetylase inhibitor valproic acid

Valproic acid (VPA) is well-known as a histone deacetylase (HDAC) inhibitor. It has been reported that HDAC inhibitors enhance basal and aryl hydrocarbon receptor (AhR) ligand-induced aryl hydrocarbon receptor-responsive gene expression. Other studies suggested that HDAC inhibition might significantly activate the NF-E2-related factor-2 (Nrf2). Moreover, VPA activates mitogen-activated protein kinases (MAPKs). MAPK pathways regulate Nrf2 transactivation domain activity. Uridine 5'-diphospho-glucuronosyltransferase (UGT) 1A6 is one of the important isoforms to affect drug pharmacokinetics. UGT1A6 gene is regulated transcriptionally by AhR and Nrf2. The present study aimed to investigate whether UGT1A6 expression was changed by VPA and to elucidate the mechanism of the alteration. Following VPA treatment for 72 h in Caco-2 cells, UGT1A6 mRNA was increased by 7.9-fold. Moreover, UGT1A6 mRNA was increased by other HDAC inhibitors, suggesting that HDAC inhibition caused the UGT1A6 mRNA induction. AhR and Nrf2 proteins in the nucleus of Caco-2 cells were increased by 1.5- and 1.7-fold, respectively, following the VPA treatment. However, VPA treatment did not activate the extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) pathways in Caco-2 cells. In conclusion, we observed that VPA induced UGT1A6 mRNA expression via AhR and Nrf2 pathways, but not via the ERK or JNK pathways.

The Antiepileptic Valproic Acid Ameliorates Charcot-Marie-Tooth 2W (CMT2W) Disease-Associated HARS1 Mutation-Induced Inhibition of Neuronal Cell Morphological Differentiation Through c-Jun N-terminal Kinase

Hereditary peripheral neuropathies called Charcot-Marie-Tooth (CMT) disease affect the sensory nerves as well as motor neurons. CMT diseases are composed of a heterogeneous group of diseases. They are characterized by symptoms such as muscle weakness and wasting. Type 2 CMT (CMT2) disease is a neuropathy with blunted or disrupted neuronal morphological differentiation phenotypes including process formation of peripheral neuronal axons. In the early stages of CMT2, demyelination that occurs in Schwann cells (glial cells) is rarely observed. CMT2W is an autosomal-dominant disease and is responsible for the gene encoding histidyl-tRNA synthetase 1 (HARS1), which is a family molecule of cytoplasmic aminoacyl-tRNA synthetases and functions by ligating histidine to its cognate tRNA. Despite increasing knowledge of the relationship of mutations on responsible genes with diseases, it still remains unclear how each mutation affects neuronal differentiation. Here we show that in neuronal N1E-115 cells, a severe Asp364-to-Tyr (D364Y) mutation of HARS1 leads to formation of small aggregates of HARS1 proteins; in contrast, wild type proteins are distributed throughout cell bodies. Expression of D364Y mutant proteins inhibited process formation whereas expression of wild type proteins possessed the normal differentiation ability to grow processes. Pretreatment with the antiepileptic valproic acid recovered inhibition of process formation by D364Y mutant proteins through the c-Jun N-terminal kinase signaling pathway. Taken together, these results indicate that the D364Y mutation of HARS1 causes HARS1 proteins to form small aggregates, inhibiting process growth, and that these effects are recovered by valproic acid. This could be a potential therapeutic drug for CMT2W at the cellular levels.

Autophagy and Lysosomal Functionality in CMT2B Fibroblasts Carrying the RAB7K126R Mutation

Charcot-Marie-Tooth type 2B (CMT2B) disease is a dominant axonal peripheral neuropathy caused by five mutations in the RAB7A gene. Autophagy and late endocytic trafficking were already characterized in CMT2B. Indeed, impairment of autophagy and an increase in lysosomal degradative activity were found in cells expressing the mutant proteins. Recently, we described a novel RAB7 mutation associated with predominantly motor CMT2 and impaired EGFR trafficking. With the aim to analyze the autophagy process and lysosomal activity in CMT2B fibroblasts carrying the p.K126R RAB7 novel mutation and to investigate further the causes of the different phenotype, we have performed Western blot, immunofluorescence and cytometric analyses monitoring autophagic markers and endocytic proteins. Moreover, we investigated lipophagy by analyzing accumulation of lipid droplets and their co-localization with endolysosomal degradative compartments. We found that cells expressing the RAB7^K126R mutant protein were characterized by impairment of autophagy and lipophagy processes and by a moderate increase in lysosomal activity compared to the previously studied cells carrying the RAB7^V162M mutation. Thus, we concluded that EGFR trafficking alterations and a moderate increase in lysosomal activity with concomitant impairment of autophagy could induce the specific predominantly motor phenotype observed in K126R patients.

Human iPSC-derived neurons reveal early developmental alteration of neurite outgrowth in the late-occurring neurodegenerative Wolfram syndrome

Recent studies indicate that neurodegenerative processes that appear during childhood and adolescence in individuals with Wolfram syndrome (WS) occur in addition to early brain development alteration, which is clinically silent. Underlying pathological mechanisms are still unknown. We have used induced pluripotent stem cell-derived neural cells from individuals affected by WS in order to reveal their phenotypic and molecular correlates. We have observed that a subpopulation of Wolfram neurons displayed aberrant neurite outgrowth associated with altered expression of axon guidance genes. Selective inhibition of the ATF6α arm of the unfolded protein response prevented the altered phenotype, although acute endoplasmic reticulum stress response-which is activated in late Wolfram degenerative processes-was not detected. Among the drugs currently tried in individuals with WS, valproic acid was the one that prevented the pathological phenotypes. These results suggest that early defects in axon guidance may contribute to the loss of neurons in individuals with WS.

Dysfunction of RAB39B-Mediated Vesicular Trafficking in Lewy Body Diseases

Intracellular vesicular trafficking is essential for neuronal development, function, and homeostasis and serves to process, direct, and sort proteins, lipids, and other cargo throughout the cell. This intricate system of membrane trafficking between different compartments is tightly orchestrated by Ras analog in brain (RAB) GTPases and their effectors. Of the 66 members of the RAB family in humans, many have been implicated in neurodegenerative diseases and impairment of their functions contributes to cellular stress, protein aggregation, and death. Critically, RAB39B loss-of-function mutations are known to be associated with X-linked intellectual disability and with rare early-onset Parkinson's disease. Moreover, recent studies have highlighted altered RAB39B expression in idiopathic cases of several Lewy body diseases (LBDs). This review contextualizes the role of RAB proteins in LBDs and highlights the consequences of RAB39B impairment in terms of endosomal trafficking, neurite outgrowth, synaptic maturation, autophagy, as well as alpha-synuclein homeostasis. Additionally, the potential for therapeutic intervention is examined via a discussion of the recent progress towards the development of specific RAB modulators. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Hypomyelinating Leukodystrophy 15 (HLD15)-Associated Mutation of EPRS1 Leads to Its Polymeric Aggregation in Rab7-Positive Vesicle Structures, Inhibiting Oligodendroglial Cell Morphological Differentiation

Pelizaeus–Merzbacher disease (PMD), also known as hypomyelinating leukodystrophy 1 (HLD1), is an X-linked recessive disease affecting in the central nervous system (CNS). The gene responsible for HLD1 encodes proteolipid protein 1 (plp1), which is the major myelin structural protein produced by oligodendroglial cells (oligodendrocytes). HLD15 is an autosomal recessive disease affecting the glutamyl-prolyl-aminoacyl-tRNA synthetase 1 (eprs1) gene, whose product, the EPRS1 protein, is a bifunctional aminoacyl-tRNA synthetase that is localized throughout cell bodies and that catalyzes the aminoacylation of glutamic acid and proline tRNA species. Here, we show that the HLD15-associated nonsense mutation of Arg339-to-Ter (R339X) localizes EPRS1 proteins as polymeric aggregates into Rab7-positive vesicle structures in mouse oligodendroglial FBD-102b cells. Wild-type proteins, in contrast, are distributed throughout the cell bodies. Expression of the R339X mutant proteins, but not the wild-type proteins, in cells induces strong signals regulating Rab7. Whereas cells expressing the wild-type proteins exhibited phenotypes with myelin web-like structures bearing processes following the induction of differentiation, cells expressing the R339X mutant proteins did not. These results indicate that HLD15-associated EPRS1 mutant proteins are localized in Rab7-positive vesicle structures where they modulate Rab7 regulatory signaling, inhibiting cell morphological differentiation. These findings may reveal some of the molecular and cellular pathological mechanisms underlying HLD15.

An altered lipid metabolism characterizes Charcot-Marie-Tooth type 2B peripheral neuropathy

Charcot-Marie Tooth type 2B (CMT2B) is a rare inherited peripheral neuropathy caused by five missense mutations in the RAB7A gene, which encodes a small GTPase of the RAB family. Currently, no cure is available for this disease. In this study, we approached the disease by comparing the lipid metabolism of CMT2B-derived fibroblasts to that of healthy controls. We found that CMT2B cells showed increased monounsaturated fatty acid level and increased expression of key enzymes of monounsaturated and polyunsaturated fatty acid synthesis. Moreover, in CMT2B cells a higher expression of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), key enzymes of de novo fatty acid synthesis, with a concomitantly increased [1-¹⁴C]acetate incorporation into fatty acids, was observed. The expression of diacylglycerol acyltransferase 2, a rate-limiting enzyme in triacylglycerol synthesis, as well as triacylglycerol levels were increased in CMT2B compared to control cells. In addition, as RAB7A controls lipid droplet breakdown and lipid droplet dynamics have been linked to diseases, we analyzed these organelles and showed that in CMT2B cells there is a strong accumulation of lipid droplets compared to control cells, thus reinforcing our data on abnormal lipid metabolism in CMT2B. Furthermore, we demonstrated that ACC and FAS expression levels changed upon RAB7 silencing or overexpression in HeLa cells, thus suggesting that metabolic modifications observed in CMT2B-derived fibroblasts can be, at least in part, related to RAB7 mutations.

Charcot-Marie-Tooth Type 2B: A New Phenotype Associated with a Novel RAB7A Mutation and Inhibited EGFR Degradation

The rare autosomal dominant Charcot-Marie-Tooth type 2B (CMT2B) is associated with mutations in the RAB7A gene, involved in the late endocytic pathway. CMT2B is characterized by predominant sensory loss, ulceromutilating features, with lesser-to-absent motor deficits. We characterized clinically and genetically a family harboring a novel pathogenic RAB7A variant and performed structural and functional analysis of the mutant protein. A 39-year-old woman presented with early-onset walking difficulties, progressive distal muscle wasting and weakness in lower limbs and only mild sensory signs. Electrophysiology demonstrated an axonal sensorimotor neuropathy. Nerve biopsy showed a chronic axonal neuropathy with moderate loss of all caliber myelinated fibers. Next-generation sequencing (NGS) technology revealed in the proband and in her similarly affected father the novel c.377A>G (p.K126R) heterozygous variant predicted to be deleterious. The mutation affects the biochemical properties of RAB7 GTPase, causes altered interaction with peripherin, and inhibition of neurite outgrowth, as for previously reported CMT2B mutants. However, it also shows differences, particularly in the epidermal growth factor receptor degradation process. Altogether, our findings indicate that this RAB7A variant is pathogenic and widens the phenotypic spectrum of CMT2B to include predominantly motor CMT2. Alteration of the receptor degradation process might explain the different clinical presentations in this family.

Alteration of the late endocytic pathway in Charcot-Marie-Tooth type 2B disease

The small GTPase RAB7A regulates late stages of the endocytic pathway and plays specific roles in neurons, controlling neurotrophins trafficking and signaling, neurite outgrowth and neuronal migration. Mutations in the RAB7A gene cause the autosomal dominant Charcot–Marie–Tooth type 2B (CMT2B) disease, an axonal peripheral neuropathy. As several neurodegenerative diseases are caused by alterations of endocytosis, we investigated whether CMT2B-causing mutations correlate with changes in this process. To this purpose, we studied the endocytic pathway in skin fibroblasts from healthy and CMT2B individuals. We found higher expression of late endocytic proteins in CMT2B cells compared to control cells, as well as higher activity of cathepsins and higher receptor degradation activity. Consistently, we observed an increased number of lysosomes, accompanied by higher lysosomal degradative activity in CMT2B cells. Furthermore, we found increased migration and increased RAC1 and MMP-2 activation in CMT2B compared to control cells. To validate these data, we obtained sensory neurons from patient and control iPS cells, to confirm increased lysosomal protein expression and lysosomal activity in CMT2B-derived neurons. Altogether, these results demonstrate that in CMT2B patient-derived cells, the endocytic degradative pathway is altered, suggesting that higher lysosomal activity contributes to neurodegeneration occurring in CMT2B.

HDAC6 inhibitors: Translating genetic and molecular insights into a therapy for axonal CMT

Histone deacetylase 6 (HDAC6) plays a central role in various processes that are key for neuronal survival. In this review, we summarize the current evidence related to disease pathways in the axonal form of Charcot-Marie-Tooth disease (CMT) and highlight the role of HDAC6 in these pathways. We hypothesize that HDAC6 might in fact actively contribute to the pathogenesis of certain forms of axonal CMT. HDAC6 plays a deacetylase activity-dependent, negative role in axonal transport and axonal regeneration, which are both processes implicated in axonal CMT. On the other hand, HDAC6 coordinates a protective response during elimination of toxic misfolded proteins, but this is mostly mediated independent of its deacetylase activity. The current mechanistic insights on these functions of HDAC6 in axonal CMT, along with the selective druggability against its deacetylase activity, make the targeting of HDAC6 particularly attractive. We elaborate on the preclinical studies that demonstrated beneficial effects of HDAC6 inhibitors in axonal CMT models and outline possible modes of action. Overall, this overview ultimately provides a rationale for the use of small-molecule HDAC6 inhibitors as a therapeutic strategy for this devastating disease.

Old but Gold: Tracking the New Guise of Histone Deacetylase 6 (HDAC6) Enzyme as a Biomarker and Therapeutic Target in Rare Diseases

Epigenetic regulation orchestrates many cellular processes and greatly influences key disease mechanisms. Histone deacetylase (HDAC) enzymes play a crucial role either as biomarkers or therapeutic targets owing to their involvement in specific pathophysiological pathways. Beyond their well-characterized role as histone modifiers, HDACs also interact with several nonhistone substrates and their increased expression has been highlighted in specific diseases. The HDAC6 isoform, due to its unique cytoplasmic localization, modulates the acetylation status of tubulin, HSP90, TGF-β, and peroxiredoxins. HDAC6 also exerts noncatalytic activities through its interaction with ubiquitin. Both catalytic and noncatalytic functions of HDACs are being actively studied in the field of specific rare disorders beyond the well-established role in carcinogenesis. This Perspective outlines the application of HDAC(6) inhibitors in rare diseases, such as Rett syndrome, inherited retinal disorders, idiopathic pulmonary fibrosis, and Charcot-Marie-Tooth disease, highlighting their therapeutic potential as innovative and targeted disease-modifying agents.

Coordination between Rac1 and Rab Proteins: Functional Implications in Health and Disease

The small GTPases of the Rho family regulate many aspects of actin dynamics, but are functionally connected to many other cellular processes. Rac1, a member of this family, besides its known function in the regulation of actin cytoskeleton, plays a key role in the production of reactive oxygen species, in gene transcription, in DNA repair, and also has been proven to have specific roles in neurons. This review focuses on the cooperation between Rac1 and Rab proteins, analyzing how the coordination between these GTPases impact on cells and how alterations of their functions lead to disease.

CMT type 2N disease-associated AARS mutant inhibits neurite growth that can be reversed by valproic acid

Charcot-Marie-Tooth (CMT) disease is composed of a heterogeneous group of hereditary peripheral neuropathies. The peripheral nervous system primarily comprises two types of cells: neuronal cells and myelinating glial Schwann cells. CMT2 N is an autosomal dominant disease and its responsible gene encodes alanyl-tRNA synthetase (AARS), which is a family of cytoplasmic aminoacyl-tRNA synthetases. CMT2 N is associated with the mutation, including a missense mutation, which is known to decrease the enzymatic activity of AARS, but whether and how its mutation affects AARS localization and neuronal process formation remains to be understood. First, we show that the AARS mutant harboring Asn71-to-Tyr (N71Y) is not localized in cytoplasm. The expression of AARS mutant proteins in COS-7 cells mainly leads to localization into lysosome, whereas the wild type is indeed localized in cytoplasm. Second, in N1E-115 cells as the neuronal cell model, cells expressing the N71Y mutant do not have the ability to grow processes. Third, pretreatment with antiepileptic valproic acid reverses the inhibitory effect of the N71Y mutant on process growth. Taken together, the N71Y mutation of AARS leads to abnormal intracellular localization, inhibiting process growth, yet this inhibition is reversed by valproic acid.

Alterations of autophagy in the peripheral neuropathy Charcot-Marie-Tooth type 2B

Charcot-Marie-Tooth type 2B (CMT2B) disease is a dominant axonal peripheral neuropathy caused by 5 mutations in the RAB7A gene, a ubiquitously expressed GTPase controlling late endocytic trafficking. In neurons, RAB7A also controls neuronal-specific processes such as NTF (neurotrophin) trafficking and signaling, neurite outgrowth and neuronal migration. Given the involvement of macroautophagy/autophagy in several neurodegenerative diseases and considering that RAB7A is fundamental for autophagosome maturation, we investigated whether CMT2B-causing mutants affect the ability of this gene to regulate autophagy. In HeLa cells, we observed a reduced localization of all CMT2B-causing RAB7A mutants on autophagic compartments. Furthermore, compared to expression of RAB7A^WT, expression of these mutants caused a reduced autophagic flux, similar to what happens in cells expressing the dominant negative RAB7A^T22N mutant. Consistently, both basal and starvation-induced autophagy were strongly inhibited in skin fibroblasts from a CMT2B patient carrying the RAB7A^V162M mutation, suggesting that alteration of the autophagic flux could be responsible for neurodegeneration.

Neuroprotective Effects of Valproic Acid in a Rat Model of Cauda Equina Injury

BACKGROUND

Histone deacetylase inhibitors, including valproic acid (VPA), are promising therapeutic interventions in neurological disorders and play an important role in synaptic activity and neuronal function.

METHODS

A total of 30 rats were randomly allocated to 3 groups: sham, control, and VPA. The rats in the VPA and control groups received laminectomy at the L4 level of the vertebrae and silicone gel implantation into the epidural spaces L5 and L6. Rats in the sham group only received laminectomy at the L4 level of vertebrae without any implantation. VPA (300 mg/kg in saline) was administered 2 hours before the surgery. After the surgery, the VPA group received further VPA injections at 300 mg/kg twice a day for 1 week. The same volume of saline was injected in the control group. Neurobehavioral tests using the Basso, Beattie, Bresnahan scale and the oblique board test were performed for 1 week starting at 2 hours before surgery up to day 7 after surgery. At day 7 after surgery, tissues from the compressed cauda equina (L5-L6) were subjected to hematoxylin and eosin, luxol fast blue, or immunofluorescence staining, whereas the terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick-end label assay staining was performed on the tissue from the dorsal root ganglions and the lumbar segment of the spinal cord proximal to the compressed cauda equina (L5-L6).

RESULTS

The behavioral results suggested a significant improvement in the lower limb motor function in the VPA group compared with controls (P < 0.05). Furthermore, histologic assessment revealed a significant reduction in nerve fibers showing Wallerian degeneration and demyelinating lesions in the VPA group, in addition to an increased myelination compared with the control group (P < 0.05). The terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick-end label assay staining revealed a significant decrease in the number of apoptotic neurons in the spinal cord anterior horn and dorsal root ganglions in the VPA group compared with controls (P < 0.05).

CONCLUSIONS

Our data demonstrated that VPA could alleviate cauda equina injury, reduce apoptotic cells, and improve motor recovery, suggesting a neuroprotective effect in acute cauda equina syndrome.

Multiple Roles of the Small GTPase Rab7

Rab7 is a small GTPase that belongs to the Rab family and controls transport to late endocytic compartments such as late endosomes and lysosomes. The mechanism of action of Rab7 in the late endocytic pathway has been extensively studied. Rab7 is fundamental for lysosomal biogenesis, positioning and functions, and for trafficking and degradation of several signaling receptors, thus also having implications on signal transduction. Several Rab7 interacting proteins have being identified leading to the discovery of a number of different important functions, beside its established role in endocytosis. Furthermore, Rab7 has specific functions in neurons. This review highlights and discusses the role and the importance of Rab7 on different cellular pathways and processes.

Charcot-Marie-Tooth 2b associated Rab7 mutations cause axon growth and guidance defects during vertebrate sensory neuron development

Background

Charcot-Marie-Tooth2b (CMT2b) is an axonal form of a human neurodegenerative disease that preferentially affects sensory neurons. CMT2b is dominantly inherited and is characterized by unusually early onset, presenting in the second or third decade of life. Five missense mutations in the gene encoding Rab7 GTPase have been identified as causative in human CMT2b disease. Although several studies have modeled CMT2b disease in cultured neurons and in Drosophila, the mechanisms by which defective Rab7 leads to disease remain poorly understood.

Results

We used zebrafish to investigate the effects of CMT2b-associated Rab7 mutations in a vertebrate model. We generated transgenic animals expressing the CMT2b-associated mutant forms of Rab7 in sensory neurons, and show that these Rab7 variants cause neurodevelopmental defects, including defects in sensory axon growth, branching and pathfinding at early developmental stages. We also find reduced axon growth and branching in neurons expressing a constitutively active form of Rab7, suggesting these defects may be caused by Rab7 gain-of-function. Further, we use high-speed, high-resolution imaging of endosome transport in vivo and find that CMT2b-associated Rab7 variants cause reduced vesicle speeds, suggesting altered transport may underlie axon development defects.

Conclusions

Our data provide new insight into how disease-associated alterations in Rab7 protein disrupt cellular function in vertebrate sensory neurons. Moreover, our findings suggest that defects in axon development may be a previously unrecognized component of CMT2b disease.

Valproic acid enhances neuronal differentiation of sympathoadrenal progenitor cells

The antiepileptic drug valproic acid (VPA) has been shown to influence the neural differentiation and neurite outgrowth of neural stem cells. Sympathoadrenal progenitor cells share properties with neural stem cells and are considered a potential cell source in the treatment of neurodegenerative diseases. The present study therefore aims at modulating the neural differentiation potential of these cells by treatment with the histone deacetylase inhibitor VPA. We studied the epigenetic effects of VPA in two culture conditions: suspension conditions aimed to expand adrenomedullary sympathoadrenal progenitors within free-floating chromospheres and adherent cell cultures optimized to derive neurons. Treatment of chromospheres with VPA may launch neuronal differentiation mechanisms and improve their neurogenic potential upon transplantation. However, also transplantation of differentiated functional neurons could be beneficial. Treating chromospheres for 7 days with clinically relevant concentrations of VPA (2 mm) revealed a decrease of neural progenitor markers Nestin, Notch2 and Sox10. Furthermore, VPA initiated catecholaminergic neuronal differentiation indicated by upregulation of the neuronal marker β-III-tubulin, the dopaminergic transcription factor Pitx3 and the catecholaminergic enzymes TH and GTPCH. In adherent neural differentiation conditions, VPA treatment improved the differentiation of sympathoadrenal progenitor cells into catecholaminergic neurons with significantly elevated levels of nor- and epinephrine. In conclusion, similar to neural stem cells, VPA launches differentiation mechanisms in sympathoadrenal progenitor cells that result in increased generation of functional neurons. Thus, data from this study will be relevant to the potential use of chromaffin progenitors in transplantation therapies of neurodegenerative diseases.

Involvement of the Tyro3 receptor and its intracellular partner Fyn signaling in Schwann cell myelination

During early development of the peripheral nervous system, Schwann cell precursors proliferate, migrate, and differentiate into premyelinating Schwann cells. After birth, Schwann cells envelop neuronal axons with myelin sheaths. Although some molecular mechanisms underlying myelination by Schwann cells have been identified, the whole picture remains unclear. Here we show that signaling through Tyro3 receptor tyrosine kinase and its binding partner, Fyn nonreceptor cytoplasmic tyrosine kinase, is involved in myelination by Schwann cells. Impaired formation of myelin segments is observed in Schwann cell neuronal cultures established from Tyro3-knockout mouse dorsal root ganglia (DRG). Indeed, Tyro3-knockout mice exhibit reduced myelin thickness. By affinity chromatography, Fyn was identified as the binding partner of the Tyro3 intracellular domain, and activity of Fyn is down-regulated in Tyro3-knockout mice, suggesting that Tyro3, acting through Fyn, regulates myelination. Ablating Fyn in mice results in reduced myelin thickness. Decreased myelin formation is observed in cultures established from Fyn-knockout mouse DRG. Furthermore, decreased kinase activity levels and altered expression of myelination-associated transcription factors are observed in these knockout mice. These results suggest the involvement of Tyro3 receptor and its binding partner Fyn in Schwann cell myelination. This constitutes a newly recognized receptor-linked signaling mechanism that can control Schwann cell myelination.

Arf6 guanine nucleotide exchange factor cytohesin-2 binds to CCDC120 and is transported along neurites to mediate neurite growth

The mechanism of neurite growth is complicated, involving continuous cytoskeletal rearrangement and vesicular trafficking. Cytohesin-2 is a guanine nucleotide exchange factor for Arf6, an Arf family molecular switch protein, controlling cell morphological changes such as neuritogenesis. Here, we show that cytohesin-2 binds to a protein with a previously unknown function, CCDC120, which contains three coiled-coil domains, and is transported along neurites in differentiating N1E-115 cells. Transfection of the small interfering RNA (siRNA) specific for CCDC120 into cells inhibits neurite growth and Arf6 activation. When neurites start to extend, vesicles containing CCDC120 and cytohesin-2 are transported in an anterograde manner rather than a retrograde one. As neurites continue extension, anterograde vesicle transport decreases. CCDC120 knockdown inhibits cytohesin-2 localization into vesicles containing CCDC120 and diffuses cytohesin-2 in cytoplasmic regions, illustrating that CCDC120 determines cytohesin-2 localization in growing neurites. Reintroduction of the wild type CCDC120 construct into cells transfected with CCDC120 siRNA reverses blunted neurite growth and Arf6 activity, whereas the cytohesin-2-binding CC1 region-deficient CCDC120 construct does not. Thus, cytohesin-2 is transported along neurites by vesicles containing CCDC120, and it mediates neurite growth. These results suggest a mechanism by which guanine nucleotide exchange factor for Arf6 is transported to mediate neurite growth.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Human Rab7 mutation mimics features of Charcot-Marie-Tooth neuropathy type 2B in Drosophila

Charcot-Marie-Tooth disease type 2B (CMT2B) is an inherited axonal peripheral neuropathy. It is characterised by prominent sensory loss, often complicated by severe ulcero-mutilations of toes or feet, and variable motor involvement. Missense mutations in RAB7A, the gene encoding the small GTPase Rab7, cause CMT2B and increase Rab7 activity. Rab7 is ubiquitously expressed and is involved in degradation through the lysosomal pathway. In the neurons, Rab7 plays a role in the long-range retrograde transport of signalling endosomes in the axons. Here we developed the first animal model of CMT2B, modelling one of the mutations (L129F) in Drosophila melanogaster. Behavioural assays show that this model recapitulates several hallmarks of the human disease. Upon expression of mutant Rab7 in the sensory neurons, larvae present with a reduction of temperature and pain perception. Furthermore, the larvae exhibit a crawling defect when the mutant protein is expressed in the motor neurons. Analysis of axonal transport of Rab7 positive vesicles in sensory neurons of Drosophila larvae and in neurites of mammalian neuroblastoma cells demonstrates that mutant vesicles pause less than their wild-type counterparts. This latter finding indicates that alterations in vesicle transport might contribute to the pathomechanism of CMT2B.

Charcot-Marie-Tooth 2B mutations in rab7 cause dosage-dependent neurodegeneration due to partial loss of function

The small GTPase Rab7 is a key regulator of endosomal maturation in eukaryotic cells. Mutations in rab7 are thought to cause the dominant neuropathy Charcot-Marie-Tooth 2B (CMT2B) by a gain-of-function mechanism. Here we show that loss of rab7, but not overexpression of rab7 CMT2B mutants, causes adult-onset neurodegeneration in a Drosophila model. All CMT2B mutant proteins retain 10–50% function based on quantitative imaging, electrophysiology, and rescue experiments in sensory and motor neurons in vivo. Consequently, expression of CMT2B mutants at levels between 0.5 and 10-fold their endogenous levels fully rescues the neuropathy-like phenotypes of the rab7 mutant. Live imaging reveals that CMT2B proteins are inefficiently recruited to endosomes, but do not impair endosomal maturation. These findings are not consistent with a gain-of-function mechanism. Instead, they indicate a dosage-dependent sensitivity of neurons to rab7-dependent degradation. Our results suggest a therapeutic approach opposite to the currently proposed reduction of mutant protein function.

DOI:http://dx.doi.org/10.7554/eLife.01064.001

Charcot-Marie-Tooth disease is an inherited disorder of the nervous system with symptoms that typically begin in adolescence or early adulthood. The sensory and motor nerves gradually degenerate, causing muscles to waste away and leading to the loss of touch sensation across the body. One subtype of the disease—Charcot-Marie-Tooth 2B—is caused by mutations in a gene called rab7, which codes for a protein that helps to regulate the breakdown of waste proteins inside cells.

Charcot-Marie-Tooth 2B is described as a genetically dominant disorder because all patients have one wild type copy and one mutant copy of the rab7 gene. Overexpression of the mutant gene in cells grown in culture alters many of the signaling pathways inside the cells, but it is unclear whether these alterations cause the pathology seen in the disease.

Now, Cherry et al. have obtained new insights into the genetics of Charcot-Marie-Tooth 2B by creating the first animal model of the disorder. Fruit flies that did not have the rab7 gene in the light-sensitive sensory neurons in their eyes were used to compare normal and mutant cells. While the two cell types were initially similar, the mutant cells gradually degenerated in the adult animal. By contrast, cells that overexpressed a mutant form of the rab7 gene continued to function normally throughout adulthood. Moreover, when mutant Rab7 proteins were introduced into the cells that lacked the rab7 gene, the proteins restored the cells’ sensitivity to light. These results suggest that mutant Rab7 proteins do not cause degeneration; instead, it is the loss of normal Rab7 function that causes problems.

At present, most research into treatment is aimed at finding ways to reduce the activity of mutant Rab7 proteins. However, the work of Cherry et al. suggests that increasing the activity of normal Rab7 proteins—or increasing the activity of alternative pathways that degrade waste proteins—may help to restore nerve function in this, and possibly other, neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.01064.002

Valproic Acid Regulates α-Synuclein Expression through JNK Pathway in Rat Primary Astrocytes

Although the role of α-synuclein aggregation on Parkinson's disease is relatively well known, the physiological role and the regulatory mechanism governing the expression of α-synuclein are unclear yet. We recently reported that α-synuclein is expressed and secreted from cultured astrocytes. In this study, we investigated the effect of valproic acid (VPA), which has been suggested to provide neuroprotection by increasing α-synuclein in neuron, on α-synuclein expression in rat primary astrocytes. VPA concentrationdependently increased the protein expression level of α-synuclein in cultured rat primary astrocytes with concomitant increase in mRNA expression level. Likewise, the level of secreted α-synuclein was also increased by VPA. VPA increased the phosphorylation of Erk1/2 and JNK and pretreatment of a JNK inhibitor SP600125 prevented the VPA-induced increase in α-synuclein. Whether the increased α-synuclein in astrocytes is involved in the reported neuroprotective effects of VPA awaits further investigation.

Defective axonal transport of Rab7 GTPase results in dysregulated trophic signaling

Retrograde trophic signaling of nerve growth factor (NGF) supports neuronal survival and differentiation. Dysregulated trophic signaling could lead to various neurological disorders. Charcot-Marie-Tooth type 2B (CMT2B) is one of the most common inherited peripheral neuropathies characterized by severe terminal axonal loss. Genetic analysis of human CMT2B patients has revealed four missense point mutations in Rab7, a small GTPase that regulates late endosomal/lysosomal pathways, but the exact pathological mechanism remains poorly understood. Here, we show that these Rab7 mutants dysregulated axonal transport and diminished the retrograde signaling of NGF and its TrkA receptor. We found that all CMT2B Rab7 mutants were transported significantly faster than Rab7(wt) in the anterograde direction, accompanied with an increased percentile of anterograde Rab7-vesicles within axons of rat E15.5 dorsal root ganglion (DRG) neurons. In PC12M cells, the CMT2B Rab7 mutants drastically reduced the level of surface TrkA and NGF binding, presumably by premature degradation of TrkA. On the other hand, siRNA knock-down of endogenous Rab7 led to the appearance of large TrkA puncta in enlarged Rab5-early endosomes within the cytoplasm, suggesting delayed TrkA degradation. We also show that CMT2B Rab7 mutants markedly impaired NGF-induced Erk1/2 activation and differentiation in PC12M cells. Further analysis revealed that CMT2B Rab7 mutants caused axonal degeneration in rat E15.5 DRG neurons. We propose that Rab7 mutants induce premature degradation of retrograde NGF-TrkA trophic signaling, which may potentially contribute to the CMT2B disease.

Regulation of neurite growth by inorganic pyrophosphatase 1 via JNK dephosphorylation

Neural cell differentiation during development is controlled by multiple signaling pathways, in which protein phosphorylation and dephosphorylation play an important role. In this study, we examined the role of pyrophosphatase1 (PPA1) in neuronal differentiation using the loss and gain of function analysis. Neuronal differentiation induced by external factors was studied using a mouse neuroblastoma cell line (N1E115). The neuronal like differentiation in N1E115 cells was determined by morphological analysis based on neurite growth length. In order to analyze the loss of the PPA1 function in N1E115, si-RNA specifically targeting PPA1 was generated. To study the effect of PPA1 overexpression, an adenoviral gene vector containing the PPA1 gene was utilized to infect N1E115 cells. To address the need for pyrophosphatase activity in PPA1, D117A PPA1, which has inactive pyrophosphatase, was overexpressed in N1E115 cells. We used valproic acid (VPA) as a neuronal differentiator to examine the effect of PPA1 in actively differentiated N1E115 cells. Si-PPA1 treatment reduced the PPA1 protein level and led to enhanced neurite growth in N1E115 cells. In contrast, PPA1 overexpression suppressed neurite growth in N1E115 cells treated with VPA, whereas this effect was abolished in D117A PPA1. PPA1 knockdown enhanced the JNK phosphorylation level, and PPA1 overexpression suppressed it in N1E115 cells. It seems that recombinant PPA1 can dephosphorylate JNK while no alteration of JNK phosphorylation level was seen after treatment with recombinant PPA1 D117A. Enhanced neurite growth by PPA1 knockdown was also observed in rat cortical neurons. Thus, PPA1 may play a role in neuronal differentiation via JNK dephosphorylation.

Vimentin phosphorylation and assembly are regulated by the small GTPase Rab7a

Intermediate filaments are cytoskeletal elements important for cell architecture. Recently it has been discovered that intermediate filaments are highly dynamic and that they are fundamental for organelle positioning, transport and function thus being an important regulatory component of membrane traffic. We have identified, using the yeast two-hybrid system, vimentin, a class III intermediate filament protein, as a Rab7a interacting protein. Rab7a is a member of the Rab family of small GTPases and it controls vesicular membrane traffic to late endosomes and lysosomes. In addition, Rab7a is important for maturation of phagosomes and autophagic vacuoles. We confirmed the interaction in HeLa cells by co-immunoprecipitation and pull-down experiments, and established that the interaction is direct using bacterially expressed recombinant proteins. Immunofluorescence analysis on HeLa cells indicate that Rab7a-positive vesicles sometimes overlap with vimentin filaments. Overexpression of Rab7a causes an increase in vimentin phosphorylation at different sites and causes redistribution of vimentin in the soluble fraction. Consistently, Rab7a silencing causes an increase of vimentin present in the insoluble fraction (assembled). Also, expression of Charcot–Marie–Tooth 2B-causing Rab7a mutant proteins induces vimentin phosphorylation and increases the amount of vimentin in the soluble fraction. Thus, modulation of expression levels of Rab7a wt or expression of Rab7a mutant proteins changes the assembly of vimentin and its phosphorylation state indicating that Rab7a is important for the regulation of vimentin function.

► We searched for new Rab7a interacting proteins and we found vimentin. ► We demonstrated that Rab7a interacts directly with vimentin. ► Rab7a influences vimentin's phosphorylation and soluble/insoluble ratio. ► Rab7a regulates vimentin organization and function.

Charcot-Marie-Tooth type 2B disease-causing RAB7A mutant proteins show altered interaction with the neuronal intermediate filament peripherin

Charcot-Marie-Tooth type 2B (CMT2B) is a peripheral ulcero-mutilating neuropathy caused by four missense mutations in the rab7a gene. CMT2B is clinically characterized by prominent sensory loss, distal muscle weakness leading to muscle atrophy, high frequency of foot ulcers and infections that often results in toe amputations. RAB7A is a ubiquitous small GTPase, which controls transport to late endocytic compartments. Although the biochemical and functional properties of disease-causing RAB7A mutant proteins have been investigated, it is not yet clear how the disease originates. To understand how mutations in a ubiquitous protein specifically affect peripheral neurons, we performed a two-hybrid screen using a dorsal root ganglia cDNA library with the purpose of identifying RAB7A interactors specific for these cells. We identified peripherin, an intermediate filament protein expressed primarily in peripheral neurons, as a putative RAB7A interacting protein. The interaction was confirmed by co-immunoprecipitation and pull-down experiments, and established that the interaction is direct using recombinant proteins. Silencing or overexpression of wild type RAB7A changed the soluble/insoluble rate of peripherin indicating that RAB7A is important for peripherin organization and function. In addition, disease-causing RAB7A mutant proteins bind more strongly to peripherin and their expression causes a significant increase in the amount of soluble peripherin. Since peripherin plays a role not only in neurite outgrowth during development but also in axonal regeneration after injury, these data suggest that the altered interaction between disease-causing RAB7A mutants and peripherin could play an important role in CMT2B neuropathy.

Neuroprotective effects of valproic acid against hemin toxicity: possible involvement of the down-regulation of heme oxygenase-1 by regulating ubiquitin-proteasomal pathway

During hemorrhagic stroke induced by intracerebral hemorrhage (ICH), brain injury occurs from the deleterious actions of hemoglobin byproducts; induction of heme oxygenase-1 (HO-1) also plays a critical role in the neurotoxicity in ICH. Valproic acid (VPA), which is a commonly used drug in the treatment of epilepsy, has been reported to have neuroprotective effects against various neuronal insults including ischemic stroke. We investigated the effect of VPA on HO-1-mediated neurotoxicity in an experimental model of ICH. We investigated the effects of VPA on HO-1 protein in primary cortical neurons: (1) the expression levels of HO-1 mRNA and protein measured by RT-PCR and Western blotting; (2) the cell viability and ROS generation by MTT reduction assay and ROS measurement; (3) the signal pathway regulated by VPA using IP-Western blotting; (4) the effects of VPA on hemin-induced cell death by hemin microinjection and immunohistochemistry in vivo. VPA treatment partially blocked cell death induced by hemin, which is released from hemoglobin during ICH, both in rat primary cortical neurons and rat brain. Treatment of VPA significantly decreased the expression of HO-1 protein both in vitro and in vivo. Hemin treatment induced HO-1 protein expression and this was partially blocked by pretreatment with VPA, which might be mediated by increased ubiquitination and degradation of HO-1 via ERK1/2 and JNK activation in primary cortical neurons. Our results indicate that VPA inhibits hemin toxicity by downregulating HO-1 protein expression, and provide a therapeutic strategy to attenuate intracerebral hemorrhagic injury.

Molecular basis of Charcot–Marie–Tooth type 2B disease

CMT2B (Charcot–Marie–Tooth type 2B) disease is an autosomal dominant peripheral neuropathy whose onset is in the second or third decade of life, thus in adolescence or young adulthood. CMT2B is clinically characterized by severe symmetric distal sensory loss, reduced tendon reflexes at ankles, weakness in the lower limbs and muscle atrophy, complicated by ulcerations that often lead to amputations. Four missense mutations in the gene encoding the small GTPase Rab7 cause the CMT2B neuropathy. Rab7 is a ubiquitous protein that regulates transport to late endosomes and lysosomes in the endocytic pathway. In neurons, Rab7 is important for endosomal trafficking and signalling of neurotrophins, and for retrograde axonal transport. Recent data on CMT2B-causing Rab7 mutant proteins show that these proteins exhibit altered koff rates and, as a consequence, they are mainly in the GTP-bound state and bind more strongly to Rab7 effector proteins. Notably, expression of CMT2B-causing Rab7 mutant proteins strongly inhibit neurite outgrowth in several cells lines and alter NGF (nerve growth factor) trafficking and signalling. These data indicate that Rab7 plays an essential role in neuronal cells and that CMT2B-causing Rab7 mutant proteins alter neuronal specific pathways, but do not fully explain why only peripheral neurons are affected in CMT2B. In the present paper, we discuss the current understanding of the molecular and cellular mechanisms underlying CMT2B, and we consider possible hypotheses in order to explain how alterations of Rab7 function lead to CMT2B.

Prenatal exposure to valproic acid increases the neural progenitor cell pool and induces macrocephaly in rat brain via a mechanism involving the GSK-3β/β-catenin pathway

Autism is a spectrum of neurodevelopmental disorders characterized by social isolation and lack of interaction. Anatomically, autism patients often show macrocephaly and high neuronal density. To investigate the mechanism underlying the higher neuronal populations seen in ASD, we subcutaneously injected VPA (400 mg/kg) into pregnant Sprague-Dawley rats on E12, an animal model often used in ASD study. Alternatively, cultured rat neural progenitor cells were treated with VPA. Until E18, VPA induced NPC proliferation and delayed neurogenesis in fetal brain, but the subsequent differentiation of NPCs to neurons increased brain neuronal density afterward. Similar findings were observed with NPCs treated with VPA in vitro. At a molecular level, VPA enhanced Wnt1 expression and activated the GSK-3β/β-catenin pathway. Furthermore, inhibition of this pathway attenuated the effects of VPA. The findings of this study suggest that an altered developmental process underlies the macrocephaly and abnormal brain structure observed in the autistic brain.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Mechanisms of disease in hereditary sensory and autonomic neuropathies

Hereditary sensory and autonomic neuropathies (HSANs) are a clinically and genetically heterogeneous group of disorders of the PNS. Progressive degeneration, predominantly of sensory and autonomic neurons, is the main pathological feature in patients with HSAN, and causes prominent sensory loss and ulcerative mutilations in combination with variable autonomic and motor disturbances. Advances in molecular genetics have enabled identification of disease-causing mutations in 12 genes, and studies on the functional effects of these mutations are underway. Although some of the affected proteins--such as nerve growth factor and its receptor--have obvious nerve-specific roles, others are ubiquitously expressed proteins that are involved in sphingolipid metabolism, vesicular transport, transcription regulation and structural integrity. An important challenge in the future will be to understand the common molecular pathways that result in HSANs. Unraveling the mechanisms that underlie sensory and autonomic neurodegeneration could assist in identifying targets for future therapeutic strategies in patients with HSAN. This Review highlights key advances in the understanding of HSANs, including insights into the molecular mechanisms of disease, derived from genetic studies of patients with these disorders.

Valproic acid improves locomotion in vivo after SCI and axonal growth of neurons in vitro

Previous studies have found that valproic acid (VPA), a histone deacetylases (HDAC) inhibitor, improves outcomes in a rat model of spinal cord injury (SCI). The study here aimed to further illuminate the neuroprotective effects of VPA against SCI, both in vivo and in vitro. First, spinal cord injury was performed in rats using NYU impactor. Delayed VPA injection (8 h following SCI) significantly accelerated locomotor recovery. VPA therapy also suppressed SCI-induced hypoacetylation of histone and promoted expressions of BDNF and GDNF. Next, the influence of VPA on axonal growth inhibited by a myelin protein was tested. Neurons from embryonic spinal cord or hippocampus were cultured on plates coated with Nogo-A peptide, and escalating concentrations of VPA were added into the cultures. VPA treatment, in a concentration dependent manner, allowed neurons to overcome Nogo-A inhibition of neurite outgrowth. Meanwhile, VPA exposure increased the level of histone acetylation and expression of BDNF in spinal neurons. Cumulatively, these findings indicate that VPA is possibly a promising medication and deserves translational trials for spinal cord injury.

Rab GTPases as regulators of endocytosis, targets of disease and therapeutic opportunities

Rab GTPases are well-recognized targets in human disease, although are underexplored therapeutically. Elucidation of how mutant or dysregulated Rab GTPases and accessory proteins contribute to organ specific and systemic disease remains an area of intensive study and an essential foundation for effective drug targeting. Mutation of Rab GTPases or associated regulatory proteins causes numerous human genetic diseases. Cancer, neurodegeneration and diabetes represent examples of acquired human diseases resulting from the up- or downregulation or aberrant function of Rab GTPases. The broad range of physiologic processes and organ systems affected by altered Rab GTPase activity is based on pivotal roles in responding to cell signaling and metabolic demand through the coordinated regulation of membrane trafficking. The Rab-regulated processes of cargo sorting, cytoskeletal translocation of vesicles and appropriate fusion with the target membranes control cell metabolism, viability, growth and differentiation. In this review, we focus on Rab GTPase roles in endocytosis to illustrate normal function and the consequences of dysregulation resulting in human disease. Selected examples are designed to illustrate how defects in Rab GTPase cascades alter endocytic trafficking that underlie neurologic, lipid storage, and metabolic bone disorders as well as cancer. Perspectives on potential therapeutic modulation of GTPase activity through small molecule interventions are provided.

A role for Rab7 in the movement of secretory granules in cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) are potent killers of virally infected and tumorigenic cells. Upon recognition of target cells, CTL undergo polarized secretion of secretory lysosomes at the immunological synapse (IS) that forms between CTL and target. However, the molecular machinery involved in the polarization of secretory lysosomes is still largely uncharacterized. In this paper, we investigated the role of Rab7 in the polarization of secretory lysosomes. We show that silencing of Rab7 by RNA interference reduces the ability of CTL to kill targets. GTP-bound Rab7 and Rab interacting lysosomal protein, RILP, interact and both localize to secretory lysosomes in CTL. Over-expression of RILP recruits dynein to the membranes of secretory lysosomes and triggers their movement toward the centrosome. Together, these results suggest that Rab7 may play a role in secretory lysosome movement toward the centrosome by interacting with RILP to recruit the minus-end motor, dynein.