The RAB39B p.G192R mutation causes X-linked dominant Parkinson's disease

Impaired Sonic Hedgehog Responsiveness of Induced Pluripotent Stem Cell-Derived Floor Plate Cells Carrying the LRRK2-I1371V Mutation Contributes to the Ontogenic Origin of Lower Dopaminergic Neuron Yield

Lower population of dopaminergic (DA) neurons is known to increase susceptibility to Parkinson's disease (PD), and our earlier study showed a lower yield of DA neurons in Leucine-Rich Repeat Kinase Isoleucine 1371 Valine (LRRK2-I1371V) mutation-carrying PD patient-derived induced Pluripotent Stem Cells (iPSCs). Although the role of Sonic Hedgehog (SHH) in DA neurogenesis of floor plate cells (FPCs) is known, the effect of LRRK2 mutations on SHH responsiveness of FPCs impacting DA neuronal yield has not been studied. We investigated SHH responsiveness of FPCs derived from LRRK2-I1371V PD patient iPSCs with regard to the expression of SHH receptors Patched1 (Ptch1) and Smoothened (Smo), in conjunction with nuclear Gli1 (glioma-associated oncogene 1) expression, intracellular Ca2+ rise, and cytosolic cyclic adenosine monophosphate (cAMP) levels upon SHH induction. In addition, we examined the mechanistic link with LRRK2-I1371V gain-of-function by assessing membrane fluidity and Rab8A and Rab10 phosphorylation in SH-SY5Y cells and healthy control (HC) FPCs overexpressing LRRK2-I1371V as well as FPCs. Although total expression of Ptch1 and Smo was comparable, receptor expression on cell surface was significantly lower in LRRK2-I1371V FPCs than in HC FPCs, with distinctly lower nuclear expression of the downstream transcription factor Gli1. HC-FPCs transfected with LRRK2-I1371V exhibited a similarly reduced cell surface expression of Ptch1 and Smo. Intracellular Ca2+ response was significantly lower with corresponding elevated cAMP levels in LRRK2-I1371V FPCs compared with HC FPCs upon SHH stimulation. The LRRK2-I1371V mutant FPCs and LRRK2-I1371V-transfected SH-SY5Y and HC FPCs too exhibited higher autophosphorylation of phospho LRRK2 (pLRRK2) serine1292 and serine935, as well as substrate phosphorylation of Rab8A and Rab10. Concurrent increase in membrane fluidity, accompanied by a decrease in membrane cholesterol, and lower expression of lipid raft marker caveolin 1 were also observed in them. These findings suggest that impaired SHH responsiveness of LRRK2-I1371V PD FPCs indeed leads to lower yield of DA neurons during ontogeny. Reduced cell surface expression of SHH receptors is influenced by alteration in membrane fluidity owing to the increased substrate phosphorylation of Rab8A and reduced membrane protein trafficking due to pRab10, both results of the LRRK2-I1371V mutation.

Novel RAB39B loss-of-function mutation in patient with typical early-onset Parkinson's disease

RAB39B mutations have been identified in X-linked developmental delays. Recently, RAB39B mutations were identified in males with early-onset parkinsonism and intellectual disability. A novel loss-of-function RAB39B mutation was found in a female patient with typical early-onset Parkinson's disease (EOPD). RAB39B mutations may cause EOPD, potentially due to a-synuclein homeostasis disruption.

X-Linked Levodopa-Responsive Parkinsonism-Epilepsy Syndrome: A Novel PGK1 Mutation and Literature Review

BACKGROUND

Genetic underpinnings in Parkinson's disease (PD) and parkinsonian syndromes are challenging, and recent discoveries regarding their genetic pathways have led to potential gene-specific treatment trials.

CASES

We report 3 X-linked levodopa (l-dopa)-responsive parkinsonism-epilepsy syndrome cases due to a hemizygous variant in the phosphoglycerate kinase 1 (PGK1) gene. The likely pathogenic variant NM_000291.4 (PGK1):c.950G > A;p.(Gly317Asp) was identified in a hemizygous state.

LITERATURE REVIEW

Only 8 previous cases have linked this phenotype to PGK1, a gene more commonly associated with hemolytic anemia and myopathy. The unusual association of epilepsy, psychiatric symptoms, action tremor, limb dystonia, cognitive symptoms, and l-dopa-responsive parkinsonism must draw attention to PGK1 mutations, especially because this gene is absent from most commercial hereditary parkinsonism panels.

CONCLUSIONS

This report aims to shed light on an overlooked gene that causes hereditary parkinsonian syndromes. Further research regarding genetic pathways in PD may provide a better understanding of its pathophysiology and open possibilities for new disease-modifying trials, such as SNCA, LRRK2, PRKN, PINK1, and DJ-1 genes.

Investigating the role of common cis-regulatory variants in modifying penetrance of putatively damaging, inherited variants in severe neurodevelopmental disorders

Recent work has revealed an important role for rare, incompletely penetrant inherited coding variants in neurodevelopmental disorders (NDDs). Additionally, we have previously shown that common variants contribute to risk for rare NDDs. Here, we investigate whether common variants exert their effects by modifying gene expression, using multi-cis-expression quantitative trait loci (cis-eQTL) prediction models. We first performed a transcriptome-wide association study for NDDs using 6987 probands from the Deciphering Developmental Disorders (DDD) study and 9720 controls, and found one gene, RAB2A, that passed multiple testing correction (p = 6.7 × 10-7). We then investigated whether cis-eQTLs modify the penetrance of putatively damaging, rare coding variants inherited by NDD probands from their unaffected parents in a set of 1700 trios. We found no evidence that unaffected parents transmitting putatively damaging coding variants had higher genetically-predicted expression of the variant-harboring gene than their child. In probands carrying putatively damaging variants in constrained genes, the genetically-predicted expression of these genes in blood was lower than in controls (p = 2.7 × 10-3). However, results for proband-control comparisons were inconsistent across different sets of genes, variant filters and tissues. We find limited evidence that common cis-eQTLs modify penetrance of rare coding variants in a large cohort of NDD probands.

Expression of human Ras-related protein Rab39B variant T168K in Caenorhabditis elegans leads to motor dysfunction and dopaminergic neuron degeneration

Human RAB39B gene is related to familial early-onset Parkinson disease. In early adulthood, men with the RAB39B c.503C > A (Thr168Lys, p. T168K) mutation develop typical tremor, bradykinesia, and alpha-synuclein accumulation. We investigated the pathological mechanism of RAB39B T168K in a Caenorhabditis elegans model. In early adult C. elegans, RAB39B T168K led to dopaminergic neuron degeneration that presented as disrupted dendrites and blunt neuronal cells. Abnormal dopamine secretion was inferred from a decline in motor function and a positive basal slowing phenotype. Dopamine-associated tests confirmed that synthesis and recycling of dopamine were normal. The RAB39B T168K mutation might impair dopamine vesicular transmission from the presynaptic membrane to the synaptic gap in dopaminergic neurons. The release-dependent feedback mechanism in neurotransmitters regulates the balance of receptor activities. Protein-protein interactions network analysis revealed that RAB39B may also function in lysosomal degradation and autophagy. Impaired disposal of misfolded α-synuclein eventually leads to protein aggregation. Thus, like other members of the Rab family, RAB39B may be involved in vesicular transport associated with dopamine secretion and α-synuclein clearance.

Novel RAB39B variant associated intellectual disability and levodopa-responsive young-onset parkinsonism

We report a 37-year-old Caucasian male with history of developmental delay, childhood onset Intellectual Disability (ID) and attention deficit hyperactivity disorder (ADHD) who presented at the age of 34 with tremor-dominant parkinsonism. Next Generation Sequencing (NGS) revealed pathogenic hemizygous sequence variant, c.200G > T, in the RAB39B gene. This report expands the number of described individuals with young onset PD associated with RAB39B mutation.

Astrocytes Differentiated from LRRK2-I1371V Parkinson's-Disease-Induced Pluripotent Stem Cells Exhibit Similar Yield but Cell-Intrinsic Dysfunction in Glutamate Uptake and Metabolism, ATP Generation, and Nrf2-Mediated Glutathione Machinery

Owing to the presence of multiple enzymatic domains, LRRK2 has been associated with a diverse set of cellular functions and signaling pathways. It also has several pathological mutant-variants, and their incidences show ethnicity biases and drug-response differences with expression in dopaminergic-neurons and astrocytes. Here, we aimed to assess the cell-intrinsic effect of the LRRK2-I1371V mutant variant, prevalent in East Asian populations, on astrocyte yield and biology, involving Nrf2-mediated glutathione machinery, glutamate uptake and metabolism, and ATP generation in astrocytes derived from LRRK2-I1371V PD patient iPSCs and independently confirmed in LRRK2-I1371V-overexpressed U87 cells. Astrocyte yield (GFAP-immunopositive) was comparable between LRRK2-I1371V and healthy control (HC) populations; however, the astrocytic capability to mitigate oxidative stress in terms of glutathione content was significantly reduced in the mutant astrocytes, along with a reduction in the gene expression of the enzymes involved in glutathione machinery and nuclear factor erythroid 2-related factor 2 (Nrf2) expression. Simultaneously, a significant decrease in glutamate uptake was observed in LRRK2-I1371V astrocytes, with lower gene expression of glutamate transporters SLC1A2 and SLC1A3. The reduction in the protein expression of SLC1A2 was also directly confirmed. Enzymes catalyzing the generation of γ glutamyl cysteine (precursor of glutathione) from glutamate and the metabolism of glutamate to enter the Krebs cycle (α-ketoglutaric acid) were impaired, with significantly lower ATP generation in LRRK2-I1371V astrocytes. De novo glutamine synthesis via the conversion of glutamate to glutamine was also affected, indicating glutamate metabolism disorder. Our data demonstrate for the first time that the mutation in the LRRK2-I1371V allele causes significant astrocytic dysfunction with respect to Nrf2-mediated antioxidant machinery, AT -generation, and glutamate metabolism, even with comparable astrocyte yields.

Sex and Brain: The Role of Sex Chromosomes and Hormones in Brain Development and Parkinson's Disease

Sex hormones and genes on the sex chromosomes are not only key factors in the regulation of sexual differentiation and reproduction but they are also deeply involved in brain homeostasis. Their action is crucial for the development of the brain, which presents different characteristics depending on the sex of individuals. The role of these players in the brain is fundamental in the maintenance of brain function during adulthood as well, thus being important also with respect to age-related neurodegenerative diseases. In this review, we explore the role of biological sex in the development of the brain and analyze its impact on the predisposition toward and the progression of neurodegenerative diseases. In particular, we focus on Parkinson's disease, a neurodegenerative disorder that has a higher incidence in the male population. We report how sex hormones and genes encoded by the sex chromosomes could protect from the disease or alternatively predispose toward its development. We finally underline the importance of considering sex when studying brain physiology and pathology in cellular and animal models in order to better understand disease etiology and develop novel tailored therapeutic strategies.

Deficiency of RAB39B Activates ER Stress-Induced Pro-apoptotic Pathway and Causes Mitochondrial Dysfunction and Oxidative Stress in Dopaminergic Neurons by Impairing Autophagy and Upregulating α-Synuclein

Deletion and missense or nonsense mutation of RAB39B gene cause familial Parkinson's disease (PD). We hypothesized that deletion and mutation of RAB39B gene induce degeneration of dopaminergic neurons by decreasing protein level of functional RAB39B and causing RAB39B deficiency. Cellular model of deletion or mutation of RAB39B gene-induced PD was prepared by knocking down endogenous RAB39B in human SH-SY5Y dopaminergic cells. Transfection of shRNA-induced 90% reduction in RAB39B level significantly decreased viability of SH-SY5Y dopaminergic neurons. Deficiency of RAB39B caused impairment of macroautophagy/autophagy, which led to increased protein levels of α-synuclein and phospho-α-synuclein^Ser129 within endoplasmic reticulum (ER) and mitochondria. RAB39B deficiency-induced increase of ER α-synuclein and phospho-α-synuclein^Ser129 caused activation of ER stress, unfolded protein response, and ER stress-induced pro-apoptotic cascade. Deficiency of RAB39B-induced increase of mitochondrial α-synuclein decreased mitochondrial membrane potential and increased mitochondrial superoxide. RAB39B deficiency-induced activation of ER stress pro-apoptotic pathway, mitochondrial dysfunction, and oxidative stress caused apoptotic death of SH-SY5Y dopaminergic cells by activating mitochondrial apoptotic cascade. In contrast to neuroprotective effect of wild-type RAB39B, PD mutant (T168K), (W186X), or (G192R) RAB39B did not prevent tunicamycin- or rotenone-induced increase of neurotoxic α-synuclein and activation of pro-apoptotic pathway. Our results suggest that RAB39B is required for survival and macroautophagy function of dopaminergic neurons and that deletion or PD mutation of RAB39B gene-induced RAB39B deficiency induces apoptotic death of dopaminergic neurons via impairing autophagy function and upregulating α-synuclein.

Genetic Evidence for Endolysosomal Dysfunction in Parkinson’s Disease: A Critical Overview

Parkinson’s disease (PD) is the second most common neurodegenerative disorder in the aging population, and no disease-modifying therapy has been approved to date. The pathogenesis of PD has been related to many dysfunctional cellular mechanisms, however, most of its monogenic forms are caused by pathogenic variants in genes involved in endolysosomal function (LRRK2, VPS35, VPS13C, and ATP13A2) and synaptic vesicle trafficking (SNCA, RAB39B, SYNJ1, and DNAJC6). Moreover, an extensive search for PD risk variants revealed strong risk variants in several lysosomal genes (e.g., GBA1, SMPD1, TMEM175, and SCARB2) highlighting the key role of lysosomal dysfunction in PD pathogenesis. Furthermore, large genetic studies revealed that PD status is associated with the overall “lysosomal genetic burden”, namely the cumulative effect of strong and weak risk variants affecting lysosomal genes. In this context, understanding the complex mechanisms of impaired vesicular trafficking and dysfunctional endolysosomes in dopaminergic neurons of PD patients is a fundamental step to identifying precise therapeutic targets and developing effective drugs to modify the neurodegenerative process in PD.

Loss of RAB39B does not alter MPTP-induced Parkinson's disease-like phenotypes in mice

Parkinson's disease (PD) is a common neurodegenerative movement disorder with undetermined etiology. A major pathological hallmark of PD is the progressive degeneration of dopaminergic neurons in the substantia nigra. Loss-of-function mutations in the RAB39B gene, which encodes a neuronal-specific small GTPase RAB39B, have been associated with X-linked intellectual disability and pathologically confirmed early-onset PD in multiple families. However, the role of RAB39B in PD pathogenesis remains elusive. In this study, we treated Rab39b knock-out (KO) mice with MPTP to explore whether RAB39B deficiency could alter MPTP-induced behavioral impairments and dopaminergic neuron degeneration. Surprisingly, we found that MPTP treatment impaired motor activity and led to loss of tyrosine hydroxylase-positive dopaminergic neurons and gliosis in both WT and Rab39b KO mice. However, RAB39B deficiency did not alter MPTP-induced impairments. These results suggest that RAB39B deficiency does not contribute to PD-like phenotypes through compromising dopaminergic neurons in mice; and its role in PD requires further scrutiny.

Homozygous CADPS2 Mutations Cause Neurodegenerative Disease with Lewy Body-like Pathology in Parrots

BACKGROUND

Several genetic models that recapitulate neurodegenerative features of Parkinson's disease (PD) exist, which have been largely based on genes discovered in monogenic PD families. However, spontaneous genetic mutations have not been linked to the pathological hallmarks of PD in non-human vertebrates.

OBJECTIVE

To describe the genetic and pathological findings of three Yellow-crowned parrot (Amazona ochrocepahala) siblings with a severe and rapidly progressive neurological phenotype.

METHODS

The phenotype of the three parrots included severe ataxia, rigidity, and tremor, while their parents were phenotypically normal. Tests to identify avian viral infections and brain imaging studies were all negative. Due to their severe impairment, they were all euthanized at age 3 months and their brains underwent neuropathological examination and proteasome activity assays. Whole genome sequencing (WGS) was performed on the three affected parrots and their parents.

RESULTS

The brains of affected parrots exhibited neuronal loss, spongiosis, and widespread Lewy body-like inclusions in many regions including the midbrain, basal ganglia, and neocortex. Proteasome activity was significantly reduced in these animals compared to a control (P < 0.05). WGS identified a single homozygous missense mutation (p.V559L) in a highly conserved amino acid within the pleckstrin homology (PH) domain of the calcium-dependent secretion activator 2 (CADPS2) gene.

CONCLUSIONS

Our data suggest that a homozygous mutation in the CADPS2 gene causes a severe neurodegenerative phenotype with Lewy body-like pathology in parrots. Although CADPS2 variants have not been reported to cause PD, further investigation of the gene might provide important insights into the pathophysiology of Lewy body disorders. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Dopaminergic Neurons Differentiated from LRRK2 I1371V-Induced Pluripotent Stem Cells Display a Lower Yield, α-Synuclein Pathology, and Functional Impairment

Being a large multidomain protein, LRRK2 has several confirmed pathological mutant variants for PD, and the incidence of these variants shows ethnicity biases. I1371V, a mutation in the GTPase domain, has been reported in East-Asian populations, but there are no studies reported on dopaminergic (DA) neurons differentiated from this variant. The aim here was to assess the yield, function, and α-synuclein pathology of DA neurons differentiated from LRRK2 I1371V iPSCs. FACS analysis of neural progenitors (NPs) showed a comparable immunopositive population of cells for neural and glial progenitor markers nestin and S100β; however, NPs from I1371V iPSCs showed lower clonogenic and proliferative capacities than healthy control NPs as determined by the neurosphere assay and Ki67 expression. Floor plate cells obtained from I1371V NPs primed with FGF8 showed distinctly lower immunopositivity for FOXA2 and CLIC5 than healthy control FPCs and similar DOC2B expression. On SHH addition, a similar mature neuronal population was obtained from both groups; however, the yield of TH-immunopositive cells was significantly lower in I1371V, with lower expression of mature DA neuronal markers En1, Nurr1, and DAT. Vesicular dopamine release and intracellular Ca²⁺ response with KCl stimulation were lower in I1371V DA neurons, along with a significantly reduced expression of resting vesicle marker VMAT2. A concurrently lower expression of PSD95/Syn-I immunopositive puncta was observed in I1371V differentiated cells. Further, higher phosphorylation of α-synuclein and aggregation of oligomeric α-synuclein in I1371V DA neurons were observed. Our data demonstrated conclusively for the first time that mutations in the I1371V allele of LRRK2 showed developmental deficit from the FPC stage and generated a lower yield/number of TH-immunopositive neurons with impairment in their function and synapse density along with increased α-synuclein pathology.

The Role of Rab Proteins in Parkinson's Disease Synaptopathy

In patients affected by Parkinson's disease (PD), the most common neurodegenerative movement disorder, the brain is characterized by the loss of dopaminergic neurons in the nigrostriatal system, leading to dyshomeostasis of the basal ganglia network activity that is linked to motility dysfunction. PD mostly arises as an age-associated sporadic disease, but several genetic forms also exist. Compelling evidence supports that synaptic damage and dysfunction characterize the very early phases of either sporadic or genetic forms of PD and that this early PD synaptopathy drives retrograde terminal-to-cell body degeneration, culminating in neuronal loss. The Ras-associated binding protein (Rab) family of small GTPases, which is involved in the maintenance of neuronal vesicular trafficking, synaptic architecture and function in the central nervous system, has recently emerged among the major players in PD synaptopathy. In this manuscript, we provide an overview of the main findings supporting the involvement of Rabs in either sporadic or genetic PD pathophysiology, and we highlight how Rab alterations participate in the onset of early synaptic damage and dysfunction.

Molecular and Functional Interactions of Alpha-Synuclein with Rab3a

Alpha-synuclein (a-Syn) is a presynaptic protein, the misfolding of which is associated with Parkinson’s disease. Rab GTPases are small guanine nucleotide binding proteins that play key roles in vesicle trafficking and have been associated with a-Syn function and dysfunction. a-Syn is enriched on synaptic vesicles, where it has been reported to interact with GTP-bound Rab3a, a master regulator of synaptic vesicle trafficking. a-Syn is known to bind weakly to Rab8a in solution via a positively charged patch, but the physiological implications of such interactions have not been explored. Here, we investigate direct interactions between a-Syn and Rab3a in solution and on lipid membranes using NMR spectroscopy. We find that the C terminus of a-Syn interacts with Rab3a in a manner similar to its previously reported interaction with Rab8a. While weak in solution, we demonstrate that this interaction becomes stronger when the proteins are bound to a membrane surface. The Rab3a binding site for a-Syn is similar to the surface that contacts the Rab3a effector rabphilin-3A, which modulates the enzymatic activity of Rab3a. Accordingly, we show that a-Syn inhibits GTP hydrolysis by Rab3a and that inhibition is more potent on the membrane surface, suggesting that their interaction may be functionally relevant. Finally, we show that phosphorylation of a-Syn residue Ser 129, a modification associated with Parkinson’s disease pathology, enhances its interactions with Rab3a and increases its ability to inhibit Rab3a GTP hydrolysis. These results represent the first observation of a functional role for synuclein-Rab interactions and for a-Syn Ser 129 phosphorylation.

Rab39 and its effector UACA regulate basolateral exosome release from polarized epithelial cells

Exosomes are small extracellular vesicles that originate from the intraluminal vesicles of multivesicular bodies (MVBs). We previously reported that polarized Madin-Darby canine kidney (MDCK) epithelial cells secrete two types of exosomes, apical and basolateral exosomes, from different MVBs. However, how these MVBs are selectively targeted to the apical or basolateral membrane remained unknown. Here, we analyze members of the Rab family small GTPases and show that different sets of Rabs mediate asymmetrical exosome release. Rab27, the best-known regulator of MVB transport for exosome release, is specifically but partially involved in apical exosome release, and Rab37, a close homolog of Rab27, is an additional apical exosome regulator. By contrast, Rab39 functions as a specific regulator of basolateral exosome release. Mechanistically, Rab39 interacts with its effector UACA, and UACA then recruits Lyspersin, a component of BLOC-1-related complex (BORC). Our findings suggest that the Rab39-UACA-BORC complex specifically mediates basolateral exosome release.

Multimodal imaging of a patient with RAB39B mutation

Mutations in RAB39B gene have been linked to intellectual deficiency associated with parkinsonism, also referred as to Waisman syndrome. As it appears to be a very rare cause of Parkinson Disease (PD), with only few cases described in the literature, the typical clinical and radiological features are yet to be determined. In this article, we report and illustrate multimodal brain imaging by computed tomography, magnetic resonance imaging, transcranial ultrasound (US), dopamine transporter single photon emission computed tomography and [¹⁸F]-fluorodeoxyglucose positron emission tomography ([¹⁸F]FDG-PET) in a 37-year-old man with PD features and mild mental retardation harboring a new RAB39B mutation. We then propose a comparison with data previously published regarding neuroimaging in this condition and present a summary of previous imaging reports. If our patient's results partly support previously described radiological features, they also highlight potential new characteristics of this rare syndrome. To the best of our knowledge, [¹⁸F]FDG-PET and transcranial US have never been reported before in this condition. This is therefore the first multimodal brain imaging description of a patient presenting RAB39B mutation.

Clinical characterization of a novel RAB39B nonstop mutation in a family with ASD and severe ID causing RAB39B downregulation and study of a Rab39b knock down mouse model

Autism spectrum disorder (ASD) and intellectual disability (ID) often exist together in patients. The RAB39B gene has been reported to be mutated in ID patients with additional clinical features ranging from ASD, macrocephaly, seizures and/or early-onset parkinsonism. Here, we describe a novel RAB39B nonstop mutation [Xq28; c.640 T > C; p.(^*214Glnext^*21)] in a family with ASD, severe ID and poor motor coordination, and we assessed the pathogenicity of the mutation. A heterologous cell system and a Rab39b knockdown (KD) murine model, which mimic the nonstop mutation, were used to validate the deleterious effect of the RAB39B mutation. The mutation led to RAB39B protein instability, resulting in its increased degradation and consequent downregulation. Using a Rab39b KD mouse model, we demonstrated that the downregulation of RAB39B led to increased GluA2 lacking Ca²⁺-permeable AMPAR composition at the hippocampal neuronal surface and increased dendritic spine density that remained in an immature filopodia-like state. These phenotypes affected behavioural performance in a disease-specific manner. Rab39b KD mice revealed impaired social behaviour but intact social recognition. They also showed normal anxiety-like, exploratory and motivational behaviours but impaired working and associative memories. In conclusion, we found a novel RAB39B nonstop variant that segregated in a family with a clinical phenotype including ID, ASD and poor motor coordination. The pathogenicity of mutations causing the downregulation of RAB39B proteins, impacting AMPAR trafficking and dendritic spine morphogenesis, reinforced the idea that AMPAR modulation and dendritic spine assets could be considered hallmarks of neurodevelopmental disorders.

Genotype-Phenotype Correlations in Monogenic Parkinson Disease: A Review on Clinical and Molecular Findings

Parkinson disease (PD) is a complex neurodegenerative disorder, usually with multifactorial etiology. It is characterized by prominent movement disorders and non-motor symptoms. Movement disorders commonly include bradykinesia, rigidity, and resting tremor. Non-motor symptoms can include behavior disorders, sleep disturbances, hyposmia, cognitive impairment, and depression. A fraction of PD cases instead is due to Parkinsonian conditions with Mendelian inheritance. The study of the genetic causes of these phenotypes has shed light onto common pathogenetic mechanisms underlying Parkinsonian conditions. Monogenic Parkinsonisms can present autosomal dominant, autosomal recessive, or even X-linked inheritance patterns. Clinical presentations vary from forms indistinguishable from idiopathic PD to severe childhood-onset conditions with other neurological signs. We provided a comprehensive description of each condition, discussing current knowledge on genotype-phenotype correlations. Despite the broad clinical spectrum and the many genes involved, the phenotype appears to be related to the disrupted cell function and inheritance pattern, and several assumptions about genotype-phenotype correlations can be made. The interest in these assumptions is not merely speculative, in the light of novel promising targeted therapies currently under development.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

A Practical Approach to Early-Onset Parkinsonism

Early-onset parkinsonism (EO parkinsonism), defined as subjects with disease onset before the age of 40 or 50 years, can be the main clinical presentation of a variety of conditions that are important to differentiate. Although rarer than classical late-onset Parkinson’s disease (PD) and not infrequently overlapping with forms of juvenile onset PD, a correct diagnosis of the specific cause of EO parkinsonism is critical for offering appropriate counseling to patients, for family and work planning, and to select the most appropriate symptomatic or etiopathogenic treatments. Clinical features, radiological and laboratory findings are crucial for guiding the differential diagnosis. Here we summarize the most important conditions associated with primary and secondary EO parkinsonism. We also proposed a practical approach based on the current literature and expert opinion to help movement disorders specialists and neurologists navigate this complex and challenging landscape.

Common X-Chromosome Variants Are Associated with Parkinson Disease Risk

OBJECTIVE

The objective of this study was to identify genetic variants on the X-chromosome associated with Parkinson disease (PD) risk.

METHODS

We performed an X-chromosome-wide association study (XWAS) of PD risk by meta-analyzing results from sex-stratified analyses. To avoid spurious associations, we designed a specific harmonization pipeline for the X-chromosome and focused on a European ancestry sample. We included 11,142 cases, 280,164 controls, and 5,379 proxy cases, based on parental history of PD. Additionally, we tested the association of significant variants with (1) PD risk in an independent replication with 1,561 cases and 2,465 controls and (2) putamen volume in 33,360 individuals from the UK Biobank.

RESULTS

In the discovery meta-analysis, we identified rs7066890 (odds ratio [OR] = 1.10, 95% confidence interval [CI] = 1.06-1.14, p = 2.2 × 10-9 ), intron of GPM6B, and rs28602900 (OR = 1.10, 95% CI = 1.07-1.14, p = 1.6 × 10-8 ) in a high gene density region including RPL10, ATP6A1, FAM50A, and PLXNA3. The rs28602900 association with PD was replicated (OR = 1.16, 95% CI = 1.03-1.30, p = 0.016) and shown to colocalize with a significant expression quantitative locus (eQTL) regulating RPL10 expression in the putamen and other brain tissues in the Genotype-Tissue Expression Project. Additionally, the rs28602900 locus was found to be associated with reduced brain putamen volume. No results reached genome-wide significance in the sex-stratified analyses.

INTERPRETATION

We report the first XWAS of PD and identify 2 genome-wide significant loci. The rs28602900 association was replicated in an independent PD dataset and showed concordant effects in its association with putamen volume. Critically, rs26802900 is a significant eQTL of RPL10. These results support a role for ribosomal proteins in PD pathogenesis and show that the X-chromosome contributes to PD genetic risk. ANN NEUROL 2021;90:22-34.

X-Linked Parkinsonism: Phenotypic and Genetic Heterogeneity

X-linked parkinsonism encompasses rare heterogeneous disorders mainly inherited as a recessive trait, therefore being more prevalent in males. Recent developments have revealed a complex underlying panorama, including a spectrum of disorders in which parkinsonism is variably associated with additional neurological and non-neurological signs. In particular, a childhood-onset encephalopathy with epilepsy and/or cognitive disability is the most common feature. Their genetic basis is also heterogeneous, with many causative genes and different mutation types ranging from "classical" coding variants to intronic repeat expansions. In this review, we provide an updated overview of the phenotypic and genetic spectrum of the most relevant X-linked parkinsonian syndromes, namely X-linked dystonia-parkinsonism (XDP, Lubag disease), fragile X-associated tremor/ataxia syndrome (FXTAS), beta-propeller protein-associated neurodegeneration (BPAN, NBIA/PARK-WDR45), Fabry disease, Waisman syndrome, methyl CpG-binding protein 2 (MeCP2) spectrum disorder, phosphoglycerate kinase-1 deficiency syndrome (PGK1) and X-linked parkinsonism and spasticity (XPDS). All clinical and radiological features reported in the literature have been reviewed. Epilepsy occasionally represents the symptom of onset, predating parkinsonism even by a few years; action tremor is another common feature along with akinetic-rigid parkinsonism. A focus on the genetic background and its pathophysiological implications is provided. The pathogenesis of these disorders ranges from well-defined metabolic alterations (PGK1) to non-specific lysosomal dysfunctions (XPDS) and vesicular trafficking alterations (Waisman syndrome). However, in other cases it still remains poorly defined. Recognition of the phenotypic and genetic heterogeneity of X-linked parkinsonism has important implications for diagnosis, management, and genetic counseling. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

RAB39B-mediated trafficking of the GluA2-AMPAR subunit controls dendritic spine maturation and intellectual disability-related behaviour

Mutations in the RAB39B gene cause X-linked intellectual disability (XLID), comorbid with autism spectrum disorders or early Parkinson's disease. One of the functions of the neuronal small GTPase RAB39B is to drive GluA2/GluA3 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) maturation and trafficking, determining AMPAR subunit composition at glutamatergic postsynaptic neuronal terminals. Taking advantage of the Rab39b knockout murine model, we show that a lack of RAB39B affects neuronal dendritic spine refinement, prompting a more Ca²⁺-permeable and excitable synaptic network, which correlates with an immature spine arrangement and behavioural and cognitive alterations in adult mice. The persistence of immature circuits is triggered by increased hypermobility of the spine, which is restored by the Ca²⁺-permeable AMPAR antagonist NASPM. Together, these data confirm that RAB39B controls AMPAR trafficking, which in turn plays a pivotal role in neuronal dendritic spine remodelling and that targeting Ca²⁺-permeable AMPARs may highlight future pharmaceutical interventions for RAB39B-associated disease conditions.

RAB39B Deficiency Impairs Learning and Memory Partially Through Compromising Autophagy

RAB39B is located on the X chromosome and encodes the RAB39B protein that belongs to the RAB family. Mutations in RAB39B are known to be associated with X-linked intellectual disability (XLID), Parkinson’s disease, and autism. However, the patho/physiological functions of RAB39B remain largely unknown. In the present study, we established Rab39b knockout (KO) mice, which exhibited overall normal birth rate and morphologies as wild type mice. However, Rab39b deficiency led to reduced anxiety and impaired learning and memory in 2 months old mice. Deletion of Rab39b resulted in impairments of synaptic structures and functions, with reductions in NMDA receptors in the postsynaptic density (PSD). RAB39B deficiency also compromised autophagic flux at basal level, which could be overridden by rapamycin-induced autophagy activation. Further, treatment with rapamycin partially rescued impaired memory and synaptic plasticity in Rab39b KO mice, without affecting the PSD distribution of NMDA receptors. Together, these results suggest that RAB39B plays an important role in regulating both autophagy and synapse formation, and that targeting autophagy may have potential for treating XLID caused by RAB39B loss-of-function mutations.

Rab8 Promotes Mutant HTT Aggregation, Reduces Neurodegeneration, and Ameliorates Behavioural Alterations in a Drosophila Model of Huntington's Disease

BACKGROUND

Altered cellular vesicle trafficking has been linked to the pathogenesis of Huntington's disease (HD), a fatal, inherited neurodegenerative disorder caused by mutation of the huntingtin (HTT) protein. The Rab GTPase family of proteins plays a key role in regulation of vesicle trafficking, with distinct Rabs helping specify membrane identity and mediating cellular processes including budding, motility and tethering of vesicles to their targets. In recent years several Rab GTPases-notably, Rab5 and Rab11-have been linked to the pathogenesis of neurodegenerative disorders, including HD.

OBJECTIVE

We investigated whether Rab8, which regulates post-Golgi vesicle trafficking, is able to improve HD-relevant phenotypes in a well-characterised model.

METHODS

We overexpressed Rab8 in a Drosophila model of HD testing cellular, behavioural, and molecular phenotypes.

RESULTS

We found that Rab8 overexpression ameliorated several disease-related phenotypes in fruit flies expressing a mutant HTT fragment throughout the nervous system, including neurodegeneration of photoreceptor neurons, reduced eclosion of the adult fly from the pupal case and shortened lifespan. Rab8 overexpression also normalised aberrant circadian locomotor behaviour in flies expressing mutant HTT in a specific population of neurons that regulate the circadian clock. Intriguingly, expression of Rab8 increased the accumulation of SDS-insoluble aggregated species of mutant HTT.

CONCLUSION

Collectively, our findings demonstrate that increased Rab8 levels protect against mutant HTT toxicity and potentiate its aggregation, likely reducing the accumulation of downstream toxic soluble species.

A novel RAB39B mutation and concurrent de novo NF1 mutation in a boy with neurofibromatosis type 1, intellectual disability, and autism: a case report

BACKGROUND

Mutations in RAB39B at Xq28 causes a rare form of X-linked intellectual disability (ID) and Parkinson's disease. Neurofibromatosis type 1 (NF1) is caused by heterozygous mutations in NF1 occurring de novo in about 50% of cases, usually due to paternal gonadal mutations. This case report describes clinical and genetic findings in a boy with the occurrence of two distinct causative mutations in NF1 and RAB39B explaining the observed phenotype.

CASE PRESENTATION

Here we report a 7-year-old boy with multiple café-au-lait macules (CALMs) and freckling, severe macrocephaly, peculiar facial gestalt, severe ID with absent speech, epilepsy, autistic traits, self-harming, and aggressiveness. Proband is an only child born to a father aged 47. Parents did not present signs of NF1, while a maternal uncle showed severe ID, epilepsy, and tremors.By RNA analysis of NF1, we identified a de novo splicing variant (NM_000267.3:c.6579+2T>C) in proband, which explained NF1 clinical features but not the severe ID, behavioral problems, and aggressiveness. Family history suggested an X-linked condition and massively parallel sequencing of X-exome identified a novel RAB39B mutation (NM_171998.2:c.436_447del) in proband, his mother, and affected maternal uncle, subsequently validated by Sanger sequencing in these and other family members.

CONCLUSIONS

The case presented here highlights how concurrent genetic defects should be considered in NF1 patients when NF1 mutations cannot reasonably explain all the observed clinical features.

RAB39B is redistributed in dementia with Lewy bodies and is sequestered within aβ plaques and Lewy bodies

Loss of function mutations within the vesicular trafficking protein Ras analogy in brain 39B (RAB39B) are associated with rare X‐linked Parkinson’s disease (PD). Physiologically, RAB39B is localized to Golgi vesicles and recycling endosomes and is required for glutamatergic receptor maturation but also for alpha‐Synuclein (aSyn) homeostasis and the inhibition of its aggregation. Despite evidence linking RAB39B to neurodegeneration, the involvement of the protein in idiopathic neurodegenerative diseases remains undetermined. Here, analysis of the spatial distribution and expression of RAB39B was conducted in post‐mortem human brain tissue from cases of dementia with Lewy bodies (DLB, n = 10), Alzheimer’s disease (AD, n = 12) and controls (n = 12). Assessment of cortical RAB39B immunoreactivity using tissue microarrays revealed an overall reduction in the area of RAB39B positive gray matter in DLB cases when compared to controls and AD cases. Strikingly, RAB39B co‐localized with beta‐amyloid (Aβ) plaques in all cases examined and was additionally present in a subpopulation of Lewy bodies (LBs) in DLB. Biochemical measures of total RAB39B levels within the temporal cortex were unchanged between DLB, AD and controls. However, upon subcellular fractionation, a reduction of RAB39B in the cytoplasmic pool was found in DLB cases, alongside an increase of phosphorylated aSyn and Aβ in whole tissue lysates. The reduction of cytoplasmic RAB39B is consistent with an impaired reserve capacity for RAB39B‐associated functions, which in turn may facilitate LB aggregation and synaptic impairment. Collectively, our data support the involvement of RAB39B in the pathogenesis of DLB and the co‐aggregation of RAB39B with Aβ in plaques suggests that age‐associated cerebral Aβ pathology may be contributory to the loss of RAB39B. Thus RAB39B, its associated functional pathways and its entrapment in aggregates may be considered as future targets for therapeutic interventions to impede the overall pathological burden and cellular dysfunction in Lewy body diseases.

RAB39B's role in membrane traffic, autophagy, and associated neuropathology

Neuropathological disorders are increasingly associated with dysfunctions in neuronal membrane traffic and autophagy, with defects among members of the Rab family of small GTPases implicated. Mutations in the human Xq28 localized gene RAB39B have been associated with X-linked neurodevelopmental defects including macrocephaly, intellectual disability, autism spectrum disorder (ASD), as well as rare cases of early-onset Parkinson's disease (PD). Despite the finding that RAB39B regulates GluA2 trafficking and could thus influence synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit composition, reasons for the wide-ranging neuropathological consequences associated with RAB39B defects have been unclear. Recent studies have now unraveled possible mechanisms underlying the neuropathological roles of this brain-enriched small GTPase. Studies in RAB39B knockout mice showed that RAB39B interacts with components of Class I phosphatidylinositol-3-kinase (PI3K) signaling. In its absence, the PI3K-AKT-mechanistic target of rapamycin signaling pathway in neural progenitor cells (NPCs) is hyperactivated, which promotes NPC proliferation, leading to macrocephaly and ASD. Pertaining to early-onset PD, a complex of C9orf72, Smith-Magenis syndrome chromosome region candidate 8 and WD repeat domain 41 that functions in autophagy has been identified as a guanine nucleotide exchange factor of RAB39B. Here, recent findings that have shed light on our mechanistic understanding of RAB39B's role in neurodevelopmental and neurodegenerative pathologies are reviewed. Caveats and unanswered questions are also discussed, and future perspectives outlined.

Parkinson's: A Disease of Aberrant Vesicle Trafficking

Parkinson's disease (PD) is a leading cause of neurodegeneration that is defined by the selective loss of dopaminergic neurons and the accumulation of protein aggregates called Lewy bodies (LBs). The unequivocal identification of Mendelian inherited mutations in 13 genes in PD has provided transforming insights into the pathogenesis of this disease. The mechanistic analysis of several PD genes, including α-synuclein (α-syn), leucine-rich repeat kinase 2 (LRRK2), PTEN-induced kinase 1 (PINK1), and Parkin, has revealed central roles for protein aggregation, mitochondrial damage, and defects in endolysosomal trafficking in PD neurodegeneration. In this review, we outline recent advances in our understanding of these gene pathways with a focus on the emergent role of Rab (Ras analog in brain) GTPases and vesicular trafficking as a common mechanism that underpins how mutations in PD genes lead to neuronal loss. These advances have led to previously distinct genes such as vacuolar protein-sorting-associated protein 35 (VPS35) and LRRK2 being implicated in a common signaling pathway. A greater understanding of these common nodes of vesicular trafficking will be crucial for linking other PD genes and improving patient stratification in clinical trials underway against α-syn and LRRK2 targets.

Genetic Analysis of RAB39B in an Early-Onset Parkinson's Disease Cohort

Pathogenic variants in the gene encoding RAB39B, resulting in the loss of protein function, lead to the development of X-linked early-onset parkinsonism. The gene is located within a chromosomal region that is susceptible to genomic rearrangement, and while an increased dosage of RAB39B was previously associated with cognitive impairment, the potential role of dosage alterations in Parkinson's disease (PD) remains to be determined. This study aimed to investigate the contribution of the genetic variation in RAB39B to the development of early-onset PD. We performed gene dosage studies and sequence analysis in a cohort of 176 individuals with early-onset PD (age of onset ≤ 50 years) of unknown genetic etiology. An assessment of the copy number variation over both coding exons and the 3' untranslated region (UTR) of RAB39B did not identify any alterations in gene dosage. An analysis of the UTRs identified two male individuals carrying single, likely benign, nucleotide variants in the 3'UTR (chrX:154489749-A-G and chrX:154489197-T-G). Furthermore, one novel variant of uncertain significance was identified in the 5'UTR, 229 bp upstream of the start codon (chrX:154493802-C-T). In silico analyses predicted that this variant disrupts a highly conserved transcription factor binding site and could impact RAB39B expression. The results of this study do not support a significant role for genetic variation in RAB39B as contributing to early-onset PD but do highlight that additional molecular studies are required to determine the mechanisms regulating RAB39B expression and their association with the disease. Genetic investigations in larger parkinsonism/PD cohorts and longitudinal studies of individuals with cognitive impairment due to an altered dosage of RAB39B will be required to fully delineate the contribution of RAB39B to parkinsonism.

Membrane trafficking in health and disease

Membrane trafficking pathways are essential for the viability and growth of cells, and play a major role in the interaction of cells with their environment. In this At a Glance article and accompanying poster, we outline the major cellular trafficking pathways and discuss how defects in the function of the molecular machinery that mediates this transport lead to various diseases in humans. We also briefly discuss possible therapeutic approaches that may be used in the future treatment of trafficking-based disorders.

Summary: This At a Glance article and poster summarise the major intracellular membrane trafficking pathways and associated molecular machineries, and describe how defects in these give rise to disease in humans.

Distribution of Parkinson's disease associated RAB39B in mouse brain tissue

Pathogenic variants in the gene encoding the small GTPase Ras analogue in Brain 39b (RAB39B) are associated with early-onset parkinsonism. In this study we investigated the expression and localization of RAB39B (RNA and protein) in mouse brain tissue to gain a better understanding of its normal physiological function(s) and role in disease.

We developed novel resources, including monoclonal antibodies directed against RAB39B and mice with Rab39b knockout, and performed real-time PCR and western blot analysis on whole brain lysates. To determine the spatial localization of Rab39b RNA and protein, we performed in-situ hybridization and immunohistochemistry on fresh frozen and fixed brain tissue. Our results show that RAB39B is localized throughout the cortex, hippocampus and substantia nigra of mice throughout postnatal life. We found high levels of RAB39B within MAP2 positive cortical and hippocampal neurons, and TH positive dopaminergic neurons in the substantia nigra pars compacta.

Our studies support and extend current knowledge of the localization of RAB39B. We validate RAB39B as a neuron-enriched protein and demonstrate that it is present throughout the mouse cortex and hippocampus. Further, we observe high levels in the substantia nigra pars compacta, the brain region most affected in Parkinson’s disease pathology. The distribution of Rab39b is consistent with human disease associations with parkinsonism and cognitive impairment. We also describe and validate novel resources, including monoclonal antibodies directed against RAB39B and mice with Rab39b knockout, both of which are valuable tools for future studies of the molecular function of RAB39B.

Ginkgo biloba Prevents Oxidative Stress-Induced Apoptosis Blocking p53 Activation in Neuroblastoma Cells

Oxidative stress has been associated to neuronal cell loss in neurodegenerative diseases. Neurons are post-mitotic cells that are very sensitive to oxidative stress—especially considering their limited capacity to be replaced. Therefore, reduction of oxidative stress, and inhibiting apoptosis, will potentially prevent neurodegeneration. In this study, we investigated the neuroprotective effect of Ginkgo biloba extract (EGb 761) against H₂O₂ induced apoptosis in SK-N-BE neuroblastoma cells. We analysed the molecular signalling pathway involved in the apoptotic cell death. H₂O₂ induced an increased acetylation of p53 lysine 382, a reduction in mitochondrial membrane potential, an increased BAX/Bcl-2 ratio and consequently increased Poly (ADP-ribose) polymerase (PARP) cleavage. All these effects were blocked by EGb 761 treatment. Thus, EGb 761, acting as intracellular antioxidant, protects neuroblastoma cells against activation of p53 mediated pathway and intrinsic mitochondrial apoptosis. Our results suggest that EGb 761, protecting against oxidative-stress induced apoptotic cell death, could potentially be used as nutraceutical for the prevention and treatment of neurodegenerative diseases.

Endosomal sorting pathways in the pathogenesis of Parkinson's disease

The identification of Parkinson's disease (PD)-associated genes has created a powerful platform to begin to understand and nominate pathophysiological disease mechanisms. Herein, we discuss the genetic and experimental evidence supporting endolysosomal dysfunction as a major pathway implicated in PD. Well-studied familial PD-linked gene products, including LRRK2, VPS35, and α-synuclein, demonstrate how disruption of different aspects of endolysosomal sorting pathways by disease-causing mutations may manifest into PD-like phenotypes in many disease models. Newly-identified PD-linked genes, including auxilin, synaptojanin-1 and Rab39b, as well as putative risk genes for idiopathic PD (endophilinA1, Rab29, GAK), further support endosomal sorting deficits as being central to PD. LRRK2 may represent a nexus by regulating many distinct features of endosomal sorting, potentially via phosphorylation of key endocytosis machinery (i.e., auxilin, synaptojanin-1, endoA1) and Rab GTPases (i.e., Rab29, Rab8A, Rab10) that function within these pathways. In turn, LRRK2 kinase activity is critically regulated by Rab29 at the Golgi complex and retromer-associated VPS35 at endosomes. Taken together, the known functions of PD-associated gene products, the impact of disease-linked mutations, and the emerging functional interactions between these proteins points to endosomal sorting pathways as a key point of convergence in the pathogenesis of PD.

Int22h1/Int22h2-mediated Xq28 duplication syndrome: de novo duplications, prenatal diagnoses, and additional phenotypic features

Int22h1/Int22h2-mediated Xq28 duplication syndrome is a relatively new X-linked intellectual disability syndrome, arising from duplications of the subregion flanked by intron 22 homologous regions 1 and 2 on the q arm of chromosome X. Its primary manifestations include variable cognitive deficits, distinct facial dysmorphia, and neurobehavioral abnormalities that mainly include hyperactivity, irritability, and autistic behavior. Affected males are hemizygous for the duplication, which explains their often more severe manifestations compared with heterozygous females. In this report, we describe the cases of nine individuals recently identified having the syndrome, highlighting unique and previously unreported findings of this syndrome. Specifically, we report for the first time in this syndrome, two cases with de novo duplications, three receiving prenatal diagnosis with the syndrome, and three others having atypical versions of the duplication. Among the latter, one proband has a shortened version spanning only the centromeric half of the typical duplication, while the other two cases have a nearly identical length duplication as the classical duplication, with the exception that their duplication's breakpoints are telomerically shifted by about 0.2 Mb. Finally, we shed light on two new manifestations in this syndrome, vertebral anomalies and multiple malignancies, which possibly expand the phenotypic spectrum of the syndrome.

Cerebral organoid and mouse models reveal a RAB39b-PI3K-mTOR pathway-dependent dysregulation of cortical development leading to macrocephaly/autism phenotypes

Dysregulation of early neurodevelopment is implicated in macrocephaly/autism disorders. However, the mechanism underlying this dysregulation, particularly in human cells, remains poorly understood. Mutations in the small GTPase gene RAB39b are associated with X-linked macrocephaly, autism spectrum disorder (ASD), and intellectual disability. The in vivo roles of RAB39b in the brain remain unknown. We generated Rab39b knockout (KO) mice and found that they exhibited cortical neurogenesis impairment, macrocephaly, and hallmark ASD behaviors, which resembled patient phenotypes. We also produced mutant human cerebral organoids that were substantially enlarged due to the overproliferation and impaired differentiation of neural progenitor cells (NPCs), which resemble neurodevelopmental deficits in KO mice. Mechanistic studies reveal that RAB39b interacts with PI3K components and its deletion promotes PI3K-AKT-mTOR signaling in NPCs of mouse cortex and cerebral organoids. The mTOR activity is robustly enhanced in mutant outer radial glia cells (oRGs), a subtype of NPCs barely detectable in rodents but abundant in human brains. Inhibition of AKT signaling rescued enlarged organoid sizes and NPC overproliferation caused by RAB39b mutations. Therefore, RAB39b mutation promotes PI3K-AKT-mTOR activity and alters cortical neurogenesis, leading to macrocephaly and autistic-like behaviors. Our studies provide new insights into neurodevelopmental dysregulation and common pathways associated with ASD across species.

Clinical and Neuropathological Features Associated With Loss of RAB39B

BACKGROUND

Pathogenic variants in the small GTPase Ras Analogue in Brain 39b (RAB39B) have been linked to the development of early-onset parkinsonism. The study was aimed at delineating the clinical and neuropathological features associated with a previously reported pathogenic variant in RAB39B (c.503C>A p.T168K) and testing for dysregulation of RAB39B in idiopathic PD.

METHODS

Clinical details of a male individual hemizygous for the T168K variant were collected by systematic review of medical records. Neuropathological studies of fixed brain tissue were performed and steady-state RAB39B levels were determined by western blot analysis.

RESULTS

Neuropathological examination showed extensive dopaminergic neuron loss, widespread Lewy pathology, and iron accumulation in the substantia nigra. Additional pathology was observed in the hippocampus and thalamus. Western blot analysis demonstrated that the T168K variant results in loss of RAB39B. In individuals with idiopathic PD (n = 10, 6 male/4 female), steady-state RAB39B was significantly reduced in the prefrontal cortex and substantia nigra.

CONCLUSIONS

T168K RAB39B is unstable in vivo and associated with dopaminergic neuron loss and Lewy pathology. Dysregulation of RAB39B in the prefrontal cortex and substantia nigra of individuals with idiopathic PD potentially implicates the protein more broadly in the pathological mechanisms underlying PD and related Lewy body disorders. © 2020 International Parkinson and Movement Disorder Society.

Vesicular Dysfunction and the Pathogenesis of Parkinson's Disease: Clues From Genetic Studies

Parkinson’s disease (PD) is a common age-related neurodegenerative disorder with disabling motor symptoms and no available disease modifying treatment. The majority of the PD cases are of unknown etiology, with both genetics and environment playing important roles. Over the past 25 years, however, genetic analysis of patients with familial history of Parkinson’s and, latterly, genome wide association studies (GWAS) have provided significant advances in our understanding of the causes of the disease. These genetic insights have uncovered pathways that are affected in both genetic and sporadic forms of PD. These pathways involve oxidative stress, abnormal protein homeostasis, mitochondrial dysfunction, and lysosomal defects. In addition, newly identified PD genes and GWAS nominated genes point toward synaptic changes involving vesicles. This review will highlight the genes that contribute PD risk relating to intracellular vesicle trafficking and their functional consequences. There is still much to investigate on this newly identified and converging pathway of vesicular dynamics and PD, which will aid in better understanding and suggest novel therapeutic strategies for PD patients.

Clinical and neuroimaging phenotypes of genetic parkinsonism from infancy to adolescence

Genetic early-onset parkinsonism presenting from infancy to adolescence (≤21 years old) is a clinically diverse syndrome often combined with other hyperkinetic movement disorders, neurological and imaging abnormalities. The syndrome is genetically heterogeneous, with many causative genes already known. With the increased use of next-generation sequencing in clinical practice, there have been novel and unexpected insights into phenotype-genotype correlations and the discovery of new disease-causing genes. It is now recognized that mutations in a single gene can give rise to a broad phenotypic spectrum and that, conversely different genetic disorders can manifest with a similar phenotype. Accurate phenotypic characterization remains an essential step in interpreting genetic findings in undiagnosed patients. However, in the past decade, there has been a marked expansion in knowledge about the number of both disease-causing genes and phenotypic spectrum of early-onset cases. Detailed knowledge of genetic disorders and their clinical expression is required for rational planning of genetic and molecular testing, as well as correct interpretation of next-generation sequencing results. In this review we examine the relevant literature of genetic parkinsonism with ≤21 years onset, extracting data on associated movement disorders as well as other neurological and imaging features, to delineate syndromic patterns associated with early-onset parkinsonism. Excluding PRKN (parkin) mutations, &gt;90% of the presenting phenotypes have a complex or atypical presentation, with dystonia, abnormal cognition, pyramidal signs, neuropsychiatric disorders, abnormal imaging and abnormal eye movements being the most common features. Furthermore, several imaging features and extraneurological manifestations are relatively specific for certain disorders and are important diagnostic clues. From the currently available literature, the most commonly implicated causes of early-onset parkinsonism have been elucidated but diagnosis is still challenging in many cases. Mutations in ∼70 different genes have been associated with early-onset parkinsonism or may feature parkinsonism as part of their phenotypic spectrum. Most of the cases are caused by recessively inherited mutations, followed by dominant and X-linked mutations, and rarely by mitochondrially inherited mutations. In infantile-onset parkinsonism, the phenotype of hypokinetic-rigid syndrome is most commonly caused by disorders of monoamine synthesis. In childhood and juvenile-onset cases, common genotypes include PRKN, HTT, ATP13A2, ATP1A3, FBX07, PINK1 and PLA2G6 mutations. Moreover, Wilson’s disease and mutations in the manganese transporter are potentially treatable conditions and should always be considered in the differential diagnosis in any patient with early-onset parkinsonism.

Rab GTPases and Membrane Trafficking in Neurodegeneration

Defects in membrane trafficking are hallmarks of neurodegeneration. Rab GTPases are key regulators of membrane trafficking. Alterations of Rab GTPases, or the membrane compartments they regulate, are associated with virtually all neuronal activities in health and disease. The observation that many Rab GTPases are associated with neurodegeneration has proven a challenge in the quest for cause and effect. Neurodegeneration can be a direct consequence of a defect in membrane trafficking. Alternatively, changes in membrane trafficking may be secondary consequences or cellular responses. The secondary consequences and cellular responses, in turn, may protect, represent inconsequential correlates or function as drivers of pathology. Here, we attempt to disentangle the different roles of membrane trafficking in neurodegeneration by focusing on selected associations with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease and selected neuropathies. We provide an overview of current knowledge on Rab GTPase functions in neurons and review the associations of Rab GTPases with neurodegeneration with respect to the following classifications: primary cause, secondary cause driving pathology or secondary correlate. This analysis is devised to aid the interpretation of frequently observed membrane trafficking defects in neurodegeneration and facilitate the identification of true causes of pathology.

Assessment of patient clinical descriptions and pathogenic variants from gene panel sequences in the CAGI-5 intellectual disability challenge

The Critical Assessment of Genome Interpretation-5 intellectual disability challenge asked to use computational methods to predict patient clinical phenotypes and the causal variant(s) based on an analysis of their gene panel sequence data. Sequence data for 74 genes associated with intellectual disability (ID) and/or autism spectrum disorders (ASD) from a cohort of 150 patients with a range of neurodevelopmental manifestations (i.e. ID, autism, epilepsy, microcephaly, macrocephaly, hypotonia, ataxia) have been made available for this challenge. For each patient, predictors had to report the causative variants and which of the seven phenotypes were present. Since neurodevelopmental disorders are characterized by strong comorbidity, tested individuals often present more than one pathological condition. Considering the overall clinical manifestation of each patient, the correct phenotype has been predicted by at least one group for 93 individuals (62%). ID and ASD were the best predicted among the seven phenotypic traits. Also, causative or potentially pathogenic variants were predicted correctly by at least one group. However, the prediction of the correct causative variant seems to be insufficient to predict the correct phenotype. In some cases, the correct prediction has been supported by rare or common variants in genes different from the causative one.

Rab GTPases: Switching to Human Diseases

Rab proteins compose the largest family of small GTPases and control the different steps of intracellular membrane traffic. More recently, they have been shown to also regulate cell signaling, division, survival, and migration. The regulation of these processes generally occurs through recruitment of effectors and regulatory proteins, which control the association of Rab proteins to membranes and their activation state. Alterations in Rab proteins and their effectors are associated with multiple human diseases, including neurodegeneration, cancer, and infections. This review provides an overview of how the dysregulation of Rab-mediated functions and membrane trafficking contributes to these disorders. Understanding the altered dynamics of Rabs and intracellular transport defects might thus shed new light on potential therapeutic strategies.

Characterization of intellectual disability and autism comorbidity through gene panel sequencing

Intellectual disability (ID) and autism spectrum disorder (ASD) are clinically and genetically heterogeneous diseases. Recent whole exome sequencing studies indicated that genes associated with different neurological diseases are shared across disorders and converge on common functional pathways. Using the Ion Torrent platform, we developed a low-cost next-generation sequencing gene panel that has been transferred into clinical practice, replacing single disease-gene analyses for the early diagnosis of individuals with ID/ASD. The gene panel was designed using an innovative in silico approach based on disease networks and mining data from public resources to score disease-gene associations. We analyzed 150 unrelated individuals with ID and/or ASD and a confident diagnosis has been reached in 26 cases (17%). Likely pathogenic mutations have been identified in another 15 patients, reaching a total diagnostic yield of 27%. Our data also support the pathogenic role of genes recently proposed to be involved in ASD. Although many of the identified variants need further investigation to be considered disease-causing, our results indicate the efficiency of the targeted gene panel on the identification of novel and rare variants in patients with ID and ASD.

Rab-dependent cellular trafficking and amyotrophic lateral sclerosis

Rab GTPases are becoming increasingly implicated in neurodegenerative disorders, although their role in amyotrophic lateral sclerosis (ALS) has been somewhat overlooked. However, dysfunction of intracellular transport is gaining increasing attention as a pathogenic mechanism in ALS. Many previous studies have focused axonal trafficking, and the extreme length of axons in motor neurons may contribute to their unique susceptibility in this disorder. In contrast, the role of transport defects within the cell body has been relatively neglected. Similarly, whilst Rab GTPases control all intracellular membrane trafficking events, their role in ALS is poorly understood. Emerging evidence now highlights this family of proteins in ALS, particularly the discovery that C9orf72 functions in intra transport in conjunction with several Rab GTPases. Here, we summarize recent updates on cellular transport defects in ALS, with a focus on Rab GTPases and how their dysfunction may specifically target neurons and contribute to pathophysiology. We discuss the molecular mechanisms associated with dysfunction of Rab proteins in ALS. Finally, we also discuss dysfunction in other modes of transport recently implicated in ALS, including nucleocytoplasmic transport and the ER-mitochondrial contact regions (MAM compartment), and speculate whether these may also involve Rab GTPases.

Role of Rab GTPases in Alzheimer's Disease

Alzheimer's disease (AD) comprises two major pathological hallmarks: extraneuronal deposition of β-amyloid (Aβ) peptides ("senile plaques") and intraneuronal aggregation of the microtubule-associated protein tau ("neurofibrillary tangles"). Aβ is derived from sequential cleavage of the β-amyloid precursor protein by β- and γ-secretases, while aggregated tau is hyperphosphorylated in AD. Mounting evidence suggests that dysregulated trafficking of these AD-related proteins contributes to AD pathogenesis. Rab proteins are small GTPases that function as master regulators of vesicular transport and membrane trafficking. Multiple Rab GTPases have been implicated in AD-related protein trafficking, and their expression has been observed to be altered in postmortem AD brain. Here we review current implicated roles of Rab GTPase dysregulation in AD pathogenesis. Further elucidation of the pathophysiological role of Rab GTPases will likely reveal novel targets for AD therapeutics.

Mitochondrial function and autophagy: integrating proteotoxic, redox, and metabolic stress in Parkinson's disease

Parkinson's disease (PD) is a movement disorder with widespread neurodegeneration in the brain. Significant oxidative, reductive, metabolic, and proteotoxic alterations have been observed in PD postmortem brains. The alterations of mitochondrial function resulting in decreased bioenergetic health is important and needs to be further examined to help develop biomarkers for PD severity and prognosis. It is now becoming clear that multiple hits on metabolic and signaling pathways are likely to exacerbate PD pathogenesis. Indeed, data obtained from genetic and genome association studies have implicated interactive contributions of genes controlling protein quality control and metabolism. For example, loss of key proteins that are responsible for clearance of dysfunctional mitochondria through a process called mitophagy has been found to cause PD, and a significant proportion of genes associated with PD encode proteins involved in the autophagy-lysosomal pathway. In this review, we highlight the evidence for the targeting of mitochondria by proteotoxic, redox and metabolic stress, and the role autophagic surveillance in maintenance of mitochondrial quality. Furthermore, we summarize the role of α-synuclein, leucine-rich repeat kinase 2, and tau in modulating mitochondrial function and autophagy. Among the stressors that can overwhelm the mitochondrial quality control mechanisms, we will discuss 4-hydroxynonenal and nitric oxide. The impact of autophagy is context depend and as such can have both beneficial and detrimental effects. Furthermore, we highlight the potential of targeting mitochondria and autophagic function as an integrated therapeutic strategy and the emerging contribution of the microbiome to PD susceptibility.

Dysregulated phosphorylation of Rab GTPases by LRRK2 induces neurodegeneration

Background

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial and sporadic Parkinson’s disease (PD). Elevated kinase activity is associated with LRRK2 toxicity, but the substrates that mediate neurodegeneration remain poorly defined. Given the increasing evidence suggesting a role of LRRK2 in membrane and vesicle trafficking, here we systemically screened Rab GTPases, core regulators of vesicular dynamics, as potential substrates of LRRK2 and investigated the functional consequence of such phosphorylation in cells and in vivo.

Methods

In vitro LRRK2 kinase assay with forty-five purified human Rab GTPases was performed to identify Rab family proteins as substrates of LRRK2. We identified the phosphorylation site by tandem mass-spectrometry and confirmed it by assessing phosphorylation in the in vitro LRRK2 kinase assay and in cells. Effects of Rab phosphorylation on neurodegeneration were examined in primary cultures and in vivo by intracranial injection of adeno-associated viral vectors (AAV) expressing wild-type or phosphomutants of Rab35.

Results

Our screening revealed that LRRK2 phosphorylated several Rab GTPases at a conserved threonine residue in the switch II region, and by using the kinase-inactive LRRK2-D1994A and the pathogenic LRRK2-G2019S along with Rab proteins in which the LRRK2 site was mutated, we verified that a subset of Rab proteins, including Rab35, were authentic substrates of LRRK2 both in vitro and in cells. We also showed that phosphorylation of Rab regulated GDP/GTP-binding property in cells. Moreover, in primary cortical neurons, mutation of the LRRK2 site in several Rabs caused neurotoxicity, which was most severely induced by phosphomutants of Rab35. Furthermore, intracranial injection of the AAV-Rab35 -T72A or AAV-Rab35-T72D into the substantia nigra substantially induced degeneration of dopaminergic neurons in vivo.

Conclusions

Here we show that a subset of Rab GTPases are authentic substrates of LRRK2 both in vitro and in cells. We also provide evidence that dysregulation of Rab phosphorylation in the LRRK2 site induces neurotoxicity in primary neurons and degeneration of dopaminergic neurons in vivo. Our study suggests that Rab GTPases might mediate LRRK2 toxicity in the progression of PD.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0240-1) contains supplementary material, which is available to authorized users.