RAB39B is a member of the RAB family of small GTPases that controls intracellular vesicular trafficking in a compartment-specific manner. Mutations in the RAB39B gene cause intellectual disability ...comorbid with autism spectrum disorder and epilepsy, but the impact of RAB39B loss of function on synaptic activity is largely unexplained. Here we show that protein interacting with C-kinase 1 (PICK1) is a downstream effector of GTP-bound RAB39B and that RAB39B-PICK1 controls trafficking from the endoplasmic reticulum to the Golgi and, hence, surface expression of GluA2, a subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). The role of AMPARs in synaptic transmission varies depending on the combination of subunits (GluA1, GluA2 and GluA3) they incorporate. RAB39B downregulation in mouse hippocampal neurons skews AMPAR composition towards non GluA2-containing Ca(2+)-permeable forms and thereby alters synaptic activity, specifically in hippocampal neurons. We posit that the resulting alteration in synaptic function underlies cognitive dysfunction in RAB39B-related disorders.
Neurons are highly polarized cells that exhibit one of the more complex morphology and function. Neuronal intracellular trafficking plays a key role in dictating the directionality and specificity of ...vesicle formation, transport and fusion, allowing the transmission of information in sophisticate cellular network. Thus, the integrity of protein trafficking and spatial organization is especially important in neuronal cells. RAB proteins, small monomeric GTPases belonging to the RAS superfamily, spatially and temporally orchestrate specific vesicular trafficking steps. In this review we summarise the known roles of RAB GTPases involved in the maintenance of neuronal vesicular trafficking in the central nervous system. In particular, we discriminate the axonal pre-synaptic trafficking and dendritic post-synaptic trafficking, to better underlie how a correct orchestration of vesicle movement is necessary to maintain neuronal polarity and then, to permit an accurate architecture and functionality of synaptic activity.
Abstract
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.
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 Ca
-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 Ca
-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 Ca
-permeable AMPARs may highlight future pharmaceutical interventions for RAB39B-associated disease conditions.
A RAS-related class of small monomeric G proteins, the RAB GTPases, is emerging as of key biological importance in compartment specific directional control of vesicles formation, transport and ...fusion. Thanks to human genetic observation and to the consequent dedicated biochemical work, substantial progress has been made on the understanding of the role played by RAB GTPases and their effector proteins on neuronal development and the shaping of cognitive functions. This review is highlighting these initial elements to broaden the current scope of research on developmental cognitive deficits and take the point of view of RAB GTPases control on membrane transport in neurons and astrocytes.
GDI1 gene encodes for αGDI, a protein controlling the cycling of small GTPases, reputed to orchestrate vesicle trafficking. Mutations in human GDI1 are responsible for intellectual disability (ID). ...In mice with ablated Gdi1, a model of ID, impaired working and associative short-term memory was recorded. This cognitive phenotype worsens if the deletion of αGDI expression is restricted to neurons. However, whether astrocytes, key homeostasis providing neuroglial cells, supporting neurons via aerobic glycolysis, contribute to this cognitive impairment is unclear.
We carried out proteomic analysis and monitored 18F-fluoro-2-deoxy-d-glucose uptake into brain slices of Gdi1 knockout and wild type control mice. d-Glucose utilization at single astrocyte level was measured by the Förster Resonance Energy Transfer (FRET)-based measurements of cytosolic cyclic AMP, d-glucose and L-lactate, evoked by agonists selective for noradrenaline and L-lactate receptors. To test the role of astrocyte-resident processes in disease phenotype, we generated an inducible Gdi1 knockout mouse carrying the Gdi1 deletion only in adult astrocytes and conducted behavioural tests.
Proteomic analysis revealed significant changes in astrocyte-resident glycolytic enzymes. Imaging 18F-fluoro-2-deoxy-d-glucose revealed an increased d-glucose uptake in Gdi1 knockout tissue versus wild type control mice, consistent with the facilitated d-glucose uptake determined by FRET measurements. In mice with Gdi1 deletion restricted to astrocytes, a selective and significant impairment in working memory was recorded, which was rescued by inhibiting glycolysis by 2-deoxy-d-glucose injection.
These results reveal a new astrocyte-based mechanism in neurodevelopmental disorders and open a novel therapeutic opportunity of targeting aerobic glycolysis, advocating a change in clinical practice.
•Mutations in human Gdi1, encoding αGDI, a protein controlling vesicle traffic, are responsible for Intellectual Disability.•Gdi1 knockout revealed significant changes in astrocyte-resident glycolytic enzymes and facilitated D-glucose utilization.•Astrocyte-selective Gdi1 deletion impairs working memory, which can be rescued by administration of 2-deoxy-D-glucose.•Astrocyte-based glycolysis is a new target to treat Intellectual Disability.
Human Mental Retardation (MR) is a common and highly heterogeneous pediatric disorder affecting around 3% of the general population; at least 215 X-linked MR (XLMR) conditions have been described, ...and mutations have been identified in 83 different genes, encoding proteins with a variety of function, such as chromatin remodeling, synaptic function, and intracellular trafficking. The small GTPases of the RAB family, which play an essential role in intracellular vesicular trafficking, have been shown to be involved in MR. We report here the identification of mutations in the small GTPase RAB39B gene in two male patients. One mutation in family X (D-23) introduced a stop codon seven amino acids after the start codon (c.21C > A; p.Y7X). A second mutation, in the MRX72 family, altered the 5′ splice site (c.215+1G > A) and normal splicing. Neither instance produced a protein. Mutations segregate with the disease in the families, and in some family members intellectual disabilities were associated with autism spectrum disorder, epileptic seizures, and macrocephaly. We show that RAB39B, a novel RAB GTPase of unknown function, is a neuronal-specific protein that is localized to the Golgi compartment. Its downregulation leads to an alteration in the number and morphology of neurite growth cones and a significant reduction in presynaptic buttons, suggesting that RAB39B is required for synapse formation and maintenance. Our results demonstrate developmental and functional neuronal alteration as a consequence of downregulation of RAB39B and emphasize the critical role of vesicular trafficking in the development of neurons and human intellectual abilities.
RAB39B pathogenic variants cause X-linked Parkinsonism associated with Intellectual Disability, known as Waisman syndrome, a very rare disorder that has been mainly identified through exome ...sequencing in large Parkinson's disease cohorts.
In this study we searched for pathogenic variants in RAB39B in two Italian families affected by X-linked early-onset Parkinsonism and Intellectual Disability.
Three patients received neurological evaluation and underwent RAB39B sequencing.
Two novel RAB39B frameshift variants were found to result in the absence of RAB39B protein (family 1: c.137dupT; family 2: c.371delA). Patients showed unilateral rest tremor and bradykinesia; one of them also displayed an early-onset postural tremor. Paramagnetic substance deposition in the substantia nigra, globus pallidi, red nucleus, putamen and pulvinar was assessed by brain imaging. Two patients also showed moderate calcification of globus pallidi.
In this study we highlight the evidence that X-linked early-onset Parkinsonism associated with Intellectual Disability occurs as a pattern of clinical and neuroimaging features attributable to RAB39B pathogenic variants.
•Loss of RAB39B may lead to X-linked Parkinsonism with Intellectual Disability.•Two novel RAB39B frameshift variants result in the absence of RAB39B protein.•We discuss the syndromic Parkinsonism's variable expression that may affect women.•We support the evidence that neuroimaging adds specific features to the syndrome.•RAB39B somatic mosaicism may be important for unravelling Parkinson's disease.
: Transoral laser microsurgery (TLM) is widely accepted for its advantages, which consist of a brief hospital stay, rapid functional recovery, low management costs and the fact that it can be easily ...repeated in cases of recurrence. However, a high incidence of positive or narrow surgical margins has been reported in the literature, even if controversy still exists on the prognostic significance of positive resection margins. The aim of the study was to evaluate the utility of toluidine blue staining in defining the resection margins of early glottic cancer (T1a-T2) treated with TLM.
: This retrospective study was conducted on patients with early glottic cancer (T1a-T2) managed by TLM. A group of patients treated between 2010 and 2014 underwent toluidine blue staining (TB group) of the lesions before starting the cordectomy by TLM, and a group of patients treated by TLM between 2006 and 2009 was considered the control group.
: A total of 44 subjects were included in this study: 41 were men, and 3 were women. The mean age was 58 ± 9.0 years (median 59.0, range 41-77). Twenty-three of the 44 patients were included in the TB group and 21 in the case control group. In the TB group, only the positivity of the deep margin was a predictor of local recurrence (
= 0.037), while in the control group, positive or close margins and the type of cordectomy were predictive factors of local recurrence (
0.049). Considering the TB group and control cases, the 5-year local recurrence-free survival was 95.6% and 80.9%, respectively (
0.14).
: From this first study, toluidine blue staining seems to be a useful modality to improve the rate of the negative resection margins of early glottic cancer (T1a-T2) treated by TLM.
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