Expression of sorting nexin 12 is regulated in developing cerebral cortical neurons

Developmental nicotine exposure and masculinization of the rat preoptic area

Nicotine is a neuroteratogenic component of tobacco smoke, e-cigarettes, and other products and can exert sex-specific effects in the developing brain, likely mediated through sex hormones. Estradiol modulates expression of nicotinic acetylcholine receptors in rats, and plays critical roles in neurodevelopmental processes, including sexual differentiation of the brain. Here, we examined the effects of developmental nicotine exposure on the sexual differentiation of the preoptic area (POA), a brain region that normally displays robust structural sexual dimorphisms and controls adult mating behavior in rodents. Using a rat model of gestational exposure, developing pups were exposed to nicotine (2 mg/kg/day) via maternal osmotic minipump (subcutaneously, sc) throughout the critical window for brain sexual differentiation. At postnatal day (PND) 4, a subset of offspring was analyzed for epigenetic effects in the POA. At PND40, all offspring were gonadectomized, implanted with a testosterone-releasing capsule (sc), and assessed for male sexual behavior at PND60. Following sexual behavior assessment, the area of the sexually dimorphic nucleus of the POA (SDN-POA) was measured using immunofluorescent staining techniques. In adults, normal sex differences in male sexual behavior and in the SDN-POA area were eliminated in nicotine-treated animals. Using novel analytical approaches to evaluate overall masculinization of the adult POA, we identified significant masculinization of the nicotine-treated female POA. In neonates (PND4), nicotine exposure induced trending alterations in methylation-dependent masculinizing gene expression and DNA methylation levels at sexually-dimorphic differentially methylated regions, suggesting that developmental nicotine exposure is capable of triggering masculinization of the rat POA via epigenetic mechanisms.

Sorting Out Sorting Nexins Functions in the Nervous System in Health and Disease

Endocytosis is a fundamental process that controls protein/lipid composition of the plasma membrane, thereby shaping cellular metabolism, sensing, adhesion, signaling, and nutrient uptake. Endocytosis is essential for the cell to adapt to its surrounding environment, and a tight regulation of the endocytic mechanisms is required to maintain cell function and survival. This is particularly significant in the central nervous system (CNS), where composition of neuronal cell surface is crucial for synaptic functioning. In fact, distinct pathologies of the CNS are tightly linked to abnormal endolysosomal function, and several genome wide association analysis (GWAS) and biochemical studies have identified intracellular trafficking regulators as genetic risk factors for such pathologies. The sorting nexins (SNXs) are a family of proteins involved in protein trafficking regulation and signaling. SNXs dysregulation occurs in patients with Alzheimer’s disease (AD), Down’s syndrome (DS), schizophrenia, ataxia and epilepsy, among others, establishing clear roles for this protein family in pathology. Interestingly, restoration of SNXs levels has been shown to trigger synaptic plasticity recovery in a DS mouse model. This review encompasses an historical and evolutionary overview of SNXs protein family, focusing on its organization, phyla conservation, and evolution throughout the development of the nervous system during speciation. We will also survey SNXs molecular interactions and highlight how defects on SNXs underlie distinct pathologies of the CNS. Ultimately, we discuss possible strategies of intervention, surveying how our knowledge about the fundamental processes regulated by SNXs can be applied to the identification of novel therapeutic avenues for SNXs-related disorders.

Neural expression of sorting nexin 25 and its regulation of tyrosine receptor kinase B trafficking

Sorting nexin 25 (SNX25) belongs to the sorting nexin superfamily, whose members are responsible for membrane attachment to organelles of the endocytic system. Recent reports point to critical roles for SNX25 as a negative regulator of transforming growth factor β signaling, but the expression patterns of SNX25 in the central nervous system (CNS) remain almost uncharacterized. Here, we show widespread neuronal expression of SNX25 protein and Snx25 mRNA using immunohistochemistry and in situ hybridization. As an exception, SNX25 was present in the Bergmann glia of the cerebellum. SNX25 immunoreactivity was found in cholinergic and catecholaminergic neurons. Moreover, SNX25 colocalized with tropomyosin receptor kinase B (TrkB) in the neurons of the cortex and hippocampus. In vitro, SNX25 can interact with full-length TrkB, but not with its C-terminal-truncated isoform. Overexpression of SNX25 accelerated degradation of full-lengh TrkB, indicating that SNX25 promotes the trafficking of TrkB for lysosomal degradation. These findings suggest that SNX25 is a new actor in endocytic signaling, perhaps contributing to the regulation of BDNF-TrkB signaling in the CNS.

Isoflurane-induced expression of miR-140-5p aggravates neurotoxicity in diabetic rats by targeting SNX12

MicroRNAs (miRNAs) are widely known as critical regulators in isoflurane-induced neurotoxicity during the development of brain. Moreover, isoflurane could aggravate cognitive impairment in diabetic rats. The present study was designed to investigate the role and mechanism of miR-140-5p on isoflurane-induced neurotoxicity in diabetic rats. Firstly, a diabetic rat model was established by injection of streptozotocin (STZ) and identified by Morris water maze test. The result indicated that isoflurane treatment exacerbated STZ-induced cognitive impairment, as demonstrated by increase of the latency to the platform and decrease of the proportion of time spent in the target quadrant. Secondly, miR-140-5p was up-regulated in diabetic rats treated with isoflurane. Functional assays revealed that knockdown of miR-140-5p attenuated neurotoxicity in diabetic rats, which was shown by a decrease of the latency to the platform and an increase of the proportion of time spent in the target quadrant. Mechanistically, we demonstrated that miR-140-5p directly bonded to SNX12 (sorting nexin 12). At last, the neuroprotective effect of miR-140-5p knockdown against isoflurane-aggravated neurotoxicity in diabetic rats was dependent on up-regulation of SNX12 and inhibition of cell apoptosis. In summary, these meaningful results demonstrated the mitigation of miR-140-5p knockdown against isoflurane-aggravated neurotoxicity in diabetic rats via SNX12, suggesting a novel target for neuroprotection in diabetes under isoflurane treatment.

The Model of Aging Acceleration Network Reveals the Correlation of Alzheimer's Disease and Aging at System Level

As the incidence of senile dementia continues to increase, researches on Alzheimer's disease (AD) have become more and more important. Several studies have reported that there is a close relationship between AD and aging. Some researchers even pointed out that if we wanted to understand AD in depth, mechanisms of AD based on accelerated aging must be studied. Nowadays, machine learning techniques have been utilized to deal with large and complex profiles, thus playing an important role in disease researches (i.e., modelling biological systems, identifying key modules based on biological networks, and so on). Here, we developed an aging predictor and an AD predictor using machine learning techniques, respectively. Both aging and AD biomarkers were identified to provide insights into genes associated with AD. Besides, aging scores were calculated to reflect the aging process of brain tissues. As a result, the aging acceleration network and the aging-AD bipartite graph were constructed to delve into the relationship between AD and aging. Finally, a series of network and enrichment analyses were also conducted to gain further insights into the mechanisms of AD based on accelerated aging. In a word, our results indicated that aging may contribute to the development of AD by affecting the function of the immune system and the energy metabolism process, where the immune system may play a more prominent role in AD.

Essential and selective role of SNX12 in transport of endocytic and retrograde cargo

The endosomal protein sorting machineries play vital roles in diverse physiologically important cellular processes. Much of the core membrane sorting apparatus are conserved in evolution, such as retromer, involved in the recycling of a diverse set of cargoes via retrograde trafficking route. Here, using a RNAi based loss of function study, we identified that SNX12 when suppressed, leads to severe blockage in CIM6PR transport and alters the morphology of the endocytic compartments. We demonstrate that SNX12 is involved in the early phase of CIM6PR transport and mediates receptor recycling upstream of the other well established SNX components of retromer. Ultra-structural analysis revealed that SNX12 resides on tubulo-vesicular structures, inspite of lacking a BAR domain. Further, we illustrate that SNX12 plays a key role in intraluminal vesicle formation and in the maturation of a sub-population of early endosomes to late endosomes thereby regulating selective endocytic transport of cargo for degradation. This study therefore provides evidence for the existence of early endosomal sub-populations, which have differential roles in sorting of the cargoes along endocytic degradative pathways.

Epitope Fingerprinting for Recognition of the Polyclonal Serum Autoantibodies of Alzheimer's Disease

Autoantibodies (aAb) associated with Alzheimer's disease (AD) have not been sufficiently characterized and their exact involvement is undefined. The use of information technology and computerized analysis with phage display technology was used, in the present research, to map the epitope of putative self-antigens in AD patients. A 12-mer random peptide library, displayed on M13 phages, was screened using IgG from AD patients with two repetitions. Seventy-one peptides were isolated; however, only 10 were positive using the Elisa assay technique (Elisa Index > 1). The results showed that the epitope regions of the immunoreactive peptides, identified by phage display analysis, were on the exposed surfaces of the proteins. The putative antigens MAST1, Enah, MAO-A, X11/MINT1, HGF, SNX14, ARHGAP 11A, APC, and CENTG3, which have been associated with AD or have functions in neural tissue, may indicate possible therapeutic targets.

Dysregulation of protein trafficking in neurodegeneration

Intracellular protein trafficking plays an important role in neuronal function and survival. Protein misfolding is a common theme found in many neurodegenerative diseases, and intracellular trafficking machinery contributes to the pathological accumulation and clearance of misfolded proteins. Although neurodegenerative diseases exhibit distinct pathological features, abnormal endocytic trafficking is apparent in several neurodegenerative diseases, such as Alzheimer’s disease (AD), Down syndrome (DS) and Parkinson’s disease (PD). In this review, we will focus on protein sorting defects in three major neurodegenerative diseases, including AD, DS and PD. An important pathological feature of AD is the presence of extracellular senile plaques in the brain. Senile plaques are composed of β-amyloid (Aβ) peptide aggregates. Multiple lines of evidence demonstrate that over-production/aggregation of Aβ in the brain is a primary cause of AD and attenuation of Aβ generation has become a topic of extreme interest in AD research. Aβ is generated from β-amyloid precursor protein (APP) through sequential cleavage by β-secretase and the γ-secretase complex. Alternatively, APP can be cleaved by α-secretase within the Aβ domain to release soluble APPα which precludes Aβ generation. DS patients display a strikingly similar pathology to AD patients, including the generation of neuronal amyloid plaques. Moreover, all DS patients develop an AD-like neuropathology by their 40 s. Therefore, understanding the metabolism/processing of APP and how these underlying mechanisms may be pathologically compromised is crucial for future AD and DS therapeutic strategies. Evidence accumulated thus far reveals that synaptic vesicle regulation, endocytic trafficking, and lysosome-mediated autophagy are involved in increased susceptibility to PD. Here we review current knowledge of endosomal trafficking regulation in AD, DS and PD.

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.

Sorting nexin 12 interacts with BACE1 and regulates BACE1-mediated APP processing

Background

β-site APP cleaving enzyme 1 (BACE1) cleaves β-amyloid precursor protein (APP) to initiate the production of β-amyloid (Aβ), the prime culprit in Alzheimer’s disease (AD). Dysregulation of the intracellular trafficking of BACE1 may affect Aβ generation, contributing to AD pathology. In this study, we investigated whether BACE1 trafficking and BACE1-mediated APP processing/Aβ generation are affected by sorting nexin 12 (SNX12), a member of the sorting nexin (SNX) family that is involved in protein trafficking regulation.

Results

Herein, we find that SNX12 is widely expressed in brain tissues and is mainly localized in the early endosomes. Overexpression of SNX12 does not affect the steady-state levels of APP, BACE1 or γ-secretase components, but dramatically reduces the levels of Aβ, soluble APPβ and APP β-carboxyl terminal fragments. Downregulation of SNX12 has the opposite effects. Modulation of SNX12 levels does not affect γ-secretase activity or in vitro β-secretase activity. Further studies reveal that SNX12 interacts with BACE1 and downregulation of SNX12 accelerates BACE1 endocytosis and decreases steady-state level of cell surface BACE1. Finally, we find that the SNX12 protein level is dramatically decreased in the brain of AD patients as compared to that of controls.

Conclusion

This study demonstrates that SNX12 can regulate the endocytosis of BACE1 through their interaction, thereby affecting β-processing of APP for Aβ production. The reduced level of SNX12 in AD brains suggests that an alteration of SNX12 may contribute to AD pathology. Therefore, inhibition of BACE1-mediated β-processing of APP by regulating SNX12 might serve as an alternative strategy in developing an AD intervention.