Seipin regulates excitatory synaptic transmission in cortical neurons

Role of Seipin in Human Diseases and Experimental Animal Models

Seipin, a protein encoded by the Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) gene, is famous for its key role in the biogenesis of lipid droplets and type 2 congenital generalised lipodystrophy (CGL2). BSCL2 gene mutations result in genetic diseases including CGL2, progressive encephalopathy with or without lipodystrophy (also called Celia’s encephalopathy), and BSCL2-associated motor neuron diseases. Abnormal expression of seipin has also been found in hepatic steatosis, neurodegenerative diseases, glioblastoma stroke, cardiac hypertrophy, and other diseases. In the current study, we comprehensively summarise phenotypes, underlying mechanisms, and treatment of human diseases caused by BSCL2 gene mutations, paralleled by animal studies including systemic or specific Bscl2 gene knockout, or Bscl2 gene overexpression. In various animal models representing diseases that are not related to Bscl2 mutations, differential expression patterns and functional roles of seipin are also described. Furthermore, we highlight the potential therapeutic approaches by targeting seipin or its upstream and downstream signalling pathways. Taken together, restoring adipose tissue function and targeting seipin-related pathways are effective strategies for CGL2 treatment. Meanwhile, seipin-related pathways are also considered to have potential therapeutic value in diseases that are not caused by BSCL2 gene mutations.

Celia's Encephalopathy (BSCL2-Gene-Related): Current Understanding

Seipin, encoded by the BSCL2 gene, is a protein that in humans is expressed mainly in the central nervous system. Uniquely, certain variants in BSCL2 can cause both generalized congenital lipodystrophy type 2, upper and/or lower motor neuron diseases, or progressive encephalopathy, with a poor prognosis during childhood. The latter, Celia's encephalopathy, which may or may not be associated with generalized lipodystrophy, is caused by the c.985C >T variant. This cytosine to thymine transition creates a cryptic splicing zone that leads to intronization of exon 7, resulting in an aberrant form of seipin, Celia seipin. It has been proposed that the accumulation of this protein, both in the endoplasmic reticulum and in the nucleus of neurons, might be the pathogenetic mechanism of this neurodegenerative condition. In recent years, other variants in BSCL2 associated with generalized lipodystrophy and progressive epileptic encephalopathy have been reported. Interestingly, most of these variants could also lead to the loss of exon 7. In this review, we analyzed the molecular bases of Celia's encephalopathy and its pathogenic mechanisms, the clinical features of the different variants, and a therapeutic approach in order to slow down the progression of this fatal neurological disorder.

Impairment of respiratory muscle strength in Berardinelli-Seip congenital lipodystrophy subjects

BACKGROUND

Berardinelli-Seip Congenital Generalized Lipodystrophy (BSCL) is an ultra-rare metabolic disease characterized by hypertriglyceridemia, hyperinsulinemia, hyperglycemia, hypoleptinemia, and diabetes mellitus. Although cardiovascular disturbances have been observed in BSCL patients, there are no studies regarding the Respiratory Muscle Strength (RMS) in this type of lipodystrophy. This study aimed to evaluate RMS in BSCL subjects compared with healthy subjects.

METHODS

Eleven individuals with BSCL and 11 healthy subjects matched for age and gender were included in this study. The Maximum Inspiratory Pressure (MIP), Maximum Expiratory Pressure (MEP), and Peripheral Muscle Strength (PMS) were measured for three consecutive years. BSCL subjects were compared to healthy individuals for MIP, MEP, and PMS. Correlations between PMS and MIP were also analyzed. The genetic diagnosis was performed, and sociodemographic and anthropometric data were also collected.

RESULTS

BSCL subjects showed significantly lower values for MIP and MEP (p <  0.0001 and p = 0.0002, respectively) in comparison to healthy subjects, but no changes in handgrip strength (p = 0.15). Additionally, we did not observe changes in MIP, MEP, and PMS two years after the first analysis, showing maintenance of respiratory dysfunction in BSCL subjects (p = 0.05; p = 0.45; p = 0.99). PMS and MIP were not correlated in these subjects (r = 0.56; p = 0.18).

CONCLUSION

BSCL subjects showed lower respiratory muscle strength when compared with healthy subjects; however, PMS was not altered. These findings were maintained at similar levels during the two years of evaluation. Our data reveal the first association of BSCL with the development of respiratory muscle weakness.

Echinacoside's nigrostriatal dopaminergic protection against 6-OHDA-Induced endoplasmic reticulum stress through reducing the accumulation of Seipin

Parkinson's disease (PD) is one of the most common neurodegenerative diseases. Recent epidemiological studies suggest that echinacoside (ECH), a phenylethanoid glycoside found in Cistanche deserticola, has a protective effect against the development of PD. However, the detailed mechanisms of how ECH suppresses neuronal death have not been fully elucidated. In this study, we confirmed that ECH protects nigrostriatal neurons against 6‐hydroxydopamine (6‐OHDA)‐induced endoplasmic reticulum stress (ERS) in vivo and in vitro. ECH rescued cell viability in damaged cells and decreased 6‐OHDA‐induced reactive oxygen species accumulation in vitro. It also rescued tyrosine hydroxylase and dopamine transporter expression in the striatum, and decreased α‐synuclein aggregation following 6‐OHDA treatment in vivo. The validated mechanism of ECH activity was the reduction in the 6‐OHDA‐induced accumulation of seipin (Berardinelli–Seip congenital lipodystrophy 2). Seipin has been shown to be a key molecule related to motor neuron disease and was tightly associated with ERS in a series of in vivo studies. ECH attenuated seipinopathy by promoting seipin degradation via ubiquitination. ERS was relieved by ECH through the Grp94/Bip‐ATF4‐CHOP signal pathway.

Brefeldin A-inhibited guanine nucleotide exchange protein 3 is localized in lysosomes and regulates GABA signaling in hippocampal neurons

ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins regulates organelle biogenesis, structure and trafficking. The functions of ARF proteins are tightly controlled by guanine nucleotide exchange factors (GEFs) containing a conserved SEC7 domain. Based on sequence similarity to brefeldin A-inhibited guanine nucleotide exchange protein (BIG)/GBF of the Arf-GEF family, we recently identified BIG3 as a novel ARF GEF protein with a non-functional catalytic motif in the SEC7 domain. BIG3 is mainly expressed in pancreatic islets and brain. In the islets, depletion of BIG3 increases insulin and glucagon secretion because of enhanced biogenesis of insulin and glucagon granules in the absence of BIG3. Here, we investigate BIG3 functions in the brain, in particular its regulation of neurotransmitter release in hippocampal neurons from wild-type and BIG3 knockout mice. In hippocampal neurons, BIG3 is mainly localized in lysosomes, and its depletion selectively impairs inhibitory synaptic transmission. Our finding provides novel insights for a cell-specific function of BIG3 in regulating neurotransmission.

Whole Exome Sequencing Reveals a BSCL2 Mutation Causing Progressive Encephalopathy with Lipodystrophy (PELD) in an Iranian Pediatric Patient

Background:

Progressive encephalopathy with or without lipodystrophy is a rare autosomal recessive childhood-onset seipin-associated neurodegenerative syndrome, leading to developmental regression of motor and cognitive skills. In this study, we introduce a patient with developmental regression and autism. The causative mutation was found by exome sequencing.

Methods:

The proband showed a generalized hypertonia and regression of all developmental milestones. Based on the advantages of next-generation sequencing (NGS), whole exome sequencing (WES) was requested. The functional significance of variants was evaluated by NGS-specific prediction servers. Sanger sequencing was used for segregation analysis in the family.

Results:

There was no specific sign in the clinical and paraclinical investigations of the patient to establish a conclusive clinical diagnosis. WES detected a known homozygous nonsense mutation in BSCL2 (NM_001122955.3:c. 985C>T; p.Arg329*). The variant is segregating in the pedigree with an autosomal recessive pattern.

Conclusion:

Exome sequencing is a robust method for identifying the candidate gene variants in Mendelian traits.

Skipped BSCL2 Transcript in Celia's Encephalopathy (PELD): New Insights on Fatty Acids Involvement, Senescence and Adipogenesis

OBJECTIVE

PELD (Progressive Encephalopathy with or without Lipodystrophy or Celia's Encephalopathy) is a fatal and rare neurodegenerative syndrome associated with the BSCL2 mutation c.985C>T, that results in an aberrant transcript without the exon 7 (Celia seipin). The aim of this study was to evaluate both the process of cellular senescence and the effect of unsaturated fatty acids on preadipocytes from a homozygous c.985C>T patient. Also, the role of aberrant seipin isoform on adipogenesis was studied in adipose-derived human mesenchymal stem cells.

MATERIAL AND METHODS

Cellular senescence was evaluated using β-galactosidase staining of preadipocytes obtained from a homozygous c.985C>T patient. Moreover, these cells were cultured during 24 hours with Intralipid, a soybean oil-based commercial lipid emulsion. The expression of the different BSCL2 transcripts was measured by qPCR. Adipose-derived human mesenchymal stem cells were differentiated to a fat lineage using StemPRO adipogenesis kit, and the expression of BSCL2 transcripts and several adipogenesis-related genes was measured by qPCR.

RESULTS

the treatment of preadipocytes with unsaturated fatty acids significantly reduced the expression of the BSCL2 transcript without exon 7 by 34 to 63%. On the other hand, at least in preadipocytes, this mutation does not disturb cellular senescence rate. Finally, during adipocyte differentiation of adipose-derived human mesenchymal stem cells, the expression of adipogenic genes (PPARG, LPIN1, and LPL) increased significantly over 14 days, and noteworthy is that the BSCL2 transcript without exon 7 was differentially expressed by 332 to 723% when compared to day 0, suggesting an underlying role in adipogenesis.

CONCLUSIONS

our results suggest that Celia seipin is probably playing an underestimated role in adipocyte maturation, but not in senescence, and its expression can be modified by exogenous factors as fatty acids.

Activation of PPARγ Ameliorates Spatial Cognitive Deficits through Restoring Expression of AMPA Receptors in Seipin Knock-Out Mice

A characteristic phenotype of congenital generalized lipodystrophy 2 (CGL2) that is caused by loss-of-function of seipin gene is mental retardation. Here, we show that seipin deficiency in hippocampal CA1 pyramidal cells caused the reduction of peroxisome proliferator-activated receptor gamma (PPARγ). Twelve-week-old systemic seipin knock-out mice and neuronal seipin knock-out (seipin-nKO) mice, but not adipose seipin knock-out mice, exhibited spatial cognitive deficits as assessed by the Morris water maze and Y-maze, which were ameliorated by the treatment with the PPARγ agonist rosiglitazone (rosi). In addition, seipin-nKO mice showed the synaptic dysfunction and the impairment of NMDA receptor-dependent LTP in hippocampal CA1 regions. The density of AMPA-induced current (IAMPA) in CA1 pyramidal cells and GluR1/GluR2 expression were significantly reduced in seipin-nKO mice, whereas the NMDA-induced current (INMDA) and NR1/NR2 expression were not altered. Rosi treatment in seipin-nKO mice could correct the decrease in expression and activity of AMPA receptor (AMPAR) and was accompanied by recovered synaptic function and LTP induction. Furthermore, hippocampal ERK2 and CREB phosphorylation in seipin-nKO mice were reduced and this could be rescued by rosi treatment. Rosi treatment in seipin-nKO mice elevated BDNF concentration. The MEK inhibitor U0126 blocked rosi-restored AMPAR expression and LTP induction in seipin-nKO mice, but the Trk family inhibitor K252a did not. These findings indicate that the neuronal seipin deficiency selectively suppresses AMPAR expression through reducing ERK-CREB activities, leading to the impairment of LTP and spatial memory, which can be rescued by PPARγ activation.

SIGNIFICANCE STATEMENT

Congenital generalized lipodystrophy 2 (CGL2), caused by loss-of-function mutation of seipin gene, is characterized by mental retardation. By the generation of systemic or neuronal seipin knock-out mice, the present study provides in vivo evidence that neuronal seipin deficiency causes deficits in spatial memory and hippocampal LTP induction. Neuronal seipin deficiency selectively suppresses AMPA receptor expression, ERK-CREB phosphorylation with the decline of PPARγ. The PPARγ agonist rosiglitazone can ameliorate spatial cognitive deficits and rescue the LTP induction in seipin knock-out mice by restoring AMPA receptor expression and ERK-CREB activities.

How Research on Human Progeroid and Antigeroid Syndromes Can Contribute to the Longevity Dividend Initiative

Although translational applications derived from research on basic mechanisms of aging are likely to enhance health spans and life spans for most of us (the longevity dividend), there will remain subsets of individuals with special vulnerabilities. Medical genetics is a discipline that describes such "private" patterns of aging and can reveal underlying mechanisms, many of which support genomic instability as a major mechanism of aging. We review examples of three classes of informative disorders: "segmental progeroid syndromes" (those that appear to accelerate multiple features of aging), "unimodal progeroid syndromes" (those that impact on a single disorder of aging), and "unimodal antigeroid syndromes," variants that provide enhanced protection against specific disorders of aging; we urge our colleagues to expand our meager research efforts on the latter, including ancillary somatic cell genetic approaches.

Clinical and Molecular Characterization of BSCL2 Mutations in a Taiwanese Cohort with Hereditary Neuropathy

BACKGROUND

A small group of patients with inherited neuropathy that has been shown to be caused by mutations in the BSCL2 gene. However, little information is available about the role of BSCL2 mutations in inherited neuropathies in Taiwan.

METHODOLOGY AND PRINCIPAL FINDINGS

Utilizing targeted sequencing, 76 patients with molecularly unassigned Charcot-Marie-Tooth disease type 2 (CMT2) and 8 with distal hereditary motor neuropathy (dHMN), who were selected from 348 unrelated patients with inherited neuropathies, were screened for mutations in the coding regions of BSCL2. Two heterozygous BSCL2 mutations, p.S90L and p.R96H, were identified, of which the p.R96H mutation is novel. The p.S90L was identified in a pedigree with CMT2 while the p.R96H was identified in a patient with apparently sporadic dHMN. In vitro studies demonstrated that the p.R96H mutation results in a remarkably low seipin expression and reduced cell viability.

CONCLUSION

BSCL2 mutations account for a small number of patients with inherited neuropathies in Taiwan. The p.R96H mutation is associated with dHMN. This study expands the molecular spectrum of BSCL2 mutations and also emphasizes the pathogenic role of BSCL2 mutations in molecularly unassigned hereditary neuropathies.

Clinical and laboratory data of a large series of patients with congenital generalized lipodystrophy

BACKGROUND

Berardinelli-Seip congenital lipodystrophy (BSCL) was initially described by Berardinelli in Brazil in 1954 and 5 years later by Seip in Norway. It is an autosomal recessive disease that leads to a generalized deficit of body fat, evolving with diabetes and hypertriglyceridemia. The aim of this study was to describe the clinical and laboratory characteristics of a large series of patients with BSCL.

METHODS

This is a cross-sectional study of patients with BSCL. A total of 54 cases of BSCL were diagnosed, treated and followed for the past 17 years. We report clinical and laboratorial data of 44 of those patients.

RESULTS

There was a predominance of female patients (27 patients), and the mean age was 21.3 ± 13.7 years old. The majority of patients (30/44; 68.2 %) were diabetic, and almost half of them (14/30 patients, 46.7 %) were on insulin. The mean body mass index was 19.6 ± 3.3 and the mean body fat measured by dual-energy X-ray absorptiometry (DEXA) was 5.4 ± 0.8 %. Acanthosis nigricans, acromegaloid facies, atrophic cheeks, prognathism, phlebomegaly, and muscle hypertrophy were the most common clinical features. Only two patients had triglyceridemia lower than 150 mg/dl without the use of lipid-lowering drugs. Hyperinsulinemia was present in the majority of patients, and leptin values were very low in all patients.

CONCLUSIONS

We report one of the largest series of patients with BSCL treated at a single medical center. Earlier identification of the mutations and a better understanding of the pathophysiology can aid to better treatment and decrease complications, potentially improving life quality and expectancy.

Hexa (ethylene glycol) derivative of benzothiazole aniline promotes dendritic spine formation through the RasGRF1-Ras dependent pathway

Our recent study demonstrated that an amyloid-β binding molecule, BTA-EG4, increases dendritic spine number via Ras-mediated signaling. To potentially optimize the potency of the BTA compounds, we synthesized and evaluated an amyloid-β binding analog of BTA-EG4 with increased solubility in aqueous solution, BTA-EG6. We initially examined the effects of BTA-EG6 on dendritic spine formation and found that BTA-EG6-treated primary hippocampal neurons had significantly increased dendritic spine number compared to control treatment. In addition, BTA-EG6 significantly increased the surface level of AMPA receptors. Upon investigation into the molecular mechanism by which BTA-EG6 promotes dendritic spine formation, we found that BTA-EG6 may exert its effects on spinogenesis via RasGRF1-ERK signaling, with potential involvement of other spinogenesis-related proteins such as Cdc42 and CDK5. Taken together, our data suggest that BTA-EG6 boosts spine and synapse number, which may have a beneficial effect of enhancing neuronal and synaptic function in the normal healthy brain.

The expression of SEIPIN in the mouse central nervous system

Immunohistochemical staining was used to investigate the expression pattern of SEIPIN in the mouse central nervous system. SEIPIN was found to be present in a large number of areas, including the motor and somatosensory cortex, the thalamic nuclei, the hypothalamic nuclei, the mesencephalic nuclei, some cranial motor nuclei, the reticular formation of the brainstem, and the vestibular complex. Double labeling with NeuN antibody confirmed that SEIPIN-positive cells in some nuclei were neurons. Retrograde tracer injections into the spinal cord revealed that SEIPIN-positive neurons in the motor and somatosensory cortex and other movement related nuclei project to the mouse spinal cord. The present study found more nuclei positive for SEIPIN than shown using in situ hybridization and confirmed the presence of SEIPIN in neurons projecting to the spinal cord. The results of this study help to explain the clinical manifestations of patients with Berardinelli-Seip congenital lipodystrophy (Bscl2) gene mutations.

Larger aggregates of mutant seipin in Celia's Encephalopathy, a new protein misfolding neurodegenerative disease

Celia's Encephalopathy (MIM #615924) is a recently discovered fatal neurodegenerative syndrome associated with a new BSCL2 mutation (c.985C>T) that results in an aberrant isoform of seipin (Celia seipin). This mutation is lethal in both homozygosity and compounded heterozygosity with a lipodystrophic BSCL2 mutation, resulting in a progressive encephalopathy with fatal outcomes at ages 6-8. Strikingly, heterozygous carriers are asymptomatic, conflicting with the gain of toxic function attributed to this mutation. Here we report new key insights about the molecular pathogenic mechanism of this new syndrome. Intranuclear inclusions containing mutant seipin were found in brain tissue from a homozygous patient suggesting a pathogenic mechanism similar to other neurodegenerative diseases featuring brain accumulation of aggregated, misfolded proteins. Sucrose gradient distribution showed that mutant seipin forms much larger aggregates as compared with wild type (wt) seipin, indicating an impaired oligomerization. On the other hand, the interaction between wt and Celia seipin confirmed by coimmunoprecipitation (CoIP) assays, together with the identification of mixed oligomers in sucrose gradient fractionation experiments can explain the lack of symptoms in heterozygous carriers. We propose that the increased aggregation and subsequent impaired oligomerization of Celia seipin leads to cell death. In heterozygous carriers, wt seipin might prevent the damage caused by mutant seipin through its sequestration into harmless mixed oligomers.

Towards a mechanistic understanding of lipodystrophy and seipin functions

CGL (Congenital generalized lipodystrophy) is a genetic disorder characterized by near complete loss of adipose tissue along with increased ectopic fat storage in other organs including liver and muscle. Of the four CGL types, BSCL2 (Berardinelli-Seip Congenital lipodystrophy type 2), resulting from mutations in the BSCL2/seipin gene, exhibits the most severe lipodystrophic phenotype with loss of both metabolic and mechanical adipose depots. The majority of Seipin mutations cause C-terminal truncations, along with a handful of point mutations. Seipin localizes to the ER and is composed of a conserved region including a luminal loop and two transmembrane domains, plus cytosolic N- and C-termini. Animal models deficient in seipin recapitulate the human lipodystrophic phenotype. Cells isolated from seipin knockout mouse models also exhibit impaired adipogenesis. Mechanistically, seipin appears to function as a scaffolding protein to bring together interacting partners essential for lipid metabolism and LD (lipid droplet) formation during adipocyte development. Moreover, cell line and genetic studies indicate that seipin functions in a cell-autonomous manner. Here we will provide a brief overview of the genetic association of the CGLs, and focus on the current understanding of differential contributions of distinct seipin domains to lipid storage and adipogenesis. We will also discuss the roles of seipin-interacting partners, including lipin 1 and 14-3-3β, in mediating seipin-dependent regulation of cellular pathways such as actin cytoskeletal remodelling.

Seipin differentially regulates lipogenesis and adipogenesis through a conserved core sequence and an evolutionarily acquired C-terminus

Homozygous mutations in BSCL2 (Berardinelli–Seip congenital lipodystrophy)/seipin cause CGL2 (congenital generalized lipodystrophy type 2). Recent data suggest that seipin regulates LD (lipid droplet) dynamics and adipocyte differentiation, but whether these roles are mechanistically linked remains unclear. To understand how seipin regulates these processes, we investigated the evolutionary changes of seipin orthologues, and studied individual domains in regulating lipid accumulation in non-adipocytes and adipocytes. Mammalian seipins comprise at least two distinct functional domains, a conserved core sequence and an evolutionarily acquired C-terminus. Despite its requirement for adipocyte formation, seipin overexpression inhibited oleate-induced LD formation and accumulation in nonadipocytes, which was mediated by the core sequence. In contrast, seipin overexpression did not inhibit LD accumulation during adipocyte differentiation or the adipogenic process in 3T3-L1 cells. However, adipogenesis and LD accumulation were impaired in 3T3-L1 cells expressing a seipin mutant lacking the C-terminus. Furthermore, expression of the same mutant without the C-terminus failed to rescue the adipogenic defects in seipin-knockdown cells, demonstrating the importance of the C-terminus for seipin's function in adipocyte development. We propose that seipin is involved in lipid homoeostasis by restricting lipogenesis and LD accumulation in non-adipocytes, while promoting adipogenesis to accommodate excess energy storage.

A new seipin-associated neurodegenerative syndrome

BACKGROUND

Seipin/BSCL2 mutations can cause type 2 congenital generalised lipodystrophy (BSCL) or dominant motor neurone diseases. Type 2 BSCL is frequently associated with some degree of intellectual impairment, but not to fatal neurodegeneration. In order to unveil the aetiology and pathogenetic mechanisms of a new neurodegenerative syndrome associated with a novel BSCL2 mutation, six children, four of them showing the BSCL features, were studied.

METHODS

Mutational and splicing analyses of BSCL2 were performed. The brain of two of these children was examined postmortem. Relative expression of BSCL2 transcripts was analysed by real-time reverse transcription-polymerase chain reaction (RT-PCR) in different tissues of the index case and controls. Overexpressed mutated seipin in HeLa cells was analysed by immunofluorescence and western blotting.

RESULTS

Two patients carried a novel homozygous c.985C>T mutation, which appeared in the other four patients in compound heterozygosity. Splicing analysis showed that the c.985C>T mutation causes an aberrant splicing site leading to skipping of exon 7. Expression of exon 7-skipping transcripts was very high with respect to that of the non-skipped transcripts in all the analysed tissues of the index case. Neuropathological studies showed severe neurone loss, astrogliosis and intranuclear ubiquitin(+) aggregates in neurones from multiple cortical regions and in the caudate nucleus.

CONCLUSIONS

Our results suggest that exon 7 skipping in the BSCL2 gene due to the c.985C>T mutation is responsible for a novel early onset, fatal neurodegenerative syndrome involving cerebral cortex and basal ganglia.