Fatty acid binding protein 4 (FABP4, also known as aP2) is a cytoplasmic fatty acid chaperone expressed primarily in adipocytes and myeloid cells and implicated in the development of insulin resistance and atherosclerosis. Here we demonstrate that FABP4 triggers the ubiquitination and subsequent proteasomal degradation of peroxisome proliferator-activated receptor g (PPARg), a master regulator of adipogenesis and insulin responsiveness. Importantly, FABP4-null mouse preadipocytes as well as macrophages exhibited increased expression of PPARg, and complementation of FABP4 in the macrophages reversed the increase in FABP4 expression. The FABP4-null preadipocytes exhibited a remarkably enhanced adipogenesis compared with wild-type cells, indicating that FABP4 regulates adipogenesis by downregulating PPARg. We found that the FABP4 level was higher and PPARg level was lower in human visceral fat and mouse epididymal fat compared with their subcutaneous fat. Furthermore, FABP4 was higher in the adipose tissues of obese diabetic individuals compared with healthy ones. Suppression of PPARg by FABP4 in visceral fat may explain the reported role of FABP4 in the development of obesity-related morbidities, including insulin resistance, diabetes, and atherosclerosis. Adiposity is closely correlated with important physiological parameters such as blood pressure, systemic insulin sensitivity, dyslipidemia, and serum triglyceride levels (1,2), rendering obesity to an independent risk factor for myocardial infarction, stroke, type 2 diabetes, and certain cancers (3). Among adipose tissues, visceral fat is more closely correlated with obesity-associated pathologies than overall adiposity (4-8).The nuclear receptor peroxisome proliferatoractivated receptor g (PPARg) is a master regulator of adipose cell differentiation, playing a critical role in systemic lipid and glucose metabolism (9). PPARg is activated by natural or synthetic agonists such as the antidiabetic thiazolidinedione (TZD) (10). Activated PPARg is a master regulator of adipogenesis, acting as a transcription factor of genes expressed in mature adipocytes, including fatty acid binding protein (FABP4), CD36, lipoprotein lipase (LPL), and adiponectin, all of which contain peroxisome proliferator response elements (PPREs) (11). PPARg is also expressed in myeloid cells, and its activation promotes an anti-inflammatory phenotype (11). Disruption of PPARg specifically in myeloid cells also predisposes METABOLISMmice to the development of diet-induced obesity, insulin resistance, and glucose intolerance (12), whereas activation of PPARg within macrophages promotes lipid efflux, thereby stabilizing atherosclerotic lesions (13). A major target gene of PPARg is the lipid transporter FABP4, also known as aP2 (14). PPARg induces FABP4 almost exclusively in adipocytes and macrophages. FABP4 acts as a fatty acids chaperone, which couples intracellular lipids to biological targets and signaling pathways (15,16). FABP4 has been implicated in several aspects of the metabolic syndro...

The N-methyl-D-aspartate receptors (NMDARs; GluNRS) are glutamate receptors, commonly located at excitatory synapses. Mutations affecting receptor function often lead to devastating neurodevelopmental disorders. We have identified two toddlers with different heterozygous missense mutations of the same, and highly conserved, glycine residue located in the ligand-binding-domain of GRIN2B: G689C and G689S. Structure simulations suggest severely impaired glutamate binding which we confirm by functional analysis. Both variants show three-orders of magnitude reductions in glutamate EC50, with G689S exhibiting the largest reductions observed in GRIN2B (~2000-fold). Moreover, variants multimerize with, and upregulate, GluN2Bwt-subunits, thus engendering a strong dominant-negative effect on mixed channels. In neurons, overexpression of the variants instigates suppression of synaptic GluNRs. Lastly, while exploring spermine potentiation as a potential treatment, we discovered that the variants fail to respond due to G689's novel role in proton-sensing. Together, we describe two unique variants with extreme effects on channel function. We employ protein-stability measures to explain why current (and future) LBD mutations in GluN2B primarily instigate Loss-of-Function.

Autism Spectrum Disorder (ASD) is mainly characterized by social and sensory-motor abnormal and repetitive behavior patterns. Over 1000 genetic variants were reported to be highly penetrant and causative of ASD. Many of these mutations cause comorbidities such as epilepsy and intellectual disabilities (ID). In this study, we measured cortical neurons derived from induced pluripotent stem cells (iPSCs) of patients with four mutations in the genes GRIN2B, SHANK3, UBTF, as well as chromosomal duplication in the 7q11.23 region. Using a whole-cell patch-clamp, we observed that the mutant cortical neurons demonstrated hyperexcitability and early maturation compared to control lines. These changes were characterized by higher sodium current, higher amplitude and rates of excitatory postsynaptic currents (EPSCs), and more evoked action potentials in response to current stimulation in early-stage cell development (3-5 weeks post differentiation). These changes that appeared in all the different mutant lines, together with previously reported data, indicate that an early maturation and hyperexcitability may be a convergent phenotype of ASD cortical neurons.

No abstract

Autism Spectrum Disorder (ASD) is characterized mainly by social and sensory-motor abnormal and repetitive behavior patterns. Over hundreds of genes and thousands of genetic variants were reported to be highly penetrant and causative of ASD. Many of these mutations cause comorbidities such as epilepsy and intellectual disabilities (ID). In this study, we measured cortical neurons derived from induced pluripotent stem cells (iPSCs) of patients with four mutations in the genes GRIN2B, SHANK3, UBTF, as well as chromosomal duplication in the 7q11.23 region and compared them to neurons derived from a first-degree relative without the mutation. Using a whole-cell patch-clamp, we observed that the mutant cortical neurons demonstrated hyperexcitability and early maturation compared to control lines. These changes were characterized by increased sodium currents, increased amplitude and rate of excitatory postsynaptic currents (EPSCs), and more evoked action potentials in response to current stimulation in early-stage cell development (3–5 weeks post differentiation). These changes that appeared in all the different mutant lines, together with previously reported data, indicate that an early maturation and hyperexcitability may be a convergent phenotype of ASD cortical neurons.