The carboxy-terminal region of the TBC1D4 (AS160) RabGAP mediates protein homodimerization

Osteoblasts induce glucose-derived ATP perturbations in chondrocytes through noncontact communication

Cartilage and subchondral bone communicate with each other through material and signal exchanges. However, direct evidence provided by experimental studies on their interactions is insufficient. In the present study, we establish a noncontact co-culture model with a transwell chamber to explore the energetic perturbations in chondrocytes influenced by osteoblasts. Our results indicate that osteoblasts induce more ATP generation in chondrocytes through an energetic shift characterized by enhanced glycolysis and impaired mitochondrial tricarboxylic acid cycle. Enhanced glycolysis is shown by an increase of secreted lactate and the upregulation of glycolytic enzymes, including glucose-6-phosphate isomerase (Gpi), liver type ATP-dependent 6-phosphofructokinase (Pfkl), fructose-bisphosphate aldolase C (Aldoc), glyceraldehyde-3-phosphate dehydrogenase (Gapdh), triosephosphate isomerase (Tpi1), and phosphoglycerate kinase 1 (Pgk1). Impaired mitochondrial tricarboxylic acid cycle is characterized by the downregulation of cytoplasmic aspartate aminotransferase (Got1) and mitochondrial citrate synthase (Cs). Osteoblasts induce the activation of Akt and P38 signaling to mediate ATP perturbations in chondrocytes. This study may deepen our understanding of the maintenance of metabolic homeostasis in the bone-cartilage unit.

Illumination of the Endogenous Insulin-Regulated TBC1D4 Interactome in Human Skeletal Muscle

Insulin-stimulated muscle glucose uptake is a key process in glycemic control. This process depends on the redistribution of glucose transporters to the surface membrane, a process that involves regulatory proteins such as TBC1D1 and TBC1D4. Accordingly, a TBC1D4 loss-of-function mutation in human skeletal muscle is associated with an increased risk of type 2 diabetes, and observations from carriers of a TBC1D1 variant associate this protein to a severe obesity phenotype. Here, we identified interactors of the endogenous TBC1D4 protein in human skeletal muscle by an unbiased proteomics approach. We detected 76 proteins as candidate TBC1D4 interactors. The binding of 12 of these interactors was regulated by insulin, including proteins known to be involved in glucose metabolism (e.g., 14-3-3 proteins and α-actinin-4 [ACTN4]). TBC1D1 also coprecipitated with TBC1D4 and vice versa in both human and mouse skeletal muscle. This interaction was not regulated by insulin or exercise in young, healthy, lean individuals. Similarly, the exercise- and insulin-regulated phosphorylation of the TBC1D1-TBC1D4 complex was intact. In contrast, we observed an altered interaction as well as compromised insulin-stimulated phosphoregulation of the TBC1D1-TBC1D4 complex in muscle of obese individuals with type 2 diabetes. Altogether, we provide a repository of TBC1D4 interactors in human and mouse skeletal muscle that serve as potential regulators of TBC1D4 function and, thus, insulin-stimulated glucose uptake in human skeletal muscle.

AKT/AMPK-mediated phosphorylation of TBC1D4 disrupts the interaction with insulin-regulated aminopeptidase

TBC1D4 is a 160 kDa multidomain Rab GTPase-activating protein (RabGAP) and a downstream target of the insulin- and contraction-activated kinases AKT and AMPK. Phosphorylation of TBC1D4 has been linked to translocation of GLUT4 from storage vesicles (GSVs) to the cell surface. However, its impact on enzymatic activity is not well understood, as previous studies mostly investigated the truncated GAP domain lacking the known phosphorylation sites. In the present study, we expressed and purified recombinant full-length TBC1D4 using a baculovirus system. Size-exclusion chromatography and coimmunoprecipitation experiments revealed that full-length TBC1D4 forms oligomers of ∼600 kDa. Compared with the truncated GAP domain, full-length TBC1D4 displayed similar substrate specificity, but had a markedly higher specific GAP activity toward Rab10. Using high-resolution mass spectrometry, we mapped 19 Ser/Thr phosphorylation sites in TBC1D4. We determined Michaelis–Menten kinetics using in vitro phosphorylation assays with purified kinases and stable isotope-labeled γ-[¹⁸O₄]-ATP. These data revealed that Ser³²⁴ (K_M ∼6 μM) and Thr⁶⁴⁹ (K_M ∼25 μM) were preferential sites for phosphorylation by AKT, whereas Ser³⁴⁸, Ser⁵⁷⁷, Ser⁵⁹⁵ (K_M ∼10 μM), Ser⁷¹¹ (K_M ∼79 μM), and Ser⁷⁶⁴ were found to be preferred targets for AMPK. Phosphorylation of TBC1D4 by AKT or AMPK did not alter the intrinsic RabGAP activity, but did disrupt interaction with insulin-regulated aminopeptidase (IRAP), a resident protein of GSVs implicated in GLUT4 trafficking. These findings provide evidence that insulin and contraction may regulate TBC1D4 function primarily by disrupting the recruitment of the RabGAP to GLUT4 vesicles.

New Classification of Macrophages in Plaques: a Revolution

PURPOSE

Macrophages play vital roles in the development of atherosclerosis in responding to lipid accumulation and inflammation. Macrophages were classified as inflammatory (M1) and alternatively activated (M2) macrophage types based on results of in vitro experiments. On the other hand, the composition of macrophages in vivo is more complex and remains unresolved. This review summarizes the transcriptional variations of macrophages in atherosclerosis plaques that were discovered by single-cell RNA sequencing (scRNA-seq) to better understand their contribution to atherosclerosis.

RECENT FINDINGS

ScRNA-seq provides a more detailed transcriptional landscape of macrophages in atherosclerosis, which challenges the traditional view. By mining the data of GSE97310, we discovered the transcriptional variations of macrophages in LDLR^-/- mice that were fed with high-fat diet (HFD) for 11 and 20 weeks. Cells were represented in a two-dimensional tSNE plane and clusters were identified and annotated via Seurat and SingleR respectively, which were R toolkits for single-cell genomics. The results showed that in healthy conditions, Trem2^hi (high expression of triggering receptors expressed on myeloid cells 2)-positive, inflammatory, and resident-like macrophages make up 68%, 18%, and 6% of total macrophages respectively. When mice were fed with HFD for 11 weeks, Trem2^hi, monocytes, and monocyte-derived dendritic cells take possession of 40%, 18%, and 17% of total macrophages respectively. After 20 weeks of HFD feeding, Trem2^hi, inflammatory, and resident-like macrophages occupied 12%, 37%, and 35% of total macrophages respectively. The phenotypes of macrophages are very different from the previous studies. In general, Trem2^hi macrophages are the most abundant population in healthy mice, while the proportion of monocytes increases after 11 weeks of HFD. Most importantly, inflammatory and resident-like macrophages make up 70% of the macrophage populations after 20 weeks of HFD. These strongly indicate that inflammatory and resident-like macrophages promote the progression of atherosclerosis plaques.

Cooperative actions of Tbc1d1 and AS160/Tbc1d4 in GLUT4-trafficking activities

AS160 and Tbc1d1 are key Rab GTPase-activating proteins (RabGAPs) that mediate release of static GLUT4 in response to insulin or exercise-mimetic stimuli, respectively, but their cooperative regulation and its underlying mechanisms remain unclear. By employing GLUT4 nanometry with cell-based reconstitution models, we herein analyzed the functional cooperative activities of the RabGAPs. When both RabGAPs are present, Tbc1d1 functionally dominates AS160, and stimuli-inducible GLUT4 release relies on Tbc1d1-evoking proximal stimuli, such as AICAR and intracellular Ca²⁺ Detailed functional assessments with varying expression ratios revealed that AS160 modulates sensitivity to external stimuli in Tbc1d1-mediated GLUT4 release. For example, Tbc1d1-governed GLUT4 release triggered by Ca²⁺ plus insulin occurred more efficiently than that in cells with little or no AS160. Series of mutational analyses revealed that these synergizing actions rely on the phosphotyrosine-binding 1 (PTB1) and calmodulin-binding domains of Tbc1d1 as well as key phosphorylation sites of both AS160 (Thr⁶⁴²) and Tbc1d1 (Ser²³⁷ and Thr⁵⁹⁶). Thus, the emerging cooperative governance relying on the multiple regulatory nodes of both Tbc1d1 and AS160, functioning together, plays a key role in properly deciphering biochemical signals into a physical GLUT4 release process in response to insulin, exercise, and the two in combination.

AKT and AMP-activated protein kinase regulate TBC1D1 through phosphorylation and its interaction with the cytosolic tail of insulin-regulated aminopeptidase IRAP

In skeletal muscle, the Rab GTPase-activating (GAP) protein TBC1D1 is phosphorylated by AKT and AMP-activated protein kinase (AMPK) in response to insulin and muscle contraction. Genetic ablation of Tbc1d1 or mutation of distinct phosphorylation sites impairs intracellular GLUT4 retention and GLUT4 traffic, presumably through alterations of the activation state of downstream Rab GTPases. Previous studies have focused on characterizing the C-terminal GAP domain of TBC1D1 that lacks the known phosphorylation sites, as well as putative regulatory domains. As a result, it has been unclear how phosphorylation of TBC1D1 would regulate its activity. In the present study, we have expressed, purified, and characterized recombinant full-length TBC1D1 in Sf9 insect cells via the baculovirus system. Full-length TBC1D1 showed RabGAP activity toward GLUT4-associated Rab8a, Rab10, and Rab14, indicating similar substrate specificity as the truncated GAP domain. However, the catalytic activity of the full-length TBC1D1 was markedly higher than that of the GAP domain. Although in vitro phosphorylation of TBC1D1 by AKT or AMPK increased 14-3-3 binding, it did not alter the intrinsic RabGAP activity. However, we found that TBC1D1 interacts through its N-terminal PTB domains with the cytoplasmic domain of the insulin-regulated aminopeptidase, a resident protein of GLUT4 storage vesicles, and this binding is disrupted by phosphorylation of TBC1D1 by AKT or AMPK. In summary, our findings suggest that other regions outside the GAP domain may contribute to the catalytic activity of TBC1D1. Moreover, our data indicate that recruitment of TBC1D1 to GLUT4-containing vesicles and not its GAP activity is regulated by insulin and contraction-mediated phosphorylation.