Backbone and sidechain 1H, 13C and 15N resonance assignments of the human brain-type fatty acid binding protein (FABP7) in its apo form and the holo forms binding to DHA, oleic acid, linoleic acid and elaidic acid

A combined computational-biophysical approach to understanding fatty acid binding to FABP7

Members of the fatty acid binding protein (FABP) family function as intracellular transporters of long-chain fatty acids and other hydrophobic molecules to different cellular compartments. Brain FABP (FABP7) exhibits ligand-directed differences in cellular transport. For example, when FABP7 binds to docosahexaenoic acid (DHA), the complex relocates to the nucleus and influences transcriptional activity, whereas FABP7 bound with monosaturated fatty acids remains in the cytosol. Preferential binding of FABP7 to polyunsaturated fatty acids like DHA has been previously observed and is thought to play a role in differential localization. However, we find that at 37°C, FABP7 does not display strong selectivity, suggesting that the conformational ensemble of FABP7 and its perturbation upon binding may be important. We use molecular dynamics simulations, NMR, and a variety of biophysical techniques to better understand the conformational ensemble of FABP7, how it is perturbed by fatty acid binding, and how this may be related to ligand-directed transport. We find that FABP7 has high degree of conformational heterogeneity that is substantially reduced upon ligand binding. We also observe substantial heterogeneity in ligand binding poses, which is consistent with our finding that ligand binding is resistant to mutations in key polar residues in the binding pocket. Our NMR experiments show that DHA binding leads to chemical shift perturbations in residues near the nuclear localization signal, which may point toward a mechanism of differential transport.

Understanding FABP7 binding to fatty acid micelles and membranes

Fatty acid-binding proteins (FABPs) are chaperones that facilitate the transport of long-chain fatty acids within the cell and can provide cargo-dependent localization to specific cellular compartments. Understanding the nature of this transport is important because lipid signaling functions are associated with metabolic pathways impacting disease pathologies including cancer, autism, and schizophrenia. FABPs often associate with cell membranes to acquire and deliver their bound cargo as part of transport. We focus on brain FABP (FABP7), which demonstrates localization to the cytoplasm and nucleus, influencing transcription and fatty acid metabolism. We use a combined biophysical-computational approach to elucidate the interaction between FABP7 and model membranes. Specifically, we use multiple experiments to demonstrate that FABP7 can bind oleic acid and docosahexaenoic acid micelles. Data from NMR and multiscale molecular dynamics simulations reveal that the interaction with micelles is through FABP7's portal region residues. Simulations suggest that binding to membranes occurs through the same residues as micelles. Simulations also capture binding events where fatty acids dissociate from the membrane and enter FABP7's binding pocket. Overall, our data shed light on the interactions between FABP7 and OA or DHA micelles and provide insight into the transport of long-chain fatty acids.

BLBP Is Both a Marker for Poor Prognosis and a Potential Therapeutic Target in Paediatric Ependymoma

Paediatric ependymomas are aggressive, treatment-resistant tumours with a tendency towards relapse, consistent with a sub-population of therapy-resistant cancer stem cells. These cells are believed to derive from brain lipid binding protein (BLBP)-expressing radial glia, hence we proposed that BLBP may be a marker for ependymoma therapy resistance. BLBP protein expression correlated with reduced overall survival (OS) in patients from two trials (CNS9204, a chemotherapy-led infant trial-5 y OS 45% vs. 80%, p = 0.011-and CNS9904, a radiotherapy-led trial-OS 38% vs. 85%, p = 0.002). All ependymoma cell lines examined by qRT-PCR expressed BLBP, with expression elevated in stem cell-enriched neurospheres. Modulation of BLBP function in 2D and 3D assays, using either peroxisome proliferator activated receptor (PPAR) antagonists or BLBP's fatty acid substrate docosahexaneoic acid (DHA), potentiated chemotherapy response and reduced cell migration and invasion in ependymoma cell lines. BLBP is therefore an independent predictor of poor survival in paediatric ependymoma, and treatment with PPAR antagonists or DHA may represent effective novel therapies, preventing chemotherapy resistance and invasion in paediatric ependymoma patients.

Hydrogen-bonded His93 as a sensitive probe for identifying inhibitors of the endocannabinoid transport protein FABP7

The human brain FABP (FABP7) has been shown to be an intracellular carrier protein that can significantly potentiate the uptake of the endocannabinoid anandamide. For this reason, there is a great interest in the discovery and development of FABP7 inhibitors for treating stress, pain, inflammation, and drug abuse. We found that in the (1) H-NMR spectrum of the protein, a well-separated downfield resonance arising from the hydrogen-bonded His93 side chain is very sensitive to ligand binding. Using this characteristic spectral marker together with another well-resolved upfield resonance from the side chain of Val84, we have identified that an adipocyte FABP (FABP4) inhibitor BMS309403 also binds tightly to FABP7. Our data demonstrated that this unique His93 downfield resonance can be used as a sensitive probe for rapidly and unambiguously identifying novel high-affinity FABP7 ligands. The findings should help accelerate the discovery of potential drug leads for the modulation of endocannabinoid transport.

Probing the interaction of brain fatty acid binding protein (B-FABP) with model membranes

Brain fatty acid-binding protein (B-FABP) interacts with biological membranes and delivers polyunsaturated fatty acids (FAs) via a collisional mechanism. The binding of FAs in the protein and the interaction with membranes involve a motif called “portal region”, formed by two small α-helices, A1 and A2, connected by a loop. We used a combination of site-directed mutagenesis and electron spin resonance to probe the changes in the protein and in the membrane model induced by their interaction. Spin labeled B-FABP mutants and lipidic spin probes incorporated into a membrane model confirmed that B-FABP interacts with micelles through the portal region and led to structural changes in the protein as well in the micelles. These changes were greater in the presence of LPG when compared to the LPC models. ESR spectra of B-FABP labeled mutants showed the presence of two groups of residues that responded to the presence of micelles in opposite ways. In the presence of lysophospholipids, group I of residues, whose side chains point outwards from the contact region between the helices, had their mobility decreased in an environment of lower polarity when compared to the same residues in solution. The second group, composed by residues with side chains situated at the interface between the α-helices, experienced an increase in mobility in the presence of the model membranes. These modifications in the ESR spectra of B-FABP mutants are compatible with a less ordered structure of the portal region inner residues (group II) that is likely to facilitate the delivery of FAs to target membranes. On the other hand, residues in group I and micelle components have their mobilities decreased probably as a result of the formation of a collisional complex. Our results bring new insights for the understanding of the gating and delivery mechanisms of FABPs.

A radial glia gene marker, fatty acid binding protein 7 (FABP7), is involved in proliferation and invasion of glioblastoma cells

Glioblastoma multiforme (GBM) is among the most deadly cancers. A number of studies suggest that a fraction of tumor cells with stem cell features (Glioma Stem-like Cells, GSC) might be responsible for GBM recurrence and aggressiveness. GSC similarly to normal neural stem cells, can form neurospheres (NS) in vitro, and seem to mirror the genetic features of the original tumor better than glioma cells growing adherently in the presence of serum. Using cDNA microarray analysis we identified a number of relevant genes for glioma biology that are differentially expressed in adherent cells and neurospheres derived from the same tumor. Fatty acid-binding protein 7 (FABP7) was identified as one of the most highly expressed genes in NS compared to their adherent counterpart. We found that down-regulation of FABP7 expression in NS by small interfering RNAs significantly reduced cell proliferation and migration. We also evaluated the potential involvement of FABP7 in response to radiotherapy, as this treatment may cause increased tumor infiltration. Migration of irradiated NS was associated to increased expression of FABP7. In agreement with this, in vivo reduced tumorigenicity of GBM cells with down-regulated expression of FABP7 was associated to decreased expression of the migration marker doublecortin. Notably, we observed that PPAR antagonists affect FABP7 expression and decrease the migration capability of NS after irradiation. As a whole, the data emphasize the role of FABP7 expression in GBM migration and provide translational hints on the timing of treatment with anti-FABP7 agents like PPAR antagonists during GBM evolution.