Lipid‐binding properties of human ApoD and Lazarillo‐related lipocalins: functional implications for cell differentiation

• Published in: The FEBS journal Vol. 280; no. 16; pp. 3928 - 3943
• Main Authors: Ruiz, Mario, Sanchez, Diego, Correnti, Colin, Strong, Roland K, Ganfornina, Maria D
• Format: Journal Article
• Language: English
• Published: England Published by Blackwell Pub. on behalf of the Federation of European Biochemical Societies 01.08.2013; Blackwell Publishing Ltd
• Subjects: acetates; Alkenes - metabolism; Amino Acid Sequence; Animals; apolipoprotein D; Apolipoproteins D - chemistry; Apolipoproteins D - metabolism; Carrier Proteins - chemistry; Carrier Proteins - genetics; Carrier Proteins - metabolism; cell differentiation; Cell Line; Cells, Cultured; cholesterol; courtship; dissociation; Drosophila; Drosophila Proteins - chemistry; Drosophila Proteins - genetics; Drosophila Proteins - metabolism; ergosterol; fatty acids; fluorescence; fluorescence titration; Humans; Insect Proteins - chemistry; Insect Proteins - genetics; Insect Proteins - metabolism; insects; ligand binding; Ligands; lipid metabolism; Lipids; Lipocalins - chemistry; Lipocalins - genetics; Lipocalins - metabolism; longevity; Membrane Glycoproteins - chemistry; Membrane Glycoproteins - genetics; Membrane Glycoproteins - metabolism; Models, Molecular; Molecular Conformation; Molecular Sequence Data; neurite extension; Neurites - metabolism; Neurogenesis; neuronal differentiation; neurons; Neurons - metabolism; NLaz; Peptide Fragments - chemistry; Peptide Fragments - genetics; Peptide Fragments - metabolism; pheromones; Recombinant Proteins - chemistry; Recombinant Proteins - metabolism; retinoic acid; Sequence Alignment; Sequence Homology, Amino Acid; sphingomyelins; stress tolerance; titration; Tretinoin - chemistry; Tretinoin - metabolism; tryptophan; vitamin A; fluorescence titration; neurite extension; NLaz; ligand binding; neuronal differentiation
• ISSN: 1742-464X; 1742-4658
• DOI: 10.1111/febs.12394

Lipocalins are a family of proteins characterized by a conserved eight‐stranded β‐barrel structure with a ligand‐binding pocket. They perform a wide range of biological functions and this functional multiplicity must relate to the lipid partner involved. Apolipoprotein D (ApoD) and its insect homologues, Lazarillo (Laz) and neural Lazarillo (NLaz), share common ancestral functions like longevity, stress resistance and lipid metabolism regulation, coexisting with very specialized functions, like courtship behavior. Using tryptophan fluorescence titration, we screened the binding of 15 potential lipid partners for NLaz, ApoD and Laz and uncovered several novel ligands with apparent dissociation constants in the low micromolar range. Retinoic acid (RA), retinol, fatty acids and sphingomyelin are shared ligands. Sterols, however, showed a species‐specific binding pattern: cholesterol did not show strong binding to human ApoD, whereas NLaz and Laz did bind ergosterol. Among the lipocalin‐specific ligands, we found that ApoD selectively binds the endocannabinoid anandamide but not 2‐acylglycerol, and that NLaz binds the pheromone 7‐tricosene, but not 7,11‐heptacosadiene or 11‐cis‐vaccenyl acetate. To test the functional relevance of lipocalin ligand binding at the cellular level, we analyzed the effect of ApoD, Laz and NLaz preloaded with RA on neuronal differentiation. Our results show that ApoD is necessary and sufficient to allow for RA differentiating activity. Both human ApoD and Drosophila NLaz successfully deliver RA to immature neurons, driving neurite outgrowth. We conclude that ApoD, NLaz and Laz bind selectively to a different but overlapping set of lipid ligands. This multispecificity can explain their varied physiological functions.