σ1B adaptin regulates adipogenesis by mediating the sorting of sortilin in adipose tissue.

Here, we describe altered sorting of sortilin in adipocytes deficient for the σ1B-containing AP-1 complex, leading to the inhibition of adipogenesis. The AP-1 complex mediates protein sorting between the trans-Golgi network and endosomes. Vertebrates express three AP1 σ1 subunit isoforms - σ1A, σ1B and σ1C (also known as AP1S1, AP1S2 and AP1S3, respectively). σ1B-deficient mice display impaired recycling of synaptic vesicles and lipodystrophy. Here, we show that sortilin is overexpressed in adipose tissue from σ1B(-/-) mice, and that its overexpression in wild-type cells is sufficient to suppress adipogenesis. σ1B-specific binding of sortilin requires the sortilin DxxD-x12-DSxxxL motif. σ1B deficiency does not lead to a block of sortilin transport out of a specific organelle, but the fraction that reaches lysosomes is reduced. Sortilin binds to the receptor DLK1, an inhibitor of adipocyte differentiation, and the overexpression of sortilin prevents DLK1 downregulation, leading to enhanced inhibition of adipogenesis. DLK1 and sortilin expression are not increased in the brain tissue of σ1B(-/-) mice, although this is the tissue with the highest expression of σ1B and sortilin. Thus, adipose-tissue-specific and σ1B-dependent routes for the transport of sortilin exist and are involved in the regulation of adipogenesis and adipose-tissue mass.

Trans-Golgi network morphology and sorting is regulated by prolyl-oligopeptidase-like protein PREPL and the AP-1 complex subunit µ1A.

The AP-1 complex recycles between membranes and the cytoplasm and dissociates from membranes during clathrin-coated-vesicle uncoating, but also independently of vesicular transport. The µ1A N-terminal 70 amino acids are involved in regulating AP-1 recycling. In a yeast two-hybrid library screen we identified the cytoplasmic prolyl-oligopeptidase-like protein PREPL as an interaction partner of this domain. PREPL overexpression leads to reduced AP-1 membrane binding, whereas reduced PREPL expression increases membrane binding and impairs AP-1 recycling. Altered AP-1 membrane binding in PREPL-deficient cells mirrors the membrane binding of the mutant AP-1* complex, which is not able to bind PREPL. Colocalisation of PREPL with residual membrane-bound AP-1 can be demonstrated. Patient cell lines deficient in PREPL have an expanded trans-Golgi network, which could be rescued by PREPL expression. These data demonstrate PREPL as an AP-1 effector that takes part in the regulation of AP-1 membrane binding. PREPL is highly expressed in brain and at lower levels in muscle and kidney. Its deficiency causes hypotonia and growth hormone hyposecretion, supporting essential PREPL functions in AP-1-dependent secretory pathways.

AP-1/sigma1B-adaptin mediates endosomal synaptic vesicle recycling, learning and memory.

Synaptic vesicle recycling involves AP-2/clathrin-mediated endocytosis, but it is not known whether the endosomal pathway is also required. Mice deficient in the tissue-specific AP-1-sigma1B complex have impaired synaptic vesicle recycling in hippocampal synapses. The ubiquitously expressed AP-1-sigma1A complex mediates protein sorting between the trans-Golgi network and early endosomes. Vertebrates express three sigma1 subunit isoforms: A, B and C. The expressions of sigma1A and sigma1B are highest in the brain. Synaptic vesicle reformation in cultured neurons from sigma1B-deficient mice is reduced upon stimulation, and large endosomal intermediates accumulate. The sigma1B-deficient mice have reduced motor coordination and severely impaired long-term spatial memory. These data reveal a molecular mechanism for a severe human X-chromosome-linked mental retardation.