Homotypic lysosome fusion in macrophages: analysis using an in vitro assay.

Lysosomes are dynamic structures capable of fusing with endosomes as well as other lysosomes. We examined the biochemical requirements for homotypic lysosome fusion in vitro using lysosomes obtained from rabbit alveolar macrophages or the cultured macrophage-like cell line, J774E. The in vitro assay measures the formation of a biotinylated HRP-avidin conjugate, in which biotinylated HRP and avidin were accumulated in lysosomes by receptor-mediated endocytosis. We determined that lysosome fusion in vitro was time- and temperature-dependent and required ATP and an N-ethylmaleimide (NEM)-sensitive factor from cytosol. The NEM-sensitive factor was NSF as purified recombinant NSF could completely replace cytosol in the fusion assay whereas a dominant-negative mutant NSF inhibited fusion. Fusion in vitro was extensive; up to 30% of purified macrophage lysosomes were capable of self-fusion. Addition of GTPgammas to the in vitro assay inhibited fusion in a concentration-dependent manner. Purified GDP-dissociation inhibitor inhibited homotypic lysosome fusion suggesting the involvement of rabs. Fusion was also inhibited by the heterotrimeric G protein activator mastoparan, but not by its inactive analogue Mas-17. Pertussis toxin, a Galphai activator, inhibited in vitro lysosome fusion whereas cholera toxin, a Galphas activator did not inhibit the fusion reaction. Addition of agents that either promoted or disrupted microtubule function had little effect on either the extent or rate of lysosome fusion. The high value of homotypic fusion was supported by in vivo experiments examining lysosome fusion in heterokaryons formed between cells containing fluorescently labeled lysosomes. In both macrophages and J774E cells, almost complete mixing of the lysosome labels was observed within 1-3 h of UV sendai-mediated cell fusion. These studies provide a model system for identifying the components required for lysosome fusion.

Pancreatic islet cell tumors.

Tumors arising from the pancreatic islet cells represent a heterogeneous group of lesions. Some tumors present with well-characterized syndromes, while others appear to be nonfunctioning. Eighty-four patients with pancreatic islet cell tumors operated on at the Cleveland Clinic during a 35-year period were reviewed. The tumor types include 21 nonfunctioning tumors, 41 insulinomas, 16 gastrinomas, two vasoactive intestinal polypeptide (VIP)-omas, two carcinoids, and two probable cases of pancreatic parathyrinoma. Eleven patients had multiple endocrine neoplasia type I syndrome. Preoperative localization was possible in 63% of patients in whom it was attempted. Complete mobilization of the head and distal pancreas with bimanual palpation of the entire gland is critical for intraoperative tumor localization. Distal pancreatectomy is favored for tumors in the body and tail. In the head of the pancreas, small, benign lesions require enucleation, and large or malignant lesions necessitate a Whipple procedure. The operative morbidity rate was 24%, and the mortality rate was 3.6%. The 10-year survival rate was 54.7% for nonfunctioning lesions, 68.4% for gastrinomas, and 92.4% for insulinomas. At this time surgery represents the only way to cure these lesions.

The Beige/Chediak-Higashi syndrome gene encodes a widely expressed cytosolic protein.

The human autosomal recessive disorder Chediak-Higashi syndrome and its murine homologue beige are associated with the formation of giant lysosomes that cluster near the perinuclear region of cells. We prepared a polyclonal antiserum against a glutathione S-transferase-Beige fusion protein and demonstrated by Western analysis that the beige gene encodes a protein of 400 kDa that is expressed in cultured murine fibroblasts as well as most mouse tissues. The protein was not detected in either cultured fibroblasts or mouse tissues from two different beige mutants. Cultured fibroblasts transformed with multiple copies of yeast artificial chromosomes that contain the full-length beige gene showed much higher levels of Beige protein than either wild type fibroblasts or mouse tissues. Subcellular fractionation experiments demonstrated that the Beige protein was cytosolic and, under the conditions of isolation, had no measurable membrane association. Cultured mouse fibroblasts in which the Beige protein was overexpressed had smaller than normal lysosomes that were more peripherally distributed than in control cells. These findings, coupled with earlier published results, suggest that the Beige protein regulates lysosomal fission.