Apolipoproteins prevent glomerular albumin permeability induced in vitro by serum from patients with focal segmental glomerulosclerosis

Glomerular albumin permeability alterations can be induced in vitro by seru m from patients with end-stage renal disease caused by primary focal segmen tal glomerulosclerosis (FSGS). It was hypothesized that inhibitory substanc es may be present in normal serum, which may prevent the permeability alter ations in isolated glomeruli, and the present study sought to isolate and c haracterize these factors. Albumin permeability was determined from the cha nge in glomerular volume induced by applying oncotic gradients across the b asement membrane of healthy isolated rat glomeruli preincubated with FSGS s erum and normal serum fractionated using standard techniques. Fractions of normal serum with inhibitory activity obtained by a multistep chromatograph ic procedure underwent two-dimensional electrophoresis and staining. Approx imately 50 protein spots were recovered, renatured, and tested for antiperm eability activity. Five of these proteins demonstrated consistent inhibitor y activity, and desorption ionization and mass spectrometry proved them to be components of high-density lipoprotein: apolipoproteins (apo) E-2 and E- 4, high-molecular-weight J and L, and a 28-kD fragment of A-IV. Polyclonal antibodies to apo E or apo J added to the whole normal serum restored the p ermeability activity of the FSGS serum in the bioassay. Commercially availa ble apo E and apo J also demonstrated antipermeability activity when added to FSGS serum. Cyanogen bromide digestion of apo A-IV produced fragments th at inhibited the permeability activity of the FSGS serum, whereas the intac t protein did not. Thus, components of high-density lipoprotein are capable of preventing glomerular albumin permeability induced by serum from patien ts with FSGS in an in vitro system. The specificity and mechanism of the in hibition remain to be determined; the alteration of normal inhibitory activ ity in vivo may be a component in the pathophysiology of FSGS.