How Does FSGS Causes proteinuria

FSGS is a pathological diagnosis of kidney disorder. Focal refers to that only some of the glomeruli are involved (as opposed to diffuse), segmental means that only part of each glomerulus is damaged (as opposed to global). Glomerular cells are responsible for blocking proteins from running out of the body. In FSGS, the damaged glomerular cells will cause an excess loss of proteins, thus causing proteinuria. Massive proteinuria as what is mentioned is a dangerous sign, for heavy loss of proteins worsen fibrosis on renal tissues.

However, how does proteinuria is formed in FSGS?

FSGS is diagnosed through renal biopsy. Protein, as the essential substances of our body, are kept in our body because of glomerular filtration membrane. Glomerular filtration membrane is composed of glomerulus which is a kind of renal intrinsic cell. Once many glomeruli are damaged, glomerular filtration is membrane is damaged and consequently, protein leak. 
Damaged glomerular filtration membrane is the real reason that large amounts of protein leak into urine. Proteinuria is just the manifestation or we can say the result of damaged glomerular filtration membrane. It is not the real reason that causes FSGS and the real treatment for FSGS is to repair damaged glomerular filtration membrane.