Human mesangial cells (HMCs) are injured by either excessive amounts or abnormal light chains (LCs), or a combination of both in patients with plasma cell dyscrasias. Consequently, these HMCs undergo phenotypic transformations. HMCs were incubated with eight different light-chains (LCs) for 96 h. These cells, in addition to 51 patient samples from patients with AL-amyloidosis (AL-Am), light-chain deposition disease (LCDD), myeloma cast nephropathy (MCN) and controls were analyzed by immunohistochemistry for CD68, musclespecific actin (MSA), smooth muscle actin (SMA), CD14, and Ham56 protein expressions. All samples were also studied using electron microscopy. Greater staining (four-and three-fold) expressions of CD68 and Ham56, respectively, were observed in the HMCs incubated with AL-Am-LCs compared to those with LCDD-LCs and control. SMA expression levels were five-fold higher in LCDD-LC-treated cells compared to the other categories of LC-treated and control cells. Similar results were obtained in the renal specimens, however, CD68 levels were 12-fold higher in the AL-Am cases compared to the LCDD cases, respectively. Conversely, MSA and SMA levels were three fold higher in the LCDD cases than in the AL-Am ones. No CD14 expression was noted in any of the samples and CD-34 staining of HMCs treated with the various LCs only showed rare positive cells. The renal glomerulus is a complex structure, consisting of a number of cell types performing specialized functions, which all culminate in the filtration of plasma and the production of urine. The glomerular mesangial cell is one of the major cell types and accounts for 30-40% of the total population of the glomerulus. 1 These cells, being of mesenchyme origin, are derived from smooth muscle cell precursors and, although being able at times to perform phagocytic functions, are not from the mononuclear phagocytic (monocytic) system. Ultrastructural and morphological evidence show the glomerular mesangial cells to be similar to smooth muscle cells and express many of their associated proteins.Ultrastructurally, the mesangial cell appears to be remarkably irregular in shape with numerous processes of varying lengths projecting into the surrounding extracellular matrix (ECM) and connecting with the glomerular basement membrane (GBM). The cell generally has an indented nucleus and a small amount of cytoplasmic organelles such as mitochondria, lysosomes, ribosomes, endoplasmic reticulum and stacked Golgi cysternae. 2 Numerous bundles of small intracellular filaments with spindle densities or dense bodies and associated attachment plaques are present in the normal mesangial

Altered expressions of MMPs play a key role in the pathogenesis of AL-Am and LCDD. MMPs were more highly expressed in AL-Am compared to LCDD.

Nodular glomerulosclerosis results from increased deposition of extracellular matrix proteins and monotypic light chains. The inability of mesangial cells to degrade abnormal levels of tenascin-C--along with the increased expression of some growth factors such as platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta)--is crucial to the pathogenesis of light chain deposition disease (LCDD). In order to study the molecular processes contributing to LCDD, we grew mesangial cells in three-dimensional matrices and incubated the cells with free light chains purified from the urine of patients with biopsy-proven LCDD, immunoglobulin-associated amyloid deposits, or myeloma cast nephropathy. Light chains of the latter two cohorts served as controls. Mesangial cells incubated with light chains from patients with LCDD show a significant increase in tenascin-C expression, centrally located within newly formed nodules, along with increased expression of PDGF and TGF-betas, compared to mesangial cells incubated with control light chains. There was less extracellular MMP-7 even though its intracellular expression is markedly increased compared to the control. Addition of active MMP-7 degraded this excess tenascin-C in vitro, a process that could be prevented by an exogenous MMP inhibitor. Our in vitro model recapitulates in vivo findings in patients with LCDD, thus allowing definition of the sequential pathologic processes associated with glomerulopathic light chain interactions with mesangial cells.

Matrix metalloproteinases are zinc dependent endopeptidases belonging to the M10 family of the metalloproteinase superfamily. They are ubiquitous enzymes, structurally and functionally related, with a high degree of sequence homology. They are primarily involved in extracellular matrix (ECM) turn-over and cell migration through their expanding repertoire of substrate affinities. Twenty three different forms of human MMPs have been described to be arranged in eight distinct structural classes. Their interactions with tissue inhibitors of metalloproteinases (TIMPs), and other indigenous inhibitors have been well documented. This manuscript reviews pertinent information available on matrix metalloproteinases and TIMPs in the literature. Light chain-mediated glomerular injury represents an excellent example of how metalloproteinases participate in altering mesangial homeostasis. Investigations regarding these conditions have shown that the physico-chemical characteristics of the light chains govern the pattern of renal damage that will ensue with the mesangium representing the critical site where pathological alterations are centered. The mesangium is either replaced or expanded depending on the light chains involved in the pathologic process.

Certain structurally abnormal light and heavy chains are known to be nephrotoxic and alter mesangial homeostasis producing pathological alterations. Many of the mechanisms involved in light chain-mesangial interactions have been deciphered using an in vitro model, providing a framework for understanding the sequence of events that leads to irreversible glomerular changes and eventually renal failure. The molecular events involved in the pathogenesis of these disorders are now for the most part well-established. These studies have delineated the sequence of steps involved and crucial events have been determined. An animal model is being developed which will undoubtedly contribute significantly to validate the information that has been obtained from the in vitro models. The present chapter will address the pathogenesis of these disorders with an emphasis on highlighting crucial steps possibly amenable to therapeutic intervention to stop or ameliorate adverse consequences leading to irreversible changes.