And you thought the age of anatomic discovery was over

Glomerular subepithelial microparticles - a footprint for podocyte injury

The aim of this study was to clarify the nature and clinical significance of glomerular subepithelial microparticles (SMPs), located between the basal surface of the podocytes and the glomerular basement membrane. Ultrastructural morphology of 79 renal biopsy samples (obtained from 25 native and 54 transplanted kidneys), showing SMPs in the last 3 years, was reevaluated with regard to the podocyte changes and clinical condition of the patients. One hundred and nine SMPs were identified, with 32.9% of the samples having two or more per glomerulus. Overall, they were most frequently located in the open capillary loops (55%). However, in the native kidney samples with mesangial deposits, 64.3% of SMPs were present in the mesangium-bound areas. Each vesicle ranged from 46.9 to 87.1 nm, and vesicles were admixed with curved strands in larger SMPs. Diffuse effacement of the foot processes and condensation of the actin filaments were present in 56.0% and 62.4% of the samples, respectively. SMPs were associated with hematuria, proteinuria of ≥ 1 gm, and immune complex deposition in the patients with native kidneys, whereas they were related to hyperglycemia and elevated serum creatinine levels in the patients with renal allografts. Patients with native and transplanted kidneys most commonly presented with IgA nephropathy and allograft rejection, respectively. Finding SMPs in the renal biopsy samples is not rare and they may act as a footprint of podocyte injury caused by diverse etiologies. Considering their size, podocyte exosomes could be a possible source of SMPs.

Detrimental effects of hypoxia on glomerular podocytes

Hypoxia-inducible factor1 (HIF1) plays a pivotal role in ensuring cells adapt to low-oxygen conditions. Depletion of oxygen, a co-substrate during hydroxylation of prolyl (P402 and P564) residues of HIF1⍺, evades HIF1⍺ ubiquitination and enables its dimerization with HIF1β to mediate global transcriptional response to hypoxia. Though HIF1 is largely considered eliciting a protective role during physiological or pathological hypoxia or ischemia, elevated HIF1 during chronic hypoxia contributes to glomerular diseases' pathology and proteinuria. The glomerulus is responsible for renal permselectivity and excretion of ultra-filtrated urine. Podocytes are the glomerulus' major cell types and are instrumental for glomerular filtration, permselectivity, and glomerular basement membrane maintenance. Podocyte injury is expected to impair the efficiency of glomerular filtration and manifestation of glomerulosclerosis and proteinuria. Accumulated evidence suggests that podocytes are susceptible to various insults during chronic hypoxia, including podocyte EMT, slit-diaphragm dysfunction, foot process effacement, and cytoskeletal derangement due to accumulation of HIF1. This review discusses how hypoxia/HIF1 signaling regulates various features and function of podocytes during exposure to chronic hypoxia or inducing HIF1 by various chemical modulators.

Glomerular filtrate affects the dynamics of podocyte detachment in a model of diffuse toxic podocytopathy

Podocyte injury and subsequent detachment are hallmarks of progressive glomerulosclerosis. In addition to cell injury, unknown mechanical forces on the injured podocyte may promote detachment. To identify the nature of these mechanical forces, we studied the dynamics of podocyte detachment using sequential ultrastructural geometry analysis by transmission electron microscopy in NEP25, a mouse model of podocytopathy induced by anti-Tac(Fv)-PE38 (LMB2), a fusion protein attached to Pseudomonas exotoxin A, targeting CD25 on podocytes. After LMB2 injection, foot process effacement occurred on day three but detachment commenced on day eight and extended to day ten, reaching toward the urinary pole in clusters. Podocyte detachment was associated with foot process effacement covering over 60% of the glomerular basement membrane length. However, approximately 25% of glomeruli with diffuse (over 80%) foot process effacement showed no detachment. Blocking glomerular filtration via unilateral ureteral obstruction resulted in diffuse foot process effacement but no pseudocysts or detachment, whereas uninephrectomy increased pseudocysts and accelerated detachment, indicating that glomerular filtrate drives podocyte detachment via pseudocyst formation as a forerunner. Additionally, more detachment was observed in juxtamedullary glomeruli than in superficial glomeruli. Thus, glomerular filtrate drives the dynamics of podocyte detachment in this model of podocytopathy. Hence, foot process effacement may be a prerequisite allowing filtrate to generate local mechanical forces that expand the subpodocyte space forming pseudocysts, promote podocyte detachment and subsequent segmental sclerosis.