An ex vivo culture model of kidney podocyte injury reveals mechanosensitive, synaptopodin-templating, sarcomere-like structures.

Chronic kidney diseases are widespread and incurable. The biophysical mechanisms underlying them are unclear, in part because material systems for reconstituting the microenvironment of relevant kidney cells are limited. A critical question is how kidney podocytes (glomerular epithelial cells) regenerate foot processes of the filtration apparatus following injury. Recently identified sarcomere-like structures (SLSs) with periodically spaced myosin IIA and synaptopodin appear in injured podocytes in vivo. We hypothesized that SLSs template synaptopodin in the initial stages of recovery in response to microenvironmental stimuli and tested this hypothesis by developing an ex vivo culture system that allows control of the podocyte microenvironment. Results supported our hypothesis. SLSs in podocytes that migrated from isolated kidney glomeruli presented periodic synaptopodin-positive clusters that nucleated peripheral, foot process-like extensions. SLSs were mechanoresponsive to actomyosin inhibitors and substrate stiffness. Results suggest SLSs as mechanobiological mediators of podocyte recovery and as potential targets for therapeutic intervention.

Opposing Roles of Dendritic Cell Subsets in Experimental GN.

Dendritic cells (DCs) are thought to form a dendritic network across barrier surfaces and throughout organs, including the kidney, to perform an important sentinel function. However, previous studies of DC function used markers, such as CD11c or CX3CR1, that are not unique to DCs. Here, we evaluated the role of DCs in renal inflammation using a CD11c reporter mouse line and two mouse lines with DC-specific reporters, Zbtb46-GFP and Snx22-GFP. Multiphoton microscopy of kidney sections confirmed that most of the dendritically shaped CD11c+ cells forming a network throughout the renal interstitium expressed macrophage-specific markers. In contrast, DCs marked by Zbtb46-GFP or Snx22-GFP were less abundant, concentrated around blood vessels, and round in shape. We confirmed this pattern of localization using imaging mass cytometry. Motility measurements showed that resident macrophages were sessile, whereas DCs were motile before and after inflammation. Although uninflamed glomeruli rarely contained DCs, injury with nephrotoxic antibodies resulted in accumulation of ZBTB46 + cells in the periglomerular region. ZBTB46 identifies all classic DCs, which can be categorized into two functional subsets that express either CD103 or CD11b. Depletion of ZBTB46 + cells attenuated the antibody-induced kidney injury, whereas deficiency of the CD103+ subset accelerated injury through a mechanism that involved increased neutrophil infiltration. RNA sequencing 7 days after nephrotoxic antibody injection showed that CD11b+ DCs expressed the neutrophil-attracting cytokine CXCL2, whereas CD103+ DCs expressed high levels of several anti-inflammatory genes. These results provide new insights into the distinct functions of the two major DC subsets in glomerular inflammation.

Role of transcription factors in podocytes.

Despite a wealth of information on structural proteins, comparatively little is known on the transcriptional regulation of podocyte structure and function. In this review we will highlight those transcription factors which, by gene inactivation or classical transgenic experiments, have been shown to be essential for podocytes or probably will turn out to be so. The tumor suppressor protein WT1 is not only indispensable for the initial stages of kidney development, but also very likely maintains the integrity of the fully differentiated podocyte. In the kidney, the LIM homeodomain transcription factor LMX1B is specifically synthesized in podocytes, and mutations in LMX1B lead to nail-patella syndrome and the associated nephropathy. Other transcription factors such as hypoxia-inducible factors and PAX2 are likely to play a role in podocytes, whereas the significance of others, e.g. of POD1 and CITED2, is more speculative at this point.