New Views of the Glomerulus: Advanced Microscopy for Advanced Diagnosis

Lessons learned from early-stage clinical trials for diabetic nephropathy

INTRODUCTION

The evolution of treatment for diabetic nephropathy illustrates how basic biochemistry and physiology have led to new agents such as SGLT2 inhibitors and mineralocorticoid blockers. Conversely, clinical studies performed with these agents have suggested new concepts for investigational drug development. We reviewed currently available treatments for diabetic nephropathy and then analyzed early clinical trials of new agents to assess the potential for future treatment modalities.

AREAS COVERED

We searched ClinicalTrials.gov for new agents under study for diabetic nephropathy in the past decade. Once we have identified investigation trials of new agents, we then used search engines and Pubmed.gov to find publications providing insight on these drugs. Current treatments have shown benefit in both cardiac and renal disease. In our review, we found 51 trials and 43 pharmaceuticals in a number of drug classes: mineralocorticoid blockers, anti-inflammatory, anti-fibrosis, nitric oxide stimulatory, and podocyte protection, and endothelin inhibitors.

EXPERT OPINION

It is difficult to predict which early phase treatments will advance to confirmatory clinical trials. Current agents are thought to improve hemodynamic function. However, the coincident benefit of both myocardial function and the glomerulus argues for primary effects at the subcellular level, and we follow the evolution of agents which modify fundamental cellular processes.

Kif21a deficiency leads to impaired glomerular filtration barrier function

The renal glomerulus represents the major filtration body of the vertebrate nephron and is responsible for urine production and a number of other functions such as metabolic waste elimination and the regulation of water, electrolyte and acid-base balance. Podocytes are highly specialized epithelial cells that form a crucial part of the glomerular filtration barrier (GFB) by establishing a slit diaphragm for semipermeable plasma ultrafiltration. Defects of the GFB lead to proteinuria and impaired kidney function often resulting in end-stage renal failure. Although significant knowledge has been acquired in recent years, many aspects in podocyte biology are still incompletely understood. By using zebrafish as a vertebrate in vivo model, we report a novel role of the Kinesin-like motor protein Kif21a in glomerular filtration. Our studies demonstrate specific Kif21a localization to the podocytes. Its deficiency resulted in altered podocyte morphology leading to podocyte foot process effacement and altered slit diaphragm formation. Finally, we proved considerable functional consequences of Kif21a deficiency by demonstrating a leaky GFB resulting in severe proteinuria. Conclusively, our data identified a novel role of Kif21a for proper GFB function and adds another piece to the understanding of podocyte architecture and regulation.

Deep learning-based segmentation and quantification of podocyte foot process morphology suggests differential patterns of foot process effacement across kidney pathologies

Morphological alterations at the kidney filtration barrier increase intrinsic capillary wall permeability resulting in albuminuria. However, automated, quantitative assessment of these morphological changes has not been possible with electron or light microscopy. Here we present a deep learning-based approach for segmentation and quantitative analysis of foot processes in images acquired with confocal and super-resolution fluorescence microscopy. Our method, Automatic Morphological Analysis of Podocytes (AMAP), accurately segments podocyte foot processes and quantifies their morphology. AMAP applied to a set of kidney diseases in patient biopsies and a mouse model of focal segmental glomerulosclerosis allowed for accurate and comprehensive quantification of various morphometric features. With the use of AMAP, detailed morphology of podocyte foot process effacement was found to differ between categories of kidney pathologies, showed detailed variability between diverse patients with the same clinical diagnosis, and correlated with levels of proteinuria. AMAP could potentially complement other readouts such as various omics, standard histologic/electron microscopy and blood/urine assays for future personalized diagnosis and treatment of kidney disease. Thus, our novel finding could have implications to afford an understanding of early phases of kidney disease progression and may provide supplemental information in precision diagnostics.

Quantitative Imaging of Podocyte Foot Processes in the Kidney Using Confocal and STED Microscopy

Morphological alterations to the kidney filter, particularly to podocyte foot processes, are seen in most types of glomerular diseases. Due to the nanoscale dimensions of the filter, visualization of such alterations has historically relied on electron microscopy. However, with recent technical development, it is now possible to also visualize podocyte foot processes, as well as other parts of the kidney filtration barrier, with light microscopy. With developments both in sample preparation, imaging, and image analysis, these new tools are becoming increasingly applied in kidney research, due to their demonstrated quantitative potential. We here present an overview of these protocols that can be applied to samples that have been fixed and stored using most standard procedures used today (i.e., PFA fixed, fresh frozen, formalin-fixed and paraffin-embedded (FFPE)). We additionally introduce tools for quantitative image analysis of foot process morphology and foot process effacement.

Three-Dimensional Super-Resolved Imaging of Paraffin-Embedded Kidney Samples

Background

Diseases of the glomeruli, the renal filtration units, are a leading cause of progressive kidney disease. Assessment of the ultrastructure of podocytes at the glomerular filtration barrier is essential for diagnosing diverse disease entities, providing insight into the disease pathogenesis, and monitoring treatment responses.

Methods

Here we apply previously published sample preparation methods together with stimulated emission depletion and confocal microscopy for resolving nanoscale podocyte substructure. The protocols are modified and optimized in order to be applied to formalin-fixed paraffin-embedded (FFPE) samples.

Results

We successfully modified our protocols to allow for deep three-dimensional stimulated emission depletion and confocal imaging of FFPE kidney tissue with similar staining and image quality compared with our previous approaches. We further show that quantitative analysis can be applied to extract morphometrics from healthy and diseased samples from both mice and humans.

Conclusions

The results from this study could increase the feasibility of implementing optical kidney imaging protocols in clinical routines because FFPE is the gold-standard method for storage of patient samples.

Complexities of the glomerular basement membrane

The glomerular basement membrane (GBM) is a key component of the glomerular capillary wall and is essential for kidney filtration. The major components of the GBM include laminins, type IV collagen, nidogens and heparan sulfate proteoglycans. In addition, the GBM harbours a number of other structural and regulatory components and provides a reservoir for growth factors. New technologies have improved our ability to study the composition and assembly of basement membranes. We now know that the GBM is a complex macromolecular structure that undergoes key transitions during glomerular development. Defects in GBM components are associated with a range of hereditary human diseases such as Alport syndrome, which is caused by defects in the genes COL4A3, COL4A4 and COL4A5, and Pierson syndrome, which is caused by variants in LAMB2. In addition, the GBM is affected by acquired autoimmune disorders and metabolic diseases such as diabetes mellitus. Current treatments for diseases associated with GBM involvement aim to reduce intraglomerular pressure and to treat the underlying cause where possible. As our understanding about the maintenance and turnover of the GBM improves, therapies to replace GBM components or to stimulate GBM repair could translate into new therapies for patients with GBM-associated disease.

Challenges in primary focal segmental glomerulosclerosis diagnosis: from the diagnostic algorithm to novel biomarkers

Primary or idiopathic focal segmental glomerulosclerosis (FSGS) is a kidney entity that involves the podocytes, leading to heavy proteinuria and in many cases progresses to end-stage renal disease. Idiopathic FSGS has a bad prognosis, as it involves young individuals who, in a considerably high proportion (∼15%), are resistant to corticosteroids and other immunosuppressive treatments as well. Moreover, the disease recurs in 30–50% of patients after kidney transplantation, leading to graft function impairment. It is suspected that this relapsing disease is caused by a circulating factor(s) that would permeabilize the glomerular filtration barrier. However, the exact pathologic mechanism is an unsettled issue. Besides its poor outcome, a major concern of primary FSGS is the complexity to confirm the diagnosis, as it can be confused with other variants or secondary forms of FSGS and also with other glomerular diseases, such as minimal change disease. New efforts to optimize the diagnostic approach are arising to improve knowledge in well-defined primary FSGS cohorts of patients. Follow-up of properly classified primary FSGS patients will allow risk stratification for predicting the response to different treatments. In this review we will focus on the diagnostic algorithm used in idiopathic FSGS both in native kidneys and in disease recurrence after kidney transplantation. We will emphasize those potential confusing factors as well as their detection and prevention. In addition, we will also provide an overview of ongoing studies that recruit large cohorts of glomerulopathy patients (Nephrotic Syndrome Study Network and Cure Glomerulonephropathy, among others) and the experimental studies performed to find novel reliable biomarkers to detect primary FSGS.

Imaging tools for assessment of myocardial fibrosis in humans: the need for greater detail

Myocardial fibrosis is recognized as a key pathological process in the development of cardiac disease and a target for future therapeutics. Despite this recognition, the assessment of fibrosis is not a part of routine clinical practice. This is primarily due to the difficulties in obtaining an accurate assessment of fibrosis non-invasively. Moreover, there is a clear discrepancy between the understandings of myocardial fibrosis clinically where fibrosis is predominately studied with comparatively low-resolution medical imaging technologies like MRI compared with the basic science laboratories where fibrosis can be visualized invasively with high resolution using molecularly specific fluorescence microscopes at the microscopic and nanoscopic scales. In this article, we will first review current medical imaging technologies for assessing fibrosis including echo and MRI. We will then highlight the need for greater microscopic and nanoscopic analysis of human tissue and how this can be addressed through greater utilization of human tissue available through endomyocardial biopsies and cardiac surgeries. We will then describe the relatively new field of molecular imaging that promises to translate research findings to the clinical practice by non-invasively monitoring the molecular signature of fibrosis in patients.

Super-resolution microscopy can identify specific protein distribution patterns in platelets incubated with cancer cells

Protein contents in platelets are frequently changed upon tumor development and metastasis. However, how cancer cells can influence protein-selective redistribution and release within platelets, thereby promoting tumor development, remains largely elusive. With fluorescence-based super-resolution stimulated emission depletion (STED) imaging we reveal how specific proteins, implicated in tumor progression and metastasis, re-distribute within platelets, when subject to soluble activators (thrombin, adenosine diphosphate and thromboxane A2), and when incubated with cancer (MCF-7, MDA-MB-231, EFO21) or non-cancer cells (184A1, MCF10A). Upon cancer cell incubation, the cell-adhesion protein P-selectin was found to re-distribute into circular nano-structures, consistent with accumulation into the membrane of protein-storing alpha-granules within the platelets. These changes were to a significantly lesser extent, if at all, found in platelets incubated with normal cells, or in platelets subject to soluble platelet activators. From these patterns, we developed a classification procedure, whereby platelets exposed to cancer cells, to non-cancer cells, soluble activators, as well as non-activated platelets all could be identified in an automatic, objective manner. We demonstrate that STED imaging, in contrast to electron and confocal microscopy, has the necessary spatial resolution and labelling efficiency to identify protein distribution patterns in platelets and can resolve how they specifically change upon different activations. Combined with image analyses of specific protein distribution patterns within the platelets, STED imaging can thus have a role in future platelet-based cancer diagnostics and therapeutic monitoring. The presented approach can also bring further clarity into fundamental mechanisms for cancer cell-platelet interactions, and into non-contact cell-to-cell interactions in general.