Overview

In the dynamic universe of the human body, the kidneys play an important role in filtering waste products and maintaining balance. Yet within this complex system, the smallest structures -microscopic fibrils- can have the most destructive effects. Fibrillary glomerulonephritis (FGN) is a rare disease affecting the kidneys that proves this point. Through the lens of electron microscopy, these small but deadly structures can be revealed to the human eye, making their study crucial to proper diagnosis and management. In this article, we will go over the disease process and electron microscopic findings in FGN.

What is Fibrillary Glomerulonephritis?

Glomerulonephritis encompasses a group of inflammatory kidney diseases affecting the glomeruli - the sites of the kidney at which blood waste products are filtered out of blood in the form of urine. FGN is a type of primary glomerulonephritis, meaning that it does not have a known underlying cause, in contrast to secondary glomerulonephritis, which is often a complication of another disease like diabetes, hypertension, hepatitis infections or autoimmune diseases.¹

Glomerulonephritis is caused by a buildup of abnormal protein fibrils inside the glomeruli, interfering with their function and causing chronic kidney damage. With time, blood cells and proteins start to leak into the urine, causing a red and foamy appearance. Once fibrillary glomerulonephritis manifests, the kidneys begin to lose their filter function, and organ failure develops rapidly with FGN patients, usually progressing to end-stage kidney disease in a matter of a few years.² To diagnose FGN, first, a surgeon would need to perform a biopsy, taking a sample of kidney tissue. The sample is later transferred to a laboratory where it is observed under a light microscope to assess any structural changes in the glomeruli, i.e. enlargement and accumulation of immune cells in the structures of the glomeruli and accompanying capillaries.³

After the initial microscopic observation, laboratories usually follow up with additional tests like negative Congo red staining to differentiate FGN from amyloidosis,⁴ and immunofluorescence to identify the presence of specific antibodies inside fibrils.

Ultimately, electron microscopy is used to confirm the diagnosis by observing the specific structure of microscopic fibrils inside FGN kidneys.⁵

How does Fibrillary Glomerulonephritis Develop?

FGN is thought to be caused by an autoimmune reaction linked to the formation of antigen-antibody complexes that deposit inside glomeruli and cause tissue damage. This friendly fire is linked to a protein known as DnaJ homolog subfamily B member 9 (DNAJB9). The latter is a member of the heat shock protein family. In patients with FGN, the DNAJB9 protein acts as an autoantigen, forming immune complexes with IgG1 autoantibodies. As these complexes are highly immunogenic, they attract immune cells and the complement system, activating an immune response that leads to progressive kidney damage and loss of filter function, resulting in severe kidney failure.⁶

Apart from DNAJB9, other potential antigens, such as HLA-DR7 and HLA-B35, could potentially play a role in the development of FGN.⁷

Electron Microscopy for Fibrillary Glomerulonephritis

What is Electron Microscopy?

Electron microscopy uses beams of high-energy electrons to provide information about the submicroscopic structure of organs, tissues, and cells. Samples are blasted with accelerated electrons by using electromagnetic fields, resulting in wavelengths much shorter than visible light. This enables much higher resolution imaging compared to light microscopy and the ability to observe nanosized structures such as proteins. Interactions with a sample lead to changes in the electrons’ energy and direction of movement, which leads to the generation of an image.

Many variations of electron microscopy exist, with transmission and scanning electron microscopy being the most used techniques. In transmission electron microscopy, electron beams penetrate through samples and generate images of the internal structure of observed cells. Scanning electron microscopy, on the other hand, images the surface architecture of observed cells and tissues.⁸

How Does Electron Microscopy Complement Other Diagnostic Procedures?

Electron microscopy provides highly detailed images of structures at a submicroscopic level. It is widely used to study the structure of viruses and subcellular mechanisms of disease and drug effects. Electron microscopy is a very useful tool to differentiate between diseases with similar symptoms and organ damage but with different molecular characteristics. Electron microscopy is highly effective where traditional light microscopy fails to deliver by shedding light on practically invisible details. It can also be used in conjunction with molecular diagnostic methods like immunohistochemistry to enhance their effectiveness. By tagging specific proteins inside cells with antibodies and subsequently applying electron microscopy, we can observe their exact location and submicroscopic structure. This technique is known as immuno-electron microscopy.⁹ In FGN, transmission electron microscopy allows for the visualisation of pathologic fibrils inside the glomerular basement membrane and the internal structure of fibrillar networks, differentiating FGN from similar diseases.¹⁰

Electron Microscopy Findings in Fibrillary Glomerulonephritis

When observed with an electron microscope, FGN presents with randomly arranged non-branching fibrils that deposit primarily in the mesangium of glomeruli but also the glomerular basement membrane. A single fibril usually has a diameter of 12-22 nm.³

Electron microscopy is used to differentiate FGN from other similar conditions involving protein deposits in glomeruli and kidney damage by comparing their specific ultrastructural characteristics. Such diseases include amyloidosis and immunotactoid glomerulopathy.

• Amyloidosis is a disease that can affect the heart, brain, kidneys, and other organs and is associated with the formation of pathological fibrils just like in FGN, however, both diseases differ in pathophysiology and treatment, so they should always be differentiated from one another by using specific tests like electron microscopy. While amyloid deposits consist of multiple protein fibrils arranged in a very specific pattern (cross-beta sheet structure) and fibrils are 7-13 nm thick, fibrils in FGN are spread out irregularly and are up to 2-3 times thicker than amyloid fibrils.¹¹ Another key difference between both diseases is the composition of fibrils. Amyloid can be formed from different proteins and peptides that have a misfolded structure - from light chains of immunoglobulins in AL amyloidosis to amyloid-beta peptides in Alzheimer’s disease.¹² On the other hand, FGN fibrils are formed specifically by the abnormal aggregation and deposition of IgG1 antibodies with antigens like DNAJB9. The latter can be identified using immunohistochemical labelling techniques¹³
• Immunotactoid glomerulopathy - another less known kidney disease associated with the formation of protein fibrils inside glomeruli is immunotactoid glomerulopathy. Unlike FGN, in this condition, protein fibrils are much thicker, measuring 30-50 nm, and often display a more orderly structure, similar to amyloid fibrils¹⁴

Drawbacks of Electron Microscopy in Fibrillary Glomerulonephritis Diagnosis

Despite being a highly specific and sensitive diagnostic test and holding the position of gold standard for the diagnosis of FGN, kidney biopsy followed by electron microscopy has several significant drawbacks:

• Electron microscopy is not a routine procedure readily available to every laboratory. It requires specialised equipment and highly trained professionals, making it difficult, timely, and costly to implement
• A kidney biopsy is an invasive procedure and carries a great risk of bleeding, infection, and pain to the patient. Healthcare professionals performing kidney biopsies should be very cautious of its possible complications
• In some cases, kidney biopsies of FGN-suspected patients may have atypical electron microscopy features, further complicating an already rare diagnosis. For instance, there are cases of FGN that present with unusual fibril diameter, Congo red positivity, or absence of immunoglobulins inside fibrils. In such cases, electron microscopy alone might give misleading results. DNAJB9 could be a highly specific and sensitive biomarker for the diagnosis of atypical FGN even without electron microscopy. Immunohistochemical staining with DNAJB9-targeting antibodies has proven to be a specific and sensitive test for FGN.¹⁵ Patients with FGN have also described elevated serum DNAJB9 levels⁷

 which puts in question the potential application of blood tests for diagnosing FGN as an alternative to the high-risk kidney biopsy used currently.

Summary

Scientific progress has granted us the opportunity to uncover a disease once invisible to the naked eye. Electron microscopy still serves as a major instrument for revealing the intricate structure of FGN fibrils, allowing us to grasp just how these tiny deposits damage the kidneys. This progress has ultimately enhanced the timely diagnosis of affected individuals. Still, the lack of well-established non-invasive and accurate diagnostic tools, as well as early screening procedures, prove to be a significant unmet medical need for patients with FGN.