WG1: Advanced Renal Physiology: capita selecta
Know the microscopic anatomy of the nephron, in particular of the arteries and capillaries.
● The nephron is the main filtration unit of the kidney.
● Blood enters the glomerulus through the afferent arteriole and leaves to glomerulus into the
tubular system through the efferent arteriole.
● The glomerular filter consists of the
endothelium of the glomerular capillaries
that form the Bowman’s capsule, the
glomerular basement membrane (GMB), and
the visceral layer of the Bowman’s capsule
that contains podocytes.
● The endothelium contains fenestrations that
restrict the passage of cellular components.
● The foot processes of podocytes that rest on
the GMB form a filtration slit that blocks
molecules size-selectively. The filtration slit is
held together by a zipper-like protein known
as nephrin.
● The mesangium also helps maintain the filter
by providing structural support and getting rid
of debris from the GMB and filtration slits
through phago/endocytosis.
Understand why the kidney is particularly susceptible to ischemic injury and know the concepts of
ischemia, mitochondrial dysfunction and tubular dysfunction.
● The kidneys receive a major part of the cardiac output as the kidney has a high energy demand
for its transport activity.
● Therefore, a reduction in renal blood flow may quickly lead to hypoxia and ischemic injury.
● In addition, due to the large amount of renal blood flow, many complements components are
circulated through the kidney, and dysfunction of the complement system contributes to tubular
microangiopathy (TMA), thus making the kidneys more susceptible to this type of damage as
well.
● During sepsis, low blood pressure leads to reduced blood flow in the kidney and hypoxia, a
phenomenon known as ischemia.
● As a result, endothelial cells die leading to inflammatory cell infiltration and damaging of tubular
cells. This in turn may lead to the accumulation of cellular debris in the tubular system, causing
tubular dysfunction.
● Mitochondria are involved in energy production through bond breaking in electron carriers
NADH, and FADH2. This energy is used to transport protons across mitochondrial membranes to
, create a membrane potential allowing ADP phosphorylation. Oxygen is consumed in this
process.
● Thus, in a hypoxic state, the kidneys cannot produce adequate energy to support transportation
activity, causing uremic toxins to accumulate and leading to mitochondrial damage and
dysfunction. As mitochondrial dysfunction progresses, there is an increased enhancement in the
progression of renal failure itself.
● Mitochondrial dysfunction initially mainly affects tubular cells in the PCT, and later on also on the
DCT.
Understand the pathophysiology of TMA.
● TMA is a pattern of injury to the microvasculature that may be induced by a number of
pathophysiological causes, such as a very high blood pressure, dysfunction of the ADAMTS13
enzyme or dysfunction of the complement system.
● If an inborn or acquired dysfunction of the complement system is the cause of TMA, the
syndrome that ensues is known as atypical haemolytic uremic syndrome (aHUS).
● TMA is characterized by thrombosis in the microvasculature in the kidneys.
● aHUS or TMA is characterized by several clinical manifestations:
○ Decreased thrombocytes count as they are consumed in thrombosis
○ Presence of schistocytes i.e. fragmented RBCs due to hemolysis
○ Increased LDH (hemolysis) and increased bilirubin (hemolysis)
○ Decreased haptoglobin (hemolysis)
○ Possibly low ADAMTS13 activity
○ Renal failure and neurological problems
● Thus, TMA is a pathology that results in thrombosis in capillaries and arterioles, due to an
endothelial injury. It may be seen in association with thrombocytopenia, anemia, purpura and
renal failure.
Understand the molecular biology that underlies a number of causes of TMA.
● Several processes underlie causes of TMA, including malignant hypertension, ADAMTS13
deficiency, factor H mutations, and infection induced shiga toxin (diarrhea associated HUS).
● Malignant hypertension increases RPF and GFR,
causing laminar flow to disappear and turbulent
flow to occur. The turbulent flow in turns causes
shear stress, which activates endothelial cells
causing fibrilar monomers and inducing coagulation
in the microcirculation, aka TMA.
● ADAMTS13 is a protein that normally cleaves von
Willebrand factor in endothelial cells.
● Thrombocytes can stick to van Willebrandt
monomers, thus by cleaving vWBF ADAMTS13
prevents TMA. Hence, in ADAMTS13 deficiency
thrombosis via vWBF is allowed to occur. ADAMTS13
, deficiency thus also causes thrombocyte deficiency and is hence involved in TPP form of TMA.
● Shiga toxin has an alpha and a beta subunit. The B-subunit can bind to glycolipids on endothelial
cells allowing it to become intracellular in endothelial cells. Once intracellular, the toxin subunits
binds to ribosomes and inhibits protein synthesis causing cellular dysfunction and damage,
ultimately leading to TMA.
● Thus, the result of any of these mutations is endothelial cell damage, causing exposure of the
subendothelial matrix. This results in a prothrombotic state with fibrin deposition and release
of von Willebrand factor, which attracts platelets to the site. Platelets will aggregate.
● The result is a full complement attack and thrombus formation. Partial or complete vessel
occlusion will result. This will produce platelet consumption and red cell damage and TMA is the
result
Understand how a factor H mutation causes aHUS and TMA.
● Factor H is an important regulatory factor in the complement system, particularly the alternative
pathways, hence loss of factor H function leads to overactivity of the complement system,
contributing to the development of aHUS and TMA.
●
● Mutated complement factor H cannot bind to cell surface C3b or the endothelial GAGs to stop
alternative complement pathway attack.
● Due to overactivation of C3, its levels will decrease causing a left read of the alternative pathway.
Understand the role of heparan sulfphate as determining pathogenic factor in complement factor H
associated diseases
● Several factors contribute to an increased susceptibility of glomeruli for damage due to
dysregulation of the complement system:
○ negative charge on the surface. Therefore complement proteins can more easily adhere
to the surface.
○ concentration of plasma due to glomerular filtration, which leads to a higher
concentration of complement factors in glomeruli than in other blood vessels.
, ○ In the gaps between endothelial cells, there is 'naked' GBM covered only by glycocalyx.
GBM does not have surface bound proteins that regulate the complement system (f.e.
MCP). Therefore glomeruli are more dependent on the glycocalyx and circulating
proteins that regulate the complement system, such as factor H (FH).
●
● Heparan sulfate (HS) modifications of C3b are tissue-specific, meaning that different tissues such
as the eyes or glomeruli have different HS modifications.
● Factor H has binding domains for these HS modifications to allow binding and activation of factor
H. Thus, if a mutation of an HS binding domain in factor H occurs, tissues can be differentially
affected, resulting in differential pathology, depending on the specific HS modification that
binds to the altered HS domain.
● Complement regulatory protein Factor H (FH) inhibits complement activity, whereas FH-related
proteins (FHRs) lack a complement regulatory domain.
● FH and FHRs compete for binding to host cell glycans, in particular heparan sulfates (HS). HS is a
glycosaminoglycan with an immense structural variability, where distinct sulfation patterns
mediate specific binding of proteins.
● Mutations in FH, FHRs, or an altered glomerular HS structure may disturb the FH : FHRs balance
on glomerular endothelial cells, thereby leading to complement activation and the subsequent
development of aHUS/C3G.
WG2: Vascular System and Sepsis
Understand the concept of sterile inflammation.
● The innate immune system can help to both fight pathogenic infections or help to recognize
tissue damage to allow tissue repair to occur.
● An inflammatory response against pathogens is mediated by pattern associated molecular
patterns (PAMPs), such as LPS on gram negative bacteria.
● When an inflammatory response occurs to tissue damage, without the presence of pathogens,
there is sterile inflammation.
● A sterile inflammatory response is mediated by Damage/danger associated molecular patterns
(DAMPs), which are endogenous ligands that can activate the innate immune response. DAMPs
are released when tissue damage occurs, and are thus often intracellular ligands such as DNA
and RNA.
