Transcript Nephrogenesis

Nephrogenesis
Maria E. Ferris, MD, MPH
September 2001
Organogenesis
• Most parenchymal epithelial organs follow a
simple scheme of embryonic organogenesis. An
epithelial sheet or tube that is derived from one
of the primordia, enters a process of sequential
branching to generate a treelike structure.
• In the kidney, the epithelial tube is to become
the arborizing nephric duct-derived collecting
duct system.
Nephrogenesis
• A series of morphogenetic and
differentiation events that starts with
inductive interactions between 2 different
primordial tissues and leads, in one of
two mainstream processes:
– formation of mesenchymal condensations
– formation of aggregates
Nephrogenic Stages
• The kidney is derived from two different
early embryonic tissue primordia:
– The nephric duct  the mesonephric duct
and continues through the Wolffian duct
stage to the ureteric bud.
– The nephrogenic cord, after inductive
signaling with the pronephric duct-derived
cells  the nephroi of mesonephros and
metanephros
Successive Bilateral Ontogenic Stages
• Pronephros
• Mesonephros
• Metanephros
Pronephros
• Occurs at the 3rd gestational week
• About 7 tubules coalesce to form the
pronephric duct
Mesonephros
•
•
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•
4th Gestational week
40 tubules that coalesce
Production of a urine-like substance
A few mesonephric tubules persist in
males to form the epididymis, duct
deferens and the ejaculatory duct
Metanephros
• Weeks 5-12 of gestation
• Lasts until 38 weeks of gestation
• Develops from 2 sources
– The ureteric bud  Ureter
– The blastema  Induces dichotomous
branching of the ureteric bud to form the
collecting ducts, calyces and renal pelvis
Nephrogenesis
• Occurs from 2 distinct embryological origins
– the ureter-derived collecting duct
– the mesenchymal blastema which will form the
nephrons (glomerulus to the junction of connecting
tubule to collecting tubules)
• The ureter-derived collecting duct is induced to
branch, while the mesenchymal blastema is
induced to enter the critical process of
mesenchyme-to-epithelium conversion or
transition (MET).
Embryonic precursors of metanephros. Rudimentary pronephros, transiently
functioning mesonephros, and permanent metanephros are sequentially induced
and formed, thus recapitulating phylogeny of excretory system. This embryonic
continuity also pertains to some transcription factors and signal molecules.
[Modified from Horster M. Physiological Reviews 1999; 79:1157-91]
Histology of early metanephrogenic organization. Section through human kidney (~20.5
mm embryo) showing structures derived from Wolffian duct and metanephrogenic
blastema in outermost zone of cortex. Peripheral branch of ureteric tree extends distally
into an ampulla. Metanephric blastema has been induced to enter nephrogenic pathway
and nephron anlage has completed mesenchyme-to-epithelium transition .
Mesenchyme-to-Epithelium Transition (MET)
• These events are:
– epithelial cell polarization and
– differentiation into the highly specialized
epithelial cell populations of the nephron
Signaling Molecules
• Each step along the metanephrogenic
pathway is initiated and organized by
signaling molecules that are locally
secreted polypeptides, encoded by
different gene families and regulated by
transcription factors
Functional complexes in transition of mesenchymal to epithelial cells (MET). Nephrogenic
mesenchymal cells (top) are induced to enter MET whereby several systems with signaling functions
are activated. CAM-mediated signals and ECM-mediated signals interact with secreted growth factors
to express epithelial phenotype (bottom).
Nephrogenesis
• Proceeds from the medulla to the outer
cortex, directed by the ductal branching
of the ureteric bud-derived collecting
tubule
Microculture of metanephrogenic unit. To study MET, a nephrogenic unit as defined by a single
ureteric bud with induced adherent mesenchyme is transferred in collagens. Schematic view
illustrates in vitro MET and early nephrogenesis.These processes are documented by electron
microscopy and by molecular analysis. Noninduced mesenchymal cells enter apoptosis, and ureteric
bud cells proliferate and migrate to form a monolayer
Nephrogenesis Model
• Set up in the 1940’s, at the NIH demonstrated
in an organ system in vitro that
– Kidney rudiments when removed at embryonic day
11 (E11mouse) follow an almost normal
developmental program in culture,
– The isolated ureteric bud cannot develop without
contact to the metanephric mesenchyme, and
– The isolated metanephrogenic mesenchyme can be
induced to go through the MET by a number of
tissues, (embryonic spinal cord & the ureteric bud)
Regulation of Developmental Pathways
• The nephrogenic mesenchyma and the
ureteric bud are regulated by
– Transcription factors and proto-oncogenes,
– Polypeptide growth factors acting as
signaling molecules, and their receptors.
Modulation of Developmental Pathways
• Modulated by cell adhesion molecule
(CAM) complexes and their associations
with the cytoskeleton, by extracellular
matrix (ECM) glycoproteins and ECM
receptor molecules such as the integrin
family, and by ECM degrading proteases.
Growth Regulation
• Proto-oncogenes that encode for receptor
tyrosine kinases are involved in mesenchymal
(nephrogenic)-epithelial (ductopenic)
interactions
• Proto-oncogene encoded tyrosine or
serine/threonine kinase, is the ureteric receptor
for signaling molecules secreted by the
metanephrogenic mesenchyme
• Proto-oncogenes regulate growth and have the
potential to gain tumorigenesis after gene
mutations (Wilms tumor).
Genetic Signals
Temporospatial expression of
signaling systems in nephrogenic
and ureteric bud morphogenic
pathways. Receptor tyrosine kinases
(Ret, Met, Ros) are encoded by protooncogenes. Growth factor signaling
molecules are expressed and
secreted as indicated. Relative
abundance of expression is specified
by bold or normal type. Induced precondensing mesenchyme is shown
on top; other stages of metanephric
nephrogenesis correspond to those
depicted highly schematically in Fig.
3, A, C, and E. PDGF, platelet-derived
growth factor.
Overview of principal events in early nephrogenesis. Ureteric bud, an offspring of
Wolffian duct, invades mesenchymal blastema (left) and initiates reciprocal signaling
(middle) between epithelial (ductal) and mesenchymal (metanephrogenic) cell types.
Receptor tyrosine kinases are expressed almost exclusively in ureteric bud cell,
whereas ligands are secreted by adjacent mesenchymal cells. Ligand for c-ros encoded
receptor is not yet known. [Horster, M. Physiological Reviews. 1999; 79: 1157-91]
Patterns of expression and repression of critical developmental genes. Genes that code for transcription
factors and for signaling molecules interact positively (expression) or negatively (repression) or
behave autoregulatory. Stages of metanephric morphogenesis require profound changes in gene
expression, for cell condensation and adhesion, MET, epithelial cell apicobasal polarization, nephron
segmental pattern formation, and acquisition of membrane transport molecules. Regulation of most
expression events and downstream gene targets remains elusive.
Gene Mutations Influencing Nephrogenesis
Gene
Expression Pattern
Renal Murine Phenotype
No ureteric bud outgrowth, no induction of
mesenchyme
Arrest of nephrogenesis at condensation stage,
Wnt-4 Mesenchymal condensate
but initial branching of ureteric bud
Induced mesenchyme, Wolffian Wolffian duct growth blocked and metanephros
Pax-2
duct, ureteric bud
deleted
c-ret Ureteric bud tip cells
Branching morphogenesis blocked
Absence of metanephros and precursors and of
lim-1 Mesenchymal condensates
Wolffian duct
Ureteric bud outgrowth blocked, no induction of
ld
Mesenchyme, ureteric bud
mesenchyme, renal agenesis
WT-1 Uninduced mesenchyme
Mouse mutants, occurring naturally or engineered genetically, have contributed much to
understanding the roles of developmental genes. Wild-type expression pattern and
phenotype after targeted mutation of a few developmental genes are listed.
GROWTH FACTORS AND
EXTRACELLULAR MATRIX
A. Growth Factors Are Signaling Molecules
in Induction and Differentiation
B. Growth Factor Families Are Expressed
in Temporospatial Patterns
C. Extracellular Matrix Proteins (ECM)
and Cells Interact in Epithelial
Morphogenesis
A. Growth Factors are Signaling
Molecules
• In addition to their mitotic (growth) action, not only
mediate motogenic (migration) & morphogenic inductive
signals, but also those for cell differentiation (polarization),
proliferation, and apoptosis.
• The roles of the growth factor families and their receptors,
(each a multigene family) is very complex.
• The best-characterized growth factors pertinent to renal
organogenesis are IGF-I and IGF-II, HGF, TGF- and FGF
B. Growth Factor Families Are
Expressed in Temporospatial Patterns
• 1. IGF-I and IGF-II
• 2. HGF (hepatocyte growth factor)/SF
• 3. TGF- (Transforming growth factor)
• 4. TGF- /EGF
• 5. PDGF (Platelet-derived Growth Factor A & B)
• 6. Bmp 7 (bone morphogenic protein)
• 7. NGF( Nerve growth factors) & neurotrophin-3
C. ECM and Cells Interact in
Epithelial Morphogenesis
• Most ECM molecules contain multiple binding domains that are
recognized to interact with integrins, their glycoproteins have signal
transducing receptors & they mediate events such as cell adhesion in
embryonic cells
– ECM molecules have morphoregulatory functions
– Laminins transfer ECM signals to the cell
– Integrin expression repertoire changes with
nephrogenesis
GENES THAT CONTROL NEHPROGENESIS
• A. Transcriptional Regulation
– Wilms Tumor Suppressor 1
– Paired-box genes (Pax-2)
– BF-2 and the stromal cell lineage
– Homeobox genes (Hoxd-3)
– Hepatocyte nuclear factors
– The myc gene family
GENES THAT CONTROL NEHPROGENESIS
• B. Signaling by Receptor Tyrosine
Kinases
– C-met
– C-ret
– C-ros
GENETIC ERRORS IN NEPHROGENESIS
• Polycystic Kidney Disease
– Dysruption of C-myc and bcl2 on ARPKD
– Polycystin may participate in the epithelial polarization process
following MET.
• Wilms Tumor
– Suppressing activity of WT-1 might be cause erroneous
blastemal growth
•
Renal Cell Carcinoma
– Reexpression of Pax-2 in mature epithelium may be a
predisposition for oncogenesis
Newborn GFR
• In utero is the cardiac output to the
kidney is low (2-4%) due to placental
function
• Increase in renal mass between 35-40 wks
allows GFR to increasew from 20 to 50
ml/min/1.73 m2
• Increases with time, reaching adult levels
by 1 year of age
Urine Flow Rate
• Fetal kidneys excrete about 10 ml/kg/hr
of urine with large Na content
• The 1st week of life there is an  urine
o.p., accounting for a 10% weight loss.
Stimuli may be increased blood flow
• Voiding: 50% at 12 hrs. of life, 92% at 24
hrs and 99% at 48 hrs of life
FeNa
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In-utero: 15%
28 weeks: 5%
33 weeks: 3%
Full term: 1-3%
Potassium
• K levels in the NB are higher than in
older babies (5.5-6.0 mEq/L)
• Renal CK in the NB = 9%
• Later in life = 15%
Acid-Base Balance
• Bicarbonate levels are lower in the
newborn (19-21 mEq/L) due to
–  endogenous acid production
–  Bicarbonate reabsorption by the proximal
tubule
–  proton secretion in the collecting duct
–  carbonic anhydrase activity
Web Reference
• http://physrev.physiology.org/cgi/content/ful
l/79/4/1157#T1