Diabetic kidney disease (DKD) is one of the most devastating complications of diabetes. Renal dysfunction develops in about one-third of patients with diabetes (1). Diabetic nephropathy is characterized by albuminuria, glomerulosclerosis, and progressive loss of renal function. Current therapies for DKD, including blood glucose control, angiotensin II receptors blockers, and ACE inhibitors, slow down, but do not halt, the progression to end-stage renal disease after overt nephropathy has been established (2). Recent studies indicate that podocyte injury and depletion play key roles in the pathogenesis of DKD (3,4). Clinical observational studies showed a strong correlation between podocyte density, albuminuria and renal function decline in patients with type 1 and type 2 diabetes (5).

Notch signaling regulates many aspects of metazoan development and tissue renewal (6). Notch is a transmembrane protein that interacts with ligands of the Jagged and Delta family (7). In mammals, there are four Notch receptors (Notch 1–4), two Jaggeds, and three delta-like ligands (8). Each of these proteins show a cell type- and tissue-specific expression. Notch is made in the endoplasmic reticulum as pre-Notch. A furin-like convertase cleaves pre-Notch to intracellular and extracellular domain. The protein is then transported to the plasma membrane. Interaction of the ligand with the Notch receptor triggers a series of proteolytic cleavage, by ADAM, a disintegrin and metalloprotease and by the γ-secretase complex. The final cleavage releases the Notch intracellular domain (NICD), which then moves to the nucleus, where it can regulate gene expressions by binding to the transcription factor CSL. The Notch pathway in the kidney is indispensable for glomerular and proximal tubule development (9). Once the development process is complete, very little Notch activity can be observed in rodent or human kidneys. Using unbiased microarray technologies, several recent studies described the expression of Notch pathway proteins in kidneys of patients with DKD (10,11). Mechanistic studies performed in mouse models and in cultured cells showed that activation of the Notch signaling in podocytes plays a key functional role in the pathogenesis of podocyte injury (11,12). Expression of Notch1 in podocytes caused the development of albuminuria and glomerulosclerosis, whereas genetic deletion or pharmacological inhibition ameliorated DKD in a mouse model of diabetes. In podocytes, Notch signaling interacts with the transforming growth factor (TGF)-β pathway (6,7). This interaction seems to form a positive feed-back loop: TGF-β transcriptionally upregulates notch ligand Jagged1 expression (Fig. 1). On the other hand, Notch activation also increases TGF-β expression. Given the potent profibrotic activity of TGF-β in glomerular disease, this suggests that Notch is an important “master regulator” of glomerulosclerosis (11,13).

FIG. 1.

The mechanism and consequence of Notch activation in podocytes. The proposed model for Notch activation in podocytes based on data from Lin et al. (indicated by blue lines) and prior published work (11,13) (black lines). In podocytes, hyperglycemia or TGF-β leads to increased Notch activity and an increase in NICD. Increased Notch activity in podocytes causes VEGF release, decrease in nephrin expression, and apoptosis. In addition, prior experiments also showed that Notch activation leads to increased TGF-β expression and p53 mediated apoptosis. (A high-quality digital representation of this figure is available in the online issue.)

FIG. 1.

The mechanism and consequence of Notch activation in podocytes. The proposed model for Notch activation in podocytes based on data from Lin et al. (indicated by blue lines) and prior published work (11,13) (black lines). In podocytes, hyperglycemia or TGF-β leads to increased Notch activity and an increase in NICD. Increased Notch activity in podocytes causes VEGF release, decrease in nephrin expression, and apoptosis. In addition, prior experiments also showed that Notch activation leads to increased TGF-β expression and p53 mediated apoptosis. (A high-quality digital representation of this figure is available in the online issue.)

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Vascular endothelial growth factor (VEGF) plays a key role in vasculogenesis and angiogenesis by stimulating endothelial cell mitosis and migration. Evidence is emerging that VEGF plays a critical role in maintaining renal homeostasis. Altered (increased or decreased) expression of VEGF leads to glomerular dysfunction and proteinuria. In cultured podocytes, VEGF induces the prosurvival phosphoinositol 3-kinase/Akt and suppresses the proapoptotic p38 mitogen-activated protein kinase pathway (14,15). In diabetic kidneys, both decreased and increased expression of VEGF has been reported (16,,,20). The reason for such discrepancy is not clear, and there might be a stage-specific regulation, i.e., an early increase followed by a decreased expression later on.

In the current issue of Diabetes, Lin et al. (21) report that the Notch and VEGF pathways interact in diabetic podocytes and drive the development of DKD. They found that hyperglycemia led to the activation of Notch signaling in cultured human podocytes, in HEK293 cells, and in kidneys from a rat model of diabetes (Fig. 1). Notch1 activation induced VEGF expression and subsequently caused a decrease in nephrin expression and induced podocyte apoptosis (Fig. 1). Lin et al. also treated diabetic rats with a γ-secretase inhibitor (DAPT), which led to a decrease in albuminuria in a dose-dependent manner. In addition, DAPT treatment also normalized VEGF and nephrin expression in vivo. This is an important translational finding suggesting that blocking Notch activation could provide a new therapeutic strategy for the cure of diabetic nephropathy. Moreover, because hyperglycemia was able to regulate Notch expression in different cell types (including podocytes and HEK293 cells), it also raises the possibility that Notch activation could be a common mechanism leading to the development of diabetes complications.

Interaction between VEGF and Notch signaling has been reported in the vascular endothelium, and it plays a key role in angiogenesis. VEGF, by inducing AKT phosphorylation, is a potent prosurvival cytokine (15). However, in the report by Lin et al., Notch and VEGF signaling appears to have a pro-apoptotic effect (21). Further studies are needed to define these key differences. In addition, in Drosophila, Notch is a strong inducer of the nephrin homologue hibris (22); however—according to Lin et al.—in podocytes, Notch appears to downregulate nephrin expression. A key confounding factor is that many of the experiments were performed in a cultured human embryonic kidney cell line (HEK293), which appears to express a key podocyte-specific protein, nephrin. Whether the expression and regulation of nephrin in this “heterologous system” are the same as in podocytes remains to be established.

Nevertheless, the study by Lin et al. (21) strongly supports the view that Notch signaling in podocytes plays a critical role in the development of albuminuria. There are several key questions to be answered in this area. First, in the Notch signaling pathway, there are differences in ligand-expressing and signal-receiving cells, and often the ligand and the receptor are expressed in different cell types. It is not clear how the Notch signaling works in the kidney. Therefore, it would be important to better characterize the expression and localization of Notch ligands and receptors in the glomerulus. Are they both expressed on the podocytes? Further studies will be needed to analyze the complex interaction between the VEGF, Notch, and the TGF-β pathways. Is Notch a common downstream regulator? Another key question will be to determine the targets of Notch activation in podocytes to understand how and why this important developmental pathway causes damage to the kidney.

Taken as a whole, the study by Lin et al. suggests that there is a critical interaction between HIF/VEGF and Notch signaling in diabetic podocytes, thus expanding our understanding of diabetic nephropathy.

See accompanying article, p. 1915.

The work was supported by a National Institutes of Health Grant (R01DK076077) and by the Juvenile Diabetes Research Foundation to K.S.

No potential conflicts of interest relevant to this article were reported.

The authors thank members of the Susztak lab for helpful discussion.

1.
Groop
PH
,
Thomas
MC
,
Moran
JL
,
Wadèn
J
,
Thorn
LM
,
Mäkinen
VP
,
Rosengård-Bärlund
M
,
Saraheimo
M
,
Hietala
K
,
Heikkilä
O
,
Forsblom
C
:
FinnDiane Study Group
.
The presence and severity of chronic kidney disease predicts all-cause mortality in type 1 diabetes
.
Diabetes
2009
;
58
:
1651
1658
2.
Kowey
PR
,
Dickson
TZ
,
Zhang
Z
,
Shahinfar
S
,
Brenner
BM
:
RENAAL Investigators
.
Losartan and end-organ protection–lessons from the RENAAL study
.
Clin Cardiol
2005
;
28
:
136
142
3.
Susztak
K
,
Raff
AC
,
Schiffer
M
,
Böttinger
EP
:
Glucose-induced reactive oxygen species cause apoptosis of podocytes and podocyte depletion at the onset of diabetic nephropathy
.
Diabetes
2006
;
55
:
225
233
4.
Szabó
C
,
Biser
A
,
Benko
R
,
Böttinger
E
,
Suszták
K
:
Poly(ADP-ribose) polymerase inhibitors ameliorate nephropathy of type 2 diabetic Leprdb/db mice
.
Diabetes
2006
;
55
:
3004
3012
5.
Pagtalunan
ME
,
Miller
PL
,
Jumping-Eagle
S
,
Nelson
RG
,
Myers
BD
,
Rennke
HG
,
Coplon
NS
,
Sun
L
,
Meyer
TW
:
Podocyte loss and progressive glomerular injury in type II diabetes
.
J Clin Invest
1997
;
99
:
342
348
6.
Artavanis-Tsakonas
S
,
Rand
MD
,
Lake
RJ
:
Notch signaling: cell fate control and signal integration in development
.
Science
1999
;
284
:
770
776
7.
Schweisguth
F
:
Notch signaling activity
.
Curr Biol
2004
;
14
:
R129
138
8.
Callahan
R
,
Egan
SE
:
Notch signaling in mammary development and oncogenesis
.
J Mammary Gland Biol Neoplasia
2004
;
9
:
145
163
9.
Cheng
HT
,
Kim
M
,
Valerius
MT
,
Surendran
K
,
Schuster-Gossler
K
,
Gossler
A
,
McMahon
AP
,
Kopan
R
:
Notch2, but not Notch1, is required for proximal fate acquisition in the mammalian nephron
.
Development
2007
;
134
:
801
811
10.
Walsh
DW
,
Roxburgh
SA
,
McGettigan
P
,
Berthier
CC
,
Higgins
DG
,
Kretzler
M
,
Cohen
CD
,
Mezzano
S
,
Brazil
DP
,
Martin
F
:
Co-regulation of Gremlin and Notch signalling in diabetic nephropathy
.
Biochim Biophys Acta
2008
;
1782
:
10
21
11.
Niranjan
T
,
Bielesz
B
,
Gruenwald
A
,
Ponda
MP
,
Kopp
JB
,
Thomas
DB
,
Susztak
K
:
The Notch pathway in podocytes plays a role in the development of glomerular disease
.
Nat Med
2008
;
14
:
290
298
12.
Waters
AM
,
Wu
MY
,
Onay
T
,
Scutaru
J
,
Liu
J
,
Lobe
CG
,
Quaggin
SE
,
Piscione
TD
:
Ectopic notch activation in developing podocytes causes glomerulosclerosis
.
J Am Soc Nephrol
2008
;
19
:
1139
1157
13.
Zavadil
J
,
Cermak
L
,
Soto-Nieves
N
,
Böttinger
EP
:
Integration of TGF-beta/Smad and Jagged1/Notch signalling in epithelial-to-mesenchymal transition
.
Embo J
2004
;
23
:
1155
1165
14.
Foster
RR
,
Hole
R
,
Anderson
K
,
Satchell
SC
,
Coward
RJ
,
Mathieson
PW
,
Gillatt
DA
,
Saleem
MA
,
Bates
DO
,
Harper
SJ
:
Functional evidence that vascular endothelial growth factor may act as an autocrine factor on human podocytes
.
Am J Physiol Renal Physiol
2003
;
284
:
F1263
1273
15.
Müller-Deile
J
,
Worthmann
K
,
Saleem
M
,
Tossidou
I
,
Haller
H
,
Schiffer
M
:
The balance of autocrine VEGF-A and VEGF-C determines podocyte survival
.
Am J Physiol Renal Physiol
2009
;
297
:
F1656
1667
16.
Cooper
ME
,
Vranes
D
,
Youssef
S
,
Stacker
SA
,
Cox
AJ
,
Rizkalla
B
,
Casley
DJ
,
Bach
LA
,
Kelly
DJ
,
Gilbert
RE
:
Increased renal expression of vascular endothelial growth factor (VEGF) and its receptor VEGFR-2 in experimental diabetes
.
Diabetes
1999
;
48
:
2229
2239
17.
Hovind
P
,
Tarnow
L
,
Oestergaard
PB
,
Parving
HH
:
Elevated vascular endothelial growth factor in type 1 diabetic patients with diabetic nephropathy
.
Kidney Int Suppl
2000
;
75
:
S56
61
18.
Sung
SH
,
Ziyadeh
FN
,
Wang
A
,
Pyagay
PE
,
Kanwar
YS
,
Chen
S
:
Blockade of vascular endothelial growth factor signaling ameliorates diabetic albuminuria in mice
.
J Am Soc Nephrol
2006
;
17
:
3093
3104
19.
Hohenstein
B
,
Hausknecht
B
,
Boehmer
K
,
Riess
R
,
Brekken
RA
,
Hugo
CP
:
Local VEGF activity but not VEGF expression is tightly regulated during diabetic nephropathy in man
.
Kidney Int
2006
;
69
:
1654
1661
20.
Lindenmeyer
MT
,
Kretzler
M
,
Boucherot
A
,
Berra
S
,
Yasuda
Y
,
Henger
A
,
Eichinger
F
,
Gaiser
S
,
Schmid
H
,
Rastaldi
MP
,
Schrier
RW
,
Schlöndorff
D
,
Cohen
CD
:
Interstitial vascular rarefaction and reduced VEGF-A expression in human diabetic nephropathy
.
J Am Soc Nephrol
2007
;
18
:
1765
1776
21.
Lin
CL
,
Wang
FS
,
Hsu
YC
,
Chen
CN
,
Tseng
MJ
,
Saleem
MA
,
Chang
PJ
,
Wang
JY
:
Modulation of Notch-1 signaling alleviates vascular endothelial growth factor—mediated diabetic nephropathy
.
Diabetes
2010
;
59
:
1915
1925
22.
Cagan
R
:
The signals that drive kidney development: a view from the fly eye
.
Curr Opin Nephrol Hypertens
2003
;
12
:
11
17
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