Dissecting novel regulatory mechanisms of S100A1 on cardiac function

6.1 Extracellular S100A1 inhibits apoptosis in ventricular cardiomyocytes:
via activation of the extracellular signal-regulated protein kinase 1/2 (ERK1/2)
Growing evidence indicates that members of the S100 protein family exert intracellular but
also extracellular effects on their target cells (Donato, 2003). Moreover, it has been shown
that S100A1 protein is released into the extracellular space in considerable amounts during
ischemic myocardial injury (Kiewitz et al., 2000). This observation prompted us to explore
the extracellular effect of S100A1 on neonatal ventricular cardiomyocytes (NVCMs).
By directly coupling human recombinant S100A1 protein with rhodamine we were
able to trace the uptake of extracellulary added S100A1 into the cytosolic compartment of
cultured NVCMs (Chaper 2). Using confocal laser scanning microscopy, we have shown
for the first time that S100A1 is internalized by NVCMs via a Ca2+-dependent pathway. The
colocalization studies documented that the pathway of S100A1 internalization is clathrindependent.
By using several inhibitors that are specific for different steps along the intracellular
signal transduction pathway, we could identify that S100A1-mediated activation of the
ERK1/2 signaling involves activation of PLC and PKC, both of which have been closely
linked to the endosomal compartment.
Taken together this part of our study demonstrated extracellular S100A1 to act as a
novel anti-apoptotic factor that enhances survival of neonatal cardiomyocytes in vitro via
activation of the PLC-PKC-MAP kinase kinase1-ERK1/2 pathway.
So far, the receptor for S100A1 internalization is still unknown. Previous studies identified
the cell surface receptor for advanced glycosylated end products (RAGE) as possible receptor
for S100 proteins (Hofmann et al., 1999; Huttunen et al., 2000). This was shown for the
isoforms S100B and S100A12 (Donato, 2003). Because our results suggest that RAGE is
not involved in the endocytosis of S100A1 in NVCM, we predict that another cell surface
receptor is responsible for S100A1 uptake in NVCMs (Figure 6.1)
In future studies we will attempt to identify a possible receptor for S100A1 by employing
GST-S100A1 fusion protein pull-down assays of plasma membranes. In addition, the cardioprotective
effect of internalized S100A1 in vitro warrants further research into the release
of S100A1 during heart failure. Moreover, our findings should initiate new perspectives on
the pathophysiological relevance of the cardioprotective effect of S100A1 protein on cardiac
cells in vivo.
6.2 Adenoviral-mediated S100A1 gene delivery rescues failing myocard:
Since S100A1 protein has been shown to be downregulated in human and animal heart
failure (HF) model (Remppis et al., 1996; Tsoporis et al., 2003), we undertook this study
to address whether S100A1 gene addition might reserve ventricular contractile dysfunction
in failing myocardium (Chapter 3). Using a postinfarct HF model in the rat, we provided
evidence that adenoviral-mediated myocardial S100A1 gene delivery can restore S100A1
protein expression in failing myocardium. As a result of restored S100A1 levels, a normalisation
of previously dysfunctional intracellular Ca2+- and Na+-handling occured. In additon
S100A1 expression abolished aberrant fetal gene expression associated with HF, and most
intringuely restored energy supply in failing myocardium.
Overall, we have shown for the first time that restoring S100A1 protein expression can
rescue defective contractile performance of failing myocardium in vitro and in vivo due to
improved cytosolic and SR (sarcoplasmic reticulum) Ca2+-cycling. This effect is
caused by both enhanced activity of SERCA (sarco(endo)plasmic reticulum Ca2+-ATPase)
and modulation of RyR (ryanodine receptor).
Our studies provide ample vidence that S100A1 is a key-regulator of cardiac function in
vitro and in vivo. The effects of restored S100A1 protein level in failing myocardium argue
for a novel clinical approach in the regimen of HF. Athough the study described first results
on the application of adenoviral S100A1 expression in failing myocardium, there exist also
limitations and open questions. For instance, we have used a first-generation adenoviral vector
that limits study duration. Thus, we will analyze chronic effects of HF rescue by S100A1
gene delivery with improved vectors. However, in the future S100A1 HF gene therapy might
proof to be a promising clinical treatment.
6.3 Distinct subcellular location of S100A1 differentially modulates Ca2+-cycling in
ventricular rat cardiomyocytes:
Because S100A1 is a Ca2+-sensor, we investigated the effect of an increasing S100A1
levels on the cycling of cytosolic Ca2+ in NVCMs by two different procedures: NVCMs were
either transduced by an adenoviral S100A1-expression construct (AdS100A1) or incubated
with recombinant human S100A1 protein (S100A1-treated) (Chapter 4). Ca2+-transient
measurements revealed an increase on Ca2+-turnover for both procedures. By using different
inhibitors of Ca2+-cycling, we have demonstrated that cells overexpressing S100A1 and cells
treated with S100A1 use different mechanisms to increase the intracellular Ca2+-cycling.
AdS100A1 cells arrived at an enhanced Ca2+-transient amplitude mainly through an increase
in systolic [Ca2+]i. In contrast, a marked decrease in diastolic Ca2+-concentrations ([Ca2+]i)
was the main cause for the enhanced Ca2+-transient amplitude in S100A1-treated NVCMs.
The decreased diastolic [Ca2+]i in S100A1-treated cells was likely the result of increased
sarcolemmal Ca2+-extrusion through the Na+/Ca2+-exchanger (NCX). Moreover, uptake of
S100A1 into the endosomal compartment triggered endosome-associated PLC and PKC,
which then activate NCX to increase sarcolemmal Ca2+-efflux.
The enhanced systolic [Ca2+]i in AdS100A1 cells was brought about by an increased activity
of RyR and SERCA. Consistently, immunofluorescence documented a colocalization of intracellular
overexpressed S100A1 and these two Ca2+-regulatory proteins.
In conclusion, we demonstrated that intracellular S100A1, depending on its subcellular location,
modulates cardiac Ca2+-turnover via different Ca2+-regulatory proteins.
After having shown that internalized S100A1 has a pro-survival effect on cultured cardiomyocytes
and that adenoviral S100A1 gene delivery rescues failing myocardium in vitro
and in vivo (Most et al., 2004), we are now facing the challenge of elucidating the effects of
internalized S100A1 on cardiac function in vivo.
6.4 The Ca2+-dependent dependent interaction of S100A1 with the F1-ATPase leads to
an increased ATP content in cardiomyoctes:
Reports on the involvement of S100A1 in energy metabolism (Zimmer et al., 1995; Zimmer
and Dubuisson, 1993), have prompted us to examine the effect of S100A1 overexpression
on cardiac energy metabolism (Chapter 5). We found that AdS100A1 cells exhibited a significantly
higher ATP content compared to control cells (Adcontrol). By using GST-S100A1
fusion protein pull-down assays we were able to identify several mitochondrial proteins,
which are all involved in energy metabolism. Based on the relationship between S100A1
and ATP content, we primarily focussed on the interaciton of S100A1 with the F1-ATPase.
Confocal and electron microscopy studies provided further evidence that S100A1 interacts
with the mitochondrial F1-ATPase. Interestingly, several groups have reported the localization
of S100A1 in mitochondria, but so far the functional consequence of this localization has
not been addressed (Haimoto and Kato, 1988; Maco et al., 2001). In order to fill this gap, we
dissected the S100A1-F1-ATPase complex using different biochemical assays. We found the
interaction to be dependent on the presence of Ca2+ and on the pH. At physiological pH
(7.4 ) and in the presence of Ca2+, S100A1 and F1-ATPase form a stable complex.
For further structural analysis of the complex, we initialized a collaboration with the laboratory
of Sir John Walkers in Cambridge. In a first step we have isolated an S100A1-F1-ATPase
complex by gel filtration chromatography. To unravel the molecular interactions of S100A1
and F1-ATPase, we ultimately plan to crystallize the complex.
This part of the projekt unveiled a novel mechanism of S100A1 on cardiac function. The
high metabolic demand of the heart requires a close coordination of energy production (ATP)
and workload (Cortassa et al., 2003). Approximately 2% of the cellular ATP is consumed
with each heartbeat and almost all of this energy is replenished by mitochondrial oxidative
phosphorylation under normoxic conditions (Das and Harris, 1991; Harris and Das, 1991).
The fact that not only the F1-ATPase but also other mitochondrial proteins that participate
oxidative phosphorylation associate with GST-S100A1 in pull-down experiments, convincingly
argues for further investigations of these interactions (Figure 6.2).
6.5 Conclusions:
The work presented in this thesis provides essential insights into the molecular mechanisms
of S100A1 on cardiac function.
First it reveals novel mechanisms of S100A1. By combining different localization techniques
and biochemical assays, the internalization of S100A1 via a Ca2+-dependent, clathrinmediated
process was discovered. Moreover, it is shown that internalized S100A1 exhibits
anti-apoptotic effects on cardiomyocytes and increases Ca2+-turnover. Different molecular
mechanisms were implicated in Ca2+-handling depending on the localization of the exogenous
S100A1. In addition, the rescue of failing myocardium in response to S100A1 gene
delivery holds promise for a treatment of heart failure by S100A1 gene therapy. Finally, the
identification of new target proteins for S100A1 gives rise to new perspectives on the involvement
of S100A1 in the energy metabolism.