Cellular mechanisms regulating the enzymatic activities of cGMP-producing natriuretic peptide receptors [Elektronische Ressource] : studies in Mus musculus cell lines / presented by Lourdes Cortes-Dericks

Cellular Mechanisms Regulating the Enzymatic Activities
of cGMP-Producing Natriuretic Peptide Receptors:
Studies in Mus musculus Cell Lines


Dissertation
for the attainment of Doctoral Degree
from the Faculty of Biology,
University of Hamburg






Presented by
Lourdes Cortes-Dericks



Hamburg
April 2005




“One never notes what has been done;
One can only see what remains to be done…”

Marie Curie, 1894





Table of Contents 3
Table of Contents
Abstract 7
1 Introduction 9
1.1 Natriuretic peptides 9
1.1.1 Structure and biosynthesis of natriuretic peptides 9
1.1.2 Proposed roles for natriuretic peptides in testicular Leydig cells and
gonadotrophs of the pituitary gland 10
1.2 The Natriuretic Peptide Receptors 10
1.2.1 Structure of cGMP-generating natriuretic peptide receptors 12
1.2.1.1 Extracellular Domain 12
1.2.1.2 Transmembrane domain 13
1.2.1.3 Kinase homology domain 13
1.2.1.4 Hinge Region 13
1.2.1.5 Guanylyl cyclase catalytic domain 14
1.3 cGMP signalling 14
1.3.1 cGMP-generating systems 15
1.3.2 cGMP-target proteins 16
1.3.2.1 cGMP-dependent protein kinase 16
1.3.2.2 cGMP-regulated PDEs 17
1.3.2.3 cGMP-regulated ion channel 17
1.3.2.4 cAMP-dependent protein kinase (PKA) 18
1.4 Regulation of cGMP-generating natriuretic peptide receptors 18
1.4.1 Regulation by Phosphorylation 18
1.4.2 Homologous Desensitization 20
1.4.3 Heterologous Desensitization 21
1.5 Lysophosphatidic acid 23
1.5.1 Structure and biosynthesis of LPA 24
Table of Contents 4
1.5.2 LPA receptors 24
1.5.3 LPA and natriuretic peptide signalling 26
1.6 Aims of the Study 26
1.6.1 Background 26
1.6.2 Experiments with MA-10 cells 27
1.6.3 Experiments with αT3-1 cells 28
2 Materials and Methods 30
2.1 Materials 30
2.1.1 Cell lines 30
2.1.2 Wistar rats 30
2.1.3 Reagents and solutions 30
2.2 Methods 32
2.2.1 Cell culture 32
2.2.1.1 Cryoconservation and thawing 32
2.2.1.2 Surface trypsination of cells 32
2.2.1.3 Preparation of cells for in vitro stimulation 33
2.2.2 Isolation, purification and culture of Leydig cells 33
2.2.3 Whole Cell Stimulation 34
2.2.3.1 cGMP assay 36
2.2.3.2 cAMP assay 37
2.2.4 Protein extraction by subcellular fractionation 37
TM2.2.5 Protein extraction with Poppers cell lysing reagent 38
2.2.6 Quantitative protein determination according to Bradford 38
2.2.7 Guanylyl cyclase assays 38
2.2.8 SDS – Polyacrylamide gel electrophoresis (SDS-PAGE) 39
2.2.9 Protein transfer to membranes (Blotting) 40
2.2.10 Immunostaining of membranes 40
2.2.11 Stripping 41
Table of Contents 5
2.2.12 Affinity crosslinking 41
2.2.13 Isolation of total RNA from cells 42
2.2.14 Reverse transcription and PCR analysis of LPA-receptors 42
2.2.15 Immunohistochemistry and confocal microscopy 44
2.2.16 Data presentation and statistical analysis 44
3 Results 45
3.1 Characterization of natriuretic peptide receptor expression in MA-10 and
ααααT3-1 cell lines 45
3.2 Characterization of the functional activities of natriuretic peptide receptors in
MA-10 and αT3-1 cells 46
3.2.1 MA-10 cells 46
3.2.2 αT3-1 cells 48
3.3 Homologous desensitization of GC-A in MA-10 cells 49
3.3.1 Preliminary examinations 49
3.3.2 Pre-treatment of MA-10 cells with ANP seems to lead to GC-A desensitization 51
3.3.3 ANP-dependent desensitization of GC-A is based on a decrease in hormone-
dependent guanylyl cyclase activity 52
3.3.4 The PKA inhibitor, H89, blocks homologous desensitization of GC-A 53
3.4 LPA-induced (“heterologous”) desensitization of GC-A in MA-10 cells 55
3.4.1 Initial studies with isolated Leydig cells 55
3.4.2 LPA exposure to MA-10 cells inhibits ANP-induced cGMP elevations in a dose-
and time- dependent manner 57
3.4.3 LPA-induced desensitization of GC-A is based on a decrease in hormone-
dependent guanylyl cyclase activity 58
3.4.4 The LPA-induced desensitization of GC-A is not mediated by PKA 59
3.4.5 Experimental variability in experiments with LPA 61
3.4.6 LPA induces ERK phosphorylation in MA-10 cells 63
3.4.7 The MEK/ERK pathway is not involved in LPA-induced desensitization of GC-A
in MA-10 cells 64
Table of Contents 6
3.4.8 Gene expression of LPA receptors in MA-10 cells 65
3.4.9 LPA induces morphological alterations in MA-10 cells 66
3.5 Regulation of natriuretic peptide receptors in αT3-1 cells 68
3.5.1 The activities of both GC-A and GC-B are unaffected by LPA 68
3.5.2 Examination of homologous desensitization 69
3.5.3 Identification of cross-reactions between GC-A and GC-B signalling 72
3.5.4 Characterization of “resensitization” of GC-A by CNP/GC-B signalling 73
3.5.4.1 The effect is based on an increase in ANP-dependent guanylyl cyclase
activity 73
3.5.4.2 CNP-induced GC-A sensitization is mediated by PKA 74
3.5.5 Experimental prove that GC-A activity in αT3-1 cells is regulated by both ANP-
and CNP- mediated signalling 75
4 Discussion 78
4.1 Experiments with MA-10 cells 78
4.1.1 ANP-induced desensitization 78
4.1.2 LPA-ization 82
4.1.3 Comparison between homologous and heterologous desensitization 87
4.1.4 Potential relevance for Leydig cell physiology 88
4.2 Experiments with ααT3-1 cells 89
αα
4.2.1 Molecular aspects 89
4.2.2 Physiological aspects 93
Abbreviations 94
References 97
List of Publications 118
Acknowledgement 119


Abstract 7
Abstract
The regulation of signalling pathways during and following their ligand-induced
activation of cell surface receptors represents an important mechanism to ensure
appropriate signal intensity and cellular response. In this regard, receptor down-
regulation via internalization/endocytosis in response to chronic ligand exposure is
known as a key reaction.
Three biologically-active peptides, designated as atrial natriuretic peptide
(ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) use
plasma membrane receptors that contain intracellular domains capable of generating
the second messenger cyclic GMP (cGMP) for eliciting effects in their target cells.
The two cardiac hormones ANP and BNP bind to and stimulate guanylyl cyclase-A
(GC-A), whereas GC-B is the specific receptor for CNP. Representing a particular
phenomenon, the ligand-responsiveness of these two receptors is regulated by
phosphorylation/dephosphorylation at intracellular residues without leaving the
plasma membrane.
Since the natriuretic peptides play pivotal roles in a broad variety of tissues and
cell types and considering that synthetic analogs have recently proven to be clinically
beneficial, an understanding of receptor regulation mechanisms and cell type-specific
phenomena is important from both a basic science and a clinical perspective.
In this thesis, I used two cell lines which either express only GC-A (MA-10
Leydig cells) or co-express GC-A and GC-B ( αT3-1 pituitary cells) to specifically
address certain questions of interest.
The experiments revealed that sustained exposure to either ANP itself or to the
biologically-active phospholipid lysophosphatidic acid (LPA) elicits GC-A
desensitization in MA-10 Leydig cells. Both reactions were found to have similar
kinetics and finally resulted in an equal decrease (by 40%) in GC-A hormone-
responsiveness. Homologous (ANP-induced), but not LPA-induced (so-called
heterologous) desensitization was blocked by a cyclic AMP-dependent protein kinase
(PKA) inhibitor, indicating distinct pathways and a crucial role for PKA in the process
of homologous desensitization, where cGMP is generated as second messenger.
LPA, but not ANP treatment enhanced extracellular signal-regulated kinase (ERK)
Abstract 8
phosphorylation and induced a striking re-organization of actin filaments. The LPA
effects suggested receptor-mediated pathways, consistent with the identification of
type 2 (LPA ) LPA receptor gene expression. At the molecular level, these findings 2
showed for the first time (a) that homologous and heterologous desensitization are
mediated by unique pathways, and (b) that PKA is essentially implicated in
homologous (cGMP-dependent) desensitization processes. In addition, they provided
novel evidence for cross-talks between natriuretic peptide and phospholipid
signalling. Physiologically, the control mechanisms identified in MA-10 cells suggest
distinct roles in native Leydig cells, related to regulation of steroidogenesis and cell
growth/differentiation. The elucidation of pronounced cellular effects in response to
LPA indicates for the first time that Leydig cells may represent targets for
phospholipid signalling in vivo.
Questions that could be specifically addressed in αT3-1 cells were based on the
co-expression of GC-A and GC-B in this cell type. The results obtained demonstrate
for the first time (a) interactions (“cross-talks”) between ANP/GC-A and CNP/GC-B
signalling at a cellular level and (b) that the hormone-responsiveness of natriuretic
peptide receptors can be enhanced (“sensit