Integrative Physiology of the Plasma Membrane Calcium Pump Isoform 4 in the Healthy and Failing Heart

Heart failure, which is a condition in which the heart is unable to pump sufficient blood throughout the body, and its early signs are a major cause of disability, reduced quality of life and death in our aging population. In order to develop better treatments we must increase our understanding of the genetic and molecular changes which lead to heart failure.
Calcium is very important for the correct functioning of the heart and it is well know that disruption of calcium levels leads to cardiovascular complications. One of the proteins involved in the movement of calcium from heart cells is the plasma membrane calcium pump (PMCA); in this project we aim to find out how PMCA regulates the working of the heart.
Specifically we will generate mice with modified expression of PMCA and determine whether this leads to changes in the structure and/or function of the heart. We use the mouse as a model system as it reflects the human heart and we are able to use tools similar to those used by clinicians to determine how well the heart is working.
This research will improve our understanding of heart failure and may lead to potential new treatment strategies.

Heart failure is one of the leading causes of mortality, but our understanding of its pathophysiology and treatment options are limited. Calcium transport anomalies are key aspects in heart failure, but in contrast to several well-characterised transport systems such as Na+-Ca2+ exchanger and SERCA, the role of the plasma membrane calcium pump (or PMCA: Plasma Membrane Calcium / Calmodulin-dependent ATPase), a system which extrudes calcium from the cytoplasm, has remained elusive. The present programme aims at elucidating the role of PMCA4 in the healthy and failing heart.

In the last 3 years in Manchester, we have achieved a major breakthrough: PMCA is a key signalling molecule, potentially in addition to having a role in relaxation.

We have shown that:
1. A major isoform, PMCA4b (also known as PMCA4CI), directly interacts with nNOS (neuronal NOS, a regulator of cardiac signalling) through a PDZ domain, tightly downregulates its activity and is tethered to the dystrophin complex via syntrophin. This suggests a role for the PMCA in dystrophin-related cardiac myopathies as well as in common forms of heart failure.
2. PMCA4 also regulates the Ras pathway, which is involved in cardiac hypertrophy and failure.
3. In contrast to the conventional hypothesis regarding PMCA function, our PMCA4 knock-out mice show enhanced cardiac relaxation, likely mediated by uninhibited nNOS function.

The present proposal is a direct continuation of this work addressing three questions:
A. What are the molecular and cellular mechanisms of PMCA action in the heart?
We will use electrophysiology of cardiomyocytes from PMCA4 knock-out and over-expressing animals as well as cells transfected with adenoviral constructs to characterise the role of PMCA4 in cardiac calcium transport and relaxation as well as in signalling processes involving Ras, nNOS and syntrophin.
B. What is the physiological function of cardiac PMCA in the whole animal?
Using invasive and non-invasive haemodynamic methods, the cardiac function of our general and heart specific knock-out models will be investigated, a pharmacological approach will determine phenotypic mechanisms.
C. What is the function of the PMCA in cardiac disease?
The proposed electrophysiological and in vivo studies will be transposed to models of heart disease.

This programme will result in a comprehensive characterisation of PMCA4 function in the heart and potentially identify the PMCA as an attractive target for future drug therapy in heart, and possibly other, diseases.