Gomez Espinosa, EF; (2017) An investigation into the characteristics of the phospholipid transfer protein RdgBβ and the phospholipid biosynthetic enzyme Cds1 in the mammalian heart. Doctoral thesis , UCL (University College London).

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Abstract

The conversion of phosphatidic acid (PA) to cytidine diphosphate-diacylglycerol (CDP-DAG) by CDP-DAG synthase (Cds) is the rate-limiting step for the biosynthesis of the mitochondrial lipid, cardiolipin (CL) and phosphatidylinositol (PI), the precursor for many signalling phosphoinositides. There are two Cds isoforms in mammals, Cds1 and Cds2. During cardiomyocyte growth and development, mitochondrial biogenesis is increased under the control of the transcription factor, PGC-1α which also regulates Cds1 expression. Mitochondria have limited capacity to synthesise their own lipids and are dependent on the endo/sarcoplasmic reticulum for many of their lipids, including PI and PA. RdgBβ (Retinal degeneration B beta) is a phosphatidylinositol transfer protein (PITP), recently found to be enriched in cardiac tissue, able to transfer both PI and PA. This investigation attempts to identify the residues within the PITP domain that are required for PA transfer and it focuses on the characterisation of the expression and distribution of RdgBβ and Cds1 in rat hearts and H9c2 cells which are derived from embryonic rat ventricles. Withdrawal of serum and the addition of retinoic acid induces differentiation of H9c2 cells towards a cardiomyocyte-like phenotype. At the protein level, proliferating H9c2 cells have low RdgBβ expression which increases dramatically following differentiation with retinoic acid. The increase in RdgBβ expression occurred in parallel with an elevation in mitochondrial content. In addition to the cytosol, RdgBβ was also located at membrane fractions, suggesting that RdgBβ may facilitate PI and PA transport between the ER and the mitochondria. The regulation of mRNA expression of Cds1 was not associated with cellular differentiation but with vasopressin stimulation of H9c2 cells. Both myoblasts and differentiated H9c2 cells showed an increase of Cds1 mRNA suggesting that Cds1 is involved in the PI cycle.

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