Carnitine Palmitoyltransferase I and Acyl-CoA Dehydrogenase 9 in Retina: Insights of Retinopathy in Mitochondrial Trifunctional Protein Defects

Progressive pigment chorioretinopathy is a major long-term complication of mitochondrial trifunctional protein (MTP) defects, disorders of mitochondrial fatty acid beta-oxidation. To better understand the pathogenesis of the retinopathy component, the authors studied expression of the main regulatory protein of the beta-oxidation pathway, carnitine palmitoyltransferase (CPT) 1, and acyl-CoA dehydrogenase (ACAD) 9 in retinal sections and cultured cells. Immunoblotting was performed with polyclonal antibodies to ACAD9 and the three isoforms of CPT1. In quantitative real-time PCR (QRT-PCR), predesigned gene-specific probes and primer sets for human CPT1 isoforms were used. In situ hybridization (ISH) and immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections of the rat and human eye. The predominant CPT1 mRNA types detected by QRT-PCR in cultured human retinal pigment epithelial cells were of the liver (CPT1A) and brain (CPT1C) isotypes. CPT1A and ACAD9 protein expression was found in cultured human and rat RPE and rat neural retinal precursor cells. ISH of rat retinal sections showed CPT1A and CPT1C expression in the retinal pigment epithelium (RPE), the inner nuclear layer, and the ganglion cell layer. CPT1A expression was also detected in the Müller cell microvilli, and CPT1C expression was detected in the photoreceptor inner segments. ACAD9 immunolabeling was detected in rat and human RPE, human photoreceptor inner segments, and ganglion cell layer. These findings imply that the mitochondrial fatty acid beta-oxidation pathway probably is active in metabolism of the RPE and certain neuroretinal cell types. Accumulation of 3-hydroxylated intermediates of long-chain fatty acids may contribute to the pathogenesis of retinopathy in MTP deficiencies.