The impairment of HCCS leads to MLS syndrome by activating a non-canonical cell death pathway in the brain and eyes.
Bottom Line: Mitochondrial-dependent (intrinsic) programmed cell death (PCD) is an essential homoeostatic mechanism that selects bioenergetically proficient cells suitable for tissue/organ development.By taking advantage of a medaka model that recapitulates the MLS phenotype we demonstrate that downregulation of hccs, an essential player of the mitochondrial respiratory chain (MRC), causes increased cell death via an apoptosome-independent caspase-9 activation in brain and eyes.We also show that the unconventional activation of caspase-9 occurs in the mitochondria and is triggered by MRC impairment and overproduction of reactive oxygen species (ROS).
Affiliation: Telethon Institute of Genetics and Medicine, Naples, Italy.Show MeSH
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Mentions: Next, to gain insight into the mechanism of defective organ development in MLS, we investigated a vertebrate animal model. Genetic inactivation of Hccs results in ES cell lethality in the mouse (Prakash et al, 2002), preventing the generation of a suitable mammalian model for MLS syndrome. We thus turned to the teleost medaka fish in which a morpholino (MO)-based knockdown approach enables the generation of morphant embryos with less dramatic phenotypes (Wittbrodt et al, 2002). Publicly available sequences were used to identify the HCCS homolog in medaka (hccs; see Supporting Information Materials and Methods section and Supporting Information Fig S2). RNA in situ hybridization (ISH) studies revealed that hccs is ubiquitously expressed at all developmental stages analysed, with higher expression levels in the eyes, CNS, heart and skeletal muscle (Fig 1). In particular, at stage (st) 19 we found high levels of expression in the optic vesicle, the presumptive retinal pigmented epithelium (RPE), the presumptive mesencephalic region and the tail (arrows in Fig 1A). At st24 high levels of expression were detected in the lens placode, the RPE and the neural retina of the optic cup, with an apparent peripheralhigh to centrallow gradient (Fig 1B). At later stages (st30, st34, st38), hccs is also expressed in the retinal progenitor cells within the ciliary marginal zone as well as in the ganglion and amacrine cells (Fig 1C–E). At st38, similarly to what reported in human and mouse (Ramskold et al, 2009; Schaefer et al, 1996; Schwarz & Cox, 2002), high expression levels were found in the CNS (Fig 1F), the heart (Fig 1G) and the skeletal muscles (Fig 1H).
Affiliation: Telethon Institute of Genetics and Medicine, Naples, Italy.