Role of porphyrin sequestration in the biogenesis of iron-laden astrocytic inclusions in primary culture

Pitfalls and fallacies interfering with correct identification of embryonic stem cells implanted into the brain after experimental traumatic injury

Cell-therapy was proposed to be a promising tool in case of death or impairment of specific cell types. Correct identification of implanted cells became crucial when evaluating the success of transplantation therapy. Various methods of cell labeling have been employed in previously published studies. The use of intrinsic signaling of green fluorescent protein (GFP) has led to a well known controversy in the field of cardiovascular research. We encountered similar methodological pitfalls after transplantation of GFP-transfected embryonic stem cells into rat brains following traumatic brain injury (TBI). As the identification of implanted graft by intrinsic autofluorescence failed, anti-GFP labeling coupled to fluorescent and conventional antibodies was needed to visualize the implanted cells. Furthermore, different cell types with strong intrinsic autofluorescence were found at the sites of injury and transplantation, thus mimicking the implanted stem cells. GFP-positive stem cells were correctly localized, using advanced histological techniques. The activation of microglia/macrophages, accompanying the transplantation post TBI, was shown to be a significant source of artefacts, interfering with correct identification of implanted stem cells. Dependent on the strategy of stem cell tracking, the phagocytosis of implanted cells as observed in this study, might also impede the interpretation of results. Critical appraisal of previously published data as well as a review of different histological techniques provide tools for a more accurate identification of transplanted stem cells.

Detecting fluorescent protein expression and co-localisation on single secretory vesicles with linear spectral unmixing

Many questions in cell biology and biophysics involve the quantitation of co-localisation and the interaction of proteins tagged with different fluorophores. However, the incomplete separation of the different colour channels due to the presence of autofluorescence, along with cross-excitation and emission "bleed-through" of one colour channel into the other, all combine to render the interpretation of multi-band images ambiguous. Here we introduce a new live-cell epifluorescence spectral imaging and linear unmixing technique for classifying resolution-limited point objects containing multiple fluorophores. We demonstrate the performance of our technique by detecting, at the single-vesicle level, the co-expression of the vesicle-associated membrane protein, VAMP-2 (also called synaptobrevin-2), linked to either enhanced green fluorescent protein (EGFP) or citrine [a less pH-sensitive variant of enhanced yellow fluorescent protein (EYFP)], in mouse cortical astrocytes. In contrast, the co-expression of VAMP-2-citrine and the lysosomal transporter sialine fused to EGFP resulted in little overlap. Spectral imaging and linear unmixing permit us to fingerprint the expression of spectrally overlapping fluorescent proteins on single secretory organelles in the presence of a spectrally broad autofluorescence. Our technique provides a robust alternative to error-prone dual- or triple colour co-localisation studies.

Heme, iron, and the mitochondrial decay of ageing

Heme, the major functional form of iron, is synthesized in the mitochondria. Although disturbed heme metabolism causes mitochondrial decay, oxidative stress, and iron accumulation, all of which are hallmarks of ageing, heme has been little studied in nutritional deficiency, in ageing, or age-related disorders such as Alzheimer's disease (AD). Biosynthesis of heme requires Vitamin B(6), riboflavin, biotin, pantothenic acid, and lipoic acid and the minerals zinc, iron, and copper, micronutrients are essential for the production of succinyl-CoA, the precursor for porphyrins, by the TCA (Krebs) cycle. Only a small fraction of the porphyrins synthesized from succinyl-CoA are converted to heme, the rest are excreted out of the body together with the degradation products of heme (e.g. bilirubin). Therefore, the heme biosynthetic pathway causes a net loss of succinyl-CoA from the TCA cycle. The mitochondrial pool of succinyl-CoA may limit heme biosynthesis in deficiencies for micronutrients (e.g. iron or biotin deficiency). Ageing and AD are also associated with hypometabolism, increase in heme oxygenase-1, loss of complex IV, and iron accumulation. Heme is a common denominator for all these changes, suggesting that heme metabolism maybe altered in age-related disorders. Heme can also be a prooxidant: it converts less reactive oxidants to highly reactive free radicals. Free heme has high affinity for different cell structures (protein, membranes, and DNA), triggering site-directed oxidative damage. This review discusses heme metabolism as related to metabolic changes seen in ageing and age-related disorders and highlights the possible role in iron deficiency.

Brain iron deposition and the free radical-mitochondrial theory of ageing

The central hypothesis of this paper states that oxidative stress, augmented iron deposition, and mitochondrial insufficiency in the ageing and degenerating CNS constitute a single neuropathological 'lesion', and that the advent of one component of this triad obligates the appearance of the others. Evidence in support of this unifying perspective is adduced from human neuropathological studies, experimental paradigms of ageing-associated neurological disorders, and a comprehensive model of astroglial senescence. A pivotal role for the enzyme, heme oxygenase-1 (HO-1) in consolidating this tripartite lesion in the ageing and diseased CNS is emphasized. The data are discussed in the context of a revised 'free radical-mitochondrial-metal' theory of brain ageing, and some scientific and clinical implications of the latter are considered.