Porphyrin synthesis in the planarian. I. Conversion of delta-aminolevulinic acid to porphobilinogen

The true colours of the flatworm: Mechanisms of pigment biosynthesis and pigment cell lineage development in planarians

Pigment cells serve a variety of important uses across the animal kingdom, and in many species can change and regenerate throughout the lifetime of the organism. The functions of these cells, as well as their origins in both embryonic development and adult regeneration, are not fully understood. Here, we review advances in the study of pigment cells in the freshwater planarian, a model system for stem cell biology and regeneration. Freshwater planarians produce at least three pigment types to generate brown eye and body colouration: melanin, porphyrin, and ommochrome. The body pigments of planarians are produced and contained by a specialized, highly dendritic cell type located in the subepidermal parenchymal space. This cell type is specifically ablated following intense light exposure, a characteristic which has been exploited to discover the gene expression and regeneration of planarian pigment cells. Regenerating pigment cells progress through an immature state marked by upregulation of pigment synthesis genes before differentiating into mature pigment cells; these two states are dynamically regulated in homeostasis to maintain constant body pigmentation. The transcription factors Albino, FoxF-1, and Ets-1, as well as an FGFR-like molecule, are required for proper maintenance of the pigment lineage in both regeneration and homeostasis. These discoveries set the stage for research into external signals that regulate the pigment lineage, as well as possible functions for pigment cells in planarians, including the extra-ocular light response. These insights will address outstanding questions about the evolutionarily-conserved biology of pigment cells.

Forkhead containing transcription factor Albino controls tetrapyrrole-based body pigmentation in planarian

Pigmentation processes occur from invertebrates to mammals. Owing to the complexity of the pigmentary system, in vivo animal models for pigmentation study are limited. Planarians are capable of regenerating any missing part including the dark-brown pigments, providing a promising model for pigmentation study. However, the molecular mechanism of planarian body pigmentation is poorly understood. We found in an RNA interference screen that a forkhead containing transcription factor, Albino, was required for pigmentation without affecting survival or other regeneration processes. In addition, the body color recovered after termination of Albino double stranded RNA feeding owing to the robust stem cell system. Further expression analysis revealed a spatial and temporal correlation between Albino and pigmentation process. Gene expression arrays revealed that the expression of three tetrapyrrole biosynthesis enzymes, ALAD, ALAS and PBGD, was impaired upon Albino RNA interference. RNA interference of PBGD led to a similar albinism phenotype caused by Albino RNA interference. Moreover, PBGD was specifically expressed in pigment cells and can serve as a pigment cell molecular marker. Our results revealed that Albino controls planarian body color pigmentation dominantly via regulating tetrapyrrole biogenesis. These results identified Albino as the key regulator of the tetrapyrrole-based planarian body pigmentation, suggesting a role of Albino during stem cell-pigment cell fate decision and provided new insights into porphyria pathogenesis.

Light-induced depigmentation in planarians models the pathophysiology of acute porphyrias

Porphyrias are disorders of heme metabolism frequently characterized by extreme photosensitivity. This symptom results from accumulation of porphyrins, tetrapyrrole intermediates in heme biosynthesis that generate reactive oxygen species when exposed to light, in the skin of affected individuals. Here we report that in addition to producing an ommochrome body pigment, the planarian flatworm Schmidtea mediterranea generates porphyrins in its subepithelial pigment cells under physiological conditions, and that this leads to pigment cell loss when animals are exposed to intense visible light. Remarkably, porphyrin biosynthesis and light-induced depigmentation are enhanced by starvation, recapitulating a common feature of some porphyrias – decreased nutrient intake precipitates an acute manifestation of the disease. Our results establish planarians as an experimentally tractable animal model for research into the pathophysiology of acute porphyrias, and potentially for the identification of novel pharmacological interventions capable of alleviating porphyrin-mediated photosensitivity or decoupling dieting and fasting from disease pathogenesis.

DOI:http://dx.doi.org/10.7554/eLife.14175.001

Porphyrias are rare diseases that involve ring-shaped molecules called porphyrins accumulating in various parts of the body. Porphyrins are produced as part of the normal process that makes an important molecule called heme, which is required to transport oxygen. However, high levels of porphyrins can be toxic. For example, porphyrins deposited in the skin can cause swelling and blistering when the skin is exposed to bright light. Other disease symptoms include neurological issues ranging from anxiety and confusion to seizures or paralysis. It has been speculated that porphyrias may have affected several historical figures, including the artist Vincent van Gogh.

In addition to their role in heme production, porphyrins also have other roles. For example, they are used as pigments in the wing feathers of some owls. Researchers are trying to understand more about how organisms regulate porphyrin production so that it might be possible to develop more effective treatments for porphyria in humans.

Here, Stubenhaus et al. studied how a flatworm called Schmidtea mediterranea makes porphyrins. A group of undergraduate students noticed that these animals – which are normally brown in color – turned white when they were exposed to sunlight for several days. Stubenhaus et al. found that S. mediterranea makes porphyrins in the pigment cells of its skin using the same genes that make porphyrins in humans. Together with other molecules called ommochromes, the porphyrins give rise to the normal color of this flatworm. However, when the animals are exposed to intense light for extended periods of time, which is unlikely to occur in the wild, porphyrin production leads to loss of the pigment cells.

The experiments also show that starvation increases the rate of pigment cell loss in light-exposed flatworms, which mirrors the worsening of disease symptoms some porphyria patients experience when they diet or fast. Stubenhaus et al. propose that flatworms are useful models in which to study the molecular processes that are responsible for porphyrias in humans. Further research is required to determine the exact chemical structure of the porphyrin and ommochrome molecules produced in different flatworm species. Stubenhaus et al. also plan to use flatworms to screen for drugs that could potentially be developed into new treatments for porphyria.

DOI:http://dx.doi.org/10.7554/eLife.14175.002

Localization of porphyrin fluorescence in planarians

Two species of planarians were studied by fluorescence microscopy. Red fluorescence of uroporphyrin was observed localized in the epidermal rhabdites and subepidermal rhabdite-containing gland cells. Fluorescence was observed in isolated rhabdites of homogenates, but was not seen in rhabdites of the living animal. The identity of rhabdites was established by their location, shape, size, and acid-ophilic staining properties.

Effect of chronic and acute lead treatment in the slug Arion ater on calcium and delta-aminolaevulinic acid dehydratase activity

Accumulation of lead in the tissues was greater after acute exposure. Most of the metal was deposited in the intestine and least in foot. After chronic exposure to lead the calcium content of the intestine decreased. After chronic and acute exposure to lead the calcium content of the digestive gland and foot was lower than control values. Lead in the digestive gland granules was higher after acute lead treatment. Less than 45% of the lead was associated with the granules. These results are discussed in view of the difference in metabolism and chemical nature of the organs investigated. ALAD activity was located in the digestive gland and it was reduced after acute treatment with lead.