The vertebrate brain has evolved, in large part, to integrate and process multiple types of sensory information (”input”), and to produce and execute a response, or coordinated program for action (”output”). The thalamus develops from the embryonic brain region called the diencephalon, which also gives rise to the retina of the eye, and to the hypothalamus, epithalamus (pineal body), and subthalamic nucleus. The thalamus is a key relay center required to connect different brain structures, such as the cerebral cortex, basal nuclei, cerebellum, and brainstem nuclei, in functional networks that allow an animal to respond to its changing environment. Although applying computing systems analogies to the mammalian brain is an oversimplification, it might be useful to think of the thalamus as a router, which forwards information from the thousands of brain CPUs required for complex parallel processing. Undoubtedly, the thalamus has its own CPUs, but a major function of this brain structure is to receive information from sensory pathways (pain, touch, position sense, vision, hearing, taste) and from independent supercomputers like the cerebellum and basal nuclei, and to pass this information to the cerebral cortex in a very organized manner. Of course, the orderly connections between neurons in the thalamus and neurons in the cerebral cortex (thalamocortical projections) must be established during development, and this embryonic wiring process is dependent on accurate pathfinding by the processes (axons) that neurons send out from their “homes” in the various nuclei and cortical layers of the brain.

Figure 1: Topographic organization of thalamocortical projections. a) in mouse, ventrolateral thalamic nucleus projects to primary motor cortex (M1); ventrobasal nucleus (VB) projects to primary somatosensory cortex (S1); lateral geniculate nucleus (LGN) projects to visual cortex (V1) b) axons from the lateral VB nucleus project to medial S1 cortex (barrel field) d) same topographic organization in human cerebral cortex (from Vanderhaeghen and Polleux, 2004)

This is Anatomy Week here at Guadalupe Storm-Petrel, and the general question addressed by Powell and colleagues is a neuroanatomical one: what cues do thalamic axons use to navigate to the correct regions of the cerebral cortex (e.g. visual, auditory, motor, somatosensory)? Not only must the axons of thalamic neurons project to the correct cortical areas, but they must also maintain spatial relationships, or topography. This precise topographic organization of axons, which exists throughout the central nervous system, allows the brain to determine precisely where in the body sensory information came from, and where motor information must be sent. Dufour and colleagues (2003) reported that ephrins and their Eph receptors act as guidance cues for thalamocortical axons, as they undergo topographic sorting in an intermediate target, the ventral telencephalon. In mice that lack the adhesion molecule, close homolog of L1 (CHL1), axons from the VB nucleus of the thalamus are misrouted from the somatosensory cortex (their normal target), to the visual cortex (Wright et al., 2007). In the PLoS Biology article to be discussed in detail here, Powell and colleagues (doi:10.1371/journal.pbio.0060016) add three more molecules to the list of those involved in the patterning of thalamocortical projections: Netrin-1, DCC (deleted in colorectal cancer), and Unc5. As summarized in Figure 2, Netrin-1 attracts axons from the rostral thalamus, and repels those emerging from the caudal thalamus.

Figure 2. Diagrams of horizontal brain sections A. Previous work from Dufour et al. (2003) showed that Ephrin-A5 and Eph receptors are involved in patterning thalamocortical connections. B. and C. Netrin-1 attracts some thalamic axons, and repels others (from Powell et al., 2008 )

To perform the studies, Powell and colleagues used a “whole-mount telencephalic assay”, which allowed them to label thalamic neurons, trace their axons growing through the ventral telencephalon, and map projections to the cerebral cortex, in an accessible organ culture preparation that maintains anatomical relationships. In particular, the researchers used the landmark of the corticostriatal boundary (CSB), between ventral and dorsal telencephalon, to monitor the topographic organization of emerging thalamic axons, and to establish that this pattern exists well before axons reach the cortex. Low molecular weight biotinylated dextran amine (BDA) was injected into the thalamus, to fill thalamocortical axons rapidly and anterogradely, such that developing connections could be mapped and counted. As thalamic axons grow to the cerebral cortex, they form a structure called the internal capsule, and as a final descriptive step, Powell and colleagues used in situ hybridization and a gene trap mouse line to show that Netrin-1 is expressed in a gradient across this region of the ventral telencephalon: high levels at the rostral (nose) end, and low levels at the caudal (tail) end. The remaining experiments address the roles of Netrin-1, and its attractive (DCC) and repulsive (Unc5) receptors, in establishing the topographic organization of thalamic axons as they project to the cortex.

Figure 3. Co-culture of wild-type rostral thalamus with wild-type (B) or Netrin-1-/- ventral telencephalon (C). In these photos, the nose end (R) of the brain is to the right, and the lateral edge of the brain slice is toward the top of the figure. Note how the DTR axons wander caudally (towards the tail) in C.

One of the advantages of working with mouse embryos is the availability of lines with targeted (”knockout”) mutations in identified genes. Although normal numbers of thalamocortical axons are present in the brains of Netrin-1-/- mouse embryos, axons emerging from the rostral part of the thalamus project more caudally into the ventral telencephalon, than they do in the brain of a wild-type embryo. Moreover, Figure 3 shows that when wild-type rostral thalamus (DTR) is combined with Netrin-1-/- ventral telencephalon (VTel), many of the axons extend more caudally than they normally would. As one might expect, axons from the rostral thalamus express DCC receptors for Netrin-1, which allow them to respond to the attractive properties of this protein. When the function of this DCC receptor was blocked with specific antibodies, the DTR axons became disorganized, and extended more caudally into the VTel, just as in the experiment with the Netrin-deficient telencephalon.

Figure 4. Co-culture of caudal thalamus with ventral telencephalon, in the presence of a control (F) or Unc5-blocking (G) antibody. Note how the DTC axons extend rostrally in G (red arrow).

So the DCC: Netrin-1 interaction can get DTR axons to the correct rostral cortical regions, but now you might be wondering how the caudal thalamic (DTC) axons are kept out of the rostral VTel, and guided instead to their more caudal cortical targets. The answer lies with the Unc5 receptor, which also binds Netrin-1, but unlike DCC, causes axons to be repelled by this protein. Unc5 is expressed in the thalamus, in a pattern complementary to DCC, such that axons from the caudal thalamus (DTC) express lots of Unc5, and only a small amount of DCC. Figure 4 shows that when the repulsive function of the Unc5: Netrin-1 interaction was blocked with Unc5 antibodies, DTC axons inappropriately invaded rostral areas of the ventral telencephalon. Finally, to wrap up this Netrin-1/DCC/Unc5 story and tie it with an elegant bow, Powell and colleagues overexpressed the Unc5 receptor in DTR neurons, causing their axons to be repelled, rather than attracted, by the high levels of Netrin-1 in the rostral VTel, and to extend more caudally than normal as a consequence.

I’ve included only a small fraction of the beautiful photomicrographs of labeled axons in the whole mount telencephalon preps-go take a look at this Open Access article to see the others!

References:

Dufour, A., Seibt, J., Passante, L., et al. (2003) Area specificity and topography of thalamocortical projections are controlled by ephrin/Eph genes. Neuron 39, 453-465.

Vanderhaeghen, P., and Polleux, F. (2004) Developmental mechanisms patterning thalamocortical projections: intrinsic, extrinsic and in between. Trends in Neurosciences 27(7), 384-391.

Wright, A.G., Demyanenko, G.P., Powell, A. (2007) Close homolog of L1 and neuropilin 1 mediate guidance of thalamocortical axons at the ventral telecephalon. J. Neuroscience 27(50), 13667-13679.

Powell, A.W., Sassa, T., Wu, Y., Tessier-Lavigne, M., Polleux, F., Ghosh, A. (2008). Topography of Thalamic Projections Requires Attractive and Repulsive Functions of Netrin-1 in the Ventral Telencephalon. PLoS Biology, 6(5), e116. DOI: 10.1371/journal.pbio.0060116

Beautiful San Francisco Bay is one of my favorite places to visit, and numerous feathered types feel the same way: the Bay supports thousands of wintering and migrating shorebirds and waterbirds each year. Unfortunately, this important estuary is contaminated by residual mercury from mining and gold extraction, and the restoration of salt evaporation ponds as tidal marsh habitats may actually increase the bioavailability of mercury for aquatic wildlife. To determine current contamination levels in locally breeding shorebirds, and to correlate these levels with foraging, roosting, and breeding sites, Ackerman and colleagues measured mercury concentrations in the blood of American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus).

Golden Gate Bridge, San Francisco CA
Photo: Barn Owl

First, study sites in the North and South Bays were chosen, with wetland habitats including salt ponds, tidal flats, tidal marsh, and diked wetlands. Avocets and stilts are the most abundant shorebird species that breed in SF Bay, and these birds were captured during the pre-breeding season, using a net-launcher or rocket nets. Blood was collected from the brachial vein for contaminant analyses, and adult birds were fitted with radio transmitters and leg bands, for space use monitoring. Birds were tracked daily from trucks, and every two weeks from fixed-wing aircraft. Blood samples were analyzed for total mercury levels by atomic absorption spectroscopy.

Black-necked Stilt, Himantopus mexicanus
Photo: Dr. Jim Smith

Over two pre-breeding seasons in 2005 and 2006, 373 avocets and 157 stilts were examined. The highest blood concentrations of mercury were discovered in birds at the Alviso salt pond complex in South SF Bay, most likely due to contaminated sediments from the historic New Almaden mercury mine; this area is the site of over 200 avocet nests, and over 300 stilt nests. At all study sites, mercury levels were higher in stilts than in avocets. Stilts tended to use managed marshes, and vegetated areas, more than did avocets, and the researchers proposed that differences in micro-habitat selection may influence mercury exposure. Females of both shorebird species had lower mercury levels than did males, a difference that can only be partially explained by the depuration of methyl mercury into eggs, a process demonstrated to occur in loons, gulls, pelicans, and terns. Although the restoration of SF Bay wetland habitats is a positive development, these efforts may have the unintended consequence of increasing levels of toxic and bioavailable methyl mercury.

References:

Ackerman, J.T., Eagles-Smith, C.A., Takekawa, J.Y., et al. (2007). Mercury concentrations and space use of pre-breeding American avocets and black-necked stilts in San Francisco Bay. Science of the Total Environment 384, 452-466.

Both critics and fans of horse racing are mourning the death of the filly Eight Belles, euthanized by the track veterinarian after sustaining fractures in both front legs. One fracture involved the condyle of the cannon bone, which is actually the third metacarpal of the equine forelimb; there are two smaller metacarpals (medial and lateral) in the horse forelimb, and the condyle of the cannon bone articulates with the first phalanx at the fetlock. Many blame the track surface at Churchill Downs for such injuries, but I’m more inclined to put the blame on irresponsible breeding and training practices in the Thoroughbred racing industry. I’ll use some horses that I know quite well to illustrate my point about the breeding: this is an unscientific study (with help, I can provide measurements in the future), with a very small “n”, but I think I can start to make a point with some photos. The two Thoroughbreds were bred for racing (which they did, unsuccessfully), whereas the warmbloods were bred for conformation.

Exhibit A is a photo of the forelegs of my 17-year-old Thoroughbred mare; pay particular attention to the diameter of her cannon bones, which you can see as separate from the tendons at the back. She also has a visible birth defect, which causes her to favor strongly one particular lead at the canter, and prevents her from being a decent dressage horse (otherwise she is quite talented at this discipline, for a Thoroughbred). Can you tell what her birth defect is? (I’ll bet Coturnix can spot it)

Exhibit B is a photo of my 9-year-old Thoroughbred gelding. He’s reasonably correct, for a Thoroughbred-perhaps a bit slab-sided, and his pasterns are a little too upright. He’s 16 hh, at the upper limit for polo and polocrosse. He’s developing “Birdcatcher spots”, a few more each year-certainly not a flaw, and rather pretty.

Now we get to the warmbloods. First, my friends’ stallion-I think he is seven or eight years old now- a Cleveland Bay, about 17 hh. Look at the diameter of the bone in his lower legs! (Exhibit C)

Exhibit D is one of the stallion’s offspring, out of a Percheron x Thoroughbred mare; this is a two-year-old filly, still growing (already 17 hh or taller…she will be somebody’s dream eventing horse).

Exhibit E is the second of the stallion’s offspring out of the Percheron x Thoroughbred mare. This is a yearling filly (almost exactly one year old)…look at the bone in her legs!

Against my better judgment, I have created another plastic bag jellyfish. The oral arms were crocheted from plastic grocery bags; the bell and the tentacles were crocheted from plastic newspaper wrappers. Photographed on my front porch.

My Earth Week series would not be balanced, without a post describing research on environmental conditions that affect millions of humans, particularly in the developing world. Approximately one billion people across the globe reside in urban slums, which were defined in 2002 by a United Nations Expert Group as settlements with the following characteristics: inadequate access to safe water and to sanitation, poor structural quality of housing, overcrowding, and insecure residential status, i.e. individuals live as squatters, and have no legal title to the land (Riley et al., 2007). In general, chronic non-communicable and communicable diseases go unrecognized and untreated in urban slum inhabitants, until they manifest as kidney failure, heart attack, stroke, pulmonary failure, or multidrug-resistant tuberculosis. In Salvador, a large Brazilian city where 60% of the inhabitants reside in slums (favelas), severe leptospirosis was diagnosed almost exclusively in slum-dwellers (95% of cases). This disease, which can cause acute, potentially fatal, kidney failure or lung hemorrhage, is entirely preventable (Riley et al., 2007).

A favela in Salvador, Brazil, showing proximity of houses to an open sewer (from Riley et al., 2007)

Leptospirosis is caused by a spirochete, which is excreted in urine by rats and other infected mammals, and transmitted in contaminated water supplies. Both subsistence farmers and urban slum-dwellers in developing countries are at a particularly high risk for contracting leptospirosis, which in its severe kidney (Weil’s disease) and lung (severe pulmonary hemorrhage syndrome, SPHS) forms is associated with 10%, and over 50% case fatality, respectively (McBride et al., 2005). In a recent article in PloS Neglected Tropical Diseases, Reis and colleagues reported on how the urban environment and social gradient affect the incidence of infection with the Leptospira spirochete, using study populations in the Pau da Lima favela community in Salvador, Brazil. The researchers chose a study site within this favela that included four valleys within an area of 0.46 square kilometers, and 1079 sample households were selected at random. Reis and colleagues focused on sewage systems, refuse collection services, rat infestation, microgeographic features, and socioeconomic status within their study site, and collected blood samples from the research subjects to look for serological evidence of prior Leptospira infection.

Panel C shows the distribution of high risk (dark red-orange) and low risk (yellow) for Leptospira infection with the study site. Panel E shows the locations of open sewer systems (tan lines) and open rainwater drainage systems (blue lines) on the same study site map. (from Reis et al., 2008 )

Of the 3,171 individuals tested, 489 (15.4%) had detectable levels of anti-Leptospira antibodies in the serological analyses, and the risk of acquiring these antibodies was correlated with residence at the bottoms of valleys within the study site. Open sewers are most often located at the bottoms of valleys, and the closer a household was to an open sewer, the greater the risk of Leptospira infection for its inhabitants. Living close to an open refuse deposit, and more frequent sightings of rats, were also associated with higher frequencies of Leptospira infection in the favela residents. These correlations, and the figures included in the paper, reminded me of Dr. John Snow’s maps of London cholera deaths and water pumps, described in Edward Tufte’s wonderful book, The Visual Display of Quantitative Information.

The risk of acquiring Leptospira antibodies (f), as associated with the continuous variables of A. per capita daily household income, and C. distance in meters to the nearest open sewer. (from Reis et al., 2008 )

The authors identified and confirmed several risk factors, associated with the infrastructure of the urban slum environment, for Leptospira infection. Since rats are the main reservoir of the spirochete in this setting, proximity to open sewers and refuse deposits, where rats have home ranges, was positively correlated with Leptospira antibody titers in the study population. Independent of these environmental factors, the researchers found that both low per capita household income and black race were risk factors for spirochete infection, indicating a role for social determinants in transmission of this disease organism. Reis and colleagues concluded that factors related to climate, geography, and urban poverty interact to influence patterns of Leptospira infection in the Salvador population, and suggest that this is likely to be the case in many other urban slums. Importantly, the researchers point out that many of the transmission factors could be addressed readily by improving sanitation in this Brazilian community, and by considering the social determinants that lead to health outcome inequalities. Recent analyses by other investigators have indicated that the stresses of low socioeconomic status and racial prejudice contribute significantly to poor health outcomes, though measurements of corticosteroid levels and other physiological stress indicators were not the focus of this study.

References:

McBride, A.J.A., Athanazio, D.A., Reis, M.G., and Ko, A.I. (2005). Leptospirosis. Current Opinion in Infectious Disease 18, 376-386.

Riley, L.W., Ko, A.I., Unger, A., and Reis, M.G. (2007). Slum health: Diseases of neglected populations. BMC International Health and Human Rights 7:2 (doi:10.1186/1742-698X-7-2)

Reis, R.B., Ribeiro, G.S., Felzemburgh, R.D., Santana, F.S., Mohr, S., Melendez, A.X., Queiroz, A., Santos, A.C., Ravines, R.R., Tassinari, W.S., Carvalho, M.S., Reis, M.G., Ko, A.I., Gurtler, R.E. (2008). Impact of Environment and Social Gradient on Leptospira Infection in Urban Slums. PLoS Neglected Tropical Diseases, 2(4), e228. DOI: 10.1371/journal.pntd.0000228

Humans cause a lot of problems for sea turtles, in spite of the status of these reptiles as charismatic megafauna: destruction of habitat, disturbance of nesting sites on beaches, egg theft, hunting, pollution, and accidental death and injuries through fishing and boating. Three Brazilian researchers (Bugoni et al., 2001) examined yet another anthropogenic impact, ingestion of plastic debris, on sea turtle populations, in the southern part of their country. They collected stranded sea turtles along the coast of the state of Rio Grande do Sul, measured the curved carapace length to determine age classification, and removed the foregut region (esophagus and stomach) of the digestive tract to analyze number and weight of plastic pieces ingested.

For the Green Turtles (Chelonia mydas) in the study, 23/38 had ingested anthropogenic debris, and even this high percentage may have been an underestimate. Plastics were by far the most frequent type of debris found, but one of the Green Turtles appeared to have ingested oil and tar from a recent spill at a nearby offloading terminal. Most of the plastic debris was in the form of ropes and bags, and the plastic bags were predominantly white (28.9%), transparent (39%), or black (18.4%); plastic bags are thought to be mistaken as jellyfish by the Green Turtles. Four of the 23 Green Turtles appeared to have died from gut obstruction by the plastic debris. Bugoni and colleagues also analyzed 10 Loggerhead Turtles (Caretta caretta), which had a lower frequency of ingested plastic debris than did Green Turtles, perhaps because of their wider digestive tracts or benthic foraging habits.

Green Sea Turtle, Chelonia mydas
Photo: NOAA Photo Library

A more recent study included two stranded sea turtles, one Green Turtle and one Olive Ridley Turtle (Lepidochelys olivacea), found along the northeastern coast of Brazil, Paraíba state. Mascarenhas and colleagues (2004) pointed out that both debris ingestion and entanglement in items such as lines, plastic ropes, and nets can cause serious injury and death in sea turtles. The Olive Ridley Turtle in this study was found with several large external wounds from a boat propeller, and nine pieces of hard plastic in the stomach. The Green Turtle was found alive, with a fractured maxilla and frontal bone, but subsequently died during the rehabilitation period. This animal had experienced an intestinal perforation, likely due to hard, sharp plastic pieces that it had ingested. From these two Brazilian studies, and similar ones in other parts of the world, it is clear that ingestion of plastic and tar is a significant cause of non-natural deaths in sea turtles.

References:

Bugoni, L., Krause, L., and Petry, M.V. (2001). Marine debris and human impacts on sea turtles in Southern Brazil. Marine Pollution Bulletin 42(12), 1330-1334.

MASCARENHAS, R. (2004). Plastic debris ingestion by sea turtle in Para�ba, Brazil. Marine Pollution Bulletin, 49(4), 354-355. DOI: 10.1016/j.marpolbul.2004.05.006

Although polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) were banned throughout much of the developed world in the 1970s, these and other organochlorine contaminants persist in the atmosphere and in the oceans. These chemicals, which were synthesized for use as pesticides and lubricants, accumulate in the lipids of animals, and biomagnify as they move up the food chain to apex predators. In addition, most organochlorines (OC) persist in the environment, and are transported with time to northern latitudes, where they precipitate out in cold air sinks. OC exposure has several well-documented effects on various animal species, and in seals and sea lions, these contaminants can suppress the immune system, act as endocrine disruptors, cause cancer, and interfere with reproduction and normal gestation. Two recent papers document OC levels in two pinniped species affected by population declines, the Steller Sea Lion and the Caspian Seal.

Caspian Seals (Pusa caspica) experienced an unusually high mortality rate beginning in spring 2000, the cause of which was determined to be infection with canine distemper virus. Since OC burden is known to increase susceptibility to viral diseases in mammals, Kajiwara and colleagues (2008 ) measured levels of these compounds in the blubber of Caspian seals that had been affected by the distemper virus, and stranded along the coasts of Iran, Azerbaijan, Dagestan Republic, Kazahkstan, and Turkmenistan between May 2000 and June 2001. In addition, OC levels in six prey species of fish for the seals were measured; compounds analyzed included PCBs, DDT, polychlorinated dibenzo-p-dioxins (PCDDs), and furans (PCDFs).

Steller Sea Lion, Aleutian Islands
Photo: Captain Harry Reed, NOAA Photo Library

The researchers found OC contaminants in the blubber of all 36 male and 17 female Caspian Seals stranded during the study period, with DDTs and PCBs as the predominant classes of OC. DDT levels were high enough to indicate heavy use, possibly illegal, and residues of this pesticide around the Caspian Sea. Although the six fish species examined showed regional differences in PCB/DDT contamination, the seals appeared to be ubiquitously exposed to OCs during their seasonal migration in the Caspian Sea. Kajiwara and colleagues found that high OC contamination levels were correlated with decreased blubber thickness, indicating that these contaminants are concentrated further as animals mobilize their lipid stores. They suggest that the period from spring to summer represents a “high risk season” for Caspian seals, during which the higher concentrations of OC contaminants in their blubber have immunosuppressive effects, and result in increased susceptibility to viral infections.

With a range from the California Channel Islands and along the North Pacific Rim to Japan, the Steller Sea Lion (Eumetopias jubatus) is a more widespread pinniped species than is the Caspian Seal, but no less susceptible to the detrimental effects of OC contaminants. Since 1976, Steller Sea Lion populations have decreased by 70-80%, with the most prominent declines in the eastern Aleutian/ western Gulf of Alaska portions of the range. To determine whether OC levels were correlated with population declines in western Alaska and the Russian Far East, Myers and colleagues (2008 ) measured PCBs and DDTs in the whole blood of Steller Sea Lion pups. Seventy six pups from western Alaskan rookeries, and 136 pups from the Russian Far East, were included in the study.

Steller Sea Lions, Alaska
Photo: Captain Bud Christman, NOAA Photo Library

Pups from the Russian rookeries exhibited higher OC contaminant levels in whole blood, than did conspecifics from Alaskan rookeries. The researchers suggest that these sea lions may be exposed to more point source pollution, or that atmospheric and oceanic transport patterns of OC contaminants may lead to higher exposures. Although OC levels appear to be higher in Steller Sea Lions than in other North Pacific pinniped species, direct comparisons are subject to confounding factors, and may be of limited use. PCB levels were higher than DDT levels in sea lions from both Russian and Alaskan rookeries, and circulating OC levels were higher in female, compared to male, pups, perhaps because of body mass differences. A significant number of the Russian Steller Sea Lion pups had blood OC levels sufficient to produce immunosuppressive effects in another pinniped species, the Harbor Seal (Phoca vitulina). Myers and colleagues concluded that OC contaminants in the Steller Sea Lion pups cannot be dismissed as a possible cause of population declines in the western Alaska and Russian Far East regions.

References:

Kajiwara, N., Watanabe, M., Wilson, S., et al. (2008). Persistent organic pollutants (POPs) in caspian seals of unusual mortality event during 2000 and 2001. Environmental Pollution 152, 431-442.

MYERS, M., YLITALO, G., KRAHN, M., BOYD, D., CALKINS, D., BURKANOV, V., ATKINSON, S. (2008). Organochlorine contaminants in endangered Steller sea lion pups (Eumetopias jubatus) from western Alaska and the Russian Far East. Science of The Total Environment DOI: 10.1016/j.scitotenv.2008.02.008

Although maritime oil spills have declined in frequency and volume in the decades since 1960, they remain a major source of ocean pollution, with a devastating impact on many species of marine birds, mammals, fish, and invertebrates. In a historical and geographical survey of major oil spills, designed in part to determine whether the apparent high incidence of oil tanker accidents in the European Atlantic reflected a genuine trend, Vieites and colleagues (2004) reported that spills correlated with all major crude production areas and maritime transport routes. Unfortunately, major oil spills overlapped with productive areas of the oceans (NOAA-defined Large Marine Ecosystems), and with zones containing vulnerable coral reef ecosystems.

The European Atlantic is not a crude oil production area, but the analyses by Vieites and colleagues revealed that it is an accident hotspot, with one-fifth of the global amount of spilled oil (1,097,359 tons, between 1960 and 2002). The English Channel and the Galician coast, site of the devastating Prestige accident in November 2002, were the most affected areas, with an oil spill incident rate of one every 2.3 years. Between 1970 and 2002, most of the world’s oceans experienced a decreasing trend in the number of oil spills, but there were nine tanker accidents in the European Atlantic during the 1990s, and a single accident in 2002 spilled 60,000 tons of oil in this region. Hopefully, the 2003 ban on single-hulled tankers in EU ports will prevent major oil spill incidents like those following the wrecks of the Prestige in 2002, and the Erika in 1999.

Marine Iguana (Amblyrhyncus cristatus).
Photo: Dr. Jim Smith

Even a small amount of oil spilled in a fragile marine ecosystem can have a major impact on local animal species. The tanker Jessica accident, which affected the Galápagos archipelago in January 2001, was considered to be a low-level oil spill. Nevertheless, the impact on one species, the marine iguana (Amblyrhyncus cristatus), was severe, with a 62% mortality rate among animals on the oil-contaminated island. Two biologists, who have studied environmental stressors and glucocorticoid levels in Galápagos marine iguanas, reported (Romero and Wikelski, 2002) that low-level oil contamination can elicit a stress response in these animals, potentially leading to a weakened immune system and declining reproductive rates.

Under natural conditions, the main cause of marine iguana deaths is starvation, due to insufficient amounts of their algae food source. Such a reduction in food supply occurred in the late 1990s, with a strong El Niño event that restricted nutrient upwelling and algal growth. Levels of the glucocorticoid corticosterone were elevated in malnourished animals, as an adaptation that increases protein catabolism to prolong survival. Corticosterone levels proved to be a predictor of marine iguana mortality rates, not only under the El Niño starvation conditions, but in the wake of the Jessica oil spill as well. Prior to the oil spill, corticosterone levels in the marine iguana populations were low, as the La Nina cycle had restored their algae forage; however, after the tanker grounded, even those animals not visibly contaminated with oil exhibited elevated corticosterone levels, indicative of stress. Romero and Wikelski predicted that mortality would be 40% on affected islands, and the actual mortality was higher on Santa Fe (62%). The researchers speculated that ingested oil killed intestinal bacteria necessary for proper digestion of algae consumed by the iguanas, and proposed that wildlife populations exposed to environmental contaminants should be monitored carefully for the stress response.

References:

Romero, L.M., and Wikelski, M. (2004). Severe effects of low-level oil contamination on wildlife predicted by the corticosterone-stress response: preliminary data and a research agenda. Spill Science and Technology Bulletin 7, 309-313.

Vieites, D.R., Nieto-Rom�n, S., Palanca, A., Ferrer, X., Vences, M. (2004). European Atlantic: the hottest oil spill hotspot worldwide. Naturwissenschaften, 91(11), 535-538. DOI: 10.1007/s00114-004-0572-2

Those of us who donate whole blood and platelets on a regular basis can do so in large part because of hematopoietic stem cells (HSC) in our bone marrow, which are capable of giving rise to all the blood cell lineages (e.g. erythrocytes, lymphocytes, thrombocytes) and can maintain their own numbers through self-renewal. Naturally, the production and balance of appropriate numbers of different blood cell types in healthy individuals, in response to infection, and following blood donation require regulated expression of genes critical for differentiation and for stem cell properties. The Polycomb Group of genes, originally identified in Drosophila, encode proteins that modify histones, thus altering chromatin structure and, ultimately, gene transcription. Homeotic genes, which influence anterior-posterior patterning in Drosophila as well as vertebrate embryos, are suppressed by Polycomb Group (PcG) proteins, and mutations in PcG genes result in changes in the identities of body segments (Lessard and Sauvageau, 2003).

More recently, the roles of PcG proteins in maintaining self-renewing mammalian stem cells have become an important research focus. PcG proteins are present at high levels in the totipotent early blastomeres and inner cell mass of mouse embryos, and decline in amount as the descendants of these cells begin to differentiate. Maintenance of adult stem cells, particularly in the hematopoietic compartment, is also likely to require silencing of differentiation genes by PcG proteins (Rajasekhar and Begemann, 2007). Null mutation of PcG genes in Polycomb repressive complex 1 (PRC1), such as Bmi-1, causes defects in hematopoeitic stem cell proliferation, leading to aplastic anemia and susceptibility to infections. Conversely, Bmi-1 can promote tumor development by cooperating with another oncogene, c-myc, to generate B- and T-cell lymphomas (Lessard and Sauvageau, 2003).

The consequences of too much or too little PRC1 function for blood cell development and leukemia clearly reflected the roles of this protein complex in maintaining the gene repression required to keep hematopoietic stem cells (HSC) in a self-renewing proliferative state. The second complex, PRC2, includes Embryonic ectoderm development (Eed), Enhancer of zeste 2 (Ezh2), and Suppressor of zeste 12 (Suz12) proteins. Mice that have reduced amounts of Eed protein (hypomorphic allele) have proliferation defects in myeloid and lymphoid lineages that progress to leukemia (Lessard and Sauvageau, 2003). This month in PLoS Biology, Majewski and colleagues reported on the consequences of reducing amounts of PRC2 on another hematopoietic lineage, platelet formation, and their studies were initiated by isolating a mutation in the PRC2 gene Suz12.

The Plt8 mutation rescues hematopoietic colony-forming ability in thrombocytopenic mice (Majewski et al., 2008 ). A. Numbers of colony-forming units in recipient spleens B. Photos of recipient mouse spleens

This paper is noteworthy for the genetic screens, functional assays, and genomic techniques employed to examine the function of PRC2, as well as for the utilization of a mammalian experimental model, and the potential for applying the findings to development of cancer treatments. The investigators were interested in finding mutations that suppress the stem cell abnormalities and/or reduced platelet counts (thrombocytopenia) in mice that have defects in thrombopoetin signaling, which is critical for normal platelet production. Through an extensive and elegant series of genetic experiments, Majewski and colleagues identified a mutation in the Suz12 gene (Plt8) that “rescued” platelet counts in c-Mpl-/- mice. The numbers of HSC were then determined, using the classic method of donor bone marrow transplantation into lethally-irradiated recipient mice. The Plt8 mutant bone marrow contributed more hematopoietic progenitors than did wild-type bone marrow in both competitive and serial transplantation experiments.

These results with the Plt8, however, reflected only a partial loss of Suz12, and so to reduce expression of this gene further, Majewski and colleagues used short hairpin RNA-mediated silencing. In this case, a nonspecific sequence shRNA could be used as a control, and again, the Suz12-deficient bone marrow contributed more colony-forming blasts and megakaryocyte progenitors in transplantation experiments. The points made in the discussion section of this paper emphasize the importance of the results to understanding PRC2 function, in the context of hematopoiesis. First, altering levels of one PRC2 component (Suz12) changed the levels of the other components (Eed and Ezh2), and reduced total amounts of the complex in hematopoietic cells (thymocytes and splenocytes). Second, the reduction in PRC2 resulted in changes in the expression of three cancer-related genes: Bex2, Bex4, and Fibulin. Third, manipulating the amount of PRC2 may have therapeutic applications for bone marrow transplantation and cancer treatment. Overall, this paper is a tour de force of mouse genetics, functional hematopoietic assays, and gene expression analyses, and my only quibble is that potentially conflicting results from Kamminga and colleagues (2006), on the overexpression of Ezh2 and inhibition of replicative senescence in HSC, are inadequately discussed.

References:

Kamminga, L.M., Bystrykh, L.V., deBoer, A., et al. (2006). The Polycomb group gene Ezh2 prevents hematopoeitic stem cell exhaustion. Blood 107, 2170-2179.

Lessard, J., and Sauvageau, G. (2003). Polycomb group genes as epigenetic regulators of normal and leukemic hemopoeisis. Experimental Hematology 31, 567-585.

Rajasekhar, V.K., and Begemann, M. (2007). Concise review: roles of Polycomb Group proteins in development and disease: a stem cell perspective. Stem Cells 25, 2498-2510.

Majewski, I.J., Blewitt, M.E., de Graaf, C.A., McManus, E.J., Bahlo, M., Hilton, A.A., Hyland, C.D., Smyth, G.K., Corbin, J.E., Metcalf, D., Alexander, W.S., Hilton, D.J., Goodell, M.A. (2008). Polycomb Repressive Complex 2 (PRC2) Restricts Hematopoietic Stem Cell Activity. PLoS Biology, 6(4), e93. DOI: 10.1371/journal.pbio.0060093

Here’s a photo of my crocheted version of the bone-eating marine polychaete, Osedax.  I don’t have access to a whale skull, so I photographed it with a cow skull.  I made this in response to a challenge on the ScienceBlog Deep Sea News.

I’ll write up the pattern and add it to this post later-it was mostly free-form crochet, so the pattern will be more of a suggestion, than detailed instructions.  Besides, it’s best to use up scraps of yarn from other projects, to make your own Osedax.