survival of the fittest

On March 5, 2007, the New York Times ran an obituary marking the passing of former US senator Thomas F. Eagleton. To most post-Baby Boomers like myself, the name meant very little. But to anyone eligible to vote in the 1972 presidential election, Eagleton was the infamous Running Mate for 18 Days. Democratic candidate George McGovern picked him as his running mate, only to discover one week later that Eagleton had struggled with clinical depression, had been hospitalized three times, and subjected to electroshock therapy. They initially tried to spin it as "nervous exhaustion," but the press had a field day (plus ca change...) with the revelations, and Eagleton eventually bowed to political pressure and stepped down, at McGovern's urging, for the sake of "party unity." Fat lot of good it did, in retrospect: McGovern suffered a crushing defeat in favor of Richard Nixon. But chances are, that would have happened anyway.

Those are the political breaks: it's not an arena you want to enter if you have secret vulnerabilities, like poor Eagleton, who was, by most accounts, an honorable man despite his all-too-human foibles. We've come a long way in our attitudes towards clinical depression; it's no longer the stigma it once was, and there are far more treatment options available (albeit with unpleasant side effects accompanying the most common drug therapies). Even McGovern, the year before Eagleton died, said publicly that he regretted asking for Eagleton's resignation as his running mate: "If I had it to do over again, I'd have kept him," he said. "I didn't know anything about mental illness. Nobody did." In fact, he listed Eagleton as being among the "10 or 12 best senators" with whom he had served.

We know much more about mental illness and depression in part because of advances made in neuroscience: specifically, in the case of depression (as well as bipolar disorder, Huntington's Disease, and schizophrenia), in our understanding of the role played by a family of proteins known as neurotrophins. This was the subject of a talk here at KITP last week by Moses Chao of New York University, who focused mostly on one neurotrophin in particular: brain-derived neurotrophic factor (BDNF). There have been several studies purporting to link low BDNF levels with depression or bipolar disorder, according to Chao, and while many of those have yet to be verified, one study by Pamela Sklar  of Harvard Medical School appears to be pretty solid, and Chao's own research with lab mice genetically altered to have low BDNF levels seems to support a link as well. Apparently antidepressants help boost BDNF expression, among other effects.

To really understand why Chao and others suspect a link between BDNF levels and depression, bipolar and Huntington's, it's helpful to have a bit of background. After Chao's talk, I did what I always do when I'm intrigued by a topic: I turned to Google. Fortunately for me, there's a wealth of useful information on neurotrophins available online, and much of it is reasonably accessible to the curious non-scientist.

Nerve growth factors are secreted proteins that induce the survival of neurons. They were discovered in the 1950s by an Italian developmental biologist named Rita Levi-Montalcini. She has a compelling personal story of conducting science under siege:  after overcoming her family's objections to a woman engaging in a professional career, she graduated from med school in 1936 with highest honors and devoted herself to the study of neurology and psychiatry. But that was the same year Mussolini issued his infamous manifesto barring academic and professional careers to non-Aryan Italian citizens (notably, those of Jewish descent, like Levi-Montalcini). She fled home to Turin in 1940, as the German army was poised to invade Belgium. The family opted not to emigrate to the US, but to hide out instead. Levi-Montalcini built a small research lab in her bedroom (and, when the bombing of Turin became too intense, in the attic of the country cottage to which her family fled), where she conducted experiments on chick embryos. That, my friends, is scientific grace under extreme pressure.

Once the war ended, she returned to Turin and resumed her academic positions, moving in 1947 to St. Louis to work with Viktor Hamburger, whose research had inspired her own wartime experiments on chick embryos. She ended up staying there through retirement, dividing her time between St. Louis and Rome. In 1952, Levi-Montalcini experimentally demonstrated that when tumors from mice were transplanted to chick embryos, they induced "potent growth of the chick embryo nervous system," specifically the sensory and sympathetic nerves. But there was no direct contact between the tumor and the embryo, leading her to conclude that there had to be a nerve-growth-promoting chemical that was released to cause such a response. She was the first to successfully extract and identify nerve growth factor. And it was very potent indeed! A mere few minutes after the addition of one billionth part of a gram of NGF (per milliliter of culture medium) caused nerve fibers to grow explosively out from the ganglion. After just one day, the ganglion looked like a sun surrounded by rays (see image above). She shared the 1986 Nobel Prize in Physiology or Medicine for this work, with Stanley Cohen (who discovered epidermal growth factor).

Differentiation among cells is critical to human development, not just the brain. We all start from a single cell containing the genetic material that determines our individual characteristics. As cells divide, and divide some more, they start to differentiate -- i.e., exhibit different characteristics, play specific roles or functions, and so forth. Scientists have linked growth factor chemicals with the regulation of cell growth and differentiation. In the case of nerve growth factors like neurotrophins, they activate the process by attaching to neuron cell receptors -- molecules that act like tiny antennas, sitting on the surfaces of cells like neurons. A good analogy is the lock and key. The receptors are the lock, and can only be opened by certain protein "keys" (known as ligands), and once opened, the receptors send a series of internal signals ricocheting through the nervous system.

Neurotrophins are keys that fit the locks of specific neuronal receptors in the brain. There are four related types of neurotrophins: BDNF, nerve growth factor (NGF), neurotrophin-3 (NT-3), and neurotrophin 4 (NT-4). Each of these correspond to a specific family of three receptors (A, B and C), known as Tyrokinase (Trk) receptors. NGF binds to TrkA; NT-3 binds to YrkC; and BDNF and NT-4 bind to Trk-B. There is a second class of receptor called p75 to which all the neurotrophins can bind, just with lower affinities. And p75 seems to play a critical role in triggering programmed cell death.

Once those "locks" have been opened, they trigger nerve growth, survival, and differentiation via a complex signaling pathway (so complex that the underlying mechanism is still not completely understood by neuroscientists). As such, they seem to play a vital role in behavior, learning and memory. BDNF acts specifically on certain neurons in the central nervous system and the peripheral nervous system (i.e., the lower spinal cord) to support the survival of existing neurons and encourage the growth and differentiation of new neurons and synapses. It isn't just found in the brain: BDNF is also expressed in the retina, motor neurons, the kidneys, and the prostate. Stress, or exposure to the stress hormone corticosterone, has been shown to decrease the expression of BDNF in laboratory rats, and if the exposure persists, and BDNF levels continue to drop, the entire hippocampus can atrophy. Similar atrophy has been shown in people suffering from clinical depression -- hence, the suspicion that there may be a critical link between that condition and low levels of BDNF.

The process is fairly complicated in the central nervous system, where there isn't as heavy a dependency on BDNF for survival, per Chao. But in the peripheral nervous system, sympathetic sensory neurons will undergo programmed cell death (a.k.a., apoptosis) when deprived of nerve growth factor. And since there are twice as many neurons competing for a limited number of neurotrophic factors, this is the primary mechanism by which neurons self-select and differentiate. Chao's group is seeking to identify "the biochemical steps that provide specificity in nerve growth factor signaling." Thus far, we know that nerve growth factors are responsible for neuronal cell survival and death via the activation of the TrkA tyrosine kinase (for survival) and the p75 neurotrophin receptor (for programmed cell death).

Chao and his colleagues have experimentally observed the effects of low BDNF levels in the lab. First, they bred mice with a mutation in the BDNF gene to eliminate the presence of the factor entirely, but per Chao, "This proved disastrous." The mice developed normally, but died within a few weeks from cardiovascular problems, before the researchers had time to train them to engage in experimental tasks. So next, they genetically engineered the mice to have 50% less BDNF. The mice survived this time, but showed signs of elevated anxiety, learning deficits, a tendency towards obesity, and among the males, there was more aggression and hyperactivity, particularly toward other male mice. Treating those mice with Prozac, intriguingly, reversed the aggressive behavior, as did increasing the amount of exercise.

This has some interesting implications for the development and evolution of the human brain, according to Chao. BDNF levels are generally low at birth and increase dramatically in the ensuing weeks in many different parts of the brain, with the strongest expression in the forebrain and cortical regions. The prefrontal cortex develops relatively late in life, sometimes not reaching full maturity until the mid 20s -- which might explain, in part, why certain mental conditions such as schizophrenia and bipolar disorder don't fully manifest until then. There seems to be a strong correlation between increased survival and growth of neurons in the hippocampus, and higher BDNF levels. As for Huntington's Disease, this tends to develop between ages 30 and 50, with associated loss of striatal neurons -- and since these depend on BDNF for survival, lowered BDNF levels (or complete absence thereof) could be a factor in the loss of those neurons. 

I was especially struck by a comment made during the Q&A by Columbia University's Stuart Firestein, another of the KITP "Brainiacs" (he specializes in olfactory receptors): "There are no switches, everything's a dial." In other words, it's not like the brain throws a switch and someone suddenly develops bipolar disorder. It's all about levels of crucial chemicals, of which BDNF seems to be one of the most critical. These chemicals are "good at the right level, bad at too low levels," and even though bipolar or schizophrenia might seem to have sudden onsets, it's more likely that levels have been falling for quite some time, and finally passed a critical threshold. Or something like that. Nobody's 100% sure. I suspect we'll have to wait for further research results before we can make a definitive call one way or the other.

What does this mean for future treatment options for those suffering from these diseases? It's a bit premature, since Chao's research is more of a fundamental variety, but certainly improving our understanding of how neurons develop (and die) in the brain, and the factors that influence their survival (or death), has important implications for the development of better drug therapies. Chao has been struggling with the challenge  of exploiting the signal transduction mechanisms of neurotrophins to treat neurodegenerative and psychiatric disorders. Apparently, even though nerve growth factors serve to protect neurons, the brain is resistant to treatments that use them. Chao attributes the failure of prior attempts to "problems with delivery and numerous side effects" in clinical trials. "Nerve growth factors are large, sticky proteins that do not diffuse very well," he said.

Chao and his colleagues have demonstrated that neurotrophin receptors in damaged motor neurons in mice, for example, can nonetheless be activated through a different, entirely unrelated receptor system (in this case, G protein-coupled receptors) through a kind of "cross-talk." Since G protein-coupled receptors are the target of many therapeutic drugs, Chao is hopeful that one day we might be able to use the "cross-talk" phenomenon to bypass the blood-brain barrier -- a major stumbling block when it comes to delivering drugs to affected areas in the brain -- without the usual side effects. However, "Ideally, we would like to treat not just the BDNF deficiency, but to be able to eliminate the mutant protein that causes those levels in the first place."

Comments

[joseph duemer]

Nixon, as it happens, almost certainly suffered from depression, but he self-medicated with alcohol and prescription drugs supplied by his physicians.

[Peter Lund]

Sounds like it could be very dangerous to experiment with NTFs. Are they broken down by saliva and tears and enzymes in the blood before they can do any damage?

What precautions did Levi-Montalcini take?

[Bradley Thomas]

Peter,

I would guess that it's not NTFs aren't too dangerous for the same reason that initial attempts to use them clinically failed--viz. NTFs don't easily cross the blood brain barrier. That's just a guess though.

[eingram]

Bravo to those conducting neuroscience experiments! As a sufferer of clinical depression for 56 years, 47 of which it went either undiagnosed or misdiagnosed (depending on which therapist was involved) I look forward to the day when a real cure is at hand. I do not believe I will live to see it, but research must continue. I still believe that our science in general is in it's infancy, contrary to what the producers of TV medical shows would have us believe. In several of Feynmans lectures, he always emphasized how much we still don't know, which convinced me he was one of the ones most knowledgeable. Just wanted to say Keep up the good work.
Silly little thing: I am confused how a tumor be implanted in an embryo and have "no contact" with it.