The Constantine-Paton Lab: Research
What We Do and Why We Do It:
The focus of our lab is activity-dependent brain development. We currently use rats and mice because of the many species-specific reagents and databases available for their study and their relatively rapid generation time. We are specifically interested in answering two questions: first, what cellular and molecular mechanisms cause young synapses to be stabilized at one position in the brain after an early period of exuberant projections where tentative contacts are made and broken before functionally optimal connections are formed? Second, what biochemical, structural, or genetic programs cause the developing brain to lose its dramatic plasticity as the brain matures? This loss of malleability is necessary for learning and memory but is it also likely to be responsible for the poor recovery of adult brains when confronted with trauma or disease. The same loss of plasticity is likely to underlie the late onset of the many neurological dysfunctions that destroy the lives of otherwise healthy people.
We are currently using a wide variety of physiological techniques including whole cell patch clamping, Ca++ imaging, biochemistry and molecular biology. We employ pharmacology, viral vectors, biolistic gene gun and electroporation to manipulate protein levels, while emploing physiology and anatomy to understand how activity in early pathways alters synaptic function and the structural development of pathways.
Most of our basic research work focuses on the visual pathway because of its accessibility for manipulation in the intact animal. Finding ways to reactivate developmental plasticity in the visual pathway promises to ameliorate the crippling effects of low, or loss of, vision in the human population. In addition, over the last several years we have begun to study the developmental bases of neurological disease. Thus we have an ongoing study focusing on amyotrophic lateral sclerosis (Lou Gherings Disease) as well as a collaboration on the physiological basis of glutamate dysfunction in schizophrenia. Our work takes advantage of genetically engineered mice as well as our ability to chronically apply pharmacological disruption of transmitter systems to assay their effect on the normal development of excitatory and inhibitory pathways in the developing brain.
A Little Bit of History
In the early part of the twentieth century scientists vigorously debated whether brains developed as hardwired structures dictated by an explicit sequence of events that inevitably led to each nerve cell finding its genetically dictated "correct" target. Most biologists and psychologists chuckle at this concept today since it is generally accepted that genetics and the environment work together to produce all complex organisms and their brains as well. Over the past twenty-five years, the explosion in molecular biology, genomics, and proteomics has produced enormously powerful tools for unraveling the molecular cascades that interface with and respond to environmental (defined broadly) demands.
Though brain science is benefiting tremendously from these advances, crucial questions remain unanswered. For example, which molecules among the elaborately interacting and highly diverse biochemical cascades within nerve cells cement a "would be" memory, learned fact, or emotional response in our minds? Why is it that children's brains absorb and produce new languages or other behavioral skills at greater rates than adults? Why is it that the early months and years of life are critical for normal human brain development? Attention from, and attachment to, other people is critical to later behavior. Also in regard to disease, why do apparently "normal" individuals develop devastating disabilities such as schizophrenia or autism when no specific subsets of genes can be identified as disrupted? Why, for that matter, do apparently normal individuals come suddenly experience neurodegenerative diseases? Even when the defective gene is known with 100% certainty, as in the case of Huntington's disease, it is still not possible to identify and potentially treat the mechanisms responsible for the dementia and chorea that accompany this terrible affliction.
A Brief History of Published Work:
Most of our work over the last decades has investigated the role of the NMDA subtype of glutamate receptor in activity-dependent nervous system development, along with the many mechanisms that regulate, or are triggered by, NMDA receptor function. For example, some of our earliest studies showed that NMDA receptors were critical to the segregation of retinal projections into eye-specifc termination stripes in frogs that normally never show this pattern of termination (Constantine-Paton and Law, 1978; Cline et al.,1987). We then showed that this same receptor system was necessary for the refinement of the visual projection to the midbrain superior colliculus of mammals, characterized the development of the glutamate and GABA receptors in this brain region, and showed that the NMDA receptor was involved in mediating a very rapid initiation of local protein synthesis (Simon et al., 1992; Shi et al., 1997; Scheetz et al., 2000).
After our move to MIT in 1999, we began to study the interaction between abrupt changes in early visual activity and the biochemical and functional make-up of visual synapses (Shi et al., 2000; Yoshii et al., 2003; Townsend et al., 2003; Lu and Constantine-Paton, 2004). We combined these approaches with quantitative confocal microscopy to specifically examine synaptic contacts (Colonnese and Constantine-Paton, 2005; 2006,). We have also moved some studies into tissue culture, showing for example that NMDA receptors in visual cortical neurons trigger BDNF activation of TrkB at synapses through a PI3K dependent pathway - this sends the glutamate receptor scaffolding protein PSD-95 to visual synapses. Also, we have recently studied long term potentiation (LTP) in the visual system showing that L-type Ca++ channels, as well as NMDA receptors, are critical to inducing LTP of visual neurons of the superior colliculus (Zhao et al., 2006), and that the NR2A subunit of the NMDA receptor is necessary for this LTP (Zhao and Constantine-Paton, 2007). This has led us to test some of our hypotheses on the activity and molecular determinants of potentiation in the well studied CA1 area of the hippocampus.
Currently of particular interest to the Constantine-Paton lab is schizophrenia, because several clues suggest that schizophrenia may involve defects in NMDA receptor signaling. For example, drugs that inhibit NMDA receptors, like PCP and ketamine, can cause psychosis and other schizophrenia-like symptoms. One natural regulator of NMDA receptors in the brain is neuregulin, and recent evidence suggests that mutations in the neuregulin gene can increase the risk of schizophrenia (for review, see Skolnick, Petryshen, and Sklar 2006). We are collaborating with researchers at the Broad Institute to identify neuregulin’s physiological function in the hippocampus, and are also pursuing other avenues of schizophrenia research.
Along the way we have enjoyed fruitful collaborations with colleagues in the McGovern and Whitehead Institutes at MIT, describing the development of the bold response in young rats (Colonnese et al., 2008) and showing that stem cells injected into the embryonic mouse brain can differentiate into functional neurons (Wernig et al., 2008). A paper currently under revision documents early hyperexcitabiliy in the SOD mutant mouse model for ALS and shows, using acute tissue slice whole cell recording, that the hyperexcitably arises from a persistent Na++ channel (Van Zundert et al., submitted). This work represents a collaboration between Bob Brown Jr at MGH a neurologist who has devoted his laboratory to finding a cure for ALS, Mark Bellingham a Professor at the University of Brisbane in Australia and our lab.