About gosh darn time.

Finally, after about 3 mostly frustrating years (give or take a few weeks) I have been able to partly answer one of the two major questions I am addressing for my thesis. I am happy, tired, excited, relieved, motivated, and depressed all at the same time. The moment seems very anti-climactic, but I am glad that it has occurred.

There are some experiments in the works that will hopefully allow me to fully answer that question, within a month or two. As for the other major question, well, it is unlikely that I will be able to easily answer it without answering the first question.

I know, I am being vague, so let me explain the biological phenomena I am investigating and the questions I seek to address. The following is an excerpt from my Thesis Research Advisory Committee (TRAC) Meeting last December. It is incredibly information dense as the whole thing is supposed to be a summary of my thesis up until that point in time crammed into the miniscule length of 5 single-spaced pages.

The sympathetic division of the autonomic nervous system is responsible for maintaining homeostatic function within the body as well as performing nonhomeostatic functions such as the "fight-or-flight" response to life threatening situations. This system uses a 2-cell circuit to relay signals from the central nervous system to the periphery. Sympathetic preganglionic neurons (SPNs), whose cell bodies are located in the thoracic level spinal cord, project axons to the prevertebral and paravertebral sympathetic chain ganglia, where they innervate postganglionic cell bodies. The postganglionic neurons innervate target organs, such as the heart, lungs, and liver.

During development, the SPNs begin as a subset of the postmitotic motor neuron precursors that migrate from the ventricular zone to the lateral horn of the thoracic spinal cord. Most of the motor neuron progenitor pool adopts a somatic motor neuron fate, while the SPNs migrate to a more dorsal location and form the intermediolateral column in mouse and the column of Terni in chick. Downregulation of the transcription factor Hb9 is required for SPN specification and their characteristic migration.

Antibody against Phospho-Smad1/5/8 (PS1) specifically marks developing SPNs in chick and mouse embryos. BMP ligands function by binding type II and type I BMP receptor complexes, initiating a cascade of phosphorylation. In vertebrates, the type I receptor phosphorylates a positive Smad which then translocates to the nucleus to regulate transcription. Therefore, the presence of PS1 is indicative of active transduction of a BMP signal.

There is a correlation between the extent of SPN axon projection and SPN PS1 immunoreactivity. PS1 appears in SPN nuclei only when some axons reach the sympathetic chain. This raises the possibility that the SPNs are not receiving a BMP signal from the spinal cord, but rather from the periphery, despite BMP5 expression by the SPNs in chick and BMP expression in the roofplate. This peripheral signal is possibly from the sympathetic postganglionic neurons (the targets of SPNs), which require BMP signals from the dorsal aorta and express BMPs themselves.

We have also confirmed that embryonic chick and mouse SPNs are responding specifically to BMP signals. The SPNs of conditional BmprIa mutant mice, which lack receptor expression in the developing spinal cord, never stain for PS1 or nNos, and Isl1 staining is significantly reduced. Additionally, when the BMP-inhibitor Noggin is misexpressed in the spinal cord of chick embryos via electroporation, PS1 staining is lost in the SPNs. Therefore, loss of function and gain of function experiments support a role for BMPs in SPN development.

Taken together these observations raise several questions about the source and identity of the BMP signal. For instance, is the BMP originating from within the spinal cord or from peripheral tissue? If the signal is local in origin, why do only SPNs respond to it? If there is a peripheral signal, is it the BMP, or might it be a requisite non-BMP cosignal? Is there a difference between initiation and maintenance of the BMP signal? If the signal is derived from the periphery, are somatic motor neurons, which also project axons outside of the spinal cord, also competent to respond?

There you have it. My two main questions are:

1) What is the source of the BMP signal?
2) Which of the (many) BMPs is it?

My recent breakthrough addresses the first question. I have determined that the BMP signal is from a peripheral source. Our biggest candidate for the source is the sympathetic ganglia, where the sympathetic neurons reside (those neurons being the target of innervation by the SPNs). The other experiments I have in the works will (hopefully) indicate whether this hypothesis is true.

If all of that doesn't make any sense, well, sorry. I know that a picture is worth a thousand words, and if I posted some drawings/photos it would help make it easier to understand. I also realize that Biology is a language that only those trained in it can speak. Someday I will translate my research into English for my friends and family who don't speak Biology and give them a short talk with pictures to illustrate my points. However, that could take months to put together if I assume no more than high school level Bio taken more than a decade ago.

If someone asks you what I study, you can say one of the two following sentences, depending on your level of understanding and/or ability to memorize:

A) He studies developmental neurobiology.

B) He studies the development of a population of cells in the spinal cord during embryogenesis.

That's as simple as I can make it, although it reveals no details whatsoever. You can also tell people that my research will help victims of spinal cord injury live longer healthier lives. That's potentially true. Perhaps. ;)