New Neurons Born in Adult Rat Cortex

NIH News
FOR IMMEDIATE RELEASE
Thursday, February 3, 2005

Recent evidence suggesting that antidepressants may act by
triggering the birth of new neurons in the adult
hippocampus,* the brain's memory hub, has heightened
interest in such adult neurogenesis and raised the
question: Could new neurons also be sprouting up in the
parts of the adult brain involved in the thinking and mood
disturbances of depression and anxiety?

Now, scientists at the National Institute of Health's (NIH)
National Institute of Mental Health (NIMH) have found newly
born neurons that communicate via the chemical messenger
GABA (gamma-aminobutyric acid) in adult rat cortex, seat of
higher order "executive" functions, and in the striatum,
site of habits, reward and motor skill learning. In the
cortex, the new neurons appear to arise from previously
unknown precursor cells native to the area, rather than
from cells migrating in from another area. NIMH's Drs.
Heather Cameron, Alexandre Dayer, and colleagues, report on
their findings in the January 31, 2005 "Journal of Cell
Biology".

Their discovery adds to the scientific debate over adult
neurogenesis, which has potential implications for
understanding a variety of brain disorders, possibly
including Alzheimer's and schizophrenia. While most
researchers agree that new neurons are generated in the
adult hippocampus and olfactory bulb, the existence of
adult neurogenesis in other brain regions remains
controversial.

The NIMH team used many more markers than previous studies
to track newborn neurons as they matured and to identify
the type of neurotransmitters they secreted. The markers
exploited antibody affinities for specific proteins to tag
particular cell types with telltale color codes, visible on
brain slices under fluorescence with a laser-powered
microscope.

The researchers found that the cortex and striatum were
giving birth to new, widely scattered small cells, called
interneurons, that make and secrete GABA, a
neurotransmitter that dampens neuronal activity. The new
interneurons closely resembled those seen in the
hippocampus and olfactory bulb and seemed to arise at
similar rates. Interneurons are thought to play a role in
regulating larger types of neurons that make long-distance
connections between brain regions and predominate in these
areas.

The NIMH team was surprised to find that the new cortex
interneurons appeared to arise from a previously unknown
class of local precursor cells rather than from cells that
migrate into the area from the subventricular zone, where
other neurons - including those seen in the striatum and
olfactory bulb - originate during adulthood. However,
during development, both the cortex and striatum precursors
likely stem from common ancestor cells that somehow retain
their ability to divide and generate new GABA interneurons,
propose the researchers.

"Since antidepressants increase neurogenesis in the adult
hippocampus, they might have similar effects in the cortex,
the region probably responsible for mood dysregulation in
depression," suggested Cameron. "But answers to such
questions about regulation and possible functions of the
new neurons must await results of future studies."

Also participating the project were Kathryn Cleaver and
Thamara Abouantoun of the NIMH Unit on Neuroplasticity. Dr.
Dayer's work was supported by the Swiss National Fund.

DNA molecules used to assemble nanoparticles

University of Michigan researchers have developed a faster, more efficient way to produce a wide variety of nanoparticle drug delivery systems, using DNA molecules to bind the particles together.

Nanometer-scaled dendrimers can be assembled in many configurations by using attached lengths of single-stranded DNA molecules, which naturally bind to other DNA strands in a highly specific fashion.
"With this approach, you can target a wide variety of molecules—drugs, contrast agents—to almost any cell," said Dr. James R. Baker Jr., the Ruth Dow Doan Professor of Nanotechnology and director of the Center for Biologic Nanotechnology at U-M.

Nanoparticle complexes can be specifically targeted to cancer cells and are small enough to enter a diseased cell, either killing it from within or sending out a signal to identify it. But making the particles is notoriously difficult and time-consuming.

The nanoparticle system used by Baker's lab is based on dendrimers, star-like synthetic polymers that can carry a vast array of molecules on the ends of their arms. It is possible to build a single dendrimer carrying many different kinds of molecules such as contrast agents and drugs, but the synthesis process is long and difficult, requiring months for each new molecule added to the dendrimer in sequential steps. And the yield of useful particles drops with each successive step of synthesis.

For a paper published Jan. 21 in the journal Chemistry and Biology, U-M Biomedical Engineering graduate student Youngseon Choi built nanoparticle clusters of two different functional dendrimers, one designed for imaging and the other for targeting cancer cells. Each of the dendrimers also carried a single-stranded, non-coding DNA synthesized by Choi.

In a solution of two different kinds of single dendrimers, these dangling lengths of DNA, typically 34-66 bases long, found complementary sequences on other dendrimers and knitted together, forming barbell shaped two-dendrimer complexes with folate on one end and fluorescence on the other end.

Folate receptors are over-expressed on the surface of cancer cells, so these dendrimer clusters would tend to flock to the diseased cells. The other end of the complex carries a fluorescent protein so that the researchers can track their movement.

A series of experiments using cell sorters, 3-D microscopes and other tools verified that these dendrimers hit their targets, were admitted into the cells and gave off their signaling light. The self-assembled dendrimer clusters were shown to be well formed and functional.

"This is the proof-of-concept experiment," Choi said. But now that the assembly system has been worked out, other forms of dendrimer clusters are in the works.

"If you wanted to make a therapeutic that targeted five drugs to five different cells, it would be 25 synthesis steps the traditional way," Baker said. At two to three months per synthesis, and a significant loss of yield for each step, that approach just wouldn't be practical.

Instead, the Baker group will create a library of single-functional dendrimers that can be synthesized in parallel, rather than sequentially, and then linked together in many different combinations with the DNA strands.
"So it's like having a shelf full of Tinker Toys," Baker said.

An array of single-functional dendrimers, such as targets, drugs, and contrast agents, and the ability to link them together quickly and easily in many different ways would enable a clinic to offer 25 different "flavors" of dendrimer with only ten synthesis steps, Baker said.

Baker foresees a nanoparticle cluster in which a single dendrimer carries three single-strands of DNA, each with a sequence specific to the DNA attached to other kinds of dendrimers. Put into solution with these other tinker toys, the molecule would self-assemble into a four-dendrimer complex carrying one drug, one target, and one fluorescent.

The original news release can be found here.

Human stem cells trigger immune attack

From http://www.nature.com/news/channels/medicalresearch.html

Most human embryonic stem-cell lines, including those available to federally funded researchers in the United States, may be useless for therapeutic applications. The body's immune defences would probably attack the cells, say US researchers. When embryonic stem cells are added to serum from human blood, antibodies stick to the cells. This suggests the cells are seen as foreign, and that transplanting them into the body would trigger the immune system to reject them. "We've found a serious problem," says Ajit Varki, a cell biologist at the University of California, San Diego. The difficulty arises from the way human embryonic stem cells are grown and maintained in the lab. Scientists grow stem cells in petri dishes containing nutrient broth and other cells. These feed the stem cells, and give them a place to attach themselves. Feeder cells are typically embryonic cells from mice and nutrient broth usually contains animal serum. These mouse cells have a molecule on their surface called N-glycolylneuraminic acid or Neu5Gc. Varki's team had already found that human embryonic stem cells take up Neu5Gc; they now show that humans react against it. Eating red meat and dairy products has sensitized people to the molecule, Varki says. The team reports its latest finding in the February issue of Nature Medicine1.

The current stem-cell lines have little clinical value, but that is "not an issue for pursuing basic research", says James Battey, chairman of the National Institutes of Health's stem-cell task force.