Not much happening upstairs

The bad news: Dr Rod Rietze's research group at the Queensland Brain Institute in Brisbane has found that the neural stem cells that renew high-maintenance regions of the brain through life are much rarer than originally thought, and their activity declines steeply with age.

The good news: Rietze and his colleagues believe that recent research showing that exercise stimulates neurogenesis in mice - an effect that almost certainly involves activation of neural stem cells - raises the exciting possibility that exercise, or drug therapy, can also make ageing human brains young again.

The new QBI findings trace to the momentous 1992 discovery by a Canadian research team that the mouse brain contains a self-renewing population of neural stem cells.

In 1887, the great Spanish neuroanatomist and Nobel laureate Ramon y Cajal proposed, that, unlike other cells, neurons cannot regenerate, and are not replaced when they die.

US researchers knew by 1990 that Cajal's conjecture was wrong, but it was a young Canadian PhD, Brent Reynolds, who discovered the elusive precursors of new neurons, in the lining of the fluid-filled ventricles towards the centre of the brain.

Isolated and grown in culture, the undifferentiated cells divided and formed small ball-like clusters, or neurospheres.

The discovery yielded the neurosphere assay (NSA), which soon became standard in neural stem cell research laboratories around the world. It was crucial to researcher's ability to isolate neural stem cells and study their activity.

However, after Reynolds joined his friend and compatriot Rietze's lab at the Queensland Brain Institute in 2004, they discovered that not all neurosphere-forming cells are stem cells.

In fact, only as few as five in 100 neurosphere-forming cells are true stem cells, with the potential to continue replicating, throughout life, and regenerate the full diversity of specialised progeny cells that form the working brain.

The rest are neural precursor cells (NPCs) that have already taken the first step towards specialisation and terminal differentiation.

---PB--- Stem cells and progenitor cells

Reynolds had dropped out of research in 1997, taking a degree in oriental medicine, and was making a living as an acupuncturist and herbal therapist on a small island off Vancouver, while at the same time working for biotech StemCell Technologies.

In 2004 Rietze enticed his friend back into research with an offer to work as a visiting scientist at QBI. Reynolds brought with him an uncompleted project, a new assay that could discriminate stem cells from their more restricted progeny, the progenitor cell.

The collaboration that ensued between Dr Sharon Louis (still with StemCell Technologies), Rietze and Reynolds (now promoted to full professor) resulted in the development of the neural colony forming cell assay (N-CFCA).

On January 24 this year, Louis, Rietze and Reynolds and their QBI and StemCell Technologies colleagues published details of their new neural stem cell assay in the on-line edition of Stem Cell Express.

When a neural stem cell divides, it can either form two new neural stem cells (symmetrical division) or a neural stem cell and an NPC (asymmetrical division).

When an NPC is formed, it migrates to a specific tissue or region of the brain where locally secreted growth factors direct it to differentiate into a specialised neuron, glial cell, oligodendrocyte or astrocyte. At each either-or node along its path towards terminal differentiation, it becomes more "mortal" - only true neural stem cells continue to divide for the individual's lifetime.

True neural stem cells and NPCs both form neurospheres, but after several weeks the growth rate of NPC-derived neurospheres declines, and they fall off the pace.

Rietze says that the new neural colony-forming cell assay (N-CFCA), distinguishes true neural stem cells from NPCs by the size of the colonies they form: NPCs give rise to small colonies, NSCs to large colonies. At 21 days, any colony less than 2mm in diameter comprises NPCs.

The new assay confirms that the number of true neural stem cells in the brain is much smaller than originally estimated.

It also suggests that some1300 research papers published since the original neurosphere assay came into use in the mid-1990s are potentially wrong, because they counted mixed populations of true neural stem cells and neural progenitor cells.

Quoting Mark Twain, Rietze observes, "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

Many previous studies used epidermal growth factor (EGF) to expand neural stem cell populations in vitro. It turns out that one of the few differences between neural stem cells in the test tube versus in the mouse is that neural stem cells do not divide in response to EGF in vivo - making a number of EGF-related stem cell therapies (based on in vitro results) misleading

---PB--- Exercise your brain

In a second paper, published in the January 21 on-line issue of Stem Cell Express, Rietze and his colleagues confirm that neural stem cells are extremely rare entities in the mouse brain - and by inference, in the human brain.

The study, led by Dr Daniel Grant, and Mohammad Golmohammadi, a visiting PhD student from Isfahan University of Medical Sciences in Iran, was the first to employ the new assay to analyse the number and distribution of neural stem cells in the mouse brain.

The QBI team made thin sections of the mouse brain, from front to back, and counted stem cells.

"There may be as few as 51 true stem cells in the entire mouse brain, when you would think there would be thousands," Rietze says.

"Research groups using an indirect measure of stem cells - a clonal lineage analysis - were reporting around 1200 stem cells in the ventricles of the mouse brain."

The latest study explains the discrepancies in reported numbers of neural stem cells from one study to another, by showing that the prevalence of the cells in the lining of the ventricles varies markedly along the neuraxis from the front to the back of the brain.

Rietze says stem cells are completely absent from the cephalic flexures, which form the boundaries between the forebrain, which is specialized for higher cognitive functions, and the midbrain and lower regions of the brain.

"Developmentally, these different stem cell populations appear to be unique, because when you infuse factors to stimulate division, they respond differently, according to where they are in the brain," he says.

"So it's not only the prevalence of neural stem cells that varies, but their competence as they age. That's astonishing - the brain ages at different rates in different regions."

Rietze says the widest variance in stem cell numbers occurs in the forebrain.

The human brain is different - for instance, Professor Arturo Alvarez-Buylla of the University of California, San Francisco, has found that neural stem cells appear to be restricted to the lateral ventricles.

"Nobody has yet published convincing evidence that they reside anywhere else, including the spinal cord, but these are still early days, and the jury is still out on that one," Rietze says.

The significance of this distribution is unclear, but Rietze said it makes sense that the cortex, the youngest and most plastic region of the human brain, would contain the most neural stem cells.

Neural stem cells are rare, and their replication rate declines with age. But the effects of ageing may not be inexorable.

The QBI researchers believe it may be possible to develop a biochemical cocktail to restore neural stem cells to youthful vigour - or even to make amendments for nature's frugality, by preserving and expanding the original endowment of stem cells in the brain.

While awaiting further developments in the brain stem cell story, readers should self-medicate with regular exercise.