Stem Cell Therapies for Neurodegenerative Disease: How Should We Push Ahead?
Mahendra Rao, with co-moderator George Martin, led this live discussion on 30 March 2004. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.
View Transcript of Live Discussion — Posted 23 August 2006
By Mahendra Rao, National Institutes of Health, Baltimore, Maryland.
As we age, the brain's capacity gradually declines. This is associated with age-related changes in the brain environment, including elevated oxidative stress and the accumulation of protein and lipid byproducts and other mitochondrial changes. There is also a progressive reduction in synaptic function and a reduction in the total number of postmitotic cells, accompanied probably by a relative gliosis (Rosenzweig et al., 2003; Miller et al., 2003). At the same time, fewer new neurons are generated in the two major neurogenic regions—the subventricular zone (SVZ) surrounding the lateral ventricles, and the subgranular layer (SGL) of the hippocampal dentate gyrus (see Kempermann et al., 2002; Jessberger and Kempermann, 2003; Shors et al., 2002; Haughey et al., 2002). The underlying cause of this declining neurogenesis is unknown, but presumably it is related to the age-related changes that occur during normal aging of the brain.
Neurodegenerative diseases such as Alzheimer's and Parkinson's may well exacerbate these age-related changes in stem cell biology, and several studies indeed suggest as much (Haughey et al., 2002; Wen et al., 2002; Feng et al., 2002). It is important to note, however, that it is unclear at this early stage what's behind this decline (see for example Jin et al., 2004). Does it reflect a reduction in absolute numbers of stem cells, a failure of stem cells to respond to cues as they mature, a decline in proliferation cues, or even defects in the migration, survival, or integration of newborn neurons? It has been difficult to separate these issues as markers delineating stem cell stages are limited, and the quantifying overall number remains a challenge in spite of advances in stereologic methods.
It is in this context that one can examine the utility of stem cell therapy and perhaps generalize to other complex degenerative disorders. I believe there are three general ways of examining how cells can be used for therapy. Whichever one pursues, I believe that taking into consideration the cellular, molecular, and environmental changes of the aging brain will be critical. One can imagine replacing stem cells in the brain so that they replenish the reservoir. One can imagine mobilizing endogenous stem cells. Or one can imagine providing trophic support to either stem or differentiated cells using any population of cells that can survive with minimal damage to the brain parenchyma. Each of these strategies has its own problems that need to be considered when assessing potential therapeutic approaches. I summarize these below:
1. Replacement of Stem Cells
- Can only work where a stem cell niche exists: Most brain regions are devoid of stem cells that respond in any significant fashion after injury or in disease.
- Surgical approach to such a niche is difficult.
- The number of stem cells present in the adult human brain is vanishingly small (Roy et al., 2000; Nunes et al., 2003).
- Adult neural stem cells do not passage well and reports suggest that cells may be transformed in as little as 10 passages (Kim and Morshead, 2003).
- Fetal stem cells themselves do not survive if placed outside the stem cell niche and, in injury models, do not differentiate into neurons (personal observation).
- Positional information ("where do I belong?") appears relatively fixed and maintained in culture (Fishell et al., 1990; Parmar et al., 2002, and references therein).
- Use ES cell-derived neural derivatives to avoid the positional limitation.
- Figure out a way to alter positional identity.
- Learn to amplify stem cells in culture.
- Use differentiated cells that have the appropriate positional identity.
- Characterize stem cells better to enhance their survival in injury and bias differentiation appropriately.
2. Mobilization of Endogenous Cells:
In principle, this is much easier. Note, however, the caveat that one can only mobilize in regions where stem cells and cues exist to direct the cells' appropriate migration, differentiation, and integration. One can enhance or modulate an existing process, but cannot graft on an entirely new pathway of differentiation with any reasonable fidelity or chance of success in the near future.
- Can potentially be done in hippocampus and olfactory bulb, and possibly in the striatum (the presence of striatal stem cells is controversial; see, however, Kovacs et al., 2001; Chmielnicki et al., 2004).
- Noninvasive methods exist: exercise, hormones, calorie restriction, antioxidants, etc. (reviewed in Lie et al., 2004).
- Growth factors and possibly their small-molecule mimics can alter cell proliferation rate as much as 10-fold.
- The number of stem cells relative to the number of lost neurons is very small. Estimates from FACS sorting data suggest approximately 30,000 stem cells in the adult human brain (Goldman SA personal communication).
- The ongoing rate of division is very low; even a 10-fold increase is possibly two orders of magnitude less than what is required.
- The time to integration and maturation in humans is very slow.
- Despite increases in stem cell proliferation, the total increase in integration is small and many of the new neurons born die by apoptosis.
- Can this ever be enough given the massive loss in forebrain and other brain regions?
3. Enhance survival of existing neurons or delay death
Not novel, this idea is the basis for many current therapies in the nervous system. The problem here—and the reason why clinical trials have failed—has always been how to deliver trophic factors, or other molecules, across the blood-brain barrier and to the target site in sufficient levels without side effects due to unacceptable levels in other regions of the brain.
- Preliminary results in other models are encouraging (reviewed in Grondin et al., 2004). The requirement for cells is simpler: All they have to do is survive and remain within a given region. Success does not depend on 100 percent survival; a small increase in absolute numbers is sufficient and more achievable.
- Stem cells may not be ideal delivery vehicles for survival factors, as they may differentiate and integrate ectopically. Designed to proliferate or become quiescent, stem cells generally lack extensive secretory machinery.
- Stem cells require specialized niches to survive and will likely die in a diseased non-niche environment.
- Mesenchymal and other non-neural cells do not appear to last well in the environment; if they transdifferentiate, they do so at such a small frequency that it is not useful (reviewed in Liu et al 2003).
- Localized surgical delivery of cells remains necessary to achieve reasonable numbers. Even if intravenous infusion worked, the numbers are simply insufficient. Current data on intravenous infusion leading to homing to an appropriate site remain conceptually difficult to understand or wrong.
On Choice of Cell:
- Possibilities for delivery include glial cells of the CNS, Schwann cells, olfactory ensheathing cells, possibly mesenchymal cells, or microglia. Any of these cell types can be obtained in large numbers from fetal and adult sources. Constituting 90 percent of brain cells, glia generally provide trophic support, and as such, would be performing a function close to their physiological role.
- Mesenchymal cells have the advantage of potential autologous therapy and can be obtained in truly large numbers.
In conclusion, possibilities for stem cell therapies in neurodegenerative diseases remain encouraging. That said, important questions need to be answered before we can to predict which strategy will be appropriate for which disease. No one-size-fits-all cell therapy approach exists. If we are to reduce the tantalizing but highly variable data currently in the literature into an effective future therapy, I recommend that we focus on testing specific hypotheses of how cells may be effective, but do so with detailed, rigorous quantification.
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