Stem Cell News: Can RNAi Get to the Htt of the Matter?

Stem cells sidle up to neurons, poke them with a membranous finger, and pump them full of short hairpin RNAs that turn off huntingtin, potentially slowing the progression of Huntington’s disease. That’s the scenario envisioned in the December 8 Molecular and Cellular Neuroscience online. A group, led by senior author Jan Nolta of the University of California, Davis, took advantage of mesenchymal stem cells’ propensity to share cellular material across membrane bridges to tune down neural huntingtin in cell culture. “The system is not perfect” in this "proof-of-concept paper," noted Nolta, who said the researchers have plenty of work ahead in optimizing the RNA transfer. Stem cell treatments are looking closer than ever this week with another report, in the January 23 Lancet online, that Advanced Cell Technologies of Marlborough, Massachusetts, successfully transplanted tissue derived from human embryonic stem cells into two people with macular degeneration.

Nolta’s group researched how to eliminate the extra-long form of the huntingtin (Htt) protein that causes neurodegeneration with RNA interference. “In transgenic mice, this works so well,” Nolta said (see ARF related news story on Harper et al., 2005; Boudreau et al., 2009; Difiglia et al., 2007). But in people, the challenge is how to deliver the silencer, in the form of short hairpin RNAs (shRNAs), across the blood-brain barrier and into the suffering striatal neurons, particularly given that the extracellular milieu is full of RNAs that would chop up the medicine.

Nolta and her team, including first author Scott Olson, who has since moved on to the University of Texas Health Science Center at Houston, thought that mesenchymal stem cells (MSCs) could chauffeur shRNAs right to the heart of the degenerating neurons. MSCs are easy to extract from bone marrow and are considered relatively safe for transplant. They home in on damaged tissue like cellular “paramedics,” Nolta said. The group has already shown that transplanted MSCs can deliver needed factors, such as enzymes and cytokines, to transgenic mice (Meyerrose et al., 2008; reviewed in Meyerrose et al., 2010) and hoped they could also make and share shRNAs. “Their job is to secrete things to other cells, and they can make a huge amount of whatever we engineer them to make,” Nolta said.

Researchers have long known that bacteria and plant cells can transfer nucleic acid from cell to cell, but only recently learned the same is possible in mammalian systems. MSCs make tunneling nanotubules—a bit like a neuron’s axons or dendrites, Nolta said—that reach out and touch other cells (Gerdes and Carvalho, 2008). “It is kind of like putting a little gas hose into the other cell and pouring things into it,” she said. Donor cells may also release exosomes that deliver nucleic acid to neighboring cells (Simons and Raposo, 2009). “The RNA never leaves the protective environment of the cell,” Nolta said.

Olson and Nolta tested the potential for MSCs to deliver huntingtin shRNAs to neurons in culture. The target cells were SH-SY5Y neuroblastoma cells. Olson infected them with a viral vector containing both green fluorescent protein (GFP) and a mutant Htt gene. They used Htt-142, which has 142 CAG repeats in the first exon compared to the normal 28 or fewer, and is associated with juvenile-onset Huntington’s. For the paramedic donors, the researchers used MSCs isolated from donated human bone marrow. They provided the MSCs with shRNA against either the first exon of Htt or a scrambled, control sequence.

Then, Olson put the MSC donors and SH-SY5Y acceptors in the same culture to allow the shRNA transfer and analyzed Htt levels via Western blotting. To account for variable amounts of the Htt-GFP vector’s uptake and expression, he calculated the ratio of the extra-long Htt protein to GFP in the co-cultures; presumably, any cell making GFP also started out making the mutant Htt. After five days, the Htt:GFP ratio dropped by about half in the cultures with the Htt RNAi. The scrambled shRNA construct did not significantly alter Htt levels.

“I have never seen anything quite like this [approach] before,” said Jeanne Loring of The Scripps Research Institute in La Jolla, California, who was not involved with the study. While she thought the baseline Htt reduction was “not impressive,” she added, “it is not really the amount that is important at this stage; it is that it does anything at all.” The team might be able to further optimize Htt downregulation in the future, she suggested. Learning more about the mechanism at work in the cultures—be it nanotubules, nanovesicles, or some other transfer technique—will also help identify the best RNAi donors, the authors wrote.

“I really hope they can move forward with it…;. My major concern is whether this would work consistently in vivo, and be sustainable,” Loring said. Nolta’s team kept the interference going for a week in culture, and they know that transplanted MSCs survive for a month in rodent brains, but they do not know how long the cells could last and keep pumping shRNAs into their neighbors in a human brain. If Nolta can succeed in perfecting the approach, she hopes it might slow disease in Huntington’s, as well as any other condition where a dominant, toxic protein is causing pathology. For example, mutant SOD1 is responsible for some cases of amyotrophic lateral sclerosis.

While a therapy based on this approach is far from the clinic, stem cell treatments are closer than ever with the Lancet publication. Though transplantation of bone marrow stem cells is mainstream, and several trials have progressed with fetal-derived stem cells, Advanced Cell Technologies’ work is the first human transplant of cells derived from human embryonic stem cells (hESCs). First author Steven Schwartz of the University of California, Los Angeles, senior author Robert Lanza of Advanced Cell Technologies, and colleagues report on how they treated two people with advanced macular degeneration with retinal pigmented epithelium derived from hESCs. Four months in, no undesirable side effects have appeared, and a hint of benefit is apparent. One person went from reading no letters on an eye chart to discerning the five biggest characters; the other went from seeing 21 letters to reading 28, including smaller characters. The researchers suspect that providing the treatment earlier in disease will increase their chances of restoring vision. “Schwartz and colleagues have realized the potential to use hESCs therapeutically in human beings,” wrote Anthony Atala of the Wake Forest University School of Medicine in Winston-Salem, North Carolina, in a comment accompanying the Lancet paper. “The report is preliminary…;but the results are impressive.”

Olson SD, Kambal A, Pollok K, Mitchell GM, Stewart H, Kalomoiris S, Cary W, Nacey C, Pepper K, Nolta JA. Examination of mesenchymal stem cell-mediated RNAi transfer to Huntington’s disease affected neuronal cells for reduction of huntingtin. Mol Cell Neurosci. 2011 Dec 8. Abstract

Schwartz SD, Hubschman JP, Heilwell G, Franco-Cardenas V, Pan CK, Ostrick RM, Mickunas E, Gay R, Klimanskaya I, Lanza R. Embryonic stem cell trials for macular degeneration: a preliminary report. Lancet. 2012 Jan 23. Abstract

Atala A. Human embryonic stem cells: early hints on safety and efficacy. Lancet. 2012 Jan 23. Abstract

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