Most people associate Huntington’s disease (HD) with the famous folksinger Woody Guthrie or the National Baseball Hall of Fame pitcher Catfish Hunter. Although it is one of the more common genetic disorders, HD is currently untreatable. For over a decade, researchers led by Drs. Abdellatif Benraiss and Steven Goldman, from the University of Rochester Medical Center, Rochester, NY, have been refining a strategy for coaxing the brain to replace cells lost or damaged by HD. They recently published the latest iteration of that strategy, and it can mitigate HD pathology and progression in the B6CBA-Tg(HDexon1)62Gpb/3J (“R6/2” 006494) mouse model of HD (Benraiss et al. 2013). Further refinement of the strategy might lead to a long-awaited HD therapy.
HD is a devastating, degenerative brain disorder caused by an autosomal dominant mutation – the expansion of a polyglutamine-encoding CAG triplet repeat stretch – in either of two copies of a gene called “Huntingtin.” A child of an affected parent has a 50% chance of inheriting the disease. More than a quarter million Americans either have or are at risk of inheriting HD. HD typically begins in mid-life, between the ages of 30 and 45, though it may begin as early as two. It is characterized by progressive loss of muscle control, cognitive decline, and psychological problems (Huntington’s Disease Society of America).
Refining a vector that induces brain cell regeneration
One of the hallmarks of HD is the loss of neurons from the cortex and midbrain. Among the neurons lost are the medium spiny neurons (MSNs), which are located in the neostriatum and play a key role in initiating and controlling body, limb, and eye movements. Although MSNs are located in the neostriatum, they originate from stem cells in another brain region – the subependyma, a cell layer below the ependyma in the lateral ventricles. The Benraiss/Goldman team had demonstrated that adenoviral vector-mediated over-expression of two proteins – brain-derived neurotrophic factor(BDNF) and noggin – induces subependymal stem cells to generate new MSNs, slows disease progression, and extends the lifespan of R6/2 mice (Benraiss et al. 2001; Chmielnicki et al. 2004). However, the adenoviral vector they used expresses BDNF and noggin for only a short time and is eventually eliminated by the host’s immune system. The team next tried a non-immunogenic, adeno-associated virus serotype 4 (AAV4) vector strategy – hereafter called “AAV4-BDNF/noggin” – and found that it can induce the over-expression of BDNF and noggin and sustain the generation of MSNs and their recruitment to the neostriatum of rats for at least four months (Benraiss et al. 2012). In their most recent study, the Benraiss/Goldman team demonstrated that AAV4-BDNF/noggin delays HD progression and significantly extends the life span of R6/2 mice. Their key findings are summarized below.
AAV4-BDNF/noggin orchestrates MSN regeneration in R6/2 mice
- Mice injected intracerebroventricularly with an AAV4 vector harboring an enhanced green fluorescent protein (EGFP) transgene express GFP only in the ependymal cells lining the ventricles, and mice injected with AAV4-BDNF/noggin express BDNF robustly in the same cells, indicating that the AAV4 vector is ependymal cell-specific
- AAV4-mediated IVC delivery induces the production of enough BDNF and noggin to generate MSNs in the adult mouse brain
- AAV4-BDNF/noggin-treated R6/2 mice generate significantly more cells with MSN markers than controls
- AAV4-BDNF/noggin-treatment corrects the high striatal neuron densities and concomitant striatal involution/atrophy of R6/2 mice
- Newly generated MSNs of AAV4-BDNF/noggin-treated R6/2 mice reconstitute each major pool of striatal projection neurons
- AAV4-BDNF/noggin-treatment delays disease onset and extends the life span or R6/2 mice
- The use of fluorescence-expressing, tamoxifen-inducible nestin Cre reporter strains confirms that new AAV4-BDNF/noggin-generated MSNs originate from nestin-expressing ependymal stem or progenitor cells
- The morphology, electrophysiology, network integration, and density of newly generated MSNs in AAV4-BDNF/noggin-treated R6/2 mice appear normal
- Newly generated MSNs in AAV4-BDNF/noggin-treated R6/2 mice contain significantly fewer and later-developing neuronal intranuclear inclusions (NIIs) than do resident MSNs
BDNF/noggin over-expression stimulates MSN regeneration in squirrel monkeys
The Benraiss/Goldman team found that, as in mice, intraventricular BDNF/noggin over-expression stimulates the generation and recruitment of mature MSNs to the appropriate brain region in squirrel monkeys.
In summary, the Benraiss/Goldman team demonstrated that AAV4-mediated over-expression of BDNF and noggin is effective, long-term, and ependymal cell-specific. It induces thegeneration, recruitment, and normal integration of morphologically and electrophysiologically normal MSNs to the appropriate neostriatal brain region, mitigates HD symptoms, and extends the life span of R6/2 mice. The encouraging results of BDNF/noggin over-expression in squirrel monkeys and R6/2 mice suggest that an ependymal cell-specific BDNF/noggin over-expression strategy may be therapeutic to humans with HD.