Differential recovery of inhibitory avoidance learning by striatal, cortical, and mesencephalic fetal grafts

Using gene or cell therapies to treat Huntington's disease

Huntington's disease is caused by a CAG repeat expansion in the first exon of the HTT gene, leading to the production of gain-of-toxic-function mutant huntingtin protein species and consequent transcriptional dysregulation and disrupted cell metabolism. The brunt of the disease process is borne by the striatum from the earliest disease stages, with striatal atrophy beginning approximately a decade prior to the onset of neurologic signs. Although the expanded CAG repeat in the HTT gene is necessary and sufficient to cause HD, other genes can influence the age at onset of symptoms and how they progress. Many of these modifier genes have roles in DNA repair and are likely to modulate the stability of the CAG repeat in somatic cells. Currently, there are no disease-modifying treatments for HD that can be prescribed to patients and few symptomatic treatments, but there is a lot of interest in therapeutics that can target the pathogenic pathways at the DNA and RNA levels, some of which have reached the stage of human studies. In contrast, cell therapies aim to replace key neural cells lost to the disease process and/or to support the host vulnerable striatum by direct delivery of cells to the brain. Ultimately it may be possible to combine gene and cell therapies to both slow disease processes and provide some level of neural repair. In this chapter we consider the current status of these therapeutic strategies along with their prospects and challenges.

Differentiation of pluripotent stem cells into striatal projection neurons: a pure MSN fate may not be sufficient

Huntington's disease (HD) is an autosomal dominant inherited disorder leading to the loss inter alia of DARPP-32 positive medium spiny projection neurons ("MSNs") in the striatum. There is no known cure for HD but the relative specificity of cell loss early in the disease has made cell replacement by neural transplantation an attractive therapeutic possibility. Transplantation of human fetal striatal precursor cells has shown "proof-of-principle" in clinical trials; however, the practical and ethical difficulties associated with sourcing fetal tissues have stimulated the need to identify alternative source(s) of donor cells that are more readily available and more suitable for standardization. We now have available the first generation of protocols to generate DARPP-32 positive MSN-like neurons from pluripotent stem cells and these have been successfully grafted into animal models of HD. However, whether these grafts can provide stable functional recovery to the level that can regularly be achieved with primary fetal striatal grafts remains to be demonstrated. Of particular concern, primary fetal striatal grafts are not homogenous; they contain not only the MSN subpopulation of striatal projection neurons but also include all the different cell types that make up the mature striatum, such as the multiple populations of striatal interneurons and striatal glia, and which certainly contribute to normal striatal function. By contrast, present protocols for pluripotent stem cell differentiation are almost entirely targeted at specifying just neurons of an MSN lineage. So far, evidence for the functionality and integration of stem-cell derived grafts is correspondingly limited. Indeed, consideration of the features of full striatal reconstruction that is achieved with primary fetal striatal grafts suggests that optimal success of the next generations of stem cell-derived replacement therapy in HD will require that graft protocols be developed to allow inclusion of multiple striatal cell types, such as interneurons and/or glia. Almost certainly, therefore, more sophisticated differentiation protocols will be necessary, over and above replacement of a specific population of MSNs. A rational solution to this technical challenge requires that we re-address the underlying question-what constitutes a functional striatal graft?

Cell-based treatments for huntington's disease

In experimental rats, mice, and monkeys, transplantation of embryonic striatal cells into the striatum can repair the damage and alleviate the functional deficits caused by striatal lesions. Such strategies have been translated to striatal repair by cell transplantation in small numbers of patients with progressive genetic striatal degeneration in Huntington's disease. In spite of some encouraging preliminary data, the clinical results are to date neither as reliable nor as compelling as the broad extend of recovery observed in the animal models across motor, cognitive, and skill and habit learning domains. Strategies to achieve immediate and long-term improvements in the clinical applications include identifying and limiting the causes of complications, standardization and quality control of preparation and delivery, appropriate patient selection to match the cellular repair to specific profiles of cell loss and degeneration in individual patients and different neurodegenerative diseases, and improving the availability of alternative sources of donor cells and tissues.

Striatal grafts alleviate bilateral striatal lesion deficits in operant delayed alternation in the rat

In order to assess the capacity of striatal grafts to alleviate cognitive deficits of the frontal type that arise following bilateral striatal lesions, control, lesion and grafted rats were tested in an operant test of delayed alternation. Bilateral striatal lesions induced a marked impairment in choice accuracy, and signal detection analysis indicated that the lesion animals were reliably impaired on both parametric and non-parametric indices of discriminative sensitivity but not of response bias. The impairment was apparent at all intertrial interval delays, including the very shortest, suggesting the deficit is one of frontal-type executive function rather than of short-term memory. The grafted animals exhibited a significant alleviation of the deficit, again apparent at all delays. Histological analyses indicated good graft survival, and injections of a dextran amine anterograde tracer bilaterally into the host prefrontal cortex indicated reformation of extensive projections into the grafted tissues. Since performance of the operant delayed alternation task is dependent upon the integrity of corticostriatal connections, which is disrupted bilaterally by the lesions and restored to the grafts in the transplanted animals, the results corroborate the hypothesis that striatal grafts can alleviate complex cognitive functions of the frontal type by a mechanism that involves functional integration of the grafted neurons into the neural circuits of the host brain.

Striatal reconstruction by striatal grafts

It is now well established that striatal lesions induce motor and cognitive deficits in rats, and that grafts of embryonic striatal tissue can survive, integrate into the lesioned host brain and alleviate the behavioural deficits in both motor and cognitive spheres. How? Since normal striatal function is dependent upon it's integration within a connected cortical-subcortical neuronal circuitry, and the deficits following striatal damage appear to reflect a "disconnexion" syndrome, the observation of recovery suggests that the grafts re-establish a connected circuitry within the host brain. Evidence to corroborate or refute this hypothesis, in comparison with a less-specific mechanism (or mechanisms) of recovery, is considered, including anatomical, electrophysiological and neurochemical demonstrations of functional circuit reconstruction in the host brain by striatal tissue transplants.