The Future of Stem Cells and Interventions: Integrating Human and Animal Knowledge
Work in animal models reveals fundamental processes of brain development that are important to generate and test hypotheses concerning the pathogenesis of human mental disorders. This opens the possibility of manipulating stem cells in clinical populations. Avenues of research are utilizing general knowledge about stem cells to (1) use manipulated stem cells in a rehabilitative capacity in patients and (2) use stem cells from patients to understand pathophysiology and test candidate drugs for reversing abnormal cellular functions. We will review briefly here how knowledge about stem cell biology from both humans and animal models is being applied to develop these new research directions.
Research on the use of stem cells for rehabilitation was originally motivated by the potential for direct development of new neurons from stem cells that might replace lost neurons that are at the center of pathophysiology. For example, in degenerative illnesses such as Parkinson's disease and Amyotrophic Lateral Sclerosis, specific populations of neurons, dopaminergic cells and spinal motor neurons respectively, are progressively lost. Stem cell research has sought to use either embryonic or other types of stem cells to generate replacement neurons that could be transplanted and incorporated into the CNS of patients (for a Review, see Hynes and Rosenthal, 2000). This work has required knowledge acquired from animal models of the transcription and signaling factors that determine dopaminergic and spinal motor neuron fate and maturation. As researchers have pursued this goal, it has been shown that stem cells in animal model transplant recipients may have an indirect trophic role, i.e., reducing cell death, increasing metabolic functions and protecting remaining neurons from insults (Jung et al., 2004; Rafuse et al., 2005; Yasuhara et al., 2006). These findings may hold promise for future application of stem cell-derived therapies to neuropsychiatric disorders. Disorders such as schizophrenia and depression may have a neurodegenerative component, similar to neurological disorders for which the supportive role of stem cells has been demonstrated. The regions of the brain involved may be different for major affective and psychotic disorders than degenerative motor diseases – e.g., cortex and hippocampus – but these regions may be even better candidates for the application of stem cell support therapy given their nearby neurogenic zones. Extrinsic factors that influence the stem cell niches, like FGF and BMPs, will likely have an important role to play in these potential therapies.
Induced pluripotent stem cells (iPSCs) also hold great promise in their application to neuropsychiatric disorders. These pluripotent stem cells can be derived from skin fibroblasts or blood lymphocytes by introducing a combination of genes that confer stem cell like properties, essentially reprogramming the differentiated cells to behave like embryonically derived stem cells (Takahashi et al., 2007; Yu et al., 2007; Muller et al., 2008). Takahashi and colleagues reported the first generation of human iPSCs in 2007. They achieved this by introducing the oct3/4, sox2, klf4, and c-myc genes into fibroblasts with retroviral vectors. As iPSCs reproduce the essential features of human embryonic stem cells (hESC) they can be coaxed to give rise to neurons. It has already been shown that hESC lines can give rise to mature, physiologically active neurons using different protocols, and many of these protocols have already been successfully applied to iPSC (Johnson et al., 2007; Chambers et al., 2009; Kriks and Studer, 2009; Soldner et al., 2009). Furthermore, by using different combinations of secreted patterning gene products such as FGFs, SHH and WNTs, different subtypes of neurons can be generated such as forebrain specific neurons, midbrain dopaminergic or GABAergic neurons (Yan et al., 2005; Eiraku et al., 2008; Chambers et al., 2009). Here again, decades of animal research in neurodevelopment is paying off, in that the accumulated knowledge in patterning and cell fate specification events in normal development will aid in the development of in vitro differentiation paradigms for generating specific neuron subtypes. More needs to be done and human and animal research must proceed in parallel.
Our inability to examine the direct consequences of gene sequence variations in the living CNS at the transcript and biological levels is a formidable challenge to the investigation of the genetic pathophysiology of psychiatric disorders. The derivation of iPSCs may allow us to investigate basic neurobiological aspects of neuropsychiatric disorders using patient-derived cell lines. For the first time, scientists will be able to use patient-derived cells to investigate developmental properties of the nervous system, such as neural stem cell proliferation, neural differentiation, and synapse formation either in vitro by using cell culture techniques, or in vivo by transplantation into a host animal, such as the developing rodent brain. One of the most promising aspects of iPSCs is the potential to examine the effects of specific human gene variants upon neural differentiation and function, by creating iPSCs from patients with particular genetic mutations. Furthermore, the gene expression profiles of neurons derived from such patients-derived neural stem cell lines can be examined. One potential complication of such studies is the large number of genetic variants present in the human genome. While on the one hand it is useful to begin to catalog such variants, associating them with particular phenotypes will require sophisticated statistical analyses and a large number of samples. Prior to the development of iPSCs, the standard approach for investigating the role of individual genes in brain development was to create a transgenic mouse model with either gene inactivation, gene overexpression, or mutations that mimic the mutations found in human disease. This remains a powerful approach, and indeed, animal work has provided useful experimental frameworks in which the functional impact of specific gene mutations can be tested. However, there are important differences in the development of the rodent brain and the human brain, which are reflected in the diversity of gene splice variants expressed during animal and human brain development (Johnson et al., 2009b). While work in animals continues to represent an essential reference point against which differences in gene expression and function found in humans can be compared, the use of iPSCs will allow us to test hypotheses about the effects of specific gene variants and mutations in developing human cells. Using iPSC models, we should be able to ask questions such as, how mutations in the disc-1 gene, together with a specific constellation of gene mutations/variants present an individual patient, lead to abnormal cell proliferation or neuronal differentiation in human NSCs? We may also be able to identify abnormalities in stem cell development in patients for which no specific genetic mutation has yet been identified. Such research may also provide means by which to test candidate pharmacological agents for their ability to normalize the function of neural cells derived from patients that may have abnormal functions.
The study of NSCs in multiple model systems will continue to inform our understanding of normal brain structure and function. The application of this information to patient populations is slowly shedding light on the pathophysiology of major neuropsychiatric diseases. The advent of new techniques for manipulating human cells may accelerate this process of discovery and may hold real promise for improving the lives of patients with mental illness.