Neurotrophic factors NGF and FGF-2 alter levels of huntingtin (IT15) in striatal neuronal cell cultures

Gene Therapy Approach with an Emphasis on Growth Factors: Theoretical and Clinical Outcomes in Neurodegenerative Diseases

The etiology of many neurological diseases affecting the central nervous system (CNS) is unknown and still needs more effective and specific therapeutic approaches. Gene therapy has a promising future in treating neurodegenerative disorders by correcting the genetic defects or by therapeutic protein delivery and is now an attraction for neurologists to treat brain disorders, like Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, spinal muscular atrophy, spinocerebellar ataxia, epilepsy, Huntington’s disease, stroke, and spinal cord injury. Gene therapy allows the transgene induction, with a unique expression in cells’ substrate. This article mainly focuses on the delivering modes of genetic materials in the CNS, which includes viral and non-viral vectors and their application in gene therapy. Despite the many clinical trials conducted so far, data have shown disappointing outcomes. The efforts done to improve outcomes, efficacy, and safety in the identification of targets in various neurological disorders are also discussed here. Adapting gene therapy as a new therapeutic approach for treating neurological disorders seems to be promising, with early detection and delivery of therapy before the neuron is lost, helping a lot the development of new therapeutic options to translate to the clinic.

Huntington's disease and cell therapies: past, present, and future

Huntington's disease (HD) is an inherited neurodegenerative disease that is characterized by movement abnormalities, cognitive impairment, and abnormal behavior as well as sleep and weight problems. It is an autosomal dominant disorder caused by a mutation in the huntingtin gene on the short arm of chromosome 4, which results in the progressive degeneration of the basal ganglia (caudate, putamen, and globus pallidus), cerebral cortex, brainstem, thalamus, and hypothalamus. This chapter considers four avenues of research: (a) the restoration of neurogenesis as an endogenous cell therapy in HD, (b) fetal tissue transplantation, (c) stem cell transplantation, and finally (d) the use of endogenous trophic factors such as brain derived neurotrophic factor.

Gene therapy in mouse models of huntington disease

Huntingtin, the protein that when mutated causes Huntington disease (HD), has many known interactors and participates in diverse cellular functions. Mutant Htt (mHtt) engages in a variety of aberrant interactions that lead to pathological gain of toxic functions as well as loss of normal functions. The broad symptomatology of HD, including diminished voluntary motor control, cognitive decline, and psychiatric disturbances, reflects the multifaceted neuropathology. Although currently available therapies for HD focus on symptom management, the autosomal dominant cause and the adult onset make this disease an ideal candidate for genetic intervention. A variety of gene therapy approaches have been tested in mouse models of HD, ranging from those aimed at ameliorating downstream pathology or replacing lost neuronal populations to more upstream strategies to reduce mHtt levels. Here the authors review the results of these preclinical trials.

Huntington's disease: from pathology and genetics to potential therapies

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease caused by a CAG trinucleotide repeat expansion encoding an abnormally long polyglutamine tract in the huntingtin protein. Much has been learnt since the mutation was identified in 1993. We review the functions of wild-type huntingtin. Mutant huntingtin may cause toxicity via a range of different mechanisms. The primary consequence of the mutation is to confer a toxic gain of function on the mutant protein and this may be modified by certain normal activities that are impaired by the mutation. It is likely that the toxicity of mutant huntingtin is revealed after a series of cleavage events leading to the production of N-terminal huntingtin fragment(s) containing the expanded polyglutamine tract. Although aggregation of the mutant protein is a hallmark of the disease, the role of aggregation is complex and the arguments for protective roles of inclusions are discussed. Mutant huntingtin may mediate some of its toxicity in the nucleus by perturbing specific transcriptional pathways. HD may also inhibit mitochondrial function and proteasome activity. Importantly, not all of the effects of mutant huntingtin may be cell-autonomous, and it is possible that abnormalities in neighbouring neurons and glia may also have an impact on connected cells. It is likely that there is still much to learn about mutant huntingtin toxicity, and important insights have already come and may still come from chemical and genetic screens. Importantly, basic biological studies in HD have led to numerous potential therapeutic strategies.

Pharmaceutical, cellular and genetic therapies for Huntington's disease

HD (Huntington's disease) is a devastating neurodegenerative disorder caused by a polyglutamine expansion in the gene encoding the huntingtin protein. Presently, there is no known cure for HD and existing symptomatic treatments are limited. However, recent advances have identified multiple pathological mechanisms involved in HD, some of which have now become the focus of therapeutic intervention. In this review, we consider progress made towards developing safe and effective pharmaceutical-, cell- and genetic-based therapies, and discuss the extent to which some of these therapies have been successfully translated into clinical trials. These new prospects offer hope for delaying and possibly halting this debilitating disease.

FGF-2 promotes neurogenesis and neuroprotection and prolongs survival in a transgenic mouse model of Huntington's disease

There is no satisfactory treatment for Huntington's disease (HD), a hereditary neurodegenerative disorder that produces chorea, dementia, and death. One potential treatment strategy involves the replacement of dead neurons by stimulating the proliferation of endogenous neuronal precursors (neurogenesis) and their migration into damaged regions of the brain. Because growth factors are neuroprotective in some settings and can also stimulate neurogenesis, we treated HD transgenic R6/2 mice from 8 weeks of age until death by s.c. administration of FGF-2. FGF-2 increased the number of proliferating cells in the subventricular zone by approximately 30% in wild-type mice, and by approximately 150% in HD transgenic R6/2 mice. FGF-2 also induced the recruitment of new neurons from the subventricular zone into the neostriatum and cerebral cortex of HD transgenic R6/2 mice. In the striatum, these neurons were DARPP-32-expressing medium spiny neurons, consistent with the phenotype of neurons lost in HD. FGF-2 was neuroprotective as well, because it blocked cell death induced by mutant expanded Htt in primary striatal cultures. FGF-2 also reduced polyglutamine aggregates, improved motor performance, and extended lifespan by approximately 20%. We conclude that FGF-2 improves neurological deficits and longevity in a transgenic mouse model of HD, and that its neuroprotective and neuroproliferative effects may contribute to this improvement.

Regulation of the proapoptotic activity of huntingtin interacting protein 1 by Dyrk1 and caspase-3 in hippocampal neuroprogenitor cells

Dual specific protein kinase Dyrks are thought to play a key role in the regulation of cell growth in a variety of cellular systems. Interestingly, human Dyrk1 is mapped to the Down's syndrome (DS) critical region on chromosome 21, and thought to be a candidate gene responsible for the mental retardation of DS patients. Huntingtin-interacting protein 1 (Hip-1), a proapoptotic mediator, is implicated as a molecular accomplice in the pathogenesis of Huntington's disease. In the present study we found that Dyrk1 selectively binds to and phosphorylates Hip-1 during the neuronal differentiation of embryonic hippocampal neuroprogenitor (H19-7) cells. The Dyrk1-mediated phosphorylation of Hip-1, in response to bFGF, resulted in the blockade of Hip-1-mediated neuronal cell death as well as the enhancement of neurite outgrowth. Furthermore, the addition of etoposide to proliferating H19-7 cells caused the diminished binding of Hip-1 to Dyrk1 and the levels of phosphorylated Hip-1 remarkably decreased. Simultaneously, the dissociated Hip-1 from Dyrk1 bound to caspase-3 in response to etoposide, which led to its activation and consequently cell death in H19-7 cells. These data suggest that the phosphorylation of Hip-1 by Dyrk1 has a dual role in regulating neuronal differentiation and cell death. The interaction between Dyrk1 and Hip-1 appeared to be differentially modulated by different kinds of stimuli, such as bFGF and etoposide in H19-7 cells.

Generalized brain and skin proteasome inhibition in Huntington's disease

Mutated intracellular huntingtin is widely expressed in tissues of Huntington's disease (HD) patients. Intraneuronal nuclear protein aggregates of mutant huntingtin are present in HD brains, suggesting a dysfunction of the ubiquitin proteasome system (UPS). Because many cells and tissues can cope with the abnormal gene effects while others dysfunction and die, we determined gene-induced effects and considered the hypothesis that the gene causes multiple intracellular problems, but severe pathology is seen only in selected brain regions. In this study, we found inhibition of UPS function in both early (0-1, with no or little neuronal loss) and late (3-4, with more severe neuronal loss) stage HD patients' cerebellum, cortex, substantia nigra and caudate-putamen brain regions. Late HD stage increases in ubiquitin levels were unique to caudate-putamen. HD patients' skin fibroblasts also had UPS inhibition similar to brain despite increases in proteasome beta-subunit expression. Gene delivery and expression of proteasome activator PA28 increased UPS function in normal but not HD fibroblasts. These generalized UPS problems are associated with severe neuronal pathology only when coupled with decreases in brain-derived neurotrophic factor levels, mitochondrial complex II/III activity, and increases of ubiquitin levels particularly as seen in the caudate-putamen of HD patients.

Effects of ethanol on neurotrophic factors, apoptosis-related proteins, endogenous antioxidants, and reactive oxygen species in neonatal striatum: relationship to periods of vulnerability

The developing central nervous system is extremely sensitive to ethanol, with well-defined temporal periods of vulnerability. Many brain regions are particularly susceptible to ethanol during the early neonatal period, corresponding to the human third trimester, which represents a dynamic period of growth and differentiation. For this study, neonatal rats were acutely exposed to ethanol or control conditions at a neonatal age when the developing striatum has been shown to be vulnerable to ethanol (postnatal day 3 [P3]), and at a later age (P14), when this developing region is relatively ethanol-resistant. We then analyzed basal levels of neurotrophic factors (NTFs), and ethanol-mediated changes in NTFs, apoptosis-related proteins, antioxidants, and reactive oxygen species (ROS) generation, which may underlie this differential temporal vulnerability. Sequential analyses were made following ethanol exposure on these two postnatal days, with assessments of NTFs nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4); apoptosis-related proteins Bcl-2, Bcl-xl, Bax, Akt and c-jun N-terminal kinase (JNK); antioxidants superoxide dismutase, glutathione reductase and catalase; and ROS. The results indicated that basal levels of BDNF, and to some degree NGF, were greater at the older age, and that ethanol exposure at the earlier age elicited considerably more pro-apoptotic and fewer pro-survival changes than those produced at the later age. Thus, differential temporal vulnerability to ethanol in this CNS region appears to be related to differences in both differential levels of protective substances (e.g. NTFs), and differential cellular responsiveness which favors apoptosis at the most sensitive age and survival at the resistant age.

Role of phosphatidylinositide 3-kinase in brain-derived neurotrophic factor-induced DARPP-32 expression in medium size spiny neurons in vitro

Brain-derived neurotrophic factor (BDNF) regulates several properties of striatal dopaminoceptive medium-sized spiny neurons (MSNs) in vivo and in vitro, including expression levels of DARPP-32 (dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa). DARPP-32 is expressed in 96% of the MSNs, and is a key modulator of dopamine actions. We investigated the intracellular signal transduction pathways activated by BDNF in MSNs and via which BDNF induces DARPP-32 expression. We found that phosphorylation of the cyclic AMP response element binding protein (CREB) is only transiently increased following stimulation of MSNs by BDNF, whereas increased phosphorylation of the extracellular signal regulated kinases 1 and 2 (Erk1/2) and Akt is sustained for longer than 4 h. Treatment of cultures with inhibitors of mitogen-activated protein kinase kinase (MEK) or phosphatidylinositide 3-kinase (PI3K) showed that the majority of the BDNF-induced increase in DARPP-32 requires the PI3K pathway. We also found that inhibition of PI3K reduces BDNF-induced Erk phosphorylation, indicating that cross-talk between these pathways may play a prominent role in MSNs.

Neuroprotective possibilities for Huntington's disease

Huntington's disease (HD) is a devastating genetic disorder. Despite the absence of effective therapy, there has been an explosion in interest for developing treatment strategies aimed at lessening or preventing the neuronal death that occurs in this disease. In large part, the renewed interest in neuroprotective strategies has been spurred by our increasing understanding of the genetic and molecular events that drive the underlying neuropathology of HD. This escalating understanding of the biological underpinnings of HD is derived from several convergent sources represented by investigators with clinical, genetic, molecular, physiological and neurobehavioural backgrounds. The diversity of data being generated has, in turn, produced a unique time in HD research where an impressive number of potential therapeutics are coming to the forefront. This review outlines several of these possibilities including the use of intracerebrally delivered neurotrophic factors, pharmacologically altering cellular energy production, the use of antiglutamatergic drugs, the use of caspase inhibitors and inhibitors of protein aggregation. This review also touches on the interesting possibility of whether or not the neurodegeneration in HD is at least partially reversible in nature. All of these possibilities are highlighted in the context that HD is a neurodegenerative disorder in which genetic detection provides a clear and unequivocal opportunity for neuroprotection.