Projection neurons in fetal striatal transplants are predominantly derived from the lateral ganglionic eminence

Cell transplantation for Huntington’s disease: practical and clinical considerations

Huntington’s disease is a dominantly inherited neurodegenerative disorder, usually starting in mid-life and leading to progressive disability and early death. There are currently no disease-modifying treatments available. Cell transplantation is being considered as a potential therapy, following proof of principle that cell transplantation can improve outcomes in another basal ganglia disorder, namely Parkinson’s disease. The principle aim is to replace the striatal medium spiny neurons lost in Huntington’s disease with new cells that are able to take over their function and reconnect the circuitry. This article reviews the experimental background and evidence from clinical studies that suggest that cell transplantation may improve function in Huntington’s disease, reviews the current status of the field and considers the current challenges to taking this experimental strategy forward to becoming a reliable therapeutic option.

Enlarged lateral ventricles and aberrant behavior in mice overexpressing PDGF-B in embryonic neural stem cells

Platelet-derived growth factor (PDGF) is important in central nervous system (CNS) development, and aberrant expression of PDGF and its receptors has been linked to developmental defects and brain tumorigenesis. We previously found that neural stem and progenitor cells in culture produce PDGF and respond to it by autocrine and/or paracrine signaling. We therefore aimed to examine CNS development after PDGF overexpression in neural stem cells in vivo. Transgenic mice were generated with PDGF-B under control of a minimal nestin enhancer element, which is specific for embryonic expression and will not drive adult expression in mice. The resulting mouse showed increased apoptosis in the developing striatum, which suggests a disturbed regulation of progenitor cells. Later in neurodevelopment, in early postnatal life, mice displayed enlarged lateral ventricles. This enlargement remained into adulthood and it was more pronounced in male mice than in transgenic female mice. Nevertheless, there was an overall normal composition of cell types and numbers in the brain and the transgenic mice were viable and fertile. Adult transgenic males, however, showed behavioral aberrations and locomotor dysfunction. Thus, a tightly regulated expression of PDGF during embryogenesis is required for normal brain development and function in mice.

Origin and molecular specification of globus pallidus neurons

The mechanisms controlling the assembly of brain nuclei are poorly understood. In the forebrain, it is typically assumed that the formation of nuclei follows a similar sequence of events that in the cortex. In this structure, projection neurons are generated sequentially from common progenitor cells and migrate radially to reach their final destination, whereas interneurons are generated remotely and arrive to the cortex through tangential migration. Using the globus pallidus as a model to study the formation of forebrain nuclei, we found that the development of this basal ganglia structure involves the generation of several distinct classes of projection neurons from relatively distant progenitor pools, which then assemble together through tangential migration. Our results thus suggest that tangential migration in the forebrain is not limited to interneurons, as previously thought, but also involves projection neurons and reveal that the assembly of forebrain nuclei is more complex than previously anticipated.

Embryonic MGE precursor cells grafted into adult rat striatum integrate and ameliorate motor symptoms in 6-OHDA-lesioned rats

We investigated a strategy to ameliorate the motor symptoms of rats that received 6-hydroxydopamine (6-OHDA) lesions, a rodent model of Parkinson's disease, through transplantation of embryonic medial ganglionic eminence (MGE) cells into the striatum. During brain development, embryonic MGE cells migrate into the striatum and neocortex where they mature into GABAergic interneurons and play a key role in establishing the balance between excitation and inhibition. Unlike most other embryonic neurons, MGE cells retain the capacity for migration and integration when transplanted into the postnatal and adult brain. We performed MGE cell transplantation into the basal ganglia of control and 6-OHDA-lesioned rats. Transplanted MGE cells survived, differentiated into GABA(+) neurons, integrated into host circuitry, and modified motor behavior in both lesioned and control rats. Our data suggest that MGE cell transplantation into the striatum is a promising approach to investigate the potential benefits of remodeling basal ganglia circuitry in neurodegenerative diseases.

LIM homeodomain transcription factor-dependent specification of bipotential MGE progenitors into cholinergic and GABAergic striatal interneurons

Coordination of voluntary motor activity depends on the generation of the appropriate neuronal subtypes in the basal ganglia and their integration into functional neuronal circuits. The largest nucleus of the basal ganglia, the striatum, contains two classes of neurons: the principal population of medium-sized dense spiny neurons (MSNs; 97-98% of all striatal neurons in rodents), which project to the globus pallidus and the substantia nigra, and the locally projecting striatal interneurons (SINs; 2-3% in rodents). SINs are further subdivided into two non-overlapping groups: those producing acetylcholine (cholinergic) and those producing gamma-amino butyric acid (GABAergic). Despite the pivotal role of SINs in integrating the output of striatal circuits and the function of neuronal networks in the ventral forebrain, the lineage relationship of SIN subtypes and the molecular mechanisms that control their differentiation are currently unclear. Using genetic fate mapping, we demonstrate here that the majority of cholinergic and GABAergic SINs are derived from common precursors generated in the medial ganglionic eminence during embryogenesis. These precursors express the LIM homeodomain protein Lhx6 and have characteristics of proto-GABAergic neurons. By combining gene expression analysis with loss-of-function and misexpression experiments, we provide evidence that the differentiation of the common precursor into mature SIN subtypes is regulated by the combinatorial activity of the LIM homeodomain proteins Lhx6, Lhx7 (Lhx8) and Isl1. These studies suggest that a LIM homeodomain transcriptional code confers cell-fate specification and neurotransmitter identity in neuronal subpopulations of the ventral forebrain.

Distinct temporal requirements for the homeobox gene Gsx2 in specifying striatal and olfactory bulb neuronal fates

The homeobox gene Gsx2 (formerly Gsh2) is known to be required for striatal and olfactory bulb neurogenesis; however, its specific role in the specification of these two neuronal subtypes remains unclear. To address this, we have employed a temporally regulated gain-of-function approach in transgenic mice and found that misexpression of Gsx2 at early stages of telencephalic neurogenesis favors the specification of striatal projection neuron identity over that of olfactory bulb interneurons. In contrast, delayed activation of the Gsx2 transgene until later stages exclusively promotes olfactory bulb interneuron identity. In a complementary approach, we have conditionally inactivated Gsx2 in a temporally progressive manner. Unlike germline Gsx2 mutants, which exhibit severe alterations in both striatal and olfactory bulb neurogenesis at birth, the conditional mutants exhibited defects restricted to olfactory bulb interneurons. These results demonstrate that Gsx2 specifies striatal projection neuron and olfactory bulb interneuron identity at distinct time points during telencephalic neurogenesis.

The developmental integration of cortical interneurons into a functional network

The central goal of this manuscript is to survey our present knowledge of how cortical interneuron subtypes are generated. To achieve this, we will first define what is meant by subtype diversity. To this end, we begin by considering the mature properties that differentiate between the different populations of cortical interneurons. This requires us to address the difficulties involved in determining which characteristics allow particular interneurons to be assigned to distinct subclasses. Having grappled with this thorny issue, we will then proceed to review the progressive events in development involved in the generation of interneuron diversity. Starting with their origin and specification within the subpallium, we will follow them up through the first postnatal weeks during their integration into a functional network. Finally, we will conclude by calling the readers attention to the devastating consequences that result from developmental failures in the formation of inhibitory circuits within the cortex.

Medium spiny neurons for transplantation in Huntington's disease

Cell-replacement therapy for Huntington's disease is one of very few therapies that has reported positive outcomes in clinical trials. However, for cell transplantation to be made more readily available, logistical, standardization and ethical issues associated with the current methodology need to be resolved. To achieve these goals, it is imperative that an alternative cell source be identified. One of the key requirements of the cells is that they are capable of acquiring an MSN (medium spiny neuron) morphology, express MSN markers such as DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa), and function in vivo in a manner that replicates those that have been lost to the disease. Developmental biology has progressed in recent years to provide a vast array of information with regard to the key signalling events involved in the proliferation, specification and differentiation of striatal-specific neurons. In the present paper, we review the rationale for cell-replacement therapy in Huntington's disease, discuss some potential donor sources and consider the value of developmental markers in the identification of cells with the potential to develop an MSN phenotype.

Ascl1 is a required downstream effector of Gsx gene function in the embryonic mouse telencephalon

Background

The homeobox gene Gsx2 (formerly Gsh2) is known to regulate patterning in the lateral ganglionic eminence (LGE) of the embryonic telencephalon. In its absence, the closely related gene Gsx1 (previously known as Gsh1) can partially compensate in the patterning and differentiation of ventral telencephalic structures, such as the striatum. However, the cellular and molecular mechanisms underlying this compensation remain unclear.

Results

We show here that in the Gsx2 mutants Gsx1 is expressed in only a subset of the ventral telencephalic progenitors that normally express Gsx2. Based on the similarities in the expression of Gsx1 and Ascl1 (Mash1) within the Gsx2 mutant LGE, we examined whether Ascl1 plays an integral part in the Gsx1-based recovery. Ascl1 mutants show only modest alterations in striatal development; however, in Gsx2;Ascl1 double mutants, striatal development is severely affected, similar to that seen in the Gsx1;Gsx2 double mutants. This is despite the fact that Gsx1 is expressed, and even expands, in the Gsx2;Ascl1 mutant LGE, comparable to that seen in the Gsx2 mutant. Finally, Notch signaling has recently been suggested to be required for normal striatal development. In spite of the fact that Notch signaling is severely disrupted in Ascl1 mutants, it actually appears to be improved in the Gsx2;Ascl1 double mutants.

Conclusion

These results, therefore, reveal a non-proneural requirement of Ascl1 that together with Gsx1 compensates for the loss of Gsx2 in a subset of LGE progenitors.

Pax6 promotes neurogenesis in human neural stem cells

During brain embryogenesis, transcription factors drive stem cells towards neuronal fate. Here we show that the transcription factor Pax6 increased in vitro generation of neurons from striatal but not cortical neural stem cells (NSCs), derived from 6 to 9 weeks old human fetuses, without affecting survival and proliferation. Overexpression of mouse Pax6 produced increased numbers of GABA+ and DARPP-32+ (characteristic of striatum) but not glutamate+ neurons (characteristic of cortex). Pax6-overexpressing cells survived and migrated to the same extent as control cells at 1 month after intrastriatal transplantation into newborn rats and generated more neuroblasts. Overexpression of mouse Pax6 in human NSCs also leads to altered levels of lineage-appropriate genes as revealed by Q-PCR. Our data suggest that Pax6 function is conserved between species since its overexpression activates similar genes in mouse and human NSCs. Also, that Pax6 overexpression in striatal NSCs increases the number of neurons but their region-specificity is maintained.

Reprogramming of neonatal SVZ progenitors by islet-1 and neurogenin-2

The subventricular zone (SVZ) lining the lateral walls of the lateral ventricles is one of the major neurogenic areas in the postnatal brain. Precursor cells in the SVZ migrate via the rostral migratory stream to the olfactory bulb where they differentiate into neurons. Cell replacement strategies utilizing the recruitment of these endogenous progenitors and their progeny to different areas of the brain hold great promise for the future, but much research is needed in order to understand the sequence of molecular signals necessary to induce proliferation, migration and site-specific differentiation of these cells. In this study we show that the SVZ cells can be redirected from their normal migration route and directed towards other brain regions when they are infected with retroviruses encoding the developmentally important transcription factors Islet-1 and Neurogenin-2. After co-transduction with these transcription factors, transduced cells could be detected in several areas of the brain. When located in the striatum, the reprogrammed cells displayed neuroblast-like morphology. Once removed from the striatal parenchyma and allowed to further differentiation in vitro they developed into beta-III-tubulin positive neurons.

Expression patterns of the three Teashirt-related genes define specific boundaries in the developing and postnatal mouse forebrain

We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1-positive cell population is part of the DLX-positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1+ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1+ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain.

Ctip2 controls the differentiation of medium spiny neurons and the establishment of the cellular architecture of the striatum

Striatal medium spiny neurons (MSN) are critically involved in motor control, and their degeneration is a principal component of Huntington's disease. We find that the transcription factor Ctip2 (also known as Bcl11b) is central to MSN differentiation and striatal development. Within the striatum, it is expressed by all MSN, although it is excluded from essentially all striatal interneurons. In the absence of Ctip2, MSN do not fully differentiate, as demonstrated by dramatically reduced expression of a large number of MSN markers, including DARPP-32, FOXP1, Chrm4, Reelin, MOR1 (mu-opioid receptor 1), glutamate receptor 1, and Plexin-D1. Furthermore, MSN fail to aggregate into patches, resulting in severely disrupted patch-matrix organization within the striatum. Finally, heterotopic cellular aggregates invade the Ctip2-/- striatum, suggesting a failure by MSN to repel these cells in the absence of Ctip2. This is associated with abnormal dopaminergic innervation of the mutant striatum and dramatic changes in gene expression, including dysregulation of molecules involved in cellular repulsion. Together, these data indicate that Ctip2 is a critical regulator of MSN differentiation, striatal patch development, and the establishment of the cellular architecture of the striatum.

Group I mGluR5 metabotropic glutamate receptors regulate proliferation of neuronal progenitors in specific forebrain developmental domains

Major classical neurotransmitters including GABA and glutamate play novel morphogenic roles during development of the mammalian CNS. During forebrain neurogenesis, glutamate regulates neuroblast proliferation in different germinal domains using receptor subtype-specific mechanisms. For example, ionotropic N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors mediate distinct proliferative effects in ventral or dorsal forebrain germinal domains, and regulate the correct number of neurons that populate the striatum or cerebral cortex. Recent work suggests metabotropic receptors may also mediate glutamate's proliferative effects. Group I mGluR5 receptor subtypes are highly expressed in forebrain germinal zones. Using in vitro and in vivo methods, we demonstrate mGluR5 receptor activation plays an important role in neuroblast proliferation in the ventral telencephalon, and helps determine the complement of striatum projection neurons. mGluR5 receptor-mediated effects on striatal neuronal progenitors are restricted mainly to early cycling populations in the ventricular zone, with little effect on secondary proliferative populations in the subventricular zone. In contrast to proliferative effects in the ventral telencephalon, mGluR5 receptors do not modulate proliferation of dorsal telencephalon-derived cortical neuroblasts. Heterogeneous domain-specific proliferative effects of glutamate-mediated by specific receptor subtypes provide an important developmental mechanism allowing generation of the correct complement of neuronal subtypes that populate the mammalian forebrain.

Cell transplantation for Huntington's disease

Cell transplantation for Huntington's disease has developed over the last decade to clinical application in pilot trials in the USA, France and the UK. Although the procedures are feasible, and under appropriate conditions safe, evidence for efficacy is still limited, which has led to some calls that further development should be discontinued. We review the background of striatal cell transplantation in experimental animal models of Huntington's disease and the rationale for applying similar strategies in the human disease, and we survey the present status of the preliminary studies that have so far been undertaken in patients. When we consider the variety of parameters and principles that remain poorly defined -- such as the optimal source, age, dissection, preparation, implantation, immunoprotection and assessment protocols -- it is not surprising that clinical efficacy is still unreliable. However, since these protocols are all tractable to experimental refinement, we consider that the potential for cell transplantation in Huntington's disease is greater than has yet been realised, and remains a therapeutic strategy worthy of investigation and pursuit.

Stem cell transplantation for Huntington's disease

By way of commentary on a recent report that transplanted adult neural progenitor cells can alleviate functional deficits in a rat lesion model of Huntington's disease [Vazey, E.M., Chen, K., Hughes, S.M., Connor, B., 2006. Transplanted adult neural progenitor cells survive, differentiate and reduce motor function impairment in a rodent model of Huntington's disease. Exp. Neurol. 199, 384-396], we review the current status of the field exploring the use of stem cells, progenitor cells and immortalised cell lines to repair the lesioned striatum in animal models of the human disease. A remarkably rich range of alternative cell types have been used in various animal models, several of which exhibit cell survival and incorporation in the host brain, leading to subsequent functional recovery. In comparing the alternatives with the 'gold standard' currently offered by primary tissue grafts, key issues turn out to be: cell survival, differentiation prior to and following implantation into striatal-like phenotypes, integration and connectivity with the host brain, the nature of the electrophysiological, motor and cognitive tests used to assess functional repair, and the mechanisms by which the grafts exert their function. Although none of the alternatives yet has the capacity to match primary fetal tissues for functional repair, that standard is itself limited, and the long term goal must be not just to match but to surpass present capabilities in order to achieve fully functional reconstruction reliably, flexibly, and on demand.

Development of DARPP-32-positive parts of fetal pig ganglionic eminence and ventral mesencephalon in organotypic slice co-cultures

Neurons from the fetal pig dopaminergic ventral mesencephalon (VM) and basal ganglia anlage (the ganglionic eminence) were co-cultured as organotypic slice cultures to study the development of the two interconnected brain areas. During a short developmental period (E35-E42), a groove separates the ganglionic eminence into a lateral and a medial part. This was used (a) to study the developmental expression of the striatal marker protein, dopamine and adenosine 3,5-monophosphate regulated phospho-protein (DARPP-32) in the two parts and (b) to compare innervations of the two parts by tyrosine hydroxylase (TH)-positive, dopaminergic fibers from co-cultured slices of the ventral mesencephalon. DARPP-32 expression was more extensive and dense in cultures of the lateral part of the striatal anlage than the medial part. The DARPP-32-positive areas moreover overlapped with areas rich in acetylcholine esterase (AChE) and were the preferred target areas for TH-positive fibers from the co-cultured VM.

Ex vivo gene delivery of GDNF using primary astrocytes transduced with a lentiviral vector provides neuroprotection in a rat model of Parkinson's disease

Astrocytes are, as normal constituents of the brain, promising vehicles for ex vivo gene delivery to the central nervous system. In the present study, we have used a lentiviral vector encoding glial cell line-derived neurotrophic factor (GDNF) to transduce rat-derived primary astrocytes, in order to evaluate their potential for long-term transgene expression in vivo and neuroprotection in a rat model of Parkinson's disease. Following transplantation of GDNF-transduced astrocytes to the intact striatum, the level of released GDNF was 2.93 +/- 0.28 ng/mg tissue at 1 week post-grafting, reduced to 0.42 +/- 0.12 ng/mg tissue at 4 weeks, and thereafter was maintained at this level throughout the experiment (12 weeks; 0.53 +/- 0.068 ng/mg tissue). Similarly, grafting to the substantia nigra (SN) resulted in a significant overexpression of GDNF ( approximately 0.20 ng/mg tissue) at 1 week. Intact animals receiving transplants of GDNF-transduced astrocytes displayed an increased contralateral turning (5.39 +/- 1.19 turns/min) in the amphetamine-induced rotation test, which significantly correlated with the GDNF tissue levels measured in the striatum, indicating a stimulatory effect of GDNF on the dopaminergic function. Transplantation of GDNF-transduced astrocytes to the SN 1 week prior to an intrastriatal 6-hydroxydopamine lesion provided a significant protection of nigral tyrosine hydroxylase-positive cells. By contrast, when the cells were transplanted to the striatum, the level of released GDNF was not sufficient to rescue the striatal fibers and, hence, to protect the nigral dopaminergic neurons. Overall, our results suggest that genetically modified astrocytes expressing GDNF can provide neuroprotection in a rat model of Parkinson's disease following transplantation to the SN.

Migration of cells from primary transplants of allo- and xenografted foetal striatal tissue in the adult rat brain

Primary neural cells derived from human xenografts migrate extensively following transplantation into the adult rat CNS. However, it is unknown whether cells from allografts have the same capability to migrate within the adult rat brain. Moreover, it is unclear whether human-derived cells migrate to this extent as an inherent property of being in a xenograft environment, or whether it is due to the large size of the developed human brain compared with the adult rat brain. In order to address these issues we have designed an experimental paradigm to investigate the potential for cells derived from grafts of primary rat, mouse and human foetal striatal tissue to migrate following intrastriatal transplantation in an adult rat model of Huntington's disease (HD). Green fluorescent protein (GFP)-expressing rat and mouse donors and an antibody specific to human nuclear antigen enabled identification of graft-derived cells within the host brain, and double-labelling with GFP and neuronal nuclear antigen or immunostaining with human-specific tau identified graft-derived neurons. Twelve weeks post-transplantation, cells had migrated throughout the host in all groups; however, human cells and neurons had migrated significantly more than rat or mouse cells. These results demonstrate that neural cells derived from allografts are capable of migrating in the adult rat CNS and that the extent of migration is most likely determined by the size of the mature donor adult brain. This has important implications for the use of allo- and xenogeneic tissue as a source for transplantation in treating diffuse neurodegenerative disorders such as HD.

Striatal neuron differentiation from neurosphere-expanded progenitors depends on Gsh2 expression

Neural stem and progenitor cells from the embryonic forebrain can be expanded under growth factor stimulation in vitro, either as free-floating aggregates called neurospheres or as attached monolayer cultures. We have previously shown that despite the maintenance of important regulatory genes such as Gsh2, in vitro expansion of cells from the lateral ganglion eminence (LGE) restricts their differentiation potential. Specifically, their ability to differentiate into striatal projection neurons is compromised. It is not clear whether this restriction is caused by loss of progenitors with the ability to generate striatal projection neurons or whether the restricted differentiation potential is caused by factors lacking during in vitro differentiation. To address this, we have set up an in vitro system, in which expanded LGE-derived cells are differentiated in coculture with primary cells isolated from different regions of the embryonic brain. We provide evidence that the primary cells supply the expanded cells with contact-mediated region-specific developmental cues. Neurosphere-expanded LGE progenitors can, when presented with these cues, differentiate into neurons with characteristics of striatal projection neurons. Furthermore, we show that the ability of the expanded LGE cells to respond to the developmental cues presented by the primary cells depends on the maintained expression of Gsh2 in the expanded cells.

CNTF/LIF/gp130 receptor complex signaling maintains a VZ precursor differentiation gradient in the developing ventral forebrain

The extrinsic signaling pathways responsible for the formation and maintenance of the unique laminar organization of the forebrain germinal zones are largely unknown. In the present study, we asked whether ciliary neurotrophic factor (CNTF)/leukemia inhibitory factor (LIF)/gp130 signaling plays a role in the development of the germinal layers in the lateral ganglionic eminence. We found that CNTF/LIF/gp130 receptor signaling promotes the self-renewal/expansion of a subpopulation of fibroblast growth factor-responsive ventricular zone (VZ) precursors in the ventral forebrain. Analysis of Lifr-/- mice suggests that CNTF/LIF/gp130 signaling maintains a subpopulation of GSH2+ VZ precursors, which are necessary for normal growth of the early ventral forebrain and for maintaining a gradient of VZ precursor differentiation in the lateral ganglionic eminence, as defined by GSH2, MASH1 and DLX2 expression. Furthermore, addition of exogenous CNTF to embryonic forebrain explant cultures deprived of choroid plexus-derived CNTF, was sufficient to promote a VZ differentiation gradient. In contrast to the forebrain, CNTF/LIF/gp130 signaling reduced, rather than enhanced, precursor self-renewal/expansion in the spinal cord. These results demonstrate a novel region-specific role for CNTF/LIF/gp130 signaling in the development of the germinal layers of the embryonic telencephalon.

Cell therapy in Huntington's disease

Huntington's disease is an autosomal dominant genetic disease, which results in progressive neuronal degeneration in the neostriatum and neocortex, and associated functional impairments in motor, cognitive, and psychiatric domains. Although the genetic mutation is identified, involving an abnormal CAG expansion within the htt gene on chromosome 4, the mechanism by which this leads to neuronal cell death and the question of why striatal neurones are targeted both remain unknown. Thus, in addition to the search for molecular and genetic strategies to inhibit development of the disease, we still need to identify effective strategies for cellular repair in affected individuals. Aspects of the human neuropathology can be well modeled by excitotoxic or metabolic lesions in experimental animals, and in transgenic mice carrying the htt mutation, providing the basis for testing alternative therapeutic strategies. The rationale and efficacy of alternative cell therapies are reviewed, including transplantation repair with embryonic striatal tissues, expansion and differentiation of striatal-like cells from stem cells, and in vivo and ex vivo gene therapy for delivery of neuroprotective growth factor molecules. Pilot and experimental clinical trials of several approaches are now also underway, and the alternative strategies are compared.

Defective neuronogenesis in the absence of Dlx5

Dlx genes play an important role in the control of the development of the central nervous system (CNS). Single or compound inactivation of Dlx1, Dlx2, or Dlx5 in the mouse causes defects of neuronal migration and differentiation. Dlx5, in particular, is essential for the correct development of the olfactory system. Targeted inactivation of Dlx1 and Dlx2 in the mouse results in abnormal neuronal differentiation in the embryonic subcortical forebrain and is associated to the loss of Dlx5 and Dlx6 expression. So far, however, it has been impossible to investigate the role of Dlx genes on late neurogenesis, as their inactivation leads to perinatal death. We have now generated cultures of neural stem cells (NSCs) derived from embryonic and newborn Dlx5-null mice, and we have compared their capacity to differentiate in vitro to that of equivalent cells derived from normal littermates. We show here that in the absence of Dlx5, NSCs derived from newborn animals have a severely reduced capacity to generate neurons. This is not the case for cells derived from E12.5 embryos. Forced expression of Dlx5 in cultures of newborn mutant NSCs fully restores their neuronogenic potential. Our data suggest that Dlx5 is essential for secondary (postnatal) neuronogenesis.

Neurogenesis and stereological morphometry of calretinin-immunoreactive GABAergic interneurons of the neostriatum

We determined the neurogenesis characteristics of a distinct subclass of rat striatum gamma-aminobutyric acidergic (GABAergic) interneurons expressing the calcium-binding protein calretinin (CR). Timed-pregnant rats were given an intraperitoneal injection of 5-bromo-2'-deoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day 12 (E12) and E21. CR-immunoreactive (-IR) neurons and BrdU-positive nuclei were labeled in the adult neostriatum by double immunohistochemistry, and the proportion of double-labeled cells was quantified. CR-IR interneurons of the neostriatum show maximum birth rates (>10% double labeling) between E14 and E17, with a peak at E15. CR-IR interneurons occupying the lateral half of the neostriatum become postmitotic prior to medial neurons. In the precomissural neostriatum, the earliest-born neurons occupy the lateral quadrants and the latest-born neurons occupy the dorsomedial sector. No significant rostrocaudal neurogenesis gradient is observed. CR-IR neurons make up 0.5% of the striatal population and are localized in both the patch and the matrix compartments. CR-IR neurons of the patch compartment are born early (E13-15), with later-born neurons (E16-18) populating mainly the matrix compartment. CR-IR cells of the neostriatum are a distinct subclass of interneurons that are born at an intermediate time during striatal development and share common neurogenesis characteristics with other interneurons and projection neurons produced in the ventral telencephalon.

Tlx controls proliferation and patterning of lateral telencephalic progenitor domains

We showed previously that the orphan nuclear receptor Tlx is required for the correct establishment of the pallio-subpallial boundary. Loss of Tlx results in a dorsal expansion of ventral markers (e.g., the homeodomain protein GSH2) into the ventralmost pallial region, i.e., the ventral pallium. We also observed a disproportionate reduction in the size of the Tlx mutant lateral ganglionic eminence (LGE) from embryonic day 14.5 onward. Here we show that this reduction is caused, at least in large part, by a proliferation defect. Interestingly, in Tlx mutants, the LGE derivatives are differentially affected. Although the development of the Tlx mutant striatum is compromised, an apparently normal number of olfactory bulb interneurons are observed. Consistent with this observation, we found that Tlx is required for the normal establishment of the ventral LGE that gives rise to striatal projection neurons. This domain is reduced by the dorsal and ventral expansion of molecular markers normally confined to progenitor domains flanking the ventral LGE. Finally, we investigated possible genetic interactions between Gsh2 and Tlx in lateral telencephalic development. Our results show that, although Gsh2 and Tlx have additive effects on striatal development, they differentially regulate the establishment of ventral pallial identity.

Neuronal differentiation following transplantation of expanded mouse neurosphere cultures derived from different embryonic forebrain regions

In vitro, expanded neurospheres exhibit multipotent properties and can differentiate into neurons, astrocytes and oligodendrocytes. In vivo, cells from neurospheres derived from mouse fetal forebrain have previously been reported to predominantly differentiate into glial cells, and not into neurons. Here we isolated stem/progenitor cells from E13.5 lateral ganglionic eminence (LGE), medial ganglionic eminence (MGE) and cortical primordium, of a green fluorescent protein (GFP)-actin transgenic mouse. Free-floating neurospheres were expanded in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) and implanted after five to six passages into the striatum, hippocampus and cortex of neonatal rats. Cell suspensions of primary LGE tissue were prepared and grafted in parallel. Grafted cells derived from the primary tissue displayed widespread incorporation into all regions, as visualized with the mouse-specific antibody M2, or mouse satellite DNA in situ hybridization, and differentiated into both neurons, astrocytes and oligodendrocytes. Grafts of neurosphere cells derived from the LGE, MGE and cortical primordium differentiated primarily into astrocytes, but contained low but significant numbers of GFP-immunoreactive neurons. Neurons derived from LGE neurospheres were of three types: cells with the morphology of medium-sized densely spiny projection neurons in the striatum; cells with interneuron-like morphologies in striatum, cortex and hippocampus; and cells integrating into SVZ and migrating along the RMS to the olfactory bulb. MGE- or cortical primordium-derived neurospheres differentiated into interneuron-like cells in both striatum and hippocampus. The results demonstrate the ability of in vitro expanded neural stem/progenitor cells to generate both neurons and glia after transplantation into neonatal recipients, and differentiate in a region-specific manner into mature neurons with morphological features characteristic for each target site.

Phenotypic and molecular identity of cells in the adult subventricular zone

We have studied the molecular identity of adult mouse SVZ cells in situ, and after isolation and expansion as neurospheres in vitro. The gene and protein expression patterns of the adult cells have been compared to that of the cells from the lateral ganglionic eminence (LGE), their putative embryonic counterparts. The LGE gives rise to both striatal projection neurons and olfactory bulb interneurons via spatially and molecularly distinct progenitor populations present in the SVZ of the LGE. These two populations are thought to have a common origin in the GSH2 expressing cells of the embryonic LGE ventricular zone. We found that a significant number of cells in the adult SVZ, and in the in vitro expanded neurospheres, derived from the adult SVZ express GSH2. However, under normal conditions, GSH2-expressing cells in the adult SVZ and in the in vitro expanded neurospheres appear to specify only olfactory bulb progenitors and not striatal progenitors.

The differentiation potential of precursor cells from the mouse lateral ganglionic eminence is restricted by in vitro expansion

We have investigated whether the differentiation potential of attached cultures derived from the mouse lateral ganglionic eminence (LGE) is influenced by in vitro expansion. Primary neuronal cultures derived from the LGE give rise to neurons expressing the striatal projection neuron markers Islet1 (ISL1) and dopamine and cAMP-regulated phosphoprotein of 32 kilodaltons (DARPP-32) as well as the olfactory bulb interneuron marker Er81. Our previous results showed that after expansion in vitro, LGE precursor cells can be induced to differentiate into neurons which exhibit molecular characteristics of the LGE, such as the homeobox transcription factors DLX and MEIS2. We show here that while attached LGE cultures maintain Er81 expression through five passages, they lose the ability to generate ISL1- or dopamine and cAMP-regulated phosphoprotein of 32 kilodaltons-expressing neurons already after the first passage. This indicates that the expansion of LGE precursor cells restricts their differentiation potential in vitro. Interestingly, the undifferentiated LGE cultures retain the expression of both the Isl1 and Er81 genes, suggesting that precursor cells for both striatal projection neurons and olfactory bulb interneurons are present in these cultures. Thus the restriction in differentiation potential of the expanded LGE cultures likely reflects deficiencies in the differentiation conditions used.

Signaling to and from radial glia

Radial glia represent the major glial cell type in the developing CNS and perform many essential functions, which range from acting as neural precursors to providing physical substrates for newborn neurons to migrate on. Previous work has shown that cell-cell signaling is important for the development of the radial glial phenotype. In particular, signals from newborn neurons appear to contribute significantly to the formation of this cell type. In addition, radial glia may be involved in reciprocal signaling roles that contribute to regional patterning and differentiation in the developing CNS.

Identification of two distinct progenitor populations in the lateral ganglionic eminence: implications for striatal and olfactory bulb neurogenesis

The lateral ganglionic eminence (LGE) is known to give rise to striatal projection neurons as well as interneurons, which migrate in the rostral migratory stream (RMS) to populate the granule cell and glomerular layers of the olfactory bulb. Because all of these neuronal subtypes express Distalless-related (DLX) homeobox proteins during their differentiation, we set out to further characterize progenitors in the Dlx-positive domain of the LGE. Previous studies have shown that the LIM homeobox protein Islet1 (ISL1) marks the LGE subventricular zone (SVZ) and differentiating striatal projection neurons. However, ISL1 is not expressed in neurons of the developing olfactory bulb or the RMS. We show here that the dorsal-most portion of the Dlx-expressing region of the LGE SVZ lacks ISL1 cells. This dorsal domain, however, contains cells that express the ETS transcription factor Er81, which is also expressed in granule and periglomerular cells of the developing and adult olfactory bulb. Moreover, the adult SVZ and RMS contain numerous Er81-positive cells. Fate-mapping studies using Dlx5/6-cre transgenic mice demonstrate that Er81-positive cells in the granule cell and glomerular layers of the olfactory bulb derive from the Dlx-expressing SVZ region. These findings suggest that the LGE SVZ contains two distinct progenitor populations: a DLX(+);ISL1(+) population representing striatal progenitors and a DLX(+);Er81(+) population comprising olfactory bulb interneuron progenitors. In support of this, mice mutant for the homeobox genes Gsh2 and Gsh1/2, which show olfactory bulb defects, exhibit dramatically reduced numbers of Er81-positive cells in the LGE SVZ as well as in the olfactory bulb mantle.

Permissive corridor and diffusible gradients direct medial ganglionic eminence cell migration to the neocortex

Young neurons born in the medial ganglionic eminence (MGE) migrate a long distance dorsally, giving rise to several types of interneurons in neocortex. The mechanisms that facilitate selective dorsal dispersion of MGE cells while restricting their movement ventrally into neighboring regions are not known. Using microtransplantation into fetal brain slices and onto dissociated substrate cells on floating filters (spot assay), we demonstrate that ventral forebrain regions neighboring the MGE are nonpermissive for MGE cell migration, whereas the dorsal regions leading to the neocortex are increasingly permissive. Spot assay experiments using filters with different pore sizes indicate that the permissive factors are not diffusible. We also show that MGE cells respond to chemoattractive and inhibitory factors diffusing from the neocortex and ventromedial forebrain, respectively. We propose that the final extent and regional specificity of MGE cell dispersion is largely dictated by contact guidance through a selectively permissive environment, flanked by nonpermissive tissues. In addition, we propose that chemotactic guidance cues superimposed over the permissive corridor facilitate efficient dorsal migration of MGE cells.

Roles of the mammalian subventricular zone in brain development

There has been enormous progress in uncovering the contributions of the subventricular zone (SVZ) to the developing brain. Here, we review the roles of four anatomically defined embryologic divisions of the SVZ of the mammalian brain: the lateral ganglionic eminence (LGE), the medial ganglionic eminence (MGE), the caudal ganglionic eminence (CGE), and the fetal neocortical SVZ (SVZn), as well as the roles of the two major anatomically defined regions of the postnatal SVZ, the anterior SVZ (SVZa) and the dorsolateral SVZ (SVZdl). We describe the types of cells within each subdivision of the SVZ, the types of brain cells that they generate during embryonic, fetal, and perinatal development, and when known the mechanisms that regulate their differentiation. This review provides a critical analysis of the literature, from which current and future studies on the SVZ can be formulated and evaluated.

The caudal ganglionic eminence is a source of distinct cortical and subcortical cell populations

During development, the mammalian ventral telencephalon is comprised of three major proliferative zones: the medial (MGE), lateral (LGE) and caudal (CGE) ganglionic eminences. Through gene expression studies, in vitro migration assays, genetic mutant analysis and in vivo fate mapping in mice, we found that the CGE is a progenitor region that is distinct from both the MGE and LGE. Notably, CGE cells showed a unique in vivo pattern of migration, and the CGE contributed cells to nuclei distinct from those populated by the MGE and LGE. Moreover, we report that the migratory fate of cells from the CGE is intrinsically determined by embryonic day 13.5 (E13.5). Together, these results provide the first insights into the development and fate of the CGE.

Ex vivo and in vitro studies of transgene expression in rat astrocytes transduced with lentiviral vectors

Implantation of cells genetically modified to express therapeutic genes into the brain has been proposed as a potential treatment for neurodegenerative diseases. In the current study embryonic rat-derived astrocytes were cultured and transduced with a lentiviral vector expressing the reporter gene green fluorescent protein (GFP) and subsequently grafted into the adult rat brain. The proportion of GFP expressing cells was stable, albeit small (1%), at all survival times, up to 6 weeks, the longest time point studied. In parallel in vitro studies, the astrocytes were lentivirally transduced to express either one of the two isoforms of glutamate decarboxylase (GAD(65) or GAD(67)) or glial cell line-derived neurotrophic factor (GDNF). When transducing 293T cells with the two GAD vectors, released GABA could be measured using high-performance liquid chromatography. Further studies of rat astrocytes transduced with the same vectors resulted in a level of GAD activity about 10 times higher than the activity of an intact rat striatum. One hundred thousand astrocytes transduced with LV-GDNF released approximately 27 ng of GDNF per hour. Thus, taken together, our observations provide support for the use of rat astrocytes in ex vivo gene transfer of these proteins in animal models of CNS disorders, e.g., Parkinson's disease or epilepsy.

The transcription factor neurogenin 2 restricts cell migration from the cortex to the striatum

The dorsal and ventral domains of the telencephalon are delineated by a unique boundary structure that restricts the migration of dorsal and ventral cells to a different extent. While many cells invade the dorsal cortex from the ventral ganglionic eminence (GE), hardly any cortical cells cross the boundary into the GE. Several molecules have been implicated in the regulation of ventral to dorsal cell migration, but so far nothing is known about the molecular mechanisms restricting cortical cell migration in vivo. Here we show that in the absence of the transcription factor neurogenin 2, cells from the cortex migrate into the GE in vitro and in vivo as detected in transgenic mice containing a lacZ gene in the neurogenin 2 locus. In contrast, the migration of cells from the GE is not affected. Molecular and cellular analysis of the cortico-striatal boundary revealed that neurogenin 2 regulates the fasciculation of the cortico-striatal boundary which may explain the non cell-autonomous nature of the migration defect as detected by in vitro transplantation. Taken together, these results show that distinct cues located in the cortico-striatal boundary restrict cells in the dorsal and ventral telencephalon.

A role for Gsh1 in the developing striatum and olfactory bulb of Gsh2 mutant mice

We have examined the role of the two closely related homeobox genes Gsh1 and Gsh2, in the development of the striatum and the olfactory bulb. These two genes are expressed in a partially overlapping pattern by ventricular zone progenitors of the ventral telencephalon. Gsh2 is expressed in both of the ganglionic eminences while Gsh1 is largely confined to the medial ganglionic eminence. Previous studies have shown that Gsh2–/– embryos suffer from an early misspecification of precursors in the lateral ganglionic eminence (LGE) leading to disruptions in striatal and olfactory bulb development. This molecular misspecification is present only in early precursor cells while at later stages the molecular identity of these cells appears to be normalized. Concomitant with this normalization, Gsh1 expression is notably expanded in the Gsh2–/– LGE. While no obvious defects in striatal or olfactory bulb development were detected in Gsh1–/– embryos, Gsh1/2 double homozygous mutants displayed more severe disruptions than were observed in the Gsh2 mutant alone. Accordingly, the molecular identity of LGE precursors in the double mutant is considerably more perturbed than in Gsh2 single mutants. These findings, therefore, demonstrate an important role for Gsh1 in the development of the striatum and olfactory bulb of Gsh2 mutant mice. In addition, our data indicate a role for Gsh genes in controlling the size of the LGE precursor pools, since decreasing copies of Gsh2 and Gsh1 alleles results in a notable decrease in precursor cell number, particularly in the subventricular zone.

The subventricular zone: source of neuronal precursors for brain repair

The subventricular zone (SVZ) is a major germinal zone which persists in the adult brain. The SVZ contains cells that self renew and continuously produce new neurons and glia. In this chapter we discuss the development, architecture and function of the adult SVZ, as well as the fate of SVZ cells after transplantation. We focus on identification of neural stem cells, factors which regulate neurogenesis and mechanisms for neuronal migration through the adult brain. Detailed understanding of these processes is necessary to utilize the SVZ as a source of neuronal and glial precursors for genetic manipulation, transplantation or brain self repair.

Genetic control of dorsal-ventral identity in the telencephalon: opposing roles for Pax6 and Gsh2

We have examined the genetic mechanisms that regulate dorsal-ventral identity in the embryonic mouse telencephalon and, in particular, the specification of progenitors in the cerebral cortex and striatum. The respective roles of Pax6 and Gsh2 in cortical and striatal development were studied in single and double loss-of-function mouse mutants. Gsh2 gene function was found to be essential to maintain the molecular identity of early striatal progenitors and in its absence the ventral telencephalic regulatory genes Mash1 and Dlx are lost from most of the striatal germinal zone. In their place, the dorsal regulators, Pax6, neurogenin 1 and neurogenin 2 are found ectopically. Conversely, Pax6 is required to maintain the correct molecular identity of cortical progenitors. In its absence, neurogenins are lost from the cortical germinal zone and Gsh2, Mash1 and Dlx genes are found ectopically. These reciprocal alterations in cortical and striatal progenitor specification lead to the abnormal development of the cortex and striatum observed in Pax6 (small eye) and Gsh2 mutants, respectively. In support of this, double homozygous mutants for Pax6 and Gsh2 exhibit significant improvements in both cortical and striatal development compared with their respective single mutants. Taken together, these results demonstrate that Pax6 and Gsh2 govern cortical and striatal development by regulating genetically opposing programs that control the expression of each other as well as the regionally expressed developmental regulators Mash1, the neurogenins and Dlx genes in telencephalic progenitors.

Origin and molecular specification of striatal interneurons

The striatum, the largest component of the basal ganglia, contains projection neurons and interneurons. Whereas there is considerable agreement that the lateral ganglionic eminence (LGE) is the origin of striatal projection neurons, less is known about the origin of striatal interneurons. Using focal injections of retrovirus into the ventral telencephalon in vitro, we demonstrate that most striatal interneurons tangentially migrate from the medial ganglionic eminence (MGE) or the adjacent preoptic/anterior entopeduncular areas (POa/AEP) and express the NKX2.1 homeodomain protein. Although the majority of striatal interneurons (cholinergic, calretinin(+), and parvalbumin(+)) maintain the expression of NKX2.1 into adulthood, most of the interneurons expressing somatostatin (SOM), neuropeptide Y (NPY), and neural nitric oxide synthase (NOS) appear to downregulate the expression of NKX2.1 as they exit the neuroepithelium. Analysis of striatal development in mice lacking Nkx2.1 suggests that this gene is required for the specification of nearly all striatal interneurons. Similar analysis of mice lacking the Mash1 basic helix-loop-helix (bHLH) or both the Dlx1 and Dlx2 homeodomain transcription factors demonstrates that these genes are required for the differentiation of striatal interneurons. Mash1 mutants primarily have a reduction in early-born striatal interneurons, whereas Dlx1/2 mutants primarily have reduced numbers of late-born striatal interneurons. We also present evidence implicating the Lhx6 and Lhx7 LIM-homeobox genes in the development of distinct interneuron subtypes. Finally, we hypothesize that, within the MGE, radially migrating cells generally become projection neurons, whereas tangentially migrating cells mainly form interneurons of the striatum and cerebral cortex.

Failure of neuroprotection by embryonic striatal grafts in a double lesion rat model of striatonigral degeneration (multiple system atrophy)

In the present experiment we studied the ability of embryonic striatal grafts to protect against striatal quinolinic acid (QA)-induced excitotoxicity in a previously established double lesion rat model of striatonigral degeneration (SND), the neuropathological substrate of parkinsonism associated with multiple system atrophy (MSA). Male Wistar rats received under halothane inhalation anesthesia a 6-hydroxydopamine 6-OHDA injection into the left medial forebrain bundle. Four to 5 weeks later apomorphine-induced rotation behavior was tested. Rats were divided into two treatment groups receiving either embryonic striatal cell suspensions or sham injections. Apomorphine-induced rotation behavior was retested 2 and 4 weeks after the grafting procedure. Following the rotation test animals of the striatal and sham graft group received a stereotaxic injection of 150 nmol QA. Again rotation behavior was assessed 2 and 4 weeks after lesioning. Brains were then processed to dopamine reuptake ([(3)H]mazindol), dopamine D1 ([(3)H]SCH23390), and D2 ([(3)H]spiperone) receptor autoradiography. Gliosis was detected using [(3)H]PK11195, a marker for peripheral benzodiazepine binding sites. Behavioral and autoradiographic analysis failed to show striatal protection in 6-OHDA prelesioned animals receiving embryonic striatal grafts. These findings indicate that beneficial protective effects of striatal grafts implanted into host striatum prior to excitotoxic insults are abolished in the presence of severe dopaminergic denervation. Our present results are relevant to future applications of neural grafting in MSA-SND.

Long-term, EGF-stimulated cultures of attached GFAP-positive cells derived from the embryonic mouse lateral ganglionic eminence: in vitro and transplantation studies

Long-term attached cultures, prepared from mouse embryonic days 15-17 lateral ganglionic eminence, were grown in a medium including epidermal growth factor and serum, and the survival, differentiation, and migration of cells from either early or late passages were analyzed following transplantation. The cultured cells had the morphology of type I astroglial cells, with the vast majority of the cells immunoreactive for glial fibrillary acidic protein (around 90%), the intermediate filament marker nestin, and also the mouse-specific neural markers M2 and M6. The cultures were kept over 25 passages (7 months). During the first 8 passages, the growth rate gradually declined, but it increased again after passage 9 and thereafter stabilized at values similar to those observed during the initial culture period. After passages 4-6 and 18, cell suspensions were implanted cross-species into the intact or lesioned striatum of adult (passages 4-5 only) or intact striatum of neonatal rats (passages 4-6 or 18). Both early and late passage cells formed M2 (and M6)-positive transplants. In the neonatal recipients, widespread migration was seen from the needle tract throughout most of the striatum, along the internal capsule, and into the globus pallidus. In the adult striatum, the cells remained mostly around the injection tract, or within 0.4-0.6 mm from the graft core. These long-term attached cultures are interesting to compare to nonattached neurosphere cultures, and might also offer a means of propagating relatively pure populations of astroglia-like cells for basic transplantation studies or for use in experimental trials with ex vivo gene transfer.

Fetal tissue transplants in animal models of Huntington's disease: the effects on damaged neuronal circuitry and behavioral deficits

Accumulating evidence indicates that grafts of embryonic neurons achieve the anatomical and functional reconstruction of damaged neuronal circuitry. The restorative capacity of grafted embryonic neural tissue is most illustrated by studies with striatal tissue transplantation in animals with striatal lesions. Striatal neurons implanted into the lesioned striatum receive some of the major striatal afferents such as the nigrostriatal dopaminergic inputs and the gluatmatergic afferents from the neocortex and thalamus. The grafted neurons also send efferents to the primary striatal targets, including the globus pallidus (GP, the rodent homologue of the external segment of the globus pallidus) and the entopeduncular nucleus (EP, the rodent homologue of the internal segment of the globus pallidus). These anatomical connections provide the reversal of the lesion-induced alterations in neuronal activities of primary and secondary striatal targets. Furthermore, intrastriatal striatal grafts improve motor and cognitive deficits seen in animals with striatal lesions. Since the grafts affect motor and cognitive behaviors that are critically dependent on the integrity of neuronal circuits of the basal ganglia, the graft-mediated recovery in these behavioral deficits is most likely attributable to the functional reconstruction of the damaged neuronal circuits. The fact that the extent of the behavioral recovery is positively correlated to the amount of grafted neurons surviving in the striatum encourages this view. Based on the animal studies, embryonic striatal tissue grafting could be a viable strategy to alleviate motor and cognitive disorders seen in patients with Huntington's disease where massive degeneration of striatal neurons occurs.

Expression of Meis and Pbx genes and their protein products in the developing telencephalon: implications for regional differentiation

The Meis and Pbx genes encode for homeodomain proteins of the TALE class and have been shown to act as co-factors for other homeodomain transcription factors (Mann and Affolter, 1998. Curr. Opin. Genet. Dev. 8, 423-429). We have studied the expression of these genes in the mouse telencephalon and found that Meis1 and Meis2 display region-specific patterns of expression from embryonic day (E)10.5 until birth, defining distinct subterritories in the developing telencephalon. The expression of the Meis genes and their proteins is highest in the subventricular zone (SVZ) and mantle regions of the ventral telencephalon. Compared to the Meis genes, Pbx genes show a broader expression within the telencephalon. However, as is the case in Drosophila (Rieckhof et al., 1997. Cell 91, 171-183; Kurrant et al., 1998. Development 125, 1037-1048; Pai et al., 1998. Genes Dev. 12, 435-446), nuclear localized PBX proteins were found to correlate highly with Meis expression. In addition, DLX proteins co-localize with nuclear PBX in distinct regions of the ventral telencephalon.

The morphology, integration, and functional efficacy of striatal grafts differ between cell suspensions and tissue pieces

In order to develop a surgical protocol for use in clinical trials of striatal transplantation in Huntington's disease (HD), the issues involved in the preparation and implantation of the embryonic striatal tissue must be addressed. Rodent models of HD offer the best experimental paradigm with which to study various aspects of striatal transplantation. In this article we present the results of an investigation of the role of trypsin and the process of trituration in the preparation of cell suspensions compared to the use of solid pieces of tissue. The embryonic material was derived from the lateral ganglionic eminence (LGE) and implanted into the excitotoxically lesioned striatum of the host rats. Twelve weeks following implantation, retrograde tracing of projections from the graft to the globus pallidus was performed. Grafts derived from cell suspensions triturated in the presence of trypsin contained larger quantities of striatal tissue within the graft and more DARPP-32-positive medium spiny neurons than grafts implanted as fragments of tissue. Afferent and efferent connectivity was also better in the trypsinized suspension graft group. Modest recovery in paw reaching was observed contralateral to the grafted side in animals implanted with solid fragments of embryonic striatal tissue. No relationship was observed between functional effect and the graft anatomy. These results suggest that local graft host interaction may also be involved in graft-mediated functional recovery.

Embryonic donor age and dissection influences striatal graft development and functional integration in a rodent model of Huntington's disease

The method of embryonic dissection and the age of the donor material remain areas of controversy in the preparation of striatal tissue for intrastriatal implantation. This study explores the relationship between these two parameters with respect to the morphology, function, and physiological integration of the resultant grafts. Tissue derived from embryos of 14 and 16 days of gestation (CRL 10-11 and 14-15 mm, respectively) was prepared as whole, lateral, and medial ganglionic eminence suspensions (WGE, LGE, and MGE, respectively). The embryonic material was implanted into the excitotoxically lesioned striatum of host rats. Grafts derived from E14 LGE attenuated drug-induced rotational bias whereas grafts from E14 MGE ameliorated contralateral deficits in paw reaching. Six months after grafting retrograde tracing of graft projections to the globus pallidus was performed followed by electrical excitation of cortical afferent fibers. Grafts derived from E14 WGE had the largest volume of striatum-like tissue and more striatal neurons compared to LGE from the same donor age. These results suggest that MGE tissue as well as LGE plays a role in the structural and functional integration of striatal grafts.

Autoradiographic study of striatal dopamine re-uptake sites and dopamine D1 and D2 receptors in a 6-hydroxydopamine and quinolinic acid double-lesion rat model of striatonigral degeneration (multiple system atrophy) and effects of embryonic ventral mesencephalic, striatal or co-grafts

The influence of embryonic mesencephalic, striatal and mesencephalic/striatal co-grafts on amphetamine- and apomorphine-induced rotation behaviour was assessed in a rat model of multiple system atrophy/striatonigral degeneration type using dopamine D1 ([3H]SCH23390) and D2 ([3H]spiperone) receptor and dopamine re-uptake ([3H]mazindol) autoradiography. Male Wistar rats subjected to a sequential unilateral 6-hydroxydopamine lesion of the medial forebrain bundle followed by a quinolinic acid lesion of the ipsilateral striatum were divided into four treatment groups, receiving either mesencephalic, striatal, mesencephalic/striatal co-grafts or sham grafts. Amphetamine- and apomorphine-induced rotation behaviour was recorded prior to and up to 10 weeks following transplantation. 6-Hydroxydopamine-lesioned animals showed ipsiversive amphetamine-induced and contraversive apomorphine-induced rotation behaviour. Amphetamine-induced rotation rates persisted after the subsequent quinolinic acid lesion, whereas rotation induced by apomorphine was decreased. In 11 of 14 animals receiving mesencephalic or mesencephalic/striatal co-grafts, amphetamine-induced rotation scores were decreased by >50% at the 10-week post-grafting time-point. In contrast, only one of 12 animals receiving non-mesencephalic (striatal or sham) grafts exhibited diminished rotation rates at this time-point. Apomorphine-induced rotation rates were significantly increased following transplantation of mesencephalic, striatal or sham grafts. The largest increase of apomorphine-induced rotation rates approaching post-6-hydroxydopamine levels were observed in animals with striatal grafts. In contrast, in the co-graft group, there was no significant increase of apomorphine-induced rotation compared to the post-quinolinic acid time-point. Morphometric analysis revealed a 63-74% reduction of striatal surface areas across the treatment groups. Striatal [3H]mazindol binding on the lesioned side (excluding the demarcated graft area) revealed a marked loss of dopamine re-uptake sites across all treatment groups, indicating missing graft-induced dopaminergic re-innervation of the host. In eight (73%) of the 11 animals with mesencephalic grafts and reduced amphetamine-induced circling, discrete areas of [3H]mazindol binding ("hot spots") were observed, indicating graft survival. Dopamine D1 and D2 receptor binding was preserved in the remaining lesioned striatum irrespective of treatment assignment, except for a significant reduction of D2 receptor binding in animals receiving mesencephalic grafts. "Hot spots" of dopamine D1 and D2 receptor binding were observed in 10 (83%) and nine (75%) of 12 animals receiving striatal grafts or co-grafts, consistent with survival of embryonic primordial striatum grafted into a severely denervated and lesioned striatum. Our study confirms that functional improvement may be obtained from embryonic neuronal grafts in a double-lesion rat model of multiple system atrophy/striatonigral degeneration type. Co-grafts appear to be required for reversal of both amphetamine- and apomorphine-induced rotation behaviour in this model. We propose that the partial reversal of amphetamine-induced rotation asymmetry in double-lesioned rats receiving mesencephalic or mesencephalic/striatal co-grafts reflects non-synaptic graft-derived dopamine release. The changes of apomorphine-induced rotation following transplantation are likely to reflect a complex interaction of graft- and host-derived striatal projection pathways and basal ganglia output nuclei. Further studies in a larger number of animals are required to determine whether morphological parameters and behavioural improvement in the neurotransplantation multiple system atrophy rat model correlate.