Effects of target deprivation on the morphology and survival of adult dorsal column nuclei neurons

Neurons require glucose uptake and glycolysis in vivo

Neurons require large amounts of energy, but whether they can perform glycolysis or require glycolysis to maintain energy remains unclear. Using metabolomics, we show that human neurons do metabolize glucose through glycolysis and can rely on glycolysis to supply tricarboxylic acid (TCA) cycle metabolites. To investigate the requirement for glycolysis, we generated mice with postnatal deletion of either the dominant neuronal glucose transporter (GLUT3cKO) or the neuronal-enriched pyruvate kinase isoform (PKM1cKO) in CA1 and other hippocampal neurons. GLUT3cKO and PKM1cKO mice show age-dependent learning and memory deficits. Hyperpolarized magnetic resonance spectroscopic (MRS) imaging shows that female PKM1cKO mice have increased pyruvate-to-lactate conversion, whereas female GLUT3cKO mice have decreased conversion, body weight, and brain volume. GLUT3KO neurons also have decreased cytosolic glucose and ATP at nerve terminals, with spatial genomics and metabolomics revealing compensatory changes in mitochondrial bioenergetics and galactose metabolism. Therefore, neurons metabolize glucose through glycolysis in vivo and require glycolysis for normal function.

Dynamin-related protein 1 is required for normal mitochondrial bioenergetic and synaptic function in CA1 hippocampal neurons

Disrupting particular mitochondrial fission and fusion proteins leads to the death of specific neuronal populations; however, the normal functions of mitochondrial fission in neurons are poorly understood, especially in vivo, which limits the understanding of mitochondrial changes in disease. Altered activity of the central mitochondrial fission protein dynamin-related protein 1 (Drp1) may contribute to the pathophysiology of several neurologic diseases. To study Drp1 in a neuronal population affected by Alzheimer's disease (AD), stroke, and seizure disorders, we postnatally deleted Drp1 from CA1 and other forebrain neurons in mice (CamKII-Cre, Drp1^lox/lox (Drp1cKO)). Although most CA1 neurons survived for more than 1 year, their synaptic transmission was impaired, and Drp1cKO mice had impaired memory. In Drp1cKO cell bodies, we observed marked mitochondrial swelling but no change in the number of mitochondria in individual synaptic terminals. Using ATP FRET sensors, we found that cultured neurons lacking Drp1 (Drp1KO) could not maintain normal levels of mitochondrial-derived ATP when energy consumption was increased by neural activity. These deficits occurred specifically at the nerve terminal, but not the cell body, and were sufficient to impair synaptic vesicle cycling. Although Drp1KO increased the distance between axonal mitochondria, mitochondrial-derived ATP still decreased similarly in Drp1KO boutons with and without mitochondria. This indicates that mitochondrial-derived ATP is rapidly dispersed in Drp1KO axons, and that the deficits in axonal bioenergetics and function are not caused by regional energy gradients. Instead, loss of Drp1 compromises the intrinsic bioenergetic function of axonal mitochondria, thus revealing a mechanism by which disrupting mitochondrial dynamics can cause dysfunction of axons.

Klf9 is necessary and sufficient for Purkinje cell survival in organotypic culture

During their phase of developmental programmed cell death (PCD), neurons depend on target-released trophic factors for survival. After this period, however, they critically change as their survival becomes target-independent. The molecular mechanisms underlying this major transition remain poorly understood. Here, we investigated, which transcription factors (TFs) might be responsible for the closure of PCD. We used Purkinje cells as a model since their PCD is restricted to the first postnatal week in the mouse cerebellum. Transcriptome analysis of Purkinje cells during or after PCD allowed the identification of Krüppel like factor 9 (Klf9) as a candidate for PCD closure, given its high increase of expression at the end of the 1st postnatal week. Klf9 function was tested in organotypic cultures, through lentiviral vector-mediated manipulation of Klf9 expression. In absence of trophic factors, the Purkinje cell survival rate is of 40%. Overexpression of Klf9 during PCD dramatically increases the Purkinje cell survival rate from 40% to 88%, whereas its down-regulation decreases it to 14%. Accordingly, in organotypic cultures of Klf9 knockout animals, Purkinje cell survival rate is reduced by half as compared to wild-type mice. Furthermore, the absence of Klf9 could be rescued by Purkinje cell trophic factors, Insulin growth factor-1 and Neurotrophin3. Altogether, our results ascribe a clear role of Klf9 in Purkinje cell survival. Thus, we propose that Klf9 might be a key molecule involved in turning off the phase of Purkinje PCD.

Intercellular (mis)communication in neurodegenerative disease

Neurodegenerative diseases have been intensively studied, but a comprehensive understanding of their pathogenesis remains elusive. An increasing body of evidence suggests that non-cell-autonomous processes play critical roles during the initiation and spatiotemporal progression or propagation of the dominant pathology. Here, we review findings highlighting the importance of pathological cell-cell communication in neurodegenerative disease. We focus primarily on the accumulating evidence suggesting dysfunctional crosstalk between neurons and astroglia, neurons and innate immune system cells, as well as cellular processes leading to transmission of pathogenic proteins between cells. Insights into the complex intercellular perturbations underlying neurodegeneration will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression in this devastating class of human diseases.

Cerebellar grafting in the oculomotor system as a model to study target influence on adult neurons

In the last decades, there have been many efforts directed to gain a better understanding on adult neuron-target cell relationships. Embryonic grafts have been used for the study of neural circuit rewiring. Thus, using several donor neuronal tissues, such as cerebellum or striatum, developing grafted cells have been shown to have the capability of substituting neural cell populations and establishing reciprocal connections with the host. In addition, different lesion paradigms have also led to a better understanding of target dependence in neuronal cells. Thus, for example, axotomy induces profound morphofunctional changes in adult neurons, including the loss of synaptic inputs and discharge alterations. These alterations are probably due to trophic factor loss in response to target disconnection. In this review, we summarize the different strategies performed to disconnect neurons from their targets, and the effects of target substitution, performed by tissue grafting, upon neural properties. Using the oculomotor system-and more precisely the abducens internuclear neurons-as a model, we describe herein the effects of disconnecting a population of central neurons from its natural target (i.e., the medial rectus motoneurons at the mesencephalic oculomotor nucleus). We also analyze target-derived influences in the structure and physiology of these neurons by using cerebellar embryonic grafts as a new target for the axotomized abducens internuclear neurons.

Fetal neural grafts for Huntington's disease: a prospective view

Intrastriatal transplantation of striatal neuroblasts from human fetuses is a promising approach for treatment of Huntington's disease, on the basis of many experimental animal studies and, most recently, pilot clinical trials. Technically, several issues remain to be resolved (e.g., the precise site of dissection of the fetal tissue; the number and location of the fetal striatal implants; or the use of immunosuppressive therapy), and await larger-scale trials and purposely designed protocols. Further clinical data must also be obtained, and preliminary promising results must be replicated in a patient group large enough to provide conclusive results. It is important to establish (1) the amount of clinical benefit provided to the patient by the grafted cells; (2) the anticipated duration of clinical benefits; and (3) the secondary rate of decline after the benefit of the graft has been overbalanced. Evaluation of these parameters will require very long-term follow-up of the patients involved, over several years after grafting, before the technique can eventually be proposed widely to patients.

Neuroprotective gene therapy for Huntington's disease using a polymer encapsulated BHK cell line engineered to secrete human CNTF

Huntington's disease (HD) is an autosomal dominant genetic disease with devastating clinical effects on cognitive, psychological, and motor functions. These clinical symptoms primarily relate to the progressive loss of medium-spiny GABA-ergic neurons of the striatum. There is no known treatment to date. Several neurotrophic factors have, however, demonstrated the capacity to protect striatal neurons in various experimental models of HD. This includes the ciliary neurotrophic factor (CNTF), the substance examined in this protocol. An ex vivo gene therapy approach based on encapsulated genetically modified BHK cells will be used for the continuous and long-term intracerebral delivery of CNTF. A device, containing up to 106 human CNTF-producing BHK cells surrounded by a semipermeable membrane, will be implanted into the right lateral ventricle of 6 patients. Capsules releasing 0.15-0.5 microg CNTF/day will be used. In this phase I study, the principal goal will be the evaluation of the safety and tolerability of the procedure. As a secondary goal, HD symptoms will be analyzed using a large battery of neuropsychological, motor, neurological, and neurophysiological tests and the striatal pathology monitored using MRI and PET-scan imaging. It is expected that the gene therapy approach described in this protocol will mitigate the side effects associated with the peripheral administration of recombinant hCNTF and allow a well-tolerated, continuous intracerebroventricular delivery of the neuroprotective factor.

MAP1B expression and microtubule stability in growing and regenerating axons

MAP1B is a microtubule-associated phosphoprotein that is particularly highly expressed in developing neurons. There is experimental evidence that it plays an important role in neuronal differentiation, especially the extension of axons and dendrites, but exactly what role is unclear. Recent experiments have shed light on the gene structure of MAP1B and identified some of the kinases that phosphorylate the protein. Implicit in these findings is the idea that MAP1B regulates the organisation of microtubules in neurites and is itself regulated in a complex way and at a number of levels.

Phenotype changes of inferior olive neurons following collateral reinnervation

Inferior olive neurons are able to enlarge or retract their axonic terminal fields in response to changes in the extension of their target domain. Following Purkinje cell loss, the retraction of target-deprived climbing fibres is accompanied by a size reduction in the inferior olive neuron cell bodies. Here, we asked whether perikaryal modifications also occur when inferior olivary neurons enlarge their terminal fields to innervate supernumerary targets. To achieve this aim, we carried out a morphometric analysis on the somatic compartment of inferior olive neurons in two experimental conditions known to induce an expansion of their terminal field, i.e. a subtotal 3-acetylpyridine inferior olive lesion in the adult and a unilateral transection of the inferior cerebellar peduncle in newborn rats. In both experimental conditions, the inferior olive neurons that survived the lesion showed a remarkable increase in cell body and nuclear size, although the latter change was less pronounced in the 3-acetylpyridine-treated animals. These results show that both developing and mature inferior olive neurons are capable of adjusting their perikaryal phenotype to match the modifications of their target size.

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.

Chronic morphine induces visible changes in the morphology of mesolimbic dopamine neurons

The mesolimbic dopamine system, which arises in the ventral tegmental area (VTA), is an important neural substrate for opiate reinforcement and addiction. Chronic exposure to opiates is known to produce biochemical adaptations in this brain region. We now show that these adaptations are associated with structural changes in VTA dopamine neurons. Individual VTA neurons in paraformaldehyde-fixed brain sections from control or morphine-treated rats were injected with the fluorescent dye Lucifer yellow. The identity of the injected cells as dopaminergic or nondopaminergic was determined by immunohistochemical labeling of the sections for tyrosine hydroxylase. Chronic morphine treatment resulted in a mean approximately 25% reduction in the area and perimeter of VTA dopamine neurons. This reduction in cell size was prevented by concomitant treatment of rats with naltrexone, an opioid receptor antagonist, as well as by intra-VTA infusion of brain-derived neurotrophic factor. In contrast, chronic morphine treatment did not alter the size of nondopaminergic neurons in the VTA, nor did it affect the total number of dopaminergic neurons in this brain region. The results of these studies provide direct evidence for structural alterations in VTA dopamine neurons as a consequence of chronic opiate exposure, which could contribute to changes in mesolimbic dopamine function associated with addiction.

Rationale for intrastriatal grafting of striatal neuroblasts in patients with Huntington's disease

Huntington's disease is a genetic disease, autosomal and dominant, that induces motor disorders, an inexorable deterioration of higher brain functions and psychiatric disturbances. At present, there are no known therapeutics against Huntington's disease. The Network of European CNS Transplantation and Restoration (NECTAR) has begun a program aimed at defining the conditions under which intrastriatal transplantation of fetal striatal cells could be attempted as an experimental treatment for Huntington's disease. This review presents the reasons why our group is considering participating in these trials. The validity of this therapeutic approach is supported by three main series of data: (i) neuropathological, clinical and imaging data indicate that Huntington's disease is, above all, a localized affection of a specific neuronal population ("medium-spiny" neurons) in the striatum; (ii) a large body of experimental results, obtained in rats and non-human primates, demonstrates that transplanted fetal striatal cells are able to integrate the host brain and to substitute for previously lesioned host striatal neurons; (iii) expertise in clinical neural transplantation has now been acquired from the treatment of patients with Parkinson's disease. These different sets of data are presented and discussed in this review. There are a number of problems which do not yet appear to be entirely resolved, nor are they likely to be using the experimental models currently available. These problems are identified and explicitly presented as working hypotheses. (1) Anatomo-functional results obtained in rodents and non-human primates with excitotoxic striatal lesions can serve as a basis for the extrapolation of what can be obtained from patients with Huntington's disease. (2). Huntington's disease can be efficiently fought by substituting degenerated striatal neurons alone. (3) Huntington's disease is due to a genetic defect which either hits the neurons that carry it directly or hits them indirectly only after several decades. Transplanted neurons, because they do not carry the gene or because they are of fetal origin, will not be rapidly affected by the ongoing disease process. Given the current state of knowledge, intracerebral transplantation appears to be the most serious opportunity (if not the only one that has been experimentally validated) for clinical improvement to be obtained in patients with Huntington's disease. The purpose of this review is to open a scientific discussion on its experimental bases before actual clinical trials start.