Shelley J Allen

Neurotrophins and neurodegeneration

There is growing evidence that reduced neurotrophic support is a significant factor in the pathogenesis of neurodegenerative diseases such as Alzheimers disease (AD), Parkinsons disease (PD) and amyotrophic lateral sclerosis (ALS). In this review we discuss the structure and functions of neurotrophins such as nerve growth factor, and the role of these proteins and their tyrosine kinase (Trk) receptors in the aetiology and therapy of such diseases. Neurotrophins regulate development and the maintenance of the vertebrate nervous system. In the mature nervous system they affect neuronal survival and also influence synaptic function and plasticity. The neurotrophins are able to bind to two different receptors: all bind to a common receptor p75NTR, and each also binds to one of a family of Trk receptors. By dimerization of the Trk receptors, and subsequent transphosphorylation of the intracellular kinase domain, signalling pathways are activated. We discuss here the structure and function of the neurotrophins and how they have been, or may be, used therapeutically in AD, PD, Huntingtons diseases, ALS and peripheral neuropathy. Neurotrophins are central to many aspects of nervous system function. However they have not truly fulfilled their therapeutic potential in clinical trials because of the difficulties of protein delivery and pharmacokinetics in the nervous system. With the recent elucidation of the structure of the neurotrophins bound to their receptors it will now be possible, using a combination of in silico technology and novel screening techniques, to develop small molecule mimetics with much improved pharmacotherapeutic profiles.

alpha- and beta-secretase: profound changes in Alzheimer's disease.

The amyloid plaque, a neuropathological hallmark of Alzheimers disease, is produced by the deposition of beta-amyloid (Abeta) peptide, which is cleaved from Amyloid Precursor Protein (APP) by the enzyme beta-secretase. Only small amounts of Abeta form in normal brain; more typically this is precluded by the processing of APP by alpha-secretase. Here, we describe a decrease in alpha-secretase (81% of normal) and a large increase in beta-secretase activity (185%) in sporadic Alzheimers disease temporal cortex. Since alpha-secretase is present principally in neurons known to be vulnerable in Alzheimers disease, and there is known competition between alpha- and beta-secretase for the substrate APP, it is significant that the majority of Alzheimer samples tested here were low in alpha-secretase. Eighty percent of Alzheimer brains examined had an increase in beta-secretase, a decrease in alpha-secretase, or both; which may account for the means by which the majority of people develop Alzheimers disease.

Coexistence of choline acetyltransferase and nerve growth factor receptors in the rat basal forebrain

Choline acetyltransferase (ChAT) and nerve growth factor (NGF) receptors have previously been shown to be expressed in magnocellular forebrain neurones in the rat. We have now examined their colocalization in these neurones. Using monoclonal antibodies raised against ChAT and NGF receptors we demonstrate here a high degree of colocalization.

Morphometric immunochemical analysis of neurons in the nucleus basalis of Meynert in Alzheimer's disease

In Alzheimers disease there is a reported loss of large cells in the cholinergic nucleus basalis of Meynert. It has been suggested, however, that there may be neurons in the nucleus basalis in Alzheimers disease which are atrophied and therefore difficult to distinguish from neuroglia by size. This has important therapeutic implications and we have attempted to clarify the situation using a neuron-specific antiserum directed against neuron-specific enolase (NSE). Sections of nucleus basalis were stained using this antiserum and the neuronal cross-sectional area was measured. A profile of neuronal distribution with area was obtained, by image analysis, and compared in controls and patients with Alzheimers disease. A significant 29% overall loss of neurons was found in Alzheimers disease with a much greater loss (61%) of large neurons and concurrent increase (59%) in small neurons. Analysis of variance showed significant reduction in mean cross-sectional neuronal area as a consequence of this shift in frequency towards a preponderance of small cells. It is suggested that in the nucleus basalis in Alzheimers disease, large neurons are not completely lost; many are shrunken and thus excluded from the previous studies of large cells counted in Nissl-stained material. That there is partial preservation of these neurons makes it more likely that cholinergic dysfunction, characteristic of Alzheimers disease, will be amenable to neurotrophic influence.

Normal β-NGF content in Alzheimer's disease cerebral cortex and hippocampus

Abstract Nerve growth factor (β-NGF) is known to have beneficial effects on cholinergic cell survival and to function both in vivo and in vitro. It has been speculated that this protein, or the lack of it, may be involved in the aetiology of Alzheimers disease (AD). We describe the measurement of β-NGF content in 4 regions of the cerebral cortex and the hippocampus in AD brain compared with brain tissue from age-matched normal subjects using a sensitive sandwich immunoassay (ELISA). There was no difference in β-NGF content in any region examined in AD compared with normal values despite the marked loss of cortical cholinergic function.

Reduced cholinergic function in normal and Alzheimer's disease brain is associated with apolipoprotein E4 genotype

Apolipoprotein E (ApoE) is a potent risk factor for Alzheimers disease. Since the loss of cholinergic function in Alzheimers disease is known to occur at an early stage in the disease we have examined this function in normal subjects with an Apoepsilon4 allele to see if the deficit occurs in the absence of Alzheimer pathology or symptoms. We report that brain tissue obtained post-mortem from normal subjects and Alzheimer patients with an Apoepsilon4 allele has a lower cholinergic activity than tissue from those subjects without this allele. This has important significance for the interpretation of the cholinergic deficits found in Alzheimers disease.

Specificity in Trk Receptor:Neurotrophin Interactions: The Crystal Structure of TrkB-d5 in Complex with Neurotrophin-4/5

BACKGROUNDnThe binding of neurotrophin ligands to their respective Trk cellular receptors initiates intracellular signals essential for the growth and survival of neurons. The site of neurotrophin binding has been located to the fifth extracellular domain of the Trk receptor, with this region regulating both the affinity and specificity of Trk receptor:neurotrophin interaction. Neurotrophin function has been implicated in a number of neurological disorders, including Alzheimers disease and Parkinsons disease.nnnRESULTSnWe have determined the 2.7 A crystal structure of neurotrophin-4/5 bound to the neurotrophin binding domain of its high-affinity receptor TrkB (TrkB-d5). As previously seen in the interaction of nerve growth factor with TrkA, neurotrophin-4/5 forms a crosslink between two spatially distant receptor molecules. The contacts formed in the TrkB-d5:neurotrophin-4/5 complex can be divided into a conserved area similar to a region observed in the TrkA-d5:NGF complex and a second site-unique in each ligand-receptor pair-formed primarily by the ordering of the neurotrophin N terminus.nnnCONCLUSIONSnTogether, the structures of the TrkB-d5:NT-4/5 and TrkA-d5:NGF complexes confirm a consistent pattern of recognition in Trk receptor:neurotrophin complex formation. In both cases, the N terminus of the neurotrophin becomes ordered only on complex formation. This ordering appears to be directed largely by the receptor surface, with the resulting complementary surfaces providing the main determinant of receptor specificity. These features provide an explanation both for the limited crossreactivity observed between the range of neurotrophins and Trk receptors and for the high-affinity binding associated with respective ligand-receptor pairs.

Immunohistochemical localization of β-nerve growth factor receptors in the forebrain of the rat ☆

Using a monoclonal antibody (192-IgG) directed against the rat beta-nerve growth factor (beta-NGF) receptor the distribution of beta-NGF receptors in the forebrain of the rat has been determined. beta-NGF receptor-containing cells were located in presumed cholinergic cells of the medial septal nucleus, vertical and horizontal limbs of the diagonal band of Broca and the nucleus basalis of Meynert.

Cloning of a non-catalytic form of human trkB and distribution of messenger RNA for trkB in human brain.

A truncated form of the human trkB gene has been cloned and sequenced. This gene is related to the trk family of tyrosine kinases, the products of which act as receptors for the neurotrophins. Of these, brain-derived neurotrophic factor and mammalian neurotrophin-4 are the known ligands for the TrkB receptor. Catalytic and non-catalytic (or truncated) forms of the trkB gene have been cloned for rat and mouse. In this study, using in situ hybridization, we describe the distribution of trkB messenger RNA in fetal and adult human brain.

Low affinity nerve growth factor receptor binding in normal and Alzheimer's disease basal forebrain

The binding characteristics of radiolabelled beta-nerve growth factor ([125I]NGF) have been determined on membrane preparations of basal forebrain from Alzheimers disease (AD) brain and age-matched normal brains. [125I]NGF binds in a specific fashion indicative of a single receptor and is not displaced with microM concentrations of cytochrome c, insulin or epidermal growth factor (EGF). The mean dissociation constant (Kd) and the mean capacity (Bmax) of the NGF receptor were not significantly different between the 5 AD and 5 normal basal forebrain samples examined. Choline acetyltransferase (ChAT) activity was significantly reduced (P less than or equal to 0.001) in AD cerebral cortical samples compared with normal tissue.