Age-related development of a heterozygous phenotype in solitary neurons of the homozygous Brattleboro rat

Somatic mutations in neurons during aging and neurodegeneration

The nervous system is composed of a large variety of neurons with a diverse array of morphological and functional properties. This heterogeneity is essential for the construction and maintenance of a distinct set of neural networks with unique characteristics. Accumulating evidence now indicates that neurons do not only differ at a functional level, but also at the genomic level. These genomic discrepancies seem to be the result of somatic mutations that emerge in nervous tissue during development and aging. Ultimately, these mutations bring about a genetically heterogeneous population of neurons, a phenomenon that is commonly referred to as "somatic brain mosaicism". Improved understanding of the development and consequences of somatic brain mosaicism is crucial to understand the impact of somatic mutations on neuronal function in human aging and disease. Here, we highlight a number of topics related to somatic brain mosaicism, including some early experimental evidence for somatic mutations in post-mitotic neurons of the hypothalamo-neurohypophyseal system. We propose that age-related somatic mutations are particularly interesting, because aging is a major risk factor for a variety of neuronal diseases, including Alzheimer's disease. We highlight potential links between somatic mutations and the development of these diseases and argue that recent advances in single-cell genomics and in vivo physiology have now finally made it possible to dissect the origins and consequences of neuronal mutations in unprecedented detail.

Tracking Down Mutations Cell by Cell

Using somatic cell nuclear transfer, Hazen et al. (2016) examined clonally expanded single neurons for mutations and found ∼100 mutations from a variety of classes. Post-mitotic mutations in individual neurons represent an exploratory direction for finding fundamental origins of neurodegeneration.

RNA editing by T7 RNA polymerase bypasses InDel mutations causing unexpected phenotypic changes

DNA-dependent T7 RNA polymerase (T7 RNAP) is the most powerful tool for both gene expression and in vitro transcription. By using a Next Generation Sequencing (NGS) approach we have analyzed the polymorphism of a T7 RNAP-generated mRNA pool of the mboIIM2 gene. We find that the enzyme displays a relatively high level of template-dependent transcriptional infidelity. The nucleotide misincorporations and multiple insertions in A/T-rich tracts of homopolymers in mRNA (0.20 and 0.089%, respectively) cause epigenetic effects with significant impact on gene expression that is disproportionally high to their frequency of appearance. The sequence-dependent rescue of single and even double InDel frameshifting mutants and wild-type phenotype recovery is observed as a result. As a consequence, a heterogeneous pool of functional and non-functional proteins of almost the same molecular mass is produced where the proteins are indistinguishable from each other upon ordinary analysis. We suggest that transcriptional infidelity as a general feature of the most effective RNAPs may serve to repair and/or modify a protein function, thus increasing the repertoire of phenotypic variants, which in turn has a high evolutionary potential.

Heritable change caused by transient transcription errors

Transmission of cellular identity relies on the faithful transfer of information from the mother to the daughter cell. This process includes accurate replication of the DNA, but also the correct propagation of regulatory programs responsible for cellular identity. Errors in DNA replication (mutations) and protein conformation (prions) can trigger stable phenotypic changes and cause human disease, yet the ability of transient transcriptional errors to produce heritable phenotypic change ('epimutations') remains an open question. Here, we demonstrate that transcriptional errors made specifically in the mRNA encoding a transcription factor can promote heritable phenotypic change by reprogramming a transcriptional network, without altering DNA. We have harnessed the classical bistable switch in the lac operon, a memory-module, to capture the consequences of transient transcription errors in living Escherichia coli cells. We engineered an error-prone transcription sequence (A9 run) in the gene encoding the lac repressor and show that this 'slippery' sequence directly increases epigenetic switching, not mutation in the cell population. Therefore, one altered transcript within a multi-generational series of many error-free transcripts can cause long-term phenotypic consequences. Thus, like DNA mutations, transcriptional epimutations can instigate heritable changes that increase phenotypic diversity, which drives both evolution and disease.

Molecular misreading: the occurrence of frameshift proteins in different diseases

Neuronal homoeostasis requires a constant balance between biosynthetic and catabolic processes. Eukaryotic cells primarily use two distinct mechanisms for degradation: the proteasome and autophagy of aggregates by the lysosomes. We focused on the UPS (ubiquitin-proteasome system). As a result of molecular misreading, misframed UBB (ubiquitin B) (UBB+1) is generated. UBB+1 accumulates in the neuritic plaques and neurofibrillary tangles in all patients with AD (Alzheimer's disease) and in the neuronal and glial hallmarks of other tauopathies and in polyglutamine diseases such as Huntington's disease. UBB+1 is not present in synucleinopathies such as Parkinson's disease. We showed that UBB+1 causes UPS dysfunction, aggregation and apoptotic cell death. UBB+1 is also present in non-neurological cells, hepatocytes of the diseased liver and in muscles during inclusion body myositis. Other frequently occurring (age-related) diseases such as Type 2 (non-insulin-dependent) diabetes mellitus are currently under investigation. These findings point to the importance of the UPS in diseases and open new avenues for target identification of the main players of the UPS. Treatment of these diseases with tools (e.g. viral RNA interference constructs) to intervene with specific targets is the next step.

Conformational diseases: an umbrella for various neurological disorders with an impaired ubiquitin-proteasome system

It is increasingly appreciated that failures in the ubiquitin-proteasome system play a pivotal role in the neuropathogenesis of many neurological disorders. This system, involved in protein quality control, should degrade misfolded proteins, but apparently during neuropathogenesis, it is unable to cope with a number of proteins that, by themselves, can consequently accumulate. Ubiquitin is essential for ATP-dependent protein degradation by the proteasome. Ubiquitin+1 (UBB+1) is generated by a dinucleotide deletion (DeltaGU) in UBB mRNA. The aberrant protein has a 19 amino acid extension and has lost the ability to ubiquitinate. Instead of targeting proteins for degradation, it has acquired a dual substrate-inhibitor function; ubiquitinated UBB+1 is a substrate for proteasomal degradation, but can at higher concentrations inhibit, proteasomal degradation. Furthermore, UBB+1 protein accumulates in neurons and glial cells in a disease-specific way, and this event is an indication for proteasomal dysfunction. Many neurological and non-neurological conformational diseases have the accumulation of misfolded proteins and of UBB+1 in common, and this combined accumulation results in the promotion of insoluble protein deposits and neuronal cell death as shown in a cellular model of Huntington's disease.

Development of the mouse hypothalamo-neurohypophysial system in the munc18-1 null mutant that lacks regulated secretion

The hypothalamo-neurohypophysial system (HNS) is composed of hypothalamic magnocellular neurons and neural lobe pituicytes that accommodate the nerve terminals. Here we have investigated if the communication of the peptidergic neurons of the HNS with neighbouring cells plays a role in the development and assembly of the HNS. We employed munc18-1-deficient mice, which completely lack neurotransmitter secretion. Morphological and immunohistological analysis of the HNS in these mutant embryos during brain development showed that this peptidergic system was formed normally during early embryogenesis. However, the development arrested at embryonal day 14.5, the stage when terminal differentiation has to take place. The peptidergic neurons targeted axons in the correct direction, but few arrived at their final location and the neurons were not maintained in later stages. The pituicytes in the neural lobe of the pituitary were generated, but failed to organize normally. Our results indicate that peptide gene expression, axon outgrowth and migration are intrinsic developmental events in these peptidergic neurons, that are initiated in the munc18-1 null mutant. The further expansion and the integration of outgrowing axon terminals with neural lobe pituicytes requires munc18-1-dependent processes, probably exocytosis, at multiple levels. Firstly, to maintain and propagate neuronal outgrowth and guidance, and secondly, to control the cellular organization of the pituicytes. Thus, the communication between the outgrowing neurons and the pituicytes could serve to integrate these two cell types to constitute a functional peptidergic system.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Adenoviral Vector-Mediated Expression of Neurotrophin-3 Increases Neuronal Survival in Suprachiasmatic Nucleus Grafts

To improve transplantation results of fetal suprachiasmatic nucleus (SCN) in SCN-lesioned (SCNX) rats, grafts were ex vivo transduced with an adenoviral vector encoding for neurotrophin-3 (AdNT-3) before implantation. Mock- and AdLacZ-transduced grafts were used as controls. First, transplants were evaluated microscopically and by image analysis for the presence of vasopressinergic (VPergic) and vasoactive intestinal polypeptidergic (VIPergic) SCN neurons at 10 weeks or later postgrafting. Ex vivo AdNT-3-transduced transplants displayed increased volume areas of VPergic and VIPergic SCN cells in comparison with those in mock- and AdLacZ-transduced transplants, but significantly improved graft-to-host VPergic and VIPergic SCN fiber growth was not reached (though AdNT-3-transduced transplants tended to grow more VPergic fibers into the brain of VP-deficient SCNX Brattleboro rat recipients, which were chosen as recipients to circumvent the presence of non-SCN VP fiber staining). Second, a small group of arrhythmic Wistar rats received AdNT-3- or control-treated SCN grafts while continuously on-line for the monitoring of overt circadian activities in the pre- and postgrafting periods. The results indicated that ex vivo transduced SCN grafts can still restore arrhythmia, but that the NT-3-mediated anatomical improvements of the grafting results were not sufficient to enhance efficacy of reinstatement of circadian rhythm in SCN-lesioned rats. However, in this group VIP staining volume area, not VP staining volume area, correlated significantly with reinstatement of circadian rhythm.

Inhibition of the ubiquitin-proteasome system in Alzheimer's disease

Alzheimer's disease is the most common cause of dementia in the elderly. Although several genetic defects have been identified in patients with a family history of this disease, the majority of cases involve individuals with no known genetic predisposition. A mutant form of ubiquitin, termed Ub(+1), has been selectively observed in the brains of Alzheimer's patients, including those with nonfamilial Alzheimer's disease, but it has been unclear why Ub(+1) expression should be deleterious. Here we show that Ub(+1) is an efficient substrate for polyubiquitination in vitro and in transfected human cells. The resulting polyubiquitin chains are refractory to disassembly by deubiquitinating enzymes and potently inhibit the degradation of a polyubiquitinated substrate by purified 26S proteasomes. Thus, expression of Ub(+1) in aging brain could result in dominant inhibition of the Ub-proteasome system, leading to neuropathologic consequences.

ENU induces mutations in the heart of lacZ transgenic mice

The use of transgenic mouse models as somatic mutation assays allows determination of mutation in all tissues of the mouse, including non-dividing tissues. In this regard, these models can be used to study the possibility that mutations can be induced in mitotically quiescent organs such as the heart. Mutations are generally thought to be associated with mitotic processes of DNA replication. Mutations, however, are also postulated to occur in the absence of mitosis as the result of DNA repair. In order to determine whether or not mutations could be induced in the heart, we analyzed the mutant frequency in the hearts of F(1) (Muta Mouse X SWR) mice that had been treated acutely with 250 mg/kg ENU and sampled at days 10, 35, and 70 post-treatment. A significant increase in mutant frequency at day 70 shows that mutations can be induced in the heart. Since the heart contains small numbers of non-muscle cells, additional mechanisms that could explain these results were also considered. The effect of ENU-induced cell proliferation or a sub-population of rapidly dividing cells is ruled out by C(14)-thymidine uptake studies which showed minimal proliferation. By the same token, the influence of ex vivo mutations (i.e., DNA adducts fixed as mutations during replication in the bacteria) is ruled out by the observed time course of mutations, as well as experimental evidence showing that such mutations are not detected in the lacZ assay.

Processing of frameshifted vasopressin precursors

Biosynthesis of the vasopressin (VP) prohormone in magnocellular neurones of the hypothalamo-neurohypophysial system comprises endoplasmic reticulum (ER) transit, sorting into the regulated secretory pathway and subsequent processing in the individual proteins VP, neurophysin and a glycoprotein. These processes are severely disrupted in the homozygous diabetes insipidus (di/di) Brattleboro rat, which expresses a mutant VP precursor due to a single nucleotide deletion in the neurophysin region of the VP gene resulting in VP deficiency. Previous studies have shown the presence of additional frameshift mutations in VP transcripts, in solitary magnocellular neurones of the di/di rat due to a GA dinucleotide deletion resulting in two different mutant VP precursors with partly restored reading frame. Frameshifted VP precursors are also expressed in several magnocellular neurones in wild-type rats. In this study, we determined if the +1 frameshifted precursors from di/di and wild-type rats can lead to biosynthesis of the hormone VP. Therefore, eukaryotic expression plasmids containing the frameshifted VP cDNAs were transiently expressed in peptidergic tumour cell lines, and cells were analysed by reversed phase high-performance liquid chromatography and specific radioimmunoassays, and by immunofluoresence. Neuro2A neuroblastoma cells expressing the +1 frameshifted precursors of di/di rats retained products in the cell body. Only precursor or insignificant quantities of neurophysin-immunoreactive products were detected. In contrast, in AtT20 cells, frameshifted VP precursors were at least partly processed to yield the VP peptide, indicating that they have access to the regulated secretory pathway. Comparison between the two cell lines showed a very slow ER transit of the wild-type prohormone combined with inefficient processing in Neuro2A cells. The results show that mutant precursors can reach the regulated secretory pathway if ER transport is sufficiently rapid as in the case of AtT20 cells. This suggests that the di/di rat may regain the capacity to biosynthesize authentic VP through these +1 frameshifted precursors in magnocellular neurones.

Molecular misreading. A new type of transcript mutation in gerontology

Molecular misreading is a novel process that causes mutations in neuronal transcripts. It is defined as the inaccurate conversion of genomic information from DNA into nonsense transcripts and the subsequent translation into mutant proteins. As a result of dinucleotide deletions (delta GA, delta GU, delta CU) in and around GAGAG motifs in mRNA the reading frame shifts to the +1 frame, and subsequently the so-called +1 proteins are synthetized. +1 Proteins have a wild-type NH2 terminus and from the site of the dinucleotide deletion onwards an aberrant, nonfunctional COOH terminus. Molecular misreading was found in the rat vasopressin gene associated with diabetes insipidus and in the human genes linked to Alzheimer's disease (AD), that is, beta-amyloid precursor protein (beta APP) and ubiquitin-B (UBB). Moreover, beta APP+1 and UBB+1 proteins accumulate in the neuropathological hallmarks of AD. Inasmuch as these +1 proteins were also found in elderly, nondemented control patients, but not in younger ones (< 72 years), molecular misreading may act as a factor that becomes manifest in aged people. A hotspot for dinucleotide deletions is GAGAG motifs. Because statistically an average of 2.1 GAGAG motifs per gene can be expected, other genes expressed in other tissues may undergo molecular misreading as well. Indeed, we recently detected +1 proteins in proliferating cells present in tissues such as the liver, epididymis, parotid gland, and neuroblastoma cell lines. Therefore, molecular misreading can be regarded as a general biological source of transcript errors that may be involved in cellular derangements in numerous age-related pathologic conditions apart from Alzheimer's disease.

Molecular misreading of genes in Down syndrome as a model for the Alzheimer type of neurodegeneration

The occurrence of +1 frameshifted proteins, such as amyloid precursor protein (APP+1) and ubiquitin-B (UBB+1) in Down syndrome (DS) has been linked to the onset of Alzheimer's disease (AD). In DS and AD patients, but also in elderly non-demented persons, these co-called +1 proteins accumulate in the neuropathological hallmarks (neurofibrillary tangles, dystrophic neurites of the neuritic plaques and neuropil threads) and may have deleterious effects on neuronal function. Frameshifts are caused by dinucleotide deletions in GAGAG motifs in messenger RNA and are now thought to be the result of unfaithful transcription of normal DNA by a novel process termed "molecular misreading". In the present review some of the critical events in molecular misreading are discussed, the emphasis being on DS.

Genetic models of vasopressin deficiency

Animal models of genetic hormone deficiency are useful as models for physiological studies of hormone deficiency and hormone action. Structure-function studies of the specific underlying gene defect may help in understanding mechanisms regulating gene expression and secretion of the peptide product. Spontaneous genetic models of vasopressin deficiency, such as the Brattleboro rat and human familial diabetes insipidus, have facilitated many studies of vasopressin. However, the Brattleboro rat may not be an ideal model of genetic vasopressin deficiency and therefore could be less useful for studies of the central nervous system or as a background strain for the introduction of new vasopressin gene constructs. The human model is appropriately limited by the constraints of human studies, so that engineered animal models of specific diseases, such as familial neurohypophysial diabetes insipidus, are required. The recent development of a vasopressin-null mouse may provide insights into the various roles of vasopressin in the stress response, cardiovascular regulation and behaviour. Additionally, animals with a complete genetic deficiency of vasopressin can serve as a background strain for introduction of novel vasopressin gene constructs to enable sophisticated studies of the regulation of vasopressin expression and the intracellular processes required for appropriate secretion of vasopressin peptide. As advanced techniques of genetic manipulation become more reliable, conditional expression of vasopressin, regulated by time or body site will permit even more detailed studies in this field.

Transcriptional slippage of p53 gene enhanced by cellular damage in rat liver: monitoring the slippage by a yeast functional assay

The Long-Evans Cinnamon (LEC) rat is a mutant strain characterized by abnormal copper metabolism and a high incidence of hepatitis and hepatoma. Using a yeast-based assay which scores mutants in p53 gene transcripts as red colonies, we detected frequent mutations in the liver of LEC rats. The majority (50-60%) of these were frameshift mutations caused by the insertion of an extra adenine (A) in the regions containing six consecutive adenines. The rate of A insertion was calculated to be 6.9-9.0% of the total p53 cDNA. Insertions of an extra adenine were found almost exclusively in the mRNA (cDNA), especially in the (A)(6) tract located at the most 5'-side (exon 4) among the three (A)(6) tracts (exons 4, 7, and 8), but rarely in the corresponding sites of genomic DNA. Wild-type p53 cDNA was transcribed in vitro into mRNA with the use of SP6 RNA polymerase and tested by the yeast functional assay. Subsequent sequencing detected A insertions at an overall rate of 1.6% in exons 7 and 8 but none in exon 4. This indicates that the A insertion in the exon 4 (A)(6) tract was an in vivo phenomenon rather than an artifact in reverse transcription or polymerase chain reaction. The percentage of red colonies increased sharply to about 20% of the liver samples in the acute hepatitis stage, and returned to control level of those in the chronic hepatitis stage, and increased again slightly to those in the neoplastic stage. The percentage of red colonies correlated with the serum GOT level (r=0.96, p<0.001) but not with the contents of copper and 8-hydroxydeoxyguanosine in the liver of LEC rats. Ethanol treatment of hepatic cell lines also increased the rate of transcriptional slippage at the (A)(6) tract. These findings indicate that cellular damage is responsible for the increase in the rate of mutation at the transcriptional level, and suggest that cellular damage degrades transcriptional fidelity, thereby further impairing cellular functions.

Anomalous expression of microtubule-associated protein 1B in the hippocampus and cortex of aged rats treated with pentylenetetrazole

The aim of the present study was to assess the age-dependent response of microtubule-associated protein 1B, a plasticity-associated protein deriving from a late gene, following administration of an epileptogenic stimulus. The effect of a single administration of the convulsant pentylenetetrazole on microtubule-associated protein 1B expression in the hippocampal formation and cortex of three-, 18- and 28-month-old rats was assessed using northern blot analysis, in situ hybridization and immunohistochemistry. In three-month-old rats, we detected initial increases in microtubule-associated protein 1B messenger RNA at 15 h following pentylenetetrazole administration in the granule cells of the dentate gyrus, in the CA3 region of the hippocampus and in layers II/III of the entorhinal cortex, and these reached a maximum at 44 h. However, in the hippocampus and cortex of 18-month-old rats, the peak occurred at 15 h, and in the brains of 28-month-old rats a blunted peak was reached at 3 h. Pentylenetetrazole treatment in young rats resulted in a robust induction of microtubule-associated protein 1B immunoreactivity in the granule cells of the dentate gyrus and in layers II/III of the entorhinal cortex, but also produced a large decrease in the retrosplenial cortex. However, following pentylenetetrazole treatment in older rats, the granule cells of the dentate gyrus were nearly devoid of microtubule-associated protein 1B immunoreactivity, whereas the retrosplenial cortex showed no changes at all, and the entorhinal cortex had an expression pattern similar to that of young rats. Aberrant immunolabeling of microtubule-associated protein 1B occurred in cortical layer VI of the aged rats where, unlike in young rats, there was heavy staining of neuronal somata. These results suggest that the regulation of the plasticity-associated protein microtubule-associated protein 1B is altered in the ageing rat brain, with the peak of expression shifted to earlier times in 18-month-old rats and blunted, variable increases at even earlier times in 28-month-old rats.

Revertant mosaicism in human genetic disorders

Somatic reversion of inherited mutations is known for many years in plant breeding, however it was recognized only recently in humans. The concept of revertant mosaicism is important in medical genetics.

The magnocellular neurons of the hypothalamo-neurohypophyseal system display remarkable neuropeptidergic phenotypes leading to novel insights in neuronal cell biology

For decades the magnocellular neurons of the hypothalamo-neurophypophyseal system (HNS), in which either vasopressin or oxytocin are produced and released into the bloodstream, have been playing a pivotal role in fundamental discoveries in the nervous system. The primary structure of vasopressin and oxytocin was the first of all neuropeptides to be published, i.e., in the 1950s by the Nobel prize laureate Du Vigneaud. Moreover, many trend-setting discoveries have their origin in the HNS, which abundantly expresses vasopressin and oxytocin, clearly displays its function and is relatively easily to manipulate. Examples are the phenomenon of coexpression of neuropeptides, patch-clamping of nerve endings, axonal transport of RNA, neuroglia interactions and the behavioral effects. An extraordinarily intriguing example is the homozygous Brattleboro rat, which lacks vasopressin by a germ-line mutation, and has disclosed many of the fundamental characteristics of peptidergic neurons, and neurons in general. In this chapter we will discuss a few of them, in particular the recent data on mutations in vasopressin RNA. It is to be expected that the HNS will retain its informative role in the next decades.

The vasopressin deficient Brattleboro rats: a natural knockout model used in the search for CNS effects of vasopressin

Behavioral neuroscience is using more and more gene knockout techniques to produce animals with a specific deletion. These studies have their precedent in nature. A mutation may result in a limited genetic defect, as seen in the vasopressin (VP) deficiency in the Brattleboro rat. The mutation is in a single pair of autosomal loci, and the sequences of VP gene from wild-type and homozygous Brattleboro rats are identical except for a single nucleotide deletion in the second exon. The deletion results in the synthesis of an altered VP precursor that is unable to enter the secretory pathway. The genetic disturbance results in a central diabetes insipidus comparable to that found in humans. Starting with our work during the early 1970s we found that the genetic defect in the availability of VP causes deficits in central nervous system (CNS) functions. Behavioral processes from cognition to drug tolerance appeared to be disturbed by the absence of VP, but not all behaviors are affected. The specificity of the absence of VP in causing behavioral deficits is shown in many cases. However, certain deficits are due to genetic factors other than the deletion of the VP gene. The picture is further complicated by differences in testing conditions, the absence of proper controls, i.e. heterozygous and wild-type Brattleboro rats, sex, compensation phenomena, and the absence of neuropeptides co-localized with VP. Interestingly, an age dependent spontaneous shunt to a heterozygous phenotype in vasopressinergic neurons might also compensate for the disturbance. Accordingly, findings in knockout animals should be interpreted with caution. One should realize that brain functions are modulated by multiple neuropeptides and that neuropeptides possess multiple CNS effects.

Effects of vasopressin and related peptides on neurons of the rat lateral septum and ventral hippocampus

The effects of vasopressin (VP), VP fragments and propressophysin glycopeptide on neuronal activities in the septum-hippocampus complex of rats were studied in vitro and in vivo. The frequency of the hippocampus theta rhythm in Brattleboro rats homozygous for diabetes insipidus was significantly slower than that of heterozygous litter mates and normal rats. Intracerebroventricular micro-injection of des-glycine-amide vasopressin corrected for several hours the frequency deficit of the theta rhythm in the homozygous Brattleboro rats and the centrally administered VP slowed down theta rhythm in normal rats. Microinotophoretically administered VP excited single neurons in the lateral septum of ventral hippocampus, and/or facilitated the responses of these neurons to glutamate and to stimulation of the glutamatergic afferent fibers in the fimbria bundle. The excitatory effects of VP vanished within seconds after termination of the peptide administration, however, the peptide-induced enhancement of glutamate and syntatically induced excitations were sustained for up to 60 min after the peptide administration. In vitro, pM concentrations of VP, VP 4-8 and C-terminus glycopeptide of propresophysin facilitated for 30-60 min the glutamate-mediated EPSPs in neurons of the lateral septum or the ventral hippocampus. The EPSPs increase in the lateral septum neurons was not prevented by pretreatment with antagonist of the V1a type of the vasopressin receptor. The resting membrane potential and input resistance were not affected by the peptides. A low-frequency electrical stimulation in the diagonal Band of Broca or in the Bed nucleus of the stria terminals, sources of the vasopressinergic innervation of the septum, facilitated the negative wave of the filed potentials responses evoked in the lateral septum by stimulating the fimbria bundle fibers in control Long-Evans and Brattleboro rats heterozygous for diabetes insipidus. The field potential increase was sustained for several hours after the stimulation, and it was not occluded by long-term potentiation elicited by high frequency stimulation of the fimbria bundle afferent fibers. Brattleboro rats homozygous for diabetes insipidus failed to show the filed potential increase after the diagonal band stimulation. It is suggested that the long-lasting facilitation of glutamate-mediated excitations might be a physiological action of the propressophysin-derived peptides in the septum-hippocampus complex which, in concert with other forms of synaptic plasticity like the long-term potentiation, facilitates the hippocampus-mediated forms of learning and memory. This action is presumably related to the memory enhancing effect of the propressophysin-derived peptides.

Vasopressin neurotransmission and the control of circadian rhythms in the suprachiasmatic nucleus

Vasopressin (VP) is one of the principal transmitters in the suprachiasmatic nucleus (SCN). Approximately 20% of neurones in the dorsomedial division of the SCN synthesize the peptide and a high proportion of SCN neurones (> 40%) are excited by VP acting through the V1 receptor. This suggests that VP may act as a feedback regulator of electrical activity within the nucleus. Such an intrinsic excitatory signal can be demonstrated by perifusion with a V1 antagonist which reduces spontaneous neural activity. As the synthesis and release of VP occurs in a circadian manner, this leads to a variable feedback excitation which may contribute to the circadian pattern of activity of the neural clock. This role in amplifying rhythmicity is supported by observations that animals deficient in VP show a reduced circadian amplitude of behavioural rhythms (e.g. locomotor and cortical electroencephalographic rhythms). VP expression declines during ageing and although aged animals show no change in the proportion of SCN neurones excited by VP, the rhythm of spontaneous electrical activity shows a progressive decline, consistent with the reduced endogenous excitatory feedback. However, the homozygous Brattleboro rat which lacks any VP expression still maintains rhythms of electrical activity, indicating that VP is not the sole factor generating circadian activity. The generation of this rhythmicity may depend upon the interaction of VP with other transmitter systems, such as the inhibitory transmitters somatostatin and GABA which show a circadian variation in efficacy. In addition to its role in feedback amplification of the endogenous rhythm of electrical activity, VP also functions as part of the efferent signal to the rest of the CNS where it potentially regulates a number of behavioural and physiological rhythms, including the circadian activity of the hypothalamo-pituitary-adrenal axis. Thus, the combined amplification and signalling functions makes VP an important component of the neuronal clock function in mammals.

Functions of the perikaryon and dendrites in magnocellular vasopressin-secreting neurons: new insights from ultrastructural studies

Magnocellular hypothalamic neurosecretory neurons secreting vasopressin or oxytocin provide a robust model system for the investigation and understanding of many aspects of peptidergic neuronal function. Many of their functions and the cellular organelles involved are well understood. However, recent ultrastructural studies have thrown new light on various aspects of magnocellular neurosecretory function which have not previously received much attention. This review concerns two of these: the effects of mutations in the vasopressin gene on the handling of the translated peptide by the rough endoplasmic reticulum; and the role of the magnocellular dendrites in the production, secretion and localisation of peptides. Investigation of the synthesis of proteins derived from vasopressin genes which have undergone various mutations has at the moment provided more answers than questions: Why do some abnormal products accumulate as masses of peptide in the rough endoplasmic reticulum while others do not? Why do accumulations in humans appear to be damaging to the neurons while those in the rat do not? Investigations of the role of dendrites in the production and release of peptides show that the dendrites have all the machinery needed for protein translation and appear to synthesize locally proteins required for dendritic function. Of particular interest is the possibility that various transmitter receptor proteins could be synthesized in the dendrites close to the synapses in which they become localized. Precisely how such membrane proteins are inserted into the synaptic complex is, however, unclear, because the most part of the dendrites lack any form of the Golgi packaging organelle that can be recognised as such either by immunocytochemistry or electron microscopy. Better established is the ability of magnocellular dendrites to secrete either vasopressin or oxytocin in response to a variety of stimuli including sex steroids. This local release of peptide into the magnocellular nuclei has important but as yet incompletely defined effects on the functioning of the neurons.

Mutations in RNA: a first example of molecular misreading in Alzheimer's disease

In the past decade, considerable progress has been made in the understanding of the neurodegenerative changes that occur in Alzheimer's disease (AD). Knowledge about this disease is based mainly on studies of inherited forms of AD, although most cases of AD are of the non-familial type. Recently, a novel type of mutation in 'vulnerable' dinucleotide repeats in messenger RNA was discovered in AD patients: in this type of mutation a mutated transcript is produced from a correct DNA sequence, a process that we call 'molecular misreading'. The resulting mutated '+1 proteins' are prominent neuropathological hallmarks of AD and they are present in most elderly non-demented people also. This suggests that the dinucleotide deletions in transcripts could be one of the earliest events in the neuropathogenesis of AD and an important factor in normal aging.

Transgene expression in rat fetal brain grafts is maintained for 7 months after ex vivo adenoviral vector-mediated gene transfer

Fetal brain tissue fragments containing the suprachiasmatic nucleus were infected with an adenoviral vector containing the marker gene LacZ encoding for beta-galactosidase, and subsequently cultured or transplanted in the third ventricle of SCN-lesioned adult Wistar rats. In previous studies we optimized the infection procedure and characterized the immunological response directed against the viral vector in this model. The present study reports on beta-gal expression for at least 7 months in neuronal and glial cells. Maturation of the transplanted fetal SCN with respect to immunoreactivity for vasoactive intestinal polypeptide and C-terminal propressophysin was not hampered by the viral infection.

Lack of translation of normal 7B2 mRNA levels in hypothalamic mutant vasopressin cells of the homozygous Brattleboro rat

The homozygous Brattleboro rat (di/di) synthesizes a vasopressin (VP) precursor with an aberrant C-terminus, which causes a hypothalamic form of diabetes insipidus. The neuroendocrine polypeptide 7B2 is present in VP and oxytocin (OT) neurons of the supraoptic and paraventricular nucleus of the hypothalamus in wild type rats. However, in the di/di rat 7B2 immunoreactivity is absent in the VP cell population, whereas 7B2 levels within the OT cells are unaffected. Remarkably, there is no obvious difference in 7B2 transcript levels between VP and OT neurons in the di/di rat hypothalamus. This study shows that the presence of mRNA does not automatically result in the subsequent synthesis of its protein. Cellular mechanisms underlying this discrepancy are discussed.

Specific expression of secretogranin II in magnocellular vasopressin neurons of the rat supraoptic and paraventricular nucleus in response to osmotic stimulation

As secretogranin II is considered to be a marker for the regulated secretory pathway, its distribution in the hypothalamo-neurohypophyseal system of salt-loaded Wistar rats was studied in detail by immunocytochemistry. Although after an osmotic challenge both vasopressin and oxytocin neurons are stimulated, secretogranin II was exclusively expressed in a subpopulation of vasopressinergic magnocellular neurons in the supraoptic and paraventricular nucleus of Wistar rats. Secretogranin II was only surely visualized after a combination of osmotic challenge and blockade of axonal transport by colchicine treatment. When these pre-treatments were not performed, only punctate fibers situated around the magnocellular neurons within the paraventricular and supraoptic nucleus were observed. Oxytocinergic magnocellular neurons never displayed any secretogranin II immunoreactivity, not even during lactation and after colchicine treatment. These findings suggest that secretogranin II is of functional importance during enhanced secretory activity within vasopressinergic neurons.

DNA turnover and mutation in resting cells

There is growing evidence that mutations can arise in non-dividing cells (both bacterial and mammalian) in the absence of chromosomal replication. The processes that are involved are still largely unknown but may include two separate mechanisms. In the first, DNA lesions resulting from the action of endogenous mutagens may give rise to RNA transcripts with miscoded bases. If these confer the ability to initiate DNA replication, the DNA lesions may have an opportunity to miscode during replication and thus could give rise to apparently 'adaptive' mutations. A second mechanism is suggested by recent work in starved bacteria, showing that there is much more turnover of chromosomal DNA than has been previously thought. This could permit polymerase errors to lead to mutations in non-dividing cells. Such cryptic DNA synthesis, which may essentially replace existing DNA rather than duplicating it, could, in principle, act as an additional source of variability on which selection may act, initially in the absence of cell division. In mammals such processes would undoubtedly have implications for germ cell mutagenesis and carcinogenesis.

Revertant mosaicism in epidermolysis bullosa caused by mitotic gene conversion

Mitotic gene conversion acting as reverse mutation has not been previously demonstrated in human. We report here that the revertant mosaicism of a compound heterozygous proband with an autosomal recessive genodermatosis, generalized atrophic benign epidermolysis bullosa, is caused by mitotic gene conversion of one of the two mutated COL17A1 alleles. Specifically, the maternal allele surrounding the mutation site on COL17A1 (1706delA) showed reversion of the mutation and loss of heterozygosity along a tract of at least 381 bp in revertant keratinocytes derived from clinically unaffected skin patches; the paternal mutation (R1226X) remained present in all cell samples. Revertant mosaicism represents a way of natural gene therapy.

Circadian rhythm of neuronal activity in suprachiasmatic nucleus slices from the vasopressin-deficient Brattleboro rat

In vitro extracellular recordings were made from tissue slices of suprachiasmatic nucleus from homozygous Brattleboro rats which are deficient in vasopressin. A high proportion (56%) of neurons were excited by application of exogenous vasopressin, indicating that the V1 receptors expressed by these neurons were functional. Basal activity of these vasopressin-sensitive neurons showed a marked circadian variation (higher during the subjective light phase) while vasopressin-insensitive neurons showed no significant variation, suggesting the presence of the V1 receptor identifies a population of highly circadian neurons. Suprachiasmatic neurons from both homozygous rats and their heterozygous (vasopressin-containing) litter mates displayed a circadian rhythm of spontaneous (basal) activity, with firing rates declining during the subjective dark phase, indicating that the endogenous pacemaker driving the circadian rhythm was not dependent upon the presence of vasopressin. However, the peak of spontaneous activity displayed during the subjective light phase was significantly lower in the vasopressin-deficient animals. These data show that the presence of endogenous vasopressin within the suprachiasmatic nucleus is not necessary for the generation of the circadian pattern of activity. However, vasopressin does function to amplify the rhythm by its excitatory effect during the light phase.

Somatic nonhomologous crossing-over between neuropeptide genes in rat hypothalamic neurons

Molecular biological and immunocytochemical data demonstrate nonhomologous crossing-over between the closely linked vasopressin (VP) and oxytocin (OT) genes in rat hypothalamic neuroendocrine neurons. Reverse transcription of hypothalamic total RNA from wild-type or homozygous Brattleboro aged rats combined with polymerase chain reaction (PCR) amplifications in the presence of appropriate 5' forward and 3' reverse primers deduced from the VP and OT cDNA sequences yielded PCR products that, upon cloning and sequencing, revealed several hybrid transcripts. They encode the N-terminal part of the VP precursor fused to the C-terminal part of the OT precursor (VP/OT transcripts) and vice versa (OT/VP transcripts). VP/OT hybrid precursor proteins have been identified immunocytochemically in enlarged cisternae of the rough endoplasmic reticulum, yet there is no evidence that the products can be secreted from affected cells. Recombination appears to be a rather frequent genetic event affecting about 0.06-0.1% of the rat vasopressinergic magnocellular neurons in aged rats.

Frameshift mutations at two hotspots in vasopressin transcripts in post-mitotic neurons

Mutations in DNA underlie carcinogenesis, inherited pathology, and aging and are generally thought to be introduced during meiosis and mitosis. Here we report that in post-mitotic neurons specific frameshift mutations occur at high frequency. These mutations were identified in vasopressin transcripts in magnocellular neurons of the homozygous Brattleboro rat and predominantly consist of a GA deletion in GAGAG motifs. Immunocytochemistry provides evidence for similar events in wild-type rats. However, the diseased state of the Brattleboro rat, resulting in a permanent activation of vasopressin neurons, enhanced the mutational rate. These data reveal hitherto unrecognized somatic mutations in nondividing neurons.

Immunocytochemical localization of neuropeptide FF (FMRF amide-like peptide) in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats by light and electron microscopy

Neuropeptide FF (F8Famide, FMRFamide-like, or morphine modulating peptide) immunoreactivity was localized by light and electron microscopy in the hypothalamo-neurohypophyseal system of Wistar and Brattleboro rats. In Wistar rats neuropeptide FF was present in part of the magnocellular neurones of the paraventricular and supraoptic nuclei in which it was coexpressed with vasopressin. Neuropeptide FF containing fibres were present in the paraventricular and the supraoptic nuclei, and in the central part of the neural lobe. At the electron microscopic level, neuropeptide FF containing nerve terminals in the neural lobe formed synaptoid contacts exclusively with pituicytes. No neuropeptide FF containing neurovascular contacts or contacts with other neuronal structures were observed. In contrast with Wistar rats, neuropeptide FF was almost completely absent in cell bodies of the paraventricular and supraoptic nuclei, and in fibres of the neural lobe in Brattleboro rats. Only a few solitary cells could be observed in these structures. The present results demonstrate that neuropeptide FF coexists with vasopressin within the hypothalamo-neurohypophyseal system. As we did not observe neuropeptide FF containing neurovascular contacts, neuropeptide FF containing nerve terminals probably have a local function within the neural lobe. Neuropeptide FF may be involved in the modulation of oxytocin and vasopressin release, with the pituicyte as an intermediate cell.

Processing endopeptidase deficiency in neurohypophysial secretory granules of the diabetes insipidus (Brattleboro) rat

The homozygote Brattleboro rat exhibits a hereditary diabetes insipidus due to a deficiency of vasopressin, the antidiuretic hormone. It has previously been shown that in this animal a single nucleotide deletion in the provasopressin gene leads to a mutant precursor with a C-terminal amino acid sequence different from that of the wild-type. However the N-terminal region including the hormone moiety, the processing signal as well as the first two-thirds of the neurophysin is entirely preserved and absence of maturation has to be explained by an additional cause. We show here that the neurohypophysis of the homozygote Brattleboro rat, in contrast to the adenohypophysis, displays a significant decrease in the Lys-Arg processing endopeptidase activity when compared to the heterozygote or the wild-type Wistar. It is suggested that hypothalamic vasopressinergic neurons of the homozygote Brattleboro rat display a deficiency in the processing enzyme in contrast to the oxytocinergic neurons in which processing of prooxytocin is normal.

A pattern of accumulation of a somatic deletion of mitochondrial DNA in aging human tissues

An assay that selectively amplifies a specific deletion of the mitochondrial genome has been used to study the extent of the deletion's accumulation in a variety of human tissues. The deletion occurs at much higher levels in nervous and muscle tissues than in all other tissues studied. The variation in deletion level between the same tissues in different persons of similar age appears to be less than the variation among tissues within an individual. Tests for artifactual explanations of the level differences were each negative. Three cellular parameters that are correlated with the level of the deletion are identified. The preferential accumulation of deleterious mitochondrial mutations in a restricted subset of aging human tissues may compound deficiencies of function in those tissues that accrue with age.

Mutant vasopressin precursor producing cells of the homozygous Brattleboro rat as a model for co-expression of neuropeptides

The homozygous Brattleboro rat (di/di) synthesizes a VP precursor with an abnormal C terminus, which is not transported from the rough endoplasmic reticulum to the Golgi apparatus. In addition, the phenotypic expression of co-existing peptides is differentially disturbed. Ang II and 7B2 are two of the peptides which are not detectable, whereas other peptides (e.g. galanin) are clearly expressed in mutant VP cells. During postnatal life a small but increasing number of solitary post-mitotic VP neurons of the di/di rat undergoes a switch to a heterozygous phenotype. At the same time Ang II and 7B2 show up again in these heterozygous cells, which suggests that for the expression of 7B2, but not for that of other peptides (e.g. galanin), a normal VP precursor is required. A possible underlying mechanism (i.e. the existence of several domains on the endoplasmic reticulum involved in the translocation of sets of neuropeptides) for this differential phenotypic expression of co-existing peptides is discussed.

Animal models for osmoregulatory disturbances

For the various types of di in humans, animal models are available. However, their value for explaining human di is for the major part an indirect one; by studying cellular mechanisms in these animal models, fundamental aspects of the cellular processes become available, which will help to understand similar processes in human di and subsequently lead to the molecular cause(s) of the various types of human di. Finally, it is to be expected that in the very near future transgenic animals will be raised in which very specific genetic information is overexpressed (or knocked out by homologous recombination; McMahon and Bradley, 1990). Recently hypervasopressinemia could be shown in transgenic mice, providing an animal model for the syndrome of the inappropriate VP secretion (Bartter and Schwartz, 1967), which is often observed in patients with lung cancers that ectopically express the VP gene (Habener et al., 1989). Furthermore it will be possible to study the exact cause(s) of human di by performing in vitro mutagenesis and to express the RNA constructs within a cell-free translation system and in oocytes (e.g., Schmale et al., 1989) and subsequently study the pattern of precursor synthesis, packaging and processing.

Peptide accretions in the endoplasmic reticulum of magnocellular neurosecretory neurons in normal and experimentally manipulated rats

Light microscopic observations using Nomarski interference contrast optics or darkfield optics on unstained aldehyde-fixed vibratome sections of hypothalami from normal young adult male and female Long Evans rats and from vasopressin-deficient Brattleboro rats, revealed the presence of cells in the supraoptic, paraventricular and periventricular nuclei which possessed yellow birefringent inclusions of globular or filamentous appearance in their somata. These inclusions were morphologically distinct from the large lipid droplets present in vasopressinergic magnocellular neurons of diabetes insipidus mice. Small portions of the vibratome sections containing the birefringent cells were excised and prepared for correlative electron microscopy. This revealed that the birefringent inclusions represented electron-dense material within cisterns of endoplasmic reticulum in magnocellular neurons. Antibodies to oxytocin or oxytocin-associated neurophysin immunolabelled the intracisternal electron-dense material and neurosecretory granules in resin-embedded ultrathin sections. Antibodies to vasopressin or vasopressin-associated neurophysin, and a panel of lectins did not label the intracisternal material. Quantitation revealed a small increase in the numbers of birefringent cells in aged rats and in rats drinking saline for 3 days. Subcutaneous injection of oestradiol benzoate for 7 days prior to fixation caused a large increase. After cessation of oestradiol administration the numbers of birefringent cells decreased; observations on the remaining cells showed that the endoplasmic reticulum cisterns were frequently fused with the plasmalemma, resulting in direct release of neurosecretory material into the extracellular spaces.

Vasopressin immunoreactive fibers and neurons in the dorsal pontine tegmentum of the rat, monkey and human

It is now well established that extensive extrahypothalamic vasopressin (VP) systems exist in the rat, monkey and human brain. There are marked differences between species, but in each case VP nuclei provide dense afferents to the dorsal pontine tegmentum. Here VP may play a role in the mechanisms exerted by the locus coeruleus (LC) neurons, possibly both as a neurotransmitter and as a neuromodulator. Although we are aware of some properties of VP systems, e.g., gonadal steroid dependency in the rat, major gaps characterize our knowledge of its anatomy. With regard to the interaction of VP with the LC in the brainstem of mammals some of the questions which stand out are: (1) Is VP really being biosynthesized and transported by LC cells and, if not, what is its function within these cells? (2) Is there a structural difference between male and female LC neurons in the rat as a consequence of the sex-dimorphic VP innervation? (3) What is the origin of VP afferents in the dorsal pontine tegmentum of the (non)human primate and are these afferents also controlled by gonadal steroids? Research strategies to answer these questions will provide us with information to resolve some of the current inconsistencies about the anatomy and the function of the VP and LC systems in the brain.

Chimeric and molecular genetic analysis of myelin-deficient (shiverer and mld) mutant mice

The shiverer and myelin-deficient (mld) mutants are two allelic mutations. Both are characterized by hypomyelination in the CNS and deficient expression of the MBP gene. Chimeric analysis of the pathogenesis of shiverer showed that shiverer mutation acts intrinsic to the oligodendrocyte, which is the only cell type expressing the MBP gene in the CNS. Molecular genetic studies by several groups demonstrated that shiverer is a deletion mutation in the MBP gene. Consequently, no MBP is produced in the shiverer mutant. In mld, however, partial expression of the MBP gene was observed. Interestingly, MBP was expressed in a mosaic fashion in the CNS of mld mice. Molecular genetic studies revealed that the mld mutant has duplicated MBP genes in tandem on a distal part (E2-4) of chromosome 18q. The reduced expression was based on the level of mRNA. A large portion is inverted in the upstream copy, and an intact copy is located downstream. We showed in mld mutants that antisense RNA corresponding to the inverted segment is transcribed, and it forms RNA duplex, with the RNA transcribed from the normal gene located downstream.