Regional distribution of presenilin-1 messenger RNA in the embryonic rat brain: comparison with beta-amyloid precursor protein messenger RNA localization

Cortical structure and the risk for Alzheimer's disease: a bidirectional Mendelian randomization study

Progressive loss of neurons in a specific brain area is one of the manifestations of Alzheimer's disease (AD). Much effort has been devoted to investigating brain atrophy and AD. However, the causal relationship between cortical structure and AD is not clear. We conducted a bidirectional two-sample Mendelian randomization analysis to investigate the causal relationship between cortical structure (surface area and thickness of the whole cortex and 34 cortical regions) and AD risk. Genetic variants used as instruments came from a large genome-wide association meta-analysis of cortical structure (33,992 participants of European ancestry) and AD (AD and AD-by-proxy, 71,880 cases, 383,378 controls). We found suggestive associations of the decreased surface area of the temporal pole (OR (95% CI): 0.95 (0.9, 0.997), p = 0.04), and decreased thickness of cuneus (OR (95% CI): 0.93 (0.89, 0.98), p = 0.006) with higher AD risk. We also found a suggestive association of vulnerability to AD with the decreased surface area of precentral (β (SE): -43.4 (21.3), p = 0.042) and isthmus cingulate (β (SE): -18.5 (7.3), p = 0.011). However, none of the Bonferroni-corrected p values of the causal relationship between cortical structure and AD met the threshold. We show suggestive evidence of an association of the atrophy of the temporal pole and cuneus with higher AD risk. In the other direction, there was a suggestive causal relationship between vulnerability to AD and the decreased surface area of the precentral and isthmus cingulate. Our findings shed light on the associations of cortical structure with the occurrence of AD.

Diverse and pervasive subcellular distributions for both coding and long noncoding RNAs

Here, Wilk et al. examined ∼8000 mRNA transcripts throughout Drosophila embryogenesis. They found that almost all RNAs, both coding and noncoding RNAs, are subcellularly localized at some stage of development, thus providing an important resource for functional gene analysis.

In a previous analysis of 2300 mRNAs via whole-mount fluorescent in situ hybridization in cellularizing Drosophila embryos, we found that 70% of the transcripts exhibited some form of subcellular localization. To see whether this prevalence is unique to early Drosophila embryos, we examined ∼8000 transcripts over the full course of embryogenesis and ∼800 transcripts in late third instar larval tissues. The numbers and varieties of new subcellular localization patterns are both striking and revealing. In the much larger cells of the third instar larva, virtually all transcripts observed showed subcellular localization in at least one tissue. We also examined the prevalence and variety of localization mechanisms for >100 long noncoding RNAs. All of these were also found to be expressed and subcellularly localized. Thus, subcellular RNA localization appears to be the norm rather than the exception for both coding and noncoding RNAs. These results, which have been annotated and made available on a recompiled database, provide a rich and unique resource for functional gene analyses, some examples of which are provided.

Presenilin 1 in migration and morphogenesis in the central nervous system

Morphogenesis of the central nervous system relies in large part upon the correct migration of neuronal cells from birthplace to final position. Two general modes of migration govern CNS morphogenesis: radial, which is mostly glia-guided and topologically relatively simple; and tangential, which often involves complex movement of neurons in more than one direction. We describe the consequences of loss of function of presenilin 1 on these fundamental processes. Previous studies of the central nervous system in presenilin 1 homozygote mutant embryos identified a premature neuronal differentiation that is transient and localized, with cortical dysplasia at later stages. We document widespread effects on CNS morphogenesis that appear strongly linked to defective neuronal migration. Loss of presenilin 1 function perturbs both radial and tangential migration in cerebral cortex, and several tangential migratory pathways in the brainstem. The inability of cells to execute their migratory trajectories affects cortical lamination, formation of the facial branchiomotor nucleus, the spread of cerebellar granule cell precursors to form the external granule layer and development of the pontine nuclei. Finally, overall morphogenesis of the mid-hindbrain region is abnormal,resulting in incomplete midline fusion of the cerebellum and overgrowth of the caudal midbrain. These observations indicate that in the absence of presenilin 1 function, the ability of a cell to move can be severely impaired regardless of its mode of migration, and, at a grosser level, brain morphogenesis is perturbed. Our results demonstrate that presenilin 1 plays a much more important role in brain development than has been assumed, consistent with a pleiotropic involvement of this molecule in cellular signaling.

Expression of presenilin-1 and Notch-1 receptor in human embryonic CNS

In vitro studies have shown that the Alzheimer's disease-related presenilin-1 protein can mediate Notch-1 receptor cleavage during signalling. In the present study, we compared the distribution of presenilin-1 and Notch-1 receptor immunoreactivities in human embryonic CNS tissue during the first trimester of development. Our aim was to gain insight into whether these proteins are likely to interact functionally during human fetal brain development. CNS material was obtained from routine abortions, cryosectioned and studied by means of immunohistochemistry with antibodies to presenilin-1 and Notch-1. At very early stages of embryonic development (four to five gestational weeks) intensive presenilin-1 immunoreactivity could be seen predominantly in neurites in the ventral horn of the spinal cord, where it overlapped with 200-kDa neurofilament immunoreactivity. Presenilin-1 immunoreactivity was also seen in neuroblasts of the ventricular zone of the tel- and mesencephalon, as well as of the brainstem. Notch-1 receptor appeared in neuronal and ependymal cells throughout the CNS. Seven- to eight-week CNS tissue showed similar patterns of presenilin-1 and Notch-1 receptor expression in the spinal cord and cerebral cortex as was seen at five weeks. Both proteins were localised in the neuroepithelial cell layer lining the ventricles, as well as in the cortical plate layer, where immunoreactivity was seen in the cell bodies. In addition, presenilin-1 immunoreactivity was seen in thin neurites in the subplate of the developing cortex. At 10 weeks, presenilin-1 immunoreactivity was reduced in the spinal cord. These results show that, although presenilin-1 and Notch-1 receptor are localised to the same differentiating cell populations in the human cerebral cortex, making a direct interaction possible, these proteins are otherwise confined to different neurons or neuronal compartments, suggesting a role for presenilin-1 during early CNS differentiation that does not involve Notch-1 receptor processing. Double staining for presenilin-1 in the endoplasmic reticulum and presenilin-1 in the Golgi showed overlap to some extent in investigated CNS regions, but not in neurites. This suggests that presenilin-1 function during neurogenesis is not exclusively correlated to protein processing within the endoplasmic reticulum and Golgi, but that presenilin-1 may also be involved in other processes, such as axonal and dendritic outgrowth or synaptic formation. In summary, our findings provide supportive evidence that the presenilin-1 protein is involved in the development and maturation of the human fetal CNS. The presence of presenilin-1 immunoreactivity in both the cell bodies and neurites of developing neurons strongly suggests divergent mechanisms of function for presenilin-1 during human brain development. These may include interactions with any of the Notch receptor proteins, as well as Notch-independent mechanisms.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Expressions of amyloid precursor protein, synaptophysin and presenilin-1 in the different areas of the developing cerebellum of rat

This study reveals the expressions of Alzheimer's disease-related amyloid precursor protein, presenilin-1, and a presynaptic marker protein, synaptophysin, in the archi-, paleo- and neocerebellum during the postnatal development of the rat. The Western blot results demonstrate a gradual increase in the soluble amyloid precursor protein level in the archicerebellum during the first 3 weeks, while in the neo- and paleocerebellum the levels reach a plateau as early as the 1st week. Immunohistochemically, the protein is present in the deep part of the external granule cell layer and the internal granule cell layer in the newborn animal, while in 3-week-old animals the staining appears mainly in the perikarya and dendrites of the Purkinje cells. The level of synaptophysin increases progressively from postnatal day 7 up to 3 weeks in the archi- and paleocerebellum, and up to 6 weeks in the neocerebellum. Immunohistochemically, the amyloid precursor protein staining appears first in the inner part of the molecular layer and in the internal granule cell layer. In a 3-week-old animal, synaptophysin staining is present in all areas of the cerebellar molecular layer and in the internal granule cell layer. The presenilin-1 immunohistochemical reaction appeared equally in the archi-, paleo- and neocerebellum. Much of the staining is present in the glial cells and Purkinje cells. Less immunoreactivity is observed in the Golgi cells and granule cells. It is concluded that the postnatal expressions of soluble and membrane-bound amyloid precursor protein, synaptophysin and presenilin-1 are regulated differently during the ontogenetical development of the archi-, paleo- and neocerebellum of rat. Further, the amyloid precursor protein and presenilin-1 may be present in cells which do not degenerate in Alzheimer's disease.

Presenilins: structural aspects and posttranslational events

Most of early-onset forms of Alzheimer's disease (AD) are caused by inherited mutations located on chromosomes 14 and 1, the gene products of which have been recently identified and referred to as presenilins 1 (PS1) and 2 (PS2), respectively. The first phenotypic alterations triggered by mutated PS were reported to be an increased production of the amyloid peptide (Abeta) and, more precisely, its 42 amino-acids long counterpart Abeta42. This overproduction is thought to be responsible for the genesis of the senile plaques that invade the cortical and subcortical areas of these AD-affected brains. The discovery of PSs has triggered numerous studies aimed at better understanding their normal physiology and the dysfunctions brought by the mutations that could explain, at least in part, the neurodegenerative process taking place in this syndrome. In this review, I will focus on the structural aspects of PS and on the various posttranscriptional events they undergo. I will also briefly discuss that current hypotheses concerning their normal functions and the influence of FAD-linked mutations.