Rac1 activation links tau hyperphosphorylation and Aß dysmetabolism in Alzheimer's disease.

One of the earliest pathological features characterizing Alzheimer's disease (AD) is the loss of dendritic spines. Among the many factors potentially mediating this loss of neuronal connectivity, the contribution of Rho-GTPases is of particular interest. This family of proteins has been known for years as a key regulator of actin cytoskeleton remodeling. More recent insights have indicated how its complex signaling might be triggered also in pathological conditions. Here, we showed that the Rho-GTPase family member Rac1 levels decreased in the frontal cortex of AD patients compared to non-demented controls. Also, Rac1 increased in plasma samples of AD patients with Mini-Mental State Examination < 18 compared to age-matched non demented controls. The use of different constitutively active peptides allowed us to investigate in vitro Rac1 specific signaling. Its activation increased the processing of amyloid precursor protein and induced the translocation of SET from the nucleus to the cytoplasm, resulting in tau hyperphosphorylation at residue pT181. Notably, Rac1 was abnormally activated in the hippocampus of 6-week-old 3xTg-AD mice. However, the total protein levels decreased at 7-months. A rescue strategy based on the intranasal administration of Rac1 active peptide at 6.5 months prevented dendritic spine loss. This data suggests the intriguing possibility of a dual role of Rac1 according to the different stages of the pathology. In an initial stage, Rac1 deregulation might represent a triggering co-factor due to the direct effect on Aß and tau. However, at a later stage of the pathology, it might represent a potential therapeutic target due to the beneficial effect on spine dynamics.

Molecular subtypes of Alzheimer's disease.

Protein misfolding and aggregation is a central feature of several neurodegenerative disorders including Alzheimer's disease (AD), in which assemblies of amyloid ß (Aß) peptides accumulate in the brain in the form of parenchymal and/or vascular amyloid. A widely accepted concept is that AD is characterized by distinct clinical and neuropathological phenotypes. Recent studies revealed that Aß assemblies might have structural differences among AD brains and that such pleomorphic assemblies can correlate with distinct disease phenotypes. We found that in both sporadic and inherited forms of AD, amyloid aggregates differ in the biochemical composition of Aß species. These differences affect the physicochemical properties of Aß assemblies including aggregation kinetics, resistance to degradation by proteases and seeding ability. Aß-amyloidosis can be induced and propagated in animal models by inoculation of brain extracts containing aggregated Aß. We found that brain homogenates from AD patients with different molecular profiles of Aß are able to induce distinct patterns of Aß-amyloidosis when injected into mice. Overall these data suggest that the assembly of mixtures of Aß peptides into different Aß seeds leads to the formation of distinct subtypes of amyloid having distinctive physicochemical and biological properties which result in the generation of distinct AD molecular subgroups.

Losing protein in the brain: the case of progranulin.

It is well known that progranulin protein is involved in wound repair, inflammation, and tumor formation. The wedding between progranulin and brain was celebrated in 2006 with the involvement of progranulin gene (GRN) in Frontotemporal lobar degeneration (FTLD), the most common form of early-onset dementia: up to date, 75 mutations have been detected in FTLD patients as well as in patients with widely variable clinical phenotypes. All pathogenic GRN mutations identified thus far cause the disease through a uniform mechanism, i.e. loss of functional progranulin or haploinsufficiency. Studies on GRN knockout mice suggest that progranulin-related neurodegenerative diseases may result from lifetime depletion of neurotrophic support together with cumulative damage in association with dysregulated inflammation, thus highlighting possible new molecular targets for GRN-related FTLD treatment. Recently, the dosage of plasma progranulin has been proposed as a useful tool for a quick and inexpensive large-scale screening of affected and unaffected carriers of GRN mutations. Before it is systematically translated into clinical practice and, more importantly, included into diagnostic criteria for dementias, further standardization of plasma progranulin test and harmonization of its use are required. Once a specific treatment becomes available for these pathologies, this test - being applicable on large scale - will represent an important step towards personalized healthcare. This article is part of a Special Issue entitled: Brain Integration.

Circulating progranulin as a biomarker for neurodegenerative diseases.

Progranulin is a growth factor involved in the regulation of multiple processes including tumorigenesis, wound repair, development, and inflammation. The recent discovery that mutations in the gene encoding for progranulin (GRN) cause frontotemporal lobar degeneration (FTLD), and other neurodegenerative diseases leading to dementia, has brought renewed interest in progranulin and its functions in the central nervous system. GRN null mutations cause protein haploinsufficiency, leading to a significant decrease in progranulin levels that can be detected in plasma, serum and cerebrospinal fluid (CSF) of mutation carriers. The dosage of circulating progranulin sped up the identification of GRN mutations thus favoring genotype-phenotype correlation studies. Researchers demonstrated that, in GRN null mutation carriers, the shortage of progranulin invariably precedes clinical symptoms and thus mutation carriers are "captured" regardless of their disease status. GRN is a particularly appealing gene for drug targeting, in the way that boosting its expression may be beneficial for mutation carriers, preventing or delaying the onset of GRN-related neurodegenerative diseases. Physiological regulation of progranulin expression level is only partially known. Progranulin expression reflects mutation status and, intriguingly, its levels can be modulated by some additional factor (i.e. genetic background; drugs). Thus, factors increasing the production and secretion of progranulin from the normal gene are promising potential therapeutic avenues. In conclusion, peripheral progranulin is a nonintrusive highly accurate biomarker for early identification of mutation carriers and for monitoring future treatments that might boost the level of this protein.

Cystatin C is released in association with exosomes: a new tool of neuronal communication which is unbalanced in Alzheimer's disease.

It has recently become clear that proteins associated with neurodegenerative disorders can be selectively incorporated into intraluminal vesicles of multivesicular bodies and subsequently released within exosomes. Multiple lines of research support a neuroprotective role for cystatin C in Alzheimer's disease (AD). Herein we demonstrate that cystatin C, a protein targeted to the classical secretory pathway by its signal peptide sequence, is also secreted by mouse primary neurons in association with exosomes. Immunoproteomic analysis using SELDI-TOF MS revealed the presence in exosomes of at least 9 different cystatin C glycoforms. Moreover, the over-expression of familial AD-associated presenilin 2 mutations (PS2 M239I and PS2 T122R) resulted in reduced levels of all cystatin C forms (native and glycosylated) and of amyloid-ß precursor protein (APP) metabolites within exosomes. A better understanding of the mechanisms involved in exosomal processing and release may have important implications for the fight against AD and other neurodegenerative diseases.

Presenilin 2 mutations alter cystatin C trafficking in mouse primary neurons.

Mutations in the presenilin genes account for the majority of familial Alzheimer disease (FAD) cases. In the present report we demonstrated that the FAD-linked presenilin 2 mutations (PS2 M239I and T122R) alter cystatin C trafficking in mouse primary neurons reducing secretion of its glycosylated form. These mutations showed a different impact on cystatin C: PS2 T122R had a much stronger effect determining a dramatic intracellular accumulation of cystatin C (native and glycosylated), followed by a reduction in the secretion of both forms. Several experimental evidences suggest that cystatin C exerts a protective role in the brain and favors stem cells proliferation. Confocal imaging showed that the effect of PS2 T122R mutation was a massive recruitment of cystatin C into the neuronal processes, in the presence of an intact cytoskeletal structure. The consequent reduction in the cystatin C extracellular levels might result in a failure of neuroregeneration. Understanding the interplay of PS2 and cystatin C in the pathogenesis of AD might highlight new therapeutic prospective.