8-month-old, intact female mouse (Mus musculus), B6.Â Cg - Tg(APPSwFlLon,PSEN1*M146L*L286V)6799Vas/Mmjax.The goal of this experiment was to test the efficacy of a novel compound. This mouse was in the control group and was administered the vehicle
without the compound by subcutaneous injection. Mice were sacrificed at the end of the study and tissues were collected to assess the effect
Complete necropsies were
performed and no gross lesions were observed.
Coronal sections of the head are provided, and include sections of the brain at the level of the cerebral cortex, hippocampus,
thalamus, midbrain, and pituitary gland, and at the level of the cerebellum and brainstem.Â Multifocally in the neuropil of the cerebral cortex, hippocampus,
thalamus, and midbrain, there are numerous aggregates of a basophilic, amorphous to finely fibrillar material, which is often surrounded by a rim of decreased
staining intensity of the neuropil.Â On Congo Red stain, this material is congophilic and shows green birefringence when observed with polarized light.
Brain, cerebral cortex, hippocampus, thalamus, and midbrain: Abundant neuritic plaques with amyloid core, multifocal.
The mouse strain B6.Cg-Tg(APPSwFlLon,PSEN1*M146L*L286V)6799Vas/Mmjax is also known by the common names 5XFAD and Tg6799.Â This strain overexpresses two mutant human genes on a C57BL/6 and SJL background.Â The first gene, APP695, contains four mutations associated
with familial Alzheimers disease (FAD): the Swedish (K670N, M671L), Florida (I716V), and London (V717I) mutations.Â The second gene, PS1, contains
two mutations also associated with FAD: M146L and L286V.Â Expression of both transgenes is regulated by neural-specific elements of the mouse Thy1 promoter to drive overexpression in the brain.Â This strain rapidly recapitulates the amyloid pathology of Alzheimers disease (AD), beginning at 2 months of
age and characterized by the formation of a heavy burden of neuritic plaques.(1) Neuritic plaques (also called senile plaques) are focal round collections of
dilated and tortuous neurites that surround an amyloid core that can be stained with Congo Red.(2) Neuritic plaques are often surrounded by a clear halo and by reactive astrocytes and microglial cells.Â In humans, these plaques range in size from 20 to 200 Î¼m and are predominantly observed in the hippocampus, amygdala, and neocortex, with relative sparing of primary motor and sensitive cortices.
The main component of the amyloid core is A, a peptide derived from cleavage of the larger protein Amyloid Precursor Protein (APP), a cell surface protein that may function as a receptor.(2) The A peptides readily aggregate and can be directly neurotoxic and can result in synaptic dysfunction and an inflammatory response.Â Other proteins that present in plaques in smaller amounts include complement proteins, pro-inflammatory cytokines, 1 -antichymotrypsin, and apolipoproteins.Â Findings in individuals affected by the familiar form of AD have supported the hypothesis that the generation of A is a critical step in the initiation of the disease.Â Some patients with familial AD have point mutations of the APP gene.Â Two other commonly affected loci are in the Presenilin 1 and 2 (PS1, PS2), which lead to a gain of function of the -secretase complex, which is involved in the cleavage of APP into A.
The other main morphologic change in AD is the formation of neurofibrillary tangles, which are composed of bundles of filaments in the cytoplasm of neurons.(2) These are seen as basophilic structures by H&E staining, and can be demonstrated by silver stain methods such as Bielschowsky.Â A major component of these filaments is a hyperphosphorylated form of the protein tau.Â Tangles are not specific to AD, as they can be found in other diseases.Â There was no evidence of neurofibrillary tangles in the brain of these mice on H&E and Bielschowsky stains.
1.Â Cerebrum, hippocampus, amygdaloid nucleus and brainstem: Neuritic plaques, numerous, diffuse, with gliosis and neuronal loss.
2.Â Brainstem nuclei and neurons: Chromatolysis, multifocal, marked, with spheroid formation.
The contributor provided a thorough description of the components of the senile (neuritic) plaques and neurofibrillary tangles (NFT), the two microscopic hallmarks of Alzheimers disease first described by Alois Alzheimer in 1906.(3) Senile plaques and NFTs represent two manifestations of abnormally folded A proteins.1 Normally the body
prevents such misfolded proteins from depositing in tissues and interfering with normal functions via several mechanisms collectively known as the
unfolded protein response (UPR).Â The UPR activates signaling pathways that: 1) increase chaperone production in an attempt to repair, refold, and return proteins to normal; 2) slow protein translation; and 3) enhance the ubiquitination and proteolysis of misfolded proteins via the ubiquitinproteasome pathway.(1,4,5) If the UPR is insufficient,
abnormal proteins may be removed by autophagy, the major degradative pathway for intracellular components.Â Failure of autophagy has been implicated in the pathogenesis of several neurodegenerative diseases, including Alzheimers disease.(5)
Three autophagic mechanisms are described in mammalian cells: chaperone-mediated autophagy, microautophagy, and macroautophagy, all which result in the lysosomal degradation of targeted cellular components.Â In chaperone-mediated autophagy, cytoplasmic proteins exposing a KFERQ-like motif are targeted directly to the lysosomes for degradation.Â Microautophagy involves the invagination of the lysosomal membrane to nonselectively engulf and degrade smal l por t ions of the cytoplasm.Â Macroautophagy is the best characterized mechanism of autophagy; thus, it is often referred to as simply autophagy. Unlike chaperone-mediated autophagy and microautophagy, macroautophagy relies on the de novo synthesis of double membrane-bound autophagosomes. The major upstream inhibitor of macroautophagy initiation is the mammalian target of rapamycin complex 1 (mTORC1), which regulates several cellular processes, to include autophagy, cell growth and proliferation, and protein synthesis.Â Various cell signaling pathways converge upon mTORC1; for instance, in response to insulin and growth factors, the class I phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway activates mTORC1 and thus suppresses autophagy.Â Conversely, energy depletion signals through the liver kinase B1 (LKB1)/AMPactivated protein kinase (AMPK) pathway to inhibit mTORC1, and thereby activate autophagy.
When mTORC1 is inhibited, autophagy-related (Atg) proteins coordinate a series of events that results in the formation of an autophagosome (i.e., vesicle nucleation, vesicle elongation, docking and fusion, and vesicle maturation).Â This series begins with the formation of a phagophore (i.e., pre-autophagosomal structure) and progresses to the formation of a double membrane enclosed autophagosome, the outer membrane of which fuses with a lysosome or a late endosome to form an autolysosome or an amphisome, respectively.Â Acidic hydrolases digest material within the autolysosome or amphisome, after which the lysosome is restored.(5)
Current research strongly suggests the disruption of proteolysis within autolysosomes is the principle mechanism underlying autophagy failure in Alzheimers disease.Â Various forms of autophagic vacuoles representing intermediate stages in autophagy (including autophagosomes, amphisomes, and autolysosomes) have been observed to accumulate in the brains of patients with Alzheimers disease and other neurodegenerative diseases.(4,5) Interestingly, attempts to restore lysosomal proteolysis and enhance autophagy in mouse models of Alzheimers disease have shown positive effects on neuronal function and cognitive performance.(5)
1.Â Frosch MP, Anthony DC, De Girolami U.Â The central nervous system.Â In: Kumar V, Abbas AK, Fausto N, Aster JC, eds.Â Robbins and Cotran Pathologic Basis of Disease 8th ed.Â Philadelphia, PA: Saunders Elsevier; 2010:1279-1344.
2.Â Oakley H, Cole SL, Logan S, et al.Â Intraneuronal amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimers disease mutations: potential factors in amyloid plaque formation.Â J Neurosci. 2006;26:10129-10140.
3.Â Gibbs G.Â Alois Alzheimer: The Man.Â http://www.unmc .edu/intmed/geriatrics/docs/alois_alzheimer.pdf Accessed 19 October 2012.
4.Â Levine B, Kroemer G.Â Autophagy in the pathogenesis of disease.Â Cell.Â 2008;132(1):27-42.
5.Â Nixon RA, Yang DS.Â Autophagy failure in Alzheimers disease-ï¿½-ï¿½Locating the Primary Defect.Â Neurobiol Dis. 2011; 43(1):38-45.