Joined: 8/20/2013 Posts: 56
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Have not see this on these boards but wondering if this a significant step forward? This appears to be a "new" theory or mechanisms to explain AD and a looks like it is a bit of a divergence from the standard Amyloid Cascade Hypothesis. Seems like we need new theories and this sounds cogent enough so one wonders if it is a large step forward?
Sorting receptor SORLA – a trafficking path to avoid Alzheimer disease
Summary Excessive proteolytic breakdown of the amyloid precursor protein (APP) to neurotoxic amyloid b peptides (Ab) by secretases in the brain is a molecular cause of Alzheimer disease (AD). According to current concepts, the complex route whereby APP moves between the secretory compartment, the cell surface and endosomes to encounter the various secretases determines its processing fate. However, the molecular mechanisms that control the intracellular trafficking of APP in neurons and their contribution to AD remain poorly understood. Here, we describe the functional elucidation of a new sorting receptor SORLA that emerges as a central regulator of trafficking and processing of APP. SORLA interacts with distinct sets of cytosolic adaptors for anterograde and retrograde movement of APP between the trans-Golgi network and early endosomes, thereby restricting delivery of the precursor to endocytic compartments that favor amyloidogenic breakdown. Defects in SORLA and its interacting adaptors result in transport defects and enhanced amyloidogenic processing of APP, and represent important risk factors for AD in patients. As discussed here, these findings uncovered a unique regulatory pathway for the control of neuronal protein transport, and provide clues as to why defects in this pathway cause neurodegenerative disease.
Loss of SORLA activity might underlie sporadic AD Given all the evidence that implicates SORLA in the cellular processing of APP, an important question concerns the genetic and pathophysiological mechanisms that predispose individuals to low receptor activity and, hence, to increased amyloidogenic processing that is causative of sporadic AD.
Because gene expression profiling in lymphoblasts has uncovered 60% less SORL1 transcripts in AD patients compared with controls, it has been proposed that sequence variations in SORL1 might affect transcript levels (Scherzer et al., 2004). In accordance with this assumption, Grear and colleagues found a correlation between lower levels of SORL1 mRNA and the presence of a SNP in the 59 intronic region of the gene in cases of AD (Grear et al., 2009). Recently, we identified the presence of two closely linked SNPs in the 39 region of SORL1, which correlate with low expression of the receptor in the diseased brain. These gene variations alter the sequence of the SORL1 transcript, and result in a change from a frequent to a rare codon usage in the SNP genotype. Further studies in cultured cells confirmed that these SNP-containing transcripts are less efficiently translated into protein and result in a 30% reduction in SORLA protein levels, which is similar to the reduction seen in AD patients (Caglayan et al., 2012).
However, regression analyses suggested that sequence variations only account for as little as 14% of variations in SORL1 mRNA levels, and therefore indicate that additional genetic or non-genetic factors modulate SORL1 expression in the brain (Rogaeva et al., 2007). One such factor is docosahexaenoic acid (DHA), an essential dietary v-3 polyunsaturated fatty acid that has been implicated in AD, because low blood levels of DHA have been associated with an increased risk of neurodegeneration in a large epidemiological study (Johnson and Schaefer, 2006). DHA was shown to markedly upregulate expression of SORLA in primary neurons and in mouse models, arguing that the increased disease risk when DHA levels are low might be explained by defects in induction of SORL1 transcription (Ma et al., 2007). Another protein that modulates SORLA levels in the brain is brain-derived neurotrophic factor (BDNF), a growth factor that provides trophic support to neurons (Chao, 2003). BDNF potently activates transcription of the SORL1 gene by more than tenfold through the extracellular-signal-regulated kinase (ERK) pathway and yet unknown downstream transcription factors (Rohe et al., 2009). It is possible that a poor trophic support to neurons, as shown in chronic insults to the nervous system, is deleterious through the associated loss of SORL1 expression (Matrone et al., 2008; Rohe et al., 2009). Because SORLA and its interacting adaptors have a pivotal role in the trafficking of APP, it comes as no surprise that additional components of this trafficking machinery emerge as genetic risk factors in AD. Foremost, this applies to the pentameric retromer complex, which is composed of VPS35, VPS29, VPS26 and the sorting nexins 1 and 2 (reviewed in Seaman, 2012). VPS35 and VPS26 are poorly expressed in the brain of AD patients (Small et al., 2005), and disruption of Vps35 in mice results in memory deficits and synaptic dysfunction, which are associated with elevated Ab levels (Muhammad et al., 200 . As retrograde transport of SORLA, which is mediated by retromer, is important for processing of APP (Fjorback et al., 2012), impaired retromer function, might be an underlying cause of neurodegenerative processes (Nothwehr et al., 2000). In a similar manner, decreased levels of GGA3 are observed in brain autopsies of AD patients (Santosa et al., 2011). Moreover, the loss of GGA3 expression enhances b-secretase activity (Tesco et al., 2007), suggesting that impaired anterograde transport of SORLA is an additional cause of neuronal dysfunction.
Conclusions Recent years witnessed the detailed elucidation of cellular trafficking pathways that control APP processing, and how they contribute to neurodegenerative processes in patients. In particular, the functional characterization of SORLA, a unique sorting receptor for APP, sheds light on the targeted transport of APP to distinct neuronal compartments that harbour the various secretase activities – a process that was previously poorly understood. Results from histopathological and epidemiological studies further substantiate the crucial role of this receptor in sporadic AD. Equally exciting is a recent report that mutations in SORL1 might even be the cause of autosomal dominant forms of early-onset AD (Pottier et al., 2012). Here, five missense mutations, one of which maps to the APP binding domain in SORLA, have been identified in familial cases of AD. Although much still needs to be learned about this sorting pathway, it certainly holds great promise as a genetic risk factor and new biomarker for diagnosis, and perhaps it might even constitute a druggable target for the treatment of AD.
http://jcs.biologists.org/content/126/13/2751.full.pdf+html
http://jcs.biologists.org/content/126/13/2751.long
So it looks like one can get a hold of DHA quite easily and also appears to be supplements with BDNF available. Do we add these to the regimine?
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