C-terminal fragments of APP have been on the radar for some time. Both caspases and their Asp664 APP cleavage products are elevated in AD brain (Gervais et al., 1999), and C31, the main cleavage product, promotes apoptosis, or programmed cell death (see ARF related news story). That this fragment plays a crucial pathological role became evident when Galvan and colleagues showed that it is essential for toxicity in transgenic mouse models of AD—abolishing the caspase site protects PDAPP mice from synaptic and cognitive losses induced by human APP, even though the animals accumulate Aβ at normal rates (see ARF related news story). But there were also hints in that study that the C-terminal fragments might have a normal role because the researchers found Asp664-cleaved fragments in young, non-diseased mouse brains as well.

Now, first author Surita Banwait and colleagues extend that last observation to human samples. Using the APPNeo antibody, a well-characterized immunoglobulin that reacts with the N-terminal end of Asp664-cleaved APP products, the researchers compared the distribution of C-terminal APP fragments in postmortem brain tissue from young, healthy humans; AD patients; and age-matched older controls. In keeping with previous observations, they found a significant increase in immunoreactivity in the hippocampus in early AD (Braak stages II and III) and a significant drop in immunoreactivity in late stages of the disease (Braak V and VI). Age-matched controls showed relatively little immunoreactivity. Remarkably, however, immunoreactivity in young controls was several orders of magnitude higher than in older, non-demented adults.

The big difference between young, normal adults and early-stage AD patients is in the distribution of the peptides. In young adults the fragments turn up in both the entorhinal cortex (EC) and the hippocampus. In the EC, the fragments appear in both soma and projections of stellate neurons and in glia. In the hippocampus, APPNeo immunoreactivity is prominent in granular cells and what appear to be mossy fiber terminals. In older, early-stage AD patients, the fragments appear mostly in nuclei and perinuclear spaces and even colocalize with chromatin. In older, non-diseased brain, the distribution is the same as in young adults, but the expression of the fragments is dramatically reduced. This gradual disappearance of the fragments with normal aging appears related to reduced processing rather than reduced expression of APP, since the researchers found that APP levels in young and older adults did not correlate with age.

What are these fragments doing in young, healthy brains? “I think an easy explanation is that they have a regulatory function at the level of the synapse, which is why we see them in terminals and in processes,” said Galvan. She went on to explain that cleavage at Asp664 renders APP incompetent for signaling in two ways. First, the cleavage deprives the APP intracellular domain (AICD) of its NPTY motif, which is essential for binding Fe65, Tip60, and the other partners that purportedly transport AICD to the nucleus, where it initiates transcriptional responses (see ARF related news story and ARF news story). Second, Asp664 cleavage also disrupts APP’s internalization signal, compromising its retrieval from the cell surface by endocytosis. Why cleavage trails off as people age is not clear, but it may reflect normal maturation of the brain. Galvan noted that some brain maturation processes, such as myelination, go on well into a person’s twenties.

Brain maturation is also the subject of a second paper this week with echoes in AD research. In the March 11 PNAS, Bradley Schlaggar and colleagues at Washington University, St. Louis, reported that the brain’s “default network” only fully matures in adults.

The default network is a concept that was evoked to explain why certain areas of the brain are active during periods of mental quiescence but deactivated when the brain focuses on a specific task, such as a face-name recognition task. The theory goes that the default network is involved in introspection or some internal narrative. Such networks are of interest to AD researchers because recent work suggests that failure to deactivate certain brain areas is indicative of learning problems (see ARF related news story), mild cognitive impairment (see Rombouts et al., 2005), and may even turn out to be a diagnostic criterion for AD (see ARF related news story and ARF news story; see also Buckner et al., 2005).

First author Damien Fair and colleagues used functional, or blood-oxygen level-dependent (BOLD), MRI to compare default network activation in 210 subjects covering three age groups: 7-9 years, 10-15 years, and 19-31 years. The researchers found that the medial prefrontal cortex is only sparsely connected to other brain regions, such as the posterior cingulate and the lateral parietal cortex, in 7-9 year olds, but that these connections gradually grow with time to be much more extensive by age 30. The authors suggest that increased myelination, which would support functional integration of distant regions, may be one reason for the developmental growth of the default network, though they note that myelination is probably not the sole driving factor.

Given that children are well able to perform introspective tasks, such as episodic memory retrieval, this study raises questions about the functional role of default networks. The authors note that episodic memory continues to improve with age and that this improvement may be less to do with encoding and retrieval mechanisms but with increasingly more complex strategies for carrying them out. The functional coherence of the default network is also associated with better executive function in older adults (see ARF related news story).—Tom Fagan.

References:
Banwait S, Galvan V, Zhang J, Gorostiza OF, Ataie M, Huang W, Crippen D, Koo EH, Bredesen DE. C-terminal cleavage of the amyloid-β protein precursor at Asp664: A switch associated with Alzheimer’s Disease. Journal of Alzheimer’s Disease 2008 March 7; 13: 1-16. Abstract

Fair DA, Cohen AL, Dosenbach NUF, Church JA, Miezin FM, Barch DM, Raichle ME, Petersen SE, Schlaggar BL. The maturing architecture of the brain’s default network. PNAS 2008. March 11;105:4028-4032. Abstract