For all the scrutiny of the amyloid precursor protein (APP) as the source of pathogenic Aβ peptides, there is little understanding of the parent protein’s normal role in neurons. Using APP knockout mice, Hui Zheng and colleagues at Baylor College of Medicine, Houston, Texas, have been studying the need for APP in synapses, and their latest work identifies a new function for the protein in regulating calcium fluxes in some neurons. Specifically, the researchers find that APP controls the levels of an L-type calcium channel, CaV1.2, specifically in GABAergic neurons. Loss of APP results in higher levels of CaV1.2 protein and activity, which leads to altered short-term plasticity at synapses, they show. While the study does not directly link any of these changes to Alzheimer disease, the results suggest that alterations in APP metabolism or function could be linked to failed calcium homeostasis and changes in synaptic plasticity, both events that occur early on in the disease. In other APP news this week, researchers identify new signaling pathways that may mediate the effects of the precursor protein (see ARF related news story).

The work follows on Zheng’s previous studies on the neuromuscular junction in APP knockout mice, where she identified problems in synapse development (see ARF related news story and Wang et al., 2009) and function, including defects in GABA-induced short-term plasticity involving aberrant activation of L-type calcium channels (Yang et al., 2007). In the new study, the investigators extended their work to the CNS by looking at the expression of various classes of calcium channels in different brain regions in APP-null mice by Western blot. When they found a specific upregulation of the CaV1.2 L-type calcium channel in the striatum, first author Li Yang followed up with electrophysiology studies on striatal neuron cultures. Yang found that the increase in CaV1.2 coincided with the presence of exaggerated calcium currents that were reversed by nifedipine, a specific L-type calcium channel blocker. The cells also showed alterations in short-term plasticity (reduced paired-pulse inhibition and increased post-tetanic potentiation), which could be corrected by nifedipine. Re-expression of APP in the neurons using a lentivirus rescued both normal channel expression and calcium currents. Thus, it appears that loss of APP led to increased CaV1.2, which was responsible for increased calcium influx and accompanying functional changes in striatal GABAergic neurons.

What about neurons in the hippocampus, the site of early changes in calcium homeostasis in AD? The researchers did not see a significant difference in overall CaV1.2 expression in the hippocampal neurons between wild-type and APP-knockout mice. At the same time, they knew from previous work (Seabrook et al., 1999) that loss of APP results in a significant reduction in paired-pulse inhibition in GABAergic synapses there. When the researchers used quantitative immunostaining to look specifically at GABAergic neurons (which make up a minority of hippocampal neurons), they did find an elevation of CaV1.2. Along with this, they found altered short-term plasticity in the GABAergic neurons, which was reversed by nifedipine. Thus, it seems that increased CaV1.2 levels and calcium currents are a general result of loss of APP in GABAergic neurons, whether in the hippocampus or striatum.

Other recent data suggest that APP regulates L-type calcium currents and calcium oscillations in rat cortical (glutamatergic) neurons as well (Santos et al., 2009). In that study, though, increased channel function resulted from overexpressing human APP, the opposite to what Zheng and colleagues report. Taken together, the two studies “support the notion that APP may affect L-type calcium channel function in multiple neurons and by multiple mechanisms,” the authors write.

How does APP regulate CaV1.2 levels? The mechanism did not seem to involve transcription, as CaV1.2 mRNA levels were unchanged in APP knockout mice compared to wild-type. That ruled out a role for the APP intracellular domain (AICD), but another possibility was Aβ, which itself modulates calcium influx in neurons. However, treating wild-type cells with a γ-secretase inhibitor had no effect on channel levels, suggesting that Aβ likewise was not involved. The researchers did find a direct and specific physical interaction between the proteins by co-immunoprecipitation, and they speculate that APP may control CaV1.2 trafficking to the membrane.

As yet, Zheng says they have no clue why APP might regulate CaV1.2 specifically in GABAergic neurons. “If you look at the APP expression pattern and the L-type calcium channel expression pattern, we did find a higher level of L-type calcium channels in inhibitory neurons, but they are expressed in all the neurons. We hypothesize that there must be some intermediate molecule that is GABAergic specific, but we have not been able to identify that particular molecule,” she told ARF.

Calcium dysregulation is one of earliest manifestations of pathology in mouse models of AD, but it is not clear how this new function of APP might relate to disease processes. Zheng told ARF that her lab is now studying APP mutants to see if they affect calcium channel levels or function.—Pat McCaffrey

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  1. The finding that APP depletion can result in aberrations in calcium-channel function and synaptic function is interesting for reasons beyond the "protein’s normal role in neurons." We reported a little over a year ago that APP is severely depleted in neuronal somata in a manner correlated with Alzheimer plaque pathology (Barger et al., 2008). The more advanced the pathology in a given region, the lower the expression of APP in the neurons; likewise, the closer the neurons were to plaques, the lower their expression. Analysis of mRNA indicated that this was not simply a reflection of increased processing/degradation of APP protein.

    The basic finding—that AD pathology is associated with a decline in APP expression—runs contrary to what many have assumed: that Aβ accumulation in AD results, at least in part, from elevated levels of its precursor. Interestingly, we did find a gradual rise in APP levels with advancing age. Thus, the drop in APP appears to reflect something peculiar to the pathogenic process of Alzheimer disease.

    References:

    . Relationships between expression of apolipoprotein E and beta-amyloid precursor protein are altered in proximity to Alzheimer beta-amyloid plaques: potential explanations from cell culture studies. J Neuropathol Exp Neurol. 2008 Aug;67(8):773-83. PubMed.

  2. The current study by Yang et al. shows a novel physiological role for APP in the regulation of the L-type voltage-gated calcium channel specifically in GABAergic neurons. These results highlight APP—and APP fragments—as important regulators of synaptic function, and give new insight into the role of neuronal APP, as well as insight into how disrupted APP proteolysis could affect synaptic plasticity in Alzheimer disease. Interestingly, there are several parallels between APP deletion and reported effects of endogenously applied Aβ peptide—both appear to augment current through the L-type channel via post translational regulation of steady-state levels. Scragg et al., 2005 showed that Aβ could promote insertion or retention of L-type channels in the plasma membrane, while here, Yang et al. show that full-length APP and the L-type channel are associated, and they hypothesize that APP could be mediating trafficking of the channel from the plasma membrane, although in an Aβ-independent fashion. It is hence interesting that multiple APP fragments can specifically affect L-type calcium channel currents and steady-state levels, but in apparent opposing roles. Of note is the specificity of these effects of APP deletion to GABAergic neurons; future studies will no doubt be required to understand the special role that APP plays in these neuronal subtypes.

    View all comments by Kim Green

References

News Citations

  1. Brushing Up on APP—Mint-y Fresh Signaling Pathways Uncovered
  2. San Diego: Exploring the Role of APP in Transit

Paper Citations

  1. . Presynaptic and postsynaptic interaction of the amyloid precursor protein promotes peripheral and central synaptogenesis. J Neurosci. 2009 Sep 2;29(35):10788-801. PubMed.
  2. . Increased asynchronous release and aberrant calcium channel activation in amyloid precursor protein deficient neuromuscular synapses. Neuroscience. 2007 Nov 23;149(4):768-78. PubMed.
  3. . Mechanisms contributing to the deficits in hippocampal synaptic plasticity in mice lacking amyloid precursor protein. Neuropharmacology. 1999 Mar;38(3):349-59. PubMed.
  4. . Expression of human amyloid precursor protein in rat cortical neurons inhibits calcium oscillations. J Neurosci. 2009 Apr 15;29(15):4708-18. PubMed.

Further Reading

Primary Papers

  1. . Amyloid precursor protein regulates Cav1.2 L-type calcium channel levels and function to influence GABAergic short-term plasticity. J Neurosci. 2009 Dec 16;29(50):15660-8. PubMed.