Down-regulation of APLP1 mRNA expression in hippocampus of pilocarpine-induced epileptic rats

Deletion of the amyloid precursor-like protein 1 (APLP1) enhances excitatory synaptic transmission, reduces network inhibition but does not impair synaptic plasticity in the mouse dentate gyrus

Amyloid precursor-like protein 1 (APLP1) is a transmembrane synaptic protein belonging to the amyloid precursor protein (APP) gene family. Although the role of this gene family-in particular of APP-has been intensely studied in the context of Alzheimer's disease, the physiological roles of its family members remain poorly understood. In particular, the function of APLP1, which is predominantly expressed in the nervous system, has remained enigmatic. Since APP has been implicated in synaptic plasticity, we wondered whether APLP1 could play a similar role. First, using in situ hybridization and laser microdissection combined with reverse transcription-quantitative polymerase chain reaction (PCR) we observed that Aplp1 mRNA is highly expressed in dentate granule cells. Having this examined, we studied synaptic plasticity at the perforant path-granule cell synapses in the dentate gyrus of APLP1-deficient mice in vivo. Analysis of field excitatory postsynaptic potentials evoked by stimulation of perforant path fibers revealed increased excitatory transmission in APLP1-deficient mice. Moreover, we observed decreased paired-pulse inhibition of population spikes indicating a decrease in network inhibition upon deletion of APLP1. In contrast, short-term presynaptic plasticity (STP) as well as long-term synaptic plasticity (LTP) was unchanged in the absence of APLP1. Based on these results we conclude that APLP1 deficiency on its own does not lead to defects in synaptic plasticity, but affects synaptic transmission and network inhibition in the dentate gyrus.

APLP1 and Rab5A interact with the II-III loop of the voltage-gated Ca-channel Ca(v)2.3 and modulate its internalization differently

BACKGROUND

Voltage gated calcium channels (VGCCs) regulate cellular activity in response to membrane depolarization by altering calcium homeostasis. Because calcium is the most versatile second messenger, regulation of the amount of VGCCs at the plasma membrane is highly critical for several essential cellular processes. Among the different types of VGCCs, the Ca(v)2.3 calcium channel and its regulation mechanisms are least understood due to Ca(v)2.3's resistance to most pharmacological agents.

METHODS

In order to study regulation and surface expression of Ca(v)2.3, a yeast two hybrid (Y2H) screen with the II-III loop of human Ca(v)2.3 as bait, was performed. APLP1, a member of the APP gene family and Rab5A, an endocytotic catalyst were identified as putative interaction partners. The interaction were confirmed by immunoprecipitation. To study the functional importance of the interaction, patch-clamp recordings in Ca(v)2.3 stably transfected HEK-293 cells (2C6) and surface biotin endocytosis assays were performed.

RESULTS

We are able to show that the II-III loop of the Ca(v)2.3 calcium channel binds APLP1 and that this binding promotes internalization of the channel. In addition, Rab5A also binds to the same loop of the channel and exerts an inhibitory effect on APLP1 mediated channel internalization.

CONCLUSIONS

This study identifies a regulation mechanism of Ca(v)2.3's surface expression, which implicates APLP1 as a regulator of calcium homeostasis. Thus APLP1 may play a crucial role in neuropathological mechanisms, which involve modulation of surface expression of voltage-gated Ca(2+) channels.