APLP2 Protein

APLP2 Protein

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">APLP2 Protein</th>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>APLP2</td>
</tr>
<tr>
<td class="label">Protein Alias</td>
<td>APLP2, amyloid precursor-like protein 2</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>15q21.3</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>Q06481</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~87 kDa (mature, after N-glycosylation)</td>
</tr>
<tr>
<td class="label">Amino Acids</td>
<td>723</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Plasma membrane, synaptic vesicles, endosomes, Golgi</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>APP protein family (APP, APLP1, APLP2)</td>
</tr>
<tr>
<td class="label">Abeta Production</td>
<td>None (lacks Abeta sequence)</td>
</tr>
<tr>
<td class="label">Protein</td>
<td>Interaction Type</td>
</tr>
<tr>
<td class="label">APP</td>
<td>Heterophilic binding (E1/E2)</td>
</tr>
<tr>
<td class="label">APLP1</td>
<td>Heterophilic binding (E1/E2)</td>
</tr>
<tr>
<td class="label">Fe65 (APBB1)</td>
<td>Tail interaction (YENPTY)</td>
</tr>
<tr>
<td class="label">X11 alpha (APBA1)</td>
<td>Tail interaction</td>
</tr>
<tr>
<td class="label">MUNC18 (STXBP1)</td>
<td>Tail interaction</td>
</tr>
<tr>
<td cl

APLP2 Protein

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">APLP2 Protein</th>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>APLP2</td>
</tr>
<tr>
<td class="label">Protein Alias</td>
<td>APLP2, amyloid precursor-like protein 2</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>15q21.3</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>Q06481</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~87 kDa (mature, after N-glycosylation)</td>
</tr>
<tr>
<td class="label">Amino Acids</td>
<td>723</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Plasma membrane, synaptic vesicles, endosomes, Golgi</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>APP protein family (APP, APLP1, APLP2)</td>
</tr>
<tr>
<td class="label">Abeta Production</td>
<td>None (lacks Abeta sequence)</td>
</tr>
<tr>
<td class="label">Protein</td>
<td>Interaction Type</td>
</tr>
<tr>
<td class="label">APP</td>
<td>Heterophilic binding (E1/E2)</td>
</tr>
<tr>
<td class="label">APLP1</td>
<td>Heterophilic binding (E1/E2)</td>
</tr>
<tr>
<td class="label">Fe65 (APBB1)</td>
<td>Tail interaction (YENPTY)</td>
</tr>
<tr>
<td class="label">X11 alpha (APBA1)</td>
<td>Tail interaction</td>
</tr>
<tr>
<td class="label">MUNC18 (STXBP1)</td>
<td>Tail interaction</td>
</tr>
<tr>
<td class="label">Heparan sulfate proteoglycans</td>
<td>E2 domain binding</td>
</tr>
<tr>
<td class="label">Copper ions</td>
<td>E1 domain (CuBD)</td>
</tr>
<tr>
<td class="label">Zn2+ ions</td>
<td>E2 domain binding</td>
</tr>
<tr>
<td class="label">APOE</td>
<td>Receptor-mediated</td>
</tr>
<tr>
<td class="label">LDLR family</td>
<td>Receptor interactions</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">30 edges</a></td>
</tr>
</table>

Overview

APLP2 (Amyloid Precursor-Like Protein 2), encoded by the APLP2 gene on chromosome 15q21.3, is a type-I transmembrane glycoprotein and the second of two known mammalian APP homologs (alongside APP and APLP1)[@mller2017]. Like [APP](/entities/app-protein) and APLP1, APLP2 contains a large extracellular region with the characteristic E1 and E2 protein interaction domains, a single transmembrane helix, and a short cytoplasmic tail containing an YENPTY motif that recruits adaptor proteins controlling endocytosis and trafficking. Unlike APP, APLP2 does not produce amyloid-beta (Abeta) peptide — the proteolytic cleavage sites that generate Abeta from APP are not conserved in APLP2.

APLP2 is broadly expressed throughout the [central nervous system](/entities/cns) and [peripheral nervous system](/entities/pns), with particularly high levels in [neurons](/entities/neurons) of the cerebral cortex, hippocampus, and cerebellum[@mller2017]. It plays essential roles in synaptic organization, trans-synaptic adhesion, neuronal excitability, and synaptic plasticity[@lee2020][@weyer2011]. Critically, APLP2 is not merely a redundant paralog of APP — it has non-overlapping and partially compensated functions that make it essential for life when all APP family members are disrupted, yet redundant with APP in specific synaptic contexts[@von1997]. Its relevance to [Alzheimer's disease](/diseases/alzheimers-disease) stems from the complex interplay within the APP family: therapies targeting APP processing inevitably affect APLP2 biology, and APLP2 compensation for APP deficiency shapes disease phenotypes and therapeutic responses[@mller2017][@chen2020a].

Gene and Protein Information

Structure

APLP2 adopts the characteristic type-I transmembrane architecture shared by the APP family[@mller2017]:

• Extracellular E1 domain (residues ~25-200): Contains a growth factor-like (GFL) region and a copper-binding domain (CuBD). The E1 domain mediates homophilic and heterophilic protein interactions within the APP family
• E2 domain (residues ~200-480): A large structured domain with a zinc-binding motif and heparin-binding sites. The E2 domain is central to APLP2's role in cell adhesion and trans-synaptic interactions
• Hinge region (residues ~480-560): Contains the alpha-secretase cleavage site (ADAM10-dependent), which releases a soluble ectodomain (sAPLP2)
• Transmembrane helix (residues ~560-580): Single-pass transmembrane domain
• Cytoplasmic tail (residues ~580-723): Contains the YENPTY motif for endocytosis, interaction with Fe65, X11/MUNC18 complexes, and other adaptor proteins. Multiple phosphorylation sites (Thr654, Tyr682) regulate adaptor recruitment and trafficking

Normal Function

Trans-synaptic Adhesion

APLP2 functions as a synaptic adhesion molecule, forming trans-synaptic complexes that organize the presynaptic and postsynaptic apparatus[@weyer2011][@troy2012]:

• Homophilic interactions: APLP2 can bind itself on opposing synaptic membranes, forming trans-synaptic dimers that bridge the synaptic cleft
• Heterophilic interactions: APLP2 interacts with other synaptic proteins including neurexin, neuroligin, and APP family members, forming a synaptic scaffold network
• Synapse maintenance: APLP2 at the synapse helps maintain synaptic stability and architecture; loss of APLP2 leads to subtle synaptic structural abnormalities over time

Synaptic Transmission and Plasticity

APLP2 regulates both basal synaptic transmission and activity-dependent plasticity[@hoey2009][@weyer2011]:

• Presynaptic function: APLP2 influences synaptic vesicle cycling, release probability, and the size of the readily releasable pool. Loss of APLP2 subtly reduces neurotransmitter release
• Postsynaptic function: APLP2 affects postsynaptic receptor trafficking and scaffold organization
• Long-term potentiation (LTP): APLP2 is required for normal LTP at hippocampal synapses; APP/APLP2 double knockout mice show profoundly impaired LTP even when APP alone is intact[@weyer2011]
• Learning and memory: APLP2 knockout mice show deficits in spatial learning and memory, particularly in water maze and fear conditioning paradigms

Neuronal Excitability

APLP2 modulates neuronal excitability through multiple mechanisms[@lee2020]:

• APLP2 influences the trafficking and function of ion channels at the plasma membrane
• Loss of APLP2 alters the balance of excitatory and inhibitory synaptic inputs
• APLP2 may interact with potassium channels and other regulators of neuronal membrane potential
• These excitability effects may be relevant to seizure susceptibility observed in some APP family models

Synaptic Vesicle Biology

APLP2 localizes to synaptic vesicles and regulates vesicle trafficking and release[@hoey2009]:

• APLP2 is present on synaptic vesicles in presynaptic terminals
• Loss of APLP2 causes subtle deficits in synaptic vesicle endocytosis and recycling
• The relationship between APLP2 and the synaptic vesicle cycle involves both presynaptic and postsynaptic compartments

Role in Neurodegenerative Disease

Alzheimer's Disease

APLP2 intersects with [Alzheimer's disease](/diseases/alzheimers-disease) through multiple mechanisms[@mller2017][@chen2020a][@suh2020]:

APP family compensation: APLP2 frequently compensates for APP deficiency, which complicates interpretation of APP-targeted therapies:

• When APP is reduced or absent, APLP2 expression is upregulated and can partially substitute for APP's synaptic functions
• This compensation means that APP-only phenotypes may be milder than expected
• In AD, where APP is abundant and producing Abeta, APLP2's compensation may also buffer against APP loss, reducing the apparent impact of APP-targeted interventions

• BACE1 inhibitors reduce not only Abeta production from APP but also APLP2 processing, potentially affecting synaptic function
• Gamma-secretase modulators affect both APP and APLP2 processing, with complex consequences
• Understanding APP:APLP1:APLP2 stoichiometry in vulnerable neurons is essential for predicting therapeutic outcomes[@chen2020a]

• APLP2 binds to APOE and other ligands, potentially modulating their clearance and signaling
• This APLP2-APOE receptor axis may influence brain lipid homeostasis and AD risk

• Stress conditions enhance APLP2 sumoylation, which protects synaptic integrity
• Dysregulated sumoylation of APLP2 may contribute to synaptic failure in AD

Other Neurodegenerative Contexts

APLP2 may also be relevant in other neurodegenerative conditions:

• Parkinson's disease: APLP2 expression is altered in PD models; it may modulate the cellular response to alpha-synuclein toxicity
• Huntington's disease: Mutant huntingtin affects APP family processing and APLP2 may be implicated in the resulting synaptic dysfunction
• Frontotemporal dementia: The overlap between FTD and AD pathology involves complex APP family interactions

Molecular Interactions

Therapeutic Implications

APLP2 is not a primary drug target for neurodegeneration, but understanding its biology is essential for[@mller2017][@chen2020a]:

• BACE1 inhibitor safety: BACE1 cleaves both APP and APLP2; long-term BACE1 inhibition may impair APLP2-dependent synaptic functions, contributing to cognitive side effects observed in trials
• Gamma-secretase modulation: Modulators affect both APP and APLP2; understanding these effects requires APLP2 readouts
• APP-targeting immunotherapy: Anti-Abeta antibodies that reduce APP processing may alter APLP2 compensation, with unpredictable effects on synaptic function
• APOE receptor targeting: APLP2's interaction with APOE receptors suggests potential for indirect modulation through lipid pathways

Animal Models

• Aplp2 single knockout mice: Viable and fertile with relatively mild phenotypes — subtle synaptic deficits, slight learning impairment[@shariati2012]
• APP/APLP2 double knockout: Severe deficits in synaptic transmission, LTP, spatial learning, and PNS function[@von1997][@weyer2011]
• APP/APLP1/APLP2 triple knockout: Neonatal death with pronounced neurological phenotypes[@von1997]
• APP/APLP2 conditional double knockout in adulthood: Adult-onset synaptic failure and cognitive decline[@chen2020a]
• APLP2 overexpression: Enhances certain synaptic functions; may modulate APP-family dynamics in disease models

See Also

• [APLP2 Gene](/genes/aplp2)
• [APP Protein](/proteins/app)
• [APLP1 Protein](/proteins/aplp1-protein)
• [APP Gene](/genes/app)
• [BACE1 Protein](/proteins/bace1)
• [Synaptic Dysfunction](/mechanisms/synaptic-dysfunction)
• [Long-Term Potentiation](/mechanisms/long-term-potentiation)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Neurotransmitter Release](/mechanisms/neurotransmitter-release)