Background & Aims
Activation of microglia in the spinal cord following peripheral nerve injury is critical for the development of long-lasting pain hypersensitivity(1). Single cell RNA sequencing of microglia revealed that Apolipoprotein E (Apoe) is the top upregulated gene in spinal cord microglia at chronic time points after peripheral nerve injury in mice(2). APOE is a lipoprotein that is essential for the regulation of neuroimmune functions, synaptic activity, and aging. In humans, there are 3 different isoforms of APOE: APOE-?2, APOE-?3 and APOE-?4(3-6). We have previously shown that carriers of APOE-?2 have significantly higher risk to develop chronic pain, whereas carriers of APOE-?4 have lower risk to develop distinct chronic pain conditions(2). We hypothesize that APOE-?2 increases the risk of developing chronic pain, whereas APOE-?4 has a protective effect against developing neuropathic pain.
Methods
To test the functional role of ApoE polymorphisms in chronic pain, we used humanized mice expressing APOE-?2, APOE-?3 and APOE-?4. To test for cell-type-specific effects in neuropathic pain, the knock-in (KI) mice were crossed to a microglia-specific Cre inducible mouse line (TMEM119CreERT2) to create a conditional knock out (cKO) model of each ApoE polymorphism in microglia. Both KI and cKO transgenic lines were subjected to the spared nerve injury (SNI) model of neuropathic pain.
Results
Behavioral testing conducted at baseline revealed that APOE-?2 mice had shorter latencies for non-reflexive tests (cold plate/hot plate) as compared to the neutral APOE-?3 mice. Following SNI, APOE-?4 mice showed less nerve injury-induced cold hypersensitivity as compared to APOE-?2 and APOE-?3. After SNI, APOE-?2: TMEM119CreERT2 cKO mice showed less mechanical hypersensitivity as compared to APOE-?2 KI mice.
Conclusions
At baseline, there was no difference between groups in reflex-based behavioral tests (von Frey, Hargreaves), but a difference was found in non-reflexive tests (cold plate and hot plate), suggesting a supraspinal influence. After SNI, APOE-?2: TMEM119 CreERT2 cKO mice showed less mechanical hypersensitivity, suggesting that APOE-?2 in microglia plays a detrimental role in neuropathic pain. These results further bolster our hypothesis that APOE-?2 increases the risk for neuropathic pain, while APOE-?4 decreases the risk.
References
1Echeverry, S., Shi, X. Q. & Zhang, J. Characterization of cell proliferation in rat spinal cord following peripheral nerve injury and the relationship with neuropathic pain. Pain 135, 37-47, doi:10.1016/j.pain.2007.05.002 (2008).
2Tansley, S. et al. Single-cell RNA sequencing reveals time- and sex-specific responses of mouse spinal cord microglia to peripheral nerve injury and links ApoE to chronic pain. Nat Commun 13, 843, doi:10.1038/s41467-022-28473-8 (2022).
3Hammond, T. R. et al. Single-Cell RNA Sequencing of Microglia throughout the Mouse Lifespan and in the Injured Brain Reveals Complex Cell-State Changes. Immunity 50, 253-271 e256, doi:10.1016/j.immuni.2018.11.004 (2019).
4Dhillon, H. & Singh, S. Role of Apolipoprotein E in the tangled mystery of pain. Med Hypotheses 114, 58-64, doi:10.1016/j.mehy.2018.02.033 (2018).
5Griffin, E. F. et al. ApoE-associated modulation of neuroprotection from Abeta-mediated neurodegeneration in transgenic Caenorhabditis elegans. Dis Model Mech 12, doi:10.1242/dmm.037218 (2019).
6Pocivavsek, A., Burns, M. P. & Rebeck, G. W. Low-density lipoprotein receptors regulate microglial inflammation through c-Jun N-terminal kinase. Glia 57, 444-453, doi:10.1002/glia.20772 (2009).
Presenting Author
Nicole Brown
Poster Authors
Nicole Brown
McGill
Lead Author
Topics
- Genetics