Contribution Of APLP2 In Spinal Cord Dorsal Horn To Neuropathic Pain

Objective:Amyloid precursor protein(APP)correlates closely with the pathogenesis of Alzheimer’s disease.In addition to Alzheimer’s disease,APP has a series of important pathophysiological functions.Previous studies have shown that APP is present in dorsal root ganglion neurons and involved in the transmission of pain information.Mammalian APP has two homologues,APLP1(Amyloid precursor-like protein 1)and APLP2.However,it is far from clear whether APP and its homologues are required for neuropathic pain.This study was conducted in spinal cord dorsal horn,a key site for pain integration and modification,and aimed to investigate the possible contribution of APP family proteins to neuropathic pain.Method:We established the spared nerve injury(SNI)model of neuropathic pain.Immunohistochemistry was performed to observe the distribution of APP family proteins in the dorsal horn of spinal cord.Western blot analysis was conducted to reveal the influence of nerve injury on APP family proteins.Intraspinal viral injection was performed in GAD2-Cre or SOM-Cre mice to knock down or overexpress APLP2,and the changes in the painful behaviors were measured.Results:(1)The spinal cord dorsal horn was enriched with APP and its two homologues APLP1 and APLP2.(2)Peripheral nerve injury specifically reduced the content of APLP2 in the spinal cord without any effects on APP and APLP1,suggesting that APLP2 was likely involved in the development of neuropathic pain.(3)The conveyance of neuropathic allodynia relies on a population of somatostatin-positive(SOM~+)excitatory nociceptive transmission neurons.Our data showed that APLP2was abundant in SOM~+neurons.However,the nerve injury produced little effect on the APLP2 expression in SOM~+neurons,suggesting that APLP2 in SOM~+neurons was dispersible for the regulation of neuropathic pain.(4)Consistent with the observation above,specific knockdown of APLP2 in SOM~+neurons affected neither the painful responses of mice nor the activities of spinal neurons,microglia or astrocytes.(5)The neuropathic allodynia is dependent on SOM~+neurons and meanwhile,subjected to the control by inhibitory GABAergic neurons.We found that APLP2 was present in GABAergic neurons,which significantly declined after the nerve injury.These data suggested that the reduction of APLP2 in GABAergic neurons was involved in neuropathic pain.(6)To consolidate the above-mentioned hypothesis,we overexpressed APLP2 in GABAergic neurons by means of intraspinal viral injection in GAD2-Cre mice.Our data showed that resuming APLP2 expression in GABAergic neurons effectively alleviated the neuropathic pain.(7)We found that spinal GABAergic neurons projected onto microglial cells,and the overexpression of APLP2 in GABAergic neurons reversed the hyperactivities of microglia,but not neurons or astrocytes,in neuropathic mice.Conclusion:The peripheral nerve injury specifically decreased the expression of APLP2 in spinal GABAergic neurons and led to the microglial activation.The overexpression of APLP2 in spinal GABAergic neurons was able to attenuate neuropathic pain.