Reduced expression of APLP2 in spinal GABAergic inhibitory neurons contributed to nerve injury-induced microglial activation and pain sensitization.

The amyloid precursor protein (APP) is critical for the pathogenesis of Alzheimer's disease (AD). The AD patients usually have lower pain sensitivity in addition to cognitive impairments. However, considerably less is known as yet about the role of APP and its two mammalian homologues, amyloid precursor-like protein 1 and 2 (APLP1, APLP2), in spinal processing of nociceptive information. Here we found that all APP family members were present in spinal cord dorsal horn of adult male C57BL/6J mice. Peripheral nerve injury specifically reduced the expression of spinal APLP2 that correlated with neuropathic mechanical allodynia. The loss of APLP2 was confined to inhibitory GABAergic interneurons. Targeted knockdown of APLP2 in GABAergic interneurons of GAD2-Cre mice evoked pain hypersensitivity by means of microglia activation. Our data showed that GABAergic terminals expressed APLP2, a putative cell adhesion protein that interacted with microglia-specific integrin molecule CD11b. Knocking down APLP2 in GAD2-positive neurons to disrupt the trans-cellular interaction led to microglia-dependent pain sensitization. Our data thus revealed an important role of APLP2 for GABAergic interneurons to control microglial activity and pain sensitivity.

Cbl-b modulated TrkA ubiquitination and function in the dorsal root ganglion of mice.

Casitas B-lineage lymphoma b (Cbl-b) is one of the E3 ubiquitin ligases that ubiquitinate Tropomyosin-related kinase A (TrkA), a key nerve growth factor receptor involved in the pathological pain. Here we found that Cbl-b was abundant in dorsal root ganglion (DRG) neurons of mice and co-localized with TrkA. Ubiquitination of TrkA by Cbl-b exerted a tonic negative control over the protein level of TrkA. Knockdown of Cbl-b caused TrkA accumulation in DRGs and evoked mechanical and heat hypersensitivity in intact mice. Our data showed that knee osteoarthritis induced by destabilization of the medial meniscus (DMM) led to the dissociation of Cbl-b with TrkA in DRG neurons, which impaired the ability of Cbl-b to ubiquitinate TrkA and served as an important mechanism to cause TrkA-dependent pain sensitization. Viral expression of constitutively active Cbl-b in DRGs of osteoarthritic mice effectively repressed TrkA protein level and more importantly, alleviated mechanical allodynia and heat hyperalgesia. Viral delivery of Cbl-b through intra-articular route generated a similar analgesic action. These data suggested that ubiquitination of TrkA by Cbl-b might represent an effective way to treat the osteoarthritic pain.