E2F1 in neurons is cleaved by calpain in an NMDA receptor‐dependent manner in a model of HIV‐induced neurotoxicity

Abstract

The transcription factor E2F1 activates gene targets required for G1-S phase progression and for apoptosis, and exhibits increased expression levels in neurons in several CNS diseases including HIV encephalitis, Alzheimer disease, and Parkinson Disease. While E2F1 is known to regulate cell viability through activation of caspases, here we present evidence supporting the involvement of E2F1 in NMDA receptor-dependent, HIV-induced neuronal death mediated by calpains. Using an in vitro model of HIV-induced neurotoxicity that is dependent on NMDA receptor and calpain activation, we have shown that cortical neurons lacking functional E2F1 are less susceptible to neuronal death. Additionally, we report that neuronal E2F1 is cleaved by calpain to a stable 55-kiloDalton fragment following NR2B-dependent NMDA receptor stimulation. This cleavage of E2F1 is protein conformation-dependent and involves at least two cleavage events, one at each terminus of the protein. Intriguingly, the stabilized E2F1 cleavage product is produced in postmitotic neurons of all ages, but fails to be stabilized in cycling cells. Finally, we show that a matching E2F1 cleavage product is produced in human fetal neurons, suggesting that calpain cleavage of E2F1 may be produced in human cortical tissue. These results suggest neuronal E2F1 is processed in a novel manner in response to NMDA receptor-mediated toxicity, a mechanism implicated in HAND pathogenesis as well as several other diseases of the CNS.

Neuroprotection mediated by inhibition of calpain during acute viral encephalitis

“…Furthermore, calpain inhibitors, including ritonavir, are well-documented as neuroprotective agents in both in vitro and in vivo models of excitotoxicity and neurotoxicity 13 39 . On this basis we favor a strong interpretation of our data: ritonavir-mediated inhibition of calpain activity protects CA1 pyramidal neurons from excitotoxicity- and inflammation-induced cell death 40 , short-circuiting the development of seizures and preserving cognitive function. However, it is possible that ritonavir acts indirectly or via an off-target mechanism to elicit the protective phenotype.…”

Gene expression patterns associated with neurological disease in human HIV infection

“…In HIV without NCI, genes related to neurotransmission were also downregulated in the frontal cortex and basal ganglia while genes related to apoptosis, such as calpain-related mechanisms, which contribute to neurodegeneration in HIV [ 63 ], were induced in the basal ganglia and frontal cortex. Conversely, no pathways showed significant dysregulations in the frontal cortex and basal ganglia in HIV patients with NCI and no HIVE.…”

Transcriptional control of stem cell fate by E2Fs and pocket proteins

“…E2fs themselves are also heavily involved in the proliferation, survival, and differentiation of distinct neuronal cell types in the retina ( Chen et al, 2007 , 2013 ). Additionally, E2F1 and hyper-phosphorylated pRB play important roles in post-mitotic neurons in the adult brain, specifically in effecting the calpain-induced neuronal cell death observed in a number of CNS neurocognitive disorders, including HIV-induced encephalitis, Alzheimer disease, Parkinson’s disease, and amyotrophic lateral sclerosis ( Giovanni et al, 2000 ; Jordan-Sciutto et al, 2001 ; Ranganathan and Bowser, 2003 ; Höglinger et al, 2007 ; Akay et al, 2011 ; Zyskind et al, 2015 ). Thus, determination and maintenance of cell fate by E2F and PPs is a key feature underlying both tissue homeostasis and disease phenotypes.…”