[Downloaded free from http://www.nrronline.org on Monday, May 31, 2021, IP: 161.111.10.229] Perspective activity has been observed early after cell Excitotoxicity-induced endocytosis death induction which is related to changes in calcium homeostasis taking place along neurodegeneration. The NMDARs are efficient as a potential target for stroke calcium channels and their overactivation, among other mechanisms, profoundly neuroprotection alters the intracellular regulation of this ion. Correspondingly, endocytosis is enhanced in excitotoxicity by a clathrin/dynamin-mediated Margarita Díaz-Guerra* mechanism that precedes neuronal death in vitro or in models of ischemia, where stressed Decreased neuronal survival-signaling and full-length receptor (TrkB-FL) and a truncated but viable neurons of the ischemic region have brain damage: Stroke is a leading cause of receptor lacking the tyrosine kinase domain been described as highly endocytic (Vaslin et al., 2009). A crosstalk between endocytosis death worldwide, the major cause of adult (TrkB-T1) (Vidaurre et al., 2012; Tejeda et al., 2+ disability and second of dementia. In spite 2016). Nonetheless, recent work from my and the activation by Ca of the protease of the social and economic importance of group has demonstrated that it is possible to calpain has been also described. After NMDAR this disorder, and after intense research, no interfere TrkB-FL degradation in stroke and, in overstimulation, cleavage by calpain of different effective drugs have yet reached the clinic. this way, decrease neuronal death and brain substrates triggers multiple mechanisms of Blood reperfusion with the thrombolytic agent damage (Tejeda et al., 2019). Interestingly, neurotoxicity, particularly enhancement of tissue plasminogen activator remains the only these results have been accomplished by calcium overload, disruption of cell structure pharmacologic treatment currently available primarily preventing TrkB-FL endocytosis, and promotion of cell-death signaling by degradation of antiapoptotic proteins or, for ischemic stroke, the major type of brain which is strongly induced by excitotoxicity and alternatively, those involved in neuroprotective infarction (> 85% of total cases). Damage precedes receptor processing (Figure 1). In signaling such as TrkB-FL (Vidaurre at al., 2012). in this situation results from thrombotic or this perspective, we will discuss the prospects The subcellular localization of calpain activation embolic occlusion of a cerebral artery causing of using the modulation of excitotoxicity- is currently under discussion. Activation a decrease of blood flow to a specific area induced endocytosis, and the subsequent is favored near plasma and endosomal of the brain parenchyma, neurons being preservation of membrane survival proteins, membranes but can also occur in microdomains particularly sensitive to a reduction of the as a neuroprotective therapeutic strategy for 2+ with high local [Ca ] as well as other cell supply of glucose and oxygen. It is thus a acute brain insults (stroke, epilepsy, or trauma) compartments (mitochondria, nucleus or Golgi priority to develop neuroprotective strategies and excitotoxicity-associated chronic disorders membranes). Notably, several components of able to preserve neurons from the ischemic (e.g., Alzheimer’s, Parkinson´s, Huntington´s diseases). the clathrin-coated vesicles, including the α- injury and, in this way, reduce brain damage and β2-subunits of the adaptor protein complex and patient disability. A promising approach 2 (AP-2), are calpain substrates and result involves rescue of the area of penumbra Dual role of endocytosis in neuronal survival and death: Endocytosis is a ubiquitous hydrolyzed in experimental ischemia and the surrounding the infarct, a region functionally brain of Alzheimer’s disease patients (Rudinskiy silent but structurally intact. However, neurons physiological process that mediates nutrient uptake, receptor internalization and signaling, et al., 2009). Cleavage of these AP-2 proteins in the penumbra can undergo a process has been suggested to be a mechanism to of delayed death known as excitotoxicity, essential events for cell growth and survival. In neurons, endocytosis is required at the reduce clathrin-coated vesicles and moderate caused by overstimulation of the N-methyl- endocytosis at late stages of necrotic cell death. D-aspartate type of excitatory glutamate synaptic cleft after neurotransmitter release for recycling of membranes and surface proteins. receptors (NMDARs). The critical role played by Prevention of TrkB-FL endocytosis is Meanwhile, in the postsynaptic membranes these receptors in synaptic plasticity, learning relevant for stroke neuroprotection of glutaminergic neurons, endocytosis in and memory, together with dual functions in : is central for the reduction in long-term vivo Since endocytosis is a requirement for neuronal survival and death (Hardingham et necrosis, general strategies to downregulate depression of surface glutamate receptors, al., 2002), underlies previous failure of NMDAR or deplete key proteins mediating different mostly of the α-amino-3-hydroxy-5-methyl- blockade as a therapeutic target in stroke. steps of the endocytic process have been 4-isoxazolepropionic acid family. NMDARs Nevertheless, the low-affinity uncompetitive devised and shown to actually interfere also undergo internalization in response to NMDAR antagonist memantine is still able to with neurodegeneration (Troulinaki and ligand binding, synapse maturation or long- improve cognitive functions and behavioral Tavernarakis, 2012). Therefore, generic term depression. While C-terminal sequences disturbances in moderate-to-severe Alzheimer’s interference of endocytosis might be of GluN2A/B subunits direct endocytosed disease, a neurodegenerative disorder also considered as a therapeutic strategy with NMDARs to recycling endosomes, conserved associated with excitotoxicity. Anyhow, for potential to reduce cell death due to acute motifs near the juxtamembrane region of stroke treatment, we are currently exploring brain insults. However, potential caveats to GluN1 and GluN2A/B drive receptors to late alternative strategies such as the inhibition this approach are the pleiotropic adverse endosomes and degradation (Scott et al., of neurotoxic proteins that act downstream effects it might have considering that 2004). Notably, trafficking of TrkB isoforms overactivated NMDARs or directed to enhance essential physiological cell processes would is similarly very important for neurotrophin neuronal survival pathways. Concerning be inhibited as well. As an alternative, we signaling in physiological conditions. Upon the latter, several laboratories have chosen propose the use of molecules able to interfere BDNF binding, both isoforms are rapidly and to analyze the promotion of neurotrophin- pathological endocytosis of specific neuronal efficiently internalized in a clathrin-dependent dependent survival pathways by treatment with survival proteins. One candidate to consider way and form signaling endosomes. However, brain-derived neurotrophic factor (BDNF) as a is the NMDAR, internalized by its own while TrkB-T1 predominantly recycles back possible strategy for neuroprotection in stroke overstimulation. In fact, endocytosis of GluN2B- to the cell surface by a default mechanism, but also other acute or chronic disorders of the containing NMDARs has been shown to mediate TrkB-FL recycling is less efficient, relies on its central nervous system. However, a potential excitotoxicity by still unknown mechanisms tyrosine kinase activity, and is regulated by caveat of this approach is that signaling (Wu et al., 2017). Blockade of this clathrin- binding of the protein Hrs (hepatocyte growth mediated by BDNF is dramatically subverted dependent endocytic process using a cell- factor-regulated tyrosine kinase substrate) by excitotoxicity, a process not only central penetrating peptide (CPP) was able to inhibit to a receptor juxtamembrane region located to stroke but, as mentioned, also associated excitotoxicity in cultured cortical neurons. This between the transmembrane and tyrosine to many other neurological disorders (Tejeda CPP (Tat-YEKL) contains a 11 aa domain of the kinase domains (Huang et al., 2009). and Diaz-Guerra, 2017). In models of stroke human immunodeficiency virus type 1 Tat and human samples, excitotoxicity induces In addition, endocytosis plays an evolutionarily protein, which allows attached cargoes to cross transcriptional and proteolytic mechanisms conserved function in cell destruction by the blood-brain barrier and plasma membrane, strongly associated with neurodegeneration necrosis, a form of death central to stroke and followed by 12 aa of the GluN2B C-terminus that alter the expression of the two major brain other diseases affecting the central nervous corresponding to the AP-2 binding motif. The isoforms of the BDNF receptor, tropomyosin- system (Troulinaki and Tavernarakis, 2012). selection of CPPs as neuroprotective tools for related kinase B (TrkB): the catalytically active A transient upregulation of the endocytic stroke therapy seems particularly appropriate 300 ｜NEURAL REGENERATION RESEARCH｜Vol 16｜No.2｜February 2021 [Downloaded free from http://www.nrronline.org on Monday, May 31, 2021, IP: 161.111.10.229] since the highly endocytic neurons of the *Correspondence to: Margarita Díaz-Guerra, PhD, ischemic area present an enhanced uptake of [email protected]. Tat-derived peptides (Vaslin et al., 2009)