A hallmark feature of Ca 2؉ /calmodulin (CaM)-dependent protein kinase II (CaMKII) regulation is the generation of Ca 2؉ -independent autonomous activity by Thr-286 autophosphorylation. CaMKII autonomy has been regarded a form of molecular memory and is indeed important in neuronal plasticity and learning/memory. Thr-286-phosphorylated CaMKII is thought to be essentially fully active (ϳ70 -100%), implicating that it is no longer regulated and that its dramatically increased Ca 2؉ / CaM affinity is of minor functional importance. However, this study shows that autonomy greater than 15-25% was the exception, not the rule, and required a special mechanism (T-site binding; by the T-substrates AC2 or NR2B). Autonomous activity toward regular R-substrates (including tyrosine hydroxylase and GluR1) was significantly further stimulated by Ca 2؉ /CaM, both in vitro and within cells. Altered K m and V max made autonomy also substrate-(and ATP) concentration-dependent, but only over a narrow range, with remarkable stability at physiological concentrations. Such regulation still allows molecular memory of previous Ca 2؉ signals, but prevents complete uncoupling from subsequent cellular stimulation. Ca 2ϩ/calmodulin (CaM) 2 -dependent protein kinase II (CaMKII) can phosphorylate a large variety of substrate proteins and is a key player in many Ca 2ϩ -regulated cellular events (for review see Refs. 1-4). However, CaMKII is best know for its regulation of long term potentiation of synaptic strength (LTP) (5, 6), likely by increasing both number (7) and single channel conductance (8, 9) of synaptic AMPA-type glutamate receptors, and possibly by stimulating BDNF production (10, 11) (for review see Refs. 1-4). CaMKII autophosphorylation at Thr-286 generates Ca 2ϩ -independent autonomous activity (12-14), a process regarded as molecular memory (for review see Ref.2) and indeed important in learning and memory (15).Phosphorylation in the activation loop is a necessary step to generate full activity of many kinases, including PKA, PKC, and several CaMKs (for review see Refs. 16,17). By contrast, CaMKII is thought to be fully activated by Ca 2ϩ /CaM alone, without requirement for phosphorylation. Its Thr-286 is not located in the activation loop, but in the N-terminal half of the autoinhibitory ␣-helix, which binds to the T-site (Thr-286-interaction site; Ref. 18) in the basal state of CaMKII (19) (Fig. 1A). The C-terminal portion of the autoinhibitory ␣-helix extends to block the substrate binding site (S-site)(19) (Fig. 1A). Ca 2ϩ /CaM binding to the autoinhibitory ␣-helix relieves the S-site block, and makes Thr-286 accessible for phosphorylation by a neighboring kinase subunit within the 12meric CaMKII holoenzyme (20 -22). Phospho-T286 then prevents complete re-binding of the autoinhibitory ␣-helix.The dual role of Ca 2ϩ /CaM in Thr-286 autophosphorylation (for kinase activation and substrate presentation) allows computation of temporal patterns in Ca 2ϩ signaling, and indeed, CaMKII autonomy is dependent on the frequency of sti...

Mild traumatic brain injury (mTBI) affects millions of people annually and is difficult to diagnose. Mild injury is insensitive to conventional imaging techniques and diagnoses are often made using subjective criteria such as self-reported symptoms. Many people who sustain a mTBI develop persistent post-concussive symptoms. Athletes and military personnel are at great risk for repeat injury which can result in second impact syndrome or chronic traumatic encephalopathy. An objective and quantifiable measure, such as a serum biomarker, is needed to aid in mTBI diagnosis, prognosis, return to play/duty assessments, and would further elucidate mTBI pathophysiology. The majority of TBI biomarker research focuses on severe TBI with few studies specific to mild injury. Most studies use a hypothesis-driven approach, screening biofluids for markers known to be associated with TBI pathophysiology. This approach has yielded limited success in identifying markers that can be used clinically, additional candidate biomarkers are needed. Innovative and unbiased methods such as proteomics, microRNA arrays, urinary screens, autoantibody identification and phage display would complement more traditional approaches to aid in the discovery of novel mTBI biomarkers.

A hallmark feature of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is generation of autonomous (Ca(2+)-independent) activity by T286 autophosphorylation. Biochemical studies have shown that "autonomous" CaMKII is ∼5-fold further stimulated by Ca(2+)/CaM, but demonstration of a physiological function for such regulation within cells has remained elusive. In this study, CaMKII-induced enhancement of synaptic strength in rat hippocampal neurons required both autonomous activity and further stimulation. Synaptic strength was decreased by CaMKIIα knockdown and rescued by reexpression, but not by mutants impaired for autonomy (T286A) or binding to NMDA-type glutamate receptor subunit 2B (GluN2B; formerly NR2B; I205K). Full rescue was seen with constitutively autonomous mutants (T286D), but only if they could be further stimulated (additional T305/306A mutation), and not with two other mutations that additionally impair Ca(2+)/CaM binding. Compared to rescue with wild-type CaMKII, the CaM-binding-impaired mutants even had reduced synaptic strength. One of these mutants (T305/306D) mimicked an inhibitory autophosphorylation of CaMKII, whereas the other one (Δstim) abolished CaM binding without introducing charged residues. Inhibitory T305/306 autophosphorylation also reduced GluN2B binding, but this effect was independent of reduced Ca(2+)/CaM binding and was not mimicked by T305/306D mutation. Thus, even autonomous CaMKII activity must be further stimulated by Ca(2+)/CaM for enhancement of synaptic strength.

Excitotoxic insults such as cerebral ischemia are thought to enhance neuronal autophagy, which is then thought to promote neuronal cell death. Excitotoxic insults indeed increase autophagy markers. Notably, however, autophagy markers can be increased either by autophagy induction (as this enhances their production) or by late-stage autophagy inhibition (as this prevents their degradation during autophagic flux). By comparing each condition with and without protease inhibitors that prevent autophagic degradation of the autophagy marker, the results of this study show that excitotoxic glutamate increases autophagy markers by a late-stage block of autophagy. Initially, this study set out to test if the CaMKII inhibitor tatCN21 mediates its post-insult neuroprotection by regulating autophagy. While tatCN21 partially inhibited basal autophagy in hippocampal neurons, it had no effects on the already blocked autophagy after excitotoxic glutamate insults, indicating that autophagy inhibition is not its neuroprotective mechanism. Additionally, while the autophagy inhibitor chloroquine had no effect, significant neuroprotection was seen instead with two drugs that enhance autophagy induction by different mechanisms, rapamycin (mTOR dependent) and trehalose (mTOR-independent). This suggests that therapeutic approaches should seek to enhance rather than inhibit autophagy, not only in neurodegenerative diseases (where such approach is widely accepted) but also after acute excitotoxic insults. Together, these findings significantly reshape the current view on the mutual cross-regulation of autophagy and excitotoxicity.

In recent years, a new neurodegenerative tauopathy labeled Chronic Traumatic Encephalopathy (CTE), has been identified that is believed to be primarily a sequela of repeated mild traumatic brain injury (TBI), often referred to as concussion, that occurs in athletes participating in contact sports (e.g. boxing, football, football, rugby, soccer, ice hockey) or in military combatants, especially after blast-induced injuries. Since the identification of CTE, and its neuropathological finding of deposits of hyperphosphorylated tau protein, mechanistic attention has been on lumping the disorder together with various other non-traumatic neurodegenerative tauopathies. Indeed, brains from suspected CTE cases that have come to autopsy have been confirmed to have deposits of hyperphosphorylated tau in locations that make its anatomical distribution distinct for other tauopathies. The fact that these individuals experienced repetitive TBI episodes during their athletic or military careers suggests that the secondary injury mechanisms that have been extensively characterized in acute TBI preclinical models, and in TBI patients, including glutamate excitotoxicity, intracellular calcium overload, mitochondrial dysfunction, free radical-induced oxidative damage and neuroinflammation, may contribute to the brain damage associated with CTE. Thus, the current review begins with an in depth analysis of what is known about the tau protein and its functions and dysfunctions followed by a discussion of the major TBI secondary injury mechanisms, and how the latter have been shown to contribute to tau pathology. The value of this review is that it might lead to improved neuroprotective strategies for either prophylactically attenuating the development of CTE or slowing its progression.