A role for PKC in mediating stress-induced prefrontal cortical structural plasticity and cognitive function.

I mpaired cognitive functioning plays an important role in several major psychiatric illnesses, including schizophrenia and bipolar disorder (BPD) (1, 2). Cognitive impairments can also appear before full-blown onset of illness (3) and affect key components of course and recovery, including illness progression, functional outcome, disability, and quality of life. Considerable interest therefore exists in elucidating the cellular mechanisms associated with these working memory deficits and developing improved therapeutics to treat them (4). Notably, in this issue of PNAS, Arnsten and colleagues (5) describe a novel role for protein kinase C (PKC) in mediating the effects of behavioral stress on both prefrontal cortical structural connectivity and prefrontal cortex (PFC)-dependent cognitive function. PKC has long been known to play a major role in regulating various forms of synaptic and neural plasticity; as we discuss below, the findings of Arnsten and colleagues, demonstrating an important role for PKC in mediating structural plasticity and prefrontal cortical function, have potentially important implications for developing novel therapeutics. Previous research assessing the impact of stress focused predominantly on the hippocampus, but there is a growing appreciation that the PFC is also impacted by stress. For instance, chronic restraint stress causes progressive retraction and loss of dendritic spines in medial PFC regions (6, 7); these effects can last for weeks after stress ceases (8) and are associated with impaired performance on attentional tasks (7, 8). Arnsten and colleagues (5) observed similar changes in the medial PFC (5). They also found that stressed rats displayed progressively impaired performance on a task that assessed working memory, but not on a spatial discrimination task known to be independent of PFC functioning. Furthermore, working memory task performance was significantly correlated with dendritic spine density. These data are particularly important because they demonstrate that behavioral stress impairs working memory function, which relies on PFC neuronal connectivity. The study further found that PKC inhibition significantly altered the outcome of the stress-induced structural modulation in the PFC and working memory. This key finding builds on prior work suggesting that PKC is involved in prefrontal cortical response to stress. Stress exposure increases norepinephrine release in the PFC (9), which stimulates noradrenergic -1 receptors ( 1Rs) and activates the phosphatidylinositol (PI) signaling pathway and PKC (10). Consistent with this finding, PFC cognitive deficits are observed after exposure to pharmacologic stress, stimulation of 1Rs, or direct activation of PKC with phorbol esters in the PFC (10, 11). Conversely, PKC inhibition restores prefrontal cognitive functioning after all of these conditions (10, 11). Effects are also observed at the single-cell level, where the firing of PFC neurons during cognitive tasks is reduced by activating PKC and restored by inhibiting it (10). In toto, the data clearly demonstrate that PKC regulates cognitive functioning in the PFC and strongly suggest that stress may impair prefrontal cortical connectivity-dependent function via a PKC pathway. The study presented by Arnsten and colleagues (5) extends these findings and shows that pretreatment of rats with chelerythrine (a selective PKC inhibitor) before the daily restraint stress portion of the experiment diminished progressive impairment of working memory task performance. This reduction was more pronounced at the end of the chronic restraint stress when impairment was most severe. Perhaps somewhat surprisingly, pretreatment with a PKC inhibitor also attenuated the loss of dendritic spine density caused by the restraint stress. Thus, the data suggest that PKC inhibition not only protects against stress-induced functional deficits, but also protects against the effect of stress on brain structure in the PFC. One possible avenue underlying the protective effects of PKC inhibition is via the blockade of myristoylated alanine-rich PKC C substrate (MARCKS) phosphorylation (Fig. 1). MARCKS is an actin filament cross-linking protein. In cultured hippocampal neurons, PKC activation causes loss and shrinkage of spines, a process abolished in neurons that overexpress nonphosphorylatable MARCKS (12). Consistent with this finding, overexpression of pseudophosphorylated MARCKS also causes loss and shrinkage of spines. It is thus possible that overactivation of PKC results in MARCKS dysfunction and spine loss. This hypothetical mechanism observed in hippocampal neurons could potentially exist in cortical neurons; however, PKC phosphorylates a number of distinct substrates and, therefore, other pathways may also play a role.