Needs Assessment: Suicide is a major public health concern. Because of the weak predictive values of psychological, social, and environmental factors, understanding of the neurobiology of suicide offers a promising approach to identifying the risk factors associated with it. This cannot only clarify the etiology but also the treatment and prevention of suicide. This article focuses on the role of the signal transduction molecule protein kinase A in the pathophysiology of suicide.


Learning Objectives:

• Explain the importance of understanding the neurobiology of suicide.
• Describe the relevance of signal transduction molecules in understanding the risk factors associated with suicide.
• List the changes in protein kinase A (PKA) in suicide subjects as revealed by postmortem brain studies.
• Describe how the changes in PKA differ in teenage versus adult suicide and how changes in PKA during stress may be related to suicide.

Target Audience: Primary care physicians and psychiatrists.

CME Accreditation Statement: This activity has been planned and implemented in accordance with the Essentials and Standards of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the Mount Sinai School of Medicine and MBL Communications, Inc. The Mount Sinai School of Medicine is accredited by the ACCME to provide continuing medical education for physicians.

Credit Designation: The Mount Sinai School of Medicine designates this educational activity for a maximum of 3 AMA PRA Category 1 Credit(s)TM. Physicians should only claim credit commensurate with the extent of their participation in the activity.

Faculty Disclosure Policy Statement: It is the policy of the Mount Sinai School of Medicine to ensure objectivity, balance, independence, transparency, and scientific rigor in all CME-sponsored educational activities. All faculty participating in the planning or implementation of a sponsored activity are expected to disclose to the audience any relevant financial relationships and to assist in resolving any conflict of interest that may arise from the relationship. Presenters must also make a meaningful disclosure to the audience of their discussions of unlabeled or unapproved drugs or devices. This information will be available as part of the course material.

This activity has been peer-reviewed and approved by Eric Hollander, MD, chair and professor of psychiatry at the Mount Sinai School of Medicine, and Norman Sussman, MD, editor of Primary Psychiatry and professor of psychiatry at New York University School of Medicine. Review Date: October 16, 2007.

Drs. Hollander and Sussman report no affiliation with or financial interest in any organization that may pose a conflict of interest.

To receive credit for this activity: Read this article and the two CME-designated accompanying articles, reflect on the information presented, and then complete the CME posttest and evaluation. To obtain credits, you should score 70% or better. Early submission of this posttest is encouraged: please submit this posttest by November 1, 2009 to be eligible for credit.

Release date: November 1, 2007. Termination date: November 30, 2009. The estimated time to complete all three articles and the posttest is 3 hours.

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Dr. Dwivedi is tenured associate professor in the Departments of Psychiatry and Pharmacology and faculty member at the Graduate College at the University of Illinois in Chicago. Dr. Pandey is professor of psychiatry in the Department of Psychiatry at the University of Illinois College of Medicine.

Disclosures: The authors report no affiliation with or financial interest in any organization that may pose a conflict of interest. Funding for this research was provided by the American Foundation for Suicide Prevention, the National Alliance for Research on Schizophrenia and Depression, and the National Institute of Mental Health.

Please direct all correspondence to: Yogesh Dwivedi, PhD, Associate Professor, Department of Psychiatry, University of Illinois at Chicago, 1601 West Taylor St, Chicago, IL 60612; Tel: 312-413-4557; Fax: 312-355-3857; E-mail: ydwivedi@psych.uic.edu.

 


Abstract

In modern society, suicide is a major public health problem, yet currently the neurobiologic risk factors associated with suicide are still poorly understood. Recent studies demonstrate that alteration in synaptic and structural plasticity is key to affective disorders and suicide. Signal transduction molecules play an important role in such plastic events. Protein kinase A (PKA) is one of the crucial signaling molecules that, by phosphorylating proteins, affects a wide array of physiologic functions in the brain. This article focuses on recent findings that indicate alterations in activation and expression of PKA may be involved in the pathophysiology of suicide. The authors critically discuss these findings in human postmortem brain and in pre-clinical models as well as attempt to elucidate how stress, one of the major risk factors in suicide, may be crucial in altering PKA. In addition, the functional significance of altered PKA in respect to its target molecules cyclic adenosine monophosphate response element binding protein and brain-derived neurotrophic factor is also discussed. These findings provide a better understanding of the neurobiology of suicide and indicate that altered PKA may be one of the important neurobiologic risk factors associated with suicide.

 

Introduction

Suicide is a major public health concern. Each year, approximately 30,000 people commit suicide in the United States, and 1 million people commit suicide worldwide.1,2 Suicide is the third leading cause of death among adolescents, after motor vehicle accidents and homicide.3 Over 90% of suicides are associated with mental illnesses4 and 60% of them occur in the context of depressive disorders.5 Because almost all psychiatric disorders are characterized by an increased risk of suicidal behavior, the existence of suicidal syndromes independently of psychiatric illnesses has been proposed.6 Recently, Mann and colleagues7 proposed a stress-diathesis model, which postulates that suicidal behavior may be understood as a function of the interplay between state-dependent factors such as illness and life events, and trait-dependent factors which include biologic markers for suicidal behavior.

Recently, research on the biologic perspective of suicide has gained a stronghold and appears to provide a promising approach to identifying potential risk factors associated with suicide. An emerging hypothesis suggests that the pathogenesis of suicide/depression involves altered plasticity of neuronal pathways.8 Even structural abnormalities in the brains of mood disorder patients and suicide subjects have been reported.9 Intracellular signaling coordinates the behavior of individual cells within the brain in various physiologic processes. This signaling requires three essential components, including a molecular signal, also known as a neurotransmitter, that sends the information from one cell to another; a receptor that receives the signal and transmits the information provided by the signal; and a target molecule that mediates the cellular responses. Numerous studies suggest the involvement of serotonergic and to some extent noradrenergic systems in suicide, which includes alterations in levels of serotonin and norepinephrine, their metabolites, and the receptors to which these neurotransmitters bind in the brain and peripheral tissues of suicide subjects.10-12 However, emerging evidence suggests that downstream abnormalities in signal transduction mechanisms, particularly at the level of protein phosphorylation (whereby proteins are activated or deactivated through the addition of a phosphate group via an enzyme), are important mediators of neural plasticity and adaptations. Given the critical importance of intracellular signaling in amplifying, integrating, and regulating physiologic processes and in mediating gene expression, recent studies have focused on the role of signaling molecules in the pathophysiology of mood disorders and suicide.13 Protein kinase A (PKA) is one such key phosphorylating enzyme, which upon activation triggers a wide variety of physiologic responses in the brain that are important for cell survival, synaptic plasticity, and activation or repression of gene transcription.14,15

This article critically discusses recent findings implicating a role for PKA in suicide. In addition, the article discusses the functional importance of changes in PKA with respect to alterations in its target molecules, in particular the transcription factor cyclic adenosine monophosphate response element binding protein (CREB) and neurotrophin brain-derived neurotrophic factor (BDNF). Such studies attempt to elucidate the neurobiologic basis of suicide and pinpoint the abnormalities in a specific gene or gene product that may serve as potential vulnerability factor(s) associated with suicide and provide target(s) for future therapeutic interventions.

 

Protein Kinase A: Role in Suicide

Protein Kinase A: General Aspects

PKA is a phosphorylating enzyme in the adenylyl cyclase-cyclic adenosine monophosphate (cAMP) signaling pathway, which is one of the major signal transduction pathways and is linked to many neurotransmitter receptors, including β-adrenergic and serotonergic receptors, either in a stimulatory or an inhibitory mode. A diagram illustrating the adenylyl cyclase-cAMP signaling pathway is presented in Figure 1. In this pathway, a neurotransmitter binding to receptors causes activation of the intermediary molecule guanine nucleotide-binding protein (G protein). These G proteins then modulate the activity of the enzyme adenylyl cyclase, which in turn causes conversion of adenosine triphosphate to cAMP, which then activates PKA. Once activated, PKA can phosphorylate proteins in the cytoplasm or can translocate into the nucleus, where it can change the transcriptional machinery involved in gene synthesis by phosphorylating and therefore activating transcription factors. These phosphorylation events modify hormonal and neurotransmitter responses, including receptor downregulation/desensitization, alteration in neurotransmitter release, neurite outgrowth, neuronal differentiation, cell survival, and activation/repression of gene expression.14,15 One of the most important targets of PKA in the nucleus is the transcription factor CREB. The activation of CREB causes alterations in expression of several genes that have been implicated in synaptic and structural plasticity. One such gene is BDNF, which is essential for the maintenance of neuronal phenotypes, cell survival, and neuronal plasticity.16

 

PKA is a holoenzyme composed of two homodimeric regulatory (R) and two catalytic (C) subunits. In the absence of cAMP, PKA is inactive and exists as a stable tetramer. After an increase in intracellular cAMP, the regulatory PKA subunits bind to cAMP in a cooperative manner, which results in the disassociation of the holoenzyme into a dimeric regulatory unit and two monomers of catalytic subunits. The free catalytic subunits then phosphorylate substrates in the cytosol or translocate into the nucleus and phosphorylate nuclear proteins (Figure 1). Thus, both the catalytic and the regulatory subunits are important in facilitating PKA-mediated functions.

On the basis of the elution profile on diethylaminoethyl exchange chromatography, two major forms of PKA have been identified, ie, Type I and Type II. These two types differ in their structure and in the regulatory subunits incorporated, termed RI or RII, whereas their catalytic subunits are either identical or very similar. Cloning studies have revealed multiple isoforms for each regulatory and catalytic subunit. Two RI subunits, termed RIα and RIβ, and two RII subunits, termed RIIα and RIIβ, have been identified. Furthermore, three distinct catalytic subunits have been identified, termed Cα, Cβ, and Cγ. Each regulatory and catalytic subunit is a separate gene product and has a distinct expression pattern in different tissues.17

 

Role of Protein Kinase A in Suicide: Evidence from Human Postmortem Brain Studies

The role of PKA in suicide was examined in a comprehensive study. Dwivedi and colleagues18 determined cAMP binding to regulatory subunits of PKA and catalytic activity of PKA in the prefrontal cortex (PFC) of adult suicide subjects and well-matched nonpsychiatric normal control subjects obtained from the Maryland Psychiatric Research Center in collaboration with the Medical Examiner’s Office in Baltimore. It was observed that cAMP binding to regulatory subunits of PKA was significantly decreased in PFC of suicide subjects; however, there were no significant differences in the affinity of cAMP binding to these subunits. In addition, it was observed that PKA activity was decreased in the PFC of adult suicide subjects compared with normal control subjects in both the presence and the absence of cAMP, which suggests that decreased activation of PKA is not dependent on less availability of cAMP but on other factors such as altered expression of certain catalytic and/or regulatory subunits. To confirm this finding, in a different cohort of brain samples, the authors studied similar paradigms in postmortem brain of suicide subjects and well-matched normal control subjects obtained from Semmelweis University, Budapest, Hungary. Dwivedi and colleagues19 found similar changes in cAMP binding and PKA activity in the PFC of suicide subjects to those observed in the Maryland cohort.

To examine if the decreases in PKA activity and cAMP binding in postmortem brain of suicide subjects are related to changes in expression of specific catalytic and/or regulatory subunits, the authors19 determined the protein and gene expression of all the regulatory and catalytic subunits of PKA in the same postmortem brain samples in which they studied PKA activity and cAMP binding. Interesting observations were noted in both cohorts of brain samples; there was a significant and selective decrease in gene and protein expression of regulatory RIIβ and catalytic Cβ subunits in the PFC of suicide subjects without any changes in expression levels of RIα, RIIα, RIβ, or Cα subunits.19 These findings were independently confirmed by Odagaki and colleagues.20 These results suggest that decreases in cAMP binding and PKA activity could be due to decreases in the gene expression of RIIβ and Cβ, respectively, and changes in expression of catalytic and regulatory subunits of PKA are quite specific.

Recently, Pandey and colleagues21 examined whether the changes in PKA in adult suicide are similar to those in the teenage population. The rationale behind this determination was that the characteristics and risk factors for teenage suicide may be similar to those of adult suicide in some respects but differed in others. One distinct difference is that teenage suicide is driven primarily by impulsive aggressive behavior.22,23 Similarly, as in the adult population, PKA activity was found to be decreased in the PFC of teenage suicide subjects. However, when the protein and gene expression levels of PKA subunits were determined, a different pattern of changes in expression of PKA emerged. Whereas the protein and messenger ribonucleic acid (mRNA) expression of RIIβ and Cβ were decreased in the PFC of adult suicide subjects, the data in the PFC of teenage suicide subjects showed that expression not of RIIβ or Cβ but of RIα and RIβ was significantly decreased. This is quite distinct from what was observed in the adult population. When teenage suicide subjects were divided into those with a history of major mental disorders and those with no such history, no significant differences in PKA activity or protein and mRNA expression of any of the subunits were noted between the two groups,21 which suggests that the observed changes were not related to specific mental disorders but to suicide. The question is why are different regulatory and catalytic subunits altered in teenage versus adult populations? Is this related to specific behavioral characteristics such as affective disorders or impulsive aggressive behavior? There is currently no explanation for such contrasting results in teenage versus adult suicide, but overall there is decreased activation of PKA in the brains of both these populations. Further molecular studies are required to delineate this issue.

Interestingly, several studies in peripheral tissues obtained from depressed patients have also shown abnormalities in PKA. For example, Perez and colleagues24 found that the level of the Type II PKA regulatory subunit was significantly lower in platelets of untreated depressed patients compared with euthymic patients or normal controls. Shelton and colleagues25 and Manier and colleagues26 reported significantly decreased β-adrenergic receptor-stimulated PKA activity in fibroblasts of depressed patients, which was present only in melancholic depressed patients.27 More recently, Akin and colleagues28 reported reduced PKA activity along with reduced expression of PKA RIIα, Cα, and Cβ subunits in fibroblasts of melancholic depressed subjects.

 

Role of Protein Kinase A in Suicide: Preclinical Studies

Effect of Stress on Protein Kinase A
Stress is known to cause changes in the hypothalamic-pituitary-adrenal (HPA) axis in both humans and non-human models. Numerous studies point to a strong relationship between a hyperactive HPA axis and suicidal behavior,29,30 as is evident from studies showing increased corticotropin releasing hormone (CRH) in CSF,31 an altered ratio of CRH-I/II receptor mRNA in the pituitary,32 downregulation of corticotropin releasing factor receptors in PFC,33 an increase in mRNA of the  precursor molecule of adrenocorticotropic hormone in the pituitary34 of suicide subjects, and an increased volume of urinary free cortisol in violent suicide attempters.35 Therefore, it is of interest to examine whether changes in PKA are related to stress.

In a detailed study, Dwivedi and Pandey36 examined the effect of glucocorticoids after bilateral adrenalectomy and supplementation with exogenous glucocorticoid, as well as the effect of endogenous glucocorticoids, on various measures of PKA in rat brain. For this, rats were given various doses of corticosterone at different time intervals, and in a separate experiment, an adrenalectomy was performed and these rats were then given various doses of corticosterone. The authors observed that 1 day of corticosterone treatment had no significant effects, but 4 days of corticosterone treatment decreased cAMP binding to the regulatory subunit of PKA and PKA catalytic activity in the rat cortex and hippocampus. These changes were much more profound after 14 days of corticosterone. These effects were also dose dependent. A higher dose of corticosterone was much more effective in causing changes in PKA than a lower dose. Adrenalectomy produced the opposite results, increasing PKA activity and cAMP binding in both cortex and hippocampus in a time-dependent manner. These changes were reversed by corticosterone treatment in a dose-dependent manner. The higher dose of corticosterone completely reversed the changes in PKA after adrenalectomy.36 Interesting results were noted when the authors examined the expression levels of regulatory and catalytic subunits; mRNA and protein expressions of RIα, RIIβ, and Cβ isoforms were significantly decreased in cortex and hippocampus after corticosterone treatment. Removal of adrenal glands increased the expression of these subunits, and corticosterone treatment of adrenalectomized rats reversed the adrenalectomy-induced changes in PKA RIα, RIIβ, and Cβ isoforms.36 These changes were very similar to what were observed in postmortem brain of adult suicide subjects.18,19 Thus, these studies suggest that the expression of specific isoforms of PKA regulatory and catalytic subunits may be under the regulation of glucocorticoids and that stress may be playing an important role in such changes in the brain of suicide subjects.

Effect of Learned Helpless Behavior on Protein Kinase A
Since the ability to cope with stress is critical for humans, parallel studies of the effects of uncontrollable stress have been performed in animals, with the result of proactive interference with the acquisition of escape/avoidance responding.37 This phenomenon is termed learned helplessness and has been used extensively as an animal model of stress-induced behavioral depression, which is a common risk factor in suicide.38,39 To examine whether alterations in PKA occur during learned helplessness behavior, PKA in the brains of learned helpless rats were studied. Dwivedi and colleagues40 found that cAMP binding and PKA activity were both significantly decreased in the brains of learned helpless rats, and this was associated with selectively decreased expression of RIIβ and Cβ subunits. These changes were well-correlated with stress-induced behavioral paradigms. For example, learned helplessness behavior dissipated 4 days after the induction of learned helplessness, and the changes in PKA also reverted to the normal level, which suggests that changes in PKA are specific to stress-induced behavioral depression. The findings in PKA in learned helplessness rats were very similar to those observed in postmortem brains of suicide subjects.

 

Functional Significance of Altered Protein Kinase A

The findings that PKA activation is reduced in postmortem brains of suicide subjects and is also dysregulated during stress suggest the interesting possibility that many physiologic functions in the brain mediated by PKA may be altered in suicide subjects. However, although it is a matter for further research, the findings that expression of specific catalytic and regulatory subunits is decreased in suicide subjects and during stress in laboratory animals provide further evidence that there is a defect in the transcription of genes corresponding to certain catalytic and regulatory subunits of PKA; as discussed above, this defect could be associated with altered levels of glucocorticoids. The question is what could be the relevance of such changes in expression of specific PKA regulatory and catalytic subunits with respect to their role in suicide and depression? The specific functions of RIIβ and Cβ are not clearly known; however, tissue distribution studies suggest that RIIβ is predominantly expressed in brain, adrenal, and adipose tissues and is the principal mediator of cAMP activity in the mammalian central nervous system (CNS),41 whereas Cβ is expressed primarily in the brain.42 Ludvig and colleagues43 showed that RIIβ immunolabeling is associated with postsynaptic structures, suggesting that this subunit is involved in several postsynaptic neuronal functions. In addition, many studies have demonstrated that RIIβ and Cβ subunits may be specifically involved in neuronal and behavioral functions. For example, Constantinescu and colleagues44 showed that the RIIβ subunit can translocate to the nucleus and induce phosphorylation of one of the most important substrates of PKA, ie, CREB. In addition, RIIβ-mutant mice exhibit defective motor behavior45 and show an increased tendency to consume ethanol.46 Cβ-mutant mice, on the other hand, show impaired hippocampal plasticity.47 Targeted disruption of the RIβ subunit gene results in mice that exhibit defects in long-term depression and depolarization, which suggests a deficit in a learning-related form of synaptic plasticity.45 It has also been shown that an RIβ deficiency produces selective defects in mossy fiber long-term potentiation.45 Whether these abnormalities are relevant to suicide is not clear at present; however, given the significance of PKA in many biologic actions in the brain, together with emerging studies demonstrating specific roles for its catalytic and regulatory subunits in physiologic and behavioral manifestations, the observations of decreased catalytic and regulatory activities and expression of specific regulatory and catalytic subunits in postmortem brains of suicide subjects suggest that abnormalities in PKA may be of critical importance in the pathophysiology of suicidal behavior.

Transcription Factor CREB in Suicide
The regulation of gene expression is a fundamental mechanism of brain development, homeostatics, and adaptation to the environment. Transcription factor CREB is a member of the leucine zipper family, which binds to the consensus motif 5’-TGACGTCA-3’, found in the promoters of many neuronally expressed genes, and thereby activates or represses the transcription of target genes.14 Phosphorylation of CREB at Ser133 by PKA is the critical step in its activation. In its active form, CREB regulates many aspects of neuronal functioning, including excitation of nerve cells, CNS development, and long-term synaptic plasticity.48 BDNF is one of the many important genes whose transcription is mediated through the activation of PKA/CREB. To determine if CREB could possibly be involved in suicide, Dwivedi and colleagues49 recently examined the mRNA and protein expression as well as cAMP response element (CRE)-deoxyribonucleic acid (DNA)–binding activity (a measure of functional characteristics) of CREB in the PFC and hippocampus of suicide victims. The protein expression of CREB was found to be significantly decreased in the nuclear fractions of PFC and hippocampus obtained from suicide victims compared with normal control subjects. It was also observed that this decrease in protein expression levels was associated with a significant decrease in the mRNA levels of CREB in both the PFC and hippocampus of suicide victims. The authors also found decreased functional activity of CREB, such that CRE-DNA binding activity was significantly decreased in the nuclear fractions of both PFC as well as hippocampus of suicide subjects. These changes in CREB were present in all suicide subjects irrespective of psychiatric diagnosis. Similar results were noted in the postmortem brains of teenage suicide subjects.50 In addition, Young and colleagues51 reported altered levels of phospho-CREB in the amygdala of suicide subjects. The results of these studies suggest that not only PKA but also expression and functional characteristics of one of the important substrates of PKA, ie, CREB, are abnormal in the brains of suicide subjects.

These findings may have important clinical implications. For example, it has been shown that all antidepressants activate or upregulate the expression of CREB in the brains of rats.52 Similar results have been shown in postmortem brains of depressed patients treated with antidepressants.53 In contrast, the phosphorylation and the expression of CREB are decreased in postmortem brain of depressed patients, which could be associated with a decrease in PKA activity.54 Given the role of CREB in various physiologic actions in the brain, the findings of its increased expression by antidepressants and of its decreased expression and functional characteristics in the brain of depressed/suicide subjects are quite important and implicate this gene in depression/suicidal behavior.

Interestingly, at the genetic level, Zubenko and colleagues55 performed a linkage analysis of six polymorphic markers located in a 15 cM region of chromosome 2q33-35 and unipolar depression. They found a significant linkage of unipolar depression to a 451 Kb region of 2q33-34, which contains the CREB1 gene. This study further suggests that the CREB1 gene may be an attractive candidate for a susceptibility gene for depression.

BDNF as a Target Gene of Protein Kinase A: Role in Suicide
Among the epigenetic factors that may influence the development and survival of neurons in the CNS are neurotrophins. The most important and widely distributed member of the neurotrophin family in the brain is BDNF. It has been shown that PKA/CREB activation increases BDNF transcription through a Ca2+/CRE within exon III of BDNF.56 BDNF-mediated activation of its cognate receptor, tropomycin-related kinase B, influences neurite outgrowth, phenotypic maturation, morphologic plasticity, and synthesis of proteins for differentiated functioning of neurons and synaptic functioning.16 Several studies suggest that BDNF is also involved in nerve regeneration, structural integrity, and maintenance of neuronal plasticity in the adult brain, including activity-dependent regulation of synaptic activity and in neurotransmitter synthesis.57 Thus, pathologic alteration of the neurotrophic factor system may lead not only to altered neural maintenance and regeneration, and therefore structural abnormalities in the brain, but also to reduced neural plasticity. The result could be an impairment of an individual’s ability to adapt to crisis situations.

The suggestion that BDNF may play a role in the pathophysiology of suicide/depression is derived from many preclinical and clinical studies. For example, Siuciak and colleagues58 found that midbrain infusion of BDNF in rats greatly reduced learned helpless behavior. Shirayama and colleagues59 reported that bilateral infusion of BDNF into the dentate gyrus of rats produced an antidepressant-like effect in animal models of depression. In contrast, acute and repeated restraint stress or glucocorticoid administration causes a rapid decrease in the expression of BDNF in hippocampus and other brain areas,60 which suggests that a hyperactive HPA axis may cause a downregulation of BDNF. Chronic treatment with antidepressants prevents the stress-induced lowering of BDNF.61 Direct evidence of the role of BDNF in depression is derived from studies showing that serum BDNF levels are significantly decreased in drug-free depressed patients and are negatively correlated with Montgomery-Asberg-Depression Rating Scale scores.62 Recently, Shimizu and colleagues63 reported that the serum level of BDNF was significantly decreased in antidepressant-free depressed patients compared with those medicated with antidepressants or normal controls. These investigators also reported that the serum BDNF level was negatively correlated with Hamilton Rating Scale for Depression scores. Sen and colleagues64 reported that the val allele of the BDNF val66met polymorphism is associated with neuroticism, a heritable risk factor for depression. Recently, Dwivedi and colleagues65 reported decreased expression of BDNF in various areas of postmortem brain of suicide subjects, which was not related to any specific psychiatric diagnosis. Similar results were found by Karege and colleagues,66 who reported a decreased level of BDNF in postmortem brains of suicide subjects. More recently, Kim and colleagues67 found that the plasma level of BDNF is reduced in suicidal patients.

The findings of lower BDNF levels in depressed and suicide subjects have important implications. For example, emerging studies suggest that stress, affective disorders, and suicide are associated with structural abnormalities in the brain, including reduced hippocampal volume, reduced density and size of cortical neurons in the dorsolateral prefrontal cortex and orbitofrontal cortex, reduced density of nonpyramidal neurons, and layer-specific reduction in interneurons in the anterior cingulate cortex and in nonpyramidal neurons in the hippocampal formation. Similarly, several studies show that stress or glucocorticoid administration causes neuronal atrophy, a decrease in the number or length of apical dendrites, and even loss of hippocampal neurons in rodents or nonhuman primates.61 A few studies have demonstrated that the size of the parahippocampal cortex and the cortical laminar thickness are reduced in suicide brain.68,69 Taken together, these studies indicate that depression and suicide could be associated with atrophy or loss of neurons and/or glia. Because of the role played by BDNF in preventing neuronal atrophy and in survival and maintenance of neurons, decreased levels of BDNF could be associated with such structural abnormalities in the brain of affective disorder patients and suicidal subjects. It is pertinent to mention that besides BDNF, there are other growth factors whose transcription is mediated by the phosphorylation of transcription factors. These growth factors may also be affected in depression and suicide. Recent studies demonstrate that fibroblast growth factor and vascular endothelial growth factors may play important roles in depression and in the mechanism of action of antidepressants.70,71 Whether the expression of these growth factors is altered in suicide needs to be studied.

 

Conclusion

This article summarized and integrated recent findings that implicate the crucial phosphorylating enzyme PKA in suicide, the results of research which ranges from human postmortem brain studies to preclinical studies in rodents. In addition, the article discussed whether changes in PKA have any functional consequences in terms of the responsiveness of the cAMP signaling cascade as well as of its signaling molecules. A summary of findings pertaining to PKA and functional response of altered PKA in postmortem brains of suicide subjects is provided in Figure 2. It appears that there is a reduction in the activation of PKA in the brains of suicide subjects. In addition, there are reductions in specific regulatory and catalytic subunits of PKA in postmortem brains of suicide subjects. An interesting point is that there is specificity in the subunits that are affected in adult versus teenage suicide. Stress and glucocorticoid treatments in rats reduce the same subunits that are altered in the adult suicide population. Does this mean that the findings in adult suicide are related more to stress factors whereas in teenage suicide some other factors may be involved, such as impulsivity and/or aggressive behavior? Another important question is whether the changes in PKA are causative or consequences of stress or depressive/suicidal behavior. At this juncture, it would be premature to speculate, and further studies are needed to clarify these issues. Additional behavioral phenotypes will shed some light on this aspect, and currently, the laboratory of this article’s authors is engaged in delineating various behavioral and molecular aspects of mutations in specific PKA regulatory subunits in mice. Interestingly, in contrast to the differences in expression of specific PKA subunits in teenage and adult populations, the downstream target molecules, namely, CREB and BDNF, were less activated and expressed in postmortem brains of both adult and teenage suicide subjects. This is to be expected, since the overall outcome of decreased PKA is a reduction in the activation of CREB and a reduction in transcription of the BDNF gene. Thus, findings of reduced activation of CREB and expression of BDNF in both adult and teenage populations are not surprising.

 

In contrast, a few studies found results that are contradictory to what the authors of this article as well as other investigators have reported in postmortem brains of suicide subjects. For example, Lowther and colleagues72 reported no change in cAMP binding to regulatory subunits in postmortem brains of depressed suicides, whereas Odagaki and colleagues20 reported increased expression of total and phosphorylated CREB (an active form of CREB) in the PFC of depressed suicide subjects. Even a few animal study findings argue against the BDNF hypothesis of depression (reviewed by Groves73). Thus, although caution is required interpreting the data, nonetheless, the majority of studies show abnormalities in PKA-CREB-BDNF in suicide and depression.

Another important issue is whether the changes in PKA, CREB, and BDNF are specific to suicide or are related to mental disorders. Most suicidal patients have one or another form of psychiatric illness. As mentioned earlier, some of the findings observed in PKA signaling in suicide have been found in depressed patients. Since a large percentage of suicidal patients suffer from depression,74 it is to be expected that such findings would be present in depressed patients as well. In fact, in the authors’ study population, >60% of the subjects had a history of major depression. Therefore, although it appears that alterations in PKA are related to suicide, the possibility that changes in PKA signaling could be associated with a depressed mood or the inability to experience pleasure cannot be ruled out.

Overall, the discussed findings of abnormalities in various intracellular signaling molecules have provided insight into the molecular mechanisms associated with suicide. In the coming years, continuing studies will further advance our understanding of the pathophysiology of suicide, for they hold much promise not only for clarifying the neurobiologic risk factors associated with suicide but also for pinpointing specific genes that may be useful for developing novel “site-specific” therapeutic interventions. Finally, it will be important to investigate whether there is a genetic linkage of these abnormalities to suicide. PP

 

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