This holiday might be full of tricks, but your research should be full of treats!
Happy Halloween from Cyagen! To celebrate and educate, our team has put together brief synopses of several genes related to fear: STMN1, P4H-TM, ANXA1, FPR1, DAGLA, and TAAR1. Read on to learn about each gene’s functions, research progress, and opportunities for discovery.
Fear is typically attributed to two causes – innate fears and learned fears. Fear memories are an essential part of survival, which causes them to be easily formed and difficult to lose. The biological basis of fear is a complex process, but advancements in genome mapping and gene association studies have revealed several fear memory-related genes. Below, we have put together a selection of such fear-related protein coding genes, summaries of related research efforts, and research applications for each.
The STMN1 (Stathmin 1) gene belongs to the stathmin family of genes. STMN1 is highly expressed in the region of the amygdala critical for fear learning, as well as in the thalamic and cortical structures that send information to the lateral nucleus (LA) of the amygdala about the conditioned (learned fear) and unconditioned stimuli (innate fear). The Stathmin protein is involved in the regulation of the microtubule (MT) filament system. Multiple transcript variants encoding different isoforms have been found for this gene. Additionally, Stathmin - an ubiquitous cytosolic phosphoprotein - is proposed to operate as an intracellular relay for regulatory signals of the cellular environment, functioning through signaling molecules known as second messengers.
Common Aliases: Stathmin-1, Leukemia-Associated Phosphoprotein P18, Oncoprotein 18
STMN1 gene knockout mice exhibit decreased memory in amygdala-dependent fear conditioning and fail to recognize danger in innately aversive environments. Initial studies have concluded “that stathmin is required for the induction of long-term potentiation (LTP) in afferent inputs to the amygdala and is essential in regulating both innate and learned fear.”1 The excess production of microtubules in Stathmin knockout mice impairs LTP - a cellular process critical for memory formation.
In 2010, follow-up research “hypothesized that in humans the gene coding for stathmin (STMN1) ... influences behavioral responses to fear and anxiety stimuli by way of two common single nucleotide polymorphisms (rs182455, SNP1; rs213641, SNP2). These polymorphisms are located within or close to the putative transcriptional control region. [They] used the acoustic startle paradigm and a standardized laboratory protocol for the induction of fear and psychosocial stress in 106 healthy volunteers to investigate the impact of stathmin gene variation on two fear‐ and anxiety‐controlling effector‐systems of the amygdala. [They] found that STMN1 genotype interacting with individuals' gender significantly impacts fear and anxiety responses as measured with the startle and cortisol stress response. We therefore conclude that STMN1 genotype has functional relevance for the acquisition and expression of basic fear and anxiety responses also in humans.”
Taking these results together, it may be possible that modeling fear extinction in mice could provide insights into how stathmin controls similar extinction processes in humans.
May lead to new treatments for a variety of mental disorders including generalized anxiety, panic, phobias, obsessive-compulsive disorder (OCD), and post-traumatic stress disorder (PTSD).
“Diseases associated with STMN1 include Acute Leukemia and Pancreatic Intraductal Papillary-Mucinous Adenoma. Among its related pathways are EGF/EGFR Signaling Pathway and MAPK signaling pathway. Gene Ontology (GO) annotations related to this gene include obsolete signal transducer activity and tubulin binding. An important paralog of this gene is STMN2.”5
The P4H-TM (Prolyl 4-Hydroxylase, Transmembrane) gene encodes a protein that belongs to the family of prolyl-4-hydroxylases, which play a pivotal role in the cellular adaptation to hypoxia. However, the P4H-TM protein differs from other family proteins in both its structure and unusual location – the endoplasmic reticulum (ER). Alternatively spliced variants encoding different isoforms have been identified.
The physiological role of the P4H-TM protein remains elusive despite years of intensive research, but it is assumed to have other effects on cellular biology besides adaptation to varying oxygen levels. The protein is known to catalyze the post-translational formation of 4-hydroxyproline in hypoxia-inducible factor (HIF) alpha proteins, and has been implicated as a transmembrane HIF prolyl 4-hydroxylase (HIF-P4H) in recent studies. Several research groups have indicated that P4H-TM may be a promising new target for anti-anxiety (anxiolytic) and antidepressant drugs.
Common Aliases: P4HTM, PH4, Proline 4-Hydroxylase, P4H with transmembrane domain, Hypoxia Inducible Factor Prolyl 4 Hydroxylase, HIF-PH4, HIFPH4
The involvement of P4H-TM in central nervous system (CNS) physiology has emerged across experimental and clinical evidence. Firstly, researchers have confirmed that P4H-TM expression is remarkably high in brain regions related to emotional and social behavior – including the amygdala, lateral septum and bed nucleus of stria terminalis. As determined by a number of behavioral tests, P4h-tm deficient mice are social, show little anxiety or behavioral despair, demonstrating notable courage and lack of learned helplessness. Further, the study found a connection between brain anatomy and the behavioral phenotype: the expression of the P4h-tm gene was especially high in the amygdala - which plays a key role in controlling emotional reactions, including fear and anxiety. Notably, P4h-tm−/− mice lacked behavioral despair response, a surrogate marker of depression, in forced swim and tail suspension tests.
Mutant mice of all other Hif-p4h isoforms lacked such a behavioral phenotype – further supporting the remarkable anatomy-physiology association between the brain expression of P4H-TM and the behavioral phenotype in P4h-tm−/− mice.1
Further studies will show whether P4H-TM is a promising new target for anxiolytic and antidepressant pharmacotherapies. Since deficiency of the P4H-TM gene results in severe developmental defects in humans, conditional gene inactivation models of P4h-tm would help determine if inactivation in adult tissues would have the same effect. Ideally, P4h-tm would be turned off only in the amygdala of an adult mouse for future experiments.2
P4HTM (Prolyl 4-Hydroxylase, Transmembrane) is a Protein Coding gene. Diseases associated with P4HTM include Hypotonia, Hypoventilation, Impaired Intellectual Development, Dysautonomia, Epilepsy, And Eye Abnormalities and Dysautonomia. Gene Ontology (GO) annotations related to this gene include calcium ion binding and iron ion binding. An important paralog of this gene is P4HA1.3
The ANXA1 (Annexin 1) gene encodes a membrane-localized protein that binds phospholipids – inhibiting phospholipase A2 – and activates FPR1-3. Annexin 1 also plays important roles in the innate immune system as effector of glucocorticoid-mediated responses and regulator of the inflammatory process. As a ligand for Fpr1 and Fpr2, Annexin 1 has been considered in research efforts to determine whether Fpr1 deficiency may modulate mouse behavior by affecting homeostatic serum corticosterone levels.
Common ANXA1 Gene Aliases: ANX1, LPC1, Annexin-1, Annexin A1, Phospholipase A2 Inhibitory Protein, Calpactin-2, LPC1, Lipocortin I,
Common FPR1 Gene Aliases: FPR, Formyl Peptide Receptor 1, FMet-Leu-Phe Receptor, FMLP Receptor, N-Formyl Peptide Receptor
Fpr1 (-/-) mice retain normal spatial memory and learning capacity, but exhibit increased exploratory activity, reduced anxiety-like behavior, and reduced fear memory. Hypocorticosteronemia in Fpr1−/−mice is a plausible mechanism to explain the reduced anxiety-like behavior and impaired fear memory that helps produce increased exploratory activity. How Fpr1 deficiency causes hypocorticosteronemia may be related to its ability to compete with Fpr2 to bind annexin-1, which is known to mediate negative feedback of the hypothalamic–pituitary–adrenal (HPA) axis by glucocorticoids. Glucocorticoids stimulate expression and release of annexin-1. Additional studies are needed to delineate the role of Fpr1 in the HPA axis, and to explore possible direct role of Fpr1 in regulating corticosterone level.
In conclusion, this study provided the first evidence that a leukocyte chemoattractant receptor may also regulate behavior. Collectively, the data supports the hypothesis that Fpr1 may be involved in modulation of anxiety-like behavior and fear memory by regulating glucocorticoid levels.1
The DAGLA gene encodes the Sn1-specific diacylglycerol lipase alpha protein, which is required for the biosynthesis of 2-arachidonoyl-glycerol (2AG) – one of the most abundant endogenous cannabinoids (endocannabinoids). Disruption of the endocannabinoid system has been linked to depression in humans and depression-like behaviors in mice.1
Common Aliases: Diacylcerol lipase alpha, DAGLALPHA, DAGLα, DGL-Alpha, Neural Stem Cell-Derived Dendrite Regulator, NSDDR, C11orf11, Chromosome 11 Open Reading Frame 11
Several studies have shown how deletion of DAGLA adversely affects the emotional state of animals and results in enhanced anxiety, stress, and fear responses. The deletion of DAGLA results in anxiety and sex-specific depressive phenotypes – reversible upon normalization of endocannabinoid levels. Furthermore, DAGLA deletion reduces CNS, but not circulating, 2-AG levels. It is also shown to impair endocannabinoid suppression of amygdala glutamate release.2
Another research group used knockout mice deficient in DAGL-α (Dagla-/-) to assess the behavioral consequences of reduced endocannabinoid levels in the brain. In this study, Dagla-/- mice demonstrated increased fear response and impaired fear extinction. Data indicated that the “anxiety- and stress-related behavioral phenotypes are entirely consistent with the observed reduction in endocannabinoid levels in the amygdala and hippocampus, two brain regions essential for normal fear and anxiety behaviors. To determine more precisely how endocannabinoid signaling modulates the neuronal circuits associated with affective behaviors will require further studies with a cell-specific Dagla deletion.”3
Development of anxiolytic and antidepressant interventions. Conditional knockout Dagla mouse models may help determine the specific cells and neuronal circuits associated with affective behaviors.
The TAAR1 gene encodes the trace amine-associated receptor 1 protein, a G-protein coupled receptor activated by endogenous trace amines. This gene primarily functions in neurologic systems, but there is growing evidence that it is also involved in immunologic and blood cell functions. TAAR1 serves as an endogenous receptor for metabolic derivatives of amino acids (phenylalanine, tyrosine, and tryptophan) as well as ephedrine.
Common Aliases: Trace amine associated receptor 1, Trace amine receptor 1, TRAR1, TAR1, TA1
Mammalian TAAR1 is a receptor for thyronamines (TAMs), a family of decarboxylated and deiodinated metabolites of the thyroid hormones 3,5,3'-triiodothyronine (T3) and thyroxine (T4). TAMs are a class of endogenous signaling compounds, which are identical in structure from thyroid hormones and deiodinated thyroid hormone derivatives – except that TAMs lack a carboxylate group.2
TAAR1 is widely expressed across the mammalian brain and central nervous system (CNS), particularly in monoaminergic and limbic areas – which are allegedly involved in mood, attention, fear, memory, and addiction. TAAR1 knockout (KO) mice are more prone to develop ethanol addiction and perform more poorly in anxiety and working memory tests. The human genes for TAARs cluster on chromosome 6 at q23 – a region whose mutations may confer susceptibility to bipolar disorder and schizophrenia. The existence of additional targets for most, if not all, known TAAR1 ligands has made it difficult to formally demonstrate that a specific endogenous mediator produces a specific effect in the CNS through TAAR1 stimulation. To date, the brain metabolism of putative TAAR1 ligands is largely unclear. Many different classes of synthetic TAAR1 agonists have been developed, and promising results have been obtained in experimental models of drug abuse, stress, depression, narcolepsy, and cognitive impairment.3
Evidence points to a significant physiological role of TAAR1 in the modulation of central nervous system function and a potential pharmacological role of TAAR1 agonists in neurology and/or psychiatry. This is supported by the significant neurological and behavioral effects produced by the administration of TAAR1 agonists. Additional investigation of the human TAAR1 single-nucleotide polymorphisms (SNPs) may reveal additional functional consequences. Experimental models allowing conditional and tissue-specific TAAR1 KO (or knockdown) are needed to obtain clear-cut answers to several of the critical questions that remain.
Additional potential for research of the sites, pathways, and regulation of TAM biosynthesis, as well as possible implications in pathophysiology and therapeutics.
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