Central nervous system effects of 5-HT7 receptors: a potential target for neurodegenerative diseases

5-HT7 receptors (5-HT7R) are the most recently identified among the family of serotonin receptors. Their role in health and disease, particularly as mediators of, and druggable targets for, neurodegenerative diseases, is incompletely understood. Unlike other serotonin receptors, for which abundant preclinical and clinical data evaluating their effect on neurodegenerative conditions exist, the available information on the role of the 5-HT7R receptor is limited. In this review, we describe the signaling pathways and cellular mechanisms implicated in the activation of the 5-HT7R; also, we analyze different mechanisms of neurodegeneration and the potential therapeutic implications of pharmacological interventions for 5-HT7R signaling.

Experimental data suggest that 5-HT 7 R may be an amenable therapeutic target in neurodegenerative disorders. However, no clinical studies have evaluated the role of 5-HT 7 R in neurodegenerative processes, although targeting other serotonin receptors (with drugs such as selective serotonin reuptake inhibitors) has shown little clinical benefit in neurodegenerative conditions (Hüll et al. 2006;Lalut et al. 2017). Here, we aimed to review the potential role of 5-HT 7 R in neurodegeneration and their potential therapeutic implications, based on different in vivo and in vitro pre-clinical studies (Table 1).

Methods
We performed a comprehensive search of the Pub-Med database English language literature to identify all original research and review articles regarding 5-HT 7 R localization, signaling pathways, effectors, its role in health in the central nervous system, and the pathology of selected neurodegenerative diseases. For that purpose, we included the following Medical Subject Headings (MeSH) and main keywords for searches: 5-HT 7 , LP-211, LP-44, LP-21, AS-19, SB-269970, SB-656104-A, 5-HT 7 R mechanism of action, 5-HT 7 R signaling pathway, 5-HT 7 R effect, 5-HT 7 R distribution, 5-HT 7 R neuroprotection, excitotoxicity, oxidative stress, apoptosis, necrosis, unfolded protein response, endoplasmic reticle stress, amyloid β, tau tangles, tau oligomers, α-synuclein, inflammation, Alzheimer's, Parkinson's, Huntington's, Frontotemporal dementia, dementia, and neurodegeneration. We also reviewed the articles cited in the reference lists of the articles identified during the search. The authors independently reviewed the selected articles. The search included articles available from 1993 (when the receptor was originally cloned) to March 2022.

Distribution in the CNS
5-HT 7 R is broadly expressed in different cell types in the CNS, including neurons, astrocytes, and microglia (Shimizu et al. 1998;Mahé et al. 2005;Guseva et al. 2014). CNS expression of 5-HT 7 R is not homogeneous; the highest expression occurs in the hippocampus, amygdala, and hypothalamus (Thomas and Hagan 2004), and comparatively lower density occurs in the dorsal raphe, caudate nucleus, putamen, and substantia nigra (Martín-Cora and Pazos 2004). In the amygdala, 5-HT 7 R are present primordially in GABAergic interneurons; in other structures, their expression in specific neurons is not clear, hence of critical importance for therapeutic purposes (Kusek et al. 2021). Of biological and clinical relevance, the regional expression of 5-HT 7 R is evolutionarily conserved in mammals (Martín-Cora and Pazos 2004). 5-HT 7 R are also expressed in the digestive tract, aorta, and other tissues, exerting immunomodulatory effects as well as other organ-specific effects (Quintero-Villegas and Valdés-Ferrer 2019).

Cellular mechanisms and signaling pathways
Like other GPCR, 5-HT 7 R are found in the cell membrane, where they form homodimers and homoligomers, with no known relevant differences between their biological functions (Smith et al. 2011;Guseva et al. 2014). In addition, 5-HT 7 R can form heterodimers and heterooligomers with 5-HT 1A receptors (5-HT 1A R), which in turn lead to diminished activity and increased internalization of 5-HT 1A receptors without a discernible effect on 5-HT 7 R signaling or activity (Renner et al. 2005;Prasad et al. 2019). This is of biological relevance, as activation of 5-HT 1A R results in activation of G αi and reducing levels of cyclic adenosine monophosphate (cAMP) and mitogen-activated protein kinase (MAPK), also known as an extracellular signal-regulated kinase (ERK), hence   antagonizing the effects of the 5-HT 7 R-Gαs signaling cascade (Zhou et al. 2019).
The activation of 5-HT 7 R leads to the initiation of two possible signaling pathways: the canonical one, described when the receptor was originally cloned (Lovenberg et al. 1983) acts via Gαs (Fig. 1). The activation of this pathway, like in other GPCRs results in the phosphorylation of different adenylyl cyclases (AC) (Baker et al. 1998). This leads to cAMP production, activation of protein kinase A, and, finally, phosphorylation of different proteins, like ERK 1 and ERK 2, Akt, and tropomyosin-related kinase B (Trkb) (Errico et al. 2001;Johnson-Farley et al. 2005;Samarajeewa et al. 2014).
The Non-canonical signaling pathway of 5-HT 7 R acts via Gα12 (Guseva et al. 2014). This leads to the activation of Rho, Rac, and cell division control protein 42 (Cdc42) all part of the Rho family of small GTPases, which in neurons promote dendrite sprouting, formation of filopodia, and synaptogenesis (Hart, et al. xxxx;Kobe et al. 2012a;Speranza et al. 2013;Speranza et al. 2015;Marin and Dityatev 2017). Of relevance, Trkb expression (a brainderived neurotrophic factor [BDNF] receptor) appears to be enhanced by both Gαs and Gα12 ( Fig. 1) (Samarajeewa et al. 2014). These signaling pathways may be of therapeutic relevance for neurodegenerative diseases, although few studies have so far evaluated these effects (Hashemi-Firouzi et al. 2017;Costa et al. 2018;).

Biopathology of neurodegeneration
The biological mechanisms leading to neurodegeneration include many neuronal and glial molecular pathways that result in neuronal damage. Below, we briefly summarize the most common mechanisms involved in neurodegeneration and how they might associate with the known 5-HT 7 R effects ( Fig. 2; Table 2).

Amyloid β-mediated neurodegeneration
Amyloid β (Aβ) is a key mediator of neurodegeneration in Alzheimer's disease (AD), inducing damage through multiple pathways (Querfurth and Laferla 2018), some of which overlap with other mechanisms discussed below. Besides AD, Aβ may also play a role in diseases such as frontotemporal dementia (FTD), cerebral amyloid angiopathy, and cerebral amyloidosis (Miller- Thomas et al. 2016). Aβ is a critical source of reactive oxygen species (ROS) and reactive nitrogen species in AD, causing neuronal lipoperoxidation in neurons and thus, Fig. 1 5-HT 7 and 5-HT 1A receptor signaling pathways and oligo/heterodimer formation. 5-HT 7 receptor monomers (in yellow) can form homodimers or homoligomers, with the same signaling pathways and cellular effects. 5-HT 7 can also form heterodimers or heteroligomers with 5-HT 1A (in teal), resulting in the inhibition of the 5-HT 1A signaling pathway, with no net effect downstream of 5-HT 7 . When activated, 5-HT 7 activates Gas (canonical pathway) with a subsequent signaling cascade that results in the activation of ERK (also known as MAPK) and Akt; in contrast, the activation of Ga12 activates mTOR and different Rho family small GTPases. As illustrated, the phosphorylation of Trkb is mediated by both G proteins. AC adenylate cyclase, cAMP cyclic adenosine monophosphate, Cdc42 cell division control protein 42 homolog, ERK extracellular signal-regulated kinases, MAPK mitogen-activated protein kinases, mTOR mammalian target of rapamycin, Trkb Tropomyosin receptor kinase B . Also, Aβ helps the formation of voltage-independent, cation channels in the lipid membranes, which could lead to excitotoxicity-mediated neurodegeneration (Arispe et al. 1993). Chronically Aβ-stimulated microglia releases multiple pro-inflammatory cytokines, such as interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)-α, which induces pathological changes in the CNS (Heppner et al. 2015). Aβ-stimulated microglia increases neuronal damage and further accumulation of Aβ, an effect mediated by receptors for advanced glycation end products (Fang et al. 2010). Although not directly associated with neuronal death, Aβ impairs synaptic function and synaptogenesis and dysregulates neurotransmitter levels in the synaptic cleft, contributing to  (1); these receptors also increase the number of neurotrophins (especially BDNF) and the affinity of its receptor Trkb (2); through ERK and Akt, 5-HT 7 decreases neuronal damage mediated by ROS (3); and reduces the excitotoxicity burden mediated by glutamate-NMDA-calcium. Cellular effects of 5-HT 7 activation. B When stimulated by serotonin, 5-HT 7 enhances dendritic sprouting and synaptogenesis, while regulating (often towards suppression) immune cells. LTD long-term depression, LTP long-term potentiation, NMDA (N-methyl-d-aspartate receptor, ROS reactive oxygen species, Trkb tropomyosin receptor kinase B Aβ induces neurotoxicity through several mechanisms including apoptosis, excitotoxicity, and oxidative stress (Querfurth and Laferla 2018;Bernal-Mondragón et al. 2013). In a streptozotocin-mediated neurodegeneration murine model for AD, the intracerebroventricular (ICV) treatment of AS-19 (a 5-HT 7 R selective agonist) reduced long-term potentiation (LTP) impairment and apoptosis in hippocampal (Hashemi-Firouzi et al. 2017).
The exact mechanism of 5-HT 7 R-mediated neuroprotection in Aβ-induced neurodegeneration is currently under investigation, an effect that is likely to be mediated through multiple mechanisms.

Excitotoxicity
This is an important cause of neuronal damage in neurodegenerative diseases, including AD, stroke, Huntington's disease (HD), and Parkinson's disease (PD) (Martire et al. 2013;Lai et al. 2014;Iovino et al. 2020). Persistent excitatory -mainly glutamatergic-stimuli lead to altered calcium homeostasis, resulting in oxidative stress, mitochondrial dysfunction, disturbances in protein turnover, inflammation, and caspase-mediated apoptosis (Binvignat and Olloquequi 2020). In vitro studies suggest that the MAPK/ERK and phosphatidylinositol-3/ Akt/Glycogen synthase kinase 3b pathways are closely associated with protection against glutamate-induced damage (Jiang et al. 2000;Pi et al. 2004;Li et al. 2005); modulation of these kinases via 5-HT 7 R Gαs could have therapeutic implications. 5-HT 7 R activation leads to a decrease in the NR2B and NR1 subunits of the N-methyl-D-aspartate (NMDA) glutamate receptors, thus protecting against glutamate-mediated excitotoxicity (Vasefi et al. 2013a). Treatment with LP-44, a 5-HT 7 R-specific agonist, protects human neuroblastoma SH-SY5Y cells against glutamate-mediated damage in an in-vitro model, also increasing superoxide dismutase and glutathione while decreasing TNF-α, and caspase-3 and -9 (Yuksel et al. 2019). Moreover, 5-HT 7 R modulate glutamate-NMDA activity in a time-dependent manner; while the acute activation of 5-HT 7 R by LP-12, a selective agonist of 5-HT 7 R, enhances NMDA activity, the chronic activation inhibits its activity (Vasefi et al. 2013b).

Oxidative stress
This can lead to membrane damage and neuronal death. (Bernal-Mondragón et al. 2013) Although, oxidative phosphorylation in mitochondria produces ROS, reactive nitrogen species, carbon-centered radicals, and sulfur-centered radicals (Pero et al. 1990), and these byproducts are considered necessary for neuronal function and development (Salim 2017); increase of their levels, beyond a physiological threshold, are considered deleterious. Multiple in vitro studies have demonstrated that  high levels of ROS reduce long-term potentiation, synaptic signaling, and brain plasticity (Salim 2017;O'Dell et al. 1991;Stevens and Wang 1993). Moreover, oxidative stress damages macromolecules, mainly lipid-rich structures such as membranes, via lipoperoxidation (Salim 2017).
Although no study so far has evaluated the potential effect of 5-HT 7 R in oxidative stress damage in the CNS, a study evaluating sepsis-induced lung injury demonstrated that 5-HT 7 R agonism decreased ROS burden (Cadirci et al. 2013). In contrast, the antagonism of the 5-HT 7 R by SB-269970 decreased oxidative burden in bleomycin-induced pulmonary fibrosis (Tawfik and Makary 2017). Also, 5-HT 7 R activation by LP-44 enhances microsome stability towards oxidative metabolism (Lacivita et al. 2016a). As mentioned before, 5-HT 7 R regulates the activation of ERK and Akt, and these kinases have biological effects on oxidative stress injury protection in PC12 cells (Ong et al. 2016). Thus, 5-HT 7 R may have therapeutic implications in ROS-induced neurodegeneration, something that needs to be experimentally assessed.

Apoptosis
Neuronal cell death is a major pathological characteristic of every neurodegenerative disease, whether via apoptosis or necrosis. In AD, both phosphorylated tau protein and Aβ aggregates induce apoptosis in vitro studies, with contradictory effects in tissular studies. Other proteins, including α-Synuclein (in PD or Lewy body dementia), or mutant huntingtin protein (in HD) also induce neuronal cell death via multiple mechanisms, including apoptosis (Chi et al. 2018).
It is also important to note, that apoptosis is closely related to excitotoxicity and oxidative stress; thus, these pathological effects are somewhat overlapped (Yuksel et al. 2019;Loh et al. 2006;Nicholls et al. 2007). The 5-HT 7 R agonist AS-19 reduces apoptosis in the streptozotocin-induced AD model (Hashemi-Firouzi et al. 2017). While no other study has yet evaluated this effect in the CNS, methiothepin maleate (a non-specific 5-HT 1/6/7 R agonist) prevents monocyte activation via ERK 1/2 and Nuclear factor-κB (Soga et al. 2007).

Long-term potentiation and long-term depression (LTD) impairment
Impairment in LTP and LTD have been extensively described in many types of dementia, such as AD (Skaper et al. 2017), PD (Marinelli et al. 2017), and HD (Filippo et al. 2007), to name a few, with a strong correlation with the cognitive symptoms in each disease. LTP and LTD are crucial for memory formation, and impairment in these are associated with amnesic and psychiatric symptoms (Loprinzi 2020).
Like other CNS effects, the role of 5-HT 7 R in LTP is controversial. Because chronically stimulated neurons by 5-HT 7 R agonists show a reduction in the expression of NMDA glutamate receptors, 5-HT 7 R has been associated with a reduction in LTP (Kobe et al. 2012b), however, 5-HT 7 R knock-out mice also display an impairment in LTP, suggesting that 5-HT 7 R receptors at a baseline state are necessary for LTP (Roberts et al. 2004). Regarding LTD, 5-HT 7 R agonism by 8-OH-DPAT (a 5-HT 1A/7 R agonist) reduced mGluR mediated LTD (Costa et al. 2012). Finally, LP-211 induces LTD on the parallel fiber-Purkinje cell synapse via Protein kinase C-MAPK pathway while impairing LTP, and pharmacological antagonism with SB-269970 decreases LTD (Lippiello et al. 2016).

Synaptogenesis and brain plasticity reduction
In many neurodegenerative diseases, reductions in synaptogenesis, brain plasticity, and dendritic sprouting are hallmarks of severity and progression (Querfurth and LaFerla 2010). So far, no treatment strategies have been shown to reverse that.

Neurotrophin depletion
Neurotrophins like BDNF and platelet-derived growth factor (PDGF)-β are necessary for the development and correct function of the CNS. Depletion or altered signaling occurs in neurodegenerative diseases (Kashyap et al. 2018) AD, PD, HD, and FTD are associated with a reduction in the expression of BDNF and other neurotrophins, where the modulation of BDNF could have a potential therapeutic role (Palasz et al. 2020;Schulte-Herbrüggen et al. 2008;Alberch et al. 2002;Huey et al. 2020). Interestingly, 5-HT 7 R modulates both neurotrophins and their receptors. 5-HT 7 R activation by LP-12 leads to an increase in PDGF-β, with increased protection against glutamate-mediated excitotoxicity (Vasefi et al. 2013a). In addition, 5-HT 7 R agonism by LP-12 increases the expression and affinity of the tropomyosin-related kinase B receptor, one of the receptors for BDNF (Samarajeewa et al. 2014).

Immune-mediated damage
The specific role of 5-HT 7 R as neuro-immune mediators is still debated. 5-HT 7 R is expressed broadly by different immune cells, including monocytes, lymphocytes, and dendritic cells, but its anti-inflammatory potential has been shown in some but not all studies (Quintero-Villegas and Valdés-Ferrer 2019).

Neuronal hyperexcitability and seizures
Epilepsy-induced neuronal damage shares pathophysiological neurodegenerative features with dementias and other CNS diseases, such as increased inflammation and excitotoxicity (Nikiforuk 2015). Epilepsy is prevalent in sufferers of CNS diseases, including AD, PD, or HD, and, when present, indicates a higher burden of neurodegeneration (Cano et al. 2021). 5-HT 7 R manipulation has shown controversial results in pre-clinical studies of epilepsy. Non-specific pharmacological blockade of 5-HT 7 R reduces the prevalence of audiogenic seizures in a DBA/2J rat model (Bourson et al. 1997).
Additionally, SB-258719, an antagonist of 5-HT 7 R showed a benefit in reducing spontaneous epileptic activity in a WAG/Rij rat model of absence seizures (Graf et al. 2004). Finally, in a pilocarpine-induced rat model of temporal lobe epilepsy, AS-19 increased epileptic activity, whereas SB-269970, an antagonist of 5-HT 7 R reduced seizure activity. Interestingly, the expression of 5-HT 7 R was higher in the epilepsy group, compared to the control group (Yang et al. 2012). However, in a pilocarpine-induced model of epilepsy, the density of 5-HT 7 R decreased in the hippocampus, especially in the dentate gyrus (Núñez-Ochoa et al. 2021). Hence, the role of 5-HT 7 R in epilepsy is still unclear but avidly explored. The controversial finding may represent that 5-HT 7 R plays different roles in different models of epilepsy. Evaluation of 5-HT 7 R tissue expression in specimens obtained from patients undergoing surgical excision of epileptic foci may help to clarify this controversy.

Mood disorders
The relationship between mood disorders and cognitive disorders is a topic of continuous research. The prevalence of depression and anxiety is higher among patients with dementia, and patients with depression have a higher prevalence of dementia (Lyketsos et al. 2002). Of therapeutic relevance, even mild levels of depression can impact substantially the functionality and quality of life of patients with dementia. Thus, treating these symptoms may be crucial in the management of dementia (Gutzmann and Qazi 2015). In a rat model of stress using forced swim and tail suspension, pharmacological and genetic blockade of 5-HT 7 R reduced depressive symptoms and improved REM sleep (Hedlund et al. 2005). 5-HT 7 R KO mice show improved mobility in the Porsolt swim test; however, the pharmacological blockade by SB-258719 only had the same results when rats were tested in the dark (Guscott et al. 2005). Similar results were observed with SB-269970 in depression and anxiety, with an effect similar to the one observed with imipramine (Wesołowska et al. 2006). In experimental depression, pharmacological blockade of 5-HT 7 R seems to have a rapid effect in reducing depressive symptoms (Mnie-Filali et al. 2011). Altogether, while more data is needed before moving to the clinical trial setting, these experimental models suggest that 5-HT 7 R may be a druggable target for depression.
The calcium-binding protein S100B interacts with 5-HT 7 R and negatively regulates inducible cAMP accumulation; its overexpression in transgenic mice is associated with depressive-like symptoms, which are reversed by the administration of SB269970 (Stroth and Svenningsson 2015).

The contradictory memory effect of 5-HT 7 R
Evidence about the effect of 5-HT 7 R on learning and memory is still contradictory. Systemic administration of LP-211 and AS-19 revert scopolamine-induced amnesia and enhanced auto-shaping training learning respectively in rats; whereas pharmacologic blockade with SB-269970 has the opposite effect (Perez-García and Meneses 2005). Accordingly, LP-211 administrated intraperitoneally improves long-term memory, with conditioned responses up to 80%, compared to 20-30% in the control groups, with no effect on short-term memory, and reverts scopolamine-induced memory impairment, something associated with increased cAMP levels (Meneses et al. 2015). Moreover, 5-HT 7 R agonism appears to revert the memory impairment mediated by 5-HT 1A R, based on the fact that the co-administration of SB-269970 and 8-OH-DPAT (a 5-HT 7 R/5-HT 1A R agonist) caused greater performance impairment in contextual learning than the administration of 8-OH-DPAT alone (Eriksson et al. 2008).
On the other hand, SB-269970 improved reference memory in a radial arm maze task (Gasbarri et al. 2008). In agreement, the administration of SB-656104-A (a 5-HT 7 R-specific antagonist) reverses dizocilpine-induced memory impairment induced, with significant differences in thigmotaxic swimming in the Morris water maze test (Horisawa et al. 2011). We have observed that intracerebroventricular injection of LP-211 reverts memory impairment induced by Aβ without a discernible effect on healthy rats ). The answer to this paradox is inconclusive with the current evidence (Meneses 2014;Stiedl et al. 2015;Zareifopoulos and Papatheodoropoulos 2016), suggesting a memory type-specific role of 5-HT 7 R.

Lack of clinical trial-derived data
No clinical trials with 5-HT 7 R-specific modulation for neurodegenerative studies have been performed. However, the potential effect of non-specific serotonin activation in neurodegenerative diseases is a highly studied topic and the outflow of human data may start shortly. Interestingly, lurasidone, a 5-HT 7 R antagonist, with an effect on other serotonin receptors and D2 dopamine receptor (Meltzer et al. 2020), has been FDA approved for the treatment of schizophrenia (Okubo et al. 2021).

Conclusion
At this point, no 5-HT 7 R-specific drugs have been evaluated for neurodegenerative diseases, and few studies have experimentally evaluated their potential therapeutic effects. Hence, elucidating the effect of 5-HT 7 R in health and (predominantly neurodegenerative) diseases may have vast translational and therapeutic implications. 5-HT 7 R agonists may be neuroprotective by acting at multiple levels, including the reduction of excitotoxicity and oxidative stress, synaptic remodeling, regulation of neurotrophic factors, or immunomodulation, to name a few. The need for more studies, both experimental and clinical, before reaching conclusions about a therapeutic role for this serotonin receptor cannot be overemphasized.