Gangliosides activate Trk receptors by inducing the release of neurotrophins

We used NIH-3T3 fibroblasts expressing the different Trk receptors to examine whether GM1 ganglioside and its semisynthetic derivative LIGA20 activate various neurotrophin receptors. GM1 induced autophosphorylation of TrkC more potently than TrkA or TrkB receptors. In contrast, LIGA20 activated TrkB tyrosine phosphorylation only. Therefore, Scatchard analysis was performed to determine whether GM1 binds to TrkC. GM1 failed to displace neurotrophin-3 binding, suggesting that this ganglioside does not act as a ligand for Trk receptors. In addition, GM1 failed to induce autophosphorylation of a chimeric receptor consisting of the extracellular domain of the tumor necrosis factor receptor and the intracellular domain of TrkA, suggesting that GM1 does not affect the tyrosine kinase domain. We next determined whether GM1 induces the release of neurotrophins from fibroblast cells. GM1 induced a rapid and significant increase in the amount of neurotrophin-3, but not other neurotrophins. This effect was independent of the presence of Trk because K252a did not prevent GM1-mediated release of neurotrophin-3. Moreover, GM1-mediated TrkC autophosphorylation was blocked by TrkC-IgG (but not TrkB-IgG) receptor bodies, further suggesting that GM1 activates TrkC by inducing the release of neurotrophin-3. This hypothesis was also tested in cultured cerebellar granule cells. GM1 induced neurotrophin-3 (but not brain-derived neurotrophic factor or nerve growth factor) release. In contrast, LIGA20 increased the secretion of brain-derived neurotrophic factor. Our data show that gangliosides may activate different Trk receptors by differentially affecting the release of neurotrophins.     

Expression of neurotrophins and Trk receptors in the developing,adult, and regenerating avian cochlea

We studied the expression of neurotrophins and their Trk receptors in the chicken cochlea. Based on in situ hybridization, brain-derived neurotrophic factor (BDNF) is the major neurotrophin there, in contrast to the mammalian cochlea, where neurotrophin-3 (NT-3) predominates. NT-3 mRNA labeling was weak and found only during a short time period in the early cochles. During embryogenesis, BDNF mRNA was first seen in early differentiating hair cells. Afferent cochlear neurons expressed trkB mRNA from the early stages of gangliogenesis onward. In accordance, in vitro, BDNF promoted survival of dissociated neurons and stimulated neuritogenesis from ganglionic explants. High levels of BDNF mRNA in hair cells and trkB mRNA in cochlear neurons persisted in the mature cochlea. In addition, mRNA for the truncated TrkB receptor was expressed in nonneuronal cells, specifically in supporting cells, located adjacent to the site of BDNF synthesis and nerve endings. Following acoustic trauma, regenerated hair cells acquired BDNF mRNA expression at early stages of differentiation. Truncated trkB mRNA was lost from supporting cells that regenerated into hair cells. High levels of BDNF mRNA persisted in surviving hair cells and trkB mRNA in cochlear neurons after noise exposure. These results suggest that in the avian cochlea, peripheral target-derived BDNF contributes to the onset and maintenance of hearing function by supporting neuronal survival and regulating the (re)innervation process. Truncated TrkB receptors may regulate the BDNF concentration available to neurites, and they might have an important role during reinnervation. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 1019–1033, 1997     

Autocrine regulation of nerve growth factor expression by Trk receptors

Activation of the neurotrophin receptor Trk induces the release of neurotrophins. However, little is known about the ability of released neurotrophins to modulate their own synthesis in an autocrine manner. As a step towards understanding the role of Trk in regulating the synthesis of neurotrophins, we exposed NIH-3T3 cells expressing TrkA or TrkC receptors to their cognate ligands as well as to GM1, a ganglioside that activates TrkA and TrkC by inducing the release of neurotrophin-3. Nerve growth factor and neurotrophin-3 synthesis were then determined by measuring the relative levels of protein and mRNA. TrkA-expressing cells exposed to human recombinant nerve growth factor exhibited higher levels of nerve growth factor mRNA. Human recombinant neurotrophin-3 evoked an increase in nerve growth factor mRNA in both TrkA and TrkC-expressing cells. GM1 elicited a time-dependent increase in nerve growth factor protein and mRNA in NIH-3T3 cells expressing TrkA or TrkC receptor but not in wild-type cells. Surprisingly, GM1 failed to change neurotrophin-3 levels. The ability of GM1 to increase nerve growth factor mRNA levels was blocked by TrkC-IgG but not by TrkB-IgG receptor body. These data suggest that released neurotrophin-3 may activate a positive autocrine loop of nerve growth factor synthesis by Trk activation.     

Neuron-derived D-serine release provides a novel means to activate N-methyl-D-aspartate receptors

D-serine is a coagonist of N-methyl-D-aspartate (NMDA) receptors that occurs at high levels in the brain. Biosynthesis of D-serine is carried out by serine racemase, which converts L- to D-serine. D-serine has been demonstrated to occur in glial cells, leading to the proposal that astrocytes are the only source of D-serine. We now report significant amounts of serine racemase and D-serine in primary neuronal cultures and neurons in vivo. Several neuronal culture types expressed serine racemase, and D-serine synthesis was comparable with that in glial cultures. Immunohistochemical staining of brain sections with new antibodies revealed the presence of serine racemase and D-serine in neurons. Cortical neurons expressing serine racemase also expressed the NR2a subunit in situ. Neuron-derived D-serine contributes to NMDA receptor activation in cortical neuronal cultures. Degradation of endogenous D-serine by addition of the recombinant enzyme D-serine deaminase diminished NMDA-elicited excitotoxicity. Release of neuronal D-serine was mediated by ionotropic glutamate receptor agonists such as NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate. Removal of either external Ca2+ or Na+ blocked D-serine release. Release of D-serine was mostly through a cytosolic route because it was insensitive to bafilomycin A1, a potent inhibitor of vesicular neurotransmitter uptake. D-serine was also not transported into purified synaptic vesicles under conditions optimal for the uptake of known transmitters. Our results suggest that neurons are a major source of D-serine. Glutamate-induced neuronal D-serine release provides a novel mechanism for activating NMDA receptors by an autocrine or paracrine way.     

Trk receptors: mediators of neurotrophin action

The four mammalian neurotrophins - NGF, BDNF, NT-3 and NT-4 - each bind and activate one or more of the Trk family of receptor tyrosine kinases. Through these receptors, neurotrophins activate many intracellular signaling pathways, including those controlled by Ras, the Cdc42/Rac/RhoG protein family, MAPK, PI3K and PLC-gamma, thereby affecting both development and function of the nervous system. During the past two years, several novel signaling pathways controlled by Trk receptors have been characterized, and it has become clear that membrane transport and sorting controls Trk-receptor-mediated signaling because key intermediates are localized to different membrane compartments. Three-dimensional structures of the Trk receptors, in one instance in association with a neurotrophin, have revealed the structural bases underlying specificity in neurotrophin signaling.     

Expression of the neurotrophin receptors Trk A and Trk B in adult human astrocytoma and glioblastoma

Neurotrophins and their receptors of the Trk family play a critical role in proliferation, differentiation and survival of the developing neurons. There are reports on their expression in neoplasms too, namely, the primitive neuroectodermal tumours of childhood, and in adult astrocytic gliomas. The involvement of Trk receptors in tumour pathogenesis, if any, is not known. With this end in view, the present study has examined 10 tumour biopsy samples (identified as astrocytoma, pilocytic astrocytoma and glioblastoma) and peritumoral brain tissue of adult patients, for the presence of Trk A and Trk B receptors, by immunohistochemistry. The nature of the tumour samples was also confirmed by their immunoreactivity (IR) to glial fibrillary acidic protein. In the peritumoral brain tissue, only neurons showed IR for Trk A and Trk B. On the contrary, in the tumour sections, the IR to both receptors was localized in the vast majority of glia and capillary endothelium. There was an obvious pattern of IR in these gliomas: high levels of IR were present in the low-grade (type I and II) astrocytoma; whereas in the advanced malignant forms (WHO grade IV giant cell glioblastoma and glio-blastoma multiforme) the IR was very weak. These findings suggest that Trk A and Trk B are involved in tumour pathogenesis, especially in the early stage, and may respond to signals that elicit glial proliferation, and thus contribute to progression towards malignancy.     

Cell survival through Trk neurotrophin receptors is differentially regulated by ubiquitination

Specificity of neurotrophin factor signaling is dictated through the action of Trk receptor tyrosine kinases. Once activated, Trk receptors are internalized and targeted for degradation. However, the mechanisms implicated in this process are incompletely understood. Here we report that the Trk receptors are multimonoubiquitinated in response to neurotrophins. We have identified an E3 ubiquitin ligase, Nedd4-2, that associates with the TrkA receptor and is phosphorylated upon NGF binding. The binding of Nedd4-2 to TrkA through a PPXY motif leads to the ubiquitination and downregulation of TrkA. Activated TrkA receptor levels and the survival of NGF-dependent sensory neurons, but not BDNF-dependent sensory neurons, are directly influenced by Nedd4-2 expression. Unexpectedly, Nedd4-2 does not bind or ubiquitinate related TrkB receptors, due to the lack of a consensus PPXY motif. Our results indicate that Trk neurotrophin receptors are differentially regulated by ubiquitination to modulate the survival of neurons.     

Distinctive features of Trk neurotrophin receptor transactivation by G protein-coupled receptors.

Ligands for G protein-coupled receptors (GPCR) are capable of activating mitogenic receptor tyrosine kinases, in addition to the mitogen-activated protein (MAP) kinase signaling pathway and classic G protein-dependent signaling pathways involving adenylyl cyclase and phospholipase. For example, receptors for epidermal growth factor (EGF), insulin-like growth-1 and platelet-derived growth factor and can be transactivated through G protein-coupled receptors. Neurotrophins, such as NGF, BDNF and NT-3 also utilize receptor tyrosine kinases, namely TrkA, TrkB and TrkC. Recently, it has been shown that activation of Trk receptor tyrosine kinases can also occur via a G protein-coupled receptor mechanism, without involvement of neurotrophins. Adenosine and adenosine agonists can activate Trk receptor phosphorylation specifically through the seven transmembrane spanning adenosine 2A (A2A) receptor. Several features of Trk receptor transactivation are noteworthy and differ significantly from other transactivation events. Trk receptor transactivation is slower and results in a selective increase in activated Akt. Unlike the biological actions of other tyrosine kinase receptors, increased Trk receptor activity by adenosine resulted in increased cell survival. This article will discuss potential mechanisms by which adenosine can activate trophic responses through Trk tyrosine kinase receptors.     

Gangliosides activate the phosphatase activity of the erythrocyte plasma membrane Ca2+-ATPase

The previous studies showed that gangliosides modulated the ATPase activity of the PMCA from porcine brain synaptosomes [Yongfang Zhao, Xiaoxuan Fan, Fuyu Yang, Xujia Zhang, Arch. Biochem. Biophys. 427 (2004) 204-212]. The effects of gangliosides on the hydrolysis of p-nitrophenyl phosphate (pNPP) catalyzed by the erythrocyte plasma membrane Ca(2+)-ATPase, which was characterized as E(2) conformer of the enzyme, were studied. The results showed that pNPPase activity was stimulated up to seven-fold, depending upon the different gangliosides used with GD1b>GM1>GM2>GM3 approximately Asialo-GM1. Under the same conditions, the ATPase activity was also activated, suggesting that gangliosides should modify both E(1) and E(2) conformer of the enzyme. The Ca(2+), which drove the enzyme to E(1) conformation, inhibited the pNPPase activity, but with the similar half-maximal inhibitory concentrations (IC(50)) in the presence and the absence of gangliosides. Moreover, the pNPPase activity was also inhibited by the raise in ATP concentrations. Gangliosides caused a large increase in V(max), but had no effect on the apparent affinity (K(m)) of the enzyme for pNPP. The kinetic analysis indicated that gangliosides could modulate the erythrocyte PMCA through stabilizing E(2) conformer.     

Role of the conformational versatility of the neurotrophin N-terminal regions in their recognition by Trk receptors

Neurotrophins (NTs) represent a family of proteins that play an important role in the survival, development, and function of neurons. Extensive efforts are currently being made to develop small molecules endowed with agonist or antagonist NT activity. The structurally versatile N-termini of these proteins are considered regions of interest for the design of new molecules. By combining experimental and computational approaches, we analyzed the intrinsic conformational preferences of the N-termini of two of the most important NTs: NGF (NGF-Nter) and NT4 (NT4-Nter). Circular dichroism spectra clearly indicate that both peptides show a preference for random coil states. Because this finding does not preclude the possibility that structured forms may occur in solution as minor conformational states, we performed molecular-dynamics simulations to gain insights into the structural features of populated species. In line with the circular dichroism analysis, the simulations show a preference for unstructured states for both peptides. However, the simulations also show that for NT4-Nter, and to a lesser extent for NGF-Nter, helical conformations, which are required for binding to the Trk receptor, are present in the repertoire of structures that are intrinsically accessible to these peptides. Accordingly, molecular recognition of NTs by the Trk receptor is accomplished by the general mechanism known as population shift. These findings provide a structural rationale for the observed activity of synthetic peptides based on these NT regions. They also suggest strategies for the development of biologically active peptide-based compounds.     

Molecular evolution of the neurotrophin family members and their Trk receptors

Neurotrophins are structurally related proteins regulating brain development and function. Molecular evolution studies of neurotrophins and their receptors are essential for understanding the mechanisms underlying the coevolution processes of these gene families and how they correlate with the increased complexity of the vertebrate nervous system. In order to improve our current knowledge of the molecular evolution of neurotrophins and receptors, we have collected all information available in the literature and analyzed the genome database for each of them. Statistical analysis of aminoacid and nucleotide sequences of the neurotrophin and Trk family genes was applied to both complete genes and mature sequences, and different phylogenetic methods were used to compare aminoacid and nucleotide sequences variability among the different species. All collected data favor a model in which several rounds of genome duplications might have facilitated the generation of the many different neurotrophins and the acquisition of specific different functions correlated with the increased complexity of the vertebrate nervous system during evolution. We report findings that refine the structure of the evolutionary trees for neurotrophins and Trk receptors families, indicate different rates of evolution for each member of the two families, and newly demonstrate that the NGF-like genes found in Fowlpox and Canarypox viruses are closely related to reptile NGF.     

Changes in retinal expression of neurotrophins and neurotrophin receptors induced by ocular hypertension

Open angle glaucoma is defined as a progressive and time-dependent death of retinal ganglion cells concomitant with high intraocular pressure, leading to loss of visual field. Because neurotrophins are a family of growth factors that support neuronal survival, we hypothesized that quantitative and qualitative changes in neurotrophins or their receptors may take place early in ocular hypertension, preceding extensive cell death and clinical features of glaucoma. We present molecular, biochemical, and phenotypic evidence that significant neurotrophic changes occur in retina, which correlate temporally with retinal ganglion cell death. After 7 days of ocular hypertension there is a transient up-regulation of retinal NGF, while its receptor TrkA is up-regulated in a sustained fashion in retinal neurons. After 28 days of ocular hypertension there is sustained up-regulation of retinal BDNF, but its receptor TrkB remains unchanged. Throughout, NT-3 levels remain unchanged but there is an early and sustained increase of its receptor TrkC in Müller cells but not in retinal ganglion cells. These newly synthesized glial TrkC receptors are truncated, kinase-dead isoforms. Expression of retinal p75 also increases late at day 28. Asymmetric up-regulation of neurotrophins and neurotrophin receptors may preclude efficient neurotrophic rescue of RGCs from apoptosis. A possible rationale for therapeutic intervention with Trk receptor agonists and p75 receptor antagonists is proposed.     

High-resolution imaging demonstrates dynein-based vesicular transport of activated Trk receptors

Target-derived neurotrophins signal from nerve endings to the cell body to influence cellular and nuclear responses. The retrograde signal is conveyed by neurotrophin receptors (Trks) themselves. To accomplish this, activated Trks may physically relocalize from nerve endings to the cell bodies. However, alternative signaling mechanisms may also be used. To identify the vehicle wherein the activated Trks are located and transported, and to identify associated motor proteins that would facilitate transport, we use activation-state specific antibodies in concert with immunoelectron microscopy and deconvolution microscopy. We show that the'activated Trks within rat sciatic nerve axons are preferentially localized to coated and uncoated vesicles. These vesicles are moving in a retrograde direction and so accumulate distal to a ligation site. The P-Trk containing vesicles, in turn, colocalize with dynein components, and not with kinesins. Collectively, these results indicate activated Trk within axons travel in vesicles and dynein is the motor that drives these vesicles towards the cell bodies.     

Regulated secretion of neurotrophins by metabotropic glutamate group I (mGluRI) and Trk receptor activation is mediated via phospholipase C signalling pathways

Neurotrophins (NTs) play an essential role in modulating activity-dependent neuronal plasticity. In this context, the site and extent of NT secretion are of crucial importance. Here, we demonstrate that the activation of phospolipase C (PLC) and the subsequent mobilization of Ca(2+) from intracellular stores are essential for NT secretion initiated by both Trk and glutamate receptor activation. Mutational analysis of tyrosine residues, highly conserved in the cytoplasmic domain of all Trk receptors, revealed that the activation of PLC-gamma in cultured hippocampal neurons and nnr5 cells is necessary to mobilize Ca(2+) from intracellular stores, the key mechanism for regulated NT secretion. A similar signalling mechanism has been identified for glutamate-mediated NT secretion-which in part depends on the activation of PLC via metabotropic receptors-leading to the mobilization of Ca(2+) from internal stores by inositol trisphosphate. Thus, PLC-mediated signal transduction pathways are the common mechanisms for both Trk- and mGluRI-mediated NT secretion.     

Vanilloid, purinergic, and CCK receptors activate glutamate release on single neurons of the nucleus tractus solitarius centralis

Baroreceptor inputs to nucleus of the tractus solitarius medialis (mNTS) neurons can be differentiated, among other features, by their response to vanilloid or purinergic agonists, active only on C- or A-fibers, respectively. A major aim of this study was to examine whether neurons of NTS centralis (cNTS), a subnucleus dominated by esophageal inputs, exhibit a similar dichotomy. Since it has been suggested that cholecystokinin (CCK), exerts its gastrointestinal (GI)-related effects via paracrine activation of vagal afferent C-fibers, we tested whether CCK-sensitive fibers impinging upon cNTS neurons are responsive to vanilloid but not purinergic agonists. Using whole cell patch-clamp recordings from cNTS, we recorded miniature excitatory postsynaptic currents (mEPSCs) to test the effects of the vanilloid agonist capsaicin, the purinergic agonist α,β-methylene-ATP (α,β-Met-ATP), and/or CCK-octapeptide (CCK-8s). α,β-Met-ATP, capsaicin; and CCK-8s increased EPSC frequency in 37, 71, and 46% of cNTS neurons, respectively. Approximately 30% of cNTS neurons were responsive to both CCK-8s and α,β-Met-ATP, to CCK-8s and capsaicin, or to α,β-Met-ATP and capsaicin, while 32% of neurons were responsive to all three agonists. All neurons responding to either α,β-Met-ATP or CCK-8s were also responsive to capsaicin. Perivagal capsaicin, which is supposed to induce a selective degeneration of C-fibers, decreased the number of cNTS neurons responding to capsaicin or CCK-8s but not those responding to α,β-Met-ATP. In summary, GI inputs to cNTS neurons cannot be distinguished on the basis of their selective responses to α,β-Met-ATP or capsaicin. Our data also indicate that CCK-8s increases glutamate release from purinergic and vanilloid responsive fibers impinging on cNTS neurons.     

Neurotrophin signal transduction by the Trk receptor

The initial event in the neuronal differentiation of PC12 cells is the binding of the neurotrophin nerve growth factor (NGF) to the Trk receptor. This interaction stimulates the intrinsic tyrosine kinase activity of TRk, initiating a signalling cascade involving the phosphorylation of intracellular proteins on tyrosine, serine, and threonine residues. These signals are then in turn propagated to other messengers, ultimately leading to differentiation, neurotrophin-dependent survival and the loss of proliferative capacity. To transmit NGF signals, NGF-activated Trk rapidly associated with the cytoplasmic proteins, SHC, PI-3 kinase, and PLC-γ1. These proteins are involved in stimulating the formation of various second messenger molecules and activating the Ras signal transduction pathway. Studies with Trk mutants indicate that the acivation of the Ras pathway is necessary for complete differentiation of PC12-derived cells and for the maintenance of the differentiated phenotype. Trk also induces the tyrosine phosphorylation of SNT, a specific target of neurotrophic factor activity in neuronal cells. This review will discuss the potential roles of Trk and the proteins of the Trk signalling pathways in NGF function, and summarize our attempts to understand the mechanisms used by Trk to generate dthe many phenotypic responses of PC12 cells to NGF. 1994 John Wiley & Sons, Inc.     

PACAP stimulates gastric acid secretion in the rat by inducing histamine release

Previous studies have shown that pituitary adenylate cyclase-activating peptide (PACAP) stimulates enterochromaffin-like (ECL) cell histamine release, but its role in the regulation of gastric acid secretion is disputed. This work examines the effect of PACAP-38 on aminopyrine uptake in enriched rat parietal cells and on histamine release and acid secretion in the isolated vascularly perfused rat stomach and the role of PACAP in vagally (2-deoxyglucose) stimulated acid secretion in the awake rat. PACAP has no direct effect on the isolated parietal cell as assessed by aminopyrine uptake. PACAP induces a concentration-dependent histamine release and acid secretion in the isolated stomach, and its effect on histamine release is additive to gastrin. The histamine H2 antagonist ranitidine potently inhibits PACAP-stimulated acid secretion without affecting histamine release. Vagally stimulated acid secretion is partially inhibited by a PACAP antagonist. The results from the present study strongly suggest that PACAP plays an important role in the neurohumoral regulation of gastric acid secretion. Its effect seems to be mediated by the release of ECL cell histamine.     

Positive allosteric modulation of alpha‐7 nicotinic receptors promotes cell death by inducing Ca2+ release from the endoplasmic reticulum

Positive allosteric modulation of α7 isoform of nicotinic acetylcholine receptors (α7‐nAChRs) is emerging as a promising therapeutic approach for central nervous system disorders such as schizophrenia or Alzheimer's disease. However, its effect on Ca²⁺ signaling and cell viability remains controversial. This study focuses on how the type II positive allosteric modulator (PAM II) PNU120596 affects intracellular Ca²⁺ signaling and cell viability. We used human SH‐SY5Y neuroblastoma cells overexpressing α7‐nAChRs (α7‐SH) and their control (C‐SH). We monitored cytoplasmic and endoplasmic reticulum (ER) Ca²⁺ with Fura‐2 and the genetically encoded cameleon targeting the ER, respectively. Nicotinic inward currents were measured using patch‐clamp techniques. Viability was assessed using methylthiazolyl blue tetrazolium bromide or propidium iodide staining. We observed that in the presence of a nicotinic agonist, PNU120596 (i) reduced viability of α7‐SH but not of C‐SH cells; (ii) significantly increased inward nicotinic currents and cytosolic Ca²⁺ concentration; (iii) released Ca²⁺ from the ER by a Ca²⁺‐induced Ca²⁺ release mechanism only in α7‐SH cells; (iv) was cytotoxic in rat organotypic hippocampal slice cultures; and, lastly, all these effects were prevented by selective blockade of α7‐nAChRs, ryanodine receptors, or IP₃ receptors. In conclusion, positive allosteric modulation of α7‐nAChRs with the PAM II PNU120596 can lead to dysregulation of ER Ca²⁺, overloading of intracellular Ca²⁺, and neuronal cell death.

     

A novel endocytic recycling signal distinguishes biological responses of Trk neurotrophin receptors

Endocytic trafficking of signaling receptors to alternate intracellular pathways has been shown to lead to diverse biological consequences. In this study, we report that two neurotrophin receptors (tropomyosin-related kinase TrkA and TrkB) traverse divergent endocytic pathways after binding to their respective ligands (nerve growth factor and brain-derived neurotrophic factor). We provide evidence that TrkA receptors in neurosecretory cells and neurons predominantly recycle back to the cell surface in a ligand-dependent manner. We have identified a specific sequence in the TrkA juxtamembrane region, which is distinct from that in TrkB receptors, and is both necessary and sufficient for rapid recycling of internalized receptors. Conversely, TrkB receptors are predominantly sorted to the degradative pathway. Transplantation of the TrkA recycling sequence into TrkB receptors reroutes the TrkB receptor to the recycling pathway. Finally, we link these divergent trafficking pathways to alternate biological responses. On prolonged neurotrophin treatment, TrkA receptors produce prolonged activation of phosphatidylinositol 3-kinase/Akt signaling as well as survival responses, compared with TrkB receptors. These results indicate that TrkA receptors, which predominantly recycle in signal-dependent manner, have unique biological properties dictated by its specific endocytic trafficking itinerary.