Rit subfamily small GTPases: regulators in neuronal differentiation and survival

Abstract

Ras family small GTPases serve as binary molecular switches to regulate a broad array of cellular signaling cascades, playing essential roles in a vast range of normal physiological processes, with dysregulation of numerous Ras-superfamily G-protein-dependent regulatory cascades underlying the development of human disease. However, the physiological function for many "orphan" Ras-related GTPases remain poorly characterized, including members of the Rit subfamily GTPases. Rit is the founding member of a novel branch of the Ras subfamily, sharing close homology with the neuronally expressed Rin and Drosophila Ric GTPases. Here, we highlight recent studies using transgenic and knockout animal models which have begun to elucidate the physiological roles for the Rit subfamily, including emerging roles in the regulation of neuronal morphology and cellular survival signaling, and discuss new genetic data implicating Rit and Rin signaling in disorders such as cancer, Parkinson's disease, autism, and schizophrenia.

Keywords: CA; CCI; CREB; CRIB; CaM; Cdc42/Rac interactive binding; DA; DAT; DN; EGF; ERK; Epac; G protein coupled receptor; GAP; GEF; GPCR; GTPase; GTPase activating protein; HSP27; IFNγ; JNK; MAPK; MAPK kinase (also MAP2K); MAPK-activated protein kinase 2 (MAPKAPK2); MEFs; MK2; MKK; MSK1/2; NGF; Neuron; PACAP38; PD; PI3K; PKA; PKC; PMA; Parkinson's disease; RGL3; ROS; RTK; RalGDS-like 3; Ras GTPase; Ras-related protein which interacted with calmodulin; Ric; Ric (Ras)-related gene expressed in neural tissues; Ric (Ras)-related gene expressed throughout the organism; Rin; Rit; SOS; Signaling; TBI; TNFα; TrkA; c-Jun N-terminal protein kinase; cAMP; cAMP response element binding protein; calmodulin; constitutively active; controlled cortical impact; cyclic adenosine monophosphate; dominant negative; dopamine; dopamine transporter; epidermal growth factor; exchange protein directly activated by cAMP; extracellular-signal regulated protein kinase; guanine nucleotide exchange factor; guanosine triphosphatase; heat shock protein 27; interferon-γ; mitogen- and stress-activated protein kinase 1/2; mitogen-activated protein kinase; mouse embryonic fibroblasts; nerve growth factor; phorbol 12-myristate 13-acetate; phosphatidylinositol 3-kinase; pituitary adenylate cyclase activated polypeptide 38; protein kinase A; protein kinase C; reactive oxygen species; receptor tyrosine kinase; son of sevenless; traumatic brain injury; tropomyosin receptor kinase A; tumor necrosis factor α.

Figure 1. Rit subfamily small GTPases

(A) Rit, Rin, and Drosophila dRic define a novel subfamily of the Ras-related small GTPases. (B) Amino acid sequence alignment of the G2 (effector) and C-terminal extension domains of human Rit and Rin, M. Racemosus Ras 3 and Ras 2, human H-, K-, N-, M-, R-Ras, RalA/B, TC21, Rap1A/B, and Rap2A/B proteins. The Rit subfamily contains a unique G2 domain (effector loop) and polybasic C-terminus but lacks classic CAAX prenylation motifs. The highlighted sequence is the calmodulin binding motif of dRIC. :, substitution of residue between Rit GTPases and the larger Ras family; *, conserved residues across the entire Ras family of GTPases; +, conserved polybasic residues within Rit family proteins.

Figure 2. Rit subfamily GTPases promote neurite outgrowth in PC6 cells

Expression of constitutively active Rit (Rit-Q79L) and Rin (Rin-Q78L) is sufficient for the induction of neurite outgrowth. Cells expressing Flag tagged-Rit^Q79L and -Rin^Q78L were visualized by anti-Flag immunostaining with FITC-conjugated secondary antibody.

Figure 3. Rit subfamily GTPase-mediated signal transduction pathways in cell transformation and neurite outgrowth

(A) Rit- and Rin-induced signaling in cell transformation and tumorigenesis. (B) Rit-dependent signaling in neurite outgrowth. Rit triggers neurite outgrowth via both ERK and p38 MAPK pathways and contributes to both NGF- and PACAP38-induced neuronal differentiation. PACAP-dependent neuronal differentiation involves PACR1-Epac-Src cascade mediated transactivation of the TrkA receptor, resulting in Rit activation and neurite elongation. (C) Rin-dependent signaling in neurite outgrowth. Rin-induced neurite outgrowth has been found to depend upon ERK, p38, calmodulin and Rac/Cdc42 signaling. The contribution of Rin to NGF-induced neurite outgrowth requires both ERK and p38 signaling, whereas PACAP38-mediated neurite elongation involves Src-dependent Rin activation, leading to cAMP-PKA-HSP27 signaling. Rin-B-Raf directed crosstalk between the ERK and p38 MAPK cascades may exist, but remains uncharacterized (dashed arrow).

Figure 4. Rit regulates p38-dependent survival signaling

Rit family GTPases function as critical regulators of an evolutionarily conserved, p38 MAPK-dependent, signaling cascade that functions as an important survival mechanism in cells responding to environmental stress. Rit-p38 pro-survival signaling has been shown to involve MK2-HSP27-Akt and MSK1/2-CREB pathways, which in part control Bcl-2 family proteins to promote survival.