Nerve injury induces a Gem-GTPase-dependent downregulation of P/Q-type Ca2+ channels contributing to neurite plasticity in dorsal root ganglion neurons

Decreased expression of GEM in osteoarthritis cartilage regulates chondrogenic differentiation via Wnt/β-catenin signaling

BACKGROUND

GEM (GTP-binding protein overexpressed in skeletal muscle) is one of the atypical small GTPase subfamily members recently identified as a regulator of cell differentiation. Abnormal chondrogenesis coupled with an imbalance in the turnover of cartilaginous matrix formation is highly relevant to the onset and progression of osteoarthritis (OA). However, how GEM regulates chondrogenic differentiation remains unexplored.

METHODS

Cartilage tissues were obtained from OA patients and graded according to the ORASI and ICRS grading systems. The expression alteration of GEM was detected in the Grade 4 cartilage compared to Grade 0 and verified in OA mimic culture systems. Next, to investigate the specific function of GEM during these processes, we generated a Gem knockdown (Gem-Kd) system by transfecting siRNA targeting Gem into ATDC5 cells. Acan, Col2a1, Sox9, and Wnt target genes of Gem-Kd ATDC5 cells were detected during induction. The transcriptomic sequencing analysis was performed to investigate the mechanism of GEM regulation. Wnt signaling pathways were verified by real-time PCR and immunoblot analysis. Finally, a rescue model generated by treating Gem-KD ATDC5 cells with a Wnt signaling agonist was established to validate the mechanism identified by RNA sequencing analysis.

RESULTS

A decreased expression of GEM in OA patients' cartilage tissues and OA mimic chondrocytes was observed. While during chondrogenesis differentiation and cartilage matrix formation, the expression of GEM was increased. Gem silencing suppressed chondrogenic differentiation and the expressions of Acan, Col2a1, and Sox9. RNA sequencing analysis revealed that Wnt signaling was downregulated in Gem-Kd cells. Decreased expression of Wnt signaling associated genes and the total β-CATENIN in the nucleus and cytoplasm were observed. The exogenous Wnt activation exhibited reversed effect on Gem loss-of-function cells.

CONCLUSION

These findings collectively validated that GEM functions as a novel regulator mediating chondrogenic differentiation and cartilage matrix formation through Wnt/β-catenin signaling.

Inactivation influences the extent of inhibition of voltage-gated Ca+2 channels by Gem-implications for channelopathies

Voltage-gated Ca2+ channels (VGCC) directly control muscle contraction and neurotransmitter release, and slower processes such as cell differentiation, migration, and death. They are potently inhibited by RGK GTP-ases (Rem, Rem2, Rad, and Gem/Kir), which decrease Ca2+ channel membrane expression, as well as directly inhibit membrane-resident channels. The mechanisms of membrane-resident channel inhibition are difficult to study because RGK-overexpression causes complete or near complete channel inhibition. Using titrated levels of Gem expression in Xenopus oocytes to inhibit WT P/Q-type calcium channels by ∼50%, we show that inhibition is dependent on channel inactivation. Interestingly, fast-inactivating channels, including Familial Hemiplegic Migraine mutants, are more potently inhibited than WT channels, while slow-inactivating channels, such as those expressed with the Cavβ2a auxiliary subunit, are spared. We found similar results in L-type channels, and, remarkably, Timothy Syndrome mutant channels were insensitive to Gem inhibition. Further results suggest that RGKs slow channel recovery from inactivation and further implicate RGKs as likely modulating factors in channelopathies.

A light-induced small G-protein gem limits the circadian clock phase-shift magnitude by inhibiting voltage-dependent calcium channels

Calcium signaling is pivotal to the circadian clockwork in the suprachiasmatic nucleus (SCN), particularly in rhythm entrainment to environmental light-dark cycles. Here, we show that a small G-protein Gem, an endogenous inhibitor of high-voltage-activated voltage-dependent calcium channels (VDCCs), is rapidly induced by light in SCN neurons via the calcium (Ca²⁺)-mediated CREB/CRE transcriptional pathway. Gem attenuates light-induced calcium signaling through its interaction with VDCCs. The phase-shift magnitude of locomotor activity rhythms by light, at night, increases in Gem-deficient (Gem^-/-) mice. Similarly, in SCN slices from Gem^-/- mice, depolarizing stimuli induce larger phase shifts of clock gene transcription rhythms that are normalized by the application of an L-type VDCC blocker, nifedipine. Voltage-clamp recordings from SCN neurons reveal that Ca²⁺ currents through L-type channels increase in Gem^-/- mice. Our findings suggest that transcriptionally activated Gem feeds back to suppress excessive light-evoked L-type VDCC activation, adjusting the light-induced phase-shift magnitude to an appropriate level in mammals.

Gene sequence screening for manganese poisoning-susceptible genes and analysis of gene interaction effects

Manganese poisoning is a common occupational disease, studies have found that the susceptibility to manganese poisoning differs in individuals. We adopted genome-wide sequencing methods to screen for susceptibility genes involved in gene-mediated metabolic pathways from the perspective of manganese poisoning. We identified 18,439 genes in this study, including 14,272 known genes and 4398 new genes. We then selected 17 differential genes using p values, of which 7 genes were down-regulated and 10 genes were up-regulated. Possible interaction genes for each differential gene were selected according to the String database. Sgk1, HCRTr1, HspB1, Rem2, Oprd1, ATF5, and TRHr identified in this study may be involved in oxidative stress mechanisms, dopamine (DA) synthesis, and neuronal survival during apoptosis and may affect susceptibility to manganese poisoning.

Rem2, a member of the RGK family of small GTPases, is enriched in nuclei of the basal ganglia

Rem2 is a member of the RGK subfamily of RAS small GTPases. Rem2 inhibits high voltage activated calcium channels, is involved in synaptogenesis, and regulates dendritic morphology. Rem2 is the primary RGK protein expressed in the nervous system, but to date, the precise expression patterns of this protein are unknown. In this study, we characterized Rem2 expression in the mouse nervous system. In the CNS, Rem2 mRNA was detected in all regions examined, but was enriched in the striatum. An antibody specific for Rem2 was validated using a Rem2 knockout mouse model and used to show abundant expression in striatonigral and striatopallidal medium spiny neurons but not in several interneuron populations. In the PNS, Rem2 was abundant in a subpopulation of neurons in the trigeminal and dorsal root ganglia, but was absent in sympathetic neurons of superior cervical ganglia. Under basal conditions, Rem2 was subject to post-translational phosphorylation, likely at multiple residues. Further, Rem2 mRNA and protein expression peaked at postnatal week two, which corresponds to the period of robust neuronal maturation in rodents. This study will be useful for elucidating the functions of Rem2 in basal ganglia physiology.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α(1) subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein-protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein-protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity.