|Alzheimer > 14. The Role of CRMP2 in Alzheimer's Disease|
Collapsin response mediator protein-2 (CRMP2) was first discovered as a principal regulator of axonal extension (Goshima et al., 1995). Since then, its functions have expanded to include dendritic branching (Niisato et al., 2013), calcium channel regulation (Khanna et al., 2012), microtubule transport (Hensley and Kursula, 2016), and dendritic spine development (Zhang et al., 2018; Jin et al., 2016). Overexpression of CRMP2 increases spine density especially the mushroom-shape spines (Zhang et al., 2018). On the contrary, elimination of CRMP2 leads to aberrant dendritic development with diminished spine density (Tobe et al., 2017). The spine is a well-established target of beta amyloid oligomers (AβOs) (Chapter 13). This chapter will show that the AβO-induced signaling cascade may lead to CRMP2 hyperphosphorylation, which could be the major cause of spine loss.
Phosphorylation Sites of CRMP2
A growing number of studies indicate that CRMP2 hyperphosphorylation is involved in AβO-induced synaptic dysfunction (Cole et al., 2007; Isono et al., 2013; Xing et al., 2016; Mokhtar et al., 2018). CRMP2 has three subtypes: A, B and C. The CRMP2B subtype contains 572 amino acids (aa) while CRMP2A comprises 677 aa. In the brain, CRMP2B is ~20 times more abundant than CRMP2A (Balastik et al., 2015). Perhaps for this reason, previous studies focused on CRMP2B. However, as discussed below, CRMP2A seems to be more engaged in Alzheimer’s disease (AD) than CRMP2B.
Phosphorylation of CRMP2 is catalyzed mainly by three protein kinases: glycogen synthase kinase-3β (GSK-3β), cyclin dependent kinase 5 (Cdk5) and Rho kinase. Their target sites are shown in Figure 14-1. T555 is the phosphorylation site of Rho kinase which can be activated by small GTPase protein such as RhoA (Arimura et al., 2005). Aβ has been shown to increase CRMP2 phosphorylation via the RhoA mechanism. Intriguingly, the expression level of CRMP2A was also increased (Petratos et al., 2008). This could be a compensatory response. In another study, by comparing the phosphorylation levels of CRMP2 among different neurodegenerative disorders (AD, Huntington's disease, fronto-temporal dementia and multiple sclerosis), Mokhtar et al. (2018) found that the phosphorylation of CRMP2A on T555 was significantly enhanced primarily in AD.
The Role of CRMP2 in Synaptic Function
CRMP2 plays a crucial role in mediating the transport of tubulin heterodimer (Kimura et al., 2005) and WAVE1/Sra1 complex (Kawano et al., 2005) along microtubules (Figure 14-2). Tubulin is the canonical binding partner of CRMP2. It has two isoforms, α and β, that usually form a heterodimer. The dendritic spine contains abundant tubulin heterodimers. In the postsynaptic density (PSD, a structure just beneath the postsynaptic membrane), the amount of α-tubulin accounts for 8% of the PSD protein mass, far exceeding the amount of PSD-95, which accounts for only ~ 0.8% (Yun-Hong et al., 2011). The synaptic function of tubulin was largely unexplored. It has been proposed that the tubulin heterodimer may form a complex (named CABT) with CRMP2 to regulate memory extinction and retrieval (see Born to Forget, Die to Remember). Therefore, impaired transport of tubulin into the spine would cause synaptic dysfunction.
WAVE1 stimulates the assembly of actin monomers into filamentous actin (F-actin) (Pollitt and Insall, 2009) which is an essential component in spines. Loss of WAVE1 reduces spine density (Soderling et al., 2007). Hence, impaired transport of WAVE1 into spines would result in spine loss.
The binding affinity of CRMP2 to tubulin is decreased by the phosphorylation of Cdk5, GSK-3β, and Rho kinase (Arimura et al., 2005). In particular, phosphorylation at T514 by GSK-3β disrupts its binding to tubulin (Yoshimura et al., 2005). However, the phosphorylation at T514 must first be primed by phosphorylation at S522, which is catalyzed by Cdk5 (Cole et al., 2006). In the AD brain, phosphorylation of CRMP2 at S522 is increased (Cole et al., 2007). This may promote T514 phosphorylation by GSK-3β, thereby impeding the CRMP2-mediated transport of tubulin heterodimers into the spine. Consistently, increased phosphorylation of CRMP2 at T514 has been found to correlate with Aβ burden and synaptic deficits in Lewy body dementia (Xing et al., 2016).
Phosphorylation of CRMP2 at T555 has been demonstrated to dissociate CRMP2 from kinesin (Mokhtar et al., 2018) which is a motor protein responsible for carrying various cargos (proteins and organelles) along microtubules (Figure 14-2). Since impaired transport of WAVE1 can cause spine loss, phosphorylation of CRMP2 at T555 should play a critical role in Aβ-mediated spine loss. In agreement, AD patients demonstrated elevated phosphorylation of CRMP2 at the T555 site (Mokhtar et al., 2018).
As mentioned above, T555 is the phosphorylation site of Rho kinase which can be activated by the small GTPase RhoA. Pyk2 may induce the RhoA signaling via GTPase activating proteins (GAPs), guanine exchange factors (GEFs) and guanine nucleotide dissociation inhibitors (GDIs) (Ying et al., 2009). In line with this mechanism, Pyk2 signaling through Graf1 (a GAP) and RhoA has been demonstrated to be required for AβO-triggered synapse loss (Lee et al., 2019).
In summary, a clear picture of AβO-induced synaptic dysfunction is now emerging. The extracellular AβO may bind prion protein (PrPc) to induce mGluR5 signaling which leads to increased Ca2+ level in the cytosol. Elevated Ca2+, in turn, may enhance the activity of Fyn and Pyk2. The Tau protein is required for AβO-induced toxicity because it may facilitate synergistic coactivation between Fyn and Pyk2 (Chapter 13). Both Fyn and Pyk2 can lead to activation of GSK-3β, resulting in impaired transport of tubulin into the spine by phosphorylating CRMP2 at T514. This would disrupt synaptic functions, but may not be sufficient to cause spine loss. In addition to GSK-3β, Pyk2 also augments the activity of Rho kinase which targets the T555 site. Phosphorylation of CRMP2 at T555 inhibits its association with the microtubule motor protein, kinesin. Consequently, the WAVE1/Sra1 complex cannot be transported into the spine. WAVE1 is a key driver for the assembly of actin monomers into F-actin. Loss of WAVE1 has been demonstrated to reduce spine density.
Author: Frank Lee