Jab1 regulates Schwann cell proliferation and axonal sorting through p27.

Axonal sorting is a crucial event in nerve formation and requires proper Schwann cell proliferation, differentiation, and contact with axons. Any defect in axonal sorting results in dysmyelinating peripheral neuropathies. Evidence from mouse models shows that axonal sorting is regulated by laminin211– and, possibly, neuregulin 1 (Nrg1)–derived signals. However, how these signals are integrated in Schwann cells is largely unknown. We now report that the nuclear Jun activation domain–binding protein 1 (Jab1) may transduce laminin211 signals to regulate Schwann cell number and differentiation during axonal sorting. Mice with inactivation of Jab1 in Schwann cells develop a dysmyelinating neuropathy with axonal sorting defects. Loss of Jab1 increases p27 levels in Schwann cells, which causes defective cell cycle progression and aberrant differentiation. Genetic down-regulation of p27 levels in Jab1-null mice restores Schwann cell number, differentiation, and axonal sorting and rescues the dysmyelinating neuropathy. Thus, Jab1 constitutes a regulatory molecule that integrates laminin211 signals in Schwann cells to govern cell cycle, cell number, and differentiation. Finally, Jab1 may constitute a key molecule in the pathogenesis of dysmyelinating neuropathies. In peripheral nerve development, the transition between bundles of growing axons surrounded by Schwann cell processes to individual axon ensheathment is termed axonal sorting (Sherman and Brophy, 2005). This event relies on extensive and regulated Schwann cell proliferation to match axon–Schwann cell number and coordinated withdrawal from the cell cycle, differentiation, and survival (Martin and Webster, 1973; Jessen and Mirsky, 2005). Furthermore, Schwann cells extend longitudinal and radial processes to sort large caliber axons from bundles, adopt a 1:1 relationship, and myelinate them (Martin and Webster, 1973; Webster et al., 1973; Nodari et al., 2007). Any defect in the process of axonal sorting results in dysmyelinating neuropathies, such as those associated with merosin-deficient congenital muscular dystrophy type 1A (MDC1A; OMIM #607855) in humans (Shorer et al., 1995) and equivalent disorders in spontaneous dystrophic (dy2J) and knockout (dy3k) mice (Miyagoe et al., 1997; Guo et al., 2003). All of these neuropathies are caused by mutations of the laminin α2 gene (LAMA2), which encodes for the α2 subunit of laminin211 (or merosin), the major component of the Schwann cell basal lamina. A hallmark of Lama2 neuropathies is impaired axonal sorting that resembles embryonic fascicles (Bradley and Jenkison, 1973; Stirling, 1975; Shorer et al., 1995). In fact, laminin211 affects axonal sorting by regulating Schwann cell proliferation and cytoskeletal remodeling. In the process, the laminin receptors αβ1 integrin and dystroglycan are recruited (Feltri et al., 2002; Berti et al., 2011), and downstream intracellular molecules such as integrin-linked kinase (Ilk; Pereira et al., 2009), focal adhesion kinase (Fak; Grove et al., 2007), and the RhoGTPase Rac1 are activated (Benninger et al., 2007; Nodari et al., 2007). Another pathway originated by neuregulin 1 (Nrg1) type III might be involved in axonal sorting (Raphael et al., 2011). Nrg1 type III is an axonally anchored molecule that interacts with ErbB2/3 receptor on Schwann cells and regulates their proliferation and survival in early development and myelination after birth (Nave and Salzer, 2006; Birchmeier and Nave, 2008). As for Laminin211, Nrg1 signaling may control radial sorting through Schwann cell proliferation and cytoskeletal remodeling (Benninger et al., 2007; Raphael et al., 2011). The molecular basis of laminin- and Nrg1-derived signals and whether they constitute distinct pathways or interact to regulate axon sorting are unclear. Studies in cancer cells showed that laminin and ErbB2 control the expression and function of Jun activation domain–binding protein 1 (Jab1; Hsu et al., 2007; Wang et al., 2011), a multifunctional protein member of the COP9 signalosome complex. Jab1, shuttling between nucleus and cytoplasm, controls many cell functions such as proliferation, gene transcription, and protein degradation, thus carefully regulating cell number, differentiation, and motility (Chamovitz and Segal, 2001; Shackleford and Claret, 2010). Recently, changes in Jab1 expression have been described in injured peripheral nerves and inversely correlated to p27KIP1 (p27), a potent cell cycle inhibitor (Cheng et al., 2013). Thus, Jab1 constitutes a good candidate to integrate laminin211- and Nrg1-derived signals in Schwann cells to regulate axonal sorting. To investigate Jab1 function in nerve development, we generated and characterized a mouse in which Jab1 was ablated in Schwann cells. Here we report that, consistent with our hypothesis, loss of Jab1 in Schwann cells causes axonal sorting defects leading to a dysmyelinating neuropathy. Our data suggest that Jab1 integrates laminin211- but not Nrg1-derived signals to control p27 levels and to regulate Schwann cell differentiation and cell number. Indeed, p27 levels are increased in Jab1 mutant nerves, and down-regulation of p27 in jab1-null mice restores Schwann cell number and axonal sorting and rescues the peripheral neuropathy.