Engrailed homeoprotein recruits the adenosine A1 receptor to potentiate ephrin A5 function in retinal growth cones.

Engrailed 1 and engrailed 2 homeoprotein transcription factors (collectively Engrailed) display graded expression in the chick optic tectum where they participate in retino-tectal patterning. In vitro, extracellular Engrailed guides retinal ganglion cell (RGC) axons and synergises with ephrin A5 to provoke the collapse of temporal growth cones. In vivo disruption of endogenous extracellular Engrailed leads to misrouting of RGC axons. Here we characterise the signalling pathway of extracellular Engrailed. Our results show that Engrailed/ephrin A5 synergy in growth cone collapse involves adenosine A1 receptor activation after Engrailed-dependent ATP synthesis, followed by ATP secretion and hydrolysis to adenosine. This is, to our knowledge, the first evidence for a role of the adenosine A1 receptor in axon guidance. Based on these results, together with higher expression of the adenosine A1 receptor in temporal than nasal growth cones, we propose a computational model that illustrates how the interaction between Engrailed, ephrin A5 and adenosine could increase the precision of the retinal projection map.

Engrailed protects mouse midbrain dopaminergic neurons against mitochondrial complex I insults.

Mice heterozygous for the homeobox gene Engrailed-1 (En1) display progressive loss of mesencephalic dopaminergic (mDA) neurons. We report that exogenous Engrailed-1 and Engrailed-2 (collectively Engrailed) protect mDA neurons from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a mitochondrial complex I toxin used to model Parkinson's disease in animals. Engrailed enhances the translation of nuclearly encoded mRNAs for two key complex I subunits, Ndufs1 and Ndufs3, and increases complex I activity. Accordingly, in vivo protection against MPTP by Engrailed is antagonized by Ndufs1 small interfering RNA. An association between Engrailed and complex I is further confirmed by the reduced expression of Ndufs1 and Ndufs3 in the substantia nigra pars compacta of En1 heterozygous mice. Engrailed also confers in vivo protection against 6-hydroxydopamine and α-synuclein-A30P. Finally, the unilateral infusion of Engrailed into the midbrain increases striatal dopamine content, resulting in contralateral amphetamine-induced turning. Therefore, Engrailed is both a survival factor for adult mDA neurons and a regulator of their physiological activity.

Cell non-autonomous functions of homeoproteins in neuroprotection in the brain.

Homeoproteins transcription factors can transfer between cells and play important roles in development. However, some of these homeoproteins are expressed in the adult, but their function is unknown. The loss of mesencephalic dopaminergic (mDA) neurons is the cause of Parkinson's disease. In mice lacking a functional allele for the Engrailed 1 homeoprotein, mDA neurons progressively die starting about 6 weeks after birth. Infusion of recombinant Engrailed stops the death of these neurons demonstrating that homeoproteins can be neuroprotective. This has been extended to retinal ganglion cell neurons (RGCs), which die in glaucoma and optic neuropathies. The homeoprotein Otx2 promotes the survival of injured adult RGCs both in vitro and in vivo. These examples raise the possibility that homeoproteins may provide neuroprotection to neurons vulnerable in other neurodegenerative diseases.

Peripheral nerve demyelination caused by a mutant Rho GTPase guanine nucleotide exchange factor, frabin/FGD4.

GTPases of the Rho subfamily are widely involved in the myelination of the vertebrate nervous system. Rho GTPase activity is temporally and spatially regulated by a set of specific guanine nucleotide exchange factors (GEFs). Here, we report that disruption of frabin/FGD4, a GEF for the Rho GTPase cell-division cycle 42 (Cdc42), causes peripheral nerve demyelination in patients with autosomal recessive Charcot-Marie-Tooth (CMT) neuropathy. These data, together with the ability of frabin to induce Cdc42-mediated cell-shape changes in transfected Schwann cells, suggest that Rho GTPase signaling is essential for proper myelination of the peripheral nervous system.

The protooncogene Ski controls Schwann cell proliferation and myelination.

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Distinct elements of the peripheral myelin protein 22 (PMP22) promoter regulate expression in Schwann cells and sensory neurons.

Genetic disease mechanisms in the demyelinating peripheral neuropathies Charcot-Marie-Tooth disease type 1A (CMTA) and hereditary neuropathy with liability to pressure palsies (HNPP) as well as transgenic animals with altered PMP22 gene dosage revealed that alterations in PMP22 gene expression have profound effects on the development and maintenance of peripheral nerves. Consequently, the regulation of PMP22 is a crucial aspect in understanding the function of this protein in health and disease. In this study, we dissected and analyzed different cis-acting elements in the 5'-flanking region of the Pmp22 gene in vivo. We found two separate elements that contribute to different aspects of Pmp22 expression. The first is located 5' distally to promoter 1 and is involved in gene regulation during late phases of myelination in development ["late myelination Schwann cell-specific element" (LMSE)] and in remyelination after injury. The second element was identified upstream of promoter 2 and guides Pmp22 expression in sensory neurons. These results suggest that multiple distinct signaling pathways regulating Pmp22 expression in myelination as well as in neurons converge on distinct segments of the PMP22 promoter region. The underlying molecular mechanisms are likely to be crucially involved in the maintenance of the integrity of myelinated peripheral nerves.