TENM3

The extracellular C terminal domain is composed of a linker region, EGF-like repeats and then a globular domain. The linker region is made up of ~200 amino acid residues and is found immediately distal to the transmembrane domain. This is followed by eight phylogenetically conserved tenascin C-type EGF-like repeats, which features the uniquely conserved replacement of a single cysteine in repeats 2 and 5 in place of the original tyrosine and phenylalanine residues respectively. Since cysteines are susceptible to forming disulfide bonds, the single cysteines at the EGF-like repeats of a teneurin molecule can facilitate the homophilic and heterophilic dimerisation of teneurin family molecules.[14] More distally is the globular domain consisting of a 700-800 amino acid residue region. There are 17 conserved cysteine residues, a region of NHL repeats, a region of 26 YD residue repeats, and then a teneurin C-associated peptide (TCAP). The YD repeats are rich in N-linked glycosylation and were previously only reported in the rhs element of bacteria.[19][20] The TCAP is the resulting peptide from cleaving a putative furin cleavage site found immediately on the N-terminal of TCAP. The furin cleavage site is rich in tyrosine residues and consists of 4 conserved cysteine residues. The 4 cysteine residues assist in protein folding, however, they are absent in Ten-m2 and Ten-m3. There are 41 amino acids in TCAPs, except for TCAP-3 from Ten-m3, which has 40.[21] TCAPs show structural homology to the CRF family molecule and appears to influence neurite outgrowth and some behaviours relating to stress and anxiety.[9][10]

The N terminal intracellular domain (ICD) consists of two proline-rich regions in the half closest to the transmembrane domain, two EF-hand-like motifs near the centre, and a number of conserved tyrosine phosphorylation sites. The proline-rich stretches are typical binding sites for SH3 proteins, which can regulate intracellular teneurin signalling pathway.[22]

At day 7.5 in mouse embryonic development (E7.5), in situ hybridisation shows Ten-m3 mRNA expression at the neural plate, particularly in the neural folds. At E8.5, Ten-m3 is expressed at the caudal forebrain, the midbrain region and structures outside of the CNS, including the pharyngeal arches and the otic vesicles. At E9.5 and 10.5, Ten-m3 expression extends from the telencephalon to the midbrain and also at the pharyngeal arches, otic vesicles, anterior somites and the limb buds. Between these stages, Ten-m3 and Ten-m4 are expressed in complementary patterns in the brain, suggesting a complementary function during development. At E12.5, Ten-m3 is higher in the midbrain compared to the caudal diencephalon and the spinal cord. It is also co-expressed with Ten-m4 in the first, second and third pharyngeal arches. At E15.5, Ten-m3 is expressed in the forebrain and facial mesenchyme, but absent from the mid- and hindbrain. It is also expressed in the developing whisker pads in mouse.[12][25]

In a 6-week-old adult mouse, Ten-m3 is co-expressed with the other three Ten-m mRNAs at the granular layer of the dentate gyrus and the stratum pyramidale of the hippocampus. It is expressed relatively weakly in the granular layer and in the stratum lacunosum moleculare, but is strongly expressed in the CA2 subfield and weakly in the CA1 subfield of the hippocampus. However, immunostaining of Ten-m3 shows weak protein expression throughout the hippocampus except for the stratum lacunosum moleculare. Ten-m3 mRNA is prominently co-expressed with Ten-m2 and Ten-m4 in the Purkinje's cell zone of the cerebellum. Ten-m3 protein is expressed in the Purkinje's cell zone, molecular and granular layers and the white matter of the cerebellum. All Ten-m mRNAs are expressed prominently between layers II and VI of the cerebrum.[25]

The Ten-m3 gene, along with Ten-m2 and Ten-m4, is expressed throughout the neocortex in a low rostral to high caudal and a high dorsal-medial to low ventral-lateral gradient from E15.5 to P2.[26] In E17 mouse, Ten-m3 mRNA is expressed in the parafascicular thalamic nucleus, a subregion of the thalamus, and in the striatum in a high dorsal-caudal to low ventral-rostral gradient. Patches of this expression can still be observed in first week postnatal mice.[27][28] Similarly, there is a graded expression of Ten-m3 in the visual pathway, especially during embryonic and early postnatal development. Expression is highest in the dorsal lateral geniculate nucleus (dLGN) and superior colliculus in the region that corresponds topographically to ventral retina.[29][18]

Ten-m3 plays an important role during early development in directing the topographic neural projection and formation of the thalamostriatal circuitry, thus critical for motor skill acquisition. Ten-m3 molecule is the first to be reported to regulate connectivity in the thalamostriatal pathway. Ten-m3 guides some of the axon projections from dorsal regions of the parafascicular nucleus (PF) of the thalamus to dorsal regions of the striatum. This creates a high dorsal to low ventral gradient topography mapping between the two structures. In Ten-m3 null mutant mice, these projections are diffuse and project ectopically to more ventral and medial regions in the striatum. Furthermore, the null mutant mice display delayed motor skill acquisition in the accelerating rotorod task.[28]

In in vivo vertebrate studies, Ten-m3 acts as an eye-specific guidance molecule during early development. Functional binocular vision requires the correct projection of ipsilateral axons from the retina to the dorsal lateral geniculate nucleus (dLGN) and primary visual cortex (V1) and to the superior colliculus (SC). Ten-m3 facilitates the retinotopic mapping of ipsilateral axons from the ventrotemporal retinal ganglion cells, which encode visual input from the binocular visual field, to the dorsomedial dLGN and to the rostromedial SC. Immunostaining reveals a cluster of high Ten-m3 protein expression in the areas involved in this ipsilateral mapping. In Ten-m3 null mutant mice, these projections are reduced and ectopic projections are expanded ventrolaterally along the dLGN and caudomedially in the SC from both eyes. The aberrant misalignment of ipsilateral axons from both eyes result in binocular vision deficits. Ten-m3 null mutant mice performed worse than wild type (WT) in behavioural tests of binocular visual function, such as vertical placement and visual cliff test. However, inactivation of inputs from one eye (i.e. inactivate binocular vision) restored visual behaviour to a level similar to WT mice under binocular condition.[29][18]

The peptide cleaved from the C terminal of Ten-m3, TCAP-3, stimulates the production of cAMP and the proliferation of neurons. It can increase the expression of its gene at high concentrations but attenuate the expression at low concentrations.[21] TCAP-1 from Ten-m1, another member of the Ten-m family, modulates stress and anxiety behaviours. TCAP-1 increases the acoustic startle response in a low-anxiety rat but decreases the response in a high anxiety rat when injected into the basolateral amygdala. It also inhibits the sensitisation of the response when injected into the lateral ventricles.[30]