Conserved subcortical and divergent cortical expression of proteins encoded by orthologs of the autism risk gene MET

Abstract

Met receptor tyrosine kinase signaling regulates the growth and development of axons and may contribute to the wiring of cortical and limbic circuits in the rodent forebrain. Whether the orthologous MET receptor functions similarly in the developing primate forebrain is not known but is of considerable interest considering the association of variant MET alleles with social and communication phenotypes in autism. To begin addressing this question, we compared Met/MET protein expression in the developing mouse and rhesus macaque forebrain. There was a strong temporal conservation of expression during the time of rapid axon development and the onset of robust synapse formation. Expression patterns of Met/MET in limbic-related structures were almost identical between species. In marked contrast, there was highly divergent expression in the neocortex. In mouse, Met was broadly distributed throughout neocortex. In the macaque, robust MET expression was largely restricted to the posterior cingulate, inferior temporal, posterior parietal, and visual cortices, including face processing regions. The pattern is consistent with the importance of vision in the social repertoire of the primate. Collectively, these data suggest a conserved developmental function of the MET receptor in wiring together limbic and neocortical circuits that facilitate species-appropriate responses, including social behavior.

Figure 1.

Conserved temporal patterns of Met/MET expression in the neocortex. Differential interference contrast photomicrographs of coronal brain sections illustrate Met/MET immunohistochemistry in mouse barrel cortex and macaque inferotemporal cortex. Labeling is predominantly seen in the outgrowing axons of cortical projection neurons in the cortex of the P0 mouse (A) and GD100 macaque (E). During axon collateralization and the onset of synaptogenesis, Met/MET labeling is readily observed in neuropil compartments in both species (B, F). Nissl staining in matched cortical regions (C, G) reveals that expression is especially heavy in the marginal zone (mz) and relatively sparse in layer IV at this developmental stage. By 3 weeks of age (D, H), early periods of axon wiring have past in both mice and macaques, corresponding with drastically decreased immunohistochemical detection of Met/MET. Scale bar = 138 μM for all images.

Figure 2.

Conserved temporal patterns of Met/MET expression in the anterior commissure. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry in mouse and macaque coronal brain sections during development. In both the mouse (A) and macaque (D), intense Met/MET staining of corticofugal axons within the anterior commissure (ac) is observed at time-points just after the end of cortical neurogenesis. Axon staining within this structure gradually decreases in intensity throughout perinatal/early postnatal development in both species (mouse B and C; macaque E and F). The body of the ac, located just inferior to a commissural division of the ST (arrows), is depicted in mouse panels (A–C), whereas the temporal limb of the ac is depicted in macaque panels D–F. The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized dorsal views of the brain. f, fornix. Scale bar = 275 μM for (A–C); 1.1 mm for (D–F).

Figure 3.

Conserved temporal patterns of Met/MET expression in the corticothalamic projection. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry in coronal brain sections during development. Axonal Met staining is evident in the internal capsule (ic, A) but not corticothalamic terminal fields (B, boxed region in A) in the dorsal thalamus in the P0 mouse. A similar pattern of expression is observed in low- (E) and high-magnification (F) images of the pulvinar in the GD100 macaque. Corresponding images in the P7 mouse (C and D) and GD150 macaque (G and H) show dramatically increased Met labeling of the thalamic neuropil, concurrent with robust periods of corticothalamic terminal arborization in each species. The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized dorsal views of the brain. 3V, third ventricle; CA1, cornu ammonis 1 of hippocampus; cc, corpus callosum; fi, fimbria of hippocampus; LGP, lateral globus pallidus; S1BF, barrel field of primary somatosensory cortex. Scale bar = 550 μM for (A) and (C); 825 μM for (E) and (G); 138 μM for (B), (D), (F), and (H).

Figure 4.

Conserved Met/Met expression in amygdaloid afferents. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry at various anteroposterior levels of the amygdala in coronal brain sections from the P7 mouse and GD150 macaque. Though Met expression is widespread in the mouse amygdala during axon collateralization, the nucleus of the lateral olfactory tract (NLOT, A) and the posterior cortical nucleus (PCo, C) exhibit exceptionally heavy Met labeling. More moderate labeling in the basolateral complex is greatest in the lateral (L) nucleus, and of decreased intensity in the basal (B), and especially accessory basal (AB) nuclei (B). In the macaque, MET staining is also enriched in the L and B nuclei (D and E) as compared with the (AB) and amygdalohippocampal (AHA) nuclei (E and F), indicating that Met/MET is differentially expressed by select amygdaloid afferents during development. The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized, dorsal views of the brain. AAA, anterior amygdaloid area; Ce, central amygdaloid nucleus; ec, external capsule; S, subiculum; ST. Scale bar = 275 μM for (A–C); 770 μM for (D–F).

Figure 5.

Conserved Met/Met expression in hippocampal afferents. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry at various anteroposterior levels of the hippocampus in coronal brain sections from the P7 mouse and GD150 macaque. In both the developing mouse (A–C) and macaque (D–F), Met/MET staining is observed in entorhinal cortical projections of the perforant pathway (pp) within the molecular layer. The perforated boundary in mouse panels encompasses the molecular layer of both the dentate gyrus (DG) and cornu ammonis (CA) subfields but bounds only that of the DG in macaque panels. Additional more intense MET labeling is focused in the region overlaying the stratum radiatum (rad) at the subiculum/CA1 boundary (asterisks in D, E, and F). Staining patterns in both species are most salient at posterior (mouse B and C; macaque E and F) as opposed to anterior (mouse A; macaque D) levels. The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized dorsal views of the brain. alv, alveus; CA1, cornu ammonis 1 of hippocampus; CA3, cornu ammonis 3 of hippocampus; fi, fimbria; or, stratum oriens; PrS, presubiculum; pyr, pyramidal cell layer; S, subiculum. Scale bar = 275 μM for (A–C); 770 μM for (D–F).

Figure 6.

Conserved Met/MET expression in efferents of the hippocampal formation. Photomicrographs illustrate Met/MET immunohistochemistry in fiber tracts and axon terminal fields in coronal forebrain sections from the P7 mouse and GD150 macaque. (A and B): Examples of Met staining in the precommissural fornix (black asterisk) and mammillary bodies (white asterisk) in the developing mouse forebrain. Corresponding MET-stained structures are observed in the macaque during a similar developmental period as shown at low- (C and D) as well as high-magnification (E, boxed region in C, black asterisk; F, boxed region in D, white asterisk). ac, anterior commissure; 3V, third ventricle; LV, lateral ventricle; ST. Scale bar = 550 μM for (A), (B) and (E), (F); 9.48 mm for (C) and (D).

Figure 7.

Conserved temporal patterns of Met/MET expression in the fornix. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry in mouse and macaque coronal brain sections during development. Met/MET staining in efferent fibers of the hippocampus decreases developmentally in the mouse (A–C) and macaque (D–F). Axons of the postcommissural fornix (f) are shown in cross-section in mouse panels (A–C). The macaque f, inferior to the corpus callosum (cc), is depicted in (D–F). Examples of select intensely stained axon bundles are indicated by arrows (D and E). The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized dorsal views of the brain. 3V, third ventricle. Scale bar = 275 μM for (A–C); 1.1 mm for (D–F).

Figure 8.

Divergent spatial patterns of neocortical Met/MET expression in the developing mouse and macaque forebrain. Differential interference contrast photomicrographs illustrate the anterior (A) to posterior (D) progression of Met/MET immunohistochemistry in coronal forebrain sections from the P0 mouse and GD100 macaque. Notably, all major fiber tracts that carry corticofugal projections as well as the subplate exhibit intense Met staining in the mouse forebrain (inset images, A–D). Robust MET expression in the macaque is largely confined to the subplate underlying cortices inferior to the superior temporal sulcus (sts) and in select corticofugal fiber tracts of the incipient temporal lobe including, most notably, the anterior commissure (ac, B) as well as the external (ec) and extreme (ex) capusules anteriorly (A). Additional staining in the cingulum (cg, A–D) likely reflects MET expression in the efferent fibers of the posterior cingulate cortex, whereas labeled axons of the corpus callosum (cc, A–D) may originate in the posterior cingulate and/or cortices inferior to the intraparietal sulcus (ips). The posteroanterior (P←→A) level of mouse (top) and macaque (bottom) sections is indicated in schematized dorsal views of the brain to the left of each figure panel. 24, cortical area 24; 25, cortical area 25; Aq, cerebral aquaduct; Cd, caudate; dhc, dorsal hippocampal commissure; lf, lateral fissure; LV, lateral ventricle; Pu, putamen; Pul, pulvinar. Scale bar = 3.15 mm for all macaque images; 2.3 mm for inset mouse images.

Figure 9.

Divergent spatial patterns of neocortical Met/MET expression in the developing mouse and macaque forebrain. Differential interference contrast photomicrographs illustrate the anterior (A) to posterior (D) progression of Met/MET immunohistochemistry in coronal forebrain sections from the P7 mouse and GD150 macaque. While Met expression in the mouse (inset images, A–D) is broadly distributed throughout the tangential domain of the neocortex, MET expression in the macaque is largely restricted to the temporal cortices (white and black asterisks, B–D) and midline cortices including the anterior cingulate cortex (cortical area 24, A–C; area 23, D) and subgenual cortex (area 25, A). MET is also differentially expressed within the macaque temporal lobe; staining is strong inferior to (white asterisks, B–D), and of modest intensity superior to (black asterisks, B–D), the superior temporal sulcus (sts). Axon staining within the cingulum (cg, A–D), anterior commissure (ac, B), and posterior regions of the corpus callosum (cc, C and D) reflect the restricted populations of neocorticocortical projection neurons that express MET. The posteroanterior (P←→A) level of mouse (top) and macaque (bottom) sections is indicated in schematized dorsal views of the brain to the left of each figure panel. Aq, cerebral aquaduct; Cd, caudate; cs, central sulcus; dhc, dorsal hippocampal commissure; ec, external capsule; ex, extreme capsule; ips, intraparietal sulcus; lf, lateral fissure; LV, lateral ventricle; ox, optic chiasm; Pu, putamen; Pul, pulvinar. Scale bar = 4.6 mm for all macaque images; 2.8 mm for inset mouse images.

Figure 10.

Met/MET expression in neocortical efferents in the developing mouse and macaque forebrain. Differential interference contrast photomicrographs illustrate Met/MET immunohistochemistry in forebrain sections from the P7 mouse and GD150 macaque. Widespread Met labeling is observed in the neuropil of the caudatoputamen (CPu) (A, B, and C) and lateral thalamus (D, E, and F) in coronal (A and D), sagittal (B and E), and horizontal (C and F) brain sections of the developing mouse forebrain, consistent with widespread Met expression in long-projecting axons of the neocortex. The distribution of MET-labeled neocortical efferents in the developing macaque striatum is much more restricted as robust staining is observed only in the olfactory tubercle (OT) (G) and the ventral putamen (Pu) and caudate (Cd) (H). Areas of lighter striatal MET staining include the nucleus accumbens (NAc) (G) and restricted regions within the dorsal Cd (G and I). MET-labeled neocortical efferents to the macaque thalamus are predominantly restricted to the reticular nucleus (Rt) (J), lateral (LPul) and inferior (IPul) pulvinar nuclei (J), and limbic thalamic nuclei including the anteroventral nucleus (AV) (I). The boxed region in schematized macaque brain sections corresponds to the photomicrograph directly above. The posteroanterior (P←→A) position of macaque sections is indicated in schematized dorsal views of the brain. 3V, third ventricle; AD, anterodorsal thalamic nucleus; cp, cerebral peduncle; DLG, dorsolateral geniculate nucleus; ec, external capsule; eml, external medullary lamina; f, fornix; fi, fimbria of hippocampus; fr, fasciculus retroflexus; ic, internal capsule; LD, laterodorsal thalamic nucleus; LGP, lateral globus palidus; LV; lateral ventricle; LSI, intermediate lateral septal nucleus; MGD, medial geniculate nucleus, dorsal part; MPul, medial pulvinar nucleus; mt, mammillothalamic tract; Po, posterior thalamic nuclear group; PV, paraventricular thalamic nucleus; SNR, substantia nigra pars reticulata; ST; VAL, ventral anterior thalamic nucleus, lateral part; VLM, ventrolateral thalamic nucleus, medial part; VPM, ventral posteromedial thalamic nucleus. Scale bar = 550 μM for (A–F); 3.39 mm for (G–J).