Patterned collagen fibers orient branching mammary epithelium through distinct signaling modules

Abstract

For decades, the work of cell and developmental biologists has demonstrated the striking ability of the mesenchyme and the stroma to instruct epithelial form and function in the mammary gland, but the role of extracellular matrix (ECM) molecules in mammary pattern specification has not been elucidated. Here, we show that stromal collagen I (Col-I) fibers in the mammary fat pad are axially oriented prior to branching morphogenesis. Upon puberty, the branching epithelium orients along these fibers, thereby adopting a similar axial bias. To establish a causal relationship from Col-I fiber to epithelial orientation, we embedded mammary organoids within axially oriented Col-I fiber gels and observed dramatic epithelial co-orientation. Whereas a constitutively active form of Rac1, a molecule implicated in cell motility, prevented a directional epithelial response to Col-I fiber orientation, inhibition of the RhoA/Rho-associated kinase (ROCK) pathway did not. However, time-lapse studies revealed that, within randomly oriented Col-I matrices, the epithelium axially aligns fibers at branch sites via RhoA/ROCK-mediated contractions. Our data provide an explanation for how the stromal ECM encodes architectural cues for branch orientation as well as how the branching epithelium interprets and reinforces these cues through distinct signaling processes.

Figure 1. Axial Collagen I (Col-I) Tracks Orient the Branching Mammary Epithelium

(A) Representative mouse mammary gland whole mounts at postnatal week 3 and 8. Morphological features were used to register images for comparison; labeled are the epithelium (EP), nipple (*), lymph node (LN), and stroma. For orientation analysis, the long axis of registered glands was specified as 0° (dashed arrow). (B) Branch orientation analysis of traced epithelia showed a structural transition from an unbiased structure at week 3 to significantly orientated along the long axis by week 8. Values are represented as mean ± SEM; n = 10-11 whole mounts. (C) Oriented Col-I tracks exist in the mammary stroma prior to branching morphogenesis. Confocal microscopy was performed on mammary glands from week 3 mice stained for Col-I at sites distal to the epithelium (schematic, dashed box). A representative z section of the distal stroma reveals persistent Col-I tracks (XZ, white arrow) as well as heterogeneity in Col-I density (High, Medium, and Low), medium- and high-density regions containing Col-I fibers. The orthogonal XZ section consists of about 110 slices at a z step of 0.677 μm/step. (D) Fiber orientation analysis of the above regions revealed a significant orientation bias toward the long axis in high- and medium-intensity regions, whereas the low-intensity region had no significant bias (inset). Values are represented as mean ± SEM; n = 5 whole mounts. (E) Col-I fibers proximal to the epithelium at week 3 extend past—and co-orient to—branches. Representative Col-I staining around an epithelial end bud. To compare Col-I orientation relative to branch, we specified the long axis of the end bud as 0° (dashed arrows). (F) Fiber orientation analysis at branch sites of week 3 mammary glands found that Col-I fibers are co-oriented with branch direction. Values are represented as mean ± SEM; n = 5 end buds. The scale bars in (A) represent 5 mm; the scale bars in (C) and (E) represent 50 μm. See also Figure S1.

Figure 2. Epithelial Cells Follow Col-I Fiber Orientation in a Rac1 Dependent Manner in Culture

(A) Method for axially orienting fibers within three-dimensional Col-I matrices. (B–C) Confocal reflection micrographs of control (B) and oriented (C) Col-I matrices with the axis of orientation indicated (green arrows). (D) Fiber alignment significantly increases upon compression. Values are represented as mean ± SEM; n = 10 matrices. (E and F) Brightfield images of branching mammary explants in random (E) and oriented (F) Col-I matrices. (G) Quantification of branch orientation between explants in control versus oriented Col-I matrices. Values are represented as mean ± SEM; n = 5 or 6 wells. (H and I) Representative branching aggregates expressing vector alone (H) or a constitutively active form of Rac1 (I). (J) Branch orientation was significantly disrupted in Rac1-CA-expressing aggregates in comparison to vector alone. Values are represented as mean ± SEM, n = 5 or 6 wells. (K and L) Representative branching aggregates expressing vector alone (K) or fascin-1 (L). (M) Branch orientation analysis showed a reproducible, although not significant, enhancement in branch orientation. Values are represented as mean ± SEM, n = 5 or 6 wells. The scale bars in (B), (C), (E), (F), (H), (I), (K), and (L) represent 50 μm. See also Figure S2 and Movie S1.

Figure 3. ROCK-Dependent Contractions Are Involved in Generating—but Not Sensing—Col-I Fiber Orientation

(A and B) Time lapse microscopy of mammary explants. Confocal reflection of Col-I proximal to a representative branch site at hour 28 (A, black inset) showed dramatic reorganization by 39 hr (B) (representative explant at 28 [A] and 39 [B] hrs, concurrent with early- and late-stage outgrowth). (C) Fiber orientation analysis at branch sites demonstrated a substantial increase in Col-I co-orientation to branch direction by the late stage. Values are represented as mean ± SEM; n = 3 branches. (D and E) Representative branch sites after the addition of either vehicle (D) or 20 μM Y-27632 (E) with Col-I (green), DAPI (blue), and phalloidin (red) shown. (F) Fiber orientation analysis found a loss of Col-I co-orientation with Y-27632 treatment. Values are represented as mean ± SEM; n = 5 branches. In (G), (H), (J), and (K), Col-I orientation sensing was assessed by embedding aggregates in patterned matrices (green arrows). (G and H) Representative aggregates treated with vehicle alone (G) or 20 μM Y-27632 (H). (I) Branch orientation analysis showed no difference in orientation between vehicle and Y-27632 treatment. Values are represented as mean ± SEM; n = 5 wells. (J and K) Representative branching aggregates expressing vector alone (J) and RhoA-DN (K). (L) Orientation analysis found no significant difference in branch orientation between vector and RhoA-DN expression. Values are represented as mean ± SEM; n = 5 wells. The scale bars in (A), (D), (E), (G), (H), (J), and (K) represent 50 μm. See also Figure S3 and Movie S2.