Deneen Wellik, Ph.D. - US grants

[unreadable] DESCRIPTION (provided by applicant): The Hox complex of genes affects both development and disease, but the pathways by which Hox transcription factors regulate downstream target genes remain largely undefined. Targeted gene disruption studies have confirmed that these genes affect anteroposterior patterning processes, but phenotypes of single mutant animals are generally mild, pleiotropic, demonstrate incomplete penetrance and have variable expressivities. Because of the nature of these defects, very little is known regarding downstream targets of Hox genes or the molecular pathways in which Hox genes operate. By removing all six functional copies of the Hoxll paralogous genes, we generated animals that have no kidneys. The metanephric blastema forms early in development, but no ureteric bud induction occurs. This phenotype occurs with 100% penetrance. By examining this phenotype molecularly, we have been able to identify factors downstream of the Hoxll paralogous genes. Six2 a member of the conserved Pax-Eya-Six pathway, and Gdnf, the inducing ligand responsible for signaling to the Wolffian duct to initiate ureteric bud induction, are not expressed in the Hoxll paralogous mutants. Our preliminary data indicate that Six2 is a direct downstream target of the Hoxll paralogs, and that Hoxll paralogs directly interact with Eyal and Pax2 to regulate Six2 expression. As each of these genes has also been implicated in the regulation of Gdnf, the inducing ligand, defining their interactions is critical to understanding the molecular basis of kidney development. Finally, complimentary new findings indicate that HoxlO paralogous genes also play a critical role in nephrogenic development by as yet undefined mechanisms. In our view, this system is ideal for identifying downstream targets of Hox genes and factors with which they regulate expression. The paralogous nature of the Hox genes make it very likely that these genes act as modifiers of disease proccesses in conjunction with other factors in their regulatory network. Mutations in Pax, Eya and Six genes have been demonstrated in human cases of Branchio-Oto-Renal Syndrome as well as Renal- Coloboma Syndrome. It is thus very likely that Hox genes influence the severity of these syndromes in humans. Using the kidney as a model organ system, we will examine the interaction between these highly conserved groups of developmental regulators and define the previously undescribed molecular relationships between Hox genes and the Pax-Eya-Six/Gdnf regulatory pathway in nephrogenesis. We hypothesize that a conserved interaction of Hox genes with the Pax-Eya-Six regulatory network specifies ureteric bud induction and patterns the developing kidney. Relevance: These studies will provide key insights into the molecular and mechanistic basis of Hox regulation in kidney development and disease. [unreadable] [unreadable] [unreadable]

[unreadable] DESCRIPTION (provided by applicant): Targeted gene disruption studies have confirmed that Hox genes affect prostate development, but phenotypes of single mutant animals have been generally mild, pleiotropic, and incompletely penetrant. Because of the nature of these defects, it has been difficult to determine the genetic function of Hox genes in the developing prostate. By removing all six functional copies of the Hox11 paralogous genes, we have generated animals with profound defects in the developing prostate. Preliminary data suggests that triple mutant animals display a very disrupted bud pattern and, while the anterior and dorsolateral lobes of the prostate bud from E18.5 urogenital sinuses, they do not branch or develop past this stage. Surviving four-allele animals show incomplete penetrance of these defects as adults. Together with previous data on single mutants in the Hox10 and Hox13 paralogous groups, it appears that the prostate develops in response to anteroposterior patterning signals from the AbdB Hox genes, with Hox10 genes patterning the anterior prostate, Hox11 genes patterning primarily the dorsolateral prostate and Hox13 genes patterning primarily the ventral prostate. The nature of these defects have important implications for understanding the development of this organ system, as well as beginning to understand potential roles for Hox genes have in disease. Results in the laboratory suggest that Hox11 proteins, together with Pax2 and Eya1, form a regulatory complex that, together can activate downstream genes, such as Six2 and Gdnf in the kidney. Both of these genes are also expressed in the developing prostate, and our preliminary data shows Gdnf functions in the developing prostate, supporting the conservation of this pathway in prostate development. We have also engineered new constructs in the Hox11 paralogous genes that produce null alleles, but also express fluorescently fusion proteins from the endogenous loci. Additionally, as triple mutant and high-allele mutants in the Hox11 colony die at newborn stages due to insufficient kidney development, we have worked out culture conditions that allow us to continue prostatic growth through early ductal morphogenesis. Using our newly generated fluorecent alleles in culture experiments will allow real-time imaging of Hox expression during early prostate organogenesis. We hypothesize that Hox11 paralogous genes contribute to patterning the dorsolateral prostate and their mesenchymal expression is necessary for branching morphogenesis and growth of this region of the developing prostate. These studies will provide key insights into the genetic and molecular basis of Hox regulation in prostate development. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Hox genes encode homeodomain-containing transcription factors that are expressed and function along the body axis in an AP-restricted manner corresponding to their position within the cluster. This pattern of global Hox expression is termed colinearity and this feature of Hox expression has been retained among vertebrate embryos. In addition to the colinearity along the main body axis, a nested, colinear expression pattern is also observed for the HoxA and HoxD paralogous group 9 through 13 genes in the early developing vertebrate limb bud. Mutations in these genes cause specific morphological perturbations along the proximodistal (PD) axis during limb development. The preponderance of evidence supports a model in which only the AbdB-related Hox genes (Hox9-13) from the HoxA and HoxD complex are required in the developing forelimb. No loss-of-function mutants in HoxB or HoxC group genes have ever been demonstrated to have forelimb defects. Similarly, no limb phenotypes have been reported with loss- of-function of non-AbdB-related genes (Hox1-8) from any of the four HoxA-D complex genes. Recent work has additionally shown that Hox10 through Hox13 genes from the HoxA and D complex, together, control early limb bud outgrowth and the initiation of Shh expression, suggesting that the same Hox compliment of genes that control later limb proximodistal patterning events, together, control early outgrowth and anterior-posterior patterning events. It has been clearly shown that colinear expression in the limb relies on regulatory elements both within and outside the Hox cluster, but the factors that control the initiation of the Shh/Hoxa10-13;Hoxd10-13 regulatory loop are unknown. We have recently generated paralogous mutant mice in the Hox5 (Hoxa5 -/-;Hoxb5 -/-;Hoxc5 - /-), and Hox9 (Hoxa9 -/-;Hoxb9 -/-;Hoxc9 -/-;Hoxd9 -/-) group genes. Loss of Hox5 function (non- AbdB-related Hox genes) results in anterior patterning defects of the forelimb skeleton that closely resemble Holt-Oram Syndrome patients, and preliminary evidence suggests Hox5 genes are involved in the Tbx5 regulatory pathway in forelimb development. Also unexpectedly, loss of all four Hox9 paralogous genes results in profound PD outgrowth and AP patterning defects that affect the entire forelimb skeleton. Preliminary results demonstrate that Shh and AbdB-related HoxA/D 10-13 genes are not initiated in Hox9 limb buds, suggesting Hox9 is a node, critical for establishment of the Hox10- 13/Shh loop in forelimb development. The emerging view from our preliminary studies supports a new model wherein expression of Hox genes from all four complexes participates in limb patterning events, a significant departure from the currently accepted paradigm. We hypothesize that Hox5 genes direct early forelimb AP patterning events by regulating the Tbx5 pathway, and that Hox9 genes are an early signaling node, critical for the establishment of the Shh/Hox10-13 regulatory loop. PUBLIC HEALTH RELEVANCE: Our new genetic results uncover two unexpected and important roles for Hox genes in early limb patterning: Hox5 genes are important for the establishment of anterior limb pattern, and preliminary results suggest they are regulators in the pathway disrupted in human Holt-Oram Syndrome. Further, Hox9 genes appear to be a node for the establishing the Shh-Abdb Hox loop;loss of these genes results in severe truncation of limb development. Analyses of these unexpected phenotypes and functions are critical for developing an understanding of early limb developmental processes and how mis-regulation results in human disease.

DESCRIPTION (provided by applicant): A significant amount of data exists regarding the formation and differentiation of muscle, bone and tendon tissue individually, but little is understood regarding molecular mechanisms that allow these tissues to make appropriate connections with one another. This knowledge is critical for establishing useful therapies to repair these tissues after disruption from injury or disease. Hox genes perform fundamental roles in patterning the skeleton, but new preliminary data shows that Hox gene expression in the developing limb is not restricted to skeletal tissue, and these genes also appear to play less anticipated roles in patterning tendon and muscle tissue as well as in the integration of all three tissue types. Recent data additionally that shows that these genes are highly up-regulated following injury, suggesting a role in the healing and remodeling that occurs in response to injury. The long-term goal of this research is to understand how Hox genes regulate the region- specific formation and integration of the musculoskeletal system and how this information can be used to inform regenerative therapies following injury or disease. The objective of this proposal is to achieve an understanding of the cellular mechanisms of Hox function in the development and integration of muscle, tendon and bone in the developing forelimb and how these mechanisms are redeployed in response to injury. The central hypothesis is that Hox11 genes function to direct the patterning and the integration of the muscle, tendon and cartilage in the developing limb and that these genes are up-regulated in response to injury to allow proper repair and remodeling in vivo. How Hox11 genes contribute to the formation and patterning of muscle groups, tendons and cartilage elements within the developing limb will be assessed using existing null alleles and fluorescent reporter lines. The spatiotemporal control of Hox-mediated patterning and the establishment of connectivity in the musculoskeletal system will be examined by conditionally ablating Hox function using spatial and temporally restricted Cre deletor lines. Finally, the roles for Hox genes in tissue repair and skeletal remodeling will be determined by conditionally ablating Hox function in adulthood (after proper musculoskeletal patterning has been achieved) and examining the effects of loss of Hox function on fracture repair and subsequent skeletal remodeling. The contribution of the proposed research is significant because critical new knowledge regarding the regional regulation musculoskeletal integration will be gained, the tissue(s) from which this information is transmitted will be defined, and how these factors are redeployed in adulthood to repair injuries will be determined. The research proposed in this application is innovative in identifying factors and cell types involved in the integration of individual components of the musculoskeletal system as well as in the novel tools generated to allow an understanding of the function of Hox genes in properly patterned adult tissue during injury repair processes. Together, the information gained from these studies will provide a paradigm-shifting understanding of Hox function in musculoskeletal biology.

DESCRIPTION (provided by applicant): Approximately 350,000 anterior cruciate ligament (ACL) reconstruction surgeries are performed each year, and billions of dollars are spent on the acute care costs involved in this procedure. The most common ACL replacement strategies involve grafts (allografts from cadavers or autografts of the patients' own patellar or hamstring tendons), although newer, more experimental treatments use engineered tissues that typically involve seeding cells on a polymeric scaffold. In these procedures, the graft or engineered tissue never fully integrates with the bone tunnel created during surgery and the joint never recovers pre-surgical level biomechanics. Not surprisingly, more than half of the patients who have undergone this procedure develop early-onset osteoarthritis (OA). With acute ACL injuries becoming increasingly prevalent in children, the number of young adults developing OA is increasing each year. The Larkin and Arruda laboratories have spent the last several years engineering an ACL replacement construct generated from bone marrow-derived stromal cells (BMSCs) that exhibits the structural and functional interface characteristics of native ACL when transplanted in vivo. The construct is composed of an engineered ligament with engineered bone at each end; it fully integrates into the recipient bone and differentiates in vivo to form a mechanically appropriate and biologically matched interface between the two tissues. Work from these laboratories demonstrates many advantages to this strategy and evidence in a large animal model supports its superiority for replacement therapy. One critical concern regarding the use of this novel and innovative therapy in humans, however, is the safety of use of stem/precursor cell-derived tissue in patients. It is imperative to demonstrate that the BMSCs used for the generation of these constructs pose no long-term threat after transplantation. The main concern in the field is the ability to ascertain that no undifferentiated cells persist in the transplanted construct that might later lead to aberrant cellular behavior such as cancer. In collaboration with Dr. Wellik's laboratory, a recent examination of the fate of the BMSC-derived BLB construct led to the surprising discovery that within several months after implantation of the BLB, donor construct cells are replaced entirely by recipient cells and donor-derived cells are no longer present. Thus, by transplanting a developmentally immature, exogenous live tissue template for replacement, adult recipient cells are induced to fully regenerate a viable and mechanically appropriate replacement ligament! This proposal seeks to confirm these preliminary results and initiate exploration into the mechanisms of this remarkable regenerative process, with the longer-term goal of translating this innovative therapy into humans and vastly improving the outcomes of acute ACL and potentially other joint and connective tissue injuries. PUBLIC HEALTH RELEVANCE: Billions of dollars are spent each year on anterior cruciate ligament (ACL) reconstructions after acute knee injury and more than half of patients who undergo this procedure develop early- onset osteoarthritis. New technology developed by this research team uses bone marrow stromal cell-derived, living bone-ligament-bone templates to replace the injured ACL. Transplanted tissue does not survive long-term in the recipient (patient), but induces the regeneration of a new ligament made entirely of the patient's own cells! The regenerating ACL integrates into bone and forms a viable and mechanically appropriate replacement ligament. If these preliminary results are confirmed, we will be one large step closer to translating these results into a vastly improved human therapy. Further, understanding the mechanisms by which this regeneration occurs will open new avenues in the regeneration of musculoskeletal tissue.

DESCRIPTION (provided by applicant): The rapid evolution of biomedical science in increasingly translational directions requires that we identify strategies to help the next generation of scientists navigate a complex research landscape in which fluent communication and collaboration between scientific disciplines is essential for success. The Center for Organogenesis at Michigan was formed in 1995 to unite basic, applied and clinical scientists with a common goal: To understand the basic mechanisms by which organs and tissues are formed and maintained, and to use this knowledge to create long-lasting artificial organs, improved stem cell therapies and effective organ transplantation systems that will correct acquired and genetic human diseases. The Training Program in Organogenesis, initiated 14 years ago, is an integral part of the educational mission of the Center. Its goals are: a) To provide intellectual and technical training in the field of organogenesis; and b) To promote interdisciplinary thinking by exposing trainees to research that crosses boundaries between the clinical, basic and applied sciences. These goals are accomplished by encouraging a two-mentor structure for research training and requiring trainees to participate in several specific training activities: a formal course in Organogenesis, Organogenesis Seminar series, Monthly Trainee Meeting, International Symposium. Trainees also actively shape the Training program, proposing new initiatives such as Bioartography, a novel combination of art, science and public education and Crosstalk, a clinical case-based forum run by one clinical and one basic scientist. Additionally, all trainees have the option to be paired with a clinical co-mentor who will facilitae their exposure clinical management of patients and their diseases. This competitive renewal requests support for 5 predoctoral and 3 postdoctoral training slots (one specifically targeted to an M.D. fellow). Trainees come primarily from the laboratories of the 36 listed mentors of the Training Program, all of whom are highly recognized scientists. Formal competitive applications, reviewed by a selection committee, are required for appointment to the Training Program. The Program is monitored by several internal mechanisms and also by two External Advisors (Dr. Blanche Capel, Duke University; Dr. Christopher Wylie, Cincinnati Children's Hospital Medical Center) to ensure its continued responsiveness to demands of a continuously evolving research environment.

ABSTRACT A fundamental question in musculoskeletal development, repair and regeneration is how mesenchymal stem/stromal cells regulate development and repair processes. Our recently published data shows that, in the skeleton, Hox genes are exclusively expressed in progenitor-enriched, bone marrow mesenchymal stem/stromal cells (BM-MSCs) and Hox11 gene expression is confined to the zeugopod limb region (radius/ulna; tibia/fibula). Loss of Hox function leads to defects in bone repair in adults as well as previously identified defects in the embryonic skeleton (Rux, et al, in press, Developmental Cell). We have also previously shown that Hox11 genes are expressed in muscle connective tissue stromal cells with the same regional restriction, and function at developmental stages to pattern the muscles of the limb. New preliminary data shows that muscle stromal expression continues through adult stages and expands in response to muscle injury, consistent with continued function in adult muscle tissue. We have generated a Hoxd11 conditional allele that will allow us to selectively remove Hox11 function at postnatal or adult stages and examine continued function at adult stages. In both the skeletal and muscle tissue, Hox genes are expressed only in the stromal progenitor cells (BM-MSCs in the skeleton) that are known to be important for proper developmental patterning (as well as repair in the skeleton) and both cell populations show regional specificity. Our Hoxa11eGFP knock-in reporter allows us to probe the biological similarities and differences between these two stromal populations as well as between controls and mutants at developmental stages and during injury repair/regeneration. The goal of this study is to investigate the continued role for Hox genes in muscle stromal cells and the nature of the signaling pathways regulated in muscle stromal cells during development and repair. A successful outcome from these analyses will demonstrate a novel function for Hox genes in adult muscle repair and will provide critical new information regarding the nature of connective tissue stromal cell regulation of muscle development and repair processes and how these stromal progenitors compare to more well-characterized skeletal MSCs. The data generated through this study will provide the basis for continued funding on this important aspect of muscle and MSC biology.

Mesenchymal multi-potent stromal/stem cells (MSCs) have historically been characterized as stromal progenitors that can differentiate into a variety of cell types including bone, cartilage, muscle and fat. In recent years, elegant genetic and molecular studies have allowed enrichment of MSCs with higher progenitor potential. In mice, among the cell surface markers used for enrichment, co-expression of PDGFR?/ CD51 results in high enrichment of stem/progenitor activity. Likewise, LepR-Cre marks a long-term, self-renewing cell population with the highest progenitor potential compared to other Cres. During our studies of Hox11 function in the zeugopod skeleton (radius and ulna), we have made the surprising discovery that Hox11 protein expression is excluded from all differentiated cell types of the zeugopod, but is found uniquely in cells that co-express PDGFR?/CD51 and with cells that retain the LepR-Cre lineage trace. While not all PDGFR?+/CD51+ or LepRCre+ cells are Hox11+, all Hox11+ cells are PDGFR?/+CD51+ and LepRCre+. When comparing the in vitro CFU-F progenitor potential of PDGFR?/+CD51+/Hoxa11eGFP+ cells to the population double-positive for PDGFR?/+CD51+, we find that the PDGFR?/+CD51+/Hoxa11eGFP+ cells have three times higher progenitor potential than the double-positive total population, consistent with enriching for higher progenitor potential. Further, triple- positive cells can differentiate into cartilage, bone and adipocytes in vitro, further supporting their MSC potential. Unlike the commonly used markers for MSCs, Hox11 genes also function in these cells. Loss of Hox11 function eliminates the chondrogenic and osteogenic potential in this regionally restricted set of MSCs in vitro. Postnatal defects arising in Hox11 compound mutants as well as impaired fracture repair responses provide in vivo support for a continued role for Hox11 in MCSs throughout the life of the animal. Importantly, defects are only observed in the zeugopod; other skeletal compartments of Hox11 mutants exhibit normal MSC tri-lineage differentiation in vitro and are able to heal normally after fracture. In this application, we will use our Hox null mutants and Hoxa11eGFP reporter, in addition to a newly generated a Hoxa11-CreERT2 and Hoxd11 conditional allele that will allow us to prove in vivo lineage analyses, self-renewal potential and origin of this population, and dissection of continued function of this MSC population through skeletal development, postnatal growth, maintenance and repair. We will also explore preliminary data that shows that ALL endochondral bones maintain differential regional Hox expression in the bone marrow MSCs and investigate global function for Hox genes in regionally restricted MSC populations. Interrogation of these questions will directly impact our knowledge of skeletal development and bone biology, but also have important implications for the isolation and use of MSCs in tissue engineering and regenerative medicine approaches.

PROJECT SUMMARY Hijacking of normal developmental programs is a common mechanism of tumorigenesis and deregulation of developmental HOX programs, in particular, contributes to the pathogenesis of leukemia, as well as some solid tumors. We recently discovered that HOX gene expression is highly deregulated in Ewing sarcoma, an aggressive bone and soft tissue tumor that has a peak incidence in adolescence. Specifically, the posterior HOX genes, HOXD10, HOXD11 and HOXD13, are over-expressed compared to other normal and malignant tissues, and down-regulation of HOXD13 in Ewing sarcoma cell lines results in a dramatic decrease in tumorigenicity. The expression of all Hox genes normally becomes restricted to distinct regions of the developing musculoskeletal (MSK) system during embryonic development, with the posterior genes becoming restricted to the lumbosacral vertebrae, pelvis and developing forelimb and hindlimb ? the dominant sites of Ewing sarcoma presentation clinically. Recent work from the Wellik laboratory has shown that Hox-expressing cells arise as mesenchymal progenitor/stem cells (MSCs) during embryonic development and persist as regionally-restricted MSCs through postnatal and adult stages. MSCs have been implicated as putative cells of origin for Ewing sarcoma, but the precise identity of the target cells, the molecular mechanisms of malignant transformation and a clear explanation of the region-specific etiology of this tumor type remain poorly understood. Important for this application, the Lawlor lab has shown that ectopic expression of the Ewing sarcoma driver oncogene, EWS/FLI1, in bulk populations of MSCs initiates tumorigenesis and disrupts HOX gene expression as a result of epigenetic deregulation. These two key findings lead us to the novel hypothesis that EWS/FLI1-dependent disruption of the posterior HOX program in MSCs where these genes are developmentally and regionally expressed (i.e. the posterior skeleton) is central to Ewing sarcoma tumorigenesis. We will determine the efficiency and latency of EWS/FLI1-induced tumor formation in Hox-expressing MSCs that are isolated from different anatomic sites and test the hypothesis that oncogenic transformation occurs more readily in MSC populations where restricted, posterior Hox gene expression has been established. We will use Hoxa11eGFP reporter mice to monitor ectopic induction of the posterior Hox program in transduced cells and their resultant tumors. Use of MSCs from control and Hox mutant mouse lines will allow us to determine if posterior Hox genes are functionally required for the transformation process in vivo as our in vitro data suggest. With the biochemical, molecular and in vivo genetic tools the two labs have developed, successful demonstration of this novel hypothesis during this funding period will provide the basis for future highly impactful and mechanistic dissection of the etiology of Ewing's sarcoma, including a molecular basis for understanding the regional specificity of this tumor in vivo.

ABSTRACT Multipotent mesenchymal stem/stromal cells (MSCs) are used in a large number of regenerative/reparative clinical applications and tissue engineering approaches. Translational outcomes from the use of these cells in various contexts vary widely and generating new musculoskeletal tissue that is both functional and able to integrate appropriately in vivo remains a major challenge. Recent research from several laboratories have refined and advanced the field's ability to enrich for MSCs/stromal cells with high progenitor potential, but a recent study from my laboratory reveals that MSCs from the bone marrow of different bones of the adult skeleton maintain the regionally restricted, unique Hox expression profile that is established during development (Rux, et al., Dev. Cell, 2016). Hox expression is only observed in progenitor-enriched MSC populations (PDGFR?+/CD51+, LepR-Cre labeled) and is not detected in differentiated skeletal cell types or in non-progenitor enriched (LepR-negative) non- endothelial stroma. Further, fracture repair studies demonstrate that Hox11 genes function in the central limb regions (zeugopod: radius/ulna, tibia/fibula) in adult MSC populations and loss of Hox11 function leads to the inability of MSC progenitors to differentiate towards cartilage and bone (but has no effect on adipose differentiation). This leads to the broader question of whether unique Hox expression patterns in MSCs are a by-product of development and function via a common mechanism after developmental patterning stages to promote general MSC potential. Alternatively, the much more interesting and biologically significant possibility is that unique Hox genes or paralogs may function differentially in spatially distinct MSCs to provide region-specific regulatory information during growth, maintenance and repair through adult stages. This has critical implications for the use of MSCs in musculoskeletal tissue engineering and regenerative approaches as the unique functional and regulatory characteristics of MSCs may differ considerably depending on their origin; this could profoundly impact their performance in clinical and tissue engineering applications.

ABSTRACT Alveologenesis occurs during postnatal development in humans and mice and this process allows for the growth of gas exchange surface area of the lung. One of the key events during this phase is the establishment of the elastin-based matrix in the distal airway. The incorporation of elastin into the existing lung matrix provide the elasticity that allows the distal airways to stretch and recoil effectively during breathing and mesodermally-derived SMA+ lung fibroblasts are key drivers of this process. Our previously published work has shown that the Hox5 genes are exclusively expressed in the mesenchyme of the lung and that loss of all three Hox5 genes leads to early, severe developmental lung defects and neonatal death. Four-allele, compound Hox5 mutant mice (Hox5 AabbCc) are born in Mendelian ratios and are phenotypically normal at birth, however, they develop alveolar simplification at postnatal stages. Consistent with a direct role for Hox5 genes in alveologenesis, the expression levels of all three Hox5 genes are highest during early postnatal stages when the bulk of alveologenesis occurs, higher than observed at any embryonic stage and these genes remain expressed through adult life. Using a newly generated conditional allele for Hoxa5, we show that conditional deletion of Hoxa5 in the lung mesenchyme beginning at birth results in an alveolar simplification phenotype postnatally. Hox5 mutant animals exhibit abnormal myofibroblast morphology and impaired function. Hox5 mutant fibroblasts demonstrate defects in cell adhesion and the expression of Integrin ?5 and ?1 are down- regulated. The continuing importance of Hox5 function at all stages is highlighted by surprising preliminary evidence that deletion of Hoxa5 at later stages (after the establishment of the elastin-based matrix) leads to rapid loss of the integrity of the elastin matrix. In this proposal, we will interrogate the cellular and molecular mechanisms of Hox5 regulation of lung mesenchyme during alveolar development, remodeling and homeostasis.

ABSTRACT Alveologenesis occurs during postnatal development in humans and mice and this process allows for the growth of gas exchange surface area of the lung. One of the key events during this phase is the establishment of the elastin-based matrix in the distal airway. The incorporation of elastin into the existing lung matrix provide the elasticity that allows the distal airways to stretch and recoil effectively during breathing and mesodermally-derived SMA+ lung fibroblasts are key drivers of this process. Our previously published work has shown that the Hox5 genes are exclusively expressed in the mesenchyme of the lung and that loss of all three Hox5 genes leads to early, severe developmental lung defects and neonatal death. Four-allele, compound Hox5 mutant mice (Hox5 AabbCc) are born in Mendelian ratios and are phenotypically normal at birth, however, they develop alveolar simplification at postnatal stages. Consistent with a direct role for Hox5 genes in alveologenesis, the expression levels of all three Hox5 genes are highest during early postnatal stages when the bulk of alveologenesis occurs, higher than observed at any embryonic stage and these genes remain expressed through adult life. Using a newly generated conditional allele for Hoxa5, we show that conditional deletion of Hoxa5 in the lung mesenchyme beginning at birth results in an alveolar simplification phenotype postnatally. Hox5 mutant animals exhibit abnormal myofibroblast morphology and impaired function. Hox5 mutant fibroblasts demonstrate defects in cell adhesion and the expression of Integrin ?5 and ?1 are down- regulated. The continuing importance of Hox5 function at all stages is highlighted by surprising preliminary evidence that deletion of Hoxa5 at later stages (after the establishment of the elastin-based matrix) leads to rapid loss of the integrity of the elastin matrix. In this proposal, we will interrogate the cellular and molecular mechanisms of Hox5 regulation of lung mesenchyme during alveolar development, remodeling and homeostasis.